Requirement of phosphatidylinositol 4,5-bisphosphate for alpha-actinin function

Defects in integrin complex formation promote CHKB-mediated muscular dystrophy

Phosphatidylcholine (PC) is the major membrane phospholipid in most eukaryotic cells. Bi-allelic loss of function variants in CHKB, encoding the first step in the synthesis of PC, is the cause of a rostrocaudal muscular dystrophy in both humans and mice. Loss of sarcolemma integrity is a hallmark of muscular dystrophies; however, how this occurs in the absence of choline kinase function is not known. We determine that in Chkb -/- mice there is a failure of the α7β1 integrin complex that is specific to affected muscle. We observed that in Chkb -/- hindlimb muscles there is a decrease in sarcolemma association/abundance of the PI(4,5)P2 binding integrin complex proteins vinculin, and α-actinin, and a decrease in actin association with the sarcolemma. In cells, pharmacological inhibition of choline kinase activity results in internalization of a fluorescent PI(4,5)P2 reporter from discrete plasma membrane clusters at the cell surface membrane to cytosol, this corresponds with a decreased vinculin localization at plasma membrane focal adhesions that was rescued by overexpression of CHKB.

Chlamydia trachomatis Subverts Alpha-Actinins To Stabilize Its Inclusion

Chlamydia trachomatis is the leading cause of sexually transmitted bacterial disease and a global health burden. As an obligate intracellular pathogen, Chlamydia has evolved many strategies to manipulate its host and establish its intracellular niche called the inclusion. C. trachomatis reorganizes the host actin cytoskeleton to form scaffolds around the inclusion and reinforce the growing inclusion membrane. To control the kinetics and formation of actin scaffolds, Chlamydia expresses the effector InaC/CT813, which activates the host GTPase RhoA. Here, we have discovered that InaC stabilizes actin scaffolds through the host actin cross-linking proteins α-actinins 1 and 4. We demonstrate that α-actinins are recruited to the inclusion membrane in an InaC-dependent manner and associate with actin scaffolds that envelop the inclusion. Small interfering RNA (siRNA)-mediated knockdown of α-actinins differentially regulate the frequency of actin scaffolds and impair inclusion stability, leaving them susceptible to rupture and to nonionic detergent extraction. Overall, our data identify new host effectors that are subverted by InaC to stabilize actin scaffolds, highlighting the versatility of InaC as a key regulator of the host cytoskeletal network during Chlamydia infection. IMPORTANCE Despite antibiotics, recurrent C. trachomatis infections cause significant damage to the genital tract in men and women. Without a preventative vaccine, it is paramount to understand the virulence mechanisms that Chlamydia employs to cause disease. In this context, manipulation of the host cytoskeleton is a critical component of Chlamydia development. Actin scaffolds reinforce the integrity of Chlamydia's infectious vacuole, which is a critical barrier between Chlamydia and the host environment. Having previously established that InaC co-opts RhoA to promote the formation of actin scaffolds around the inclusion, we now show that Chlamydia hijacks a new class of host effectors, α-actinins, to cross-link these scaffolds and further stabilize the inclusion. We also establish that a core function of the chlamydial effector InaC is the regulation of cytoskeletal stability during Chlamydia infection. Ultimately, this work expands our understanding of how bacterial pathogens subvert the actin cytoskeleton by targeting fundamental host effector proteins.

Getting cells into shape by calcium-dependent actin cross-linking proteins

The actin cytoskeleton represents a highly dynamic filament system providing cell structure and mechanical forces to drive a variety of cellular processes. The dynamics of the actin cytoskeleton are controlled by a number of conserved proteins that maintain the pool of actin monomers, promote actin nucleation, restrict the length of actin filaments and cross-link filaments into networks or bundles. Previous work has been established that cytoplasmic calcium is an important signal to rapidly relay information to the actin cytoskeleton, but the underlying mechanisms remain poorly understood. Here, we summarize new recent perspectives on how calcium fluxes are transduced to the actin cytoskeleton in a physiological context. In this mini-review we will focus on three calcium-binding EF-hand-containing actin cross-linking proteins, α-actinin, plastin and EFHD2/Swiprosin-1, and how these conserved proteins affect the cell's actin reorganization in the context of cell migration and wound closure in response to calcium.

Plasma membrane phosphatidylinositol (4,5)-bisphosphate is critical for determination of epithelial characteristics

Epithelial cells provide cell-cell adhesion that is essential to maintain the integrity of multicellular organisms. Epithelial cell-characterizing proteins, such as epithelial junctional proteins and transcription factors are well defined. However, the role of lipids in epithelial characterization remains poorly understood. Here we show that the phospholipid phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂] is enriched in the plasma membrane (PM) of epithelial cells. Epithelial cells lose their characteristics upon depletion of PM PI(4,5)P₂, and synthesis of PI(4,5)P₂ in the PM results in the development of epithelial-like morphology in osteosarcoma cells. PM localization of PARD3 is impaired by depletion of PM PI(4,5)P₂ in epithelial cells, whereas expression of the PM-targeting exocyst-docking region of PARD3 induces osteosarcoma cells to show epithelial-like morphological changes, suggesting that PI(4,5)P₂ regulates epithelial characteristics by recruiting PARD3 to the PM. These results indicate that a high level of PM PI(4,5)P₂ plays a crucial role in the maintenance of epithelial characteristics.

Epithelial cells provide cell-cell adhesion to maintain the integrity of multicellular organisms. Here the authors show that phospholipid phosphatidylinositol (4,5)-bisphosphate is critical for the maintenance of epithelial characteristics.

