Drebrin controls neuronal migration through the formation and alignment of the leading process

ARHGEF5 binds Drebrin and affects α-tubulin acetylation to direct neuronal morphogenesis and migration during mouse brain development

Rho guanine nucleotide exchange factors (Rho GEFs) activate Rho GTPases, which act as molecular switches regulating various essential cellular functions. This study investigated the role of ARHGEF5, a Rho GEF known for its involvement in cell migration and invasion processes, in the context of brain development. We found that ARHGEF5 is essential for dendrite development during the early stages of neuronal growth. We also discovered that ARHGEF5 binds to Drebrin E, which is vital for coordinating actin and microtubule dynamics, and facilitates the interaction between Drebrin E and Cyclin-dependent kinase 5, which phosphorylates Drebrin E. Notably, ARHGEF5 deficiency resulted in a decrease in acetylated α-tubulin levels, and the expression of an α-tubulin acetylation mimetic mutant (K40Q) rescued the defects in dendrite development and neuronal migration, suggesting ARHGEF5's role in modulating microtubule stability. Additionally, ARHGEF5 was shown to influence Golgi positioning in the leading processes of migrating cortical neurons during brain development. Our study suggests that ARHGEF5 plays a crucial role in integrating cytoskeletal dynamics with neuronal morphogenesis and migration processes during brain development.

Junctional Adhesion Molecule (JAM)-C recruitment of Pard3 and drebrin to cell contacts initiates neuron-glia recognition and layer-specific cell sorting in developing cerebella

Sorting maturing neurons into distinct layers is critical for brain development, with disruptions leading to neurological disorders and pediatric cancers. Lamination coordinates where, when, and how cells interact, facilitating events that direct migrating neurons to their destined positions within emerging neural networks and control the wiring of connections in functional circuits. While the role of adhesion molecule expression and presentation in driving adhesive recognition during neuronal migration along glial fibers is recognized, the mechanisms by which the spatial arrangement of these molecules on the cell surface dictates adhesive specificity and translates contact-based external cues into intracellular responses like polarization and cytoskeletal organization remain largely unexplored. We used the cerebellar granule neuron (CGN) system to demonstrate that JAM-C receptor cis-binding on the same cell and trans-binding to neighboring cells controls the recruitment of the Pard3 polarity protein and drebrin microtubule-actin crosslinker at CGN to glial adhesion sites, complementing previous studies that showed Pard3 controls JAM-C exocytic surface presentation. Leveraging advanced imaging techniques, specific probes for cell recognition, and analytical methods to dissect adhesion dynamics, our findings reveal: 1) JAM-C cis or trans mutants result in reduced adhesion formation between CGNs and cerebellar glia, 2) these mutants exhibit delayed recruitment of Pard3 at the adhesion sites, and 3) CGNs with JAM-C mutations experience postponed sorting and entry into the cerebellar molecular layer (ML). By developing a conditional system to image adhesion components from two different cells simultaneously, we made it possible to investigate the dynamics of cell recognition on both sides of neuron-glial contacts and the subsequent recruitment of proteins required for CGN migration. This system and an approach that calculates local correlation based on convolution kernels at the cell adhesions site revealed that CGN to CGN JAM recognition preferentially recruits higher levels of Pard3 and drebrin than CGN to glia JAM recognition. The long latency time of CGNs in the inner external germinal layer (EGL) can be attributed to the combined strength of CGN-CGN contacts and the less efficient Pard3 recruitment by CGN-BG contacts, acting as gatekeepers to ML entry. As CGNs eventually transition to glia binding for radial migration, our research demonstrates that establishing permissive JAM-recognition sites on glia via cis and trans interactions of CGN JAM-C serves as a critical temporal checkpoint for sorting at the EGL to ML boundary. This mechanism integrates intrinsic and extrinsic cellular signals, facilitating heterotypic cell sorting into the ML and dictating the precise spatial organization within the cerebellar architecture.

