BMP6 is axonally transported by motoneurons and supports their survival in vitro

The regulation of motoneuron survival is only partially elucidated. We have sought new survival factors for motoneuron by analyzing which receptors they produce. We report here that the type II bone morphogenetic receptor (BMPRII) mRNA is one of the most abundant receptor mRNAs in laser microdissected motoneurons. Motoneurons were intensely stained by an anti-BMPRII antibody, indicating the presence of BMPRII protein. One of its ligands (BMP6) supported the survival of motoneurons in vitro. BMP6 was produced by myotubes and mature Schwann cells and was retrogradely transported in mature motor axons. BMP6 thus joins a list of known Schwann-cell-derived regulators of motoneurons, which includes GDNF, CNTF, LIF and TGF-beta2. The control of the production of these factors by Schwann cells and the direction of their movement in motor axons is diverse. This suggests that the multiplicity of motoneuron factors is because cells use different factors to regulate different aspects of motoneuron function.