Implication of Ceramide Kinase/C1P in Cancer Development and Progression

Cancer cells rewire their metabolic programs to favor biological processes that promote cell survival, proliferation, and dissemination. Among this relevant reprogramming, sphingolipid metabolism provides metabolites that can favor or oppose these hallmarks of cancer. The sphingolipid ceramide 1-phosphate (C1P) and the enzyme responsible for its biosynthesis, ceramide kinase (CERK), are well established regulators of cell growth and survival in normal, as well as malignant cells through stress-regulated signaling pathways. This metabolite also promotes cell survival, which has been associated with the feedback regulation of other antitumoral sphingolipids or second messengers. C1P also regulates cancer cell invasion and migration of different types of cancer, including lung, breast, pancreas, prostate, or leukemia cells. More recently, CERK and C1P have been implicated in the control of inflammatory responses. The present review provides an updated view on the important role of CERK/C1P in the regulation of cancer cell growth, survival, and dissemination.

Mechanobiology of muscle and myofibril morphogenesis

Muscles generate forces for animal locomotion. The contractile apparatus of muscles is the sarcomere, a highly regular array of large actin and myosin filaments linked by gigantic titin springs. During muscle development many sarcomeres assemble in series into long periodic myofibrils that mechanically connect the attached skeleton elements. Thus, ATP-driven myosin forces can power movement of the skeleton. Here we review muscle and myofibril morphogenesis, with a particular focus on their mechanobiology. We describe recent progress on the molecular structure of sarcomeres and their mechanical connections to the skeleton. We discuss current models predicting how tension coordinates the assembly of key sarcomeric components to periodic myofibrils that then further mature during development. This requires transcriptional feedback mechanisms that may help to coordinate myofibril assembly and maturation states with the transcriptional program. To fuel the varying energy demands of muscles we also discuss the close mechanical interactions of myofibrils with mitochondria and nuclei to optimally support powerful or enduring muscle fibers.

Signaling Enzymes and Ion Channels Being Modulated by the Actin Cytoskeleton at the Plasma Membrane

A cell should deal with the changing external environment or the neighboring cells. Inevitably, the cell surface receives and transduces a number of signals to produce apt responses. Typically, cell surface receptors are activated, and during this process, the subplasmalemmal actin cytoskeleton is often rearranged. An intriguing point is that some signaling enzymes and ion channels are physically associated with the actin cytoskeleton, raising the possibility that the subtle changes of the local actin cytoskeleton can, in turn, modulate the activities of these proteins. In this study, we reviewed the early and new experimental evidence supporting the notion of actin-regulated enzyme and ion channel activities in various cell types including the cells of immune response, neurons, oocytes, hepatocytes, and epithelial cells, with a special emphasis on the Ca²⁺ signaling pathway that depends on the synthesis of inositol 1,4,5-trisphosphate. Some of the features that are commonly found in diverse cells from a wide spectrum of the animal species suggest that fine-tuning of the activities of the enzymes and ion channels by the actin cytoskeleton may be an important strategy to inhibit or enhance the function of these signaling proteins.

A novel frameshift ACTN2 variant causes a rare adult-onset distal myopathy with multi-minicores

Introduction

Distal myopathies are a group of rare muscle disorders characterized by selective or predominant weakness in the feet and/or hands. In 2019, ACTN2 gene was firstly identified to be a cause of a new adult‐onset distal muscular dystrophy calling actininopathy and another distinctly different myopathy, named multiple structured core disease (MsCD). Thus, the various phenotypes and limited mutations in ACTN2‐related myopathy make the genotype‐phenotype correlation hard to understand.

Aims

To investigate the clinical features and histological findings in a Chinese family with distal myopathy. Whole exome sequencing and several functional studies were performed to explore the pathogenesis of the disease.

Results

We firstly identified a novel frameshift variant (c.2504delT, p.Phe835Serfs*66) within ACTN2 in a family including three patients. The patients exhibited adult‐onset distal myopathy with multi‐minicores, which, interestingly, was more like a combination of MsCD and actininopathy. Moreover, functional analysis using muscle samples revealed that the variant significantly increased the expression level of α‐actinin‐2 and resulted in abnormal Z‐line organization of muscle fiber. Vitro studies suggested aggregate formations might be involved in the pathogenesis of the disease.

Conclusion

Our results expanded the phenotypes of ACTN2‐related myopathy and provided helpful information to clarify the molecular mechanisms.

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.

Calcium modulates the domain flexibility and function of an α-actinin similar to the ancestral α-actinin

The actin cytoskeleton, a dynamic network of actin filaments and associated F-actin-binding proteins, is fundamentally important in eukaryotes. α-Actinins are major F-actin bundlers that are inhibited by Ca²⁺ in nonmuscle cells. Here we report the mechanism of Ca²⁺-mediated regulation of Entamoeba histolytica α-actinin-2 (EhActn2) with features expected for the common ancestor of Entamoeba and higher eukaryotic α-actinins. Crystal structures of Ca²⁺-free and Ca²⁺-bound EhActn2 reveal a calmodulin-like domain (CaMD) uniquely inserted within the rod domain. Integrative studies reveal an exceptionally high affinity of the EhActn2 CaMD for Ca²⁺, binding of which can only be regulated in the presence of physiological concentrations of Mg²⁺ Ca²⁺ binding triggers an increase in protein multidomain rigidity, reducing conformational flexibility of F-actin-binding domains via interdomain cross-talk and consequently inhibiting F-actin bundling. In vivo studies uncover that EhActn2 plays an important role in phagocytic cup formation and might constitute a new drug target for amoebic dysentery.

