Summary Gradual changes in steady-state levels of beta amyloid peptides (Aβ) in brain are considered an initial step in the amyloid cascade hypothesis of Alzheimer's disease. Aβ is a product of the secretase cleavage of amyloid precursor protein (APP). There is evidence that the membrane lipid environment may modulate secretase activity and alters its function. Cleavage of APP strongly depends on membrane properties. Since Aβ perturbs cell membrane fluidity, the cell membrane may be the location where the neurotoxic cascade of Aβ is initiated. Therefore, we tested effects of oligomeric Aβ on membrane fluidity of whole living cells, the impact of exogenous and cellular Aβ on the processing of APP and the role of GM-1 ganglioside. We present evidence that oligoAβ(1-40) stimulates the amyloidogenic processing of APP by reducing membrane fluidity and complexing with GM-1 ganglioside. This dynamic action of Aβ may start a vicious circle, where endogenous Aβ stimulates its own production. Based on our novel findings, we propose that oligoAβ(1-40) accelerates the proteolytic cleavage of APP by decreasing membrane fluidity.

PDX-positive cells are lost in the urine in disease states that require podocyte regeneration and are a useful non-invasive marker for glomerular disease activity. These cells are possibly derived from the parietal epithelial layer.

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Defects in podocyte signaling are the basis of many inherited glomerular diseases leading to glomerulosclerosis. CD2-associated protein (CD2AP) is highly expressed in podocytes and is considered to play an important role in the maintenance of the glomerular slit diaphragm. Mice deficient for CD2AP (CD2AP ؊/؊ ) appear normal at birth but develop a rapid onset nephrotic syndrome at 3 weeks of age. We demonstrate that impaired intracellular signaling with subsequent podocyte damage is the reason for this delayed podocyte injury in CD2AP ؊/؊ mice. We document that CD2AP deficiency in podocytes leads to diminished signal initiation and termination of signaling pathways mediated by receptor tyrosine kinases (RTKs). In addition, we demonstrate that CIN85, a paralog of CD2AP, is involved in termination of RTK signaling in podocytes. CIN85 protein expression is increased in CD2AP ؊/؊ podocytes in vitro. Stimulation of CD2AP؊/؊ podocytes with various growth factors, including insulin-like growth factor 1, vascular endothelial growth factor, and fibroblast growth factor, resulted in a significantly decreased phosphatidylinositol 3-kinase/AKT and ERK signaling response. Moreover, increased CIN85 protein is detectable in podocytes in diseased CD2AP ؊/؊ mice, leading to decreased base-line activation of ERK and decreased phosphorylation after growth factor stimulation in vivo. Because repression of CIN85 protein leads to a restored RTK signaling response, our results support an important role of CD2AP/ CIN85 protein balance in the normal signaling response of podocytes. Phosphatidylinositol 3-kinase (PI3K)2 and Ras/ERK mitogen-activated protein kinase signaling pathways are key factors for determining the specificity of cellular responses, including cell proliferation, cell differentiation, and cell survival (1, 2). We and others have previously demonstrated that the PI3K/AKT signaling response plays an important role for podocyte survival in particular in the presence of active transforming growth factor ␤ (3, 4). We recently demonstrated that the PI3K/AKT response is directly targeted by cytokine cross-talk and subsequently influences the cellular outcome (5). The adaptor molecules CD2-associated protein (CD2AP) and CIN85 belong to a family of adaptor molecules that selectively control the spatial and temporal assembly of multiprotein complexes that transmit intracellular signals. For both molecules various interaction partners have been described placing them at the center of regulatory pathways involving signaling (6, 7), cytoskeletal arrangement (8, 9), vesicular trafficking (10), and endocytosis (11,12). In this study we demonstrate that CD2AP and CIN85 contribute to the balance of RTK signaling in podocytes. The anatomical localization of the podocyte exposes this highly specialized cell type to a variety of cellular stressors like stretch force, reactive oxygen species, osmotic milieu changes, cytokines and chemokines, filtrated toxins, and waste products. Therefore, this location requires the cells to have a highly e...

Podocyte apoptosis initiates progressive glomerulosclerosis in TGF-␤1 transgenic and CD2AP-knockout (CD2AP؊/؊) mice. It was previously shown that in both mouse models, activation of the TGF-␤ pathway is the key event during development of podocyte apoptosis. Furthermore, CD2AP is an important modifier of TGF-␤-induced survival signaling via activation of the phosphoinositol 3-kinase/AKT signaling pathway. This article presents IGF-binding protein-3 (IGFBP-3) as a new modulator of apoptosis and survival signaling in glomerular podocytes. High expression of IGFBP-3 protein in the urine of diseased CD2AP؊/؊ mice was discovered, and IGFBP-3 expression in glomerular podocytes and parietal cells was detected. IGFBP-3 can induce changes in podocyte actin cytoskeleton, leads to apoptosis in cultured murine podocytes, and can enhance TGF-␤1-induced apoptosis in vitro. For studying this process on a molecular level, proapoptotic p38 mitogen-activated protein kinase pathways and antiapoptotic phosphoinositol 3-kinase/AKT pathways were examined in cultured murine podocytes. It was found that IGFBP-3 increments the level of TGF-␤1-induced phosphorylated p38 mitogen-activated protein kinase and decreases the phosphorylation of antiapoptotic AKT. This effect is specific for the co-stimulation of IGFBP-3 with TGF-␤1 because a combination of IGFBP-3 with bone morphogenic protein-7 (BMP-7), another member of the TGF-␤ superfamily, results in apoptosis opposing signaling effects with a strong increase of phosphorylated AKT and subsequent functional effects. These results demonstrate that the IGF/IGFBP axis plays an important role in the development of podocyte apoptosis by modulation of TGF-␤ and BMP-7-induced pro-and antiapoptotic signals.