An epidermal plakin that integrates actin and microtubule networks at cellular junctions

Focal adhesions contain three specialized actin nanoscale layers

Focal adhesions (FAs) connect inner workings of cell to the extracellular matrix to control cell adhesion, migration and mechanosensing. Previous studies demonstrated that FAs contain three vertical layers, which connect extracellular matrix to the cytoskeleton. By using super-resolution iPALM microscopy, we identify two additional nanoscale layers within FAs, specified by actin filaments bound to tropomyosin isoforms Tpm1.6 and Tpm3.2. The Tpm1.6-actin filaments, beneath the previously identified α-actinin cross-linked actin filaments, appear critical for adhesion maturation and controlled cell motility, whereas the adjacent Tpm3.2-actin filament layer beneath seems to facilitate adhesion disassembly. Mechanistically, Tpm3.2 stabilizes ACF-7/MACF1 and KANK-family proteins at adhesions, and hence targets microtubule plus-ends to FAs to catalyse their disassembly. Tpm3.2 depletion leads to disorganized microtubule network, abnormally stable FAs, and defects in tail retraction during migration. Thus, FAs are composed of distinct actin filament layers, and each may have specific roles in coupling adhesions to the cytoskeleton, or in controlling adhesion dynamics.

Comprehensive ceRNA network for MACF1 regulates osteoblast proliferation

BACKGROUND

Previous studies have shown that microtubule actin crosslinking factor 1 (MACF1) can regulate osteoblast proliferation and differentiation through non-coding RNA (ncRNA) in bone-forming osteoblasts. However, the role of MACF1 in targeting the competing endogenous RNA (ceRNA) network to regulate osteoblast differentiation remains poorly understood. Here, we profiled messenger RNA (mRNA), microRNA (miRNA), and long ncRNA (lncRNA) expression in MACF1 knockdown MC3TC‑E1 pre‑osteoblast cells.

RESULTS

In total, 547 lncRNAs, 107 miRNAs, and 376 mRNAs were differentially expressed. Significantly altered lncRNAs, miRNAs, and mRNAs were primarily found on chromosome 2. A lncRNA-miRNA-mRNA network was constructed using a bioinformatics computational approach. The network indicated that mir-7063 and mir-7646 were the most potent ncRNA regulators and mef2c was the most potent target gene. Pathway enrichment analysis showed that the fluid shear stress and atherosclerosis, p53 signaling, and focal adhesion pathways were highly enriched and contributed to osteoblast proliferation. Importantly, the fluid shear stress and atherosclerosis pathway was co-regulated by lncRNAs and miRNAs. In this pathway, Dusp1 was regulated by AK079370, while Arhgef2 was regulated by mir-5101. Furthermore, Map3k5 was regulated by AK154638 and mir-466q simultaneously. AK003142 and mir-3082-5p as well as Ak141402 and mir-446 m-3p were identified as interacting pairs that regulate target genes.

CONCLUSION

This study revealed the global expression profile of ceRNAs involved in the differentiation of MC3TC‑E1 osteoblasts induced by MACF1 deletion. These results indicate that loss of MACF1 activates a comprehensive ceRNA network to regulate osteoblast proliferation.

Regulation of the COPII secretory machinery via focal adhesions and extracellular matrix signaling

Proteins that enter the secretory pathway are transported from their place of synthesis in the endoplasmic reticulum to the Golgi complex by COPII-coated carriers. The networks of proteins that regulate these components in response to extracellular cues have remained largely elusive. Using high-throughput microscopy, we comprehensively screened 378 cytoskeleton-associated and related proteins for their functional interaction with the coat protein complex II (COPII) components SEC23A and SEC23B. Among these, we identified a group of proteins associated with focal adhesions (FERMT2, MACF1, MAPK8IP2, NGEF, PIK3CA, and ROCK1) that led to the downregulation of SEC23A when depleted by siRNA. Changes in focal adhesions induced by plating cells on ECM also led to the downregulation of SEC23A and decreases in VSVG transport from ER to Golgi. Both the expression of SEC23A and the transport defect could be rescued by treatment with a focal adhesion kinase inhibitor. Altogether, our results identify a network of cytoskeleton-associated proteins connecting focal adhesions and ECM-related signaling with the gene expression of the COPII secretory machinery and trafficking.

Actin Crosslinking Family Protein 7 Deficiency Does Not Impair Hearing in Young Mice

To enable hearing, the sensory hair cell contains specialized subcellular structures at its apical region, including the actin-rich cuticular plate and circumferential band. ACF7 (actin crosslinking family protein 7), encoded by the gene Macf1 (microtubule and actin crosslinking factor 1), is a large cytoskeletal crosslinking protein that interacts with microtubules and filamentous actin to shape cells. ACF7 localizes to the cuticular plate and the circumferential band in the hair cells of vertebrates. The compelling expression pattern of ACF7 in hair cells, combined with conserved roles of this protein in the cytoskeleton of various cell types in invertebrates and vertebrates, led to the hypothesis that ACF7 performs a key function in the subcellular architecture of hair cells. To test the hypothesis, we conditionally target Macf1 in the inner ears of mice. Surprisingly, our data show that in young, but mature, conditional knockout mice cochlear hair cell survival, planar cell polarity, organization of the hair cells within the organ of Corti, and capacity to hear are not significantly impacted. Overall, these results fail to support the hypothesis that ACF7 is an essential hair cell protein in young mice, and the purpose of ACF7 expression in the hair cell remains to be understood.

Strength Through Unity: The Power of the Mega-Scaffold MACF1

The tight coordination of diverse cytoskeleton elements is required to support several dynamic cellular processes involved in development and tissue homeostasis. The spectraplakin-family of proteins are composed of multiple domains that provide versatility to connect different components of the cytoskeleton, including the actin microfilaments, microtubules and intermediates filaments. Spectraplakins act as orchestrators of precise cytoskeletal dynamic events. In this review, we focus on the prototypical spectraplakin MACF1, a protein scaffold of more than 700 kDa that coordinates the crosstalk between actin microfilaments and microtubules to support cell-cell connections, cell polarity, vesicular transport, proliferation, and cell migration. We will review over two decades of research aimed at understanding the molecular, physiological and pathological roles of MACF1, with a focus on its roles in developmental and cancer. A deeper understanding of MACF1 is currently limited by technical challenges associated to the study of such a large protein and we discuss ideas to advance the field.

Microtubule-associated proteins (MAPs) in microtubule cytoskeletal dynamics and spermatogenesis

The microtubule (MT) cytoskeleton in Sertoli cells, a crucial cellular structure in the seminiferous epithelium of adult mammalian testes that supports spermatogenesis, was studied morphologically decades ago. However, its biology, in particular the involving regulatory biomolecules and the underlying mechanism(s) in modulating MT dynamics, are only beginning to be revealed in recent years. This lack of studies in delineating the biology of MT cytoskeletal dynamics undermines other studies in the field, in particular the plausible therapeutic treatment and management of male infertility and fertility since studies have shown that the MT cytoskeleton is one of the prime targets of toxicants. Interestingly, much of the information regarding the function of actin-, MT- and intermediate filament-based cytoskeletons come from studies using toxicant models including some genetic models. During the past several years, there have been some advances in studying the biology of MT cytoskeleton in the testis, and many of these studies were based on the use of pharmaceutical/toxicant models. In this review, we summarize the results of these findings, illustrating the importance of toxicant/pharmaceutical models in unravelling the biology of MT dynamics, in particular the role of microtubule-associated proteins (MAPs), a family of regulatory proteins that modulate MT dynamics but also actin- and intermediate filament-based cytoskeletons. We also provide a timely hypothetical model which can serve as a guide to design functional experiments to study how the MT cytoskeleton is regulated during spermatogenesis through the use of toxicants and/or pharmaceutical agents.

