Cell biology: watching the first steps of podosome formation

Functional Role of the L396R Mutation of Tks5 Identified by an Exome-Wide Association Study in Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common cardiac arrhythmia; however, the current treatment strategies for AF have limited efficacy. Thus, a better understanding of the mechanisms underlying AF is important for future therapeutic strategy. A previous study (Exome-Wide Association Study (ExWAS)) identified a rare variant, rs202011870 (MAF=0.00036, GenomAD), which is highly associated with AF (OR=3.617, P<0.0001). rs202011870 results in the replacement of Leu at 396 with Arg (L396R) in a molecule, Tks5; however, the mechanism of how rs202011870 links to AF is completely unknown.

METHODS AND RESULTS

The association of rs202011870 with AF was examined in 3,378 participants (641 control and 2,737 AF cases) from 4 independent cohorts by using an Invader assay. Consequences of rs202011870 in migration ability, podosome formation, and expression of inflammation-related molecules in macrophages were examined using RAW264.7 cells with a trans-well assay, immunocytochemistry, and qPCR assay. Validation of the association of rs202011870 with AF was successful. In vitro studies showed that RAW264.7 cells with L396R-Tks5 increased trans-well migration ability, and enhanced podosome formation. RAW264.7 cells with L396R-Tks5 also increased the expression of several inflammatory cytokines and inflammation-related molecules.

CONCLUSIONS

L396R mutation in Tks5 associated with AF enhances migration of macrophages and their inflammatory features, resulting in enhanced susceptibility to AF.

Septin2 mediates podosome maturation and endothelial cell invasion associated with angiogenesis

Podosomes are compartmentalized actin-rich adhesions, defined by their ability to locally secrete proteases and remodel extracellular matrix. Matrix remodeling by endothelial podosomes facilitates invasion and thereby vessel formation. However, the mechanisms underlying endothelial podosome formation and function remain unclear. Here, we demonstrate that Septin2, Septin6, and Septin7 are required for maturation of nascent endothelial podosomes into matrix-degrading organelles. We show that podosome development occurs through initial mobilization of the scaffolding protein Tks5 and F-actin accumulation, followed by later recruitment of Septin2. Septin2 localizes around the perimeter of podosomes in close proximity to the basolateral plasma membrane, and phosphoinositide-binding residues of Septin2 are required for podosome function. Combined, our results suggest that the septin cytoskeleton forms a diffusive barrier around nascent podosomes to promote their maturation. Finally, we show that Septin2-mediated regulation of podosomes is critical for endothelial cell invasion associated with angiogenesis. Therefore, targeting of Septin2-mediated podosome formation is a potentially attractive anti-angiogenesis strategy.

Signaling Determinants of Glioma Cell Invasion

Tumor cell invasiveness is a critical challenge in the clinical management of glioma patients. In addition, there is accumulating evidence that current therapeutic modalities, including anti-angiogenic therapy and radiotherapy, can enhance glioma invasiveness. Glioma cell invasion is stimulated by both autocrine and paracrine factors that act on a large array of cell surface-bound receptors. Key signaling elements that mediate receptor-initiated signaling in the regulation of glioblastoma invasion are Rho family GTPases, including Rac, RhoA and Cdc42. These GTPases regulate cell morphology and actin dynamics and stimulate cell squeezing through the narrow extracellular spaces that are typical of the brain parenchyma. Transient attachment of cells to the extracellular matrix is also necessary for glioblastoma cell invasion. Interactions with extracellular matrix components are mediated by integrins that initiate diverse intracellular signalling pathways. Key signaling elements stimulated by integrins include PI3K, Akt, mTOR and MAP kinases. In order to detach from the tumor mass, glioma cells secrete proteolytic enzymes that cleave cell surface adhesion molecules, including CD44 and L1. Key proteases produced by glioma cells include uPA, ADAMs and MMPs. Increased understanding of the molecular mechanisms that control glioma cell invasion has led to the identification of molecular targets for therapeutic intervention in this devastating disease.

