Biology subjects

Davidyan, A.

Publications and source records attributed to Davidyan, A..

6 recordsLinked to original sources

bioRxiv2024

Combining in vivo 2-photon imaging with photoactivatable fluorescent labeling reveals strikingly low rates of mitochondrial dynamics in skeletal muscle.

Mitochondrial dynamics involve two distinct and opposing processes, fusion and fission. Traditionally we assess fusion and fission by snapshots of protein markers at distinct time points or in vitro models to infer outcomes in vivo. Recent technological advancements enable visualization of mitochondrial dynamics in vivo using fluorescent microscopy. Our study modified this technique to evaluate mitochondrial dynamics in skeletal muscle, comparing young (6mo) and old (24mo) mice in vivo and contrasting this to ex vivo and in vitro models. We hypothesized that in vitro and ex vivo models would have higher rates of dynamics than in vivo models and that young animals would have higher rates than old animals. We electroporated mitochondrial matrix-targeted photo-activatable GFP into the tibialis anterior (TA) of young and old C57Bl6 mice and imaged using multiphoton microscopy. We also measured rates of mitochondrial dynamics using single fibers isolated from the TA of the electroporated mice, as well as C2C12 myotubes transfected with the same plasmids. We found that the rates of dynamic events in vivo are slower than previously indicated, with the C2C12 myoblasts having the fastest rates of dynamic events across all models. We also observed that dynamic rates are slower in old animals compared to young animals. Finally, we found that rates of dynamic events were higher in old animals after an acute bout of exercise. Our data demonstrate it is possible to directly measure rates of mitochondrial dynamics in vivo. This technique provides a powerful tool to answer experimental questions about mitochondrial dynamics of skeletal muscle.

physiology↗

bioRxiv2024

Metformin treatment results in distinctive skeletal muscle mitochondrial remodeling in rats with different intrinsic aerobic capacities

The rationale for the use of metformin as a treatment to slow aging was largely based on data collected from metabolically unhealthy individuals. For healthspan extension metformin will also be used in periods of good health. To understand potential context specificity of metformin treatment on skeletal muscle, we used a rat model (HCR/LCR) with a divide in intrinsic aerobic capacity. Outcomes of metformin treatment differed based on baseline intrinsic mitochondrial function, oxidative capacity of the muscle (gastroc vs soleus), and the mitochondrial population (IMF vs SS). Metformin caused lower ADP-stimulated respiration in LCRs, with less of a change in HCRs. However, a washout of metformin resulted in an unexpected doubling of respiratory capacity in HCRs. These improvements in respiratory capacity were accompanied by mitochondrial remodeling that included increases in protein synthesis and changes in morphology. Our findings raise questions about whether the positive findings of metformin treatment are broadly applicable.

physiology↗

bioRxiv2023

Fibroblast growth factor-21 induces skeletal muscle atrophy and increases plasma amino acids in female mice: a potential role for glucocorticoids

BackgroundFibroblast growth factor-21 (FGF21) is an intercellular signaling molecule secreted by metabolic organs, including skeletal muscle, in response to intracellular stress. FGF21 crosses the blood brain barrier and acts via the nervous system to coordinate aspects of the adaptive starvation response, including increased lipolysis, gluconeogenesis, hepatic fatty acid oxidation, and activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Given its beneficial effects for hepatic lipid metabolism, pharmaceutical FGF21 analogues are in clinical trials treatment of fatty liver disease. We predicted pharmacologic treatment with FGF21 in-creases HPA axis activity and skeletal muscle glucocorticoid signaling and induces skeletal muscle atrophy in mice. MethodsWe treated male and female mice with FGF21 or saline, delivered either pe-ripherally or directly to the brain, to determine its effect on skeletal muscle. To identify metabolic pathways affected by FGF21, we analyzed untargeted primary metabolites measured in plasma by GCTOF-MS. To determine mechanisms underlying sex-and FGF21-dependent changes in muscle mass, we measured hormonal and molecular mediators of muscle protein synthesis and degradation. We performed stable isotope labeling with deuterium oxide to directly measure muscle protein synthesis. ResultsA short course of systemic FGF21 treatment decreased muscle protein synthe-sis (P < 0.001) and reduced tibialis anterior weight (P < 0.05); this was driven primarily by its effect in female mice (P < 0.05). Similarly, intracerebroventricular FGF21 reduced TA muscle fiber cross sectional area (P < 0.01); this was more apparent among female mice compared to male littermates (P < 0.05). In agreement with the reduced muscle mass, the topmost enriched meta-bolic pathways in FGF21-treated females were related to amino acid metabolism, and the relative abundance of plasma proteinogenic amino acids were increased up to three-fold (P < 0.05). FGF21 treatment increased hypothalamic Crh mRNA (P < 0.01), plasma corticosterone (P < 0.01), and adrenal weight (P < 0.05), and increased expression of glucocorticoid receptor target genes known to reduce muscle protein synthesis and/or promote degradation including Foxo1, Redd1, and Klf15 (P < 0.05). Again, these changes were driven primarily by effects of FGF21 in females (P < 0.05). ConclusionsFGF21 increased plasma amino acids and decreased skeletal muscle mass, together with activation of the HPA axis and glucocorticoid receptor target genes in skeletal muscle--and female mice were more sensitive to all these outcomes. Given the proposed use of FGF21 analogues for the treatment of metabolic disease, the study is both physiologically relevant and may have important clinical implications.

