Association of CD2AP with dynamic actin on vesicles in podocytes

Synaptic tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer's disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.

Up-regulation of APP endocytosis by neuronal aging drives amyloid dependent-synapse loss

Neuronal aging increases the risk of late-onset Alzheimer's disease. During normal aging, synapses decline, and β-amyloid (Aβ) accumulates intraneuronally. However, little is known about the underlying cell biological mechanisms. We studied normal neuronal aging using normal aged brain and aged mouse primary neurons that accumulate lysosomal lipofuscin and show synapse loss. We identify the up-regulation of amyloid precursor protein (APP) endocytosis as a neuronal aging mechanism that potentiates APP processing and Aβ production in vitro and in vivo. The increased APP endocytosis may contribute to the observed early endosomes enlargement in the aged brain. Mechanistically, we show that clathrin-dependent APP endocytosis requires F-actin and that clathrin and endocytic F-actin increase with neuronal aging. Finally, Aβ production inhibition reverts synaptic decline in aged neurons while Aβ accumulation, promoted by endocytosis up-regulation in younger neurons, recapitulates aging-related synapse decline. Overall, we identify APP endocytosis up-regulation as a potential mechanism of neuronal aging and, thus, a novel target to prevent late-onset Alzheimer's disease.

Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease characterized by progressive memory loss. Although AD neuropathological hallmarks are extracellular amyloid plaques and intracellular tau tangles, the best correlate of disease progression is synapse loss. What causes synapse loss has been the focus of several researchers in the AD field. Synapses become dysfunctional before plaques and tangles form. Studies based on early-onset familial AD (eFAD) models have supported that synaptic transmission is depressed by β-amyloid (Aβ) triggered mechanisms. Since eFAD is rare, affecting only 1% of patients, research has shifted to the study of the most common late-onset AD (LOAD). Intracellular trafficking has emerged as one of the pathways of LOAD genes. Few studies have assessed the impact of trafficking LOAD genes on synapse dysfunction. Since endocytic traffic is essential for synaptic function, we reviewed Aβ-dependent and independent mechanisms of the earliest synaptic dysfunction in AD. We have focused on the role of intraneuronal and secreted Aβ oligomers, highlighting the dysfunction of endocytic trafficking as an Aβ-dependent mechanism of synapse dysfunction in AD. Here, we reviewed the LOAD trafficking genes APOE4, ABCA7, BIN1, CD2AP, PICALM, EPH1A, and SORL1, for which there is a synaptic link. We conclude that in eFAD and LOAD, the earliest synaptic dysfunctions are characterized by disruptions of the presynaptic vesicle exo- and endocytosis and of postsynaptic glutamate receptor endocytosis. While in eFAD synapse dysfunction seems to be triggered by Aβ, in LOAD, there might be a direct synaptic disruption by LOAD trafficking genes. To identify promising therapeutic targets and biomarkers of the earliest synaptic dysfunction in AD, it will be necessary to join efforts in further dissecting the mechanisms used by Aβ and by LOAD genes to disrupt synapses.

LIM and SH3 protein 1 (LASP-1): A novel link between the slit membrane and actin cytoskeleton dynamics in podocytes

The foot processes of podocytes exhibit a dynamic actin cytoskeleton, which maintains their complex cell structure and antagonizes the elastic forces of the glomerular capillary. Interdigitating secondary foot processes form a highly selective filter for proteins in the kidney, the slit membrane. Knockdown of slit membrane components such as Nephrin or Neph1 and cytoskeletal adaptor proteins such as CD2AP in mice leads to breakdown of the filtration barrier with foot process effacement, proteinuria, and early death of the mice. Less is known about the crosstalk between the slit membrane-associated proteins and cytoskeletal components inside the podocyte foot processes. Our study shows that LASP-1, an actin-binding protein, is highly expressed in podocytes. Electron microscopy studies demonstrate that LASP-1 is found at the slit membrane suggesting a role in anchoring slit membrane components to the actin cytoskeleton. Live cell imaging experiments with transfected podocytes reveal that LASP-1 is either part of a highly dynamic granular complex or a static, actin cytoskeleton-bound protein. We identify CD2AP as a novel LASP-1 binding partner that regulates its association with the actin cytoskeleton. Activation of the renin-angiotensin-aldosterone system, which is crucial for podocyte function, leads to phosphorylation and altered localization of LASP-1. In vivo studies using the Drosophila nephrocyte model indicate that Lasp is necessary for the slit membrane integrity and functional filtration.

cindr, the Drosophila Homolog of the CD2AP Alzheimer's Disease Risk Gene, Is Required for Synaptic Transmission and Proteostasis

The Alzheimer's disease (AD) susceptibility gene, CD2-associated protein (CD2AP), encodes an actin binding adaptor protein, but its function in the nervous system is largely unknown. Loss of the Drosophila ortholog cindr enhances neurotoxicity of human Tau, which forms neurofibrillary tangle pathology in AD. We show that Cindr is expressed in neurons and present at synaptic terminals. cindr mutants show impairments in synapse maturation and both synaptic vesicle recycling and release. Cindr associates and genetically interacts with 14-3-3ζ, regulates the ubiquitin-proteasome system, and affects turnover of Synapsin and the plasma membrane calcium ATPase (PMCA). Loss of cindr elevates PMCA levels and reduces cytosolic calcium. Studies of Cd2ap null mice support a conserved role in synaptic proteostasis, and CD2AP protein levels are inversely related to Synapsin abundance in human postmortem brains. Our results reveal CD2AP neuronal requirements with relevance to AD susceptibility, including for proteostasis, calcium handling, and synaptic structure and function.

