Sieck, Gary C.

AbstractStructural plasticity in motoneurons may be influenced by activation history and motoneuron–muscle fiber interactions. The goal of this study was to examine the morphological adaptations of phrenic motoneurons following imposed motoneuron inactivity while controlling for diaphragm muscle inactivity. Well‐characterized rat models were used including unilateral C2 spinal hemisection (SH; ipsilateral phrenic motoneurons and diaphragm muscle are inactive) and tetrodotoxin phrenic nerve blockade (TTX; ipsilateral diaphragm muscle is paralyzed while phrenic motoneuron activity is preserved). We hypothesized that inactivity of phrenic motoneurons would result in a decrease in motoneuron size, consistent with a homeostatic increase in excitability. Phrenic motoneurons were retrogradely labeled by ipsilateral diaphragm muscle injection of fluorescent dextrans or cholera toxin subunit B. Following 2 weeks of diaphragm muscle paralysis, morphological parameters of labeled ipsilateral phrenic motoneurons were assessed quantitatively using fluorescence confocal microscopy. Compared to controls, phrenic motoneuron somal volumes and surface areas decreased with SH, but increased with TTX. Total phrenic motoneuron surface area was unchanged by SH, but increased with TTX. Dendritic surface area was estimated from primary dendrite diameter using a power equation obtained from three‐dimensional reconstructed phrenic motoneurons. Estimated dendritic surface area was not significantly different between control and SH, but increased with TTX. Similarly, TTX significantly increased total phrenic motoneuron surface area. These results suggest that ipsilateral phrenic motoneuron morphological adaptations are consistent with a normalization of motoneuron excitability following prolonged alterations in motoneuron activity. Phrenic motoneuron structural plasticity is likely more dependent on motoneuron activity (or descending input) than muscle fiber activity.

New Findings What is the central question of this study? Absence of hypothermia‐induced cardiac arrest is a strong predictor for a favourable outcome after rewarming. Nevertheless, detailed knowledge of preferences in organ blood flow during rewarming with spontaneous circulation is largely unknown. What is the main finding and its importance? In a porcine model of accidental hypothermia, we find, despite a significantly reduced cardiac output during rewarming, normal blood flow and O2 supply in vital organs owing to patency of adequate physiological compensatory responses. In critical care medicine, active rewarming must aim at supporting the spontaneous circulation and maintaining spontaneous autonomous vascular control. AbstractThe absence of hypothermia‐induced cardiac arrest is one of the strongest predictors for a favourable outcome after rewarming from accidental hypothermia. We studied temperature‐dependent changes in organ blood flow and O2 delivery () in a porcine model with spontaneous circulation during 3 h of hypothermia at 27°C followed by rewarming. Anaesthetized pigs (n = 16, weighing 20–29 kg) were randomly assigned to one of two groups: (i) hypothermia/rewarming (n = 10), immersion cooled to 27°C and maintained for 3 h before being rewarmed by pleural lavage; and (ii) time‐matched normothermic (38°C) control animals (n = 6), immersed for 6.5 h, the last 2 h with pleural lavage. Regional blood flow was measured using a neutron‐labelled microsphere technique. Simultaneous measurements of and O2 consumption () were made. During hypothermia, there was a reduction in organ blood flow, and . After rewarming, there was a 40% reduction in stroke volume and cardiac output, causing a global reduction in ; nevertheless, blood flow to the brain, heart, stomach and small intestine returned to prehypothermic values. Blood flow in the liver and kidneys was significantly reduced. Cerebral and returned to control values. After hypothermia and rewarming there is a significant lowering of owing to heart failure. However, compensatory mechanisms preserve O2 transport, blood flow and in most organs. Nevertheless, these results indicate that hypothermia‐induced heart failure requires therapeutic intervention.

Midcervical spinal cord contusion injury results in tissue damage, disruption of spinal pathways, and motor neuron loss. Unilateral C4 contusion results in loss of 40%–50% of phrenic motor neurons ipsilateral to the injury (~25% of the total phrenic motor neuron pool). Over time after unilateral C4 contusion injury, diaphragm muscle (DIAm) electromyogram activity increases both contralateral and ipsilateral to the side of injury in rats, suggesting compensation because of increased activation of the surviving motor neurons. However, the impact of contusion injury on DIAm force generation is less clear. Transdiaphragmatic pressure (Pdi) was measured across motor behaviors over time after unilateral C4 contusion injury in adult male Sprague-Dawley rats. Maximum Pdi (Pdimax) was elicited by bilateral phrenic nerve stimulation at 7 days postinjury. We hypothesized that Pdimaxis reduced following unilateral C4 contusion injury, whereas ventilatory behaviors of the DIAm are unimpaired. In support of our hypothesis, Pdimaxwas reduced by ~25% after unilateral C4 contusion, consistent with the extent of phrenic motor neuron loss following contusion injury. One day after contusion injury, the Pdi amplitude during airway occlusion was reduced from ~30 to ~20 cmH2O, but this reduction was completely reversed by 7 days postinjury. Ventilatory behaviors (~10 cmH2O), DIAm-specific force, and muscle fiber cross-sectional area did not differ between the laminectomy and contusion groups. These results indicate that the large reserve capacity for DIAm force generation allows for higher-force motor behaviors to be accomplished despite motor neuron loss, likely reflecting changes in motor unit recruitment.NEW & NOTEWORTHY Respiratory muscles such as the diaphragm generate the pressures necessary to accomplish a variety of motor behaviors ranging from ventilation to near-maximal expulsive behaviors. However, the impact of contusion injury on diaphragm pressure generation across behaviors is not clear. The present study shows that contusion injury impairs maximal pressure generation while preserving the ability of the diaphragm to accomplish lower-force motor behaviors, likely reflecting changes in diaphragm motor unit recruitment.