Drebrin Regulates Collateral Axon Branching in Cortical Layer II/III Somatosensory Neurons

Proper cortical lamination is essential for cognition, learning, and memory. Within the somatosensory cortex, pyramidal excitatory neurons elaborate axon collateral branches in a laminar-specific manner that dictates synaptic partners and overall circuit organization. Here, we leverage both male and female mouse models, single-cell labeling and imaging approaches to identify intrinsic regulators of laminar-specific collateral, also termed interstitial, axon branching. We developed new approaches for the robust, sparse, labeling of Layer II/III pyramidal neurons to obtain single-cell quantitative assessment of axon branch morphologies. We combined these approaches with cell-autonomous loss-of-function (LOF) and overexpression (OE) manipulations in an in vivo candidate screen to identify regulators of cortical neuron axon branch lamination. We identify a role for the cytoskeletal binding protein drebrin (Dbn1) in regulating Layer II/III cortical projection neuron (CPN) collateral axon branching in vitro LOF experiments show that Dbn1 is necessary to suppress the elongation of Layer II/III CPN collateral axon branches within Layer IV, where axon branching by Layer II/III CPNs is normally absent. Conversely, Dbn1 OE produces excess short axonal protrusions reminiscent of nascent axon collaterals that fail to elongate. Structure-function analyses implicate Dbn1S142 phosphorylation and Dbn1 protein domains known to mediate F-actin bundling and microtubule (MT) coupling as necessary for collateral branch initiation upon Dbn1 OE. Taken together, these results contribute to our understanding of the molecular mechanisms that regulate collateral axon branching in excitatory CPNs, a key process in the elaboration of neocortical circuit formation.SIGNIFICANCE STATEMENT Laminar-specific axon targeting is essential for cortical circuit formation. Here, we show that the cytoskeletal protein drebrin (Dbn1) regulates excitatory Layer II/III cortical projection neuron (CPN) collateral axon branching, lending insight into the molecular mechanisms that underlie neocortical laminar-specific innervation. To identify branching patterns of single cortical neurons in vivo, we have developed tools that allow us to obtain detailed images of individual CPN morphologies throughout postnatal development and to manipulate gene expression in these same neurons. Our results showing that Dbn1 regulates CPN interstitial axon branching both in vivo and in vitro may aid in our understanding of how aberrant cortical neuron morphology contributes to dysfunctions observed in autism spectrum disorder and epilepsy.

Neuronal paxillin and drebrin mediate BDNF-induced force transduction and growth cone turning in a soft-tissue-like environment

Soft tissue environments govern neuronal morphogenesis. However, the precise molecular mechanisms underlying chemotropism-directed axonal growth cone movement in extremely soft environments remain unclear. Here, we show that drebrin, a growth cone T-zone protein, modulates growth cone turning in response to brain-derived neurotrophic factor (BDNF) coated on a soft substrate. Structurally, axonal growth cones of rodent hippocampal neurons grown on 0.1 kPa hydrogels possess an expanded T zone in which drebrin is highly integrated with both F-actin and microtubules. Biochemically, we identify paxillin as interacting with drebrin in cells grown on 0.1 kPa hydrogels but not on glass coverslips. When grown on 0.1 kPa substrates, growth cones asymmetrically exposed to BDNF-bound stripes exhibit enhanced paxillin-drebrin interaction on the side facing the stripes, an activity that is PKA and AAK1 dependent but independent of Src kinase. Functionally, we show that BDNF-induced growth cone turning and force generation on soft substrates require drebrin phosphorylation and paxillin-drebrin association.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

The drebrin/EB3 pathway regulates cytoskeletal dynamics to drive neuritogenesis in embryonic cortical neurons

