Skeletal muscle sympathetic denervation disrupts the neuromuscular junction postterminal organization: A single-cell quantitative approach.

The sympathetic nervous system (SNS) regulates skeletal muscle motor innervation and stabilizes the NMJ in health, disease and aging. Previous studies using both chemical (6-hydroxydopamine, 6-OHDA) and microsurgically-induced sympathetic denervation examined the NMJ organization and transmission in the mouse; however, a detailed quantification of the postterminal on larger hindlimb muscles involved in gait mechanics and posture is lacking. The purpose of this study was to determine whether targets of the sympathetic neuron (SN) exhibiting different intrinsic composition such as the fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus muscles differ in their response to SN deprivation, and to develop a strategy to accurately quantify the impact of sympathectomy on the NMJ postterminal including those fibers located deeper in the muscle. This approach included muscle fixed ex vivo or through transcardial perfusion in mice treated with 6-OHDA or control ascorbic acid. We measured NMJ postterminal mean terminal total area, number of postterminal fragments, mean fragment area, and mean distance between fragments in free-floating alpha-bungarotoxin-stained in 1038 isolated muscle fibers. We found that muscle fiber sympathetic innervation plays a crucial role in the structural organization of the motorneuron-myofiber synapse postterminal and its deprivation leads to AChR cluster dispersion or shrinking as described in various neuromuscular diseases and aging.

Postganglionic sympathetic neurons, but not locus coeruleus optostimulation, activates neuromuscular transmission in the adult mouse in vivo.

Recent work demonstrated that sympathetic neurons innervate the skeletal muscle near the neuromuscular junction (NMJ), and muscle sympathectomy and sympathomimetic agents strongly influence motoneuron synaptic vesicle release ex vivo. Moreover, reports attest that the pontine nucleus locus coeruleus (LC) projects to preganglionic sympathetic neurons and regulates human mobility and skeletal muscle physiology. Thus, we hypothesized that peripheral and central sympathetic neurons projecting directly or indirectly to the skeletal muscle regulate NMJ transmission. The aim of this study was to define the specific neuronal groups in the peripheral and central nervous systems that account for such regulation in adult mice in vivo by using optogenetics and NMJ transmission recordings in 3-5-month-old, male and female ChR2(H134R/EYFP)/TH-Cre mice. After detecting ChR2(H134R)/EYFP fluorescence in the paravertebral ganglia and LC neurons, we tested whether optostimulating the plantar nerve near the lumbricalis muscle or LC neurons effectively modulates motor nerve terminal synaptic vesicle release in living mice. Nerve optostimulation increased motor synaptic vesicle release in vitro and in vivo, while the presynaptic adrenoceptor blockers propranolol (ß1/ß2) and atenolol (ß1) prevented this outcome. The effect is primarily presynaptic since miniature end-plate potential (MEPP) kinetics remained statistically unmodified after stimulation. In contrast, optostimulation of LC neurons did not regulate NMJ transmission. In summary, we conclude that postganglionic sympathetic neurons, but not LC neurons, increased NMJ transmission by acting on presynaptic ß1-adrenergic receptors in vivo.

Sympathomimetics regulate neuromuscular junction transmission through TRPV1, P/Q- and N-type Ca2+ channels.

Increasing evidence indicates that, first, the sympathetic nervous system interacts extensively with both vasculature and skeletal muscle fibers near neuromuscular junctions (NMJs) and, second, its neurotransmitter, noradrenaline, influences myofiber molecular composition and function and motor innervation. Since sympathomimetic agents have been reported to improve NMJ transmission, we examined whether two in clinical use, salbutamol and clenbuterol, affect the motor axon terminal via extracellular Ca2+ and molecular targets, such as TRPV1 and P/Q- and N-type voltage-activated Ca2+ channels. Electrophysiological recordings in ex-vivo preparations of peroneal nerves and lumbricalis muscles from young adult mice focused on spontaneous miniature end-plate potentials and singly and repetitively evoked end-plate potentials. Adding one dose of salbutamol or clenbuterol to the nerve/muscle preparation or repeatedly administering salbutamol to a mouse for 4 weeks increased spontaneous and evoked synaptic vesicle release but induced a steep decline in EPP amplitude in response to repetitive nerve stimulation. These effects were mediated primarily by ω-agatoxin IVA-sensitive P/Q-type and secondarily by ω-conotoxin GVIA-sensitive N-type Ca2+ channels. Presynaptic arvanil-sensitive TRPV1 channels seem to regulate Ca2+ at the motor neuron terminal at rest, while putative presynaptic ß-adrenergic receptors may mediate sympathomimetic and catecholamine effects on presynaptic Ca2+ channels during NMJ activation.