Signaling determinants of glioma cell invasion

Tumor cell invasiveness is a critical challenge in the clinical management of glioma patients. In addition, there is accumulating evidence that current therapeutic modalities, including anti-angiogenic therapy and radiotherapy, can enhance glioma invasiveness. Glioma cell invasion is stimulated by both autocrine and paracrine factors that act on a large array of cell surface-bound receptors. Key signaling elements that mediate receptor-initiated signaling in the regulation of glioblastoma invasion are Rho family GTPases, including Rac, RhoA and Cdc42. These GTPases regulate cell morphology and actin dynamics and stimulate cell squeezing through the narrow extracellular spaces that are typical of the brain parenchyma. Transient attachment of cells to the extracellular matrix is also necessary for glioblastoma cell invasion. Interactions with extracellular matrix components are mediated by integrins that initiate diverse intracellular signalling pathways. Key signaling elements stimulated by integrins include PI3K, Akt, mTOR and MAP kinases. In order to detach from the tumor mass, glioma cells secrete proteolytic enzymes that cleave cell surface adhesion molecules, including CD44 and L1. Key proteases produced by glioma cells include uPA, ADAMs and MMPs. Increased understanding of the molecular mechanisms that control glioma cell invasion has led to the identification of molecular targets for therapeutic intervention in this devastating disease.

3' Phosphatase activity toward phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] by voltage-sensing phosphatase (VSP)

Voltage-sensing phosphatases (VSPs) consist of a voltage-sensor domain and a cytoplasmic region with remarkable sequence similarity to phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor phosphatase. VSPs dephosphorylate the 5' position of the inositol ring of both phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] upon voltage depolarization. However, it is unclear whether VSPs also have 3' phosphatase activity. To gain insights into this question, we performed in vitro assays of phosphatase activities of Ciona intestinalis VSP (Ci-VSP) and transmembrane phosphatase with tensin homology (TPTE) and PTEN homologous inositol lipid phosphatase (TPIP; one human ortholog of VSP) with radiolabeled PI(3,4,5)P(3). TLC assay showed that the 3' phosphate of PI(3,4,5)P(3) was not dephosphorylated, whereas that of phosphatidylinositol 3,4-bisphosphate [PI(3,4)P(2)] was removed by VSPs. Monitoring of PI(3,4)P(2) levels with the pleckstrin homology (PH) domain from tandem PH domain-containing protein (TAPP1) fused with GFP (PH(TAPP1)-GFP) by confocal microscopy in amphibian oocytes showed an increase of fluorescence intensity during depolarization to 0 mV, consistent with 5' phosphatase activity of VSP toward PI(3,4,5)P(3). However, depolarization to 60 mV showed a transient increase of GFP fluorescence followed by a decrease, indicating that, after PI(3,4,5)P(3) is dephosphorylated at the 5' position, PI(3,4)P(2) is then dephosphorylated at the 3' position. These results suggest that substrate specificity of the VSP changes with membrane potential.

p53 regulation of podosome formation and cellular invasion in vascular smooth muscle cells

The p53 transcription factor, discovered in 1979 ( 1;2) , is well known as a potent suppressor of tumor development by inhibiting cell cycle progression, and promoting senescence or apoptosis, when the genome is compromised or under oncogenic stress ( 3) . Accumulating evidence has pointed to an alternative role of p53 in the curtailment of tumor progression and colonization of secondary sites by negatively regulating tumor cell metastasis ( 4;5) . Recently, we have found that p53 suppresses Src-induced formation of podosomes and associated invasive phenotypes in fibroblasts and vascular smooth muscle cells (VSMC) ( 6;7) . In this review, I will focus on some recent studies that have identified p53 as a suppressor of cell migration and invasion in general, and VSMC podosome formation and ECM degradation in particular.

