Drebrin is a widespread actin-associating protein enriched at junctional plaques, defining a specific microfilament anchorage system in polar epithelial cells

The role of actin cytoskeleton CFL1 and ADF/cofilin superfamily in inflammatory response

Actin remodeling proteins are important in immune diseases and regulate cell cytoskeletal responses. These responses play a pivotal role in maintaining the delicate balance of biological events, protecting against acute or chronic inflammation in a range of diseases. Cofilin (CFL) and actin depolymerization factor (ADF) are potent actin-binding proteins that cut and depolymerize actin filaments to generate actin cytoskeleton dynamics. Although the molecular mechanism by which actin induces actin cytoskeletal reconstitution has been studied for decades, the regulation of actin in the inflammatory process has only recently become apparent. In this paper, the functions of the actin cytoskeleton and ADF/cofilin superfamily members are briefly introduced, and then focus on the role of CFL1 in inflammatory response.

Cofilin Acts as a Booster for Progression of Malignant Tumors Represented by Glioma

Cofilin, as a depolymerization factor of actin filaments, has been widely studied. Evidences show that cofilin has a role in actin structural reorganization and dynamic regulation. In recent years, several studies have demonstrated a regulatory role for cofilin in the migration and invasion mediated by cell dynamics and epithelial to mesenchymal transition (EMT)/EMT-like process, apoptosis, radiotherapy resistance, immune escape, and transcriptional dysregulation of malignant tumor cells, particularly glioma cells. On this basis, it is practical to evaluate cofilin as a biomarker for predicting tumor metastasis and prognosis. Targeting cofilin regulating kinases, Lin11, Isl-1 and Mec-3 kinases (LIM kinases/LIMKs) and their major upstream molecules inhibits tumor cell migration and invasion and targeting cofilin-mediated mitochondrial pathway induces apoptosis of tumor cells represent effective options for the development of novel anti-malignant tumor drug, especially anti-glioma drugs. This review explores the structure, general biological function, and regulation of cofilin, with an emphasis on the critical functions and prospects for clinical therapeutic applications of cofilin in malignant tumors represented by glioma.

Drebrin, an actin-binding protein, is required for lens morphogenesis and growth

BACKGROUND

Lens morphogenesis, architecture, and clarity are known to be critically dependent on actin cytoskeleton organization and cell adhesive interactions. There is limited knowledge, however regarding the identity and role of key proteins regulating actin cytoskeletal organization in the lens. This study investigated the role of drebrin, a developmentally regulated actin-binding protein, in mouse lens development by generating and characterizing a conditional knockout (cKO) mouse model using the Cre-LoxP recombination approach.

RESULTS

Drebrin E, a splice variant of DBN1 is a predominant isoform expressed in the mouse lens and exhibits a maturation-dependent downregulation. Drebrin co-distributes with actin in both epithelium and fibers. Conditional deficiency (both haploinsufficiency and complete absence) of drebrin results in disrupted lens morphogenesis leading to cataract and microphthalmia. The drebrin cKO lens reveals a dramatic decrease in epithelial height and width, E-cadherin, and proliferation, and increased apoptotic cell death and expression of α-smooth muscle actin, together with severely impaired fiber cell organization, polarity, and cell-cell adhesion.

CONCLUSIONS

This study demonstrates the requirement of drebrin in lens development and growth, with drebrin deficiency leading to impaired lens morphogenesis and microphthalmia.

Drebrin Regulates Acetylcholine Receptor Clustering and Organization of Microtubules at the Postsynaptic Machinery

Proper muscle function depends on the neuromuscular junctions (NMJs), which mature postnatally to complex "pretzel-like" structures, allowing for effective synaptic transmission. Postsynaptic acetylcholine receptors (AChRs) at NMJs are anchored in the actin cytoskeleton and clustered by the scaffold protein rapsyn, recruiting various actin-organizing proteins. Mechanisms driving the maturation of the postsynaptic machinery and regulating rapsyn interactions with the cytoskeleton are still poorly understood. Drebrin is an actin and microtubule cross-linker essential for the functioning of the synapses in the brain, but its role at NMJs remains elusive. We used immunohistochemistry, RNA interference, drebrin inhibitor 3,5-bis-trifluoromethyl pyrazole (BTP2) and co-immunopreciptation to explore the role of this protein at the postsynaptic machinery. We identify drebrin as a postsynaptic protein colocalizing with the AChRs both in vitro and in vivo. We also show that drebrin is enriched at synaptic podosomes. Downregulation of drebrin or blocking its interaction with actin in cultured myotubes impairs the organization of AChR clusters and the cluster-associated microtubule network. Finally, we demonstrate that drebrin interacts with rapsyn and a drebrin interactor, plus-end-tracking protein EB3. Our results reveal an interplay between drebrin and cluster-stabilizing machinery involving rapsyn, actin cytoskeleton, and microtubules.

The Significant Role of the Microfilament System in Tumors

Actin is the structural protein of microfilaments, and it usually exists in two forms: monomer and polymer. Among them, monomer actin is a spherical molecule composed of a polypeptide chain, also known as spherical actin. The function of actin polymers is to produce actin filaments, so it is also called fibroactin. The actin cytoskeleton is considered to be an important subcellular filament system. It interacts with numerous relevant proteins and regulatory cells, regulating basic functions, from cell division and muscle contraction to cell movement and ensuring tissue integrity. The dynamic reorganization of the actin cytoskeleton has immense influence on the progression and metastasis of cancer as well. This paper explores the significance of the microfilament network, the dynamic changes of its structure and function in the presence of a tumor, the formation process around the actin system, and the relevant proteins that may be target molecules for anticancer drugs so as to provide support and reference for interlinked cancer treatment research in the future.

Physical interaction between BAALC and DBN1 induces chemoresistance in leukemia

BAALC is identified as a leukemia-associated gene and is highly expressed in CD34-positive hematopoietic stem cells. High BAALC expression is associated with poor prognosis in several types of acute myeloid leukemia. We explored binding partner proteins of BAALC by means of mass spectrometry and analyzed biological properties of BAALC-expressing leukemic cells. We found that BAALC physically interacts with DBN1, which is an actin-binding protein and promotes cell adhesion and mobility by forming cell membrane spines during cell-cell interactions. Drebrin1 downregulation impeded cell adhesion to bone marrow stromal cells, resulting in improvement of sensitivity to cytarabine. Taken together, our findings suggest that BAALC-DBN1 interaction contributes to the anchoring of BAALC-expressing cells in the bone marrow, which in turn leads to resistance to cytotoxic drugs. Therefore, the BAALC-DBN1 interaction provides us with an opportunity to overcome the chemotherapy resistance in BAALC-activated leukemia via functional blockage of these genes.

