Transcriptomic Regulation of Muscle Mitochondria and Calcium Signaling by Insulin/IGF-1 Receptors Depends on FoxO Transcription Factors

The Roles of Forkhead Box O3a (FOXO3a) in Bone and Cartilage Diseases - A Narrative Review

Bone and cartilage diseases are significantly associated with musculoskeletal disability. However, no effective drugs are available to cure them. FOXO3a, a member of the FOXO family, has been implicated in cell proliferation, ROS detoxification, autophagy, and apoptosis. The biological functions of FOXO3a can be modulated by post-translational modifications (PTMs), such as phosphorylation and acetylation. Several signaling pathways, such as MAPK, NF-κB, PI3K/AKT, and AMPK/Sirt1 pathways, have been implicated in the development of bone and cartilage diseases by mediating the expression of FOXO3a. In particular, FOXO3a acts as a transcriptional factor in mediating the expression of various genes, such as MnSOD, CAT, BIM, BBC3, and CDK6. FOXO3a plays a critical role in the metabolism of bone and cartilage. In this article, we mainly discussed the biological functions of FOXO3a in bone and cartilage diseases, such as osteoporosis (OP), osteoarthritis (OA), rheumatoid arthritis (RA), ankylosing spondylitis (AS), and intervertebral disc degeneration (IDD). FOXO3a can promote osteogenic differentiation, induce osteoblast proliferation, inhibit osteoclast activity, suppress chondrocyte apoptosis, and reduce inflammatory responses. Collectively, up-regulation of FOXO3a expression shows beneficial effects, and FOXO3a has become a potential target for bone and cartilage diseases.

Temporal phosphoproteomics reveals circuitry of phased propagation in insulin signaling

Insulin is a pleiotropic hormone that elicits its metabolic and mitogenic actions through numerous rapid and reversible protein phosphorylations. The temporal regulation of insulin's intracellular signaling cascade is highly complex and insufficiently understood. We conduct a time-resolved analysis of the global insulin-regulated phosphoproteome of differentiated human primary myotubes derived from satellite cells of healthy donors using high-resolution mass spectrometry. Identification and tracking of ~13,000 phosphopeptides over time reveal a highly complex and coordinated network of transient phosphorylation and dephosphorylation events that can be allocated to time-phased regulation of distinct and non-overlapping subcellular pathways. Advanced network analysis combining protein-protein-interaction (PPI) resources and investigation of donor variability in relative phosphosite occupancy over time identifies novel putative candidates in non-canonical insulin signaling and key regulatory nodes that are likely essential for signal propagation. Lastly, we find that insulin-regulated phosphorylation of the pre-catalytic spliceosome complex is associated with acute alternative splicing events in the transcriptome of human skeletal muscle. Our findings highlight the temporal relevance of protein phosphorylations and suggest that synchronized contributions of multiple signaling pathways form part of the circuitry for propagating information to insulin effector sites.

Skeletal Muscle mRNA Splicing Variants Association With Four Different Fitness and Energetic Measures in the GESTALT Study

BACKGROUND

Physical activity is essential for maintaining muscle mitochondrial function and aerobic capacity. The molecular mechanisms underlying such protective effects are incompletely understood, in part because it is difficult to separate the effects of disease status and physical activity. We explored the association of human skeletal muscle transcriptomic with four measures of energetics and mitochondria oxidative capacity in healthy individuals.

METHODS

Using RNA sequencing of vastus lateralis muscle biopsies from 82 GESTALT participants (52 males, aged 22-89 years), we explored gene and splicing variant expression profiles associated with self-reported physical activity, peak oxygen consumption (VO2 peak), muscle oxidative capacity (kPCr) and mitochondrial respiration (Mit-O2 flux). The effect of aging on gene expression was examined in participants with low and high VO2 peak.

RESULTS

The four measures of energetics were negative correlated with age and generally intercorrelated. We identified protein-coding genes associated with four energetic measures adjusting for age, muscle fiber-ratio, sex and batch effect. Mitochondrial pathways were overrepresented across all energetic variables, albeit with little overlap at the gene level. Alternative spliced transcript isoforms associated with energetics were primarily enriched for cytoplasmic ribonucleoprotein granules. The splicing pathway was up-regulated with aging in low but not in high fitness participants, and transcript isoforms detected in the low fitness group pertain to processes such as cell cycle regulation, RNA/protein localization, nuclear transport and catabolism.

