Identification of novel interaction partners for the conserved membrane proximal region of alpha-integrin cytoplasmic domains

Untangling huge literature to disinter genetic underpinnings of Alzheimer's Disease: A systematic review and meta-analysis

Drug discovery for Alzheimer's Disease (AD) is channeled towards unravelling key disease specific drug targets/genes to predict promising therapeutic candidates. Though enormous literature on AD genetics is available, there exists dearth in data pertinent to drug targets and crucial pathological pathways intertwined in disease progression. Further, the research findings revealing genetic associations failed to demonstrate consistency across different studies. This scenario prompted us to initiate a systematic review and meta-analysis with an aim of unearthing significant genetic hallmarks of AD. Initially, a Boolean search strategy was developed to retrieve case-control studies from PubMed, Cochrane, ProQuest, Europe PMC, grey literature and HuGE navigator. Subsequently, certain inclusion and exclusion criteria were framed to shortlist the relevant studies. These studies were later critically appraised using New Castle Ottawa Scale and Q-Genie followed by data extraction. Later, meta-analysis was performed only for those Single Nucleotide Polymorphisms (SNPs) which were evaluated in at least two different ethnicities from two different reports. Among, 204,351 studies retrieved, 820 met our eligibility criteria and 117 were processed for systematic review after critical appraisal. Ultimately, meta-analysis was performed for 23 SNPs associated with 15 genes which revealed significant associations of rs3865444 (CD33), rs7561528 (BIN1) and rs1801133 (MTHFR) with AD risk.

The Progresses of Spermatogonial Stem Cells Sorting Using Fluorescence-Activated Cell Sorting

In recent years, the research on stem cells has been more and more in-depth, and many achievements have been made in application. However, due to the small number of spermatogonial stem cells (SSCs) and deficiency of efficient markers, it is difficult to obtain very pure SSCs, which results in the research on them being hindered. In fact, many methods have been developed to isolate and purify SSCs, but these methods have their shortcomings. Fluorescence-activated cell sorting (FACS), as a method to enrich SSCs with the help of specific surface markers, has the characteristics of high efficiency and accuracy in enrichment of SSCs, thus it is widely accepted as an effective method for purification of SSCs. This review summarizes the recent studies on the application of FACS in SSCs, and introduces some commonly used markers of effective SSCs sorting, aiming to further optimize the FACS sorting method for SSCs, so as to promote the research of germline stem cells and provide new ideas for the research of reproductive biology.

The Mechanistic Role of Bridging Integrator 1 (BIN1) in Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of dementia. The majority of AD cases are late-onset, multifactorial cases. Genome-wide association studies have identified more than 30 loci associated with sporadic AD (SAD), one of which is Bridging integrator 1 (BIN1). For the past few years, there has been a consensus that BIN1 is second only to APOE as the strongest genetic risk factor for SAD. Therefore, many researchers have put great effort into studying the mechanism by which BIN1 might be involved in the pathogenetic process of AD. To date, plenty of evidence has shown that BIN1 may participate in several pathways in AD, including tau and amyloid pathology. In addition, BIN1 has been indicated to take part in other relevant pathways such as inflammation, apoptosis, and calcium homeostasis. In this review, we systemically summarize the research progress on how BIN1 participates in the development of AD, with the expectation of providing promising perspectives for future research.

Molecular mechanism of Fast Endophilin-Mediated Endocytosis

Endocytosis mediates the cellular uptake of micronutrients and cell surface proteins. Clathrin-mediated endocytosis (CME) is the housekeeping pathway in resting cells but additional Clathrin-independent endocytic (CIE) routes, including Fast Endophilin-Mediated Endocytosis (FEME), internalize specific cargoes and support diverse cellular functions. FEME is part of the Dynamin-dependent subgroup of CIE pathways. Here, we review our current understanding of the molecular mechanism of FEME. Key steps are: (i) priming, (ii) cargo selection, (iii) membrane curvature and carrier formation, (iv) membrane scission and (v) cytosolic transport. All steps are controlled by regulatory mechanisms mediated by phosphoinositides and by kinases such as Src, LRRK2, Cdk5 and GSK3β. A key feature of FEME is that it is not constitutively active but triggered upon the stimulation of selected cell surface receptors by their ligands. In resting cells, there is a priming cycle that concentrates Endophilin into clusters on discrete locations of the plasma membrane. In the absence of receptor activation, the patches quickly abort and new cycles are initiated nearby, constantly priming the plasma membrane for FEME. Upon activation, receptors are swiftly sorted into pre-existing Endophilin clusters, which then bud to form FEME carriers within 10 s. We summarize the hallmarks of FEME and the techniques and assays required to identify it. Next, we review similarities and differences with other CIE pathways and proposed cargoes that may use FEME to enter cells. Finally, we submit pending questions and future milestones and discuss the exciting perspectives that targeting FEME may boost treatments against cancer and neurodegenerative diseases.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Evaluation of the Common Molecular Basis in Alzheimer's and Parkinson's Diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative disorders related to aging. Though several risk factors are shared between these two diseases, the exact relationship between them is still unknown. In this paper, we analyzed how these two diseases relate to each other from the genomic, epigenomic, and transcriptomic viewpoints. Using an extensive literature mining, we first accumulated the list of genes from major genome-wide association (GWAS) studies. Based on these GWAS studies, we observed that only one gene (HLA-DRB5) was shared between AD and PD. A subsequent literature search identified a few other genes involved in these two diseases, among which SIRT1 seemed to be the most prominent one. While we listed all the miRNAs that have been previously reported for AD and PD separately, we found only 15 different miRNAs that were reported in both diseases. In order to get better insights, we predicted the gene co-expression network for both AD and PD using network analysis algorithms applied to two GEO datasets. The network analysis revealed six clusters of genes related to AD and four clusters of genes related to PD; however, there was very low functional similarity between these clusters, pointing to insignificant similarity between AD and PD even at the level of affected biological processes. Finally, we postulated the putative epigenetic regulator modules that are common to AD and PD.

Overview and Current Status of Alzheimer's Disease in Bangladesh

Alzheimer’s disease (AD) is a complex neurological disorder with economic, social, and medical burdens which is acknowledged as leading cause of dementia marked by the accumulation and aggregation of amyloid-β peptide and phosphorylated tau (p-tau) protein and concomitant dementia, neuron loss and brain atrophy. AD is the most prevalent neurodegenerative brain disorder with sporadic etiology, except for a small fraction of cases with familial inheritance where familial forms of AD are correlated to mutations in three functionally related genes: the amyloid-β protein precursor and presenilins 1 and 2, two key γ-secretase components. The common clinical features of AD are memory impairment that interrupts daily life, difficulty in accomplishing usual tasks, confusion with time or place, trouble understanding visual images and spatial relationships. Age is the most significant risk factor for AD, whereas other risk factors correlated with AD are hypercholesterolemia, hypertension, atherosclerosis, coronary heart disease, smoking, obesity, and diabetes. Despite decades of research, there is no satisfying therapy which will terminate the advancement of AD by acting on the origin of the disease process, whereas currently available therapeutics only provide symptomatic relief but fail to attain a definite cure and prevention. This review also represents the current status of AD in Bangladesh.

