CD2AP links cortactin and capping protein at the cell periphery to facilitate formation of lamellipodia

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.

Endosomal actin branching, fission, and receptor recycling require FCHSD2 recruitment by MICAL-L1

Endosome fission is required for the release of carrier vesicles and the recycling of receptors to the plasma membrane. Early events in endosome budding and fission rely on actin branching to constrict the endosomal membrane, ultimately leading to nucleotide hydrolysis and enzymatic fission. However, our current understanding of this process is limited, particularly regarding the coordination between the early and late steps of endosomal fission. Here we have identified a novel interaction between the endosomal scaffolding protein, MICAL-L1, and the human homologue of the Drosophila Nervous Wreck (Nwk) protein, FCH and double SH3 domains protein 2 (FCHSD2). We demonstrate that MICAL-L1 recruits FCHSD2 to the endosomal membrane, where it is required for ARP2/3-mediated generation of branched actin, endosome fission and receptor recycling to the plasma membrane. Because MICAL-L1 first recruits FCHSD2 to the endosomal membrane, and is subsequently responsible for recruitment of the ATPase and fission protein EHD1 to endosomes, our findings support a model in which MICAL-L1 orchestrates endosomal fission by connecting between the early actin-driven and subsequent nucleotide hydrolysis steps of the process.

Human cytomegalovirus pUL135 protein affects endothelial cell function via CD2AP in Kawasaki disease

BACKGROUND

Kawasaki disease (KD) is a kind of pediatric vasculitis, whose pathogenesis has not been elucidated until now. Many scholars believe that KD is one type of infectious diseases in the susceptible groups. However, no recognized pathogens are confirmed. Human cytomegalovirus (HCMV) is a ubiquitous human herpes virus, which can infect varieties of cells including endothelial cells. Studies reported that the viral protein pUL135 is very important for virus replication, reactivation and immune escape. Therefore, we hypothesize that HCMV pUL135 may have a pathogenic effect on KD.

METHODS

We first determined pUL135 levels in the serum from KD patients. Next, we examined the effects and mechanisms of pUL135 on endothelial cell proliferation and migration. Finally, we assessed the effect of pUL135 on cardiac inflammation in a KD murine model.

RESULTS

Data showed that pUL135 level was significantly increased in the serum from KD patients compared with the healthy and fever controls. And pUL135 expression in endothelial cells remarkably inhibited cell proliferation, migration and tube formation. Moreover, expression of pUL135 obviously affected actin cytoskeleton. Mechanism investigation substantiated that pUL135 mediated endothelial cell dysfunction via regulating CD2AP. Ultimately, we found that HCMV pUL135 aggravated coronary arteritis in the Candida albicans cell wall extracts (CAWS)-induced KD mouse model.

CONCLUSION

Our findings imply that HCMV pUL135-mediated endothelial dysfunction plays an important role in exacerbating coronary artery injury in KD conditions.

Unveiling epithelial plasticity regulation in lung cancer: Exploring the cross-talk among Tks4 scaffold protein partners

The epithelial-to-mesenchymal transition (EMT) represents a hallmark event in the evolution of lung cancer. This work aims to study a recently described EMT-regulating protein, Tks4, and to explore its potential as a prognostic biomarker in non-small cell lung cancer. In this study, we used CRISPR/Cas9 method to knockout (KO) Tks4 to study its functional roles in invadopodia formation, migration, and regulation of EMT marker expressions and we identified Tks4-interacting proteins. Tks4-KO A549 cells exhibited an EMT-like phenotype characterized by elongated morphology and increased expression of EMT markers. Furthermore, analyses of a large-scale lung cancer database and a patient-derived tissue array data revealed that the Tks4 mRNA level was decreased in more aggressive lung cancer stages. To understand the regulatory role of Tks4 in lung cancer, we performed a Tks4-interactome analysis via Tks4 immunoprecipitation-mass spectrometry on five different cell lines and identified CAPZA1 as a novel Tks4 partner protein. Thus, we propose that the absence of Tks4 leads to disruption of a connectome of multiple proteins and that the resulting undocking and likely mislocalization of signaling molecules impairs actin cytoskeleton rearrangement and activates EMT-like cell fate switches, both of which likely influence disease severity.

Endosomal actin branching, fission and receptor recycling require FCHSD2 recruitment by MICAL-L1

Endosome fission is required for the release of carrier vesicles and the recycling of receptors to the plasma membrane. Early events in endosome budding and fission rely on actin branching to constrict the endosomal membrane, ultimately leading to nucleotide hydrolysis and enzymatic fission. However, our current understanding of this process is limited, particularly regarding the coordination between the early and late steps of endosomal fission. Here we have identified a novel interaction between the endosomal scaffolding protein, MICAL-L1, and the human homolog of the Drosophila Nervous Wreck (Nwk) protein, FCH and double SH3 domains protein 2 (FCHSD2). We demonstrate that MICAL-L1 recruits FCHSD2 to the endosomal membrane, where it is required for ARP2/3-mediated generation of branched actin, endosome fission and receptor recycling to the plasma membrane. Since MICAL-L1 first recruits FCHSD2 to the endosomal membrane, and is subsequently responsible for recruitment of the ATPase and fission protein EHD1 to endosomes, our findings support a model in which MICAL-L1 orchestrates endosomal fission by connecting between the early actin-driven and subsequent nucleotide hydrolysis steps of the process.

Mechanism of actin capping protein recruitment and turnover during clathrin-mediated endocytosis

Clathrin-mediated endocytosis depends on polymerization of a branched actin network to provide force for membrane invagination. A key regulator in branched actin network formation is actin capping protein (CP), which binds to the barbed end of actin filaments to prevent the addition or loss of actin subunits. CP was thought to stochastically bind actin filaments, but recent evidence shows CP is regulated by a group of proteins containing CP-interacting (CPI) motifs. Importantly, how CPI motif proteins function together to regulate CP is poorly understood. Here, we show Aim21 and Bsp1 work synergistically to recruit CP to the endocytic actin network in budding yeast through their CPI motifs, which also allosterically modulate capping strength. In contrast, twinfilin works downstream of CP recruitment, regulating the turnover of CP through its CPI motif and a non-allosteric mechanism. Collectively, our findings reveal how three CPI motif proteins work together to regulate CP in a stepwise fashion during endocytosis.

