Novel espin actin-bundling proteins are localized to Purkinje cell dendritic spines and bind the Src homology 3 adapter protein insulin receptor substrate p53

Tunnelling nanotube formation is driven by Eps8/IRSp53-dependent linear actin polymerization

Tunnelling nanotubes (TNTs) connect distant cells and mediate cargo transfer for intercellular communication in physiological and pathological contexts. How cells generate these actin-mediated protrusions to span lengths beyond those attainable by canonical filopodia remains unknown. Through a combination of micropatterning, microscopy, and optical tweezer-based approaches, we demonstrate that TNTs formed through the outward extension of actin achieve distances greater than the mean length of filopodia and that branched Arp2/3-dependent pathways attenuate the extent to which actin polymerizes in nanotubes, thus limiting their occurrence. Proteomic analysis using epidermal growth factor receptor kinase substrate 8 (Eps8) as a positive effector of TNTs showed that, upon Arp2/3 inhibition, proteins enhancing filament turnover and depolymerization were reduced and Eps8 instead exhibited heightened interactions with the inverted Bin/Amphiphysin/Rvs (I-BAR) domain protein IRSp53 that provides a direct connection with linear actin polymerases. Our data reveals how common protrusion players (Eps8 and IRSp53) form tunnelling nanotubes, and that when competing pathways overutilizing such proteins and monomeric actin in Arp2/3 networks are inhibited, processes promoting linear actin growth dominate to favour tunnelling nanotube formation.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

Studies of functional properties of espin 1: Its interaction to actin filaments

Actin is a multifunctional biomolecule that forms not only basic structural bodies such as filopodia and lamellipodia, but also large microvilli-like organelles like stereocilia. Actin consists of four sub-domains (S1, S2, S3, and S4), and the “target-binding groove” formed between S1 and S3 is the major binding site for various actin binding proteins. Actin filament dynamics are regulated by numerous actin binding proteins with different mechanisms of actin binding, assembly, and disassembly such as actin severing, branching, and bundling. Ectoplasmic specialization protein 1 (espin 1) is an actin binding and bundling protein that is specifically implicated in the elongation and stabilization of stereocilia as a binding partner with myosin III. However, little is known about the molecular structure, actin bundling, and stabilizing mechanism of espin 1; hence, we investigated the interaction between actin and espin 1 through structural data. In this study, we first purified human espin 1 in an E. coli system following a new detergent-free approach and then demonstrated the 2D structure of full-length espin 1 using transmission electron microscopy along with Nickel nitrilotriacetic acid nanogold labeling and 2D averaging using SPIDER. Furthermore, we also determined the espin 1 binding domain of actin using a co-sedimentation assay along with gelsolin and myosin S1. These findings are not only beneficial for understanding the actin binding and bundling mechanism of espin 1, but also shed light on its elongation, stabilization, and tip-localization mechanisms with myosin III. This study thus provides a basis for understanding the molecular structure of espin 1 and can contribute to various hearing-related diseases, such as hearing loss and vestibular dysfunction.

Loss of Baiap2l2 destabilizes the transducing stereocilia of cochlear hair cells and leads to deafness

KEY POINTS

Mechanoelectrical transduction at auditory hair cells requires highly specialized stereociliary bundles that project from their apical surface, forming a characteristic graded 'staircase' structure. The morphogenesis and maintenance of these stereociliary bundles is a tightly regulated process requiring the involvement of several actin-binding proteins, many of which are still unidentified. We identify a new stereociliary protein, the I-BAR protein BAIAP2L2, which localizes to the tips of the shorter transducing stereocilia in both inner and outer hair cells (IHCs and OHCs). We find that Baiap2l2 deficient mice lose their second and third rows of stereocilia, their mechanoelectrical transducer current, and develop progressive hearing loss, becoming deaf by 8 months of age. We demonstrate that BAIAP2L2 localization to stereocilia tips is dependent on the motor protein MYO15A and its cargo EPS8. We propose that BAIAP2L2 is a new key protein required for the maintenance of the transducing stereocilia in mature cochlear hair cells.

ABSTRACT

The transduction of sound waves into electrical signals depends upon mechanosensitive stereociliary bundles that project from the apical surface of hair cells within the cochlea. The height and width of these actin-based stereocilia is tightly regulated throughout life to establish and maintain their characteristic staircase-like structure, which is essential for normal mechanoelectrical transduction. Here, we show that BAIAP2L2, a member of the I-BAR protein family, is a newly identified hair bundle protein that is localized to the tips of the shorter rows of transducing stereocilia in mouse cochlear hair cells. BAIAP2L2 was detected by immunohistochemistry from postnatal day 2.5 (P2.5) throughout adulthood. In Baiap2l2 deficient mice, outer hair cells (OHCs), but not inner hair cells (IHCs), began to lose their third row of stereocilia and showed a reduction in the size of the mechanoelectrical transducer current from just after P9. Over the following post-hearing weeks, the ordered staircase structure of the bundle progressively deteriorates, such that, by 8 months of age, both OHCs and IHCs of Baiap2l2 deficient mice have lost most of the second and third rows of stereocilia and become deaf. We also found that BAIAP2L2 interacts with other key stereociliary proteins involved in normal hair bundle morphogenesis, such as CDC42, RAC1, EPS8 and ESPNL. Furthermore, we show that BAIAP2L2 localization to the stereocilia tips depends on the motor protein MYO15A and its cargo EPS8. We propose that BAIAP2L2 is key to maintenance of the normal actin structure of the transducing stereocilia in mature mouse cochlear hair cells.

