Regulation of mitochondria distribution by RhoA and formins

Lysosomal TPC2 channels disrupt Ca2+ entry and dopaminergic function in models of LRRK2-Parkinson's disease

Parkinson's disease results from degeneration of dopaminergic neurons in the midbrain, but the underlying mechanisms are unclear. Here, we identify novel crosstalk between depolarization-induced entry of Ca2+ and lysosomal cation release in maintaining dopaminergic neuronal function. The common disease-causing G2019S mutation in LRRK2 selectively exaggerated Ca2+ entry in vitro. Chemical and molecular strategies inhibiting the lysosomal ion channel TPC2 reversed this. Using Drosophila, which lack TPCs, we show that the expression of human TPC2 phenocopied LRRK2 G2019S in perturbing dopaminergic-dependent vision and movement in vivo. Mechanistically, dysfunction required an intact pore, correct subcellular targeting and Rab interactivity of TPC2. Reducing Ca2+ permeability with a novel biased TPC2 agonist corrected deviant Ca2+ entry and behavioral defects. Thus, both inhibition and select activation of TPC2 are beneficial. Functional coupling between lysosomal cation release and Ca2+ influx emerges as a potential druggable node in Parkinson's disease.

Host factor DIAPH1 regulates pseudorabivirus replication by modulating the dynamics of cytoskeleton

As obligate parasites, viruses exploit host cell organelles and molecular components to complete their life cycle. Among which, viruses firstly hijack the cytoskeleton of host cells to ensure their efficiently cell entry and replication. Although formin family members play a key role in both microfilament and microtubule cytoskeletal remodeling, few studies addressed the detailed function and mechanism of formins in the process of viral infection. Here, we showed that sus scrofa DIAPH1 was involved in the regulation of cytoskeletal dynamics during PRV replication. Firstly, we found that DIAPH1 showed significant changes in the expression level and intracellular localization during PRV infection of PK-15 cells. Next, inhibition of DIAPH1 by RNA interference or small molecular inhibitor SMIFH2 was found to diminish the outcome of PRV infection. Besides, DIAPH1 partially co-localized with actin and tubulin in PRV-infected cells. Cross-talk occurred between microfilaments and microfilaments, which also had an influence on the intracellular localization of DIAPH1. What's more, inhibition of DIAPH1 induced the reorganization of microfilament and the stability of microtubule. These results suggested that DIAPH1 regulated PRV infection by remodeling microfilament and microtubule cytoskeletal dynamics.

Trafficking in cancer: from gene deregulation to altered organelles and emerging biophysical properties

Intracellular trafficking supports all cell functions maintaining the exchange of material between membrane-bound organelles and the plasma membrane during endocytosis, cargo sorting, and exocytosis/secretion. Several proteins of the intracellular trafficking machinery are deregulated in diseases, particularly cancer. This complex and deadly disease stays a heavy burden for society, despite years of intense research activity. Here, we give an overview about trafficking proteins and highlight that in addition to their molecular functions, they contribute to the emergence of intracellular organelle landscapes. We review recent evidence of organelle landscape alterations in cancer. We argue that focusing on organelles, which represent the higher-order, cumulative behavior of trafficking regulators, could help to better understand, describe and fight cancer. In particular, we propose adopting a physical framework to describe the organelle landscape, with the goal of identifying the key parameters that are crucial for a stable and non-random organelle organization characteristic of healthy cells. By understanding these parameters, we may gain insights into the mechanisms that lead to a pathological organelle spatial organization, which could help explain the plasticity of cancer cells.

Acutely Modifying Phosphatidylinositol Phosphates on Endolysosomes Using Chemically Inducible Dimerization Systems

Phosphoinositides are rare membrane lipids that mediate cell signaling and membrane dynamics. PI(4)P and PI(3)P are two major phosphoinositides crucial for endolysosomal functions and dynamics, making them the lipids of interest in many studies. The acute modulation of phosphoinositides at a given organelle membrane can reveal important insights into their cellular function. Indeed, the localized depletion of PI(4)P and PI(3)P is a viable tool to assess the importance of these phosphoinositides in various experimental conditions. Here, we describe a live imaging method to acutely deplete PI(4)P and PI(3)P on endolysosomes. The depletion assay utilizes the GAI-GID1 or the FRB-FKBP inducible dimerization system to target the catalytic domain of the PI(4)P phosphatase, Sac1, or the PI(3)P phosphatase domain of MTM1 to the endolysosome for localized depletion of these phosphoinositides. By using the fluorescently tagged biosensors, 2xP4M and PX, we can validate and monitor the depletion of PI(4)P and PI(3)P, respectively, on endolysosomes in real-time. We discuss a method for normalizing the fluorescence measurements to appropriate the relative amount of these phosphoinositides in the organellar membranes (endolysosomes), which is required for monitoring PI(4)P or PI(3)P levels during the acute depletion assay. Since the localization of the dimerization partners is specified by the membrane targeting signal, our protocol will be useful for studying the signaling and functions of phosphoinositides at any membrane. Key features • Acute depletion and real-time monitoring of PI(3)P and PI(4)P on the endolysosomal membrane using chemically inducible dimerization systems. • Modifiable and adaptable to modulate other phosphoinositides on different organellar membranes.

Transmembrane formins as active cargoes of membrane trafficking

Formins are a large, evolutionarily old family of cytoskeletal regulators whose roles include actin capping and nucleation, as well as modulation of microtubule dynamics. The plant class I formin clade is characterized by a unique domain organization, as most of its members are transmembrane proteins with possible cell wall-binding motifs exposed to the extracytoplasmic space-a structure that appears to be a synapomorphy of the plant kingdom. While such transmembrane formins are traditionally considered mainly as plasmalemma-localized proteins contributing to the organization of the cell cortex, we review, from a cell biology perspective, the growing evidence that they can also, at least temporarily, reside (and in some cases also function) in endomembranes including secretory and endocytotic pathway compartments, the endoplasmic reticulum, the nuclear envelope, and the tonoplast. Based on this evidence, we propose that class I formins may thus serve as 'active cargoes' of membrane trafficking-membrane-embedded proteins that modulate the fate of endo- or exocytotic compartments while being transported by them.

FMNL2 regulates actin for endoplasmic reticulum and mitochondria distribution in oocyte meiosis

During mammalian oocyte meiosis, spindle migration and asymmetric cytokinesis are unique steps for the successful polar body extrusion. The asymmetry defects of oocytes will lead to the failure of fertilization and embryo implantation. In present study, we reported that an actin nucleating factor Formin-like 2 (FMNL2) played critical roles in the regulation of spindle migration and organelle distribution in mouse and porcine oocytes. Our results showed that FMNL2 mainly localized at the oocyte cortex and periphery of spindle. Depletion of FMNL2 led to the failure of polar body extrusion and large polar bodies in oocytes. Live-cell imaging revealed that the spindle failed to migrate to the oocyte cortex, which caused polar body formation defects, and this might be due to the decreased polymerization of cytoplasmic actin by FMNL2 depletion in the oocytes of both mice and pigs. Furthermore, mass spectrometry analysis indicated that FMNL2 was associated with mitochondria and endoplasmic reticulum (ER)-related proteins, and FMNL2 depletion disrupted the function and distribution of mitochondria and ER, showing with decreased mitochondrial membrane potential and the occurrence of ER stress. Microinjecting Fmnl2-EGFP mRNA into FMNL2-depleted oocytes significantly rescued these defects. Thus, our results indicate that FMNL2 is essential for the actin assembly, which further involves into meiotic spindle migration and ER/mitochondria functions in mammalian oocytes.