TBC1D8B Loss-of-Function Mutations Lead to X-Linked Nephrotic Syndrome via Defective Trafficking Pathways

Steroid-resistant nephrotic syndrome (SRNS) is characterized by high-range proteinuria and most often focal and segmental glomerulosclerosis (FSGS). Identification of mutations in genes causing SRNS has improved our understanding of disease mechanisms and highlighted defects in the podocyte, a highly specialized glomerular epithelial cell, as major factors in disease pathogenesis. By exome sequencing, we identified missense mutations in TBC1D8B in two families with an X-linked early-onset SRNS with FSGS. TBC1D8B is an uncharacterized Rab-GTPase-activating protein likely involved in endocytic and recycling pathways. Immunofluorescence studies revealed TBC1D8B presence in human glomeruli, and affected individual podocytes displayed architectural changes associated with migration defects commonly found in FSGS. In zebrafish we demonstrated that both knockdown and knockout of the unique TBC1D8B ortholog-induced proteinuria and that this phenotype was rescued by human TBC1D8B mRNA injection, but not by either of the two mutated mRNAs. We also showed an interaction between TBC1D8B and Rab11b, a key protein in vesicular recycling in cells. Interestingly, both internalization and recycling processes were dramatically decreased in affected individuals' podocytes and fibroblasts, confirming the crucial role of TBC1D8B in the cellular recycling processes, probably as a Rab11b GTPase-activating protein. Altogether, these results confirmed that pathogenic variations in TBC1D8B are involved in X-linked podocytopathy and points to alterations in recycling processes as a mechanism of SRNS.

CD2-Associated Protein Contributes to Hepatitis C, Virus Propagation and Steatosis by Disrupting Insulin Signaling

Chronic hepatitis C virus (HCV) infection can result in steatosis, a condition displaying aberrant accumulation of neutral lipid vesicles, the component of lipid droplets (LDs), which are essential for HCV assembly. However, the interplay between HCV infection and steatosis remains unclear. Here, we show that HCV-infected cells have higher levels of CD2-associated protein (CD2AP), which plays two distinct, yet tightly linked, roles in HCV pathogenesis: Elevated CD2AP binds to nonstructural protein 5A (NS5A) and participates in the transport of NS5A to LDs to facilitate viral assembly; Up-regulated CD2AP also interacts with casitas B-lineage lymphoma (b) (Cbl/Cbl-b) E3 ligases to degrade insulin receptor substrate 1 (IRS1), which, in turn, disrupts insulin signaling and increases LD accumulation through the IRS1/protein kinase B (Akt)/adenosine monophosphate-activated protein kinase (AMPK)/hormone-sensitive lipase (HSL) signaling axis to accommodate viral assembly. In the HCV-infected mouse model, CD2AP expression is up-regulated during the chronic infection stage and this up-regulation correlates well with liver steatosis. Importantly, CD2AP up-regulation was also detected in HCV-infected human liver biopsies showing steatosis compared to non-HCV-infected controls. Conclusion: CD2AP is indicated as a protein up-regulated by HCV infection, which, in turn, stimulates HCV propagation and steatosis by disrupting insulin signaling; targeting CD2AP may offer an opportunity for alleviating HCV infection and its associated liver pathology. (Hepatology 2018;XX:XXX-XXX.).

Endocytic Trafficking at the Mature Podocyte Slit Diaphragm

Endocytic trafficking couples cell signaling with the cytoskeletal dynamics by organizing a crosstalk between protein networks in different subcellular compartments. Proteins residing in the plasma membrane are internalized and transported as cargo in endocytic vesicles (i.e., endocytosis). Subsequently, cargo proteins can be delivered to lysosomes for degradation or recycled back to the plasma membrane. The slit diaphragm is a modified tight junction connecting foot processes of the glomerular epithelial cells, podocytes. Signaling at the slit diaphragm plays a critical role in the kidney while its dysfunction leads to glomerular protein loss (proteinuria), manifesting as nephrotic syndrome, a rare condition with an estimated incidence of 2-4 new cases per 100,000 each year. Relatively little is known about the role of endocytic trafficking in podocyte signaling and maintenance of the slit diaphragm integrity. This review will focus on the role of endocytic trafficking at the mature podocyte slit diaphragm.