Myosin XVI Regulates Actin Cytoskeleton Dynamics in Dendritic Spines of Purkinje Cells and Affects Presynaptic Organization

The actin cytoskeleton is crucial for function and morphology of neuronal synapses. Moreover, altered regulation of the neuronal actin cytoskeleton has been implicated in neuropsychiatric diseases such as autism spectrum disorder (ASD). Myosin XVI is a neuronally expressed unconventional myosin known to bind the WAVE regulatory complex (WRC), a regulator of filamentous actin (F-actin) polymerization. Notably, the gene encoding the myosin’s heavy chain (MYO16) shows genetic association with neuropsychiatric disorders including ASD. Here, we investigated whether myosin XVI plays a role for actin cytoskeleton regulation in the dendritic spines of cerebellar Purkinje cells (PCs), a neuronal cell type crucial for motor learning, social cognition and vocalization. We provide evidence that both myosin XVI and the WRC component WAVE1 localize to PC spines. Fluorescence recovery after photobleaching (FRAP) analysis of GFP-actin in cultured PCs shows that Myo16 knockout as well as PC-specific Myo16 knockdown, lead to faster F-actin turnover in the dendritic spines of PCs. We also detect accelerated F-actin turnover upon interference with the WRC, and upon inhibition of Arp2/3 that drives formation of branched F-actin downstream of the WRC. In contrast, inhibition of formins that are responsible for polymerization of linear actin filaments does not cause faster F-actin turnover. Together, our data establish myosin XVI as a regulator of the postsynaptic actin cytoskeleton and suggest that it is an upstream activator of the WRC-Arp2/3 pathway in PC spines. Furthermore, ultra-structural and electrophysiological analyses of Myo16 knockout cerebellum reveals the presence of reduced numbers of synaptic vesicles at presynaptic terminals in the absence of the myosin. Therefore, we here define myosin XVI as an F-actin regulator important for presynaptic organization in the cerebellum.

Regulation of the apical extension morphogenesis tunes the mechanosensory response of microvilliated neurons

Multiple types of microvilliated sensory cells exhibit an apical extension thought to be instrumental in the detection of sensory cues. The investigation of the mechanisms underlying morphogenesis of sensory apparatus is critical to understand the biology of sensation. Most of what we currently know comes from the study of the hair bundle of the inner ear sensory cells, but morphogenesis and function of other sensory microvilliated apical extensions remain poorly understood. We focused on spinal sensory neurons that contact the cerebrospinal fluid (CSF) through the projection of a microvilliated apical process in the central canal, referred to as cerebrospinal fluid-contacting neurons (CSF-cNs). CSF-cNs respond to pH and osmolarity changes as well as mechanical stimuli associated with changes of flow and tail bending. In vivo time-lapse imaging in zebrafish embryos revealed that CSF-cNs are atypical neurons that do not lose their apical attachment and form a ring of actin at the apical junctional complexes (AJCs) that they retain during differentiation. We show that the actin-based protrusions constituting the microvilliated apical extension arise and elongate from this ring of actin, and we identify candidate molecular factors underlying every step of CSF-cN morphogenesis. We demonstrate that Crumbs 1 (Crb1), Myosin 3b (Myo3b), and Espin orchestrate the morphogenesis of CSF-cN apical extension. Using calcium imaging in crb1 and espin mutants, we further show that the size of the apical extension modulates the amplitude of CSF-cN sensory response to bending of the spinal cord. Based on our results, we propose that the apical actin ring could be a common site of initiation of actin-based protrusions in microvilliated sensory cells. Furthermore, our work provides a set of actors underlying actin-based protrusion elongation shared by different sensory cell types and highlights the critical role of the apical extension shape in sensory detection.

A study of differentiating spinal sensory neurons in vivo uncovers critical factors required for the morphogenesis of sensory microvilli and reveals fine modulation of mechanosensory responses by microvillus length.

Building and repairing the stereocilia cytoskeleton in mammalian auditory hair cells

Despite all recent achievements in identification of the molecules that are essential for the structure and mechanosensory function of stereocilia bundles in the auditory hair cells of mammalian species, we still have only a rudimentary understanding of the mechanisms of stereocilia formation, maintenance, and repair. Important molecular differences distinguishing mammalian auditory hair cells from hair cells of other types and species have been recently revealed. In addition, we are beginning to solve the puzzle of the apparent life-long stability of the stereocilia bundles in these cells. New data link the stability of the cytoskeleton in the mammalian auditory stereocilia with the normal activity of mechanotransduction channels. These data suggest new ideas on how a terminally-differentiated non-regenerating hair cell in the mammalian cochlea may repair and tune its stereocilia bundle throughout the life span of the organism.

Lhx1/5 control dendritogenesis and spine morphogenesis of Purkinje cells via regulation of Espin

In the cerebellar cortex, Purkinje cells (PCs) receive signals from different inputs through their extensively branched dendrites and serve as an integration centre. Defects in the dendritic development of PCs thus disrupt cerebellar circuitry and cause ataxia. Here we report that specific inactivation of both Lhx1 and Lhx5 in postnatal PCs results in ataxic mutant mice with abnormal dendritic development. The PCs in the mutants have reduced expression of Espin, an F-actin cytoskeleton regulator. We show that Espin expression is transcriptionally activated by Lhx1/5. Downregulation of Espin leads to F-actin mislocalization, thereby impairing dendritogenesis and dendritic spine maturation in the PCs. The mutant PCs therefore fail to form proper synapses and show aberrant electrophysiological properties. By overexpressing Espin, we can successfully rescue the defects in the mutant PCs. Our findings suggest that Lhx1/5, through regulating Espin expression, control dendritogenesis and spine morphogenesis in postnatal PCs.

