Overlapping function of Lmx1a and Lmx1b in anterior hindbrain roof plate formation and cerebellar growth.

The roof plate is an organizing center in the dorsal CNS that controls specification and differentiation of adjacent neurons through secretion of the BMP and WNT signaling molecules. Lmx1a, a member of the LIM-homeodomain (LIM-HD) transcription factor family, is expressed in the roof plate and its progenitors at all axial levels of the CNS and is necessary and sufficient for roof plate formation in the spinal cord. In the anterior CNS, however, a residual roof plate develops in the absence of Lmx1a. Lmx1b, another member of the LIM-HD transcription factor family which is highly related to Lmx1a, is expressed in the roof plate in the anterior CNS. Although Lmx1b-null mice do not show a substantial deficiency in hindbrain roof plate formation, Lmx1a/Lmx1b compound-null mutants fail to generate hindbrain roof plate. This observation indicates that both genes act in concert to direct normal hindbrain roof plate formation. Since the requirement of Lmx1b function for normal isthmic organizer at the mid-hindbrain boundary complicates analysis of a distinct dorsal patterning role of this gene, we also used a conditional knock-out strategy to specifically delete dorsal midline Lmx1b expression. Phenotypic analysis of single and compound conditional mutants confirmed overlapping roles for Lmx1 genes in regulating hindbrain roof plate formation and growth and also revealed roles in regulating adjacent cerebellar morphogenesis. Our data provides the first evidence of overlapping function of the Lmx1 genes during embryonic CNS development.