Studying the Association of TKS4 and CD2AP Scaffold Proteins and Their Implications in the Partial Epithelial-Mesenchymal Transition (EMT) Process

Colon cancer is a leading cause of death worldwide. Identification of new molecular factors governing the invasiveness of colon cancer holds promise in developing screening and targeted therapeutic methods. The Tyrosine Kinase Substrate with four SH3 domains (TKS4) and the CD2-associated protein (CD2AP) have previously been linked to dynamic actin assembly related processes and cancer cell migration, although their co-instructive role during tumor formation remained unknown. Therefore, this study was designed to investigate the TKS4-CD2AP interaction and study the interdependent effect of TKS4/CD2AP on oncogenic events. We identified CD2AP as a novel TKS4 interacting partner via co-immunoprecipitation-mass spectrometry methods. The interaction was validated via Western blot (WB), immunocytochemistry (ICC) and proximity ligation assay (PLA). The binding motif of CD2AP was explored via peptide microarray. To uncover the possible cooperative effects of TKS4 and CD2AP in cell movement and in epithelial-mesenchymal transition (EMT), we performed gene silencing and overexpressing experiments. Our results showed that TKS4 and CD2AP form a scaffolding protein complex and that they can regulate migration and EMT-related pathways in HCT116 colon cancer cells. This is the first study demonstrating the TKS4-CD2AP protein-protein interaction in vitro, their co-localization in intact cells, and their potential interdependent effects on partial-EMT in colon cancer.

The indispensability of macrophage adaptor proteins in chronic inflammatory diseases

Adaptor proteins represent key signalling molecules involved in regulating immune responses. The host's innate immune system recognizes pathogens via various surface and intracellular receptors. Adaptor molecules are centrally involved in different receptor-mediated signalling pathways, acting as bridges between the receptors and other molecules. The presence of adaptors in major signalling pathways involved in the pathogenesis of various chronic inflammatory diseases has drawn attention toward the role of these proteins in such diseases. In this review, we summarize the importance and roles of different adaptor molecules in macrophage-mediated signalling in various chronic disease states. We highlight the mechanistic roles of adaptors and how they are involved in protein-protein interactions (PPI) via different domains to carry out signalling. Hence, we also provide insights into how targeting these adaptor proteins can be a good therapeutic strategy against various chronic inflammatory diseases.

The Rab GTPase-binding protein EHBP1L1 and its interactors CD2AP/CIN85 negatively regulate the length of primary cilia via actin remodeling

Primary cilia are organelles consisting of axonemal microtubules and plasma membranes, and they protrude from the cell surface to the extracellular region and function in signal sensing and transduction. The integrity of cilia, including the length and structure, is associated with signaling functions; however, factors involved in regulating the integrity of cilia have not been fully elucidated. Here, we showed that the Rab GTPase-binding protein EHBP1L1 and its newly identified interactors CD2AP and CIN85, known as adaptor proteins of actin regulators, are involved in ciliary length control. Immunofluorescence microscopy showed that EHBP1L1 and CD2AP/CIN85 are localized to the ciliary sheath. EHBP1L1 depletion caused mislocalization of CD2AP/CIN85, suggesting that CD2AP/CIN85 localization to the ciliary sheath is dependent on EHBP1L1. Additionally, we determined that EHBP1L1- and CD2AP/CIN85-depleted cells had elongated cilia. The aberrantly elongated cilia phenotype and the ciliary localization defect of CD2AP/CIN85 in EHBP1L1-depleted cells were rescued by the expression of WT EHBP1L1, although this was not observed in the CD2AP/CIN85-binding-deficient mutant, indicating that the EHBP1L1-CD2AP/CIN85 interaction is crucial for controlling ciliary length. Furthermore, EHBP1L1- and CD2AP/CIN85-depleted cells exhibited actin nucleation and branching defects around the ciliary base. Taken together, our data demonstrate that the EHBP1L1-CD2AP/CIN85 axis negatively regulates ciliary length via actin network remodeling around the basal body.

Cortactin in Lung Cell Function and Disease

Cortactin (CTTN) is an actin-binding and cytoskeletal protein that is found in abundance in the cell cortex and other peripheral structures of most cell types. It was initially described as a target for Src-mediated phosphorylation at several tyrosine sites within CTTN, and post-translational modifications at these tyrosine sites are a primary regulator of its function. CTTN participates in multiple cellular functions that require cytoskeletal rearrangement, including lamellipodia formation, cell migration, invasion, and various other processes dependent upon the cell type involved. The role of CTTN in vascular endothelial cells is particularly important for promoting barrier integrity and inhibiting vascular permeability and tissue edema. To mediate its functional effects, CTTN undergoes multiple post-translational modifications and interacts with numerous other proteins to alter cytoskeletal structures and signaling mechanisms. In the present review, we briefly describe CTTN structure, post-translational modifications, and protein binding partners and then focus on its role in regulating cellular processes and well-established functional mechanisms, primarily in vascular endothelial cells and disease models. We then provide insights into how CTTN function affects the pathophysiology of multiple lung disorders, including acute lung injury syndromes, COPD, and asthma.

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.

Hexagonal patterning of the Drosophila eye

A complex network of transcription factor interactions propagates across the larval eye disc to establish columns of evenly-spaced R8 precursor cells, the founding cells of Drosophila ommatidia. After the recruitment of additional photoreceptors to each ommatidium, the surrounding cells are organized into their stereotypical pattern during pupal development. These support cells - comprised of pigment and cone cells - are patterned to encapsulate the photoreceptors and separate ommatidia with an hexagonal honeycomb lattice. Since the proteins and processes essential for correct eye patterning are conserved, elucidating how these function and change during Drosophila eye patterning can substantially advance our understanding of transcription factor and signaling networks, cytoskeletal structures, adhesion complexes, and the biophysical properties of complex tissues during their morphogenesis. Our understanding of many of these aspects of Drosophila eye patterning is largely descriptive. Many important questions, especially relating to the regulation and integration of cellular events, remain.

