Bailey, Jeffrey P.

Neuregulin (NRG) is part of the epidermal growth factor family of proteins. Previous studies have shown that NRG activates PI3K/Akt pathways, leading to regulation of protein synthesis via downstream regulation of translation initiators. The principal translation initiator involved in protein synthesis is probably eIF4E, and the principal transcription factor regulating protein degradation is FOXO. We hypothesized that NRG signaling involves activation of the PI3K/Akt pathway, thus leading to an increase in expression of eIF4E and a decrease in expression of FOXO. An increase in eIF4E expression increases protein synthesis, and a decrease in FOXO expression reduces protein degradation. In this study we used dexamethasone, a synthetic glucocorticoid, to induce atrophy and cause an imbalance between protein synthesis and degradation in L6 myotubes. As expected, we found that after 48 hours of treatment with 10–100nM dexamethasone, eIF4E expression decreased and FOXO expression increased in a dose‐dependent manner. We found that treatment with 200 ng/ml NRG increased eIF4E expression. When L6 cells were treated with both NRG and dexamethasone for 48 hours, NRG blunted the catabolic effects of dexamethasone, thus increasing eIF4E expression and decreasing FOXO expression. These results indicate that NRG promotes protein synthesis and prevents protein degradation. Supported by NIH grant AR51173.

Brain‐derived neurotrophic factor (BDNF) acting through full‐length tropomyosin related kinase receptor subtype B (TrkB.FL) plays an important role in neuroplasticity following spinal cord injury. BDNF/TrkB.FL signaling in phrenic motoneurons is both necessary and sufficient to enhance spontaneous recovery of ipsilateral rhythmic phrenic activity after C2 spinal cord hemisection (SH), where premotor drive to phrenic motoneurons is removed. Phrenic motoneuron expression of the excitatory glutamatergic NMDA receptor correlates with the time course of spontaneous functional recovery post‐SH. We hypothesized that increased BDNF/TrkB.FL signaling in phrenic motoneurons enhances recovery of rhythmic phrenic activity after SH via regulation of glutamatergic receptor expression. Rats were injected intrapleurally with AAV7 expressing human TrkB.FL‐FLAG at 3 weeks prior to SH. FLAG was present in the cervical ventral horn region, and AAV7‐GFP expression was specifically in phrenic motoneurons. At 14 days post‐SH, quantitative real‐time RT‐PCR verified human TrkB.FL expression in microdissected phrenic motoneurons. Phrenic motoneuron expression of NMDA increased ~10 fold compared to control, suggesting that increased BDNF/TrkB.FL signaling magnified SH‐induced changes in phrenic motoneuron NMDA expression. Supported by HL96750 and T32‐HL105355, and Paralyzed Veterans of America.

In previous studies, we found that after cervical spinal cord hemisection at C2 (SH), neuromuscular transmission improved in the rat diaphragm muscle (DIAm). We also found that exogenous brain‐derived neurotrophic factor (BDNF) treatment enhanced neuromuscular transmission, whereas inhibiting activation of its receptor (TrkB) worsened neuromuscular transmission. We hypothesized that expression of BDNF and/or its receptor TrkB at phrenic motoneurons would increase following SH. In the present study, adult male rats underwent SH or sham surgery. Phrenic motoneurons were labeled by injection of cholera toxin B into the DIAm and retrogradely‐labeled phrenic motoneurons were sampled by laser capture microdissection (LCM). BDNF and TrkB receptor (truncated – T1 and full length – FL) mRNA were measured in LCM‐microdissected motoneurons using quantitative real‐time RT‐PCR and expressed as a ratio to ribosomal S16 RNA. We found that BDNF mRNA expression in phrenic motoneurons increased by 3 days after SH (~2‐fold). Similarly, both TrkB.T1 and TrkB.FL mRNA increased by 3 days after SH (~17‐ and ~11‐fold, respectively). It is possible that the relative expression of neurotrophins and their receptors (truncated or full‐length) may determine the time‐course of restoration of phrenic activity following SH. Supported by NIH grants AR51173 and HL37680 and the Mayo Foundation.