Arctigenin attenuates diabetic kidney disease through the activation of PP2A in podocytes

Arctigenin (ATG) is a major component of Fructus Arctii, a traditional herbal remedy that reduced proteinuria in diabetic patients. However, whether ATG specifically provides renoprotection in DKD is not known. Here we report that ATG administration is sufficient to attenuate proteinuria and podocyte injury in mouse models of diabetes. Transcriptomic analysis of diabetic mouse glomeruli showed that cell adhesion and inflammation are two key pathways affected by ATG treatment, and mass spectrometry analysis identified protein phosphatase 2 A (PP2A) as one of the top ATG-interacting proteins in renal cells. Enhanced PP2A activity by ATG reduces p65 NF-κB-mediated inflammatory response and high glucose-induced migration in cultured podocytes via interaction with Drebrin-1. Importantly, podocyte-specific Pp2a deletion in mice exacerbates DKD injury and abrogates the ATG-mediated renoprotection. Collectively, our results demonstrate a renoprotective mechanism of ATG via PP2A activation and establish PP2A as a potential target for DKD progression.

Elevated Plasma Levels of Drebrin in Glaucoma Patients With Neurodegeneration

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs). Aberrations in several cytoskeletal proteins, such as tau have been implicated in the pathogenesis of neurodegenerative diseases, could be initiating factors in glaucoma progression and occurring prior to axon degeneration. Developmentally regulated brain protein (Drebrin or DBN1) is an evolutionarily conserved actin-binding protein playing a prominent role in neurons and is implicated in neurodegenerative diseases. However, the relationship between circulating DBN1 levels and RGC degeneration in glaucoma patients remains unclear. In our preliminary study, we detected drebrin protein in the plasma of glaucoma patients using proteomic analysis. Subsequently, we recruited a total of 232 patients including primary angle-closure glaucoma (PACG), primary open-angle glaucoma (POAG) and Posner-Schlossman syndrome (PS) and measured its DBN1 plasma levels. We observed elevated DBN1 plasma levels in patients with primary glaucoma but not in patients with PS compared to nonaxonopathic controls. Interestingly, in contrast to tau plasma levels increased in all groups of patients, elevated drebrin plasma levels correlated with retinal nerve fiber layer defect (RNFLD) in glaucoma patients. To further explore the expression of DBN1 in neurodegeneration, we conducted experiment of optic nerve crush (ONC) models, and observed increased expression of DBN1 in the serum as well as in the retina and then decreased after ONC. This result reinforces the potentiality of circulating DBN1 levels are increased in glaucoma patients with neurodegeneration. Taken together, our findings suggest that circulating DBN1 levels correlated with RNFLD and may reflect the severity of RGCs injury in glaucoma patients. Combining measurement of circulating drebrin and tau levels may be a useful indicator for monitoring progression of neurodegenerative diseases.

The cell-cell junctions of mammalian testes: II. The lamellar smooth muscle monolayer cells of the peritubular wall are laterally connected by vertical adherens junctions-a novel architectonic cell-cell junction system

The testes of sexually mature males of six mammalian species (men, bulls, boars, rats, mice, guinea pigs) have been studied using biochemical as well as light and electron microscopical techniques, in particular immunolocalizations. In these tissues, the peritubular walls represent lamellar encasement structures wrapped around the seminiferous tubules as a bandage system of extracellular matrix layers, alternating with monolayers of very flat polyhedral "lamellar smooth muscle cells" (LSMCs), the number of which varies in different species from 1 to 5 or 6. These LSMCs are complete SMCs containing smooth muscle α-actin (SMA), myosin light and heavy chains, α-actinin, tropomyosin, smoothelin, intermediate-sized filament proteins desmin and/or vimentin, filamin, talin, dystrophin, caldesmon, calponin, and protein SM22α, often also cytokeratins 8 and 18. In the monolayers, the LSMCs are connected by adherens junctions (AJs) based on cadherin-11, in some species also with P-cadherin and/or E-cadherin, which are anchored in cytoplasmic plaques containing β-catenin and other armadillo proteins, in some species also striatin family proteins, protein myozap and/or LUMA. The LSMC cytoplasm is rich in myofilament bundles, which in many regions are packed in paracrystalline arrays, as well as in "dense bodies," "focal adhesions," and caveolae. In addition to some AJ-like end-on-end contacts, the LSMCs are laterally connected by numerous vertical AJ-like junctions located in variously sized and variously shaped, overlapping (alter super alterum) lamelliform cell protrusions. Consequently, the LSMCs of the peritubular wall monolayers are SMCs sensu stricto which are laterally connected by a novel architectonic system of arrays of vertical AJs located in overlapping cell protrusions.

Drebrin's Role in the Maintenance of Endothelial Integrity

The human endothelium forms a permeable barrier between the blood stream and surrounding tissues, strictly governing the passage of immune cells, fluids and metabolites. The regulation of cell-cell contact dynamics between endothelial cells is essential for this function and thus for the maintenance of vascular integrity. Intercellular adhesion within the endothelium is mainly dependent on adherens junctions, composed of cell-cell adhesion proteins such as VE-cadherin and nectin, and their associated proteins. Recent research points to a critical role of the actin cytoskeleton in endothelial integrity, by providing anchorage of adhesion complexes to the cell cortex. We could show that the F-actin-binding protein drebrin is a critical regulator of endothelial integrity, by linking nectin to the cortical actin cytoskeleton. In particular, the knockdown of drebrin leads to functional impairment of endothelial cells, characterized by rupturing of endothelial monolayers cultured under conditions mimicking vascular flow. This weakening of cell-cell contacts upon drebrin depletion is based on the destabilization of nectin at adherens junctions, followed by internalization and degradation in lysosomes. Conducting interaction studies, we showed that drebrin binds to nectin's interaction partner afadin, thus linking the nectin/afadin system to the cortical F-actin network. Drebrin, containing binding sites for both afadin and F-actin, is thus uniquely equipped to stabilize nectin at adherens junctions, thereby preserving endothelial integrity. Collectively, these results contribute to the current understanding of cell-cell junction regulation, introducing a new function of drebrin as a stabilizer of endothelial integrity.

