Advances and challenges in skeletal muscle angiogenesis

Circular RNAs and host genes act synergistically in regulating cellular processes and functions in skeletal myogenesis

Circular RNAs (circRNAs) are post-transcriptional regulators generated from backsplicing of pre-mRNAs of host genes. A major circRNA regulatory mechanism involves microRNA (miRNA) sequestering, relieving miRNA-blocked mRNAs for translation and functions. To investigate possible circRNA-host gene relationship, skeletal myogenesis is chosen as a study model for its developmental importance and for readily available muscle tissues from farm animals for studies at different myogenic stages. This review aims to provide an integrated interpretations on methodologies, regulatory mechanisms and possible host gene-circRNA synergistic functional relationships in skeletal myogenesis, focusing on myoblast differentiation and proliferation, core drivers of muscle formation in myogenesis, while other myogenic processes that play supportive roles in the structure, maintenance and function of muscle tissues are also briefly discussed. On literature review,thirty-two circRNAs derived from thirty-one host genes involved in various myogenic stages are identified; twenty-two (68.6 %) of these circRNAs regulate myogenesis by sequestering miRNAs to engage PI3K/AKT and other signaling pathways while four (12.5 %) are translated into proteins for functions. In circRNA-host gene relationship,ten (32.3 %) host genes are shown to regulate myogenesis,nine (29.0 %) are specific to skeletal muscle functions,and twelve (38.8 %) are linked to skeletal muscle disorders.Our analysis of skeletal myogenesis suggests that circRNAs and host genes act synergistically to regulate cellular functions. Such circRNA-host gene functional synergism may also be found in other major cellular processes. CircRNAs may have evolved later than miRNAs to counteract the suppressive effects of miRNAs and to augment host gene functions to further fine-tune gene regulation.

Charting the Molecular Terrain of Exercise: Energetics, Exerkines, and the Future of Multiomic Mapping

Physical activity plays a fundamental role in human health and disease. Exercise has been shown to improve a wide variety of disease states, and the scientific community is committed to understanding the precise molecular mechanisms that underlie the exquisite benefits. This review provides an overview of molecular responses to acute exercise and chronic training, particularly energy mobilization and generation, structural adaptation, inflammation, and immune regulation. Furthermore, it offers a detailed discussion of known molecular signals and systemic regulators activated during various forms of exercise and their role in orchestrating health benefits. Critically, the increasing use of multiomic technologies is explored with an emphasis on how multiomic and multitissue studies contribute to a more profound understanding of exercise biology. These data inform anticipated future advancement in the field and highlight the prospect of integrating exercise with pharmacology for personalized disease prevention and treatment.

Platelet-derived mitochondria attenuate muscle atrophy following rotator cuff tears in a rat model

BACKGROUND

Rotator cuff tears (RCTs) often result in muscle atrophy, compromising surgical outcomes and recovery. Mitochondrial dysfunction is implicated in this process, suggesting potential for mitochondria-based therapies. This study aimed to investigate the effects of platelet-derived mitochondria (Plt-Mito) administration into the supraspinatus muscle (SSP) following RCTs.

METHODS

Seventy-two male Sprague-Dawley rats were allocated into 3 distinct groups: (1) a sham surgery group, (2) a group with RCTs treated with Plt-Mito, and (3) a group with RCTs treated with phosphate-buffered saline. Treatments were administered every 2 weeks. After 12 weeks, the SSPs were analyzed for wet muscle weight ratio, muscle fiber cross-sectional area, fibrosis, antioxidant activity, mitochondrial markers, capillary density, and mitochondrial structure.

RESULTS

Plt-Mito successfully incorporated into SSP, maintaining functional integrity. Compared to the phosphate-buffered saline group, Plt-Mito treatment significantly preserved wet muscle weight, increased mean muscle fiber cross-sectional area, promoted muscle regeneration, reduced fibrosis, enhanced antioxidant activity (increased superoxide dismutase activity and decreased malondialdehyde activity), improved muscle vascularity (increased platelet endothelial cell adhesion molecule-1 and α-smooth muscle actin), increased expression of mitochondrial markers (C oxidase subunit IV and uncoupling protein 1) and maintained mitochondrial density and structure.

CONCLUSIONS

Our findings demonstrated Plt-Mito administration effectively halted muscle atrophy and fibrosis, while attenuating mitochondrial damage and dysfunction following RCTs.

Nicotinamide mononucleotide restores impaired metabolism, endothelial cell proliferation and angiogenesis in old sedentary male mice

Aging is accompanied by a decline in neovascularization potential and increased susceptibility to ischemic injury. Here, we confirm the age-related impaired neovascularization following ischemic leg injury and impaired angiogenesis. The age-related deficits in angiogenesis arose primarily from diminished EC proliferation capacity, but not migration or VEGF sensitivity. Aged EC harvested from the mouse skeletal muscle displayed a pro-angiogenic gene expression phenotype, along with considerable changes in metabolic genes. Metabolomics analysis and 13C glucose tracing revealed impaired ATP production and blockade in glycolysis and TCA cycle in late passage HUVECs, which occurred at nicotinamide adenine dinucleotide (NAD⁺)-dependent steps, along with NAD+ depletion. Supplementation with nicotinamide mononucleotide (NMN), a precursor of NAD⁺, enhances late-passage EC proliferation and sprouting angiogenesis from aged mice aortas. Taken together, our study illustrates the importance of NAD+-dependent metabolism in the maintenance of EC proliferation capacity with age, and the therapeutic potential of NAD precursors.

Temporal Tissue Remodeling in Volumetric Muscle Injury with Endothelial Cell-Laden Patterned Nanofibrillar Constructs

A primary challenge following severe musculoskeletal trauma is incomplete muscle regeneration. Current therapies often fail to heal damaged muscle due to dysregulated healing programs and insufficient revascularization early in the repair process. There is a limited understanding of the temporal changes that occur during the early stages of muscle remodeling in response to engineered therapies. Previous work demonstrated that nanotopographically patterned scaffolds provide cytoskeletal guidance and direct endothelial angiogenic and anti-inflammatory phenotypes. The aim of this study was to evaluate how endothelial cell (EC) patterning guides temporal and histomorphological muscle remodeling after muscle injury. In the current study, mice were treated with EC-laden engineered constructs that exhibited either aligned or random patterning of collagen nanofibrils, following a volumetric muscle loss injury (VML). Remodeling was evaluated at 2, 7, and 21 days post injury. Over the 21-day study, all groups (Acellular Aligned, EC Aligned, EC Random) demonstrated similar significant increases in vascular density and myogenesis. Animals treated with acellular controls demonstrated a two-fold decrease in muscle cross-sectional area between days 2 and 21 post injury, consistent with VML-induced muscle atrophy; however, animals treated with patterned EC-laden constructs exhibited preservation of muscle mass. The implantation of an EC-laden construct led to a 50% increase in the number of animals exhibiting areas of fibrous remodeling adjacent to the construct, along with greater collagen deposition (p < 0.01) compared to acellular controls 21 days post injury. These findings suggest that nanotopographically patterned EC-laden constructs may guide early muscle-protective programs that support muscle mass retention through myo-vascular independent pathways.

Prazosin improves insulin-induced anabolic signaling by protecting capillary regression in the soleus muscle of hindlimb-unloaded rats

Purpose

Reduced capillary number in skeletal muscle due to disuse can hinder the delivery of insulin and amino acid delivery to muscle cells, diminishing insulin activity and muscle protein synthesis, ultimately contributing to anabolic resistance. However, it remains unknown whether mitigating capillary regression during inactivity improves anabolic resistance. This study aimed to investigate the effect of increasing capillary number through the administration of prazosin, which can increase blood flow and prevent capillary regression, on anabolic resistance in skeletal muscle induced by disuse.

Methods

Male Sprague Dawley rats were divided into control and hindlimb unloading (HU) groups, with half of each group receiving prazosin (50 mg/L) in their drinking water for 2 weeks. Histological analysis of the soleus muscles was conducted to measure the capillary-to-fiber (C/F) ratio, while western blotting was performed to measure the activation of the Akt/mTORC1 muscle protein synthesis pathway before and after insulin stimulation.

Results

The C/F ratios were significantly lower in the HU and HU + Prz groups than in the control group but were significantly higher in the HU + Prz group than in the HU group. Following insulin stimulation, the phosphorylation levels of Akt, p70S6K, and S6RP increased in all groups, with a significantly greater increase observed in the HU + Prz group compared to the HU group, indicating improved molecular signaling related to muscle protein synthesis.

Conclusion

Administration of prazosin during hindlimb unloading mitigated capillary regression and enhanced insulin-stimulated muscle protein synthesis response. These findings suggest that enhancing capillary number may reduce the anabolic resistance caused by muscle disuse.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-024-01454-y.

The Potential Interplay Between HIF-1α, Angiogenic, and Autophagic Signaling During Intermittent Hypoxic Exposure and Exercise

Berkemeier, Quint N., Michael R. Deyhle, James J. McCormick, Kurt A. Escobar, and Christine M. Mermier. The potential interplay between HIF-1α, angiogenic, and autophagic signaling during intermittent hypoxic exposure and exercise High Alt Med Biol. 25:326-336, 2024.-Environmental hypoxia as a result of decreased barometric pressure upon ascent to high altitudes (>2,500 m) presents increased physiological demands compared with low altitudes, or normoxic environments. Competitive athletes, mountaineers, wildland firefighters, military personnel, miners, and outdoor enthusiasts commonly participate in, or are exposed to, forms of exercise or physical labor at moderate to high altitudes. However, the majority of research on intermittent hypoxic exposure is centered around hematological markers, and the skeletal muscle cellular responses to exercise in hypoxic environments remain largely unknown. Two processes that may be integral for the maintenance of cellular health in skeletal muscle include angiogenesis, or the formation of new blood vessels from preexisting vasculature and autophagy, a process that removes and recycles damaged and dysfunctional cellular material in the lysosome. The purpose of this review is to is to examine the current body of literature and highlight the potential interplay between low-oxygen-sensing pathways, angiogenesis, and autophagy during acute and prolonged intermittent hypoxic exposure in conjunction with exercise. The views expressed in this paper are those of the authors and do not reflect the official policy of the Department of Army, DOD, DOE, ORAU/ORISE or U.S. Government.