Pericytes are heterogeneous in their origin within the same tissue.

Pericytes heterogeneity is based on their morphology, distribution, and markers. It is well known that pericytes from different organs may have distinct embryonic sources. Yamazaki et al. (2017) using several transgenic mouse model reveal by cell-lineage tracing that pericytes are even more heterogeneous than previously appreciated. This study shows that pericytes from within the same tissue may be heterogeneous in their origin. Remarkably, a subpopulation of embryonic dermal pericytes derives from the hematopoietic lineage, an unexpected source. Reconstructing the lineage of pericytes is central to understanding development, and also for the diagnosis and treatment of diseases in which pericytes play important roles.

Troponin T3 regulates nuclear localization of the calcium channel Cavß1a subunit in skeletal muscle.

The voltage-gated calcium channel (Cav) ß1a subunit (Cavß1a) plays an important role in excitation-contraction coupling (ECC), a process in the myoplasm that leads to muscle-force generation. Recently, we discovered that the Cavß1a subunit travels to the nucleus of skeletal muscle cells where it helps to regulate gene transcription. To determine how it travels to the nucleus, we performed a yeast two-hybrid screening of the mouse fast skeletal muscle cDNA library and identified an interaction with troponin T3 (TnT3), which we subsequently confirmed by co-immunoprecipitation and co-localization assays in mouse skeletal muscle in vivo and in cultured C2C12 muscle cells. Interacting domains were mapped to the leucine zipper domain in TnT3 COOH-terminus (160-244 aa) and Cavß1a NH2-terminus (1-99 aa), respectively. The double fluorescence assay in C2C12 cells co-expressing TnT3/DsRed and Cavß1a/YFP shows that TnT3 facilitates Cavß1a nuclear recruitment, suggesting that the two proteins play a heretofore unknown role during early muscle differentiation in addition to their classical role in ECC regulation.

Raptor ablation in skeletal muscle decreases Cav1.1 expression and affects the function of the excitation-contraction coupling supramolecular complex.

The protein mammalian target of rapamycin (mTOR) is a serine/threonine kinase regulating a number of biochemical pathways controlling cell growth. mTOR exists in two complexes termed mTORC1 and mTORC2. Regulatory associated protein of mTOR (raptor) is associated with mTORC1 and is essential for its function. Ablation of raptor in skeletal muscle results in several phenotypic changes including decreased life expectancy, increased glycogen deposits and alterations of the twitch kinetics of slow fibres. In the present paper, we show that in muscle-specific raptor knockout (RamKO), the bulk of glycogen phosphorylase (GP) is mainly associated in its cAMP-non-stimulated form with sarcoplasmic reticulum (SR) membranes. In addition, 3[H]-ryanodine and 3[H]-PN200-110 equilibrium binding show a ryanodine to dihydropyridine receptors (DHPRs) ratio of 0.79 and 1.35 for wild-type (WT) and raptor KO skeletal muscle membranes respectively. Peak amplitude and time to peak of the global calcium transients evoked by supramaximal field stimulation were not different between WT and raptor KO. However, the increase in the voltage sensor-uncoupled RyRs leads to an increase of both frequency and mass of elementary calcium release events (ECRE) induced by hyper-osmotic shock in flexor digitorum brevis (FDB) fibres from raptor KO. The present study shows that the protein composition and function of the molecular machinery involved in skeletal muscle excitation-contraction (E-C) coupling is affected by mTORC1 signalling.

Pericytes at the intersection between tissue regeneration and pathology.

Perivascular multipotent cells, pericytes, contribute to the generation and repair of various tissues in response to injury. They are heterogeneous in their morphology, distribution, origin and markers, and elucidating their molecular and cellular differences may inform novel treatments for disorders in which tissue regeneration is either impaired or excessive. Moreover, these discoveries offer novel cellular targets for therapeutic approaches to many diseases. This review discusses recent studies that support the concept that pericyte subtypes play a distinctive role in myogenesis, neurogenesis, adipogenesis, fibrogenesis and angiogenesis.

Type-2 pericytes participate in normal and tumoral angiogenesis.