Alzheimer's disease risk genes and mechanisms of disease pathogenesis

We review the genetic risk factors for late-onset Alzheimer's disease (AD) and their role in AD pathogenesis. More recent advances in understanding of the human genome-technologic advances in methods to analyze millions of polymorphisms in thousands of subjects-have revealed new genes associated with AD risk, including ABCA7, BIN1, CASS4, CD33, CD2AP, CELF1, CLU, CR1, DSG2, EPHA1, FERMT2, HLA-DRB5-DBR1, INPP5D, MS4A, MEF2C, NME8, PICALM, PTK2B, SLC24H4-RIN3, SORL1, and ZCWPW1. Emerging technologies to analyze the entire genome in large data sets have also revealed coding variants that increase AD risk: PLD3 and TREM2. We review the relationship between these AD risk genes and the cellular and neuropathologic features of AD. Understanding the mechanisms underlying the association of these genes with risk for disease will provide the most meaningful targets for therapeutic development to date.

Fluorescence- and magnetic-activated cell sorting strategies to isolate and enrich human spermatogonial stem cells

OBJECTIVE

To determine the molecular characteristics of human spermatogonia and optimize methods to enrich spermatogonial stem cells (SSCs).

DESIGN

Laboratory study using human tissues.

SETTING

Research institute.

PATIENT(S)

Healthy adult human testicular tissue.

INTERVENTION(S)

Human testicular tissue was fixed or digested with enzymes to produce a cell suspension. Human testis cells were fractionated by fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS).

MAIN OUTCOME MEASURE(S)

Immunostaining for selected markers, human-to-nude mouse xenotransplantation assay.

RESULT(S)

Immunohistochemistry costaining revealed the relative expression patterns of SALL4, UTF1, ZBTB16, UCHL1, and ENO2 in human undifferentiated spermatogonia as well as the extent of overlap with the differentiation marker KIT. Whole mount analyses revealed that human undifferentiated spermatogonia (UCHL1+) were typically arranged in clones of one to four cells whereas differentiated spermatogonia (KIT+) were typically arranged in clones of eight or more cells. The ratio of undifferentiated-to-differentiated spermatogonia is greater in humans than in rodents. The SSC colonizing activity was enriched in the THY1dim and ITGA6+ fractions of human testes sorted by FACS. ITGA6 was effective for sorting human SSCs by MACS; THY1 and EPCAM were not.

CONCLUSION(S)

Human spermatogonial differentiation correlates with increased clone size and onset of KIT expression, similar to rodents. The undifferentiated-to-differentiated developmental dynamics in human spermatogonia is different than rodents. THY1, ITGA6, and EPCAM can be used to enrich human SSC colonizing activity by FACS, but only ITGA6 is amenable to high throughput sorting by MACS.

Integrin α6A splice variant regulates proliferation and the Wnt/β-catenin pathway in human colorectal cancer cells

Integrin α6Aβ4 is up-regulated in colorectal cancers. Knockdown of α6A in adenocarcinoma cell lines revealed a sustained reduction of cell growth both in cellulo and in xenografts as well as a repression of a number of Wnt/β-catenin pathway end points.

The integrin α6 subunit pre-messenger RNA undergoes alternative splicing to generate two different splice variants, named α6A and α6B, having distinct cytoplasmic domains. In the human colonic gland, these splice variants display different patterns of expression suggesting specific functions for each variant. We have previously found an up-regulation of the α6β4 integrin in colon adenocarcinomas as well as an increase in the α6A/α6B ratio, but little is known about the involvement of α6Aβ4 versus α6Bβ4 in this context. The aim of this study was to elucidate the function of the α6Aβ4 integrin in human colorectal cancer (CRC) cells. Expression studies on a panel of primary CRCs confirmed that the up-regulation of the α6 subunit in CRC is a direct consequence of the increase of the α6A variant. To investigate the functional significance of an α6A up-regulation in CRC, we specifically knocked down its expression in well-established CRC cell lines using a small-hairpin RNA approach. Results showed a growth rate reduction in all α6A knockdown CRC cell lines studied. The α6A silencing was also found to be associated with a significant repression of a number of Wnt/β-catenin pathway end points. Moreover, it was accompanied by a reduction in the capacity of these cells to develop tumours in xenografts. Taken together, these results demonstrate that the α6A variant is a pro-proliferative form of the α6 integrin subunit in CRC cells and appears to mediate its effects through the Wnt/β-catenin pathway.

Bridging integrator 1 (BIN1): form, function, and Alzheimer's disease

The bridging integrator 1 (BIN1) gene, also known as amphiphysin 2, has recently been identified as the most important risk locus for late onset Alzheimer's disease (LOAD), after apolipoprotein E (APOE). Here, we summarize the known functions of BIN1 and discuss the polymorphisms associated with LOAD, as well as their possible physiological effects. Emerging data suggest that BIN1 affects AD risk primarily by modulating tau pathology, but other affected cellular functions are discussed, including endocytosis/trafficking, inflammation, calcium homeostasis, and apoptosis. Epigenetic modifications are important for AD pathogenesis, and we review data that suggests the possible DNA methylation of the BIN1 promoter. Finally, given the potential contributions of BIN1 to AD pathogenesis, targeting BIN1 might present novel opportunities for AD therapy.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Mss4 protein is a regulator of stress response and apoptosis

Mss4 (mammalian suppressor of Sec4) is an evolutionarily highly conserved protein and shows high sequence and structural similarity to nucleotide exchange factors. Although Mss4 tightly binds a series of exocytic Rab GTPases, it exercises only a low catalytic activity. Therefore Mss4 was proposed to work rather as a chaperone, protecting nucleotide free Rabs from degradation than as a nucleotide exchange factor. Here we provide further evidence for chaperone-like properties of Mss4. We show that expression levels of cellular Mss4 mRNA and protein are rapidly changed in response to a broad range of extracellular stress stimuli. The alterations are regulated mostly via the (c-jun NH₂-terminal kinase) JNK stress MAPK signaling pathway and the mode of regulation resembles that of heat shock proteins. Similar to heat shock proteins, upregulation of Mss4 after stress stimulation functions protectively against the programmed cell death. Molecular analysis of the Mss4-mediated inhibition of apoptosis showed that interaction of Mss4 with eIF3f (eukaryotic translation initiation factor 3 subunit f), a member of the translation initiation complex and a protein with distinct pro-apoptotic properties, is the critical event in the anti-apoptotic action of Mss4.

Alzheimer's Disease: APP, Gamma Secretase, APOE, CLU, CR1, PICALM, ABCA7, BIN1, CD2AP, CD33, EPHA1, and MS4A2, and Their Relationships with Herpes Simplex, C. Pneumoniae, Other Suspect Pathogens, and the Immune System

Alzheimer's disease susceptibility genes, APP and gamma-secretase, are involved in the herpes simplex life cycle, and that of other suspect pathogens (C. pneumoniae, H. pylori, C. neoformans, B. burgdorferri, P. gingivalis) or immune defence. Such pathogens promote beta-amyloid deposition and tau phosphorylation and may thus be causative agents, whose effects are conditioned by genes. The antimicrobial effects of beta-amyloid, the localisation of APP/gamma-secretase in immunocompetent dendritic cells, and gamma secretase cleavage of numerous pathogen receptors suggest that this network is concerned with pathogen disposal, effects which may be abrogated by the presence of beta-amyloid autoantibodies in the elderly. These autoantibodies, as well as those to nerve growth factor and tau, also observed in Alzheimer's disease, may well be antibodies to pathogens, due to homology between human autoantigens and pathogen proteins. NGF or tau antibodies promote beta-amyloid deposition, neurofibrillary tangles, or cholinergic neuronal loss, and, with other autoantibodies, such as anti-ATPase, are potential agents of destruction, whose formation is dictated by sequence homology between pathogen and human proteins, and thus by pathogen strain and human genes. Pathogen elimination in the ageing population and removal of culpable autoantibodies might reduce the incidence and offer hope for a cure in this affliction.

