Search

The Ras homologous (Rho) protein family of GTPases (RhoA, RhoB and RhoC) are the members of the Ras superfamily and regulate cellular processes such as cell migration, proliferation, polarization, adhesion, gene transcription and cytoskeletal structure. Rho GTPases function as molecular switches that cycle between GTP-bound (active state) and GDP-bound (inactive state) forms. Leukaemia-associated RhoGEF (LARG) is a guanine nucleotide exchange factor (GEF) that activates RhoA subfamily GTPases by promoting the exchange of GDP for GTP. LARG is selective for RhoA subfamily GTPases and is an essential regulator of cell migration and invasion. Here, we describe the mechanisms by which LARG is regulated to facilitate the understanding of how LARG mediates functions like cell motility and to provide insight for better therapeutic targeting of these functions.

Endosomal trafficking of cell surface receptors is essential to their function. Integrins are transmembrane receptors that integrate adhesion to the extracellular matrix with engagement of the cytoskeleton. Ligated integrins mediate diverse signals that regulate matrix assembly, cell survival, cell morphology, and cell motility. Endosomal trafficking of integrins modulates these signals and contributes to cell motility and is required for cancer cell invasion. The phosphoprotein PEA-15 modulates integrin activation and ERK MAP Kinase signaling. To elucidate novel PEA-15 functions we utilized an unbiased proteomics approach. We identified several binding partners for PEA-15 in the endosome including clathrin and AP-2 as well as integrin β1 and other focal adhesion complex proteins. We confirmed these interactions using proximity ligation analysis, immunofluorescence imaging, pull-down and co-immunoprecipitation. We further found that PEA-15 is enriched in endosomes and was required for efficient endosomal internalization of α5β1 integrin and cellular migration. Importantly, PEA-15 promotion of migration was dependent on PEA-15 phosphorylation at serines 104 and 116. These data support a novel endosomal role for PEA-15 in control of endosomal trafficking of integrins through an association with the β1 integrin and clathrin complexes, and thereby regulation of cell motility.

Significance Cell motility is a dynamic process that requires the directed application of force and continuous coordinated changes in cell adhesion and cytoskeletal architecture often in response to extracellular stimuli. Here we have defined a mechanism by which RSK2 can promote cell migration and invasion in response to promotility stimuli. We show that in response to these signals RSK2 directly binds the RhoGEF LARG and phosphorylates it, thereby promoting LARG activation of RhoA GTPases. Moreover, we find that RSK2 is important for epidermal growth factor activation of Rho GTPases. These results advance our understanding of cell motility, RSK kinase function, and LARG/RhoA activation by revealing that these pathways are integrated and the precise mechanism by which that is accomplished., Directed migration is essential for cell motility in many processes, including development and cancer cell invasion. RSKs (p90 ribosomal S6 kinases) have emerged as central regulators of cell migration; however, the mechanisms mediating RSK-dependent motility remain incompletely understood. We have identified a unique signaling mechanism by which RSK2 promotes cell motility through leukemia-associated RhoGEF (LARG)-dependent Rho GTPase activation. RSK2 directly interacts with LARG and nucleotide-bound Rho isoforms, but not Rac1 or Cdc42. We further show that epidermal growth factor or FBS stimulation induces association of endogenous RSK2 with LARG and LARG with RhoA. In response to these stimuli, RSK2 phosphorylates LARG at Ser1288 and thereby activates RhoA. Phosphorylation of RSK2 at threonine 577 is essential for activation of LARG-RhoA. Moreover, RSK2-mediated motility signaling depends on RhoA and -B, but not RhoC. These results establish a unique RSK2-dependent LARG-RhoA signaling module as a central organizer of directed cell migration and invasion.