CpSmt3, an ortholog of small ubiquitin-like modifier, is essential for growth, organelle function, virulence, and antiviral defense in Cryphonectria parasitica

Introduction

SUMOylation is an important post-translational modification that regulates the expression, localization, and activity of substrate proteins, thereby participating in various important cellular processes such as the cell cycle, cell metabolism, gene transcription, and antiviral activity. However, the function of SUMOylation in phytopathogenic fungi has not yet been adequately explored.

Methods

A comprehensive analysis composed of proteomics, affinity pull-down, molecular and cellular approaches was performed to explore the roles of SUMOylation in Cryphonectria parasitica, the fungal pathogen responsible for chestnut blight.

Results and discussion

CpSmt3, the gene encoding the SUMO protein CpSmt3 in C. parasitica was identified and characterized. Deletion of the CpSmt3 gene resulted in defects in mycelial growth and hyphal morphology, suppression of sporulation, attenuation of virulence, weakening of stress tolerance, and elevated accumulation of hypovirus dsRNA. The ΔCpSmt3 deletion mutant exhibited an increase in mitochondrial ROS, swollen mitochondria, excess autophagy, and thickened cell walls. About 500 putative SUMO substrate proteins were identified by affinity pull-down, among which many were implicated in the cell cycle, ribosome, translation, and virulence. Proteomics and SUMO substrate analyses further revealed that deletion of CpSmt3 reduced the accumulation of CpRho1, an important protein that is involved in TOR signal transduction. Silencing of CpRho1 resulted in a phenotype similar to that of ΔCpSmt3, while overexpression of CpRho1 could partly rescue some of the prominent defects in ΔCpSmt3. Together, these findings demonstrate that SUMOylation by CpSmt3 is vitally important and provide new insights into the SUMOylation-related regulatory mechanisms in C. parasitica.

Armcx1 Reduces Neurological Damage Via a Mitochondrial Transport Pathway Involving Miro1 After Traumatic Brain Injury

Armcx1 is a member of the ARMadillo repeat-Containing protein on the X chromosome (ARMCX) family, which is recognized to have evolutionary conserved roles in regulating mitochondrial transport and dynamics. Previous research has shown that Armcx1 is expressed at higher levels in mice after axotomy and in adult retinal ganglion cells after crush injury, and this protein increases neuronal survival and axonal regeneration. However, its role in traumatic brain injury (TBI) is unclear. Therefore, the aim of this study was to assess the expression of Armcx1 after TBI and to explore possible related mechanisms by which Armcx1 is involved in TBI. We used C57BL/6 male mice to model TBI and evaluated the role of Armcx1 in TBI by transfecting mice with Armcx1 small interfering RNA (siRNA) to inhibit Armcx1 expression 24 h before TBI modeling. Western blotting, immunofluorescence, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining, Nissl staining, transmission electron microscopy, adenosine triphosphate (ATP) level measurement, neuronal apoptosis analysis, neurological function scoring and the Morris water maze were performed. The results demonstrated that Armcx1 protein expression was elevated after TBI and that the Armcx1 protein was localized in neurons and astroglial cells in cortical tissue surrounding the injury site. In addition, inhibition of Armcx1 expression further led to impaired mitochondrial transport, abnormal morphology, reduced ATP levels, aggravation of neuronal apoptosis and neurological dysfunction, and decrease Miro1 expression. In conclusion, our findings indicate that Armcx1 may exert neuroprotective effects by ameliorating neurological injury after TBI through a mitochondrial transport pathway involving Miro1.

Unique Role of Vimentin in the Intermediate Filament Proteins Family

Intermediate filaments (IFs), being traditionally the least studied component of the cytoskeleton, have begun to receive more attention in recent years. IFs are found in different cell types and are specific to them. Accumulated data have shifted the paradigm about the role of IFs as structures that merely provide mechanical strength to the cell. In addition to this role, IFs have been shown to participate in maintaining cell shape and strengthening cell adhesion. The data have also been obtained that point out to the role of IFs in a number of other biological processes, including organization of microtubules and microfilaments, regulation of nuclear structure and activity, cell cycle control, and regulation of signal transduction pathways. They are also actively involved in the regulation of several aspects of intracellular transport. Among the intermediate filament proteins, vimentin is of particular interest for researchers. Vimentin has been shown to be associated with a range of diseases, including cancer, cataracts, Crohn's disease, rheumatoid arthritis, and HIV. In this review, we focus almost exclusively on vimentin and the currently known functions of vimentin intermediate filaments (VIFs). This is due to the structural features of vimentin, biological functions of its domains, and its involvement in the regulation of a wide range of basic cellular functions, and its role in the development of human diseases. Particular attention in the review will be paid to comparing the role of VIFs with the role of intermediate filaments consisting of other proteins in cell physiology.

DIAPH1-MFN2 interaction regulates mitochondria-SR/ER contact and modulates ischemic/hypoxic stress

Inter-organelle contact and communication between mitochondria and sarco/endoplasmic reticulum (SR/ER) maintain cellular homeostasis and are profoundly disturbed during tissue ischemia. We tested the hypothesis that the formin Diaphanous-1 (DIAPH1), which regulates actin dynamics, signal transduction and metabolic functions, contributes to these processes. We demonstrate that DIAPH1 interacts directly with Mitofusin-2 (MFN2) to shorten mitochondria-SR/ER distance, thereby enhancing mitochondria-ER contact in cells including cardiomyocytes, endothelial cells and macrophages. Solution structure studies affirm the interaction between the Diaphanous Inhibitory Domain and the cytosolic GTPase domain of MFN2. In male rodent and human cardiomyocytes, DIAPH1-MFN2 interaction regulates mitochondrial turnover, mitophagy, and oxidative stress. Introduction of synthetic linker construct, which shorten the mitochondria-SR/ER distance, mitigated the molecular and functional benefits of DIAPH1 silencing in ischemia. This work establishes fundamental roles for DIAPH1-MFN2 interaction in the regulation of mitochondria-SR/ER contact networks. We propose that targeting pathways that regulate DIAPH1-MFN2 interactions may facilitate recovery from tissue ischemia.