CONCLUSIONS

A consistent mitochondrial signature emerged across all energetic measures. Alternative splicing was enhanced in older, low fitness participants supporting the energy-splicing axis hypothesis. The identified splicing variants were enriched in pathways involving the accumulation of ribonucleoproteins in cytoplasmic granules, whose function remains unclear. Further research is needed to understand the function of these proteoforms in promoting adaptation to low energy availability.

Inhibitory Regulation of FOXO1 in PPARδ Expression Drives Mitochondrial Dysfunction and Insulin Resistance

Forkhead box O1 (FOXO1) regulates muscle growth, but the metabolic role of FOXO1 in skeletal muscle and its mechanisms remain unclear. To explore the metabolic role of FOXO1 in skeletal muscle, we generated skeletal muscle-specific Foxo1 inducible knockout (mFOXO1 iKO) mice and fed them a high-fat diet to induce obesity. We measured insulin sensitivity, fatty acid oxidation, mitochondrial function, and exercise capacity in obese mFOXO1 iKO mice and assessed the correlation between FOXO1 and mitochondria-related protein in the skeletal muscle of patients with diabetes. Obese mFOXO1 iKO mice exhibited improved mitochondrial respiratory capacity, which was followed by attenuated insulin resistance, enhanced fatty acid oxidation, and improved skeletal muscle exercise capacity. Transcriptional inhibition of FOXO1 in peroxisome proliferator-activated receptor δ (PPARδ) expression was confirmed in skeletal muscle, and deletion of PPARδ abolished the beneficial effects of FOXO1 deficiency. FOXO1 protein levels were higher in the skeletal muscle of patients with diabetes and negatively correlated with PPARδ and electron transport chain protein levels. These findings highlight FOXO1 as a new repressor in PPARδ gene expression in skeletal muscle and suggest that FOXO1 links insulin resistance and mitochondrial dysfunction in skeletal muscle via PPARδ.

ARTICLE HIGHLIGHTS

Contrasting consequences of podocyte insulin-like growth factor 1 receptor inhibition

Insulin signaling to the glomerular podocyte via the insulin receptor (IR) is critical for kidney function. In this study we show that near-complete knockout of the closely related insulin-like growth factor 1 receptor (IGF1R) in podocytes is detrimental, resulting in albuminuria in vivo and podocyte cell death in vitro. In contrast, partial podocyte IGF1R knockdown confers protection against doxorubicin-induced podocyte injury. Proteomic analysis of cultured podocytes revealed that while near-complete loss of podocyte IGF1R results in the downregulation of mitochondrial respiratory complex I and DNA damage repair proteins, partial IGF1R inhibition promotes respiratory complex expression. This suggests that altered mitochondrial function and resistance to podocyte stress depends on the level of IGF1R suppression, the latter determining whether receptor inhibition is protective or detrimental. Our work suggests that the partial suppression of podocyte IGF1R could have therapeutic benefits in treating albuminuric kidney disease.

Normobaric hypoxia shows enhanced FOXO1 signaling in obese mouse gastrocnemius muscle linked to metabolism and muscle structure and neuromuscular innervation