The maternal coordinate system: Molecular-genetics of embryonic axis formation and patterning in the zebrafish

Axis specification of the zebrafish embryo begins during oogenesis and relies on proper formation of well-defined cytoplasmic domains within the oocyte. Upon fertilization, maternally-regulated cytoplasmic flow and repositioning of dorsal determinants establish the coordinate system that will build the structure and developmental body plan of the embryo. Failure of specific genes that regulate the embryonic coordinate system leads to catastrophic loss of body structures. Here, we review the genetic principles of axis formation and discuss how maternal factors orchestrate axis patterning during zebrafish early embryogenesis. We focus on the molecular identity and functional contribution of genes controlling critical aspects of oogenesis, egg activation, blastula, and gastrula stages. We examine how polarized cytoplasmic domains form in the oocyte, which set off downstream events such as animal-vegetal polarity and germ line development. After gametes interact and form the zygote, cytoplasmic segregation drives the animal-directed reorganization of maternal determinants through calcium- and cell cycle-dependent signals. We also summarize how maternal genes control dorsoventral, anterior-posterior, mesendodermal, and left-right cell fate specification and how signaling pathways pattern these axes and tissues during early development to instruct the three-dimensional body plan. Advances in reverse genetics and phenotyping approaches in the zebrafish model are revealing positional patterning signatures at the single-cell level, thus enhancing our understanding of genotype-phenotype interactions in axis formation. Our emphasis is on the genetic interrogation of novel and specific maternal regulatory mechanisms of axis specification in the zebrafish.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

MACF1 promotes preosteoblast migration by mediating focal adhesion turnover through EB1

Microtubule actin crosslinking factor 1 (MACF1) is a widely expressed cytoskeletal linker and plays an essential role in various cells' functions by mediating cytoskeleton organization and dynamics. However, the role of MACF1 on preosteoblast migration is not clear. Here, by using MACF1 knockdown and overexpressed MC3T3-E1 cells, we found MACF1 positively regulated preosteoblast migration induced by cell polarization. Furthermore, immunofluorescent staining showed that MACF1 increased end-binding protein (EB1) distribution on microtubule (MT), and decreased EB1 distribution on focal adhesion (FA) complex. Moreover, upregulation of MACF1 activated Src level and enhanced the colocalization of EB1 with activated Src. In addition, MACF1 diminished colocalization of EB1 with adenomatous polyposis coli (APC), which induced EB1 release from FA and promoted FA turnover. These results indicated an important role and mechanism of MACF1 in regulating preosteoblast migration through promoting FA turnover by mediating EB1 colocalization with Src and APC, which inferred that MACF1 might be a potential target for preventing and treating bone disorders.

Microtubule actin crosslinking factor 1 (MACF1) knockdown inhibits RANKL-induced osteoclastogenesis via Akt/GSK3β/NFATc1 signalling pathway

Osteoclasts are responsible for bone resorption and play essential roles in causing bone diseases such as osteoporosis. Microtubule actin crosslinking factor 1 (MACF1) is a large spectraplakin protein that has been implicated in regulating cytoskeletal distribution, cell migration, cell survival and cell differentiation. However, whether MACF1 regulates the differentiation of osteoclasts has not been elucidated. In this study, we found that the expression of MACF1 was increased in primary bone marrow-derived monocytes (BMMs) of osteoporotic mice and was downregulated during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis of pre-osteoclast cell lines RAW264.7 cells. RAW264.7 cells were transfected with shMACF1 using a lentiviral vector to study the role of MACF1 in osteoclastogenic differentiation. Knockdown of MACF1 in RAW264.7 cells inhibited the formation of multinucleated osteoclasts and decreased the expression of osteoclast-marker genes (Ctsk, Acp5, Mmp9 and Oscar) during RANKL-induced osteoclastogenesis. Additionally, knockdown of MACF1 disrupted actin ring formation in osteoclasts and further blocked the bone resorption activity of osteoclasts by reducing the area and depth of pits. Knockdown of MACF1 had no effect on the survival of pre-osteoclasts and mature osteoclasts. We further established that knockdown of MACF1 attenuated the phosphorylation of Akt and GSK3β and inhibited the expression of its downstream target NFATc1. Akt activator rescued the inhibition of osteoclast differentiation by MACF1 knockdown. These data demonstrate that MACF1 positively regulates osteoclast differentiation via the Akt/GSK3β/NFATc1 signalling pathway, suggesting that targeting MACF1 may be a novel therapeutic approach against osteoporosis.

Microtubule-Associated Proteins: Structuring the Cytoskeleton

Microtubule-associated proteins (MAPs) were initially discovered as proteins that bind to and stabilize microtubules. Today, an ever-growing number of MAPs reveals a more complex picture of these proteins as organizers of the microtubule cytoskeleton that have a large variety of functions. MAPs enable microtubules to participate in a plethora of cellular processes such as the assembly of mitotic and meiotic spindles, neuronal development, and the formation of the ciliary axoneme. Although some subgroups of MAPs have been exhaustively characterized, a strikingly large number of MAPs remain barely characterized other than their interactions with microtubules. We provide a comprehensive view on the currently known MAPs in mammals. We discuss their molecular mechanisms and functions, as well as their physiological role and links to pathologies.

The cytoskeletal crosslinking protein MACF1 is dispensable for thrombus formation and hemostasis

Coordinated reorganization of cytoskeletal structures is critical for key aspects of platelet physiology. While several studies have addressed the role of microtubules and filamentous actin in platelet production and function, the significance of their crosstalk in these processes has been poorly investigated. The microtubule-actin cross-linking factor 1 (MACF1; synonym: Actin cross-linking factor 7, ACF7) is a member of the spectraplakin family, and one of the few proteins expressed in platelets, which possess actin and microtubule binding domains thereby facilitating actin-microtubule interaction and regulation. We used megakaryocyte- and platelet-specific Macf1 knockout (Macf1^fl/fl, Pf4-Cre) mice to study the role of MACF1 in platelet production and function. MACF1 deficient mice displayed comparable platelet counts to control mice. Analysis of the platelet cytoskeletal ultrastructure revealed a normal marginal band and actin network. Platelet spreading on fibrinogen was slightly delayed but platelet activation and clot traction was unaffected. Ex vivo thrombus formation and mouse tail bleeding responses were similar between control and mutant mice. These results suggest that MACF1 is dispensable for thrombopoiesis, platelet activation, thrombus formation and the hemostatic function in mice.