Purkinje cells (PCs) receive signals from different inputs through their extensively branched dendrites and dysregulation of this process leads to ataxia and other diseases. Here the authors show that the LIM-homeodomain transcription factors Lhx1 and Lhx5 govern dendritogenesis and dendritic spine morphogenesis in postnatal PCs through regulating Espin expression.

Stereocilia-staircase spacing is influenced by myosin III motors and their cargos espin-1 and espin-like

Hair cells tightly control the dimensions of their stereocilia, which are actin-rich protrusions with graded heights that mediate mechanotransduction in the inner ear. Two members of the myosin-III family, MYO3A and MYO3B, are thought to regulate stereocilia length by transporting cargos that control actin polymerization at stereocilia tips. We show that eliminating espin-1 (ESPN-1), an isoform of ESPN and a myosin-III cargo, dramatically alters the slope of the stereocilia staircase in a subset of hair cells. Furthermore, we show that espin-like (ESPNL), primarily present in developing stereocilia, is also a myosin-III cargo and is essential for normal hearing. ESPN-1 and ESPNL each bind MYO3A and MYO3B, but differentially influence how the two motors function. Consequently, functional properties of different motor-cargo combinations differentially affect molecular transport and the length of actin protrusions. This mechanism is used by hair cells to establish the required range of stereocilia lengths within a single cell.

Myosin III-mediated cross-linking and stimulation of actin bundling activity of Espin

Class III myosins (Myo3) and actin-bundling protein Espin play critical roles in regulating the development and maintenance of stereocilia in vertebrate hair cells, and their defects cause hereditary hearing impairments. Myo3 interacts with Espin1 through its tail homology I motif (THDI), however it is not clear how Myo3 specifically acts through Espin1 to regulate the actin bundle assembly and stabilization. Here we discover that Myo3 THDI contains a pair of repeat sequences capable of independently and strongly binding to the ankyrin repeats of Espin1, revealing an unexpected Myo3-mediated cross-linking mechanism of Espin1. The structures of Myo3 in complex with Espin1 not only elucidate the mechanism of the binding, but also reveal a Myo3-induced release of Espin1 auto-inhibition mechanism. We also provide evidence that Myo3-mediated cross-linking can further promote actin fiber bundling activity of Espin1.

IRSp53/BAIAP2 in dendritic spine development, NMDA receptor regulation, and psychiatric disorders

IRSp53 (also known as BAIAP2) is a multi-domain scaffolding and adaptor protein that has been implicated in the regulation of membrane and actin dynamics at subcellular structures, including filopodia and lamellipodia. Accumulating evidence indicates that IRSp53 is an abundant component of the postsynaptic density at excitatory synapses and an important regulator of actin-rich dendritic spines. In addition, IRSp53 has been implicated in diverse psychiatric disorders, including autism spectrum disorders, schizophrenia, and attention deficit/hyperactivity disorder. Mice lacking IRSp53 display enhanced NMDA (N-methyl-d-aspartate) receptor function accompanied by social and cognitive deficits, which are reversed by pharmacological suppression of NMDA receptor function. These results suggest the hypothesis that defective actin/membrane modulation in IRSp53-deficient dendritic spines may lead to social and cognitive deficits through NMDA receptor dysfunction. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Postsynaptic distribution of IRSp53 in spiny excitatory and inhibitory neurons

The 53 kDa insulin receptor substrate protein (IRSp53) is highly enriched in the brain. Despite evidence that links mutations of IRSp53 with autism and other neuropsychiatric problems, the functional significance of this protein remains unclear. We used light and electron microscopic immunohistochemistry to demonstrate that IRSp53 is expressed throughout the adult rat brain. Labeling concentrated selectively in dendritic spines, where it was associated with the postsynaptic density (PSD). Surprisingly, its organization within the PSD of spiny excitatory neurons of neocortex and hippocampus differed from that within spiny inhibitory neurons of neostriatum and cerebellar cortex. The present data support previous suggestions that IRSp53 is involved in postsynaptic signaling, while hinting that its signaling role may differ in different types of neurons.

Mechanism of IRSp53 inhibition and combinatorial activation by Cdc42 and downstream effectors

The Rho family GTPase effector IRSp53 has essential roles in filopodia formation and neuronal development, but its regulatory mechanism is poorly understood. IRSp53 contains a membrane-binding BAR domain followed by an unconventional CRIB motif that overlaps with a proline-rich region (CRIB-PR) and an SH3 domain that recruits actin cytoskeleton effectors. Using a fluorescence reporter assay, we show that human IRSp53 adopts a closed inactive conformation that opens synergistically with the binding of human Cdc42 to the CRIB-PR and effector proteins, such as the tumor-promoting factor Eps8, to the SH3 domain. The crystal structure of Cdc42 bound to the CRIB-PR reveals a new mode of effector binding to Rho family GTPases. Structure-inspired mutations disrupt autoinhibition and Cdc42 binding in vitro and decouple Cdc42- and IRSp53-dependent filopodia formation in cells. The data support a combinatorial mechanism of IRSp53 activation.