Kinetics of phagosome maturation is coupled to their intracellular motility

Immune cells degrade internalized pathogens in phagosomes through sequential biochemical changes. The degradation must be fast enough for effective infection control. The presumption is that each phagosome degrades cargos autonomously with a distinct but stochastic kinetic rate. However, here we show that the degradation kinetics of individual phagosomes is not stochastic but coupled to their intracellular motility. By engineering RotSensors that are optically anisotropic, magnetic responsive, and fluorogenic in response to degradation activities in phagosomes, we monitored cargo degradation kinetics in single phagosomes simultaneously with their translational and rotational dynamics. We show that phagosomes that move faster centripetally are more likely to encounter and fuse with lysosomes, thereby acidifying faster and degrading cargos more efficiently. The degradation rates increase nearly linearly with the translational and rotational velocities of phagosomes. Our results indicate that the centripetal motion of phagosomes functions as a clock for controlling the progression of cargo degradation.

RhoA Signaling in Neurodegenerative Diseases

Ras homolog gene family member A (RhoA) is a small GTPase of the Rho family involved in regulating multiple signal transduction pathways that influence a diverse range of cellular functions. RhoA and many of its downstream effector proteins are highly expressed in the nervous system, implying an important role for RhoA signaling in neurons and glial cells. Indeed, emerging evidence points toward a role of aberrant RhoA signaling in neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. In this review, we summarize the current knowledge of RhoA regulation and downstream cellular functions with an emphasis on the role of RhoA signaling in neurodegenerative diseases and the therapeutic potential of RhoA inhibition in neurodegeneration.

Mitochondria-actin cytoskeleton crosstalk in cell migration

Mitochondria perform diverse functions in the cell and their roles during processes such as cell survival, differentiation, and migration are increasingly being appreciated. Mitochondrial and actin cytoskeletal networks not only interact with each other, but this multifaceted interaction shapes their functional dynamics. The interrelation between mitochondria and the actin cytoskeleton extends far beyond the requirement of mitochondrial ATP generation to power actin dynamics, and impinges upon several major aspects of cellular physiology. Being situated at the hub of cell signaling pathways, mitochondrial function can alter the activity of actin regulatory proteins and therefore modulate the processes downstream of actin dynamics such as cellular migration. As we will discuss, this regulation is highly nuanced and operates at multiple levels allowing mitochondria to occupy a strategic position in the regulation of migration, as well as pathological events that rely on aberrant cell motility such as cancer metastasis. In this review, we summarize the crosstalk that exists between mitochondria and actin regulatory proteins, and further emphasize on how this interaction holds importance in cell migration in normal as well as dysregulated scenarios as in cancer.

A novel mechanism of bulk cytoplasmic transport by cortical dynein in Drosophila ovary

Cytoplasmic dynein, a major minus-end directed microtubule motor, plays essential roles in eukaryotic cells. Drosophila oocyte growth is mainly dependent on the contribution of cytoplasmic contents from the interconnected sister cells, nurse cells. We have previously shown that cytoplasmic dynein is required for Drosophila oocyte growth and assumed that it simply transports cargoes along microtubule tracks from nurse cells to the oocyte. Here, we report that instead of transporting individual cargoes along stationary microtubules into the oocyte, cortical dynein actively moves microtubules within nurse cells and from nurse cells to the oocyte via the cytoplasmic bridges, the ring canals. This robust microtubule movement is sufficient to drag even inert cytoplasmic particles through the ring canals to the oocyte. Furthermore, replacing dynein with a minus-end directed plant kinesin linked to the actin cortex is sufficient for transporting organelles and cytoplasm to the oocyte and driving its growth. These experiments show that cortical dynein performs bulk cytoplasmic transport by gliding microtubules along the cell cortex and through the ring canals to the oocyte. We propose that the dynein-driven microtubule flow could serve as a novel mode of fast cytoplasmic transport.

Single-phagosome imaging reveals that homotypic fusion impairs phagosome degradative function

Immune cells degrade internalized pathogens in vesicle compartments called phagosomes. Many intracellular bacteria induce homotypic phagosome fusion to survive in host cells, but the fusion interaction between phagosomes and its consequence for phagosome function have scarcely been studied. Here, we characterize homotypic fusion between phagosomes in macrophages and identify how such interactions impact the degradative capacity of phagosomes. By developing a series of particle sensors for measuring biochemical changes of single phagosomes, we show that phagosomes undergo stable fusion, transient "kiss-and-run" fusion, or both in succession. Super-resolution three-dimensional fluorescence microscopy revealed that stably fused phagosomes are connected by membrane "necks" with submicron-sized fusion pores. Furthermore, we demonstrate that, after stable fusion, phagosomes have leaky membranes and thereby impaired degradative functions. Our findings, based on phagosomes that contain synthetic particles, illustrate that homotypic fusion is not exclusive to phagosomes that encapsulate pathogens, as previously believed. The physical process of homotypic fusion is alone sufficient to perturb the degradative functions of phagosomes.

Mitofusin-2 in the Nucleus Accumbens Regulates Anxiety and Depression-like Behaviors Through Mitochondrial and Neuronal Actions

BACKGROUND

Emerging evidence points to a central role of mitochondria in psychiatric disorders. However, little is known about the molecular players that regulate mitochondria in neural circuits regulating anxiety and depression and about how they impact neuronal structure and function. Here, we investigated the role of molecules involved in mitochondrial dynamics in medium spiny neurons (MSNs) from the nucleus accumbens (NAc), a hub of the brain's motivation system.

METHODS

We assessed how individual differences in anxiety-like (measured via the elevated plus maze and open field tests) and depression-like (measured via the forced swim and saccharin preference tests) behaviors in outbred rats relate to mitochondrial morphology (electron microscopy and 3-dimensional reconstructions) and function (mitochondrial respirometry). Mitochondrial molecules were measured for protein (Western blot) and messenger RNA (quantitative reverse transcriptase polymerase chain reaction, RNAscope) content. Dendritic arborization (Golgi Sholl analyses), spine morphology, and MSN excitatory inputs (patch-clamp electrophysiology) were characterized. MFN2 overexpression in the NAc was induced through an AAV9-syn1-MFN2.

RESULTS

Highly anxious animals showed increased depression-like behaviors, as well as reduced expression of the mitochondrial GTPase MFN2 in the NAc. They also showed alterations in mitochondria (i.e., respiration, volume, and interactions with the endoplasmic reticulum) and MSNs (i.e., dendritic complexity, spine density and typology, and excitatory inputs). Viral MFN2 overexpression in the NAc reversed all of these behavioral, mitochondrial, and neuronal phenotypes.

CONCLUSIONS

Our results implicate a causal role for accumbal MFN2 on the regulation of anxiety and depression-like behaviors through actions on mitochondrial and MSN structure and function. MFN2 is posited as a promising therapeutic target to treat anxiety and associated behavioral disturbances.

