Identification of MYO18A as a novel interacting partner of the PAK2/betaPIX/GIT1 complex and its potential function in modulating epithelial cell migration

Emerging concepts of myosin 18A isoform mechanobiology in organismal and immune system physiology, development, and function

Alternative and combinatorial splicing of myosin 18A (MYO18A) gene transcripts results in expression of MYO18A protein isoforms and isoform variants with different membrane and subcellular localizations, and functional properties. MYO18A proteins are members of the myosin superfamily consisting of a myosin-like motor domain, an IQ motif, and a coiled-coil domain. MYO18A isoforms, however, lack the ability to hydrolyze ATP and do not perform ATP-dependent motor activity. MYO18A isoforms are distinguished by different amino- and carboxy-terminal extensions and domains. The domain organization and functions of MYO18Aα, MYO18Aβ, and MYO18Aγ have been studied experimentally. MYO18Aα and MYO18Aβ have a common carboxy-terminal extension but differ by the presence or absence of an amino-terminal KE repeat and PDZ domain, respectively. The amino- and carboxy-terminal extensions of MYO18Aγ contain unique proline and serine-rich domains. Computationally predicted MYO18Aε and MYO18Aδ isoforms contain the carboxy-terminal serine-rich extension but differ by the presence or absence of the amino-terminal KE/PDZ extension. Additional isoform variants within each category arise by alternative utilization or inclusion/exclusion of small exons. MYO18Aα variants are expressed in somatic cells and mature immune cells, whereas MYO18Aβ variants occur mainly in myeloid and natural killer cells. MYO18Aγ expression is selective to cardiac and skeletal muscle. In the present review perspective, we discuss current and emerging concepts of the functional specialization of MYO18A proteins in membrane and cytoskeletal dynamics, cellular communication and signaling, endocytic and exocytic organelle movement, viral infection, and as the SP-R210 receptor for surfactant protein A.

Are the class 18 myosins Myo18A and Myo18B specialist sarcomeric proteins?

Initially, the two members of class 18 myosins, Myo18A and Myo18B, appeared to exhibit highly divergent functions, complicating the assignment of class-specific functions. However, the identification of a striated muscle-specific isoform of Myo18A, Myo18Aγ, suggests that class 18 myosins may have evolved to complement the functions of conventional class 2 myosins in sarcomeres. Indeed, both genes, Myo18a and Myo18b, are predominantly expressed in the heart and somites, precursors of skeletal muscle, of developing mouse embryos. Genetic deletion of either gene in mice is embryonic lethal and is associated with the disorganization of cardiac sarcomeres. Moreover, Myo18Aγ and Myo18B localize to sarcomeric A-bands, albeit the motor (head) domains of these unconventional myosins have been both deduced and biochemically demonstrated to exhibit negligible ATPase activity, a hallmark of motor proteins. Instead, Myo18Aγ and Myo18B presumably coassemble with thick filaments and provide structural integrity and/or internal resistance through interactions with F-actin and/or other proteins. In addition, Myo18Aγ and Myo18B may play distinct roles in the assembly of myofibrils, which may arise from actin stress fibers containing the α-isoform of Myo18A, Myo18Aα. The β-isoform of Myo18A, Myo18Aβ, is similar to Myo18Aα, except that it lacks the N-terminal extension, and may serve as a negative regulator through heterodimerization with either Myo18Aα or Myo18Aγ. In this review, we contend that Myo18Aγ and Myo18B are essential for myofibril structure and function in striated muscle cells, while α- and β-isoforms of Myo18A play diverse roles in nonmuscle cells.

Tanc1/2 TPR domain interacts with Myo18a C-terminus and undergoes liquid-liquid phase separation

Tanc1 and its homologous protein Tanc2 are critical synaptic scaffold proteins which regulate synaptic spine densities and excitatory synapse strength. Recent studies indicated TANC1 and TANC2 are candidate genes of several neurodevelopmental disorders (NDDs). In this study, we identified and characterized a novel interaction between Tanc1/2 and Myo18a, mediated by the Tanc1/2 TPR domains and Myo18a coiled-coil domain and C-extension (CCex). Sequence analysis and size exclusion chromatography experiments reveal that high salt disrupts the interaction between Myo18a and Tanc1/2, indicating that the interaction is primarily driven by charge-charge interactions. More importantly, we found that the Tanc1-TPR/Myo18a CCex interaction could undergo liquid-liquid phase separation (LLPS) in both cultured cells and test tubes, which provides the biochemical basis and potential molecular mechanisms for the LLPS-mediated interactions between Myo18a and Tanc1/2.

Proteomic and yeast 2-hybrid screens to identify PTEN binding partners

PTEN is a phosphoinositide lipid phosphatase and an important tumour suppressor protein. PTEN function is reduced or lost in around a third of all human cancers through diverse mechanisms, from gene deletion to changes in the function of proteins which regulate PTEN through direct protein binding. Here we present data from SILAC (Stable Isotope Labelling by Amino acids in Cell culture) proteomic screens to identify proteins which bind to PTEN. These experiments using untransformed epithelial cells and glioma cells identified several novel candidate proteins in addition to many previously identified PTEN binding partners and many proteins which are recognised as common false positives using these methods. From subsequent co-expression pull-down experiments we provide further evidence supporting the physical interaction of PTEN with MMP1, Myosin 18A and SHROOM3. We also performed yeast two-hybrid screens which identify the previously recognised PTEN binding partner MSP58 in addition to the nuclear import export receptor TNPO3. These experiments identify several novel candidate binding partners of PTEN and provide further data addressing the set of proteins that interact with this important tumour suppressor.

3D matrix adhesion feedback controls nuclear force coupling to drive invasive cell migration

Cell invasion is a multi-step process, initiated by the acquisition of a migratory phenotype and the ability to move through complex 3D extracellular environments. We determine the composition of cell-matrix adhesion complexes of invasive breast cancer cells in 3D matrices and identify an interaction complex required for invasive migration. βPix and myosin18A (Myo18A) drive polarized recruitment of non-muscle myosin 2A (NM2A) to adhesion complexes at the tips of protrusions. Actomyosin force engagement then displaces the Git1-βPix complex from paxillin, establishing a feedback loop for adhesion maturation. We observe active force transmission to the nucleus during invasive migration that is needed to pull the nucleus forward. The recruitment of NM2A to adhesions creates a non-muscle myosin isoform gradient, which extends from the protrusion to the nucleus. We postulate that this gradient facilitates coupling of cell-matrix interactions at the protrusive cell front with nuclear movement, enabling effective invasive migration and front-rear cell polarity.

Small extracellular vesicles promote invadopodia activity in glioblastoma cells in a therapy-dependent manner

PURPOSE

The therapeutic efficacy of radiotherapy/temozolomide treatment for glioblastoma (GBM) is limited by the augmented invasiveness mediated by invadopodia activity of surviving GBM cells. As yet, however the underlying mechanisms remain poorly understood. Due to their ability to transport oncogenic material between cells, small extracellular vesicles (sEVs) have emerged as key mediators of tumour progression. We hypothesize that the sustained growth and invasion of cancer cells depends on bidirectional sEV-mediated cell-cell communication.