Characterization and regulation of an additional actin-filament-binding site in large isoforms of the stereocilia actin-bundling protein espin

The espin actin-bundling proteins, which are produced as isoforms of different sizes from a single gene, are required for the growth of hair cell stereocilia. We have characterized an additional actin-filament-binding site present in the extended amino-termini of large espin isoforms. Constitutively active in espin 2, the site increased the size of actin bundles formed in vitro and inhibited actin fluorescence recovery in microvilli. In espin 1, which has an N-terminal ankyrin repeat domain, the site was autoinhibited by binding between the ankyrin repeat domain and a peptide near the actin-binding site. Deletion of this peptide from espin 1 activated its actin-binding site. The peptide resembled tail homology domain I of myosin III, a ligand of the ankyrin repeat domain localized with espin 1 at the tip of stereocilia. A myosin III tail homology domain I peptide, but not scrambled control peptides, inhibited internal binding of the ankyrin repeat domain and released the espin 1 actin-binding site from autoinhibition. Thus, this regulation could result in local activation of the additional actin-binding site of espin 1 by myosin III in stereocilia.

Membrane binding properties of IRSp53-missing in metastasis domain (IMD) protein

The 53-kDa insulin receptor substrate protein (IRSp53) organizes the actin cytoskeleton in response to stimulation of small GTPases, promoting the formation of cell protrusions such as filopodia and lamellipodia. IMD is the N-terminal 250 amino acid domain (IRSp53/MIM Homology Domain) of IRSp53 (also called I-BAR), which can bind to negatively charged lipid molecules. Overexpression of IMD induces filopodia formation in cells and purified IMD assembles finger-like protrusions in reconstituted lipid membranes. IMD was shown by several groups to bundle actin filaments, but other groups showed that it also binds to membranes. IMD binds to negatively charged lipid molecules with preference to clusters of PI(4,5)P2. Here, we performed a range of different in vitro fluorescence experiments to determine the binding properties of the IMD to phospholipids. We used different constructs of large unilamellar vesicles (LUVETs), containing neutral or negatively charged phospholipids. We found that IMD has a stronger binding interaction with negatively charged PI(4,5)P2 or PS lipids than PS/PC or neutral PC lipids. The equilibrium dissociation constant for the IMD-lipid interaction falls into the 78-170μM range for all the lipids tested. The solvent accessibility of the fluorescence labels on the IMD during its binding to lipids is also reduced as the lipids become more negatively charged. Actin affects the IMD-lipid interaction, depending on its polymerization state. Monomeric actin partially disrupts the binding, while filamentous actin can further stabilize the IMD-lipid interaction.

The neuronal p35 activator of Cdk5 is a novel F-actin binding and bundling protein

The neuronal Cdk5 activator p35 is involved in a multitude of neuronal activities, including cytoskeletal organization. We show here that p35 directly interacts with filamentous actin (F-actin) but not with monomeric actin (G-actin). Through binding, p35 induces the formation of actin bundles and stabilizes F-actin against dilution-induced depolymerization. p35 forms intermolecular self-associations, suggesting that p35 cross-links actin filaments into bundles via its intermolecular self-association. p35 dimerization and association with F-actin occur at the N-terminal region that is absent in the calpain-cleaved product p25, indicating that such p35 properties are lost by its truncation induced under neurotoxic conditions. Using p35 phosphorylated by Cdk5 and a mutational approach, we demonstrate that the phosphorylation of p35 promotes its homodimerization and p35-induced formation of F-actin bundles. In addition, the phosphorylation regulates p35 distribution to microtubule and actin cytoskeletons. Together, these observations define a novel function for p35 in cytoskeletal regulation.

Roles of the espin actin-bundling proteins in the morphogenesis and stabilization of hair cell stereocilia revealed in CBA/CaJ congenic jerker mice

Hearing and vestibular function depend on mechanosensory staircase collections of hair cell stereocilia, which are produced from microvillus-like precursors as their parallel actin bundle scaffolds increase in diameter and elongate or shorten. Hair cell stereocilia contain multiple classes of actin-bundling protein, but little is known about what each class contributes. To investigate the roles of the espin class of actin-bundling protein, we used a genetic approach that benefited from a judicious selection of mouse background strain and an examination of the effects of heterozygosity. A congenic jerker mouse line was prepared by repeated backcrossing into the inbred CBA/CaJ strain, which is known for excellent hearing and minimal age-related hearing loss. We compared stereocilia in wild-type CBA/CaJ mice, jerker homozygotes that lack espin proteins owing to a frameshift mutation in the espin gene, and jerker heterozygotes that contain reduced espin levels. The lack of espins radically impaired stereociliary morphogenesis, resulting in stereocilia that were abnormally thin and short, with reduced differential elongation to form a staircase. Mean stereociliary diameter did not increase beyond ∼0.10–0.14 µm, making stereocilia ∼30%–60% thinner than wild type and suggesting that they contained ∼50%–85% fewer actin filaments. These characteristics indicate a requirement for espins in the appositional growth and differential elongation of the stereociliary parallel actin bundle and fit the known biological activities of espins in vitro and in transfected cells. The stereocilia of jerker heterozygotes showed a transient proximal-distal tapering suggestive of haploinsufficiency and a slowing of morphogenesis that revealed previously unrecognized assembly steps and intermediates. The lack of espins also led to a region-dependent degeneration of stereocilia involving shortening and collapse. We conclude that the espin actin-bundling proteins are required for the assembly and stabilization of the stereociliary parallel actin bundle.

