Interaction between microtubules and the Drosophila formin Cappuccino and its effect on actin assembly

Formin tails act as a switch, inhibiting or enhancing processive actin elongation

Formins are large, multidomain proteins that nucleate new actin filaments and accelerate elongation through a processive interaction with the barbed ends of filaments. Their actin assembly activity is generally attributed to their eponymous formin homology (FH) 1 and 2 domains; however, evidence is mounting that regions outside of the FH1FH2 stretch also tune actin assembly. Here, we explore the underlying contributions of the tail domain, which spans the sequence between the FH2 domain and the C terminus of formins. Tails vary in length from ∼0 to >200 residues and contain a number of recognizable motifs. The most common and well-studied motif is the ∼15-residue-long diaphanous autoregulatory domain. This domain mediates all or nothing regulation of actin assembly through an intramolecular interaction with the diaphanous inhibitory domain in the N-terminal half of the protein. Multiple reports demonstrate that the tail can enhance both nucleation and processivity. In this study, we provide a high-resolution view of the alternative splicing encompassing the tail in the formin homology domain (Fhod) family of formins during development. While four distinct tails are predicted, we found significant levels of only two of these. We characterized the biochemical effects of the different tails. Surprisingly, the two highly expressed Fhod-tails inhibit processive elongation and diminish nucleation, while a third supports activity. These findings demonstrate a new mechanism of modulating actin assembly by formins and support a model in which splice variants are specialized to build distinct actin structures during development.

Antagonistic Activities of Fmn2 and ADF Regulate Axonal F-Actin Patch Dynamics and the Initiation of Collateral Branching

Interstitial collateral branching of axons is a critical component in the development of functional neural circuits. Axon collateral branches are established through a series of cellular processes initiated by the development of a specialized, focal F-actin network in axons. The formation, maintenance and remodeling of this F-actin patch is critical for the initiation of axonal protrusions that are subsequently consolidated to form a collateral branch. However, the mechanisms regulating F-actin patch dynamics are poorly understood. Fmn2 is a formin family member implicated in multiple neurodevelopmental disorders. We find that Fmn2 regulates the initiation of axon collateral protrusions in chick spinal neurons and in zebrafish motor neurons. Fmn2 localizes to the protrusion-initiating axonal F-actin patches and regulates the lifetime and size of these F-actin networks. The F-actin nucleation activity of Fmn2 is necessary for F-actin patch stability but not for initiating patch formation. We show that Fmn2 insulates the F-actin patches from disassembly by the actin-depolymerizing factor, ADF, and promotes long-lived, larger patches that are competent to initiate axonal protrusions. The regulation of axonal branching can contribute to the neurodevelopmental pathologies associated with Fmn2 and the dynamic antagonism between Fmn2 and ADF may represent a general mechanism of formin-dependent protection of Arp2/3-initiated F-actin networks from disassembly.SIGNIFICANCE STATEMENT Axonal branching is a key process in the development of functional circuits and neural plasticity. Axon collateral branching is initiated by the elaboration of F-actin filaments from discrete axonal F-actin networks. We show that the neurodevelopmental disorder-associated formin, Fmn2, is a critical regulator of axon collateral branching. Fmn2 localizes to the collateral branch-inducing F-actin patches in axons and regulates the stability of these actin networks. The F-actin nucleation activity of Fmn2 protects the patches from ADF-mediated disassembly. Opposing activities of Fmn2 and ADF exert a dynamic regulatory control on axon collateral branch initiation and may underly the neurodevelopmental defects associated with Fmn2.

