An analysis of splicing, actin-binding properties, heterodimerization and molecular interactions of the non-muscle α-actinins

Structural and functional insights into α-actinin isoforms and their implications in cardiovascular disease

α-actinin (ACTN) is a pivotal member of the actin-binding protein family, crucial for the anchoring and organization of actin filaments within the cytoskeleton. Four isoforms of α-actinin exist: two non-muscle isoforms (ACTN1 and ACTN4) primarily associated with actin stress fibers and focal adhesions, and two muscle-specific isoforms (ACTN2 and ACTN3) localized to the Z-disk of the striated muscle. Although these isoforms share structural similarities, they exhibit distinct functional characteristics that reflect their specialized roles in various tissues. Genetic variants in α-actinin isoforms have been implicated in a range of pathologies, including cardiomyopathies, thrombocytopenia, and non-cardiovascular diseases, such as nephropathy. However, the precise impact of these genetic variants on the α-actinin structure and their contribution to disease pathogenesis remains poorly understood. This review provides a comprehensive overview of the structural and functional attributes of the four α-actinin isoforms, emphasizing their roles in actin crosslinking and sarcomere stabilization. Furthermore, we present detailed structural modeling of select ACTN1 and ACTN2 variants to elucidate mechanisms underlying disease pathogenesis, with a particular focus on macrothrombocytopenia and hypertrophic cardiomyopathy. By advancing our understanding of α-actinin's role in both normal cellular function and disease states, this review lays the groundwork for future research and the development of targeted therapeutic interventions.

From stress fiber to focal adhesion: a role of actin crosslinkers in force transmission

The contractile apparatus, stress fiber (SF), is connected to the cell adhesion machinery, focal adhesion (FA), at the termini of SF. The SF-FA complex is essential for various mechanical activities of cells, including cell adhesion to the extracellular matrix (ECM), ECM rigidity sensing, and cell migration. This mini-review highlights the importance of SF mechanics in these cellular activities. Actin-crosslinking proteins solidify SFs by attenuating myosin-driven flows of actin and myosin filaments within the SF. In the solidified SFs, viscous slippage between actin filaments in SFs and between the filaments and the surrounding cytosol is reduced, leading to efficient transmission of myosin-generated contractile force along the SFs. Hence, SF solidification via actin crosslinking ensures exertion of a large force to FAs, enabling FA maturation, ECM rigidity sensing and cell migration. We further discuss intracellular mechanisms for tuning crosslinker-modulated SF mechanics and the potential relationship between the aberrance of SF mechanics and pathology including cancer.

Identification of key genes involved in collagen hydrogel-induced chondrogenic differentiation of mesenchymal stem cells through transcriptome analysis: the role of m6A modification

Collagen hydrogel has been shown promise as an inducer for chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), contributing to the repair of cartilage defects. However, the precise molecular mechanism underlying this phenomenon remains poorly elucidated. Here, we induced chondrogenic differentiation of BMSCs using collagen hydrogel and identified 4451 differentially expressed genes (DEGs) through transcriptomic sequencing. Our analysis revealed that DEGs were enriched in the focal adhesion pathway, with a notable decrease in expression levels in the collagen hydrogel group compared to the control group. Protein-protein interaction network analysis suggested that actinin alpha 1 (ACTN1) and actinin alpha 4 (ACTN4), two proteins also involved in cytoskeletal recombination, may be crucial in collagen hydrogel-induced chondrogenic differentiation of BMSCs. Additionally, we found that N6-methyladenosine RNA methylation (m6A) modification was involved in collagen hydrogel-mediated chondrogenic differentiation, with fat mass and obesity-associated protein (FTO) implicated in regulating the expression of ACTN1 and ACTN4. These findings suggest that collagen hydrogel might regulate focal adhesion and actin cytoskeletal signaling pathways through down-regulation of ACTN1 and ACTN4 mRNA via FTO-mediated m6A modification, ultimately driving chondrogenic differentiation of BMSCs. In conclusion, our study provides valuable insights into the molecular mechanisms of collagen hydrogel-induced chondrogenic differentiation of BMSCs, which may aid in developing more effective strategies for cartilage regeneration.

Multi-domain interaction mediated strength-building in human α-actinin dimers unveiled by direct single-molecule quantification

α-Actinins play crucial roles in cytoskeletal mechanobiology by acting as force-bearing structural modules that orchestrate and sustain the cytoskeletal framework, serving as pivotal hubs for diverse mechanosensing proteins. The mechanical stability of α-actinin dimer, a determinant of its functional state, remains largely unexplored. Here, we directly quantify the force-dependent lifetimes of homo- and hetero-dimers of human α-actinins, revealing an ultra-high mechanical stability of the dimers associated with > 100 seconds lifetime within 40 pN forces under shear-stretching geometry. Intriguingly, we uncover that the strong dimer stability is arisen from much weaker sub-domain pair interactions, suggesting the existence of distinct dimerized functional states of the dimer, spanning a spectrum of mechanical stability, with the spectrin repeats (SRs) in folded or unfolded conformation. In essence, our study supports a potent mechanism for building strength in biomolecular dimers through weak, multiple sub-domain interactions, and illuminates multifaceted roles of α-actinin dimers in cytoskeletal mechanics and mechanotransduction.