The Adaptor Protein CD2AP Is a Coordinator of Neurotrophin Signaling-Mediated Axon Arbor Plasticity

Growth of intact axons of noninjured neurons, often termed collateral sprouting, contributes to both adaptive and pathological plasticity in the adult nervous system, but the intracellular factors controlling this growth are largely unknown. An automated functional assay of genes regulated in sensory neurons from the rat in vivo spared dermatome model of collateral sprouting identified the adaptor protein CD2-associated protein (CD2AP; human CMS) as a positive regulator of axon growth. In non-neuronal cells, CD2AP, like other adaptor proteins, functions to selectively control the spatial/temporal assembly of multiprotein complexes that transmit intracellular signals. Although CD2AP polymorphisms are associated with increased risk of late-onset Alzheimer's disease, its role in axon growth is unknown. Assessments of neurite arbor structure in vitro revealed CD2AP overexpression, and siRNA-mediated knockdown, modulated (1) neurite length, (2) neurite complexity, and (3) growth cone filopodia number, in accordance with CD2AP expression levels. We show, for the first time, that CD2AP forms a novel multiprotein complex with the NGF receptor TrkA and the PI3K regulatory subunit p85, with the degree of TrkA:p85 association positively regulated by CD2AP levels. CD2AP also regulates NGF signaling through AKT, but not ERK, and regulates long-range signaling though TrkA(+)/RAB5(+) signaling endosomes. CD2AP mRNA and protein levels were increased in neurons during collateral sprouting but decreased following injury, suggesting that, although typically considered together, these two adult axonal growth processes are fundamentally different. These data position CD2AP as a major intracellular signaling molecule coordinating NGF signaling to regulate collateral sprouting and structural plasticity of intact adult axons.

SIGNIFICANCE STATEMENT

Growth of noninjured axons in the adult nervous system contributes to adaptive and maladaptive plasticity, and dysfunction of this process may contribute to neurologic pathologies. Functional screening of genes regulated during growth of noninjured axons revealed CD2AP as a positive regulator of axon outgrowth. A novel association of CD2AP with TrkA and p85 suggests a distinct intracellular signaling pathway regulating growth of noninjured axons. This may also represent a novel mechanism of generating specificity in multifunctional NGF signaling. Divergent regulation of CD2AP in different axon growth conditions suggests that separate mechanisms exist for different modes of axon growth. CD2AP is the first signaling molecule associated with adult sensory axonal collateral sprouting, and this association may offer new insights for NGF/TrkA-related Alzheimer's disease mechanisms.

Podocyte-actin dynamics in health and disease

Genetic studies of hereditary forms of nephrotic syndrome have identified several proteins that are involved in regulating the permselective properties of the glomerular filtration system. Further extensive research has elucidated the complex molecular basis of the glomerular filtration barrier and clearly established the pivotal role of podocytes in the pathophysiology of glomerular diseases. Podocyte architecture is centred on focal adhesions and slit diaphragms - multiprotein signalling hubs that regulate cell morphology and function. A highly interconnected actin cytoskeleton enables podocytes to adapt in order to accommodate environmental changes and maintain an intact glomerular filtration barrier. Actin-based endocytosis has now emerged as a regulator of podocyte integrity, providing an impetus for understanding the precise mechanisms that underlie the steady-state control of focal adhesion and slit diaphragm components. This Review outlines the role of actin dynamics and endocytosis in podocyte biology, and discusses how molecular heterogeneity in glomerular disorders could be exploited to deliver more rational therapeutic interventions, paving the way for targeted medicine in nephrology.

The adaptor protein Cindr regulates JNK activity to maintain epithelial sheet integrity

Epithelia are essential barrier tissues that must be appropriately maintained for their correct function. To achieve this a plethora of protein interactions regulate epithelial cell number, structure and adhesion, and differentiation. Here we show that Cindr (the Drosophila Cin85 and Cd2ap ortholog) is required to maintain epithelial integrity. Reducing Cindr triggered cell delamination and movement. Most delaminating cells died. These behaviors were consistent with JNK activation previously associated with loss of epithelial integrity in response to ectopic oncogene activity. We confirmed a novel interaction between Cindr and Drosophila JNK (dJNK), which when perturbed caused inappropriate JNK signaling. Genetically reducing JNK signaling activity suppressed the effects of reducing Cindr. Furthermore, ectopic JNK signaling phenocopied loss of Cindr and was partially rescued by concomitant cindr over-expression. Thus, correct Cindr-dJNK stoichiometry is essential to maintain epithelial integrity and disturbing this balance may contribute to the pathogenesis of disease states, including cancer.

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

The adapter protein CD2-associated protein (CD2AP) functions in various signaling and vesicle trafficking pathways, including endosomal sorting and/or trafficking and degradation pathways. Here, we investigated the role of CD2AP in insulin-dependent glucose transporter 4 (Glut4, also known as SLC2A4) trafficking and glucose uptake. Glucose uptake was attenuated in CD2AP(-/-) podocytes compared with wild-type podocytes in the basal state, and CD2AP(-/-) podocytes failed to increase glucose uptake in response to insulin. Live-cell imaging revealed dynamic trafficking of HA-Glut4-GFP in wild-type podocytes, whereas in CD2AP(-/-) podocytes, HA-Glut4-GFP clustered perinuclearly. In subcellular membrane fractionations, CD2AP co-fractionated with Glut4, IRAP (also known as LNPEP) and sortilin, constituents of Glut4 storage vesicles (GSVs). We further found that CD2AP forms a complex with GGA2, a clathrin adaptor, which sorts Glut4 to GSVs, suggesting a role for CD2AP in this process. We also found that CD2AP forms a complex with clathrin and connects clathrin to actin in the perinuclear region. Furthermore, clathrin recycling back to trans-Golgi membranes from the vesicular fraction containing GSVs was defective in the absence of CD2AP. This leads to reduced insulin-stimulated trafficking of GSVs and attenuated glucose uptake into CD2AP(-/-) podocytes.