Distinct fission signatures predict mitochondrial degradation or biogenesis

Mitochondrial fission is a highly regulated process that, when disrupted, can alter metabolism, proliferation and apoptosis^1-3. Dysregulation has been linked to neurodegeneration^3,4, cardiovascular disease³ and cancer⁵. Key components of the fission machinery include the endoplasmic reticulum⁶ and actin⁷, which initiate constriction before dynamin-related protein 1 (DRP1)⁸ binds to the outer mitochondrial membrane via adaptor proteins^9-11, to drive scission¹². In the mitochondrial life cycle, fission enables both biogenesis of new mitochondria and clearance of dysfunctional mitochondria through mitophagy^1,13. Current models of fission regulation cannot explain how those dual fates are decided. However, uncovering fate determinants is challenging, as fission is unpredictable, and mitochondrial morphology is heterogeneous, with ultrastructural features that are below the diffraction limit. Here, we used live-cell structured illumination microscopy to capture mitochondrial dynamics. By analysing hundreds of fissions in African green monkey Cos-7 cells and mouse cardiomyocytes, we discovered two functionally and mechanistically distinct types of fission. Division at the periphery enables damaged material to be shed into smaller mitochondria destined for mitophagy, whereas division at the midzone leads to the proliferation of mitochondria. Both types are mediated by DRP1, but endoplasmic reticulum- and actin-mediated pre-constriction and the adaptor MFF govern only midzone fission. Peripheral fission is preceded by lysosomal contact and is regulated by the mitochondrial outer membrane protein FIS1. These distinct molecular mechanisms explain how cells independently regulate fission, leading to distinct mitochondrial fates.

Principles and Applications of Single Particle Tracking in Cell Research

It is a tough challenge for many decades to decipher the complex relationships between cell behaviors and cellular physical properties. Single particle tracking (SPT) with high spatial and temporal resolution has been applied extensively in cell research to understand physicochemical properties of cells and their bio-functions by tracking endogenous or exogenous probes. This review describes the fundamental principles of SPT as well as its applications in intracellular mechanics, membrane dynamics, organelles distribution, and processes of internalization and transport. Finally, challenges and future directions of SPT are also discussed.

Loss of DIAPH1 causes SCBMS, combined immunodeficiency, and mitochondrial dysfunction

BACKGROUND

Homozygous loss of DIAPH1 results in seizures, cortical blindness, and microcephaly syndrome (SCBMS). We studied 5 Finnish and 2 Omani patients with loss of DIAPH1 presenting with SCBMS, mitochondrial dysfunction, and immunodeficiency.

OBJECTIVE

We sought to further characterize phenotypes and disease mechanisms associated with loss of DIAPH1.

METHODS

Exome sequencing, genotyping and haplotype analysis, B- and T-cell phenotyping, in vitro lymphocyte stimulation assays, analyses of mitochondrial function, immunofluorescence staining for cytoskeletal proteins and mitochondria, and CRISPR-Cas9 DIAPH1 knockout in heathy donor PBMCs were used.

RESULTS

Genetic analyses found all Finnish patients homozygous for a rare DIAPH1 splice-variant (NM_005219:c.684+1G>A) enriched in the Finnish population, and Omani patients homozygous for a previously described pathogenic DIAPH1 frameshift-variant (NM_005219:c.2769delT;p.F923fs). In addition to microcephaly, epilepsy, and cortical blindness characteristic to SCBMS, the patients presented with infection susceptibility due to defective lymphocyte maturation and 3 patients developed B-cell lymphoma. Patients' immunophenotype was characterized by poor lymphocyte activation and proliferation, defective B-cell maturation, and lack of naive T cells. CRISPR-Cas9 knockout of DIAPH1 in PBMCs from healthy donors replicated the T-cell activation defect. Patient-derived peripheral blood T cells exhibited impaired adhesion and inefficient microtubule-organizing center repositioning to the immunologic synapse. The clinical symptoms and laboratory tests also suggested mitochondrial dysfunction. Experiments with immortalized, patient-derived fibroblasts indicated that DIAPH1 affects the amount of complex IV of the mitochondrial respiratory chain.

CONCLUSIONS

Our data demonstrate that individuals with SCBMS can have combined immune deficiency and implicate defective cytoskeletal organization and mitochondrial dysfunction in SCBMS pathogenesis.

Antiangiogenic Effect of Graphene Oxide in Primary Human Endothelial Cells

In this work, we exploited an integrated approach combining systematic analysis of cytotoxicity, angiogenic potential, and metabolomics to shed light on the effects of graphene oxide (GO) on primary human endothelial Huvec cells. Contrary to the outcomes observed in immortalized cell lines able to internalize a similar amount of GO, significant toxicity was found in Huvec cells at high GO concentrations (25 and 50 μg/mL). In particular, we found that the steric hindrance of GO intracellular aggregates perturbed the correct assembly of cytoskeleton and distribution of mitochondria. This was found to be primarily associated with oxidative stress and impairment of cell migration, affecting the formation of capillary-like structures. In addition, preliminary metabolomics characterization demonstrated that GO affects the consumption of niacinamide, a precursor of energy carriers, and several amino acids involved in the regulation of angiogenesis. Our findings suggest that GO acts at different cellular levels, both directly and indirectly. More precisely, the combination of the physical hindrance of internalized GO aggregates, induction of oxidative stress, and alteration of some metabolic pathways leads to a significant antiangiogenic effect in primary human endothelial cells.

The Bacterial Toxin CNF1 Protects Human Neuroblastoma SH-SY5Y Cells against 6-Hydroxydopamine-Induced Cell Damage: The Hypothesis of CNF1-Promoted Autophagy as an Antioxidant Strategy

Several chronic neuroinflammatory diseases, including Parkinson’s disease (PD), have the so-called ‘redox imbalance’ in common, a dynamic system modulated by various factors. Among them, alteration of the mitochondrial functionality can cause overproduction of reactive oxygen species (ROS) with the consequent induction of oxidative DNA damage and apoptosis. Considering the failure of clinical trials with drugs that eliminate ROS directly, research currently focuses on approaches that counteract redox imbalance, thus restoring normal physiology in a neuroinflammatory condition. Herein, we used SH-SY5Y cells treated with 6-hydroxydopamine (6-OHDA), a neurotoxin broadly employed to generate experimental models of PD. Cells were pre-treated with the Rho-modulating Escherichia coli cytotoxic necrotizing factor 1 (CNF1), before the addition of 6-OHDA. Then, cell viability, mitochondrial morphology and dynamics, redox profile as well as autophagic markers expression were assessed. We found that CNF1 preserves cell viability and counteracts oxidative stress induced by 6-OHDA. These effects are accompanied by modulation of the mitochondrial network and an increase in macroautophagic markers. Our results confirm the Rho GTPases as suitable pharmacological targets to counteract neuroinflammatory diseases and evidence the potentiality of CNF1, whose beneficial effects on pathological animal models have been already proven to act against oxidative stress through an autophagic strategy.