Challenges facing quantitative large-scale optical super-resolution, and some simple solutions

Optical super-resolution microscopy (SRM) has enabled biologists to visualize cellular structures with near-molecular resolution, giving unprecedented access to details about the amounts, sizes, and spatial distributions of macromolecules in the cell. Precisely quantifying these molecular details requires large datasets of high-quality, reproducible SRM images. In this review, we discuss the unique set of challenges facing quantitative SRM, giving particular attention to the shortcomings of conventional specimen preparation techniques and the necessity for optimal labeling of molecular targets. We further discuss the obstacles to scaling SRM methods, such as lengthy image acquisition and complex SRM data analysis. For each of these challenges, we review the recent advances in the field that circumvent these pitfalls and provide practical advice to biologists for optimizing SRM experiments.

Twinfilin uncaps filament barbed ends to promote turnover of lamellipodial actin networks

Coordinated polymerization of actin filaments provides force for cell migration, morphogenesis and endocytosis. Capping protein (CP) is a central regulator of actin dynamics in all eukaryotes. It binds to actin filament (F-actin) barbed ends with high affinity and slow dissociation kinetics to prevent filament polymerization and depolymerization. However, in cells, CP displays remarkably rapid dynamics within F-actin networks, but the underlying mechanism remains unclear. Here, we report that the conserved cytoskeletal regulator twinfilin is responsible for CP's rapid dynamics and specific localization in cells. Depletion of twinfilin led to stable association between CP and cellular F-actin arrays, as well as to its retrograde movement throughout leading-edge lamellipodia. These were accompanied by diminished F-actin turnover rates. In vitro single-filament imaging approaches revealed that twinfilin directly promotes dissociation of CP from filament barbed ends, while enabling subsequent filament depolymerization. These results uncover a bipartite mechanism that controls how actin cytoskeleton-mediated forces are generated in cells.

Lysine acetylation of cytoskeletal proteins: Emergence of an actin code

Reversible lysine acetylation of nuclear proteins such as histones is a long-established important regulatory mechanism for chromatin remodeling and transcription. In the cytoplasm, acetylation of a number of cytoskeletal proteins, including tubulin, cortactin, and the formin mDia2, regulates both cytoskeletal assembly and stability. More recently, acetylation of actin itself was revealed to regulate cytoplasmic actin polymerization through the formin INF2, with downstream effects on ER-to-mitochondrial calcium transfer, mitochondrial fission, and vesicle transport. This finding raises the possibility that actin acetylation, along with other post-translational modifications to actin, might constitute an "actin code," similar to the "histone code" or "tubulin code," controlling functional shifts to these central cellular proteins. Given the multiple roles of actin in nuclear functions, its modifications might also have important roles in gene expression.

CD2AP links actin to PI3 kinase activity to extend epithelial cell height and constrain cell area

Epithelial cells are categorized as cuboidal versus squamous based on the height of the lateral membrane. Wang and Brieher show that CD2AP links PI3K activity to actin assembly to extend the height of the lateral membrane.

Maintaining the correct ratio of apical, basal, and lateral membrane domains is important for epithelial physiology. Here, we show that CD2AP is a critical determinant of epithelial membrane proportions. Depletion of CD2AP or phosphoinositide 3-kinase (PI3K) inhibition results in loss of F-actin and expansion of apical–basal domains, which comes at the expense of lateral membrane height in MDCK cells. We demonstrate that the SH3 domains of CD2AP bind to PI3K and are necessary for PI3K activity along lateral membranes and constraining cell area. Tethering the SH3 domains of CD2AP or p110γ to the membrane is sufficient to rescue CD2AP-knockdown phenotypes. CD2AP and PI3K are both upstream and downstream of actin polymerization. Since CD2AP binds to both actin filaments and PI3K, CD2AP might bridge actin assembly to PI3K activation to form a positive feedback loop to support lateral membrane extension. Our results provide insight into the squamous to cuboidal to columnar epithelial transitions seen in complex epithelial tissues in vivo.

CD2 Immunobiology

The glycoprotein CD2 is a costimulatory receptor expressed mainly on T and NK cells that binds to LFA3, a cell surface protein expressed on e.g., antigen-presenting cells. CD2 has an important role in the formation and organization of the immunological synapse that is formed between T cells and antigen-presenting cells upon cell-cell conjugation and associated intracellular signaling. CD2 expression is upregulated on memory T cells as well as activated T cells and plays an important role in activation of memory T cells despite the coexistence of several other costimulatory pathways. Anti-CD2 monoclonal antibodies have been shown to induce immune modulatory effects in vitro and clinical studies have proven the safety and efficacy of CD2-targeting biologics. Investigators have highlighted that the lack of attention to the CD2/LFA3 costimulatory pathway is a missed opportunity. Overall, CD2 is an attractive target for monoclonal antibodies intended for treatment of pathologies characterized by undesired T cell activation and offers an avenue to more selectively target memory T cells while favoring immune regulation.

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.

Comparative Analysis of CPI-Motif Regulation of Biochemical Functions of Actin Capping Protein

The heterodimeric actin capping protein (CP) is regulated by a set of proteins that contain CP-interacting (CPI) motifs. Outside of the CPI motif, the sequences of these proteins are unrelated and distinct. The CPI motif and surrounding sequences are conserved within a given protein family, when compared to those of other CPI-motif protein families. Using biochemical assays with purified proteins, we compared the ability of CPI-motif-containing peptides from different protein families (a) to bind to CP, (b) to allosterically inhibit barbed-end capping by CP, and (c) to allosterically inhibit interaction of CP with V-1, another regulator of CP. We found large differences in potency among the different CPI-motif-containing peptides, and the different functional assays showed different orders of potency. These biochemical differences among the CPI-motif peptides presumably reflect interactions between CP and CPI-motif peptides involving amino acid residues that are conserved but are not part of the strictly defined consensus, as it was originally identified in comparisons of sequences of CPI motifs across all protein families [Hernandez-Valladares, M., et al. (2010) Structural characterization of a capping protein interaction motif defines a family of actin filament regulators. Nat. Struct. Mol. Biol. 17, 497-503; Bruck, S., et al. (2006) Identification of a Novel Inhibitory Actin-capping Protein Binding Motif in CD2-associated Protein. J. Biol. Chem. 281, 19196-19203]. These biochemical differences may be important for conserved distinct functions of CPI-motif protein families in cells with respect to the regulation of CP activity and actin assembly near membranes.