Skeletal muscle relies on mitochondria for sustainable ATP production, which may be impacted by reduced oxygen availability (hypoxia). Compared with long-term hypoxia, the mechanistic in vivo response to acute hypoxia remains elusive. Therefore, we aimed to provide an integrated description of the Musculus gastrocnemius response to acute hypoxia. Fasted male C57BL/6JOlaHsd mice, fed a 40en% fat diet for six weeks, were exposed to 12% O2 normobaric hypoxia or normoxia (20.9% O2) for six hours (n = 12 per group). Whole-body energy metabolism and the transcriptome response of the M. gastrocnemius were analyzed and confirmed by acylcarnitine determination and Q-PCR. At the whole-body level, six hours of hypoxia reduced energy expenditure, increased blood glucose and tended to decreased the respiratory exchange ratio (RER). Whole-genome transcriptome analysis revealed upregulation of forkhead box-O (FOXO) signalling, including an increased expression of tribbles pseudokinase 3 (Trib3). Trib3 positively correlated with blood glucose levels. Upregulated carnitine palmitoyltransferase 1A negatively correlated with the RER, but the significantly increased in tissue C14-1, C16-0 and C18-1 acylcarnitines supported that β-oxidation was not regulated. The hypoxia-induced FOXO activation could also be connected to altered gene expression related to fiber-type switching, extracellular matrix remodeling, muscle differentiation and neuromuscular junction denervation. Our results suggest that a six-hour exposure of obese mice to 12% O2 normobaric hypoxia impacts M. gastrocnemius via FOXO1, initiating alterations that may contribute to muscle remodeling of which denervation is novel and warrants further investigation. The findings support an early role of hypoxia in tissue alterations in hypoxia-associated conditions such as aging and obesity.

Polyamines and Physical Activity in Musculoskeletal Diseases: A Potential Therapeutic Challenge

Autophagy dysregulation is commonplace in the pathogenesis of several invalidating diseases, such as musculoskeletal diseases. Polyamines, as spermidine and spermine, are small aliphatic cations essential for cell growth and differentiation, with multiple antioxidant, anti-inflammatory, and anti-apoptotic effects. Remarkably, they are emerging as natural autophagy regulators with strong anti-aging effects. Polyamine levels were significantly altered in the skeletal muscles of aged animals. Therefore, supplementation of spermine and spermidine may be important to prevent or treat muscle atrophy. Recent in vitro and in vivo experimental studies indicate that spermidine reverses dysfunctional autophagy and stimulates mitophagy in muscles and heart, preventing senescence. Physical exercise, as polyamines, regulates skeletal muscle mass inducing proper autophagy and mitophagy. This narrative review focuses on the latest evidence regarding the efficacy of polyamines and exercise as autophagy inducers, alone or coupled, in alleviating sarcopenia and aging-dependent musculoskeletal diseases. A comprehensive description of overall autophagic steps in muscle, polyamine metabolic pathways, and effects of the role of autophagy inducers played by both polyamines and exercise has been presented. Although literature shows few data in regard to this controversial topic, interesting effects on muscle atrophy in murine models have emerged when the two "autophagy-inducers" were combined. We hope these findings, with caution, can encourage researchers to continue investigating in this direction. In particular, if these novel insights could be confirmed in further in vivo and clinical studies, and the two synergic treatments could be optimized in terms of dose and duration, then polyamine supplementation and physical exercise might have a clinical potential in sarcopenia, and more importantly, implications for a healthy lifestyle in the elderly population.

Molecular targets and mechanisms involved in the action of Banxia Shumi decoction in insomnia treatment

Insomnia is a common sleep-wake rhythm disorder, which is closely associated with the occurrence of many serious diseases. Recent researches suggest that circadian rhythms play an important role in regulating sleep duration and sleep quality. Banxia Shumi decoction (BSXM) is a well-known Chinese formula used to treat insomnia in China. However, the overall molecular mechanism behind this therapeutic effect has not yet been fully elucidated. This study aimed to identify the molecular targets and mechanisms involved in the action of BSXM during the treatment of insomnia. Using network pharmacology and molecular docking methods, we investigated the molecular targets and underlying mechanisms of action of BSXM in insomnia therapy. We identified 8 active compounds from Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform and the traditional Chinese medicine integrative database that corresponded to 26 target genes involved in insomnia treatment. The compound-differentially expressed genes of the BXSM network indicated that cavidine and gondoic acid could potentially become key components of drugs used for insomnia treatment. Further analysis revealed that GSK3B, MAPK14, IGF1R, CCL5, and BCL2L11 were core targets significantly associated with the circadian clock. Pathway enrichment analysis of Kyoto Encyclopedia of Genes and Genomes revealed that epidermal growth factor receptor tyrosine kinase inhibitor resistance was the most prominently enriched pathway for BSXM in the insomnia treatment. The forkhead box O signaling pathway was also found to be significantly enriched. These targets were validated using the Gene Expression Omnibus dataset. Molecular docking studies were performed to confirm the binding of cavidine and gondoic acid to the identified core targets. To our knowledge, our study confirmed for the first time that the multi-component, multi-target, and multi-pathway characteristics of BXSM may be the potential mechanism for treating insomnia with respect to the circadian clock gene. The results of this study provided theoretical guidance for researchers to further explore its mechanism of action.