Phosphatidylinositol 4,5-bisphosphate is localized in the plasma membrane outer leaflet and regulates cell adhesion and motility

Phospholipids are distributed asymmetrically in the plasma membrane (PM) of mammalian cells. Phosphatidylinositol (PI) and its phosphorylated forms are primarily located in the inner leaflet of the PM. Among them, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is a well-known substrate for phospholipase C (PLC) or phosphoinositide-3 kinase, and is also a regulator for the actin cytoskeleton or ion channels. Although functions of PI(4,5)P₂ in the inner leaflet are well characterized, those in the outer leaflet are poorly understood. Here, PI(4,5)P₂ was detected in the cell surface of non-permeabilized cells by anti-PI(4,5)P₂ antibodies and the pleckstrin-homology (PH) domain of PLCδ1 that specifically binds PI(4,5)P₂. Cell surface PI(4,5)P₂ signal was universally detected in various cell lines and freshly isolated mouse bone marrow cells and showed a punctate pattern in a cholesterol, sphingomyelin, and actin polymerization-dependent manner. Furthermore, blocking cell surface PI(4,5)P₂ by the addition of anti-PI(4,5)P₂ antibody or the PH domain of PLCδ1 inhibited cell attachment, spreading, and migration. Taken together, these results indicate a unique localization of PI(4,5)P₂ in the outer leaflet that may have a crucial role in cell attachment, spreading, and migration.

miRNA-mRNA crosstalk in myocardial ischemia induced by calcified aortic valve stenosis

Aortic valve stenosis is the most common cause of morbidity and mortality in valvular heart disease in aged people. Both microRNA (miRNA) and mRNA are potential targets for the diagnosis and therapeutic intervention of myocardial ischemia induced by calcified aortic valve stenosis (CAVS), with unclear mechanisms. Here, 3 gene expression profiles of 47 male participants were applied to generate shared differentially expressed genes (DEGs) with significant major biological functions. Moreover, 20 hub genes were generated by a Weighted Genes Co-Expression Network Analysis (WGCNA) and were cross-linked to miRNA based on miRanda/miRwalk2 databases. Integrated miRNA/mRNA analysis identified several novel miRNAs and targeted genes as diagnostic/prognostic biomarkers or therapeutic targets in CAVS patients. In addition, the clinical data suggested that myocardial hypertrophy and myocardial ischemia in CAVS patients are likely associated with hub genes and the upstream regulatory miRNAs. Together, our data provide evidence that miRNAs and their targeted genes play an important role in the pathogenesis of myocardial hypertrophy and ischemia in patients with CAVS.

Cytoskeleton and Nucleotide Signaling in Glioma C6 Cells

This chapter describes signaling pathways, stimulated by the P2Y₂ nucleotide receptor (P2Y₂R), that regulate cellular processes dependent on actin cytoskeleton dynamics in glioma C6 cells. P2Y₂R coupled with G-proteins, in response to ATP or UTP, regulates the level of iphosphatidylinositol-4,5-bisphosphate (PIP₂) which modulates a variety of actin binding proteins and is involved in calcium response and activates Rac1 and RhoA proteins. The RhoA/ROCK signaling pathway plays an important role in contractile force generation needed for the assembly of stress fibers, focal adhesions and for tail retraction during cell migration. Blocking of this pathway by a specific Rho-kinase inhibitor induces changes in F-actin organization and cell shape and decreases the level of phosphorylated myosin II and cofilin. In glioma C6 cells these changes are reversed after UTP stimulation of P2Y₂R. Signaling pathways responsible for this compensation are calcium signaling which regulates MLC kinase activation via calmodulin, and the Rac1/PAK/LIMK cascade. Stimulation of the Rac1 mediated pathway via G_o proteins needs additional interaction between α_vβ₅ integrins and P2Y₂Rs. Calcium free medium, or growing of the cells in suspension, prevents Gα_o activation by P2Y₂ receptors. Rac1 activation is necessary for cofilin phosphorylation as well as integrin activation needed for focal complexes formation and stabilization of lamellipodium. Inhibition of positive Rac1 regulation prevents glioma C6 cells from recovery of control cell like morphology.

Phosphoinositide phosphatases in cancer cell dynamics-Beyond PI3K and PTEN

Phosphoinositides are a group of lipids that regulate intracellular signaling and subcellular biological events. The signaling by phosphatidylinositol-3,4,5-trisphosphate and Akt mediates the action of growth factors that are essential for cell proliferation, gene transcription, cell migration, and polarity. The hyperactivation of this signaling has been identified in different cancer cells; and, it has been implicated in oncogenic transformation and cancer cell malignancy. Recent studies have argued the role of phosphoinositides in cancer cell dynamics, including actin cytoskeletal rearrangement at the plasma membrane and the organization of intracellular compartments. The focus of this review is to summarize the impact of the activities of phosphoinositide phosphatases on intracellular signaling related to cancer cell dynamics and to discuss how the abnormalities in the activities of the enzymes alter the levels of phosphoinositides in cancer cells.

Regulation of actin assembly by PI(4,5)P2 and other inositol phospholipids: An update on possible mechanisms

Actin cytoskeleton dynamics depend on a tight regulation of actin filament formation from an intracellular pool of monomers, followed by their linkage to each other or to cell membranes, followed by their depolymerization into a fresh pool of actin monomers. The ubiquitous requirement for continuous actin remodeling that is necessary for many cellular functions is orchestrated in large part by actin binding proteins whose affinity for actin is altered by inositol phospholipids, most prominently PI(4,5)P2 (phosphatidylinositol 4,5-bisphosphate). The kinetics of PI(4,5)P2 synthesis and hydrolysis, its lateral distribution within the lipid bilayer, and coincident detection of PI(4,5)P2 and another signal, all play a role in determining when and where a particular PI(4,5)P2-regulated protein is inactivated or activated to exert its effect on the actin cytoskeleton. This review summarizes a range of models that have been developed to explain how PI(4,5)P2 might function in the complex chemical and structural environment of the cell based on a combination of experiment and computational studies.