CD2AP inhibits metastasis in gastric cancer by promoting cellular adhesion and cytoskeleton assembly

Diffuse gastric cancer (DGC) is a lethal malignancy lacking effective systemic therapy. Among the most provocative recent results in DGC has been that the alter of the cellular cytoskeleton and intercellular adhesion. CD2‐associated protein (CD2AP) is one of the critical proteins regulating cytoskeleton assembly and intercellular adhesion. However, no study has investigated the expression and biological significance of CD2AP in gastric cancer (GC) to date. Therefore, the aim of our study was to explore if the expression of CD2AP is associated with any clinical features of GC and to elucidate the underlying mechanism. Immunohistochemistry of 620 patient tissue samples indicated that the expression of CD2AP is downregulated in DGC. Moreover, a low CD2AP level was indicative of poor patient prognosis. In vitro, forced expression of CD2AP caused a significant decrease in the migration and invasion of GC cells, whereas depletion of CD2AP had the opposite effect. Immunofluorescence analysis indicated that CD2AP promoted cellular adhesion and influenced cell cytoskeleton assembly via interaction with the F‐actin capping protein CAPZA1. Overall, the upregulation of CD2AP could attenuate GC metastasis, suggesting CD2AP as a novel biomarker for the prognosis and treatment of patients with GC.

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.

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.

CD2-associated protein (CD2AP) overexpression accelerates amyloid precursor protein (APP) transfer from early endosomes to the lysosomal degradation pathway

A hallmark of Alzheimer's disease (AD) pathology is the appearance of senile plaques, which are composed of β-amyloid (Aβ) peptides. Aβ is produced by sequential cleavages of amyloid precursor protein (APP) by β- and γ-secretases. These cleavages take place in endosomes during intracellular trafficking of APP through the endocytic and recycling pathways. Genome-wide association studies have identified several risk factors for late-onset AD, one of which is CD2-associated protein (CD2AP), an adaptor molecule that regulates membrane trafficking. Although CD2AP's involvement in APP trafficking has recently been reported, how APP trafficking is regulated remains unclear. We sought to address this question by investigating the effect of CD2AP overexpression or knockdown on the intracellular APP distribution and degradation of APP in cultured COS-7 and HEK293 cells. We found that overexpression of CD2AP increases the localization of APP to Rab7-positive late endosomes, and decreases its localization to Rab5-positive early endosomes. CD2AP overexpression accelerated the onset of APP degradation without affecting its degradation rate. Furthermore, nutrient starvation increased the localization of APP to Rab7-positive late endosomes, and CD2AP overexpression stimulated starvation-induced lysosomal APP degradation. Moreover, the effect of CD2AP on the degradation of APP was confirmed by CD2AP overexpression and knockdown in primary cortical neurons from mice. We conclude that CD2AP accelerates the transfer of APP from early to late endosomes. This transfer in localization stimulates APP degradation by reducing the amount of time before degradation initiation. Taken together, these results may explain why impaired CD2AP function is a risk factor for AD.

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.

AIF1L regulates actomyosin contractility and filopodial extensions in human podocytes

Podocytes are highly-specialized epithelial cells essentially required for the generation and the maintenance of the kidney filtration barrier. This elementary function is directly based on an elaborated cytoskeletal apparatus establishing a complex network of primary and secondary processes. Here, we identify the actin-bundling protein allograft-inflammatory-inhibitor 1 like (AIF1L) as a selectively expressed podocyte protein in vivo. We describe the distinct subcellular localization of AIF1L to actin stress fibers, focal adhesion complexes and the nuclear compartment of podocytes in vitro. Genetic deletion of AIF1L in immortalized human podocytes resulted in an increased formation of filopodial extensions and decreased actomyosin contractility. By the use of SILAC based quantitative proteomics analysis we describe the podocyte specific AIF1L interactome and identify several components of the actomyosin machinery such as MYL9 and UNC45A as potential AIF1L interaction partners. Together, these findings indicate an involvement of AIF1L in the stabilization of podocyte morphology by titrating actomyosin contractility and membrane dynamics.

Impact of late-onset Alzheimer's genetic risk factors on beta-amyloid endocytic production

The increased production of the 42 aminoacids long beta-amyloid (Aβ42) peptide has been established as a causal mechanism of the familial early onset Alzheimer's disease (AD). In contrast, the causal mechanisms of the late-onset AD (LOAD), that affects most AD patients, remain to be established. Indeed, Aβ42 accumulation has been detected more than 30 years before diagnosis. Thus, the mechanisms that control Aβ accumulation in LOAD likely go awry long before pathogenesis becomes detectable. Early on, APOE4 was identified as the biggest genetic risk factor for LOAD. However, since APOE4 is not present in all LOAD patients, genome-wide association studies of thousands of LOAD patients were undertaken to identify other genetic variants that could explain the development of LOAD. PICALM, BIN1, CD2AP, SORL1, and PLD3 are now with APOE4 among the identified genes at highest risk in LOAD that have been implicated in Aβ42 production. Recent evidence indicates that the regulation of the endocytic trafficking of the amyloid precursor protein (APP) and/or its secretases to and from sorting endosomes is determinant for Aβ42 production. Thus, here, we will review the described mechanisms, whereby these genetic risk factors can contribute to the enhanced endocytic production of Aβ42. Dissecting causal LOAD mechanisms of Aβ42 accumulation, underlying the contribution of each genetic risk factor, will be required to identify therapeutic targets for novel personalized preventive strategies.

Identification and characterisation of a Theileria annulata proline-rich microtubule and SH3 domain-interacting protein (TaMISHIP) that forms a complex with CLASP1, EB1, and CD2AP at the schizont surface

Theileria annulata is an apicomplexan parasite that modifies the phenotype of its host cell completely, inducing uncontrolled proliferation, resistance to apoptosis, and increased invasiveness. The infected cell thus resembles a cancer cell, and changes to various host cell signalling pathways accompany transformation. Most of the molecular mechanisms leading to Theileria-induced immortalization of leukocytes remain unknown. The parasite dissolves the surrounding host cell membrane soon after invasion and starts interacting with host proteins, ensuring its propagation by stably associating with the host cell microtubule network. By using BioID technology together with fluorescence microscopy and co-immunoprecipitation, we identified a CLASP1/CD2AP/EB1-containing protein complex that surrounds the schizont throughout the host cell cycle and integrates bovine adaptor proteins (CIN85, 14-3-3 epsilon, and ASAP1). This complex also includes the schizont membrane protein Ta-p104 together with a novel secreted T. annulata protein (encoded by TA20980), which we term microtubule and SH3 domain-interacting protein (TaMISHIP). TaMISHIP localises to the schizont surface and contains a functional EB1-binding SxIP motif, as well as functional SH3 domain-binding Px(P/A)xPR motifs that mediate its interaction with CD2AP. Upon overexpression in non-infected bovine macrophages, TaMISHIP causes binucleation, potentially indicative of a role in cytokinesis.

