Spatiotemporal regulation of cell proliferation is necessary for normal tissue development. The molecular mechanisms, especially the signaling pathways controlling the cell cycle machinery, remain largely unknown. Here, we demonstrate a negative relationship between the spatiotemporal patterns of jumonji (jmj) expression and cardiac myocyte proliferation. cyclin D1 expression and cell proliferation are enhanced in the cardiac myocytes of jmj-deficient mutant embryos. In contrast, jmj overexpression represses cyclin D1 expression in cardiac cells, and Jmj protein binds to cyclin D1 promoter in vivo and represses its transcriptional activity. cyclin D1 overexpression causes hyperproliferation in the cardiac myocytes, but the absence of cyclin D1 in jmj mutant embryos rescues the hyperproliferation. Therefore, Jmj might control cardiac myocyte proliferation and consequently cardiac morphogenesis by repressing cyclin D1 expression.

No abstract

BIT (a brain immunoglobulin-like molecule with tyrosine-based activation motifs) is a brain-specific membrane protein which has two cytoplasmic TAMs (tyrosine-based activation motifs). Using the Far Western blotting technique, we detected association of a 70-kDa protein with the tyrosine-phosphorylated TAMs of BIT. A mouse brain cDNA library in gt11 was screened for this association, and two positive clones encoding tyrosine phosphatase SH-PTP2 were isolated. SH-PTP2 has two SH2 domains and is believed to function as a positive mediator in receptor tyrosine kinase signaling. SH-PTP2 and BIT were coimmunoprecipitated from phosphorylated rat brain lysate, and BIT was a major tyrosine-phosphorylated protein associated with SH-PTP2 in this lysate. This interaction was also observed in Jurkat T cells transfected with BIT cDNA depending on tyrosine phosphorylation of BIT. Bisphosphotyrosyl peptides corresponding to BIT-TAMs stimulated SH-PTP2 activity 33-35-fold in vitro, indicating that two SH2 domains of SH-PTP2 simultaneously interact with two phosphotyrosines of BIT-TAM. Our findings suggest that the tyrosine phosphorylation of BIT results in stimulation of the signal transduction pathway promoted by SH-PTP2 and that BIT is probably a major receptor molecule in the brain located just upstream of SH-PTP2.Protein tyrosine phosphorylation plays an important role in signal transduction and regulates a wide range of cellular processes. Protein-tyrosine kinases and protein-tyrosine phosphatases are highly expressed in the central nervous system, consistent with the importance of tyrosine phosphorylation in neural function (1).BIT 1 (a brain immunoglobulin-like molecule with tyrosinebased activation motifs) is a novel immune antigen receptorlike molecule of the brain.2 This molecule is composed of an antigen receptor-like extracellular domain, a transmembrane domain, and a cytoplasmic region containing two variants of TAM (tyrosine-based activation motif) that was recently designated ITAM (immunoreceptor TAM). This cytoplasmic motif contains two tyrosine phosphorylation sites. TAM was originally described in the immune system where it plays a crucial role in the activation responses of B and T cells (2-5). BIT is one of major substrates of protein-tyrosine kinase(s) in crude brain suspensions 2 and is widely distributed in the brain in synapse-rich regions and in some nerve fibers.3 These findings suggest that the tyrosine phosphorylation of TAMs in BIT may be involved in neural signal transduction. Recent studies in the immune system have demonstrated that the oligomerization of TAMs allows the phosphorylation of two tyrosine residues found in this motif and these phosphotyrosine residues act as a bidentate docking site for the paired Src homology 2 (SH2) domains present in the cytoplasmic tyrosine kinases, Syk and ZAP-70, believed to be indispensable for initiation of the signaling cascade (6 -8). From these investigations, we predicted that TAMs of BIT may recruit tyrosine kinases containing paired SH2 domains to the ...