Drebrin at Junctional Plaques

Adhesion, segregation, and cellular plasticity are regulated by actin filaments anchored at the plaques of adherens junctions, sites of mechanical stabilization, and interfaces of multiple signaling networks. Drebrins were originally identified in neuronal cells, but the isoform drebrin E was also detected at adherens junctions of a wide range of non-neuronal cells, including polarized epithelia, endothelia, and fibroblasts. Here the protein is enriched at actin filament bundles associated with junctional plaques. Polarized epithelial cells contain two types of actin-associated complexes, one comprising drebrin but not vinculin and the other involving vinculin, but not drebrin. At gap junctions drebrin interacts with connexin 43, stabilizes this protein at membranes, and links it to the actin cytoskeleton. In vivo drebrin is widespread in diverse non-neuronal tissues of epithelial, endothelial, and smooth muscle origin, but not ubiquitous. In intestinal cells it is involved in cell compaction, linking of actin filaments to microtubules and formation and stabilization of the terminal web. Upregulation of drebrin was noted in several types of cancers, e.g., basal cell carcinomas for which it may serve as marker, liver metastases of colon carcinomas, and bladder cancer, suggesting that it is involved in regulating actin dynamics during tumor development, progression, and metastasis.

Drebrin in Renal Glomeruli

The central function of renal glomeruli is plasma ultrafiltration for primary urine production. The glomerular filtration barrier consists of a fenestrated endothelium, the glomerular basement membrane and podocytes, mesenchymal-like cells with actin filament-rich protrusions, the "foot processes." Their architecture and function are maintained and regulated by actin and several actin-binding proteins, mutations of which can be causative of glomerular diseases. Since initial immunostaining experiments had demonstrated intense drebrin reactions in renal glomeruli, the distribution of this protein was studied in detail in the kidneys of diverse mammalian species. Double-label confocal laser scanning microscopy revealed drebrin enrichment in mesangial cells of human, bovine, murine, and rat kidneys. In Thy-1.1 nephritic rat glomeruli, the protein was concentrated in mesangial cell processes and upregulated during their formation and remodeling. In adult human and bovine kidneys, drebrin was additionally accumulated in the foot processes of podocytes, a finding confirmed by immunoelectron microscopy. By contrast, podocytes of rodent glomeruli contained significant amounts of drebrin only during early developmental stages. In cultured murine podocytes induced to form cell processes, however, drebrin was concentrated in these protrusions, partly in colocalization with other actin-binding proteins. Protein extracts from human and bovine kidneys comprised 20 S-complexes of drebrin and actin, so-called drebrosomes. In summary, drebrin has to be added to the list of actin-binding proteins regulating actin dynamics of mesangial cell processes and foot processes of podocytes. It will be important to determine its role in hereditary and acquired glomerulopathies.

Drebrin and Spermatogenesis

Drebrin is a family of actin-binding proteins with two known members called drebrin A and E. Apart from the ability to stabilize F-actin microfilaments via their actin-binding domains near the N-terminus, drebrin also regulates multiple cellular functions due to its unique ability to recruit multiple binding partners to a specific cellular domain, such as the seminiferous epithelium during the epithelial cycle of spermatogenesis. Recent studies have illustrated the role of drebrin E in the testis during spermatogenesis in particular via its ability to recruit branched actin polymerization protein known as actin-related protein 3 (Arp3), illustrating its involvement in modifying the organization of actin microfilaments at the ectoplasmic specialization (ES) which includes the testis-specific anchoring junction at the Sertoli-spermatid (apical ES) interface and at the Sertoli cell-cell (basal ES) interface. These data are carefully evaluated in light of other recent findings herein regarding the role of drebrin in actin filament organization at the ES. We also provide the hypothetical model regarding its involvement in germ cell transport during the epithelial cycle in the seminiferous epithelium to support spermatogenesis.

Role of Drebrin at the Immunological Synapse

Although drebrin was first described in neurons, it is also expressed in cells of the immune system, such as T lymphocytes and mast cells. Another member of the drebrin family of proteins, mammalian actin-binding protein 1 (mAbp-1) is more widely expressed and plays important roles in the function of macrophages, polymorphonuclear neutrophils, and B lymphocytes. We will briefly discuss on the function of mAbp-1 and drebrin in immune cells with emphasis on T cells. Specifically, drebrin enables the immune responses of CD4⁺ T lymphocytes. T cells are activated after the recognition of an antigen presented by antigen-presenting cells through cognate cell-cell contacts called immunological synapses (IS). In CD4⁺ T cells, drebrin associates with the chemokine receptor CXCR4, and both molecules redistribute to the IS displaying similar dynamics. Through its interaction with CXCR4 and the actin cytoskeleton, drebrin regulates T cell activation. CD4⁺ T cells are one of the main targets for the human immunodeficiency virus (HIV)-1. This virus utilizes the IS structure to be transmitted to uninfected cells, forming cell-cell contacts called virological synapses (VS). Interestingly, drebrin negatively regulates HIV-1 infection of CD4⁺ T lymphocytes, by regulating actin polymerization at the VS.

Cell biology of mesangial cells: the third cell that maintains the glomerular capillary

The renal glomerulus consists of glomerular endothelial cells, podocytes, and mesangial cells, which cooperate with each other for glomerular filtration. We have produced monoclonal antibodies against glomerular cells in order to identify different types of glomerular cells. Among these antibodies, the E30 clone specifically recognizes the Thy1.1 molecule expressed on mesangial cells. An injection of this antibody into rats resulted in mesangial cell-specific injury within 15 min, and induced mesangial proliferative glomerulonephritis in a reproducible manner. We examined the role of mesangial cells in glomerular function using several experimental tools, including an E30-induced nephritis model, mesangial cell culture, and the deletion of specific genes. Herein, we describe the characterization of E30-induced nephritis, formation of the glomerular capillary network, mesangial matrix turnover, and intercellular signaling between glomerular cells. New molecules that are involved in a wide variety of mesangial cell functions are also introduced.

Drebrin regulates neuroblast migration in the postnatal mammalian brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). This migration is crucial for the proper integration of newborn neurons in a pre-existing synaptic network and is believed to play a key role in infant human brain development. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. Here, we have investigated the function of drebrin, an actin-binding protein highly expressed in the RMS of the postnatal mammalian brain. Neuroblast migration was monitored both in culture and in brain slices obtained from electroporated mice by time-lapse spinning disk confocal microscopy. Depletion of drebrin using distinct RNAi approaches in early postnatal mice affects neuroblast morphology and impairs neuroblast migration and orientation in vitro and in vivo. Overexpression of drebrin also impairs migration along the RMS and affects the distribution of neuroblasts at their final destination, the OB. Drebrin phosphorylation on Ser142 by Cyclin-dependent kinase 5 (Cdk5) has been recently shown to regulate F-actin-microtubule coupling in neuronal growth cones. We also investigated the functional significance of this phosphorylation in RMS neuroblasts using in vivo postnatal electroporation of phosphomimetic (S142D) or non-phosphorylatable (S142A) drebrin in the SVZ of mouse pups. Preventing or mimicking phosphorylation of S142 in vivo caused similar effects on neuroblast dynamics, leading to aberrant neuroblast branching. We conclude that drebrin is necessary for efficient migration of SVZ-derived neuroblasts and propose that regulated phosphorylation of drebrin on S142 maintains leading process stability for polarized migration along the RMS, thus ensuring proper neurogenesis.

