Evolutionary tuning of barbed end competition allows simultaneous construction of architecturally distinct actin structures.

How cells simultaneously assemble actin structures of distinct sizes, shapes, and filamentous architectures is still not well understood. Here, we used budding yeast as a model to investigate how competition for the barbed ends of actin filaments might influence this process. We found that while vertebrate capping protein (CapZ) and formins can simultaneously associate with barbed ends and catalyze each other's displacement, yeast capping protein (Cap1/2) poorly displaces both yeast and vertebrate formins. Consistent with these biochemical differences, in vivo formin-mediated actin cable assembly was strongly attenuated by the overexpression of CapZ but not Cap1/2. Multiwavelength live cell imaging further revealed that actin patches in cap2∆ cells acquire cable-like features over time, including recruitment of formins and tropomyosin. Together, our results suggest that the activities of S. cerevisiae Cap1/2 have been tuned across evolution to allow robust cable assembly by formins in the presence of high cytosolic levels of Cap1/2, which conversely limit patch growth and shield patches from formins.

Exosome-Delivered circSTAU2 Inhibits the Progression of Gastric Cancer by Targeting the miR-589/CAPZA1 Axis.

Background: Circular RNAs (circRNAs) are endogenous noncoding RNAs that play vital roles in many biological processes, particularly in human cancer. Recent studies indicate that circRNAs play an important role in tumor progression through exosomes. However, the specific functions of gastric cancer-derived exosomes and the role of circSTAU2 in gastric cancer (GC) remain largely unknown. Methods: Differentially expressed circRNAs in GC were identified by circRNA microarrays analysis and quantitative real-time polymerase chain reaction (qRT-PCR). The role of circSTAU2 in GC was verified by circSTAU2 knockdown and overexpression with functional assays both in vitro and in vivo. Fluorescence in situ hybridization (FISH), immunofluorescence, RNA immunoprecipitation (RIP), dual-luciferase reporter assay, qRT-PCR and Western blot were adopted to evaluate the expression and regulatory mechanism of MBNL1, circSTAU2, miR-589 and CAPZA1. Furthermore, the role of exosomes was demonstrated by transmission electron microscopy and nano-sight particle tracking analysis. Results: CircSTAU2, mainly localized in the cytoplasm, was significantly downregulated in GC. CircSTAU2 overexpression inhibited GC cell proliferation, invasion and migration both in vitro and in vivo, while circSTAU2 knockdown had the inverse effect. CircSTAU2 could be wrapped in exosomes and delivered to recipient cells, and functioned as a sponge for miR-589 to relieve its inhibitory effect on CAPZA1, thus inhibiting GC progression. Furthermore, MBNL1 acted as the upstream RNA-binding protein of circSTAU2 and significantly influenced the circularization and expression of circSTAU2. Conclusion: Exosome-delivered circSTAU2 may act as a tumor suppressor that restrains GC progression via miR-589/CAPZA1 axis, which demonstrates a potential therapeutic target for GC.

Genetic dissection of novel myopathy models reveals a role of CapZα and Leiomodin 3 during myofibril elongation.

Myofibrils within skeletal muscle are composed of sarcomeres that generate force by contraction when their myosin-rich thick filaments slide past actin-based thin filaments. Although mutations in components of the sarcomere are a major cause of human disease, the highly complex process of sarcomere assembly is not fully understood. Current models of thin filament assembly highlight a central role for filament capping proteins, which can be divided into three protein families, each ascribed with separate roles in thin filament assembly. CapZ proteins have been shown to bind the Z-disc protein α-actinin to form an anchoring complex for thin filaments and actin polymerisation. Subsequent thin filaments extension dynamics are thought to be facilitated by Leiomodins (Lmods) and thin filament assembly is concluded by Tropomodulins (Tmods) that specifically cap the pointed end of thin filaments. To study thin filament assembly in vivo, single and compound loss-of-function zebrafish mutants within distinct classes of capping proteins were analysed. The generated lmod3- and capza1b-deficient zebrafish exhibited aspects of the pathology caused by variations in their human orthologs. Although loss of the analysed main capping proteins of the skeletal muscle, capza1b, capza1a, lmod3 and tmod4, resulted in sarcomere defects, residual organised sarcomeres were formed within the assessed mutants, indicating that these proteins are not essential for the initial myofibril assembly. Furthermore, detected similarity and location of myofibril defects, apparent at the peripheral ends of myofibres of both Lmod3- and CapZα-deficient mutants, suggest a function in longitudinal myofibril growth for both proteins, which is molecularly distinct to the function of Tmod4.