Peptidyl-tRNA hydrolase 2 (PTRH2) regulates integrin-mediated pro-survival and apoptotic signaling. PTRH2 is critical in muscle development and regulates myogenic differentiation. In humans a biallelic mutation in the PTRH2 gene causes infantile-onset multisystem disease with progressive muscle weakness. We report here that the Ptrh2 knockout mouse model recapitulates the progressive congenital muscle pathology observed in patients. Ptrh2 null mice demonstrate multiple degenerating and regenerating muscle fibers, increased central nuclei, elevated creatine kinase activity and endomysial fibrosis. This progressive muscle pathology resembles the muscular dystrophy phenotype in humans and mice lacking the α7 integrin. We demonstrate that in normal muscle Ptrh2 associates in a complex with the α7β1 integrin at the sarcolemma and Ptrh2 expression is decreased in α7 integrin null muscle. Furthermore, Ptrh2 expression is altered in skeletal muscle of classical congenital muscular dystrophy mouse models. Ptrh2 levels were up-regulated in dystrophin deficient mdx muscle, which correlates with the elevated levels of the α7β1 integrin observed in mdx muscle and Duchenne muscular dystrophy patients. Similar to the α7 integrin, Ptrh2 expression was decreased in laminin-α2 dyW null gastrocnemius muscle. Our data establishes a PTRH2 mutation as a novel driver of congenital muscle degeneration and identifies a potential novel target to treat muscle myopathies.

The Myomatrix 2012 conference held April 22–24th, 2012 at the University of Nevada, Reno convened 73 international participants to discuss the dynamic relationship between muscle and its matrix in muscular dystrophy with a specific focus on congenital muscular dystrophy. Seven sessions over 2½ days defined three central themes: (1) the role of extracellular matrix proteins and compartments in development and specifically in congenital muscular dystrophy (CMD) (2) the role of extracellular matrix signaling and adhesion to membrane receptors and (3) the balance and interplay between inflammation and fibrosis as drivers of altered matrix stiffness, impaired regeneration and progressive dystrophy. This report highlights major conference findings and the translational roadmap as defined by conference attendees.

Background: RSK2 alters cell migration and metastasis, but the mechanism is incompletely understood. Results: RSK2 inactivates integrins, increases filamin binding to integrin tails, alters actin distribution, increases migration, and decreases fibronectin matrix assembly. Conclusion: RSK2 mediates inactivation of integrins and thus modulates integrin functions. Significance: These findings provide a novel mechanism by which RSK2 affects migration and may lead to more selective ways to inhibit RSK2-dependent metastasis., Modulation of integrin activation is important in many cellular functions including adhesion, migration, and assembly of the extracellular matrix. RSK2 functions downstream of Ras/Raf and promotes tumor cell motility and metastasis. We therefore investigated whether RSK2 affects integrin function. We report that RSK2 mediates Ras/Raf inactivation of integrins. As a result, we find that RSK2 impairs cell adhesion and integrin-mediated matrix assembly and promotes cell motility. Active RSK2 appears to affect integrins by reducing actin stress fibers and disrupting focal adhesions. Moreover, RSK2 co-localizes with the integrin activator talin and is present at integrin cytoplasmic tails. It is thereby in a position to modulate integrin activation and integrin-mediated migration. Activation of RSK2 promotes filamin phosphorylation and binding to integrins. We also find that RSK2 is activated in response to integrin ligation to fibronectin. Thus, RSK2 could participate in a feedback loop controlling integrin function. These results reveal RSK2 as a key regulator of integrin activity and provide a novel mechanism by which it may promote cell migration and cancer metastasis.

Loss of properly regulated cell death and cell survival pathways can contribute to the development of cancer and cancer metastasis. Cell survival signals are modulated by many different receptors, including integrins. Bit-1 is an effector of anoikis (cell death due to loss of attachment) in suspended cells. The anoikis function of Bit-1 can be counteracted by integrin-mediated cell attachment. Here, we explored integrin regulation of Bit-1 in adherent cells. We show that knockdown of endogenous Bit-1 in adherent cells decreased cell survival and re-expression of Bit-1 abrogated this effect. Furthermore, reduction of Bit-1 promoted both staurosporine and serum-deprivation induced apoptosis. Indeed knockdown of Bit-1 in these cells led to increased apoptosis as determined by caspase-3 activation and positive TUNEL staining. Bit-1 expression protected cells from apoptosis by increasing phospho-IκB levels and subsequently bcl-2 gene transcription. Protection from apoptosis under serum-free conditions correlated with bcl-2 transcription and Bcl-2 protein expression. Finally, Bit-1-mediated regulation of bcl-2 was dependent on focal adhesion kinase, PI3K, and AKT. Thus, we have elucidated an integrin-controlled pathway in which Bit-1 is, in part, responsible for the survival effects of cell-ECM interactions.