Rbfox1 controls alternative splicing of focal adhesion genes in cardiac muscle cells

Alternative splicing is one of the major cellular processes that determine the tissue-specific expression of protein variants. However, it remains challenging to identify physiologically relevant and tissue-selective proteins that are generated by alternative splicing. Hence, we investigated the target spectrum of the splicing factor Rbfox1 in the cardiac muscle context in more detail. By using a combination of in silico target prediction and in-cell validation, we identified several focal adhesion proteins as alternative splicing targets of Rbfox1. We focused on the alternative splicing patterns of vinculin (metavinculin isoform) and paxillin (extended paxillin isoform) and identified both as potential Rbfox1 targets. Minigene analyses suggested that both isoforms are promoted by Rbfox1 due to binding in the introns. Focal adhesions play an important role in the cardiac muscle context, since they mainly influence cell shape, cytoskeletal organization, and cell-matrix association. Our data confirmed that depletion of Rbfox1 changed cardiomyoblast morphology, cytoskeletal organization, and multinuclearity after differentiation, which might be due to changes in alternative splicing of focal adhesion proteins. Hence, our results indicate that Rbfox1 promotes alternative splicing of focal adhesion genes in cardiac muscle cells, which might contribute to heart disease progression, where downregulation of Rbfox1 is frequently observed.

Induction of filopodia formation by α-Actinin-2 via RelA with a feedforward activation loop promoting overt bone marrow metastasis of gastric cancer

BACKGROUND

Bone marrow metastasis (BMM) is underestimated in gastric cancer (GC). GC with BMM frequently complicate critical hematological abnormalities like diffused intravascular coagulation and microangiopathic hemolytic anemia, which constitute a highly aggressive GC (HAGC) subtype. HAGC present a very poor prognosis with peculiar clinical and pathological features when compared with not otherwise specified advanced GC (NAGC). But the molecular mechanisms underlying BMM from GC remain rudimentary.

METHODS

The transcriptomic difference between HAGC and NAGC were analyzed. Genes that were specifically upregulated in HAGC were identified, and their effect on cell migration and invasion was studied. The function of ACTN2 gene were confirmed by GC cell lines, bone-metastatic animal model and patients' tissues. Furthermore, the molecular mechanism of ACTN2 derived-BMM was explored by multiple immunofluorescence staining, western blot, chromatin immunoprecipitation, and luciferase reporter assays.

RESULTS

We elucidated the key mechanisms of BMM depending on the transcriptomic difference between HAGC and NAGC. Five genes specifically upregulated in HAGC were assessed their effect on cell migration and invasion. The ACTN2 gene encoding protein α-Actinin-2 was detected enhanced the metastatic capability and induced BMM of GC cells in mouse models. Mechanically, α-Actinin-2 was involved in filopodia formation where it promoted the Actin filament cross-linking by replacing α-Actinin-1 to form α-Actinin-2:α-Actinin-4 complexes in GC cells. Moreover, NF-κB subunit RelA and α-Actinin-2 formed heterotrimers in the nuclei of GC cells. As a direct target of RelA:α-Actinin-2 heterotrimers, the ACTN2 gene was a positive auto-regulatory loop for α-Actinin-2 expression.

CONCLUSIONS

We demonstrated a link between filopodia, BMM and ACTN2 activation, where a feedforward activation loop between ACTN2 and RelA is established via actin in response to distant metastasis. Given the novel filopodia formation function and the new mechanism of BMM in GC, we propose ACTN2 as a druggable molecular vulnerability that may provide potential therapeutic benefit against BMM of GC.

Case report and literature review: A de novo pathogenic missense variant in ACTN4 gene caused rapid progression to end-stage renal disease

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a histopathological diagnosis of the sclerosis of glomeruli and the damage to renal podocytes. FSGS affects the filtration function of the kidneys and results in nephrotic syndrome (NS) in children and adults. FSGS is a clinically and genetically heterogeneous disorder. FSGS-1 [OMIM #603278] is one of the progressive hereditary renal diseases. It is caused by heterozygous variants of the actinin alpha 4 (ACTN4) [OMIM*604638] gene on chromosome 19q13.2 in a dominant inheritance (AD) manner. With the recent development of whole-exome sequencing (WES), 22 (including our case) pathogenic or likely pathogenic variants have been identified in ACTN4 gene.