METHODS

Invadopodia assays and zymography gels were used to examine the invadopodia activity capacity of GBM cells. Differential ultracentrifugation was utilized to isolate sEVs from conditioned medium and proteomic analyses were conducted on both GBM cell lines and their sEVs to determine the cargo present within the sEVs. In addition, the impact of radiotherapy and temozolomide treatment of GBM cells was studied.

RESULTS

We found that GBM cells form active invadopodia and secrete sEVs containing the matrix metalloproteinase MMP-2. Subsequent proteomic studies revealed the presence of an invadopodia-related protein sEV cargo and that sEVs from highly invadopodia active GBM cells (LN229) increase invadopodia activity in sEV recipient GBM cells. We also found that GBM cells displayed increases in invadopodia activity and sEV secretion post radiation/temozolomide treatment. Together, these data reveal a relationship between invadopodia and sEV composition/secretion/uptake in promoting the invasiveness of GBM cells.

CONCLUSIONS

Our data indicate that sEVs secreted by GBM cells can facilitate tumour invasion by promoting invadopodia activity in recipient cells, which may be enhanced by treatment with radio-chemotherapy. The transfer of pro-invasive cargos may yield important insights into the functional capacity of sEVs in invadopodia.

Brain-specific deletion of GIT1 impairs cognition and alters phosphorylation of synaptic protein networks implicated in schizophrenia susceptibility

Despite tremendous effort, the molecular and cellular basis of cognitive deficits in schizophrenia remain poorly understood. Recent progress in elucidating the genetic architecture of schizophrenia has highlighted the association of multiple loci and rare variants that may impact susceptibility. One key example, given their potential etiopathogenic and therapeutic relevance, is a set of genes that encode proteins that regulate excitatory glutamatergic synapses in brain. A critical next step is to delineate specifically how such genetic variation impacts synaptic plasticity and to determine if and how the encoded proteins interact biochemically with one another to control cognitive function in a convergent manner. Towards this goal, here we study the roles of GPCR-kinase interacting protein 1 (GIT1), a synaptic scaffolding and signaling protein with damaging coding variants found in schizophrenia patients, as well as copy number variants found in patients with neurodevelopmental disorders. We generated conditional neural-selective GIT1 knockout mice and found that these mice have deficits in fear conditioning memory recall and spatial memory, as well as reduced cortical neuron dendritic spine density. Using global quantitative phospho-proteomics, we revealed that GIT1 deletion in brain perturbs specific networks of GIT1-interacting synaptic proteins. Importantly, several schizophrenia and neurodevelopmental disorder risk genes are present within these networks. We propose that GIT1 regulates the phosphorylation of a network of synaptic proteins and other critical regulators of neuroplasticity, and that perturbation of these networks may contribute specifically to cognitive deficits observed in schizophrenia and neurodevelopmental disorders.

Mechanotransduction in Skin Inflammation

In the process of mechanotransduction, the cells in the body perceive and interpret mechanical stimuli to maintain tissue homeostasis and respond to the environmental changes. Increasing evidence points towards dysregulated mechanotransduction as a pathologically relevant factor in human diseases, including inflammatory conditions. Skin is the organ that constantly undergoes considerable mechanical stresses, and the ability of mechanical factors to provoke inflammatory processes in the skin has long been known, with the Koebner phenomenon being an example. However, the molecular mechanisms and key factors linking mechanotransduction and cutaneous inflammation remain understudied. In this review, we outline the key players in the tissue’s mechanical homeostasis, the available data, and the gaps in our current understanding of their aberrant regulation in chronic cutaneous inflammation. We mainly focus on psoriasis as one of the most studied skin inflammatory diseases; we also discuss mechanotransduction in the context of skin fibrosis as a result of chronic inflammation. Even though the role of mechanotransduction in inflammation of the simple epithelia of internal organs is being actively studied, we conclude that the mechanoregulation in the stratified epidermis of the skin requires more attention in future translational research.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Control of cell signaling by Arf GTPases and their regulators: Focus on links to cancer and other GTPase families

The ADP-ribosylation factors (Arfs) comprise a family of regulatory GTP binding proteins. The Arfs regulate membrane trafficking and cytoskeleton remodeling, processes critical for eukaryotes and which have been the focus of most studies on Arfs. A more limited literature describes a role in signaling and in integrating several signaling pathways to bring about specific cell behaviors. Here, we will highlight work describing function of Arf1, Arf6 and several effectors and regulators of Arfs in signaling.

Gene Families With Stochastic Exclusive Gene Choice Underlie Cell Adhesion in Mammalian Cells

Exclusive stochastic gene choice combines precision with diversity. This regulation enables most T-cells to express exactly one T-cell receptor isoform chosen from a large repertoire, and to react precisely against diverse antigens. Some cells express two receptor isoforms, revealing the stochastic nature of this process. A similar regulation of odorant receptors and protocadherins enable cells to recognize odors and confer individuality to cells in neuronal interaction networks, respectively. We explored whether genes in other families are expressed exclusively by analyzing single-cell RNA-seq data with a simple metric. This metric can detect exclusivity independently of the mean value and the monoallelic nature of gene expression. Chromosomal segments and gene families are more likely to express genes concurrently than exclusively, possibly due to the evolutionary and biophysical aspects of shared regulation. Nonetheless, gene families with exclusive gene choice were detected in multiple cell types, most of them are membrane proteins involved in ion transport and cell adhesion, suggesting the coordination of these two functions. Thus, stochastic exclusive expression extends beyond the prototypical families, permitting precision in gene choice to be combined with the diversity of intercellular interactions.

Multifaceted Function of Myosin-18, an Unconventional Class of the Myosin Superfamily

Myosin is a diverse superfamily of motor proteins responsible for actin-based motility and contractility in eukaryotic cells. Myosin-18 family, including myosin-18A and myosin-18B, belongs to an unconventional class of myosin, which lacks ATPase motor activity, and the investigations on their functions and molecular mechanisms in vertebrate development and diseases have just been initiated in recent years. Myosin-18A is ubiquitously expressed in mammalian cells, whereas myosin-18B shows strong enrichment in striated muscles. Myosin-18 family is important for cell motility, sarcomere formation, and mechanosensing, mostly by interacting with other cytoskeletal proteins and cellular apparatus. Myosin-18A participates in several intracellular transport processes, such as Golgi trafficking, and has multiple roles in focal adhesions, stress fibers, and lamellipodia formation. Myosin-18B, on the other hand, participates in actomyosin alignment and sarcomere assembly, thus relating to cell migration and muscle contractility. Mutations of either Myo18a or Myo18b cause cardiac developmental defects in mouse, emphasizing their crucial role in muscle development and cardiac diseases. In this review, we revisit the discovery history of myosin-18s and summarize the evolving understanding of the molecular functions of myosin-18A and myosin-18B, with an emphasis on their separate yet closely related functions in cell motility and contraction. Moreover, we discuss the diseases tightly associated with myosin-18s, especially cardiovascular defects and cancer, as well as highlight the unanswered questions and potential future research perspectives on myosin-18s.

