Crossinhibitory activities of Ngn1 and Math1 allow specification of distinct dorsal interneurons.

Distinct classes of neurons are generated from progenitor cells distributed in characteristic dorsoventral patterns in the developing spinal neural tube. We define restricted neural progenitor populations by the discrete, nonoverlapping expression of Ngn1, Math1, and Mash1. Crossinhibition between these bHLH factors is demonstrated and provides a mechanism for the generation of discrete bHLH expression domains. This precise control of bHLH factor expression is essential for proper neural development since as demonstrated in both loss- and gain-of-function experiments, expression of Math1 or Ngn1 in dorsal progenitor cells determines whether LH2A/B- or dorsal Lim1/2-expressing interneurons will develop. Together, the data suggest that although Math1 and Ngn1 appear to be redundant with respect to neurogenesis, they have distinct functions in specifying neuronal subtype in the dorsal neural tube.

Overexpression of MATH1 disrupts the coordination of neural differentiation in cerebellum development.

An essential role for the bHLH transcription factor MATH1 in the formation of cerebellar granule cells was previously demonstrated in a Math1 null mouse. The function of regulated levels of MATH1 in granule cell development is investigated here using a gain-of-function paradigm. Overexpression of Math1 in its normal domain in transgenic mice leads to early postnatal lethality and perturbs cerebellar development. The cerebellum of the (B)MATH1 transgenic neonate is smaller with less foliation, particularly in the central vermal regions, when compared to wild-type cerebella. A detailed analysis of multiple molecular markers in brains overexpressing Math1 has revealed defects in the differentiation of cerebellar granule cells. NeuroD and doublecortin, markers normally distinguishing the discrete layered organization of granule cell maturation in the inner EGL, are aberrantly expressed in the outer EGL where MATH1-positive, proliferating cells reside. In contrast, TAG-1, a later marker of developing granule cells that labels parallel fibers, is severely diminished. The elevated MATH1 levels appear to drive expression of a subset of early differentiation markers but are insufficient for development of a mature TAG-1-expressing granule cell. Thus, balanced levels of MATH1 are essential for the correct coordination of differentiation events in granule cell development.

Lineage-specific regulation of the neural differentiation gene MASH1.

Mash1 is a transcription factor required during embryogenesis for the development of multiple neural lineages. It is expressed in restricted domains at specific stages in the developing central and peripheral nervous systems and in the developing olfactory epithelium. We have investigated the regulation of Mash1 expression during embryogenesis using transgenic mice containing Mash1/lacZ reporter constructs. Cis-acting regulatory elements controlling Mash1 expression in the central nervous system are located within an 8-kb sequence upstream of the Mash1 coding region. This 8-kb sequence does not contain elements directing expression to the peripheral nervous system, olfactory epithelium, or retina. Sequences outside this 8 kb but within 36 kb of the Mash1 locus contain elements responsible for expression in the autonomic division of the peripheral nervous system. However, transgene expression in embryos containing the 36-kb sequence was never detected in the olfactory epithelium and retina. Thus, regulatory elements driving expression in these lineages may be at even greater distances from the Mash1 coding region. These data provide evidence for complex regulation of Mash1 expression in which multiple lineage-specific cis-acting regulatory regions span greater than 36 kb of the Mash1 locus. Further characterization of these regions will facilitate the study of factors that regulate the temporal and spatial expression of Mash1 during development. In addition, the regulatory sequences identified here can direct expression of heterologous genes to developing neural lineages that normally express Mash1, thus providing an important tool for examining the function of candidate regulatory genes in mammalian nervous system development.