Capping protein regulates endosomal trafficking by controlling F-actin density around endocytic vesicles and recruiting RAB5 effectors.

Actin filaments (F-actin) have been implicated in various steps of endosomal trafficking, and the length of F-actin is controlled by actin capping proteins, such as CapZ, which is a stable heterodimeric protein complex consisting of α and ß subunits. However, the role of these capping proteins in endosomal trafficking remains elusive. Here, we found that CapZ docks to endocytic vesicles via its C-terminal actin-binding motif. CapZ knockout significantly increases the F-actin density around immature early endosomes, and this impedes fusion between these vesicles, manifested by the accumulation of small endocytic vesicles in CapZ-knockout cells. CapZ also recruits several RAB5 effectors, such as Rabaptin-5 and Rabex-5, to RAB5-positive early endosomes via its N-terminal domain, and this further activates RAB5. Collectively, our results indicate that CapZ regulates endosomal trafficking by controlling actin density around early endosomes and recruiting RAB5 effectors.

Mechanosignaling pathways alter muscle structure and function by post-translational modification of existing sarcomeric proteins to optimize energy usage.

A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca2+ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.

Actin-binding protein profilin1 promotes aggressiveness of clear-cell renal cell carcinoma cells.

Clear-cell renal cell carcinoma (ccRCC), the most common subtype of renal cancer, has a poor clinical outcome. A hallmark of ccRCC is genetic loss-of-function of VHL (von Hippel-Lindau) that leads to a highly vascularized tumor microenvironment. Although many ccRCC patients initially respond to antiangiogenic therapies, virtually all develop progressive, drug-refractory disease. Given the role of dysregulated expressions of cytoskeletal and cytoskeleton-regulatory proteins in tumor progression, we performed analyses of The Cancer Genome Atlas (TCGA) transcriptome data for different classes of actin-binding proteins to demonstrate that increased mRNA expression of profilin1 (Pfn1), Arp3, cofilin1, Ena/VASP, and CapZ, is an indicator of poor prognosis in ccRCC. Focusing further on Pfn1, we performed immunohistochemistry-based classification of Pfn1 staining in tissue microarrays, which indicated Pfn1 positivity in both tumor and stromal cells; however, the vast majority of ccRCC tumors tend to be Pfn1-positive selectively in stromal cells only. This finding is further supported by evidence for dramatic transcriptional up-regulation of Pfn1 in tumor-associated vascular endothelial cells in the clinical specimens of ccRCC. In vitro studies support the importance of Pfn1 in proliferation and migration of RCC cells and in soluble Pfn1's involvement in vascular endothelial cell tumor cell cross-talk. Furthermore, proof-of-concept studies demonstrate that treatment with a novel computationally designed Pfn1-actin interaction inhibitor identified herein reduces proliferation and migration of RCC cells in vitro and RCC tumor growth in vivo Based on these findings, we propose a potentiating role for Pfn1 in promoting tumor cell aggressiveness in the setting of ccRCC.

Cytoskeleton regulators CAPZA2 and INF2 associate with CFTR to control its plasma membrane levels under EPAC1 activation.