Myosin 18Aα targets the guanine nucleotide exchange factor β-Pix to the dendritic spines of cerebellar Purkinje neurons and promotes spine maturation

Myosin 18Aα is a myosin 2-like protein containing unique N- and C-terminal protein interaction domains that co-assembles with myosin 2. One protein known to bind to myosin 18Aα is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42 that has been shown to promote dendritic spine maturation by activating the assembly of actin and myosin filaments in spines. Here, we show that myosin 18A⍺ concentrates in the spines of cerebellar Purkinje neurons via co-assembly with myosin 2 and through an actin binding site in its N-terminal extension. miRNA-mediated knockdown of myosin 18A⍺ results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18A⍺. Importantly, β-Pix co-localizes with myosin 18A⍺ in spines, and its spine localization is lost upon myosin 18A⍺ knockdown or when its myosin 18A⍺ binding site is deleted. Finally, we show that the spines of myosin 18A⍺ knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data argue that mixed filaments of myosin 2 and myosin 18A⍺ form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix's GEF activity.

Intratumor Heterogeneity of MYO18A and FBXW7 Variants Impact the Clinical Outcome of Stage III Colorectal Cancer

Many studies failed to demonstrate benefit from the addition of targeted agents to current standard adjuvant FOLFOX chemotherapy in stage III colorectal cancer (CRC) patients. Intratumor heterogeneity may foster the resistant subclones and leads to cancer recurrence. Here, we built a cancer evolution model and applied machine learning analysis to identify potential therapeutic targets. Among 78 CRC cases, whole-genome (WGS) and deep targeted sequencing data generated from paired blood and primary tumor were used for phylogenetic tree reconstruction. Genetic alterations in the PI3K/AKT, and RTK oncogenic signaling pathways were commonly detected in founding clones. The dominant subclones frequently exhibited dysregulations in the TP53, FBXW7/NOTCH1 tumor suppression, and DNA repair pathways. Fourteen genetic mutations were simultaneously selected by random forest and LASSO methods. The logistic regression model had better accuracy (79%), precision (70%), and recall (65%) and area under the curve (AUC) (82%) for cancer recurrence prediction. Three genes, including MYO18A in the founding clone, FBXW7, and ATM in the dominant subclone, affected the prognosis were selected simultaneously by different feature sets. The in vitro studies, HCT-116 cells transfected with MYO18A siRNA demonstrated a significant reduction in cell migration activity by 20-40%. These results indicate that MYO18A plays a crucial role in the migration of human CRC cells. The cancer evolution model revealed the critical mutations in the founding and dominant subclones. They can be used to predict clinical outcomes and the development of novel therapeutic targets for stage III CRC.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Integrin-mediated adhesions in regulation of cellular senescence

Bioinformatic and functional data link integrin-mediated cell adhesion to cellular senescence; however, the significance of and molecular mechanisms behind these connections are unknown. We now report that the focal adhesion-localized βPAK-interacting exchange factor (βPIX)-G protein-coupled receptor kinase interacting protein (GIT) complex controls cellular senescence in vitro and in vivo. βPIX and GIT levels decline with age. βPIX knockdown induces cellular senescence, which was prevented by reexpression. Loss of βPIX induced calpain cleavage of the endocytic adapter amphiphysin 1 to suppress clathrin-mediated endocytosis (CME); direct competition of GIT1/2 for the calpain-binding site on paxillin mediates this effect. Decreased CME and thus integrin endocytosis induced abnormal integrin signaling, with elevated reactive oxygen species production. Blocking integrin signaling inhibited senescence in human fibroblasts and mouse lungs in vivo. These results reveal a central role for integrin signaling in cellular senescence, potentially identifying a new therapeutic direction.

Myosin XVIII

Class XVIII myosins represent a branch of the myosin family tree characterized by the presence of large N- and C-terminal extensions flanking a generic myosin core. These myosins display the highest sequence similarity to conventional class II muscle myosins and are compatible with but not restricted to myosin-2 contractile structures. Instead, they fulfill their functions at diverse localities, such as lamella, actomyosin bundles, the Golgi apparatus, focal adhesions, the cell membrane, and within sarcomeres. Sequence comparison of active-site residues and biochemical data available thus far indicate that this myosin class lacks active ATPase-driven motor activity, suggesting that its members function as structural myosins. An emerging body of evidence indicates that this structural capability is essential for the organization, maturation, and regulation of the contractile machinery in both muscle and nonmuscle cells. This is supported by the clear association of myosin-18A (Myo18A) and myosin-18B (Myo18B) dysregulation with diseases such as cancer and various myopathies.

MicroRNA-760 acts as a tumor suppressor in gastric cancer development via inhibiting G-protein-coupled receptor kinase interacting protein-1 transcription

BACKGROUND

Gastric cancer (GC) has become a serious threat to people's health. Accumulative evidence reveals that dysregulation of numerous microRNAs (miRNAs) has been found during malignant formation. So far, the role of microRNA-760 (miR-760) in the development of GC is largely unknown.

AIM

To measure the expression level of miR-760 in GC and investigate its role in gastric tumorigenesis.

METHODS

Real-time quantitative polymerase chain reaction and Western blot analysis were used to measure the expression of miR-760 and G-protein-coupled receptor kinase interacting protein-1 (GIT1). Cell growth was detected by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) and cell colony formation assays. Apoptosis was assessed by flow cytometric analysis. The relationship between miR-760 and GIT1 was verified by luciferase reporter assay.

RESULTS

The results showed that the expression of miR-760 was decreased in GC and associated with poor clinical outcomes in GC patients. Furthermore, miR-760 restrained cell proliferation and cell colony formation and induced apoptosis in GC cells. In addition, miR-760 directly targeted GIT1 and negatively regulated its expression in GC. GIT1 was upregulated in GC and predicted a worse prognosis in GC patients. We also found that upregulation of GIT1 weakened the inhibitory effect of miR-760 in GC.

CONCLUSION

In conclusion, miR-760 targets GIT1 to inhibit cell growth and promote apoptosis in GC cells. Our data demonstrate that miR-760 may be a potential target for the treatment of GC.

IGF1R signaling drives antiestrogen resistance through PAK2/PIX activation in luminal breast cancer

Antiestrogen resistance in estrogen receptor positive (ER⁺) breast cancer is associated with increased expression and activity of insulin-like growth factor 1 receptor (IGF1R). Here, a kinome siRNA screen has identified 10 regulators of IGF1R-mediated antiestrogen with clinical significance. These include the tamoxifen resistance suppressors BMPR1B, CDK10, CDK5, EIF2AK1, and MAP2K5, and the tamoxifen resistance inducers CHEK1, PAK2, RPS6KC1, TTK, and TXK. The p21-activated kinase 2, PAK2, is the strongest resistance inducer. Silencing of the tamoxifen resistance inducing genes, particularly PAK2, attenuates IGF1R-mediated resistance to tamoxifen and fulvestrant. High expression of PAK2 in ER⁺ metastatic breast cancer patients is correlated with unfavorable outcome after first-line tamoxifen monotherapy. Phospho-proteomics has defined PAK2 and the PAK-interacting exchange factors PIXα/β as downstream targets of IGF1R signaling, which are independent from PI3K/ATK and MAPK/ERK pathways. PAK2 and PIXα/β modulate IGF1R signaling-driven cell scattering. Targeting PIXα/β entirely mimics the effect of PAK2 silencing on antiestrogen re-sensitization. These data indicate PAK2/PIX as an effector pathway in IGF1R-mediated antiestrogen resistance.