CASE PRESENTATION

We reported a 17-year-old Chinese girl who was hospitalized with foamy urine, nausea and vomiting. Laboratory tests revealed increased levels of serum creatinine and urea nitrogen. Ultrasonography demonstrated bilaterally reduced size of kidneys. The primary diagnoses were NS and chronic kidney disease stage 5 (CKD5). The hemodialysis was initiated in 48 h after admission. After 4 months of treatment, the patient received an allogeneic kidney transplantation from her father. A novel heterozygous missense variant c.494C > T (p.A165V) in the ACTN4 gene was found by WES in the patient. This variant was confirmed by Sanger sequencing. The computational simulation of the stability of mutant protein (p.A165V) was decreased. Interatomic interactions of the p.A165V site were increased, and it might be associated with the increased ubiquitylation in the vicinity of the mutant site.

CONCLUSION

As per the guidelines of the American College of Medical Genetics and Genomics for interpreting sequence variants, the novel heterozygous missense variant was pathogenic (PS2 + PM1 + PM2 + PP3 + PP4). It should be noted that the early onset of severe proteinuria with a poor prognosis is an important and universal symptom for most genetic FSGS. If necessary, genetic screening is recommended.

The Importance of Alpha-Actinin Proteins in Platelet Formation and Function, and Their Causative Role in Congenital Macrothrombocytopenia

The actin cytoskeleton plays a central role in platelet formation and function. Alpha-actinins (actinins) are actin filament crosslinking proteins that are prominently expressed in platelets and have been studied in relation to their role in platelet activation since the 1970s. However, within the past decade, several groups have described mutations in ACTN1/actinin-1 that cause congenital macrothrombocytopenia (CMTP)-accounting for approximately 5% of all cases of this condition. These findings are suggestive of potentially novel functions for actinins in platelet formation from megakaryocytes in the bone marrow and/or platelet maturation in circulation. Here, we review some recent insights into the well-known functions of actinins in platelet activation before considering possible roles for actinins in platelet formation that could explain their association with CMTP. We describe what is known about the consequences of CMTP-linked mutations on actinin-1 function at a molecular and cellular level and speculate how these changes might lead to the alterations in platelet count and morphology observed in CMTP patients. Finally, we outline some unanswered questions in this area and how they might be addressed in future studies.

Distinct forms of the actin cross-linking protein α-actinin support macropinosome internalization and trafficking

The α-actinin family of actin cross-linking proteins have been implicated in driving tumor cell metastasis through regulation of the actin cytoskeleton; however, there has been little investigation into whether these proteins can influence tumor cell growth. We demonstrate that α-actinin 1 and 4 are essential for nutrient uptake through the process of macropinocytosis in pancreatic ductal adenocarcinoma (PDAC) cells, and inhibition of these proteins decreases tumor cell survival in the presence of extracellular protein. The α-actinin proteins play essential roles throughout the macropinocytic process, where α-actinin 4 stabilizes the actin cytoskeleton on the plasma membrane to drive membrane ruffling and macropinosome internalization and α-actinin 1 localizes to actin tails on macropinosomes to facilitate trafficking to the lysosome for degradation. In addition to tumor cell growth, we also observe that the α-actinin proteins can influence uptake of chemotherapeutics and extracellular matrix proteins through macropinocytosis, suggesting that the α-actinin proteins can regulate multiple tumor cell properties through this endocytic process. In summary, these data demonstrate a critical role for the α-actinin isoforms in tumor cell macropinocytosis, thereby affecting the growth and invasive potential of PDAC tumors.

Cell response to substrate rigidity is regulated by active and passive cytoskeletal stress

Morphogenesis, tumor formation, and wound healing are regulated by tissue rigidity. Focal adhesion behavior is locally regulated by stiffness; however, how cells globally adapt, detect, and respond to rigidity remains unknown. Here, we studied the interplay between the rheological properties of the cytoskeleton and matrix rigidity. We seeded fibroblasts onto flexible microfabricated pillar arrays with varying stiffness and simultaneously measured the cytoskeleton organization, traction forces, and cell-rigidity responses at both the adhesion and cell scale. Cells adopted a rigidity-dependent phenotype whereby the actin cytoskeleton polarized on stiff substrates but not on soft. We further showed a crucial role of active and passive cross-linkers in rigidity-sensing responses. By reducing myosin II activity or knocking down α-actinin, we found that both promoted cell polarization on soft substrates, whereas α-actinin overexpression prevented polarization on stiff substrates. Atomic force microscopy indentation experiments showed that this polarization response correlated with cell stiffness, whereby cell stiffness decreased when active or passive cross-linking was reduced and softer cells polarized on softer matrices. Theoretical modeling of the actin network as an active gel suggests that adaptation to matrix rigidity is controlled by internal mechanical properties of the cytoskeleton and puts forward a universal scaling between nematic order of the actin cytoskeleton and the substrate-to-cell elastic modulus ratio. Altogether, our study demonstrates the implication of cell-scale mechanosensing through the internal stress within the actomyosin cytoskeleton and its coupling with local rigidity sensing at focal adhesions in the regulation of cell shape changes and polarity.

