A monopartite nuclear localization sequence regulates nuclear targeting of the actin binding protein myopodin

Synaptopodin-2: a potential tumor suppressor

Initially identified as an actin-binding protein containing a PSD95-DLG-ZO1 Domain (PZD domain), Synaptopodin 2 (SYNPO2) has long been considered a structural protein ubiquitously expressed in muscular tissues. However, emerging evidence suggests that SYNPO2 performs diverse functions in cancers in addition to its role in microfilament assembly. In most cancers, high SYNPO2 expression is positively correlated with a good prognosis, suggesting its role as a novel tumor suppressor. Abnormal SYNPO2 expression affects autophagy generation, particularly mitophagy induced by low oxidation or viral infection, as well as chaperone-mediated autophagy triggered by microfilament damage. Mechanically, SYNPO2 regulates tumor growth, metastasis, and invasion via activating the PI3K/AKT/mTOR signal and Hippo signaling pathways. Moreover, the subcellular localization, promoter methylation and single nucleotide polymorphism (SNP) of SYNPO2 have been associated with cancer progression and clinical outcomes, highlighting its potential as a prognostic or diagnostic target for this patient population. This review focuses on the role of SYNPO2 in cancer, including its generation, epigenetic modification, subcellular localization, and biological function.

Isoform-specific functions of synaptopodin-2 variants in cytoskeleton stabilization and autophagy regulation in muscle under mechanical stress

Protein homeostasis (proteostasis) in multicellular organisms depends on the maintenance of force-bearing and force-generating cellular structures. Within myofibrillar Z-discs of striated muscle, isoforms of synaptopodin-2 (SYNPO2/myopodin) act as adapter proteins that are engaged in proteostasis of the actin-crosslinking protein filamin C (FLNc) under mechanical stress. SYNPO2 directly binds F-actin, FLNc and α-actinin and thus contributes to the architectural features of the actin cytoskeleton. By its association with autophagy mediating proteins, i.e. BAG3 and VPS18, SYNPO2 is also engaged in protein quality control and helps to target mechanical unfolded and damaged FLNc for degradation. Here we show that deficiency of all SYNPO2-isoforms in myotubes leads to decreased myofibrillar stability and deregulated autophagy under mechanical stress. In addition, isoform-specific proteostasis functions were revealed. The PDZ-domain containing variant SYNPO2b and the shorter, PDZ-less isoform SYNPO2e both localize to Z-discs. Yet, SYNPO2e is less stably associated with the Z-disc than SYNPO2b, and is dynamically transferred into FLNc-containing myofibrillar lesions under mechanical stress. SYNPO2e also recruits BAG3 into these lesions via interaction with the WW domain of BAG3. Our data provide evidence for a role of myofibrillar lesions as a transient quality control compartment essential to prevent and repair contraction-induced myofibril damage in muscle and indicate an important coordinating activity for SYNPO2 therein.

Myopodin methylation is a prognostic biomarker and predicts antiangiogenic response in advanced kidney cancer

Myopodin is a cytoskeleton protein that shuttles to the nucleus depending on the cellular differentiation and stress. It has shown tumor suppressor functions. Myopodin methylation status was useful for staging bladder and colon tumors and predicting clinical outcome. To our knowledge, myopodin has not been tested in kidney cancer to date. The purpose of this study was to evaluate whether myopodin methylation status could be clinically useful in renal cancer (1) as a prognostic biomarker and 2) as a predictive factor of response to antiangiogenic therapy in patients with metastatic disease. Methylation-specific polymerase chain reactions (MS-PCR) were used to evaluate myopodin methylation in 88 kidney tumors. These belonged to patients with localized disease and no evidence of disease during follow-up (n = 25) (group 1), and 63 patients under antiangiogenic therapy (sunitinib, sorafenib, pazopanib, and temsirolimus), from which group 2 had non-metastatic disease at diagnosis (n = 32), and group 3 showed metastatic disease at diagnosis (n = 31). Univariate and multivariate Cox analyses were utilized to assess outcome and response to antiangiogenic agents taking progression, disease-specific survival, and overall survival as clinical endpoints. Myopodin was methylated in 50 out of the 88 kidney tumors (56.8 %). Among the 88 cases analyzed, 10 of them recurred (11.4 %), 51 progressed (57.9 %), and 40 died of disease (45.4 %). Myopodin methylation status correlated to MSKCC Risk score (p = 0.050) and the presence of distant metastasis (p = 0.039). Taking all patients, an unmethylated myopodin identified patients with shorter progression-free survival, disease-specific survival, and overall survival. Using also in univariate and multivariate models, an unmethylated myopodin predicted response to antiangiogenic therapy (groups 2 and 3) using progression-free survival, disease-specific, and overall survival as clinical endpoints. Myopodin was revealed hypermethylated in kidney cancer. Myopodin methylation status identified which patients showed a more aggressive clinical behavior and predicted antiangiogenic response. These observations support the clinical utility of an unmethylated myopodin as a prognostic and predictive biomarker in kidney cancer.

