Impaired proteostatic mechanisms other than decreased protein synthesis limit old skeletal muscle recovery after disuse atrophy

BACKGROUND

Skeletal muscle mass and strength diminish during periods of disuse but recover upon return to weight bearing in healthy adults but are incomplete in old muscle. Efforts to improve muscle recovery in older individuals commonly aim at increasing myofibrillar protein synthesis via mammalian target of rapamycin (mTOR) stimulation despite evidence demonstrating that old muscle has chronically elevated levels of mammalian target of rapamycin complex 1 (mTORC1) activity. We hypothesized that protein synthesis is higher in old muscle than adult muscle, which contributes to a proteostatic stress that impairs recovery.

METHODS

We unloaded hindlimbs of adult (10-month) and old (28-month) F344BN rats for 14 days to induce atrophy, followed by reloading up to 60 days with deuterium oxide (D2 O) labelling to study muscle regrowth and proteostasis.

RESULTS

We found that old muscle has limited recovery of muscle mass during reloading despite having higher translational capacity and myofibrillar protein synthesis (0.029 k/day ± 0.002 vs. 0.039 k/day ± 0.002, P < 0.0001) than adult muscle. We showed that collagen protein synthesis was not different (0.005 k (1/day) ± 0.0005 vs. 0.004 k (1/day) ± 0.0005, P = 0.15) in old compared to adult, but old muscle had higher collagen concentration (4.5 μg/mg ± 1.2 vs. 9.8 μg/mg ± 0.96, P < 0.01), implying that collagen breakdown was slower in old muscle than adult muscle. This finding was supported by old muscle having more insoluble collagen (4.0 ± 1.1 vs. 9.2 ± 0.9, P < 0.01) and an accumulation of advanced glycation end products (1.0 ± 0.06 vs. 1.5 ± 0.08, P < 0.001) than adult muscle during reloading. Limited recovery of muscle mass during reloading is in part due to higher protein degradation (0.017 1/t ± 0.002 vs. 0.028 1/t ± 0.004, P < 0.05) and/or compromised proteostasis as evidenced by accumulation of ubiquitinated insoluble proteins (1.02 ± 0.06 vs. 1.22 ± 0.06, P < 0.05). Last, we showed that synthesis of individual proteins related to protein folding/refolding, protein degradation and neural-related biological processes was higher in old muscle during reloading than adult muscle.

CONCLUSIONS

Our data suggest that the failure of old muscle to recover after disuse is not due to limitations in the ability to synthesize myofibrillar proteins but because of other impaired proteostatic mechanisms (e.g., protein folding and degradation). These data provide novel information on individual proteins that accumulate in protein aggregates after disuse and certain biological processes such as protein folding and degradation that likely play a role in impaired recovery. Therefore, interventions to enhance regrowth of old muscle after disuse should be directed towards the identified impaired proteostatic mechanisms and not aimed at increasing protein synthesis.

Increased seizure sensitivity in pregnant mice with genetic knockdown of acid sensing ion channel 2a is associated with impaired hippocampal inflammatory response

Acid sensing ion channels (ASICs) are mechano- and chemo-receptor channels that are activated by drops in extracellular pH as occurs after neurotransmission. In our previous study, we demonstrated that mice subjected to reduced utero-placental perfusion pressure during pregnancy, to mimic the pregnancy complication of preeclampsia, have reduced hippocampal expression of ASIC2a protein. We also showed that pregnant mice with heterozygous expression of ASIC2a (+/-) had increased sensitivity and severity to pentylenetetrazol-induced seizures; however, the mechanisms by which this occurs remain unclear. The purpose of this study was to investigate key molecular targets involving neurotransmission and inflammation that are differentially changed following seizure exposure in pregnant ASIC2a +/- mice. On gestational day 18.5, ASIC2a wild-type (+/+, n = 7) and +/- (n = 14) mice were injected with 40 mg/kg pentylenetetrazol and monitored for 30 min. Western blot and ELISA analysis revealed no difference in hippocampal synaptosome glutamate-related proteins but an increase in GABA concentration in pregnant +/- mice. Using ELISA and multiplex assays, we found a significant decrease in serum TNFα, and a decreased concentration of pro-inflammatory cytokines and chemokines in hippocampal cytosolic fraction. Significant reductions in IL-1β, IL-3, IL-12 (p70), eotaxin, interferon gamma, and macrophage inflammatory protein (MIP-1β), in the hippocampal cytosolic fractions of +/- mice were observed compared to +/+ mice. Additionally, there was no difference in hippocampal microglia density or activation in pregnant ASIC2a+/+ vs. +/- mice. These results support the hypothesis that pregnant mice with reduced ASIC2a may not be able to mount an inflammatory response following acute seizure exposure.

Sex differences in anatomic plasticity of gut neuronal-mast cell interactions

The gut wall houses mast cells that are anatomically situated near enteric neuronal fibers. Roles of specific neuropeptides in modulating function of immune components like mast cells in response to challenge with bacterial components are relatively unknown. Investigating such interactions requires models that include diverse cellular elements in native anatomic arrangements. Using an organotypic slice model that maintains gut wall cellular diversity ex vivo, the present study compared responses between tissues derived from male and female mice to examine neural-immune signaling in the gut wall after selected treatments. Ileum slices were treated with pharmacological reagents that block neuronal function (e.g., tetrodotoxin) or vasoactive intestinal peptide (VIP) receptors prior to challenge with lipopolysaccharide (LPS) to assess their influence on anatomic plasticity of VIP fibers and activation of mast cells. Sex differences were observed in the number of mucosal mast cells (c-kit/ACK2 immunoreactive) at baseline, regardless of treatment, with female ileum tissue having 46% more ACK2-IR mast cells than males. After challenge with LPS, male mast cell counts rose to female levels. Furthermore, sex differences were observed in the percentage of ACK2-IR cells within 1 µm of a VIP+ neuronal fiber, and mast cell size, a metric previously tied to activation, with females having larger cells at baseline. Male mast cell sizes reached female levels after LPS challenge. This study suggests sex differences in neural-immune plasticity and in mast cell activation both basally and in response to challenge with LPS. These sex differences could potentially impact functional neuroimmune response to pathogens.