Integrated genomic analysis identifies subclasses and prognosis signatures of kidney cancer

PURPOSE

To define robust miRNA-based molecular classifiers for human clear cell renal cell carcinoma (ccRCC) subgrouping and prognostication.

EXPERIMENTAL DESIGN

Multidimensional data of over 500 clear cell renal cell carcinoma (ccRCC) patients were retrieved from The Cancer Genome Atlas (TCGA) archive. Data analysis was based on a novel computational approach that selectively considers patients with extreme expression values of miRNAs to detect survival-associated molecular signatures.

RESULTS

Our in silico analysis unveiled a novel ccRCC-specific 5-miRNA (miR-10b, miR-21, miR-143, miR-183, and miR-192) signature able, when combined with information from conventional TNM staging and the age of the patient, to prognosticate ccRCC outcome more accurately than known ccRCC miRNA signatures or TNM staging alone. Furthermore, our approach revealed the existence of 6 distinct subgroups of ccRCC characterized by discrete differences in overall survival, tumor stage, and mutational spectra in key ccRCC tumor suppressor genes. It also demonstrated that BAP1 mutations correlate with tumor progression rather than overall survival.

CONCLUSION

Integrated analysis of multidimensional data from the TCGA archive allowed to draw a portrait of distinct molecular subclasses of human ccRCC and to define signatures for prognosticating disease outcome. Together, these results offer new prospects for more accurate stratification and prognostication of ccRCC.

Actin-binding protein drebrin E is involved in junction dynamics during spermatogenesis

The actin-based cytoskeleton plays a critical role in the seminiferous epithelium during spermatogenesis by conferring cell shape, adhesion, structural support and cell polarity to both Sertoli and developing germ cells, which are essential for spermatogonial stem cell renewal, maintenance of the stem cell niche, cell cycle progression, mitosis, meiosis, spermiogenesis and spermiation. However, few functional studies are found in the literature, which explore the functional significance of actin dynamics in these events. This by and large is due to a lack of information on the proteins that regulate actin dynamics. Herein, we report drebrin E is an integrated component of the apical ectoplasmic specialization (apical ES) and the basal ES at the blood-testis barrier (BTB) in the seminiferous epithelium of the adult rat testis. Using immunohistochemistry and dual-labeled immunofluorescence analysis, drebrin E was found to display a stage-specific localization at the apical ES, as well as at the basal ES at the BTB during the seminiferous epithelial cycle of spermatogenesis. Drebrin E was first detected in stage V tubules at the basal ES with the highest expression at the BTB at stages V and VI, but it diminished considerably by stages VII and VIII and was almost non-detectable until stage IV. At the apical ES, drebrin E was also first detected at stage V, surrounding the entire head of the elongating spermatid, but by stage VI its localization had "shifted" to localize most intensely and almost exclusively to the concave side of the spermatid head. In stage VII tubules, drebrin E co-localized with actin, as well as with two other actin regulatory proteins Eps8 (epidermal growth factor receptor pathway substrate 8, an actin capping and bundling protein) and Arp3 (actin-related protein 3, a component of the Arp2/3 complex known to regulate actin nucleation and branching). The localization of drebrin E at the apical ES was compromised following treatment of rats with adjudin, which is known to exert its destructive effects primarily at the apical ES by inducing premature loss of elongating/elongated spermatids from the epithelium, mimicking "spermiation." Instead of being restricted to the concave side of spermatid heads, drebrin E was found to be mis-localized in the seminiferous epithelium of adjudin-treated rats; it was also present on the convex side of elongating spermatids, but these cells were mis-oriented so that their heads no longer pointed toward the basement membrane. The expression of drebrin E by Sertoli cells was also found to be modulated by TGFβ3 and TNFα. Since Arp3, but not Eps8, was found to bind drebrin E; and cytokines were also shown to affect the cellular distribution of drebrin E and enhance the interaction between drebrin E and Arp3, these findings illustrate that cytokines may regulate BTB dynamics during the epithelial cycle by recruiting drebrin E and Arp3 to the BTB microenvironment to induce changes in the configuration of actin filament bundles at the basal ES. In summary, these findings illustrate drebrin E is working in concert with Arp3 to regulate actin filament bundles at both the apical and the basal ES in the testis, conferring adhesion and cell polarity at both sites during spermatogenesis.

Actin binding proteins and spermiogenesis: Some unexpected findings

Drebrin E, an actin-binding protein lacking intrinsic activity in the regulation of actin dynamics (e.g., polymerization, capping, nucleation, branching, cross-linking, bundling and severing), is known to recruit actin regulatory proteins to a specific cellular site. Herein, we critically evaluate recent findings in the field which illustrate that drebrin E works together with two other actin-binding proteins, namely Arp3 (actin-related protein 3, a component of the Arp2/3 complex that simultaneously controls actin nucleation for polymerization and branching of actin filaments) and Eps8 (epidermal growth factor receptor pathway substrate 8 that controls capping of the barbed ends of actin filaments, as well as actin filament bundling) to regulate the homeostasis of F-actin filament bundles at the ectoplasmic specialization (ES), a testis-specific atypical adherens junction (AJ) in the seminiferous epithelium. This is mediated by the strict temporal and spatial expression of these three actin-binding proteins at the apical and basal ES at the Sertoli cell-spermatid (step 8-19) and Sertoli-Sertoli cell interface, respectively, during the seminiferous epithelial cycle of spermatogenesis. In this Commentary, we put forth a possible model by which drebrin E may be acting as a platform upon which proteins (e.g., Arp3) that are needed to alter the conformation of actin filament bundles at the ES can be recruited to the site, thus facilitating changes in cell shape and cell position in the epithelium during spermiogenesis and spermiation. In short, drebrin E may be acting as a "logistic" distribution center to manage different regulatory proteins at the apical ES, thereby regulating the dynamics of actin filament bundles and modulating the plasticity of the apical ES. This would allow adhesion to be altered continuously throughout the epithelial cycle to accommodate spermatid movement in the seminiferous epithelium during spermiogenesis and spermiation. We also describe a hypothetical model, upon which functional studies can be designed in the future.