Diet and exercise in frailty and sarcopenia. Molecular aspects

Function declines throughout life although phenotypical manifestations in terms of frailty or disability are only seen in the later periods of our life. The causes underlying lifelong function decline are the aging process "per se", chronic diseases, and lifestyle factors. These three etiological causes result in the deterioration of several organs and systems which act synergistically to finally produce frailty and disability. Regardless of the causes, the skeletal muscle is the main organ affected by developing sarcopenia. In the first section of the manuscript, as an introduction, we review the quantitative and qualitative age-associated skeletal muscle changes leading to frailty and sarcopenia and their impact in the quality of life and independence in the elderly. The reversibility of frailty and sarcopenia are discussed in the second and third sections of the manuscript. The most effective intervention to delay and even reverse frailty is exercise training. We review the role of different training programs (resistance exercise, cardiorespiratory exercise, multicomponent exercise, and real-life interventions) not only as a preventive but also as a therapeutical strategy to promote healthy aging. We also devote a section in the text to the sexual dimorphic effects of exercise training interventions in aging. How to optimize the skeletal muscle anabolic response to exercise training with nutrition is also discussed in our manuscript. The concept of anabolic resistance and the evidence of the role of high-quality protein, essential amino acids, creatine, vitamin D, β-hydroxy-β-methylbutyrate, and Omega-3 fatty acids, is reviewed. In the last section of the manuscript, the main genetic interventions to promote robustness in preclinical models are discussed. We aim to highlight the molecular pathways that are involved in frailty and sarcopenia. The possibility to effectively target these signaling pathways in clinical practice to delay muscle aging is also discussed.

Leg immobilization and subsequent recovery resistance training affect skeletal muscle angiogenesis related markers in young healthy adults regardless of prior resistance training experience

We recently reported that resistance trained (T, n=10) and untrained (UT, n=11) young adults experience vastus lateralis (VL) muscle atrophy following two weeks of disuse, and 8 weeks of recovery resistance training (RT) promotes VL hypertrophy in both participant cohorts. However, angiogenesis targets and muscle capillary number were not examined and currently no human studies that have sought to determine if disuse followed by recovery RT affects these outcomes. Thus, we examined whether disuse and/or recovery RT affected these outcomes. All participants underwent two weeks of left leg immobilization using locking leg braces and crutches followed by eight weeks (3d/week) of knee extensor focused progressive RT. VL biopsies were obtained at baseline (PRE), immediately after disuse (MID), and after RT (POST). Western blotting was used to assay angiogenesis markers and immunohistochemistry was performed in 16/21 participants to determine type I and II muscle fiber capillary number. Significant main effects of time (p<0.05) were observed for protein levels of VEGF (MID0.100). Although disuse and recovery RT affect skeletal muscle angiogenesis-related protein targets, prior training history does not differentially affect these outcomes.

NEW AND NOTEWORTHY

This is the first study to examine how limb immobilization and recovery resistance training affect molecular outcomes related to angiogenesis in younger adults with or without a prior training history. Regardless of resistance training history, the molecular responses are largely similar between participant cohorts and is suggestive of a reduced (pre-mid) and increased (mid-post) angiogenic response, with disuse and subsequent recovery resistance training.

Computational Insights into the Interplay of Mechanical Forces in Angiogenesis

This study employs a meshless computational model to investigate the impacts of compression and traction on angiogenesis, exploring their effects on vascular endothelial growth factor (VEGF) diffusion and subsequent capillary network formation. Three distinct initial domain geometries were defined to simulate variations in endothelial cell sprouting and VEGF release. Compression and traction were applied, and the ensuing effects on VEGF diffusion coefficients were analysed. Compression promoted angiogenesis, increasing capillary network density. The reduction in the VEGF diffusion coefficient under compression altered VEGF concentration, impacting endothelial cell migration patterns. The findings were consistent across diverse simulation scenarios, demonstrating the robust influence of compression on angiogenesis. This computational study enhances our understanding of the intricate interplay between mechanical forces and angiogenesis. Compression emerges as an effective mediator of angiogenesis, influencing VEGF diffusion and vascular pattern. These insights may contribute to innovative therapeutic strategies for angiogenesis-related disorders, fostering tissue regeneration and addressing diseases where angiogenesis is crucial.

Making the case for resistance training in improving vascular function and skeletal muscle capillarization

Through decades of empirical data, it has become evident that resistance training (RT) can improve strength/power and skeletal muscle hypertrophy. Yet, until recently, vascular outcomes have historically been underemphasized in RT studies, which is underscored by several exercise-related reviews supporting the benefits of endurance training on vascular measures. Several lines of evidence suggest large artery diameter and blood flow velocity increase after a single bout of resistance exercise, and these events are mediated by vasoactive substances released from endothelial cells and myofibers (e.g., nitric oxide). Weeks to months of RT can also improve basal limb blood flow and arterial diameter while lowering blood pressure. Although several older investigations suggested RT reduces skeletal muscle capillary density, this is likely due to most of these studies being cross-sectional in nature. Critically, newer evidence from longitudinal studies contradicts these findings, and a growing body of mechanistic rodent and human data suggest skeletal muscle capillarity is related to mechanical overload-induced skeletal muscle hypertrophy. In this review, we will discuss methods used by our laboratories and others to assess large artery size/function and skeletal muscle capillary characteristics. Next, we will discuss data by our groups and others examining large artery and capillary responses to a single bout of resistance exercise and chronic RT paradigms. Finally, we will discuss RT-induced mechanisms associated with acute and chronic vascular outcomes.

Treatment with Cobra Venom Factor Decreases Ischemic Tissue Damage in Mice

Tissue ischemia, caused by the blockage of blood vessels, can result in substantial damage and impaired tissue performance. Information regarding the functional contribution of the complement system in the context of ischemia and angiogenesis is lacking. To investigate the influence of complement activation and depletion upon femoral artery ligation (FAL), Cobra venom factor (CVF) (that functionally resembles C3b, the activated form of complement component C3) was applied in mice in comparison to control mice. Seven days after induction of muscle ischemia through FAL, gastrocnemius muscles of mice were excised and subjected to (immuno-)histological analyses. H&E and apoptotic cell staining (TUNEL) staining revealed a significant reduction in ischemic tissue damage in CVF-treated mice compared to controls. The control mice, however, exhibited a significantly higher capillary-to-muscle fiber ratio and a higher number of proliferating endothelial cells (CD31+/CD45-/BrdU+). The total number of leukocytes (CD45+) substantially decreased in CVF-treated mice versus control mice. Moreover, the CVF-treated group displayed a shift towards the M2-like anti-inflammatory and regenerative macrophage phenotype (CD68+/MRC1+). In conclusion, our findings suggest that treatment with CVF leads to reduced ischemic tissue damage along with decreased leukocyte recruitment but increased numbers of M2-like polarized macrophages, thereby enhancing tissue regeneration, repair, and healing.

Plxnd1-mediated mechanosensing of blood flow controls the caliber of the Dorsal Aorta via the transcription factor Klf2

The cardiovascular system generates and responds to mechanical forces. The heartbeat pumps blood through a network of vascular tubes, which adjust their caliber in response to the hemodynamic environment. However, how endothelial cells in the developing vascular system integrate inputs from circulatory forces into signaling pathways to define vessel caliber is poorly understood. Using vertebrate embryos and in vitro-assembled microvascular networks of human endothelial cells as models, flow and genetic manipulations, and custom software, we reveal that Plexin-D1, an endothelial Semaphorin receptor critical for angiogenic guidance, employs its mechanosensing activity to serve as a crucial positive regulator of the Dorsal Aorta's (DA) caliber. We also uncover that the flow-responsive transcription factor KLF2 acts as a paramount mechanosensitive effector of Plexin-D1 that enlarges endothelial cells to widen the vessel. These findings illuminate the molecular and cellular mechanisms orchestrating the interplay between cardiovascular development and hemodynamic forces.

Peripheral limitations for performance: Muscle capillarization

Sufficient delivery of oxygen and metabolic substrates, together with removal of waste products, are key elements of muscle performance. Capillaries are the primary site for this exchange in skeletal muscle and the degree of muscle capillarization affects diffusion conditions by influencing mean transit time, capillary surface area and diffusion distance. Muscle capillarization may thus represent a limiting factor for performance. Exercise training increases the number of capillaries per muscle fiber by about 10%-20% within a few weeks in untrained subjects, whereas capillary growth progresses more slowly in well-trained endurance athletes. Studies show that capillaries are tortuous, situated along and across the length of the fibers with an arrangement related to muscle fascicles. Although direct data is lacking, it is possible that years of training not only enhances capillary density but also optimizes the positioning of capillaries, to further improve the diffusion conditions. Muscle capillarization has been shown to increase oxygen extraction during exercise in humans, but direct evidence for a causal link between increased muscle capillarization and performance is scarce. This review covers current knowledge on the implications of muscle capillarization for oxygen and glucose uptake as well as performance. A brief overview of the process of capillary growth and of physical factors, inherent to exercise, which promote angiogenesis, provides the foundation for a discussion on how different training modalities may influence muscle capillary growth. Finally, we identify three areas for future research on the role of capillarization for exercise performance.

