Unanticipated temporal and spatial effects of sarcomeric alpha-actinin peptides expressed in PtK2 cells

Polycystins and intercellular mechanotransduction: A precise dosage of polycystin 2 is necessary for alpha-actinin reinforcement of junctions upon mechanical stimulation

Polycystins 1 and 2, which are mutated in Autosomal Polycystic Kidney Disease, are involved in mechanotransduction through various mechanisms. In renal cells, polycystins not only have an important mechanotransductive role in primary cilia but are also present in intercellular contacts but their role there remains unclear. Here, we address the hypothesis that polycystins are involved in mechanotransduction via intercellular junctions, which would be expected to have consequences on tissue organization. We focused on the role of polycystin 2, which could be involved in mechanical organization at junctions either by its channel activity or by the direct recruitment of cytoskeleton components such as the F-actin cross-linker α-actinin. After mechanical stimulation of intercellular junctions in MDCK renal epithelial cells, α-actinin is rapidly recruited but this is inhibited upon overexpression of PC2 or the D509V mutant that lacks channel activity, and is also decreased upon PC2 silencing. This suggests that a precise dosage of PC2 is necessary for an adequate mechanosensitive α-actinin recruitment at junctions. At the multicellular level, a change in PC2 expression was associated with changes in velocity in confluent epithelia and during wound healing together with a loss of orientation. This study suggests that the mechanosensitive regulation of cytoskeleton by polycystins in intercellular contacts may be important in the context of ADPKD.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

MicroRNA-1 targets Slug and endows lung cancer A549 cells with epithelial and anti-tumorigenic properties

MicroRNA-1 (miR-1) has recently been suggested to function as a tumor suppressor. Its functional relevance was assessed by exploring structural and tumorigenic properties of lung cancer A549 cells stably transduced with retrovirus containing pre-miR-1. A549 cells overexpressing miR-1 exhibited a significant morphological change from a mesenchymal to an epithelial phenotype characterized by cell polarization and intercellular junctions. The cells showed increased expression of E-cadherin, which colocalized with cortical actin filaments and vinculin to form typical adherens junction at the apical regions of intercellular borders. Additionally, they exhibited occludin-positive tight junctions at similar apical regions. Moreover, their migratory and invasive activities were inhibited, and their sensitivity to doxorubicin was increased slightly compared to control mock-infected cells. These structural and tumorigenic properties induced by miR-1 were associated with the reduced expression of Slug, which was a transcriptional repressor of E-cadherin or an inducer of epithelial-to-mesenchymal transition. Consistently, Slug was identified as a miR-1 target by bioinformatics and a luciferase reporter assay with plasmids containing luciferase-Slug 3'UTR. Collectively, the data presented here suggest that re-expression of miR-1 may be an effective therapy that prevents cancer malignancy by converting cells from a mesenchymal phenotype to an epithelial phenotype via the downregulation of Slug.

Plectin 1 links intermediate filaments to costameric sarcolemma through β-synemin, α-dystrobrevin and actin

In skeletal muscles, the sarcolemma is possibly stabilized and protected against contraction-imposed stress by intermediate filaments (IFs) tethered to costameric sarcolemma. Although there is emerging evidence that plectin links IFs to costameres through dystrophin-glycoprotein complexes (DGC), the molecular organization from plectin to costameres still remains unclear. Here, we show that plectin 1, a plectin isoform expressed in skeletal muscle, can interact with β-synemin, actin and a DGC component, α-dystrobrevin, in vitro. Ultrastructurally, β-synemin molecules appear to be incorporated into costameric dense plaques, where they seem to serve as actin-associated proteins rather than IF proteins. In fact, they can bind actin and α-dystrobrevin in vitro. Moreover, in vivo immunoprecipitation analyses demonstrated that β-synemin- and plectin-immune complexes from lysates of muscle light microsomes contained α-dystrobrevin, dystrophin, nonmuscle actin, metavinculin, plectin and β-synemin. These findings suggest a model in which plectin 1 interacts with DGC and integrin complexes directly, or indirectly through nonmuscle actin and β-synemin within costameres. The DGC and integrin complexes would cooperate to stabilize and fortify the sarcolemma by linking the basement membrane to IFs through plectin 1, β-synemin and actin. Besides, the two complexes, together with plectin and IFs, might have their own functions as platforms for distinct signal transduction.

