Shank2 Regulates Renal Albumin Endocytosis

Downregulation of the Autism Spectrum Disorder Gene Shank2 Decreases Bone Mass in Male Mice

Mutations of the postsynaptic scaffold protein Shank2 lead to autism spectrum disorders (ASD). These patients frequently suffer from higher fracture risk. Here, we investigated whether Shank2 directly regulates bone mass. We show that Shank2 is expressed in bone and that Shank2 levels are increased during osteoblastogenesis. Knockdown of Shank2 by siRNA targeting the encoding regions for PDZ and SAM domain inhibits osteoblastogenesis of primary murine calvarial osteoblasts. Shank2 knockout mice (Shank2 ^-/-) have a decreased bone mass due to reduced osteoblastogenesis and bone formation, whereas bone resorption remains unaffected. Induced pluripotent stem cells (iPSCs)-derived osteoblasts from a loss-of-function Shank2 mutation in a patient showed a significantly reduced osteoblast differentiation potential. Moreover, silencing of known Shank2 interacting proteins revealed that a majority of them promote osteoblast differentiation. From this we conclude that Shank2 and interacting proteins known from the central nervous system are decisive regulators in osteoblast differentiation. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Podocyte-specific knockout of the neonatal Fc receptor (FcRn) results in differential protection depending on the model of glomerulonephritis

Podocytes have been proposed to be antigen presenting cells (APCs). In traditional APCs, the neonatal Fc receptor (FcRn) is required for antigen presentation and global knockout of FcRn protects against glomerulonephritis. Since podocytes express FcRn, we sought to determine whether the absence of podocyte FcRn ameliorates immune-mediated disease. We examined MHCII and costimulatory markers expression in cultured wild type (WT) and FcRn knockout (KO) podocytes. Interferon gamma (IFNγ) induced MHCII expression in both WT and KO podocytes but did not change CD80 expression. Neither WT nor KO expressed CD86 or inducible costimulatory ligand (ICOSL) at baseline or with IFNγ. Using an antigen presentation assay, WT podocytes but not KO treated with immune complexes induced a modest increase in IL-2. Induction of the anti-glomerular basement membrane (anti-GBM) model resulted in a significant decrease in glomerular crescents in podocyte-specific FcRn knockout mouse (podFcRn KO) versus controls but the overall percentage of crescents was low. To examine the effects of the podocyte-specific FcRn knockout in a model with a longer autologous phase, we used the nephrotoxic serum nephritis (NTS) model. We found that the podFcRn KO mice had significantly reduced crescent formation and glomerulosclerosis compared to control mice. This study demonstrates that lack of podocyte FcRn is protective in immune mediated kidney disease that is dependent on an autologous phase. This study also highlights the difference between the anti-GBM model and NTS model of disease.

Knockout of the neonatal Fc receptor in cultured podocytes alters IL-6 signaling and the actin cytoskeleton

The neonatal Fc receptor (FcRn) has been shown to be required for antigen presentation in dendritic cells, and global knockout of FcRn attenuates immune-mediated kidney disease. Podocytes express interleukin-6 (IL-6) receptor and produce IL-6 under proinflammatory conditions. Here we examined the role of FcRn in the IL-6-mediated inflammatory response in podocytes. We examined IL-6 production by ELISA and expression by qPCR in wild type (WT) and FcRn knockout (KO) podocytes after treatment with proinflammatory stimuli as well as IL-6-mediated signaling via the JAK/STAT pathway. We also examined podocyte motility in cultured WT and KO podocytes after a proinflammatory challenge. We found that FcRn KO podocytes produced minimal amount of IL-6 after treatment with albumin, IgG, or immune complexes whereas WT podocytes had a robust response. FcRn KO podocytes also had minimal expression of IL-6 compared with WT. By Western blotting, there was significantly less phosphorylated STAT3 in KO podocytes after treatment with IFNγ or immune complexes. In a scratch assay, FcRn KO podocytes showed increased motility comparted KO, suggesting a defect in actin dynamics. Cultured FcRn KO podocytes also demonstrated abnormal stress fibers compared with WT and the defect could be rescued by IL-6 treatment. This study shows that in podocytes, FcRn modulates the IL-6 mediated response to proinflammatory stimuli and regulates podocytes actin structure, motility and synaptopodin expression.