A Comparative Study of the Role of Formins in Drosophila Embryonic Dorsal Closure

Dorsal closure is a late embryogenesis process required to seal the epidermal hole on the dorsal side of the Drosophila embryo. This process involves the coordination of several forces generated in the epidermal cell layer and in the amnioserosa cells, covering the hole. Ultimately, these forces arise due to cytoskeletal rearrangements that induce changes in cell shape and result in tissue movement. While a number of cytoskeleton regulatory proteins have already been linked to dorsal closure, here we expand this list by demonstrating that four of the six Drosophila formin type actin assembly factors are needed to bring about the proper fusion of the epithelia. An analysis of the morphological and dynamic properties of dorsal closure in formin mutants revealed a differential contribution for each formin, although we found evidence for functional redundancies as well. Therefore, we propose that the four formins promote the formation of several, and only partly identical, actin structures each with a specific role in the mechanics of dorsal closure.

Molecular Dissection of DAAM Function during Axon Growth in Drosophila Embryonic Neurons

Axonal growth is mediated by coordinated changes of the actin and microtubule (MT) cytoskeleton. Ample evidence suggests that members of the formin protein family are involved in the coordination of these cytoskeletal rearrangements, but the molecular mechanisms of the formin-dependent actin–microtubule crosstalk remains largely elusive. Of the six Drosophila formins, DAAM was shown to play a pivotal role during axonal growth in all stages of nervous system development, while FRL was implicated in axonal development in the adult brain. Here, we aimed to investigate the potentially redundant function of these two formins, and we attempted to clarify which molecular activities are important for axonal growth. We used a combination of genetic analyses, cellular assays and biochemical approaches to demonstrate that the actin-processing activity of DAAM is indispensable for axonal growth in every developmental condition. In addition, we identified a novel MT-binding motif within the FH2 domain of DAAM, which is required for proper growth and guidance of the mushroom body axons, while being dispensable during embryonic axon development. Together, these data suggest that DAAM is the predominant formin during axonal growth in Drosophila, and highlight the contribution of multiple formin-mediated mechanisms in cytoskeleton coordination during axonal growth.

New twists in actin-microtubule interactions

Actin filaments and microtubules are cytoskeletal polymers that participate in many vital cell functions including division, morphogenesis, phagocytosis, and motility. Despite the persistent dogma that actin filament and microtubule networks are distinct in localization, structure, and function, a growing body of evidence shows that these elements are choreographed through intricate mechanisms sensitive to either polymer. Many proteins and cellular signals that mediate actin–microtubule interactions have already been identified. However, the impact of these regulators is typically assessed with actin filament or microtubule polymers alone, independent of the other system. Further, unconventional modes and regulators coordinating actin–microtubule interactions are still being discovered. Here we examine several methods of actin–microtubule crosstalk with an emphasis on the molecular links between both polymer systems and their higher-order interactions.

Formin 3 directs dendritic architecture via microtubule regulation and is required for somatosensory nociceptive behavior

Dendrite shape impacts functional connectivity and is mediated by organization and dynamics of cytoskeletal fibers. Identifying the molecular factors that regulate dendritic cytoskeletal architecture is therefore important in understanding the mechanistic links between cytoskeletal organization and neuronal function. We identified Formin 3 (Form3) as an essential regulator of cytoskeletal architecture in nociceptive sensory neurons in Drosophila larvae. Time course analyses reveal that Form3 is cell-autonomously required to promote dendritic arbor complexity. We show that form3 is required for the maintenance of a population of stable dendritic microtubules (MTs), and mutants exhibit defects in the localization of dendritic mitochondria, satellite Golgi, and the TRPA channel Painless. Form3 directly interacts with MTs via FH1-FH2 domains. Mutations in human inverted formin 2 (INF2; ortholog of form3) have been causally linked to Charcot-Marie-Tooth (CMT) disease. CMT sensory neuropathies lead to impaired peripheral sensitivity. Defects in form3 function in nociceptive neurons result in severe impairment of noxious heat-evoked behaviors. Expression of the INF2 FH1-FH2 domains partially recovers form3 defects in MTs and nocifensive behavior, suggesting conserved functions, thereby providing putative mechanistic insights into potential etiologies of CMT sensory neuropathies.