Pan-Cancer Analysis Reveals the Functional Importance of Protein Lysine Modification in Cancer Development

Large-scale tumor genome sequencing projects have revealed a complex landscape of genomic mutations in multiple cancer types. A major goal of these projects is to characterize somatic mutations and discover cancer drivers, thereby providing important clues to uncover diagnostic or therapeutic targets for clinical treatment. However, distinguishing only a few somatic mutations from the majority of passenger mutations is still a major challenge facing the biological community. Fortunately, combining other functional features with mutations to predict cancer driver genes is an effective approach to solve the above problem. Protein lysine modifications are an important functional feature that regulates the development of cancer. Therefore, in this work, we have systematically analyzed somatic mutations on seven protein lysine modifications and identified several important drivers that are responsible for tumorigenesis. From published literature, we first collected more than 100,000 lysine modification sites for analysis. Another 1 million non-synonymous single nucleotide variants (SNVs) were then downloaded from TCGA and mapped to our collected lysine modification sites. To identify driver proteins that significantly altered lysine modifications, we further developed a hierarchical Bayesian model and applied the Markov Chain Monte Carlo (MCMC) method for testing. Strikingly, the coding sequences of 473 proteins were found to carry a higher mutation rate in lysine modification sites compared to other background regions. Hypergeometric tests also revealed that these gene products were enriched in known cancer drivers. Functional analysis suggested that mutations within the lysine modification regions possessed higher evolutionary conservation and deleteriousness. Furthermore, pathway enrichment showed that mutations on lysine modification sites mainly affected cancer related processes, such as cell cycle and RNA transport. Moreover, clinical studies also suggested that the driver proteins were significantly associated with patient survival, implying an opportunity to use lysine modifications as molecular markers in cancer diagnosis or treatment. By searching within protein-protein interaction networks using a random walk with restart (RWR) algorithm, we further identified a series of potential treatment agents and therapeutic targets for cancer related to lysine modifications. Collectively, this study reveals the functional importance of lysine modifications in cancer development and may benefit the discovery of novel mechanisms for cancer treatment.

rbpms2 functions in Balbiani body architecture and ovary fate

The most prominent developmental regulators in oocytes are RNA-binding proteins (RNAbps) that assemble their targets into ribonucleoprotein granules where they are stored, transported and translationally regulated. RNA-binding protein of multiple splice forms 2, or Rbpms2, interacts with molecules that are essential to reproduction and egg patterning, including bucky ball, a key factor for Bb formation. Rbpms2 is localized to germ granules in primordial germ cells (PGCs) and to the Balbiani body (Bb) of oocytes, although the mechanisms regulating Rbpms2 localization to these structures are unknown. Using mutant Rbpms2 proteins, we show that Rbpms2 requires distinct protein domains to localize within germ cells and somatic cells. Accumulation and localization to subcellular compartments in the germline requires an intact RNA binding domain. Whereas in zebrafish somatic blastula cells, the conserved C-terminal domain promotes localization to the bipolar centrosomes/spindle. To investigate Rbpms2 functions, we mutated the duplicated and functionally redundant zebrafish rbpms2 genes. The gonads of rbpms2a;2b (rbpms2) mutants initially contain early oocytes, however definitive oogenesis ultimately fails during sexual differentiation and, rbpms2 mutants develop as fertile males. Unlike other genes that promote oogenesis, failure to maintain oocytes in rbpms2 mutants was not suppressed by mutation of Tp53. These findings reveal a novel and essential role for rbpms2 in oogenesis. Ultrastructural and immunohistochemical analyses revealed that rbpms2 is not required for the asymmetric accumulation of mitochondria and Buc protein in oocytes, however its absence resulted in formation of abnormal Buc aggregates and atypical electron-dense cytoplasmic inclusions. Our findings reveal novel and essential roles for rbpms2 in Buc organization and oocyte differentiation.

Oocyte development relies on posttranscriptional regulation by RNA binding proteins (RNAbps). RNAbps form large multi-molecular structures called RNPs (ribonucleoproteins) that further aggregate into regulatory granules within germ cells. In zebrafish primary oocytes, a large transient RNP aggregate called the Balbiani body (Bb) is essential for localizing patterning molecules and germline determinants within oocytes. RNA-binding protein of multiple splice forms 2, or Rbpms2, localizes to germ granules and the Bb, and interacts with bucky ball, a key factor for Bb formation. We show that Rbpms2 requires RNA binding for localization within germ cells, and that the C-term and RRM contribute to Rbpms2 subcellular localization in distinct somatic cell types. To investigate Rbpms2 functions we mutated the duplicated zebrafish rbpms2 genes. Consistent with redundant functions, rbpms2a and rbpms2b gene expression overlaps, and single mutants have no discernible phenotypes. Although rbpms2a;2b double mutants have cardiac phenotypes, those that reach adulthood are exclusively fertile males. Genetic analysis shows that rbpms2 mutant oocytes are not maintained even when Tp53, a regulator of cell death is absent. Initial oocyte polarity is established in rbpms2 mutants based on asymmetric distribution of Buc protein and mitochondria; however, abnormal Buc structures and atypical cytoplasmic inclusions form. This work reveals independent Rbpms2 functions in promoting Bb integrity, and as a novel regulator of ovary fate.

Spindly is required for rapid migration of human cells

Dynein is the sole processive minus-end-directed microtubule motor found in animals. It has roles in cell division, membrane trafficking, and cell migration. Together with dynactin, dynein regulates centrosomal orientation to establish and maintain cell polarity, controls focal adhesion turnover and anchors microtubules at the leading edge. In higher eukaryotes, dynein/dynactin requires additional components such as Bicaudal D to form an active motor complex and for regulating its cellular localization. Spindly is a protein that targets dynein/dynactin to kinetochores in mitosis and can activate its motility in vitro However, no role for Spindly in interphase dynein/dynactin function has been found. We show that Spindly binds to the cell cortex and microtubule tips and colocalizes with dynein/dynactin at the leading edge of migrating U2OS cells and primary fibroblasts. U2OS cells that lack Spindly migrated slower in 2D than control cells, although centrosome polarization appeared to happen properly in the absence of Spindly. Re-expression of Spindly rescues migration, but the expression of a mutant, which is defective for dynactin binding, failed to rescue this defect. Taken together, these data demonstrate that Spindly plays an important role in mediating a subset of dynein/dynactin's function in cell migration.

Analysis of subcellular structural tension in axonal growth of neurons

The growth and regeneration of axons are the core processes of nervous system development and functional recovery. They are also related to certain physiological and pathological conditions. For decades, it has been the consensus that a new axon is formed by adding new material at the growth cone. However, using the existing technology, we have studied the structural tension of the nerve cell, which led us to hypothesize that some subcellular structural tensions contribute synergistically to axonal growth and regeneration. In this review, we classified the subcellular structural tension, osmotic pressure, microfilament and microtubule-dependent tension involved controllably in promoting axonal growth. A squeezing model was built to analyze the mechanical mechanism underlying axonal elongation, which may provide a new view of axonal growth and inspire further research.

