Prolyl Hydroxylase Domain-2 Inhibition Improves Skeletal Muscle Regeneration in a Male Murine Model of Obesity

Severely Damaged Freeze-Injured Skeletal Muscle Reveals Functional Impairment, Inadequate Repair, and Opportunity for Human Stem Cell Application

BACKGROUND

The regeneration of severe traumatic muscle injuries is an unsolved medical need that is relevant for civilian and military medicine. In this work, we produced a critically sized nonhealing muscle defect in a mouse model to investigate muscle degeneration/healing phases.

MATERIALS AND METHODS

We caused a freeze injury (FI) in the biceps femoris of C57BL/6N mice. From day 1 to day 25 post-injury, we conducted histological/morphometric examinations, an analysis of the expression of genes involved in inflammation/regeneration, and an in vivo functional evaluation.

RESULTS

We found that FI activates cytosolic DNA sensing and inflammatory responses. Persistent macrophage infiltration, the prolonged expression of eMHC, the presence of centrally nucleated myofibers, and the presence of PAX7+ satellite cells at late time points and with chronic physical impairment indicated inadequate repair. By looking at stem-cell-based therapeutic protocols of muscle repair, we investigated the crosstalk between M1-biased macrophages and human amniotic mesenchymal stem cells (hAMSCs) in vitro. We demonstrated their reciprocal paracrine effects where hAMSCs induced a shift of M1 macrophages into an anti-inflammatory phenotype, and M1 macrophages promoted an increase in the expression of hAMSC immunomodulatory factors.

CONCLUSIONS

Our findings support the rationale for the future use of our injury model to exploit the full potential of in vivo hAMSC transplantation following severe traumatic injuries.

Endothelial Transient Receptor Potential Canonical Channel Regulates Angiogenesis and Promotes Recovery After Myocardial Infarction

Background

Transient receptor potential canonical (TRPC) channels play a role in angiogenesis. However, the involvement of TRPC1 in myocardial infarction (MI) remains unclear. The present study was aimed at investigating whether TRPC1 can improve the recovery of cardiac function via prompting angiogenesis following MI.

Methods and Results

In vitro, coronary artery endothelial cells from floxed TRPC1 mice and endothelial cell‐specific TRPC1 channel knockout mice were cultured to access EC angiogenesis. Both EC tube formation and migration were significantly suppressed in mouse coronary artery endothelial cells from endothelial cell‐specific TRPC1 channel knockout mice. In vivo, coronary artery endothelial cells from floxed TRPC1 and endothelial cell‐specific TRPC1 channel knockout mice were subjected to MI, then echocardiography, triphenyltetrazolium chloride staining and immunofluorescence were performed to assess cardiac repair on day 28. Endothelial cell‐specific TRPC1 channel knockout mice had higher ejection fraction change, larger myocardial infarct size, and reduced capillary density in the infarct area compared with coronary artery endothelial cells from floxed TRPC1 mice. Furthermore, we found underlying regulation by HIF‐1α (hypoxic inducible factor‐1α) and MEK‐ERK (mitogen‐activated protein kinase/extracellular signal‐regulated kinase) that could be the mechanism for the angiogenetic action of TRPC1. Significantly, treatment with dimethyloxaloylglycine, an activator of HIF‐1α, induced cardiac improvement via the HIF‐1α‐TRPC1‐MEK/ERK pathway in MI mice.

Conclusions

Our study demonstrated TRPC1 improves cardiac function after MI by increasing angiogenesis via the upstream regulator HIF‐1α and downstream MEK/ERK, and dimethyloxaloylglycine treatment has protective effect on MI through the HIF‐1α‐TRPC1‐MEK/ERK pathway.