Dynamin 2 interacts with α-actinin 4 to drive tumor cell invasion

The large GTPase Dynamin 2 (Dyn2) is known to increase the invasiveness of pancreatic cancer tumor cells, but the mechanisms by which Dyn2 regulates changes in the actin cytoskeleton to drive cell migration are still unclear. Here we report that a direct interaction between Dyn2 and the actin-bundling protein alpha-actinin (α-actinin) 4 is critical for tumor cell migration and remodeling of the extracellular matrix in pancreatic ductal adenocarcinoma (PDAC) cells. The direct interaction is mediated through the C-terminal tails of both Dyn2 and α-actinin 4, and these proteins interact at invasive structures at the plasma membrane. While Dyn2 binds directly to both α-actinin 1 and α-actinin 4, only the interaction with α-actinin 4 is required to promote tumor cell invasion. Specific disruption of the Dyn2-α-actinin 4 interaction blocks the ability of PDAC cells to migrate in either two dimensions or invade through extracellular matrix as a result of impaired invadopodia stability. Analysis of human PDAC tumor tissue additionally reveals that elevated α-actinin 4 or Dyn2 expression are predictive of poor survival. Overall, these data demonstrate that Dyn2 regulates cytoskeletal dynamics, in part, by interacting with the actin-binding protein α-actinin 4 during tumor cell invasion.

Role of ACTN4 in Tumorigenesis, Metastasis, and EMT

The actin-binding protein ACTN4 belongs to a family of actin-binding proteins and is a non-muscle alpha-actinin that has long been associated with cancer development. Numerous clinical studies showed that changes in ACTN4 gene expression are correlated with aggressiveness, invasion, and metastasis in certain tumors. Amplification of the 19q chromosomal region where the gene is located has also been reported. Experimental manipulations with ACTN4 expression further confirmed its involvement in cell proliferation, motility, and epithelial-mesenchymal transition (EMT). However, both clinical and experimental data suggest that the effects of ACTN4 up- or down-regulation may vary a lot between different types of tumors. Functional studies demonstrated its engagement in a number of cytoplasmic and nuclear processes, ranging from cytoskeleton reorganization to regulation of different signaling pathways. Such a variety of functions may be the reason behind cell type and cell line specific responses. Herein, we will review research progress and controversies regarding the prognostic and functional significance of ACTN4 for tumorigenesis.

Novel ACTN1 variants in cases of thrombocytopenia

The ACTN1 gene has been implicated in inherited macrothrombocytopenia. To decipher the spectrum of variants and phenotype of ACTN1‐related thrombocytopenia, we sequenced the ACTN1 gene in 272 cases of unexplained chronic or familial thrombocytopenia. We identified 15 rare, monoallelic, nonsynonymous and likely pathogenic ACTN1 variants in 20 index cases from 20 unrelated families. Thirty‐one family members exhibited thrombocytopenia. Targeted sequencing was carried out on 12 affected relatives, which confirmed presence of the variant. Twenty‐eight of 32 cases with monoallelic ACTN1 variants had mild to no bleeding complications. Eleven cases harbored 11 different unreported ACTN1 variants that were monoallelic and likely pathogenic. Nine variants were located in the α‐actinin‐1 (ACTN1) rod domain and were predicted to hinder dimer formation. These variants displayed a smaller increase in platelet size compared with variants located outside the rod domain. In vitro expression of the new ACTN1 variants induced actin network disorganization and led to increased thickness of actin fibers. These findings expand the repertoire of ACTN1 variants associated with thrombocytopenia and highlight the high frequency of ACTN1‐related thrombocytopenia cases. The rod domain, like other ACTN1 functional domains, may be mutated resulting in actin disorganization in vitro and thrombocytopenia with normal platelet size in most cases.

Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia

Actinin-1 mutations cause dominantly inherited congenital macrothrombocytopenia (CMTP), with mutations in the actin-binding domain increasing actinin's affinity for F-actin. In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro. Mutations within the calmodulin-like domain decrease its thermal stability slightly but do not dramatically affect calcium binding, with mutant proteins retaining calcium-dependent regulation of filament bundling in vitro. The G764S and E769K mutations increase cytoskeletal association of actinin in cells, and all mutant proteins colocalize with F-actin in cultured HeLa cells. Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP.