Catecholaminergic (dopaminergic, noradrenergic, and adrenergic) transmitter phenotypes require the cooperative actions of four biosynthetic enzymes: tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine ␤-hydroxylase, and phenylethanolamine N-methyltransferase. Mechanisms that control expression of these enzymes in a transmitter phenotype-specific manner, however, are poorly understood. Here, we provide evidence that overexpression of a novel cdc10/ SWI6 motif-containing protein, V-1, elicits the coordinate up-regulation of tyrosine hydroxylase, aromatic Lamino acid decarboxylase, and dopamine ␤-hydroxylase mRNAs in the neuronal cell line PC12D, and as a result, catecholamine levels are increased. Furthermore, V-1 is strongly expressed in the cytoplasm of rat chromaffin cells of adrenal medulla. Thus, V-1 may act as a cytoplasmic protein/protein adapter and be involved in control of the catecholaminergic phenotype expression via an intracellular pathway signaling to the nucleus.Abnormalities in catecholamine biosynthesis and neurotransmission have been implicated as the cause of neurological and psychiatric diseases (1). Signal transduction pathways via protein kinase, including cAMP-dependent protein kinases, Ca 2ϩ /phospholipid-dependent protein kinase, and Ca 2ϩ /calmodulin-dependent protein kinase, that regulate the activity of TH 1 and the transcription of genes encoding catecholamine biosynthesizing enzymes have been identified and extensively characterized (2-17). In addition, Mash1 and Phox2 transcription factors have been recently shown by gene targeting to be required for development of subpopulations of central nervous system and peripheral nervous system catecholaminergic neurons (18,19). Little is known, however, about mechanisms that control expression of genes encoding the four enzymes that determine the catecholaminergic phenotype.V-1 is a novel soluble protein consisting of 117 amino acids that contains 2.5 tandem repeats of the cdc10/SWI6 motif, also known as the ankyrin repeat (see Fig. 1A) (20,21). This motif has been demonstrated to be crucial for protein-protein interactions (22,23). Our recent studies have revealed a characteristic temporal profile for the expression of the V-1 gene in developing murine brain. During embryonic stages, expression of the V-1 gene is detectable but weak. After birth, expression of V-1 mRNA gradually increases to reach a maximal level during the first to second postnatal weeks, declining thereafter to adult levels by postnatal day 28.2 However, strong expression of the V-1 gene persists in regions of synaptic plasticity even after the second postnatal week (24). We have established stable transfectants that overexpress V-1 using the catecholamine-producing neuronal cell line PC12D to examine functional roles of V-1 in neuronal cells and analyzed the neuronal phenotypes of these cells using techniques of molecular biology, neurochemistry, biochemistry, and electrophysiology. In this study, we also demonstrate that the V-1 protein is intensely co-e...

SUMMARYIn general, cell proliferation and differentiation show an inverse relationship, and are regulated in a coordinated manner during development. Embryonic cardiomyocytes must support embryonic life by functional differentiation such as beating, and proliferate actively to increase the size of the heart. Therefore, progression of both proliferation and differentiation is indispensable. It remains unknown whether proliferation and differentiation are related in these embryonic cardiomyocytes. We focused on abnormal phenotypes, such as hyperproliferation, inhibition of differentiation and enhanced expression of cyclin D1 in cardiomyocytes of mice with mutant jumonji (Jmj, Jarid2), which encodes the repressor of cyclin D1. Analysis of Jmj/cyclin D1 double mutant mice showed that Jmj was required for normal differentiation and normal expression of GATA4 protein through cyclin D1. Analysis of transgenic mice revealed that enhanced expression of cyclin D1 decreased GATA4 protein expression and inhibited the differentiation of cardiomyocytes in a CDK4/6-dependent manner, and that exogenous expression of GATA4 rescued the abnormal differentiation. Finally, CDK4 phosphorylated GATA4 directly, which promoted the degradation of GATA4 in cultured cells. These results suggest that CDK4 activated by cyclin D1 inhibits differentiation of cardiomyocytes by degradation of GATA4, and that initiation of Jmj expression unleashes the inhibition by repression of cyclin D1 expression and allows progression of differentiation, as well as repression of proliferation. Thus, a Jmj-cyclin D1 pathway coordinately regulates proliferation and differentiation of cardiomyocytes.