Co-ordinating the dynamic behaviour of actin filaments (F-actin) and microtubules in filopodia is an important underlying process in neuritogenesis, but the molecular pathways involved are ill-defined. The drebrin/end-binding protein 3 (EB3) pathway is a candidate pathway for linking F-actin to microtubules in filopodia. Drebrin binds F-actin and, simultaneously, the microtubule-binding protein EB3 when bound to microtubule plus-ends. We assessed the effect on neuritogenesis of gain- or loss-of-function of proteins in the drebrin/EB3 pathway in rat embryonic cortical neurons in culture. Loss-of-function of drebrin by gene editing or pharmacological inhibition of drebrin binding to F-actin reduced the number of dynamic microtubules in the cell periphery and simultaneously delayed the initiation of neuritogenesis, whereas over-expression of drebrin induced supernumerary neurites. Similarly, loss of EB3 inhibited neuritogenesis, whereas loss of end-binding protein 1 (EB1), a related protein that does not bind to drebrin, did not affect neuritogenesis. Over-expression of EB3, but not EB1, induced supernumerary neurites. We discovered that EB3 is more proximally located at dynamic microtubule plus-ends than EB1 in growth cone filopodia allowing for continuous microtubule elongation as the drebrin/EB3 pathway zippers microtubules to F-actin in filopodia. Finally, we showed that preventing the entry of dynamic microtubules into filopodia using a pharmacological inhibitor of microtubule dynamics is associated with a loss of EB3, but not EB1, from microtubule plus-ends and a concurrent attenuation of neuritogenesis. Collectively, these findings support the idea that neuritogenesis depends on microtubule/F-actin zippering in filopodia orchestrated by the drebrin/EB3 pathway.

Axonal Growth Abnormalities Underlying Ocular Cranial Nerve Disorders

Abnormalities in cranial motor nerve development cause paralytic strabismus syndromes, collectively referred to as congenital cranial dysinnervation disorders, in which patients cannot fully move their eyes. These disorders can arise through one of two mechanisms: (a) defective motor neuron specification, usually by loss of a transcription factor necessary for brainstem patterning, or (b) axon growth and guidance abnormalities of the oculomotor, trochlear, and abducens nerves. This review focuses on our current understanding of axon guidance mechanisms in the cranial motor nerves and how disease-causing mutations disrupt axon targeting. Abnormalities of axon growth and guidance are often limited to a single nerve or subdivision, even when the causative gene is ubiquitously expressed. Additionally, when one nerve is absent, its normal target muscles attract other motor neurons. Study of these disorders highlights the complexities of axon guidance and how each population of neurons uses a unique but overlapping set of axon guidance pathways. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Drebrin controls scar formation and astrocyte reactivity upon traumatic brain injury by regulating membrane trafficking

The brain of mammals lacks a significant ability to regenerate neurons and is thus particularly vulnerable. To protect the brain from injury and disease, damage control by astrocytes through astrogliosis and scar formation is vital. Here, we show that brain injury in mice triggers an immediate upregulation of the actin-binding protein Drebrin (DBN) in astrocytes, which is essential for scar formation and maintenance of astrocyte reactivity. In turn, DBN loss leads to defective astrocyte scar formation and excessive neurodegeneration following brain injuries. At the cellular level, we show that DBN switches actin homeostasis from ARP2/3-dependent arrays to microtubule-compatible scaffolds, facilitating the formation of RAB8-positive membrane tubules. This injury-specific RAB8 membrane compartment serves as hub for the trafficking of surface proteins involved in astrogliosis and adhesion mediators, such as β1-integrin. Our work shows that DBN-mediated membrane trafficking in astrocytes is an important neuroprotective mechanism following traumatic brain injury in mice.

