Formin-like 2 drives amoeboid invasive cell motility downstream of RhoC. Oncogene. 2010;29:2441-2448. [PMID: 20101212 DOI: 10.1038/onc.2009.515]

Divergence of Rho residue 43 impacts GEF activity

RhoA, RhoB and RhoC GTPases are over 85% identical at the amino acid level, with RhoA and RhoC differing at only one residue (43) across the initial two-thirds of their sequences. A putative regulatory distinction between the molecules is their capacity to be uniquely activated by guanine nucleotide exchange factors (GEFs). We hypothesize that variation of amino acid residue 43 between RhoA/B (valine) and RhoC (isoleucine) impacts GEF activity. Direct participation of residue 43 in GEF-catalyzed exchange was confirmed by the observation that mutation of this position to a threonine reduced GEF-catalyzed nucleotide exchange activity in vitro (Vav2, XPLN, GEFT, Dbl and Dbs) and greatly depressed RhoA and RhoC GTP-loading profiles in cell lysates. Using a residue swap approach, substitution of RhoA Val 43 with an Ile was found to significantly promote basal nucleotide exchange activity and enhance GTP-loading in cells. Substitution of Val 43 with an Ile in RhoB negatively affected nucleotide exchange in vitro. Substitution of RhoC Ile 43 with a Val increased GEF-catalyzed exchange in vitro. In addition, RhoC-I43V was more efficacious at driving ovarian cancer cell invasion through matrigrel than wild-type RhoC, RhoC-I43T, wild-type RhoA, RhoA-V43I or RhoA-V43T GTPases. These findings suggest that a divergence between RhoA/B and RhoC at residue 43 impacts basal and GEF-stimulated nucleotide exchange activity.

Differing and isoform-specific roles for the formin DIAPH3 in plasma membrane blebbing and filopodia formation

Plasma membrane (PM) blebs are dynamic actin-rich cell protrusions that occur, e.g., during cytokinesis, amoeboid cell motility and cell attachment. Using a targeted siRNA screen against 21 actin nucleation factors, we identify a novel and essential role of the human diaphanous formin DIAPH3 in PM blebbing during cell adhesion. Suppression of DIAPH3 inhibited blebbing to promote rapid cell spreading involving β1-integrin. Multiple isoforms of DIAPH3 were detected on the mRNA and protein level of which isoforms 3 and 7 were the largest and most abundant isoforms that however did not induce formation of actin-rich protrusions. Rather, PM blebbing specifically involved the low abundance isoform 1 of DIAPH3 and activation of isoform 7 by deletion of the diaphanous-autoregulatory domain caused the formation of filopodia. Dimerization and actin assembly activity were essential for induction of specific cell protrusions by DIAPH3 isoforms 1 and 7. Our data suggest that the N-terminal region comprising the GTPase-binding domain determined the subcellular localization of the formin as well as its protrusion activity between blebs and filopodia. We propose that isoform-selective actin assembly by DIAPH3 exerts specific and differentially regulated functions during cell adhesion and motility.

Genome-wide association analysis of ischemic stroke in young adults

Ischemic stroke (IS) is among the leading causes of death in Western countries. There is a significant genetic component to IS susceptibility, especially among young adults. To date, research to identify genetic loci predisposing to stroke has met only with limited success. We performed a genome-wide association (GWA) analysis of early-onset IS to identify potential stroke susceptibility loci. The GWA analysis was conducted by genotyping 1 million SNPs in a biracial population of 889 IS cases and 927 controls, ages 15–49 years. Genotypes were imputed using the HapMap3 reference panel to provide 1.4 million SNPs for analysis. Logistic regression models adjusting for age, recruitment stages, and population structure were used to determine the association of IS with individual SNPs. Although no single SNP reached genome-wide significance (P < 5 × 10⁻⁸), we identified two SNPs in chromosome 2q23.3, rs2304556 (in FMNL2; P = 1.2 × 10⁻⁷) and rs1986743 (in ARL6IP6; P = 2.7 × 10⁻⁷), strongly associated with early-onset stroke. These data suggest that a novel locus on human chromosome 2q23.3 may be associated with IS susceptibility among young adults.