ATP competes with PIP2 for binding to gelsolin

Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP2) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP2 leading to filament elongation. The process by which gelsolin is removed from PIP2 at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP2-gelsolin complex dynamics. Fluorophore-labeled PIP2 and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP2-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP2 for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP2-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP2 to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.

Microparticles in Hematological Malignancies: Role in Coagulopathy and Tumor Pathogenesis

Microparticles (MP) are submicron vesicles released from various cells in response to activation, injury or apoptosis. They contain different structural and functional proteins and RNAs, which contribute to physiological intercellular "crosstalk" and to the pathogenesis of various diseases including cancer. In hematological malignancies, these MPs participate in the initiation and propagation of thrombosis through different pathways. They have a role in the angiogenesis, malignant cell survival and metastasis. MPs act as a mediator of resistance of leukemic cells to chemotherapy. The number of MPs is one of the prognostic factors following stem cell transplant, and studies have also found they contribute to the pathogenesis of graft versus host disease. MPs are being tested as therapeutic options in leukemias and graft versus host disease. Future studies should help us understand the interactions between MPs and cancer cells better, thereby opening new approaches for treatment of hematological malignancies.

Molecular dissection of the actin-binding ability of the fission yeast α-actinin, Ain1, in vitro and in vivo

A contractile ring (CR) is involved in cytokinesis in animal and yeast cells. Although several types of actin-bundling proteins associate with F-actin in the CR, their individual roles in the CR have not yet been elucidated in detail. Ain1 is the sole α-actinin homologue in the fission yeast Schizosaccharomyces pombe and specifically localizes to the CR with a high turnover rate. S. pombe cells lacking the ain1+ gene show defects in cytokinesis under stress conditions. We herein investigated the biochemical activity and cellular localization mechanisms of Ain1. Ain1 showed weaker affinity to F-actin in vitro than other actin-bundling proteins in S. pombe. We identified a mutation that presumably loosened the interaction between two calponin-homology domains constituting the single actin-binding domain (ABD) of Ain1, which strengthened the actin-binding activity of Ain1. This mutant protein induced a deformation in the ring shape of the CR. Neither a truncated protein consisting only of an N-terminal ABD nor a truncated protein lacking a C-terminal region containing an EF-hand motif localized to the CR, whereas the latter was involved in the bundling of F-actin in vitro. We herein propose detailed mechanisms for how each part of the molecule is involved in the proper cellular localization and function of Ain1.

Arf6 in lymphatic endothelial cells regulates lymphangiogenesis by controlling directional cell migration

The small GTPase Arf6 plays pivotal roles in a wide variety of cellular events such as endocytosis, exocytosis, and actin cytoskeleton reorganization. However, the physiological functions of Arf6 at the whole animal level have not yet been thoroughly understood. Here, we show that Arf6 regulates developmental and tumor lymphangiogenesis in mice. Lymphatic endothelial cell (LEC)-specific Arf6 conditional knockout (LEC-Arf6 cKO) mouse embryos exhibit severe skin edema and impairment in the formation of lymphatic vessel network at the mid-gestation stage. Knockdown of Arf6 in human LECs inhibits in vitro capillary tube formation and directed cell migration induced by vascular endothelial growth factor-C (VEGF-C) by inhibiting VEGF-C-induced internalization of β1 integrin. Finally, we found that LEC-Arf6 cKO mice transplanted with B16 melanoma cells attenuated tumor lymphangiogenesis and progression. Collectively, these results demonstrate that Arf6 in LECs plays a crucial role in physiological and pathological lymphangiogenesis.

The fifth sense: Mechanosensory regulation of alpha-actinin-4 and its relevance for cancer metastasis

Metastatic cancer cells invading through dense tumor stroma experience internal and external forces that are sensed through a variety of mechanosensory proteins that drive adaptations for specific environments. Alpha-actinin-4 (ACTN4) is a member of the α-actinin family of actin crosslinking proteins that is upregulated in several types of cancers. It shares 86% protein similarity with α-actinin-1, another non-muscle ACTN isoform, which appears to have a more modest role, if any, in cancer progression. While they share regulatory mechanisms, such as phosphorylation, calcium binding, phosphatidyl inositol binding, and calpain cleavage, α-actinin-4 exhibits a unique mechanosensory regulation that α-actinin-1 does not. This behavior is mediated, at least in part, by each protein's actin-binding affinity as well as the catch-slip-bond behavior of the actin binding domains. We will discuss currently known modes of ACTN4 regulation, their interactions, and how mechanosensation may provide major therapeutic targeting potential for cancer metastasis.

The sarcomeric cytoskeleton: from molecules to motion

Highly ordered organisation of striated muscle is the prerequisite for the fast and unidirectional development of force and motion during heart and skeletal muscle contraction. A group of proteins, summarised as the sarcomeric cytoskeleton, is essential for the ordered assembly of actin and myosin filaments into sarcomeres, by combining architectural, mechanical and signalling functions. This review discusses recent cell biological, biophysical and structural insight into the regulated assembly of sarcomeric cytoskeleton proteins and their roles in dissipating mechanical forces in order to maintain sarcomere integrity during passive extension and active contraction. α-Actinin crosslinks in the Z-disk show a pivot-and-rod structure that anchors both titin and actin filaments. In contrast, the myosin crosslinks formed by myomesin in the M-band are of a ball-and-spring type and may be crucial in providing stable yet elastic connections during active contractions, especially eccentric exercise.

