Lack of CD2AP disrupts Glut4 trafficking and attenuates glucose uptake in podocytes

The Alzheimer's Disease Risk Gene CD2AP Functions in Dendritic Spines by Remodeling F-Actin

CD2-associated protein (CD2AP) was identified as a genetic risk factor for late-onset Alzheimer's disease (LOAD). However, it is unclear how CD2AP contributes to LOAD synaptic dysfunction underlying AD memory deficits. We have shown that loss of CD2AP function increases β-amyloid (Aβ) endocytic production, but it is unknown whether it contributes to synapse dysfunction. As CD2AP is an actin-binding protein, it may also function in F-actin-rich dendritic spines, which are the excitatory postsynaptic compartments. Here, we demonstrate that CD2AP colocalizes with F-actin in dendritic spines of primary mouse cortical neurons of both sexes. Cell-autonomous depletion of CD2AP specifically reduces spine density and volume, resulting in a functional decrease in synapse formation and neuronal network activity. Postsynaptic reexpression of CD2AP, but not blocking Aβ production, is sufficient to rescue spine density. CD2AP overexpression increases spine density, volume, and synapse formation, while a rare LOAD CD2AP mutation induces aberrant F-actin spine-like protrusions without functional synapses. CD2AP controls postsynaptic actin turnover, with the LOAD mutation in CD2AP decreasing F-actin dynamicity. Our data support that CD2AP risk variants could contribute to LOAD synapse dysfunction by disrupting spine formation and growth by deregulating actin dynamics.

GLUT4 dispersal at the plasma membrane of adipocytes: a super-resolved journey

In adipose tissue, insulin stimulates glucose uptake by mediating the translocation of GLUT4 from intracellular vesicles to the plasma membrane. In 2010, insulin was revealed to also have a fundamental impact on the spatial distribution of GLUT4 within the plasma membrane, with the existence of two GLUT4 populations at the plasma membrane being defined: (1) as stationary clusters and (2) as diffusible monomers. In this model, in the absence of insulin, plasma membrane-fused GLUT4 are found to behave as clusters. These clusters are thought to arise from exocytic events that retain GLUT4 at their fusion sites; this has been proposed to function as an intermediate hub between GLUT4 exocytosis and re-internalisation. By contrast, insulin stimulation induces the dispersal of GLUT4 clusters into monomers and favours a distinct type of GLUT4-vesicle fusion event, known as fusion-with-release exocytosis. Here, we review how super-resolution microscopy approaches have allowed investigation of the characteristics of plasma membrane-fused GLUT4 and further discuss regulatory step(s) involved in the GLUT4 dispersal machinery, introducing the scaffold protein EFR3 which facilitates localisation of phosphatidylinositol 4-kinase type IIIα (PI4KIIIα) to the cell surface. We consider how dispersal may be linked to the control of transporter activity, consider whether macro-organisation may be a widely used phenomenon to control proteins within the plasma membrane, and speculate on the origin of different forms of GLUT4-vesicle exocytosis.

Dehydration Accelerates Cytogenesis and Cyst Growth in Pkd1-/- Mice by Regulating Macrophage M2 Polarization

Adult autosomal dominant polycystic kidney disease (ADPKD) has been shown to be related as a "third hit" to the occurrence of acute or chronic kidney injury. Here, we examined whether dehydration, as a common kidney risk factor, could cause cystogenesis in chronic-onset Pkd1-/- mice by regulating macrophage activation. First, we confirmed that dehydration accelerated cytogenesis in Pkd1-/- mice and that macrophages infiltrated the kidney tissues even earlier than macroscopic cyst formation. Then, microarray analysis suggested that glycolysis pathway may be involved in macrophage activation in Pkd1-/- kidneys under conditions of dehydration. Further, we confirmed glycolysis pathway was activated and lactic acid (L-LA) was overproduced in the Pkd1-/- kidney under conditions of dehydration. We have already proved that L-LA strongly stimulated M2 macrophage polarization and overproduction of polyamine in macrophage in vitro, and in the present study, we further discovered that M2 polarization-induced polyamine production shortened the primary cilia length by disrupting the PC1/PC2 complex. Finally, the activation of L-LA-arginase 1-polyamine pathway contributed to cystogenesis and progressive cyst growth in Pkd1-/- mice recurrently exposed to dehydration.