Mutation of CD2AP and SH3KBP1 Binding Motif in Alphavirus nsP3 Hypervariable Domain Results in Attenuated Virus

Infection by Chikungunya virus (CHIKV) of the Old World alphaviruses (family Togaviridae) in humans can cause arthritis and arthralgia. The virus encodes four non-structural proteins (nsP) (nsP1, nsp2, nsP3 and nsP4) that act as subunits of the virus replicase. These proteins also interact with numerous host proteins and some crucial interactions are mediated by the unstructured C-terminal hypervariable domain (HVD) of nsP3. In this study, a human cell line expressing EGFP tagged with CHIKV nsP3 HVD was established. Using quantitative proteomics, it was found that CHIKV nsP3 HVD can bind cytoskeletal proteins, including CD2AP, SH3KBP1, CAPZA1, CAPZA2 and CAPZB. The interaction with CD2AP was found to be most evident; its binding site was mapped to the second SH3 ligand-like element in nsP3 HVD. Further assessment indicated that CD2AP can bind to nsP3 HVDs of many different New and Old World alphaviruses. Mutation of the short binding element hampered the ability of the virus to establish infection. The mutation also abolished ability of CD2AP to co-localise with nsP3 and replication complexes of CHIKV; the same was observed for Semliki Forest virus (SFV) harbouring a similar mutation. Similar to CD2AP, its homolog SH3KBP1 also bound the identified motif in CHIKV and SFV nsP3.

PAWS1 controls cytoskeletal dynamics and cell migration through association with the SH3 adaptor CD2AP

Our previous studies of PAWS1 (protein associated with SMAD1; also known as FAM83G) have suggested that this molecule has roles beyond BMP signalling. To investigate these roles, we have used CRISPR/Cas9 to generate PAWS1-knockout U2OS osteosarcoma cells. Here, we show that PAWS1 plays a role in the regulation of the cytoskeletal machinery, including actin and focal adhesion dynamics, and cell migration. Confocal microscopy and live cell imaging of actin in U2OS cells indicate that PAWS1 is also involved in cytoskeletal dynamics and organization. Loss of PAWS1 causes severe defects in F-actin organization and distribution as well as in lamellipodial organization, resulting in impaired cell migration. PAWS1 interacts in a dynamic fashion with the actin/cytoskeletal regulator CD2AP at lamellae, suggesting that its association with CD2AP controls actin organization and cellular migration. Genetic ablation of CD2AP from U2OS cells instigates actin and cell migration defects reminiscent of those seen in PAWS1-knockout cells.

This article has an associated First Person interview with the first authors of the paper.

Summary: PAWS1 (also known as FAM83G) controls cell migration by influencing the organization of F-actin and focal adhesions and the distribution of the actin stress fibre network through its association with CD2AP.

CARMIL family proteins as multidomain regulators of actin-based motility

CARMILs are large multidomain proteins that regulate the actin-binding activity of capping protein (CP), a major capper of actin filament barbed ends in cells. CARMILs bind directly to CP and induce a conformational change that allosterically decreases but does not abolish its actin-capping activity. The CP-binding domain of CARMIL consists of the CP-interaction (CPI) and CARMIL-specific interaction (CSI) motifs, which are arranged in tandem. Many cellular functions of CARMILs require the interaction with CP; however, a more surprising result is that the cellular function of CP in cells appears to require binding to a CARMIL or another protein with a CPI motif, suggesting that CPI-motif proteins target CP and modulate its actin-capping activity. Vertebrates have three highly conserved genes and expressed isoforms of CARMIL with distinct and overlapping localizations and functions in cells. Various domains of these CARMIL isoforms interact with plasma membranes, vimentin intermediate filaments, SH3-containing class I myosins, the dual-GEF Trio, and other adaptors and signaling molecules. These biochemical properties suggest that CARMILs play a variety of membrane-associated functions related to actin assembly and signaling. CARMIL mutations and variants have been implicated in several human diseases. We focus on roles for CARMILs in signaling in addition to their function as regulators of CP and actin.

MoCAP proteins regulated by MoArk1-mediated phosphorylation coordinate endocytosis and actin dynamics to govern development and virulence of Magnaporthe oryzae

Actin organization is a conserved cellular process that regulates the growth and development of eukaryotic cells. It also governs the virulence process of pathogenic fungi, such as the rice blast fungus Magnaporthe oryzae, with mechanisms not yet fully understood. In a previous study, we found that actin-regulating kinase MoArk1 displays conserved functions important in endocytosis and actin organization, and MoArk1 is required for maintaining the growth and full virulence of M. oryzae. To understand how MoArk1 might function, we identified capping protein homologs from M. oryzae (MoCAP) that interact with MoArk1 in vivo. MoCAP is heterodimer consisting of α and β subunits MoCapA and MoCapB. Single and double deletions of MoCAP subunits resulted in abnormal mycelial growth and conidia formation. The ΔMocap mutants also exhibited reduced appressorium penetration and invasive hyphal growth within host cells. Furthermore, the ΔMocap mutants exhibited delayed endocytosis and abnormal cytoskeleton assembly. Consistent with above findings, MoCAP proteins interacted with MoAct1, co-localized with actin during mycelial development, and participated in appressorial actin ring formation. Further analysis revealed that the S85 residue of MoCapA and the S285 residue of MoCapB were subject to phosphorylation by MoArk1 that negatively regulates MoCAP functions. Finally, the addition of exogenous phosphatidylinositol 4,5-bisphosphate (PIP₂) failed to modulate actin ring formation in ΔMocap mutants, in contrast to the wild-type strain, suggesting that MoCAP may also mediate phospholipid signaling in the regulation of the actin organization. These results together demonstrate that MoCAP proteins whose functions are regulated by MoArk1 and PIP₂ are important for endocytosis and actin dynamics that are directly linked to growth, conidiation and pathogenicity of M. oryzae.