Cease-fire at the leading edge: new perspectives on actin filament branching, debranching, and cross-linking

Membrane protrusion at the leading edge of migrating cells is driven by the polymerization of actin. Recent studies using advanced imaging techniques raised a lively controversy about the morphology of these filaments; however, common ground between the two sides now appears to have been found. Here we discuss how the controversy has led to a deeper consideration of the architecture of actin networks underlying cell migration, and has helped define new challenges that lie ahead.

Defining the earliest transcriptional steps of chondrogenic progenitor specification during the formation of the digits in the embryonic limb

The characterization of genes involved in the formation of cartilage is of key importance to improve cell-based cartilage regenerative therapies. Here, we have developed a suitable experimental model to identify precocious chondrogenic events in vivo by inducing an ectopic digit in the developing embryo. In this model, only 12 hr after the implantation of a Tgfβ bead, in the absence of increased cell proliferation, cartilage forms in undifferentiated interdigital mesoderm and in the course of development, becomes a structurally and morphologically normal digit. Systematic quantitative PCR expression analysis, together with other experimental approaches allowed us to establish 3 successive periods preceding the formation of cartilage. The “pre-condensation stage”, occurring within the first 3 hr of treatment, is characterized by the activation of connective tissue identity transcriptional factors (such as Sox9 and Scleraxis) and secreted factors (such as Activin A and the matricellular proteins CCN-1 and CCN-2) and the downregulation of the galectin CG-8. Next, the “condensation stage” is characterized by intense activation of Smad 1/5/8 BMP-signaling and increased expression of extracellular matrix components. During this period, the CCN matricellular proteins promote the expression of extracellular matrix and cell adhesion components. The third period, designated the “pre-cartilage period”, precedes the formation of molecularly identifiable cartilage by 2–3 hr and is characterized by the intensification of Sox 9 gene expression, along with the stimulation of other pro-chondrogenic transcription factors, such as HifIa. In summary, this work establishes a temporal hierarchy in the regulation of pro-chondrogenic genes preceding cartilage differentiation and provides new insights into the relative roles of secreted factors and cytoskeletal regulators that direct the first steps of this process in vivo.

Dynamic remodeling of the actin cytoskeleton by FMNL1γ is required for structural maintenance of the Golgi complex

Formin-like 1 (FMNL1) is a member of the formin family of actin nucleators, and is one of the few formins for which in vitro activities have been well characterized. However, the functional roles of this mammalian formin remain ill-defined. In particular, it is unclear how the unique in vitro biochemical properties of FMNL1 relate to its regulation of cellular processes. Here, we demonstrate that FMNL1 depletion caused a dramatic increase in cellular F-actin content, which resulted in Golgi complex fragmentation. Moreover, increased F-actin and maintenance of Golgi structure were distinctly regulated by the gamma isoform of FMNL1, which required binding to actin. Importantly, in addition to Golgi fragmentation, increased F-actin content in the absence of FMNL1 also led to cation-independent mannose 6-phosphate receptor dispersal, lysosomal enlargement and missorting of cathepsin D. Taken together, our data support a model in which FMNL1 regulates cellular F-actin levels required to maintain structural integrity of the Golgi complex and lysosomes.

Identification and characterization of a set of conserved and new regulators of cytoskeletal organization, cell morphology and migration

BACKGROUND

Cell migration is essential during development and in human disease progression including cancer. Most cell migration studies concentrate on known or predicted components of migration pathways.

RESULTS

Here we use data from a genome-wide RNAi morphology screen in Drosophila melanogaster cells together with bioinformatics to identify 26 new regulators of morphology and cytoskeletal organization in human cells. These include genes previously implicated in a wide range of functions, from mental retardation, Down syndrome and Huntington's disease to RNA and DNA-binding genes. We classify these genes into seven groups according to phenotype and identify those that affect cell migration. We further characterize a subset of seven genes, FAM40A, FAM40B, ARC, FMNL3, FNBP3/FBP11, LIMD1 and ZRANB1, each of which has a different effect on cell shape, actin filament distribution and cell migration. Interestingly, in several instances closely related isoforms with a single Drosophila homologue have distinct phenotypes. For example, FAM40B depletion induces cell elongation and tail retraction defects, whereas FAM40A depletion reduces cell spreading.

CONCLUSIONS

Our results identify multiple regulators of cell migration and cytoskeletal signalling that are highly conserved between Drosophila and humans, and show that closely related paralogues can have very different functions in these processes.