Targeting of GIT1 by miR-149* in breast cancer suppresses cell proliferation and metastasis in vitro and tumor growth in vivo

Breast cancer remains a major cause of cancer-related death in women worldwide. Dysregulation of microRNAs (miRNAs) is involved in the initiation and progression of breast cancer. Moreover, it was found that GIT1 was widely involved in the development of many human cancers. Herein, we aimed to investigate the expression changes of miR-149* and GIT1 and the functional effects of miR-149*/GIT1 link in breast cancer. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western blot (WB) were used to examine the expression levels of miR-149* and GIT1. Dual luciferase reporter assay was utilized to confirm the target interaction between miR-149* and GIT1. The biological functions, including cell proliferation, invasion, and migration, of miR-149* and GIT1 were determined by MTT assay and Transwell assays, respectively. Eventually, the tumor xenograft model in nude mice injected with stable transfected MDA-MB-231 cells was established to verify the effects of miR-149* and GIT1 on tumor growth. Our results showed that miR-149* expression was decreased, whereas GIT1 expression was increased in clinical samples of breast cancer. Based on the inverse expression trend between miR-149* and GIT1, we further demonstrated that miR-149* indeed directly targets GIT1. Subsequently, it was observed that inhibition of miR-149* significantly promoted cell proliferation, invasion, and migration, but the ability of cell proliferation, invasion, and migration was obviously declined after silencing of GIT1 in MDA-MB-231 cells transfected with miR-149* mimic and/or si-GIT1. Finally, it was also found that elevated miR-149* decelerated the tumor growth, while restored GIT1 accelerated the tumor growth in nude mice after 35 days of tumor xenograft. Collectively, these findings concluded that miR-149* might exert a tumor suppressive role in breast cancer by targeting GIT1.

MYO18A: An unusual myosin

MYO18A is a divergent member of the myosin family characterized by the presence of an amino-terminal PDZ domain. MYO18A has been found in a few different complexes involved in intracellular transport processes. MYO18A is found in a complex with LURAP1 and MRCK that functions in retrograde treadmilling of actin. It also has been found in a complex with PAK2, βPIX, and GIT1, functioning to transport that protein complex from focal adhesions to the leading edge. Finally, a high proportion of MYO18A is found in complex with GOLPH3 at the trans Golgi, where it functions to promote vesicle budding for Golgi-to-plasma membrane trafficking. Interestingly, MYO18A has been implicated as a cancer driver, as have other components of the GOLPH3 pathway. It remains uncertain as to whether or not MYO18A has intrinsic motor activity. While many questions remain, MYO18A is a fascinatingly unique myosin that is essential in higher organisms.

Quantitative Phosphoproteomic Analysis Reveals Key Mechanisms of Cellular Proliferation in Liver Cancer Cells

Understanding the mechanisms of uncontrolled proliferation in cancer cells provides valuable insights into tumor development and is benefit for discovering efficient methods in cancer treatment. In this study, we identified and quantified 2,057 phosphoproteins and 9,824 unique phosphosites in three liver cell lines with high (QGY, Hep3B) and low (L02) proliferative potentials and disclosed the wide variations in phosphorylation sites and levels among them. We found that the number of identified phosphoproteins and phosphosites in these cells were negatively correlated with their proliferative abilities. The function analysis suggested that the aberrant phosphorylation of SR proteins and activation of MAPK pathway might be two critical factors to promote cancer cell proliferation. Meanwhile, the phosphorylation status of mini-chromosome maintenance (MCM) and nuclear pore (NPC) complexes are significantly different between cell lines with high and low proliferative potentials. Furthermore, the phosphosites targeted by kinase families of CDK, STE and HIPK in the proteins coded by cancer driver genes showed distinct profiles between caner and normal cell lines. These results present key phosphorylation networks involving in abnormal proliferation of cancer cells and uncovered potential molecular markers for estimating the proliferation ability of liver cancer cells.

MeCP2, a target of miR-638, facilitates gastric cancer cell proliferation through activation of the MEK1/2-ERK1/2 signaling pathway by upregulating GIT1

Methyl-CpG binding protein 2 (MeCP2) is involved in the carcinogenesis and progression of multiple types of cancer. However, its precise role in gastric cancer (GC) and the relevant molecular mechanism remain unknown. In the present study, we found that miR-638 levels were lower in GC tissues and GC cell lines than in adjacent normal tissues and normal gastric epithelial cell lines, respectively. Low miR-638 levels were associated with poor tumor differentiation, tumor size and lymph node metastasis. MeCP2 expression levels were higher in GC tissues than in adjacent normal tissues. It was found that miR-638 inhibited GC cell proliferation, colony formation, G1–S transition and tumor growth, and induced cell apoptosis by directly targeting MeCP2. MeCP2 promoted GC cell proliferation, colony formation and G1–S cell-cycle transition, and suppressed apoptosis. Molecular mechanistic investigations were performed using an integrated approach with a combination of microarray analysis, chromatin immunoprecipitation sequencing and a reporter gene assay. The results showed that MeCP2 bound to the methylated CpG islands of G-protein-coupled receptor kinase-interacting protein 1 (GIT1) promoter and upregulated its expression, thereby activating the MEK1/2–ERK1/2 signaling pathway and promoting GC cell proliferation. Taken together, our study demonstrates that MeCP2, a target of miR-638, facilitates GC cell proliferation and induces cell-cycle progression through activation of the MEK1/2–ERK1/2 signaling pathway by upregulating GIT1. The findings suggest that MeCP2 plays a significant role in GC progression, and may serve as a potential target for GC therapy.

Re-evaluating the roles of myosin 18Aα and F-actin in determining Golgi morphology

The peri-centrosomal localization and morphology of the Golgi apparatus depends largely on the microtubule cytoskeleton and the microtubule motor protein dynein. Recent studies proposed that myosin 18Aα (M18Aα) also contributes to Golgi morphology by binding the Golgi protein GOLPH3 and walking along adjacent actin filaments to stretch the Golgi into its classic ribbon structure. Biochemical analyses have shown, however, that M18A is not an actin-activated ATPase and lacks motor activity. Our goal, therefore, was to define the precise molecular mechanism by which M18Aα determines Golgi morphology. We show that purified M18Aα remains inactive in the presence of GOLPH3, arguing against the Golgi-specific activation of the myosin. Using M18A-specific antibodies and expression of GFP-tagged M18Aα, we find no evidence that it localizes to the Golgi. Moreover, several cell lines with reduced or eliminated M18Aα expression exhibited normal Golgi morphology. Interestingly, actin filament disassembly resulted in a marked reduction in lateral stretching of the Golgi in both control and M18Aα-deficient cells. Importantly, this reduction was accompanied by an expansion of the Golgi in the vertical direction, vertical movement of the centrosome, and increases in the height of both the nucleus and the cell. Collectively, our data indicate that M18Aα does not localize to the Golgi or play a significant role in determining its morphology, and suggest that global F-actin disassembly alters Golgi morphology indirectly by altering cell shape.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