Identification and characterisation of novel Mss4-binding Rab GTPases

The Mss4 (mammalian suppressor of yeast Sec4) is an evolutionarily highly conserved protein and is expressed in all mammalian tissues. Although its precise biological function is still elusive, it has been shown to associate with a subset of secretory Rab proteins (Rab1b, Rab3a, Rab8a, Rab10) and to possess a rather low guanine nucleotide exchange factor (GEF) activity towards them in vitro (Rab1, Rab3a and Rab8a). By screening a human placenta cDNA library with Mss4 as bait, we identified several Rab GTPases (Rab12, Rab13 and Rab18) as novel Mss4-binding Rab proteins. Only exocytic but no endocytic Rab GTPases were found in our search. The binding of Mss4 to Rab proteins was confirmed by direct yeast two-hybrid interaction, by co-immunoprecipitation from lysates of mammalian cells, by immunofluorescence colocalisation as well as by direct in vitro binding studies. Analysis of Mss4 catalytic activity towards different Rab substrates confirmed that it is a somewhat inefficient GEF. These data, together with our mutational analysis of Mss4-Rab binding capacity, support the already proposed idea that Mss4 functions rather as a chaperone for exocytic Rab GTPases than as a GEF.

Integrin α6β4 in colorectal cancer

The ability of cells to interact with extracellular matrix macromolecules is at the forefront of the regulation of cell phenotype and organization. Indeed most if not all cells bear specific cell surface receptors for these molecules, namely the integrins, which are specific for the ligation of various macromolecules such as the laminins, fibronectins and tenascins. It is now well established that integrins can regulate a variety of biological activities, most notably cell cycle and tissue-specific gene expression. In the intestine, several observations suggest functional roles for cell-matrix interactions in the regulation of epithelial cell functions. This article focuses on integrin α6β4 as a paradigm to illustrate the importance as well as the complexity of integrins in the mediation of cell-matrix interactions. Indeed, α6β4 has been well-characterized for its involvement as a link between the cytoskeleton and extracellular matrix molecules as well as in the activation of a variety of intracellular signalization processes in cooperation with growth factor receptors. Furthermore, recent studies show that distinct forms of α6 and β4 subunits are expressed in the human intestine and, more importantly, recent work provides experimental evidence that various forms of α6β4 can differentially regulate intestinal epithelial cell functions under both normal and pathological conditions. For instance, it has been discovered that colorectal cancer cells express a hybrid form of α6β4 that is never seen in normal cells. Although further work is needed, integrin α6β4 is emerging as a key regulator of intestinal functions in both intestinal health and disease.

Advances and perspectives of the architecture of hemidesmosomes: lessons from structural biology

Hemidesmosomes (HD) are adhesive protein complexes that mediate stable attachment of basal epithelial cells to the underlying basement membrane. The organization of HDs relies on a complex network of protein-protein interactions, in which integrin alpha6beta4 and plectin play an essential role. Here we summarize the current knowledge of the structure of hemidesmosomal proteins, which includes the structures of the first and second fibronectin type III (FnIII) domains and the calx-beta domain of the integrin beta4 subunit, the actin binding domain of plectin, and two non-overlapping pairs of spectrin repeats of plectin and BPAG1e. Binding of plectin to the beta4 subunit is critical for the formation and the stability of HDs. The recent 3D structure of the primary complex between the integrin beta4 subunit and plectin has provided a first insight into the macromolecular recognition mechanisms responsible for HD assembly. Two missense mutations in beta4 linked to non lethal forms of epidermolysis bullosa map on the plectin-binding surface. Finally, the formation of the beta4-plectin complex induces conformational changes in beta4 and plectin, suggesting that their interaction may be subject to allosteric regulation.

The coordination between actin filaments and adhesion in mesenchymal migration

Mesenchymal cell motility is characterized by a polarized distribution of actin filaments, with a network of short branched actin filaments at the leading edge, and polymers of actin filaments arranged into distinct classes of actin stress fibers behind the leading edge. Importantly, the distinct actin filaments are characteristically associated with discrete adhesion structures and both the adhesions and the actin filaments are co-ordinately regulated during cell migration. While it has long been known that these macromolecular structures are intimately linked in cells, precisely how they are co-ordinately regulated is presently unknown. Live imaging data now suggests that the focal adhesions may act as sites of actin polymerization resulting in the generation of tension-bearing actin bundles of actin filaments (stress fibers). Moreover, a picture is emerging to suggest that the tropomyosin family of proteins that can determine actin filament dynamics may also play a key role in determining the transition between adhesion states. Molecules such as the tropomyosins are therefore tantalizing candidates to orchestrate the coordination of actin and adhesion dynamics during mesenchymal cell migration.

Integrin alpha6Bbeta4 inhibits colon cancer cell proliferation and c-Myc activity

Background

Integrins are known to be important contributors to cancer progression. We have previously shown that the integrin β4 subunit is up-regulated in primary colon cancer. Its partner, the integrin α6 subunit, exists as two different mRNA splice variants, α6A and α6B, that differ in their cytoplasmic domains but evidence for distinct biological functions of these α6 splice variants is still lacking.

Methods

In this work, we first analyzed the expression of integrin α6A and α6B at the protein and transcript levels in normal human colonic cells as well as colorectal adenocarcinoma cells from both primary tumors and established cell lines. Then, using forced expression experiments, we investigated the effect of α6A and α6B on the regulation of cell proliferation in a colon cancer cell line.

Results

Using variant-specific antibodies, we observed that α6A and α6B are differentially expressed both within the normal adult colonic epithelium and between normal and diseased colonic tissues. Proliferative cells located in the lower half of the glands were found to predominantly express α6A, while the differentiated and quiescent colonocytes in the upper half of the glands and surface epithelium expressed α6B. A relative decrease of α6B expression was also identified in primary colon tumors and adenocarcinoma cell lines suggesting that the α6A/α6B ratios may be linked to the proliferative status of colonic cells. Additional studies in colon cancer cells showed that experimentally restoring the α6A/α6B balance in favor of α6B caused a decrease in cellular S-phase entry and repressed the activity of c-Myc.

Conclusion

The findings that the α6Bβ4 integrin is expressed in quiescent normal colonic cells and is significantly down-regulated in colon cancer cells relative to its α6Aβ4 counterpart are consistent with the anti-proliferative influence and inhibitory effect on c-Myc activity identified for this α6Bβ4 integrin. Taken together, these findings point out the importance of integrin variant expression in colon cancer cell biology.