Stereocilia are the fingerlike projections of inner ear hair cells that detect sound and motion. Stereocilia grow to specific lengths and diameters and form staircase-like arrays. The changes in size appear to be driven by matching alterations in the dimensions of an underlying molecular scaffold consisting of a bundle of actin filaments cross-linked by actin-bundling proteins. To elucidate the roles of the espin actin-bundling proteins in hair cell stereocilia, we carry out an in-depth accounting of stereociliary size and shape in the jerker mutant mouse, which lacks the espin proteins because of a mutation in the espin gene. We examine a new and improved jerker mouse with a genetic background known for high-quality lifelong hearing. We find that, in the absence of espins, stereocilia do not increase in diameter or complete their elongation, but instead bend, shorten, and disappear. Although the specifics vary according to inner ear region, the stereociliary defects are profound and can readily account for the deafness and balance problems of jerker mice and humans with certain espin gene mutations. Even reducing espin levels by one-half leads to temporary defects in stereociliary diameter. Thus, espins play crucial roles in the formation and maintenance of hair cell stereocilia.

The postsynaptic density protein, IQ-ArfGEF/BRAG1, can interact with IRSp53 through its proline-rich sequence

IQ-ArfGEF/BRAG1, a guanine nucleotide exchange factor for Arf1 and Arf6, is localized at the postsynaptic density (PSD) and interacts with PSD-95. In this study, we identified a novel interaction of IQ-ArfGEF/BRAG1 with insulin receptor tyrosine kinase substrate of 53 kDa (IRSp53), also known as brain-specific angiogenesis inhibitor 1-associated protein 2. The interaction was mediated by the binding of the C-terminal proline-rich sequence of IQ-ArfGEF/BRAG1 to the SH3 domain of IRSp53. IQ-ArfGEF/BRAG1 and IRSp53 were colocalized at the PSD of excitatory synapses of certain neuronal populations. Our present findings suggest that IQ-ArfGEF/BRAG1 may play roles downstream of NMDA receptors through the interaction with multivalent PSD proteins such as IRSp53 and PSD-95.

Espin actin-cytoskeletal proteins are in rat type I spiral ganglion neurons and include splice-isoforms with a functional nuclear localization signal

The espins are Ca(2+)-resistant actin-bundling proteins that are enriched in hair cell stereocilia and sensory cell microvilli. Here, we report a novel localization of espins to a large proportion of rat type I spiral ganglion neurons (SGNs) and their projections to the cochlear nucleus (CN). Moreover, we show that a fraction of these espins is in the nucleus of SGNs owing to the presence of splice-isoforms that contain a functional nuclear localization signal (NLS). Espin antibody labeled approximately 83% of type I SGNs, and the labeling intensity increased dramatically during early postnatal development. Type II SGNs and vestibular ganglion neurons were unlabeled. In the CN, espin-positive auditory nerve fibers showed a projection pattern typical of type I SGNs, with intense labeling in the nerve root region and posteroventral CN (PVCN). The anteroventral CN (AVCN) showed moderate labeling, whereas the dorsal CN showed weak labeling that was restricted to the deep layer. Espin-positive synaptic terminals were enriched around nerve root neurons and octopus cells in the PVCN and were also found on globular bushy cells and multipolar neurons in the PVCN and AVCN. SGNs expressed multiple espin transcripts and proteins, including splice-isoforms that contain a nonapeptide, which is rich in positively charged amino acids and creates a bipartite NLS. The nonapeptide was necessary to target espin isoforms to the nucleus and was sufficient to target an unrelated protein to the nucleus when joined with the upstream di-arginine-containing octapeptide. The presence of cytoplasmic and nuclear espins in SGNs suggests additional roles for espins in auditory neuroscience.

The Cdc42 effector IRSp53 generates filopodia by coupling membrane protrusion with actin dynamics

The Cdc42 effector IRSp53 is a strong inducer of filopodia formation and consists of an Src homology domain 3 (SH3), a potential WW-binding motif, a partial-Cdc42/Rac interacting binding region motif, and an Inverse-Bin-Amphiphysins-Rvs (I-BAR) domain. We show that IRSp53 interacts directly with neuronal Wiskott-Aldrich syndrome protein (N-WASP) via its SH3 domain and furthermore that N-WASP is required for filopodia formation as IRSp53 failed to induce filopodia formation in N-WASP knock-out (KO) fibroblasts. IRSp53-induced filopodia formation can be reconstituted in N-WASP KO fibroblasts by full-length N-WASP, by N-WASPDeltaWA (a mutant unable to activate the Arp2/3 complex), and by N-WASPH208D (a mutant unable to bind Cdc42). IRSp53 failed to induce filopodia in mammalian enabled (Mena)/VASP KO cells, and N-WASP failed to induce filopodia when IRSp53 was knocked down with RNA interference. The IRSp53 I-BAR domain alone induces dynamic membrane protrusions that lack actin and are smaller than normal filopodia ("partial-filopodia") in both wild-type N-WASP and N-WASP KO cells. We propose that IRSp53 generates filopodia by coupling membrane protrusion through its I-BAR domain with actin dynamics through SH3 domain binding partners, including N-WASP and Mena.