Coupling of dynamic microtubules to F-actin by Fmn2 regulates chemotaxis of neuronal growth cones

Dynamic co-regulation of the actin and microtubule subsystems enables the highly precise and adaptive remodelling of the cytoskeleton necessary for critical cellular processes, such as axonal pathfinding. The modes and mediators of this interpolymer crosstalk, however, are inadequately understood. We identify Fmn2, a non-diaphanous-related formin associated with cognitive disabilities, as a novel regulator of cooperative actin-microtubule remodelling in growth cones of both chick and zebrafish neurons. We show that Fmn2 stabilizes microtubules in the growth cones of cultured spinal neurons and in vivo. Super-resolution imaging revealed that Fmn2 facilitates guidance of exploratory microtubules along actin bundles into the chemosensory filopodia. Using live imaging, biochemistry and single-molecule assays, we show that a C-terminal domain in Fmn2 is necessary for the dynamic association between microtubules and actin filaments. In the absence of the cross-bridging function of Fmn2, filopodial capture of microtubules is compromised, resulting in destabilized filopodial protrusions and deficits in growth cone chemotaxis. Our results uncover a critical function for Fmn2 in actin-microtubule crosstalk in neurons and demonstrate that the modulation of microtubule dynamics via associations with F-actin is central to directional motility.

Plant biology: Plant formins roll out the welcome wagon for microbes

The interactions of microbes with plant cells can radically change plant-cell form and function. A new study shows how a specialised formin protein paves the way for nitrogen-fixing bacteria to make homes in legumes.

Epithelial morphogenesis, tubulogenesis and forces in organogenesis

As multi-cellular organisms evolved from small clusters of cells to complex metazoans, biological tubes became essential for life. Tubes are typically thought of as mainly playing a role in transport, with the hollow space (lumen) acting as a conduit to distribute nutrients and waste, or for gas exchange. However, biological tubes also provide a platform for physiological, mechanical, and structural functions. Indeed, tubulogenesis is often a critical aspect of morphogenesis and organogenesis. C. elegans is made up of tubes that provide structural support and protection (the epidermis), perform the mechanical and enzymatic processes of digestion (the buccal cavity, pharynx, intestine, and rectum), transport fluids for osmoregulation (the excretory system), and execute the functions necessary for reproduction (the germline, spermatheca, uterus and vulva). Here we review our current understanding of the genetic regulation, molecular processes, and physical forces involved in tubulogenesis and morphogenesis of the epidermal, digestive and excretory systems in C. elegans.

AtFH14 crosslinks actin filaments and microtubules in different manners

BACKGROUND INFORMATION

In many cellular processes including cell division, the synergistic dynamics of actin filaments and microtubules play vital roles. However, the regulatory mechanisms of these synergistic dynamics are not fully understood. Proteins such as formins are involved in actin filament-microtubule interactions and Arabidopsis thaliana formin 14 (AtFH14) may function as a crosslinker between actin filaments and microtubules in cell division, but the molecular mechanism underlying such crosslinking remains unclear.

RESULTS

Without microtubules, formin homology (FH) 1/FH2 of AtFH14 nucleated actin polymerisation from actin monomers and capped the barbed end of actin filaments. However, in the presence of microtubules, quantitative analysis showed that the binding affinity of AtFH14 FH1FH2 to microtubules was higher than that to actin filaments. Moreover, microtubule-bound AtFH14 FH1FH2 neither nucleated actin polymerisation nor inhibited barbed end elongation. In contrast, tubulin did not affect AtFH14 FH1FH2 to nucleate actin polymerisation and inhibit barbed end elongation. Nevertheless, microtubule-bound AtFH14 FH1FH2 bound actin filaments and the bound actin filaments slid and elongated along the microtubules or elongated away from the microtubules, which induced bundling or crosslinking of actin filaments and microtubules. Pharmacological analyses indicated that AtFH14 FH1FH2 promoted crosslinking of actin filaments and microtubules in vivo. Additionally, co-sedimentation and fluorescent dye-labelling experiments of AtFH14 FH2-truncated proteins in vitro revealed the essential motifs of bundling actin filaments or microtubules, which were 63-92 aa and 42-62 aa in the AtFH14 FH2 N-terminal, respectively, and 42-62 aa was the essential motif to crosslink actin filaments and microtubules.

