The enzyme Activation Induced Deaminase (AID) triggers antibody diversification in B-cells by catalyzing deamination and consequently mutation of immunoglobulin genes. To minimize off-target deamination, AID is restrained by several regulatory mechanisms including nuclear exclusion, thought to be mediated exclusively by active nuclear export. Here we identify two other mechanisms involved in controlling AID subcellular localization. AID is unable to passively diffuse into the nucleus, despite its small size, its nuclear entry requiring active import mediated by a conformational nuclear localization sequence (NLS). We also identify a determinant for AID cytoplasmic retention in its Cterminus, which hampers diffusion to the nucleus, competes with nuclear import and is critical for maintaining the predominantly cytoplasmic localization of AID in steady-state conditions. Blocking nuclear import alters the balance between these processes in favor of cytoplasmic retention, resulting in reduced isotype class switching.3

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The canonical WNT signaling pathway plays a crucial role in patterning of the embryo during development, but little is known about the specific developmental events which are under WNT control. To understand more about how the WNT pathway orchestrates mammalian organogenesis, we studied the canonical ␤-catenin-mediated WNT signaling pathway in kidneys of mice bearing a ␤-catenin-responsive TCF/␤Gal reporter transgene. In metanephric kidney, intense canonical WNT signaling was evident in epithelia of the branching ureteric bud and in nephrogenic mesenchyme during its transition into renal tubules. WNT signaling activity is rapidly downregulated in maturing nephrons and becomes undetectable in postnatal kidney. Sites of TCF/␤Gal activity are in proximity to the known sites of renal WNT2b and WNT4 expression, and these WNTs stimulate TCF reporter activity in kidney cell lines derived from ureteric bud and metanephric mesenchyme lineages. When fetal kidney explants from HoxB7/GFP mice were exposed to the canonical WNT signaling pathway inhibitor, Dickkopf-1, arborization of the ureteric bud was significantly reduced. We conclude that restricted zones of intense canonical WNT signaling drive branching nephrogenesis in fetal kidney. nephrogenesis; ␤-catenin; branching morphogenesis THE WNT FAMILY is comprised of 19 secreted glycoproteins which act as short-range intercellular signaling molecules, recognizing one of the 10 frizzled receptors expressed at the surface of nearby target cells. The canonical signaling pathway is activated by WNTs which bind to cognate frizzled receptors heterodimerized with LRP5 or LRP6 coreceptors (2). Activated receptors recruit dishevelled protein (Dvl) and inhibit degradation of cytoplasmic ␤-catenin via the GSK3␤-axin-APC complex (11). When its degradation is blocked, cytoplasmic ␤-catenin is available to translocate to the nucleus, dimerize with partners belonging to the T-cell factor (TCF) family, and activate target genes. TCF recognition motifs have been well-studied, allowing design of vectors (e.g., TOPFlash) which drive transcription of reporter genes in response to canonical WNT signaling activity (32). In general, canonical ␤-catenin/TCF signaling is thought to activate gene targets (e.g., c-myc) involved in cell proliferation (3, 27).More than 35 years ago, Unsworth and Grobstein (33) reported that tissue from spinal cord could induce formation of renal tubules when cocultured with isolated metanephric mesenchyme. In 1994, Herzlinger et al. (12) found that WNT1-expressing NIH3T3 cells were also able to induce tubule formation in the coculture assay, suggesting that the canonical (␤-catenin-mediated) WNT signaling pathway is essential for mammalian nephrogenesis. However, the precise function of canonical WNT signaling in renal development is unknown. Surprisingly, WNT1 is not present in the developing kidney, but numerous other WNTs are transiently expressed in specific cell lineages (34). Several of these are able to activate the canonical signaling pathway in other context...

Hsp90 stabilizes and prevents degradation of cytoplasmic activation-induced deaminase.

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