The actin-regulating kinase MoArk1 plays a conserved function in endocytosis and actin organization and is also essential for growth and full virulence of the rice blast fungus Magnaporthe oryzae. To understand how MoArk1 functions, we identified the F-actin capping protein α (MoCapA) and β (MoCapB) subunits that interact with MoArk1. We showed that single and double deletions of MoCAPA and MoCAPB result in slowed growth, reduced conidia production, abnormal morphogenesis, and attenuated virulence. We found that ΔMocap mutants are defective in endocytosis and actin organization and that MoCAP proteins are subject to regulation by MoArk1 through protein phosphorylation. Finally, we provided evidence demonstrating that MoCAP proteins modulate actin dynamics in response to phosphatidylinositol 4,5-biphosphate (PIP₂). These combined results suggest that MoCAP proteins play an important role in endocytosis, actin organization, and virulence. Further studies of MoCAP proteins could lead to a better understanding of the connections between actin organization and host infection by M. oryzae.

Endothelial CD2AP Binds the Receptor ICAM-1 To Control Mechanosignaling, Leukocyte Adhesion, and the Route of Leukocyte Diapedesis In Vitro

Inflammation is driven by excessive transmigration (diapedesis) of leukocytes from the blood to the tissue across the endothelial cell monolayer that lines blood vessels. Leukocyte adhesion, crawling, and transmigration are regulated by clustering of the endothelial mechanosensitive receptor intercellular adhesion molecule-1 (ICAM-1). Whereas several proteins are known to promote ICAM-1 function, the molecular mechanisms that limit ICAM-1-mediated adhesion to prevent excessive leukocyte transmigration remain unknown. We identify the endothelial actin-binding protein CD2-associated protein (CD2AP) as a novel interaction partner of ICAM-1. Loss of CD2AP stimulates the dynamics of ICAM-1 clustering, which facilitates the formation of ICAM-1 complexes on the endothelial cell surface. Consequently, neutrophil adhesion is increased, but crawling is decreased. In turn, this promotes the neutrophil preference for the transcellular over the paracellular transmigration route. Mechanistically, CD2AP is required for mechanosensitive ICAM-1 downstream signaling toward activation of the PI3K, and recruitment of F-actin and of the actin-branching protein cortactin. Moreover, CD2AP is necessary for ICAM-1-induced Rac1 recruitment and activation. Mechanical force applied on ICAM-1 impairs CD2AP binding to ICAM-1, suggesting that a tension-induced negative feedback loop promotes ICAM-1-mediated neutrophil crawling and paracellular transmigration. To our knowledge, these data show for the first time that the mechanoreceptor ICAM-1 is negatively regulated by an actin-binding adaptor protein, i.e., CD2AP, to allow a balanced and spatiotemporal control of its adhesive function. CD2AP is important in kidney dysfunction that is accompanied by inflammation. Our findings provide a mechanistic basis for the role of CD2AP in inflamed vessels, identifying this adaptor protein as a potential therapeutic target.

Correlative STED and Atomic Force Microscopy on Live Astrocytes Reveals Plasticity of Cytoskeletal Structure and Membrane Physical Properties during Polarized Migration

The plasticity of the cytoskeleton architecture and membrane properties is important for the establishment of cell polarity, adhesion and migration. Here, we present a method which combines stimulated emission depletion (STED) super-resolution imaging and atomic force microscopy (AFM) to correlate cytoskeletal structural information with membrane physical properties in live astrocytes. Using STED compatible dyes for live cell imaging of the cytoskeleton, and simultaneously mapping the cell surface topology with AFM, we obtain unprecedented detail of highly organized networks of actin and microtubules in astrocytes. Combining mechanical data from AFM with optical imaging of actin and tubulin further reveals links between cytoskeleton organization and membrane properties. Using this methodology we illustrate that scratch-induced migration induces cytoskeleton remodeling. The latter is caused by a polarization of actin and microtubule elements within astroglial cell processes, which correlates strongly with changes in cell stiffness. The method opens new avenues for the dynamic probing of the membrane structural and functional plasticity of living brain cells. It is a powerful tool for providing new insights into mechanisms of cell structural remodeling during physiological or pathological processes, such as brain development or tumorigenesis.

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.

Computational spatiotemporal analysis identifies WAVE2 and cofilin as joint regulators of costimulation-mediated T cell actin dynamics

Fluorescence microscopy is one of the most important tools in cell biology research because it provides spatial and temporal information to investigate regulatory systems inside cells. This technique can generate data in the form of signal intensities at thousands of positions resolved inside individual live cells. However, given extensive cell-to-cell variation, these data cannot be readily assembled into three- or four-dimensional maps of protein concentration that can be compared across different cells and conditions. We have developed a method to enable comparison of imaging data from many cells and applied it to investigate actin dynamics in T cell activation. Antigen recognition in T cells by the T cell receptor (TCR) is amplified by engagement of the costimulatory receptor CD28. We imaged actin and eight core actin regulators to generate over a thousand movies of T cells under conditions in which CD28 was either engaged or blocked in the context of a strong TCR signal. Our computational analysis showed that the primary effect of costimulation blockade was to decrease recruitment of the activator of actin nucleation WAVE2 (Wiskott-Aldrich syndrome protein family verprolin-homologous protein 2) and the actin-severing protein cofilin to F-actin. Reconstitution of WAVE2 and cofilin activity restored the defect in actin signaling dynamics caused by costimulation blockade. Thus, we have developed and validated an approach to quantify protein distributions in time and space for the analysis of complex regulatory systems.

Kv3.3 Channels Bind Hax-1 and Arp2/3 to Assemble a Stable Local Actin Network that Regulates Channel Gating

Mutations in the Kv3.3 potassium channel (KCNC3) cause cerebellar neurodegeneration and impair auditory processing. The cytoplasmic C terminus of Kv3.3 contains a proline-rich domain conserved in proteins that activate actin nucleation through Arp2/3. We found that Kv3.3 recruits Arp2/3 to the plasma membrane, resulting in formation of a relatively stable cortical actin filament network resistant to cytochalasin D that inhibits fast barbed end actin assembly. These Kv3.3-associated actin structures are required to prevent very rapid N-type channel inactivation during short depolarizations of the plasma membrane. The effects of Kv3.3 on the actin cytoskeleton are mediated by the binding of the cytoplasmic C terminus of Kv3.3 to Hax-1, an anti-apoptotic protein that regulates actin nucleation through Arp2/3. A human Kv3.3 mutation within a conserved proline-rich domain produces channels that bind Hax-1 but are impaired in recruiting Arp2/3 to the plasma membrane, resulting in growth cones with deficient actin veils in stem cell-derived neurons.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

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.