CONCLUSIONS AND SIGNIFICANCE

Our results aid in explaining how AtFH14 functions as a crosslinker between actin filaments and microtubules to regulate their dynamics via different manners during cell division. They also facilitate further understanding of the molecular mechanisms of the interactions between actin filaments and microtubules.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Arabidopsis Class II Formins AtFH13 and AtFH14 Can Form Heterodimers but Exhibit Distinct Patterns of Cellular Localization

Formins are evolutionarily conserved multi-domain proteins participating in the control of both actin and microtubule dynamics. Angiosperm formins form two evolutionarily distinct families, Class I and Class II, with class-specific domain layouts. The model plant Arabidopsis thaliana has 21 formin-encoding loci, including 10 Class II members. In this study, we analyze the subcellular localization of two A. thaliana Class II formins exhibiting typical domain organization, the so far uncharacterized formin AtFH13 (At5g58160) and its distant homolog AtFH14 (At1g31810), previously reported to bind microtubules. Fluorescent protein-tagged full length formins and their individual domains were transiently expressed in Nicotiana benthamiana leaves under the control of a constitutive promoter and their subcellular localization (including co-localization with cytoskeletal structures and the endoplasmic reticulum) was examined using confocal microscopy. While the two formins exhibit distinct and only partially overlapping localization patterns, they both associate with microtubules via the conserved formin homology 2 (FH2) domain and with the periphery of the endoplasmic reticulum, at least in part via the N-terminal PTEN (Phosphatase and Tensin)-like domain. Surprisingly, FH2 domains of AtFH13 and AtFH14 can form heterodimers in the yeast two-hybrid assay-a first case of potentially biologically relevant formin heterodimerization mediated solely by the FH2 domain.

Spire stimulates nucleation by Cappuccino and binds both ends of actin filaments

The actin nucleators Spire and Cappuccino synergize to promote actin assembly, but the mechanism of their synergy is controversial. Together these proteins promote the formation of actin meshes, which are conserved structures that regulate the establishment of oocyte polarity. Direct interaction between Spire and Cappuccino is required for oogenesis and for in vitro synergistic actin assembly. This synergy is proposed to be driven by elongation and the formation of a ternary complex at filament barbed ends, or by nucleation and interaction at filament pointed ends. To mimic the geometry of Spire and Cappuccino in vivo, we immobilized Spire on beads and added Cappuccino and actin. Barbed ends, protected by Cappuccino, grow away from the beads while pointed ends are retained, as expected for nucleation-driven synergy. We found that Spire is sufficient to bind barbed ends and retain pointed ends of actin filaments near beads and we identified Spire’s barbed-end binding domain. Loss of barbed-end binding increases nucleation by Spire and synergy with Cappuccino in bulk pyrene assays and on beads. Importantly, genetic rescue by the loss-of-function mutant indicates that barbed-end binding is not necessary for oogenesis. Thus, increased nucleation is a critical element of synergy both in vitro and in vivo.

