Promotion of importin α–mediated nuclear import by the phosphorylation-dependent binding of cargo protein to 14-3-3

14-3-3 proteins are phosphoserine/threonine-binding proteins that play important roles in many regulatory processes, including intracellular protein targeting. 14-3-3 proteins can anchor target proteins in the cytoplasm and in the nucleus or can mediate their nuclear export. So far, no role for 14-3-3 in mediating nuclear import has been described. There is also mounting evidence that nuclear import is regulated by the phosphorylation of cargo proteins, but the underlying mechanism remains elusive. Myopodin is a dual-compartment, actin-bundling protein that functions as a tumor suppressor in human bladder cancer. In muscle cells, myopodin redistributes between the nucleus and the cytoplasm in a differentiation-dependent and stress-induced fashion. We show that importin α binding and the subsequent nuclear import of myopodin are regulated by the serine/threonine phosphorylation-dependent binding of myopodin to 14-3-3. These results establish a novel paradigm for the promotion of nuclear import by 14-3-3 binding. They provide a molecular explanation for the phosphorylation-dependent nuclear import of nuclear localization signal-containing cargo proteins

Differentiation- and stress-dependent nuclear cytoplasmic redistribution of myopodin, a novel actin-bundling protein

We report the cloning and functional characterization of myopodin, the second member of the synaptopodin gene family. Myopodin shows no significant homology to any known protein except synaptopodin. Northern blot analysis resulted in a 3.6-kb transcript for mouse skeletal and heart muscle. Western blots showed an 80-kD signal for skeletal and a 95-kD signal for heart muscle. Myopodin contains one PPXY motif and multiple PXXP motifs. Myopodin colocalizes with α-actinin and is found at the Z-disc as shown by immunogold electron microscopy. In myoblasts, myopodin shows preferential nuclear localization. During myotube differentiation, myopodin binds to stress fibers in a punctuated pattern before incorporation into the Z-disc. Myopodin can directly bind to actin and contains a novel actin binding site in the center of the protein. Myopodin has actin-bundling activity as shown by formation of latrunculin-A–sensitive cytosolic actin bundles and nuclear actin loops in transfected cells expressing green fluorescent protein–myopodin. Under stress conditions, myopodin accumulates in the nucleus and is depleted from the cytoplasm. Nuclear export of myopodin is sensitive to leptomycin B, despite the absence of a classical nuclear export sequence. We propose a dual role for myopodin as a structural protein also participating in signaling pathways between the Z-disc and the nucleus

Effects of high glucose and TGF-β1 on the expression of collagen IV and vascular endothelial growth factor in mouse podocytes

Effects of high glucose and TGF-β1 on the expression of collagen IV and vascular endothelial growth factor in mouse podocytes.BackgroundThe podocyte takes center stage in the pathogenesis of glomerular basement membrane (GBM) thickening and proteinuria in diabetic glomerulopathy. In part, GBM thickening may occur when the podocyte synthesizes increased amounts of collagen IV. Proteinuria may develop if the podocyte secretes excessive amounts of vascular endothelial growth factor (VEGF), which may increase the glomerular permeability to macromolecules. The augmented production of collagen IV and VEGF may be caused by metabolic mediators of diabetes such as hyperglycemia and transforming growth factor-β (TGF-β).MethodsThe effects of high glucose and exogenous TGF-β1 were examined on a mouse podocyte cell line that retains its differentiated phenotype. The gene expression and protein production of certain alpha chains of collagen IV, the major isoforms of VEGF, and components of the TGF-β system were assayed. An inhibitor of TGF-β signaling was used to determine whether some of the high glucose effects might be mediated by the TGF-β system.ResultsCompared with normal glucose (5.5 mmol/L), high glucose (HG, 25 mmol/L) for 14 days stimulated [3H]-proline incorporation, a measure of collagen production, by 1.8-fold, and exogenous TGF-β1 (2 ng/mL) for 24 hours stimulated proline incorporation by 2.4-fold. Northern analysis showed that exposure to HG for 14 days increased the mRNA level of α1(IV) collagen by 51% and α5(IV) by 90%, whereas treatment with TGF-β1 (2 ng/mL) for 24 hours decreased the mRNA level of α1(IV) by 36% and α5(IV) by 40%. Consistent with these effects on mRNA expression, Western blotting showed that HG increased α1(IV) protein by 44% and α5(IV) by 28%, while TGF-β1 decreased α1(IV) protein by 29% and α5(IV) by 7%. In contrast to their opposing actions on α1 and α5(IV), both HG and exogenous TGF-β1 increased α3(IV) collagen and VEGF, with TGF-β1 having the greater effect. An inhibitor of the TGF-β type I receptor (ALK5) was able to prevent the stimulation of α3(IV) and VEGF proteins by HG. Unlike in other renal cell types, HG did not increase TGF-β1 mRNA or protein in the podocyte, but HG did induce the expression of the ligand-binding TGF-β type II receptor (TβRII). Because HG had up-regulated TβRII after two weeks, the addition of physiological-dose TGF-β1 (0.010 ng/mL) for 24 hours stimulated the production of α3(IV) and VEGF proteins to a greater extent in high than in normal glucose. Up-regulation of TβRII in the podocyte was corroborated by immunohistochemistry of the kidney cortex in the db/db mouse, a model of type 2 diabetes.ConclusionsHigh glucose and exogenous TGF-β1 exert disparate effects on the expression of α1 and α5(IV) collagen. However, high glucose and TGF-β1 coordinately induce the production of α3(IV) collagen and VEGF in the podocyte. The HG-induced increases in α3(IV) collagen and VEGF proteins are mediated by the TGF-β system. By increasing the expression of TβRII, high glucose may augment the response of the podocyte to ambient levels of TGF-β1