CPI motif interaction is necessary for capping protein function in cells

Capping protein (CP) has critical roles in actin assembly in vivo and in vitro. CP binds with high affinity to the barbed end of actin filaments, blocking the addition and loss of actin subunits. Heretofore, models for actin assembly in cells generally assumed that CP is constitutively active, diffusing freely to find and cap barbed ends. However, CP can be regulated by binding of the 'capping protein interaction' (CPI) motif, found in a diverse and otherwise unrelated set of proteins that decreases, but does not abolish, the actin-capping activity of CP and promotes uncapping in biochemical experiments. Here, we report that CP localization and the ability of CP to function in cells requires interaction with a CPI-motif-containing protein. Our discovery shows that cells target and/or modulate the capping activity of CP via CPI motif interactions in order for CP to localize and function in cells.

The Alzheimer's disease risk factor CD2AP maintains blood-brain barrier integrity

CD2-associated protein (CD2AP) is a leading genetic risk factor for Alzheimer's disease, but little is known about the function of CD2AP in the brain. We studied CD2AP(-/-) mice to address this question. Because CD2AP(-/-) mice normally die by 6 weeks from nephrotic syndrome, we used mice that also express a CD2AP transgene in the kidney, but not brain, to attenuate this phenotype. CD2AP-deficient mice had no behavioral abnormalities except for mild motor and anxiety deficits in a subset of CD2AP(-/-) mice exhibiting severe nephrotic syndrome, associated with systemic illness. Pentylenetetrazol (PTZ)-induced seizures occurred with shorter latency in CD2AP(-/-) mice, but characteristics of these seizures on electroencephalography were not altered. As CD2AP is expressed in brain-adjacent endothelial cells, we hypothesized that the shorter latency to seizures without detectably different seizure characteristics may be due to increased penetration of PTZ related to compromised blood-brain barrier integrity. Using sodium fluorescein extravasation, we found that CD2AP(-/-) mice had reduced blood-brain barrier integrity. Neither seizure severity nor blood-brain barrier integrity was correlated with nephrotic syndrome, indicating that these effects are dissociable from the systemic illness associated with CD2AP deficiency. Confirming this dissociation, wild-type mice with induced nephrotic syndrome maintained an intact blood-brain barrier. Taken together, our results support a role of CD2AP in mediating blood-brain barrier integrity and suggest that cerebrovascular roles of CD2AP could contribute to its effects on Alzheimer's disease risk.

Epithelial junctions and Rho family GTPases: the zonular signalosome

The establishment and maintenance of epithelial cell-cell junctions is crucially important to regulate adhesion, apico-basal polarity and motility of epithelial cells, and ultimately controls the architecture and physiology of epithelial organs. Junctions are supported, shaped and regulated by cytoskeletal filaments, whose dynamic organization and contractility are finely tuned by GTPases of the Rho family, primarily RhoA, Rac1 and Cdc42. Recent research has identified new molecular mechanisms underlying the cross-talk between these GTPases and epithelial junctions. Here we briefly summarize the current knowledge about the organization, molecular evolution and cytoskeletal anchoring of cell-cell junctions, and we comment on the most recent advances in the characterization of the interactions between Rho GTPases and junctional proteins, and their consequences with regards to junction assembly and regulation of cell behavior in vertebrate model systems. The concept of “zonular signalosome” is proposed, which highlights the close functional relationship between proteins of zonular junctions (zonulae occludentes and adhaerentes) and the control of cytoskeletal organization and signaling through Rho GTPases, transcription factors, and their effectors.

Differential Recognition Preferences of the Three Src Homology 3 (SH3) Domains from the Adaptor CD2-associated Protein (CD2AP) and Direct Association with Ras and Rab Interactor 3 (RIN3)

CD2AP is an adaptor protein involved in membrane trafficking, with essential roles in maintaining podocyte function within the kidney glomerulus. CD2AP contains three Src homology 3 (SH3) domains that mediate multiple protein-protein interactions. However, a detailed comparison of the molecular binding preferences of each SH3 remained unexplored, as well as the discovery of novel interactors. Thus, we studied the binding properties of each SH3 domain to the known interactor Casitas B-lineage lymphoma protein (c-CBL), conducted a peptide array screen based on the recognition motif PxPxPR and identified 40 known or novel candidate binding proteins, such as RIN3, a RAB5-activating guanine nucleotide exchange factor. CD2AP SH3 domains 1 and 2 generally bound with similar characteristics and specificities, whereas the SH3-3 domain bound more weakly to most peptide ligands tested yet recognized an unusually extended sequence in ALG-2-interacting protein X (ALIX). RIN3 peptide scanning arrays revealed two CD2AP binding sites, recognized by all three SH3 domains, but SH3-3 appeared non-functional in precipitation experiments. RIN3 recruited CD2AP to RAB5a-positive early endosomes via these interaction sites. Permutation arrays and isothermal titration calorimetry data showed that the preferred binding motif is Px(P/A)xPR. Two high-resolution crystal structures (1.65 and 1.11 Å) of CD2AP SH3-1 and SH3-2 solved in complex with RIN3 epitopes 1 and 2, respectively, indicated that another extended motif is relevant in epitope 2. In conclusion, we have discovered novel interaction candidates for CD2AP and characterized subtle yet significant differences in the recognition preferences of its three SH3 domains for c-CBL, ALIX, and RIN3.

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.

Genetic causes of proteinuria and nephrotic syndrome: impact on podocyte pathobiology

In the past 20 years, multiple genetic mutations have been identified in patients with congenital nephrotic syndrome (CNS) and both familial and sporadic focal segmental glomerulosclerosis (FSGS). Characterization of the genetic basis of CNS and FSGS has led to the recognition of the importance of podocyte injury to the development of glomerulosclerosis. Genetic mutations induce injury due to effects on the podocyte's structure, actin cytoskeleton, calcium signaling, and lysosomal and mitochondrial function. Transgenic animal studies have contributed to our understanding of podocyte pathobiology. Podocyte endoplasmic reticulum stress response, cell polarity, and autophagy play a role in maintenance of podocyte health. Further investigations related to the effects of genetic mutations on podocytes may identify new pathways for targeting therapeutics for nephrotic syndrome.

