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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...

Src family kinases play a key role in mitogenesis. The exquisitely tissue-specific distribution of different Src family members suggests that a fine tuning of their expression might be a key prerequisite for cell homeostasis. We tested B cells from patients affected by B-cell chronic lymphocytic leukemia (B-CLL) for expression of Src family kinases. The T-cell–specific tyrosine kinase Lck was found to be expressed at significant levels in CLL B-cells. This finding could be accounted for either by ectopic expression of Lck in B-CLL or by specific expression of this kinase in normal B-1 cells, which are believed to be the normal counterpart of CLL B cells. To answer this question B cells from different sources, characterized by a different size of the B-1 subpopulation, were tested for Lck expression. The results show that Lck expression is a feature of CD5+, B-1 cells, suggesting a potential role for Lck in the self-renewal capacity of this B-cell subpopulation and supporting the notion that B-1 cells are the subset undergoing oncogenic transformation in B-CLL. Furthermore, we show that the CD5−, B-2 subpopulation, while normally lacking Lck expression, acquires the capacity to express Lck ectopically upon transformation by EBV.