Identification of novel MYO18A interaction partners required for myoblast adhesion and muscle integrity

The unconventional myosin MYO18A that contains a PDZ domain is required for muscle integrity during zebrafish development. However, the mechanism by which it functions in myofibers is not clear. The presence of a PDZ domain suggests that MYO18A may interact with other partners to perform muscle-specific functions. Here we performed double-hybrid screening and co-immunoprecipitation to identify MYO18A-interacting proteins, and have identified p190RhoGEF and Golgin45 as novel partners for the MYO18A PDZ domain. We have also identified Lurap1, which was previously shown to bind MYO18A. Functional analyses indicate that, similarly as myo18a, knockdown of lurap1, p190RhoGEF and Golgin45 by morpholino oligonucleotides disrupts dystrophin localization at the sarcolemma and produces muscle lesions. Simultaneous knockdown of myo18a with either of these genes severely disrupts myofiber integrity and dystrophin localization, suggesting that they may function similarly to maintain myofiber integrity. We further show that MYO18A and its interaction partners are required for adhesion of myoblasts to extracellular matrix, and for the formation of the Golgi apparatus and organization of F-actin bundles in myoblast cells. These findings suggest that MYO18A has the potential to form a multiprotein complex that links the Golgi apparatus to F-actin, which regulates muscle integrity and function during early development.

GIT1 promotes lung cancer cell metastasis through modulating Rac1/Cdc42 activity and is associated with poor prognosis

G-protein-coupled receptor kinase interacting protein 1 (GIT1) is participated in cell movement activation, which is a fundamental process during tissue development and cancer progression. GIT1/PIX forming a functional protein complex that contributes to Rac1/Cdc42 activation, resulting in increasing cell mobility. Although the importance of Rac1/Cdc42 activation is well documented in cancer aggressiveness, the clinical importance of GIT1 remains largely unknown. Here, we investigated the clinical significance of GIT1 expression in non-small-cell lung cancer (NSCLC) and also verified the importance of GIT1-Rac1/Cdc42 axis in stimulating NSCLC cell mobility. The result indicated higher GIT1 expression patients had significantly poorer prognoses in disease-free survival (DFS) and overall survival (OS) compared with lower GIT1 expression patients. Higher GIT1 expression was an independent prognostic factor by multivariate analysis and associated with migration/invasion of NSCLC cells in transwell assay. In vivo studies indicated that GIT1 promotes metastasis of NSCLC cells. Finally, GIT1 was found to stimulate migration/invasion by altering the activity of Rac1/Cdc42 in NSCLC cells. Together, the GIT1 expression is associated with poor prognosis in patients with NSCLC. GIT1 is critical for the invasiveness of NSCLC cells through stimulating the activity of Rac1/Cdc42.

MicroRNA-34c Suppresses Breast Cancer Migration and Invasion by Targeting GIT1

Abnormal expression of microRNAs plays important role in tumor metastasis. Migration and invasion of cancer cells accord for the metastasis and deterioration of breast cancer. However, the regulatory role of microRNAs in the invasion and migration of breast cancer cells has not completely understood yet. Here we found that microRNA-34c (miR-34c) was significantly downregulated in metastatic tissue of breast cancer. In vitro study showed that miR-34c negatively regulated GIT1 protein expression by binding to the 3'UTR of GIT1 mRNA. Consistently, GIT1 protein expression was found upregulated significantly in metastatic breast cancer. Moreover, miR-34c overexpression suppressed the expression of GIT1 protein, and this effect was restored by AMO-miR-34c in breast cancer cells. Overexpression of miR-34c suppressed cell migration and invasion in both MCF-7 and MDA-MD-231 breast cancer cells. Furthermore, knockdown of endogenous GIT1 expression reduced the migration and invasion of both two breast cancer cells. Collectively, miR-34c downregulation in breast cancer cells resulted in the upregulation of GIT1, which in turn enhanced the migration and invasion of breast cancer. This study highlights molecular mechanism of migration and invasion of breast cancer cells.

p21-activated kinase 2 regulates HSPC cytoskeleton, migration, and homing via CDC42 activation and interaction with β-Pix

Cytoskeletal remodeling of hematopoietic stem and progenitor cells (HSPCs) is essential for homing to the bone marrow (BM). The Ras-related C3 botulinum toxin substrate (Rac)/cell division control protein 42 homolog (CDC42) effector p21-activated kinase (Pak2) has been implicated in HSPC homing and engraftment. However, the molecular pathways mediating Pak2 functions in HSPCs are unknown. Here, we demonstrate that both Pak2 kinase activity and its interaction with the PAK-interacting exchange factor-β (β-Pix) are required to reconstitute defective ITALIC! Pak2 (ITALIC! Δ/Δ)HSPC homing to the BM. Pak2 serine/threonine kinase activity is required for stromal-derived factor-1 (SDF1α) chemokine-induced HSPC directional migration, whereas Pak2 interaction with β-Pix is required to regulate the velocity of HSPC migration and precise F-actin assembly. Lack of SDF1α-induced filopodia and associated abnormal cell protrusions seen in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs were rescued by wild-type (WT) Pak2 but not by a Pak2-kinase dead mutant (KD). Expression of a β-Pix interaction-defective mutant of Pak2 rescued filopodia formation but led to abnormal F-actin bundles. Although CDC42 has previously been considered an upstream regulator of Pak2, we found a paradoxical decrease in baseline activation of CDC42 in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs, which was rescued by expression of Pak2-WT but not by Pak2-KD; defective homing of ITALIC! Pak2-deleted HSPCs was rescued by constitutive active CDC42. These data demonstrate that both Pak2 kinase activity and its interaction with β-Pix are essential for HSPC filopodia formation, cytoskeletal integrity, and homing via activation of CDC42. Taken together, we provide mechanistic insights into the role of Pak2 in HSPC migration and homing.

The ezrin-radixin-moesin family of proteins in the regulation of B-cell immune response

Dynamic reorganization of the cortical cytoskeleton is essential for numerous cellular processes, including B- and T-cell activation and migration. The ezrin-radixin-moesin (ERM) family of proteins plays structural and regulatory roles in the rearrangement of plasma membrane flexibility and protrusions through its members' reversible interaction with cortical actin filaments and the plasma membrane. Recent studies demonstrated that ERM proteins not only are involved in cytoskeletal organization but also offer a platform for the transmission of signals in response to a variety of extracellular stimuli through their ability to cross-link transmembrane receptors with downstream signaling components. In this review, we summarize present knowledge relating to ERMs and recent progress made toward elucidating a novel role for them in the regulation of B-cell function in health and disease.

Identification of CD245 as myosin 18A, a receptor for surfactant A: A novel pathway for activating human NK lymphocytes

CD245 is a human surface antigen expressed on peripheral blood lymphocytes, initially delineated by two monoclonal antibodies DY12 and DY35. Until now, CD245 molecular and functional characteristics remained largely unknown. We combined immunological and proteomic approaches and identified CD245 as the unconventional myosin 18A, a highly conserved motor enzyme reported as a receptor for the surfactant protein A (SP-A), that plays a critical role in cytoskeleton organization and Golgi budding. We report that the recruitment of CD245 strongly enhanced NK cell cytotoxicity. Further, we show that the enhancement of the NK lymphocytes killing ability toward CD137-ligand expressing target cells could result from the induction of CD137 expression following CD245 engagement. The SP-A receptor could therefore represent a novel and promising target in cancer immunotherapy.