Integrin alpha3beta1 interacts with I1PP2A/lanp and phosphatase PP1

Integrin alpha3beta1 is a receptor for the extracellular matrix component laminin 5. To elucidate possible signaling pathways induced by integrin alpha3beta1, we looked for proteins that interact with the cytoplasmic part of the alpha3A integrin subunit. We identified several multifunctional proteins by affinity chromatography and subsequent MALDI-TOF-MS and focused on the inhibitor 1 of serine/threonine phosphatase PP2A (I1PP2A, synonym: lanp) which also plays a role during the development of the mouse cerebellum. I1PP2A/lanp colocalizes with the alpha3A integrin subunit in differentiated PC12 cells in the cell body and in neurites as well as in Purkinje cells of mouse cerebellum. Overexpression of GFP-I1PP2A/lanp in PC12 cells leads to markedly reduced neurite length on laminin 5 after induction with nerve growth factor. By affinity chromatography the protein phosphatase PP1 can also be identified as a alpha3A/cyto-binding protein. PP1 and integrin alpha3beta1 can be pulled down by GST-I1PP2A/lanp from cell lysates of differentiated and undifferentiated PC12 cells. The phosphatase binds to the cytoplasmic membrane-proximal conserved GFFKR motif of the alpha integrin subunit, whereas I1PP2A/lanp requires a longer sequence for binding. PP1 but not PP2A is able to dephosphorylate precipitated integrin alpha3beta1 in vitro. Furthermore, PP1 releases phosphate from T1046 of phosphopeptides that mimic the phosphorylation consensus sequence in the cytoplasmic part of the alpha3A integrin subunit. These data suggest that I1PP2A/lanp forms a complex with PP1 and the alpha3A integrin subunit and might possibly regulate the phosphorylation status of integrin alpha3beta1 and/or integrin downstream targets.

Integrin-mediated adhesion: tipping the balance between chemosensitivity and chemoresistance

The integrin family of extracellular matrix receptors plays an important role in normal development, epithelial morphogenesis, angiogenesis, and in tumor progression and metastasis. Integrins cooperate with growth factor receptors to control many cellular functions including proliferation and cell survival. Integrin-mediated adhesion regulates many of the cell cycle checkpoints including activation of cyclin D/cdk4/6 complexes, expression of cyclin D genes, and regulation of levels of cyclin-dependent kinase inhibitors. In addition, integrin-mediated cell adhesion regulates apoptosis by modulating the activity of both the mitochondrial pathway and the death receptor pathways. Therefore, integrin-mediated adhesion modulates the decision of life or death. A role for tumor-matrix interactions in the acquisition of drug resistance has been reported for many cancers including breast cancer. Recent evidence suggests that integrin-mediated adhesion to the ECM may undermine the response of tumors to chemotherapeutic agents. Integrins have been shown to be readily accessible drug targets and are therefore attractive potential targets for combined modality chemotherapy.

The binding of Mss4 to alpha-integrin subunits regulates matrix metalloproteinase activation and fibronectin remodeling

In four independent yeast two-hybrid screens with the integrin alpha-subunits alpha3A, alpha6A, alpha7A, and alpha7B, we identified the Mss4 protein, a nucleotide exchange factor for exocytic Rab GTPases, as a novel integrin interacting protein. We have previously shown that it binds to the conserved KXGFFKR region of integrin alpha-subunits located directly beneath the cell membrane. Here we show that the binding site for integrins on Mss4 is overlapping with those for Rab GTPases. Functional analysis of the Mss4/integrin interaction revealed its importance for activation of matrix metalloproteinases (MMPs) and remodeling of secreted extracellular matrix (ECM) proteins. The exocytosis of all the proteins analyzed, however, was unaffected. Furthermore, our data suggest that Mss4 drives the coordinated action of the MT1-MMP/integrin protein complex, thus regulating the presence and activation of MT1-MMP at newly formed filopodia and lamellipodia. This in turn facilitates the conversion of pro-MMPs to MMPs, resulting in cleavage and remodeling of ECM proteins. C2C12 myoblasts with stably down-regulated Mss4 showed a disturbed fibronectin remodeling during differentiation, resulting in malfunctioned myotube formation.

Hermansky-Pudlak syndrome: a disease of protein trafficking and organelle function

The Hermansky-Pudlak syndrome (HPS) is a collection of related autosomal recessive disorders which are genetically heterogeneous. There are eight human HPS subtypes, characterized by oculocutaneous albinism and platelet storage disease; prolonged bleeding, congenital neutropenia, pulmonary fibrosis, and granulomatous colitis can also occur. HPS is caused primarily by defects in intracellular protein trafficking that result in the dysfunction of intracellular organelles known as lysosome-related organelles. HPS gene products are all ubiquitously expressed and all associate in various multi-protein complexes, yet HPS has cell type-specific disease expression. Impairment of specialized secretory cells such as melanocytes, platelets, lung alveolar type II epithelial cells and cytotoxic T cells are observed in HPS. This review summarizes recent molecular, biochemical and cell biological analyses together with clinical studies that have led to the correlation of molecular pathology with clinical manifestations and led to insights into such diverse disease processes such as albinism, fibrosis, hemorrhage, and congenital neutropenia.

Nucleotide exchange via local protein unfolding--structure of Rab8 in complex with MSS4

Rab GTPases function as essential regulators of vesicle transport in eukaryotic cells. MSS4 was shown to stimulate nucleotide exchange on Rab proteins associated with the exocytic pathway and to have nucleotide-free-Rab chaperone activity. A detailed kinetic analysis of MSS4 interaction with Rab8 showed that MSS4 is a relatively slow exchange factor that forms a long-lived nucleotide-free complex with RabGTPase. In contrast to other characterized exchange factor-GTPase complexes, MSS4:Rab8 complex binds GTP faster than GDP, but still ca. 3 orders of magnitude more slowly than comparable complexes. The crystal structure of the nucleotide-free MSS4:Rab8 complex revealed that MSS4 binds to the Switch I and interswitch regions of Rab8, forming an intermolecular beta-sheet. Complex formation results in dramatic structural changes of the Rab8 molecule, leading to unfolding of the nucleotide-binding site and surrounding structural elements, facilitating nucleotide release and slowing its rebinding. Coupling of nucleotide exchange activity to a cycle of GTPase unfolding and refolding represents a novel nucleotide exchange mechanism.

Genetic effects of anorganic bovine bone (Bio-Oss®) on osteoblast-like MG63 cells

Bio-Oss (Geistlich, Wolhusen, Switzerland) is composed by anorganic bovine bone and is widely used in several bone regeneration procedures in oral surgery. How this biomaterial alters osteoblast gene expression to promote bone formation is poorly understood. We therefore attempted to address this question by using microarray techniques to identify genes that are differentially regulated in osteoblasts exposed to Bio-Oss. By using DNA microarrays containing 20,000 genes, we identified in osteoblast-like cells line (MG-63) cultured with Bio-Oss several genes which expression was significantly up- and down-regulated. The differentially expressed genes cover a broad range of functional activities: (a) signaling transduction, (b) transcription, (c) cell cycle regulation, (d) vesicular transport, (e) apoptosis, and (f) immunity. These results could explain the reported bioaffinity of Bio-Oss to host animals, its biological affinity to osteogenic cells and its capability to stimulate osteoblastic differentiation. The data reported are, to our knowledge, the first genetic portrait of Bio-Oss effects. They can be relevant to our improved understanding of the molecular mechanism underlying bone regenerative procedures and as a model for comparing other materials with similar clinical effects.

