?-actinin revisited: A fresh look at an old player

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.

The NPIY motif in the integrin beta1 tail dictates the requirement for talin-1 in outside-in signaling

Protein interactions with the integrin beta-subunit cytoplasmic domain (beta-tail) are essential for adhesion-dependent processes, including cell spreading and the connection of integrins with actin filaments at adhesion sites. Talin-1 binds to the conserved membrane-proximal NPxY motif of beta-tails (NPIY in beta1 integrin) promoting the inside-out activation of integrins and providing a linkage between integrins and the actin cytoskeleton. Here, we characterize the role of interactions between talin-1 and beta-tail downstream of integrin activation, in the context of recombinant integrins containing either the wild type (WT) or the (YA) mutant beta1A tail, with a tyrosine to alanine substitution in the NPIY motif. In addition to inhibiting integrin activation, the YA mutation suppresses cell spreading, integrin signaling, focal adhesion and stress-fiber formation, as well as microtubule assembly. Constitutive activation of the mutant integrin restores these integrin-dependent processes, bringing into question the importance of the NPIY motif downstream of integrin activation. Depletion of talin-1 using TLN1 siRNA demonstrated that talin-1 is required for cell spreading, focal adhesion and stress-fiber formation, as well as microtubule assembly, even when cells are adhered by constitutively activated WT integrins. Depletion of talin-1 does not inhibit these processes when cells are adhered by constitutively activated mutant integrins, suggesting that the binding of an inhibitory protein to the NPIY motif negatively regulates integrin function when talin-1 is depleted. We identified filamin A (FLNa) as this inhibitory protein; it binds to the beta1A tail in an NPIY-dependent manner and inhibition of FLNa expression in talin-1-depleted cells restores integrin function when cells are adhered by constitutively activated WT integrins. FLNa binds FilGAP, which is a negative regulator of Rac activation. Expression of the dominant inhibitory mutant, FilGAP(DeltaGAP), which lacks GAP activity restores spreading in cells adhered by constitutively activated integrins containing the beta1A tail, but not by integrins containing the beta1D tail, which is known to bind poorly to FLNa. Together, these results suggest that the binding of talin-1 to the NPIY motif is required downstream of integrin activation to promote cell spreading by preventing the inappropriate recruitment of FLNa and FilGAP to the beta1A tail. Our studies emphasize the importance of understanding the mechanisms that regulate the differential binding FLNa and talin-1 to the beta1 tail downstream of integrin activation in promoting integrin function.

Molecular mechanisms underlying nucleocytoplasmic shuttling of actinin-4

In addition to its well-known role as a crosslinker of actin filaments at focal-adhesion sites, actinin-4 is known to be localized to the nucleus. In this study, we reveal the molecular mechanism underlying nuclear localization of actinin-4 and its novel interactions with transcriptional regulators. We found that actinin-4 is imported into the nucleus through the nuclear pore complex in an importin-independent manner and is exported by the chromosome region maintenance-1 (CRM1)-dependent pathway. Nuclear actinin-4 levels were significantly increased in the late G2 phase of the cell cycle and were decreased in the G1 phase, suggesting that active release from the actin cytoskeleton was responsible for increased nuclear actinin-4 in late G2. Nuclear actinin-4 was found to interact with the INO80 chromatin-remodeling complex. It also directs the expression of a subset of cell-cycle-related genes and interacts with the upstream-binding factor (UBF)-dependent rRNA transcriptional machinery in the M phase. These findings provide molecular mechanisms for both nucleocytoplasmic shuttling of proteins that do not contain a nuclear-localization signal and cell-cycle-dependent gene regulation that reflects morphological changes in the cytoskeleton.

Connexins, cell motility, and the cytoskeleton

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

The evolution of skeletal muscle performance: gene duplication and divergence of human sarcomeric alpha-actinins

In humans, there are two skeletal muscle alpha-actinins, encoded by ACTN2 and ACTN3, and the ACTN3 genotype is associated with human athletic performance. Remarkably, approximately 1 billion people worldwide are deficient in alpha-actinin-3 due to the common ACTN3 R577X polymorphism. The alpha-actinins are an ancient family of actin-binding proteins with structural, signalling and metabolic functions. The skeletal muscle alpha-actinins diverged approximately 250-300 million years ago, and ACTN3 has since developed restricted expression in fast muscle fibres. Despite ACTN2 and ACTN3 retaining considerable sequence similarity, it is likely that following duplication there was a divergence in function explaining why alpha-actinin-2 cannot completely compensate for the absence of alpha-actinin-3. This paper focuses on the role of skeletal muscle alpha-actinins, and how possible changes in functions between these duplicates fit in the context of gene duplication paradigms.

Cathepsin X cleaves the beta2 cytoplasmic tail of LFA-1 inducing the intermediate affinity form of LFA-1 and alpha-actinin-1 binding

The motility of T cells depends on the dynamic spatial regulation of integrin-mediated adhesion and de-adhesion. Cathepsin X, a cysteine protease, has been shown to regulate T-cell migration by interaction with lymphocyte function associated antigen-1 (LFA-1). LFA-1 adhesion to the ICAM-1 is controlled by the association of actin-binding proteins with the cytoplasmic tail of the beta(2) chain of LFA-1. Cleavage by cathepsin X of the amino acid residues S(769), E(768) and A(767) from the C-terminal of the beta(2) cytoplasmic tail of LFA-1 is shown to promote binding of the actin-binding protein alpha-actinin-1. Furthermore, cathepsin X overexpression reduced LFA-1 clustering and induced an intermediate affinity LFA-1 conformation that is known to associate with alpha-actinin-1. Increased levels of intermediate affinity LFA-1 resulted in augmented cell spreading due to reduced attachment of T cells to the ICAM-1-coated surface. Gradual cleavage of LFA-1 by cathepsin X enables the transition between intermediate and high affinity LFA-1, an event that is crucial for effective T-cell migration.

Sequence and expression of the zebrafish alpha-actinin gene family reveals conservation and diversification among vertebrates

Alpha-actinins are actin microfilament crosslinking proteins. Vertebrate actinins fall into two classes: the broadly-expressed actinins 1 and 4 (actn1 and actn4) and muscle-specific actinins, actn2 and actn3. Members of this family have numerous roles, including regulation of cell adhesion, cell differentiation, directed cell motility, intracellular signaling, and stabilization of f-actin at the sarcomeric Z-line in muscle. Here we identify five zebrafish actinin genes including two paralogs of ACTN3. We describe the temporal and spatial expression patterns of these genes through embryonic development. All zebrafish actinin genes have unique expression profiles, indicating specialization of each gene. In particular, the muscle actinins display preferential expression in different domains of axial, pharyngeal, and cranial musculature. There is no identified avian actn3 and approximately 16% of humans are null for ACTN3. Duplication of actn3 in the zebrafish indicates that variation in actn3 expression may promote physiological diversity in muscle function among vertebrates.