Tissue growth and function depend on vascularization, and vascular insufficiency or excess exacerbates many human diseases. Identification of the biological processes involved in angiogenesis will dictate strategies to modulate reduced or excessive vessel formation. We examine the essential role of pericytes. Their heterogeneous morphology, distribution, origins, and physiology have been described. Using double-transgenic Nestin-GFP/NG2-DsRed mice, we identified two pericyte subsets. We found that Nestin-GFP(-)/NG2-DsRed(+) (type-1) and Nestin-GFP(+)/NG2-DsRed(+) (type-2) pericytes attach to the walls of small and large blood vessels in vivo; in vitro, type-2, but not type-1, pericytes spark endothelial cells to form new vessels. Matrigel assay showed that only type-2 pericytes participate in normal angiogenesis. Moreover, when cancer cells were transplanted into Nestin-GFP/NG2-DsRed mice, type-1 pericytes did not penetrate the tumor, while type-2 pericytes were recruited during its angiogenesis. As inhibition of angiogenesis is a promising strategy in cancer therapy, type-2 pericytes may provide a cellular target susceptible to signaling and pharmacological manipulation in treating malignancy. This work also reports the potential of type-2 pericytes to improve blood perfusion in ischemic hindlimbs, indicating their potential for treating ischemic illnesses.

Skeletal muscle neural progenitor cells exhibit properties of NG2-glia.

Reversing brain degeneration and trauma lesions will depend on cell therapy. Our previous work identified neural precursor cells derived from the skeletal muscle of Nestin-GFP transgenic mice, but their identity, origin, and potential survival in the brain are only vaguely understood. In this work, we show that Nestin-GFP+ progenitor cells share morphological and molecular markers with NG2-glia, including NG2, PDGFRα, O4, NGF receptor (p75), glutamate receptor-1(AMPA), and A2B5 expression. Although these cells exhibit NG2, they do not express other pericyte markers, such as α-SMA or connexin-43, and do not differentiate into the muscle lineage. Patch-clamp studies displayed outward potassium currents, probably carried through Kir6.1 channels. Given their potential therapeutic application, we compared their abundance in tissues and concluded that skeletal muscle is the richest source of predifferentiated neural precursor cells. We found that these cells migrate toward the neurogenic subventricular zone displaying their typical morphology and nestin-GFP expression two weeks after brain injection. For translational purposes, we sought to identify these neural progenitor cells in wild-type species by developing a DsRed expression vector under Nestin-Intron II control. This approach revealed them in nonhuman primates and aging rodents throughout the lifespan.

Design and application of a class of sensors to monitor Ca2+ dynamics in high Ca2+ concentration cellular compartments.

Quantitative analysis of Ca(2+) fluctuations in the endoplasmic/sarcoplasmic reticulum (ER/SR) is essential to defining the mechanisms of Ca(2+)-dependent signaling under physiological and pathological conditions. Here, we developed a unique class of genetically encoded indicators by designing a Ca(2+) binding site in the EGFP. One of them, calcium sensor for detecting high concentration in the ER, exhibits unprecedented Ca(2+) release kinetics with an off-rate estimated at around 700 s(-1) and appropriate Ca(2+) binding affinity, likely attributable to local Ca(2+)-induced conformational changes around the designed Ca(2+) binding site and reduced chemical exchange between two chromophore states. Calcium sensor for detecting high concentration in the ER reported considerable differences in ER Ca(2+) dynamics and concentration among human epithelial carcinoma cells (HeLa), human embryonic kidney 293 cells (HEK-293), and mouse myoblast cells (C2C12), enabling us to monitor SR luminal Ca(2+) in flexor digitorum brevis muscle fibers to determine the mechanism of diminished SR Ca(2+) release in aging mice. This sensor will be invaluable in examining pathogenesis characterized by alterations in Ca(2+) homeostasis.

Accelerated sarcopenia precedes learning and memory impairments in the P301S mouse model of tauopathies and Alzheimer's disease.