Generating and working with Drosophila cell cultures: Current challenges and opportunities

The use of Drosophila cell cultures has positively impacted both fundamental and biomedical research. The most widely used cell lines: Schneider, Kc, the CNS and imaginal disc lines continue to be the choice for many applications. Drosophila cell lines provide a homogenous source of cells suitable for biochemical experimentations, transcriptomics, functional genomics, and biomedical applications. They are amenable to RNA interference and serve as a platform for high-throughput screens to identify relevant candidate genes or drugs for any biological process. Currently, CRISPR-based functional genomics are also being developed for Drosophila cell lines. Even though many uniquely derived cell lines exist, cell genetic techniques such the transgenic UAS-GAL4-based Ras^V12 oncogene expression, CRISPR-Cas9 editing and recombination mediated cassette exchange are likely to drive the establishment of many more lines from specific tissues, cells, or genotypes. However, the pace of creating new lines is hindered by several factors inherent to working with Drosophila cell cultures: single cell cloning, optimal media formulations and culture conditions capable of supporting lines from novel tissue sources or genotypes. Moreover, even though many Drosophila cell lines are morphologically and transcriptionally distinct it may be necessary to implement a standard for Drosophila cell line authentication, ensuring the identity and purity of each cell line. Altogether, recent advances and a standardized authentication effort should improve the utility of Drosophila cell cultures as a relevant model for fundamental and biomedical research. This article is categorized under: Technologies > Analysis of Cell, Tissue, and Animal Phenotypes.

Dissecting cellular mechanics: Implications for aging, cancer, and immunity

Cells are dynamic structures that must respond to complex physical and chemical signals from their surrounding environment. The cytoskeleton is a key mediator of a cell's response to the signals of both the extracellular matrix and other cells present in the local microenvironment and allows it to tune its own mechanical properties in response to these cues. A growing body of evidence suggests that altered cellular viscoelasticity is a strong indicator of disease state; including cancer, laminopathy (genetic disorders of the nuclear lamina), infection, and aging. Here, we review recent work on the characterization of cell mechanics in disease and discuss the implications of altered viscoelasticity in regulation of immune responses. Finally, we provide an overview of techniques for measuring the mechanical properties of cells deeply embedded within tissues.

Extracellular Matrix Induction of Intracellular Reactive Oxygen Species

SIGNIFICANCE

The extracellular matrix (ECM) is the noncellular component secreted by cells and is present within all tissues and organs. The ECM provides the structural support required for tissue integrity and also contributes to diseases, including cancer. Many diseases rich in ECM are characterized by changes in reactive oxygen species (ROS) levels that have been shown to have important context-dependent functions. Recent Advances: Many studies have found that the ECM affects ROS production through integrins. The activation of integrins by ECM ligands results in stimulation of multiple pathways that can generate ROS. Furthermore, control of ECM-integrin interaction by matricellular proteins is an underappreciated pathway that functions as an ROS rheostat in remodeling tissues.

CRITICAL ISSUES

A better understanding of how the ECM affects the generation of intracellular ROS is required for advances in the development of therapeutic strategies that affect or exploit oxidative stress.

FUTURE DIRECTIONS

Targeting ROS generation can be therapeutic or can promote disease progression in a context-dependent manner. Many ECM proteins can impact ROS generation. However, given the breadth of different proteins that constitute the ECM and the cell surface receptors that interact with ECM proteins, there are likely many tissue and microenvironmental-specific ROS-generating pathways that have yet to be investigated in depth. Identifying canonical pathways of ECM-induced ROS generation should be a priority for the field. Antioxid. Redox Signal. 27, 774-784.

Mammalian Diaphanous-related formin-1 restricts early phases of influenza A/NWS/33 virus (H1N1) infection in LLC-MK2 cells by affecting cytoskeleton dynamics

Viruses depend on cellular machinery to efficiently replicate. The host cytoskeleton is one of the first cellular systems hijacked by viruses in order to ensure their intracellular transport and promote the development of infection. Our previous results demonstrated that stable microfilaments and microtubules interfered with human influenza A/NWS/33 virus (H1N1) infection in semi-permissive LLC-MK2 cells. Although formins play a key role in cytoskeletal remodelling, few studies addressed a possible role of these proteins in development of viral infection. Here, we have demonstrated that mammalian Diaphanous-related formin-1 (mDia1) is involved in the control of cytoskeleton dynamics during human influenza A virus infection. First, by employing cytoskeleton-perturbing drugs, we evidenced a cross-talk occurring between microtubules and microfilaments that also has implications on the intracellular localization of mDia1. In influenza A/NWS/33 virus-infected LLC-MK2 cells, mDia1 showed a highly dynamic intracellular localization and partially co-localized with actin and tubulin. A depletion of mDia1 by RNA-mediated RNA interference was found to improve the outcome of influenza A/NWS/33 virus infection and to increase the dynamics of microfilament and microtubule networks in LLC-MK2 cells. Consistent with these findings, observations made in epithelial respiratory cells from paediatric patients with acute respiratory disease assessed that the expression of mDia1 is stimulated by influenza A virus but not by respiratory syncytial virus. Taken together, the obtained results suggest that mDia1 restricts the initiation of influenza A/NWS/33 virus infection in LLC-MK2 cells by counteracting cytoskeletal dynamics.

Using Confocal Microscopy to Investigate Intracellular Trafficking of Toll-Like Receptors

Toll-like receptors (TLR) survey the extracellular space, cytoplasm, and endosomal compartments for signs of infection or tissue injury. Over the past decade, it has become evident that TLR activation and signal transduction can be regulated by subcellular compartmentalization of both the receptors and their downstream signaling components. Immunofluorescence and/or overexpression of fluorescently "tagged"' proteins teamed with confocal microscopy presents a powerful technique for studying the spatial organization of TLRs, their signaling mediators, and the dynamic processes they activate. This chapter details the common methods for determining the subcellular location of TLRs in both live and fixed cells.

Transporting mitochondria in neurons

Neurons demand vast and vacillating supplies of energy. As the key contributors of this energy, as well as primary pools of calcium and signaling molecules, mitochondria must be where the neuron needs them, when the neuron needs them. The unique architecture and length of neurons, however, make them a complex system for mitochondria to navigate. To add to this difficulty, mitochondria are synthesized mainly in the soma, but must be transported as far as the distant terminals of the neuron. Similarly, damaged mitochondria-which can cause oxidative stress to the neuron-must fuse with healthy mitochondria to repair the damage, return all the way back to the soma for disposal, or be eliminated at the terminals. Increasing evidence suggests that the improper distribution of mitochondria in neurons can lead to neurodegenerative and neuropsychiatric disorders. Here, we will discuss the machinery and regulatory systems used to properly distribute mitochondria in neurons, and how this knowledge has been leveraged to better understand neurological dysfunction.

Mitochondria-cytoskeleton associations in mammalian cytokinesis

Background

The role of the cytoskeleton in regulating mitochondrial distribution in dividing mammalian cells is poorly understood. We previously demonstrated that mitochondria are transported to the cleavage furrow during cytokinesis in a microtubule-dependent manner. However, the exact subset of spindle microtubules and molecular machinery involved remains unknown.

Methods

We employed quantitative imaging techniques and structured illumination microscopy to analyse the spatial and temporal relationship of mitochondria with microtubules and actin of the contractile ring during cytokinesis in HeLa cells.

Results

Superresolution microscopy revealed that mitochondria were associated with astral microtubules of the mitotic spindle in cytokinetic cells. Dominant-negative mutants of KIF5B, the heavy chain of kinesin-1 motor, and of Miro-1 disrupted mitochondrial transport to the furrow. Live imaging revealed that mitochondrial enrichment at the cell equator occurred simultaneously with the appearance of the contractile ring in cytokinesis. Inhibiting RhoA activity and contractile ring assembly with C3 transferase, caused mitochondrial mislocalisation during division.

