Kopper, Leo A.

In rats, there is a strong association between the emergence of mature diaphragm muscle (DIAm) myosin heavy chain isoform expression and the expansion of phrenic motor neuron (PhMN) size (from ~21-28 days postnatal (P)), alongside the manifestation of mature morphological and functional properties. In the Spastic ( Spa) mutant mouse, mutated glycine receptors lead to a spasticity phenotype emerging between P14-28, concomitant with DIAm and PhMN development. In adult Spa mice, DIAm weakness, fiber atrophy and loss of larger PhMNs is evident. We aim to characterize whether these DIAm and PhMN deficits occur contemporaneously to spasticity. Using wildtype (WT) and Spa mice at P14, P21, P28, and P90 (maturity) we evaluated DIAm and PhMN physiologic properties. Ex vivo DIAm strips were used for functional assessments of maximum specific force (Po) and fatigue. DIAm strips were also fresh-frozen, and 8 μm cross-sections were cut and stained with H&E to evaluate cross-sectional area (CSA) of individual DIAm fibers. PhMNs were labelled via rhodamine phrenic nerve dip, processed for longitudinal cryosectioning (70 μm) and prepared for confocal imaging and PhMN assessment. In WT mice, DIAm specific force increased at each age (P14, P21, P28 and P90), whilst in Spa mice, DIAm specific force was reduced from P21 compared to WT. Similarly, DIAm fiber CSA was reduced in Spa compared to WT mice at P21, P28 and P90. The number of PhMNs was similar between genotypes at P21, with a reduction in the number of PhMNs in Spa mice compared to WT evident at P28 continuing to P90. In Spa mice, PhMN size was smaller compared to WT at P21, unchanged at P28 and reduced at P90. The changes in Spa DIAm and PhMNs during the first postnatal month are consistent with the emergence of spasticity from P14 to P28. As most changes occur at ~P21, we are optimistic that interventions prior to weaning may promote the normal growth of DIAm and survival of PhMNs and may stave off later life DIAm behavioral deficits. HL166204 and HL146114. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

During early postnatal development, BDNF/TrkB signaling promotes PhMN survival. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact PhMN size. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact the number or CSA of DIAm fibers.