Mechanical unloading reduces microtubule actin crosslinking factor 1 expression to inhibit β-catenin signaling and osteoblast proliferation

Mechanical unloading was considered a major threat to bone homeostasis, and has been shown to decrease osteoblast proliferation although the underlying mechanism is unclear. Microtubule actin crosslinking factor 1 (MACF1) is a cytoskeletal protein that regulates cellular processes and Wnt/β-catenin pathway, an essential signaling pathway for osteoblasts. However, the relationship between MACF1 expression and mechanical unloading, and the function and the associated mechanisms of MACF1 in regulating osteoblast proliferation are unclear. This study investigated effects of mechanical unloading on MACF1 expression levels in cultured MC3T3-E1 osteoblastic cells and in femurs of mice with hind limb unloading; and it also examined the role and potential action mechanisms of MACF1 in osteoblast proliferation in MACF1-knockdown, overexpressed or control MC3T3-E1 cells treated with or without the mechanical unloading condition. Results showed that the mechanical unloading condition inhibited osteoblast proliferation and MACF1 expression in MC3T3-E1 osteoblastic cells and mouse femurs. MACF1 knockdown decreased osteoblast proliferation, while MACF1 overexpression increased it. The inhibitory effect of mechanical unloading on osteoblast proliferation also changed with MACF1 expression levels. Furthermore, MACF1 was found to enhance β-catenin expression and activity, and mechanical unloading decreased β-catenin expression through MACF1. Moreover, β-catenin was found an important regulator of osteoblast proliferation, as its preservation by treatment with its agonist lithium attenuated the inhibitory effects of MACF1-knockdown or mechanical unloading on osteoblast proliferation. Taken together, mechanical unloading decreases MACF1 expression, and MACF1 up-regulates osteoblast proliferation through enhancing β-catenin signaling. This study has thus provided a mechanism for mechanical unloading-induced inhibited osteoblast proliferation.

Development of a microfluidic platform integrating high-resolution microstructured biomaterials to study cell-material interactions

Microfluidic screening platforms offer new possibilities for performing in vitro cell-based assays with higher throughput and in a setting that has the potential to closely mimic the physiological microenvironment. Integrating functional biomaterials into such platforms is a promising approach to obtain a deeper insight into the interactions occurring at the cell-material interface. The success of such an approach is, however, largely dependent on the ability to miniaturize the biomaterials as well as on the choice of the assay used to study the cell-material interactions. In this work, we developed a microfluidic device, the main component of which is made of a widely used biocompatible polymer, polylactic acid (PLA). This device enabled cell culture under different fluidic regimes, including perfusion and diffusion. Through a combination of photolithography, two-photon polymerization and hot embossing, it was possible to microstructure the surface of the cell culture chamber of the device with highly defined geometrical features. Furthermore, using pyramids with different heights and wall microtopographies as an example, adhesion, morphology and distribution of human MG63 osteosarcoma cells were studied. The results showed that both the height of the topographical features and the microstructural properties of their walls affected cell spreading and distribution. This proof-of-concept study shows that the platform developed here is a useful tool for studying interactions between cells and clinically relevant biomaterials under controlled fluidic regimes.

Microtubule actin cross-linking factor 1, a novel potential target in cancer

Cancer is a polygenic disease characterized by uncontrolled growth of normal body cells, deregulation of the cell cycle as well as resistance to apoptosis. The spectraplakin protein microtubule actin cross-linking factor 1 (MACF1) plays an essential function in various cellular processes, including cell proliferation, migration, signaling transduction and embryo development. MACF1 is also involved in processes such as metastatic invasion in which cytoskeleton organization is a critical element that contributes to tumor progression in various human cancers. Aberrant expression of MACF1 initiates the tumor cell proliferation, and migration and metastasis in numerous cancers, such as breast cancer, colon cancer, lung cancer and glioblastoma. In this review, we summarized the current knowledge of MACF1 and its critical role in different human cancers. This will be helpful for researchers to investigate the novel functional role of MACF1 in human cancers and as a potential target to enhance the efficacy of therapeutic treatment modalities.

Microtubule-actin crosslinking factor 1 (Macf1) domain function in Balbiani body dissociation and nuclear positioning

Animal-vegetal (AV) polarity of most vertebrate eggs is established during early oogenesis through the formation and disassembly of the Balbiani Body (Bb). The Bb is a structure conserved from insects to humans that appears as a large granule, similar to a mRNP granule composed of mRNA and proteins, that in addition contains mitochondria, ER and Golgi. The components of the Bb, which have amyloid-like properties, include germ cell and axis determinants of the embryo that are anchored to the vegetal cortex upon Bb disassembly. Our lab discovered in zebrafish the only gene known to function in Bb disassembly, microtubule-actin crosslinking factor 1a (macf1a). Macf1 is a conserved, giant multi-domain cytoskeletal linker protein that can interact with microtubules (MTs), actin filaments (AF), and intermediate filaments (IF). In macf1a mutant oocytes the Bb fails to dissociate, the nucleus is acentric, and AV polarity of the oocyte and egg fails to form. The cytoskeleton-dependent mechanism by which Macf1a regulates Bb mRNP granule dissociation was unknown. We found that disruption of AFs phenocopies the macf1a mutant phenotype, while MT disruption does not. We determined that cytokeratins (CK), a type of IF, are enriched in the Bb. We found that Macf1a localizes to the Bb, indicating a direct function in regulating its dissociation. We thus tested if Macf1a functions via its actin binding domain (ABD) and plectin repeat domain (PRD) to integrate cortical actin and Bb CK, respectively, to mediate Bb dissociation at the oocyte cortex. We developed a CRISPR/Cas9 approach to delete the exons encoding these domains from the macf1a endogenous locus, while maintaining the open reading frame. Our analysis shows that Macf1a functions via its ABD to mediate Bb granule dissociation and nuclear positioning, while the PRD is dispensable. We propose that Macf1a does not function via its canonical mechanism of linking two cytoskeletal systems together in dissociating the Bb. Instead our results suggest that Macf1a functions by linking one cytoskeletal system, cortical actin, to another structure, the Bb, where Macf1a is localized. Through this novel linking process, it dissociates the Bb at the oocyte cortex, thus specifying the AV axis of the oocyte and future egg. To our knowledge, this is also the first study to use genome editing to unravel the module-dependent function of a cytoskeletal linker.

The totipotent egg of most vertebrates is polarized in a so called animal-vegetal axis that is essential to early embryonic development. The animal-vegetal axis is established in the early oocyte by the dissociation of the Balbiani Body (Bb). The Bb is a large RNA-protein granule, conserved from insects to mammals, that forms next to the oocyte nucleus and dissociates later at the oocyte cortex. Importantly, Bb dissociation at the oocyte cortex defines the future vegetal pole of the egg. Macf1a, a cytolinker, is the only factor known to regulate Bb dissociation. However, how the giant Macf1a protein with multiple functional domains can interact with the cytoskeleton to regulate Bb disassembly is unknown. Here, we unravel Macf1a function via interrogating, for the first time, individual macf1a-encoded domains of the gene in its normal chromosomal location for their requirement in Bb dissociation and ultimately in egg polarity establishment. The method presented here is applicable to other cytolinkers involved in human disease.

