Histidine-containing dipeptides (HCDs) are abundantly expressed in striated muscles. Although important properties have been ascribed to HCDs, including H + buffering, regulation of Ca 2+ transients and protection against oxidative stress, it remains unknown whether they play relevant functions in vivo . To investigate the in vivo roles of HCDs, we developed the first carnosine synthase knockout (CARNS1 −/− ) rat strain to investigate the impact of an absence of HCDs on skeletal and cardiac muscle function. Male wild-type (WT) and knockout rats (4 months-old) were used. Skeletal muscle function was assessed by an exercise tolerance test, contractile function in situ and muscle buffering capacity in vitro . Cardiac function was assessed in vivo by echocardiography and cardiac electrical activity by electrocardiography. Cardiomyocyte contractile function was assessed in isolated cardiomyocytes by measuring sarcomere contractility, along with the determination of Ca 2+ transient. Markers of oxidative stress, mitochondrial function and expression of proteins were also evaluated in cardiac muscle. Animals were supplemented with carnosine (1.8% in drinking water for 12 weeks) in an attempt to rescue tissue HCDs levels and function. CARNS1 −/− resulted in the complete absence of carnosine and anserine, but it did not affect exercise capacity, skeletal muscle force production, fatigability or buffering capacity in vitro , indicating that these are not essential for pH regulation and function in skeletal muscle. In cardiac muscle, however, CARNS1 −/− resulted in a significant impairment of contractile function, which was confirmed both in vivo and ex vivo in isolated sarcomeres. Impaired systolic and diastolic dysfunction were accompanied by reduced intracellular Ca 2+ peaks and slowed Ca 2+ removal, but not by increased markers of oxidative stress or impaired mitochondrial respiration. No relevant increases in muscle carnosine content were observed after carnosine supplementation. Results show that a primary function of HCDs in cardiac muscle is the regulation of Ca 2+ handling and excitation-contraction coupling.

The high capacity of the skeletal muscle to regenerate is due to the presence of muscle stem cells (MuSCs, or satellite cells). The E3 ubiquitin ligase Parkin is a key regulator of mitophagy and is recruited to mitochondria during differentiation of mouse myoblast cell line. However, the function of mitophagy during regeneration has not been investigated in vivo. Here, we have utilized Parkin deficient (Parkin–/–) mice to investigate the role of Parkin in skeletal muscle regeneration. We found a persistent deficiency in skeletal muscle regeneration in Parkin–/– mice after cardiotoxin (CTX) injury with increased area of fibrosis and decreased cross-sectional area (CSA) of myofibres post-injury. There was also a significant modulation of MuSCs differentiation and mitophagic markers, with altered mitochondrial proteins during skeletal muscle regeneration in Parkin–/– mice. Our data suggest that Parkin-mediated mitophagy plays a key role in skeletal muscle regeneration and is necessary for MuSCs differentiation.

Diabetic peripheral neuropathy (DPN) is a nervous disorder caused by diabetes mellitus, affecting about 50% of patients in clinical medicine. Chronic pain is one of the major and most unpleasant symptoms developed by those patients, and conventional available treatments for the neuropathy, including the associated pain, are still unsatisfactory and benefit only a small number of patients. Photobiomodulation (PBM) has been gaining clinical acceptance once it is able to promote early nerve regeneration resulting in significant improvement in peripheral nerves disabilities. In this work, the effects of PBM (660 nm, 30 mW, 1.6 J/cm , 0.28 cm , 15 s in a continuous frequency) on treating DPN-induced pain and nerve damage were evaluated in an experimental model of diabetic-neuropathy induced by streptozotocin in mice. PBM-induced antinociception in neuropathic-pain mice was dependent on central opioids release. After 21 consecutive applications, PBM increased nerve growth factor levels and induced structural recovery increasing mitochondrial content and regulating Parkin in the sciatic nerve of DPN-mice. Taking together, these data provide new insights into the mechanisms involved in the effects of PBM-therapy emphasizing its therapeutic potential in the treatment of DPN.

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