BACKGROUND: Alzheimer's disease (AD) impairs cognitive functions and peripheral systems, including skeletal muscles. The PS19 mouse, expressing the human tau P301S mutation, shows cognitive and muscular pathologies, reflecting the central and peripheral atrophy seen in AD. METHODS: We analysed skeletal muscle morphology and neuromuscular junction (NMJ) through immunohistochemistry and advanced image quantification. A factorial Analysis of Variance assessed muscle weight, NCAM expression, NMJ, myofibre type distribution, cross-sectional areas, expression of single or multiple myosin heavy-chain isoforms, and myofibre grouping in PS19 and wild type (WT) mice over their lifespan (1-12 months). RESULTS: Significant weight differences in extensor digitorum longus (EDL) and soleus muscles between WT and PS19 mice were noted by 7-8 months. For EDL muscle in females, WT weighed 0.0113 ± 0.0005 compared with PS19's 0.0071 ± 0.0008 (P < 0.05), and in males, WT was 0.0137 ± 0.0001 versus PS19's 0.0069 ± 0.0006 (P < 0.005). Similarly, soleus muscle showed significant differences; females (WT: 0.0084 ± 0.0004; PS19: 0.0057 ± 0.0005, P < 0.005) and males (WT: 0.0088 ± 0.0003; PS19: 0.0047 ± 0.0004, P < 0.0001). Analysis of the NMJ in PS19 mice revealed a marked reduction in myofibre innervation at 5 months, with further decline by 10 months. NMJ pre-terminals in PS19 mice became shorter and simpler by 5 months, showing a steep decline by 10 months. Genotype and age strongly influenced muscle NCAM immunoreactivity, denoting denervation as early as 5-6 months in EDL muscle Type II fibres, with earlier effects in soleus muscle Type I and II fibres at 3-4 months. Muscle denervation and subsequent myofibre atrophy were linked to a reduction in Type IIB fibres in the EDL muscle and Type IIA fibres in the soleus muscle, accompanied by an increase in hybrid fibres. The EDL muscle showed Type IIB fibre atrophy with WT females at 1505 ± 110 µm2 versus PS19's 1208 ± 94 µm2, and WT males at 1731 ± 185 µm2 versus PS19's 1227 ± 116 µm2. Similarly, the soleus muscle demonstrated Type IIA fibre atrophy from 5 to 6 months, with WT females at 1194 ± 52 µm2 versus PS19's 858 ± 62 µm2, and WT males at 1257 ± 43 µm2 versus PS19's 1030 ± 55 µm2. Atrophy also affected Type IIX, I + IIA, and IIA + IIX fibres in both muscles. The timeline for both myofibre and overall muscle atrophy in PS19 mice was consistent, indicating a simultaneous decline. CONCLUSIONS: Progressive and accelerated neurogenic sarcopenia may precede and potentially predict cognitive deficits observed in AD.

Sex differences in single neuron function and proteomics profiles examined by patch-clamp and mass spectrometry in the locus coeruleus of the adult mouse.

AIMS: This study aimed to characterize the properties of locus coeruleus (LC) noradrenergic neurons in male and female mice. We also sought to investigate sex-specific differences in membrane properties, action potential generation, and protein expression profiles to understand the mechanisms underlying neuronal excitability variations. METHODS: Utilizing a genetic mouse model by crossing Dbhcre knock-in mice with tdTomato Ai14 transgenic mice, LC neurons were identified using fluorescence microscopy. Neuronal functional properties were assessed using patch-clamp recordings. Proteomic analyses of individual LC neuron soma was conducted using mass spectrometry to discern protein expression profiles. Data are available via ProteomeXchange with identifier PXD045844. RESULTS: Female LC noradrenergic neurons displayed greater membrane capacitance than those in male mice. Male LC neurons demonstrated greater spontaneous and evoked action potential generation compared to females. Male LC neurons exhibited a lower rheobase and achieved higher peak frequencies with similar current injections. Proteomic analysis revealed differences in protein expression profiles between sexes, with male mice displaying a notably larger unique protein set compared to females. Notably, pathways pertinent to protein synthesis, degradation, and recycling, such as EIF2 and glucocorticoid receptor signaling, showed reduced expression in females. CONCLUSIONS: Male LC noradrenergic neurons exhibit higher intrinsic excitability compared to those from females. The discernible sex-based differences in excitability could be ascribed to varying protein expression profiles, especially within pathways that regulate protein synthesis and degradation. This study lays the groundwork for future studies focusing on the interplay between proteomics and neuronal function examined in individual cells.

JP-45/JSRP1 variants affect skeletal muscle excitation-contraction coupling by decreasing the sensitivity of the dihydropyridine receptor.