Exogenous spermine attenuates diabetic kidney injury in rats by inhibiting AMPK/mTOR signaling pathway

Diabetic nephropathy (DN) is the primary cause of end‑stage renal disease, which is closely associated with dysfunction of the podocytes, the main component of the glomerular filtration membrane; however, the exact underlying mechanism is unknown. Polyamines, including spermine, spermidine and putrescine, have antioxidant and anti‑aging properties that are involved in the progression of numerous diseases, but their role in DN has not yet been reported. The present study aimed to explore the role of polyamines in DN, particularly in podocyte injury, and to reveal the molecular mechanism underlying the protective effect of exogenous spermine. Streptozotocin intraperitoneal injection‑induced type 1 diabetic (T1D) rat models and high glucose (HG)‑stimulated podocyte injury models were established. It was found that in T1D rat kidneys and HG‑induced podocytes, ornithine decarboxylase (a key enzyme for polyamine synthesis) was downregulated, while spermidine/spermine N1‑acetyltransferase (a key enzyme for polyamines degradation) was upregulated, which suggested that reduction of the polyamine metabolic pool particularly decreased spermine content, is a major factor in DN progression. In addition, hyperglycemia can induce an increased rat kidney weight ratio, serum creatinine, urea, urinary albumin excretion and glomerular cell matrix levels, and promote mesangial thickening and loss or fusion of podocytes. The expression levels of podocyte marker proteins (nephrin, CD2‑associated protein and podocin) and autophagy‑related proteins [autophagy protein 5, microtube‑associated proteins 1A/1B light chain 3 (LC3)II/LC3I, Beclin 1 and phosphorylated (p)‑AMPK] were downregulated, while cleaved caspase‑3, P62 and p‑mTOR were increased. These changes could be improved by pretreatment with exogenous spermine or rapamycin (autophagic agonist). In conclusion, spermine may have the potential to prevent diabetic kidney injury in rats by promoting autophagy via regulating the AMPK/mTOR signaling pathway.

PolyMet-HA nanocomplexs regulates glucose uptake by inhibiting SHIP2 activity

Metformin, the first-line drug to treat type 2 diabetes, inhibits mitochondrial glycerolphosphate dehydrogenase in the liver to suppress gluconeogenesis. The major adverse effects caused by metformin were lactic acidosis and gastrointestinal discomfort. Therefore, there is need to develop a strategy with excellent permeability and appropriate retention effects.In this study, we synthesized a simple and biocompatible PolyMetformin (denoted as PolyMet) through conjugation of PEI_1.8K with dicyandiamide, and then formed PolyMet-hyaluronic acid (HA) nanocomplexs by electrostatic self-assembly of the polycationic PolyMet and polyanionic hyaluronic acid (HA). Similar to metformin, the PolyMet-HA nanocomplexs could reduce the catalytic activity of the recombinant SHIP2 phosphatase domain in vitro. In SHIP2-overexpressing myotubes, PolyMet-HA nanocomplexes ameliorated glucose uptake by downregulating glucose transporter 4 endocytosis. PolyMet-HA nanocomplexes also could restore Akt signaling and protect the podocyte from apoptosis induced by SHIP2 overexpression. In essence, the PolyMet-HA nanocomplexes act similarly to metformin and increase glucose uptake, and maybe have a potential role in the treatment of type 2 diabetes.

Novel Sulfonanilide Inhibitors of SHIP2 Enhance Glucose Uptake into Cultured Myotubes

A series of substituted sulfonanilide analogs were prepared and evaluated as novel potent inhibitors of SH2 domain-containing inositol polyphosphate 5'-phosphatase 2 (SHIP2). SHIP2 has been shown to be a new attractive target for the treatment of insulin resistance in type 2 diabetes mellitus (T2D), which can lead to life-threatening diabetic kidney disease (DKD). Amongst the synthesized compounds, the two most promising candidates, 10 and 11, inhibited SHIP2 significantly. Additionally, these compounds induced Akt activation in a dose-dependent manner, increased the presence of glucose transporter 4 at the plasma membrane, and enhanced glucose uptake in cultured myotubes in vitro at lower concentrations than metformin, the most widely used antidiabetic drug. These results show that the novel SHIP2 inhibitors have insulin sensitizing capacity and provide prototypes for further drug development for T2D and DKD.

SHIPping out diabetes-Metformin, an old friend among new SHIP2 inhibitors

SHIP2 (Src homology 2 domain‐containing inositol 5′‐phosphatase 2) belongs to the family of 5′‐phosphatases. It regulates the phosphoinositide 3‐kinase (PI3K)‐mediated insulin signalling cascade by dephosphorylating the 5′‐position of PtdIns(3,4,5)P3 to generate PtdIns(3,4)P2, suppressing the activity of the pathway. SHIP2 mouse models and genetic studies in human propose that increased expression or activity of SHIP2 contributes to the pathogenesis of the metabolic syndrome, hypertension and type 2 diabetes. This has raised great interest to identify SHIP2 inhibitors that could be used to design new treatments for metabolic diseases. This review summarizes the central mechanisms associated with the development of diabetic kidney disease, including the role of insulin resistance, and then moves on to describe the function of SHIP2 as a regulator of metabolism in mouse models. Finally, the identification of SHIP2 inhibitors and their effects on metabolic processes in vitro and in vivo are outlined. One of the newly identified SHIP2 inhibitors is metformin, the first‐line medication prescribed to patients with type 2 diabetes, further boosting the attraction of SHIP2 as a treatment target to ameliorate metabolic disorders.