Muscle differentiation requires a complex signaling cascade that leads to the production of multinucleated myofibers. Genes regulating the intrinsic mitochondrial apoptotic pathway also function in controlling cell differentiation. How such signaling pathways are regulated during differentiation is not fully understood. Bit‐1 (also known as PTRH2 ) mutations in humans cause infantile‐onset multisystem disease with muscle weakness. We demonstrate here that Bit‐1 controls skeletal myogenesis through a caspase‐mediated signaling pathway. Bit‐1 ‐null mice exhibit a myopathy with hypotrophic myofibers. Bit‐1 ‐null myoblasts prematurely express muscle‐specific proteins. Similarly, knockdown of Bit‐1 expression in C2C12 myoblasts promotes early differentiation, whereas overexpression delays differentiation. In wild‐type mice, Bit‐1 levels increase during differentiation. Bit‐1 ‐null myoblasts exhibited increased levels of caspase 9 and caspase 3 without increased apoptosis. Bit‐1 re‐expression partially rescued differentiation. In Bit‐1 ‐null muscle, Bcl‐2 levels are reduced, suggesting that Bcl‐2‐mediated inhibition of caspase 9 and caspase 3 is decreased. Bcl‐2 re‐expression rescued Bit‐1‐mediated early differentiation in Bit‐1 ‐null myoblasts and C2C12 cells with knockdown of Bit‐1 expression. These results support an unanticipated yet essential role for Bit‐1 in controlling myogenesis through regulation of Bcl‐2.

Integrin-mediated cell adhesion is necessary for the survival of many cell types, and loss of adhesion causes apoptosis. We have previously shown that the alpha5beta1 integrin supports cell survival on fibronectin and increases Bcl-2 protein expression. Here we show that bcl-2 transcription is elevated in cells that attach to fibronectin through alpha(v)beta1 or to vitronectin through alpha(v)beta3 but is not elevated in cells attaching through the alpha(v)beta1 integrin. Bcl-2 protein expression and protection from apoptosis under serum-free conditions correlated with bcl-2 transcription. This integrin-mediated regulation of bcl-2 is Shc- and FAK-dependent, and activation of Ras by FAK is required. Furthermore, Ras mediates this up-regulation of bcl-2 by activating the phosphatidylinositol 3-kinase-AKT pathway. Mitogen-activated protein kinase did not appear to be necessary for the activation of bcl-2 transcription. Therefore, our work characterizes the pathway that mediates the effect of integrins on bcl-2 transcription and cell survival.

The tyrosine kinase called pp125FAK is believed to play an important role in integrin-mediated signal transduction. pp125FAK is associated both functionally and spatially with integrins, which are the cell surface receptors for extracellular matrix components. Although the precise function of pp125FAK is not known, two possibilities have been proposed: pp125FAK may regulate the assembly of focal adhesions in spreading or migrating cells, or pp125FAK may participate in a signal transduction cascade to inform the nucleus that the cell is anchored. To test these models in living cells, a peptide representing the focal adhesion kinase (FAK)-binding site of the beta 1 tail was coupled to carrier protein and injected into cultured cells to competitively inhibit the binding of pp125FAK to endogenous integrin, thus inhibiting activation of pp125FAK on a cell-by-cell basis. In addition, an antibody directed against an epitope adjacent to the focal adhesion targeting sequence on pp125FAK was microinjected, as an alternative means of inhibiting pp125FAK activation. It was observed that when rounded cells were injected with either the integrin peptide or the anti-FAK antibody, the cells rapidly began to apoptose, within 4 h after injection. These results indicate that pp125FAK may play a critical role in suppressing apoptosis in fibroblasts.

