BAG3 (Bcl-2-Associated Athanogene-3) Coding Variant in Mice Determines Susceptibility to Ischemic Limb Muscle Myopathy by Directing Autophagy

METTL3-dependent m6A modification of SNAP29 induces "autophagy-mitochondrial crisis" in the ischemic microenvironment after soft tissue transplantation

Necrosis at the ischemic distal end of flap transplants increases patients' pain and economic burden. Reactive oxygen species (ROS) and mitochondrial damage are crucial in regulating parthanatos, but the mechanisms linking disrupted macroautophagic/autophagic flux to parthanatos in ischemic flaps remain unclear. The results of western blotting, immunofluorescence staining, and a proteomic analysis revealed that the autophagic protein SNAP29 was deficient in ischemic flaps, resulting in disrupted autophagic flux, increased ROS-induced parthanatos, and aggravated ischemic flap necrosis. The use of AAV vector to restore SNAP29 in vivo mitigated the disruption of autophagic flux and parthanatos. Additionally, quantification of the total m6A level and RIP-qPCR, MeRIP-qPCR, and RNA stability assessments were performed to determine differential Snap29 mRNA m6A methylation levels and mRNA stability in ischemic flaps. Various in vitro and in vivo tests were conducted to verify the ability of METTL3-mediated m6A methylation to promote SNAP29 depletion and disrupt autophagic flux. Finally, we concluded that restoring SNAP29 by inhibiting METTL3 and YTHDF2 reversed the "autophagy-mitochondrial crisis", defined for the first time as disrupted autophagic flux, mitochondrial damage, mitochondrial protein leakage, and the occurrence of parthanatos. The reversal of this crisis ultimately promoted the survival of ischemic flaps.Abbreviations: AAV = adeno-associated virus; ACTA2/α-SMA = actin alpha 2, smooth muscle, aorta; AIFM/AIF = apoptosis-inducing factor, mitochondrion-associated; ALKBH5 = alkB homolog, RNA demythelase; Baf A1 = bafilomycin A1; CQ = chloroquine; DHE = dihydroethidium; ECs = endothelial cells; F-CHP = 5-FAM-conjugated collagen-hybridizing peptide; GO = gene ontology; HUVECs = human umbilical vein endothelial cells; KEGG = Kyoto Encyclopedia of Genes and Genomes; LC-MS/MS = liquid chromatography-tandem mass spectrometry; LDBF = laser doppler blood flow; m6A = N6-methyladenosine; MAP1LC3/LC3 = microtubule-associated protein 1 light chain 3; MeRIP = methylated RNA immunoprecipitation; METTL3 = methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit; NAC = N-acetylcysteine; OGD = oxygen glucose deprivation; PAR = poly (ADP-ribose); PARP1 = poly (ADP-ribose) polymerase family, member 1; PECAM1/CD31 = platelet/endothelial cell adhesion molecule 1; ROS = reactive oxygen species; RT-qPCR = reverse transcription quantitative polymerase chain reaction; RIP = RNA immunoprecipitation; SNAP29 = synaptosomal-associated protein 29; SNARE = soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SQSTM1 = sequestosome 1; SRAMP = sequence-based RNA adenosine methylation site predicting; STX17 = syntaxin 17; TMT = tandem mass tag; TUNEL = terminal deoxynucleotidyl transferase dUTP nick end labeling; VAMP8 = vesicle-associated membrane protein 8; WTAP = WT1 associating protein; YTHDF2 = YTH N6-methyladenosine RNA binding protein 2; 3' UTR = 3'-untranslated region.

A 6-Minute Limb Function Assessment for Therapeutic Testing in Experimental Peripheral Artery Disease Models

In this study, we present a novel 6-minute limb function test that allows for the congruent assessment of muscular performance and hemodynamics in preclinical models of peripheral artery disease. Using several experimental conditions, the results demonstrate the superior efficacy of the 6-minute limb function test to detect differences in the response to hindlimb ischemia across several interventions, including where traditional perfusion recovery, capillary density, and muscle strength measures were unable to detect interventional differences, thus allowing for more rigorous assessment of preclinical therapies before clinical translation.

Autophagy in Muscle Regeneration: Mechanisms, Targets, and Therapeutic Perspective

Autophagy maintains the stability of eukaryotic cells by degrading unwanted components and recycling nutrients and plays a pivotal role in muscle regeneration by regulating the quiescence, activation, and differentiation of satellite cells. Effective muscle regeneration is vital for maintaining muscle health and homeostasis. However, under certain disease conditions, such as aging, muscle regeneration can fail due to dysfunctional satellite cells. Dysregulated autophagy may limit satellite cell self-renewal, hinder differentiation, and increase susceptibility to apoptosis, thereby impeding muscle regeneration. This review explores the critical role of autophagy in muscle regeneration, emphasizing its interplay with apoptosis and recent advances in autophagy research related to diseases characterized by impaired muscle regeneration. Additionally, we discuss new approaches involving autophagy regulation to promote macrophage polarization, enhancing muscle regeneration. We suggest that utilizing cell therapy and biomaterials to modulate autophagy could be a promising strategy for supporting muscle regeneration. We hope that this review will provide new insights into the treatment of muscle diseases and promote muscle regeneration.

Universal Adapter Protein Bag3 and Small Heat Shock Proteins

Bag3 (Bcl-2-associated athanogene 3) protein contains a number of functional domains and interacts with a wide range of different partner proteins, including small heat shock proteins (sHsps) and heat shock protein Hsp70. The ternary Bag3-sHsp-and Hsp70 complex binds denatured proteins and transports them to phagosomes, thus playing a key role in the chaperone-assisted selective autophagy (CASA). This complex also participates in the control of formation and disassembly of stress granules (granulostasis) and cytoskeleton regulation. As Bag3 and sHsps participate in multiple cellular processes, mutations in these proteins are often associated with neurodegenerative diseases and cardiomyopathy. The review discusses the role of sHsps in different processes regulated by Bag3.

A 6-minute Limb Function Assessment for Therapeutic Testing in Experimental Peripheral Artery Disease Models

Background

The translation of promising therapies from pre-clinical models of hindlimb ischemia (HLI) to patients with peripheral artery disease (PAD) has been inadequate. While this failure is multifactorial, primary outcome measures in preclinical HLI models and clinical trials involving patients with PAD are not aligned well. For example, laser Doppler perfusion recovery measured under resting conditions is the most used outcome in HLI studies, whereas clinical trials involving patients with PAD primarily assess walking performance. Here, we sought to develop a 6-min limb function test for preclinical HLI models that assess muscular performance and hemodynamics congruently.

Methods

We developed an in situ 6-min limb function test that involves repeated isotonic (shortening) contractions performed against a submaximal load. Continuous measurement of muscle blood flow was performed using laser Doppler flowmetry. Quantification of muscle power, work, and perfusion are obtained across the test. To assess the efficacy of this test, we performed HLI via femoral artery ligation on several mouse strains: C57BL6J, BALBc/J, and MCK-PGC1α (muscle-specific overexpression of PGC1α). Additional experiments were performed using an exercise intervention (voluntary wheel running) following HLI.

Results

The 6-min limb function test was successful at detecting differences in limb function of C57BL6/J and BALBc/J mice subjected to HLI with effect sizes superior to laser Doppler perfusion recovery. C57BL6/J mice randomized to exercise therapy following HLI had smaller decline in muscle power, greater hyperemia, and performed more work across the 6-min limb function test compared to non-exercise controls with HLI. Mice with muscle-specific overexpression of PGC1α had no differences in perfusion recovery in resting conditions, but exhibited greater capillary density, increased muscle mass and absolute force levels, and performed more work across the 6-min limb function test compared to their wildtype littermates without the transgene.

Conclusion

These results demonstrate the efficacy of the 6-min limb function test to detect differences in the response to HLI across several interventions including where traditional perfusion recovery, capillary density, and muscle strength measures were unable to detect therapeutic differences.

Deletion of the aryl hydrocarbon receptor in endothelial cells improves ischemic angiogenesis in chronic kidney disease

Chronic kidney disease (CKD) is a strong risk factor for peripheral artery disease (PAD) that is associated with worsened clinical outcomes. CKD leads to the accumulation of tryptophan metabolites that are associated with adverse limb events in PAD and are ligands of the aryl hydrocarbon receptor (AHR), which may regulate ischemic angiogenesis. To test if endothelial cell-specific deletion of the AHR (AHRecKO) alters ischemic angiogenesis and limb function in mice with CKD subjected to femoral artery ligation. Male AHRecKO mice with CKD displayed better limb perfusion recovery and enhanced ischemic angiogenesis compared with wild-type mice with CKD. However, the improved limb perfusion did not result in better muscle performance. In contrast to male mice, deletion of the AHR in female mice with CKD had no impact on perfusion recovery or angiogenesis. With the use of primary endothelial cells from male and female mice, treatment with indoxyl sulfate uncovered sex-dependent differences in AHR activating potential and RNA sequencing revealed wide-ranging sex differences in angiogenic signaling pathways. Endothelium-specific deletion of the AHR improved ischemic angiogenesis in male, but not female, mice with CKD. There are sex-dependent differences in Ahr activating potential within endothelial cells that are independent of sex hormones.NEW & NOTEWORTHY This study provides novel insights into the mechanisms by which chronic kidney disease worsens ischemic limb outcomes in an experimental model of peripheral artery disease. Deletion of the aryl hydrocarbon receptor (AHR) in the endothelium improved ischemic angiogenesis suggesting that AHR inhibition could be a viable therapeutic target; however, this effect was only observed in male mice. Subsequent analysis in primary endothelial cells reveals sex differences in Ahr activating potential independent of sex hormones.