JP-45 (also JP45; encoded by JSRP1) is an integral protein constituent of the skeletal muscle sarcoplasmic reticulum junctional face membrane interacting with Ca(v) 1.1 (the α.1 subunit of the voltage-sensing dihydropyridine receptor, DHPR) and the luminal calcium-binding protein calsequestrin. Two JSRP1 variants have been found in the human population: c.323C>T (p.P108L) in exon 5 and c.449G>C (p.G150A) in exon 6, but nothing is known concerning the incidence of these polymorphisms in the general population or in patients with neuromuscular diseases nor the impact of the polymorphisms on excitation-contraction (EC) coupling. In the present report, we investigated the frequencies of these two JSRP1 polymorphisms in the Swiss malignant hyperthermia population and studied the functional impact of the variants on EC coupling. Our results show that the polymorphisms are equally distributed among malignant hyperthermia negative, malignant hyperthermia equivocal, and malignant hyperthermia susceptible individuals. Interestingly, however, the presence of either one of these JP-45 variants decreased the sensitivity of the DHPR to activation. The presence of a JSRP1 variant may explain the variable phenotype seen in patients with malignant hyperthermia carrying the same mutation and, more importantly, may counteract the hypersensitivity of EC coupling caused by mutations in the RYR1 gene.

Type-1 pericytes participate in fibrous tissue deposition in aged skeletal muscle.

In older adults, changes in skeletal muscle composition are associated with increased fibrosis, loss of mass, and decreased force, which can lead to dependency, morbidity, and mortality. Understanding the biological mechanisms responsible is essential to sustaining and improving their quality of life. Compared with young mice, aged mice take longer to recover from muscle injury; their tissue fibrosis is more extensive, and regenerated myofibers are smaller. Strong evidence indicates that cells called pericytes, embedded in the basement membrane of capillaries, contribute to the satellite-cell pool and muscle growth. In addition to their role in skeletal muscle repair, after tissue damage, they detach from capillaries and migrate to the interstitial space to participate in fibrosis formation. Here we distinguish two bona fide pericyte subtypes in the skeletal muscle interstitium, type-1 (Nestin-GFP(-)/NG2-DsRed(+)) and type-2 (Nestin-GFP(+)/NG2-DsRed(+)), and characterize their heretofore unknown specific roles in the aging environment. Our in vitro results show that type-1 and type-2 pericytes are either fibrogenic or myogenic, respectively. Transplantation studies in young animals indicate that type-2 pericytes are myogenic, while type-1 pericytes remain in the interstitial space. In older mice, however, the muscular regenerative capacity of type-2 pericytes is limited, and type-1 pericytes produce collagen, contributing to fibrous tissue deposition. We conclude that in injured muscles from aging mice, the pericytes involved in skeletal muscle repair differ from those associated with scar formation.

A critical role for muscle ring finger-1 in acute lung injury-associated skeletal muscle wasting.

RATIONALE: Acute lung injury (ALI) is a debilitating condition associated with severe skeletal muscle weakness that persists in humans long after lung injury has resolved. The molecular mechanisms underlying this condition are unknown. OBJECTIVES: To identify the muscle-specific molecular mechanisms responsible for muscle wasting in a mouse model of ALI. METHODS: Changes in skeletal muscle weight, fiber size, in vivo contractile performance, and expression of mRNAs and proteins encoding muscle atrophy-associated genes for muscle ring finger-1 (MuRF1) and atrogin1 were measured. Genetic inactivation of MuRF1 or electroporation-mediated transduction of miRNA-based short hairpin RNAs targeting either MuRF1 or atrogin1 were used to identify their role in ALI-associated skeletal muscle wasting. MEASUREMENTS AND MAIN RESULTS: Mice with ALI developed profound muscle atrophy and preferential loss of muscle contractile proteins associated with reduced muscle function in vivo. Although mRNA expression of the muscle-specific ubiquitin ligases, MuRF1 and atrogin1, was increased in ALI mice, only MuRF1 protein levels were up-regulated. Consistent with these changes, suppression of MuRF1 by genetic or biochemical approaches prevented muscle fiber atrophy, whereas suppression of atrogin1 expression was without effect. Despite resolution of lung injury and down-regulation of MuRF1 and atrogin1, force generation in ALI mice remained suppressed. CONCLUSIONS: These data show that MuRF1 is responsible for mediating muscle atrophy that occurs during the period of active lung injury in ALI mice and that, as in humans, skeletal muscle dysfunction persists despite resolution of lung injury.

Vitamin D Supplementation and Muscle Power, Strength and Physical Performance in Older Adults: A Randomized Controlled Trial.