Specific Myosins Control Actin Organization, Cell Morphology, and Migration in Prostate Cancer Cells

We investigated the myosin expression profile in prostate cancer cell lines and found that Myo1b, Myo9b, Myo10, and Myo18a were expressed at higher levels in cells with high metastatic potential. Moreover, Myo1b and Myo10 were expressed at higher levels in metastatic tumors. Using an siRNA-based approach, we found that knockdown of each myosin resulted in distinct phenotypes. Myo10 knockdown ablated filopodia and decreased 2D migration speed. Myo18a knockdown increased circumferential non-muscle myosin 2A-associated actin filament arrays in the lamella and reduced directional persistence of 2D migration. Myo9b knockdown increased stress fiber formation, decreased 2D migration speed, and increased directional persistence. Conversely, Myo1b knockdown increased numbers of stress fibers but did not affect 2D migration. In all cases, the cell spread area was increased and 3D migration potential was decreased. Therefore, myosins not only act as molecular motors but also directly influence actin organization and cell morphology, which can contribute to the metastatic phenotype.

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Myo1b, Myo9b, Myo10, and Myo18a are highly expressed in metastatic prostate cancer

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Knockdown of individual myosins distinctly affects the cytoskeleton and cell migration

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Myosins act in concert to directly influence actin organization and cell migration

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Misregulation of myosin expression may drive the metastatic phenotype

SP-R210 (Myo18A) Isoforms as Intrinsic Modulators of Macrophage Priming and Activation

The surfactant protein (SP-A) receptor SP-R210 has been shown to increase phagocytosis of SP-A-bound pathogens and to modulate cytokine secretion by immune cells. SP-A plays an important role in pulmonary immunity by enhancing opsonization and clearance of pathogens and by modulating macrophage inflammatory responses. Alternative splicing of the Myo18A gene results in two isoforms: SP-R210S and SP-R210L, with the latter predominantly expressed in alveolar macrophages. In this study we show that SP-A is required for optimal expression of SP-R210L on alveolar macrophages. Interestingly, pre-treatment with SP-A prepared by different methods either enhances or suppresses responsiveness to LPS, possibly due to differential co-isolation of SP-B or other proteins. We also report that dominant negative disruption of SP-R210L augments expression of receptors including SR-A, CD14, and CD36, and enhances macrophages' inflammatory response to TLR stimulation. Finally, because SP-A is known to modulate CD14, we used a variety of techniques to investigate how SP-R210 mediates the effect of SP-A on CD14. These studies revealed a novel physical association between SP-R210S, CD14, and SR-A leading to an enhanced response to LPS, and found that SP-R210L and SP-R210S regulate internalization of CD14 via distinct macropinocytosis-like mechanisms. Together, our findings support a model in which SP-R210 isoforms differentially regulate trafficking, expression, and activation of innate immune receptors on macrophages.

Myosin 18A coassembles with nonmuscle myosin 2 to form mixed bipolar filaments

Class-18 myosins are most closely related to conventional class-2 nonmuscle myosins (NM2). Surprisingly, the purified head domains of Drosophila, mouse, and human myosin 18A (M18A) lack actin-activated ATPase activity and the ability to translocate actin filaments, suggesting that the functions of M18A in vivo do not depend on intrinsic motor activity. M18A has the longest coiled coil of any myosin outside of the class-2 myosins, suggesting that it might form bipolar filaments similar to conventional myosins. To address this possibility, we expressed and purified full-length mouse M18A using the baculovirus/Sf9 system. M18A did not form large bipolar filaments under any of the conditions tested. Instead, M18A formed an ∼ 65-nm-long bipolar structure with two heads at each end. Importantly, when NM2 was polymerized in the presence of M18A, the two myosins formed mixed bipolar filaments, as evidenced by cosedimentation, electron microscopy, and single-molecule imaging. Moreover, super-resolution imaging of NM2 and M18A using fluorescently tagged proteins and immunostaining of endogenous proteins showed that NM2 and M18A are present together within individual filaments inside living cells. Together, our in vitro and live-cell imaging data argue strongly that M18A coassembles with NM2 into mixed bipolar filaments. M18A could regulate the biophysical properties of these filaments and, by virtue of its extra N- and C-terminal domains, determine the localization and/or molecular interactions of the filaments. Given the numerous, fundamental cellular and developmental roles attributed to NM2, our results have far-reaching biological implications.

Integrin-β1 regulates chondrocyte proliferation and apoptosis through the upregulation of GIT1 expression

Chondrocytes play a critical role in the repair process of osteoarthritis, which is also known as degenerative arthritis. Integrins, as the key family of cell surface receptors, are responsible for the regulation of chondrocyte proliferation, differentiation, survival and apoptosis through the recruitment and activation of downstream adaptor proteins. Moreover, G-protein-coupled receptor kinase interacting protein-1 (GIT1) exerts its effects on cell proliferation and migration through interaction with various cytokines. It has been previously suggested that GIT1 acts as a vital protein downstream of the integrin-mediated pathway. In the present study, we investigated the effects of integrin-β1 on cell proliferation and apoptosis, as well as the underlying mechanisms in chondrocytes in vitro. Following transfection with a vector expressing integrin-β1, our results revealed that the overexpression of integrin-β1 enhanced GIT1 expression, whereas the knockdown of integrin-β1 by siRNA suppressed GIT1 expression. However, no significant effect was observed on integrin-β1 expression following the enforced overexpression of GIT1, which suggests that GIT1 is localized downstream of integrin-β1. In other words, integrin-β1 regulates the expression of GIT1. Furthermore, this study demonstrated that integrin-β1 and GIT1 increased the expression levels of aggrecan and type II collagen, thus promoting chondrocyte proliferation; however, they inhibited chondrocyte apoptosis. Taken together, our data demonstrate that integrin-β1 plays a vital role in chondrocyte proliferation, differentiation and apoptosis. GIT1 exerts effects similar to those of integrin-β1 and is a downstream target of integrin-β1.

Non-muscle myosins in tumor progression, cancer cell invasion, and metastasis

The actin cytoskeleton, which regulates cell polarity, adhesion, and migration, can influence cancer progression, including initial acquisition of malignant properties by normal cells, invasion of adjacent tissues, and metastasis to distant sites. Actin-dependent molecular motors, myosins, play key roles in regulating tumor progression and metastasis. In this review, we examine how non-muscle myosins regulate neoplastic transformation and cancer cell migration and invasion. Members of the myosin superfamily can act as either enhancers or suppressors of tumor progression. This review summarizes the current state of knowledge on how mutations or epigenetic changes in myosin genes and changes in myosin expression may affect tumor progression and patient outcomes and discusses the proposed mechanisms linking myosin inactivation or upregulation to malignant phenotype, cancer cell migration, and metastasis.