Cell membrane expression of cardiac sodium channel Na(v)1.5 is modulated by alpha-actinin-2 interaction

Cardiac sodium channel Na(v)1.5 plays a critical role in heart excitability and conduction. The molecular mechanism that underlies the expression of Na(v)1.5 at the cell membrane is poorly understood. Previous studies demonstrated that cytoskeleton proteins can be involved in the regulation of cell surface expression and localization of several ion channels. We performed a yeast two-hybrid screen to identify Na(v)1.5-associated proteins that may be involved in channel function and expression. We identified alpha-actinin-2 as an interacting partner of the cytoplasmic loop connecting domains III and IV of Na(v)1.5 (Na(v)1.5/LIII-IV). Co-immunoprecipitation and His(6) pull-down assays confirmed the physical association between Na(v)1.5 and alpha-actinin-2 and showed that the spectrin-like repeat domain is essential for binding of alpha-actinin-2 to Na(v)1.5. Patch-clamp studies revealed that the interaction with alpha-actinin-2 increases sodium channel density without changing their gating properties. Consistent with these findings, coexpression of alpha-actinin-2 and Na(v)1.5 in tsA201 cells led to an increase in the level of expression of Na(v)1.5 at the cell membrane as determined by cell surface biotinylation. Lastly, immunostaining experiments showed that alpha-actinin-2 was colocalized with Na(v)1.5 along the Z-lines and in the plasma membrane. Our data suggest that alpha-actinin-2, which is known to regulate the functional expression of the potassium channels, may play a role in anchoring Na(v)1.5 to the membrane by connecting the channel to the actin cytoskeleton network.

Combining mechanical and optical approaches to dissect cellular mechanobiology

Mechanical force modulates a wide array of cell physiological processes. Cells sense and respond to mechanical stimuli using a hierarchy of structural complexes spanning multiple length scales, including force-sensitive molecules and cytoskeletal networks. Understanding mechanotransduction, i.e., the process by which cells convert mechanical inputs into biochemical signals, has required the development of novel biophysical tools that allow for probing of cellular and subcellular components at requisite time, length, and force scales and technologies that track the spatio-temporal dynamics of relevant biomolecules. In this review, we begin by discussing the underlying principles and recent applications of atomic force microscopy, magnetic twisting cytometry, and traction force microscopy, three tools that have been widely used for measuring the mechanical properties of cells and for probing the molecular basis of cellular mechanotransduction. We then discuss how such tools can be combined with advanced fluorescence methods for imaging biochemical processes in living cells in the context of three specific problem spaces. We first focus on fluorescence resonance energy transfer, which has enabled imaging of intra- and inter-molecular interactions and enzymatic activity in real time based on conformational changes in sensor molecules. Next, we examine the use of fluorescence methods to probe force-dependent dynamics of focal adhesion proteins. Finally, we discuss the use of calcium ratiometric signaling to track fast mechanotransductive signaling dynamics. Together, these studies demonstrate how single-cell biomechanical tools can be effectively combined with molecular imaging technologies for elucidating mechanotransduction processes and identifying mechanosensitive proteins.

Isoform-specific contributions of alpha-actinin to glioma cell mechanobiology

Glioblastoma Multiforme (GBM) is a malignant astrocytic tumor associated with low survival rates because of aggressive infiltration of tumor cells into the brain parenchyma. Expression of the actin binding protein alpha-actinin has been strongly correlated with the invasive phenotype of GBM in vivo. To probe the cellular basis of this correlation, we have suppressed expression of the nonmuscle isoforms alpha-actinin-1 and alpha-actinin-4 and examined the contribution of each isoform to the structure, mechanics, and motility of human glioma tumor cells in culture. While subcellular localization of each isoform is distinct, suppression of either isoform yields a phenotype that includes dramatically reduced motility, compensatory upregulation and redistribution of vinculin, reduced cortical elasticity, and reduced ability to adapt to changes in the elasticity of the extracellular matrix (ECM). Mechanistic studies reveal a relationship between alpha-actinin and non-muscle myosin II in which depletion of either alpha-actinin isoform reduces myosin expression and maximal cell-ECM tractional forces. Our results demonstrate that both alpha-actinin-1 and alpha-actinin-4 make critical and distinct contributions to cytoskeletal organization, rigidity-sensing, and motility of glioma cells, thereby yielding mechanistic insight into the observed correlation between alpha-actinin expression and GBM invasiveness in vivo.

Up-regulated alpha-actin expression is associated with cell adhesion ability in 3-D cultured myocytes subjected to mechanical stimulation

This study was aimed to investigate the alteration of alpha-actin in three-dimensionally (3-D) cultured myocytes under cyclic tensile stress loading. Myocytes were collected from neonatal SD rat's lateral pterygoid muscle for primary cell culture. The third-passage cells were implanted and 3-D cultured in poly lactic-co-glycolic acid (PLGA) scaffold, and then subjected to cyclic tensile stress (0.5 Hz, 2,000 microstrain) for 0, 2, 4, 8, 12, and 24 h through a four-point bending strain system. The alpha-actin mRNA was investigated by semi-quantitative RT-PCR. The alpha-actin protein expression was examined by immunofluorescent cytochemistry, laser confocal scanning microscopy (LCSM), and image analysis technology. The dynamic adhesion of myocytes to PLGA scaffolds was investigated by fluorescence microscope and the viability of the myocytes was measured by MTT assay. After mechanical loading, the alpha-actin mRNA increased at 2 h and then declined. The alpha-actin protein expression kept increased until peaked at 12 h, but declined at 24 h. The time course changing of alpha-actin protein expression parallelled with that of cell adhesion ability. It is concluded that alpha-actin expression is probably associated with cell adhesion ability in myocytes subjected to mechanical stimulation.