Hypertrophic cardiomyopathy mutations in the calponin-homology domain of ACTN2 affect actin binding and cardiomyocyte Z-disc incorporation

α-Actinin-2 (ACTN2) is the only muscle isoform of α-actinin expressed in cardiac muscle. Mutations in this protein have been implicated in mild to moderate forms of hypertrophic cardiomyopathy (HCM). We have investigated the effects of two mutations identified from HCM patients, A119T and G111V, on the secondary and tertiary structure of a purified actin binding domain (ABD) of ACTN2 by circular dichroism and X-ray crystallography, and show small but distinct changes for both mutations. We also find that both mutants have reduced F-actin binding affinity, although the differences are not significant. The full length mEos2 tagged protein expressed in adult cardiomyocytes shows that both mutations additionally affect Z-disc localization and dynamic behaviour. Overall, these two mutations have small effects on structure, function and behaviour, which may contribute to a mild phenotype for this disease.

The Calponin Family Member CHDP-1 Interacts with Rac/CED-10 to Promote Cell Protrusions

Eukaryotic cells extend a variety of surface protrusions to direct cell motility. Formation of protrusions is mediated by coordinated actions between the plasma membrane and the underlying actin cytoskeleton. Here, we found that the single calponin homology (CH) domain-containing protein CHDP-1 induces the formation of cell protrusions in C. elegans. CHDP-1 is anchored to the cortex through its amphipathic helix. CHDP-1 associates through its CH domain with the small GTPase Rac1/CED-10, which is a key regulator of the actin cytoskeleton. CHDP-1 preferentially binds to the GTP-bound active form of the CED-10 protein and preserves the membrane localization of GTP-CED-10. Hence, by coupling membrane expansion to Rac1-mediated actin dynamics, CHDP-1 promotes the formation of cellular protrusions in vivo.

ACAP3 regulates neurite outgrowth through its GAP activity specific to Arf6 in mouse hippocampal neurons

ACAP3 (ArfGAP with coiled-coil, ankyrin repeat and pleckstrin homology domains 3) belongs to the ACAP family of GAPs (GTPase-activating proteins) for the small GTPase Arf (ADP-ribosylation factor). However, its specificity to Arf isoforms and physiological functions remain unclear. In the present study, we demonstrate that ACAP3 plays an important role in neurite outgrowth of mouse hippocampal neurons through its GAP activity specific to Arf6. In primary cultured mouse hippocampal neurons, knockdown of ACAP3 abrogated neurite outgrowth, which was rescued by ectopically expressed wild-type ACAP3, but not by its GAP activity-deficient mutant. Ectopically expressed ACAP3 in HEK (human embryonic kidney)-293T cells showed the GAP activity specific to Arf6. In support of this observation, the level of GTP-bound Arf6 was significantly increased by knockdown of ACAP3 in hippocampal neurons. In addition, knockdown and knockout of Arf6 in mouse hippocampal neurons suppressed neurite outgrowth. These results demonstrate that ACAP3 positively regulates neurite outgrowth through its GAP activity specific to Arf6. Furthermore, neurite outgrowth suppressed by ACAP3 knockdown was rescued by expression of a fast cycle mutant of Arf6 that spontaneously exchanges guanine nucleotides on Arf6, but not by that of wild-type, GTP- or GDP-locked mutant Arf6. Thus cycling between active and inactive forms of Arf6, which is precisely regulated by ACAP3 in concert with a guanine-nucleotide-exchange factor(s), seems to be required for neurite outgrowth of hippocampal neurons.

Control of diverse subcellular processes by a single multi-functional lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is a multi-functional lipid that regulates several essential subcellular processes in eukaryotic cells. In addition to its well-established function as a substrate for receptor-activated signalling at the plasma membrane (PM), it is now recognized that distinct PI(4,5)P2 pools are present at other organelle membranes. However, a long-standing question that remains unresolved is the mechanism by which a single lipid species, with an invariant functional head group, delivers numerous functions without loss of fidelity. In the present review, we summarize studies that have examined the molecular processes that shape the repertoire of PI(4,5)P2 pools in diverse eukaryotes. Collectively, these studies indicate a conserved role for lipid kinase isoforms in generating functionally distinct pools of PI(4,5)P2 in diverse metazoan species. The sophistication underlying the regulation of multiple functions by PI(4,5)P2 is also shaped by mechanisms that regulate its availability to enzymes involved in its metabolism as well as molecular processes that control its diffusion at nanoscales in the PM. Collectively, these mechanisms ensure the specificity of PI(4,5)P2 mediated signalling at eukaryotic membranes.

Phosphatidylinositol 4-Phosphate 5-Kinases in the Regulation of T Cell Activation

Phosphatidylinositol 4,5-biphosphate kinases (PIP5Ks) are critical regulators of T cell activation being the main enzymes involved in the synthesis of phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is indeed a pivotal regulator of the actin cytoskeleton, thus controlling T cell polarization and migration, stable adhesion to antigen-presenting cells, spatial organization of the immunological synapse, and co-stimulation. Moreover, PIP2 also serves as a precursor for the second messengers inositol triphosphate, diacylglycerol, and phosphatidylinositol 3,4,5-triphosphate, which are essential for the activation of signaling pathways regulating cytokine production, cell cycle progression, survival, metabolism, and differentiation. Here, we discuss the impact of PIP5Ks on several T lymphocyte functions with a specific focus on the role of CD28 co-stimulation in PIP5K compartimentalization and activation.

Phosphatidylinositol 4-Phosphate 5-Kinase β Controls Recruitment of Lipid Rafts into the Immunological Synapse

Phosphatidylinositol 4,5-biphosphate (PIP2) is critical for T lymphocyte activation serving as a substrate for the generation of second messengers and the remodeling of actin cytoskeleton necessary for the clustering of lipid rafts, TCR, and costimulatory receptors toward the T:APC interface. Spatiotemporal analysis of PIP2 synthesis in T lymphocytes suggested that distinct isoforms of the main PIP2-generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K), play a differential role on the basis of their distinct localization. In this study, we analyze the contribution of PIP5Kβ to T cell activation and show that CD28 induces the recruitment of PIP5Kβ to the immunological synapse, where it regulates filamin A and lipid raft accumulation, as well as T cell activation, in a nonredundant manner. Finally, we found that Vav1 and the C-terminal 83 aa of PIP5Kβ are pivotal for the PIP5Kβ regulatory functions in response to CD28 stimulation.