Loss of FoxOs in muscle increases strength and mitochondrial function during aging

BACKGROUND

Muscle mitochondrial decline is associated with aging-related muscle weakness and insulin resistance. FoxO transcription factors are targets of insulin action and deletion of FoxOs improves mitochondrial function in diabetes. However, disruptions in proteostasis and autophagy are hallmarks of aging and the effect of chronic inhibition of FoxOs in aged muscle is unknown. This study investigated the role of FoxOs in regulating muscle strength and mitochondrial function with age.

METHODS

We measured muscle strength, cross-sectional area, muscle fibre-type, markers of protein synthesis/degradation, central nuclei, glucose/insulin tolerance, and mitochondrial bioenergetics in 4.5-month (Young) and 22-24-month-old (Aged) muscle-specific FoxO1/3/4 triple KO (TKO) and littermate control (Ctrl) mice.

RESULTS

Lean mass was increased in Aged TKO compared with both Aged Ctrl and younger groups by 26-33% (P < 0.01). Muscle strength, measured by max force of tibialis anterior (TA) contraction, was 20% lower in Aged Ctrl compared with Young Ctrls (P < 0.01) but was not decreased in Aged TKOs. Increased muscle strength in Young and Aged TKO was associated with 18-48% increased muscle weights compared with Ctrls (P < 0.01). Muscle cross-sectional analysis of TA, soleus, and plantaris revealed increases in fibre size distribution and a 2.5-10-fold increase in central nuclei in Young and Aged TKO mice, without histologic signs of muscle damage. Age-dependent increases in Gadd45a and Ube4a expression as well accumulation of K48 polyubiquitinated proteins were observed in quad and TA but were prevented by FoxO deletion. Young and Aged TKO muscle showed minimal changes in autophagy flux and no accumulation of autophagosomes compared with Ctrl groups. Increased strength in Young and Aged TKO was associated with a 10-20% increase in muscle mitochondrial respiration using glutamate/malate/succinate compared with controls (P < 0.05). OXPHOS subunit expression and complex I activity were decreased 16-34% in Aged Ctrl compared with Young Ctrl but were prevented in Aged TKO. Both Aged Ctrl and Aged TKO showed impaired glucose tolerance by 33% compared to young groups (P < 0.05) indicating improved strength and mitochondrial respiration are not due to improved glycemia.

CONCLUSIONS

FoxO deletion increases muscle strength even during aging. Deletion of FoxOs maintains muscle strength in part by mild suppression of atrophic pathways, including inhibition of Gadd45a and Ube4a expression, without accumulation of autophagosomes in muscle. Deletion of FoxOs also improved mitochondrial function by maintenance of OXPHOS in both young and aged TKO.

An integrated study of hormone-related sarcopenia for modeling and comparative transcriptome in rats

Sarcopenia is a senile disease with high morbidity, serious complications and limited clinical treatments. Menopause increases the risk of sarcopenia in females, while the exact pathogenesis remains unclear. To systematically investigate the development of hormone-related sarcopenia, we established a model of sarcopenia by ovariectomy and recorded successive characteristic changes. Furthermore, we performed the transcriptome RNA sequencing and bioinformatics analysis on this model to explore the underlying mechanism. In our study, we identified an integrated model combining obesity, osteoporosis and sarcopenia. Functional enrichment analyses showed that most of the significantly enriched pathways were down-regulated and closely correlated with endocrine and metabolism, muscle dysfunction, cognitive impairment and multiple important signaling pathways. We finally selected eight candidate genes to verify their expression levels. These findings confirmed the importance of estrogen in the maintenance of skeletal muscle function and homeostasis, and provided potential targets for further study on hormone-related sarcopenia.