Intravital microscopy in mammalian multicellular organisms

Imaging subcellular processes in live animals is no longer a dream. The development of Intravital Subcellular Microscopy (ISMic) combined with the astounding repertoire of available mouse models makes it possible to investigate processes such as membrane trafficking in mammalian living tissues under native conditions. This has provided the unique opportunity to answer questions that cannot be otherwise addressed in reductionist model systems and to link cell biology to tissue pathophysiology.

Differential trafficking of albumin and IgG facilitated by the neonatal Fc receptor in podocytes in vitro and in vivo

Proteinuria is strongly associated with kidney disease progression but the mechanisms underlying podocyte handling of serum proteins such as albumin and IgG remain to be elucidated. We have previously shown that albumin and IgG are transcytosed by podocytes in vitro. In other epithelial cells, the neonatal Fc receptor (FcRn) is required to salvage albumin and IgG from the degradative pathway thereby allowing these proteins to be transcytosed or recycled. Here we directly examine the role of FcRn in albumin and IgG trafficking in podocytes by studying handling of these proteins in FcRn knockout (KO) podocytes in vitro and in a podocyte-specific FcRn knockout mice in vivo. In vitro, we find that knockout of FcRn leads to IgG accumulation in podocytes but does not alter albumin trafficking. Similarly, in vivo, podocyte-specific knockout of FcRn does not result in albumin accumulation in podocytes in vivo as measured by mean albumin fluorescence intensity whereas these mice demonstrate significant intraglomerular accumulation of IgG over time. In addition we find that podocyte-specific FcRn KO mice demonstrate mesangial expansion as they age and activation of mesangial cells as demonstrated by increased expression of α-smooth muscle actin. Taken together, these results suggest that trafficking pathways for albumin and IgG differ in podocytes and that sustained disruption of trafficking of plasma proteins alters glomerular structure.

Albumin uptake in human podocytes: a possible role for the cubilin-amnionless (CUBAM) complex

Albumin re-uptake is a receptor-mediated pathway located in renal proximal tubuli. There is increasing evidence of glomerular protein handling by podocytes, but little is known about the mechanism behind this process. In this study, we found that human podocytes in vitro are committed to internalizing albumin through a receptor-mediated mechanism even after exposure to low doses of albumin. We show that these cells express cubilin, megalin, ClC-5, amnionless and Dab2, which are partners in the tubular machinery. Exposing human podocytes to albumin overload prompted an increase in CUBILIN, AMNIONLESS and CLCN5 gene expression. Inhibiting cubilin led to a reduction in albumin uptake, highlighting its importance in this mechanism. We demonstrated that human podocytes are committed to performing endocytosis via a receptor-mediated mechanism even in the presence of low doses of albumin. We also disclosed that protein overload first acts on the expression of the cubilin-amnionless (CUBAM) complex in these cells, then involves the ClC-5 channel, providing the first evidence for a possible role of the CUBAM complex in albumin endocytosis in human podocytes.

Receptor-Mediated Endocytosis in the Proximal Tubule

Cells lining the proximal tubule (PT) of the kidney are highly specialized for apical endocytosis of filtered proteins and small bioactive molecules from the glomerular ultrafiltrate to maintain essentially protein-free urine. Compromise of this pathway results in low molecular weight (LMW) proteinuria that can progress to end-stage kidney disease. This review describes our current understanding of the endocytic pathway and the multiligand receptors that mediate LMW protein uptake in PT cells, how these are regulated in response to physiologic cues, and the molecular basis of inherited diseases characterized by LMW proteinuria.