Drebrin regulates cytoskeleton dynamics in migrating neurons through interaction with CXCR4

Stromal cell-derived factor-1 (SDF-1) and chemokine receptor type 4 (CXCR4) are regulators of neuronal migration (e.g., GnRH neurons, cortical neurons, and hippocampal granule cells). However, how SDF-1/CXCR4 alters cytoskeletal components remains unclear. Developmentally regulated brain protein (drebrin) stabilizes actin polymerization, interacts with microtubule plus ends, and has been proposed to directly interact with CXCR4 in T cells. The current study examined, in mice, whether CXCR4 under SDF-1 stimulation interacts with drebrin to facilitate neuronal migration. Bioinformatic prediction of protein-protein interaction highlighted binding sites between drebrin and crystallized CXCR4. In migrating GnRH neurons, drebrin, CXCR4, and the microtubule plus-end binding protein EB1 were localized close to the cell membrane. Coimmunoprecipitation (co-IP) confirmed a direct interaction between drebrin and CXCR4 using wild-type E14.5 whole head and a GnRH cell line. Analysis of drebrin knockout (DBN1 KO) mice showed delayed migration of GnRH cells into the brain. A decrease in hippocampal granule cells was also detected, and co-IP confirmed a direct interaction between drebrin and CXCR4 in PN4 hippocampi. Migration assays on primary neurons established that inhibiting drebrin (either pharmacologically or using cells from DBN1 KO mice) prevented the effects of SDF-1 on neuronal movement. Bioinformatic prediction then identified binding sites between drebrin and the microtubule plus end protein, EB1, and super-resolution microscopy revealed decreased EB1 and drebrin coexpression after drebrin inhibition. Together, these data show a mechanism by which a chemokine, via a membrane receptor, communicates with the intracellular cytoskeleton in migrating neurons during central nervous system development.

Cadherins regulate nuclear topography and function of developing ocular motor circuitry

In the vertebrate central nervous system, groups of functionally related neurons, including cranial motor neurons of the brainstem, are frequently organised as nuclei. The molecular mechanisms governing the emergence of nuclear topography and circuit function are poorly understood. Here we investigate the role of cadherin-mediated adhesion in the development of zebrafish ocular motor (sub)nuclei. We find that developing ocular motor (sub)nuclei differentially express classical cadherins. Perturbing cadherin function in these neurons results in distinct defects in neuronal positioning, including scattering of dorsal cells and defective contralateral migration of ventral subnuclei. In addition, we show that cadherin-mediated interactions between adjacent subnuclei are critical for subnucleus position. We also find that disrupting cadherin adhesivity in dorsal oculomotor neurons impairs the larval optokinetic reflex, suggesting that neuronal clustering is important for co-ordinating circuit function. Our findings reveal that cadherins regulate distinct aspects of cranial motor neuron positioning and establish subnuclear topography and motor function.

Migrating Schwann cells direct axon regeneration within the peripheral nerve bridge

Schwann cells within the peripheral nervous system possess a remarkable regenerative potential. Current research shows that peripheral nerve-associated Schwann cells possess the capacity to promote repair of multiple tissues including peripheral nerve gap bridging, skin wound healing, digit tip repair as well as tooth regeneration. One of the key features of the specialized repair Schwann cells is that they become highly motile. They not only migrate into the area of damaged tissue and become a key component of regenerating tissue but also secrete signaling molecules to attract macrophages, support neuronal survival, promote axonal regrowth, activate local mesenchymal stem cells, and interact with other cell types. Currently, the importance of migratory Schwann cells in tissue regeneration is most evident in the case of a peripheral nerve transection injury. Following nerve transection, Schwann cells from both proximal and distal nerve stumps migrate into the nerve bridge and form Schwann cell cords to guide axon regeneration. The formation of Schwann cell cords in the nerve bridge is key to successful peripheral nerve repair following transection injury. In this review, we first examine nerve bridge formation and the behavior of Schwann cell migration in the nerve bridge, and then discuss how migrating Schwann cells direct regenerating axons into the distal nerve. We also review the current understanding of signals that could activate Schwann cell migration and signals that Schwann cells utilize to direct axon regeneration. Understanding the molecular mechanism of Schwann cell migration could potentially offer new therapeutic strategies for peripheral nerve repair.