The role of actin-binding proteins in the control of endothelial barrier integrity

The endothelial barrier of the vasculature is of utmost importance for separating the blood stream from underlying tissues. This barrier is formed by tight and adherens junctions (TJ and AJ) that form intercellular endothelial contacts. TJ and AJ are integral membrane structures that are connected to the actin cytoskeleton via various adaptor molecules. Consequently, the actin cytoskeleton plays a crucial role in regulating the stability of endothelial cell contacts and vascular permeability. While a circumferential cortical actin ring stabilises junctions, the formation of contractile stress fibres, e. g. under inflammatory conditions, can contribute to junction destabilisation. However, the role of actin-binding proteins (ABP) in the control of vascular permeability has long been underestimated. Naturally, ABP regulate permeability via regulation of actin remodelling but some actin-binding molecules can also act independently of actin and control vascular permeability via various signalling mechanisms such as activation of small GTPases. Several studies have recently been published highlighting the importance of actin-binding molecules such as cortactin, ezrin/radixin/moesin, Arp2/3, VASP or WASP for the control of vascular permeability by various mechanisms. These proteins have been described to regulate vascular permeability under various pathophysiological conditions and are thus of clinical relevance as targets for the development of treatment strategies for disorders that are characterised by vascular hyperpermeability such as sepsis. This review highlights recent advances in determining the role of ABP in the control of endothelial cell contacts and vascular permeability.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Desmoglein 2 depletion leads to increased migration and upregulation of the chemoattractant secretoneurin in melanoma cells

During development and progression of malignant melanoma, an important role has been attributed to alterations of cell-cell adhesions, in particular, to a “cadherin switch” from E- to N-cadherin. We have previously shown that a subtype of melanoma cells express the desmosomal cadherin desmoglein 2 as non-junction-bound cell surface protein in addition to classical cadherins. To study the role of desmoglein 2 in melanoma cells, melanoma lines containing high endogenous amounts of desmoglein 2 were depleted of the protein by RNA interference. Transwell migration and scratch wounding assays showed markedly increased migration upon desmoglein 2 suppression whereas proliferation and viability remained unaltered. In gene expression profiles, desmoglein 2 depletion was associated with overexpression of migration-related genes. Strongest overexpression was found for secretogranin II which has not been reported in melanoma cells before. The bioactive peptide derived from secretogranin II, secretoneurin, is known to exert chemoattractive functions and was demonstrated here to stimulate melanoma cell migration. In summary, we show that desmoglein 2 expression attenuates migration of melanoma cells. The mechanism of desmoglein 2 impaired cell migration is mediated by downregulation of secretogranin II. Loss of desmoglein 2 increases expression of secretogranin II, followed by an enhanced migratory activity of melanoma cells. Our data add a new pathway of regulating melanoma cell migration related to a desmoglein 2 – secretogranin II axis.

Regulation of actin dynamics and protein trafficking during spermatogenesis--insights into a complex process

In the mammalian testis, extensive restructuring takes place across the seminiferous epithelium at the Sertoli-Sertoli and Sertoli-germ cell interface during the epithelial cycle of spermatogenesis, which is important to facilitate changes in the cell shape and morphology of developing germ cells. However, precise communications also take place at the cell junctions to coordinate the discrete events pertinent to spermatogenesis, namely spermatogonial renewal via mitosis, cell cycle progression and meiosis, spermiogenesis and spermiation. It is obvious that these cellular events are intimately related to the underlying actin-based cytoskeleton which is being used by different cell junctions for their attachment. However, little is known on the biology and regulation of this cytoskeleton, in particular its possible involvement in endocytic vesicle-mediated trafficking during spermatogenesis, which in turn affects cell adhesive function and communication at the cell-cell interface. Studies in other epithelia in recent years have shed insightful information on the intimate involvement of actin dynamics and protein trafficking in regulating cell adhesion and communications. The goal of this critical review is to provide an updated assessment of the latest findings in the field on how these complex processes are being regulated during spermatogenesis. We also provide a working model based on the latest findings in the field including our laboratory to provide our thoughts on an apparent complicated subject, which also serves as the framework for investigators in the field. It is obvious that this model will be rapidly updated when more data are available in future years.

Drebrin preserves endothelial integrity by stabilizing nectin at adherens junctions

Regulation of cell-cell contacts is essential for integrity of the vascular endothelium. Here, a critical role of the F-actin binding protein drebrin in maintaining endothelial integrity is revealed under conditions mimicking vascular flow. Drebrin knockdown leads to weakening of cell-cell contacts, characterized by loss of nectin from adherens junctions and its subsequent lysosomal degradation. Immunoprecipitation, FRAP and mitochondrial retargeting experiments show that nectin stabilization occurs through a chain of interactions: drebrin binding to F-actin, interaction of drebrin and afadin through their polyproline and PR1-2 regions, and recruitment of nectin through afadin's PDZ region. Key elements are drebrin's modules that confer binding to afadin and F-actin. Evidence is provided by constructs containing afadin's PDZ region coupled to drebrin's F-actin binding region or to lifeact, which restore junctional nectin under knockdown of drebrin or of both drebrin and afadin. Drebrin, containing binding sites for both afadin and F-actin, is thus uniquely equipped to stabilize nectin at endothelial junctions and to preserve endothelial integrity under vascular flow.

The plaque protein myozap identified as a novel major component of adhering junctions in endothelia of the blood and the lymph vascular systems

Recently the protein myozap, a 54-kD polypeptide which is not a member of any of the known cytoskeletal and junctional protein multigene families, has been identified as a constituent of the plaques of the composite junctions in the intercalated disks connecting the cardiomyocytes of mammalian hearts. Using a set of novel, highly sensitive and specific antibodies we now report that myozap is also a major constituent of the cytoplasmic plaques of the adherens junctions (AJs) connecting the endothelial cells of the mammalian blood and lymph vascular systems, including the desmoplakin-containing complexus adhaerentes of the virgultar cells of lymph node sinus. In light and electron microscopic immunolocalization experiments we show that myozap colocalizes with several proteins of desmosomal plaques as well as with AJ-specific transmembrane molecules, including VE-cadherin. In biochemical analyses, rigorous immunoprecipitation experiments have revealed N-cadherin, desmoplakin, desmoglein-2, plakophilin-2, plakoglobin and plectin as very stably bound complex partners. We conclude that myozap is a general component of cell-cell junctions not only in the myocardium but also in diverse endothelia of the blood and lymph vascular systems of adult mammals, suggesting that this protein not only serves a specific role in the heart but also a broader set of functions in the vessel systems. We also propose to use myozap as an endothelial cell type marker in diagnoses.

