The role of podocytic and tubular β-catenin in proteinuric kidney disease

肾的功能亚单位，即肾单位，由一个被称为肾小球的滤体和一个小管组成，后者阻止血液中的高分子量蛋白质的过滤，而小管则回收初级尿液中的小分子和水。足细胞是肾小球滤器的上皮细胞。它们的损伤会导致滤器渗漏，导致血清蛋白流失到尿液中，这种状态称为蛋白尿。相应的人类疾病被称为肾病综合征。已确立的激素治疗方法区分了激素敏感型和激素抵抗型肾病综合征。虽然类固醇对足细胞的作用机制已有几十年的研究，但至今仍不清楚。WNT/β-连环蛋白信号转导途径是一种生物学上高度保守的细胞信号转导途径，在胚胎发生、细胞增殖、细胞分化和迁移中具有重要功能。虽然WNT/β-连环蛋白过去常与肾功能损害和足细胞损伤有关，但最近的研究表明，WNT/β-连环蛋白信号在细胞更新中起着有益的作用。已有研究表明，Wnt/β-Catenin通路在急性肾损伤肾小管损伤的再生和修复中起着至关重要的作用。相反，在慢性肾脏疾病中，Wnt/β-catenin信号的持续激活会导致疾病的进展，因此在这种情况下，β-catenin的抑制可能具有保护作用。因此，Wnt/β-catenin信号对肾脏疾病的影响取决于激活时间和强度。此外，我们已经在足细胞中证明了β-连环蛋白在细胞内的位置也很重要，细胞膜上结合的β-连环蛋白具有保护作用，但核β-连环蛋白会导致细胞去分化和肾小球功能障碍。最近，我们发现斑马鱼中MAGI2基因的缺失会导致一种肾病综合征，这种综合征对激素治疗的反应自相矛盾，治疗后疾病进展加快。在这一建议下，我们旨在研究如何促进β-Catenin在肾小球和肾小管中的有益功能。首先，我们将在足细胞培养模型中测试化合物增强足细胞保护膜β-连环蛋白定位的能力，以揭示肾病综合征潜在的新治疗方案。其次，我们将使用小鼠和斑马鱼MAGI2基因敲除模型，以更好地了解类固醇对足细胞作用的机制，并最终找到副作用更少的更具体的药物。最后，我们将使用肾小管细胞培养模型和蛋白尿小鼠模型来确定蛋白尿如何激活肾小管中的β-连环蛋白。这些见解可能有助于我们在肾病综合征的治疗中建立第二个肾小管作用点。为了证明这些发现的临床相关性，其中一些将在临床人体标本中得到证实。

The functional subunit of kidney, the nephron, consists of a filter body called the glomerulus which prevents filtration of high molecular weight proteins from the blood, and the tubule which recycles smaller molecules and water from the primary urine. The podocytes are the epithelial cells of the glomerular kidney filter. Their impairment leads to leakage of the filter with loss of serum proteins into the urine, a state called proteinuria. The respective human disease is called nephrotic syndrome. The established therapy with steroids distinguishes between steroid sensitive and steroid resistant nephrotic syndrome. Although used since decades, the mechanism behind steroid action on podocytes remains unclear to date.Wnt/β-catenin signaling is a biologically highly conserved cellular signal transduction pathway that has important functions in embryogenesis, cell proliferation, cell differentiation and migration. While Wnt/β-catenin was often associated with functional impairment of the kidney and damage of podocytes in the past, recent publications indicate a beneficial role of Wnt/β-catenin signaling in cellular renewal. It has been shown that the Wnt/β-catenin pathway is essential for regeneration and repair of tubular damage in acute kidney injury. In contrast, constant activation of Wnt/β-catenin signaling in chronic kidney disease leads to progression of the disease, so that in this case β-catenin inhibition may have protective effects. Thus, Wnt/β-catenin signaling can have differential influences on kidney diseases dependent on activation time and intensity. Moreover, we have shown previously in podocytes that the location of β-catenin within the cell is important as well, with cell membrane bound β-catenin being protective but nuclear β-catenin causing cellular dedifferentiation and dysfunction of the glomerulus. Recently, we established that loss of the gene MAGI2 in zebrafish causes a type of nephrotic syndrome that reacts paradoxically on steroid treatment with accelerated disease progression upon treatment.With this proposal, we aim to investigate how we can promote the beneficial function of β-catenin in both glomerulus and tubule. Firstly, we will test compounds in a podocyte cell culture model for their ability to strengthen the protective membranous β-catenin localization in podocytes in order to reveal potential novel treatment options in nephrotic syndrome. Secondly, we will employ mouse and zebrafish knock-out models of the gene MAGI2 to better understand the mechanism behind steroid action on podocytes and to ultimately find more specific drugs with less side effects. Lastly, we will use tubular cell culture models and proteinuric mouse models to determine how proteinuria activates β-catenin in the tubule. These insights may help us to establish a second tubular point of action in treatment of nephrotic syndrome. To prove the clinical relevance of these findings, some of them will be confirmed in clinical human specimen.