Spindle-F-actin interactions in mitotic spindles in an intact vertebrate epithelium

Mitotic spindles are well known to be assembled from and dependent on microtubules. In contrast, whether actin filaments (F-actin) are required for or are even present in mitotic spindles has long been controversial. Here we have developed improved methods for simultaneously preserving F-actin and microtubules in fixed samples and exploited them to demonstrate that F-actin is indeed associated with mitotic spindles in intact Xenopus laevis embryonic epithelia. We also find that there is an “F-actin cycle,” in which the distribution and organization of spindle F-actin changes over the course of the cell cycle. Live imaging using a probe for F-actin reveals that at least two pools of F-actin are associated with mitotic spindles: a relatively stable internal network of cables that moves in concert with and appears to be linked to spindles, and F-actin “fingers” that rapidly extend from the cell cortex toward the spindle and make transient contact with the spindle poles. We conclude that there is a robust endoplasmic F-actin network in normal vertebrate epithelial cells and that this network is also a component of mitotic spindles. More broadly, we conclude that there is far more internal F-actin in epithelial cells than is commonly believed.

Formins, Golgi, and the Centriole

Formin homology proteins (formins) are a highly conserved family of cytoskeletal remodeling proteins that are involved in a diverse array of cellular functions. Formins are best known for their ability to regulate actin dynamics, but the same functional domains also govern stability and organization of microtubules. It is thought that this dual activity allows them to coordinate the activity of these two major cytoskeletal networks and thereby influence cellular architecture. Golgi ribbon assembly is dependent upon cooperative interactions between actin filaments and cytoplasmic microtubules originating both at the Golgi itself and from the centrosome. Similarly, centrosome assembly, centriole duplication, and centrosome positioning are also reliant on a dialogue between both cytoskeletal networks. As presented in this chapter, a growing body of evidence suggests that multiple formin proteins play essential roles in these central cellular processes.

Mechanisms of formin-mediated actin assembly and dynamics

Cellular viability requires tight regulation of actin cytoskeletal dynamics. Distinct families of nucleation-promoting factors enable the rapid assembly of filament nuclei that elongate and are incorporated into diverse and specialized actin-based structures. In addition to promoting filament nucleation, the formin family of proteins directs the elongation of unbranched actin filaments. Processive association of formins with growing filament ends is achieved through continuous barbed end binding of the highly conserved, dimeric formin homology (FH) 2 domain. In cooperation with the FH1 domain and C-terminal tail region, FH2 dimers mediate actin subunit addition at speeds that can dramatically exceed the rate of spontaneous assembly. Here, I review recent biophysical, structural, and computational studies that have provided insight into the mechanisms of formin-mediated actin assembly and dynamics.

Drosophila and human FHOD family formin proteins nucleate actin filaments

Formins are a conserved group of proteins that nucleate and processively elongate actin filaments. Among them, the formin homology domain-containing protein (FHOD) family of formins contributes to contractility of striated muscle and cell motility in several contexts. However, the mechanisms by which they carry out these functions remain poorly understood. Mammalian FHOD proteins were reported not to accelerate actin assembly in vitro; instead, they were proposed to act as barbed end cappers or filament bundlers. Here, we show that purified Drosophila Fhod and human FHOD1 both accelerate actin assembly by nucleation. The nucleation activity of FHOD1 is restricted to cytoplasmic actin, whereas Drosophila Fhod potently nucleates both cytoplasmic and sarcomeric actin isoforms. Drosophila Fhod binds tightly to barbed ends, where it slows elongation in the absence of profilin and allows, but does not accelerate, elongation in the presence of profilin. Fhod antagonizes capping protein but dissociates from barbed ends relatively quickly. Finally, we determined that Fhod binds the sides of and bundles actin filaments. This work establishes that Fhod shares the capacity of other formins to nucleate and bundle actin filaments but is notably less effective at processively elongating barbed ends than most well studied formins.