Podocyte endocytosis in the regulation of the glomerular filtration barrier

Severe defects in the glomerular filtration barrier result in nephrotic syndrome, which is characterized by massive proteinuria. The podocyte, a specialized epithelial cell with interdigitating foot processes separated by a slit diaphragm, plays a vital role in regulating the passage of proteins from the capillary lumen to Bowman's space. Recent findings suggest a critical role for endocytosis in podocyte biology as highlighted by genetic mouse models of disease and human genetic mutations that result in the loss of the integrity of the glomerular filtration barrier. In vitro podocyte studies have also unraveled a plethora of constituents that are differentially internalized to maintain homeostasis. These observations provide a framework and impetus for understanding the precise regulation of podocyte endocytic machinery in both health and disease.

Involvement of microtubular network and its motors in productive endocytic trafficking of mouse polyomavirus

Infection of non-enveloped polyomaviruses depends on an intact microtubular network. Here we focus on mouse polyomavirus (MPyV). We show that the dynamics of MPyV cytoplasmic transport reflects the characteristics of microtubular motor-driven transport with bi-directional saltatory movements. In cells treated with microtubule-disrupting agents, localization of MPyV was significantly perturbed, the virus was retained at the cell periphery, mostly within membrane structures resembling multicaveolar complexes, and at later times post-infection, only a fraction of the virus was found in Rab7-positive endosomes and multivesicular bodies. Inhibition of cytoplasmic dynein-based motility by overexpression of dynamitin affected perinuclear translocation of the virus, delivery of virions to the ER and substantially reduced the numbers of infected cells, while overexpression of dominant-negative form of kinesin-1 or kinesin-2 had no significant impact on virus localization and infectivity. We also found that transport along microtubules was important for MPyV-containing endosome sequential acquisition of Rab5, Rab7 and Rab11 GTPases. However, in contrast to dominant-negative mutant of Rab7 (T22N), overexpression of dominant-negative mutant Rab11 (S25N) did not affect the virus infectivity. Altogether, our study revealed that MPyV cytoplasmic trafficking leading to productive infection bypasses recycling endosomes, does not require the function of kinesin-1 and kinesin-2, but depends on functional dynein-mediated transport along microtubules for translocation of the virions from peripheral, often caveolin-positive compartments to late endosomes and ER – a prerequisite for efficient delivery of the viral genome to the nucleus.

Visualization of cytoskeletal dynamics in podocytes using adenoviral vectors

Glomerular visceral epithelial cells (podocytes) play a key role in maintaining selective protein filtration in the kidney. Podocytes have a complex cell shape characterized by the presence of numerous actin-rich processes, which cover the surface of glomerular capillaries and are connected by specialized cell-cell adhesion complexes (slit diaphragms). Human genetic studies and experiments in knockout mouse models show that actin filaments and actin-associated proteins are indispensable for the maintenance of podocyte shape, slit diaphragm integrity, and normal glomerular filtration. The ability to examine cytoskeletal protein organization and dynamics in podocytes and to test the effects of disease-associated mutations on protein localization provides valuable information for researchers aiming to dissect the molecular mechanisms of podocyte dysfunction. We describe how adenovirus-mediated transduction of cultured podocytes with DNA constructs can be used to reliably introduce fluorescently tagged cytoskeletal markers for live cell imaging with high efficiency and low toxicity. This technique can be used to study the dynamic reorganization of the podocyte cytoskeleton and to test the effects of novel mutations on podocyte cytoskeletal dynamics.

CD2-associated protein regulates plasmacytoid dendritic cell migration, but is dispensable for their development and cytokine production

Plasmacytoid dendritic cells (pDCs) are a dendritic cell subset that secrete type I IFNs in response to microbial stimuli. The scaffold protein, CD2-associated protein (CD2AP), is a marker of human pDCs as it is highly expressed in this cell type. Recently, in human pDCs, decreased CD2AP expression appeared to enhance the production of type I IFNs via an inhibitory receptor-induced signaling cascade. In this study, we sought to determine the role of CD2AP in murine pDCs using CD2AP knockout (KO) mice. CD2AP was dispensable for the development of pDCs and for the upregulation of activation markers following stimulation. Loss of CD2AP expression did not affect the production of type I IFNs stimulated by TLR ligation, and only slightly impaired type I IFN production when inhibitory pathways were engaged in vitro. This was also confirmed by showing that CD2AP deficiency did not influence type I IFN production by pDCs in vivo. Because CD2AP plays a role in regulating actin dynamics, we examined the actin cytoskeleton in pDCs and found that activated CD2AP KO pDCs had significantly higher levels of actin polymerization than wild-type pDCs. Using two different inflammation models, we found that CD2AP KO pDCs have a defect in lymph node migration, correlating with the defects in actin dynamics. Our work excludes a role for CD2AP in the regulation of type I IFNs in pDCs, and suggests that the major function of CD2AP is on the actin cytoskeleton, affecting migration to local lymph nodes under conditions of inflammation.

The function of endocytosis in podocytes

PURPOSE OF REVIEW

In this review, we discuss the role of endocytosis, a fundamental process internalizing molecules from the plasma membrane, and its critical importance in podocyte biology.