Spectraplakin family proteins - cytoskeletal crosslinkers with versatile roles

The different cytoskeletal networks in a cell are responsible for many fundamental cellular processes. Current studies have shown that spectraplakins, cytoskeletal crosslinkers that combine features of both the spectrin and plakin families of crosslinkers, have a critical role in integrating these different cytoskeletal networks. Spectraplakin genes give rise to a variety of isoforms that have distinct functions. Importantly, all spectraplakin isoforms are uniquely able to associate with all three elements of the cytoskeleton, namely, F-actin, microtubules and intermediate filaments. In this Review, we will highlight recent studies that have unraveled their function in a wide range of different processes, from regulating cell adhesion in skin keratinocytes to neuronal cell migration. Taken together, this work has revealed a diverse and indispensable role for orchestrating the function of different cytoskeletal elements in vivo.

BPAG1 in muscles: Structure and function in skeletal, cardiac and smooth muscle

BPAG1, also known as Dystonin or BP230, belongs to the plakin family of proteins, which has multiple cytoskeleton-binding domains. Several BPAG1 isoforms are produced by a single BPAG1 genomic locus using different promoters and exons. For example, BPAG1a, BPAG1b, and BPAG1e are predominantly expressed in the nervous system, muscle, and skin, respectively. Among BPAG1 isoforms, BPAG1e is well studied because it was first identified as an autoantigen in patients with bullous pemphigoid, an autoimmune skin disease. BPAG1e is a component of hemidesmosomes, the adhesion complexes that promote dermal-epidermal cohesion. In the nervous system, the role of BPAG1a is also well studied because disruption of BPAG1a results in a phenotype identical to that of Dystonia musculorum (dt) mutants, which show progressive motor disorder. However, the expression and function of BPAG1 in muscles is not well studied. The aim of this review is to provide an overview of and highlight some recent findings on the expression and function of BPAG1 in muscles, which can assist future studies designed to delineate the role and regulation of BPAG1 in the dt mouse phenotype and in human hereditary sensory and autonomic neuropathy type 6 (HSAN6).

Structure of the ACF7 EF-Hand-GAR Module and Delineation of Microtubule Binding Determinants

Spectraplakins are large molecules that cross-link F-actin and microtubules (MTs). Mutations in spectraplakins yield defective cell polarization, aberrant focal adhesion dynamics, and dystonia. We present the 2.8 Å crystal structure of the hACF7 EF1-EF2-GAR MT-binding module and delineate the GAR residues critical for MT binding. The EF1-EF2 and GAR domains are autonomous domains connected by a flexible linker. The EF1-EF2 domain is an EFβ-scaffold with two bound Ca²⁺ ions that straddle an N-terminal α helix. The GAR domain has a unique α/β sandwich fold that coordinates Zn²⁺. While the EF1-EF2 domain is not sufficient for MT binding, the GAR domain is and likely enhances EF1-EF2-MT engagement. Residues in a conserved basic patch, distal to the GAR domain's Zn²⁺-binding site, mediate MT binding.

The role of MACF1 in nervous system development and maintenance

Microtubule-actin crosslinking factor 1 (MACF1), also known as actin crosslinking factor 7 (ACF7), is essential for proper modulation of actin and microtubule cytoskeletal networks. Most MACF1 isoforms are expressed broadly in the body, but some are exclusively found in the nervous system. Consequentially, MACF1 is integrally involved in multiple neural processes during development and in adulthood, including neurite outgrowth and neuronal migration. Furthermore, MACF1 participates in several signaling pathways, including the Wnt/β-catenin and GSK-3 signaling pathways, which regulate key cellular processes, such as proliferation and cell migration. Genetic mutation or dysregulation of the MACF1 gene has been associated with neurodevelopmental and neurodegenerative diseases, specifically schizophrenia and Parkinson's disease. MACF1 may also play a part in neuromuscular disorders and have a neuroprotective role in the optic nerve. In this review, the authors seek to synthesize recent findings relating to the roles of MACF1 within the nervous system and explore potential novel functions of MACF1 not yet examined.

MACF1, versatility in tissue-specific function and in human disease

Spectraplakins are a family of evolutionarily conserved gigantic proteins and play critical roles in many cytoskeleton-related processes. Microtubule actin crosslinking factor 1 (MACF1) is one of the most versatile spectraplakin with multiple isoforms. As a broadly expressed mammalian spectraplakin, MACF1 is important in maintaining normal functions of many tissues. The loss-of-function studies using knockout mouse models reveal the pivotal roles of MACF1 in embryo development, skin integrity maintenance, neural development, bone formation, and colonic paracellular permeability. Mutation in the human MACF1 gene causes a novel myopathy genetic disease. In addition, abnormal expression of MACF1 is associated with schizophrenia, Parkinson's disease, cancer and osteoporosis. This demonstrates the crucial roles of MACF1 in physiology and pathology. Here, we review the research advances of MACF1's roles in specific tissue and in human diseases, providing the perspectives of MACF1 for future studies.

A Novel Frizzled-Based Screening Tool Identifies Genetic Modifiers of Planar Cell Polarity in Drosophila Wings

Most mutant alleles in the Fz-PCP pathway genes were discovered in classic Drosophila screens looking for recessive loss-of-function (LOF) mutations. Nonetheless, although Fz-PCP signaling is sensitive to increased doses of PCP gene products, not many screens have been performed in the wing under genetically engineered Fz overexpression conditions, mostly because the Fz phenotypes were strong and/or not easy to score and quantify. Here, we present a screen based on an unexpected mild Frizzled gain-of-function (GOF) phenotype. The leakiness of a chimeric Frizzled protein designed to be accumulated in the endoplasmic reticulum (ER) generated a reproducible Frizzled GOF phenotype in Drosophila wings. Using this genotype, we first screened a genome-wide collection of large deficiencies and found 16 strongly interacting genomic regions. Next, we narrowed down seven of those regions to finally test 116 candidate genes. We were, thus, able to identify eight new loci with a potential function in the PCP context. We further analyzed and confirmed krasavietz and its interactor short-stop as new genes acting during planar cell polarity establishment with a function related to actin and microtubule dynamics.

Loss of MACF1 Abolishes Ciliogenesis and Disrupts Apicobasal Polarity Establishment in the Retina

Microtubule actin crosslinking factor 1 (MACF1) plays a role in the coordination of microtubules and actin in multiple cellular processes. Here, we show that MACF1 is also critical for ciliogenesis in multiple cell types. Ablation of Macf1 in the developing retina abolishes ciliogenesis, and basal bodies fail to dock to ciliary vesicles or migrate apically. Photoreceptor polarity is randomized, while inner retinal cells laminate correctly, suggesting that photoreceptor maturation is guided by polarity cues provided by cilia. Deletion of MACF1 in adult photoreceptors causes reversal of basal body docking and loss of outer segments, reflecting a continuous requirement for MACF1 function. MACF1 also interacts with the ciliary proteins MKKS and TALPID3. We propose that a disruption of trafficking across microtubles to actin filaments underlies the ciliogenesis defect in cells lacking MACF1 and that MKKS and TALPID3 are involved in the coordination of microtubule and actin interactions.