Cystic Fibrosis (CF), the most common lethal autosomic recessive disorder among Caucasians, is caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein, a cAMP-regulated chloride channel expressed at the apical surface of epithelial cells. Cyclic AMP regulates both CFTR channel gating through a protein kinase A (PKA)-dependent process and plasma membane (PM) stability through activation of the exchange protein directly activated by cAMP1 (EPAC1). This cAMP effector, when activated promotes the NHERF1:CFTR interaction leading to an increase in CFTR at the PM by decreasing its endocytosis. Here, we used protein interaction profiling and bioinformatic analysis to identify proteins that interact with CFTR under EPAC1 activation as possible regulators of this CFTR PM anchoring. We identified an enrichment in cytoskeleton related proteins among which we characterized CAPZA2 and INF2 as regulators of CFTR trafficking to the PM. We found that CAPZA2 promotes wt-CFTR trafficking under EPAC1 activation at the PM whereas reduction of INF2 levels leads to a similar trafficking promotion effect. These results suggest that CAPZA2 is a positive regulator and INF2 a negative one for the increase of CFTR at the PM after an increase of cAMP and concomitant EPAC1 activation. Identifying the specific interactions involving CFTR and elicited by EPAC1 activation provides novel insights into late CFTR trafficking, insertion and/or stabilization at the PM and highlighs new potential therapeutic targets to tackle CF disease.

Comparative Analysis of CPI-Motif Regulation of Biochemical Functions of Actin Capping Protein.

The heterodimeric actin capping protein (CP) is regulated by a set of proteins that contain CP-interacting (CPI) motifs. Outside of the CPI motif, the sequences of these proteins are unrelated and distinct. The CPI motif and surrounding sequences are conserved within a given protein family, when compared to those of other CPI-motif protein families. Using biochemical assays with purified proteins, we compared the ability of CPI-motif-containing peptides from different protein families (a) to bind to CP, (b) to allosterically inhibit barbed-end capping by CP, and (c) to allosterically inhibit interaction of CP with V-1, another regulator of CP. We found large differences in potency among the different CPI-motif-containing peptides, and the different functional assays showed different orders of potency. These biochemical differences among the CPI-motif peptides presumably reflect interactions between CP and CPI-motif peptides involving amino acid residues that are conserved but are not part of the strictly defined consensus, as it was originally identified in comparisons of sequences of CPI motifs across all protein families [Hernandez-Valladares, M., et al. (2010) Structural characterization of a capping protein interaction motif defines a family of actin filament regulators. Nat. Struct. Mol. Biol. 17, 497-503; Bruck, S., et al. (2006) Identification of a Novel Inhibitory Actin-capping Protein Binding Motif in CD2-associated Protein. J. Biol. Chem. 281, 19196-19203]. These biochemical differences may be important for conserved distinct functions of CPI-motif protein families in cells with respect to the regulation of CP activity and actin assembly near membranes.

Cancer Stem-Cell Marker CD44v9-Positive Cells Arise From Helicobacter pylori-Infected CAPZA1-Overexpressing Cells.

BACKGROUND & AIMS: CD44 variant 9 (CD44v9)-positive cancer stem-like cells strongly contribute to the development and recurrence of gastric cancer. However, the origin of CD44v9-positive cells is uncertain. METHODS: CD44v9, ß-catenin, and epithelial splicing regulatory protein 1 signals were assessed by real-time reverse-transcription polymerase chain reaction, immunoblot analysis, or immunofluorescence microscopy. Capping actin protein of muscle Z-line α subunit 1 (CAPZA1) expression was assessed by immunoblot analysis or immunohistochemical analysis of Mongolian gerbils' gastric mucosa or human biopsy specimens. Levels of oxidative stress were assessed by measuring malondialdehyde and protein carbonylation. Histone H3 acetylation levels in the CAPZA1 proximal promoter region were measured by using chromatin immunoprecipitation analysis with an antibody against the acetylated histone H3 in human gastric carcinoma cell line (AGS) cells. RESULTS: CD44v9 is expressed in CAPZA1-overexpressing cells in human gastric cancer tissues. CAPZA1 overexpression enhanced expression of ß-catenin, which is a transcription factor for CD44, and epithelial splicing regulatory protein 1, which increases alternative splicing of CD44 to generate CD44v9. CAPZA1-overexpressing cells after cytotoxin-associated gene A accumulation showed CD44v9 expression by inducing nuclear accumulation of ß-catenin, concomitant with the enhancement of expression of Sal-like protein 4 and Krüppel-like factor 5, which encode reprogramming factors. Oxidative stress increased the CAPZA1 expression in AGS cells through the enhancement of histone H3 acetylation of CAPZA1 promoter. CAPZA1 expression was increased depending on oxidative stress in H pylori-infected gastric mucosa. CONCLUSIONS: CD44v9 expression is evoked from CAPZA1-overexpressing cells after accumulation of cytotoxin-associated gene A. Our findings provide important insights into the mechanisms underlying the development of CD44v9-positive cells.