The formin DAAM is required for coordination of the actin and microtubule cytoskeleton in axonal growth cones

Directed axonal growth depends on correct coordination of the actin and microtubule cytoskeleton in the growth cone. However, despite the relatively large number of proteins implicated in actin-microtubule crosstalk, the mechanisms whereby actin polymerization is coupled to microtubule stabilization and advancement in the peripheral growth cone remained largely unclear. Here, we identified the formin Dishevelled-associated activator of morphogenesis (DAAM) as a novel factor playing a role in concerted regulation of actin and microtubule remodeling in Drosophilamelanogaster primary neurons. In vitro, DAAM binds to F-actin as well as to microtubules and has the ability to crosslink the two filament systems. Accordingly, DAAM associates with the neuronal cytoskeleton, and a significant fraction of DAAM accumulates at places where the actin filaments overlap with that of microtubules. Loss of DAAM affects growth cone and microtubule morphology, and several aspects of microtubule dynamics; and biochemical and cellular assays revealed a microtubule stabilization activity and binding to the microtubule tip protein EB1. Together, these data suggest that, besides operating as an actin assembly factor, DAAM is involved in linking actin remodeling in filopodia to microtubule stabilization during axonal growth.

The activities of the C-terminal regions of the formin protein disheveled-associated activator of morphogenesis (DAAM) in actin dynamics

Disheveled-associated activator of morphogenesis (DAAM) is a diaphanous-related formin protein essential for the regulation of actin cytoskeleton dynamics in diverse biological processes. The conserved formin homology 1 and 2 (FH1-FH2) domains of DAAM catalyze actin nucleation and processively mediate filament elongation. These activities are indirectly regulated by the N- and C-terminal regions flanking the FH1-FH2 domains. Recently, the C-terminal diaphanous-autoregulatory domain (DAD) and the C terminus (CT) of formins have also been shown to regulate actin assembly by directly interacting with actin. Here, to better understand the biological activities of DAAM, we studied the role of DAD-CT regions of Drosophila DAAM in its interaction with actin with in vitro biochemical and in vivo genetic approaches. We found that the DAD-CT region binds actin in vitro and that its main actin-binding element is the CT region, which does not influence actin dynamics on its own. However, we also found that it can tune the nucleating activity and the filament end-interaction properties of DAAM in an FH2 domain-dependent manner. We also demonstrate that DAD-CT makes the FH2 domain more efficient in antagonizing with capping protein. Consistently, in vivo data suggested that the CT region contributes to DAAM-mediated filopodia formation and dynamics in primary neurons. In conclusion, our results demonstrate that the CT region of DAAM plays an important role in actin assembly regulation in a biological context.

Dynamic microtubules regulate cellular contractility during T-cell activation

T-cell receptor (TCR) triggering and subsequent T-cell activation are essential for the adaptive immune response. Recently, multiple lines of evidence have shown that force transduction across the TCR complex is involved during TCR triggering, and that the T cell might use its force-generation machinery to probe the mechanical properties of the opposing antigen-presenting cell, giving rise to different signaling and physiological responses. Mechanistically, actin polymerization and turnover have been shown to be essential for force generation by T cells, but how these actin dynamics are regulated spatiotemporally remains poorly understood. Here, we report that traction forces generated by T cells are regulated by dynamic microtubules (MTs) at the interface. These MTs suppress Rho activation, nonmuscle myosin II bipolar filament assembly, and actin retrograde flow at the T-cell-substrate interface. Our results suggest a novel role of the MT cytoskeleton in regulating force generation during T-cell activation.

Cytoplasmic Streaming in the Drosophila Oocyte

Objects are commonly moved within the cell by either passive diffusion or active directed transport. A third possibility is advection, in which objects within the cytoplasm are moved with the flow of the cytoplasm. Bulk movement of the cytoplasm, or streaming, as required for advection, is more common in large cells than in small cells. For example, streaming is observed in elongated plant cells and the oocytes of several species. In the Drosophila oocyte, two stages of streaming are observed: relatively slow streaming during mid-oogenesis and streaming that is approximately ten times faster during late oogenesis. These flows are implicated in two processes: polarity establishment and mixing. In this review, I discuss the underlying mechanism of streaming, how slow and fast streaming are differentiated, and what we know about the physiological roles of the two types of streaming.