Conclusions

Taken together, the data suggest a model in which mitochondria are transported by a microtubule-mediated mechanism involving equatorial astral microtubules, Miro-1, and KIF5B to the nascent actomyosin contractile ring in cytokinesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13008-016-0015-4) contains supplementary material, which is available to authorized users.

Myo19 is an outer mitochondrial membrane motor and effector of starvation-induced filopodia

Mitochondria respond to environmental cues and stress conditions. Additionally, the disruption of the mitochondrial network dynamics and its distribution is implicated in a variety of neurodegenerative diseases. Here, we reveal a new function for Myo19 in mitochondrial dynamics and localization during the cellular response to glucose starvation. Ectopically expressed Myo19 localized with mitochondria to the tips of starvation-induced filopodia. Corollary to this, RNA interference (RNAi)-mediated knockdown of Myo19 diminished filopodia formation without evident effects on the mitochondrial network. We analyzed the Myo19-mitochondria interaction, and demonstrated that Myo19 is uniquely anchored to the outer mitochondrial membrane (OMM) through a 30-45-residue motif, indicating that Myo19 is a stably attached OMM molecular motor. Our work reveals a new function for Myo19 in mitochondrial positioning under stress.

Intracellular expression of Tat alters mitochondrial functions in T cells: a potential mechanism to understand mitochondrial damage during HIV-1 replication

Background

HIV-1 replication results in mitochondrial damage that is enhanced during antiretroviral therapy (ART). The onset of HIV-1 replication is regulated by viral protein Tat, a 101-residue protein codified by two exons that elongates viral transcripts. Although the first exon of Tat (aa 1–72) forms itself an active protein, the presence of the second exon (aa 73–101) results in a more competent transcriptional protein with additional functions.

Results

Mitochondrial overall functions were analyzed in Jurkat cells stably expressing full-length Tat (Tat101) or one-exon Tat (Tat72). Representative results were confirmed in PBLs transiently expressing Tat101 and in HIV-infected Jurkat cells. The intracellular expression of Tat101 induced the deregulation of metabolism and cytoskeletal proteins which remodeled the function and distribution of mitochondria. Tat101 reduced the transcription of the mtDNA, resulting in low ATP production. The total amount of mitochondria increased likely to counteract their functional impairment. These effects were enhanced when Tat second exon was expressed.

Conclusions

Intracellular Tat altered mtDNA transcription, mitochondrial content and distribution in CD4+ T cells. The importance of Tat second exon in non-transcriptional functions was confirmed. Tat101 may be responsible for mitochondrial dysfunctions found in HIV-1 infected patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0203-3) contains supplementary material, which is available to authorized users.

Vimentin is involved in regulation of mitochondrial motility and membrane potential by Rac1

In this study we show that binding of mitochondria to vimentin intermediate filaments (VIF) is regulated by GTPase Rac1. The activation of Rac1 leads to a redoubling of mitochondrial motility in murine fibroblasts. Using double-mutants Rac1(G12V, F37L) and Rac1(G12V, Y40H) that are capable to activate different effectors of Rac1, we show that mitochondrial movements are regulated through PAK1 kinase. The involvement of PAK1 kinase is also confirmed by the fact that expression of its auto inhibitory domain (PID) blocks the effect of activated Rac1 on mitochondrial motility. The observed effect of Rac1 and PAK1 kinase on mitochondria depends on phosphorylation of the Ser-55 of vimentin. Besides the effect on motility Rac1 activation also decreases the mitochondrial membrane potential (MMP) which is detected by ∼20% drop of the fluorescence intensity of mitochondria stained with the potential sensitive dye TMRM. One of important consequences of the discovered regulation of MMP by Rac1 and PAK1 is a spatial differentiation of mitochondria in polarized fibroblasts: at the front of the cell they are less energized (by ∼25%) than at the rear part.

Mitochondrial membrane potential is regulated by vimentin intermediate filaments

This study demonstrates that the association of mitochondria with vimentin intermediate filaments (VIFs) measurably increases their membrane potential. This increase is detected by quantitatively comparing the fluorescence intensity of mitochondria stained with the membrane potential-sensitive dye tetramethylrhodamine-ethyl ester (TMRE) in murine vimentin-null fibroblasts with that in the same cells expressing human vimentin (∼35% rise). When vimentin expression is silenced by small hairpin RNA (shRNA) to reduce vimentin by 90%, the fluorescence intensity of mitochondria decreases by 20%. The increase in membrane potential is caused by specific interactions between a subdomain of the non-α-helical N terminus (residues 40 to 93) of vimentin and mitochondria. In rho 0 cells lacking mitochondrial DNA (mtDNA) and consequently missing several key proteins in the mitochondrial respiratory chain (ρ(0) cells), the membrane potential generated by an alternative anaerobic process is insensitive to the interactions between mitochondria and VIF. The results of our studies show that the close association between mitochondria and VIF is important both for determining their position in cells and their physiologic activity.

The Mechanobiology of Aging

Aging is a complex, multifaceted process that induces a myriad of physiological changes over an extended period of time. Aging is accompanied by major biochemical and biomechanical changes at macroscopic and microscopic length scales that affect not only tissues and organs but also cells and subcellular organelles. These changes include transcriptional and epigenetic modifications; changes in energy production within mitochondria; and alterations in the overall mechanics of cells, their nuclei, and their surrounding extracellular matrix. In addition, aging influences the ability of cells to sense changes in extracellular-matrix compliance (mechanosensation) and to transduce these changes into biochemical signals (mechanotransduction). Moreover, following a complex positive-feedback loop, aging is accompanied by changes in the composition and structure of the extracellular matrix, resulting in changes in the mechanics of connective tissues in older individuals. Consequently, these progressive dysfunctions facilitate many human pathologies and deficits that are associated with aging, including cardiovascular, musculoskeletal, and neurodegenerative disorders and diseases. Here, we critically review recent work highlighting some of the primary biophysical changes occurring in cells and tissues that accompany the aging process.

Regulatory functions of microtubules

This mini-review summarizes literature and original data about the role of microtubules in interphase animal cells. Recent data have shown that functioning of microtubules is essential for such diverse phenomena as directional cell movements, distribution of organelles in the cytoplasm, and neuronal memory in the central nervous system. It is suggested that microtubules can act as an important regulatory system in eukaryotic cells. Possible mechanisms of these functions are discussed.

Enhancement of mitochondrial ATP production by the Escherichia coli cytotoxic necrotizing factor 1

Mitochondria are dynamic organelles that constantly change shape and structure in response to different stimuli and metabolic demands of the cell. The Escherichia coli protein toxin cytotoxic necrotizing factor 1 (CNF1) has recently been reported to influence mitochondrial activity in a mouse model of Rett syndrome and to increase ATP content in the brain tissue of an Alzheimer's disease mouse model. In the present work, the ability of CNF1 to influence mitochondrial activity was investigated in IEC-6 normal intestinal crypt cells. In these cells, the toxin was able to induce an increase in cellular ATP content, probably due to an increment of the mitochondrial electron transport chain. In addition, the CNF1-induced Rho GTPase activity also caused changes in the mitochondrial architecture that mainly consisted in the formation of a complex network of elongated mitochondria. The involvement of the cAMP-dependent protein kinase A signaling pathway was postulated. Our results demonstrate that CNF1 positively affects mitochondria by bursting their energetic function and modifying their morphology.