physiology↗

bioRxiv2023

17α-estradiol Alleviates High-Fat Diet-Induced Inflammatory and Metabolic Dysfunction in Skeletal Muscle of Male and Female Mice

Skeletal muscle has a central role in maintaining metabolic homeostasis. 17-estradiol (17-E2), a naturally-occurring non-feminizing diastereomer of 17{beta}-estradiol that demonstrates efficacy for improving metabolic outcomes in male, but not female, mice. Despite several lines of evidence showing that 17-E2 treatment improves metabolic parameters in middle-aged obese and old male mice through effects in brain, liver, and white adipose tissue little is known about how 17-E2 alters skeletal muscle metabolism, and what role this may play in mitigating metabolic declines. Therefore, this study aimed to determine if 17-E2 treatment improves metabolic outcomes in skeletal muscle from obese male and female mice following chronic high fat diet (HFD) administration. We hypothesized that male, but not female, mice, would benefit from 17-E2 treatment during HFD. To test this hypothesis, we used a multi-omics approach to determine changes in lipotoxic lipid intermediates, metabolites, and proteins related to metabolic homeostasis. In male mice, we show that 17-E2 alleviates HFD-induced metabolic detriments of skeletal muscle by reducing the accumulation of diacylglycerol (DAGs) and ceramides, inflammatory cytokine levels, and reduced the abundance of most of the proteins related to lipolysis and beta-oxidation. In contrast to males, 17-E2 treatment in female mice had little effect on the DAGs and ceramides content, muscle inflammatory cytokine levels, or changes to the relative abundance of proteins involved in beta-oxidation. These data support to the growing evidence that 17-E2 treatment could be beneficial for overall metabolic health in male mammals.

physiology↗

bioRxiv2022

Skeletal muscle nuclei in mice are not post-mitotic

The skeletal muscle research field generally accepts that nuclei in skeletal muscle fibers (i.e., myonuclei) are post-mitotic and unable to proliferate. Because our deuterium oxide (D2O) labeling studies showed DNA synthesis in skeletal muscle tissue, we hypothesized that resident myonuclei can replicate in vivo. To test this hypothesis, we used a mouse model that temporally labeled myonuclei with GFP followed by D2O labeling during normal cage activity, functional overload, and with satellite cell ablation. During normal cage activity, we observed deuterium enrichment into myonuclear DNA in 7 out of 7 plantaris (PLA), 6 out of 6 tibialis anterior (TA), 5 out of 7 gastrocnemius (GAST) and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR) of DNA in myonuclei were: 0.0202 {+/-} 0.0093 in PLA, 0.0239 {+/-} 0.0040 in TA, 0.0076 {+/-} 0. 0058 in GAST, and 0.0138 {+/-} 0.0039 in QUAD, while there was no replication in myonuclei from EDL. These FSR values were largely reproduced in the overload and satellite cell ablation conditions although there were higher synthesis rates in the overloaded PLA muscle. We further provided evidence that myonuclear replication is through endoreplication that results in polyploidy. These novel findings contradict the dogma that skeletal muscle nuclei are post-mitotic and open potential avenues to harness the intrinsic replicative ability of myonuclei for muscle maintenance and growth. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=141 SRC="FIGDIR/small/513426v2_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@1e4d210org.highwire.dtl.DTLVardef@19b52daorg.highwire.dtl.DTLVardef@5c9a53org.highwire.dtl.DTLVardef@197dc0d_HPS_FORMAT_FIGEXP M_FIG C_FIG

physiology↗

bioRxiv2020

The Maintenance of Muscle Mass Is Independent of Testosterone in Adult Male Mice

Testosterone is considered a potent anabolic agent in skeletal muscle with a well-established role in adolescent growth and development in males. However, alterations in the role of testosterone in the regulation of skeletal muscle mass and function throughout the lifespan has yet to be established. While some studies suggest that testosterone is important for the maintenance of skeletal muscle mass, an understanding of the role this hormone plays in young, adult, and old males with normal and low serum testosterone levels is lacking. We investigated the role testosterone plays in the maintenance of muscle mass by examining the effect of orchiectomy-induced testosterone depletion in C57Bl6 male mice at ages ranging from early postnatal through old age; the age groups we used included 1.5-, 5-, 12-, and 24-month old mice. Following 28 days of testosterone depletion, we assessed mass and fiber cross-sectional-area (CSA) of the tibialis anterior, gastrocnemius, and quadriceps muscles. In addition, we measured global rates of protein synthesis and degradation using the SuNSET method, western blots, and enzyme activity assays. 28 days of testosterone depletion resulted in smaller muscle mass in the two youngest cohorts but had no effect in the two older ones. Mean CSA decreased only in the youngest cohort and only in the tibialis anterior muscle. Testosterone depletion resulted in a general increase in proteasome activity at all ages. We did not detect changes in protein synthesis at the terminal time point. This data suggest that within physiological serum concentrations, testosterone is not important for the maintenance of muscle mass in mature male mice; however, in young mice testosterone is crucial for normal growth.

physiology↗