Abstract
NFAT (Nuclear Factor of Activated T cells) is a family of calcium-induced, calcineurin-dependent transcription factors, well characterized as central regulators of inducible gene expression in T lymphocytes but now known to function also in several other cell types in various adaptation and differentiation processes. Activation of NFAT by the phosphatase calcineurin is counteracted by several inhibitory kinases and can be completely blocked by the immunosuppressant Cyclosporin A. The Down syndrome critical region 1 (DSCR1; also termed CSP1, MCIP1 or RCAN1) gene belongs to the calcipressin family of endogenous calcineurin inhibitors and is expressed in several isoforms, one of which (isoform C, coded by exons 4–7) has been described to be a transcriptional target for NFAT in striated muscle, endothelial, and neural cells. The DSCR1 gene is located within the Down syndrome critical region of human chromosome 21 and is, together with 200–300 other genes, overexpressed about 1.5-fold in patients with Down syndrome (DS). Previously, dysregulation of NFAT signaling by overexpression of DSCR1 has been implicated in causing various of the pathophysiological features observed in DS patients. Children with DS also suffer from an about 500-fold increased incidence of acute megakaryocytic leukemia; the respective roles of NFAT or DSCR1 in megakaryocytes of either normal individuals or those with DS, however, has not yet been established. Here we show that DSCR1 is upregulated during megakaryocytic differentiation in a lineage-specific manner, and in mature megakaryocytes is further strongly induced by calcineurin stimulation. DSCR1 expression in megakaryocytes is regulated by NFAT, since overexpression of NFATc2 enhances, while overexpression of the specific inhibitor of NFAT activation, VIVIT, suppresses expression of the gene. We further demonstrate that DSCR1 does not only represent an NFAT target in megakaryocytes, but itself acts an inhibitor of NFAT signaling in these cells. Overexpression of DSCR1 in CMK cells as well as in primary megakaryocytes by retroviral transduction profoundly suppressed ionomycin-induced dephosphorylation and nuclear translocation of NFATc2, as well as transactivation of an NFAT-dependent promoter construct. Finally, overexpression of DSCR1 in megakaryocytes markedly downregulated both the constitutive and induced expression of Fas Ligand, a pro-apoptotic gene recently established as a NFAT target in megakaryocytes. Together, these results suggest that DSCR1 acts as an NFAT-induced NFAT inhibitor in megakaryocytes and, when overexpressed, interferes with the expression of NFAT-dependent megakaryocytic genes.
The Down syndrome critical region protein TTC3 inhibits neuronal differentiation via RhoA and Citron kinase