Activation of the Small G Protein Arf6 by Dynamin2 through Guanine Nucleotide Exchange Factors in Endocytosis

The small G protein Arf6 and the GTPase dynamin2 (Dyn2) play key roles in clathrin-mediated endocytosis (CME). However, their functional relationship remains obscure. Here, we show that Arf6 functions as a downstream molecule of Dyn2 in CME. Wild type of Dyn2 overexpressed in HeLa cells markedly activates Arf6, while a GTPase-lacking Dyn2 mutant does not. Of the Arf6-specific guanine nucleotide exchange factors, EFA6A, EFA6B, and EFA6D specifically interact with Dyn2. Furthermore, overexpression of dominant negative mutants or knockdown of EFA6B and EFA6D significantly inhibit Dyn2-induced Arf6 activation. Finally, overexpression of the binding region peptide of EFA6B for Dyn2 or knockdown of EFA6B and EFA6D significantly suppresses clathrin-mediated transferrin uptake. These results provide evidence for a novel Arf6 activation mechanism by Dyn2 through EFA6B and EFA6D in CME in a manner dependent upon the GTPase activity of Dyn2.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

The structure and regulation of human muscle α-actinin

The spectrin superfamily of proteins plays key roles in assembling the actin cytoskeleton in various cell types, crosslinks actin filaments, and acts as scaffolds for the assembly of large protein complexes involved in structural integrity and mechanosensation, as well as cell signaling. α-actinins in particular are the major actin crosslinkers in muscle Z-disks, focal adhesions, and actin stress fibers. We report a complete high-resolution structure of the 200 kDa α-actinin-2 dimer from striated muscle and explore its functional implications on the biochemical and cellular level. The structure provides insight into the phosphoinositide-based mechanism controlling its interaction with sarcomeric proteins such as titin, lays a foundation for studying the impact of pathogenic mutations at molecular resolution, and is likely to be broadly relevant for the regulation of spectrin-like proteins.

•

Structure of human α-actinin-2 in an autoinhibited closed conformation

•

Facilitation of PIP2-induced allosteric modulation for opening and titin binding

•

Essentiality of structural flexibility for crosslinking antiparallel F-actin

•

Relevance for the intramolecular pseudoligand regulation mechanism of the spectrin family

The non-muscle functions of actinins: an update

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

MICAL-family proteins: Complex regulators of the actin cytoskeleton

SIGNIFICANCE

The molecules interacting with CasL (MICAL) family members participate in a multitude of activities, including axonal growth cone repulsion, membrane trafficking, apoptosis, and bristle development in flies. An interesting feature of MICAL proteins is the presence of an N-terminal flavo-mono-oxygenase domain. This mono-oxygenase domain generates redox potential with which MICALs can either oxidize proteins or produce reactive oxygen species (ROS). Actin is one such protein that is affected by MICAL function, leading to dramatic cytoskeletal rearrangements. This review describes the MICAL-family members, and discusses their mechanisms of actin-binding and regulation of actin cytoskeleton organization.

RECENT ADVANCES

Recent studies show that MICALs directly induce oxidation of actin molecules, leading to actin depolymerization. ROS production by MICALs also causes oxidation of collapsin response mediator protein-2, a microtubule assembly promoter, which subsequently undergoes phosphorylation.

CRITICAL ISSUES

MICAL proteins oxidize proteins through two mechanisms: either directly by oxidizing methionine residues or indirectly via the production of ROS. It remains unclear whether MICAL proteins employ both mechanisms or whether the activity of MICAL-family proteins might vary with different substrates.

FUTURE DIRECTIONS

The identification of additional substrates oxidized by MICAL will shed new light on MICAL protein function. Additional directions include expanding studies toward the MICAL-like homologs that lack flavin adenine dinucleotide domains and oxidation activity.

Mechanotransduction pathways linking the extracellular matrix to the nucleus

Cells contain several mechanosensing components that transduce mechanical signals into biochemical cascades. During cell-ECM adhesion, a complex network of molecules mechanically couples the extracellular matrix (ECM), cytoskeleton, and nucleoskeleton. The network comprises transmembrane receptor proteins and focal adhesions, which link the ECM and cytoskeleton. Additionally, recently identified protein complexes extend this linkage to the nucleus by linking the cytoskeleton and the nucleoskeleton. Despite numerous studies in this field, due to the complexity of this network, our knowledge of the mechanisms of cell-ECM adhesion at the molecular level remains remarkably incomplete. Herein, we present a review of the structures of key molecules involved in cell-ECM adhesion, along with an evaluation of their predicted roles in mechanical sensing. Additionally, specific binding events prompted by force-induced conformational changes of each molecule are discussed. Finally, we propose a model for the biomechanical events prominent in cell-ECM adhesion.