Sirtuin1 Maintains Actin Cytoskeleton by Deacetylation of Cortactin in Injured Podocytes

Recent studies have highlighted the renoprotective effect of sirtuin1 (SIRT1), a deacetylase that contributes to cellular regulation. However, the pathophysiologic role of SIRT1 in podocytes remains unclear. Here, we investigated the function of SIRT1 in podocytes. We first established podocyte-specific Sirt1 knockout (SIRT1(pod-/-)) mice. We then induced glomerular disease by nephrotoxic serum injection. The increase in urinary albumin excretion and BUN and the severity of glomerular injury were all significantly greater in SIRT1(pod-/-) mice than in wild-type mice. Western blot analysis and immunofluorescence showed a significant decrease in podocyte-specific proteins in SIRT1(pod-/-) mice, and electron microscopy showed marked exacerbation of podocyte injury, including actin cytoskeleton derangement in SIRT1(pod-/-) mice compared with wild-type mice. Protamine sulfate-induced podocyte injury was also exacerbated by podocyte-specific SIRT1 deficiency. In vitro, actin cytoskeleton derangement in H2O2-treated podocytes became prominent when the cells were pretreated with SIRT1 inhibitors. Conversely, this H2O2-induced derangement was ameliorated by SIRT1 activation. Furthermore, SIRT1 activation deacetylated the actin-binding and -polymerizing protein cortactin in the nucleus and facilitated deacetylated cortactin localization in the cytoplasm. Cortactin knockdown or inhibition of the nuclear export of cortactin induced actin cytoskeleton derangement and dissociation of cortactin from F-actin, suggesting the necessity of cytoplasmic cortactin for maintenance of the actin cytoskeleton. Taken together, these findings indicate that SIRT1 protects podocytes and prevents glomerular injury by deacetylating cortactin and thereby, maintaining actin cytoskeleton integrity.

Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

Quantitative analysis of single-molecule superresolution images

This review highlights the quantitative capabilities of single-molecule localization-based superresolution imaging methods. In addition to revealing fine structural details, the molecule coordinate lists generated by these methods provide the critical ability to quantify the number, clustering, and colocalization of molecules with 10-50 nm resolution. Here we describe typical workflows and precautions for quantitative analysis of single-molecule superresolution images. These guidelines include potential pitfalls and essential control experiments, allowing critical assessment and interpretation of superresolution images.

The actin-driven spatiotemporal organization of T-cell signaling at the system scale

T cells are activated through interaction with antigen-presenting cells (APCs). During activation, receptors and signaling intermediates accumulate in diverse spatiotemporal distributions. These distributions control the probability of signaling interactions and thus govern information flow through the signaling system. Spatiotemporally resolved system-scale investigation of signaling can extract the regulatory information thus encoded, allowing unique insight into the control of T-cell function. Substantial technical challenges exist, and these are briefly discussed herein. While much of the work assessing T-cell spatiotemporal organization uses planar APC substitutes, we focus here on B-cell APCs with often stark differences. Spatiotemporal signaling distributions are driven by cell biologically distinct structures, a large protein assembly at the interface center, a large invagination, the actin-supported interface periphery as extended by smaller individual lamella, and a newly discovered whole-interface actin-driven lamellum. The more than 60 elements of T-cell activation studied to date are dynamically distributed between these structures, generating a complex organization of the signaling system. Signal initiation and core signaling prefer the interface center, while signal amplification is localized in the transient lamellum. Actin dynamics control signaling distributions through regulation of the underlying structures and drive a highly undulating T-cell/APC interface that imposes substantial constraints on T-cell organization. We suggest that the regulation of actin dynamics, by controlling signaling distributions and membrane topology, is an important rheostat of T-cell signaling.

FSGS3/CD2AP is a barbed-end capping protein that stabilizes actin and strengthens adherens junctions

By combining in vitro reconstitution biochemistry with a cross-linking approach, we have identified focal segmental glomerulosclerosis 3/CD2-associated protein (FSGS3/CD2AP) as a novel actin barbed-end capping protein responsible for actin stability at the adherens junction. FSGS3/CD2AP colocalizes with E-cadherin and α-actinin-4 at the apical junction in polarized Madin-Darby canine kidney (MDCK) cells. Knockdown of FSGS3/CD2AP compromised actin stability and decreased actin accumulation at the adherens junction. Using a novel apparatus to apply mechanical stress to cell-cell junctions, we showed that knockdown of FSGS3/CD2AP compromised adhesive strength, resulting in tearing between cells and disruption of barrier function. Our results reveal a novel function of FSGS3/CD2AP and a previously unrecognized role of barbed-end capping in junctional actin dynamics. Our study underscores the complexity of actin regulation at cell-cell contacts that involves actin activators, inhibitors, and stabilizers to control adhesive strength, epithelial behavior, and permeability barrier integrity.

Structure of the kidney slit diaphragm adapter protein CD2-associated protein as determined with electron microscopy

CD2-associated protein (CD2AP) is a multidomain scaffolding protein that has a critical role in renal function. CD2AP is expressed in glomerular podocytes at the slit diaphragm, a modified adherens junction that comprises the protein filtration barrier of the kidney, and interacts with a number of protein ligands involved in cytoskeletal remodeling, membrane trafficking, cell motility, and cell survival. The structure of CD2AP is unknown. We used electron microscopy and single particle image analysis to determine the three-dimensional structure of recombinant full-length CD2AP and found that the protein is a tetramer in solution. Image reconstruction of negatively stained protein particles generated a structure at 21 Å resolution. The protein assumed a roughly spherical, very loosely packed structure. Analysis of the electron density map revealed that CD2AP consists of a central coiled-coil domain, which forms the tetramer interface, surrounded by four symmetry-related motifs, each containing three globular domains corresponding to the three SH3 domains. The spatial organization exposes the binding sites of all 12 SH3 domains in the tetramer, allowing simultaneous binding to multiple targets. Determination of the structure of CD2AP provides novel insights into the biology of this slit diaphragm protein and lays the groundwork for characterizing the interactions between key molecules of the slit diaphragm that control glomerular filtration.