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In the last few years there has been a veritable explosion of knowledge about cyclic nucleotide phosphodiesterases. In particular, the accumulating data showing that there are a large number of different phosphodiesterase isozymes have triggered an equally large increase in interest about these enzymes. At least seven different gene families of cyclic nucleotide phosphodiesterase are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the more important aspects about the structure, cellular localization, and regulation of each family of phosphodiesterases. Particular emphasis is placed on new information obtained in the last few years about how differential expression and regulation of individual phosphodiesterase isozymes relate to their function(s) in the body. A substantial discussion of the currently accepted nomenclature is also included. Finally, a brief discussion is included about how the differences among distinct phosphodiesterase isozymes are beginning to be used as the basis for developing therapeutic agents.

Although cyclic nucleotide phosphodiesterases (PDEs) were described soon after the discovery of cAMP, their complexity and functions in signaling is only recently beginning to become fully realized. We now know that at least 100 different PDE proteins degrade cAMP and cGMP in eukaryotes. A complex PDE gene organization and a large number of PDE splicing variants serve to fine-tune cyclic nucleotide signals and contribute to specificity in signaling. Here we review some of the major concepts related to our understanding of PDE function and regulation including: (a) the structure of catalytic and regulatory domains and arrangement in holoenzymes; (b) PDE integration into signaling complexes; (c) the nature and function of negative and positive feedback circuits that have been conserved in PDEs from prokaryotes to human; (d) the emerging association of mutant PDE alleles with inherited diseases; and (e) the role of PDEs in generating subcellular signaling compartments.

Regulatory CD4 1 T cells (T R cells), the development of which is critically dependent on X-linked transcription factor Foxp3 (forkhead box P3), prevent self-destructive immune responses 1 . Despite its important role, molecular and functional features conferred by Foxp3 to T R precursor cells remain unknown. It has been suggested that Foxp3 expression is required for both survival of T R precursors as well as their inability to produce interleukin (IL)-2 and independently proliferate after T-cell-receptor engagement, raising the possibility that such 'anergy' and T R suppressive capacity are intimately linked 2-4 . Here we show, by dissociating Foxp3-dependent features from those induced by the signals preceding and promoting its expression in mice, that the latter signals include several functional and transcriptional hallmarks of T R cells. Although its function is required for T R cell suppressor activity, Foxp3 to a large extent amplifies and fixes pre-established molecular features of T R cells, including anergy and dependence on paracrine IL-2. Furthermore, Foxp3 solidifies T R cell lineage stability through modification of cell surface and signalling molecules, resulting in adaptation to the signals required to induce and maintain T R cells. This adaptation includes Foxp3-dependent repression of cyclic nucleotide phosphodiesterase 3B, affecting genes responsible for T R cell homeostasis.In males, Foxp3 deficiency results in fatal early-onset systemic autoimmune disease 5 . In heterozygote Foxp3 wt/null females only one-half of T cells harbours the mutant Foxp3 allele due to random X-chromosome inactivation, whereas autoimmunity is controlled by a normal T R population expressing the Foxp3 wild-type allele. Thus, we were able to genetically mark cells actively transcribing a Foxp3 null allele, yet lacking Foxp3 protein (hereafter called T FN for Foxp3 nullexpressing T cells), through an in-frame insertion of GFP into a stop-codon-disrupted Foxp3 locus (Foxp3 gfpko ) and investigate their features in mice ( Fig. 1a; see also Supplementary Figs 1 and 2a). Female Foxp3 gfpko/wt mice were healthy, whereas male Foxp3 gfpko mice developed the same severity of autoimmunity as Foxp3 knockout (Foxp3 null ) mice 6 , resulting in death at ,4 weeks of age. Thymocyte and peripheral lymphoid organ cellularity did not differ between Foxp3 gfpko/wt and Foxp3 gfp/gfp mice, nor did the proportion of Foxp3 1 T R cells and Foxp3 2 CD4 1 T cells (data not shown). As our main focus was to characterize T FN cells in healthy Foxp3 gfpko/wt mice, analysis of autoimmune male Foxp3 gfpko mice is included as Supplementary Fig. 2.T FN cells constituted ,1-3% of mature CD4 1 thymocytes and peripheral CD4 1 T cells, indicating that Foxp3 is not required to rescue T R precursors from negative selection (Fig. 1b, c). This is consistent with a reported abundance of T-cell receptors (TCRs) characteristic of T R cells in Foxp3 null mice 7 . As ectopic expression of Foxp3 has been shown to induce a state of hyporesponsiveness in CD4 1 T cell...

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Since the discovery in 1957 that cyclic AMP acts as a second messenger for the hormone adrenaline, interest in this molecule and its companion, cyclic GMP, has grown. Over a period of nearly 50 years, research into second messengers has provided a framework for understanding transmembrane signal transduction, receptor-effector coupling, protein-kinase cascades and downregulation of drug responsiveness. The breadth and impact of this work is reflected by five different Nobel prizes.

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