The nuclear factor of activated T-cells (NFAT) family transcription factors play a key role in the control of cytokine gene expression in T-cells. Although initially identified in T-cells, recent data have unveiled unanticipated roles for NFATs in the development, proliferation, and differentiation of other tissues. Here we report the identification, cDNA cloning, and functional characterization of a new isoform of NFAT1 highly expressed in mouse brain. This isoform, which we named NFAT1-D, is identical to NFAT1 throughout the N-terminal regulatory domain and the portion of the Rel domain which includes the minimal region required for specific binding to DNA and interaction with AP-1. The homology stops sharply upstream of the 3-boundary of the Rel homology domain and is followed by a short unique C-terminal region. NFAT1-D was expressed at high levels in all brain districts and was found as a constitutively active transcription complex. Transfection of a NFAT/luciferase reporter in the neuronal cell line PC12, which also expresses NFAT1-D, showed that these cells expressed a constitutive NFAT activity that was enhanced after nerve growth factor-induced differentiation but was resistant to the immunosuppressant cyclosporin A. NFAT1-D was, however, inducibly activated in a cyclosporin A-sensitive manner when expressed in Tcells, suggesting that the activity of NFAT proteins might be controlled by their specific cellular context.
Initially described as a transcriptional complex that bound a T-cell antigen receptor (TCR)1 response element on the interleukin (IL)-2 gene enhancer, nuclear factor of activated T-cells (NFAT) is a family of transcription factors crucially involved in the regulation of cytokine gene expression in T-cells (1). NFAT activity is strongly unregulated after TCR triggering; however, receptor engagement can be bypassed by a combination of phorbol esters and calcium ionophores, which activate protein kinase C and induce a rise in intracellular calcium ions, respectively. This dual requirement reflects the subunit composition of NFAT factors, which includes a cytoplasmic and a nuclear component. In resting cells, NFAT is found as a cytosolic protein phosphorylated on serine residues. After an elevation in intracellular calcium ions, the calmodulin-dependent phosphatase calcineurin is activated and dephosphorylates NFAT, exposing a nuclear localization sequence near the N terminus of NFAT and resulting in its translocation to the nucleus (2-5). This process is exquisitely sensitive to the immunosuppressants cylosporin A (CsA) and FK506, which interact with specific cytosolic receptors and form complexes that bind with high affinity to calcineurin and lock it in an inactive conformation (6 -8). In the nucleus NFAT assembles in cooperative DNAbinding complexes with dimers of the AP-1 family of transcription factors (4, 9). Complementation studies using constitutively active mutants of signaling proteins have shown that the calcium/calcineurin and the Ras/protein kinase C/mitogen-activated protein ki...