An mDia1-INF2 formin activation cascade facilitated by IQGAP1 regulates stable microtubules in migrating cells

The formin INF2 is required for stable Glu microtubule formation and inhibition of microtubule dynamics in NIH3T3 cells downstream of mDia1 and LPA. Evidence also shows that the formation of an mDia1/INF2 complex is necessary for microtubule stabilization stimulated by LPA and is regulated by IQGAP1.

Multiple formins regulate microtubule (MT) arrays, but whether they function individually or in a common pathway is unknown. Lysophosphatidic acid (LPA) stimulates the formation of stabilized detyrosinated MTs (Glu MTs) in NIH3T3 fibroblasts through RhoA and the formin mDia1. Here we show that another formin, INF2, is necessary for mDia1-mediated induction of Glu MTs and regulation of MT dynamics and that mDia1 can be bypassed by activating INF2. INF2 localized to MTs after LPA treatment in an mDia1-dependent manner, suggesting that mDia1 regulates INF2. Mutants of either formin that disrupt their interaction failed to rescue MT stability in cells depleted of the respective formin, and the mDia1-interacting protein IQGAP1 regulated INF2’s localization to MTs and the induction of Glu MTs by either formin. The N-terminus of IQGAP1 associated with the C-terminus of INF2 directly, suggesting the possibility of a tripartite complex stimulated by LPA. Supporting this, the interaction of mDia1 and INF2 was induced by LPA and dependent on IQGAP1. Our data highlight a unique mechanism of formin action in which mDia1 and INF2 function in series to stabilize MTs and point to IQGAP1 as a scaffold that facilitates the activation of one formin by another.

Reorganization of actin filaments by ADF/cofilin is involved in formation of microtubule structures during Xenopus oocyte maturation

We examined the reorganization of actin filaments and microtubules during Xenopus oocyte maturation. Surrounding the germinal vesicle (GV) in immature oocytes, the cytoplasmic actin filaments reorganized to accumulate beneath the vegetal side of the GV, where the microtubule-organizing center and transient microtubule array (MTOC-TMA) assembled, just before GV breakdown (GVBD). Immediately after GVBD, both Xenopus ADF/cofilin (XAC) and its phosphatase Slingshot (XSSH) accumulated into the nuclei and intranuclear actin filaments disassembled from the vegetal side with the shrinkage of the GV. As the MTOC-TMA developed well, cytoplasmic actin filaments were retained at the MTOC-TMA base region. Suppression of XAC dephosphorylation by anti-XSSH antibody injection inhibited both actin filament reorganization and proper formation and localization of both the MTOC-TMA and meiotic spindles. Stabilization of actin filaments by phalloidin also inhibited formation of the MTOC-TMA and disassembly of intranuclear actin filaments without affecting nuclear shrinkage. Nocodazole also caused the MTOC-TMA and the cytoplasmic actin filaments at its base region to disappear, which further impeded disassembly of intranuclear actin filaments from the vegetal side. XAC appears to reorganize cytoplasmic actin filaments required for precise assembly of the MTOC and, together with the MTOC-TMA, regulate the intranuclear actin filament disassembly essential for meiotic spindle formation.

Connecting the cytoskeleton to the endoplasmic reticulum and Golgi

A tendency in cell biology is to divide and conquer. For example, decades of painstaking work have led to an understanding of endoplasmic reticulum (ER) and Golgi structure, dynamics, and transport. In parallel, cytoskeletal researchers have revealed a fantastic diversity of structure and cellular function in both actin and microtubules. Increasingly, these areas overlap, necessitating an understanding of both organelle and cytoskeletal biology. This review addresses connections between the actin/microtubule cytoskeletons and organelles in animal cells, focusing on three key areas: ER structure and function; ER-to-Golgi transport; and Golgi structure and function. Making these connections has been challenging for several reasons: the small sizes and dynamic characteristics of some components; the fact that organelle-specific cytoskeletal elements can easily be obscured by more abundant cytoskeletal structures; and the difficulties in imaging membranes and cytoskeleton simultaneously, especially at the ultrastructural level. One major concept is that the cytoskeleton is frequently used to generate force for membrane movement, with two potential consequences: translocation of the organelle, or deformation of the organelle membrane. While initially discussing issues common to metazoan cells in general, we subsequently highlight specific features of neurons, since these highly polarized cells present unique challenges for organellar distribution and dynamics.