RECENT FINDINGS

Endocytic clathrin and nonclathrin-coated pits have been visualized in podocytes using electron microscopy, but the functional biological relevance has not been well defined. Recent evidence suggests that loss of key clathrin endocytic regulatory apparatus, such as dynamin, synaptojanin 1 or endophilin, in genetic mouse models of disease results in severe proteinuria and foot process effacement. In addition, several genes implicated in human nephrotic syndrome directly or indirectly associate with these endocytic proteins, thus creating a protein network that is linked in actin dynamics, signalling and endocytosis.

SUMMARY

This review summarizes our current understanding of membrane trafficking specifically in podocytes, thus giving further novel insights into the molecular mechanisms and pathogenesis of nephrotic syndrome.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Role of dynamin, synaptojanin, and endophilin in podocyte foot processes

Podocytes are specialized cells that play an integral role in the renal glomerular filtration barrier via their foot processes. The foot processes form a highly organized structure, the disruption of which causes nephrotic syndrome. Interestingly, several similarities have been observed between mechanisms that govern podocyte organization and mechanisms that mediate neuronal synapse development. Dynamin, synaptojanin, and endophilin are functional partners in synaptic vesicle recycling via interconnected actions in clathrin-mediated endocytosis and actin dynamics in neurons. A role of dynamin in the maintenance of the kidney filtration barrier via an action on the actin cytoskeleton of podocytes was suggested. Here we used a conditional double-KO of dynamin 1 (Dnm1) and Dnm2 in mouse podocytes to confirm dynamin's role in podocyte foot process maintenance. In addition, we demonstrated that while synaptojanin 1 (Synj1) KO mice and endophilin 1 (Sh3gl2), endophilin 2 (Sh3gl1), and endophilin 3 (Sh3gl3) triple-KO mice had grossly normal embryonic development, these mutants failed to establish a normal filtration barrier and exhibited severe proteinuria due to abnormal podocyte foot process formation. These results strongly implicate a protein network that functions at the interface between endocytosis and actin at neuronal synapses in the formation and maintenance of the kidney glomerular filtration barrier.

Morphology and migration of podocytes are affected by CD151 levels

CD151, a member of the tetraspanin family of membrane proteins, is crucially involved in the formation of the glomerular filtration barrier in humans and mice. However, the role of CD151 in podocytes has not been investigated so far. In the present study, we utilized a conditionally immortalized mouse podocyte cell line to characterize CD151 in podocytes and to examine the consequences of manipulating CD151 expression levels. Mouse podocytes endogenously express CD151 as determined by RT-PCR and Western blotting. GFP-CD151 fusion protein localized to the cell membrane, to cell protrusions and cell-cell contacts, colocalizing with actin, β1-integrin, zonula occludens-1, and CD9. The expression of GFP-CD151 in cultured podocytes resulted in a marked increase in the presence of thin arborized protrusions (TAPs). TAPs are distinct from filopodia by increased length, protein composition, branched morphology, and slower dynamics. Furthermore, the migration rate of pEGFP-CD151-transfected podocytes was reduced in a wound assay. Fluorescence recovery after photo bleaching measurements revealed a half-time of 3 s for GFP-CD151 consistent with a high mobility of CD151 in the membrane and cytosol. CD151 knockdown in podocytes reduced β1-integrin expression and podocyte cell area, indicating diminished adherence and/or spreading. Our results indicate that CD151 importantly modulates podocyte function.

Cortactin: a multifunctional regulator of cellular invasiveness

Branched actin assembly is critical for a variety of cellular processes that underlie cell motility and invasion, including cellular protrusion formation and membrane trafficking. Activation of branched actin assembly occurs at various subcellular locations via site-specific activation of distinct WASp family proteins and the Arp2/3 complex. A key branched actin regulator that promotes cell motility and links signaling, cytoskeletal and membrane trafficking proteins is the Src kinase substrate and Arp2/3 binding protein cortactin. Due to its frequent overexpression in advanced, invasive cancers and its general role in regulating branched actin assembly at multiple cellular locations, cortactin has been the subject of intense study. Recent studies suggest that cortactin has a complex role in cellular migration and invasion, promoting both on-site actin polymerization and modulation of autocrine secretion. Diverse cellular activities may derive from the interaction of cortactin with site-specific binding partners.

Slit diaphragms contain tight junction proteins

Slit diaphragms are essential components of the glomerular filtration apparatus, as changes in these junctions are the hallmark of proteinuric diseases. Slit diaphragms, considered specialized adherens junctions, contain both unique membrane proteins (e.g., nephrin, podocin, and Neph1) and typical adherens junction proteins (e.g., P-cadherin, FAT, and catenins). Whether slit diaphragms also contain tight junction proteins is unknown. Here, immunofluorescence, immunogold labeling, and cell fractionation demonstrated that rat slit diaphragms contain the tight junction proteins JAM-A (junctional adhesion molecule A), occludin, and cingulin. We found these proteins in the same protein complexes as nephrin, podocin, CD2AP, ZO-1, and Neph1 by cosedimentation, coimmunoprecipitation, and pull-down assays. PAN nephrosis increased the protein levels of JAM-A, occludin, cingulin, and ZO-1 several-fold in glomeruli and loosened their attachment to the actin cytoskeleton. These data extend current information about the molecular composition of slit diaphragms by demonstrating the presence of tight junction proteins, although slit diaphragms lack the characteristic morphologic features of tight junctions. The contribution of these proteins to the assembly of slit diaphragms and potential signaling cascades requires further investigation.