Isoforms, structures, and functions of versatile spectraplakin MACF1

Spectraplakins are crucially important communicators, linking cytoskeletal components to each other and cellular junctions. Microtubule actin crosslinking factor 1 (MACF1), also known as actin crosslinking family 7 (ACF7), is a member of the spectraplakin family. It is expressed in numerous tissues and cells as one extensively studied spectraplakin. MACF1 has several isoforms with unique structures and well-known function to be able to crosslink F-actin and microtubules. MACF1 is one versatile spectraplakin with various functions in cell processes, embryo development, tissue-specific functions, and human diseases. The importance of MACF1 has become more apparent in recent years. Here, we summarize the current knowledge on the presence and function of MACF1 and provide perspectives on future research of MACF1 based on our studies and others. [BMB Reports 2016; 49(1): 37-44]

The CAMSAP3-ACF7 Complex Couples Noncentrosomal Microtubules with Actin Filaments to Coordinate Their Dynamics

For adaptation to complex cellular functions, dynamic cytoskeletal networks are required. There are two major components of the cytoskeleton, microtubules and actin filaments, which form an intricate network maintaining an exquisite cooperation to build the physical basis for their cellular function. However, little is known about the molecular mechanism underlying their synergism. Here, we show that in Caco2 epithelial cells, noncentrosomal microtubules crosstalk with F-actin through their minus ends and contribute to the regulation of focal adhesion size and cell migration. We demonstrate that ACF7, a member of the spectraplakin family of cytoskeletal crosslinking proteins, interacts with Nezha (also called CAMSAP3) at the minus ends of noncentrosomal microtubules and anchors them to actin filaments. Those noncentrosomal microtubules cooperate with actin filaments through retrograde flow to keep their length and orientation perpendicular to the cell edge as well as regulate focal adhesion size and cell migration.

Knockdown of microtubule actin crosslinking factor 1 inhibits cell proliferation in MC3T3-E1 osteoblastic cells

Microtubule actin crosslinking factor 1 (MACF1), a widely expressed cytoskeletal linker, plays important roles in various cells by regulating cytoskeleton dynamics. However, its role in osteoblastic cells is not well understood. Based on our previous findings that the association of MACF1 with F-actin and microtubules in osteoblast-like cells was altered under magnetic force conditions, here, by adopting a stable MACF1-knockdown MC3T3-E1 osteoblastic cell line, we found that MACF1 knockdown induced large cells with a binuclear/multinuclear structure. Further, immunofluorescence staining showed disorganization of F-actin and microtubules in MACF1-knockdown cells. Cell counting revealed significant decrease of cell proliferation and cell cycle analysis showed an S phase cell cycle arrest in MACF1-knockdown cells. Moreover and interestingly, MACF1 knockdown showed a potential effect on cellular MTT reduction activity and mitochondrial content, suggesting an impact on cellular metabolic activity. These results together indicate an important role of MACF1 in regulating osteoblastic cell morphology and function. [BMB Reports 2015; 48(10): 583-588]

Exome sequencing for bipolar disorder points to roles of de novo loss-of-function and protein-altering mutations

Although numerous genetic studies have been conducted for bipolar disorder (BD), its genetic architecture remains elusive. Here we perform, to the best of our knowledge, the first trio-based exome sequencing study for BD to investigate potential roles of de novo mutations in the disease etiology. We identified 71 de novo point mutations and one de novo copy-number mutation in 79 BD probands. Among the genes hit by de novo loss-of-function (LOF; nonsense, splice site or frameshift) or protein-altering (LOF, missense and inframe indel) mutations, we found significant enrichment of genes highly intolerant (first percentile of intolerant genes assessed by Residual Variation Intolerance Score) to protein-altering variants in general population, an observation that is also reported in autism and schizophrenia. When we performed a joint analysis using the data of schizoaffective disorder in published studies, we found global enrichment of de novo LOF and protein-altering mutations in the combined group of bipolar I and schizoaffective disorders. Considering relationship between de novo mutations and clinical phenotypes, we observed significantly earlier disease onset among the BD probands with de novo protein-altering mutations when compared with non-carriers. Gene ontology enrichment analysis of genes hit by de novo protein-altering mutations in bipolar I and schizoaffective disorders did not identify any significant enrichment. These results of exploratory analyses collectively point to the roles of de novo LOF and protein-altering mutations in the etiology of bipolar disorder and warrant further large-scale studies.

In vivo epidermal migration requires focal adhesion targeting of ACF7

Turnover of focal adhesions allows cell retraction, which is essential for cell migration. The mammalian spectraplakin protein, ACF7 (Actin-Crosslinking Factor 7), promotes focal adhesion dynamics by targeting of microtubule plus ends towards focal adhesions. However, it remains unclear how the activity of ACF7 is regulated spatiotemporally to achieve focal adhesion-specific guidance of microtubule. To explore the potential mechanisms, we resolve the crystal structure of ACF7’s NT (amino-terminal) domain, which mediates F-actin interactions. Structural analysis leads to identification of a key tyrosine residue at the calponin homology (CH) domain of ACF7, whose phosphorylation by Src/FAK (focal adhesion kinase) complex is essential for F-actin binding of ACF7. Using skin epidermis as a model system, we further demonstrate that the phosphorylation of ACF7 plays an indispensable role in focal adhesion dynamics and epidermal migration in vitro and in vivo. Together, our findings provide critical insights into the molecular mechanisms underlying coordinated cytoskeletal dynamics during cell movement.

The spectraplakin protein ACF7 binds to actin at focal adhesions and targets microtubule plus ends to focal adhesions, promoting their disassembly. Here the authors reveal that ACF7 is phosphorylated by Src/FAK, and this regulates actin binding and focal adhesion dynamics in vitro and in vivo.

Cytoskeletal Linker Protein Dystonin Is Not Critical to Terminal Oligodendrocyte Differentiation or CNS Myelination

Oligodendrocyte differentiation and central nervous system myelination require massive reorganization of the oligodendrocyte cytoskeleton. Loss of specific actin- and tubulin-organizing factors can lead to impaired morphological and/or molecular differentiation of oligodendrocytes, resulting in a subsequent loss of myelination. Dystonin is a cytoskeletal linker protein with both actin- and tubulin-binding domains. Loss of function of this protein results in a sensory neuropathy called Hereditary Sensory Autonomic Neuropathy VI in humans and dystonia musculorum in mice. This disease presents with severe ataxia, dystonic muscle and is ultimately fatal early in life. While loss of the neuronal isoforms of dystonin primarily leads to sensory neuron degeneration, it has also been shown that peripheral myelination is compromised due to intrinsic Schwann cell differentiation abnormalities. The role of this cytoskeletal linker in oligodendrocytes, however, remains unclear. We sought to determine the effects of the loss of neuronal dystonin on oligodendrocyte differentiation and central myelination. To address this, primary oligodendrocytes were isolated from a severe model of dystonia musculorum, Dstdt-27J, and assessed for morphological and molecular differentiation capacity. No defects could be discerned in the differentiation of Dstdt-27J oligodendrocytes relative to oligodendrocytes from wild-type littermates. Survival was also compared between Dstdt-27J and wild-type oligodendrocytes, revealing no significant difference. Using a recently developed migration assay, we further analysed the ability of primary oligodendrocyte progenitor cell motility, and found that Dstdt-27J oligodendrocyte progenitor cells were able to migrate normally. Finally, in vivo analysis of oligodendrocyte myelination was done in phenotype-stage optic nerve, cerebral cortex and spinal cord. The density of myelinated axons and g-ratios of Dstdt-27J optic nerves was normal, as was myelin basic protein expression in both cerebral cortex and spinal cord. Together these data suggest that, unlike Schwann cells, oligodendrocytes do not have an intrinsic requirement for neuronal dystonin for differentiation and myelination.