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

The carboxyl tail of alpha-actinin-4 regulates its susceptibility to m-calpain and thus functions in cell migration and spreading

Alpha-actinin-4 links the cytoskeleton to sites of adhesion and has been shown to be modulated to enable cell migration. Such focal adhesions must be labile to accomplish migration, with this detachment occurring at least in part via m-calpain activation (Glading et al., 2001, 2002; Xie et al., 1998). In this study, we report that alpha-actinin-4 is initially cleaved by m-calpain between tyrosine 13 and glycine. Removal of the first 13 amino acids does not affect alpha-actinin-4 binding to actin filaments and its localization within fibroblasts but drives cell migration with less persistence. Binding of phosphoinositides PI(4,5)P2, PI(3,4,5)P3 and PI(3,4)P2 to alpha-actinin-4, as well as binding of alpha-actinin-4 to actin filaments all inhibit m-calpain cleavage of ACTN4 between tyrosine 13 and glycine 14. Interestingly, the carboxyl terminus of alpha-actinin-4 including its calcium binding motifs, is inhibitory for a secondary cleavage of alpha-actinin-4 between lysine 283 and valine 284. The minimal length of inhibitory domain is mapped to the last 11 amino acids of alpha-actinin-4. The C-terminal tail of alpha-actinin-4 is essential for maintaining its normal actin binding activity and localization within cytoplasm and also its colocalization with actin in the lamellipodia of locomoting fibroblasts. Live cell imaging reveals that the 1-890 fragment fails to rescue neither the basal or growth factor-stimulated migration nor the revert the spread area of fibroblasts to the level of NR6WT. These findings suggest that the C-terminal tail of alpha-actinin-4 is essential for its function in cell migration and adhesion to substratum.

Cytoskeleton and nucleotide signaling in glioma C6 cells

This chapter describes signaling pathways stimulated by the P2Y(2) nucleotide receptor (P2Y(2)R), that regulate cellular processes dependent on actin cytoskeleton dynamics in glioma C6 cells. P2Y(2)R coupled with G-proteins, in response to ATP or UTP, regulates the level of phosphatidylinositol-4,5-bisphosphate (PIP(2)) which modulates a variety of actin binding proteins and is involved in calcium response and activates Rac1 and RhoA proteins. The RhoA/ROCK signaling pathway plays an important role in contractile force generation needed for the assembly of stress fibers, focal adhesions and for tail retraction during cell migration. Blocking of this pathway by a specific Rho-kinase inhibitor induces changes in F-actin organization and cell shape and decreases the level of phosphorylated myosin II and cofilin. In glioma C6 cells these changes are reversed after UTP stimulation of P2Y(2)R. Signaling pathways responsible for this compensation are connected with calcium signaling. Stimulation of the Rac1 mediated pathway via G(o) proteins needs additional interaction between α(v)β(5) integrins and P2Y(2)Rs. Rac1 activation is necessary for cofilin phosphorylation as well as integrin activation needed for focal complexes formation and stabilization of lamellipodium. Inhibition of positive Rac1 regulation prevents glioma C6 cells from recovery of control cell like morphology.

Regulation of directional cell migration by membrane-induced actin bundling

During embryonic development and in metastatic cancers, cells detach from the epithelium and migrate with persistent directionality. Directional cell migration is also crucial for the regeneration and maintenance of the epithelium and impaired directional migration is linked to chronic inflammatory diseases. Despite its significance, the mechanisms controlling epithelial cell migration remain poorly understood. Villin is an epithelial-cell-specific actin modifying protein that regulates epithelial cell plasticity and motility. In motile cells villin is associated with the highly branched and the unbranched actin filaments of lamellipodia and filopodia, respectively. In this study we demonstrate for the first time that villin regulates directionally persistent epithelial cell migration. Functional characterization of wild-type and mutant villin proteins revealed that the ability of villin to self-associate and bundle actin as well as its direct interaction with phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] regulates villin-induced filopodial assembly and directional cell migration. Our findings suggest that convergence of different signaling cascades could spatially restrict villin activity to areas of high PtdIns(4,5)P(2) and F-actin concentration to assemble filopodia. Furthermore, our data reveal the ability of villin to undergo actin- and PtdIns(4,5)P(2)-induced self-association, which may be particularly suited to coalesce and reorganize actin bundles within the filopodia.

MICALs in control of the cytoskeleton, exocytosis, and cell death

MICALs form an evolutionary conserved family of multidomain signal transduction proteins characterized by a flavoprotein monooxygenase domain. MICALs are being implicated in the regulation of an increasing number of molecular and cellular processes including cytoskeletal dynamics and intracellular trafficking. Intriguingly, some of these effects are dependent on the MICAL monooxygenase enzyme and redox signaling, while other functions rely on other parts of the MICAL protein. Recent breakthroughs in our understanding of MICAL signaling identify the ability of MICALs to bind and directly modify the actin cytoskeleton, link MICALs to the docking and fusion of exocytotic vesicles, and uncover MICALs as anti-apoptotic proteins. These discoveries could lead to therapeutic advances in neural regeneration, cancer, and other diseases.

Limited digestion of α-actinin in the presence of F-actin

N-terminal actin-binding domain of α-actinin is connected to central rod domain through flexible neck region that is susceptible to proteolysis. It is suggested that the neck region assumes variable orientations by actin binding. In order to examine the effect of actin binding to α-actinin, we carried out limited digestion of α-actinin by chymotrypsin in the presence and absence of F-actin. Although the cleavage process was retarded when bound to F-actin, digestion to 32 kDa-head and 55 kDa-rod domains occurred through the same intermediate products as the digestion in the absence of F-actin. N-terminal sequencing of 55 kDa-fragment showed the neck region was cleaved at 276-Leu. The cleavage site was not affected by binding to F-actin nor ionic strength of the solvent. It was also indicated that α-actinin was cleaved at 15-Tyr by chymotrypsin. Quantitation of the cleavage products by densitometry of the SDS-gels suggested the conformational change of α-actinin at domain-connecting regions by F-actin binding.

Identification and expression analysis of mical family genes in zebrafish

Mical (molecule interacting with CasL) represent a conserved family of cytosolic multidomain proteins that has been shown to be associated with a variety of cellular processes, including axon guidance, cell movement, cell-cell junction formation, vesicle trafficking and cancer cell metastasis. However, the expression and function of these genes during embryonic development have not been comprehensively characterized, especially in vertebrate species, although some limited in vivo studies have been carried out in neural and musculature systems of Drosophila and in neural systems of vertebrates. So far, no mical family homologs have been reported in zebrafish, an ideal vertebrate model for the study of developmental processes. Here we report eight homologs of mical family genes in zebrafish and their expression profiles during embryonic development. Consistent with the findings in Drosophila and mammals, most zebrafish mical family genes display expression in neural and musculature systems. In addition, five mical homologs are detected in heart, and one, micall2a, in blood vessels. Our data established an important basis for further functional studies of mical family genes in zebrafish, and suggest a possible role for mical genes in cardiovascular development.