Phosphorylation of alpha-actinin 4 upon epidermal growth factor exposure regulates its interaction with actin

The ubiquitously expressed family of alpha-actinins bridges actin filaments to stabilize adhesions, a process disrupted during growth factor-induced migration of cells. During the dissolution of the actin cytoskeleton, actinins are phosphorylated on tyrosines, although the consequences of this are unknown. We expressed the two isoforms of human alpha-actinin in murine fibroblasts that express human epidermal growth factor receptor (EGFR) and found that both alpha-actinin 1 (ACTN1) and alpha-actinin 4 (ACTN4) were phosphorylated on tyrosine residues after stimulation with EGF, although ACTN4 was phosphorylated to the greater extent. This required the activation of Src protein-tyrosine kinase and p38-MAPK (and phosphoinositide trisphosphate kinase in part) but not MEK/ERK or Rac1, as determined by inhibitors. The EGF-induced phosphorylation sites of ACTN4 were mapped to tyrosine 4, the major site, and tyrosine 31, the minor one. Truncation mutagenesis showed that the C-terminal domains of ACTN4 (amino acids 300-911), which cross-link the actin binding head domains, act as an inhibitory domain for both actin binding and EGF-mediated phosphorylation. These two properties were mutually exclusive; removal of the C terminus enhanced actin binding of ACTN4 mutants while limiting EGF-induced phosphorylation, and conversely EGF-stimulated phosphorylation of ACTN4 decreased its affinity to actin. Interestingly, a phosphomimetic of tyrosine 265 (which can be found in carcinoma cells and lies near the K255E mutation that causes focal segmental glomerulosclerosis) demonstrated increased actin binding activity and susceptibility of ACTN4 to calpain-mediated cleavage; this variant also retarded cell spreading. Remarkably, either treatment of cells with low concentrations of latrunculin A, which has been shown to depolymerize F-actin, or the deletion of the actin binding domain (100-252 amino acids) of ACTN4Y265E restored EGF-induced phosphorylation. An F-actin binding assay in vitro showed that Y4E/Y31E, a mimetic of diphosphorylated ACTN4, bound F-actin slightly compared with wild type (WT). Importantly, the EGF-mediated phosphorylation of ACTN4 at tyrosine 4 and 31 significantly inhibited multinucleation of proliferating NR6WT fibroblasts that overexpress ACTN4. These results suggest that EGF regulates the actin binding activity of ACTN4 by inducing tyrosyl-directed phosphorylation.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Adenosine receptors interacting proteins (ARIPs): Behind the biology of adenosine signaling

Adenosine is a well known neuromodulator in the central nervous system. As a consequence, adenosine can be beneficial in certain disorders and adenosine receptors will be potential targets for therapy in a variety of diseases. Adenosine receptors are G protein-coupled receptors, and are also expressed in a large variety of cells and tissues. Using these receptors as a paradigm of G protein-coupled receptors, the present review focus on how protein-protein interactions might contribute to neurotransmitter/neuromodulator regulation, based on the fact that accessory proteins impinge on the receptor/G protein interaction and therefore modulate receptor functioning. Besides affecting receptor signaling, these accessory components also play a key role in receptor trafficking, internalization and desensitization, as it will be reviewed here. In conclusion, the finding of an increasing number of adenosine receptors interacting proteins, and specially the molecular and functional integration of these accessory proteins into receptorsomes, will open new perspectives in the understanding of particular disorders where these receptors have been proved to be involved.

The vertebrate muscle Z-disc: sarcomere anchor for structure and signalling

The Z-disc, appearing as a fine dense line forming sarcomere boundaries in striated muscles, when studied in detail reveals crosslinked filament arrays that transmit tension and house myriads of proteins with diverse functions. At the Z-disc the barbed ends of the antiparallel actin filaments from adjoining sarcomeres interdigitate and are crosslinked primarily by layers of α-actinin. The Z-disc is therefore the site of polarity reversal of the actin filaments, as needed to interact with the bipolar myosin filaments in successive sarcomeres. The layers of α-actinin determine the Z-disc width: fast fibres have narrow (~30–50 nm) Z-discs and slow and cardiac fibres have wide (~100 nm) Z-discs. Comprehensive reviews on the roles of the numerous proteins located at the Z-disc in signalling and disease have been published; the aim here is different, namely to review the advances in structural aspects of the Z-disc.

Cytoplasmic Ig-domain proteins: cytoskeletal regulators with a role in human disease

Immunoglobulin domains are found in a wide variety of functionally diverse transmembrane proteins, and also in a smaller number of cytoplasmic proteins. Members of this latter group are usually associated with the actin cytoskeleton, and most of them bind directly to either actin or myosin, or both. Recently, studies of inherited human disorders have identified disease-causing mutations in five cytoplasmic Ig-domain proteins: myosin-binding protein C, titin, myotilin, palladin, and myopalladin. Together with results obtained from cultured cells and mouse models, these clinical studies have yielded novel insights into the unexpected roles of Ig domain proteins in mechanotransduction and signaling to the nucleus. An emerging theme in this field is that cytoskeleton-associated Ig domain proteins are more than structural elements of the cell, and may have evolved to fill different needs in different cellular compartments. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Cell mechanics: dissecting the physical responses of cells to force

It is now widely appreciated that normal tissue morphology and function rely upon cells' ability to sense and generate forces appropriate to their correct tissue context. Although the effects of forces on cells have been studied for decades, our understanding of how those forces propagate through and act on different cell substructures remains at an early stage. The past decade has seen a resurgence of interest, with a variety of different micromechanical methods in current use that probe cells' dynamic deformation in response to a time-varying force. The ability of researchers to carefully measure the mechanical properties of cells subjected to a variety of pharmacological and genetic interventions, however, currently outstrips our ability to quantitatively interpret the data in many cases. Despite these challenges, the stage is now set for the development of detailed models for cell deformability, motility, and mechanosensing that are rooted at the molecular level.

LPS-induced genes in intestinal tissue of the sea cucumber Holothuria glaberrima

Metazoan immunity is mainly associated with specialized cells that are directly involved with the immune response. Nevertheless, both in vertebrates and invertebrates other organs might respond to immune activation and participate either directly or indirectly in the ongoing immune process. However, most of what is known about invertebrate immunity has been restricted to immune effector cells and little information is available on the immune responses of other tissues or organs. We now focus on the immune reactions of the intestinal tissue of an echinoderm. Our study employs a non-conventional model, the echinoderm Holothuria glaberrima, to identify intestinal molecules expressed after an immune challenge presented by an intra-coelomic injection of lipopolysaccharides (LPS). The expression profiles of intestinal genes expressed differentially between LPS-injected animals and control sea water-injected animals were determined using a custom-made Agilent microarray with 7209 sea cucumber intestinal ESTs. Fifty (50) unique sequences were found to be differentially expressed in the intestine of LPS-treated sea cucumbers. Seven (7) of these sequences represented homologues of known proteins, while the remaining (43) had no significant similarity with any protein, EST or RNA database. The known sequences corresponded to cytoskeletal proteins (Actin and alpha-actinin), metabolic enzymes (GAPDH, Ahcy and Gnmt), metal ion transport/metabolism (major yolk protein) and defense/recognition (fibrinogen-like protein). The expression pattern of 11 genes was validated using semi-quantitative RT-PCR. Nine of these corroborated the microarray results and the remaining two showed a similar trend but without statistical significance. Our results show some of the molecular events by which the holothurian intestine responds to an immune challenge and provide important information to the study of the evolution of the immune response.