Cell lines

We review the properties and uses of cell lines in Drosophila research, emphasizing the variety of lines, the large body of genomic and transcriptional data available for many of the lines, and the variety of ways the lines have been used to provide tools for and insights into the developmental, molecular, and cell biology of Drosophila and mammals.

A role for myosin II in mammalian mitochondrial fission

Mitochondria are dynamic organelles, undergoing both fission and fusion regularly in interphase cells. Mitochondrial fission is thought to be part of a quality-control mechanism whereby damaged mitochondrial components are segregated from healthy components in an individual mitochondrion, followed by mitochondrial fission and degradation of the damaged daughter mitochondrion. Fission also plays a role in apoptosis. Defects in mitochondrial dynamics can lead to neurodegenerative diseases such as Alzheimer's disease. Mitochondrial fission requires the dynamin GTPase Drp1, which assembles in a ring around the mitochondrion and appears to constrict both outer and inner mitochondrial membranes. However, mechanisms controlling Drp1 assembly on mammalian mitochondria are unclear. Recent results show that actin polymerization, driven by the endoplasmic reticulum-bound formin protein INF2, stimulates Drp1 assembly at fission sites. Here, we show that myosin II also plays a role in fission. Chemical inhibition by blebbistatin or small interfering RNA (siRNA)-mediated suppression of myosin IIA or myosin IIB causes an increase in mitochondrial length in both control cells and cells expressing constitutively active INF2. Active myosin II accumulates in puncta on mitochondria in an actin- and INF2-dependent manner. In addition, myosin II inhibition decreases Drp1 association with mitochondria. Based on these results, we propose a mechanistic model in which INF2-mediated actin polymerization leads to myosin II recruitment and constriction at the fission site, enhancing subsequent Drp1 accumulation and fission.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.

Mitochondrial dynamics in aging and disease

Mitochondria are self-replicating organelles but nevertheless strongly depend on supply coded in nuclear genes. They serve many physiological demands in living cells. Supply of the cytoplasm with ATP and engagement in Ca(2+) regulation belong to the main functions of mitochondria. In large eukaryotic cells, in particular in neurons, with their long dendrites and axons, mitochondria have to move to the sites of their action. This trafficking involves several motor molecules and mechanisms to sense the sites of requirements of mitochondria. With aging and as a consequence of some diseases, mitochondrial components may be rendered dysfunctional, and mtDNA mutations arise during the course of replication and by the action of reactive oxygen species. Mutants in motor molecules engaged in trafficking and in the machinery of fusion and fission are causing severe deficiencies on the cellular level; they support neurodegeneration and, thus, cause many diseases. Frequent fusion and fission events mediate the elimination of impaired parts from mitochondria which finally will be degraded by autophagosomes. Extensive fusion provides a basis for functional complementation. Mobility of proteins and small molecules within the mitochondria is necessary to reach the functional goals of fusion and fission, although cristae and a large fraction of proteins of the respiratory complexes proved to be stable for hours after fusion and perform slow exchange of material.

GEF-H1 controls microtubule-dependent sensing of nucleic acids for antiviral host defenses

Detailed understanding of the signaling intermediates that confer the sensing of intracellular viral nucleic acids for induction of type I interferons is critical for strategies to curtail viral mechanisms that impede innate immune defenses. Here we show that the activation of the microtubule-associated guanine nucleotide exchange factor GEF-H1, encoded by Arhgef2, is essential for sensing of foreign RNA by RIG-I-like receptors. Activation of GEF-H1 controls RIG-I and Mda5-dependent phosphorylation of IRF3 and induction of interferon-β expression in macrophages. Generation of Arhgef2^−/− mice revealed a pronounced signaling defect that prevented antiviral host responses to encephalomyocarditis virus and influenza A virus. Microtubule networks sequester GEF-H1 that upon activation is released to enable antiviral signaling by intracellular nucleic acid detection pathways.

Myosin IIA is critical for organelle distribution and F-actin organization in megakaryocytes and platelets

During proplatelet formation, a relatively homogeneous content of organelles is transported from the megakaryocyte (MK) to the nascent platelets along microtubule tracks. We found that platelets from Myh9(-/-) mice and a MYH9-RD patient were heterogeneous in their organelle content (granules and mitochondria). In addition, Myh9(-/-) MKs have an abnormal cytoplasmic clustering of organelles, suggesting that the platelet defect originates in the MKs. Myosin is not involved in the latest stage of organelle traffic along microtubular tracks in the proplatelet shafts as shown by confocal observations of proplatelet buds. By contrast, it is required for the earlier distribution of organelles within the large MK preplatelet fragments shed into the sinusoid circulation before terminal proplatelet remodeling. We show here that F-actin is abnormally clustered in the cytoplasm of Myh9(-/-) MKs and actin polymerization is impaired in platelets. Myosin IIA is required for normal granule motility and positioning within MKs, mechanisms that may be dependent on organelle traveling and tethering onto F-actin cytoskeleton tracks. Altogether, our results indicate that the distribution of organelles within platelets critically depends on a homogeneous organelle distribution within MKs and preplatelet fragments, which requires myosin IIA.

Mitochondrial trafficking in neurons

Neurons, perhaps more than any other cell type, depend on mitochondrial trafficking for their survival. Recent studies have elucidated a motor/adaptor complex on the mitochondrial surface that is shared between neurons and other animal cells. In addition to kinesin and dynein, this complex contains the proteins Miro (also called RhoT1/2) and milton (also called TRAK1/2) and is responsible for much, although not necessarily all, mitochondrial movement. Elucidation of the complex has permitted inroads for understanding how this movement is regulated by a variety of intracellular signals, although many mysteries remain. Regulating mitochondrial movement can match energy demand to energy supply throughout the extraordinary architecture of these cells and can control the clearance and replenishing of mitochondria in the periphery. Because the extended axons of neurons contain uniformly polarized microtubules, they have been useful for studying mitochondrial motility in conjunction with biochemical assays in many cell types.

Regulation of adrenocortical steroid hormone production by RhoA-diaphanous 1 signaling and the cytoskeleton

The production of glucocorticoids and aldosterone in the adrenal cortex is regulated at multiple levels. Biosynthesis of these hormones is initiated when cholesterol, the substrate, enters the inner mitochondrial membrane for conversion to pregnenolone. Unlike most metabolic pathways, the biosynthesis of adrenocortical steroid hormones is unique because some of the enzymes are localized in mitochondria and others in the endoplasmic reticulum (ER). Although much is known about the factors that control the transcription and activities of the proteins that are required for steroid hormone production, the parameters that govern the exchange of substrates between the ER and mitochondria are less well understood. This short review summarizes studies that have begun to provide insight into the role of the cytoskeleton, mitochondrial transport, and the physical interaction of the ER and mitochondria in the production of adrenocortical steroid hormones.