Fluoride-induced alterations of synapse-related proteins in the cerebral cortex of ICR offspring mouse brain

Fluoride (F) exposure causes cognitive dysfunction in humans and animals. However, the precise molecular mechanisms by which fluoride exerts its neurotoxic effects are poorly understood. In this study, an animal model of fluoride exposure was created by providing ICR mice were treated with vehicle F at a dose of 0 (control group), 50 (low-fluoride group) or 100 mg/L (high-fluoride group) in water for one month. After the mice mated, parents and offspring were treated and maintained under these conditions. The cognitive abilities of the mice were examined using a Morris water maze test. Results indicated that fluoride exposure significantly prolonged the escape latency period and decreased the number of crossings in a particular zone. Histopathologic analysis revealed the shrinkage and fragmentation of glial cells in the fluoride-treated groups. Pyramidal cells in the cerebral cortices of fluoride-treated groups were fewer than those of the control group. The expression of microtubule-associated protein 2 (MAP2) and synaptic proteins of the cerebral cortex in mouse offspring was assayed using RT-PCR and Western blot. Fluoride exposure possibly induced a significantly decreased expression of MAP2, synaptophysin (SYP) and developmentally regulated brain protein (Dbn) at protein and mRNA levels. Glutamate receptor (N-methyl-d-aspartate receptor, NMDAR) was also expressed, and this finding was consistent with the reduced MAP2, SYP and Dbn expression. Therefore, fluoride-mediated reduction in cognitive dysfunction is likely caused by the disruption of the expression of these synapse-associated proteins, resulting in attenuated neuronal functioning.

Drebrin in Neuronal Migration and Axonal Growth

During development, production of neurons from neural stem cells, migration of neurons from their birthplace to their final location, and extension of neurites, axons, and dendrites are important for the formation of functional neuronal circuits. The actin cytoskeleton has major roles in the morphological development of neurons. In this chapter, we focused on the distribution and function of the actin-binding protein, drebrin, to elucidate the importance of drebrin-bound F-actin in neurons during early developmental stages of neurons in embryonic, postnatal, and adult brains. There are three major isoforms of drebrin in the chicken brain (E1, E2, and A) and two major isoforms in the mammalian brain (E and A). Among these drebrin isoforms, drebrin E1 and E2 in chicken and drebrin E in the mammalian brain are involved in these neuronal stages. In migrating neurons of the developing and adult brain, drebrin is localized at the base of filopodia of leading processes, to regulate neuronal migration. In axonal growth cones, drebrin is localized in the transitional zone to regulate axonal growth by inhibiting actomyosin interactions and mediating the interactions between F-actin and microtubules. For axonal collateral branching, drebrin is localized at axonal actin patches and the base of filopodia, to accelerate the transition from actin patches to filopodia and stabilize the filopodia.

Motor neuron migration and positioning mechanisms: New roles for guidance cues

Motor neurons differentiate from progenitor cells and cluster as motor nuclei, settling next to the floor plate in the brain stem and spinal cord. Although precise positioning of motor neurons is critical for their functional input and output, the molecular mechanisms that guide motor neurons to their proper positions remain poorly understood. Here, we review recent evidence of motor neuron positioning mechanisms, highlighting situations in which motor neuron cell bodies can migrate, and experiments that show that their migration is regulated by axon guidance cues. The view that emerges is that motor neurons are actively trapped or restricted in static positions, as the cells balance a push in the dorsal direction by repulsive Slit/Robo cues and a pull in the ventral direction by attractive Netrin-1/DCC cues. These new functions of guidance cues are necessary fine-tuning to set up patterns of motor neurons at their proper positions in the neural tube during embryogenesis.