Regulation of myotube formation by the actin-binding factor drebrin

Background

Myogenic differentiation involves cell-cycle arrest, activation of the muscle-specific transcriptome, and elongation, alignment and fusion of myoblasts into multinucleated myotubes. This process is controlled by promyogenic transcription factors and regulated by signaling pathways in response to extracellular cues. The p38 mitogen-activated protein kinase (p38 MAPK) pathway promotes the activity of several such transcription factors, including MyoD and MEF2, thereby controlling the muscle-specific transcription program. However, few p38-regulated genes that play a role in the regulation of myogenesis have been identified.

Methods

RNA interference (RNAi), chemical inhibition and immunofluorescence approaches were used to assess the role of drebrin in differentiation of primary mouse myoblasts and C2C12 cells.

Results

In a search for p38-regulated genes that promote myogenic differentiation, we identified Dbn1, which encodes the actin-binding protein drebrin. Drebrin is an F-actin side-binding protein that remodels actin to facilitate the change of filopodia into dendritic spines during synaptogenesis in developing neurons. Dbn1 mRNA and protein are induced during differentiation of primary mouse and C2C12 myoblasts, and induction is substantially reduced by the p38 MAPK inhibitor SB203580. Primary myoblasts and C2C12 cells depleted of drebrin by RNAi display reduced levels of myogenin and myosin heavy chain and form multinucleated myotubes very inefficiently. Treatment of myoblasts with BTP2, a small-molecule inhibitor of drebrin, produces a phenotype similar to that produced by knockdown of drebrin, and the inhibitory effects of BTP2 are rescued by expression of a mutant form of drebrin that is unable to bind BTP2. Drebrin in myoblasts is enriched in cellular projections and cell cortices and at regions of cell-cell contact, all sites where F-actin, too, was concentrated.

Conclusions

Our findings reveal that Dbn1 expression is a target of p38 MAPK signaling during myogenesis and that drebrin promotes myoblast differentiation.

F-actin-binding protein drebrin regulates CXCR4 recruitment to the immune synapse

The adaptive immune response depends on the interaction of T cells and antigen-presenting cells at the immune synapse. Formation of the immune synapse and the subsequent T-cell activation are highly dependent on the actin cytoskeleton. In this work, we describe that T cells express drebrin, a neuronal actin-binding protein. Drebrin colocalizes with the chemokine receptor CXCR4 and F-actin at the peripheral supramolecular activation cluster in the immune synapse. Drebrin interacts with the cytoplasmic tail of CXCR4 and both proteins redistribute to the immune synapse with similar kinetics. Drebrin knockdown in T cells impairs the redistribution of CXCR4 and inhibits actin polymerization at the immune synapse as well as IL-2 production. Our data indicate that drebrin exerts an unexpected and relevant functional role in T cells during the generation of the immune response.

Protein p0071, a major plaque protein of non-desmosomal adhering junctions, is a selective cell-type marker

Protein p0071, which originally was introduced as a member of the p120-subfamily of armadillo proteins, common to desmosomes and adhaerens junctions (AJs) and to several other cell structures (centrosomes, midbodies), has been localized by using a series of novel mono- and polyclonal antibodies generated against various domains of the molecule. By protein analysis and immunolocalization techniques, protein p0071 has been localized as a plaque protein in AJs of diverse epithelia and certain vascular endothelia, in the composite junctions (areal compositae) of the intercalated disks of cardiomyocytes, and in the punctate or more extended AJs of the vast majority of cell culture types examined, including mitotic states. Using these antibodies, we have also shown that this AJ protein occurs only rarely or is even absent in tissues such as skeletal and smooth muscles, in a series of mesenchymal tissue cells, and in specific desmosome-rich cells such as those of the upper layers of the epidermis and certain other stratified epithelia and Hassall corpuscles of the thymus. We have also demonstrated that p0071 is absent from desmosomes. The occurrence of two major subtypes of lymphatic endothelial cells, one with AJs containing p0071 and one without detectable p0071, is emphasized. Possible structural and functional roles of p0071 are discussed in light of these new findings regarding its localization, and the addition of p0071 to the armamentarium of cytodiagnostic cell-type markers is recommended.

Subtypes of melanocytes and melanoma cells distinguished by their intercellular contacts: heterotypic adherens junctions, adhesive associations, and dispersed desmoglein 2 glycoproteins

In the tissue integration of melanocytes and melanoma cells, an important role is attributed to cell adhesion molecules, notably the cadherins. In cultured melanoma cells, we have previously described a more heterogeneous repertoire of cadherins than normal, including some melanoma subtypes synthesizing the desmosomal cadherin, desmoglein 2, out of the desmosomal context. Using biochemical and immunological characterization of junctional molecules, confocal laser scanning, and electron and immunoelectron microscopy, we now demonstrate homo- and heterotypic cell-cell adhesions of normal epidermal melanocytes. In human epidermis, both in situ and in cell culture, melanocytes and keratinocytes are connected by closely aligned membranes that are interspersed by small puncta adhaerentia containing heterotypic complexes of E- and P-cadherin. Moreover, melanocytes growing in culture often begin to synthesize desmoglein 2, which is dispersed over extended areas of intimate adhesive cell-cell associations. As desmoglein 2 is not found in melanocytes in situ, we hypothesize that its synthesis is correlated with cell proliferation. Indeed, in tissue microarrays, desmoglein 2 has been demonstrated in a sizable subset of nevi and primary melanomas. The biological meanings of these cell-cell adhesion molecule arrangements, the possible diagnostic and prognostic significance of these findings, and the implications of the heterogeneity types of melanomas are discussed.

Drebrin a content correlates with spine head size in the adult mouse cerebral cortex

Synaptic activities alter synaptic strengths at the axospinous junctions, and such changes are often accompanied by changes in the size of the postsynaptic spines. We have been exploring the idea that drebrin A, a neuron-specific actin-binding protein localized on the postsynaptic side of excitatory synapses, may be a molecule that links synaptic activity to the shape and content of spines. Here, we performed electron microscopic immunocytochemistry with the nondiffusible gold label to explore the relationship among levels of drebrin A, the NR2A subunit of N-methyl-D-aspartate receptors, and the size of spines in the perirhinal cortex of adult mouse brains. In contrast to the membranous localization within neonatal spines, most immunogold particles for drebrin A were localized to the cytoplasmic core region of spines in mature spines. This distribution suggests that drebrin within adult spines may reorganize the F-actin network at the spine core, in addition to its known neonatal role in spine formation. Drebrin A-immunopositive (DIP) spines exhibited larger spine head areas and longer postsynaptic densities (PSDs) than drebrin A-immunonegative (DIN) spines (P < 0.001). Furthermore, spine head area and PSD lengths correlated positively with drebrin A levels (r = 0.47 and 0.40). The number of synaptic NR2A immunolabels was also higher in DIP spines than in DIN spines, whereas their densities per unit lengths of PSD were not significantly different. These differences between the DIP and the DIN spines indicate that spine sizes and synaptic protein composition of mature brains are regulated, at least in part, by drebrin A levels.