Regulation of otic vesicle and hair cell stereocilia morphogenesis by Ena/VASP-like (Evl) in Xenopus

The inner ear is derived from a thickening in the embryonic ectoderm, called the otic placode. This structure undergoes extensive morphogenetic movements throughout its development and gives rise to all components of the inner ear. Ena/VASP-like (Evl) is an actin binding protein involved in the regulation of cytoskeletal dynamics and organization. We have examined the role of Evl during the morphogenesis of the Xenopus inner ear. Evl (hereafter referred to as Xevl) is expressed throughout otic vesicle formation and is enriched in the neuroblasts that delaminate to form the vestibulocochlear ganglion and in hair cells that possess mechanosensory stereocilia. Knockdown of Xevl perturbs epithelial morphology and intercellular adhesion in the otic vesicle and disrupts formation of the vestibulocochlear ganglion, evidenced by reduction of ganglion size, disorganization of the ganglion, and defects in neurite outgrowth. Later in embryogenesis, Xevl is required for development of mechanosensory hair cells. In Xevl knockdown embryos, hair cells of the ventromedial sensory epithelium display multiple abnormalities including disruption of the cuticular plate at the base of stereocilia and disorganization of the normal staircase appearance of stereocilia. Based on these data, we propose that Xevl plays an integral role in regulating morphogenesis of the inner ear epithelium and the subsequent development of the vestibulocochlear ganglion and mechanosensory hair cells.

Postsynaptic enrichment of Eps8 at dendritic shaft synapses of unipolar brush cells in rat cerebellum

Epidermal growth factor receptor pathway substrate 8 (Eps8) is a widely expressed multidomain signaling protein that coordinates two disparate GTPase-dependent mechanisms: actin reorganization via Ras/Rac pathways and receptor trafficking via Rab5. Expression of Eps8, the gene encoding the founding member of the Eps8 family of proteins, was found in cerebellum by virtual Northern analysis and in situ hybridization. Because the cerebellum has a well-known cellular architecture and is a favored model to study synaptic plasticity and actin dynamics, we sought to analyze Eps8 localization in rat cerebellar neurons and synapses by light and electron microscopy. Specificity of Eps8-antibody was demonstrated by immunoblots and in brain sections. In cerebellum, unipolar brush cells (UBCs) were densely Eps8 immunopositive and granule cells were moderately immunostained. In both types of neuron immunoreaction product was localized to the somatodendritic and axonal compartments. Postsynaptic immunostained foci were demonstrated in the glomeruli in correspondence of the synapses formed by mossy fiber terminals with granule cell and UBC dendrites. These foci appeared especially evident in the UBC brush, which contains an extraordinary postsynaptic apparatus of actin microfilaments facing synaptic junctions of the long and segmented varieties. Eps8 immunoreactivity was conspicuously absent in Purkinje cells and their actin-rich dendritic spines, in all types of inhibitory interneurons of the cerebellum, cerebellar nuclei neurons, and astrocytes. In conclusion, Eps8 protein in cerebellum is expressed exclusively by excitatory cortical interneurons and is intracellularly compartmentalized in a cell-class specific manner. This is the first demonstration of the presence of a member of the Eps8 protein family in UBCs and its enrichment at postsynaptic sites.

Espin cytoskeletal proteins in the sensory cells of rodent taste buds

Espins are multifunctional actin-bundling proteins that are highly enriched in the microvilli of certain chemosensory and mechanosensory cells, where they are believed to regulate the integrity and/or dimensions of the parallel-actin-bundle cytoskeletal scaffold. We have determined that, in rats and mice, affinity purified espin antibody intensely labels the lingual and palatal taste buds of the oral cavity and taste buds in the pharyngo-laryngeal region. Intense immunolabeling was observed in the apical, microvillar region of taste buds, while the level of cytoplasmic labeling in taste bud cells was considerably lower. Taste buds contain tightly packed collections of sensory cells (light, or type II plus type III) and supporting cells (dark, or type I), which can be distinguished by microscopic features and cell type-specific markers. On the basis of results obtained using an antigen-retrieval method in conjunction with double immunofluorescence for espin and sensory taste cell-specific markers, we propose that espins are expressed predominantly in the sensory cells of taste buds. In confocal images of rat circumvallate taste buds, we counted 21.5 +/- 0.3 espin-positive cells/taste bud, in agreement with a previous report showing 20.7 +/- 1.3 light cells/taste bud when counted at the ultrastructural level. The espin antibody labeled spindle-shaped cells with round nuclei and showed 100% colocalization with cell-specific markers recognizing all type II [inositol 1,4,5-trisphosphate receptor type III (IP(3)R(3))(,) alpha-gustducin, protein-specific gene product 9.5 (PGP9.5)] and a subpopulation of type III (IP(3)R(3), PGP9.5) taste cells. On average, 72%, 50%, and 32% of the espin-positive taste cells were labeled with antibodies to IP(3)R(3), alpha-gustducin, and PGP9.5, respectively. Upon sectional analysis, the taste buds of rat circumvallate papillae commonly revealed a multi-tiered, espin-positive apical cytoskeletal apparatus. One espin-positive zone, a collection of approximately 3 mum-long microvilli occupying the taste pore, was separated by an espin-depleted zone from a second espin-positive zone situated lower within the taste pit. This latter zone included espin-positive rod-like structures that occasionally extended basally to a depth of 10-12 mum into the cytoplasm of taste cells. We propose that the espin-positive zone in the taste pit coincides with actin bundles in association with the microvilli of type II taste cells, whereas the espin-positive microvilli in the taste pore are the single microvilli of type III taste cells.