Functional analysis of nuclear localization signals in VP1-2 homologues from all herpesvirus subfamilies

The herpes simplex virus (HSV) tegument protein VP1-2 contains an N-terminal nuclear localization signal (NLS) that is critical for capsid routing to the nuclear pore. Here we analyzed positionally conserved determinants in VP1-2 homologues from each of the alpha, beta, and gamma classes of human herpesviruses. The overall architectures of the VP1-2s were similar, with a conserved N-terminal ubiquitin-specific protease domain separated from an internal region by a linker that was quite poorly conserved in length and sequence. Within this linker region all herpesviruses contained a conserved, highly basic motif which nevertheless exhibited distinct class-specific features. The motif in HSV functioned as a monopartite NLS, while in varicella-zoster virus (VZV) activity required an adjacent basic section defining the motif as a bipartite NLS. Neither the beta- nor gammaherpesvirus VP1-2 motifs were identified by prediction algorithms, but they nevertheless functioned as efficient NLS motifs both in heterologous transfer assays and in HSV VP1-2. Furthermore, though with different efficiencies and with the exception of human herpesvirus 8 (HHV-8), these chimeric variants rescued the replication defect of an HSV mutant lacking its NLS motif. We demonstrate that the lysine at position 428 of HSV is critical for replication, with a single alanine substitution being sufficient to abrogate NLS function and virus growth. We conclude that the basic motifs of each of the VP1-2 proteins are likely to confer a similar function in capsid entry in the homologous setting and that while there is flexibility in the exact type of motif employed, specific individual residues are critical for function.

IMPORTANCE

To successfully infect cells, all herpesviruses, along with many other viruses, e.g., HIV, hepatitis B virus, and influenza virus, must navigate through the cytoplasmic environment and dock with nuclear pores for transport of their genomes into the nucleus. However, we still have a limited understanding of the detailed mechanisms involved. Insight into these events is needed and could offer opportunities for therapeutic intervention. This work investigated the role of a specific determinant in the structural protein VP1-2 in herpesvirus entry. We examined this determinant in representative VP1-2s from all herpesvirus subfamilies, demonstrated NLS function, dissected key residues, and showed functional relevance in rescuing replication of the mutant blocked in capsid navigation to the pore. The results are important and strongly support our conclusions of the generality that these motifs are crucial for entry of all herpesviruses. They also facilitate future analysis on selective host interactions and possible routes to disrupt function.

Prostate cancer cell migration induced by myopodin isoforms is associated with formation of morphologically and biochemically distinct actin networks

Myopodin is an actin-binding protein that promotes cancer cell migration in response to serum stimulation and is associated with invasive tumor development. To determine whether enhanced migration reflects changes in actin cytoskeleton remodeling, fluorescence confocal microscopy was used to examine the composition and morphology of filamentous actin structures in mock-transduced cells vs. stably transduced PC3 cells expressing human myopodin isoforms, and the chemokinetic response of cells was quantified using transwell assays. The same approaches were used to analyze the effects of external migration stimuli, actin polymerization inhibitors or deletion of the isoform-specific amino- and/or carboxy termini on cell migration and actin bundle formation. Results indicate that the termini of the myopodin isoforms differentially alter the formation of morphologically distinct F-actin networks that also differ in their myosin and myopodin staining patterns. Furthermore, enhanced cell migration was reduced by >50% when actin bundle formation was impaired by myopodin-truncation, low concentrations of an actin polymerization inhibitor, or in the absence of an external migration stimulus. Human myopodin isoforms are therefore potent regulators of stress fiber formation, inducing the formation of biochemically and morphologically distinct F-actin networks in the cell body whose presence directly correlates with increased cell migration.