RhoA and RhoC have distinct roles in migration and invasion by acting through different targets

Although closely related, RhoA and RhoC have distinct molecular targets and functional roles in cell migration and invasion.

Several studies suggest that RhoA and RhoC, despite their sequence similarity, have different roles in cell migration and invasion, but the molecular basis for this is not known. Using RNAi, we show that RhoA-depleted cells became elongated and extended multiple Rac1-driven narrow protrusions in 2D and 3D environments, leading to increased invasion. These phenotypes were caused by combined but distinct effects of the Rho-regulated kinases ROCK1 and ROCK2. Depletion of ROCK2 induced multiple delocalized protrusions and reduced migratory polarity, whereas ROCK1 depletion selectively led to cell elongation and defective tail retraction. In contrast, RhoC depletion increased cell spreading and induced Rac1 activation around the periphery in broad lamellipodia, thereby inhibiting directed migration and invasion. These effects of RhoC depletion are mediated by the formin FMNL3, which we identify as a new target of RhoC but not RhoA. We propose that RhoA contributes to migratory cell polarity through ROCK2-mediated suppression of Rac1 activity in lamellipodia, whereas RhoC promotes polarized migration through FMNL3 by restricting lamellipodial broadening.

Discovery of a novel tumour metastasis-promoting gene, NVM-1

We have previously reported that over-expression of a panel of 119 genes correlates with the metastatic potential of pancreatic carcinoma cells. We sought to identify and functionally characterize candidate tumour metastasis promoting genes among this library using a secondary phenotype-assisted screen. Here we report the discovery of the metastasis-promoting function of a hitherto not characterized gene located on chromosome 14 (ORF138), which we have named 'novel metastasis-promoting gene 1' (NVM-1). The NVM-1 transcript is extensively alternatively spliced, is expressed endogenously in a number of different tissues, and is strongly over-expressed at the protein level in a variety of human tumour types. Importantly, NVM-1 expression stimulates the migratory and invasive behaviour of tumour cells and promotes metastasis formation in experimental animals in vivo. Up-regulation of FMNL2 and MT1E and down-regulation of TIMP4 and MHC-I is observed as a consequence of NVM-1 expression. Together these data identify NVM-1 as a gene that is functionally involved in tumour metastasis, and suggest that NVM-1 may constitute a promising therapeutic target for inhibition of tumour metastasis.

FMNL2 is a positive regulator of cell motility and metastasis in colorectal carcinoma

FMNL2 is a member of diaphanous-related formins which act as effectors of Rho family GTPases and control the actin-dependent processes such as cell motility or invasion. We previously found that FMNL2 overexpression in metastatic cell lines and tissues of colorectal carcinoma is associated with more aggressive tumour behaviour. Here we used gain-of-function and loss-of-function approaches to investigate the effects of FMNL2 on colorectal carcinoma in vitro and in vivo. Forced expression of FMNL2 caused a significant increase in tumour cell proliferation, motility, invasion in vitro, and metastasis in vivo, whereas FMNL2 depletion showed opposite effects. We examined gene expression profiles following knockdown of FMNL2 in SW480/M5 cells. Expression of 323 genes was up-regulated by more than two-fold, whereas 222 genes were down-regulated by more than two-fold in FMNL2-depleting SW480/M5 cells. Gene ontology analysis showed that most of genes belong to functional categories such as cell cycle, cytoskeleton, transcription factor, and G-protein modulator. Pathway analysis revealed that cytoskeletal regulation by the Rho GTPase pathway, the Wnt pathway, the G-protein pathway, and the P53 pathway were affected by FMNL2. Many of these genes are in functional networks associated with cell proliferation, metastasis, Wnt or the Rho signalling pathway involved in the regulation of FMNL2. The expression of five differentially expressed genes including CXXC4, CD200, VAV1, CSF1, and EPHA2 was validated by real-time PCR and western blot analysis. Thus, FMNL2 is a positive regulator of cell motility, invasion, and metastasis of colorectal carcinoma.