Perfusability and immunogenicity of implantable pre-vascularized tissues recapitulating features of native capillary network

Vascularization is a key pre-requisite to engineered anatomical scale three dimensional (3-D) constructs to ensure their nutrient and oxygen supply upon implantation. Presently, engineered pre-vascularized 3-D tissues are limited to only micro-scale hydrogels, which meet neither the anatomical scale needs nor the complexity of natural extracellular matrix (ECM) environments. Anatomical scale perfusable constructs are critically needed for translational applications. To overcome this challenge, we previously developed pre-vascularized ECM sheets with long and oriented dense microvascular networks. The present study further evaluated the patency, perfusability and innate immune response toward these pre-vascularized constructs. Macrophage-co-cultured pre-vascularized constructs were evaluated in vitro to confirm micro-vessel patency and perturbations in macrophage metabolism. Subcutaneously implanted pre-vascularized constructs remained viable and formed a functional anastomosis with host vasculature within 3 days of implantation. This completely biological pre-vascularized construct holds great potential as a building block to engineer perfusable anatomical scale tissues.

Pharmacological modulation of Sonic Hedgehog signaling pathways in Angiogenesis: A mechanistic perspective

The Sonic hedgehog (SHh) signaling pathway is an imperative operating network that helps in regulates the critical events during the development processes like multicellular embryo growth and patterning. Disruptions in SHh pathway regulation can have severe consequences, including congenital disabilities, stem cell renewal, tissue regeneration, and cancer/tumor growth. Activation of the SHh signal occurs when SHh binds to the receptor complex of Patch (Ptc)-mediated Smoothened (Smo) (Ptc-smo), initiating downstream signaling. This review explores how pharmacological modulation of the SHh pathway affects angiogenesis through canonical and non-canonical pathways. The canonical pathway for angiogenesis involves the activation of angiogenic cytokines such as fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), placental growth factor (PGF), hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF), stromal cell-derived factor 1α, transforming growth factor-β1 (TGF-β1), and angiopoietins (Ang-1 and Ang-2), which facilitate the process of angiogenesis. The Non-canonical pathway includes indirect activation of certain pathways like iNOS/Netrin-1/PKC, RhoA/Rock, ERK/MAPK, PI3K/Akt, Wnt/β-catenin, Notch signaling pathway, and so on. This review will provide a better grasp of the mechanistic approach of SHh in mediating angiogenesis, which can aid in the suppression of certain cancer and tumor growths.

Skeletal muscle and heart failure - What is the relationship between central versus peripheral affections?

BACKGROUND AND AIM

Heart failure is considered as a systemic disease as beside the heart, skeletal muscle is affected.

METHODS AND RESULTS

In this retrospective case-control study 64 men and 15 women with heart failure as well as an individually pairwise matched sample by sex, age and body mass index of healthy individuals from the COmPLETE cohort study performed an exhaustive cardiopulmonary exercise test, strength tests and anthropometric measurements. V̇O2peak was 28.6% lower in male and 24.6% lower in female patients with heart failure as compared to healthy controls. Strength parameters are significantly higher for counter movement jump in male subjects. In females, significant differences were detected for mid-thigh pull in healthy versus patients with heart failure. Skeletal muscle mass of patients was in male as well as female 3.7% lower than in controls. Furthermore, the function of skeletal muscle seems impaired as the ability to accelerate is significantly lower in affected male with a heart pathology.

CONCLUSION

It seems that severe affections (approx. 25 to 30%) on cardiocirculatory level are associated with moderate to low affections on functional and structural capacity on skeletal muscle level. Further, as in the male cohort with a heart pathology acceleration meaning 'fast' contracting is impaired, it is suggested, that the central limitations respectively the low perfusion of skeletal muscle over years yield to adaptions on muscle cell level ingoing with a decreased ability of fast contracting. It is therefore suggested, that the central circulatory limitations in patients with heart failure, respectively the low perfusion of skeletal muscle over years, promote maladaptation's in the periphery.

Diabetes diminishes muscle precursor cell-mediated microvascular angiogenesis

The skeletal muscles of Type II diabetic (T2D) patients can be characterized by a reduced vessel density, corresponding to deficiencies in microvascular angiogenesis. Interestingly, T2D also inhibits the function of many myogenic cells resident within skeletal muscle, including satellite cells, which are well-known for the role they play in maintaining homeostasis. The current study was undertaken to gain a better understanding of the mechanisms whereby satellite cell progeny, muscle precursor cells (MPCs), influence microvascular angiogenesis. Network growth and the expression of genes associated with angiogenesis were reduced when microvessels were treated with conditioned media generated by proliferating MPCs isolated from diabetic, as compared to control rat skeletal muscle, a phenomenon that was also observed when myoblasts from control or diabetic human skeletal muscle were used. When only exosomes derived from diabetic or control MPCs were used to treat microvessels, no differences in microvascular growth were observed. An evaluation of the angiogenesis factors in control and diabetic MPCs revealed differences in Leptin, vascular endothelial growth factor (VEGF), IL1-β, interleukin 10, and IP-10, and an evaluation of the MPC secretome revealed differences in interleukin 6, MCP-1, VEGF, and interleukin 4 exist. Angiogenesis was also reduced in tissue-engineered skeletal muscles (TE-SkM) containing microvessels when they were generated from MPCs isolated from diabetic as compared to control skeletal muscle. Lastly, the secretome of injured control, but not diabetic, TE-SkM was able to increase VEGF and increase microvascular angiogenesis. This comprehensive analysis of the interaction between MPCs and microvessels in the context of diabetes points to an area for alleviating the deleterious effects of diabetes on skeletal muscle.

Kinin receptors regulate skeletal muscle regeneration: differential effects for B1 and B2 receptors

OBJECTIVE AND DESIGN

After traumatic skeletal muscle injury, muscle healing is often incomplete and produces extensive fibrosis. Bradykinin (BK) reduces fibrosis in renal and cardiac damage models through the B2 receptor. The B1 receptor expression is induced by damage, and blocking of the kallikrein-kinin system seems to affect the progression of muscular dystrophy. We hypothesized that both kinin B1 and B2 receptors could play a differential role after traumatic muscle injury, and the lack of the B1 receptor could produce more cellular and molecular substrates for myogenesis and fewer substrates for fibrosis, leading to better muscle healing.

MATERIAL AND METHODS

To test this hypothesis, tibialis anterior muscles of kinin receptor knockout animals were subjected to traumatic injury. Myogenesis, angiogenesis, fibrosis, and muscle functioning were evaluated.

RESULTS

Injured B1KO mice showed a faster healing progression of the injured area with a larger amount of central nucleated fiber post-injury when compared to control mice. In addition, they exhibited higher neovasculogenic capacity, maintaining optimal tissue perfusion for the post-injury phase; had higher amounts of myogenic markers with less inflammatory infiltrate and tissue destruction. This was followed by higher amounts of SMAD7 and lower amounts of p-SMAD2/3, which resulted in less fibrosis. In contrast, B2KO and B1B2KO mice showed more severe tissue destruction and excessive fibrosis. B1KO animals had better results in post-injury functional tests compared to control animals.

CONCLUSIONS

We demonstrate that injured skeletal muscle tissues have a better repair capacity with less fibrosis in the presence of B2 receptor and absence of B1 receptor, including better performances in functional tests.

Microvascular Skeletal-Muscle Crosstalk in Health and Disease

As an organ system, skeletal muscle is essential for the generation of energy that underpins muscle contraction, plays a critical role in controlling energy balance and insulin-dependent glucose homeostasis, as well as vascular well-being, and regenerates following injury. To achieve homeostasis, there is requirement for "cross-talk" between the myogenic and vascular components and their regulatory factors that comprise skeletal muscle. Accordingly, this review will describe the following: [a] the embryonic cell-signaling events important in establishing vascular and myogenic cell-lineage, the cross-talk between endothelial cells (EC) and myogenic precursors underpinning the development of muscle, its vasculature and the satellite-stem-cell (SC) pool, and the EC-SC cross-talk that maintains SC quiescence and localizes ECs to SCs and angio-myogenesis postnatally; [b] the vascular-myocyte cross-talk and the actions of insulin on vasodilation and capillary surface area important for the uptake of glucose/insulin by myofibers and vascular homeostasis, the microvascular-myocyte dysfunction that characterizes the development of insulin resistance, diabetes and hypertension, and the actions of estrogen on muscle vasodilation and growth in adults; [c] the role of estrogen in utero on the development of fetal skeletal-muscle microvascularization and myofiber hypertrophy required for metabolic/vascular homeostasis after birth; [d] the EC-SC interactions that underpin myofiber vascular regeneration post-injury; and [e] the role of the skeletal-muscle vasculature in Duchenne muscular dystrophy.

Skeletal muscle adaptation to indirect electrical stimulation: divergence between microvascular and metabolic adaptations

NEW FINDINGS

What is the central question of this study? Can we manipulate muscle recruitment to differentially enhance skeletal muscle fatigue resistance? What is the main finding and its importance? Through manipulation of muscle activation patterns, it is possible to promote distinct microvascular growth. Enhancement of fatigue resistance is closely associated with the distribution of the capillaries within the muscle, not necessarily with quantity. Additionally, at the acute stages of remodelling in response to indirect electrical stimulation, the improvement in fatigue resistance appears to be primarily driven by vascular remodelling, with metabolic adaptation of secondary importance.