The role of CD2AP in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by irreversible decline in cognition with unclear pathogenesis. Recently, accumulating evidence has revealed that CD2 associated protein (CD2AP), a scaffolding molecule regulates signal transduction and cytoskeletal molecules, is implicated in AD pathogenesis. Several single nucleotide polymorphisms (SNPs) in CD2AP gene are associated with higher risk for AD and mRNA levels of CD2AP are decreased in peripheral lymphocytes of sporadic AD patients. Furthermore, CD2AP loss of function is linked to enhanced Aβ production, Tau-induced neurotoxicity, abnormal neurite structure modulation and reduced blood-brain barrier integrity. This review is to summarize the recent discoveries about the genetics and known functions of CD2AP. The recent evidence concerning the roles of CD2AP in the AD pathogenesis is summarized and CD2AP can be a promising therapeutic target for AD.

Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity

Metformin, the first-line drug to treat type 2 diabetes (T2D), inhibits mitochondrial glycerolphosphate dehydrogenase in the liver to suppress gluconeogenesis. However, the direct target and the underlying mechanisms by which metformin increases glucose uptake in peripheral tissues remain uncharacterized. Lipid phosphatase Src homology 2 domain-containing inositol-5-phosphatase 2 (SHIP2) is upregulated in diabetic rodent models and suppresses insulin signaling by reducing Akt activation, leading to insulin resistance and diminished glucose uptake. Here, we demonstrate that metformin directly binds to and reduces the catalytic activity of the recombinant SHIP2 phosphatase domain in vitro. Metformin inhibits SHIP2 in cultured cells and in skeletal muscle and kidney of db/db mice. In SHIP2-overexpressing myotubes, metformin ameliorates reduced glucose uptake by slowing down glucose transporter 4 endocytosis. SHIP2 overexpression reduces Akt activity and enhances podocyte apoptosis, and both are restored to normal levels by metformin. SHIP2 activity is elevated in glomeruli of patients with T2D receiving nonmetformin medication, but not in patients receiving metformin, compared with people without diabetes. Furthermore, podocyte loss in kidneys of metformin-treated T2D patients is reduced compared with patients receiving nonmetformin medication. Our data unravel a novel molecular mechanism by which metformin enhances glucose uptake and acts renoprotectively by reducing SHIP2 activity.-Polianskyte-Prause, Z., Tolvanen, T. A., Lindfors, S., Dumont, V., Van, M., Wang, H., Dash, S. N., Berg, M., Naams, J.-B., Hautala, L. C., Nisen, H., Mirtti, T., Groop, P.-H., Wähälä, K., Tienari, J., Lehtonen, S. Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity.

BioID identifies proteins involved in the cell biology of caveolae

The mechanisms controlling the abundance and sub-cellular distribution of caveolae are not well described. A first step towards determining such mechanisms would be identification of relevant proteins that interact with known components of caveolae. Here, we applied proximity biotinylation (BioID) to identify a list of proteins that may interact with the caveolar protein cavin1. Screening of these candidates using siRNA to reduce their expression revealed that one of them, CSDE1, regulates the levels of mRNAs and protein expression for multiple components of caveolae. A second candidate, CD2AP, co-precipitated with cavin1. Caveolar proteins were observed in characteristic and previously un-described linear arrays adjacent to cell-cell junctions in both MDCK cells, and in HeLa cells overexpressing an active form of the small GTPase Rac1. CD2AP was required for the recruitment of caveolar proteins to these linear arrays. We conclude that BioID will be useful in identification of new proteins involved in the cell biology of caveolae, and that interaction between CD2AP and cavin1 may have an important role in regulating the sub-cellular distribution of caveolae.

Single-molecule labeling for studying trafficking of renal transporters

The ability to detect and track single molecules presents the advantage of visualizing the complex behavior of transmembrane proteins with a time and space resolution that would otherwise be lost with traditional labeling and biochemical techniques. Development of new imaging probes has provided a robust method to study their trafficking and surface dynamics. This mini-review focuses on the current technology available for single-molecule labeling of transmembrane proteins, their advantages, and limitations. We also discuss the application of these techniques to the study of renal transporter trafficking in light of recent research.