F-actin dynamics regulates mammalian organ growth and cell fate maintenance.

BACKGROUND & AIMS: In vitro, cell function can be potently regulated by the mechanical properties of cells and of their microenvironment. Cells measure these features by developing forces via their actomyosin cytoskeleton, and respond accordingly by regulating intracellular pathways, including the transcriptional coactivators YAP/TAZ. Whether mechanical cues are relevant for in vivo regulation of adult organ homeostasis, and whether this occurs through YAP/TAZ, remains largely unaddressed. METHODS: We developed Capzb conditional knockout mice and obtained primary fibroblasts to characterize the role of CAPZ in vitro. In vivo functional analyses were carried out by inducing Capzb inactivation in adult hepatocytes, manipulating YAP/Hippo activity by hydrodynamic tail vein injections, and treating mice with the ROCK inhibitor, fasudil. RESULTS: We found that the F-actin capping protein CAPZ restrains actomyosin contractility: Capzb inactivation alters stress fiber and focal adhesion dynamics leading to enhanced myosin activity, increased traction forces, and increased liver stiffness. In vitro, this rescues YAP from inhibition by a small cellular geometry; in vivo, it induces YAP activation in parallel to the Hippo pathway, causing extensive hepatocyte proliferation and leading to striking organ overgrowth. Moreover, Capzb is required for the maintenance of the differentiated hepatocyte state, for metabolic zonation, and for gluconeogenesis. In keeping with changes in tissue mechanics, inhibition of the contractility regulator ROCK, or deletion of the Yap1 mechanotransducer, reverse the phenotypes emerging in Capzb-null livers. CONCLUSIONS: These results indicate a previously unsuspected role for CAPZ in tuning the mechanical properties of cells and tissues, which is required in hepatocytes for the maintenance of the differentiated state and to regulate organ size. More generally, it indicates for the first time that mechanotransduction has a physiological role in maintaining liver homeostasis in mammals. LAY SUMMARY: The mechanical properties of cells and tissues (i.e. whether they are soft or stiff) are thought to be important regulators of cell behavior. Herein, we found that inactivation of the protein CAPZ alters the mechanical properties of cells and liver tissues, leading to YAP hyperactivation. In turn, this profoundly alters liver physiology, causing organ overgrowth, defects in liver cell differentiation and metabolism. These results reveal a previously uncharacterized role for mechanical signals in the maintenance of adult liver homeostasis.

Hypoxia induces actin cytoskeleton remodeling by regulating the binding of CAPZA1 to F-actin via PIP2 to drive EMT in hepatocellular carcinoma.

Studies have shown that hypoxia can induce cytoskeletal injury and remodeling through the activation of the RhoA/ROCK signaling pathway by hypoxia-inducible factor-1α (HIF-1α). Our previous study confirmed that CAPZA1 can modulate EMT by regulating actin cytoskeleton remodeling. However, the relationship between HIF-1α and CAPZA1 has not been illustrated. Therefore, this study aimed to investigate the mechanism by which hypoxia induces the remodeling of the actin cytoskeleton by regulating CAPZA1 in hepatocellular carcinoma (HCC) cells. In the present study, we showed that the low expression of CAPZA1 promotes HCC cell invasion and migration in vitro and in vivo by regulating actin cytoskeleton remodeling to drive EMT. Furthermore, we found that the combination of PIP2 and CAPZA1 enables CAPZA1 to be released from the barbed end of F-actin, which in turn drives the remodeling of the actin cytoskeleton. Finally, we confirmed that hypoxia increases PIP2 levels and its binding to CAPZA1 in HCC cells via the HIF-1α/RhoA/ROCK1 pathway. Thus, CAPZA1 and PIP2 could be therapeutic targets to inhibit the invasion and migration promoted by hypoxia in HCC cells.