Skeletal muscle regeneration failure in ischemic-damaged limbs is associated with pro-inflammatory macrophages and premature differentiation of satellite cells

BACKGROUND

Chronic limb-threatening ischemia (CLTI), a severe manifestation of peripheral arterial disease (PAD), is associated with a 1-year limb amputation rate of approximately 15-20% and substantial mortality. A key feature of CLTI is the compromised regenerative ability of skeletal muscle; however, the mechanisms responsible for this impairment are not yet fully understood. In this study, we aim to delineate pathological changes at both the cellular and transcriptomic levels, as well as in cell-cell signaling pathways, associated with compromised muscle regeneration in limb ischemia in both human tissue samples and murine models of CLTI.

METHODS

We performed single-cell transcriptome analysis of ischemic and non-ischemic muscle from the same CLTI patients and from a murine model of CLTI. In both datasets, we analyzed gene expression changes in macrophage and muscle satellite cell (MuSC) populations as well as differential cell-cell signaling interactions and differentiation trajectories.

RESULTS

Single-cell transcriptomic profiling and immunofluorescence analysis of CLTI patient skeletal muscle demonstrated that ischemic-damaged tissue displays a pro-inflammatory macrophage signature. Comparable results were observed in a murine CLTI model. Moreover, integrated analyses of both human and murine datasets revealed premature differentiation of MuSCs to be a key feature of failed muscle regeneration in the ischemic limb. Furthermore, in silico inferences of intercellular communication and in vitro assays highlight the importance of macrophage-MuSC signaling in ischemia induced muscle injuries.

CONCLUSIONS

Collectively, our research provides the first single-cell transcriptome atlases of skeletal muscle from CLTI patients and a murine CLTI model, emphasizing the crucial role of macrophages and inflammation in regulating muscle regeneration in CLTI through interactions with MuSCs.

Single-Nuclei RNA-Sequencing of the Gastrocnemius Muscle in Peripheral Artery Disease

BACKGROUND

Lower extremity peripheral artery disease (PAD) is a growing epidemic with limited effective treatment options. Here, we provide a single-nuclei atlas of PAD limb muscle to facilitate a better understanding of the composition of cells and transcriptional differences that comprise the diseased limb muscle.

METHODS

We obtained gastrocnemius muscle specimens from 20 patients with PAD and 12 non-PAD controls. Nuclei were isolated and single-nuclei RNA-sequencing was performed. The composition of nuclei was characterized by iterative clustering via principal component analysis, differential expression analysis, and the use of known marker genes. Bioinformatics analysis was performed to determine differences in gene expression between PAD and non-PAD nuclei, as well as subsequent analysis of intercellular signaling networks. Additional histological analyses of muscle specimens accompany the single-nuclei RNA-sequencing atlas.

RESULTS

Single-nuclei RNA-sequencing analysis indicated a fiber type shift with patients with PAD having fewer type I (slow/oxidative) and more type II (fast/glycolytic) myonuclei compared with non-PAD, which was confirmed using immunostaining of muscle specimens. Myonuclei from PAD displayed global upregulation of genes involved in stress response, autophagy, hypoxia, and atrophy. Subclustering of myonuclei also identified populations that were unique to PAD muscle characterized by metabolic dysregulation. PAD muscles also displayed unique transcriptional profiles and increased diversity of transcriptomes in muscle stem cells, regenerating myonuclei, and fibro-adipogenic progenitor cells. Analysis of intercellular communication networks revealed fibro-adipogenic progenitors as a major signaling hub in PAD muscle, as well as deficiencies in angiogenic and bone morphogenetic protein signaling which may contribute to poor limb function in PAD.

CONCLUSIONS

This reference single-nuclei RNA-sequencing atlas provides a comprehensive analysis of the cell composition, transcriptional signature, and intercellular communication pathways that are altered in the PAD condition.

BAG3 regulates bone marrow mesenchymal stem cell proliferation by targeting INTS7

Background

BAG3 is an essential regulator of cell survival and has been investigated in the context of heart disease and cancer. Our previous study used immunoprecipitation-liquid chromatography-tandem mass spectrometry to show that BAG3 might directly interact with INTS7 and regulate bone marrow mesenchymal stem cell (BMMSCs) proliferation. However, whether BAG3 bound INTS7 directly and how it regulated BMMSCs expansion was unclear.

Methods

BAG3 expression was detected by quantitative real-time PCR in BMMSCs after siRNA-mediated BAG3 knockdown. BMMSC proliferation was determined using the CCK-8 and colony formation assays. The transwell migration, flow cytometry and TUNEL assays were performed to measure BMMSC migration, cell cycle and apoptosis, respectively. Moreover, co-immunoprecipitation, protein half-life assay and western blotting analyses were used to determine the regulatory mechanism underlying the BAG3-mediated increase in BMMSC proliferation.

Results

The results showed that knocking down BAG3 in BMMSCs markedly decreased their proliferative activity, colony formation and migratory capacity, and induced cell apoptosis as well as cell cycle arrest. Meanwhile, overexpression of BAG3 had the opposite effect. Bioinformatics and BAG3-INTS7 co-immunoprecipitation analyses revealed that BAG3 directly interacted with INTS7. Moreover, the downregulation of BAG3 inhibited the expression of INTS7 and promoted its ubiquitination. We also observed that BAG3 knockdown increased the levels of reactive oxygen species and the extent of DNA damage in BMMSCs. Notably, the upregulation of INTS7 or the addition of an antioxidant scavenger could rescue the BMMSC phenotype induced by BAG3 downregulation.

Conclusions

BAG3 directly interacts with INTS7 and promotes BMMSC expansion by reducing oxidative stress.

Activation of the Aryl Hydrocarbon Receptor in Endothelial Cells Impairs Ischemic Angiogenesis in Chronic Kidney Disease

Rationale

Chronic kidney disease (CKD) is a strong risk factor for peripheral artery disease (PAD) that is associated with worsened clinical outcomes. CKD leads to accumulation of tryptophan metabolites that associate with adverse limb events in PAD and are ligands of the aryl hydrocarbon receptor (AHR) which may regulate ischemic angiogenesis.

Objectives

To test if endothelial cell-specific deletion of the AHR (AHRecKO) alters ischemic angiogenesis and limb function in mice with CKD subjected to femoral artery ligation.

Findings

Male AHRecKO mice with CKD displayed better limb perfusion recovery and enhanced ischemic angiogenesis compared to wildtype mice with CKD. However, the improved limb perfusion did not result in better muscle performance. In contrast to male mice, deletion of the AHR in female mice with CKD had no impact on perfusion recovery or angiogenesis. Using primary endothelial cells from male and female mice, treatment with indoxyl sulfate uncovered sex-dependent differences in AHR activating potential and RNA sequencing revealed wide ranging sex-differences in angiogenic signaling pathways.

Conclusion

Endothelium-specific deletion of the AHR improved ischemic angiogenesis in male, but not female, mice with CKD. There are sex-dependent differences in Ahr activating potential within endothelial cells that are independent of sex hormones.

IGF-1 Therapy Improves Muscle Size and Function in Experimental Peripheral Arterial Disease

Lower-extremity peripheral arterial disease (PAD) has increased in prevalence, yet therapeutic development has remained stagnant. Skeletal muscle health and function has been strongly linked to quality of life and medical outcomes in patients with PAD. Using a rodent model of PAD, this study demonstrates that treatment of the ischemic limb with insulin-like growth factor (IGF)-1 significantly increases muscle size and strength without improving limb hemodynamics. Interestingly, the effect size of IGF1 therapy was larger in female mice than in male mice, highlighting the need to carefully examine sex-dependent effects in experimental PAD therapies.

Pro-inflammatory macrophages impair skeletal muscle regeneration in ischemic-damaged limbs by inducing precocious differentiation of satellite cells

Chronic limb-threatening ischemia (CLTI), representing the end-stage of peripheral arterial disease (PAD), is associated with a one-year limb amputation rate of ∼15-20% and significant mortality. A key characteristic of CLTI is the failure of the innate regenerative capacity of skeletal muscle, though the underlying mechanisms remain unclear. Here, single-cell transcriptome analysis of ischemic and non-ischemic muscle from the same CLTI patients demonstrated that ischemic-damaged tissue is enriched with pro-inflammatory macrophages. Comparable results were also observed in a murine CLTI model. Importantly, integrated analyses of both human and murine data revealed premature differentiation of muscle satellite cells (MuSCs) in damaged tissue and indications of defects in intercellular signaling communication between MuSCs and their inflammatory niche. Collectively, our research provides the first single-cell transcriptome atlases of skeletal muscle from CLTI patients and murine models, emphasizing the crucial role of macrophages and inflammation in regulating muscle regeneration in CLTI through interactions with MuSCs.