Control of apico-basal epithelial polarity by the microtubule minus-end binding protein CAMSAP3 and spectraplakin ACF7

The microtubule cytoskeleton regulates cell polarity by spatially organizing membrane trafficking and signaling processes. In epithelial cells, microtubules form parallel arrays aligned along the apico-basal axis, and recent work has demonstrated that the members of CAMSAP/Patronin family control apical tethering of microtubule minus ends. Here, we show that in mammalian intestinal epithelial cells, the spectraplakin ACF7 specifically binds to CAMSAP3 and is required for the apical localization of CAMSAP3-decorated microtubule minus ends. Loss of ACF7 but not of CAMSAP3 or its homologue CAMSAP2 affected the formation of polarized epithelial cysts in 3D cultures. In short-term epithelial polarization assays, the knock-out of CAMSAP3, but not of CAMSAP2 caused microtubule re-organization into a more radial centrosomal array, redistribution of Rab11 endosomes from the apical cell surface to the pericentrosomal region and inhibition of actin brush border formation at the apical side of the cell. We conclude that ACF7 is an important regulator of apico-basal polarity in mammalian intestinal cells and that a radial centrosome-centered microtubule organization can act as an inhibitor of epithelial polarity.

Connections between cadherin-catenin proteins, spindle misorientation, and cancer

Cadherin-catenin mediated adhesion is an important determinant of tissue architecture in multicellular organisms. Cancer progression and maintenance is frequently associated with loss of their expression or functional activity, which not only leads to decreased cell-cell adhesion, but also to enhanced tumor cell proliferation and loss of differentiated characteristics. This review is focused on the emerging implications of cadherin-catenin proteins in the regulation of polarized divisions through their connections with the centrosomes, cytoskeleton, tissue tension and signaling pathways; and illustrates how alterations in cadherin-catenin levels or functional activity may render cells susceptible to transformation through the loss of their proliferation-differentiation balance.

Epithelial junctions and Rho family GTPases: the zonular signalosome

The establishment and maintenance of epithelial cell-cell junctions is crucially important to regulate adhesion, apico-basal polarity and motility of epithelial cells, and ultimately controls the architecture and physiology of epithelial organs. Junctions are supported, shaped and regulated by cytoskeletal filaments, whose dynamic organization and contractility are finely tuned by GTPases of the Rho family, primarily RhoA, Rac1 and Cdc42. Recent research has identified new molecular mechanisms underlying the cross-talk between these GTPases and epithelial junctions. Here we briefly summarize the current knowledge about the organization, molecular evolution and cytoskeletal anchoring of cell-cell junctions, and we comment on the most recent advances in the characterization of the interactions between Rho GTPases and junctional proteins, and their consequences with regards to junction assembly and regulation of cell behavior in vertebrate model systems. The concept of “zonular signalosome” is proposed, which highlights the close functional relationship between proteins of zonular junctions (zonulae occludentes and adhaerentes) and the control of cytoskeletal organization and signaling through Rho GTPases, transcription factors, and their effectors.

GSK3- and PRMT-1-dependent modifications of desmoplakin control desmoplakin-cytoskeleton dynamics

Intermediate filament (IF) attachment to intercellular junctions is required for skin and heart integrity, but how the strength and dynamics of this attachment are modulated during normal and pathological remodeling is poorly understood. We show that glycogen synthase kinase 3 (GSK3) and protein arginine methyltransferase 1 (PRMT-1) cooperate to orchestrate a series of posttranslational modifications on the IF-anchoring protein desmoplakin (DP) that play an essential role in coordinating cytoskeletal dynamics and cellular adhesion. Front-end electron transfer dissociation mass spectrometry analyses of DP revealed six novel serine phosphorylation sites dependent on GSK3 signaling and four novel arginine methylation sites including R2834, the mutation of which has been associated with arrhythmogenic cardiomyopathy (AC). Inhibition of GSK3 or PRMT-1 or overexpression of the AC-associated mutant R2834H enhanced DP-IF associations and delayed junction assembly. R2834H blocked the GSK3 phosphorylation cascade and reduced DP-GSK3 interactions in cultured keratinocytes and in the hearts of transgenic R2834H DP mice. Interference with this regulatory machinery may contribute to skin and heart diseases.

Gravitational Effects on Human Physiology

Physical working capacity decreases with age and also in microgravity. Regardless of age, increased physical activity can always improve the physical adaptability of the body, although the mechanisms of this adaptability are unknown. Physical exercise produces various mechanical stimuli in the body, and these stimuli may be essential for cell survival in organisms. The cytoskeleton plays an important role in maintaining cell shape and tension development, and in various molecular and/or cellular organelles involved in cellular trafficking. Both intra and extracellular stimuli send signals through the cytoskeleton to the nucleus and modulate gene expression via an intrinsic property, namely the "dynamic instability" of cytoskeletal proteins. αB-crystallin is an important chaperone for cytoskeletal proteins in muscle cells. Decreases in the levels of αB-crystallin are specifically associated with a marked decrease in muscle mass (atrophy) in a rat hindlimb suspension model that mimics muscle and bone atrophy that occurs in space and increases with passive stretch. Moreover, immunofluorescence data show complete co-localization of αB-crystallin and the tubulin/microtubule system in myoblast cells. This association was further confirmed in biochemical experiments carried out in vitro showing that αB-crystallin acts as a chaperone for heat-denatured tubulin and prevents microtubule disassembly induced by calcium. Physical activity induces the constitutive expression of αB-crystallin, which helps to maintain the homeostasis of cytoskeleton dynamics in response to gravitational forces. This relationship between chaperone expression levels and regulation of cytoskeletal dynamics observed in slow anti-gravitational muscles as well as in mammalian striated muscles, such as those in the heart, diaphragm and tongue, may have been especially essential for human evolution in particular. Elucidation of the intrinsic properties of the tubulin/microtubule and chaperone αB-crystallin protein complex systems is expected to provide valuable information for high-pressure bioscience and gravity health science.

Neuromuscular synapse integrity requires linkage of acetylcholine receptors to postsynaptic intermediate filament networks via rapsyn-plectin 1f complexes

P1f, a specific isoform of the cytolinker protein plectin, bridges AChRs to the desmin IF network of myofibers via direct interaction with the AChR-scaffolding protein rapsyn. P1f-mediated IF linkage is crucial for the formation and maintenance of AChR clusters, postsynaptic organization of the NMJ, and body locomotion.