Drosophila Cappuccino alleles provide insight into formin mechanism and role in oogenesis

During Drosophila development, the formin actin nucleator Cappuccino (Capu) helps build a cytoplasmic actin mesh throughout the oocyte. Loss of Capu leads to female sterility, presumably because polarity determinants fail to localize properly in the absence of the mesh. To gain deeper insight into how Capu builds this actin mesh, we systematically characterized seven capu alleles, which have missense mutations in Capu's formin homology 2 (FH2) domain. We report that all seven alleles have deleterious effects on fly fertility and the actin mesh in vivo but have strikingly different effects on Capu's biochemical activity in vitro. Using a combination of bulk and single- filament actin-assembly assays, we find that the alleles differentially affect Capu's ability to nucleate and processively elongate actin filaments. We also identify a unique "loop" in the lasso region of Capu's FH2 domain. Removing this loop enhances Capu's nucleation, elongation, and F-actin-bundling activities in vitro. Together our results on the loop and the seven missense mutations provides mechanistic insight into formin function in general and Capu's role in the Drosophila oocyte in particular.

The role of formin tails in actin nucleation, processive elongation, and filament bundling

Formins are multidomain proteins that assemble actin in a wide variety of biological processes. They both nucleate and remain processively associated with growing filaments, in some cases accelerating filament growth. The well conserved formin homology 1 and 2 domains were originally thought to be solely responsible for these activities. Recently a role in nucleation was identified for the Diaphanous autoinhibitory domain (DAD), which is C-terminal to the formin homology 2 domain. The C-terminal tail of the Drosophila formin Cappuccino (Capu) is conserved among FMN formins but distinct from other formins. It does not have a DAD domain. Nevertheless, we find that Capu-tail plays a role in filament nucleation similar to that described for mDia1 and other formins. Building on this, replacement of Capu-tail with DADs from other formins tunes nucleation activity. Capu-tail has low-affinity interactions with both actin monomers and filaments. Removal of the tail reduces actin filament binding and bundling. Furthermore, when the tail is removed, we find that processivity is compromised. Despite decreased processivity, the elongation rate of filaments is unchanged. Again, replacement of Capu-tail with DADs from other formins tunes the processive association with the barbed end, indicating that this is a general role for formin tails. Our data show a role for the Capu-tail domain in assembling the actin cytoskeleton, largely mediated by electrostatic interactions. Because of its multifunctionality, the formin tail is a candidate for regulation by other proteins during cytoskeletal rearrangements.

Bacterial subversion of host cytoskeletal machinery: hijacking formins and the Arp2/3 complex

The host actin nucleation machinery is subverted by many bacterial pathogens to facilitate their entry, motility, replication, and survival. The majority of research conducted in the past primarily focused on exploitation of a host actin nucleator, the Arp2/3 complex, by bacterial pathogens. Recently, new studies have begun to explore the role of formins, another family of host actin nucleators, in bacterial pathogenesis. This review provides an overview of recent advances in the study of the exploitation of the Arp2/3 complex and formins by bacterial pathogens. Secreted bacterial effector proteins seem to manipulate the regulation of these actin nucleators or functionally mimic them to drive bacterial entry, motility and survival within host cells. An enhanced understanding of how formins are exploited will provide us with greater insight into how a fundamental eurkaryotic cellular process is utilized by bacteria and will also advance our knowledge of host-pathogen interactions.