Genome-wide pathway analysis of a genome-wide association study on Alzheimer's disease

The aims of this study were to identify candidate single nucleotide polymorphisms (SNPs) and mechanisms of Alzheimer's disease (AD) and to generate SNP to gene to pathway hypotheses. An AD genome-wide association study (GWAS) dataset that included 370,542 SNPs in 1,000 cases and 1,000 controls of European descent was used in this study. Identify Candidate Causal SNPs and Pathway (ICSNPathway) analysis was applied to the GWAS dataset. ICSNPathway analysis identified 3 candidate SNPs and 2 pathways, which provided 3 hypothetical biological mechanisms. The strongest hypothetical biological mechanism was rs8076604 [non-synonymous coding (deleterious)] to MYO18A to negative regulation of programmed cell death [nominal P < 0.001, false discovery rate (FDR) <0.043]. The second was rs2811226 (regulatory region) to ANXA1 to negative regulation of programmed cell death (nominal P < 0.001, FDR 0.043). The third was rs3734166 (non-synonymous coding) to CDC25C to M phase of the mitotic cell cycle (nominal P < 0.001, FDR 0.049). By applying the ICSNPathway analysis to the AD GWAS meta-analysis data, three candidate SNPs, three genes (MYO18A, ANXA1, CDC25C), 2 pathways involving negative regulation of programmed cell death and 1 pathway involving the M phase of the mitotic cell cycle were identified, which may contribute to AD susceptibility.

The PDZ-containing unconventional myosin XVIIIA regulates embryonic muscle integrity in zebrafish

Myosin XVIIIA, or MYO18A, is a unique PDZ domain-containing unconventional myosin and is evolutionarily conserved from Drosophila to vertebrates. Although there is evidence indicating its expression in the somites, whether it regulates muscle function remains unclear. We show that the two zebrafish myo18a genes (myo18aa and myo18ab) are predominantly expressed at somite borders during early developmental stages. Knockdown of these genes or overexpression of the MYO18A PDZ domain disrupts myofiber integrity, induces myofiber lesions, and compromises the localization of dystrophin, α-dystroglycan (α-DG) and laminin at the myotome boundaries. Cell transplantation experiments indicate that myo18a morphant myoblasts fail to form elongated myofibers in the myotomes of wild-type embryos, which can be rescued by the full-length MYO18A protein. These results suggest that MYO18A likely functions in the adhesion process that maintains the stable attachment of myofibers to ECM (extracellular matrix) and muscle integrity during early development.

Platelet‑derived growth factor promotes osteoblast proliferation by activating G‑protein‑coupled receptor kinase interactor‑1

Platelet‑derived growth factor (PDGF) has been reported to stimulate bone fracture‑healing. Multiple studies have demonstrated that PDGF has a critical role in osteoblast or osteoprogenitor cell activation, although the underlying mechanism remains unclear. Studies have found that G‑protein‑coupled receptor kinase interactor‑1 (GIT1) is activated by PDGF and described as an important factor in bone metabolism. In the present study, the effects of PDGF and GIT1 on the proliferation and apoptosis of osteoblasts were investigated in cultured osteoblasts isolated from rat calvaria with PDGF stimulation and GIT1 small interfering RNA transfection. The results demonstrated that PDGF rapidly stimulated GTI1 expression in osteoblasts, increased osteoblast proliferation and inhibited cell apoptosis. Furthermore, cyclin D1 expression was significantly upregulated, the number of cells in the G0/G1 phase was decreased, while the number in the S phase was increased. In cells with knockdown of GIT1, the change stimulated by PDGF was not evident. The results indicate that, PDGF stimulated GIT1 activation of cyclin D1 expression, thereby promoting osteoblasts to enter the S phase from the stationary G0/G1 phase, leading to the proliferation of osteoblasts.

Binding of the extreme carboxyl-terminus of PAK-interacting exchange factor β (βPIX) to myosin 18A (MYO18A) is required for epithelial cell migration

The PAK2/βPIX/GIT1 (p21-activated kinase 2/PAK-interacting exchange factor-β/G protein-coupled receptor kinase-interactor 1) complex has been shown to distribute to both membrane ruffles and focal adhesions of cells, where it plays an important role in regulating focal adhesion turnover. However, the detailed mechanism underlying this regulation is largely unknown. We previously reported that MYO18Aα interacts via its carboxyl terminus with the PAK2/βPIX/GIT1 complex through direct binding to βPIX, and that knockdown of MYO18Aα in epithelial cells causes accumulation of the complex in focal adhesions and decreased cell migration ability (Hsu et al., 2010). The current study characterized the detailed MYO18Aα-βPIX interaction mechanism and the biological significance of this interaction. We found that deletion of the carboxyl-terminal globular domain of MYO18Aα profoundly altered the cellular localization of βPIX and inhibited cell migration. βPIX interacts through its most carboxyl-terminus, PAWDETNL (639-646), with MYO18Aα and partially colocalized with MYO18Aα in membrane ruffles of cells, whereas βPIX(1-638), a mutant with deletion of PAWDETNL, accumulated in focal adhesions. Both focal adhesion numbers and area in βPIX(1-638)-expressing cells were greater than those in cells expressing wild-type βPIX(FL). Further experiments using deletion mutants of MYO18A and βPIX showed that disruption of MYO18A-βPIX interaction not only impaired cell motility but also decreased Rac1 activity. Collectively, our data unravel the interaction regions between MYO18A and βPIX and provide evidence for the critical role of this interaction in regulating cellular localization of βPIX, Rac1 activity, and adhesion and migration in epithelial cells.

miRNA-491-5p and GIT1 serve as modulators and biomarkers for oral squamous cell carcinoma invasion and metastasis

MicroRNAs offer tools to identify and treat invasive cancers. Using highly invasive isogenic oral squamous cell carcinoma (OSCC) cells, established using in vitro and in vivo selection protocols from poorly invasive parental cell populations, we used microarray expression analysis to identify a relative and specific decrease in miR-491-5p in invasive cells. Lower expression of miR-491-5p correlated with poor overall survival of patients with OSCCs. miR-491-5p overexpression in invasive OSCC cells suppressed their migratory behavior in vitro and lung metastatic behavior in vivo. We defined the G-protein-coupled receptor kinase-interacting protein 1 (GIT1)-as a direct target gene for miR-491-5p control. GIT1 overexpression was sufficient to rescue miR-491-5p-mediated inhibition of migration/invasion and lung metastasis. Conversely, GIT1 silencing phenocopied the ability of miR-491-5p to inhibit migration/invasion and metastasis of OSCC cells. Mechanistic investigations indicated that miR-491-5p overexpression or GIT1 attenuation reduced focal adhesions, with a concurrent decrease in steady-state levels of paxillin, phospho-paxillin, phospho-FAK, EGF/EGFR-mediated extracellular signal-regulated kinase (ERK1/2) activation, and MMP2/9 levels and activities. In clinical specimens of OSCCs, GIT1 levels were elevated relative to paired normal tissues and were correlated with lymph node metastasis, with expression levels of miR-491-5p and GIT1 correlated inversely in OSCCs, where they informed tumor grade. Together, our findings identify a functional axis for OSCC invasion that suggests miR-491-5p and GIT1 as biomarkers for prognosis in this cancer.

