Dynamin-mediated Nephrin phosphorylation regulates glucose-stimulated insulin release in pancreatic beta cells.

We have previously demonstrated a role for Nephrin in glucose stimulated insulin release (GSIR). We now hypothesize that Nephrin phosphorylation is required for GSIR and that Dynamin influences Nephrin phosphorylation and function. MIN6-C3 Nephrin-deficient pancreatic beta cells and human islets were transfected with WT-Nephrin or with a mutant Nephrin in which the tyrosine residues responsible for SH2 domain binding were substituted with phenylalanine (3YF-Nephrin). GSIR and live images of Nephrin and vesicle trafficking were studied. Immunoprecipitation experiments and overexpression of WT-Dynamin or dominant negative Dynamin mutant (K44A-Dynamin) in WT-Nephrin, 3YF-Nephrin, or Nephrin siRNA-transfected cells were utilized to study Nephrin-Dynamin interaction. In contrast to WT-Nephrin or to single tyrosine mutants, 3YF-Nephrin did not positively affect GSIR and led to impaired cell-cell contacts and vesicle trafficking. K44A-Dynamin prevented the effect of Nephrin on GSIR in the absence of protein-protein interaction between Nephrin and Dynamin. Nephrin gene silencing abolished the positive effects of WT-Dynamin on GSIR. The effects of protamine sulfate and vanadate on Nephrin phosphorylation and GSIR were studied in MIN6 cells and human islets. WT-Nephrin phosphorylation after glucose occurred at Tyr-1176/1193 and resulted in improved GSIR. On the contrary, protamine sulfate-induced phosphorylation at Tyr-1176/1193/1217 was associated with Nephrin degradation and impaired GSIR. Vanadate, which prevented Nephrin dephosphorylation after glucose stimulation, improved GSIR in human islets and MIN6 cells. In conclusion, Dynamin-dependent Nephrin phosphorylation occurs in response to glucose and is necessary for Nephrin-mediated augmentation of GSIR. Pharmacological modulation of Nephrin phosphorylation may thus facilitate pancreatic beta cell function.

Rituximab is a safe and effective long-term treatment for children with steroid and calcineurin inhibitor-dependent idiopathic nephrotic syndrome.

In children with idiopathic nephrotic syndrome, rituximab can maintain short-term remission with withdrawal of prednisone and calcineurin inhibitors. Long-term effects including the number of repeated infusions to maintain remission are unknown. To test this, we treated 46 consecutive children with idiopathic nephrotic syndrome lasting for at least 1 year (mean 6.3 years), maintained in remission with oral prednisone and calcineurin inhibitors. They received 1-5 rituximab courses during a median follow-up of 3 years. Oral agents were tapered after each infusion, and completely withdrawn within 45 days. Rituximab was well tolerated. Six-month probabilities of remission were 48% after the first infusion and 37% after subsequent infusions. One- and 2-year-remission probabilities were, respectively, 20 and 10%. Median time intervals between complete oral-agent withdrawal and relapse were 5.6 and 8.5 months, respectively, following the first and subsequent courses. The time to reconstitution of CD20 cells correlated with the duration of remission, but was not associated with variation in FcyR, CD20, or SMPDL-3B polymorphisms. Podocyte Src phosphorylation was normal. Thus, rituximab can be safely and repeatedly used as a prednisone and calcineurin inhibitor-sparing therapy in a considerable proportion of children with dependent forms of idiopathic nephrotic syndrome. Further study is needed to identify patients who will benefit most from rituximab therapy.

Podocyte-specific GLUT4-deficient mice have fewer and larger podocytes and are protected from diabetic nephropathy.

Podocytes are a major component of the glomerular filtration barrier, and their ability to sense insulin is essential to prevent proteinuria. Here we identify the insulin downstream effector GLUT4 as a key modulator of podocyte function in diabetic nephropathy (DN). Mice with a podocyte-specific deletion of GLUT4 (G4 KO) did not develop albuminuria despite having larger and fewer podocytes than wild-type (WT) mice. Glomeruli from G4 KO mice were protected from diabetes-induced hypertrophy, mesangial expansion, and albuminuria and failed to activate the mammalian target of rapamycin (mTOR) pathway. In order to investigate whether the protection observed in G4 KO mice was due to the failure to activate mTOR, we used three independent in vivo experiments. G4 KO mice did not develop lipopolysaccharide-induced albuminuria, which requires mTOR activation. On the contrary, G4 KO mice as well as WT mice treated with the mTOR inhibitor rapamycin developed worse adriamycin-induced nephropathy than WT mice, consistent with the fact that adriamycin toxicity is augmented by mTOR inhibition. In summary, GLUT4 deficiency in podocytes affects podocyte nutrient sensing, results in fewer and larger cells, and protects mice from the development of DN. This is the first evidence that podocyte hypertrophy concomitant with podocytopenia may be associated with protection from proteinuria.

Cyclodextrin protects podocytes in diabetic kidney disease.