Molecular cloning and expression profile analysis of porcine TCAP gene

The gradually discovered sarcomeric proteins play important roles for structural integrity and signal transduction of sarcomere during myofibril genesis. TCAP (also described as telethonin, T-cap), one of the sarcomeric protein genes, is regulated developmentally. In this study, we reported the molecular characteristics of porcine TCAP gene. A 979 bp TCAP cDNA nucleotide sequence was obtained in pig and the deduced amino acid sequence had 92 and 91% identity to those of human and mouse homologous genes, respectively. One SNP was discovered and the allele frequency analysis showed that G allele frequency was low among 221 unrelated pigs from seven breeds. The tissue distribution patterns revealed that TCAP mRNA was expressed abundantly in skeletal and heart muscle tissue. Real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) results displayed TCAP mRNA was up-regulated in both Tongcheng and Landrace pigs during prenatal skeletal muscle development stages. This study suggested that TCAP gene might be a prospective candidate gene affecting muscle mass and meat quality traits in the pig, and also implicated the possible significance of TCAP on sarcomere assembly.

Meningococcal interactions with the host

Neisseria meningitidis interacts with host tissues through hierarchical, concerted and co-ordinated actions of a number of adhesins; many of which undergo antigenic and phase variation, a strategy that helps immune evasion. Three major structures, pili, Opa and Opc predominantly influence bacterial adhesion to host cells. Pili and Opa proteins also determine host and tissue specificity while Opa and Opc facilitate efficient cellular invasion. Recent studies have also implied a role of certain adhesin-receptor pairs in determining increased host susceptibility to infection. This chapter examines our current knowledge of meningococcal adhesion and invasion mechanisms particularly related to human epithelial and endothelial cells which are of primary importance in the disease process.

Mechanosensing in actin stress fibers revealed by a close correlation between force and protein localization

The mechanics of the actin cytoskeleton have a central role in the regulation of cells and tissues, but the details of how molecular sensors recognize deformations and forces are elusive. By performing cytoskeleton laser nanosurgery in cultured epithelial cells and fibroblasts, we show that the retraction of stress fibers (SFs) is restricted to the proximity of the cut and that new adhesions form at the retracting end. This suggests that SFs are attached to the substrate. A new computational model for SFs confirms this hypothesis and predicts the distribution and propagation of contractile forces along the SF. We then analyzed the dynamics of zyxin, a focal adhesion protein present in SFs. Fluorescent redistribution after laser nanosurgery and drug treatment shows a high correlation between the experimentally measured localization of zyxin and the computed localization of forces along SFs. Correlative electron microscopy reveals that zyxin is recruited very fast to intermediate substrate anchor points that are highly tensed upon SF release. A similar acute localization response is found if SFs are mechanically perturbed with the cantilever of an atomic force microscope. If actin bundles are cut by nanosurgery in living Drosophila egg chambers, we also find that zyxin redistribution dynamics correlate to force propagation and that zyxin relocates at tensed SF anchor points, demonstrating that these processes also occur in living organisms. In summary, our quantitative analysis shows that force and protein localization are closely correlated in stress fibers, suggesting a very direct force-sensing mechanism along actin bundles.

Combinatorial morphogenesis of dendritic spines and filopodia by SPAR and alpha-actinin2

Rap small GTPases regulate excitatory synaptic strength and morphological plasticity of dendritic spines. Changes in spine structure are mediated by the F-actin cytoskeleton, but the link between Rap activity and actin dynamics is unclear. Here, we report a novel interaction between SPAR, a postsynaptic inhibitor of Rap, and alpha-actinin, a family of actin-cross-linking proteins. SPAR and alpha-actinin engage in bidirectional structural plasticity of dendritic spines: SPAR promotes spine head enlargement, whereas increased alpha-actinin2 expression favors dendritic spine elongation and thinning. Surprisingly, SPAR and alpha-actinin2 can function in an additive rather than antagonistic fashion at the same dendritic spine, generating combination spine/filopodia hybrids. These data identify a molecular pathway bridging the actin cytoskeleton and Rap at synapses, and suggest that formation of spines and filopodia are not necessarily opposing forms of structural plasticity.

Characterization of interaction between CLP36 and palladin

CLP36 is a member of the PDZ-LIM family of proteins, which associates with alpha-actinin and localizes to the actin cytoskeleton. CLP36 is involved in the formation of stress fibers and focal adhesions; however, the molecular mechanism of how CLP36 regulates stress fiber formation is still unknown. To investigate the physiological function of CLP36, we performed yeast two-hybrid screening, and found that CLP36 interacts with palladin. Palladin is an important structural element of the actin cytoskeleton that is ubiquitously expressed and associates with alpha-actinin. The interaction was dependent on the PDZ domain of CLP36 and the C-terminus of palladin, and silencing of palladin suppressed localization of CLP36 to stress fibers. Overexpression of the PDZ domain of CLP36 also inhibited the localization of palladin to stress fibers, suggesting that the association of CLP36 and palladin is important for the localization of both proteins to stress fibers. Our experimental results indicate that alpha-actinin, CLP36 and palladin form a protein complex and contribute to regulation of the actin cytoskeleton.

Analysis of tarantula skeletal muscle protein sequences and identification of transcriptional isoforms

Background

Tarantula has been used as a model system for studying skeletal muscle structure and function, yet data on the genes expressed in tarantula muscle are lacking.

Results

We constructed a cDNA library from Aphonopelma sp. (Tarantula) skeletal muscle and got 2507 high-quality 5'ESTs (expressed sequence tags) from randomly picked clones. EST analysis showed 305 unigenes, among which 81 had more than 2 ESTs. Twenty abundant unigenes had matches to skeletal muscle-related genes including actin, myosin, tropomyosin, troponin-I, T and C, paramyosin, muscle LIM protein, muscle protein 20, a-actinin and tandem Ig/Fn motifs (found in giant sarcomere-related proteins). Matches to myosin light chain kinase and calponin were also identified. These results support the existence of both actin-linked and myosin-linked regulation in tarantula skeletal muscle.

We have predicted full-length as well as partial cDNA sequences both experimentally and computationally for myosin heavy and light chains, actin, tropomyosin, and troponin-I, T and C, and have deduced the putative peptides. A preliminary analysis of the structural and functional properties was also carried out. Sequence similarities suggested multiple isoforms of most myofibrillar proteins, supporting the generality of multiple isoforms known from previous muscle sequence studies. This may be related to a mix of muscle fiber types.