The gastric epithelial progenitor cell niche and differentiation of the zymogenic (chief) cell lineage

In the mammalian gastrointestinal tract, the cell fate decisions that specify the development of multiple, diverse lineages are governed in large part by interactions of stem and early lineage progenitor cells with their microenvironment, or niche. Here, we show that the gastric parietal cell (PC) is a key cellular component of the previously undescribed niche for the gastric epithelial neck cell, the progenitor of the digestive enzyme secreting zymogenic (chief) cell (ZC). Genetic ablation of PCs led to failed patterning of the entire zymogenic lineage: progenitors showed premature expression of differentiated cell markers, and fully differentiated ZCs failed to develop. We developed a separate mouse model in which PCs localized not only to the progenitor niche, but also ectopically to the gastric unit base, which is normally occupied by terminally differentiated ZCs. Surprisingly, these mislocalized PCs did not maintain adjacent zymogenic lineage cells in the progenitor state, demonstrating that PCs, though necessary, are not sufficient to define the progenitor niche. We induced this PC mislocalization by knocking out the cytoskeleton-regulating gene Cd2ap in Mist1(-/-) mice, which led to aberrant E-cadherin localization in ZCs, irregular ZC-ZC junctions, and disruption of the ZC monolayer by PCs. Thus, the characteristic histology of the gastric unit, with PCs in the middle and ZCs in the base, may depend on establishment of an ordered adherens junction network in ZCs as they migrate into the base.

CD2AP and Cbl-3/Cbl-c constitute a critical checkpoint in the regulation of ret signal transduction

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are critical for nervous system development and maintenance. GFLs promote survival and growth via activation of the receptor tyrosine kinase (RTK) Ret. In sympathetic neurons, the duration of Ret signaling is governed by how rapidly Ret is degraded after its activation. In an effort to elucidate mechanisms that control the half-life of Ret, we have identified two novel Ret interactors, CD2-associated protein (CD2AP) and Cbl-3. CD2AP, an adaptor molecule involved in the internalization of ubiquitinated RTKs, is associated with Ret under basal, unstimulated conditions in neurons. After Ret activation by GDNF, CD2AP dissociates. Similarly, the E3-ligase Cbl-3 interacts with unphosphorylated Ret and dissociates from Ret after Ret activation. In contrast to their dissociation from autophosphorylated Ret, an interaction between CD2AP and Cbl-3 is induced by GDNF stimulation of sympathetic neurons, suggesting that CD2AP and Cbl-3 dissociate from Ret as a complex. In neurons, the overexpression of CD2AP enhances the degradation of Ret and inhibits GDNF-dependent survival, and gene silencing of CD2AP blocks Ret degradation and promotes GDNF-mediated survival. Surprisingly, Cbl-3 overexpression dramatically stabilizes activated Ret and enhances neuronal survival, even though Cbl-family E3 ligases normally function to trigger RTK downregulation. In combination with CD2AP, however, Cbl-3 promotes Ret degradation rapidly and almost completely blocks survival promotion by GDNF, suggesting that Cbl-3 acts as a switch that is triggered by CD2AP and oscillates between inhibition and promotion of Ret degradation. Consistent with the hypothesis, Cbl-3 silencing in neurons only inhibited Ret degradation and enhanced neuronal survival in combination with CD2AP silencing. CD2AP and Cbl-3, therefore, constitute a checkpoint that controls the extent of Ret downregulation and, thereby, the sensitivity of neurons to GFLs.

The Drosophila CD2AP/CIN85 orthologue Cindr regulates junctions and cytoskeleton dynamics during tissue patterning

Developing tissues require cells to undergo intricate processes to shift into appropriate niches. This requires a functional connection between adhesion-mediating events at the cell surface and a cytoskeletal reorganization to permit directed movement. A small number of proteins are proposed to link these processes. Here, we identify one candidate, Cindr, the sole Drosophila melanogaster member of the CD2AP/CIN85 family (this family has been previously implicated in a variety of processes). Using D. melanogaster retina, we demonstrate that Cindr links cell surface junctions (E-cadherin) and adhesion (Roughest) with multiple components of the actin cytoskeleton. Reducing cindr activity leads to defects in local cell movement and, consequently, tissue patterning and cell death. Cindr activity is required for normal localization of Drosophila E-cadherin and Roughest, and we show additional physical and functional links to multiple components of the actin cytoskeleton, including the actin-capping proteins capping protein alpha and capping protein beta. Together, these data demonstrate that Cindr is involved in dynamic cell rearrangement in an emerging epithelium.