Biochemical and bioinformatic analysis of the myosin-XIX motor domain

Mitochondrial dynamics are dependent on both the microtubule and actin cytoskeletal systems. Evidence for the involvement of myosin motors has been described in many systems, and until recently a candidate mitochondrial myosin transport motor had not been described in vertebrates. Myosin-XIX (MYO19) was predicted to represent a novel class of myosin and had previously been shown to bind to mitochondria and increase mitochondrial network dynamics when ectopically expressed. Our analyses comparing ∼40 MYO19 orthologs to ∼2000 other myosin motor domain sequences identified instances of homology well-conserved within class XIX myosins that were not found in other myosin classes, suggesting MYO19-specific mechanochemistry. Steady-state biochemical analyses of the MYO19 motor domain indicate that Homo sapiens MYO19 is a functional motor. Insect cell-expressed constructs bound calmodulin as a light chain at the predicted stoichiometry and displayed actin-activated ATPase activity. MYO19 constructs demonstrated high actin affinity in the presence of ATP in actin-co-sedimentation assays, and translocated actin filaments in gliding assays. Expression of GFP-MYO19 containing a mutation impairing ATPase activity did not enhance mitochondrial network dynamics, as occurs with wild-type MYO19, indicating that myosin motor activity is required for mitochondrial motility. The measured biochemical properties of MYO19 suggest it is a high-duty ratio motor that could serve to transport mitochondria or anchor mitochondria, depending upon the cellular microenvironment.

Microtubule plus-ends within a mitotic cell are 'moving platforms' with anchoring, signalling and force-coupling roles

The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.

Synaptic defects in spinal muscular atrophy animal models

Proximal spinal muscular atrophy, the most frequent genetic cause of childhood lethality, is caused by homozygous loss or mutation of the SMN1 gene on human chromosome 5, which codes for the survival motor neuron (SMN) protein. SMN plays a role in the assembly of small nuclear ribonucleoproteins and, additionally, in synaptic function. SMN deficiency produces defects in motor neuron β-actin mRNA axonal transport, neurofilament dynamics, neurotransmitter release, and synapse maturation. The underlying molecular mechanisms and, in particular, the role of the cytoskeleton on the pathogenesis of this disease are starting to be revealed.

Molecular characterization of Toxoplasma gondii formin 3, an actin nucleator dispensable for tachyzoite growth and motility

Toxoplasma gondii belongs to the phylum Apicomplexa, a group of obligate intracellular parasites that rely on gliding motility to enter host cells. Drugs interfering with the actin cytoskeleton block parasite motility, host cell invasion, and egress from infected cells. Myosin A, profilin, formin 1, formin 2, and actin-depolymerizing factor have all been implicated in parasite motility, yet little is known regarding the importance of actin polymerization and other myosins for the remaining steps of the parasite lytic cycle. Here we establish that T. gondii formin 3 (TgFRM3), a newly described formin homology 2 domain (FH2)-containing protein, binds to Toxoplasma actin and nucleates rabbit actin assembly in vitro. TgFRM3 expressed as a transgene exhibits a patchy localization at several distinct structures within the parasite. Disruption of the TgFRM3 gene by double homologous recombination in a ku80-ko strain reveals no vital function for tachyzoite propagation in vitro, which is consistent with its weak level of expression in this life stage. Conditional stabilization of truncated forms of TgFRM3 suggests that different regions of the molecule contribute to distinct localizations. Moreover, expression of TgFRM3 lacking the C-terminal domain severely affects parasite growth and replication. This work provides a first insight into how this specialized formin, restricted to the group of coccidia, completes its actin-nucleating activity.

Calcium regulation of tip growth: new genes for old mechanisms

We review the recent advances on Ca(2+) in tip-growing cells, with a special focus on pollen tubes. New genes for Ca(2+) pumps, channels and sensing proteins have been recently described, with special emphasis on cyclic nucleotide gated channels (CNGCs) and glutamate receptor-like channels (GLRs). We also review the current state of knowledge in what concerns Ca(2+) sensor and relay proteins, where the knowledge of the cell models is less advanced. While these newly described genes offer promise to a better understanding of the spatial and temporal patterns of Ca(2+) signalling that may be relevant for the formation of the phenotype, we discuss the necessity to investigate further links in the network downstream of the Ca(2+) signature, with a special need for mechanisms of feed-back that might render functional feed-back loops approachable by modelling and genetics. Given the available literature, we conclude on the need to investigate more on the role of two specific classes of proteins, the calcium binding protein kinases (CPKs) and the Calcineurin B-like proteins (CBLs) and their regulatory relationships to ion channels (summarized in Figure 3b).

SMN requirement for synaptic vesicle, active zone and microtubule postnatal organization in motor nerve terminals

Low levels of the Survival Motor Neuron (SMN) protein produce Spinal Muscular Atrophy (SMA), a severe monogenetic disease in infants characterized by muscle weakness and impaired synaptic transmission. We report here severe structural and functional alterations in the organization of the organelles and the cytoskeleton of motor nerve terminals in a mouse model of SMA. The decrease in SMN levels resulted in the clustering of synaptic vesicles (SVs) and Active Zones (AZs), reduction in the size of the readily releasable pool (RRP), and the recycling pool (RP) of synaptic vesicles, a decrease in active mitochondria and limiting of neurofilament and microtubule maturation. We propose that SMN is essential for the normal postnatal maturation of motor nerve terminals and that SMN deficiency disrupts the presynaptic organization leading to neurodegeneration.

Lysosomal membrane protein composition, acidic pH and sterol content are regulated via a light-dependent pathway in metazoan cells

In metazoans, lysosomes are characterized by a unique tubular morphology, acidic pH, and specific membrane protein (LAMP) and lipid (cholesterol) composition as well as a soluble protein (hydrolases) composition. Here we show that perturbation to the eye-color gene, light, results in impaired lysosomal acidification, sterol accumulation, altered endosomal morphology as well as compromised lysosomal degradation. We find that Drosophila homologue of Vps41, Light, regulates the fusion of a specific subset of biosynthetic carriers containing characteristic endolysosomal membrane proteins, LAMP1, V0-ATPase and the cholesterol transport protein, NPC1, with the endolysosomal system, and is then required for the morphological progression of the multivesicular endosome. Inhibition of Light results in accumulation of biosynthetic transport intermediates that contain these membrane cargoes, whereas under similar conditions, endosomal delivery of soluble hydrolases, previously shown to be mediated by Dor, the Drosophila homologue of Vps18, is not affected. Unlike Dor, Light is recruited to endosomes in a PI3P-sensitive fashion wherein it facilitates fusion of these biosynthetic cargoes with the endosomes. Depletion of the mammalian counterpart of Light, hVps41, in a human cell line also inhibits delivery of hLAMP to endosomes, suggesting an evolutionarily conserved pathway in metazoa.