The survival of motor neurons is controlled by multiple factors that regulate different aspects of their physiology. The identification of these factors is important because of their relationship to motor neuron disease. We investigate here whether Mullerian Inhibiting Substance (MIS) is a motor neuron survival factor. We find that motor neurons from adult mice synthesize MIS and express its receptors, suggesting that mature motor neurons use MIS in an autocrine fashion or as a way to communicate with each other. MIS was observed to support the survival and differentiation of embryonic motor neurons in vitro. During development, male-specific MIS may have a hormone effect because the blood-brain barrier has yet to form, raising the possibility that MIS participates in generating sex-specific differences in motor neurons.Mullerian Inhibiting Substance type II receptor M otor neurons are particularly prone to age-related deterioration (1-3), which, in the extreme, leads to motor neuron disease and to death by paralysis. The survival of motor neurons is controlled by multiple factors, each of which appears to have a different physiological role. Motor neurons are, for instance, regulated by skeletal muscle fibers and Schwann cells via cardiotrophin-1 (4), TGF-␤2 (5, 6), and glial-cell-line-derived neurotrophic factor (GDNF) (7,8). Motor neurons also receive protection against viral-and hypoxic-induced damage through IL-6 (9) and VEGF (10, 11), respectively. Variations in the VEGF gene cause adult-onset motor-neuron degeneration in some mice and have been linked to ALS in some human populations (10, 11). These findings have renewed interest in identifying nonclassical neuronal survival factors.Mullerian Inhibiting Substance (MIS) is examined herein as a motor-neuron survival factor given that we found high expression of ligand and receptors in motor neurons. MIS is a member of the TGF-␤ superfamily, which includes motor-neuron survival factors, such as GDNF and TGF-␤2. The known physiological actions of MIS are thought to be limited to sexual differentiation of males and to the function of mature reproductive tissues of both sexes (12). These studies introduce a possible function for this interesting molecule and its known signaling pathway.TGF-␤ superfamily members signal through a complex of type I and type II receptors (13). MIS has a unique type II receptor (MISRII) but shares type I receptors with other members of the superfamily (12, 13). Genetic, organ culture, and cellular evidence implicates activin receptor-like kinase 3 (ALK3) (14) and ALK2 (Y. Zhan, D.T.M., and P.K.D., unpublished data) (15) as type I receptors for MIS in murine sexual differentiation, although ALK6 is likely to be involved in other cellular contexts (12, 16).We find that adult motor neurons from male and female mice synthesize MIS and its receptors, with the MIS receptor mRNA in motor neurons being much more abundant than the mRNAs for the GDNF and TGF-␤ receptors. Our experiments show that MIS supports the survival of embryonic motor n...

MyoD belongs to a family of helix-loop-helix proteins that control myogenic differentiation. Transfection of various non-myogenic cell lines with MyoD transforms them into myogenic cells. In normal embryonic development MyoD is upregulated at the time when the hypaxial musculature begins to form, but its role in the function of adult muscle remains to be elucidated. In this study we examined the cellular locations of MyoD protein in normal and abnormal muscles to see whether the presence of MyoD protein is correlated with a particular cellular behaviour and to assess the usefulness of MyoD as a marker for satellite cells. Adult rats were anaesthetised and their tibialis anterior or soleus muscles either denervated, tenotomised, freeze lesioned, lesioned and denervated, or lesioned and tenotomised. At various intervals after the operations the rats were killed and their muscles removed, snap frozen, and sectioned with a cryostat along with muscles from unoperated neonatal and adult rats. The sections were processed for immunohistochemistry using a rabbit affinity-purified antibody to recombinant MyoD. MyoD proved to be an excellent marker for active satellite cells; satellite cells in neohatal and regenerating muscles contained high levels of MyoD protein. MyoD positive cells were not observed in the muscles of old adults, in which the satellite cells are fully quiescent. MyoD immunoreactivity was rapidly lost from satellite cell nuclei after they fused into myotubes and was not detected in either sub-synaptic or non-synaptic nuclei of mature fibers. Denervation, and to a lesser extent tenotomy, of lesioned muscles induced expression of MyoD in myotubal nuclei. Denervation of normal muscles also upregulated MyoD in muscle fiber nuclei, an effect which was maximal after 3 days. We conclude that MyoD protein is neurally regulated in both myotubes and muscle fibers.