Characterization of Lhx9, a novel LIM/homeobox gene expressed by the pioneer neurons in the mouse cerebral cortex

In order to explain the phenotype observed in Lhx2 mutant embryos, we previ ously proposed that an Lhx2 related gene might exist. We now have cloned a new LIM/homeobox gene called Lhx9. Lhx9 is closely related to Lhx2 and is e xpressed in the developing central nervous system (CNS). Lhx9 and Lhx2 have expression patterns that overlap in some areas but are distinct in others. Thus, in some developmental domains these two highly related proteins may be functionally redundant. Lhx9 is expressed in the pioneer neurons of the cerebral cortex, while Lhx2 is expressed throughout the cortical layers. Po stnatally, Lhx9 is expressed in the inner nuclei of the cerebellum, while L hx2 is in the granular layer. In the developing limbs, both genes are highl y expressed in a similar pattern. Based on the expression pattern and the d evelopmental regulation of Lhx9, we propose that Lhx9 may be involved in th e specification or function of the pioneer neurons of the cerebral cortex. We show that both Lhx9 and Lhx2 bind the LIM domain binding protein Ldb1/Nl i1/Clim2. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.