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Postnatal Phrenic Motor Neuron and Diaphragm Maturation in the Mouse
Author(s) -
Brandenburg Joline E.,
Fogarty Matthew J.,
Davis Leah A.,
Sieck Gary C.
Publication year - 2022
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.2022.36.s1.0r541
Subject(s) - diaphragm muscle , diaphragm (acoustics) , biology , zoology , in vivo , anatomy , medicine , respiratory system , genetics , physics , acoustics , loudspeaker
In rats, there is a strong association between the expansion of phrenic motor neuron (PhMN) size (from ~21‐28 days postnatal), the emergence of mature diaphragm muscle (DIAm) myosin heavy chain (MyHC 2X and MyHC 2B ) isoform expression, alongside the manifestation of mature morphological and functional properties. Despite mice models being de rigueur for developmental investigations, the postnatal development of mouse PhMN and DIAm properties have not been comprehensively characterized. Using wildtype C57 mice at ages P7, P14, P21, P28, and >3 months (maturity) we evaluated PhMN and DIAm physiologic properties. PhMNs were labelled via rhodamine phrenic nerve dip, processed for longitudinal cryosectioning (70 µm) and prepared for confocal imaging and PhMN assessment. Ex vivo DIAm strips were used for functional assessments of maximum specific force (P o ) and fatigue. DIAm strips were also fresh‐frozen, with 8 µm cross‐sections immuno‐labelled for MyHC slow , MyHC 2A , MyHC 2X and MyHC 2X and processed for fluorescent assessment. The number of PhMNs remained stable throughout postnatal life (~195), while PhMN surface area increased steadily, ~10% from P14 to P21, P21 to P28, and P28 to maturity, with overall ~30% increase from P14 to maturity. The biggest changes in number of primary dendrites occurred between P14 and P28 and between P28 and maturity, 25% decrease in primary dendrites between each group. DIAm maximum P o increased in two phases, 3.5‐fold from P7 to P14 and remained similar until P21. The second large increase was ~3‐fold from P21 to P28 and remained steady to maturity. The fatiguability of the DIAm increased with age in two phases, with fatigability being similar from P7 and P14 and increasing by ~20% at P21. Following, fatigability of the DIAm was similar from P21 to P28 and increased by ~35% at maturity. These major PhMN and DIAm changes coincide with the steady growth of PhMN and the emergence of MyHC 2X and MyHC 2B expression in DIAm from P14 to P21 onwards. The maturation of orderly PhMN recruitment likely depends on an interplay between PhMN size properties and network inputs to a refined dendritic tree. We show that DIAm muscle fiber maturation in mice hinges on the expression of MyHC 2X and MyHC 2B isoforms that is associated with an ~10 fold increase in DIAm force‐generation capacity.