Myotonic dystrophy type 1 (DM1) is a RNA gain of function disorder due to the abnormal expansion of CTG repeats in the 3’UTR of the DMPK gene. Toxic mutated RNA aggregates in the nucleus and sequester MBNL RNA binding proteins leading to RNA metabolism abnormalities. The principal aim of this research project is to analyze the role of the motor unit and particularly the communication between motor neurons (MN) and the skeletal muscle via neuromuscular junctions in muscle dysfunction in DM1. Using mice and cellular models, associated with CLIP and high throughput RNA sequencing, we will identify molecular targets that are regulated by MBNL in MN and determine the consequences of MBNL functional loss in MN on motor unit development and function knowing that MN controls skeletal muscle activity. The proposal will a better understanding of MBNL loss of function in MN and its contribution to DM1 muscle pathophysiology.