Conclusion

The present study serves as a basis for defining the transcriptome of tarantula skeletal muscle, for future in vitro expression of tarantula proteins, and for interpreting structural and functional observations in this model species.

Neisseria meningitidis Opc invasin binds to the cytoskeletal protein alpha-actinin

Neisseria meningitidis Opc protein is an effective invasin for human endothelial cells. We have investigated novel human endothelial receptors targeted by Opc and observed that Opc-expressing bacteria interacted with a 100 kDa protein in whole-cell lysates of human endothelial and epithelial cells. The identity of the protein was established as alpha-actinin by mass spectrometry. Opc expression was essential for the recognition of alpha-actinin whether provided in a purified form or in cell extracts. The interaction of the two proteins did not involve intermediate molecules. As there was no demonstrable expression of alpha-actinin on the surfaces of any of the eight cell lines studied, the likelihood of the interactions after meningococcal internalization was examined. Confocal imaging demonstrated considerable colocalization of N. meningitidis with alpha-actinin especially after a prolonged period of internalization. This may imply that bacteria and alpha-actinin initially occur in separate compartments and co-compartmentalization occurs progressively over the 8 h infection period used. In conclusion, these studies have identified a novel and an intracellular target for the N. meningitidis Opc invasin. Since alpha-actinin is a modulator of a variety of signalling pathways and of cytoskeletal functions, its targeting by Opc may enable bacteria to survive/translocate across endothelial barriers.

alpha-Actinin interacts with rapsyn in agrin-stimulated AChR clustering

AChR is concentrated at the postjunctional membrane at the neuromuscular junction. However, the underlying mechanism is unclear. We show that α-actinin, a protein known to cross-link F-actin, interacts with rapsyn, a scaffold protein essential for neuromuscular junction formation. α-Actinin, rapsyn, and surface AChR form a ternary complex. Moreover, the rapsyn-α-actinin interaction is increased by agrin, a factor known to stimulate AChR clustering. Downregulation of α-actinin expression inhibits agrin-mediated AChR clustering. Furthermore, the rapsyn-α-actinin interaction can be disrupted by inhibiting Abl and by cholinergic stimulation. Together these results indicate a role for α-actinin in AChR clustering.

Photoactivated localization microscopy (PALM) of adhesion complexes

Key to understanding a protein's biological function is the accurate determination of its spatial distribution inside a cell. Although fluorescent protein markers allow the targeting of specific proteins with molecular precision, much of this information is lost when the resultant fusion proteins are imaged with conventional, diffraction-limited optics. In response, several imaging modalities that are capable of resolution below the diffraction limit (approximately 200 nm) have emerged. Here, both single- and dual-color superresolution imaging of biological structures using photoactivated localization microscopy (PALM) are described. The examples discussed focus on adhesion complexes: dense, protein-filled assemblies that form at the interface between cells and their substrata. A particular emphasis is placed on the instrumentation and photoactivatable fluorescent protein (PA-FP) tags necessary to achieve PALM images at approximately 20 nm resolution in 5 to 30 min in fixed cells.

Ultrastructural localization of actin and actin-binding proteins in the nucleus

Nuclear actin plays an important role in such processes as chromatin remodeling, transcriptional regulation, RNA processing, and nuclear export. Recent research has demonstrated that actin in the nucleus probably exists in dynamic equilibrium between monomeric and polymeric forms, and some of the actin-binding proteins, known to regulate actin dynamics in cytoplasm, have been also shown to be present in the nucleus. In this paper, we present ultrastructural data on distribution of actin and various actin-binding proteins (alpha-actinin, filamin, p190RhoGAP, paxillin, spectrin, and tropomyosin) in nuclei of HeLa cells and resting human lymphocytes. Probing extracts of HeLa cells for the presence of actin-binding proteins also confirmed their presence in nuclei. We report for the first time the presence of tropomyosin and p190RhoGAP in the cell nucleus, and the spatial colocalization of actin with spectrin, paxillin, and alpha-actinin in the nucleolus.

Expression and tissue localization of beta-catenin, alpha-actinin and chondroitin sulfate proteoglycan 6 is modulated during rat and human left ventricular hypertrophy

Left ventricular hypertrophy (LVH) correlates with chronic renal failure and is one of the most important causes of cardiac mortality. The understanding of the molecular complexity of the disease will help to find biomarkers that open new perspectives about early diagnosis and therapy. This work describes the identification of mediators during pathogenesis relevant for structural remodeling processes of cardiac tissue in uremic LVH. An established rat model of chronic renal failure allowed whole-genome transcriptome analyses as well as the investigation of differential expressed proteins in uremic LVH. The localization of potential biomarkers encoded by candidate genes was done by immunohistochemical analyses of cardiac tissue of the animal model as well as cardiac sections of LVH diseased patients. In addition, the induction of human cardiac fibroblasts (HCF) and human umbilical vein endothelial cells (HUVEC) with the LVH mediator angiotensin II enabled us to investigate uremic LVH progression in vitro. These results point to alterations of myocardial intercellular and cell-matrix contacts in hypertrophic cardiac tissue. Obviously, structural changes of the extracellular matrix are significantly modulated by beta-catenin associated signaling pathways. Interestingly, intracellular translocation of beta-catenin, alpha-actinin and chondroitin sulfate proteoglycan 6 (CSPG6/SMC3) was observed in the animal model and in LVH patients. Our results show that the parallel investigation of rat and human cardiac tissue as well as human cellular models in vitro represents a promising strategy to identify reliable biomarkers of LVH.

The cytoskeletal protein alpha-actinin regulates acid-sensing ion channel 1a through a C-terminal interaction

The acid-sensing ion channel 1a (ASIC1a) is widely expressed in central and peripheral neurons where it generates transient cation currents when extracellular pH falls. ASIC1a confers pH-dependent modulation on postsynaptic dendritic spines and has critical effects in neurological diseases associated with a reduced pH. However, knowledge of the proteins that interact with ASIC1a and influence its function is limited. Here, we show that α-actinin, which links membrane proteins to the actin cytoskeleton, associates with ASIC1a in brain and in cultured cells. The interaction depended on an α-actinin-binding site in the ASIC1a C terminus that was specific for ASIC1a versus other ASICs and for α-actinin-1 and -4. Co-expressing α-actinin-4 altered ASIC1a current density, pH sensitivity, desensitization rate, and recovery from desensitization. Moreover, reducing α-actinin expression altered acid-activated currents in hippocampal neurons. These findings suggest that α-actinins may link ASIC1a to a macromolecular complex in the postsynaptic membrane where it regulates ASIC1a activity.

