The role of Sema3-Npn-1 signaling during diaphragm innervation and muscle development

Abstract

Correct innervation of the main respiratory muscle in mammals, namely the thoracic diaphragm, is a crucial pre-requisite for the functionality of this muscle and the viability of the entire organism. Systemic impairment of Sema3A-Npn-1 signaling causes excessive branching of phrenic nerves in the diaphragm and into the central tendon region, where the majority of misguided axons innervate ectopic musculature. To elucidate whether these ectopic muscles are a result of misguided myoblast precursors due to the loss of Sema3A-Npn-1 signaling, we conditionally ablated Npn-1 in somatic motor neurons which leads to a similar phenotype of phrenic nerve defasciculation and, intriguingly, also formation of innervated ectopic muscles. We therefore hypothesize that ectopic myocyte fusion is caused by additional factors released by misprojecting growth cones. Slit2 and its Robo receptors are expressed by phrenic motor axons and migrating myoblasts, respectively, during innervation of the diaphragm. In vitro analyses revealed a chemo-attractive effect of Slit2 on primary diaphragm myoblasts. Thus, we postulate an influence of factors released by motor neuron growth cones on the migration properties of myoblasts during establishment of the diaphragm.