Piezo1 (Piezo-Type Mechanosensitive Ion Channel Component 1)-Mediated Mechanosensation in Macrophages Impairs Perfusion Recovery After Hindlimb Ischemia in Mice

BACKGROUND

Angiogenesis is a promising strategy for those with peripheral artery disease. Macrophage-centered inflammation is intended to govern the deficiency of the angiogenic response after hindlimb ischemia. However, little is known about the mechanism of macrophage activation beyond signals from cytokines and chemokines. We sought to identify a novel mechanical signal from the ischemic microenvironment that provokes macrophages and the subsequent inflammatory cascade and to investigate the potential role of Piezo-type mechanosensitive ion channels (Piezo) on macrophages during this process.

METHODS

Myeloid cell-specific Piezo1 (Piezo-type mechanosensitive ion channel component 1) knockout (Piezo1^ΔMΦ) mice were generated by crossing Piezo1^fl/fl (LysM-Cre^-/-; Piezo1 flox/flox) mice with LysM-Cre transgenic mice to assess the roles of Piezo1 in macrophages after hindlimb ischemia. Furthermore, in vitro studies were carried out in bone marrow-derived macrophages to decipher the underlying mechanism.

RESULTS

We found that tissue stiffness gradually increased after hindlimb ischemia, as indicated by Young's modulus. Compared to Piezo2, Piezo1 expression and activation were markedly upregulated in macrophages from ischemic tissues in concurrence with increased tissue stiffness. Piezo1^ΔMΦ mice exhibited improved perfusion recovery by enhancing angiogenesis. Matrigel tube formation assays revealed that Piezo1 deletion promoted angiogenesis by enhancing FGF2 (fibroblast growth factor-2) paracrine signaling in macrophages. Conversely, activation of Piezo1 by increased stiffness or the agonist Yoda1 led to reduced FGF2 production in bone marrow-derived macrophages, which could be blocked by Piezo1 silencing. Mechanistically, Piezo1 mediated extracellular Ca²⁺ influx and activated Ca²⁺-dependent CaMKII (calcium/calmodulin-dependent protein kinase II)/ETS1 (ETS proto-oncogene 1) signaling, leading to transcriptional inactivation of FGF2.

CONCLUSIONS

This study uncovers a crucial role of microenvironmental stiffness in exacerbating the macrophage-dependent deficient angiogenic response. Deletion of macrophage Piezo1 promotes perfusion recovery after hindlimb ischemia through CaMKII/ETS1-mediated transcriptional activation of FGF2. This provides a promising therapeutic strategy to enhance angiogenesis in ischemic diseases.

Hypoxia Resistance Is an Inherent Phenotype of the Mouse Flexor Digitorum Brevis Skeletal Muscle

The various functions of skeletal muscle (movement, respiration, thermogenesis, etc.) require the presence of oxygen (O2). Inadequate O2 bioavailability (ie, hypoxia) is detrimental to muscle function and, in chronic cases, can result in muscle wasting. Current therapeutic interventions have proven largely ineffective to rescue skeletal muscle from hypoxic damage. However, our lab has identified a mammalian skeletal muscle that maintains proper physiological function in an environment depleted of O2. Using mouse models of in vivo hindlimb ischemia and ex vivo anoxia exposure, we observed the preservation of force production in the flexor digitorum brevis (FDB), while in contrast the extensor digitorum longus (EDL) and soleus muscles suffered loss of force output. Unlike other muscles, we found that the FDB phenotype is not dependent on mitochondria, which partially explains the hypoxia resistance. Muscle proteomes were interrogated using a discovery-based approach, which identified significantly greater expression of the transmembrane glucose transporter GLUT1 in the FDB as compared to the EDL and soleus. Through loss-and-gain-of-function approaches, we determined that GLUT1 is necessary for the FDB to survive hypoxia, but overexpression of GLUT1 was insufficient to rescue other skeletal muscles from hypoxic damage. Collectively, the data demonstrate that the FDB is uniquely resistant to hypoxic insults. Defining the mechanisms that explain the phenotype may provide insight towards developing approaches for preventing hypoxia-induced tissue damage.

BAG3: Nature's Quintessential Multi-Functional Protein Functions as a Ubiquitous Intra-Cellular Glue

BAG3 is a 575 amino acid protein that is found throughout the animal kingdom and homologs have been identified in plants. The protein is expressed ubiquitously but is most prominent in cardiac muscle, skeletal muscle, the brain and in many cancers. We describe BAG3 as a quintessential multi-functional protein. It supports autophagy of both misfolded proteins and damaged organelles, inhibits apoptosis, maintains the homeostasis of the mitochondria, and facilitates excitation contraction coupling through the L-type calcium channel and the beta-adrenergic receptor. High levels of BAG3 are associated with insensitivity to chemotherapy in malignant cells whereas both loss of function and gain of function variants are associated with cardiomyopathy.

Muscle progenitor cells are required for skeletal muscle regeneration and prevention of adipogenesis after limb ischemia

Skeletal muscle injury in peripheral artery disease (PAD) has been attributed to vascular insufficiency, however evidence has demonstrated that muscle cell responses play a role in determining outcomes in limb ischemia. Here, we demonstrate that genetic ablation of Pax7+ muscle progenitor cells (MPCs) in a model of hindlimb ischemia (HLI) inhibited muscle regeneration following ischemic injury, despite a lack of morphological or physiological changes in resting muscle. Compared to control mice (Pax7WT), the ischemic limb of Pax7-deficient mice (Pax7Δ) was unable to generate significant force 7 or 28 days after HLI. A significant increase in adipose was observed in the ischemic limb 28 days after HLI in Pax7Δ mice, which replaced functional muscle. Adipogenesis in Pax7Δ mice corresponded with a significant increase in PDGFRα+ fibro/adipogenic progenitors (FAPs). Inhibition of FAPs with batimastat decreased muscle adipose but increased fibrosis. In vitro, Pax7Δ MPCs failed to form myotubes but displayed increased adipogenesis. Skeletal muscle from patients with critical limb threatening ischemia displayed increased adipose in more ischemic regions of muscle, which corresponded with fewer satellite cells. Collectively, these data demonstrate that Pax7+ MPCs are required for muscle regeneration after ischemia and suggest that muscle regeneration may be an important therapeutic target in PAD.

BAG3 Attenuates Ischemia-Induced Skeletal Muscle Necroptosis in Diabetic Experimental Peripheral Artery Disease

Peripheral artery disease (PAD) is characterized by impaired blood flow to the lower extremities, resulting in ischemic limb injuries. Individuals with diabetes and PAD typically have more severe ischemic limb injuries and limb amputations, but the mechanisms involved are poorly understood. Previously, we identified BAG3 as a gene within a mouse genetic locus termed limb salvage QTL1 on mouse chromosome 7 that determined the extent of limb necrosis following ischemic injury in C57Bl/6 mice. Whether BAG3 deficiency plays a role in the severe ischemic injury observed in diabetic PAD is not known. In vitro, we found simulated ischemia enhanced BAG3 expression in primary human skeletal muscle cells, whereas BAG3 knockdown increased necroptosis markers and decreased cell viability. In vivo, ischemic skeletal muscles from hind limbs of high-fat diet (HFD)-fed mice showed poor BAG3 expression compared to normal chow diet (NCD)-fed mice, and this was associated with increased limb amputations. BAG3 overexpression in ischemic skeletal muscles from hind limbs of HFD mice rescued limb amputation and improved autophagy, necroptosis, skeletal muscle function and regeneration. Therefore, BAG3 deficiency in ischemic skeletal muscles contributes to the severity of ischemic limb injury in diabetic PAD, likely through autophagy and necroptosis pathways.

pH/Thermosensitive dual-responsive hydrogel based sequential delivery for site-specific acute limb ischemia treatment

Acute limb ischemia (ALI) is the most severe manifestation of peripheral artery disease, accompanied by pH/temperature-microenvironment changes in two different phases. In the acute phase, temperature and pH are significantly decreased, and reactive oxygen species (ROS) are excessively generated owing to the sharp reduction of blood perfusion. Afterwards, in the chronic phase, although the temperature gradually recovers, angiogenesis is delayed due to chronic vascular injury, skeletal muscle cell apoptosis and endothelial cell dysfunction. Current therapeutic strategies mainly focus on recanalization; however, their effects on scavenging ROS in the acute phase and promoting angiogenesis in the chronic phase are quite limited. Herein, an injectable pH and temperature dual-responsive poloxamer 407 (PF127)/hydroxymethyl cellulose (HPMC)/sodium alginate (SA)-derived hydrogel (FHSgel), encapsulating melatonin and diallyl trisulfide-loaded biodegradable hollow mesoporous silica nanoparticles (DATS@dHMSNs), is developed, which can intelligently respond to the different phases of ALI. In the acute phase of ischemia, the decreased pH results in the rapid release of melatonin to scavenge excessive ischemia-induced ROS. On the other hand, in the chronic repair phase, the recovered temperature triggers the sustained release of DATS@dHMSNs from the FHSgel, thus generating hydrogen sulfide (H₂S) to enhance the angiogenesis and microcirculation reconstruction of ischemic limbs.