Diagnostic and prognostic utility of methylation and protein expression patterns of myopodin in colon cancer

Myopodin is an actin-binding protein believed to play a tumor suppressor role in several solid neoplasias. We evaluated the potential differential myopodin methylation and expression and their clinical relevance in colon cancer. The epigenetic silencing of myopodin by hypermethylation was tested in colon cancer cells (n = 5) before and after azacitidine treatment. Myopodin methylation status was evaluated by methylation-specific PCR in colon cancer cells and colorectal tissues (n = 210) grouped in a training set (n = 62) and two independent validation series (n = 100 and n = 48) collected at independent clinical settings. Myopodin expression patterns were analyzed by immunohistochemistry on tissue arrays. Myopodin hypermethylation correlated with gene and protein expression loss, being increased in vitro by azacitidine. Myopodin was frequently methylated in colon cancer cells (four out of five). Methylation rates were 90.3%, 70.0%, and 47.8% in the training and validation sets, respectively. Myopodin methylation rendered a diagnostic accuracy of 83.9% (p < 0.0005). Cytoplasmic myopodin expression was significantly higher in non-neoplastic biopsies compared to colon tumors (p < 0.0005). Loss of myopodin expression correlated with increasing tumor stage (p = 0.011), methylation (p = 0.005), and poor overall survival (p = 0.003). In the first validation set (n = 100), myopodin methylation predicted disease-free (p = 0.046) and overall survival (p = 0.031). In the second validation cohort, myopodin methylation and protein expression patterns predicted disease-specific (p = 0.012 and p = 0.001, respectively) and overall survival (p = 0.009 and p = 0.043, respectively). Thus, myopodin was revealed to be epigenetically modified in colon cancer. The diagnostic and prognostic clinical utility of myopodin methylation and expression patterns suggest considering their assessment for the clinical management of colon cancer patients.

Synaptopodin family of natively unfolded, actin binding proteins: physical properties and potential biological functions

The synaptopodin family of proteins consists of at least 3 members: synaptopodin, the synaptopodin 2 proteins, and the synaptopodin 2-like proteins. Each family member has at least 3 isoforms that are produced by alternative splicing. Synaptopodin family members are basic proteins that are rich in proline and have little regular 2° or 3° structure at physiological temperature, pH and ionic strength. Like other natively unfolded proteins, synaptopodin family members have multiple binding partners including actin and other actin-binding proteins. Several members of the synaptopodin family have been shown to stimulate actin polymerization and to bundle actin filaments either on their own or in collaboration with other proteins. Synaptopodin 2 has been shown to accelerate nucleation of actin filament formation and to induce actin bundling. The actin polymerization activity is inhibited by Ca²⁺-calmodulin. Synaptopodin 2 proteins are localized in Z-bands of striated and heart muscle and dense bodies of smooth muscle cells. Depending on the developmental status and stress, at least one member of the synaptopodin family can occupy nuclei of some cells. Members of the synaptopodin 2 subfamily have been implicated in cancers.

Myopodin methylation is associated with clinical outcome in patients with T1G3 bladder cancer

PURPOSE

Bacillus Calmette-Guerin is standard treatment to decrease tumor recurrence and delay progression of high risk, nonmuscle invasive bladder tumors. However, it is not yet clear which T1G3 cases are more prone to more aggressive clinical behavior or susceptible to respond to bacillus Calmette-Guerin. We evaluated the role of myopodin methylation as a clinical outcome prognosticator and predictive biomarker for the bacillus Calmette-Guerin response in patients with T1G3 bladder tumors.

MATERIALS AND METHODS

We analyzed the methylation status of myopodin in tumor specimens from 170 patients with T1G3 bladder cancer, including a subset of 108 who underwent bacillus Calmette-Guerin treatment. Myopodin methylation was assessed by methylation specific polymerase chain reactions. Recurrence, progression to muscle invasive tumors and disease specific overall survival were analyzed using competing risks regression analysis.