CapZ integrates several signaling pathways in response to mechanical stiffness.

Muscle adaptation is a response to physiological demand elicited by changes in mechanical load, hormones, or metabolic stress. Cytoskeletal remodeling processes in many cell types are thought to be primarily regulated by thin filament formation due to actin-binding accessory proteins, such as the actin-capping protein. Here, we hypothesize that in muscle, the actin-capping protein (named CapZ) integrates signaling by a variety of pathways, including phosphorylation and phosphatidylinositol 4,5-bisphosphate (PIP2) binding, to regulate muscle fiber growth in response to mechanical load. To test this hypothesis, we assess mechanotransduction signaling that regulates muscle growth using neonatal rat ventricular myocytes cultured on substrates with the stiffness of the healthy myocardium (10 kPa), fibrotic myocardium (100 kPa), or glass. We investigate how PIP2 signaling affects CapZ using the PIP2 sequestering agent neomycin and the effect of PKC-mediated CapZ phosphorylation using the PKC-activating drug phorbol 12-myristate 13-acetate (PMA). Molecular simulations suggest that close interactions between PIP2 and the ß-tentacle of CapZ are modified by phosphorylation at T267. Fluorescence recovery after photobleaching (FRAP) demonstrates that the kinetic binding constant of CapZ to sarcomeric thin filaments in living muscle cells increases with stiffness or PMA treatment but is diminished by PIP2 reduction. Furthermore, CapZ with a deletion of the ß-tentacle that lacks the phosphorylation site T267 shows increased FRAP kinetics with lack of sensitivity to PMA treatment or PIP2 reduction. Förster resonance energy transfer (FRET) probes the molecular interactions between PIP2 and CapZ, which are decreased by PIP2 availability or by the ß-tentacle truncation. These data suggest that CapZ is bound to actin tightly in the idle, locked state, with little phosphorylation or PIP2 binding. However, this tight binding is loosened in growth states triggered by mechanical stimuli such as substrate stiffness, which may have relevance to fibrotic heart disease.

Force-history dependence and cyclic mechanical reinforcement of actin filaments at the single molecular level.

The actin cytoskeleton is subjected to dynamic mechanical forces over time and the history of force loading may serve as mechanical preconditioning. While the actin cytoskeleton is known to be mechanosensitive, the mechanisms underlying force regulation of actin dynamics still need to be elucidated. Here, we investigated actin depolymerization under a range of dynamic tensile forces using atomic force microscopy. Mechanical loading by cyclic tensile forces induced significantly enhanced bond lifetimes and different force-loading histories resulted in different dissociation kinetics in G-actin-G-actin and G-actin-F-actin interactions. Actin subunits at the two ends of filaments formed bonds with distinct kinetics under dynamic force, with cyclic mechanical reinforcement more effective at the pointed end compared to that at the barbed end. Our data demonstrate force-history dependent reinforcement in actin-actin bonds and polarity of the actin depolymerization kinetics under cyclic tensile forces. These properties of actin may be important clues to understanding regulatory mechanisms underlying actin-dependent mechanotransduction and mechanosensitive cytoskeletal dynamics.This article has an associated First Person interview with the first author of the paper.

CAPZA1 determines the risk of gastric carcinogenesis by inhibiting Helicobacter pylori CagA-degraded autophagy.

