Calmodulin-driven Nuclear Trafficking linked to diabetes and insulin signaling

调节核转运对维持转录因子、核激酶和复制因子的水平和活性至关重要。我们发现Ca+2/钙调蛋白是核输入的激活剂，并提出Ca+2在细胞激活过程中对核输入的调节作用。这种进化上古老的，钙依赖的进口途径被提出，以促进在细胞活化过程中核蛋白的不同子集的摄取。钙调素依赖的核输入缺陷是人类某些性别逆转的基础，这一发现使钙调素调节的转运途径的生理意义变得清晰起来。HMG-box结构转录因子SRY和SOX9必须进入支持细胞的细胞核并与靶DNA紧密结合才能正常发育雄性性腺。因此，SRY是男性化的主要触发因素；释放这个触发器需要SRY的核转运。在一部分常染色体性逆转患者（Swyers综合征，Campomelic dysplasia）中，SRY和SOX9的核转运被阻断。与这些缺陷相关的突变位于SRY和SOX9氨基酸末端附近的保守钙调素结合基序中。因此，SOX转录因子家族似乎使用Ca+2/钙调蛋白作为输入受体和允许核输入和DNA结合的分子开关。当三种胰岛素相关受体在小鼠中被切除时，也会发生类似的常染色体性逆转表型，这表明细胞内信号级联可能影响SRY和SOX9的正常功能。最近，编码TCF7L2的基因被证明是冰岛人群中糖尿病的易感性位点。该转录因子是HMG-box家族的成员，含有钙调素结合基序。钙调蛋白对核蛋白的输入，虽然在功能上是冗余的，但受到细胞内Ca+2动员的独立调节。钙调素依赖性输入异常是常染色体性逆转患者的主要缺陷。这些性别逆转综合征是导致人类疾病的核输入途径缺陷的第一个直接例子。其他涉及sox家族转录因子的人类疾病可能在钙调蛋白依赖的核输入方面有类似的缺陷。除了分析哺乳动物中HMG-box转录因子的输入外，我们最近还证明了钙调素依赖的核输入途径在酵母中起作用。我们目前正在采用遗传方法来确定这种新型进口途径的其他成分，并寻求确定该过程的小分子抑制剂。由于SOX2蛋白对干细胞多能性特别重要，我们目前的重点是了解核输入在其调控中的作用。在线虫中，pop-1是与bar-1相关的关键结构转录因子，bar-1是β -连环蛋白的线虫同源物。这些研究是在酵母和秀丽隐杆线虫系统中进行的。

Regulated nuclear transport is critical for the maintenance of the levels and activities of transcription factors, nuclear kinases, and replication factors. We identified Ca+2/calmodulin as an activator of nuclear import and suggested a role for Ca+2 in the regulation of nuclear import during cell activation. This evolutionarily ancient, calcium-dependent import pathway was proposed to facilitate uptake of a distinct subset of nuclear proteins during cell activation. The physiological significance of the Ca+2-calmodulin-regulated transport pathway became clear with the finding that defects in calmodulin-dependent nuclear import underlie certain forms of human sex reversal. The HMG-box architectural transcription factors SRY and SOX9 must enter the nucleus of Sertoli cells and bind tightly to target DNA for proper male gonad development. Thus, SRY acts as the primary trigger for maleness; nuclear transport of SRY is required to release this trigger. In a subset of human patients with autosomal sex reversal (Swyers syndrome, Campomelic dysplasia) nuclear transport of SRY and SOX9 is blocked. The mutations associated with these defects reside in a conserved calmodulin-binding motif near the amino terminus of SRY and SOX9. Thus, the SOX family of transcription factors appears to use Ca+2/calmodulin both as import receptor and molecular switch allowing for nuclear import and DNA binding. A similar autosomal sex reversal phenotype occurs when three insulin-related receptors are ablated in mice suggesting that intracellular signaling cascades may impact the normal functions of SRY and SOX9. More recently, the gene encoding TCF7L2 was shown to be a diabetes susceptibility locus in the Icelandic population. This transcription factor is a member of the HMG-box family containing a calmodulin-binding motif. The import of nuclear proteins by calmodulin, while functionally redundant with the canonic Ran-dependent pathway, is subject to independent regulation by intracellular Ca+2 mobilization. Abnormalities in calmodulin-dependent import emerge as the primary defect in patients with autosomal sex-reversal. These sex-reversal syndromes represent the first direct examples of a defect in a nuclear import pathway leading to human disease. Other human disorders involving SOX-family transcription factors may have similar defects in calmodulin dependent nuclear import. In addition to analysis of the import of the HMG-box transcription factors in mammals, we have recently shown that the calmodulin-dependent nuclear import pathway functions in yeast. We are currently taking genetic approaches to define additional components of this novel import pathway and seek to identify small molecule inhibitors of the process. Because the SOX2 protein is particularly important for Stem Cell pluripotency, our focus is currently on understanding the role of nuclear import in its regulation. In the nematode, pop-1 is a key architectural transcription factor associated with bar-1, the worm homolog of Beta-catenin. These studies are being carried out in the yeast and C. elegans systems.