ABSTRACT

Exercise involves a complex interaction of factors influencing muscle performance, where variations in recruitment pattern (e.g., endurance vs. resistance training) may differentially modulate the local tissue environment (i.e., oxygenation, blood flow, fuel utilization). These exercise stimuli are potent drivers of vascular and metabolic change. However, their relative contribution to adaptive remodelling of skeletal muscle and subsequent performance is unclear. Using implantable devices, indirect electrical stimulation (ES) of locomotor muscles of rat at different pacing frequencies (4, 10 and 40 Hz) was used to differentially recruit hindlimb blood flow and modulate fuel utilization. After 7 days, ES promoted significant remodelling of microvascular composition, increasing capillary density in the cortex of the tibialis anterior by 73%, 110% and 55% for the 4 Hz, 10 and 40 Hz groups, respectively. Additionally, there was remodelling of the whole muscle metabolome, including significantly elevated amino acid turnover, with muscle kynurenic acid levels doubled by pacing at 10 Hz (P < 0.05). Interestingly, the fatigue index of skeletal muscle was only significantly elevated in 10 Hz (58% increase) and 40 Hz (73% increase) ES groups, apparently linked to improved capillary distribution. These data demonstrate that manipulation of muscle recruitment pattern may be used to differentially expand the capillary network prior to altering the metabolome, emphasising the importance of local capillary supply in promoting exercise tolerance.

nNOS Increases Fiber Type-Specific Angiogenesis in Skeletal Muscle of Mice in Response to Endurance Exercise

We studied the relationship between neuronal NO synthase (nNOS) expression and capillarity in the tibialis anterior (TA) muscle of mice subjected to treadmill training. The mRNA (+131%) and protein (+63%) levels of nNOS were higher (p ≤ 0.05) in the TA muscle of C57BL/6 mice undergoing treadmill training for 28 days than in those of littermates remaining sedentary, indicating an up-regulation of nNOS by endurance exercise. Both TA muscles of 16 C57BL/6 mice were subjected to gene electroporation with either the pIRES2-ZsGreen1 plasmid (control plasmid) or the pIRES2-ZsGreen1-nNOS gene-inserted plasmid (nNOS plasmid). Subsequently, one group of mice (n = 8) underwent treadmill training for seven days, while the second group of mice (n = 8) remained sedentary. At study end, 12-18% of TA muscle fibers expressed the fluorescent reporter gene ZsGreen1. Immunofluorescence for nNOS was 23% higher (p ≤ 0.05) in ZsGreen1-positive fibers than ZsGreen1-negative fibers from the nNOS-transfected TA muscle of mice subjected to treadmill training. Capillary contacts around myosin heavy-chain (MHC)-IIb immunoreactive fibers (14.2%; p ≤ 0.05) were only higher in ZsGreen1-positive fibers than ZsGreen1-negative fibers in the nNOS-plasmid-transfected TA muscles of trained mice. Our observations are in line with an angiogenic effect of quantitative increases in nNOS expression, specifically in type-IIb muscle fibers after treadmill training.

Aerobic exercise and scaffolds with hierarchical porosity synergistically promote functional recovery post volumetric muscle loss

Volumetric muscle loss (VML), which refers to a composite skeletal muscle defect, most commonly heals by scarring and minimal muscle regeneration but substantial fibrosis. Current surgical interventions and physical therapy techniques are limited in restoring muscle function following VML. Novel tissue engineering strategies may offer an option to promote functional muscle recovery. The present study evaluates a colloidal scaffold with hierarchical porosity and controlled mechanical properties for the treatment of VML. In addition, as VML results in an acute decrease in insulin-like growth factor 1 (IGF-1), a myogenic factor, the scaffold was designed to slowly release IGF-1 following implantation. The foam-like scaffold is directly crosslinked onto remnant muscle without the need for suturing. In situ 3D printing of IGF-1-releasing porous muscle scaffold onto VML injuries resulted in robust tissue ingrowth, improved muscle repair, and increased muscle strength in a murine VML model. Histological analysis confirmed regeneration of new muscle in the engineered scaffolds. In addition, the scaffolds significantly reduced fibrosis and increased the expression of neuromuscular junctions in the newly regenerated tissue. Exercise training, when combined with the engineered scaffolds, augmented the treatment outcome in a synergistic fashion. These data suggest highly porous scaffolds and exercise therapy, in combination, may be a treatment option following VML.

Limited effect of over-the-counter doses of ibuprofen on mechanisms regulating muscle hypertrophy during resistance training in young adults

We have previously shown that maximal over-the-counter doses of ibuprofen, compared with low doses of acetylsalicylic acid, reduce muscle hypertrophy in young individuals after 8 wk of resistance training. Because the mechanism behind this effect has not been fully elucidated, we here investigated skeletal muscle molecular responses and myofiber adaptations in response to acute and chronic resistance training with concomitant drug intake. Thirty-one young (aged 18-35 yr) healthy men (n = 17) and women (n = 14) were randomized to receive either ibuprofen (IBU; 1,200 mg daily; n = 15) or acetylsalicylic acid (ASA; 75 mg daily; n = 16) while undergoing 8 wk of knee extension training. Muscle biopsies from the vastus lateralis were obtained before, at week 4 after an acute exercise session, and after 8 wk of resistance training and analyzed for mRNA markers and mTOR signaling, as well as quantification of total RNA content (marker of ribosome biogenesis) and immunohistochemical analysis of muscle fiber size, satellite cell content, myonuclear accretion, and capillarization. There were only two treatment × time interaction in selected molecular markers after acute exercise (atrogin-1 and MuRF1 mRNA), but several exercise effects. Muscle fiber size, satellite cell and myonuclear accretion, and capillarization were not affected by chronic training or drug intake. RNA content increased comparably (∼14%) in both groups. Collectively, these data suggest that established acute and chronic hypertrophy regulators (including mTOR signaling, ribosome biogenesis, satellite cell content, myonuclear accretion, and angiogenesis) were not differentially affected between groups and therefore do not explain the deleterious effects of ibuprofen on muscle hypertrophy in young adults.NEW & NOTEWORTHY Here we show that mTOR signaling, fiber size, ribosome biogenesis, satellite cell content, myonuclear accretion, and angiogenesis were not differentially affected between groups undergoing 8 wk of resistance training with concomitant anti-inflammatory medication (ibuprofen versus low-dose aspirin). Atrogin-1 and MuRF-1 mRNA were more downregulated after acute exercise in the low-dose aspirin group than in the ibuprofen group. Taken together it appears that these established hypertrophy regulators do not explain the previously reported deleterious effects of high doses of ibuprofen on muscle hypertrophy in young adults.

Peroxisome proliferator-activated receptor γ coactivator 1-α overexpression improves angiogenic signalling potential of skeletal muscle-derived extracellular vesicles

NEW FINDINGS

What is the central question of this study? Skeletal muscle extracellular vesicles likely act as pro-angiogenic signalling factors: does overexpression of peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) alter skeletal muscle myotube extracellular vesicle release, contents and angiogenic potential? What is the main finding and its importance? Overexpression of PGC-1α results in secretion of extracellular vesicles that elevate measures of angiogenesis and protect against acute oxidative stress in vitro. Skeletal muscle with high levels of PGC-1α expression, commonly associated with exercise induced angiogenesis and high basal capillarization, may secrete extracellular vesicles that support capillary growth and maintenance.

ABSTRACT

Skeletal muscle capillarization is proportional to muscle fibre mitochondrial content and oxidative capacity. Skeletal muscle cells secrete many factors that regulate neighbouring capillary endothelial cells (ECs), including extracellular vesicles (SkM-EVs). Peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) regulates mitochondrial biogenesis and the oxidative phenotype in skeletal muscle. Skeletal muscle PGC-1α also regulates secretion of multiple angiogenic factors, but it is unknown whether PGC-1α regulates SkM-EV release, contents and angiogenic signalling potential. PGC-1α was overexpressed via adenovirus in primary human myotubes. EVs were collected from PGC-1α-overexpressing myotubes (PGC-EVs) as well as from green fluorescent protein-overexpressing myotubes (GFP-EVs), and from untreated myotubes. EV release and select mRNA contents were measured from EVs. Additionally, ECs were treated with EVs to measure angiogenic potential of EVs in normal conditions and following an oxidative stress challenge. PGC-1α overexpression did not impact EV release but did elevate EV content of mRNAs for several antioxidant proteins (nuclear factor erythroid 2-related factor 2, superoxide dismutase 2, glutathione peroxidase). PGC-EV treatment of cultured human umbilical vein endothelial cells (HUVECs) increased their proliferation (+36.6%), tube formation (length: +28.1%; number: +25.7%) and cellular viability (+52.9%), and reduced reactive oxygen species levels (-41%) compared to GFP-EVs. Additionally, PGC-EV treatment protected against tube formation impairments and induction of cellular senescence following acute oxidative stress. Overexpression of PGC-1α in human myotubes increases the angiogenic potential of SkM-EVs. These angiogenic benefits coincided with increased anti-oxidative capacity of recipient HUVECs. High PGC-1α expression in skeletal muscle may prompt the release of SkM-EVs that support vascular redox homeostasis and angiogenesis.

Endothelial-Specific Expression of CIDEC Improves High-Fat Diet-Induced Vascular and Metabolic Dysfunction

Cell death-inducing DNA fragmentation factor-α-like effector C (CIDEC), originally identified to be a lipid droplet-associated protein in adipocytes, positively associates with insulin sensitivity. Recently, we discovered that it is expressed abundantly in human endothelial cells and regulates vascular function. The current study was designed to characterize the physiological effects and molecular actions of endothelial CIDEC in the control of vascular phenotype and whole-body glucose homeostasis. To achieve this, we generated a humanized mouse model expressing endothelial-specific human CIDEC (E-CIDECtg). E-CIDECtg mice exhibited protection against high-fat diet-induced glucose intolerance, insulin resistance, and dyslipidemia. Moreover, these mice displayed improved insulin signaling and endothelial nitric oxide synthase activation, enhanced endothelium-dependent vascular relaxation, and improved vascularization of adipose tissue, skeletal muscle, and heart. Mechanistically, we identified a novel interplay of CIDEC-vascular endothelial growth factor A (VEGFA)-vascular endothelial growth factor receptor 2 (VEGFR2) that reduced VEGFA and VEGFR2 degradation, thereby increasing VEGFR2 activation. Overall, our results demonstrate a protective role of endothelial CIDEC against obesity-induced metabolic and vascular dysfunction, in part, by modulation of VEGF signaling. These data suggest that CIDEC may be investigated as a potential future therapeutic target for mitigating obesity-related cardiometabolic disease.