AAV-mediated expression of PFKFB3 in myofibers, but not endothelial cells, improves ischemic muscle function in mice with critical limb ischemia

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) is a powerful driver of angiogenesis through its modulation of glycolytic metabolism within endothelial cells. Recent work has demonstrated that PFKFB3 modulates the response to muscle ischemia, however the cell specificity of these effects is not fully understood. In this study, we tested the impact of viral mediated expression of PFKFB3, driven by gene promoters specific for myofibers or endothelial cells, on ischemic hindlimb revascularization and muscle function. We hypothesized that both endothelium- and muscle-specific expression of PFKFB3 would attenuate limb pathology following femoral artery ligation. Male and female BALB/cJ mice were injected with adeno-associated virus encoding the either a green fluorescent protein (GFP) or PFKFB3 driven by either the human skeletal actin (ACTA1) or cadherin-5 (Cdh5) promoters. Four weeks after AAV treatment, mice were subjected to unilateral femoral artery ligation and limb perfusion and muscle function were assessed. Both endothelium- and muscle-specific PFKFB3 expression resulted in significantly more perfused capillaries within the ischemic limb muscle, but neither changed myofiber size/area. Muscle-specific, but not endothelium-specific, PFKFB3 expression significantly improved maximal force production in ischemic muscle (P = 0.0005). Notably, there was a significant effect of sex on maximal force levels in both cohorts of mice (P = 0.0075 and P = 0.0481), indicating that female mice had higher ischemic muscle strength compared with male mice, regardless of treatment group. Taken together, these data demonstrate that although both muscle- and endothelium-specific expression of PFKFB3 enhanced ischemic revascularization, only muscle-specific PFKFB3 expression improved muscle function.NEW & NOTEWORTHY Critical limb ischemia (CLI) carries a significant risk for limb amputation, and treatment options remain limited. We tested the impact of expression of PFKFB3 in myofibers or endothelial cells on limb pathology in mice with CLI. Although both muscle and endothelium-specific PFKFB3 expression increased perfused capillary density, only muscle-specific PFKFB3 expression improve contractile function. Regardless of treatment, female mice demonstrated better recovery from limb ischemic compared with male mice.

Dysregulated autophagy-related genes in septic cardiomyopathy: Comprehensive bioinformatics analysis based on the human transcriptomes and experimental validation

Septic cardiomyopathy (SCM) is severe organ dysfunction caused by sepsis that is associated with poor prognosis, and its pathobiological mechanisms remain unclear. Autophagy is a biological process that has recently been focused on SCM, yet the current understanding of the role of dysregulated autophagy in the pathogenesis of SCM remains limited and uncertain. Exploring the molecular mechanisms of disease based on the transcriptomes of human pathological samples may bring the closest insights. In this study, we analyzed the differential expression of autophagy-related genes in SCM based on the transcriptomes of human septic hearts, and further explored their potential crosstalk and functional pathways. Key functional module and hub genes were identified by constructing a protein–protein interaction network. Eight key genes (CCL2, MYC, TP53, SOD2, HIF1A, CTNNB1, CAT, and ADIPOQ) that regulate autophagy in SCM were identified after validation in a lipopolysaccharide (LPS)-induced H9c2 cardiomyoblast injury model, as well as the autophagic characteristic features. Furthermore, we found that key genes were associated with abnormal immune infiltration in septic hearts and have the potential to serve as biomarkers. Finally, we predicted drugs that may play a protective role in SCM by regulating autophagy based on our results. Our study provides evidence and new insights into the role of autophagy in SCM based on human septic heart transcriptomes, which would be of great benefit to reveal the molecular pathological mechanisms and explore the diagnostic and therapeutic targets for SCM.

Multivalent protein-protein interactions are pivotal regulators of eukaryotic Hsp70 complexes

Heat shock protein 70 (Hsp70) is a molecular chaperone and central regulator of protein homeostasis (proteostasis). Paramount to this role is Hsp70's binding to client proteins and co-chaperones to produce distinct complexes, such that understanding the protein-protein interactions (PPIs) of Hsp70 is foundational to describing its function and dysfunction in disease. Mounting evidence suggests that these PPIs include both "canonical" interactions, which are universally conserved, and "non-canonical" (or "secondary") contacts that seem to have emerged in eukaryotes. These two categories of interactions involve discrete binding surfaces, such that some clients and co-chaperones engage Hsp70 with at least two points of contact. While the contributions of canonical interactions to chaperone function are becoming increasingly clear, it can be challenging to deconvolute the roles of secondary interactions. Here, we review what is known about non-canonical contacts and highlight examples where their contributions have been parsed, giving rise to a model in which Hsp70's secondary contacts are not simply sites of additional avidity but are necessary and sufficient to impart unique functions. From this perspective, we propose that further exploration of non-canonical contacts will generate important insights into the evolution of Hsp70 systems and inspire new approaches for developing small molecules that tune Hsp70-mediated proteostasis.

SS31 Ameliorates Oxidative Stress via the Restoration of Autophagic Flux to Protect Aged Mice From Hind Limb Ischemia

Background

Oxidative stress and impaired autophagic flux play important roles in the development of peripheral artery disease (PAD). SS31 is considered an important antioxidant peptide and autophagy regulator. We aimed to investigate the role of SS31 in PAD myopathy and its possible mechanism both in vivo and in vitro.

Methods

A hind limb ischemia (HLI) model was established with old C57BL/6 (14-month-old) mice. Mice in the SS31 group were intraperitoneally injected with SS31 (3 mg/kg) for 4 weeks. We examined skeletal muscle function and histomorphology, autophagy-related protein levels and reactive oxygen species (ROS) content. For the in vitro experiments, after C2C12 myotubes were treated with CoCl₂, SS31, and chloroquine (CQ) or rapamycin (RAPA), we measured ROS content, autophagy-related protein levels and antioxidant enzyme expression.

Results

SS31 treatment effectively enhanced the recovery of skeletal muscle function, alleviated skeletal muscle injury and suppressed mitochondrial ROS production in ischemic limbs. SS31 reduced apoptosis and oxidative stress, and SS31 restored impaired autophagic flux by inhibiting the AKT-mTOR pathway. In vitro studies showed that SS31 restored autophagic flux and improved oxidative stress in C2C12 cells. Moreover, phosphorylated AKT (p-AKT) and phosphorylated mTOR (p-mTOR) levels were reduced.

Conclusion

These experiments indicated that SS31 can inhibit oxidative stress by restoring autophagic flux to reverse hypoxia-induced injury in vivo and in vitro.

MicroRNA regulation of BAG3

B-cell lymphoma 2 (Bcl-2)-associated athanogene 3 (BAG3) protein is a member of BAG family of co-chaperones that modulates major biological processes, including apoptosis, autophagy, and development to promote cellular adaptive responses to stress stimuli. Although BAG3 is constitutively expressed in several cell types, its expression is also inducible and is regulated by microRNAs (miRNAs). miRNAs are small non-coding RNAs that mostly bind to the 3'-UTR (untranslated region) of mRNAs to inhibit their translation or to promote their degradation. miRNAs can potentially regulate over 50% of the protein-coding genes in a cell and therefore are involved in the regulation of all major functions, including cell differentiation, growth, proliferation, apoptosis, and autophagy. Dysregulation of miRNA expression is associated with pathogenesis of numerous diseases, including peripheral artery disease (PAD). BAG3 plays a critical role in regulating the response of skeletal muscle cells to ischemia by its ability to regulate autophagy. However, the biological role of miRNAs in the regulation of BAG3 in biological processes has only been elucidated recently. In this review, we discuss how miRNA may play a key role in regulating BAG3 expression under normal and pathological conditions.

Chronic aryl hydrocarbon receptor activity phenocopies smoking-induced skeletal muscle impairment

BACKGROUND

Chronic obstructive pulmonary disease (COPD) patients exhibit skeletal muscle atrophy, denervation, and reduced mitochondrial oxidative capacity. Whilst chronic tobacco smoke exposure is implicated in COPD muscle impairment, the mechanisms involved are ambiguous. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that activates detoxifying pathways with numerous exogenous ligands, including tobacco smoke. Whereas transient AHR activation is adaptive, chronic activation can be toxic. On this basis, we tested the hypothesis that chronic smoke-induced AHR activation causes adverse muscle impact.

METHODS

We used clinical patient muscle samples, and in vitro (C2C12 myotubes) and in vivo models (mouse), to perform gene expression, mitochondrial function, muscle and neuromuscular junction morphology, and genetic manipulations (adeno-associated virus-mediated gene transfer).

RESULTS

Sixteen weeks of tobacco smoke exposure in mice caused muscle atrophy, neuromuscular junction degeneration, and reduced oxidative capacity. Similarly, smoke exposure reprogrammed the muscle transcriptome, with down-regulation of mitochondrial and neuromuscular junction genes. In mouse and human patient specimens, smoke exposure increased muscle AHR signalling. Mechanistically, experiments in cultured myotubes demonstrated that smoke condensate activated the AHR, caused mitochondrial impairments, and induced an AHR-dependent myotube atrophy. Finally, to isolate the role of AHR activity, expression of a constitutively active AHR mutant without smoke exposure caused atrophy and mitochondrial impairments in cultured myotubes, and muscle atrophy and neuromuscular junction degeneration in mice.

CONCLUSIONS

These results establish that chronic AHR activity, as occurs in smokers, phenocopies the atrophy, mitochondrial impairment, and neuromuscular junction degeneration caused by chronic tobacco smoke exposure.