Laminin-332-beta1 integrin interactions negatively regulate invadopodia

Adhesion of epithelial cells to basement membranes (BM) occurs through two major structures: actin-associated focal contacts and keratin-associated hemidesmosomes, both of which form on laminin-332 (Ln-332). In epithelial-derived cancer cells, additional actin-linked structures with putative adhesive properties, invadopodia, are frequently present and mediate BM degradation. A recent study proposed that BM invasion requires a proper combination of focal contacts and invadopodia for invading cells to gain traction through degraded BM, and suggested that these structures may compete for common molecular components such as Src kinase. In this study, we tested the role of the Ln-332 in regulating invadopodia in 804G rat bladder carcinoma cells, a cell line that secretes Ln-332 and forms all three types of adhesions. Expression of shRNA to Ln-332 gamma2 chain (gamma2-kd) led to increased numbers of invadopodia and enhanced extracellular matrix degradation. Replating gamma2-kd cells on Ln-332 or collagen-I fully recovered cell spreading and inhibition of invadopodia. Inhibition of alpha3 or beta1, but not alpha6 or beta4, phenocopied the effect of gamma2-kd, suggesting that alpha3beta1-mediated focal contacts, rather than alpha6beta4-mediated hemidesmosome pathways, intersect with invadopodia regulation. gamma2-kd cells exhibited alterations in focal contact-type structures and in activation of focal adhesion kinase (FAK) and Src kinase. Inhibition of FAK also increased invadopodia number, which was reversible with Src inhibition. These data are consistent with a model whereby actin-based adhesions can limit the availability of active Src that is capable of invadopodia initiation and identifies Ln-332-beta1 interactions as a potent upstream regulator that limits cell invasion.

The podosomal-adaptor protein SH3PXD2B is essential for normal postnatal development

Podosome-type adhesions are actin-based membrane protrusions involved in cell-matrix adhesion and extracellular matrix degradation. Despite growing knowledge of many proteins associated with podosome-type adhesions, much remains unknown concerning the function of podosomal proteins at the level of the whole animal. In this study, the spontaneous mouse mutant nee was used to identify a component of podosome-type adhesions that is essential for normal postnatal growth and development. Mice homozygous for the nee allele exhibited runted growth, craniofacial and skeletal abnormalities, ocular anterior segment dysgenesis, and hearing impairment. Adults also exhibited infertility and a form of lipodystrophy. Using genetic mapping and DNA sequencing, the cause of nee phenotypes was identified as a 1-bp deletion within the Sh3pxd2b gene on mouse Chromosome 11. Whereas the wild-type Sh3pxd2b gene is predicted to encode a protein with one PX domain and four SH3 domains, the nee mutation is predicted to cause a frameshift and a protein truncation altering a portion of the third SH3 domain and deleting all of the fourth SH3 domain. The SH3PXD2B protein is believed to be an important component of podosomes likely to mediate protein-protein interactions with membrane-spanning metalloproteinases. Testing this directly, SH3PXD2B localized to podosomes in constitutively active Src-transfected fibroblasts and through its last SH3 domain associated with a transmembrane member of a disintegrin and metalloproteinase family of proteins, ADAM15. These results identify SH3PXD2B as a podosomal-adaptor protein required for postnatal growth and development, particularly within physiologic contexts involving extracellular matrix regulation.

Nck adaptor proteins link Tks5 to invadopodia actin regulation and ECM degradation

Invadopodia are actin-based projections enriched with proteases, which invasive cancer cells use to degrade the extracellular matrix (ECM). The Phox homology (PX)-Src homology (SH)3 domain adaptor protein Tks5 (also known as SH3PXD2A) cooperates with Src tyrosine kinase to promote invadopodia formation but the underlying pathway is not clear. Here we show that Src phosphorylates Tks5 at Y557, inducing it to associate directly with the SH3-SH2 domain adaptor proteins Nck1 and Nck2 in invadopodia. Tks5 mutants unable to bind Nck show reduced matrix degradation-promoting activity and recruit actin to invadopodia inefficiently. Conversely, Src- and Tks5-driven matrix proteolysis and actin assembly in invadopodia are enhanced by Nck1 or Nck2 overexpression and inhibited by Nck1 depletion. We show that clustering at the plasma membrane of the Tks5 inter-SH3 region containing Y557 triggers phosphorylation at this site, facilitating Nck recruitment and F-actin assembly. These results identify a Src-Tks5-Nck pathway in ECM-degrading invadopodia that shows parallels with pathways linking several mammalian and pathogen-derived proteins to local actin regulation.