Low-Load Resistance Exercise with Blood Flow Restriction Increases Hypoxia-Induced Angiogenic Genes Expression

The aim of the study was to determine whether low-load exercise (LL) with blood flow restriction (LL-BFR) would induce similar changes in expression of genes involved in hypoxia and angiogenesis compared to LL and high-load exercise (HL). Twenty-four males (age: 21.3 ± 1.9 years, body height: 1.74 ± 0.8 m, body mass: 73 ± 1.8 kg) were allocated into three groups: low-load exercise (LL), low-load exercise with blood-flow restriction (LL-BFR), and high-load exercise (HL). For the LL-BFR group a pneumatic cuff was inflated at 80% of the arterial occlusion pressure. All participants performed bilateral knee extension exercise, twice a week, for 8 weeks. LL and LL-BFR groups performed 3-4 sets of 15 reps at 20% 1RM, whilst the HL group performed 3-4 sets of 8-10 reps at 80% 1RM with a 60-s rest interval between sets. The hypoxia-inducible factor-1 alpha (HIF-1α) and beta (HIF-1β), vascular endothelial growth factor (VEGF), neuronal (nNOS), and inducible nitric oxide synthase (iNOS) genes expression were assessed before and after training. HIF-1α and HIF-1β mRNA levels significantly increased in the LL-BFR group and exceeded those elicited by HL and LL groups (p < .0001). VEGF gene expression was increased in both LL-BFR and HL groups, however, LL-BFR elicited a greater increase than LL (p < .0001). nNOS and iNOS genes expression significantly increased in all groups with greatest increases being observed in the LL-BFR group (p < .0001). The findings suggest that LL-BFR induces greater increases in genes expression related to hypoxia and angiogenesis than traditional resistance training.

Effects of Exercise Training Intensity and Duration on Skeletal Muscle Capillarization in Healthy Subjects: A Meta-analysis

PURPOSE

This study aimed to investigate the effect of intensity and duration of continuous and interval exercise training on capillarization in skeletal muscle of healthy adults.

METHODS

PubMed and Web of Science were searched from inception to June 2021. Eligibility criteria for studies were endurance exercise training >2 wk in healthy adults, and the capillary to fiber ratio (C:F) and/or capillary density (CD) reported. Meta-analyses were performed, and subsequent subgroup analyses were conducted by the characteristics of participants and training scheme.

RESULTS

Fifty-seven trials from 38 studies were included (10%/90%, athletic/sedentary). C:F was measured in 391 subjects from 47 trials, whereas CD was measured in 428 subjects from 50 trials. Exercise training increased C:F (mean difference, 0.33 (95% confidence interval, 0.30-0.37)) with low heterogeneity ( I2 = 45.08%) and CD (mean difference, 49.8 (36.9-62.6) capillaries per millimeter squared) with moderate heterogeneity ( I2 = 68.82%). Compared with low-intensity training (<50% of maximal oxygen consumption (V̇O 2max )), 21% higher relative change in C:F was observed after continuous moderate-intensity training (50%-80% of V̇O 2max ) and 54% higher change after interval training with high intensity (80%-100% of V̇O 2max ) in sedentary subjects. The magnitude of capillary growth was not dependent on training intervention duration. In already trained subjects, no additional increase in capillarization was observed with various types of training.

CONCLUSIONS

In sedentary subjects, continuous moderate-intensity training and interval training with high intensity lead to increases in capillarization, whereas low-intensity training has less effect. Within the time frame studied, no effect on capillarization was established regarding training duration in sedentary subjects. The meta-analysis highlights the need for further studies in athlete groups to discern if increased capillarization can be obtained, and if so, which combination is optimal (time vs intensity).

Longitudinal analyses of serum neurofilament light and associations with obesity indices and bioelectrical impedance parameters

Neurofilament light is a constituent of the neuronal cytoskeleton and released into the blood following neuro-axonal damage. It has previously been reported that NfL measured in blood serum is inversely related to body mass index. However, no reports exist with regard to body composition assessed using bioelectrical impedance analysis or other indicators of obesity beyond BMI. We analyzed the relationship between sNfL and body composition according to the three compartment model. Additionally, associations between sNfL, body shape index, waist-to-height ratio, and BMI were examined. The sample consisted of 769 participants assessed during the baseline examination and 693 participants examined in the course of the follow-up of the BiDirect Study. Associations between sNfL, BMI, BSI, and WtHR were separately analyzed using linear mixed models. Body compartments operationalized as fat mass, extracellular cell mass, and body cell mass were derived using BIA and the relationship with sNfL was analyzed with a linear mixed model. Lastly, we also analyzed the association between total body water and sNfL. We found significant inverse associations of sNfL with BMI and WtHR. The analysis of the three compartment model yielded significant inverse associations between sNfL, body cell mass and body fat mass, but not extracellular mass. Furthermore, total body water was also inversely related to sNfL. A potential mechanism could involve body cell mass and body fat mass as highly adaptive body constituents that either directly absorb sNfL, or promote the formation of new vasculature and thereby increase blood volume.

Hemodynamic Changes in the Masseter and Superior Orbicularis Oris Muscles before and after Exercise Load: A Comparison between Young Adult Women and Middle-Aged to Old Adult Women

Background

The vascularity index (VI) is useful for measuring the hemodynamics on ultrasound imaging. However, there are no reports concerning the application of the VI to facial muscles.

Objective

The aim of this study was (1) to establish a method of measuring the hemodynamics in facial muscles in a constant way and (2) to evaluate the hemodynamic changes in the masseter and superior orbicularis oris muscles (SOOMs) before and after exercise load in two subject groups of females of different ages.

Methods

(1) The VI in the SOOM was calculated, and the test-retest reliability was assessed in seven healthy adults. (2) The VIs in the left-side masseter and SOOM were calculated in 3 sessions: before exercise loading (T0), immediately after loading (T1), and 5 minutes after T1 (T2) for the young adult group (YAG, n = 20; age range, 20–35 years) and the middle-aged to old group (MOG, n = 20; age range, 50–70 years). Tasks were gum chewing for the masseter muscle and lip sealing for the SOOM. The differences in the mean peak flows between two sessions were examined.

Results

(1) Significant differences were not noted for the repeatedly measured average volumes of blood flow with good test-retest agreement (intraclass correlation coefficient = 0.81). (2) In both muscles of the YAG, there were a significant increase in T1 compared with T0 and a significant decrease in T2 compared with T1 (all p < 0.05). In both muscles of the MOG, no significant differences were noted in either comparison.

Conclusions

A method of measuring the hemodynamics in facial muscles was developed and showed good reliability. Changes in the blood flow after exercise load in these muscles may vary with age in women.

VEGF-A and FGF4 Engineered C2C12 Myoblasts and Angiogenesis in the Chick Chorioallantoic Membrane

Angiogenesis is the formation of new blood vessels from pre-existing vessels. Adequate oxygen transport and waste removal are necessary for tissue homeostasis. Restrictions in blood supply can lead to ischaemia which can contribute to disease pathology. Vascular endothelial growth factor (VEGF) is essential in angiogenesis and myogenesis, making it an ideal candidate for angiogenic and myogenic stimulation in muscle. We established C2C12 mouse myoblast cell lines which stably express elevated levels of (i) human VEGF-A and (ii) dual human FGF4-VEGF-A. Both stably transfected cells secreted increased amounts of human VEGF-A compared to non-transfected cells, with the latter greater than the former. In vitro, conditioned media from engineered cells resulted in a significant increase in endothelial cell proliferation, migration, and tube formation. In vivo, this conditioned media produced a 1.5-fold increase in angiogenesis in the chick chorioallantoic membrane (CAM) assay. Delivery of the engineered myoblasts on Matrigel demonstrated continued biological activity by eliciting an almost 2-fold increase in angiogenic response when applied directly to the CAM assay. These studies qualify the use of genetically modified myoblasts in therapeutic angiogenesis for the treatment of muscle diseases associated with vascular defects.

Hypoxic Training Ameliorates Skeletal Muscle Microcirculation Vascular Function in a Sirt3-Dependent Manner

Hypoxic training improves the microcirculation function of human skeletal muscle, but its mechanism is still unclear. Silent information regulator 2 homolog 3 (Sirt3) can improve mitochondrial function and oxidative status. We aimed to examine the role of Sirt3 in the process of hypoxic training, which affects skeletal muscle microcirculation. C57BL/6 mice were assigned to control (C), hypoxic training (HT), Sirt3 inhibitor 3-(1H-1,2,3-triazol-4-yl) pyridine (3-TYP), and 3-TYP + hypoxic training (3-TYP + HT) groups (n = 6/group). Sirt3 inhibition was induced by intraperitoneal injection of Sirt3 inhibitor 3-TYP. After 6 weeks of intervention, microcirculatory capillary formation and vasomotor capacity were evaluated using immunofluorescence, Western blot, biochemical tests, and transmission electron microscopy (TEM). Laser Doppler flowmetry was used to evaluate skeletal muscle microcirculation blood flow characteristics. Six weeks of hypoxic training enhanced skeletal muscle microcirculation function and increased microcirculatory vasodilation capacity and capillary formation. After the pharmacological inhibition of Sirt3, the reserve capacity of skeletal muscle microcirculation was reduced to varying degrees. After the inhibition of Sirt3, mice completed the same hypoxic training, and we failed to observe the microcirculation function adaptation like that observed in hypoxic training alone. The microcirculation vasodilation and the capillaries number did not improve. Hypoxic training improved skeletal muscle microcirculation vasodilation capacity and increased skeletal muscle microcirculation capillary density. Sirt3 is involved in the adaptation of skeletal muscle microcirculation induced by hypoxic training.