Targeted wild-type and jerker espins reveal a novel, WH2-domain-dependent way to make actin bundles in cells

The espin actin-bundling proteins, which are the target of deafness mutations, are present in the parallel actin bundles of stereocilia and microvilli and appear to increase their steady-state length. Here, we report a new activity of the espins, one that depends on their enigmatic WH2 domain: the ability to assemble a large actin bundle when targeted to a specific subcellular location. This activity was observed for wild-type espins targeted to the centrosome in transfected neuronal cells and for jerker espins targeted to the nucleolus in a wide variety of transfected cells as a result of the frameshifted peptide introduced into the espin C-terminus by the jerker deafness mutation. This activity, which appears specific to espins, requires two espin F-actin-binding sites and the actin-monomer-binding activity of the espin WH2 domain, but can be mimicked by adding a WH2 domain to an unrelated actin-bundling protein, villin. Espins do not activate the Arp2/3 complex in vitro, and bundle assembly is not indicative of in-vitro nucleation activity. Our results suggest a novel way to build actin bundles at specific sites in cells.

Differential expression of espin isoforms during epithelial morphogenesis, stereociliogenesis and postnatal maturation in the developing inner ear

The espins are a family of multifunctional actin cytoskeletal proteins. They are present in hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction. Here, we demonstrate that the different espin isoforms are expressed in complex spatiotemporal patterns during inner ear development. Espin 3 isoforms were prevalent in the epithelium of the otic pit, otocyst and membranous labyrinth as they underwent morphogenesis. This espin was down-regulated ahead of hair cell differentiation and during neuroblast delamination. Espin also accumulated in the epithelium of branchial clefts and pharyngeal pouches and during branching morphogenesis in other embryonic epithelial tissues, suggesting general roles for espins in epithelial morphogenesis. Espin reappeared later in inner ear development in differentiating hair cells. Its levels and compartmentalization to stereocilia increased during the formation and maturation of stereociliary bundles. Late in embryonic development, espin was also present in a tail-like process that emanated from the hair cell base. Increases in the levels of espin 1 and espin 4 isoforms correlated with stereocilium elongation and maturation in the vestibular system and cochlea, respectively. Our results suggest that the different espin isoforms play specific roles in actin cytoskeletal regulation during epithelial morphogenesis and hair cell differentiation.

TLS facilitates transport of mRNA encoding an actin-stabilizing protein to dendritic spines

TLS (translocation in liposarcoma), an RNA-binding protein, was originally identified as a heterogeneous ribonuclear protein (hnRNP). Recently, we showed that TLS is localized in neuronal dendrites of mouse hippocampal neurons and is translocated to the spines, where local translation takes place, in an mGluR5 activation-dependent manner. However, the specific role of TLS has not been clarified. TLS-null neurons display abnormal spine morphology, suggesting that TLS-deficiency may impair activity-dependent actin reorganization in spines. To address this issue, we screened for mouse brain transcripts by their in vitro binding to TLS, and identified RNAs that associate with TLS, including mRNAs encoding actin-related proteins such as actin-stabilizing protein Nd1-L. Nd1-L transcripts were increased in the dendrites upon mGluR activation and significantly reduced in TLS-null dendrites. Overexpression of Nd1-L in mouse hippocampal neurons prevented damage to spine structure caused by actin destabilization. Our results demonstrate that TLS associates with mRNA encoding an actin-related protein and may be involved in actin reorganization in spines.

Tiam1-IRSp53 complex formation directs specificity of rac-mediated actin cytoskeleton regulation

The exchange factor Tiam1 regulates multiple cellular functions by activating the Rac GTPase. Active Rac has various effects in cells, including alteration of actin cytoskeleton and gene expression, via binding to and modulating the activity of diverse effector proteins. How individual Rac effectors are selected for activation and regulated in response to upstream signals is not well understood. We find that Tiam1 contributes to both of these processes by binding to IRSp53, an adaptor protein that is an effector for both Rac and Cdc42. Tiam1 directs IRSp53 to Rac signaling by enhancing IRSp53 binding to both active Rac and the WAVE2 scaffold. Moreover, Tiam1 promotes IRSp53 localization to Rac-induced lamellipodia rather than Cdc42-induced filopodia. Finally, IRSp53 depletion from cells prevents Tiam1-dependent lamellipodia induced by Tiam1 overexpression or platelet-derived growth factor stimulation. These findings indicate that Tiam1 not only activates Rac but also contributes to Rac signaling specificity through binding to IRSp53.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

Growth of dendritic spines: a continuing story

Dendritic spines, which are present at the vast majority of excitatory synapses in the central nervous system, have a specialized cytoskeleton of dynamic actin filaments that makes them capable of rapid morphological plasticity. During development, structural remodeling of nascent spines is an important factor in experience-dependent shaping of neuronal circuits, whereas in the adult brain spines maintain a balance between morphological stability and plasticity.

