Role of skeletal muscle in the epigenetic shaping of motor neuron fate choices.

We study the role of muscle in the epigenetic (N.B., we use this term with the broader and more integrative meaning) shaping of developing motor neuron fate choices employing an approach based on mouse mutagenesis and pathology. The developmental role of skeletal muscle is studied in the whole mouse embryo by knocking out myogenic regulatory factors Myf5 and MyoD, to obtain an embryo without any skeletal musculature (Rudnicki et al., 1993). Our goal is to find muscle-provided trigger(s) of motor neuron death relevant to motor neuron diseases such as amyotrophic lateral sclerosis. The reason for this kind of thinking is the fact that a complete absence of lower and upper motor neurons, which is the pathological definition of amyotrophic lateral sclerosis, is only achieved in the complete absence of the muscle (Kablar and Rudnicki, 1999). Mutual embryonic inductive interactions between different tissue types and organs, between individual cell types belonging to the same or different lineages, and between various kinds of molecular players, are only some examples of the complex machinery that operates to connect genotype and phenotype. So far, our studies indicate that some aspects of this interplay can indeed be studied as proposed in this review article, suggesting the role of skeletal muscle in the epigenetic shaping of motor neuron fate choices. We will therefore continue this investigation as outlined to gain more insight into the nature of the epigenetic events that lead to the emergent properties of a phenotype.

Microarray analysis of Myf5-/-:MyoD-/- hypoplastic mouse lungs reveals a profile of genes involved in pneumocyte differentiation.

Fetal breathing-like movements (FBMs) are important in normal lung growth and pneumocyte differentiation. In amyogenic mouse embryos (designated as Myf5-/-:MyoD-/-, entirely lacking skeletal musculature and FBMs), type II pneumocytes fail to differentiate into type I pneumocytes, the cells responsible for gas exchange, and the fetuses die from asphyxia at birth. Using oligonucleotide microarrays, we compared gene expression in the lungs of Myf5-/-:MyoD-/- embryos to that in normal lungs at term. Nine genes were found to be up-regulated and 54 down-regulated at least 2-fold in the lungs of double-mutant embryos. Since many down-regulated genes are involved in lymphocyte function, immunohistochemistry was employed to study T- and B-cell maturity in the thymus and spleen. Our findings of normal lymphocyte maturity implied that the down-regulation was specific to the double-mutant lung phenotype and not to its immune system. Immunostaining also revealed altered distribution of transcription and growth factors (SATB1, c-Myb, CTGF) from down-regulated genes whose knockouts are now known to undergo embryonic or neonatal death secondary to respiratory failure. Together, it appears that microarray analysis has identified a profile of genes potentially involved in pneumocyte differentiation and therefore in the mechanisms that may be implicated in the mechanochemical signal transduction pathways underlying FBMs-dependent pulmonary hypoplasia.