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Increased α-actinin-1 destabilizes E-cadherin-based adhesions and associates with poor prognosis in basal-like breast cancer

The controlled formation and stabilization of E-cadherin-based adhesions is vital for epithelial integrity. This requires co-operation between the E-cadherin-based adhesions and the associated actin cytoskeleton. In cancer, this co-operation often fails, predisposing cells to migration through molecular mechanisms that have only been partially characterized. Here, we demonstrate that the actin filament cross-linker α-actinin-1 is frequently increased in human breast cancer. In mammary epithelial cells, the increased α-actinin-1 levels promote cell migration and induce disorganized acini-like structures in Matrigel. This is accompanied by a major reorganization of the actin cytoskeleton and the associated E-cadherin-based adhesions. Increased expression of α-actinin-1 is particularly noted in basal-like breast cancer cell lines, and in breast cancer patients it associates with poor prognosis in basal-like subtypes. Downregulation of α-actinin-1 in E-cadherin expressing basal-like breast cancer cells demonstrate that α-actinin-1-assembled actin fibers destabilize E-cadherin-based adhesions. Taken together, these results indicate that increased α-actinin-1 expression destabilizes E-cadherin-based adhesions, which is likely to promote the migratory potential of breast cancer cells. Furthermore, our results identify α-actinin-1 as a candidate prognostic biomarker in basal-like breast cancer.

Proteomic analysis reveals novel ligands and substrates for LNX1 E3 ubiquitin ligase

Ligand of Numb protein X1 (LNX1) is an E3 ubiquitin ligase that contains a catalytic RING (Really Interesting New Gene) domain and four PDZ (PSD-95, DlgA, ZO-1) domains. LNX1 can ubiquitinate Numb, as well as a number of other ligands. However, the physiological relevance of these interactions in vivo remain unclear. To gain functional insights into the LNX family, we have characterised the LNX1 interactome using affinity purification and mass spectrometry. This approach identified a large number of novel LNX1-interacting proteins, as well as confirming known interactions with NUMB and ERC2. Many of the novel interactions mapped to the LNX PDZ domains, particularly PDZ2, and many showed specificity for LNX1 over the closely related LNX2. We show that PPFIA1 (liprin-α1), KLHL11, KIF7 and ERC2 are substrates for ubiquitination by LNX1. LNX1 ubiquitination of liprin-α1 is dependent on a PDZ binding motif containing a carboxyl terminal cysteine that binds LNX1 PDZ2. Surprisingly, the neuronally-expressed LNX1p70 isoform, that lacks the RING domain, was found to promote ubiquitination of PPFIA1 and KLHL11, albeit to a lesser extent than the longer RING-containing LNX1p80 isoform. Of several E3-ligases identified in the LNX1 interactome we confirm interactions of LNX1 with MID2/TRIM1 and TRIM27. On this basis we propose a model whereby LNX1p70, despite lacking a catalytic RING domain, may function as a scaffold to promote ubiquitination of its ligands through recruitment of other E3-ligases. These findings provide functional insights into the LNX protein family, particularly the neuronal LNX1p70 isoform.

Actinin-1 and actinin-4 play essential but distinct roles in invadopodia formation by carcinoma cells

Invadopodia are ventral membrane protrusions formed by cancer cells that degrade the extracellular matrix (ECM) during tumor invasion and metastasis. Formation of invadopodia is initiated by the assembly of actin filaments (F-actin) that results from the coordinated activation of several actin regulatory proteins. Actinin-1 and actinin-4 are actin bundling proteins expressed in non-muscle cells and actinin-4 is preferentially associated with malignant phenotypes of carcinoma cells. In this study, we investigated the role of actinin-1 and -4 in invadopodia formation. Expression of both actinin-1 and -4 tended to be higher in invasive and metastatic breast carcinoma cell lines than in non-invasive ones. Immunofluorescence analysis revealed that actinin-1 and -4 colocalized at core actin structures of invadopodia. Time-lapse imaging showed that appearance of both actinins at invadopodia is concomitant with the assembly of F-actin. Knockdown of either actinin-1 or actinin-4 suppressed the formation of invadopodia and degradation of the ECM by carcinoma cells. Interestingly, overexpression of actinin-4, but not actinin-1, significantly promoted the formation of invadopodia and this activity required the actin binding domains and the unique N-terminal motif that exists only in actinin-4. These results demonstrate that both actinin-1 and actinin-4 participate in the assembly of F-actin at invadopodia. Additionally, actinin-4 may have a selective advantage in accelerating invadopodia-mediated invasion of carcinoma cells.

EFA6 regulates lumen formation through alpha-actinin 1

A key step of epithelial morphogenesis is the creation of the lumen. Luminogenesis by hollowing proceeds through the fusion of apical vesicles at cell-cell contact. The small nascent lumens grow through extension, coalescence and enlargement coordinated with cell division to give rise to a single central lumen. Here, using MDCK cells grown in 3D-culture, we show that EFA6A participates in luminogenesis. EFA6A recruits α-actinin 1 (ACTN1) through direct binding. In polarized cells, ACTN1 was found to be enriched at the tight junction where it acts as a primary effector of EFA6A for normal luminogenesis. Both proteins are essential for the lumen extension and enlargement, where they mediate their effect by regulating the cortical acto-myosin contractility. Finally, ACTN1 was also found to act as an effector for the isoform EFA6B in the human mammary tumoral MCF7 cell line. EFA6B restored the glandular morphology of this tumoral cell line in an ACTN1-dependent manner. Thus, we identified new regulators of cyst luminogenesis essential for the proper maturation of a newly-formed lumen into a single central lumen.