Inter-dependent apical microtubule and actin dynamics orchestrate centrosome retention and neuronal delamination

Detachment of newborn neurons from the neuroepithelium is required for correct neuronal architecture and functional circuitry. This process, also known as delamination, involves adherens-junction disassembly and acto-myosin-mediated abscission, during which the centrosome is retained while apical/ciliary membranes are shed. Cell-biological mechanisms mediating delamination are, however, poorly understood. Using live-tissue and super-resolution imaging, we uncover a centrosome-nucleated wheel-like microtubule configuration, aligned with the apical actin cable and adherens-junctions within chick and mouse neuroepithelial cells. These microtubules maintain adherens-junctions while actin maintains microtubules, adherens-junctions and apical end-foot dimensions. During neuronal delamination, acto-myosin constriction generates a tunnel-like actin-microtubule configuration through which the centrosome translocates. This movement requires inter-dependent actin and microtubule activity, and we identify drebrin as a potential coordinator of these cytoskeletal dynamics. Furthermore, centrosome compromise revealed that this organelle is required for delamination. These findings identify new cytoskeletal configurations and regulatory relationships that orchestrate neuronal delamination and may inform mechanisms underlying pathological epithelial cell detachment.

How does calcium interact with the cytoskeleton to regulate growth cone motility during axon pathfinding?

The precision with which neurons form connections is crucial for the normal development and function of the nervous system. The development of neuronal circuitry in the nervous system is accomplished by axon pathfinding: a process where growth cones guide axons through the embryonic environment to connect with their appropriate synaptic partners to form functional circuits. Despite intense efforts over many years to understand how this process is regulated, the complete repertoire of molecular mechanisms that govern the growth cone cytoskeleton and hence motility, remain unresolved. A central tenet in the axon guidance field is that calcium signals regulate growth cone behaviours such as extension, turning and pausing by regulating rearrangements of the growth cone cytoskeleton. Here, we provide evidence that not only the amplitude of a calcium signal is critical for growth cone motility but also the source of calcium mobilisation. We provide an example of this idea by demonstrating that manipulation of calcium signalling via L-type voltage gated calcium channels can perturb sensory neuron motility towards a source of netrin-1. Understanding how calcium signals can be transduced to initiate cytoskeletal changes represents a significant gap in our current knowledge of the mechanisms that govern axon guidance, and consequently the formation of functional neural circuits in the developing nervous system.

Effective expression of Drebrin in hippocampus improves cognitive function and alleviates lesions of Alzheimer's disease in APP (swe)/PS1 (ΔE9) mice

AIMS

Alzheimer's disease (AD), a progressive development dementia, is increasingly impacting patients' living conditions worldwide. Despite medical care and funding support, there are still no highly individualized drugs and practical strategies for clinical prevention and treatment. Developmentally regulated brain protein (abbreviated as Drebrin or Dbn, also known as Dbn1 in mouse) exists in neurons, especially in dendrites, and is an actin-binding protein that modulates synaptic morphology and long-term memory. However, the majority of previous studies have focused on its upstream proteins and neglected the impact Drebrin has on behavior and AD in vivo.

METHODS

Here, we tracked the behavioral performances of 4-, 8-, 12-, and 16-month-old AD mice and investigated the expression level of Drebrin in their hippocampi. A Pearson correlation analysis between Drebrin levels and behavioral data was performed. Subsequently, 2-month-old AD mice were injected with rAAV-zsGreen-Dbn1 vector, composing the APP/PS1-Dbn1 group, and sex- and age-matched AD mice were injected with rAAV-tdTomato vector to serve as the control group. All mice were conducted behavioral tests and molecular detection 6 months later.

RESULTS

(i) The expression of Drebrin is decreased in the hippocampus of aged AD mice compared with that of age-matched WT and young adult AD mice; (ii) cognitive ability of APP/PS1 mice decreases with age; (iii) Drebrin protein expression in the hippocampus correlates with behavioral performance in different aged AD mice; (iv) cognitive ability improved significantly in APP/PS1-Dbn1 mice; (v) the expression level of Drebrin in APP/PS1-Dbn1 mouse hippocampus was significantly increased; (vi) the pathological lesion of AD was alleviated in APP/PS1-Dbn1 mice; (vii) the filamentous actin (F-actin) and microtubule-associated protein 2(MAP-2) in APP/PS1-Dbn1 mice were notably more than control mice.

