Muscle-Derived BMP4 Regulates Morphology and Function of Endplates on Extrafusal and Intrafusal Muscle Fibers in Adult Mice

Understanding factors contributing to neuromuscular junction (NMJ) stability postdevelopment will shed light on how this stability is lost during aging and in neuromuscular diseases. Previous work in <i>Drosophila</i> suggests that morphogens within the bone morphogenic protein (BMP) family are potential candidates because the BMP homolog, <i>gbb</i>, along with its receptor, <i>wit</i>, have key roles in NMJ structure, stability, and function. Whether BMPs have similar roles at vertebrate NMJs is currently unknown. To examine this question, we generated doxycycline-inducible, muscle-specific BMP4 null mice, referred to here as HSA^CreBMP4^fl/fl mice. Motor behavior tasks were examined pre- and postinduction while electrophysiological and morphological characteristics were examined 4 months later in mice of both sexes. Soleus muscles from HSA^CreBMP4^fl/fl mice had significantly reduced contractile force compared with wild-type (WT) littermates. Cross-sectional areas of type I, but not type IIa, muscle fibers were reduced. NMJs were also larger in HSA^CreBMP4^fl/fl muscles compared with controls due to a significant increase in acetylcholine receptor fragment number and distribution. HSA^CreBMP4^fl/fl NMJs displayed reduced amplitude and frequency of miniature endplate potentials (mEPPs), evoked EPP amplitude, and quantal content and had increased failure rates when stimulating at high frequencies. Behaviorally, HSA^CreBMP4^fl/fl mice performed increasingly worse over time on the rotarod after doxycycline administration compared with their WT littermates. Finally, muscle spindle structure and proprioceptive function were significantly compromised in HSA^CreBMP4^fl/fl mice. These results indicate that muscle-derived BMP4 regulates morphological and electrophysiological attributes of the NMJ in adult mice as well as the structure and function of muscle spindles.