RESULTS

Of the 170 cases analyzed 72 recurred (42.4%) and 36 progressed (21.2%). A total of 24 patients (14.1%) died of the disease. Univariate and multivariate survival analysis revealed that myopodin methylation was significantly associated with an increased recurrence rate (p = 0.004), progression (p = 0.002) and shorter disease specific overall survival (p = 0.020). In a subset treated with bacillus Calmette-Guerin myopodin methylation was also related to an increased recurrence rate (p = 0.011), progression (p = 0.030) and shorter disease specific overall survival (p = 0.028).

CONCLUSIONS

Epigenetic analysis revealed that myopodin methylation was associated with tumor aggressiveness and clinical outcome in patients with T1G3 disease. Myopodin methylation distinguished patients responding to bacillus Calmette-Guerin from those who may require a more aggressive therapeutic approach.

The sarcomeric Z-disc component myopodin is a multiadapter protein that interacts with filamin and alpha-actinin

Here we introduce myopodin as a novel filamin C binding partner. Corroborative yeast two-hybrid and biochemical analyses indicate that the central part of myopodin that shows high homology to the closely related protein synaptopodin and that is common to all its currently known or predicted variants interacts with filamin C immunoglobulin-like domains 20-21. A detailed characterization of the previously described interaction between myopodin and alpha-actinin demonstrates for the first time that myopodin contains three independent alpha-actinin-binding sites. Newly developed myopodin-specific antibodies reveal expression at the earliest stages of in vitro differentiation of human skeletal muscle cells preceding the expression of sarcomeric alpha-actinin. Myopodin colocalizes with filamin and alpha-actinin during all stages of muscle development. By contrast, colocalization with its previously identified binding partner zyxin is restricted to early developmental stages. Genetic and cellular analyses of skeletal muscle provided direct evidence for an alternative transcriptional start site in exon three, corroborating the expression of a myopodin variant lacking the PDZ domain encoded by exons 1 and 2 in skeletal muscle. We conclude that myopodin is a multiadapter protein of the sarcomeric Z-disc that links nascent myofibrils to the sarcolemma via zyxin, and might play a role in early assembly and stabilization of the Z-disc. Mutations in FLNC, ACTN2 and several other genes encoding Z-disc-related proteins cause myopathy and cardiomyopathy. Its localization and its association with the myopathy-associated proteins filamin C and alpha-actinin make myopodin an interesting candidate for a muscle disease gene.

CHAP is a newly identified Z-disc protein essential for heart and skeletal muscle function

In recent years, the perception of Z-disc function has changed from a passive anchor for myofilaments that allows transmission of force, to a dynamic multicomplex structure, capable of sensing and transducing extracellular signals. Here, we describe a new Z-disc protein, which we named CHAP (cytoskeletal heart-enriched actin-associated protein), expressed in differentiating heart and skeletal muscle in vitro and in vivo. Interestingly, in addition to its sarcomeric localization, CHAP was also able to translocate to the nucleus. CHAP was associated with filamentous actin in the cytoplasm and the nucleus when expressed ectopically in vitro, but in rat neonatal cardiomyocytes, CHAP disrupted the subcellular localization of alpha-actinin, another Z-disc protein. More importantly, knockdown of CHAP in zebrafish resulted in aberrant cardiac and skeletal muscle development and function. These findings suggest that CHAP is a critical component of the sarcomere with an important role in muscle development.