Helicobacter pylori-derived CagA, a type IV secretion system effector, plays a role as an oncogenic driver in gastric epithelial cells. However, upon delivery into gastric epithelial cells, CagA is usually degraded by macroautophagy/autophagy. Hence, the induction of autophagy in H. pylori-infected epithelial cells is an important host-protective ability against gastric carcinogenesis. However, the mechanisms by which autophagosome-lysosome fusion is regulated, are unknown. Here, we report that enhancement of LAMP1 (lysosomal associated membrane protein 1) expression is necessary for autolysosome formation. LAMP1 expression is induced by nuclear translocated LRP1 (LDL receptor related protein 1) intracellular domain (LRP1-ICD) binding to the proximal LAMP1 promoter region. Nuclear translocation of LRP1-ICD is enhanced by H. pylori infection. In contrast, CAPZA1 (capping actin protein of muscle Z-line alpha subunit 1) inhibits LAMP1 expression via binding to LRP1-ICD in the nuclei. The binding of CAPZA1 to LRP1-ICD prevents LRP1-ICD binding to the LAMP1 proximal promoter. Thus, in CAPZA1-overexpressing gastric epithelial cells infected with H. pylori, autolysosome formation is inhibited and CagA escapes autophagic degradation. These findings identify CAPZA1 as a novel negative regulator of autolysosome formation and suggest that deregulation of CAPZA1 expression leads to increased risk of gastric carcinogenesis. Abbreviations: CagA: cytotoxin-associated gene A; CAPZA1: capping actin protein of muscle Z-line alpha subunit 1; ChIP: chromatin immunoprecipitation; GTF2I: general transcription factor IIi; HDAC: histone deacetylase; LAMP1: lysosomal associated membrane protein 1; LRP1: LDL receptor related protein 1; LRP1-ICD: CagA intracellular domain; qPCR: quantitative polymerase chain reaction; VacA: vacuolating cytotoxin.

Contact inhibition controls cell survival and proliferation via YAP/TAZ-autophagy axis.

Contact inhibition enables noncancerous cells to cease proliferation and growth when they contact each other. This characteristic is lost when cells undergo malignant transformation, leading to uncontrolled proliferation and solid tumor formation. Here we report that autophagy is compromised in contact-inhibited cells in 2D or 3D-soft extracellular matrix cultures. In such cells, YAP/TAZ fail to co-transcriptionally regulate the expression of myosin-II genes, resulting in the loss of F-actin stress fibers, which impairs autophagosome formation. The decreased proliferation resulting from contact inhibition is partly autophagy-dependent, as is their increased sensitivity to hypoxia and glucose starvation. These findings define how mechanically repressed YAP/TAZ activity impacts autophagy to contribute to core phenotypes resulting from high cell confluence that are lost in various cancers.

PKC epsilon signaling effect on actin assembly is diminished in cardiomyocytes when challenged to additional work in a stiff microenvironment.

The stiffness of the microenvironment surrounding a cell can result in cytoskeletal remodeling, leading to altered cell function and tissue macrostructure. In this study, we tuned the stiffness of the underlying substratum on which neonatal rat cardiomyocytes were grown in culture to mimic normal (10 kPa), pathological stiffness of fibrotic myocardium (100 kPa), and a nonphysiological extreme (glass). Cardiomyocytes were then challenged by beta adrenergic stimulation through isoproterenol treatment to investigate the response to acute work demand for cells grown on surfaces of varying stiffness. In particular, the PKCɛ signaling pathway and its role in actin assembly dynamics were examined. Significant changes in contractile metrics were seen on cardiomyocytes grown on different surfaces, but all cells responded to isoproterenol treatment, eventually reaching similar time to peak tension. In contrast, the assembly rate of actin was significantly higher on stiff surfaces, so that only cells grown on soft surfaces were able to respond to acute isoproterenol treatment. Förster Resonance Energy Transfer of immunofluorescence on the cytoskeletal fraction of cardiomyocytes confirmed that the molecular interaction of PKCɛ with the actin capping protein, CapZ, was very low on soft substrata but significantly increased with isoproterenol treatment, or on stiff substrata. Therefore, the stiffness of the culture surface chosen for in vitro experiments might mask the normal signaling and affect the ability to translate basic science more effectively into human therapy.

SPATC1L maintains the integrity of the sperm head-tail junction.