CD2-associated protein is widely expressed and differentially regulated during embryonic development

CD2-associated protein (CD2AP) is an adapter protein that is involved in various signaling and vesicular trafficking processes and also functions as a linker between plasma membrane proteins and the actin cytoskeleton. The protein is known to have important functions in T cells and glomerular podocytes, but it is also expressed by many other adult-type tissues and cells. Here we analyzed the expression of the protein during early embryonic development and organogenesis of the mouse. The results showed differential tissue-specific regulation of CD2AP in developing and maturing organs. In oocytes and pre-implantation embryos, CD2AP was located diffusely in the cytoplasm, whereas in late blastocysts it was concentrated to the intercellular contacts. During organogenesis, CD2AP was distinctly upregulated upon, e.g., the pretubular aggregation of metanephric mesenchyme cells and the appearance of the osteoblastic rim around cartilages during endochondral ossification. High CD2AP expression was also observed during epithelial-like conversion of some highly specialized secretory cell types such as the odontoblasts, the cells of the choroid plexus and the decidualized cells of the endometrial stroma. In other instances, such as the development of the proximal tubuli of the kidney and the flat alveolar epithelium of the lung, the protein was downregulated upon differentiation and maturation of the cells. Finally, certain cells, e.g., glomerular podocytes, those forming the collecting ducts of the kidney, and the urothelium of the kidney pelvis, expressed CD2AP throughout their differentiation and maturation. Multiple molecules and complex pathways regulate embryogenesis, and scaffolding proteins apparently have pivotal roles in targeting and finetuning, e.g., growth factor- or hormone-induced processes. The cell-type specific spatio-temporal regulation of CD2AP during development suggests that this adapter protein is a key regulatory partner in many signaling pathways and cellular processes governing differentiation and morphogenesis.

Evidence for restriction of fluid and solute movement across the glomerular capillary wall by the subpodocyte space

The glomerular filtration barrier (GFB) is generally considered to consist of three layers: fenestrated glomerular endothelium, glomerular basement membrane, and filtration slits between adjacent podocyte foot processes. Detailed anatomic examination of the GFB has revealed a novel abluminal structure, the subpodocyte space (SPS), identified as the labyrinthine space between the underside of podocyte cell body/primary processes and the foot processes. The SPS covers 50–65% of the filtration surface of the GFB, indicating that SPS may influence glomerular permeability. We have examined the contribution of the SPS to the permeability characteristics of the GFB using multiphoton microscopy techniques in isolated, perfused glomeruli and in the intact kidney in vivo. SPS were identified using this technique, with comparable dimensions to SPS examined with electron microscopy. The passage of the intermediate-weight molecule rhodamine-conjugated 10-kDa dextran, but not the low-weight molecule lucifer yellow (≈450 Da), accumulated in SPS-covered regions of the GFB, compared with GFB regions not covered by SPS (“naked regions”). Net lucifer yellow flux (taken to indicate fluid flux) through identifiable SPS regions was calculated to be 66–75% of that occurring through naked regions. These observations indicate both ultrafiltration and hydraulic resistance imparted by the SPS, demonstrating the potential physiological contribution of the SPS to glomerular permeability.

Podocin participates in the assembly of tight junctions between foot processes in nephrotic podocytes

The predominant type of cellular junction between normal podocyte foot processes is the slit diaphragm. Under nephrotic conditions,however, foot process effacement leads to the loss of slit diaphragms and the new formationof tight junctions composed of the proteins coxsackievirus and adenovirus receptor (CAR) and zonula occludens 1 (ZO-1). Podocin, a protein that plays a key role in maintaining the integrity of the slit diaphragm, has also been localized to these tight junctions, but its function at this site is unknown. In this study, we confirmed that podocin colocalizes with CAR and ZO-1 at the tight junction between foot processes in nephrotic rats. Using primary cultures of rat podocytes, as well as cell lines that co-expressed podocin and CAR, we observed that podocin was recruited to sites of cell-cell contact and that it co-localized with CAR and ZO-1. Immunoprecipitation suggested that these three junctional proteins from a multi-protein complex. Consistent with this, we found that podociin facilitated the coalescence of preassembled lipid rafts containing CAR and restricted their lateral mobility, the latter likely a result of dynamic actin reorganization and subsequent tethering of CAR-podocin complexes to the cytoskeleton. In conclusion, in addition to serving as a structural protein of the slit diaphragm of normal podocytes, our data suggest that podocin may also serve as a scaffold that links tight junction proteins to the actin cytoskeleton in nephrotic foot processes.

Structure and function analysis of the CMS/CIN85 protein family identifies actin-bundling properties and heterotypic-complex formation

Members of the CMS/CIN85 protein family participate in clathrin-mediated endocytosis and play a crucial role in maintaining the kidney filtration barrier. The CMS protein structure includes three Src homology 3 (SH3) domains and a proline-rich (PR) region that is connected by a `linker' sequence to a coiled-coil (CC) domain. We show that CMS is a component of special actin-rich adhesion structures – podosomes – and demonstrate specific actin-binding properties of CMS. We have found that the entire C-terminal half of CMS is necessary for efficient binding to filamentous actin (F-actin). CMS and CIN85 can crosslink F-actin into bundles, a function that depends on the PR region and the CC domain. Removal of these domains reduces migration. CMS can also form heterotypic complexes with CIN85. CIN85 is expressed as multiple isoforms that share the CC domain, suggesting that heterotypic interactions with CMS provides a mechanism to regulate CMS binding to F-actin and thus for modulating dynamic rearrangements of the cytoskeleton.