Memo-RhoA-mDia1 signaling controls microtubules, the actin network, and adhesion site formation in migrating cells

Actin assembly at the cell front drives membrane protrusion and initiates the cell migration cycle. Microtubules (MTs) extend within forward protrusions to sustain cell polarity and promote adhesion site turnover. Memo is an effector of the ErbB2 receptor tyrosine kinase involved in breast carcinoma cell migration. However, its mechanism of action remained unknown. We report in this study that Memo controls ErbB2-regulated MT dynamics by altering the transition frequency between MT growth and shortening phases. Moreover, although Memo-depleted cells can assemble the Rac1-dependent actin meshwork and form lamellipodia, they show defective localization of lamellipodial markers such as alpha-actinin-1 and a reduced number of short-lived adhesion sites underlying the advancing edge of migrating cells. Finally, we demonstrate that Memo is required for the localization of the RhoA guanosine triphosphatase and its effector mDia1 to the plasma membrane and that Memo-RhoA-mDia1 signaling coordinates the organization of the lamellipodial actin network, adhesion site formation, and MT outgrowth within the cell leading edge to sustain cell motility.

easyExon--a Java-based GUI tool for processing and visualization of Affymetrix exon array data

Background

Alternative RNA splicing greatly increases proteome diversity and thereby contribute to species- or tissue-specific functions. The possibility to study alternative splicing (AS) events on a genomic scale using splicing-sensitive microarrays, including the Affymetrix GeneChip Exon 1.0 ST microarray (exon array), has appeared very recently. However, the application of this new technology is hindered by the lack of free and user-friendly software devoted to these novel platforms.

Results

In this study we present a Java-based freeware, easyExon , to process, filtrate and visualize exon array data with an analysis pipeline. This tool implements the most commonly used probeset summarization methods as well as AS-orientated filtration algorithms, e.g. MIDAS and PAC, for the detection of alternative splicing events. We include a biological filtration function according to GO terms, and provide a module to visualize and interpret the selected exons and transcripts. Furthermore, easyExon can integrate with other related programs, such as Integrate Genome Browser (IGB) and Affymetrix Power Tools (APT), to make the whole analysis more comprehensive. We applied easyExon on a public accessible colon cancer dataset as an example to illustrate the analysis pipeline of this tool.

Conclusion

EasyExon can efficiently process and analyze the Affymetrix exon array data. The simplicity, flexibility and brevity of easyExon make it a valuable tool for AS event identification in genomic research.

Meta-analysis and profiling of cardiac expression modules

Heart failure is a complex, complicated disease that is not yet fully understood. We used the Module Map algorithm to uncover groups of genes that have a similar pattern of expression under various conditions of heart stress. These groups of genes are called modules and may serve as computational predictions of biological pathways for the various clinical situations. The Module Map algorithm allows a large-scale analysis of genes expressed. We applied this algorithm to 700 different mouse experiments downloaded from the Gene Expression Omnibus database, which identified 884 modules. The analysis reconstructed partially known principles that play a role in governing the response of heart to stress, thus demonstrating the strength of the method. We have shown a role of genes related to the immune system in conditions of heart remodeling and failure. We have also shown changes in the expression of genes involved with energy metabolism and changes in the expression of contractile proteins of the heart following myocardial infarction. When focusing on another module we noted a new correlation between genes related to osteogenesis and heart failure, including Runx2 and Ahsg, whose role in heart failure was unknown so far. Despite a lack of prior biological knowledge, the Module Map algorithm has reconstructed known pathways, which demonstrates the strength of this new method for analyzing gene profiles related to clinical phenomenon. The method and the analysis presented are a new avenue to uncover the correlation of clinical conditions to the molecular level.

Differential proteomics in the aging Noble rat ventral prostate

Incidence of prostatic diseases increases dramatically with age which may be related to a decline in androgen support. However, the key mechanisms underlying prostate aging remain unclear. In the present study, we investigated the aging process in the ventral prostate (VP) of Noble rats by identifying differentially expressed prostate proteins between 3- and 16-month-old animals using ICAT and MS. In total, 472 proteins were identified with less than a 1% false positive rate, among which 34 were determined to have a greater than two-fold increase or 1.7-fold decrease in expression in the aged VPs versus their younger counterparts. The majority of the differentially expressed proteins identified have not been previously reported to be associated with prostate aging, and they fall into specific functional categories, including oxidative stress/detoxification, chaperones, protein biosynthesis, vesicle transport, and intracellular trafficking. The expression of GST, ferritin, clusterin, kininogen, oxygen regulated protein 150, spermidine synthase, ADP ribosylation factor, and cyclophilin B was verified by Western blot analyses on samples used for the ICAT study, as well as on those obtained from an independent group of animals comprised of three age groups. To the best of our knowledge, this is the first study on the proteome of the aging rat prostate.

Actin cytoskeletal dynamics in smooth muscle: a new paradigm for the regulation of smooth muscle contraction

A growing body of data supports a view of the actin cytoskeleton of smooth muscle cells as a dynamic structure that plays an integral role in regulating the development of mechanical tension and the material properties of smooth muscle tissues. The increase in the proportion of filamentous actin that occurs in response to the stimulation of smooth muscle cells and the essential role of stimulus-induced actin polymerization and cytoskeletal dynamics in the generation of mechanical tension has been convincingly documented in many smooth muscle tissues and cells using a wide variety of experimental approaches. Most of the evidence suggests that the functional role of actin polymerization during contraction is distinct and separately regulated from the actomyosin cross-bridge cycling process. The molecular basis for the regulation of actin polymerization and its physiological roles may vary in diverse types of smooth muscle cells and tissues. However, current evidence supports a model for smooth muscle contraction in which contractile stimulation initiates the assembly of cytoskeletal/extracellular matrix adhesion complex proteins at the membrane, and proteins within this complex orchestrate the polymerization and organization of a submembranous network of actin filaments. This cytoskeletal network may serve to strengthen the membrane for the transmission of force generated by the contractile apparatus to the extracellular matrix, and to enable the adaptation of smooth muscle cells to mechanical stresses. Better understanding of the physiological function of these dynamic cytoskeletal processes in smooth muscle may provide important insights into the physiological regulation of smooth muscle tissues.

The role of palladin in actin organization and cell motility

Palladin is a widely expressed protein found in stress fibers, focal adhesions, growth cones, Z-discs, and other actin-based subcellular structures. It belongs to a small gene family that includes the Z-disc proteins myopalladin and myotilin, all of which share similar Ig-like domains. Recent advances have shown that palladin shares with myotilin the ability to bind directly to F-actin, and to crosslink actin filaments into bundles, in vitro. Studies in a variety of cultured cells suggest that the actin-organizing activity of palladin plays a central role in promoting cell motility. Correlative evidence also supports this hypothesis, as palladin levels are typically up-regulated in cells that are actively migrating: in developing vertebrate embryos, in cells along a wound edge, and in metastatic cancer cells. Recently, a mutation in the human palladin gene was implicated in an unusually penetrant form of inherited pancreatic cancer, which has stimulated new ideas about the role of palladin in invasive cancer.