Structural basis of filopodia formation induced by the IRSp53/MIM homology domain of human IRSp53

The scaffolding protein insulin receptor tyrosine kinase substrate p53 (IRSp53), a ubiquitous regulator of the actin cytoskeleton, mediates filopodia formation under the control of Rho-family GTPases. IRSp53 comprises a central SH3 domain, which binds to proline-rich regions of a wide range of actin regulators, and a conserved N-terminal IRSp53/MIM homology domain (IMD) that harbours F-actin-bundling activity. Here, we present the crystal structure of this novel actin-bundling domain revealing a coiled-coil domain that self-associates into a 180 A-long zeppelin-shaped dimer. Sedimentation velocity experiments confirm the presence of a single molecular species of twice the molecular weight of the monomer in solution. Mutagenesis of conserved basic residues at the extreme ends of the dimer abrogated actin bundling in vitro and filopodia formation in vivo, demonstrating that IMD-mediated actin bundling is required for IRSp53-induced filopodia formation. This study promotes an expanded view of IRSp53 as an actin regulator that integrates scaffolding and effector functions.

Don't get too excited: mechanisms of glutamate-mediated Purkinje cell death

Purkinje cells (PCs) present a unique cellular profile in both the cerebellum and the brain. Because they represent the only output cell of the cerebellar cortex, they play a vital role in the normal function of the cerebellum. Interestingly, PCs are highly susceptible to a variety of pathological conditions that may involve glutamate-mediated 'excitotoxicity', a term coined to describe an excessive release of glutamate, and a subsequent over-activation of excitatory amino acid (NMDA, AMPA, and kainite) receptors. Mature PCs, however, lack functional NMDA receptors, the means by which Ca(2+) enters the cell in classic hippocampal and cortical models of excitotoxicity. In PCs, glutamate predominantly mediates its effects, first via a rapid influx of Ca(2+)through voltage-gated calcium channels, caused by the depolarization of the membrane after AMPA receptor activation (and through Ca(2+)-permeable AMPA receptors themselves), and second, via a delayed release of Ca(2+) from intracellular stores. Although physiological levels of intracellular free Ca(2+) initiate vital second messenger signaling pathways in PCs, excessive Ca(2+) influx can detrimentally alter dendritic spine morphology via interactions with the neuronal cytoskeleton, and thus can perturb normal synaptic function. PCs possess various calcium-binding proteins, such as calbindin-D28K and parvalbumin, and glutamate transporters, in order to prevent glutamate from exerting deleterious effects. Bergmann glia are gaining recognition as key players in the clearance of extracellular glutamate; these cells are also high in S-100beta, a protein with both neurodegenerative and neuroprotective abilities. In this review, we discuss PC-specific mechanisms of glutamate-mediated excitotoxic cell death, the relationship between Ca(2+) and cytoskeleton, and the implications of glutamate, and S-100beta for pathological conditions, such as traumatic brain injury.

A novel actin bundling/filopodium-forming domain conserved in insulin receptor tyrosine kinase substrate p53 and missing in metastasis protein

Insulin receptor tyrosine kinase substrate p53 (IRSp53) has been identified as an SH3 domain-containing adaptor that links Rac1 with a Wiskott-Aldrich syndrome family verprolin-homologous protein 2 (WAVE2) to induce lamellipodia or Cdc42 with Mena to induce filopodia. The recruitment of these SH3-binding partners by IRSp53 is thought to be crucial for F-actin rearrangements. Here, we show that the N-terminal predicted helical stretch of 250 amino acids of IRSp53 is an evolutionarily conserved F-actin bundling domain involved in filopodium formation. Five proteins including IRSp53 and missing in metastasis (MIM) protein share this unique domain and are highly conserved in vertebrates. We named the conserved domain IRSp53/MIM homology domain (IMD). The IMD has domain relatives in invertebrates but does not show obvious homology to any known actin interacting proteins. The IMD alone, derived from either IRSp53 or MIM, induced filopodia in HeLa cells and the formation of tightly packed parallel F-actin bundles in vitro. These results suggest that IRSp53 and MIM belong to a novel actin bundling protein family. Furthermore, we found that filopodium-inducing IMD activity in the full-length IRSp53 was regulated by active Cdc42 and Rac1. The SH3 domain was not necessary for IMD-induced filopodium formation. Our results indicate that IRSp53, when activated by small GTPases, participates in F-actin reorganization not only in an SH3-dependent manner but also in a manner dependent on the activity of the IMD.

Espin cross-links cause the elongation of microvillus-type parallel actin bundles in vivo

The espin actin-bundling proteins, which are the target of the jerker deafness mutation, caused a dramatic, concentration-dependent lengthening of LLC-PK1-CL4 cell microvilli and their parallel actin bundles. Espin level was also positively correlated with stereocilium length in hair cells. Villin, but not fascin or fimbrin, also produced noticeable lengthening. The espin COOH-terminal peptide, which contains the actin-bundling module, was necessary and sufficient for lengthening. Lengthening was blocked by 100 nM cytochalasin D. Espin cross-links slowed actin depolymerization in vitro less than twofold. Elimination of an actin monomer-binding WASP homology 2 domain and a profilin-binding proline-rich domain from espin did not decrease lengthening, but made it possible to demonstrate that actin incorporation was restricted to the microvillar tip and that bundles continued to undergo actin treadmilling at ∼1.5 s⁻¹ during and after lengthening. Thus, through relatively subtle effects on actin polymerization/depolymerization reactions in a treadmilling parallel actin bundle, espin cross-links cause pronounced barbed-end elongation and, thereby, make a longer bundle without joining shorter modules.