Effects of obesity and acute resistance exercise on skeletal muscle angiogenic communication pathways

NEW FINDINGS

What are the central questions of this study? Do obesity and acute resistance exercise alter the regulation of muscle intercellular communication pathways consistent with inadequate compensatory angiogenesis in response to muscle loading present in individuals with obesity? What is the main finding and its importance? Obesity is associated with differences in both pro- and anti-angiogenic signaling consistent with lower muscle capillarization. Acute resistance exercise increases the release of skeletal muscle small extracellular vesicles independent of body mass. These results identify novel cellular factors associated with impaired angiogenesis in obesity and the positive effects of acute resistance exercise in lean and obese skeletal muscle.

ABSTRACT

Introduction Obesity (OB) impairs cell-to-cell communication signaling. Small extracellular vesicles (EVs), which includes exosomes, are released by skeletal muscle and participate in cell-to-cell communications including the regulation of angiogenesis. Resistance exercise (REx) increases muscle fiber size and capillarization. However, while obesity increases muscle fiber size, there is an inadequate increase in capillarization such that capillary density is reduced. It was hypothesized that REx induced angiogenic signaling and EV biogenesis would be lower with obesity. Methods Sedentary lean (LN) and individuals with obesity (OB) (n = 8/group) performed three sets of single leg, knee extension REx at 80% of maximum. Muscle biopsies were obtained at rest, 15 min, and 3 hr post-exercise and analyzed for angiogenic and EV biogenesis mRNA and protein. Results In OB, muscle fiber size was ∼20% greater and capillary density with type II fibers was ∼25% lower compared to LN (p<0.001) . In response to REx, increased vascular endothelial growth factor (VEGF) mRNA (pro-angiogenic) was similar (3-fold) between groups, while thrombospondin-1 (TSP-1) mRNA (anti-angiogenic) increased ∼2.5-fold in OB only (p = 0.010). miR-130a (pro-angiogenic) was ∼1.4-fold (p = 0.011) and miR-503 (anti-angiogenic) was ∼1.8-fold (p = 0.017) greater in OB compared to LN across all time points. In both groups acute REx decreased the EV surface protein Alix ∼50% consistent with the release of exosomes (p = 0.016). Conclusion Acute resistance exercise appears to induce the release of skeletal muscle small EVs independent of body mass. However, with obesity there is predominantly impaired angiogenic signaling consistent with inadequate angiogenesis in response to basal muscle hypertrophy. This article is protected by copyright. All rights reserved.

Matrix anisotropy promotes angiogenesis in a density-dependent manner

Angiogenesis is necessary for wound healing, tumorigenesis, implant inosculation, and homeostasis. In each situation, matrix structure and mechanics play a role in determining whether new vasculatures can establish transport to new or hypoxic tissues. Neovessel growth and directional guidance are sensitive to three-dimensional (3-D) matrix anisotropy and density, although the individual and integrated roles of these matrix features have not been fully recapitulated in vitro. We developed a tension-based method to align 3-D collagen constructs seeded with microvessel fragments in matrices of three levels of collagen fibril anisotropy and two levels of collagen density. The extent and direction of neovessel growth from the parent microvessel fragments increased with matrix anisotropy and decreased with density. The proangiogenic effects of anisotropy were attenuated at higher matrix densities. We also examined the impact of matrix anisotropy in an experimental model of neovessel invasion across a tissue interface. Matrix density was found to dictate the success of interface crossing, whereas interface curvature and fibril alignment were found to control directional guidance. Our findings indicate that complex configurations of matrix density and alignment can facilitate or complicate the establishment or maintenance of vascular networks in pathological and homeostatic angiogenesis. Furthermore, we extend preexisting methods for tuning collagen anisotropy in thick constructs. This approach addresses gaps in tissue engineering and cell culture by supporting the inclusion of large multicellular structures in prealigned constructs.NEW & NOTEWORTHY Matrix anisotropy and density have a considerable effect on angiogenic vessel growth and directional guidance. However, the current literature relies on 2-D and simplified models of angiogenesis (e.g., tubulogenesis and vasculogenesis). We present a method to align 3-D collagen scaffolds embedded with microvessel fragments to different levels of anisotropy. Neovessel growth increases with anisotropy and decreases with density, which may guide angiogenic neovessels across tissue interfaces such as during implant inosculation and tumorigenesis.

Repeated sprint exercise in hypoxia stimulates HIF-1-dependent gene expression in skeletal muscle

PURPOSE

Our aim was to determine the effect of repeated sprint exercise in hypoxia on HIF-1 and HIF-1-regulated genes involved in glycolysis, mitochondrial turnover and oxygen transport. We also determined whether genes upregulated by exercise in hypoxia were dependent on the activation of HIF-1 in an in vitro model of exercise in hypoxia.

METHODS

Eight endurance athletes performed bouts of repeated sprint exercise in control and hypoxic conditions. Skeletal muscle was sampled pre, post and 3 h post-exercise. HIF-1α protein and HIF1A, PDK1, GLUT4, VEGFA, BNIP3, PINK1 and PGC1A mRNA were measured. C2C12 myotubes were exposed to hypoxia and muscle contraction following treatment with a HIF-1α inhibitor to determine whether hypoxia-sensitive gene expression was dependent on HIF-1α.

RESULTS

Sprint exercise in hypoxia increased HIF-1α protein expression immediately post-exercise [fold change (FC) = 3.5 ± 2.0]. Gene expression of PDK1 (FC = 2.1 ± 1.2), BNIP3 (FC = 2.4 ± 1.4) and VEGFA (FC = 2.7 ± 1.7) increased 3 h post-exercise in hypoxia but not control. PGC1A mRNA increased 3 h post-exercise in control (FC = 5.16) and hypoxia (FC = 5.7 ± 4.1) but there was no difference between the trials. Results from the in vitro experiment showed that hypoxia plus contraction also increased PDK1, BNIP3, and VEGFA gene expression. These responses were inhibited when HIF-1 protein activity was suppressed.

CONCLUSION

Repeated sprint exercise in hypoxia upregulates some genes involved in glycolytic metabolism, mitochondrial turnover, and oxygen transport. HIF-1α is necessary for the expression of these genes in skeletal muscle cells.

The Effect of Blood Flow Restriction Exercise on Angiogenesis-Related Factors in Skeletal Muscle Among Healthy Adults: A Systematic Review and Meta-Analysis

Background

Blood flow restriction (BFR) exercise may be a potential exercise program to promote angiogenesis. This review aims to compare the effects of exercise with and without BFR on angiogenesis-related factors in skeletal muscle among healthy adults.

Methodology

Searches were made in Web of Science, Scopus, PubMed, and EBSCO databases from January 2001 to June 2021. Studies were screened, quality was evaluated, and data were extracted. The review protocol was registered at PROSPERO (PROSPERO registration number: CRD42021261367). Standardized mean differences (SMD) of vascular endothelial growth factor (VEGF), vascular endothelial growth factor receptor 2 (VEGFR-2), hypoxia inducible factor 1α (HIF-1α), peroxisome proliferator-activated receptorγcoactivator-1α (PGC-1α) and endothelial nitric oxide synthase (eNOS) were analyzed using Revman 5.4 software with a 95% confidence interval (95% CI).

Results

Ten studies fulfilled the inclusion criteria with a total of 75 participants for BFR group and 77 for CON group. BFR exercise elicits greater expression of VEGF (heterogeneity test, P = 0.09, I2 = 44%; SMD, 0.93 [0.38, 1.48], P &lt; 0.05), VEGFR-2 (heterogeneity test, P = 0.81, I2 = 0%; SMD, 0.64 [0.08, 1.21], P &lt; 0.05), HIF-1α (heterogeneity test, P = 0.67, I2 = 0%; SMD, 0.43 [0.03, 0.82], P &lt; 0.05), PGC-1α (heterogeneity test, P = 0.02, I2 = 54%; SMD, 0.74 [0.21, 1.28], P &lt; 0.05) and eNOS (heterogeneity test, P = 0.88, I2 = 0%; SMD, 0.60 [0.04, 1.17], P &lt; 0.05) mRNA than non-BFR exercise. In the sub-group analysis, resistance exercise with BFR elicits greater expression of VEGF (heterogeneity test, P = 0.36, I2 = 6%; SMD, 1.66 [0.97, 2.35], P &lt; 0.05) and HIF-1α (heterogeneity test, P = 0.56, I2 = 0%; SMD, 0.51 [0.01, 1.02], P &lt; 0.05) mRNA than aerobic exercise with BFR.

Conclusion

Exercise with BFR elicited more angiogenesis-related factors mRNA expression than exercise without BFR, but not VEGF and PGC-1α protein expression. Therefore, BFR training may be a potential training program to improve vascular function.

Systematic Review Registration

[https://www.crd.york.ac.uk/prospero/], identifier [CRD42021261367].

Pro-angiogenic approach for skeletal muscle regeneration

The angiogenesis process is a phenomenon in which numerous molecules participate in the stimulation of the new vessels' formation from pre-existing vessels. Angiogenesis is a crucial step in tissue regeneration and recovery of organ and tissue function. Muscle diseases affect millions of people worldwide overcome the ability of skeletal muscle to self-repair. Pro-angiogenic therapies are key in skeletal muscle regeneration where both myogenesis and angiogenesis occur. These therapies have been based on mesenchymal stem cells (MSCs), exosomes, microRNAs (miRs) and delivery of biological factors. The use of different calls of biomaterials is another approach, including ceramics, composites, and polymers. Natural polymers are use due its bioactivity and biocompatibility in addition to its use as scaffolds and in drug delivery systems. One of these polymers is the natural rubber latex (NRL) which is biocompatible, bioactive, versatile, low-costing, and capable of promoting tissue regeneration and angiogenesis. In this review, the advances in the field of pro-angiogenic therapies are discussed.