SGLT2 inhibitor dapagliflozin limits podocyte damage in proteinuric nondiabetic nephropathy

Sodium-glucose cotransporter 2 (SGLT2) inhibitors have pleiotropic properties beyond blood glucose-lowering effects and modify important nonglycemic pathways, leading to end-organ protection. SGLT2 inhibitors display renoprotective effects in diabetic kidney disease, which creates a rationale for testing the therapeutic potential of this drug class in nondiabetic chronic kidney disease. Here, we have shown that dapagliflozin provided glomerular protection in mice with protein-overload proteinuria induced by bovine serum albumin (BSA), to a similar extent as an ACE inhibitor used as standard therapy for comparison. Dapagliflozin limited proteinuria, glomerular lesions, and podocyte dysfunction and loss. We provide the observation that SGLT2 was expressed in podocytes and upregulated after BSA injections. Through in vitro studies with cultured podocytes loaded with albumin we have identified what we believe to be a novel mechanism of action for SGLT2 inhibitor that directly targets podocytes and relies on the maintenance of actin cytoskeleton architecture. Whether SGLT2 inhibitors represent a possible future therapeutic option for some patients with proteinuric glomerular disease who do not have as yet an effective treatment will require ad hoc clinical studies.

Glucose Transporters in Diabetic Kidney Disease-Friends or Foes?

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes and a common cause of end-stage renal disease worldwide. DKD manifests as an increased urinary protein excretion (albuminuria). Multiple studies have shown that insulin resistance correlates with the development of albuminuria in non-diabetic and diabetic patients. There is also accumulating evidence that glomerular epithelial cells or podocytes are insulin sensitive and that insulin signaling in podocytes is essential for maintaining normal kidney function. At the cellular level, the mechanisms leading to the development of insulin resistance include mutations in the insulin receptor gene, impairments in the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway, or perturbations in the trafficking of glucose transporters (GLUTs), which mediate the uptake of glucose into cells. Podocytes express several GLUTs, including GLUT1, GLUT2, GLUT3, GLUT4, and GLUT8. Of these, the most studied ones are GLUT1 and GLUT4, both shown to be insulin responsive in podocytes. In the basal state, GLUT4 is preferentially located in perinuclear and cytosolic vesicular structures and to a lesser extent at the plasma membrane. After insulin stimulation, GLUT4 is sorted into GLUT4-containing vesicles (GCVs) that translocate to the plasma membrane. GCV trafficking consists of several steps, including approaching of the GCVs to the plasma membrane, tethering, and docking, after which the lipid bilayers of the GCVs and the plasma membrane fuse, delivering GLUT4 to the cell surface for glucose uptake into the cell. Studies have revealed novel molecular regulators of the GLUT trafficking in podocytes and unraveled unexpected roles for GLUT1 and GLUT4 in the development of DKD, summarized in this review. These findings pave the way for better understanding of the mechanistic pathways associated with the development and progression of DKD and aid in the development of new treatments for this devastating disease.

PACSIN2 accelerates nephrin trafficking and is up-regulated in diabetic kidney disease

Nephrin is a core component of podocyte (glomerular epithelial cell) slit diaphragm and is required for kidney ultrafiltration. Down-regulation or mislocalization of nephrin has been observed in diabetic kidney disease (DKD), characterized by albuminuria. Here, we investigate the role of protein kinase C and casein kinase 2 substrate in neurons 2 (PACSIN2), a regulator of endocytosis and recycling, in the trafficking of nephrin and development of DKD. We observe that PACSIN2 is up-regulated and nephrin mislocalized in podocytes of obese Zucker diabetic fatty (ZDF) rats that have altered renal function. In cultured podocytes, PACSIN2 and nephrin colocalize and interact. We show that nephrin is endocytosed in PACSIN2-positive membrane regions and that PACSIN2 overexpression increases both nephrin endocytosis and recycling. We identify rabenosyn-5, which is involved in early endosome maturation and endosomal sorting, as a novel interaction partner of PACSIN2. Interestingly, rabenosyn-5 expression is increased in podocytes in obese ZDF rats, and, in vitro, its overexpression enhances the association of PACSIN2 and nephrin. We also show that palmitate, which is elevated in diabetes, enhances this association. Collectively, PACSIN2 is up-regulated and nephrin is abnormally localized in podocytes of diabetic ZDF rats. In vitro, PACSIN2 enhances nephrin turnover apparently via a mechanism involving rabenosyn-5. The data suggest that elevated PACSIN2 expression accelerates nephrin trafficking and associates with albuminuria.—Dumont, V., Tolvanen, T. A., Kuusela, S., Wang, H., Nyman, T. A., Lindfors, S., Tienari, J., Nisen, H., Suetsugu, S., Plomann, M., Kawachi, H., Lehtonen, S. PACSIN2 accelerates nephrin trafficking and is up-regulated in diabetic kidney disease.