Actin and alpha-actinin orchestrate the assembly and maturation of nascent adhesions in a myosin II motor-independent manner

Using two-colour imaging and high resolution TIRF microscopy, we investigated the assembly and maturation of nascent adhesions in migrating cells. We show that nascent adhesions assemble and are stable within the lamellipodium. The assembly is independent of myosin II but its rate is proportional to the protrusion rate and requires actin polymerization. At the lamellipodium back, the nascent adhesions either disassemble or mature through growth and elongation. Maturation occurs along an alpha-actinin-actin template that elongates centripetally from nascent adhesions. Alpha-Actinin mediates the formation of the template and organization of adhesions associated with actin filaments, suggesting that actin crosslinking has a major role in this process. Adhesion maturation also requires myosin II. Rescue of a myosin IIA knockdown with an actin-bound but motor-inhibited mutant of myosin IIA shows that the actin crosslinking function of myosin II mediates initial adhesion maturation. From these studies, we have developed a model for adhesion assembly that clarifies the relative contributions of myosin II and actin polymerization and organization.

The parafibromin tumor suppressor protein interacts with actin-binding proteins actinin-2 and actinin-3

Background

Germline and somatic inactivating mutations in the HRPT2 gene occur in the inherited hyperparathyroidism-jaw tumor syndrome, in some cases of parathyroid cancer and in some cases of familial hyperparathyroidism. HRPT2 encodes parafibromin. To identify parafibromin interacting proteins we used the yeast two-hybrid system for screening a heart cDNA library with parafibromin as the bait.

Results

Fourteen parafibromin interaction positive preys representing 10 independent clones encoding actinin-2 were isolated. Parafibromin interacted with muscle alpha-actinins (actinin-2 and actinin-3), but not with non-muscle alpha-actinins (actinin-1 and actinin-4). The parafibromin-actinin interaction was verified by yeast two-hybrid, GST pull-down, and co-immunoprecipitation. Yeast two-hybrid analysis revealed that the N-terminal region of parafibromin interacted with actinins. In actin sedimentation assays parafibromin did not dissociate skeletal muscle actinins from actin filaments, but interestingly, parafibromin could also bundle/cross-link actin filaments. Parafibromin was predominantly nuclear in undifferentiated proliferating myoblasts (C2C12 cells), but in differentiated C2C12 myotubes parafibromin co-localized with actinins in the cytoplasmic compartment.

Conclusion

These data support a possible contribution of parafibromin outside the nucleus through its interaction with actinins and actin bundling/cross-linking. These data also suggest that actinins (and actin) participate in sequestering parafibromin in the cytoplasmic compartment.

Over-expression of alpha-actinin with a GFP fusion protein is sufficient to increase whole-cell stiffness in human osteoblasts

Osteoblasts respond to shear stress by simultaneously increasing their whole-cell stiffness and up-regulating the cytoskeletal crosslinking protein alpha-actinin. The stiffness of reconstituted cytoskeletal networks increases following the addition of alpha-actinin, but the effect of alpha-actinin on whole-cell mechanical behavior has not been investigated. The hypothesis of this study was that increasing alpha-actinin in the cytoskeleton would be sufficient to increase whole-cell stiffness. hFOB osteoblasts were transfected with a plasmid for GFP-tagged alpha-actinin, resulting in a 150% increase in the amount of alpha-actinin. The GFP-alpha-actinin fusion protein co-fractionated with the cytoskeleton and co-localized to the same regions of the cytoskeleton as endogenous alpha-actinin. Whole-cell mechanical behavior was measured by atomic force microscopy using a 25 mum diameter microsphere as an indenter. The whole-cell stiffness of cells over-expressing GFP-alpha-actinin was 60% higher than cells expressing only endogenous alpha-actinin (p < 0.002), which was within the range of mechanical behavior observed in osteoblastic cells exposed to 1 and 2 Pa of fluid shear. These results indicate that the up-regulation of alpha-actinin synthesis in osteoblasts is sufficient to alter the whole-cell mechanical behavior and highlights the potential role of alpha-actinin to reinforce cells against mechanical loads.

Regulation of actin assembly associated with protrusion and adhesion in cell migration

To migrate, a cell first extends protrusions such as lamellipodia and filopodia, forms adhesions, and finally retracts its tail. The actin cytoskeleton plays a major role in this process. The first part of this review (sect. II) describes the formation of the lamellipodial and filopodial actin networks. In lamellipodia, the WASP-Arp2/3 pathways generate a branched filament array. This polarized dendritic actin array is maintained in rapid treadmilling by the concerted action of ADF, profilin, and capping proteins. In filopodia, formins catalyze the processive assembly of nonbranched actin filaments. Cell matrix adhesions mechanically couple actin filaments to the substrate to convert the treadmilling into protrusion and the actomyosin contraction into traction of the cell body and retraction of the tail. The second part of this review (sect. III) focuses on the function and the regulation of major proteins (vinculin, talin, tensin, and alpha-actinin) that control the nucleation, the binding, and the barbed-end growth of actin filaments in adhesions.

The talin rod IBS2 alpha-helix interacts with the beta3 integrin cytoplasmic tail membrane-proximal helix by establishing charge complementary salt bridges

Talin establishes a major link between integrins and actin filaments and contains two distinct integrin binding sites: one, IBS1, located in the talin head domain and involved in integrin activation and a second, IBS2, that maps to helix 50 of the talin rod domain and is essential for linking integrin beta subunits to the cytoskeleton ( Moes, M., Rodius, S., Coleman, S. J., Monkley, S. J., Goormaghtigh, E., Tremuth, L., Kox, C., van der Holst, P. P., Critchley, D. R., and Kieffer, N. (2007) J. Biol. Chem. 282, 17280-17288 ). Through the combined approach of mutational analysis of the beta3 integrin cytoplasmic tail and the talin rod IBS2 site, SPR binding studies, as well as site-specific antibody inhibition experiments, we provide evidence that the integrin beta3-talin rod interaction relies on a helix-helix association between alpha-helix 50 of the talin rod domain and the membrane-proximal alpha-helix of the beta3 integrin cytoplasmic tail. Moreover, charge complementarity between the highly conserved talin rod IBS2 lysine residues and integrin beta3 glutamic acid residues is necessary for this interaction. Our results support a model in which talin IBS2 binds to the same face of the beta3 subunit cytoplasmic helix as the integrin alphaIIb cytoplasmic tail helix, suggesting that IBS2 can only interact with the beta3 subunit following integrin activation.

