Enhancing Respiratory Motor Function after Spinal Cord Injury

ABSTRACT The proposed studies focus on the role of BDNF/TrkB signaling in promoting phrenic motor neuron (PhMN) survival and pre- and postsynaptic neuroplasticity to enhance functional recovery after cervical spinal cord injury (cSCI). We will leverage our extensive experience using two well-established models of incomplete cSCI, i.e., C2 hemisection - C2SH and C4 contusion injury - C4CI to explore neuroplasticity at PhMNs. We will also bring years of experience examining diaphragm muscle (DIAm) motor unit physiology, recognizing the importance of PhMN size in the recruitment of DIAm motor units. Stratifying the effects of BDNF/TrkB signaling on pre- and postsynaptic neuroplasticity in PhMNs of varying size is conceptually innovative and provides valuable mechanistic insight. The “Size Principle” is a key concept in neuromotor control physiology, but until now it has been difficult to directly include motor neuron size as a critical variable. We hypothesize that there are size- dependent differences in BDNF/TrkB signaling effects on pre- and postsynaptic neuroplasticity after cSCI. The proposed studies will provide novel information about PhMN neuroplasticity underlying functional recovery of inspiratory and post-inspiratory DIAm activity that will inform development of novel therapeutic approaches to enhance recovery of DIAm function in patients with incomplete cSCI. The proposed studies will utilize a comprehensive array of innovative techniques, many developed in our lab. In all 3 specific aims, we will examine the effects of modulating BDNF/TrkB signaling using a gain/loss of function approach utilizing intrathecal BDNF or TrkB-Fc treatment and a novel TrkBF616A rat with a 1NMPP1 sensitive knock-in allele that rapidly and selectively inhibits TrkB kinase activity. In Aim 1, we will use high resolution confocal imaging to explore the role of BDNF/TrkB signaling on the recovery of Glu and Gly/GABA presynaptic terminals on retrogradely labeled PhMNs of varying size following C2SH. In Aim 2, we will employ a novel RNAscope in situ hybridization technique to simultaneously explore changes in NMDA, Gly and GABAA receptor mRNA expression in PhMNs of varying size. In Aim 3, we will examine the neuroprotective effects (PhMN survival) of BDNF/TrkB signaling following C4CI. In addition, we will examine the effect of BDNF/TrkB signaling on Glu and Gly/GABA synaptic input and NMDA, Gly and GABAA receptor mRNA expression (pre- and postsynaptic neuroplasticity) in surviving PhMNs above (i.e., C3) and below (i.e., C5) the C4CI injury.