PHD3 mediates denervation skeletal muscle atrophy through Nf-κB signal pathway

Skeletal muscle is the largest organ of the body, the development of skeletal muscle is very important for the health of the animal body. Prolyl hydroxylases (PHDs) are the classical regulator of the hypoxia inducible factor (HIF) signal pathway, many researchers found that PHDs are involved in the muscle fiber type transformation, muscle regeneration, and myocyte differentiation. However, whether PHDs can impact the protein turnover of skeletal muscle is poorly understood. In this study, we constructed denervated muscle atrophy mouse model and found PHD3 was highly expressed in the atrophic muscles and there was a significant correlation between the expression level of PHD3 and skeletal muscle weight which was distinct from PHD1 and PHD2. Then, the similar results were getting from the different weight muscles of normal mice. To further verify the relationship between PHD3 and skeletal muscle protein turnover, we established a PHD3 interference model by injecting PHD3 sgRNA virus into tibialis anterior muscle (TA) muscle of MCK-Cre-cas9 mice and transfecting PHD3 shRNA lentivirus into primary satellite cells. It was found that the Knock-out of PHD3 in vivo led to a significant increase in muscle weight and muscle fiber area (P < .05). Besides, the activity of protein synthesis signal pathway increased significantly, while the protein degradation pathway was inhibited evidently (P < .05). In vitro, the results of 5-ethynyl-2'-deoxyuridine (EdU) and tetramethylrhodamine ethyl ester (TMRE) fluorescence detection showed that PHD3 interference could lead to a decrease in cell proliferation and an increase of cell apoptosis. After the differentiation of satellite cells, the production of puromycin in the interference group was higher than that in the control group, and the content of 3-methylhistidine in the interference group was lower than that in the control group (P < .05) which is consistent with the change of protein turnover signal pathway in the cell. Mechanistically, there is an interaction between PHD3, NF-κB, and IKBα which was detected by immunoprecipitation. With the interfering of PHD3, the expression of the inflammatory signal pathway also significantly decreased (P < .05). These results suggest that PHD3 may affect protein turnover in muscle tissue by mediating inflammatory signal pathway. Finally, we knocked out PHD3 in denervated muscle atrophy mice and LPS-induced myotubes atrophy model. Then, we found that the decrease of PHD3 protein level could alleviate the muscle weight and muscle fiber reduction induced by denervation in mice. Meanwhile, the protein level of the inflammatory signal pathway and the content of 3-methylhistidine in denervated atrophic muscle were also significantly reduced (P < .05). In vitro, PHD3 knock-out could alleviate the decrease of myotube diameter induced by LPS, and the expression of protein synthesis pathway was also significantly increased (P < .05). On the contrary, the expression level of protein degradation and inflammatory signal pathway was significantly decreased (P < .05). Through these series of studies, we found that the increased expression of PHD3 in denervated muscle might be an important regulator in inducing muscle atrophy, and this process is likely to be mediated by the inflammatory NF-κB signal pathway.

Muscle Cryoinjury and Quantification of Regenerating Myofibers in Mice

Cryoinjury, or injury due to freezing, is a method of creating reproducible, local injuries in skeletal muscle. This method allows studying the regenerative response following muscle injuries in vivo, thus enabling the evaluation of local and systemic factors that influence the processes of myofiber regeneration. Cryoinjuries are applicable to the study of various modalities of muscle injury, particularly non-traumatic and traumatic injuries, without a loss of substantial volume of muscle mass. Cryoinjury requires only simple instruments and has the advantage over other methods that the extent of the lesion can be easily adjusted and standardized according to the duration of contact with the freezing instrument. The regenerative response can be evaluated histologically by the average maturity of regenerating myofibers as indicated by the cross-sectional areas of myofibers with centrally located nuclei. Accordingly, cryoinjury is regarded as one of the most reliable and easily accessible methods for simulating muscle injuries in studies of muscle regeneration.

Hypoxia-inducible factors and diabetes

Hypoxia can be defined as a relative deficiency in the amount of oxygen reaching the tissues. Hypoxia-inducible factors (HIFs) are critical regulators of the mammalian response to hypoxia. In normal circumstances, HIF-1α protein turnover is rapid, and hyperglycemia further destabilizes the protein. In addition to their role in diabetes pathogenesis, HIFs are implicated in development of the microvascular and macrovascular complications of diabetes. Improving glucose control in people with diabetes increases HIF-1α protein and has wide-ranging benefits, some of which are at least partially mediated by HIF-1α. Nevertheless, most strategies to improve diabetes or its complications via regulation of HIF-1α have not currently proven to be clinically useful. The intersection of HIF biology with diabetes is a complex area in which many further questions remain, especially regarding the well-conducted studies clearly describing discrepant effects of different methods of increasing HIF-1α, even within the same tissues. This Review presents a brief overview of HIFs; discusses the range of evidence implicating HIFs in β cell dysfunction, diabetes pathogenesis, and diabetes complications; and examines the differing outcomes of HIF-targeting approaches in these conditions.