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) binds to alpha-actinin 1: novel pathways in skeletal muscle?

Background

Hereditary inclusion body myopathy (HIBM) is a rare neuromuscular disorder caused by mutations in GNE, the key enzyme in the biosynthetic pathway of sialic acid. While the mechanism leading from GNE mutations to the HIBM phenotype is not yet understood, we searched for proteins potentially interacting with GNE, which could give some insights about novel putative biological functions of GNE in muscle.

Methodology/Principal Findings

We used a Surface Plasmon Resonance (SPR)-Biosensor based assay to search for potential GNE interactors in anion exchanged fractions of human skeletal muscle primary culture cell lysate. Analysis of the positive fractions by in vitro binding assay revealed α-actinin 1 as a potential interactor of GNE. The direct interaction of the two proteins was assessed in vitro by SPR-Biosensor based kinetics analysis and in a cellular environment by a co-immunoprecipitation assay in GNE overexpressing 293T cells. Furthermore, immunohistochemistry on stretched mouse muscle suggest that both GNE and α-actinin 1 localize to an overlapping but not identical region of the myofibrillar apparatus centered on the Z line.

Conclusions/Significance

The interaction of GNE with α-actinin 1 might point to its involvement in α-actinin mediated processes. In addition these studies illustrate for the first time the expression of the non-muscle form of α-actinin, α-actinin 1, in mature skeletal muscle tissue, opening novel avenues for its specific function in the sarcomere. Although no significant difference could be detected in the binding kinetics of α-actinin 1 with either wild type or mutant GNE in our SPR biosensor based analysis, further investigation is needed to determine whether and how the interaction of GNE with α-actinin 1 in skeletal muscle is relevant to the putative muscle-specific function of α-actinin 1, and to the muscle-restricted pathology of HIBM.

Targets of caspase-6 activity in human neurons and Alzheimer disease

Caspase-6 activation occurs early in Alzheimer disease and sometimes precedes the clinical manifestation of the disease in aged individuals. The active Caspase-6 is localized in neuritic plaques, in neuropil threads, and in neurofibrillary tangles containing neurons that are not morphologically apoptotic in nature. To investigate the potential consequences of the activation of Caspase-6 in neurons, we conducted a proteomics analysis of Caspase-6-mediated cleavage of human neuronal proteins. Proteins from the cytosolic and membrane subcellular compartments were treated with recombinant active Caspase-6 and compared with undigested proteins by two-dimensional gel electrophoresis. LC/MS/MS analyses of the proteins that were cleaved identified 24 different potential protein substrates. Of these, 40% were cytoskeleton or cytoskeleton-associated proteins. We focused on the cytoskeleton proteins because these are critical for neuronal structure and function. Caspase-6 cleavage of alpha-Tubulin, alpha-Actinin-4, Spinophilin, and Drebrin was confirmed. At least one Caspase-6 cleavage site was identified for Drebrin, Spinophilin, and alpha-Tubulin. A neoepitope antiserum to alpha-Tubulin cleaved by Caspase-6 immunostained neurons, neurofibrillary tangles, neuropil threads, and neuritic plaques in Alzheimer disease and co-localized with active Caspase-6. These results imply that the early and neuritic activation of Caspase-6 in Alzheimer disease could disrupt the cytoskeleton network of neurons, resulting in impaired neuronal structure and function in the absence of cell death. This study provides novel insights into the pathophysiology of Alzheimer disease.

Investigating complexity of protein-protein interactions in focal adhesions

The formation of focal adhesions governs cell shape and function; however, there are few measurements of the binding kinetics of focal adhesion proteins in living cells. Here, we used the fluorescence recovery after photobleaching (FRAP) technique, combined with mathematical modeling and scaling analysis to quantify dissociation kinetics of focal adhesion proteins in capillary endothelial cells. Novel experimental protocols based on mathematical analysis were developed to discern the rate-limiting step during FRAP. Values for the dissociation rate constant k(OFF) ranged over an order of magnitude from 0.009+/-0.001/s for talin to 0.102+/-0.010/s for FAK, indicating that talin is bound more strongly than other proteins in focal adhesions. Comparisons with in vitro measurements reveal that multiple focal adhesion proteins form a network of bonds, rather than binding in a pair-wise manner in these anchoring structures in living cells.

Dynamic regulation of endothelial NOS mediated by competitive interaction with alpha-actinin-4 and calmodulin

Alpha-actinins are critical components of the actin cytoskeleton. Here we show that alpha-actinins serve another important biological function by binding to and competitively inhibiting calcium-dependent activation of endothelial NOS (eNOS). Alpha-actinin-2 was found to associate with eNOS in a yeast two-hybrid screen. In vascular endothelial cells, which only express alpha-actinin-1 and -4, alpha-actinin-4 interacted and colocalized with eNOS. Addition of alpha-actinin-4 directly inhibited eNOS recombinant protein, and overexpression of alpha-actinin-4 inhibited eNOS activity in eNOS-transfected COS-7 cells and bovine aortic endothelial cells (BAECs). In contrast, knockdown of alpha-actinin-4 by siRNA increased eNOS activity in BAECs. The alpha-actinin-4-binding site on eNOS was mapped to a central region comprising the calmodulin-binding domain, and the eNOS-binding site on alpha-actinin-4 was mapped to the fourth spectrin-like rod domain, R4. Treatment of endothelial cells with a calcium ionophore, A23187, decreased alpha-actinin-4-eNOS interaction, leading to translocation of alpha-actinin-4 from plasma membrane to cytoplasm. Indeed, addition of calmodulin displaced alpha-actinin-4 binding to eNOS and increased eNOS activity. These findings indicate that eNOS activity in vascular endothelial cells is tonically and dynamically regulated by competitive interaction with alpha-actinin-4 and calmodulin.

Expression of alpha-actinin-1 in human glomerular mesangial cells in vivo and in vitro

Recent studies have demonstrated important roles of alpha-actinins in glomerular disease, while little information is known about the expression profile of alpha-actinins in human glomerular mesangial cells. Here, immunofluorescence and confocal microscopy showed that alpha-actinin-1 exclusively distributed along mesangial cells in human glomeruli of IgA nephropathy. RT-PCR and Western blot further confirmed the expression of alpha-actinin-1 in primary cultured human mesangial cells. We also found that transforming growth factor-beta 1 (TGF-beta 1) stimulated ACTN1 gene transcription and that transiently transfected alpha-actinin-1 significantly increased TGF-beta 1-induced plasminogen activator inhibitor-1 (PAI-1) promoter activity in human mesangial cells. These findings suggest that alpha-actinin-1 may play a role in human glomerular disease.