Alternative Splicing May Not Be the Key to Proteome Complexity

Alternative splicing is commonly believed to be a major source of cellular protein diversity. However, although many thousands of alternatively spliced transcripts are routinely detected in RNA-seq studies, reliable large-scale mass spectrometry-based proteomics analyses identify only a small fraction of annotated alternative isoforms. The clearest finding from proteomics experiments is that most human genes have a single main protein isoform, while those alternative isoforms that are identified tend to be the most biologically plausible: those with the most cross-species conservation and those that do not compromise functional domains. Indeed, most alternative exons do not seem to be under selective pressure, suggesting that a large majority of predicted alternative transcripts may not even be translated into proteins.

Actinin-1 binds to the C-terminus of A2B adenosine receptor (A2BAR) and enhances A2BAR cell-surface expression

A2BAR (A2B adenosine receptor) has been implicated in several physiological conditions, such as allergic or inflammatory disorders, vasodilation, cell growth and epithelial electrolyte secretion. For mediating the protein-protein interactions of A2BAR, the receptor's C-terminus is recognized to be crucial. In the present study, we unexpectedly found that two point mutations in the A2BAR C-terminus (F297A and R298A) drastically impaired the expression of A2BAR protein by accelerating its degradation. Thus we tested the hypothesis that these two point mutations disrupt A2BAR's interaction with a protein essential for A2BAR stability. Our results show that both mutations disrupted the interaction of A2BAR with actinin-1, an actin-associated protein. Furthermore, actinin-1 binding stabilized the global and cell-surface expression of A2BAR. By contrast, actinin-4, another non-muscle actinin isoform, did not bind to A2BAR. Thus our findings reveal a previously unidentified regulatory mechanism of A2BAR abundance.

The F-actin bundler α-actinin Ain1 is tailored for ring assembly and constriction during cytokinesis in fission yeast

The actomyosin contractile ring is a network of cross-linked actin filaments that facilitates cytokinesis in dividing cells. Contractile ring formation has been well characterized in Schizosaccharomyces pombe, in which the cross-linking protein α-actinin SpAin1 bundles the actin filament network. However, the specific biochemical properties of SpAin1 and whether they are tailored for cytokinesis are not known. Therefore we purified SpAin1 and quantified its ability to dynamically bind and bundle actin filaments in vitro using a combination of bulk sedimentation assays and direct visualization by two-color total internal reflection fluorescence microscopy. We found that, while SpAin1 bundles actin filaments of mixed polarity like other α-actinins, SpAin1 has lower bundling activity and is more dynamic than human α-actinin HsACTN4. To determine whether dynamic bundling is important for cytokinesis in fission yeast, we created the less dynamic bundling mutant SpAin1(R216E). We found that dynamic bundling is critical for cytokinesis, as cells expressing SpAin1(R216E) display disorganized ring material and delays in both ring formation and constriction. Furthermore, computer simulations of initial actin filament elongation and alignment revealed that an intermediate level of cross-linking best facilitates filament alignment. Together our results demonstrate that dynamic bundling by SpAin1 is important for proper contractile ring formation and constriction.

Congenital macrothrombocytopenia-linked mutations in the actin-binding domain of α-actinin-1 enhance F-actin association

Mutations in the actin cross-linking protein actinin-1 were recently linked to dominantly inherited congenital macrothrombocytopenia. Here, we report that several disease-associated mutations that are located within the actinin-1 actin-binding domain cause increased binding of actinin-1 to actin filaments and enhance filament bundling in vitro. These actinin-1 mutants are also more stably associated with the cytoskeleton in cultured cells, as assessed by biochemical fractionation and fluorescence recovery after photobleaching experiments. For two mutations the disruption of contacts between the calponin homology domains within the actinin actin-binding domain may explain increased filament binding--providing mechanistic and structural insights into the basis of actinin-1 dysfunction in congenital macrothrombocytopenia.

The actinin family of actin cross-linking proteins - a genetic perspective

Actinins are one of the major actin cross-linking proteins found in virtually all cell types and are the ancestral proteins of a larger family that includes spectrin, dystrophin and utrophin. Invertebrates have a single actinin-encoding ACTN gene, while mammals have four. Mutations in all four human genes have now been linked to heritable diseases or traits. ACTN1 mutations cause macrothrombocytopenia, a platelet disorder characterized by excessive bleeding. ACTN2 mutations have been linked to a range of cardiomyopathies, and ACTN4 mutations cause a kidney condition called focal segmental glomerulosclerosis. Intriguingly, approximately 16 % of people worldwide are homozygous for a nonsense mutation in ACTN3 that abolishes actinin-3 protein expression. This ACTN3 null allele has undergone recent positive selection in specific human populations, which may be linked to improved endurance and adaptation to colder climates. In this review we discuss the human genetics of the ACTN gene family, as well as ACTN gene knockout studies in several model organisms. Observations from both of these areas provide insights into the evolution and cellular functions of actinins.