Role of actin cytoskeleton in dendritic spine morphogenesis

Dendritic spines are the postsynaptic receptive regions of most excitatory synapses, and their morphological plasticity play a pivotal role in higher brain functions, such as learning and memory. The dynamics of spine morphology is due to the actin cytoskeleton concentrated highly in spines. Filopodia, which are thin and headless protrusions, are thought to be precursors of dendritic spines. Drebrin, a spine-resident side-binding protein of filamentous actin (F-actin), is responsible for recruiting F-actin and PSD-95 into filopodia, and is suggested to govern spine morphogenesis. Interestingly, some recent studies on neurological disorders accompanied by cognitive deficits suggested that the loss of drebrin from dendritic spines is a common pathognomonic feature of synaptic dysfunction. In this review, to understand the importance of actin-binding proteins in spine morphogenesis, we first outline the well-established knowledge pertaining to the actin cytoskeleton in non-neuronal cells, such as the mechanism of regulation by small GTPases, the equilibrium between globular actin (G-actin) and F-actin, and the distinct roles of various actin-binding proteins. Then, we review the dynamic changes in the localization of drebrin during synaptogenesis and in response to glutamate receptor activation. Because side-binding proteins are located upstream of the regulatory pathway for actin organization via other actin-binding proteins, we discuss the significance of drebrin in the regulatory mechanism of spine morphology through the reorganization of the actin cytoskeleton. In addition, we discuss the possible involvement of an actin-myosin interaction in the morphological plasticity of spines.

Homo- and heterotypic cell contacts in malignant melanoma cells and desmoglein 2 as a novel solitary surface glycoprotein

During progression of melanomas, a crucial role has been attributed to alterations of cell-cell adhesions, specifically, to a "cadherin switch" from E- to N-cadherin (cad). We have examined the adhesion of melanoma cells to each other and to keratinocytes. When different human melanoma cell lines were studied by protein analysis and immunofluorescence microscopy, six of eight lines contained N-cad, three E-cad, and five P-cad, and some lines had more than one cad. Surprisingly, two N-cad-positive lines, MeWo and C32, also contained desmoglein 2 (Dsg2), a desmosomal cad previously not reported for melanomas, whereas other desmosome-specific proteins were absent. This finding was confirmed by reverse transcriptase-PCR, immunoprecipitation, and matrix-assisted laser desorption ionization-time of flight analyses. Double-label confocal and immunoelectron microscopy showed N-cad, alpha- and beta-catenin in plaque-bearing puncta adhaerentia, whereas Dsg2 was distributed rather diffusely over the cell surface. In cocultures with HaCaT keratinocytes Dsg2 was found in heterotypic cell contact regions. Correspondingly, immunohistochemistry revealed Dsg2 in five of 10 melanoma metastases. Together, we show that melanoma cell adhesions are more heterogeneous than expected and that certain cells devoid of desmosomes contain Dsg2 in a non-junction-restricted form. Future studies will have to clarify the diagnostic and prognostic significance of these different adhesion protein subtypes.

The area composita of adhering junctions connecting heart muscle cells of vertebrates – III: Assembly and disintegration of intercalated disks in rat cardiomyocytes growing in culture

For cell and molecular biological studies of heart formation and function cell cultures of embryonal, neonatal or adult hearts of various vertebrates, notably rat and chicken, have been widely used. As the myocardium-specific cell-cell junctions, the intercalated disks (ID), have recently been found to be particularly sensitive to losses of - or mutations in - certain cytoskeletal proteins, resulting in cardiac damages, we have examined the ID organization in primary cultures of cardiomyocytes obtained from neonatal rats. Using immunofluorescence and immunoelectron microscopy, we have studied the major ID components for up to 2 weeks in culture, paying special attention to spontaneously beating, individual cardiomyocytes and myocardial cell colonies. While our results demonstrate the formation of some ID-like cardiomyocyte-connecting junction arrays, they also reveal a variety of structural disorders such as rather extended, junction-free ID regions, sac-like invaginations and endocytotic blebs as well as accumulations of intracytoplasmic structures suggestive of endocytosed forms of junction-derived vesicles or of junction fragments resembling fascia adhaerens elements. Moreover, we have noticed a novel type of small, obviously plaque-free cytoplasmic vesicles containing one or both of the desmosomal cadherins, desmocollin Dsc2 and desmoglein Dsg2. We conclude that cardiomyocyte cultures are useful model systems for studies of certain aspects of myocardiac differentiation and functions but, on the other hand, show progressive disintegration and deterioration. The potential value of molecular markers and reagents in studies of myocardial pathology as well as in the monitoring of myocardial differentiation of so-called stem cells is discussed.

Many faces of drebrin: from building dendritic spines and stabilizing gap junctions to shaping neurite-like cell processes

In this review we consider the multiple functions of developmentally regulated brain protein (drebrin), an actin-binding protein, in the formation of cellular polarity in different cell types. Drebrin has a well-established role in the morphogenesis, patterning and maintenance of dendritic spines in neurons. We have recently shown that drebrin also stabilizes Connexin-43 containing gap junctions at the plasma membrane. The latest literature and our own data suggest that drebrin may be broadly involved in shaping cell processes and in the formation of stabilized plasma membrane domains, an effect that is likely to be of crucial significance for formation of cell polarity in both neuronal and non-neuronal types.

Involvement of mesangial cells expressing alpha-smooth muscle actin during restorative glomerular remodeling in Thy-1.1 nephritis

The function of actin cytoskeleton in mesangial cells (MCs) during the recovering process of injured glomeruli is not fully understood. MCs in injured glomeruli express alpha-smooth muscle actin (alpha-SMA), which is not detected in normal glomeruli. We focused on the localization of alpha-SMA in MCs of Thy-1.1 nephritic rat. Expression of alpha-SMA in the injured glomeruli peaked at day 5 after antibody injection and then declined gradually. At day 5, MCs, where alpha-SMA was localized at their cytoplasmic processes situated in various positions, occupied the expanded mesangium. MCs expressing alpha-SMA tended to be located at the peripheral region close to the glomerular basement membrane (GBM) or endothelial cells at day 8. Localization of alpha-SMA within the peripheral MCs was restricted to the cytoplasmic processes radiating toward the GBM and touching it with their tips at day 8. These alpha-SMA-containing processes are suitable to transmit the contractile force to GBM and may contribute to normalize the expanded glomerular volume. In addition, an actin-binding protein, drebrin, was localized in all MC processes extending toward various directions throughout the course of nephritis, suggesting that drebrin is involved in the formation of MC processes.