The cytoplasmic tail of the alpha3 integrin subunit promotes neurite outgrowth in PC12 cells

Binding of integrins to proteins of the extracellular matrix (ECM) provides structural and signaling information for biological processes such as cell proliferation, migration, neurite outgrowth, and differentiation. Integrins represent a family of heterodimeric transmembrane cell surface receptors. Besides connecting the ECM with the cytoskeleton, integrins also induce various signaling pathways in response to ligand binding. Integrin ligation leads to cytoplasmic protein-protein interactions requiring both integrin cytoplasmic tails. These sequences are initiation points for focal adhesion formation and subsequent signal transduction cascades. In this study, we addressed the question of whether the short cytoplasmic tail of the alpha(3) integrin subunit of alpha(3)beta(1) integrin is required for alpha(3)beta(1) integrin-dependent processes. For this purpose, cDNA representing the extracellular and transmembrane domain of the interleukin 2 receptor (IL2R) alpha subunit and the cytoplasmic sequence of the alpha(3) integrin subunit was transfected into PC12 cells. Autonomous expression of the cytoplasmic alpha(3) tail does not affect attachment but leads to inhibition of neuronal differentiation on laminin 5. This indicates that the cytoplasmic alpha(3) sequence is not required for cell attachment but is necessary for long-term adhesion and for the reorganization of the cytoskeleton that precedes neuronal differentiation. Inhibition of neurite outgrowth by chimeric IL2R-alpha(3) can be rescued by treatment of transfected cells with the pharmacological inhibitor Y27632, which inhibits the RhoA downstream effector Rho kinase alpha.

In utero angiopoietin-2 gene delivery remodels placental blood vessel phenotype: a murine model for studying placental angiogenesis

Angiopoietin (Ang)-2, the natural antagonist of the Ang1/Tie2 receptor is a complex regulator of blood vessel plasticity that plays a pivotal role in both vessel sprouting [in the presence of vascular endothelial growth factor (VEGF)-A] and vessel regression (in the absence of VEGF-A). Based on the spatial and temporal expression of Ang2 throughout human gestation, we recently suggested that the Ang2 may play a pivotal role in placental angiogenesis. Further, to examine this tenet we have developed a novel murine model system in which in utero Ang2 gene delivery via a non-replicating adenoviral expression vector has the potential to manipulate the blood vessel phenotype in vivo during pregnancy. Ang2 overexpression selectively and rapidly remodels the labyrinth perivascular extracellular matrix, subsequently promoting plasticity of the maternal and fetal vessels, which appear honeycombed due to a 2-fold increase in blood vessel luminal area. High levels of Ang2 impair endothelial cell adhesiveness, leading to vascular leakiness with perivascular oedema, which increases placental weight. These observations suggest that the Ang2 overexpression may play a key role in placental vascular remodelling. Furthermore, we suggest a novel new model to study the pathobiology of placental vascularization and the effect of placental blood vessels on fetal phenotype.

Cellular, molecular and clinical characterization of patients with Hermansky-Pudlak syndrome type 5

Hermansky-Pudlak syndrome (HPS) is a disorder of lysosome-related organelles such as melanosomes and platelet dense granules. Seven genes are now associated with HPS in humans. An accurate diagnosis of each HPS subtype has important prognostic and treatment implications. Here we describe the cellular, molecular, and clinical aspects of the recently identified HPS-5 subtype. We first analyzed the genomic organization and the RNA expression pattern of HPS5, located on chromosome 11p14, and demonstrated tissue-specific expression of at least three alternatively spliced HPS5 mRNA transcripts, coding for HPS5A and HPS5B proteins, that differ at their 5'-ends. Genetic screening of 15 unassigned HPS patients yielded six new HPS5 mutations in four patients. Clinically, our HPS-5 patients exhibited iris transillumination, variable hair and skin pigmentation, and absent platelet dense bodies, but not pulmonary fibrosis or granulomatous colitis. In two patients with homozygous missense mutations, hemizygosity was ruled out by gene-dosage multiplex polymerase chain reaction, and immunocytochemical analyses of their fibroblasts supported the HPS-5 diagnosis. Specifically, LAMP-3 distribution was restricted to the perinuclear region in HPS-5 fibroblasts, in contrast to the normal LAMP-3 distribution, which extended to the periphery. This specific intracellular vesicle distribution in fibroblasts, in combination with the clinical features, will improve the characterization of the HPS-5 subtype.

The building BLOC(k)s of lysosomes and related organelles

Hermansky-Pudlak syndrome defines a group of genetic disorders characterized by defects in organelles of the endosomal-lysosomal system, most notably melanosomes and platelet-dense granules. About a dozen genes have been implicated in the pathogenesis of the disease in humans and mice. Most of these genes encode novel polypeptides that are not conserved in unicellular eukaryotes. Recent studies have revealed that these polypeptides are stable components of at least three distinct, ubiquitously expressed protein complexes, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2 and -3. These findings provide a framework for studies on the function of these proteins and the pathogenesis of Hermansky-Pudlak syndrome.

Characterization of recombinant and natural forms of the human LIM domain-containing protein FHL2

FHL2 (Four and a Half LIM domain-containing protein 2) is a member of a small family of proteins with four LIM domains and an N-terminal half LIM domain. It is an intracellular protein thought to function as an adaptor in the formation of multi-protein complexes involved in signaling. To obtain human FHL2 in amounts allowing further characterization, we evaluated different expression systems and chose to express FHL2 with a His6 tag in insect cells using the baculovirus system. The recombinant protein was highly expressed and could be purified to >98% homogeneity as judged by SDS-PAGE analysis. Purified recombinant FHL2 was used to generate antibodies allowing detection and immunoprecipitation of FHL2 from human cells. Both recombinant and natural FHL2 were characterized by SDS-PAGE and MALDI-TOF mass spectrometry. The molecular mass of the recombinant His6-tagged protein obtained by mass spectrometry was 36,995Da, in good agreement with the apparent mass of 36kDa in SDS-PAGE and slightly higher than the 35,981Da calculated from the sequence of the construct. The measured molecular mass of natural human FHL2 was 32,742Da and the calculated mass was 32,192Da. However, the apparent molecular mass in SDS-PAGE is 41kDa, indicating that the natural protein has an abnormal electrophoretic mobility. The results show that both the recombinant and the natural proteins are post-translationally modified and indicate that such modifications may lead to an abnormal electrophoretic behavior of natural human FHL2.

Analysis of the adaptor function of the LIM domain-containing protein FHL2 using an affinity chromatography approach

Containing four LIM domains and an N-terminal half LIM domain, FHL2 has been predicted to have an adaptor function in the formation of higher order molecular complexes in the nucleus and the cytoplasm of cells. We expressed recombinant FHL2 in insect cells using the baculovirus system and used it to isolate direct or indirect interaction partners from the cytosolic fraction of fibroblasts by affinity chromatography. These were identified by their peptide mass fingerprints using MALDI-TOF mass spectrometry. Cytoskeleton-associated proteins present among the bound proteins were shown to co-localise with FHL2 in cell lamellipodia by indirect immunofluorescence staining.