CONCLUSION

In this study, an effective expression of Drebrin improves cognitive abilities and alleviates lesions in an AD mouse model. These results may provide some valid resources for therapy and research of AD.

The role of drebrin in neurons

Drebrin is an actin-binding protein that changes the helical pitch of actin filaments (F-actin), and drebrin-decorated F-actin shows slow treadmilling and decreased rate of depolymerization. Moreover, the characteristic morphology of drebrin-decorated F-actin enables it to respond differently to the same signals from other actin cytoskeletons. Drebrin consists of two major isoforms, drebrin E and drebrin A. In the developing brain, drebrin E appears in migrating neurons and accumulates in the growth cones of axons and dendrites. Drebrin E-decorated F-actin links lamellipodium F-actin to microtubules in the growth cones. Then drebrin A appears at nascent synapses and drebrin A-decorated F-actin facilitates postsynaptic molecular assembly. In the adult brain, drebrin A-decorated F-actin is concentrated in the central region of dendritic spines. During long-term potentiation initiation, NMDA receptor-mediated Ca²⁺ influx induces the transient exodus of drebrin A-decorated F-actin via myosin II ATPase activation. Because of the unique physical characteristics of drebrin A-decorated F-actin, this exodus likely contributes to the facilitation of F-actin polymerization and spine enlargement. Additionally, drebrin reaccumulation in dendritic spines is observed after the exodus. In our drebrin exodus model of structure-based synaptic plasticity, reestablishment of drebrin A-decorated F-actin is necessary to keep the enlarged spine size during long-term potentiation maintenance. In this review, we introduce the genetic and biochemical properties of drebrin and the roles of drebrin in early stage of brain development, synaptic formation and synaptic plasticity. Further, we discuss the pathological relevance of drebrin loss in Alzheimer's disease. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

It takes a village to raise a branch: Cellular mechanisms of the initiation of axon collateral branches

The formation of axon collateral branches from the pre-existing shafts of axons is an important aspect of neurodevelopment and the response of the nervous system to injury. This article provides an overview of the role of the cytoskeleton and signaling mechanisms in the formation of axon collateral branches. Both the actin filament and microtubule components of the cytoskeleton are required for the formation of axon branches. Recent work has begun to shed light on how these two elements of the cytoskeleton are integrated by proteins that functionally or physically link the cytoskeleton. While a number of signaling pathways have been determined as having a role in the formation of axon branches, the complexity of the downstream mechanisms and links to specific signaling pathways remain to be fully determined. The regulation of intra-axonal protein synthesis and organelle function are also emerging as components of signal-induced axon branching. Although much has been learned in the last couple of decades about the mechanistic basis of axon branching we can look forward to continue elucidating this complex biological phenomenon with the aim of understanding how multiple signaling pathways, cytoskeletal regulators and organelles are coordinated locally along the axon to give rise to a branch.

The drebrin/EB3 pathway drives invasive activity in prostate cancer

Prostate cancer is the most common cancer in men and the metastatic form of the disease is incurable. We show here that the drebrin/EB3 pathway, which co-ordinates dynamic microtubule/actin filament interactions underlying cell shape changes in response to guidance cues, plays a role in prostate cancer cell invasion. Drebrin expression is restricted to basal epithelial cells in benign human prostate but is upregulated in luminal epithelial cells in foci of prostatic malignancy. Drebrin is also upregulated in human prostate cancer cell lines and co-localizes with actin filaments and dynamic microtubules in filopodia of pseudopods of invading cells under a chemotactic gradient of the chemokine CXCL12. Disruption of the drebrin/EB3 pathway using BTP2, a small molecule inhibitor of drebrin binding to actin filaments, reduced the invasion of prostate cancer cell lines in 3D in vitro assays. Furthermore, gain- or loss-of-function of drebrin or EB3 by over-expression or siRNA-mediated knockdown increases or decreases invasion of prostate cancer cell lines in 3D in vitro assays, respectively. Finally, expression of a dominant-negative construct that competes with EB3 binding to drebrin, also inhibited invasion of prostate cancer cell lines in 3D in vitro assays. Our findings show that co-ordination of dynamic microtubules and actin filaments by the drebrin/EB3 pathway drives prostate cancer cell invasion and is therefore implicated in disease progression.

