More than the clock: distinct regulation of muscle function and metabolism by PER2 and RORα.

Circadian rhythms, governed by the dominant central clock, in addition to various peripheral clocks, regulate almost all biological processes, including sleep-wake cycles, hormone secretion and metabolism. In certain contexts, the regulation and function of the peripheral oscillations can be decoupled from the central clock. However, the specific mechanisms underlying muscle-intrinsic clock-dependent modulation of muscle function and metabolism remain unclear. We investigated the outcome of perturbations of the primary and secondary feedback loops of the molecular clock in skeletal muscle by specific gene ablation of Period circadian regulator 2 (Per2) and RAR-related orphan receptor alpha (Rorα), respectively. In both models, a dampening of core clock gene oscillation was observed, while the phase was preserved. Moreover, both loops seem to be involved in the homeostasis of amine groups. Highly divergent outcomes were seen for overall muscle gene expression, primarily affecting circadian rhythmicity in the PER2 knockouts and non-oscillating genes in the RORα knockouts, leading to distinct outcomes in terms of metabolome and phenotype. These results highlight the entanglement of the molecular clock and muscle plasticity and allude to specific functions of different clock components, i.e. the primary and secondary feedback loops, in this context. The reciprocal interaction between muscle contractility and circadian clocks might therefore be instrumental to determining a finely tuned adaptation of muscle tissue to perturbations in health and disease. KEY POINTS: Specific perturbations of the primary and secondary feedback loop of the molecular clock result in specific outcomes on muscle metabolism and function. Ablation of Per2 (primary loop) or Rorα (secondary loop) blunts the amplitude of core clock genes, in absence of a shift in phase. Perturbation of the primary feedback loop by deletion of PER2 primarily affects muscle gene oscillation. Knockout of RORα and the ensuing modulation of the secondary loop results in the aberrant expression of a large number of non-clock genes and proteins. The deletion of PER2 and RORα affects muscle metabolism and contractile function in a circadian manner, highlighting the central role of the molecular clock in modulating muscle plasticity.

RNA-bound PGC-1α controls gene expression in liquid-like nuclear condensates.

Plasticity of cells, tissues, and organs is controlled by the coordinated transcription of biological programs. However, the mechanisms orchestrating such context-specific transcriptional networks mediated by the dynamic interplay of transcription factors and coregulators are poorly understood. The peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is a prototypical master regulator of adaptive transcription in various cell types. We now uncovered a central function of the C-terminal domain of PGC-1α to bind RNAs and assemble multiprotein complexes including proteins that control gene transcription and RNA processing. These interactions are important for PGC-1α recruitment to chromatin in transcriptionally active liquid-like nuclear condensates. Notably, such a compartmentalization of active transcription mediated by liquid-liquid phase separation was observed in mouse and human skeletal muscle, revealing a mechanism by which PGC-1α regulates complex transcriptional networks. These findings provide a broad conceptual framework for context-dependent transcriptional control of phenotypic adaptations in metabolically active tissues.

BDNF is a mediator of glycolytic fiber-type specification in mouse skeletal muscle.

Brain-derived neurotrophic factor (BDNF) influences the differentiation, plasticity, and survival of central neurons and likewise, affects the development of the neuromuscular system. Besides its neuronal origin, BDNF is also a member of the myokine family. However, the role of skeletal muscle-derived BDNF in regulating neuromuscular physiology in vivo remains unclear. Using gain- and loss-of-function animal models, we show that muscle-specific ablation of BDNF shifts the proportion of muscle fibers from type IIB to IIX, concomitant with elevated slow muscle-type gene expression. Furthermore, BDNF deletion reduces motor end plate volume without affecting neuromuscular junction (NMJ) integrity. These morphological changes are associated with slow muscle function and a greater resistance to contraction-induced fatigue. Conversely, BDNF overexpression promotes a fast muscle-type gene program and elevates glycolytic fiber number. These findings indicate that BDNF is required for fiber-type specification and provide insights into its potential modulation as a therapeutic target in muscle diseases.

Characterization of regulatory transcriptional mechanisms in hepatocyte lipotoxicity.