Rewiring neuronal microcircuits of the brain via spine head protrusions-a role for synaptopodin and intracellular calcium stores

Neurological diseases associated with neuronal death are also accompanied by axonal denervation of connected brain regions. In these areas, denervation leads to a decrease in afferent drive, which may in turn trigger active central nervous system (CNS) circuitry rearrangement. This rewiring process is important therapeutically, since it can partially recover functions and can be further enhanced using modern rehabilitation strategies. Nevertheless, the cellular mechanisms of brain rewiring are not fully understood. We recently reported a mechanism by which neurons remodel their local connectivity under conditions of network-perturbance: hippocampal pyramidal cells can extend spine head protrusions (SHPs), which reach out toward neighboring terminals and form new synapses. Since this form of activity-dependent rewiring is observed only on some spines, we investigated the required conditions. We speculated, that the actin-associated protein synaptopodin, which is involved in several synaptic plasticity mechanisms, could play a role in the formation and/or stabilization of SHPs. Using hippocampal slice cultures, we found that ~70 % of spines with protrusions in CA1 pyramidal neurons contained synaptopodin. Analysis of synaptopodin-deficient neurons revealed that synaptopodin is required for the stability but not the formation of SHPs. The effects of synaptopodin could be linked to its role in Ca2+ homeostasis, since spines with protrusions often contained ryanodine receptors and synaptopodin. Furthermore, disrupting Ca2+ signaling shortened protrusion lifetime. By transgenically reintroducing synaptopodin on a synaptopodin-deficient background, SHP stability could be rescued. Overall, we show that synaptopodin increases the stability of SHPs, and could potentially modulate the rewiring of microcircuitries by making synaptic reorganization more efficient. Electronic supplementary material The online version of this article (doi:10.1186/s40478-016-0311-x) contains supplementary material, which is available to authorized users

Control of signaling-mediated clearance of apoptotic cells by the tumor suppressor p53

The inefficient clearance of dying cells can lead to abnormal immune responses, such as unresolved inflammation and autoimmune conditions. We show that tumor suppressor p53 controls signaling-mediated phagocytosis of apoptotic cells through its target, Death Domain1α (DD1α), which suggests that p53 promotes both the proapoptotic pathway and postapoptotic events. DD1α appears to function as an engulfment ligand or receptor that engages in homophilic intermolecular interaction at intercellular junctions of apoptotic cells and macrophages, unlike other typical scavenger receptors that recognize phosphatidylserine on the surface of dead cells. DD1α-deficient mice showed in vivo defects in clearing dying cells, which led to multiple organ damage indicative of immune dysfunction. p53-induced expression of DD1α thus prevents persistence of cell corpses and ensures efficient generation of precise immune responses

IQGAP1 Interacts with Components of the Slit Diaphragm Complex in Podocytes and Is Involved in Podocyte Migration and Permeability In Vitro

