"Objectives : Characterizing the consequences of Muscleblind-like (MBNL) functional loss in motor neurons (MN) on motor unit physiology and its latter contribution to myotonic dystrophy type 1 (DM1) muscle alterations. DM1 is a neuromuscular disease characterised by myotonia, progressive muscle weakness and atrophy, cardiac defects as well as cognitive impairments. DM1 is caused by the expression of CTG repeat expansions in the 3’UTR of the DMPK gene; subsequent mutant transcripts, containing expanded CUG repeats, aggregate as ribonuclear inclusions that sequester MBNL RNA-binding proteins, impairing their functions in various tissues. RNA metabolism abnormalities observed in affected DM1 tissues are mainly attributable to MBNL family loss-of-function: MBNL1 is mostly expressed in skeletal muscles but also found in the brain; MBNL2 is predominantly expressed in the brain and MBNL3 during embryonic development. While most studies on DM1 muscle pathology have focused on the skeletal muscle itself, prior reports suggest an impaired communication between MN and skeletal muscle. To assess the role of MBNL in MN, we have generated a conditional mouse model invalidated for Mbnl1 and Mbnl2 in MN (MN-MBNL-dKO). These mice develop progressive motor abnormalities but also neuromuscular junction (NMJ) alterations indicating that MBNL compound loss-of-function in MN significantly affects NMJ structure and function. As NMJ are unaltered in young MN-MBNL-dKO mice, deficiency in NMJ maintenance rather than development may be responsible for NMJ abnormalities observed in older mice. Identification of molecular alterations in MN of this mouse model as well as MBNL RNA targets in MN is ongoing. Altogether our work will help for a better knowledge of DM1 physiopathology. "