Interventional- and amputation-stage muscle proteomes in the chronically threatened ischemic limb

BACKGROUND

Despite improved surgical approaches for chronic limb-threatening ischemia (CLTI), amputation rates remain high and contributing tissue-level factors remain unknown. The purpose of this study was twofold: (1) to identify differences between the healthy adult and CLTI limb muscle proteome, and (2) to identify differences in the limb muscle proteome of CLTI patients prior to surgical intervention or at the time of amputation.

METHODS AND RESULTS

Gastrocnemius muscle was collected from non-ischemic controls (n = 19) and either pre-interventional surgery (n = 10) or at amputation outcome (n = 29) CLTI patients. All samples were subjected to isobaric tandem-mass-tag-assisted proteomics. The mitochondrion was the primary classification of downregulated proteins (> 70%) in CLTI limb muscles and paralleled robust functional mitochondrial impairment. Upregulated proteins (> 38%) were largely from the extracellular matrix. Across the two independent sites, 39 proteins were downregulated and 12 upregulated uniformly. Pre-interventional CLTI muscles revealed a robust upregulation of mitochondrial proteins but modest functional impairments in fatty acid oxidation as compared with controls. Comparison of pre-intervention and amputation CLTI limb muscles revealed mitochondrial proteome and functional deficits similar to that between amputation and non-ischemic controls. Interestingly, these observed changes occurred despite 62% of the amputation CLTI patients having undergone a prior surgical intervention.

CONCLUSIONS

The CLTI proteome supports failing mitochondria as a phenotype that is unique to amputation outcomes. The signature of pre-intervention CLTI muscle reveals stable mitochondrial protein abundance that is insufficient to uniformly prevent functional impairments. Taken together, these findings support the need for future longitudinal investigations aimed to determine whether mitochondrial failure is causally involved in amputation outcomes from CLTI.

S100A8 and S100A9 are elevated in chronically threatened ischemic limb muscle and induce ischemic mitochondrial pathology in mice

Objective

The objective of the present study was to determine whether elevated levels of S100A8 and S100A9 (S100A8/A9) alarmins contribute to ischemic limb pathology.

Methods

Gastrocnemius muscle was collected from control patients without peripheral arterial disease (PAD; n = 14) and patients with chronic limb threatening limb ischemia (CLTI; n = 14). Mitochondrial function was assessed in permeabilized muscle fibers, and RNA and protein analyses were used to quantify the S100A8/A9 levels. Additionally, a mouse model of hindlimb ischemia with and without exogenous delivery of S100A8/A9 was used.

Results

Compared with the non-PAD control muscles, CLTI muscles displayed significant increases in the abundance of S100A8 and S100A9 at both mRNA and protein levels (P < .01). The CLTI muscles also displayed significant impairment in mitochondrial oxidative phosphorylation and increased mitochondrial hydrogen peroxide production compared with the non-PAD controls. The S100A8/A9 levels correlated significantly with the degree of muscle mitochondrial dysfunction (P < .05 for all). C57BL6J mice treated with recombinant S100A8/A9 displayed impaired perfusion recovery and muscle mitochondrial impairment compared with the placebo-treated mice after hindlimb ischemia surgery. These mitochondrial deficits observed after S100A8/A9 treatment were confirmed in the muscle cell culture system under normoxic conditions.

Conclusions

The S100A8/A9 levels were increased in CLTI limb muscle specimens compared with the non-PAD control muscle specimens, and the level of accumulation was associated with muscle mitochondrial impairment. Elevated S100A8/A9 levels in mice subjected to hindlimb ischemia impaired perfusion recovery and mitochondrial function. Together, these findings suggest that the inflammatory mediators S100A8/A9 might be directly involved in ischemic limb pathology.

Despite improvements in the surgical management of chronic limb threatening limb ischemia (CLTI), the rates of major adverse limb events have remained high. Skeletal muscle has emerged as a strong predictor of outcomes in peripheral arterial disease (PAD)/CLTI; however, a complete understanding of muscle pathology in CLTI is lacking. This study identified elevated S100A8 and S100A9 alarmin proteins as a characteristic of CLTI muscle specimens and that the S100A8/A9 levels are associated with the degree of mitochondrial impairment in patient limb muscle specimens. Using a mouse model of PAD, treatment with S100A8/A9 exacerbated ischemic limb pathology, including impaired limb perfusion recovery and muscle mitochondrial impairment. Taken together, these findings connect the inflammatory milieu in the CLTI limb to exacerbated limb muscle outcomes via mitochondrial alterations.

Low Intrinsic Aerobic Capacity Limits Recovery Response to Hindlimb Ischemia

Introduction: In this study, we determined the influence of intrinsic exercise capacity on the vascular adaptive responses to hind limb ischemia. High Capacity Running, HCR; Low Capacity Running, LCR, rats were used to assess intrinsic aerobic capacity effects on adaptive responses to ischemia.

Methods: Muscle samples from both ischemic and non-ischemic limb in both strains were compared, histologically for the muscle-capillary relationship, and functionally using microspheres to track blood flow and muscle stimulation to test fatigability. PCR was used to identify the differences in gene expression between the phenotypes following occlusive ischemia.

Results: Prior to ligation, there were not significant differences between the phenotypes in the exhaustion time with high frequency pacing. Following ligation, LCR decreased significantly in the exhaustion time compare with HCRs (437 ± 47 vs. 824 ± 56, p < 0.001). The immediate decrease in flow was significantly more severe in LCRs than HCRs (52.5 vs. 37.8%, p < 0.001). VEGF, eNOS, and ANG2 (but not ANG1) gene expression were decreased in LCRs vs. HCRs before occlusion, and increased significantly in LCRs 14D after occlusion, but not in HCRs. LCR capillary density (CD) was significantly lower at all time points after occlusion (LCR 7D = 564.76 ± 40.5, LCR 14D = 507.48 ± 54.2, both p < 0.05 vs. HCR for respective time point). NCAF increased significantly in HCR and LCR in response to ischemia.

Summary: These results suggest that LCR confers increased risk for ischemic injury and is subject to delayed and less effective adaptive response to ischemic stress.

Chronic intermittent hypobaric hypoxia attenuates ischemic limb injury by promoting angiogenesis in mice

This study aimed to evaluate the protective effect of chronic intermittent hypobaric hypoxia (CIHH) against limb ischemic injury. C57BL/6 mice were randomly divided into three groups: limb ischemic injury group (Ischemia, induced by ligation and excision of the left femoral artery), limb ischemia following CIHH pretreatment group (CIHH+Ischemia, simulated a 5000 m altitude hypoxia, 6 h per day for 28 days, before induction of hind-limb ischemia), and sham group (Sham). The blood flow in the mouse models of hind-limb ischemia was examined using laser doppler imaging. The functional and morphological performance of ischemic muscle was evaluated using contraction force and hematoxylin-eosin and Masson's trichrome staining. Angiogenesis was determined by immunohistochemistry staining of the endothelial markers CD31 and CD34. The protein expressions of angiogenesis-related genes were detected using Western blot assay. Chronic ischemia resulted in reduced blood perfusion, decreased contraction tension, and morphological destruction in gastrocnemius muscle. CIHH pretreatment increased the contractile force and muscle fiber diameter and decreased necrosis and fibrosis of the ischemic muscle. Also, CIHH significantly increased the density of CD31+ and CD34+ cells and promoted the expression of angiogenesis-related molecules in ischemic muscle. These data demonstrate that CIHH has a protective effect against chronic limb ischemia by promoting angiogenesis.

Therapeutic targeting of BAG3: considering its complexity in cancer and heart disease

Bcl2-associated athanogene-3 (BAG3) is expressed ubiquitously in humans, but its levels are highest in the heart, the skeletal muscle, and the central nervous system; it is also elevated in many cancers. BAG3's diverse functions are supported by its multiple protein-protein binding domains, which couple with small and large heat shock proteins, members of the Bcl2 family, other antiapoptotic proteins, and various sarcomere proteins. In the heart, BAG3 inhibits apoptosis, promotes autophagy, couples the β-adrenergic receptor with the L-type Ca2+ channel, and maintains the structure of the sarcomere. In cancer cells, BAG3 binds to and supports an identical array of prosurvival proteins, and it may represent a therapeutic target. However, the development of strategies to block BAG3 function in cancer cells may be challenging, as they are likely to interfere with the essential roles of BAG3 in the heart. In this Review, we present the current knowledge regarding the biology of this complex protein in the heart and in cancer and suggest several therapeutic options.

Murine allele and transgene symbols: ensuring unique, concise, and informative nomenclature

In addition to naturally occurring sequence variation and spontaneous mutations, a wide array of technologies exist for modifying the mouse genome. Standardized nomenclature, including allele, transgene, and other mutation nomenclature, as well as persistent unique identifiers (PUID) are critical for effective scientific communication, comparison of results, and integration of data into knowledgebases such as Mouse Genome Informatics (MGI), Alliance for Genome Resources, and International Mouse Strain Resource (IMSR). As well as being the authoritative source for mouse gene, allele, and strain nomenclature, MGI integrates published and unpublished genomic, phenotypic, and expression data while linking to other online resources for a complete view of the mouse as a valuable model organism. The International Committee on Standardized Genetic Nomenclature for Mice has developed allele nomenclature rules and guidelines that take into account the number of genes impacted, the method of allele generation, and the nature of the sequence alteration. To capture details that cannot be included in allele symbols, MGI has further developed allele to gene relationships using sequence ontology (SO) definitions for mutations that provide links between alleles and the genes affected. MGI is also using (HGVS) variant nomenclature for variants associated with alleles that will enhance searching for mutations and will improve cross-species comparison. With the ability to assign unique and informative symbols as well as to link alleles with more than one gene, allele and transgene nomenclature rules and guidelines provide an unambiguous way to represent alterations in the mouse genome and facilitate data integration among multiple resources such the Alliance of Genome Resources and International Mouse Strain Resource.