Functional characterization of human myosin-18A and its interaction with F-actin and GOLPH3

Molecular motors of the myosin superfamily share a generic motor domain region. They commonly bind actin in an ATP-sensitive manner, exhibit actin-activated ATPase activity, and generate force and movement in this interaction. Class-18 myosins form heavy chain dimers and contain protein interaction domains located at their unique N-terminal extension. Here, we characterized human myosin-18A molecular function in the interaction with nucleotides, F-actin, and its putative binding partner, the Golgi-associated phosphoprotein GOLPH3. We show that myosin-18A comprises two actin binding sites. One is located in the KE-rich region at the start of the N-terminal extension and appears to mediate ATP-independent binding to F-actin. The second actin-binding site resides in the generic motor domain and is regulated by nucleotide binding in the absence of intrinsic ATP hydrolysis competence. This core motor domain displays its highest actin affinity in the ADP state. Electron micrographs of myosin-18A motor domain-decorated F-actin filaments show a periodic binding pattern independent of the nucleotide state. We show that the PDZ module mediates direct binding of myosin-18A to GOLPH3, and this interaction in turn modulates the actin binding properties of the N-terminal extension. Thus, myosin-18A can act as an actin cross-linker with multiple regulatory modulators that targets interacting proteins or complexes to the actin-based cytoskeleton.

G-protein-coupled receptor kinase interacting protein-1 mediates intima formation by regulating vascular smooth muscle proliferation, apoptosis, and migration

OBJECTIVE

The G-protein-coupled receptor kinase interacting protein-1 (GIT1) is a scaffold protein that is important for phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling induced by angiotensin II and epidermal growth factor. Because GIT1 regulates signaling by several vascular smooth muscle cell (VSMC) growth factors, we hypothesized that intima formation would be inhibited by GIT1 depletion.

APPROACH AND RESULTS

Complete carotid ligation was performed on GIT1 wild-type and knockout (KO) mice. We compared changes between GIT1 wild-type and KO mice in carotid vascular remodeling, VSMC proliferation, and apoptosis in vivo and in vitro. Our data demonstrated that GIT1 deficiency significantly decreased intima formation after carotid ligation as a result of both reduced VSMC proliferation and enhanced apoptosis. To confirm the effects of GIT1 in vitro, we performed proliferation and apoptosis assays in VSMC. In mouse aortic smooth muscle cells (MASM), we found that the growth rate and [3H]-thymidine incorporation of the GIT1 KO MASM were significantly decreased compared with the wild-type MASM. Cyclin D1, which is a key cell cycle regulator, was significantly decreased in GIT1 KO cells. Serum deprivation of GIT1 KO MASM increased apoptosis 3-fold compared with wild-type MASM. Treatment of rat aortic smooth muscle cells with GIT1 small interfering RNA impaired cell migration. Both phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling were required for GIT1-dependent VSMC proliferation and migration, whereas only phospholipase Cγ was involved in GIT1-mediated VSMC apoptosis.

CONCLUSIONS

GIT1 is a novel mediator of vascular remodeling by regulating VSMC proliferation, migration, and apoptosis through phospholipase Cγ and extracellular signal-regulated kinase 1/2 signaling pathways.

Mammalian myosin-18A, a highly divergent myosin

The Mus musculus myosin-18A gene is expressed as two alternatively spliced isoforms, α and β, with reported roles in Golgi localization, in maintenance of cytoskeleton, and as receptors for immunological surfactant proteins. Both myosin-18A isoforms feature a myosin motor domain, a single predicted IQ motif, and a long coiled-coil reminiscent of myosin-2. The myosin-18Aα isoform, additionally, has an N-terminal PDZ domain. Recombinant heavy meromyosin- and subfragment-1 (S1)-like constructs for both myosin-18Aα and -18β species were purified from the baculovirus/Sf9 cell expression system. These constructs bound both essential and regulatory light chains, indicating an additional noncanonical light chain binding site in the neck. Myosin-18Aα-S1 and -18Aβ-S1 molecules bound actin weakly with Kd values of 4.9 and 54 μm, respectively. The actin binding data could be modeled by assuming an equilibrium between two myosin conformations, a competent and an incompetent form to bind actin. Actin binding was unchanged by presence of nucleotide. Both myosin-18A isoforms bound N-methylanthraniloyl-nucleotides, but the rate of ATP hydrolysis was very slow (<0.002 s(-1)) and not significantly enhanced by actin. Phosphorylation of the regulatory light chain had no effect on ATP hydrolysis, and neither did the addition of tropomyosin or of GOLPH3, a myosin-18A binding partner. Electron microscopy of myosin-18A-S1 showed that the lever is strongly angled with respect to the long axis of the motor domain, suggesting a pre-power stroke conformation regardless of the presence of ATP. These data lead us to conclude that myosin-18A does not operate as a traditional molecular motor in cells.

Proteomic analysis of α4β1 integrin adhesion complexes reveals α-subunit-dependent protein recruitment

Integrin adhesion receptors mediate cell-cell and cell-extracellular matrix interactions, which control cell morphology and migration, differentiation, and tissue integrity. Integrins recruit multimolecular adhesion complexes to their cytoplasmic domains, which provide structural and mechanosensitive signaling connections between the extracellular and intracellular milieux. The different functions of specific integrin heterodimers, such as α4β1 and α5β1, have been attributed to distinct signal transduction mechanisms that are initiated by selective recruitment of adhesion complex components to integrin cytoplasmic tails. Here, we report the isolation of ligand-induced adhesion complexes associated with wild-type α4β1 integrin, an activated α4β1 variant in the absence of the α cytoplasmic domain (X4C0), and a chimeric α4β1 variant with α5 leg and cytoplasmic domains (α4Pα5L), and the cataloguing of their proteomes by MS. Using hierarchical clustering and interaction network analyses, we detail the differential recruitment of proteins and highlight enrichment patterns of proteins to distinct adhesion complexes. We identify previously unreported components of integrin adhesion complexes and observe receptor-specific enrichment of molecules with previously reported links to cell migration and cell signaling processes. Furthermore, we demonstrate colocalization of MYO18A with active integrin in migrating cells. These datasets provide a resource for future studies of integrin receptor-specific signaling events.

Group I PAKs function downstream of Rac to promote podosome invasion during myoblast fusion in vivo

Group I p21-activated kinases organize actin filaments in myoblasts into dense foci, which promote podosome invasion and subsequent myoblast fusion.

The p21-activated kinases (PAKs) play essential roles in diverse cellular processes and are required for cell proliferation, apoptosis, polarity establishment, migration, and cell shape changes. Here, we have identified a novel function for the group I PAKs in cell–cell fusion. We show that the two Drosophila group I PAKs, DPak3 and DPak1, have partially redundant functions in myoblast fusion in vivo, with DPak3 playing a major role. DPak3 is enriched at the site of fusion colocalizing with the F-actin focus within a podosome-like structure (PLS), and promotes actin filament assembly during PLS invasion. Although the small GTPase Rac is involved in DPak3 activation and recruitment to the PLS, the kinase activity of DPak3 is required for effective PLS invasion. We propose a model whereby group I PAKs act downstream of Rac to organize the actin filaments within the PLS into a dense focus, which in turn promotes PLS invasion and fusion pore initiation during myoblast fusion.