Dynamics of the actin-binding protein drebrin in motile cells and definition of a juxtanuclear drebrin-enriched zone

The actin-binding protein (ABP) drebrin, isoform E2, is involved in remodelling of the actin cytoskeleton and in formation of cell processes, but its role in cell migration has not yet been investigated. Therefore, we have studied the organization of drebrin in motile cultured cells such as murine B16F1 melanoma and human SV80 fibroblast cells, using live cell confocal microscopy. In cells overexpressing DNA constructs encoding drebrin linked to EGFP, numerous long, branched cell processes were formed which slowly retracted and extended, whereas forward movement was halted. In contrast, stably transfected B16F1 cells containing drebrin-EGFP at physiological levels displayed lamellipodia and were able to migrate on laminin. Surprisingly, in such cells, drebrin was absent from anterior lamellipodia but was enriched in a specific juxtanuclear zone, the "drebrin-enriched zone" (DZ), and in the tail. In leading edges of SV80 cells, characterized by pronounced actin microspikes, drebrin was specifically enriched along posterior portions of the microspikes, together with tropomyosin. Drebrin knock-down by small interfering RNAs did not impair movements of SV80 cells. Our results confirm the role of drebrin E2 in the formation of branching processes and further indicate that during cell migration, the protein contributes to retraction of the cell body and the tail but not to lamellipodia formation. In particular, the novel, sizable juxtanuclear DZ structure will have to be characterized in future experiments with respect to its molecular assembly and cell biological functions.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. II. Colocalizations of desmosomal and fascia adhaerens molecules in the intercalated disk

Using immunofluorescence histochemistry and immunoelectron microscopy on sections through myocardiac tissues of diverse mammalian (human, cow, rat, mouse) and fish species we show that both desmosomal and fascia adhaerens proteins identified by gel electrophoresis and immunoblot occur in the area composita, the by far major type of plaque-bearing junctions of the intercalated disks (IDs) connecting cardiomyocytes. Specifically, we demonstrate that desmoplakin and the other desmosomal proteins occur in these junctions, together with N-cadherin, cadherin-11, alpha- and beta-catenin as well as vinculin, afadin and proteins p120(ctn), ARVCF, p0071, and ZO-1, suggestive of colocalization. We conclude that the predominant type of adhering junction present in IDs is a junction sui generis, termed area composita, that is characterized by an unusually high molecular complexity and an intimate association of molecules of both ensembles, the desmosomal one and the fascia adhaerens category. We discuss possible myocardium-specific, complex-forming interactions between members of the two ensembles and the relevance of our findings for the formation and functioning of the heart and for the understanding of hereditary and other cardiomyopathies. We further propose to use this highly characteristic area composita ensemble of molecules as cardiomyocyte markers for the monitoring of cardiomyogenesis, cardiomyocyte regeneration and possible cardiomyocyte differentiation from mesenchymal stem cells.

Activation of N-methyl-d-aspartate receptor induces a shift of drebrin distribution: Disappearance from dendritic spines and appearance in dendritic shafts

Drebrin is a major actin-filament-binding protein localized in mature dendritic spines. A recent in vivo immunoelectron microscopic study suggests that drebrin content at each dendritic spine is regulated by some unknown mechanisms. In the present in vitro study, we examined whether glutamate stimulation alters drebrin content in dendritic spines. Glutamate stimulation induced disappearance of drebrin immunostaining from dendritic spines but led to appearance of drebrin immunostaining in dendritic shafts and somata. The glutamate-induced shift of drebrin immunostaining was blocked by an NMDA receptor antagonist. Immunoblot analyses showed that both the total and the cytosolic drebrin remained unchanged and revealed that the drebrin shift was not due to drebrin degradation. These findings indicate that NMDA receptor activation induces a shift in subcellular distribution of drebrin associated with actin filaments, and that the shift might be a molecular basis for actin reorganization accompanied with synaptic plasticity.

The complexus adhaerens of mammalian lymphatic endothelia revisited: a junction even more complex than hitherto thought

The significance of a special kind of VE-cadherin-based, desmoplakin- and plakoglobin-containing adhering junction, originally identified in certain endothelial cells of the mammalian lymphatic system (notably the retothelial cells of the lymph node sinus and a subtype of lining endothelial cells of peripheral lymphatic vessels), has been widely confirmed and its importance in the formation of blood and lymph vessels has been demonstrated in vivo and in vitro. We have recently extended the molecular and structural characterization of the complexus adhaerens and can now report that it represents a rare and special combination of components known from three other major types of cell junction. It comprises zonula adhaerens proteins (VE-cadherin, alpha- and beta-catenin, protein p120(ctn), and afadin), desmosomal plaque components (desmoplakin and plakoglobin), and tight-junction proteins (claudin-5 and ZO-1) and forms junctions that vary markedly in size and shape. The special character and the possible biological roles of the complexus adhaerens and its unique ensemble of molecules in angiogenesis, immunology, and oncology are discussed. The surprising finding of claudin-5 and protein ZO-1 in substructures of retothelial cell-cell bridges, i.e. structures that do not separate different tissues or cell layer compartments, suggests that such tight-junction molecules are involved in functions other than the "fence" and "barrier" roles of zonulae occludentes.

Drebrin, an Actin-Binding, Cell-Type Characteristic Protein: Induction and Localization in Epithelial Skin Tumors and Cultured Keratinocytes

Isoform E2 of drebrin, an actin-binding protein originally identified in neuronal cells, has recently been identified in diverse non-neuronal cells, mostly in association with cell processes and intercellular junctions. Here, we report on the presence of drebrin in normal human skin, epithelial skin cancers, and cultured keratinocytes. Keratinocytes of normal epidermis contain almost no drebrin but the protein is readily seen in hair follicles. By immunohistochemistry and immunoblot, basal cell carcinomas (BCC) are rich in drebrin, and confocal laser scanning and immunoelectron microscopy show accumulation at adhering junctions, in co-localization with actin and partially with plaque proteins. In squamous cell carcinomas, keratoacanthomas, and in epidermal precancers, drebrin is heterogeneously distributed, appearing as mosaics. Primary keratinocyte cultures contain significant amounts of drebrin enriched at adhering junctions. When epithelium-derived cells devoid of drebrin are transfected with drebrin-enhanced green fluorescent protein, constructs accumulate in the cell periphery, and immunoprecipitation shows complexes with actin. During epidermal growth factor induced formation of cell processes, drebrin retains this junction association, as observed by live cell microscopy. Our results suggest novel functions of drebrin such as an involvement in cell-cell adhesion and tumorigenesis and a potential value in diagnosis of BCC.