The LIM-only proteins FHL2 and FHL3 interact with alpha- and beta-subunits of the muscle alpha7beta1 integrin receptor

FHL1, FHL2, and FHL3 are members of the four and one-half LIM domain protein subclass that are expressed in striated muscles. Here we show that FHL2 and FHL3 are novel alpha(7)beta(1) integrin-interacting proteins. They bind both the alpha- and the beta-subunit as well as different splice isoforms. The minimal binding sites for FHL2 and FHL3 on beta(1A)-chain overlap, whereas on alpha(7A) and alpha(7B) subunits they are situated adjacent. Determining the binding sites for integrins on FHL2 or FHL3 revealed that the suprastructure of the whole molecule is important for these associations, rather than any single LIM domain. Immunofluorescence studies with cells expressing full-length FHL proteins or their deletion mutants showed that FHL2 and FHL3 but not FHL1 colocalize with integrins at cell adhesion sites. Further, their recruitment to the membrane results from binding to either the alpha- or the beta-chain of the integrin receptor. The association of FHL2 or FHL3 with integrin receptors neither influences attachment of cells to different substrates nor changes their migration capacity. However, in cardiac and skeletal muscles, FHL2 and FHL3, respectively, are colocalized with alpha(7)beta(1) integrin receptor at the periphery of Z-discs, suggesting a role in mechanical stabilization of muscle cells.

Bauplan of Urmetazoa: Basis for Genetic Complexity of Metazoa

Sponges were first grouped to the animal-plants or plant-animals then to the Zoophyta or Mesozoa and finally to the Parazoa. Only after the application of molecular biological techniques was it possible to place the Porifera monophyletically with the other metazoan phyla, justifying a unification of all multicellular animals to only one kingdom, the Metazoa. The first strong support came from the discovery that cell-cell and cell-matrix adhesion molecules that were cloned from sponges and were subsequently expressed share a high DNA sequence and protein function similarity with the corresponding molecules of other metazoans. Besides these evolutionary novelties for Metazoa, sponges also have morphogens and transcription factors in common with other metazoan phyla. Surprisingly, even those elements exist in Porifera, which are characteristic for pattern and axis formation. Recent studies showed that epithelial layers of sponges are sealed against the extracellular milieu through tight-junction proteins. The cell culture system from sponges, the primmorphs, was suitable for understanding morphogenetic events. Finally, stem cell marker genes were isolated, which underscored that sponge cells have the capacity to differentiate. In the relatively short period of time, approximately 200 million years, the basic pathways had to be established that allowed the transition for multicellular organisms to a colonial system through the formation of adhesion molecules; based on the development of a complex immune system and the apoptotic machinery of an integrated system, the Urmetazoa, which evolved approximately 800 million years ago, could be reached. Hence, the Bauplan of the hypothetical Urmetazoa can now be constructed according to genomic regulatory systems similar to those found in higher Metazoa. These data caused a paradigmatic change; the Porifera are complex and simple but by far not primitive.

Ru2 and Ru encode mouse orthologs of the genes mutated in human Hermansky-Pudlak syndrome types 5 and 6

Hermansky-Pudlak syndrome (HPS) is a genetically heterogeneous disease involving abnormalities of melanosomes, platelet dense granules and lysosomes. Here we have used positional candidate and transgenic rescue approaches to identify the genes mutated in ruby-eye 2 and ruby-eye mice (ru2 and ru, respectively), two 'mimic' mouse models of HPS. We also show that these genes are orthologs of the genes mutated in individuals with HPS types 5 and 6, respectively, and that their protein products directly interact. Both genes are previously unknown and are found only in higher eukaryotes, and together represent a new class of genes that have evolved in higher organisms to govern the synthesis of highly specialized lysosome-related organelles.

The PDZ domain of TIP-2/GIPC interacts with the C-terminus of the integrin alpha5 and alpha6 subunits

Different cDNA libraries were screened by the yeast two-hybrid system using as a bait the cytoplasmic sequence of integrin alpha6A or alpha6B subunits. Surprisingly, the same PDZ domain-containing protein, TIP-2/GIPC, was isolated with either of the variants, although their sequences are different. Direct interaction assays with the cytoplasmic domain of the integrin alpha1--7 subunits revealed that in addition to alpha6A and alpha6B, TIP-2/GIPC reacted also with alpha5, but not other alpha integrin subunits. The specificity of the interaction was confirmed by in vitro protein binding assays with purified peptides corresponding to integrin cytoplasmic domains. Further analysis with either truncation fragments of TIP-2/GIPC or mutated integrin cytoplasmic domains indicated that the interaction occurs between the PDZ domain of TIP-2/GIPC and a consensus PDZ domain-binding sequence, SDA, present at the C-terminus of the integrin alpha5 and alpha6A subunits. The integrin alpha6B subunit terminates with a different sequence, SYS, which may represent a new PDZ domain-binding motif.

Phospholipase Cgamma binds alpha1beta1 integrin and modulates alpha1beta1 integrin-specific adhesion

Integrin adhesion receptors have been implicated in bidirectional signal transduction. The dynamic regulation of integrin affinity and avidity as well as post-ligand effects involved in outside-in signaling depends on the interaction of integrins with cytoskeletal and signaling proteins. In this study, we attempted to identify cytoplasmic binding partners of alpha(1)beta(1) integrin. We were able to show that cell adhesion to alpha(1)beta(1)-specific substrates results in the association of phospholipase Cgamma (PLCgamma) with the alpha(1)beta(1) integrin independent of PLCgamma tyrosine phosphorylation. Using peptide-binding assays, the membrane proximal sequences within the alpha(1)beta(1) integrin subunits were identified as binding sites for PLCgamma. In particular, the conserved sequence of beta(1) subunit binds the enzyme very efficiently. Because purified PLCgamma also binds the integrin peptides, binding seems to be direct. Inhibition of PLC by leads to reduced cell adhesion on alpha(1)beta(1)-specific substrates. Cells lacking the conserved domain of the alpha(1) subunit fail to respond to the PLC inhibition, indicating that this domain is necessary for PLC-dependent adhesion modulation of alpha(1)beta(1) integrin.

Activation of the extracellular signal-regulated protein kinase (ERK) cascade by membrane-type-1 matrix metalloproteinase (MT1-MMP)

The mechanisms underlying membrane-type-1 matrix metalloproteinase (MT1-MMP)-dependent induction of cell migration were investigated. Overexpression of MT1-MMP induced a marked increase in cell migration, this increase being dependent on the presence of the cytoplasmic domain of the protein. MT1-MMP-dependent migration was inhibited by a mitogen-activated protein kinase kinase 1 inhibitor, suggesting the involvement of the extracellular signal-regulated protein kinase (ERK) cascade in the induction of migration. Accordingly, MT1-MMP overexpression induced the activation of ERK, this process being also dependent on the presence of its cytoplasmic domain. MT1-MMP-induced activation of both migration and ERK required the catalytic activity of the enzyme as well as attachment of the cells to matrix proteins. The MT1-MMP-dependent activation of ERK was correlated with the activation of transcription through the serum response element, whereas other promoters were unaffected. Taken together, these results indicate that MT1-MMP trigger important changes in cellular signal transduction events, leading to cell migration and to gene transcription, and that these signals possibly originate from the cytoplasmic domain of the protein.

