Drosophila adult muscle precursor cells contribute to motor axon pathfinding and proper innervation of embryonic muscles

Adult Muscle Precursors (AMPs), the Drosophila muscle stem cells, arise from the asymmetric cell divisions of a subset of muscle progenitors ([1]) and are characterized by the persistent expression of the myogenic transcription factor Twist ([2]) and activation of the Notch pathway ([3]; [4]). They occupy stereotyped positions in the vicinity of developing body wall muscles, stay quiescent and undifferentiated during embryonic life and are reactivated during second larval instar ([5]; [6]) to generate muscles of the adult fly. Strikingly, AMPs are also located in the path of intersegmental (ISN) and segmental (SN) motor neuron branches ([2]; [7]). However, their role and interactions with the motor neurons have not yet been analyzed in details. Here, using AMP sensor line revealing cell membrane extensions we show that the navigating ISN first contacts the dorso-lateral (DL-AMPs) and then the dorsal AMP (D-AMP) marking the end of its trajectory. In parallel, the segmental nerve SNa innervating lateral muscles targets the lateral AMPs (L-AMPs). In vivo analyses of AMPs behavior highlight an active filopodial dynamic of AMPs toward the ISN and SNa suggesting they could guide motor axons and contribute to muscle innervations. Indeed, our data show that loss or mispositioning of L-AMPs affect the SNa motor axons pathfinding and branching, leading to loss or aberrant muscle innervation. The finding of a transient expression of the guidance molecule Sidestep in L-AMPs suggests its implication in this process. Thus, proper muscle innervation does not only rely on the dialogue between the motor neurons and the muscles, but also on the AMP cells. AMPs represent spatial landmarks for navigating motor neurons and their positioning is critical for the muscles innervation in the lateral region.