Systems biology of angiogenesis signaling: Computational models and omics

Angiogenesis is a highly regulated multiscale process that involves a plethora of cells, their cellular signal transduction, activation, proliferation, differentiation, as well as their intercellular communication. The coordinated execution and integration of such complex signaling programs is critical for physiological angiogenesis to take place in normal growth, development, exercise, and wound healing, while its dysregulation is critically linked to many major human diseases such as cancer, cardiovascular diseases, and ocular disorders; it is also crucial in regenerative medicine. Although huge efforts have been devoted to drug development for these diseases by investigation of angiogenesis‐targeted therapies, only a few therapeutics and targets have proved effective in humans due to the innate multiscale complexity and nonlinearity in the process of angiogenic signaling. As a promising approach that can help better address this challenge, systems biology modeling allows the integration of knowledge across studies and scales and provides a powerful means to mechanistically elucidate and connect the individual molecular and cellular signaling components that function in concert to regulate angiogenesis. In this review, we summarize and discuss how systems biology modeling studies, at the pathway‐, cell‐, tissue‐, and whole body‐levels, have advanced our understanding of signaling in angiogenesis and thereby delivered new translational insights for human diseases.

This article is categorized under:

Cardiovascular Diseases > Computational Models

Cancer > Computational Models

Time course of capillary regression and an expression balance between vascular endothelial growth factor-A and thrombospondin-1 in the soleus muscle of hindlimb unloaded rats

INTRODUCTION/AIMS

Skeletal muscle capillaries regress with disuse; however, information on time-dependent changes in the expression of pro- and anti-angiogenic factors in disused muscle is limited. This study aimed to clarify time-dependent changes in skeletal muscle capillarization, pro-angiogenic vascular endothelial growth factor-A (VEGF-A), and anti-angiogenic thrombospondin-1 (TSP-1) in the soleus muscle of hindlimb unloaded rat.

METHODS

Eight-week-old male Sprague Dawley rats were randomly divided into four groups corresponding to different hindlimb unloading (HU) duration at 0, 1, 2, and 3 weeks.

RESULTS

Muscle atrophy and capillary regression worsened in the soleus muscle with longer periods of HU. The VEGF-A protein expression level was lower at week 1 than at week 0. In addition, the value at week 3 was also lower than those at weeks 0, 1, and 2. The TSP-1 protein expression level was higher at week 1 than that at week 0 but was similar at weeks 2 and 3. Moreover, reactive oxygen species, assessed by dihydroethidium fluorescence intensity on cryosection, were higher at weeks 2 and 3 than that at week 0.

DISCUSSION

Depending on the HU period, VEGF-A and TSP-1 showed different expression patterns. In the early HU phase, TSP-1 may play an important role in capillary regression. However, when HU extends for a longer period, decreased VEGF-A, and/or increased oxidative stress may be more involved in capillary regression. This article is protected by copyright. All rights reserved.

Impact of C57BL/6J and SV-129 Mouse Strain Differences on Ischemia-Induced Postnatal Angiogenesis and the Associated Leukocyte Infiltration in a Murine Hindlimb Model of Ischemia

Strain-related differences in arteriogenesis in inbred mouse strains have already been studied excessively. However, these analyses missed evaluating the mouse strain-related differences in ischemia-induced angiogenic capacities. With the present study, we wanted to shed light on the different angiogenic potentials and the associated leukocyte infiltration of C57BL/6J and SV-129 mice to facilitate the comparison of angiogenesis-related analyses between these strains. For the induction of angiogenesis, we ligated the femoral artery in 8-12-week-old male C57BL/6J and SV-129 mice and performed (immuno-) histological analyses on the ischemic gastrocnemius muscles collected 24 h or 7 days after ligation. As evidenced by hematoxylin and eosin staining, C57BL/6J mice showed reduced tissue damage but displayed an increased capillary-to-muscle fiber ratio and an elevated number of proliferating capillaries (CD31⁺/BrdU⁺ cells) compared to SV-129 mice, thus showing improved angiogenesis. Regarding the associated leukocyte infiltration, we found increased numbers of neutrophils (MPO⁺ cells), NETs (MPO⁺/CitH3⁺/DAPI⁺), and macrophages (CD68⁺ cells) in SV-129 mice, whereas macrophage polarization (MRC1^- vs. MRC1⁺) and total leukocyte infiltration (CD45⁺ cells) did not differ between the mouse strains. In summary, we show increased ischemia-induced angiogenic capacities in C57BL/6J mice compared to SV-129 mice, with the latter showing aggravated tissue damage, inflammation, and impaired angiogenesis.

Effect of Microgravity Environment on Gut Microbiome and Angiogenesis

Microgravity environments are known to cause a plethora of stressors to astronauts. Recently, it has become apparent that gut microbiome composition of astronauts is altered following space travel, and this is of significance given the important role of the gut microbiome in human health. Other changes observed in astronauts comprise reduced muscle strength and bone fragility, visual impairment, endothelial dysfunction, metabolic changes, behavior changes due to fatigue or stress and effects on mental well-being. However, the effects of microgravity on angiogenesis, as well as the connection with the gut microbiome are incompletely understood. Here, the potential association of angiogenesis with visual impairment, skeletal muscle and gut microbiome is proposed and explored. Furthermore, metabolites that are effectors of angiogenesis are deliberated upon along with their connection with gut bacterial metabolites. Targeting and modulating the gut microbiome may potentially have a profound influence on astronaut health, given its impact on overall human health, which is thus warranted given the likelihood of increased human activity in the solar system, and the determination to travel to Mars in future missions.

Altitude, Exercise, and Skeletal Muscle Angio-Adaptive Responses to Hypoxia: A Complex Story

Hypoxia, defined as a reduced oxygen availability, can be observed in many tissues in response to various physiological and pathological conditions. As a hallmark of the altitude environment, ambient hypoxia results from a drop in the oxygen pressure in the atmosphere with elevation. A hypoxic stress can also occur at the cellular level when the oxygen supply through the local microcirculation cannot match the cells’ metabolic needs. This has been suggested in contracting skeletal myofibers during physical exercise. Regardless of its origin, ambient or exercise-induced, muscle hypoxia triggers complex angio-adaptive responses in the skeletal muscle tissue. These can result in the expression of a plethora of angio-adaptive molecules, ultimately leading to the growth, stabilization, or regression of muscle capillaries. This remarkable plasticity of the capillary network is referred to as angio-adaptation. It can alter the capillary-to-myofiber interface, which represent an important determinant of skeletal muscle function. These angio-adaptive molecules can also be released in the circulation as myokines to act on distant tissues. This review addresses the respective and combined potency of ambient hypoxia and exercise to generate a cellular hypoxic stress in skeletal muscle. The major skeletal muscle angio-adaptive responses to hypoxia so far described in this context will be discussed, including existing controversies in the field. Finally, this review will highlight the molecular complexity of the skeletal muscle angio-adaptive response to hypoxia and identify current gaps of knowledges in this field of exercise and environmental physiology.

The Absence of Extracellular Cold-Inducible RNA-Binding Protein (eCIRP) Promotes Pro-Angiogenic Microenvironmental Conditions and Angiogenesis in Muscle Tissue Ischemia

Extracellular Cold-inducible RNA-binding protein (eCIRP), a damage-associated molecular pattern, is released from cells upon hypoxia and cold-stress. The overall absence of extra- and intracellular CIRP is associated with increased angiogenesis, most likely induced through influencing leukocyte accumulation. The aim of the present study was to specifically characterize the role of eCIRP in ischemia-induced angiogenesis together with the associated leukocyte recruitment. For analyzing eCIRPs impact, we induced muscle ischemia via femoral artery ligation (FAL) in mice in the presence or absence of an anti-CIRP antibody and isolated the gastrocnemius muscle for immunohistological analyses. Upon eCIRP-depletion, mice showed increased capillary/muscle fiber ratio and numbers of proliferating endothelial cells (CD31⁺/CD45⁻/BrdU⁺). This was accompanied by a reduction of total leukocyte count (CD45⁺), neutrophils (MPO⁺), neutrophil extracellular traps (NETs) (MPO⁺CitH3⁺), apoptotic area (ascertained via TUNEL assay), and pro-inflammatory M1-like polarized macrophages (CD68⁺/MRC1⁻) in ischemic muscle tissue. Conversely, the number of regenerative M2-like polarized macrophages (CD68⁺/MRC1⁺) was elevated. Altogether, we observed that eCIRP depletion similarly affected angiogenesis and leukocyte recruitment as described for the overall absence of CIRP. Thus, we propose that eCIRP is mainly responsible for modulating angiogenesis via promoting pro-angiogenic microenvironmental conditions in muscle ischemia.

The Role of the Skeletal Muscle Secretome in Mediating Endurance and Resistance Training Adaptations

Exercise, in the form of endurance or resistance training, leads to specific molecular and cellular adaptions not only in skeletal muscles, but also in many other organs such as the brain, liver, fat or bone. In addition to direct effects of exercise on these organs, the production and release of a plethora of different signaling molecules from skeletal muscle are a centerpiece of systemic plasticity. Most studies have so far focused on the regulation and function of such myokines in acute exercise bouts. In contrast, the secretome of long-term training adaptation remains less well understood, and the contribution of non-myokine factors, including metabolites, enzymes, microRNAs or mitochondrial DNA transported in extracellular vesicles or by other means, is underappreciated. In this review, we therefore provide an overview on the current knowledge of endurance and resistance exercise-induced factors of the skeletal muscle secretome that mediate muscular and systemic adaptations to long-term training. Targeting these factors and leveraging their functions could not only have broad implications for athletic performance, but also for the prevention and therapy in diseased and elderly populations.