Calcium affinity of human α-actinin 1

Due to alternative splicing, the human ACTN1 gene codes for three different transcripts of α-actinin; one isoform that is expressed only in the brain and two with a more general expression pattern. The sequence difference is located to the C-terminal domains and the EF-hand motifs. Therefore, any functional or structural distinction should involve this part of the protein. To investigate this further, the calcium affinities of these three isoforms of α-actinin 1 have been determined by isothermal calorimetry.

ACTN4 and the pathways associated with cell motility and adhesion contribute to the process of lung cancer metastasis to the brain

Background

The aim of this study was to identify critical gene pathways that are associated with lung cancer metastasis to the brain.

Methods

The RNA-Seq approach was used to establish the expression profiles of a primary lung cancer, adjacent benign tissue, and metastatic brain tumor from a single patient. The expression profiles of these three types of tissues were compared to define differentially expressed genes, followed by serial-cluster analysis, gene ontology analysis, pathway analysis, and knowledge-driven network analysis. Reverse transcription–polymerase chain reaction (RT-PCR) was used to validate the expression of essential candidate genes in tissues from ten additional patients.

Results

Differential gene expression among these three types of tissues was classified into multiple clusters according to the patterns of their alterations. Further bioinformatic analysis of these expression profile data showed that the network of the signal transduction pathways related to actin cytoskeleton reorganization, cell migration, and adhesion was associated with lung cancer metastasis to the brain. The expression of ACTN4 (actinin, alpha 4), a cytoskeleton protein gene essential for cytoskeleton organization and cell motility, was significantly elevated in the metastatic brain tumor but not in the primary lung cancer tissue.

Conclusions

The signaling pathways involved in the regulation of cytoskeleton reorganization, cell motility, and focal adhesion play a role in the process of lung cancer metastasis to the brain. The contribution of ACTN4 to the process of lung cancer metastasis to the brain could be mainly through regulation of actin cytoskeleton reorganization, cell motility, and focal adhesion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1295-9) contains supplementary material, which is available to authorized users.

α-Actinin-4 enhances colorectal cancer cell invasion by suppressing focal adhesion maturation

α-Actinins (ACTNs) are known to crosslink actin filaments at focal adhesions in migrating cells. Among the four isoforms of mammalian ACTNs, ACTN1 and ACTN4 are ubiquitously expressed. Recently, ACTN4 was reported to enhance cancer cell motility, invasion, and metastasis. However, the mechanism by which ACTN4 drives these malignant phenotypes remains unclear. Here, we show that ACTN4, but not ACTN1, induces the formation of immature focal adhesions in DLD-1 cells, leading to the rapid turnover of focal adhesions. Interestingly, zyxin (ZYX) assembly to focal adhesions was markedly decreased in ACTN4-expressing DLD-1 cells, while the recruitment of paxillin (PAX) occurred normally. On the other hand, in ACTN1-expressing DLD-1 cells, PAX and ZYX were normally recruited to focal adhesions, suggesting that ACTN4 specifically impairs focal adhesion maturation by inhibiting the recruitment of ZYX to focal complexes. Using purified recombinant proteins, we found that ZYX binding to ACTN4 was defective under conditions where ZYX binding to ACTN1 was observed. Furthermore, Matrigel invasion of SW480 cells that express high endogenous levels of ACTN4 protein was inhibited by ectopic expression of ACTN1. Altogether, our results suggest that ZYX defective binding to ACTN4, which occupies focal adhesions instead of ACTN1, induces the formation of immature focal adhesions, resulting in the enhancement of cell motility and invasion.

α-Actinin-4 is required for amoeboid-type invasiveness of melanoma cells

α-Actinin-4 (ACTN4), a key regulator of the actin cytoskeleton, is up-regulated in melanoma, though its role in melanoma remains speculative. We have discovered that in WM1158, a highly aggressive melanoma cell line, down-regulation of ACTN4 by shRNA induces a collagen I-dependent amoeboidal-to-mesenchymal transition. Re-expression of low levels of WT ACTN4 but not similar expression levels of ACTN1 successfully restores the amoeboidal morphology and limits collagen I gel compaction. A truncated ACTN4 mutant 1-890, which lacks the C-terminal tail, fails to rescue the amoeboidal morphology and to compact collagen I gel. Interestingly, in three-dimensional collagen I gels, ACTN4 KD cells are more polarized compared with cells in which scrambled shRNA is expressed. Surprisingly, ACTN4 KD cells migrate faster than the ones expressing the scrambled shRNA on a collagen I gel (two-dimensional) although these two cell lines migrate similarly on tissue culture. Most importantly, down-regulation of ACTN4 significantly reduced invasion of WM1158 cells into the three-dimensional collagen I gel, a representative of the dermis. Taken together, these findings suggest that ACTN4 plays an important role in maintaining the amoeboidal morphology of invasive melanoma and thus promoting dissemination through collagen-rich matrices.