Interaction of annexin A6 with alpha actinin in cardiomyocytes

Background

Annexins are calcium dependent phospholipid binding proteins that are expressed in a wide variety of tissues and implicated in various extra- and intracellular processes. In myocardial tissue, annexins A2, A5 and A6 are particularly abundant, of which the expression levels of annexin A6 has been found to be maximal. Conflicting reports from transgenic mice overexpressing annexin A6 or null mice lacking annexin A6 showed imbalances in intracellular calcium turnover and disturbed cardiac contractility. However, few studies have focussed on the signalling module of annexin A6 in the heart either in normal or in pathological state.

Results

To identify the putative binding partners of annexin A6 in the heart, ventricular extracts were subjected to glutathione S-transferase (GST)- annexin A6 pull down assay and the GST- annexin A6 bound proteins were identified by mass spectrometry. The pull down fractions of ventricular extracts with GST-full length annexin A6 as well as GST-C terminus deleted annexin A6 when immunoblotted with anti sarcomeric alpha (α)-actinin antibody showed the presence of α-actinin in the immunoblot which was absent when GST-N terminus deleted annexin A6 was used for pull down. Overexpression of green fluorescent protein (GFP) tagged full length annexin A6 showed z-line like appearance in cardiomyocytes whereas GFP-N termimus deleted annexin A6 was mostly localized to the nucleus. Overexpression of GFP-C terminus deleted annexin A6 in cardiomyocytes showed aggregate like appearance in the cytoplasm. Double immunofluorescent staining of cardiomyocytes with anti annexin A6 and anti sarcomeric α-actinin antibodies showed perfect co-localization of these two proteins with annexin A6 appearing like a component of sarcomere. Transient knockdown of annexin A6 in cardiomyocytes by shRNA significantly enhances the contractile functions but does not affect the z-band architecture, as revealed by α-actinin immunostaining in shRNA treated cells.

Conclusions

In overall, the present study demonstrated for the first time that annexin A6 physically interacts with sarcomeric α-actinin and alters contractility of cardiomyocytes suggesting that it might play important role in excitation and contraction process.

ARF6-mediated endocytic recycling impacts cell movement, cell division and lipid homeostasis

A wide range of cellular activities depends upon endocytic recycling. ARF6, a small molecular weight GTPase, regulates the processes of endocytosis and endocytic recycling in concert with various effector molecules and other small GTPases. This review highlights three critical processes that involve ARF6-mediated endosomal membrane trafficking-cell motility, cytokinesis, and cholesterol homeostasis. In each case, the function of ARF6-mediated trafficking varies-including localization of specific protein and lipid cargo, regulation of bulk membrane movement, and modulation of intracellular signaling. As described in this review, mis-regulation of endocytic traffic can result in human disease when it compromises the cell's ability to regulate cell movement and invasion, cell division, and lipid homeostasis.

Protein kinase Cdelta-mediated phosphorylation of phospholipase D controls integrin-mediated cell spreading

Integrin signaling plays critical roles in cell adhesion, spreading, and migration, and it is generally accepted that to regulate these integrin functions accurately, localized actin remodeling is required. However, the molecular mechanisms that control the targeting of actin regulation molecules to the proper sites are unknown. We previously demonstrated that integrin-mediated cell spreading and migration on fibronectin are dependent on the localized activation of phospholipase D (PLD). However, the mechanism underlying PLD activation by integrin is largely unknown. Here we demonstrate that protein kinase Cδ (PKCδ) is required for integrin-mediated PLD signaling. After integrin stimulation, PKCδ is activated and translocated to the edges of lamellipodia, where it colocalizes with PLD2. The abrogation of PKCδ activity inhibited integrin-induced PLD activation and cell spreading. Finally, we show that Thr566 of PLD2 is directly phosphorylated by PKCδ and that PLD2 mutation in this region prevents PLD2 activation, PLD2 translocation to the edge of lamellipodia, Rac translocation, and cell spreading after integrin activation. Together, these results suggest that PKCδ is a primary regulator of integrin-mediated PLD activation via the direct phosphorylation of PLD, which is essential for directing integrin-induced cell spreading.

Phospholipase C is a key enzyme regulating intracellular calcium and modulating the phosphoinositide balance

Spatial and temporal activation of phosphoinositide turnover enables eukaryotic cells to perform various functions such as cell proliferation/differentiation, fertilization, neuronal functions, and cell motility. In this system, phospholipase C (PLC) is a key enzyme, which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) into two second messengers, inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and diacylglycerol (DAG). Ins(1,4,5)P(3) triggers the release of calcium from intracellular stores, and DAG mediates the activation of protein kinase C (PKC). In parallel, PI(4,5)P(2) also directly regulates a variety of cellular functions, including cytoskeletal remodeling, cytokinesis, phagocytosis, membrane dynamics, and channel activity, in addition to its role as a substrate for PLC and phosphatidylinositol 3-kinase (PI3K), which generates PI(3,4,5)P(3). An imbalance of these phosphoinositides contributes to the pathogeneses of various human diseases. Therefore, strict regulation of the levels of PI(4,5)P(2) and PI(3,4,5)P(3) by PLC or other interconverting enzymes is necessary for cellular functions. In this review, we focus on the roles of PLC as a calcium-regulating enzyme and as a modulator of the phosphoinositide balance.

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.