Basement membrane-adherent type II alveolar cells isolated from lung assemble into lumen-containing cellular spheres which retain the correct polarity and thereby approximate the earliest fetal stage of alveolar morphogenesis. The molecular basis of this process, determined in initial experiments to be attributable mainly to the large heterotrimeric glycoprotein laminin, was probed with laminin proteolytic fragments, antibodies, and synthetic peptides. The carboxy-terminal fragment E8, but not equimolar amounts of fragment P1, blocked alveolar formation. To pursue this observation, we used several anti-E8 antibodies and identified one, prepared against A chain residues 2179-2198 ("SN-peptide") from the first loop of the G domain, as inhibitory. These results were confirmed by use of SN-peptide alone and further defined by trypsin digestion of SN-peptide to the sequence SINNNR. This conserved site promoted divalent cation dependent adhesion of both type II alveolar and HT1080 cells, was inhibitable with equimolar amounts of fragment E8 but not P1, and derives from a form of laminin present in fetal alveolar basement membranes. These studies point to an important novel cell adhesion site in the laminin E8 region with a key role in lung alveolar morphogenesis.

Background Changes in cell adhesion and migration in the tumor microenvironment are key in the initiation and progression of metastasis. R-Ras is one of several small GTPases that regulate cell adhesion and migration on the extracellular matrix, however the mechanism has not been completely elucidated. Using a yeast two-hybrid approach we sought to identify novel R-Ras binding proteins that might mediate its effects on integrins. Methods and findings We identified Filamin A (FLNa) as a candidate interacting protein. FLNa is an actin-binding scaffold protein that also binds to integrin beta1, beta2 and beta7 tails and is associated with diverse cell processes including cell migration. Indeed, M2 melanoma cells require FLNa for motility. We further show that R-Ras and FLNa interact in co-immunoprecipitations and pull-down assays. Deletion of FLNa repeat 3 (FLNaDelta3) abrogated this interaction. In M2 melanoma cells active R-Ras co-localized with FLNa but did not co-localize with FLNa lacking repeat 3. Thus, activated R-Ras binds repeat 3 of FLNa. The functional consequence of this interaction was that active R-Ras and FLNa coordinately increased cell migration. In contrast, co-expression of R-Ras and FLNaDelta3 had a significantly reduced effect on migration. While there was enhancement of integrin activation and fibronectin matrix assembly, cell adhesion was not altered. Finally, siRNA knockdown of endogenous R-Ras impaired FLNa-dependent fibronectin matrix assembly. Conclusions These data support a model in which R-Ras functionally associates with FLNa and thereby regulates integrin-dependent migration. Thus in melanoma cells R-Ras and FLNa may cooperatively promote metastasis by enhancing cell migration.

The amyloid-beta peptide (Abeta) can mediate cell attachment by binding to beta1 integrins through an arg-his-asp sequence. We show here that the alpha5beta1 integrin, a fibronectin receptor, is an efficient binder of Abeta, and mediates cell attachment to nonfibrillar Abeta. Cells engineered to express alpha5beta1 internalized and degraded more added Abeta1-40 than did alpha5beta1-negative control cells. Deposition of an insoluble Abeta1-40 matrix around the alpha5beta1-expressing cells was reduced, and the cells showed less apoptosis than the control cells. Thus, the alpha5beta1 integrin may protect against Abeta deposition and toxicity, which is a course of Alzheimer's disease lesions.

Cell adhesion to amino acids 2179-2198 (SN-peptide) of the laminin-1 alpha1-chain is required for lung alveolar formation in vitro (M. L. Matter and G. W. Laurie. J. Cell Biol. 124: 1083-1090, 1994). The nature of the SN-peptide receptor(s) was probed with neutralizing anti-integrin monoclonal antibodies (MAb), cells lacking integrin subunits, soluble heparin, and SN-peptide columns. Cell adhesion and spreading studies confirmed the specificity of SN-peptide and revealed adhesion to be unaffected by inclusion of anti-beta1-, anti-alpha(2-6)- or anti-alpha(V)beta5-integrin MAb. Cells lacking beta1- or alpha6-integrin subunits were fully adherent. Adhesion was heparin, but not chondroitin sulfate or heparinase, sensitive, much as is alpha-dystroglycan-laminin-1 binding. Heparin eluted approximately 155- and 180-kDa cell-surface proteins from SN-peptide columns. An additional approximately 91-kDa protein was eluted by EDTA. All were unrecognized by anti-beta1-integrin MAb. SN-peptide therefore interacts with three cell-surface proteins for which the identity remains to be determined.