Spermatogenesis is a tightly regulated process involving germ cell-specific and germ cell-predominant genes. Here we investigate a novel germ cell-specific gene, Spatc1l (spermatogenesis and centriole associated 1 like). Expression analyses show that SPATC1L is expressed in mouse and human testes. We find that mouse SPATC1L localizes to the neck region in testicular sperm. Moreover, SPATC1L associates with the regulatory subunit of protein kinase A (PKA). Using CRISPR/Cas9-mediated genome engineering, we generate mice lacking SPATC1L. Disruption of Spatc1l in mice leads to male sterility owing to separation of sperm heads from tails. The lack of SPATC1L is associated with a reduction in PKA activity in testicular sperm, and we identify capping protein muscle Z-line beta as a candidate target of phosphorylation by PKA in testis. Taken together, our results implicate the SPATC1L-PKA complex in maintaining the stability of the sperm head-tail junction, thereby revealing a new molecular basis for sperm head-tail integrity.

Mutation of CD2AP and SH3KBP1 Binding Motif in Alphavirus nsP3 Hypervariable Domain Results in Attenuated Virus.

Infection by Chikungunya virus (CHIKV) of the Old World alphaviruses (family Togaviridae) in humans can cause arthritis and arthralgia. The virus encodes four non-structural proteins (nsP) (nsP1, nsp2, nsP3 and nsP4) that act as subunits of the virus replicase. These proteins also interact with numerous host proteins and some crucial interactions are mediated by the unstructured C-terminal hypervariable domain (HVD) of nsP3. In this study, a human cell line expressing EGFP tagged with CHIKV nsP3 HVD was established. Using quantitative proteomics, it was found that CHIKV nsP3 HVD can bind cytoskeletal proteins, including CD2AP, SH3KBP1, CAPZA1, CAPZA2 and CAPZB. The interaction with CD2AP was found to be most evident; its binding site was mapped to the second SH3 ligand-like element in nsP3 HVD. Further assessment indicated that CD2AP can bind to nsP3 HVDs of many different New and Old World alphaviruses. Mutation of the short binding element hampered the ability of the virus to establish infection. The mutation also abolished ability of CD2AP to co-localise with nsP3 and replication complexes of CHIKV; the same was observed for Semliki Forest virus (SFV) harbouring a similar mutation. Similar to CD2AP, its homolog SH3KBP1 also bound the identified motif in CHIKV and SFV nsP3.

Heterodimeric capping protein is required for stereocilia length and width regulation.

Control of the dimensions of actin-rich processes like filopodia, lamellipodia, microvilli, and stereocilia requires the coordinated activity of many proteins. Each of these actin structures relies on heterodimeric capping protein (CAPZ), which blocks actin polymerization at barbed ends. Because dimension control of the inner ear's stereocilia is particularly precise, we studied the CAPZB subunit in hair cells. CAPZB, present at ∼100 copies per stereocilium, concentrated at stereocilia tips as hair cell development progressed, similar to the CAPZB-interacting protein TWF2. We deleted Capzb specifically in hair cells using Atoh1-Cre, which eliminated auditory and vestibular function. Capzb-null stereocilia initially developed normally but later shortened and disappeared; surprisingly, stereocilia width decreased concomitantly with length. CAPZB2 expressed by in utero electroporation prevented normal elongation of vestibular stereocilia and irregularly widened them. Together, these results suggest that capping protein participates in stereocilia widening by preventing newly elongating actin filaments from depolymerizing.

An RNAi Screen in a Novel Model of Oriented Divisions Identifies the Actin-Capping Protein Z ß as an Essential Regulator of Spindle Orientation.

Oriented cell divisions are controlled by a conserved molecular cascade involving Gαi, LGN, and NuMA. We developed a new cellular model of oriented cell divisions combining micropatterning and localized recruitment of Gαi and performed an RNAi screen for regulators acting downstream of Gαi. Remarkably, this screen revealed a unique subset of dynein regulators as being essential for spindle orientation, shedding light on a core regulatory aspect of oriented divisions. We further analyze the involvement of one novel regulator, the actin-capping protein CAPZB. Mechanistically, we show that CAPZB controls spindle orientation independently of its classical role in the actin cytoskeleton by regulating the assembly, stability, and motor activity of the dynein/dynactin complex at the cell cortex, as well as the dynamics of mitotic microtubules. Finally, we show that CAPZB controls planar divisions in vivo in the developing neuroepithelium. This demonstrates the power of this in cellulo model of oriented cell divisions to uncover new genes required in spindle orientation in vertebrates.