BACKGROUND: Low 25-hydroxyvitamin D (25[OH]D) concentrations (<30 ng/mL [<50 nmol/L]) have been associated with muscle weakness and impaired physical performance in observational studies. However, the effect of vitamin D supplementation on changes in muscle strength and physical performance in randomized controlled trials has been mixed. OBJECTIVES: To determine the effect of daily vitamin D supplementation on leg power, strength, and physical performance in low-functioning older adults with 25(OH)D concentrations of 18 to <30 ng/mL. METHODS: In this double-blind, randomized controlled trial, 136 low-functioning [Short Physical Performance Battery (SPPB) scores ≤10] adults aged 65-89 y with 25(OH)D concentrations of 18 to <30 ng/mL were randomly assigned to 2000 IU/d vitamin D3 or placebo for 12 mo. Lower-extremity leg power (primary outcome), leg and grip strength, SPPB, timed up and go (TUG), postural sway, and gait velocity and spatiotemporal parameters (secondary outcomes) were assessed at baseline, 4 and 12 mo. A subset (n = 37) also underwent a muscle biopsy at baseline and 4 mo and muscle fiber composition and contractile properties were assessed. RESULTS: Participants' mean ± SD age and SPPB scores at baseline were 73.4 ± 6.3 y and 7.8 ± 1.8, respectively. Mean ± SD 25(OH)D concentrations at baseline and 12 mo were 19.4 ± 4.2 ng/mL and 28.6 ± 6.7 ng/mL in the vitamin D group and 19.9 ± 4.9 ng/mL and 20.2 ± 5.0 ng/mL in the placebo group for a mean ± SE difference of 9.1 ± 1.1 ng/mL (P < 0.0001). However, there were no differences in change in leg power, leg or grip strength, SPPB score, TUG, postural sway, or gait velocity and spatiotemporal parameters by intervention group over 12 mo or muscle fiber composition and contractile properties over 4 mo. CONCLUSIONS: In low-functioning older adults with 25(OH)D concentrations of 18 to <30 ng/mL, randomization to 2000 IU/d vitamin D3 did not result in improvements in leg power, strength, or physical performance or muscle fiber composition and contractile properties. This trial was registered at clinicaltrials.gov as NCT02015611.

The association between aging-related monocyte transcriptional networks and comorbidity burden: the Multi-Ethnic Study of Atherosclerosis (MESA).

Translating our knowledge of the biological aging from animal models to humans may give rise to novel approaches of targeting multiple aging-related diseases simultaneously and increasing health span. Here, for the first time, we use transcriptomic signatures of monocytes to identify biological aging pathways underlying multiple aging-related diseases in humans. The ordinal logistic regression was used to cross-sectionally investigate transcriptomics of the comorbidity index in 1264 community-based Multi-Ethnic Study of Atherosclerosis (MESA) adults, 47% Caucasian, 32% Hispanic, 21% African American, and 51% female, aged 55-94 years. The comorbidity index was defined as the number of prevalent aging-related diseases including cardiovascular disease, type-2 diabetes, hypertension, cancer, dementia, chronic kidney disease, chronic obstructive pulmonary disease, and hip fracture. We identified 708 gene transcripts associated with the comorbidity index (FDR < 0.05) after adjusting for age, sex, ethnicity, and study site. In a weighted gene co-expression network analysis, as postulated, aging-related declines in apoptosis/autophagy (OR = 1.21 per SD increment, p = 0.0006) and ribosome/mitochondrion (OR = 0.90 per SD increment, p = 0.05) were positively associated with the comorbidity index. After adjusting for multiple comparisons, we identified 10 comorbidity-associated modules (FDR < 0.05), including the module of apoptosis/autophagy. There were three inter-correlated modules of these 10 involved in the complement subcomponent C1q, Fc gamma receptor I, and Fc gamma receptor III of the immune system, respectively. Aging-related upregulation of these three modules was positively associated with the comorbidity index. The odds of comorbidity increased with more of these modules acting together in a dose-response fashion. In conclusion, transcriptomic analysis of human immune cells may identify biomarker panels indicative of comprehensive biological mechanisms, especially immune signaling pathways, contributing to health aging.

Residual sarcoplasmic reticulum Ca2+ concentration after Ca2+ release in skeletal myofibers from young adult and old mice.