Cloning and expression of the putative aggregation factor from the marine sponge Geodia cydonium

Sponges (phylum Porifera) have extensively been used as a model system to study cell-cell interaction on molecular level. Recently, we identified and cloned the putative aggregation receptor (AR) of the sponge Geodia cydonium, which interacts in a heterophilic way with the aggregation factor (AF) complex. In the present study, antibodies against this complex have been raised that abolish the adhesion function of the enriched sponge AF, the AF-Fraction 6B. Using this antibody as a tool, a complete 1.7 kb long cDNA, GEOCYAF, could be isolated from a cDNA library that encodes the putative AF. Its deduced aa sequence in the N-terminal section comprises high similarity to amphiphysin/BIN1 sequences found in Protostomia and Deuterostomia. However, the C-terminal portion of the sponge sequence lacks the SH3 domain characteristic for amphiphysin/BIN1. The polypeptide with a calculated size of 47 kDa was expressed in Escherichia coli. The recombinant, soluble 36 kDa putative AF was prepared and found to compete with the AF complex-associated adhesion protein of the AF-Fraction 6B for the binding sites at the cell surface. Furthermore, the recombinant putative AF was recognized by the antibody used to screen the cDNA library by western blotting. In addition, there is evidence that the recombinant putative AF binds to the G. cydonium galectin. It is concluded that the putative G. cydonium AF - a further autapomorphic molecule characteristic for Metazoa - binds to the AR present on the cell surface in association with the homologous galectin.

Specific residues within the alpha 2 integrin subunit cytoplasmic domain regulate migration and cell cycle progression via distinct MAPK pathways

The alpha(2) integrin subunit cytoplasmic domain is necessary for epidermal growth factor (EGF)-stimulated chemotactic migration and insulin-dependent entry into S-phase of mammary epithelial cells adherent to type I collagen. Truncation mutants revealed that the seven amino acids, KYEKMTK, in addition to the GFFKR motif were sufficient for these functions. Mutation of tyrosine 1134 to alanine inhibited the ability of the cells to phosphorylate p38 MAPK and to migrate in response to EGF but had only a modest effect on the ability of the cells to induce sustained phosphorylation of the ERK MAPK, to up-regulate cyclin E and cdk2 expression, and to enter S-phase when adherent to type I collagen. Conversely, mutation of the lysine 1136 inhibited the ability of the cells to increase cyclin E and cdk2 expression, to maintain long term phosphorylation of the ERK MAPK, and to enter S-phase but had no effect on the ability of the cells to phosphorylate the p38 MAPK or to migrate on type I collagen in response to EGF. Methionine 1137 was essential for both migration and entry into S-phase. Thus, distinctly different structural elements of the alpha(2) integrin cytoplasmic domain are required to engage the signaling pathways leading to cell migration or cell cycle progression.

Functional comparison of the alpha3A and alpha3B cytoplasmic domain variants of the chicken alpha3 integrin subunit

Integrin alpha3beta1 can be alternatively spliced to generate alpha3A and alpha3B cytoplasmic domain variants that are conserved among vertebrates. To identify distinct functions of these variants, we transfected cells with intact alpha3 integrins or chimeric receptors. alpha3Abeta1 and alpha3Bbeta1 each localized to focal contacts in keratinocytes on an extracellular matrix rich in laminin-5, to which both are known to bind with high affinity. However, alpha3B accumulated intracellularly in keratinocytes on collagen, suggesting that laminin binding may stabilize alpha3Bbeta1 surface expression. Neither alpha3 cytoplasmic domain affected recruitment of chimeric alpha5 integrins to fibronectin-induced focal contacts, and either substituted for the alpha5 cytoplasmic domain in alpha5beta1-mediated cell migration. However, the alpha5/alpha3B chimera localized to cell-cell borders in MDCK or CHO cells to a lesser extent than did the alpha5/alpha3A chimera. To determine whether the alpha3 cytoplasmic domains conferred distinct localization to a nonintegrin protein, we transfected cells with interleukin-2 receptor (IL-2R) chimeras containing the alpha3 cytoplasmic domains. The IL-2R/alpha3A chimera was expressed efficiently on the cell surface, while the IL-2R/alpha3B chimera accumulated intracellularly. Our findings suggest that the alpha3B cytoplasmic domain harbors a retention signal that is regulated in an intact integrin and can alter cell surface expression and distribution of alpha3beta1.

alpha1 Integrin cytoplasmic domain is involved in focal adhesion formation via association with intracellular proteins

Integrins are heterodimeric adhesion receptors consisting of alpha- and beta-subunits capable of binding extracellular matrix molecules as well as other adhesion receptors on neighbouring cells. These interactions induce various signal transduction pathways in many cell types, leading to cytoskeletal reorganization, phosphorylation and induction of gene expression. Integrin ligation leads to cytoplasmic protein-protein interactions requiring both integrin cytoplasmic domains, and these domains are initiation points for focal adhesion formation and subsequent signal transduction cascades. In previous studies we have shown that the very short cytoplasmic alpha1 tail is required for post-ligand events, such as cell spreading as well as actin stress-fibre formation. In the present paper we report that cells lacking the cytoplasmic domain of the alpha1 integrin subunit are unable to form proper focal adhesions and that phosphorylation on tyrosine residues of focal adhesion components is reduced on alpha1beta1-specific substrates. The alpha1 cytoplasmic sequence is a specific recognition site for focal adhesion components like paxillin, talin, alpha-actinin and pp125FAK. It seems to account for alpha1-specific signalling, since when peptides that mimic the cytoplasmic domain of alpha1 are transferred into cells, they influence alpha1beta1-specific adhesion, presumably by competing for binding partners. For alpha1 integrin/protein binding, the conserved Lys-Ile-Gly-Phe-Phe-Lys-Arg motif and, in particular, the two lysine residues, are important.

Deoxycytidyl transferase activity of the human REV1 protein is closely associated with the conserved polymerase domain

The REV1 protein is a member of the growing family of translesion DNA polymerases. A cDNA of the human REV1 gene that we had originally isolated encoded 1250 amino acids residues, which was one amino acid shorter than previously reported ones. The shorter form of REV1 was named REV1S. All individuals examined expressed equivalent amounts of REV1S and REV1 mRNA, suggesting that the REV1S mRNA is a splicing variant. We show that the REV1S protein also possesses deoxycytidyl transferase activity that inserts a dCMP opposite a DNA template apurinic/apyrimidinic site. Deletion and point mutation analysis of the REV1S protein revealed that the domain required for deoxycytidyl transferase and DNA binding activities of the REV1S protein are located in a conserved domain of translesion DNA polymerases. This result indicates that the structure of the catalytic site of the deoxycytidyl transferase closely resembles that of the translesion DNA polymerases. Therefore, the molecular mechanism of the dCMP transfer reaction of the REV1S protein and maybe also the REV1 protein might be the same as that of the dNTP transfer reaction of the translesion DNA polymerases.

The LIM-only protein DRAL/FHL2 binds to the cytoplasmic domain of several alpha and beta integrin chains and is recruited to adhesion complexes

LIM proteins contain one or more double zinc finger structures (LIM domains) mediating specific contacts between proteins that participate in the formation of multiprotein complexes. We report that the LIM-only protein DRAL/FHL2, with four and a half LIM domains, can associate with alpha(3A), alpha(3B), alpha(7A), and several beta integrin subunits as shown in yeast two-hybrid assays as well as after overexpression in human cells. The amino acid sequence immediately following the conserved membrane-proximal region in the integrin alpha subunits or the C-terminal region with the conserved NXXY motif of the integrin beta subunits are critical for binding DRAL/FHL2. Furthermore, the DRAL/FHL2 associates with itself and with other molecules that bind to the cytoplasmic domain of integrin alpha subunits. Deletion analysis of DRAL/FHL2 revealed that particular LIM domains or LIM domain combinations bind the different proteins. These results, together with the fact that full-length DRAL/FHL2 is found in cell adhesion complexes, suggest that it is an adaptor/docking protein involved in integrin signaling pathways.