Non-alcoholic fatty liver disease is a continuum of disorders among which non-alcoholic steatohepatitis (NASH) is particularly associated with a negative prognosis. Hepatocyte lipotoxicity is one of the main pathogenic factors of liver fibrosis and NASH. However, the molecular mechanisms regulating this process are poorly understood. The main aim of this study was to dissect transcriptional mechanisms regulated by lipotoxicity in hepatocytes. We achieved this aim by combining transcriptomic, proteomic and chromatin accessibility analyses from human liver and mouse hepatocytes. This integrative approach revealed several transcription factor networks deregulated by NASH and lipotoxicity. To validate these predictions, genetic deletion of the transcription factors MAFK and TCF4 was performed, resulting in hepatocytes that were better protected against saturated fatty acid oversupply. MAFK- and TCF4-regulated gene expression profiles suggest a mitigating effect against cell stress, while promoting cell survival and growth. Moreover, in the context of lipotoxicity, some MAFK and TCF4 target genes were to the corresponding differentially regulated transcripts in human liver fibrosis. Collectively, our findings comprehensively profile the transcriptional response to lipotoxicity in hepatocytes, revealing new molecular insights and providing a valuable resource for future endeavours to tackle the molecular mechanisms of NASH.

Interleukin-6 potentiates endurance training adaptation and improves functional capacity in old mice.

BACKGROUND: Interventions to preserve functional capacities at advanced age are becoming increasingly important. So far, exercise provides the only means to counteract age-related decrements in physical performance and muscle function. Unfortunately, the effectiveness of exercise interventions in elderly populations is hampered by reduced acceptance and compliance as well as disuse complications. We therefore studied whether application of interleukin-6 (IL-6), a pleiotropic myokine that is induced by skeletal muscle activity and exerts broad systemic effects in response to exercise, affects physical performance and muscle function alone or in combination with training in aged mice. METHODS: Sedentary old male mice (Sed+Saline, n = 15) were compared with animals that received recombinant IL-6 (rIL-6) in an exercise-mimicking pulsatile manner (Sed+IL-6, n = 16), were trained with a moderate-intensity, low-volume endurance exercise regimen (Ex+Saline, n = 13), or were exposed to a combination of these two interventions (Ex+IL-6, n = 16) for 12 weeks. Before and at the end of the intervention, mice underwent a battery of tests to quantify endurance performance, muscle contractility in situ, motor coordination, and gait and metabolic parameters. RESULTS: Mice exposed to enhanced levels of IL-6 during endurance exercise bouts showed superior improvements in endurance performance (33% more work and 12% greater peak power compared with baseline), fatigue resistance in situ (P = 0.0014 vs. Sed+Saline; P = 0.0199 vs. Sed+IL-6; and P = 0.0342 vs. Ex+Saline), motor coordination (rotarod performance, P = 0.0428), and gait (gait speed, P = 0.0053) following training. Pulsatile rIL-6 treatment in sedentary mice had only marginal effects on glucose tolerance and some gait parameters. No increase in adverse events or mortality related to rIL-6 treatment was observed. CONCLUSIONS: Administration of rIL-6 paired with treadmill running bouts potentiates the adaptive response to a moderate-intensity low-volume endurance exercise regimen in old mice, while being safe and well tolerated.

PGC-1ß-expressing POMC neurons mediate the effect of leptin on thermoregulation in the mouse.

The arcuate nucleus (ARC) of the hypothalamus is a key regulator of food intake, brown adipose tissue (BAT) thermogenesis, and locomotor activity. Whole-body deficiency of the transcriptional coactivator peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1ß (PGC-1ß) disrupts mouse circadian locomotor activity and BAT-regulated thermogenesis, in association with altered gene expression at the central level. We examined whether PGC-1ß expression in the ARC is required for proper energy balance and locomotor behavior by generating mice lacking the PGC-1ß gene specifically in pro-opiomelanocortin (POMC) neurons. POMC neuron-specific deletion of PGC-1ß did not impact locomotor behavior, food intake, body composition, energy fuel utilization and metabolic rate in fed, 24-h fasted and 24-h refed conditions. In contrast, in the fed state, deletion of PGC-1ß in POMC cells elevated core body temperature during the nighttime period. Importantly, this higher body temperature is not associated with changes in BAT function and gene expression. Conversely, we provide evidence that mice lacking PGC-1ß in POMC neurons are more sensitive to the effect of leptin on heat dissipation. Our data indicate that PGC-1ß-expressing POMC neurons are part of a circuit controlling body temperature homeostasis and that PGC-1ß function in these neurons is involved in the thermoregulatory effect of leptin.