Racial differences in the limb skeletal muscle transcriptional programs of patients with critical limb ischemia

Critical limb ischemia (CLI) is the most severe manifestation of peripheral artery disease (PAD) and is characterized by high rates of morbidity and mortality. As with most severe cardiovascular disease manifestations, Black individuals disproportionately present with CLI. Accordingly, there remains a clear need to better understand the reasons for this discrepancy and to facilitate personalized therapeutic options specific for this population. Gastrocnemius muscle was obtained from White and Black healthy adult volunteers and patients with CLI for whole transcriptome shotgun sequencing (WTSS) and enrichment analysis was performed to identify alterations in specific Reactome pathways. When compared to their race-matched healthy controls, both White and Black patients with CLI demonstrated similar reductions in nuclear and mitochondrial encoded genes and mitochondrial oxygen consumption across multiple substrates, indicating a common bioenergetic paradigm associated with amputation outcomes regardless of race. Direct comparisons between tissues of White and Black patients with CLI revealed hemostasis, extracellular matrix organization, platelet regulation, and vascular wall interactions to be uniquely altered in limb muscles of Black individuals. Among traditional vascular growth factor signaling targets, WTSS revealed only Tie1 to be significantly altered from White levels in Black limb muscle tissues. Quantitative reverse transcription polymerase chain reaction validation of select identified targets verified WTSS directional changes and supports reductions in MMP9 and increases in NUDT4P1 and GRIK2 as unique to limb muscles of Black patients with CLI. This represents a critical first step in better understanding the transcriptional program similarities and differences between Black and White patients in the setting of amputations related to CLI and provides a promising start for therapeutic development in this population.

Unique Metabolomic Profile of Skeletal Muscle in Chronic Limb Threatening Ischemia

Chronic limb threatening ischemia (CLTI) is the most severe manifestation of peripheral atherosclerosis. Patients with CLTI have poor muscle quality and function and are at high risk for limb amputation and death. The objective of this study was to interrogate the metabolome of limb muscle from CLTI patients. To accomplish this, a prospective cohort of CLTI patients undergoing either a surgical intervention (CLTI Pre-surgery) or limb amputation (CLTI Amputation), as well as non-peripheral arterial disease (non-PAD) controls were enrolled. Gastrocnemius muscle biopsy specimens were obtained and processed for nuclear magnetic resonance (NMR)-based metabolomics analyses using solution state NMR on extracted aqueous and organic phases and ¹H high-resolution magic angle spinning (HR-MAS) on intact muscle specimens. CLTI Amputation specimens displayed classical features of ischemic/hypoxic metabolism including accumulation of succinate, fumarate, lactate, alanine, and a significant decrease in the pyruvate/lactate ratio. CLTI Amputation muscle also featured aberrant amino acid metabolism marked by elevated branched chain amino acids. Finally, both Pre-surgery and Amputation CLTI muscles exhibited pronounced accumulation of lipids, suggesting the presence of myosteatosis, including cholesterol, triglycerides, and saturated fatty acids. Taken together, these metabolite differences add to a growing body of literature that have characterized profound metabolic disturbance’s in the failing ischemic limb of CLTI patients.

Skeletal Muscle Mitochondrial Dysfunction and Oxidative Stress in Peripheral Arterial Disease: A Unifying Mechanism and Therapeutic Target

Peripheral artery disease (PAD) is caused by atherosclerosis in the lower extremities, which leads to a spectrum of life-altering symptomatology, including claudication, ischemic rest pain, and gangrene requiring limb amputation. Current treatments for PAD are focused primarily on re-establishing blood flow to the ischemic tissue, implying that blood flow is the decisive factor that determines whether or not the tissue survives. Unfortunately, failure rates of endovascular and revascularization procedures remain unacceptably high and numerous cell- and gene-based vascular therapies have failed to demonstrate efficacy in clinical trials. The low success of vascular-focused therapies implies that non-vascular tissues, such as skeletal muscle and oxidative stress, may substantially contribute to PAD pathobiology. Clues toward the importance of skeletal muscle in PAD pathobiology stem from clinical observations that muscle function is a strong predictor of mortality. Mitochondrial impairments in muscle have been documented in PAD patients, although its potential role in clinical pathology is incompletely understood. In this review, we discuss the underlying mechanisms causing mitochondrial dysfunction in ischemic skeletal muscle, including causal evidence in rodent studies, and highlight emerging mitochondrial-targeted therapies that have potential to improve PAD outcomes. Particularly, we will analyze literature data on reactive oxygen species production and potential counteracting endogenous and exogenous antioxidants.

PFKFB3-mediated glycolysis rescues myopathic outcomes in the ischemic limb

Compromised muscle mitochondrial metabolism is a hallmark of peripheral arterial disease, especially in patients with the most severe clinical manifestation - critical limb ischemia (CLI). We asked whether inflexibility in metabolism is critical for the development of myopathy in ischemic limb muscles. Using Polg mtDNA mutator (D257A) mice, we reveal remarkable protection from hind limb ischemia (HLI) due to a unique and beneficial adaptive enhancement of glycolytic metabolism and elevated ischemic muscle PFKFB3. Similar to the relationship between mitochondria from CLI and claudicating patient muscles, BALB/c muscle mitochondria are uniquely dysfunctional after HLI onset as compared with the C57BL/6 (BL6) parental strain. AAV-mediated overexpression of PFKFB3 in BALB/c limb muscles improved muscle contractile function and limb blood flow following HLI. Enrichment analysis of RNA sequencing data on muscle from CLI patients revealed a unique deficit in the glucose metabolism Reactome. Muscles from these patients express lower PFKFB3 protein, and their muscle progenitor cells possess decreased glycolytic flux capacity in vitro. Here, we show supplementary glycolytic flux as sufficient to protect against ischemic myopathy in instances where reduced blood flow-related mitochondrial function is compromised preclinically. Additionally, our data reveal reduced glycolytic flux as a common characteristic of the failing CLI patient limb skeletal muscle.

Host BAG3 Is Degraded by Pseudorabies Virus pUL56 C-Terminal 181L-185L and Plays a Negative Regulation Role during Viral Lytic Infection

Bcl2-associated athanogene (BAG) 3, which is a chaperone-mediated selective autophagy protein, plays a pivotal role in modulating the life cycle of a wide variety of viruses. Both positive and negative modulations of viruses by BAG3 were reported. However, the effects of BAG3 on pseudorabies virus (PRV) remain unknown. To investigate whether BAG3 could modulate the PRV life cycle during a lytic infection, we first identified PRV protein UL56 (pUL56) as a novel BAG3 interactor by co-immunoprecipitation and co-localization analyses. The overexpression of pUL56 induced a significant degradation of BAG3 at protein level via the lysosome pathway. The C-terminal mutations of ¹⁸¹L/A, ¹⁸⁵L/A, or ¹⁸¹L/A-¹⁸⁵L/A in pUL56 resulted in a deficiency in pUL56-induced BAG3 degradation. In addition, the pUL56 C-terminal mutants that lost Golgi retention abrogated pUL56-induced BAG3 degradation, which indicates a Golgi retention-dependent manner. Strikingly, BAG3 was not observed to be degraded in either wild-type or UL56-deleted PRV infected cells as compared to mock infected ones, whereas the additional two adjacent BAG3 cleaved products were found in the infected cells in a species-specific manner. Overexpression of BAG3 significantly suppressed PRV proliferation, while knockdown of BAG3 resulted in increased viral yields in HEK293T cells. Thus, these data indicated a negative regulation role of BAG3 during PRV lytic infection. Collectively, our findings revealed a novel molecular mechanism on host protein degradation induced by PRV pUL56. Moreover, we identified BAG3 as a host restricted protein during PRV lytic infection in cells.

Autophagy: an essential but limited cellular process for timely skeletal muscle recovery from injury

Macroautophagy/autophagy induction, i.e., the formation of autophagosomes, is robust following many forms of muscle injury. Autophagy inhibition studies strongly indicate that autophagy is necessary for successful muscle fiber recovery. Now, there are accumulating pieces of evidence indicating that autophagosome clearance, i.e., autophagy flux, does not increase to match the burden of accumulating damaged proteins and organelles after muscle fiber damage, creating a bottleneck effect. Some potential consequences of the bottleneck effect are reduced regenerative capacity marked by the inadequate activation of muscle stem cells (i.e., satellite cells) and a lesser commitment toward differentiation due to a deficiency in energetic substrates and/or molecular signaling pathways. These findings highlight an emerging area of investigation for both autophagy and muscle regeneration fields. The identification of the molecular mechanisms governing autophagy and autophagy flux may serve as targets for future therapies to enhance the recovery of its function in healthy and diseased muscle.