Early endosomes associated with dynamic F-actin structures are required for late trafficking of H. pylori VacA toxin

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are endocytosed by a clathrin- independent pathway into vesicles named GPI-AP–enriched early endosomal compartments (GEECs). We recently showed that the vacuolating toxin VacA secreted by Helicobacter pylori is endocytosed into the GEECs (Gauthier, N.C., P. Monzo, V. Kaddai, A. Doye, V. Ricci, and P. Boquet. 2005. Mol. Biol. Cell. 16:4852–4866). Unlike GPI-APs that are mostly recycled back to the plasma membrane, VacA reaches early endosomes (EEs) and then late endosomes (LEs), where vacuolation occurs.

In this study, we used VacA to study the trafficking pathway between GEECs and LEs. We found that VacA routing from GEECs to LEs required polymerized actin. During this trafficking, VacA was transferred from GEECs to EEs associated with polymerized actin structures. The CD2-associated protein (CD2AP), a docking protein implicated in intracellular trafficking, bridged the filamentous actin (F-actin) structures with EEs containing VacA. CD2AP regulated those F-actin structures and was required to transfer VacA from GEECs to LEs. These results demonstrate that sorting from GEECs to LEs requires dynamic F-actin structures on EEs.

Synaptic vesicle protein 2B is expressed in podocyte, and its expression is altered in proteinuric glomeruli

Synaptic vesicle protein 2B (SV2B) was identified by the subtraction hybridization technique as a molecule of which mRNA expression was decreased in puromycin aminonucleoside (PAN) nephropathy by glomerular cDNA subtraction assay. The expression of SV2B was detected in glomerular lysate with Western blot analysis. Dual-labeling immunofluorescence studies with glomerular cell markers demonstrated that SV2B is expressed in glomerular visceral epithelial cells (podocytes). The expression of SV2B is detected also in cultured podocyte and in human kidney section as podocytic pattern. The decrease of SV2B mRNA was already detected before the onset of proteinuria in PAN nephropathy. The mRNA expression of SV2B clearly is altered not only in PAN nephropathy but also in another proteinuric state that is caused by an antibody against nephrin, a functional molecule of the slit diaphragm. The decreased intensity in SV2B staining was already detected before the peak of proteinuria in both models with immunofluorescence study. A reduced amount of SV2B was detected in both models also with Western blot analysis. CD2AP, another functional molecule of the slit diaphragm, was observed in cytoplasm, including the processes area of the cultured podocyte, and when the podocyte was treated with small interfering RNA for SV2B, CD2AP staining at the process area was not detected. These results suggest that SV2B is a functional molecule of podocyte, and SV2B may play a role in the expression and proper localization of CD2AP.

Clinical impact of research on the podocyte slit diaphragm

This Review summarizes recent research on the podocyte slit diaphragm. A growing number of molecules that function at the slit diaphragm have been identified in patients with inherited and sporadic nephrotic syndromes. Genetic deletion of nearly all of these molecules results in proteinuria and effacement of foot processes. Nephrin, Neph1 and podocin seem to form a multifunctional receptor complex at the slit diaphragm. Most of the other components of the slit diaphragm interact directly with this complex, in many cases coupling slit diaphragm components to the podocyte's actin cytoskeleton. These molecular findings are being applied to patients with glomerular disease. Over the next decade, these data might help to improve disease classification and prediction of which patients will respond to immunosuppressive treatment.

Stretch, tension and adhesion – Adaptive mechanisms of the actin cytoskeleton in podocytes

Podocytes form an epithelial layer on the outer aspect of the basement membrane of glomerular capillaries. The interdigitating pattern of podocyte foot processes (PFPs) generates a unique and extremely long cell-cell contact area - the filtration slit. Thus, the interdigitating PFPs are the morphological basis for the high hydraulic conductivity of the glomerular capillaries. Any disturbance in this interdigitating pattern results in a drop of glomerular filtration rate impairing renal function. PFPs are based on the actin cytoskeleton, consisting of a subplasmalemmal network and a central core of filament bundles. Besides giving PFPs their morphology, the actin cytoskeleton anchors cell-cell contact and cell-matrix proteins in podocytes. Several human genetic diseases as well as transgenic mouse models provide evidence for the crucial role of the actin cytoskeleton in podocytes. Varying flow rates of the filtrate, increased glomerular capillary pressure in glomerular hypertension, and varying activation states of contractile proteins in PFPs impose a mechanical load on the actin cytoskeleton, challenging the intricate arrangement of PFPs and podocyte adhesion. Here we review data about the actin cytoskeleton of podocytes and the response of podocytes to mechanical load. From these data possible mechanisms are emerging how the actin cytoskeleton may allow podocytes to adapt to states of increased mechanical load.

Imaging podocyte dynamics

Cytoskeletal dynamics play important roles during normal podocyte development, in maintenance of the healthy glomerular filter, and in glomerular pathology. During the past few years, essential progress has been made in understanding the molecular mechanisms that govern podocyte cytoskeletal dynamics, nevertheless this knowledge remains incomplete. Examination of the podocyte cytoskeleton presents unique challenges to the experimentalist who is faced with attempting to model a morphologically complex cell that is situated in a complex microenvironment. This review will summarize recent progress in understanding cytoskeletal dynamics in the podocyte and will review modern imaging techniques relevant to studying this area of podocyte biology.