Down-regulation of formin-like 2 predicts poor prognosis in hepatocellular carcinoma

The formin-like 2 protein is a member of the diaphanous-related formin family that controls actin-dependent processes such as cell motility and invasion. This study aimed at clarifying formin-like 2 expression in hepatocellular carcinoma and its correlation with clinicopathologic features and also exploring the effects of formin-like 2 transfection on cell motility and invasion in vitro. Real-time polymerase chain reaction and Western blotting showed that expression of formin-like 2 was lower in 4 hepatocellular carcinoma cell lines than those in normal hepatic epithelial cells (P < .05). Surgical hepatocellular carcinoma samples were taken from patients who had been followed for 5 years. Immunohistochemical analysis revealed down-expression of formin-like 2 in 86 (71.7%) of 120 cases. The expression of formin-like 2 was significantly lower in hepatocellular carcinoma tissues than in adjacent cirrhotic or normal livers (P < .01). Statistical analysis showed that formin-like 2 expression correlated positively with tumor differentiation (P = .046) and vascular invasion (P = .008). Patients whose tumors had lower formin-like 2 expression had shorter overall survival times (P = .040). Multivariate analysis suggested that formin-like 2 expression was a significant and independent prognostic indicator (P = .041). Transient transfection of formin-like 2 suppressed motility and invasion of hepatocellular carcinoma cells in vitro. Our results suggest that formin-like 2 is a valuable marker for the progression of hepatocellular carcinoma. Down-regulation is associated with poor overall survival.

Nucleating actin for invasion

The invasion of cancer cells into the surrounding tissue is a prerequisite and initial step in metastasis, which is the leading cause of death from cancer. Invasive cell migration requires the formation of various structures, such as invadopodia and pseudopodia, which require actin assembly that is regulated by specialized actin nucleation factors. There is a large variety of different actin nucleators in human cells, such as formins, spire and Arp2/3-regulating proteins, and the list is likely to grow. Studies of the mechanisms of various actin nucleation factors that are involved in cancer cell function may ultimately provide new treatments for invasive and metastatic disease.

The plasticity of cytoskeletal dynamics underlying neoplastic cell migration

Due to the use of intra-vital imaging techniques and assays for cell migration into 3D matrices there has recently been much interest in different modes of tumour cell migration. Individually moving tumour cells can move either in an elongated-protrusive manner or in rounded, so-called 'amoeboid' modes. This review summarises ongoing efforts to delineate the cell signalling pathways that underlie these different forms of movement.

Characterization of Diaphanous-related formin FMNL2 in human tissues

Background

Diaphanous-related formins govern actin-based processes involved in many cellular functions, such as cell movement and invasion. Possible connections to developmental processes and cellular changes associated with malignant phenotype make them interesting study targets. In spite of this, very little is known of the tissue distribution and cellular location of any mammalian formin. Here we have carried out a comprehensive analysis of the formin family member formin -like 2 (FMNL2) in human tissues.

Results

An FMNL2 antibody was raised and characterized. The affinity-purified FMNL2 antibody was validated by Western blotting, Northern blotting, a peptide competition assay and siRNA experiments. Bioinformatics-based mRNA profiling indicated that FMNL2 is widely expressed in human tissues. The highest mRNA levels were seen in central and peripheral nervous systems. Immunohistochemical analysis of 26 different human tissues showed that FMNL2 is widely expressed, in agreement with the mRNA profile. The widest expression was detected in the central nervous system, since both neurons and glial cells expressed FMNL2. Strong expression was also seen in many epithelia. However, the expression in different cell types was not ubiquitous. Many mesenchymal cell types showed weak immunoreactivity and cells lacking expression were seen in many tissues. The subcellular location of FMNL2 was cytoplasmic, and in some tissues a strong perinuclear dot was detected. In cultured cells FMNL2 showed mostly a cytoplasmic localization with perinuclear accumulation consistent with the Golgi apparatus. Furthermore, FMNL2 co-localized with F-actin to the tips of cellular protrusions in WM164 human melanoma cells. This finding is in line with FMNL2's proposed function in the formation of actin filaments in cellular protrusions, during amoeboid cellular migration.

Conclusion

FMNL2 is expressed in multiple human tissues, not only in the central nervous system. The expression is especially strong in gastrointestinal and mammary epithelia, lymphatic tissues, placenta, and in the reproductive tract. In cultured melanoma cells, FMNL2 co-localizes with F-actin dots at the tips of cellular protrusions.