ABBREVIATIONS

BNIP3: BCL2/adenovirus E1B interacting protein 3; CQ: chloroquine; DMD: Duchenne muscular dystrophy; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; ULK1: unc-51 like kinase 1.

The Role of Autophagy in the Innate Immune Response to Fungal Keratitis Caused by Aspergillus fumigatus Infection

Purpose

To determine the role of autophagy in the innate immune response to fungal keratitis (FK) caused by Aspergillus fumigatus infection.

Methods

Corneal samples obtained from patients and mice with FK were visualized via transmission electron microscopy (TEM). Autophagy-related proteins LC3B-II, Beclin-1, LAMP-1, and p62 in A. fumigatus-infected corneas of C57BL/6 mice were tested by Western blot. After treatment with autophagy inhibitors 3-methyladenine (3-MA), chloroquine (CQ), or inducer rapamycin, autophagy-related proteins were detected by Western blot. Corneas were photographed with slit lamp microscopy and pathological changes were observed by hematoxylin and eosin staining. Polymorphonuclear neutrophilic leukocytes (PMNs) were assessed by immunofluorescent staining and observed under TEM. The levels of CXCL-1, IL-1β, HMGB1, IL-18, TNF-α, and IL-10 were tested by reverse transcription polymerase chain reaction and Western blot. The quantification of fungal loads was detected and photographed.

Results

The accumulation of autophagosomes in corneas of patients and mice with FK was observed with TEM. The expression of LC3B-II, Beclin-1, and LAMP-1 was elevated in corneas after fungal infection, whereas p62 was reduced. Treatment with 3-MA or CQ upregulated clinical scores, pathological changes, and the expression of CXCL-1, IL-1β, HMGB1, IL-18, and TNF-α except IL-10. The morphology of PMNs was changed and PMN recruitment was increased in mice corneas treated with 3-MA or CQ, whereas rapamycin reduced the inflammatory response to keratitis. These results were statistically significant.

Conclusions

A. fumigatus infection increases the expression of autophagy in corneas. Autophagy plays an anti-inflammatory role in the innate immune response to A. fumigatus keratitis.

Chronic kidney disease exacerbates ischemic limb myopathy in mice via altered mitochondrial energetics

Chronic kidney disease (CKD) substantially increases the severity of peripheral arterial disease (PAD) symptomology, however, the biological mechanisms remain unclear. The objective herein was to determine the impact of CKD on PAD pathology in mice. C57BL6/J mice were subjected to a diet-induced model of CKD by delivery of adenine for six weeks. CKD was confirmed by measurements of glomerular filtration rate, blood urea nitrogen, and kidney histopathology. Mice with CKD displayed lower muscle force production and greater ischemic lesions in the tibialis anterior muscle (78.1 ± 14.5% vs. 2.5 ± 0.5% in control mice, P < 0.0001, N = 5-10/group) and decreased myofiber size (1661 ± 134 μm2 vs. 2221 ± 100 μm2 in control mice, P < 0.01, N = 5-10/group). This skeletal myopathy occurred despite normal capillary density (516 ± 59 vs. 466 ± 45 capillaries/20x field of view) and limb perfusion. CKD mice displayed a ~50-65% reduction in muscle mitochondrial respiratory capacity in ischemic muscle, whereas control mice had normal mitochondrial function. Hydrogen peroxide emission was modestly higher in the ischemic muscle of CKD mice, which coincided with decreased oxidant buffering. Exposure of cultured myotubes to CKD serum resulted in myotube atrophy and elevated oxidative stress, which were attenuated by mitochondrial-targeted therapies. Taken together, these findings suggest that mitochondrial impairments caused by CKD contribute to the exacerbation of ischemic pathology.

The Hippo network kinase STK38 contributes to protein homeostasis by inhibiting BAG3-mediated autophagy

Chaperone-assisted selective autophagy (CASA) initiated by the cochaperone Bcl2-associated athanogene 3 (BAG3) represents an important mechanism for the disposal of misfolded and damaged proteins in mammalian cells. Under mechanical stress, the cochaperone cooperates with the small heat shock protein HSPB8 and the cytoskeleton-associated protein SYNPO2 to degrade force-unfolded forms of the actin-crosslinking protein filamin. This is essential for muscle maintenance in flies, fish, mice and men. Here, we identify the serine/threonine protein kinase 38 (STK38), which is part of the Hippo signaling network, as a novel interactor of BAG3. STK38 was previously shown to facilitate cytoskeleton assembly and to promote mitophagy as well as starvation and detachment induced autophagy. Significantly, our study reveals that STK38 exerts an inhibitory activity on BAG3-mediated autophagy. Inhibition relies on a disruption of the functional interplay of BAG3 with HSPB8 and SYNPO2 upon binding of STK38 to the cochaperone. Of note, STK38 attenuates CASA independently of its kinase activity, whereas previously established regulatory functions of STK38 involve target phosphorylation. The ability to exert different modes of regulation on central protein homeostasis (proteostasis) machineries apparently allows STK38 to coordinate the execution of diverse macroautophagy pathways and to balance cytoskeleton assembly and degradation.

Dual specificity phosphatase 5 regulates perfusion recovery in experimental peripheral artery disease

Peripheral artery disease (PAD) is caused by atherosclerotic occlusions of vessels outside the heart, particularly those of the lower extremities. Angiogenesis is one critical physiological response to vessel occlusion in PAD, but our understanding of the molecular mechanisms involved in angiogenesis is incomplete. Dual specificity phosphatase 5 (DUSP5) has been shown to play a key role in embryonic vascular development, but its role in post-ischemic angiogenesis is not known. We induced hind limb ischemia in mice and found robust upregulation of Dusp5 expression in ischemic hind limbs. Moreover, in vivo knockdown of Dusp5 resulted in impaired perfusion recovery in ischemic limbs and was associated with increased limb necrosis. In vitro studies showed upregulation of DUSP5 in human endothelial cells exposed to ischemia, and knockdown of DUSP5 in these ischemic endothelial cells resulted in impaired endothelial cell proliferation and angiogenesis, but did not alter apoptosis. Finally, we show that these effects of DUSP5 on post-ischemic angiogenesis are a result of DUSP5-dependent decrease in ERK1/2 phosphorylation and p21 protein expression. Thus, we have identified a role of DUSP5 in post-ischemic angiogenesis and implicated a DUSP5-ERK-p21 pathway that may serve as a therapeutic target for the modulation of post-ischemic angiogenesis in PAD.

Heat therapy improves soleus muscle force in a model of ischemia-induced muscle damage

Leg muscle ischemia in patients with peripheral artery disease (PAD) leads to alterations in skeletal muscle morphology and reduced leg strength. We tested the hypothesis that exposure to heat therapy (HT) would improve skeletal muscle function in a mouse model of ischemia-induced muscle damage. Male 42-wk-old C57Bl/6 mice underwent ligation of the femoral artery and were randomly assigned to receive HT (immersion in a water bath at 37°C, 39°C, or 41°C for 30 min) or a control intervention for 3 wk. At the end of the treatment, the animals were anesthetized and the soleus and extensor digitorum longus (EDL) muscles were harvested for the assessment of contractile function and examination of muscle morphology. A subset of animals was used to examine the impact of a single HT session on the expression of genes involved in myogenesis and the regulation of muscle mass. Relative soleus muscle mass was significantly higher in animals exposed to HT at 39°C compared with the control group (control: 0.36 ± 0.01 mg/g versus 39°C: 0.40 ± 0.01 mg/g, P = 0.024). Maximal absolute force of the soleus was also significantly higher in animals treated with HT at 37°C and 39°C (control: 274.7 ± 6.6 mN; 37°C: 300.1 ± 7.7 mN; 39°C: 299.5 ± 10 mN, P < 0.05). In the soleus, but not the EDL muscle, a single session of HT enhanced the mRNA expression of myogenic factors as well as of both positive and negative regulators of muscle mass. These findings suggest that the beneficial effects of HT are muscle specific and dependent on the treatment temperature in a model of PAD. NEW & NOTEWORTHY This is the first study to comprehensively examine the impact of temperature and muscle fiber type composition on the adaptations to repeated heat stress in a model of ischemia-induced muscle damage. Exposure to heat therapy (HT) at 37°C and 39°C, but not at 41°C, improved force development of the isolated soleus muscle. These results suggest that HT may be a practical therapeutic tool to restore muscle mass and strength in patients with peripheral artery disease.

Disparate expression of autophagy in corneas of C57BL/6 mice and BALB/c mice after Aspergillus fumigatus infection

AIM

To determine the disparate expression of autophagy in the Aspergillus fumigatus (A. fumigatus) keratitis between susceptible C57BL/6 mice and resistant BALB/c mice.

METHODS

C57BL/6 and BALB/c mice were used to establish fungal keratitis models. Disease severity and inflammatory response were observed by slit lamp microscopy in A. fumigatus-infected corneas of C57BL/6 and BALB/c mice at 1, 3 and 5d. Hematoxylin-eosin (H&E) staining was used to detect pathological changes of corneas. The expression of autophagy-related proteins Beclin-1, LC3, SQSTM1/p62, and LAMP-1 was assessed by Western blot in C57BL/6 and BALB/c mice at 1, 3 and 5d post infection (p.i.). Immunofluorescent staining was used to test the expression of LC3 in corneas after A. fumigatus infection.