Mutations in the cytolinker protein plectin lead to grossly distorted morphology of neuromuscular junctions (NMJs) in patients suffering from epidermolysis bullosa simplex (EBS)-muscular dystrophy (MS) with myasthenic syndrome (MyS). Here we investigated whether plectin contributes to the structural integrity of NMJs by linking them to the postsynaptic intermediate filament (IF) network. Live imaging of acetylcholine receptors (AChRs) in cultured myotubes differentiated ex vivo from immortalized plectin-deficient myoblasts revealed them to be highly mobile and unable to coalesce into stable clusters, in contrast to wild-type cells. We found plectin isoform 1f (P1f) to bridge AChRs and IFs via direct interaction with the AChR-scaffolding protein rapsyn in an isoform-specific manner; forced expression of P1f in plectin-deficient cells rescued both compromised AChR clustering and IF network anchoring. In conditional plectin knockout mice with gene disruption in muscle precursor/satellite cells (Pax7-Cre/cKO), uncoupling of AChRs from IFs was shown to lead to loss of postsynaptic membrane infoldings and disorganization of the NMJ microenvironment, including its invasion by microtubules. In their phenotypic behavior, mutant mice closely mimicked EBS-MD-MyS patients, including impaired body balance, severe muscle weakness, and reduced life span. Our study demonstrates that linkage to desmin IF networks via plectin is crucial for formation and maintenance of AChR clusters, postsynaptic NMJ organization, and body locomotion.

Actin-microtubule coordination at growing microtubule ends

To power dynamic processes in cells, the actin and microtubule cytoskeletons organize into complex structures. Although it is known that cytoskeletal coordination is vital for cell function, the mechanisms by which cross-linking proteins coordinate actin and microtubule activities remain poorly understood. In particular, it is unknown how the distinct mechanical properties of different actin architectures modulate the outcome of actin-microtubule interactions. To address this question, we engineered the protein TipAct, which links growing microtubule ends via end-binding proteins to actin filaments. We show that growing microtubules can be captured and guided by stiff actin bundles, leading to global actin-microtubule alignment. Conversely, growing microtubule ends can transport, stretch and bundle individual actin filaments, thereby globally defining actin filament organization. Our results provide a physical basis to understand actin-microtubule cross-talk, and reveal that a simple cross-linker can enable a mechanical feedback between actin and microtubule organization that is relevant to diverse biological contexts.

CLASP2 interacts with p120-catenin and governs microtubule dynamics at adherens junctions

The microtubule plus end–binding protein CLASP2 localizes to adherens junctions via direct interaction with p120-catenin and is required for adherens junction stability.

Classical cadherins and their connections with microtubules (MTs) are emerging as important determinants of cell adhesion. However, the functional relevance of such interactions and the molecular players that contribute to tissue architecture are still emerging. In this paper, we report that the MT plus end–binding protein CLASP2 localizes to adherens junctions (AJs) via direct interaction with p120-catenin (p120) in primary basal mouse keratinocytes. Reductions in the levels of p120 or CLASP2 decreased the localization of the other protein to cell–cell contacts and altered AJ dynamics and stability. These features were accompanied by decreased MT density and altered MT dynamics at intercellular junction sites. Interestingly, CLASP2 was enriched at the cortex of basal progenitor keratinocytes, in close localization to p120. Our findings suggest the existence of a new mechanism of MT targeting to AJs with potential functional implications in the maintenance of proper cell–cell adhesion in epidermal stem cells.

Microtubules CLASP to Adherens Junctions in epidermal progenitor cells

Cadherin-mediated cell adhesion at Adherens Junctions (AJs) and its dynamic connections with the microtubule (MT) cytoskeleton are important regulators of cellular architecture. However, the functional relevance of these interactions and the molecular players involved in different cellular contexts and cellular compartments are still not completely understood. Here, we comment on our recent findings showing that the MT plus-end binding protein CLASP2 interacts with the AJ component p120-catenin (p120) specifically in progenitor epidermal cells. Absence of either protein leads to alterations in MT dynamics and AJ functionality. These findings represent a novel mechanism of MT targeting to AJs that may be relevant for the maintenance of proper epidermal progenitor cell homeostasis. We also discuss the potential implication of other MT binding proteins previously associated to AJs in the wider context of epithelial tissues. We hypothesize the existence of adaptation mechanisms that regulate the formation and stability of AJs in different cellular contexts to allow the dynamic behavior of these complexes during tissue homeostasis and remodeling.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

Geometric control of vimentin intermediate filaments

Significant efforts have addressed the role of vimentin intermediate filaments (VIF) in cell motility, shape, adhesion and their connections to microfilaments (MF) and microtubules (MT). The present work uses micropatterned substrates to control the shapes of mouse fibroblasts and demonstrates that the cytoskeletal elements are dependent on each other and that unlike MF, VIF are globally controlled. For example, both square and circle shaped cells have a similar VIF distribution while MF distributions in these two shapes are quite different and depend on the curvature of the shape. Furthermore, in asymmetric and polarized shaped cells VIF avoid the sharp edges where MF are highly localized. Experiments with vimentin null mouse embryonic fibroblasts (MEFs) adherent to polarized (teardrop) and un-polarized (dumbbell) patterns show that the absence of VIF alters microtubule organization and perturbs cell polarity. The results of this study also demonstrate the utility of patterned substrates for quantitative studies of cytoskeleton organization in adherent cells.

Microtubule Actin Cross-linking Factor 1 regulates cardiomyocyte microtubule distribution and adaptation to hemodynamic overload

Aberrant cardiomyocyte microtubule growth is a feature of pressure overload induced cardiac hypertrophy believed to contribute to left ventricular (LV) dysfunction. Microtubule Actin Cross-linking Factor 1 (MACF1/Acf7) is a 600 kd spectraplakin that stabilizes and guides microtubule growth along actin filaments. MACF1 is expressed in the heart, but its impact on cardiac microtubules, and how this influences cardiac structure, function, and adaptation to hemodynamic overload is unknown. Here we used inducible cardiac-specific MACF1 knockout mice (MACF1 KO) to determine the impact of MACF1 on cardiac microtubules and adaptation to pressure overload (transverse aortic constriction (TAC).In adult mouse hearts, MACF1 expression was low under basal conditions, but increased significantly in response to TAC. While MACF1 KO had no observable effect on heart size or function under basal conditions, MACF1 KO exacerbated TAC induced LV hypertrophy, LV dilation and contractile dysfunction. Interestingly, subcellular fractionation of ventricular lysates revealed that MACF1 KO altered microtubule distribution in response to TAC, so that more tubulin was associated with the cell membrane fraction. Moreover, TAC induced microtubule redistribution into this cell membrane fraction in both WT and MACF1 KO mice correlated strikingly with the level of contractile dysfunction (r² = 0.786, p<.001). MACF1 disruption also resulted in reduction of membrane caveolin 3 levels, and increased levels of membrane PKCα and β1 integrin after TAC, suggesting MACF1 function is important for spatial regulation of several physiologically relevant signaling proteins during hypertrophy. Together, these data identify for the first time, a role for MACF1 in cardiomyocyte microtubule distribution and in adaptation to hemodynamic overload.