Diabetic kidney disease (DKD) remains the most common cause of end-stage kidney disease despite multifactorial intervention. We demonstrated that increased cholesterol in association with downregulation of ATP-binding cassette transporter ABCA1 occurs in normal human podocytes exposed to the sera of patients with type 1 diabetes and albuminuria (DKD(+)) when compared with diabetic patients with normoalbuminuria (DKD(-)) and similar duration of diabetes and lipid profile. Glomerular downregulation of ABCA1 was confirmed in biopsies from patients with early DKD (n = 70) when compared with normal living donors (n = 32). Induction of cholesterol efflux with cyclodextrin (CD) but not inhibition of cholesterol synthesis with simvastatin prevented podocyte injury observed in vitro after exposure to patient sera. Subcutaneous administration of CD to diabetic BTBR (black and tan, brachiuric) ob/ob mice was safe and reduced albuminuria, mesangial expansion, kidney weight, and cortical cholesterol content. This was followed by an improvement of fasting insulin, blood glucose, body weight, and glucose tolerance in vivo and improved glucose-stimulated insulin release in human islets in vitro. Our data suggest that impaired reverse cholesterol transport characterizes clinical and experimental DKD and negatively influences podocyte function. Treatment with CD is safe and effective in preserving podocyte function in vitro and in vivo and may improve the metabolic control of diabetes.

Rituximab targets podocytes in recurrent focal segmental glomerulosclerosis.

Focal segmental glomerulosclerosis (FSGS) is a glomerular disease characterized by proteinuria, progression to end-stage renal disease, and recurrence of proteinuria after kidney transplantation in about one-third of patients. It has been suggested that rituximab might treat recurrent FSGS through an unknown mechanism. Rituximab not only recognizes CD20 on B lymphocytes, but might also bind sphingomyelin phosphodiesterase acid-like 3b (SMPDL-3b) protein and regulate acid sphingomyelinase (ASMase) activity. We hypothesized that rituximab prevents recurrent FSGS and preserves podocyte SMPDL-3b expression. We studied 41 patients at high risk for recurrent FSGS, 27 of whom were treated with rituximab at time of kidney transplant. SMPDL-3b protein, ASMase activity, and cytoskeleton remodeling were studied in cultured normal human podocytes that had been exposed to patient sera with or without rituximab. Rituximab treatment was associated with lower incidence of posttransplant proteinuria and stabilization of glomerular filtration rate. The number of SMPDL-3b(+) podocytes in postreperfusion biopsies was reduced in patients who developed recurrent FSGS. Rituximab partially prevented SMPDL-3b and ASMase down-regulation that was observed in podocytes treated with the sera of patients with recurrent FSGS. Overexpression of SMPDL-3b or treatment with rituximab was able to prevent disruption of the actin cytoskeleton and podocyte apoptosis induced by patient sera. This effect was diminished in cultured podocytes where SMPDL-3b was silenced. Our study suggests that treatment of high-risk patients with rituximab at time of kidney transplant might prevent recurrent FSGS by modulating podocyte function in an SMPDL-3b-dependent manner.

The actin cytoskeleton of kidney podocytes is a direct target of the antiproteinuric effect of cyclosporine A.

The immunosuppressive action of the calcineurin inhibitor cyclosporine A (CsA) stems from the inhibition of nuclear factor of activated T cells (NFAT) signaling in T cells. CsA is also used for the treatment of proteinuric kidney diseases. As it stands, the antiproteinuric effect of CsA is attributed to its immunosuppressive action. Here we show that the beneficial effect of CsA on proteinuria is not dependent on NFAT inhibition in T cells, but rather results from the stabilization of the actin cytoskeleton in kidney podocytes. CsA blocks the calcineurin-mediated dephosphorylation of synaptopodin, a regulator of Rho GTPases in podocytes, thereby preserving the phosphorylation-dependent synaptopodin-14-3-3 beta interaction. Preservation of this interaction, in turn, protects synaptopodin from cathepsin L-mediated degradation. These results represent a new view of calcineurin signaling and shed further light on the treatment of proteinuric kidney diseases. Novel calcineurin substrates such as synaptopodin may provide promising starting points for antiproteinuric drugs that avoid the serious side effects of long-term CsA treatment.

The role of neural crest during cardiac development in a mouse model of DiGeorge syndrome.

The velo-cardio-facial syndrome (VCFS)/DiGeorge syndrome (DGS) is a genetic disorder characterized by phenotypic abnormalities of the derivatives of the pharyngeal arches, including cardiac outflow tract defects. Neural crest cells play a major role in the development of the pharyngeal arches, and defects in these cells are likely responsible for the syndrome. Most patients are hemizygous for a 1.5- to 3.0-Mb region of 22q11, that is suspected to be critical for normal pharyngeal arch development. Mice hemizygous for a 1.5-Mb homologous region of chromosome 16 (Lgdel/+) exhibit conotruncal cardiac defects similar to those seen in affected VCFS/DGS patients. To investigate the role of Lgdel genes in neural crest development, we fate mapped neural crest cells in Lgdel/+ mice and we performed hemizygous neural crest-specific inactivation of Lgdel. Hemizygosity of the Lgdel region does not eliminate cardiac neural crest migration to the forming aortic arches. However, neural crest cells do not differentiate appropriately into smooth muscle in both fourth and sixth aortic arches and the affected aortic arch segments develop abnormally. Tissue-specific hemizygous inactivation of Lgdel genes in neural crest results in normal cardiovascular development. Based on our studies, we propose that Lgdel genes are required for the expression of soluble signals that regulate neural crest cell differentiation.