The brain is composed of diverse types of neurons that fulfill distinct roles in neuronal circuits, as manifested by the hippocampus, where pyramidal neurons and granule cells constitute functionally distinct domains: cornu ammonis (CA) and dentate gyrus (DG), respectively. Little is known about how these two types of neuron differentiate during hippocampal development, although a set of transcription factors that is expressed in progenitor cells is known to be required for the survival of granule cells. Here, we demonstrate in mice that Prox1, a transcription factor constitutively expressed in the granule cell lineage, postmitotically functions to specify DG granule cell identity. Postmitotic elimination of Prox1 caused immature DG neurons to lose the granule cell identity and in turn terminally differentiate into the pyramidal cell type manifesting CA3 neuronal identity. By contrast, Prox1 overexpression caused opposing effects on presumptive hippocampal pyramidal cells. These results indicate that the immature DG cell has the potential to become a granule cell or a pyramidal cell, and Prox1 defines the granule cell identity. This bi-potency is lost in mature DG cells, although Prox1 is still required for correct gene expression in DG granule cells. Thus, our data indicate that Prox1 acts as a postmitotic cell fate determinant for DG granule cells over the CA3 pyramidal cell fate and is crucial for maintenance of the granule cell identity throughout the life.

Meltrin ␣ (ADAM12) is a metalloprotease-disintegrin whose specific expression patterns during development suggest that it is involved in myogenesis and the development of other organs. To determine the roles Meltrin ␣ plays in vivo, we generated Meltrin ␣-deficient mice by gene targeting. Although the number of homozygous embryos are close to the expected Mendelian ratio at embryonic days 17 to 18, ca. 30% of the null pups born die before weaning, mostly within 1 week of birth. The viable homozygous mutants appear normal and are fertile. Most of the muscles in the homozygous mutants appear normal, and regeneration in experimentally damaged skeletal muscle is unimpeded. In some Meltrin ␣-deficient pups, the interscapular brown adipose tissue is reduced, although the penetrance of this phenotype is low. Impaired formation of the neck and interscapular muscles is also seen in some homozygotes. These observations suggest Meltrin ␣ may be involved in regulating adipogenesis and myogenesis through a linked developmental pathway. Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a candidate substrate of Meltrin ␣, and we found that TPA (12-O-tetradecanoylphorbol-13-acetate)-induced ectodomain shedding of HB-EGF is markedly reduced in embryonic fibroblasts prepared from Meltrin ␣-deficient mice. We also report here the chromosomal locations of Meltrin ␣ in the mouse and rat.

The cloning of the full-length cDNA encoding meltrin beta (ADAM19), one of the metalloprotease-disintegrins expressed in mouse myogenic cells, revealed that the meltrin beta gene encodes a membrane protein closely related to meltrin alpha (ADAM12) which participates in myotube formation in vitro. To delineate the functions of meltrin alpha and beta, we examined the expression patterns of their transcripts during embryogenesis. The meltrin alpha gene is activated in condensed mesenchymal cells that give rise to skeletal muscle, bones and visceral organs. Meltrin beta mRNA, in contrast, is markedly expressed in craniofacial and dorsal root ganglia and ventral horns of the spinal cord, where peripheral neuronal cell lineages differentiate. Heart, skeletal muscle, intestine and lung also express meltrin beta mRNA transiently. Although the meltrin alpha and beta transcripts exhibit distinct expression patterns during embryogenesis, both genes are mainly activated in mesenchymal cells that are derived from both mesoderm and ectoderm.

Morphogenesis of the heart requires development of the endocardial cushion tissue that gives rise to the membranous septa and valves. Here we show that Meltrin beta/ADAM19, a novel metalloprotease-disintegrin, participates in the development of the endocardial cushion. Mice lacking Meltrin beta exhibit ventricular septal defect (VSD) and immature valves, and most of the animals die soon after birth. During development of the endocardial cushion, epithelial-mesenchymal transformation (EMT) of endocardial epithelial cells generates most of the cushion mesenchymes that constitute the main components of the septa and valves. Meltrin beta is expressed in both the epithelia and the mesenchymes of the endocardial cushion. In the absence of Meltrin beta, the cushion is small or thin in the septum-forming region and show poor remodeling of cardiac jelly components; both of these characteristics suggest impaired growth and differentiation of the endocardial cushion. When embryonic fibroblasts are cultured sparsely, Meltrin beta-lacking cells exhibit aberrant ectodomain shedding of type I Neuregulin, one of the ErbB ligands expressed in endocardial cells. These results suggest the necessity of proteolytic regulation of ErbB ligands by Meltrin beta for proper heart development.

Evolutionally conserved Nanos RNA-binding proteins play crucial roles in germ cell development. While a mammalian Nanos family protein, NANOS2, is required for sexual differentiation of male (XY) germ cells in mice, the underlying mechanisms and the identities of its target RNAs in vivo remain elusive. Using comprehensive microarray analysis and a bacterial artificial chromosome transgenic system, here we identify Dazl, a germ cell-specific gene encoding an RNA-binding protein implicated in translation, as a crucial target of NANOS2. Importantly, removal of the Dazl 3′-untranslated region in XY germ cells stabilizes the Dazl mRNA, resulting in elevated meiotic gene expression, abnormal resumption of the cell cycle and impaired processing-body formation, reminiscent of Nanos2-knockout phenotypes. Furthermore, our data suggest that NANOS2 acts as an antagonist of the DAZL protein. We propose a dual system of NANOS2-mediated suppression of Dazl expression as a pivotal molecular mechanism promoting sexual differentiation of XY germ cells.