Loss of ARNT in skeletal muscle limits muscle regeneration in aging

The ability of skeletal muscle to regenerate declines significantly with aging. The expression of aryl hydrocarbon receptor nuclear translocator (ARNT), a critical component of the hypoxia signaling pathway, was less abundant in skeletal muscle of old (23-25 months old) mice. This loss of ARNT was associated with decreased levels of Notch1 intracellular domain (N1ICD) and impaired regenerative response to injury in comparison to young (2-3 months old) mice. Knockdown of ARNT in a primary muscle cell line impaired differentiation in vitro. Skeletal muscle-specific ARNT deletion in young mice resulted in decreased levels of whole muscle N1ICD and limited muscle regeneration. Administration of a systemic hypoxia pathway activator (ML228), which simulates the actions of ARNT, rescued skeletal muscle regeneration in both old and ARNT-deleted mice. These results suggest that the loss of ARNT in skeletal muscle is partially responsible for diminished myogenic potential in aging and activation of hypoxia signaling holds promise for rescuing regenerative activity in old muscle.

Loss of CD36 protects against diet-induced obesity but results in impaired muscle stem cell function, delayed muscle regeneration and hepatic steatosis

AIM

The prevalence of obesity is a major risk factor for cardiovascular and metabolic diseases including impaired skeletal muscle regeneration. Since skeletal muscle regenerative capacity is regulated by satellite cells, we aimed to investigate whether a high-fat diet impairs satellite cell function and whether this is linked to fatty acid uptake via CD36. We also aimed to determine whether loss of CD36 impacts on muscle redox homeostasis and skeletal muscle regenerative capacity.

METHODS

We studied the impact of a high-fat diet and CD36 deficiency on murine skeletal muscle morphology, redox homeostasis, satellite cell function, bioenergetics and lipid accumulation in the liver. We also determined the effect of CD36 deficiency on skeletal muscle regeneration.

RESULTS

High-fat diet increased body weight, intramuscular lipid accumulation and oxidative stress in wild-type mice that were significantly mitigated in CD36-deficient mice. High-fat diet and CD36 deficiency independently attenuated satellite cell function on single fibres and myogenic capacity on primary satellite cells. CD36 deficiency resulted in delayed skeletal muscle regeneration following acute injury with cardiotoxin. CD36-deficient and wild-type primary satellite cells had distinct bioenergetic profiles in response to palmitate. High-fat diet induced hepatic steatosis in both genotypes that was more pronounced in the CD36-deficient mice.

CONCLUSION

This study demonstrates that CD36 deficiency protects against diet-induced obesity, intramuscular lipid deposition and oxidative stress but results in impaired muscle satellite cell function, delayed muscle regeneration and hepatic steatosis. CD36 is a key mediator of fatty acid uptake in skeletal muscle, linking obesity with satellite cell function and muscle regeneration.

High Fat With High Sucrose Diet Leads to Obesity and Induces Myodegeneration

Skeletal muscle utilizes both free fatty acids (FFAs) and glucose that circulate in the blood stream. When blood glucose levels acutely increase, insulin stimulates muscle glucose uptake, oxidation, and glycogen synthesis. Under these conditions, skeletal muscle preferentially oxidizes glucose while the oxidation of fatty acids (FAs) oxidation is reciprocally decreased. In metabolic disorders associated with insulin resistance, such as diabetes and obesity, both glucose uptake, and utilization muscle are significantly reduced causing FA oxidation to provide the majority of ATP for metabolic processes and contraction. Although the causes of this metabolic inflexibility or disrupted "glucose-fatty acid cycle" are largely unknown, a diet high in fat and sugar (HFS) may be a contributing factor. This metabolic inflexibility observed in models of obesity or with HFS feeding is detrimental because high rates of FA oxidation in skeletal muscle can lead to the buildup of toxic metabolites of fat metabolism and the accumulation of pro-inflammatory cytokines, which further exacerbate the insulin resistance. Further, HFS leads to skeletal muscle atrophy with a decrease in myofibrillar proteins and phenotypically characterized by loss of muscle mass and strength. Overactivation of ubiquitin proteasome pathway, oxidative stress, myonuclear apoptosis, and mitochondrial dysfunction are some of the mechanisms involved in muscle atrophy induced by obesity or in mice fed with HFS. In this review, we will discuss how HFS diet negatively impacts the various physiological and metabolic mechanisms in skeletal muscle.