The c-Myc-interacting adaptor protein Bin1 activates a caspase-independent cell death program

Cell death processes are progressively inactivated during malignant development, in part by loss of tumor suppressors that can promote cell death. The Bin1 gene encodes a nucleocytosolic adaptor protein with tumor suppressor properties, initially identified through its ability to interact with and inhibit malignant transformation by c-Myc and other oncogenes. Bin1 is frequently missing or functionally inactivated in breast and prostate cancers and in melanoma. In this study, we show that Bin1 engages a caspase-independent cell death process similar to type II apoptosis, characterized by cell shrinkage, substratum detachment, vacuolated cytoplasm, and DNA degradation. Cell death induction was relieved by mutation of the BAR domain, a putative effector domain, or by a missplicing event that occurs in melanoma and inactivates suppressor activity. Cells in all phases of the cell cycle were susceptible to death and p53 and Rb were dispensable. Notably, Bin1 did not activate caspases and the broad spectrum caspase inhibitor ZVAD.fmk did not block cell death. Consistent with the lack of caspase involvement, dying cells lacked nucleosomal DNA cleavage and nuclear lamina degradation. Moreover, neither Bcl-2 or dominant inhibition of the Fas pathway had any effect. In previous work, we showed that Bin1 could not suppress cell transformation by SV40 large T antigen. Consistent with this finding, we observed that T antigen suppressed the death program engaged by Bin1. This observation was interesting in light of emerging evidence that T antigen has roles in cell immortalization and human cell transformation beyond Rb and p53 inactivation. In support of a link to c-Myc-induced death processes, AEBSF, a serine protease inhibitor that inhibits apoptosis by c-Myc, potently suppressed DNA degradation by Bin1. Our findings suggest that the tumor suppressor activity of Bin1 reflects engagement of a unique cell death program. We propose that loss of Bin1 may promote malignancy by blunting death penalties associated with oncogene activation.

Loss of heterozygosity and tumor suppressor activity of Bin1 in prostate carcinoma

The genetic events underlying the development of prostate cancer are poorly defined. c-Myc is often activated in tumors that have progressed to metastatic status, so events that promote this process may be important. Bin1 is a nucleocytoplasmic adaptor protein with features of a tumor suppressor that was identified through its ability to interact with and inhibit malignant transformation by c-Myc. We investigated a role for Bin1 loss or inactivation in prostate cancer because the human Bin1 gene is located at chromosome 2q14 within a region that is frequently deleted in metastatic prostate cancer but where no tumor suppressor candidate has been located. A novel polymorphic microsatellite marker located within intron 5 of the human Bin1 gene was used to demonstrate loss of heterozygosity and coding alteration in 40% of informative cases of prostate neoplasia examined. RNA and immunohistochemical analyses indicated that Bin1 was expressed in most primary tumors, even at slightly elevated levels relative to benign tissues, but that it was frequently missing or inactivated by aberrant splicing in metastatic tumors and androgen-independent tumor cell lines. Ectopic expression of Bin1 suppressed the growth of prostate cancer lines in vitro. Our findings support the candidacy of Bin1 as the chromosome 2q prostate tumor suppressor gene.

The function of the human homolog of Saccharomyces cerevisiae REV1 is required for mutagenesis induced by UV light

In Saccharomyces cerevisiae, most mutations induced by a wide range of mutagens arise during translesion replication employing the REV1 gene product and DNA polymerase zeta. As part of an effort to investigate mammalian mutagenic mechanisms, we have identified cDNA clones of the human homologs of the yeast REV genes and examined their function in UV mutagenesis. Previously, we described the isolation of a human homolog of yeast REV3, the catalytic subunit of pol zeta, and here report the identification and sequence of a human homolog of yeast REV1. This gene was isolated by identifying an expressed sequence tag encoding a peptide with similarity to the C terminus of yeast Rev1p, followed by sequencing of the clone and retrieval of the remaining cDNA by 5' rapid amplification of cDNA ends. The human gene encodes an expected protein of 1,251 residues, compared with 985 residues in the yeast protein. The proteins share two amino-terminal regions of approximately 100 residues with 41% and 20% identity, a region of approximately 320 residues with 31% identity, and a central motif in which 11 of 13 residues are identical. Human cells expressing high levels of an hREV1 antisense RNA grew normally, and were not more sensitive to the cytotoxic effect of 254 nm UV radiation than cells lacking antisense RNA. However, the frequencies of 6-thioguanine resistance mutants induced by UV in the cells expressing antisense hREV1 RNA were significantly lower than in the control (P = 0.01), suggesting that the human gene has a function similar to that of the yeast homolog.

The cytoplasmic domain of the alpha1 integrin subunit influences stress fiber formation via the conserved GFFKR motif

Integrins are heterodimeric transmembrane proteins that mediate substrate adhesion and migration but also the bidirectional transfer of information across the plasma membrane via their cytoplasmic domains. We addressed the question of whether the very short cytoplasmic tail of the alpha1 integrin subunit of alpha1beta1 integrin is required for alpha1beta1-specific adhesion, spreading, and migration. For this purpose we transfected the alpha1 integrin subunit and two cytoplasmically truncated alpha1 subunits into Chinese hamster ovary (CHO) cells. Elimination of the entire cytoplasmic domain of the alpha1 subunit does not affect adhesion but leads to inhibition of spreading and stress fiber formation. The defect in spreading could not be rescued by lysophosphatidic acid, which has been reported to stimulate actin stress fiber formation via Rho. Additionally, deletion of the entire cytoplasmic domain of the alpha1 subunit abolishes migration toward alpha1beta1-specific substrates. Migration and stress fiber formation are similar in CHO-alpha1 cells and CHO cells carrying an alpha1 subunit still containing the conserved GFFKR motif. So, the GFFKR motif of the alpha1 subunit is essential and sufficient for these processes.

Interaction between the cytodomains of the alpha 3 and beta 1 integrin subunits regulates remodelling of adhesion complexes on laminin

The first step of laminin 1-induced signal transduction is initiated by the formation of alpha 6 beta 1 integrin-specific adhesion complexes. In contrast, on other laminin isoforms the adhesion complexes are alpha 3 beta 1 integrin-specific due to a transdominant regulation of the alpha 6 beta 1 integrin by the alpha 3 beta 1 integrin. To determine the mechanism of this regulation, peptides representing the cytoplasmic domain of the alpha 3 or alpha 6 integrin subunits were microinjected together with recombinant enhanced green fluorescence protein into live fibroblasts. Microinjection of the alpha 3 integrin peptide to laminin 1-adherent cells displaying alpha 6 beta 1 integrin-specific adhesion complexes resulted in the disengagement of the alpha 6 beta 1 integrin, while microinjection of green fluorescence protein alone or in combination with the alpha 6 integrin cytodomain had no effect. Further surface plasmon resonance studies revealed that the cytodomain of the beta 1 integrin subunit interacts with low affinity with the cytoplasmic tail of the alpha 3 integrin subunit, but not with that of several other alpha subunits including alpha 6. These results imply that the cytoplasmic tails of the integrin alpha subunits play a critical role in the regulation of integrin-induced signal transduction. In particular, the intracellular tail of the alpha 3 integrin subunit controls the formation of adhesion complexes in cells adhering to laminins.