IQGAP1 is a scaffold protein that interacts with proteins of the cytoskeleton and the intercellular adhesion complex. In podocytes, IQGAP1 is associated with nephrin in the glomerular slit diaphragm (SD) complex, but its role remains ill-defined. In this work, we investigated the interaction of IQGAP1 with the cytoskeleton and SD proteins in podocytes in culture, and its role in podocyte migration and permeability. Expression, localization, and interactions between IQGAP1 and SD or cytoskeletal proteins were determined in cultured human podocytes by Western blot (WB), immunocytolocalization (IC), immunoprecipitation (IP), and In situ Proximity Ligation assay (IsPL). Involvement of IQGAP1 in migration and permeability was also assessed. IQGAP1 expression in normal kidney biopsies was studied by immunohistochemistry. IQGAP1 expression by podocytes increased during their in vitro differentiation. IC, IP, and IsPL experiments showed colocalizations and/or interactions between IQGAP1 and SD proteins (nephrin, MAGI-1, CD2AP, NCK 1/2, podocin), podocalyxin, and cytoskeletal proteins (α-actinin-4). IQGAP1 silencing decreased podocyte migration and increased the permeability of a podocyte layer. Immunohistochemistry on normal human kidney confirmed IQGAP1 expression in podocytes and distal tubular epithelial cells and also showed an expression in glomerular parietal epithelial cells. In summary, our results suggest that IQGAP1, through its interaction with components of SD and cytoskeletal proteins, is involved in podocyte barrier properties

New perspectives on the renal slit diaphragm protein podocin

Podocin is a critical component of the glomerular filtration barrier, its mutations causing recessive steroid-resistant nephrotic syndrome. A GenBank analysis of the human podocin (NPHS2) gene resulted in the possible existence of a new splice variant of podocin in the kidney, missing the in-frame of exon 5, encoding the prohibitin homology domain. Using RT–polymerase chain reaction and immunoblotting followed by sequence analysis, we are for the first time able to prove the expression of a novel podocin isoform (isoform 2), exclusively and constitutively expressed in human podocytes. Furthermore, we reveal singular extrarenal podocin expression in human and murine testis. Our data show the Sertoli cells of the seminiferous tubules to be the origin of testicular podocin. Confocal laser microscopy illustrates the co-localization of podocin with filamentous actin within Sertoli cells, suggesting a role of podocin in the blood/testis barrier. These results led to the rationale to examine podocin expression in testes of men with Sertoli cell-only syndrome, a disorder characterized by azoospermia. Interestingly, we observed a complete down-regulation of podocin mRNA in Sertoli cell-only syndrome, indicating a possible role of podocin in the pathogenesis of this germinal aplasia. Men with Sertoli cell-only syndrome show normal renal podocin expression, suggesting an alternate regulation of the testicular promoter. Our findings may change the perception of podocin and give new insights into the ultrastructure of glomerular slit diaphragm and the blood/testis barrier

Metalloprotease Meprinβ in Rat Kidney: Glomerular Localization and Differential Expression in Glomerulonephritis

Meprin (EC 3.4.24.18) is an oligomeric metalloendopeptidase found in microvillar membranes of kidney proximal tubular epithelial cells. Here, we present the first report on the expression of meprinβ in rat glomerular epithelial cells and suggest a potential involvement in experimental glomerular disease. We detected meprinβ in glomeruli of immunostained rat kidney sections on the protein level and by quantitative RT-PCR of laser-capture microdissected glomeruli on the mRNA level. Using immuno-gold staining we identified the membrane of podocyte foot processes as the main site of meprinβ expression. The glomerular meprinβ expression pattern was altered in anti-Thy 1.1 and passive Heymann nephritis (PHN). In addition, the meprinβ staining pattern in the latter was reminiscent of immunostaining with the sheep anti-Fx1A antiserum, commonly used in PHN induction. Using Western blot and immunoprecipitation assays we demonstrated that meprinβ is recognized by Fx1A antiserum and may therefore represent an auto-antigen in PHN. In anti-Thy 1.1 glomerulonephritis we observed a striking redistribution of meprinβ in tubular epithelial cells from the apical to the basolateral side and the cytosol. This might point to an involvement of meprinβ in this form of glomerulonephritis