Contrasting information suggests either almost complete depletion of sarcoplasmic reticulum (SR) Ca(2+) or significant residual Ca(2+) concentration after prolonged depolarization of the skeletal muscle fiber. The primary obstacle to resolving this controversy is the lack of genetically encoded Ca(2+) indicators targeted to the SR that exhibit low-Ca(2+) affinity, a fast biosensor: Ca(2+) off-rate reaction, and can be expressed in myofibers from adult and older adult mammalian species. This work used the recently designed low-affinity Ca(2+) sensor (Kd = 1.66 mM in the myofiber) CatchER (calcium sensor for detecting high concentrations in the ER) targeted to the SR, to investigate whether prolonged skeletal muscle fiber depolarization significantly alters residual SR Ca(2+) with aging. We found CatchER a proper tool to investigate SR Ca(2+) depletion in young adult and older adult mice, consistently tracking SR luminal Ca(2+) release in response to brief and repetitive stimulation. We evoked SR Ca(2+) release in whole-cell voltage-clamped flexor digitorum brevis muscle fibers from young and old FVB mice and tested the maximal SR Ca(2+) release by directly activating the ryanodine receptor (RyR1) with 4-chloro-m-cresol in the same myofibers. Here, we report for the first time that the Ca(2+) remaining in the SR after prolonged depolarization (2 s) in myofibers from aging (~220 µM) was larger than young (~132 µM) mice. These experiments indicate that SR Ca(2+) is far from fully depleted under physiological conditions throughout life, and support the concept of excitation-contraction uncoupling in functional senescent myofibers.

Brainstem noradrenergic neurons: Identifying a hub at the intersection of cognition, motility, and skeletal muscle regulation.

Brainstem noradrenergic neuron clusters form a node integrating efferents projecting to distinct areas such as those regulating cognition and skeletal muscle structure and function, and receive dissimilar afferents through established circuits to coordinate organismal responses to internal and environmental challenges. Genetic lineage tracing shows the remarkable heterogeneity of brainstem noradrenergic neurons, which may explain their varied functions. They project to the locus coeruleus, the primary source of noradrenaline in the brain, which supports learning and cognition. They also project to pre-ganglionic neurons, which lie within the spinal cord and form synapses onto post-ganglionic neurons. The synapse between descending brainstem noradrenergic neurons and pre-ganglionic spinal neurons, and these in turn with post-ganglionic noradrenergic neurons located at the paravertebral sympathetic ganglia, support an anatomical hierarchy that regulates skeletal muscle innervation, neuromuscular transmission, and muscle trophism. Whether any noradrenergic neuron subpopulation is more susceptible to damaged protein deposit and death with ageing and neurodegeneration is a relevant question that answer will help us to detect neurodegeneration at an early stage, establish prognosis, and anticipate disease progression. Loss of muscle mass and strength with ageing, termed sarcopenia, may predict impaired cognition with ageing and neurodegeneration and establish an early time to start interventions aimed at reducing central noradrenergic neurons hyperactivity. Complex multidisciplinary approaches, including genetic tracing, specific circuit labelling, optogenetics and chemogenetics, electrophysiology, and single-cell transcriptomics and proteomics, are required to test this hypothesis pre-clinical.

The emerging role of the sympathetic nervous system in skeletal muscle motor innervation and sarcopenia.

Examining neural etiologic factors'role in the decline of neuromuscular function with aging is essential to our understanding of the mechanisms underlying sarcopenia, the age-dependent decline in muscle mass, force and power. Innervation of the skeletal muscle by both motor and sympathetic axons has been established, igniting interest in determining how the sympathetic nervous system (SNS) affect skeletal muscle composition and function throughout the lifetime. Selective expression of the heart and neural crest derivative 2 gene in peripheral SNs increases muscle mass and force regulating skeletal muscle sympathetic and motor innervation; improving acetylcholine receptor stability and NMJ transmission; preventing inflammation and myofibrillar protein degradation; increasing autophagy; and probably enhancing protein synthesis. Elucidating the role of central SNs will help to define the coordinated response of the visceral and neuromuscular system to physiological and pathological challenges across ages. This review discusses the following questions: (1) Does the SNS regulate skeletal muscle motor innervation? (2) Does the SNS regulate presynaptic and postsynaptic neuromuscular junction (NMJ) structure and function? (3) Does sympathetic neuron (SN) regulation of NMJ transmission decline with aging? (4) Does maintenance of SNs attenuate aging sarcopenia? and (5) Do central SN group relays influence sympathetic and motor muscle innervation?