Identification of protein biomarkers for cervical cancer using human cervicovaginal fluid

Objectives

Cervicovaginal fluid (CVF) can be considered as a potential source of biomarkers for diseases of the lower female reproductive tract. The fluid can easily be collected, thereby offering new opportunities such as the development of self tests. Our objective was to identify a CVF protein biomarker for cervical cancer or its precancerous state.

Methods

A differential proteomics study was set up using CVF samples from healthy and precancerous women. Label-free spectral counting was applied to quantify protein abundances.

Results

The proteome analysis revealed 16 candidate biomarkers of which alpha-actinin-4 (p = 0.001) and pyruvate kinase isozyme M1/M2 (p = 0.014) were most promising. Verification of alpha-actinin-4 by ELISA (n = 28) showed that this candidate biomarker discriminated between samples from healthy and both low-risk and high-risk HPV-infected women (p = 0.009). Additional analysis of longitudinal samples (n = 29) showed that alpha-actinin-4 levels correlated with virus persistence and clearing, with a discrimination of approximately 18 pg/ml.

Conclusions

Our results show that CVF is an excellent source of protein biomarkers for detection of lower female genital tract pathologies and that alpha-actinin-4 derived from CVF is a promising candidate biomarker for the precancerous state of cervical cancer. Further studies regarding sensitivity and specificity of this biomarker will demonstrate its utility for improving current screening programs and/or its use for a cervical cancer self-diagnosis test.

Tight, cell type-specific control of LNX expression in the nervous system, at the level of transcription, translation and protein stability

LNX1 and LNX2 are E3 ubiquitin ligases that can interact with Numb - a key regulator of neurogenesis and neuronal differentiation. LNX1 can target Numb for proteasomal degradation, and Lnx mRNAs are prominently expressed in the nervous system, suggesting that LNX proteins play a role in neural development. This hypothesis remains unproven, however, largely because LNX proteins are present at very low levels in vivo. Here, we demonstrate expression of both LNX1 and LNX2 proteins in the brain for the first time. We clarify the cell-type specific expression of LNX isoforms in both the CNS and PNS, and identify a novel LNX1 isoform. Using luciferase reporter assays, we show that the 5' untranslated region of the Lnx1_variant 2 mRNA, that generates the LNX1p70 isoform, strongly suppresses protein production. This effect is mediated in part by the presence of upstream open reading frames (uORFs), but also by a sequence element that decreases both mRNA levels and translational efficiency. By contrast, uORFs do not negatively regulate LNX1p80 or LNX2 expression. Instead, we find some evidence that protein turnover via proteasomal degradation may influence LNX1p80 levels in cells. These observations provide plausible explanations for the low levels of LNX1 proteins detected in vivo.

Molecular network analysis enhances understanding of the biology of mental disorders

We provide an introduction to network theory, evidence to support a connection between molecular network structure and neuropsychiatric disease, and examples of how network approaches can expand our knowledge of the molecular bases of these diseases. Without systematic methods to derive their biological meanings and inter-relatedness, the many molecular changes associated with neuropsychiatric disease, including genetic variants, gene expression changes, and protein differences, present an impenetrably complex set of findings. Network approaches can potentially help integrate and reconcile these findings, as well as provide new insights into the molecular architecture of neuropsychiatric diseases. Network approaches to neuropsychiatric disease are still in their infancy, and we discuss what might be done to improve their prospects.

The non-muscle functions of actinins: an update

α-Actinins are a major class of actin filament cross-linking proteins expressed in virtually all cells. In muscle, actinins cross-link thin filaments from adjacent sarcomeres. In non-muscle cells, different actinin isoforms play analogous roles in cross-linking actin filaments and anchoring them to structures such as cell-cell and cell-matrix junctions. Although actinins have long been known to play roles in cytokinesis, cell adhesion and cell migration, recent studies have provided further mechanistic insights into these functions. Roles for actinins in synaptic plasticity and membrane trafficking events have emerged more recently, as has a 'non-canonical' function for actinins in transcriptional regulation in the nucleus. In the present paper we review recent advances in our understanding of these diverse cell biological functions of actinins in non-muscle cells, as well as their roles in cancer and in genetic disorders affecting platelet and kidney physiology. We also make two proposals with regard to the actinin nomenclature. First, we argue that naming actinin isoforms according to their expression patterns is problematic and we suggest a more precise nomenclature system. Secondly, we suggest that the α in α-actinin is superfluous and can be omitted.