RESULTS

Keratitis severity was higher in C57BL/6 mice versus BALB/c mice at 1, 3 and 5d p.i. H&E staining showed that the number of inflammatory cells was larger and the severity of ulcer was higher in C57BL/6 mice than in BALB/c mice after stimulation with A. fumigatus. Higher expression of LAMP-1, Beclin-1, and LC3 was shown in C57BL/6 mice corneas than in BALB/c mice corneas at 1, 3 and 5d p.i., while the expression of p62 was lower in C57BL/6 mice. The fluorescence of LC3 was significantly increased in corneas of C57BL/6 mice compared with BALB/c mice after A. fumigatus infection.

CONCLUSION

The expression of autophagy is higher in corneas of C57BL/6 mice than in BALB/c mice after A. fumigatus infection. Autophagy may be positively correlated with keratitis severity and pathological changes.

BAG3 promotes autophagy and glutaminolysis via stabilizing glutaminase

Bcl-2 associated athanogene 3 (BAG3) is an important molecule that maintains oncogenic features of cancer cells via diverse mechanisms. One of the important functions assigned to BAG3 is implicated in selective macroautophagy/autophagy, which attracts much attention recently. However, the mechanism underlying regulation of autophagy by BAG3 has not been well defined. Here, we describe that BAG3 enhances autophagy via promotion of glutamine consumption and glutaminolysis. Glutaminolysis initiates with deamination of glutamine by glutaminase (GLS), by which yields glutamate and ammonia in mitochondria. The current study demonstrates that BAG3 stabilizes GLS via prohibition its interaction with SIRT5, thereby hindering its desuccinylation at Lys158 and Lys164 sites. As an underlying molecular mechanism, we demonstrate that BAG3 interacts with GLS and decreases SIRT5 expression. The current study also demonstrates that occupation by succinyl at Lys158 and Lys164 sites prohibits its Lys48-linked ubiquitination, thereby preventing its subsequent proteasomal degradation. Collectively, the current study demonstrates that BAG3 enhances autophagy via stabilizing GLS and promoting glutaminolysis. For the first time, this study reports that succinylation competes with ubiquitination to regulate proteasomal GLS degradation.

Extensive skeletal muscle cell mitochondriopathy distinguishes critical limb ischemia patients from claudicants

The most severe manifestation of peripheral arterial disease (PAD) is critical limb ischemia (CLI). CLI patients suffer high rates of amputation and mortality; accordingly, there remains a clear need both to better understand CLI and to develop more effective treatments. Gastrocnemius muscle was obtained from 32 older (51-84 years) non-PAD controls, 27 claudicating PAD patients (ankle-brachial index [ABI] 0.65 ± 0.21 SD), and 19 CLI patients (ABI 0.35 ± 0.30 SD) for whole transcriptome sequencing and comprehensive mitochondrial phenotyping. Comparable permeabilized myofiber mitochondrial function was paralleled by both similar mitochondrial content and related mRNA expression profiles in non-PAD control and claudicating patient tissues. Tissues from CLI patients, despite being histologically intact and harboring equivalent mitochondrial content, presented a unique bioenergetic signature. This signature was defined by deficits in permeabilized myofiber mitochondrial function and a unique pattern of both nuclear and mitochondrial encoded gene suppression. Moreover, isolated muscle progenitor cells retained both mitochondrial functional deficits and gene suppression observed in the tissue. These findings indicate that muscle tissues from claudicating patients and non-PAD controls were similar in both their bioenergetics profile and mitochondrial phenotypes. In contrast, CLI patient limb skeletal muscles harbor a unique skeletal muscle mitochondriopathy that represents a potentially novel therapeutic site for intervention.

Strain-Dependent Variation in Acute Ischemic Muscle Injury

Limited efficacy of clinical interventions for peripheral arterial disease necessitates a better understanding of the environmental and genetic determinants of tissue pathology. Existing research has largely ignored the early skeletal muscle injury response during hind limb ischemia (HLI). We compared the hind limb muscle response, after 6 hours of ischemia, in two mouse strains that differ dramatically in their postischemic extended recovery: C57BL/6J and BALB/cJ. Perfusion, measured by laser Doppler and normalized to the control limb, differed only slightly between strains after HLI (<12% across all measures). Similar (<10%) effect sizes in lectin-perfused vessel area and no differences in tissue oxygen saturation measured by reflectance spectroscopy were also found. Muscles from both strains were functionally impaired after HLI, but greater muscle necrosis and loss of dystrophin-positive immunostaining were observed in BALB/cJ muscle compared with C57BL/6J. Muscle cell-specific dystrophin loss and reduced viability were also detected in additional models of ischemia that were independent of residual perfusion differences. Our results indicate that factors other than the completeness of ischemia alone (ie, background genetics) influence the magnitude of acute ischemic muscle injury. These findings may have implications for future development of therapeutic interventions for limb ischemia and for understanding the phasic etiology of chronic and acute ischemic muscle pathophysiology.

The Multifunctional Protein BAG3: A Novel Therapeutic Target in Cardiovascular Disease

The B-cell lymphoma 2–associated anthanogene (BAG3) protein is expressed most prominently in the heart, the skeletal muscle, and in many forms of cancer. In the heart, it serves as a co-chaperone with heat shock proteins in facilitating autophagy; binds to B-cell lymphoma 2, resulting in inhibition of apoptosis; attaches actin to the Z disk, providing structural support for the sarcomere; and links the α-adrenergic receptor with the L-type Ca²⁺ channel. When BAG3 is overexpressed in cancer cells, it facilitates prosurvival pathways that lead to insensitivity to chemotherapy, metastasis, cell migration, and invasiveness. In contrast, in the heart, mutations in BAG3 have been associated with a variety of phenotypes, including both hypertrophic/restrictive and dilated cardiomyopathy. In murine skeletal muscle and vasculature, a mutation in BAG3 leads to critical limb ischemia after femoral artery ligation. An understanding of the biology of BAG3 is relevant because it may provide a therapeutic target in patients with both cardiac and skeletal muscle disease.

miR-206 Inhibits Cell Proliferation, Migration, and Invasion by Targeting BAG3 in Human Cervical Cancer

miR-206 and Bcl-2-associated athanogene 3 (BAG3) have been suggested as important regulators in various cancer types. However, the biological role of miR-206 and BAG3 in cervical cancer (CC) remains unclear. We investigated the expressions and mechanisms of miR-206 and BAG3 in CC using in vitro and in vivo assays. In the present study, miR-206 expression was expressed at a lower level in CC tissues and cells than adjacent normal tissues and NEECs. By contrast, BAG3 mRNA and protein were expressed at higher levels in CC tissues and cells. Furthermore, miR-206 overexpression repressed cell proliferation, migration, and invasion in vitro, and the 3′-untranslated region (3′-UTR) of BAG3 was a direct target of miR-206. miR-206 overexpression also inhibited EGFR, Bcl-2, and MMP2/9 protein expression, but promoted Bax protein expression. Besides, BAG3 overexpression partially abrogated miR-206-inhibited cell proliferation and invasion, while BAG3 silencing enhanced miR-206-mediated inhibition. In vivo assay revealed that miR-206 repressed tumor growth in nude mice xenograft model. In conclusion, miR-206 inhibits cell proliferation, migration, and invasion by targeting BAG3 in human CC. Thus, miR-206-BAG3 can be used as a useful target for CC.

Role of genetics in peripheral arterial disease outcomes; significance of limb-salvage quantitative locus-1 genes

Peripheral artery disease is a major health care problem with significant morbidity and mortality. Humans with peripheral artery disease exhibit two major and differential clinical manifestations - intermittent claudication and critical limb ischemia. Individuals with intermittent claudication or critical limb ischemia have overlapping risk factors and objective measures of blood flow. Hence, we hypothesized that variation in genetic make-up may be an important determinant in the severity of peripheral artery disease. Previous studies have identified polymorphism in genes, contributing to extent of atherosclerosis but much less is known about polymorphisms associated with genes that can influence peripheral artery disease severity. This review outlines some of the progress made up-to-date to unravel the molecular mechanisms underlining differential peripheral artery disease severity. By exploring the recovery phenotype of different mouse strains following experimental peripheral artery disease, our group identified the limb salvage-associated quantitative trait locus 1 on mouse chromosome 7 as the first genetic modifier of perfusion recovery and tissue necrosis phenotypes. Furthermore, a number of genes within LSq-1, such as ADAM12, IL-21Rα, and BAG3 were identified as genetic modifiers of peripheral artery disease severity that function through preservation of endothelial and skeletal muscle cells during ischemia. Taken together, these studies suggest manipulation of limb salvage-associated quantitative trait locus 1 genes show great promise as therapeutic targets in the management of peripheral artery disease. Impact statement Peripheral artery disease (PAD) is a major health care problem with significant morbidity and mortality. Individuals with similar atherosclerosis burden do display different severity of disease. This review outlines some of the progress made up-to-date in unraveling the molecular mechanisms underlining differential PAD severity with a focus on the role of the Limb Salvage-associated Quantitative trait locus 1 (LSq-1), a key locus in adaptation to ischemia in PAD.