Discovery of myopodin methylation in bladder cancer

Myopodin is an actin-binding protein that shuttles between the nucleus and the cytoplasm. After identifying an enriched CpG island encompassing the transcription site of myopodin, we aimed at evaluating the potential relevance of myopodin methylation in bladder cancer. The epigenetic silencing of myopodin by hypermethylation was tested in bladder cancer cells (n=12) before and after azacytidine treatment. Myopodin hypermethylation was associated with gene expression, being increased in vitro by this demethylating agent. The methylation status of myopodin promoter was then evaluated by methylation-specific polymerase chain reaction (MS-PCR) analyses. Myopodin was revealed to be frequently methylated in a large series of 466 bladder tumours (68.7%). Myopodin methylation was significantly associated with tumour stage (p<0.0005) and tumour grade (p=0.037). Myopodin expression patterns were analysed by immunohistochemistry on tissue arrays containing bladder tumours for which myopodin methylation was assessed (n=177). The presence of low nuclear myopodin expression alone (p = 0.031) or combined with myopodin methylation (p=0.008) was associated with poor survival. Moreover, myopodin methylation in 164 urinary specimens distinguished patients with bladder cancer from controls with a sensitivity of 65.0%, a specificity of 79.8%, and a global accuracy of 75.3%. Thus, myopodin was identified to be epigenetically modified in bladder cancer. The association of myopodin methylation and nuclear expression patterns with cancer progression and clinical outcome, together with its ability to detect bladder cancer patients using urinary specimens, suggests the utility of incorporating myopodin methylation assessment in the clinical management of patients affected by uroepithelial neoplasias.

Multiple isoforms of the tumor suppressor myopodin are simultaneously transcribed in cancer cells

Expression of myopodin, an actin associated protein, is frequently lost in invasive prostate cancers due to partial or complete deletion of the gene. Screening of public databases reveals that two human myopodin isoforms have been proposed. Remarkably both isoforms deviate profoundly from the human or mouse isoforms examined to date. Here, we investigated expression of human myopodin. Rapid amplification of cDNA ends revealed a new myopodin transcript, hitherto unpredicted by public databases. RT-PCR analysis indicates that the new isoform (Myo2), in addition to the two predicted isoforms (Myo1 and Myo3), are transcribed in various mammalian cell lines. The three isoforms (Myo1-3) are translated into full length proteins of 1093, 1109, and 1261 amino acids, respectively, when expressed in cells. Thus, mammalian cells simultaneously express at least three myopodin isoforms with a common N-terminal PDZ domain, but a dissimilar carboxy-terminal amino acid tract. These findings shed new light on the expression of this tumor suppressor gene and necessitate closer examination of both mouse and human myopodin polypeptides currently under study.

A new role for nuclear transport factor 2 and Ran: nuclear import of CapG

The small GTPase Ran plays a central role in nucleocytoplasmic transport. Nuclear transport of Ran itself depends on nuclear transport factor 2 (NTF2). Here, we report that NTF2 and Ran control nuclear import of the filamentous actin capping protein CapG. In digitonin-permeabilized cells, neither GTPgammaS nor the GTP hydrolysis-deficient Ran mutant RanQ69L affect transit of CapG to the nucleus in the presence of cytosol. Obstruction of nucleoporins prevents nuclear transport of CapG, and we show that CapG binds to nucleoporin62. In addition, CapG interacts with NTF2, associates with Ran and is furthermore able to bind the NTF2-Ran complex. NTF2-Ran interaction is required for CapG nuclear import. This is corroborated by a NTF2 mutant with reduced affinity for Ran and a Ran mutant that does not bind NTF2, both of which prevent CapG import. Thus, a ubiquitously expressed protein shuttles to the nucleus through direct association with NTF2 and Ran. The role of NTF2 may therefore not be solely confined to sustaining the Ran gradient in cells.

Interaction between importin 13 and myopodin suggests a nuclear import pathway for myopodin

Importin 13 is a member of the importin beta superfamily of nuclear transport proteins and is expressed in multiple tissues at high levels both in humans and rodents, including fetal lung, brain, and heart. In order to elucidate potential functions of imp13 in the heart, we have used rat imp13 as bait to screen a human heart cDNA library and identified an interaction with the C-terminal peptide of myopodin (a.a. 360-698), an actin-bundling protein, associated with tumor-suppressor activity that localizes to both the cytoplasm and the nucleus. We have used GST-pull down assays and co-immunoprecipitation experiments to demonstrate an interaction between imp13 and full-length myopodin and observed that RanGTP dissociates the myopodin-imp13 complex. In studies of cultured cells, we show that both imp13 siRNA and a C-terminal fragment of imp13 protein prevent nuclear localization of myopodin. We, therefore, conclude that imp13 functions in myopodin import and we suggest that the regulation of these events is critical for normal and abnormal cellular differentiation.