Abstract To assess cellular mechanisms that could contribute to muscle weakness in the context of cancer cachexia, we evaluated the effect of conditioned medium (CM) from highly tumorigenic Lewis lung carcinoma (LLC1) cells, CM from less tumorigenic MLE12 cells and non-CM on human skeletal muscle mitochondrial morphology and function. We assessed mitochondrial oxygen consumption, morphology, enzyme activity and gene expression in human skeletal muscle cells in the presence of tumor conditioned medium or control medium. We examined the secretome of LLC1 cells for mediators of skeletal muscle changes. We demonstrate that LLC1 CM inhibits cellular O2 consumption rate (OCR) and increases the proton production rate in human skeletal muscle cells. Mitochondrial fragmentation and the expression of the fission mediator, FIS1, are increased, whereas the expression of fusion mediators, OPA1 and MFN2 are reduced by LLC1 CM. LLC1 CM reduces pyruvate dehydrogenase (PDH) activity thereby limiting the production of acetyl CoA. Reduced PDH activity is associated with reduced PDHA1 sub-unit expression. The amount inactive phospho-PDH is increased in association with reduced PDP2 expression and increased PDK4 and SIRT4 expression in human skeletal muscle cells treated with LLC1 CM. The expression of several signaling pathways is altered by LLC1 CM in human skeletal muscle cells. When compared to MLE12 cells, LLC1 cells show increased expression of mRNAs encoding several potential mediators of altered muscle function such as Gdf15, TGFβ-induced protein, IL11, Il27, Il33, Il34, and PTH-like protein. We show that the active vitamin D metabolite, 1α,25(OH)2D3, that we previously demonstrated increases myoblast mitochondrial oxygen consumption and PDH activity, mitigates LLC1 CM effects on hSkMC OCR, mitochondrial fragmentation and mediators thereof. Mitochondrial oxygen consumption, morphology and enzyme activity in human skeletal muscle cells are inhibited or altered by tumor cell conditioned medium, and are reversed by 1α,25(OH)2D3, the active metabolite of vitamin D. Citation Format: Zachary C. Ryan, Theodore A. Craig, Xuewei Wang, Philippe Delmotte, Jeffrey L. Salisbury, Ian R. Lanza, Gary C. Sieck, Rajiv Kumar. 1,25-Dihydroxyvitamin D3 mitigates lung cancer cell mediated mitochondrial dysfunction in human skeletal muscle [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5232.

Motor neuron (MN) development in early onset spasticity is poorly understood. For example, spastic cerebral palsy (sCP), the most common motor disability of childhood, is poorly predicted by brain imaging, yet research remains focused on the brain. By contrast, MNs, via the motor unit and neurotransmitter signaling, are the target of most therapeutic spasticity treatments and are the final common output of motor control. MN development in sCP is a critical knowledge gap, because the late embryonic and postnatal periods are not only when the supposed brain injury occurs but also are critical times for spinal cord neuromotor development. Using an animal model of early onset spasticity [ spa mouse (B6.Cg- Glrbspa/J) with a glycine (Gly) receptor mutation], we hypothesized that removal of effective glycinergic neurotransmitter inputs to MNs during development will influence MN pruning (including primary dendrites) and MN size. Spa (Glrb−/−) and wild-type (Glrb+/+) mice, ages 4–9 wk, underwent unilateral retrograde labeling of the tibialis anterior muscle MNs via peroneal nerve dip in tetramethylrhodamine. After 3 days, mice were euthanized and perfused with 4% paraformaldehyde, and the spinal cord was excised and processed for confocal imaging. Spa mice had ~61% fewer lumbar tibialis anterior MNs ( P < 0.01), disproportionately affecting larger MNs. Additionally, a ~23% reduction in tibialis anterior MN somal surface area ( P < 0.01) and a 12% increase in primary dendrites ( P = 0.046) were observed. Thus MN pruning and MN somal surface area are abnormal in early onset spasticity. Fewer and smaller MNs may contribute to the spastic phenotype.NEW & NOTEWORTHY Motor neuron (MN) development in early onset spasticity is poorly understood. In an animal model of early onset spasticity, spa mice, we found ~61% fewer lumbar tibialis anterior MNs compared with controls. This MN loss disproportionately affected larger MNs. Thus number and heterogeneity of the MN pool are decreased in spa mice, likely contributing to the spastic phenotype.

Autophagy is a cellular digestion process that contributes to cellular homeostasis and adaptation by the elimination of proteins and damaged organelles. Evidence suggests that dysregulation of autophagy plays a role in neurodegenerative diseases, including motor neuron disorders. Herein, we review emerging evidence indicating the roles of autophagy in physiological motor neuron processes and its function in specific compartments. Moreover, we discuss the involvement of autophagy in the pathogenesis of motor neuron diseases, including spinal cord injury and aging, and recent developments that offer promising therapeutic approaches to mitigate effects of dysregulated autophagy in health and disease.