Functional role of alpha-actinin, PI 3-kinase and MEK1/2 in insulin-like growth factor I receptor kinase regulated motility of human breast carcinoma cells

Within epithelial tissue, cells are held together by specialized lateral junctions. At particular stages of development and in pathological processes such as metastasis, cells break down the intercellular junctions, separate from the epithelial sheet and migrate individually. Despite the importance of these processes, little is understood about the regulatory mechanisms of active cell separation. In view of the effects of insulin-like growth factor I (IGF-I) on mammary gland development and cancer, we developed a model using MCF-7 human breast cancer cells in which the process of cell separation can be induced by IGF-I. The separation was enhanced in MCF-7 cells overexpressing the IGF-IR and blocked in the cells expressing a dead-kinase mutant of this receptor. Activation of the IGF-IR resulted in a rapid formation of motile actin microspikes at the regions of cell-cell contacts, disorganization of mature adherens junctions and the onset of cell migration. In cell separation, the signaling between the IGF-IR kinase and actin required phosphatidylinositol 3 (PI 3)-kinase-generated phospholipids but not MAP kinases and was mediated by alpha-actinin. The activity of MEK1/2 kinases was needed for consecutive cell migration. This work also defined a new function for alpha-actinin. Upon IGF-IR activation, green fluorescence protein (GFP)-labeled alpha-actinin concentrated at the base of actin microspikes. Deletion of the N-terminal actin-binding domain of alpha-actinin prevented this redistribution, indicating that this domain is necessary. Detection of the C-terminal tail of alpha-actinin reduced the number of microspikes, showing that alpha-actinin has a role in the development of microspikes and is not passively reorganized with filamentous actin. We suggest that the signaling pathway from the IGF-IR kinase through the PI-3 kinase to alpha-actinin participates in the rapid organization of actin into microspikes at the cell-cell junctions and leads to active cell separation, whereas signaling through ERK1/2 MAP kinases controls cell migration following cell separation.

Distinct families of Z-line targeting modules in the COOH-terminal region of nebulin

To learn how nebulin functions in the assembly and maintenance of I-Z-I bands, MYC- and GFP- tagged nebulin fragments were expressed in primary cultured skeletal myotubes. Their sites of incorporation were visualized by double staining with anti-MYC, antibodies to myofibrillar proteins, and FITC- or rhodamine phalloidin. Contrary to expectations based on in vitro binding studies, none of the nebulin fragments expressed in maturing myotubes were incorporated selectively into I-band approximately 1.0-micrometer F-alpha-actin-containing thin filaments. Four of the MYC/COOH-terminal nebulin fragments were incorporated exclusively into periodic approximately 0.1-micrometer Z-bands. Whereas both anti-MYC and Rho-phalloidin stained intra-Z-band F-alpha-actin oligomers, only the latter stained the pointed ends of the polarized approximately 1.0-micrometer thin filaments. Z-band incorporation was independent of the nebulin COOH-terminal Ser or SH3 domains. In vitro cosedimentation studies also demonstrated that nebulin SH3 fragments did not bind to F-alpha-actin or alpha-actinin. The remaining six fragments were not incorporated into Z-bands, but were incorporated (a) diffusely throughout the sarcoplasm and into (b) fibrils/patches of varying lengths and widths nested among normal striated myofibrils. Over time, presumably in response to the mediation of muscle-specific homeostatic controls, many of the ectopic MYC-positive structures were resorbed. None of the tagged nebulin fragments behaved as dominant negatives; they neither blocked the assembly nor induced the disassembly of mature striated myofibrils. Moreover, they were not cytotoxic in myotubes, as they were in the fibroblasts and presumptive myoblasts in the same cultures.

Initiation and maturation of I-Z-I bodies in the growth tips of transfected myotubes