Exercise Training Enhances Angiogenesis-Related Gene Responses in Skeletal Muscle of Patients with Chronic Heart Failure

Peripheral myopathy consists of a hallmark of heart failure (HF). Exercise enhanced skeletal muscle angiogenesis, and thus, it can be further beneficial towards the HF-induced myopathy. However, there is limited evidence regarding the exercise type that elicits optimum angiogenic responses of skeletal muscle in HF patients. This study aimed to (a) compare the effects of a high-intensity-interval-training (HIIT) or combined HIIT with strength training (COM) exercise protocol on the expression of angiogenesis-related factors in skeletal muscle of HF patients, and (b) examine the potential associations between the expression of those genes and capillarization in the trained muscles. Thirteen male patients with chronic HF (age: 51 ± 13 y; BMI: 27 ± 4 kg/m2) were randomly assigned to a 3-month exercise program that consisted of either HIIT (N = 6) or COM training (N = 7). Vastus lateralis muscle biopsies were performed pre- and post-training. RT-PCR was used to quantify the fold changes in mRNA expression of vascular endothelial growth factor (VEGF), vascular endothelial growth factor receptor 2 (VEGFR-2), hypoxia-inducible factor 1 alpha (HIF-1α), angiopoietin 1 (Ang-1), angiopoietin 2 (Ang-2), angiopoietin receptor (Tie2), and matrix metallopeptidase 9 (MMP-9), and immunohistochemistry to assess capillarization in skeletal muscle post-training. There was an overall increase in the expression levels of VEGF, VEGFR-2, HIF-1α, Ang2, and MMP9 post-training, while these changes were not different among groups. Changes in capillary-to-fibre ratio were found to be strongly associated with Tie2 and HIF-1α expression. This was the first study demonstrating that both HIIT and combined HIIT with strength training enhanced similarly the expression profile of angiogenic factors in skeletal muscle of HF patients, possibly driving the angiogenic program in the trained muscles, although those gene expression increases were found to be only partially related with muscle capillarization.

Bone mesenchymal stem cells stimulation by magnetic nanoparticles and a static magnetic field: release of exosomal miR-1260a improves osteogenesis and angiogenesis

Background

The therapeutic potential of exosomes derived from stem cells has attracted increasing interest recently, because they can exert similar paracrine functions of stem cells and overcome the limitations of stem cells transplantation. Exosomes derived from bone mesenchymal stem cells (BMSC-Exos) have been confirmed to promote osteogenesis and angiogenesis. The magnetic nanoparticles (eg. Fe₃O₄, γ-Fe₂O₃) combined with a static magnetic field (SMF) has been commonly used to increase wound healing and bone regeneration. Hence, this study aims to evaluate whether exosomes derived from BMSCs preconditioned with a low dose of Fe₃O₄ nanoparticles with or without the SMF, exert superior pro-osteogenic and pro-angiogenic activities in bone regeneration and the underlying mechanisms involved.

Methods

Two novel types of exosomes derived from preconditioned BMSCs that fabricated by regulating the contents with the stimulation of magnetic nanoparticles and/or a SMF. Then, the new exosomes were isolated by ultracentrifugation and characterized. Afterwards, we conducted in vitro experiments in which we measured osteogenic differentiation, cell proliferation, cell migration, and tube formation, then established an in vivo critical-sized calvarial defect rat model. The miRNA expression profiles were compared among the exosomes to detect the potential mechanism of improving osteogenesis and angiogenesis. At last, the function of exosomal miRNA during bone regeneration was confirmed by utilizing a series of gain- and loss-of-function experiments in vitro.

Results

50 µg/mL Fe₃O₄ nanoparticles and a 100 mT SMF were chosen as the optimum magnetic conditions to fabricate two new exosomes, named BMSC-Fe₃O₄-Exos and BMSC-Fe₃O₄-SMF-Exos. They were both confirmed to enhance osteogenesis and angiogenesis in vitro and in vivo compared with BMSC-Exos, and BMSC-Fe₃O₄-SMF-Exos had the most marked effect. The promotion effect was found to be related to the highly riched miR-1260a in BMSC-Fe₃O₄-SMF-Exos. Furthermore, miR-1260a was verified to enhance osteogenesis and angiogenesis through inhibition of HDAC7 and COL4A2, respectively.

Conclusion

These results suggest that low doses of Fe₃O₄ nanoparticles combined with a SMF trigger exosomes to exert enhanced osteogenesis and angiogenesis and that targeting of HDAC7 and COL4A2 by exosomal miR-1260a plays a crucial role in this process. This work could provide a new protocol to promote bone regeneration for tissue engineering in the future.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00958-6.

Regenerated Microvascular Networks in Ischemic Skeletal Muscle

Skeletal muscle is the largest organ in humans. The viability and performance of this metabolically demanding organ are exquisitely dependent on the integrity of its microcirculation. The architectural and functional attributes of the skeletal muscle microvasculature are acquired during embryonic and early postnatal development. However, peripheral vascular disease in the adult can damage the distal microvasculature, together with damaging the skeletal myofibers. Importantly, adult skeletal muscle has the capacity to regenerate. Understanding the extent to which the microvascular network also reforms, and acquires structural and functional competence, will thus be critical to regenerative medicine efforts for those with peripheral artery disease (PAD). Herein, we discuss recent advances in studying the regenerating microvasculature in the mouse hindlimb following severe ischemic injury. We highlight new insights arising from real-time imaging of the microcirculation. This includes identifying otherwise hidden flaws in both network microarchitecture and function, deficiencies that could underlie the progressive nature of PAD and its refractoriness to therapy. Recognizing and overcoming these vulnerabilities in regenerative angiogenesis will be important for advancing treatment options for PAD.

Impaired skeletal muscle fatigue resistance during cardiac hypertrophy is prevented by functional overload- or exercise-induced functional capillarity

KEY POINTS

Capillary rarefaction is hypothesized to contribute to impaired exercise tolerance in cardiovascular disease, but it remains a poorly exploited therapeutic target for improving skeletal muscle performance. Using an abdominal aortic coarctation rat model of compensatory cardiac hypertrophy, we determine the efficacy of aerobic exercise for the prevention of, and mechanical overload for, restoration of hindlimb muscle fatigue resistance and microvascular impairment in the early stages of heart disease. Impaired muscle fatigue resistance was found after development of cardiac hypertrophy, but this impairment was prevented by low-intensity aerobic exercise and recovered after mechanical stretch due to muscle overload. Changes in muscle fatigue resistance were closely related to functional (i.e. perfused) microvascular density, independent of arterial blood flow, emphasizing the critical importance of optimal capillary diffusion for skeletal muscle function. Pro-angiogenic therapies are an important tool for improving skeletal muscle function in the incipient stages of heart disease.

ABSTRACT

Microvascular rarefaction may contribute to declining skeletal muscle performance in cardiac and vascular diseases. It remains uncertain to what extent microvascular rarefaction occurs in the earliest stages of these conditions, if impaired blood flow is an aggravating factor and whether angiogenesis restores muscle performance. To investigate this, the effects of aerobic exercise (voluntary wheel running) and functional muscle overload on the performance, femoral blood flow (FBF) and microvascular perfusion of the extensor digitorum longus (EDL) were determined in a chronic rat model of compensatory cardiac hypertrophy (CCH, induced by surgically imposed abdominal aortic coarctation). CCH was associated with hypertension (P = 0.001 vs. Control) and increased relative heart mass (P < 0.001). Immediately upon placing the aortic band (i.e. before development of CCH), post-fatigue test FBF was reduced (P < 0.003), coinciding with attenuated fatigue resistance (P = 0.039) indicating an acute arterial perfusion constraint on muscle performance. While FBF was normalized during CCH in chronic groups (P > 0.05) fatigue resistance remained reduced (P = 0.039) and was associated with reduced (P = 0.009) functional capillarity after development of CCH without intervention, indicating a microvascular limitation to muscle performance. Normalization of functional capillarity after aerobic exercise (P = 0.065) and overload (P = 0.329) in CCH coincided with restoration to control levels of muscle fatigue resistance (P > 0.999), although overload-induced EDL hypertrophy (P = 0.027) and wheel-running velocity and duration (both P < 0.05) were attenuated after aortic banding. These data show that reductions in skeletal muscle performance during CCH can be countered by improving functional capillarity, providing a therapeutic target to improve skeletal muscle function in chronic diseases.

Functionalizing biomaterials to promote neurovascular regeneration following skeletal muscle injury

During embryogenesis, blood vessels and nerves develop with similar branching structure in response to shared signaling pathways guiding network growth. With both systems integral to physiological homeostasis, dual targeting of blood vessels and nerves to promote neurovascular regeneration following injury is an emerging therapeutic approach in biomedical engineering. A limitation to this strategy is that the nature of cross talk between emergent vessels and nerves during regeneration in an adult is poorly understood. Following peripheral nerve transection, intraneural vascular cells infiltrate the site of injury to provide a migratory pathway for mobilized Schwann cells of regenerating axons. As Schwann cells demyelinate, they secrete vascular endothelial growth factor, which promotes angiogenesis. Recent advances point to concomitant restoration of neurovascular architecture and function through simultaneous targeting of growth factors and guidance cues shared by both systems during regeneration. In the context of traumatic injury associated with volumetric muscle loss, we consider the nature of biomaterials used to engineer three-dimensional scaffolds, functionalization of scaffolds with molecular signals that guide and promote neurovascular growth, and seeding scaffolds with progenitor cells. Physiological success is defined by each tissue component of the bioconstruct (nerve, vessel, muscle) becoming integrated with that of the host. Advances in microfabrication, cell culture techniques, and progenitor cell biology hold great promise for engineering bioconstructs able to restore organ function after volumetric muscle loss.