While over a dozen I-Z-I proteins are expressed in postmitotic myoblasts and myotubes it is unclear how, when, or where these first assemble into transitory I-Z-I bodies (thin filament/Z-band precursors) and, a short time later, into definitive I-Z-I bands. By double-staining the growth tips of transfected myotubes expressing (a) MYC-tagged s-alpha-actinins (MYC/s-alpha-actinins) or (b) green fluorescent protein-tagged titin cap (GFP/T-cap) with antibodies against MYC and I-Z-I band proteins, we found that the de novo assembly of I-Z-I bodies and their maturation into I-Z-I bands involved relatively concurrent, cooperative binding and reconfiguration of, at a minimum, 5 integral Z-band molecules. These included s-alpha-actinin, nebulin, titin, T-cap and alpha-actin. Resolution of the approximately 1.0 microm polarized alpha-actin/nebulin/tropomyosin/troponin thin filament complexes occurred subsequent to the maturation of Z-bands into a dense tetragonal configuration. Of particular interest is finding that mutant MYC/s-alpha-actinin peptides (a) lacking spectrin-like repeats 1–4, or consisting of spectrin-like repeats 1–4 only, as well as (b) mutants/fragments lacking titin or alpha-actin binding sites, were promptly and exclusively incorporated into de novo assembling I-Z-I bodies and definitive I-Z-I bands as was exogenous full length MYC/s-alpha-actinin or GFP/T-cap.

Functional dissection of nebulette demonstrates actin binding of nebulin-like repeats and Z-line targeting of SH3 and linker domains

Nebulette, a 107 kDa protein associated with the I-Z-I complex of cardiac myofibrils, may play an important role in the assembly of the Z-line. Determination of the complete primary structure of 1011 residue human fetal nebulette reveals a four-domain layout similar to skeletal muscle nebulin: a short N-terminal domain, followed by 22 nebulin-like repeats that are linked to a C-terminal Src homology 3 (SH3) domain via a short linker domain. To elucidate the mechanisms of assembly for nebulette in the Z-line, the complete coding sequence or fusions of nebulette domains with green fluorescent protein (GFP) were expressed in cardiomyocytes and fibroblasts. The complete protein localized to Z-lines in cardiac cells and to dense bodies in nonmuscle cells. The GFP-repeat domain forms bundles that are associated with actin filaments in both cell types and disrupts the microfilament network. In contrast, the GFP-repeat plus linker shows limited interaction with dense bodies in nonmuscle cells and the Z-lines of cardiomyocytes. Interestingly, the tagged linker or SH3 is diffusely distributed in nonmuscle cells, but localizes to the Z-lines in cardiomyocytes. Supporting the cellular localization work, recombinant nebulette fragments bind to actin, tropomyosin, and alpha-actinin in in vitro binding assays. These results suggest the repeat domain contains actin binding functions and that the linker domain may target this interaction to Z-lines and dense bodies. Our data also indicate that the linker and SH3 domains can distinguish between dense bodies and Z-lines, suggesting that the ligands for their interactions are specific to these muscular substructures.

Epitope tagging

Epitope tagging is a recombinant DNA method by which a protein encoded by a cloned gene is made immunoreactive to a known antibody. This review discusses the major advantages and limitations of epitope tagging and describes a number of recent applications. Major areas of application include monitoring protein expression, localizing proteins at the cellular and subcellular levels, and protein purification, as well as the analysis of protein topology, dynamics and interactions. Recently the method has also found use in transgenic and gene therapy studies and in the emerging fields of functional genomics and proteomics.

Dispensability of the actin-binding site and spectrin repeats for targeting sarcomeric alpha-actinin into maturing Z bands in vivo: implications for in vitro binding studies

To explore the roles of specific domains of sarcomeric alpha-actinin (s-alpha-actinin) in the assembly and maintenance of striated myofibrils, myogenic cultures were transfected with four MYC-tagged s-alpha-actinin peptides. They were: (1) full-length sarcomeric alpha-actinin, (2) an N-terminal deletion that removed the actin-binding site only (MYC/A-), (3) a peptide that consisted of the actin-binding site only (MYC/A+), and (4) an N-terminal deletion that removed the EF-hands and titin-binding domains (MYC/EFT-). While cytotoxic in replicating myogenic cells, as they were in PtK2 cells, the four MYC peptides were not cytotoxic in postmitotic myotubes. In myotubes each of the four different MYC peptides were promptly and selectively incorporated into normal Z bands. The incorporation of MYC/A-, MYC/A+, and MYC/EFT- into Z bands suggests that (a) the actin-binding site, (b) the spectrin-repeats believed to be responsible for anti-parallel dimerization, and (c) the C-terminal EF-hands and titin-binding domains are each dispensable for targeting s-alpha-actinin/MYC peptides into Z bands. These findings could not have been predicted from the behavior of alpha-actinin (a) in binding assays in cell-free systems or (b) when expressed in transfected nonmuscle cells.