Drebrin-mediated microtubule-actomyosin coupling steers cerebellar granule neuron nucleokinesis and migration pathway selection

Neuronal migration from a germinal zone to a final laminar position is essential for the morphogenesis of neuronal circuits. While it is hypothesized that microtubule–actomyosin crosstalk is required for a neuron's ‘two-stroke' nucleokinesis cycle, the molecular mechanisms controlling such crosstalk are not defined. By using the drebrin microtubule–actin crosslinking protein as an entry point into the cerebellar granule neuron system in combination with super-resolution microscopy, we investigate how these cytoskeletal systems interface during migration. Lattice light-sheet and structured illumination microscopy reveal a proximal leading process nanoscale architecture wherein f-actin and drebrin intervene between microtubules and the plasma membrane. Functional perturbations of drebrin demonstrate that proximal leading process microtubule–actomyosin coupling steers the direction of centrosome and somal migration, as well as the switch from tangential to radial migration. Finally, the Siah2 E3 ubiquitin ligase antagonizes drebrin function, suggesting a model for control of the microtubule–actomyosin interfaces during neuronal differentiation.

Neuronal migration is vital for neuronal circuit morphogenesis and is thought to rely on microtubule-actomyosin crosstalk. Here, the authors use super-resolution imaging and the drebrin microtubule-actin crosslinking protein to show that microtubule-actomyosin coupling controls the direction of centrosome and somal motility.

Contralateral migration of oculomotor neurons is regulated by Slit/Robo signaling

BACKGROUND

Oculomotor neurons develop initially like typical motor neurons, projecting axons out of the ventral midbrain to their ipsilateral targets, the extraocular muscles. However, in all vertebrates, after the oculomotor nerve (nIII) has reached the extraocular muscle primordia, the cell bodies that innervate the superior rectus migrate to join the contralateral nucleus. This motor neuron migration represents a unique strategy to form a contralateral motor projection. Whether migration is guided by diffusible cues remains unknown.

METHODS

We examined the role of Slit chemorepellent signals in contralateral oculomotor migration by analyzing mutant mouse embryos.

RESULTS

We found that the ventral midbrain expresses high levels of both Slit1 and 2, and that oculomotor neurons express the repellent Slit receptors Robo1 and Robo2. Therefore, Slit signals are in a position to influence the migration of oculomotor neurons. In Slit 1/2 or Robo1/2 double mutant embryos, motor neuron cell bodies migrated into the ventral midbrain on E10.5, three days prior to normal migration. These early migrating neurons had leading projections into and across the floor plate. In contrast to the double mutants, embryos which were mutant for single Slit or Robo genes did not have premature migration or outgrowth on E10.5, demonstrating a cooperative requirement of Slit1 and 2, as well as Robo1 and 2. To test how Slit/Robo midline repulsion is modulated, we found that the normal migration did not require the receptors Robo3 and CXCR4, or the chemoattractant, Netrin 1. The signal to initiate contralateral migration is likely autonomous to the midbrain because oculomotor neurons migrate in embryos that lack either nerve outgrowth or extraocular muscles, or in cultured midbrains that lacked peripheral tissue.

CONCLUSION

Overall, our results demonstrate that a migratory subset of motor neurons respond to floor plate-derived Slit repulsion to properly control the timing of contralateral migration.

Actin filament-microtubule interactions in axon initiation and branching

Neurons begin life as spherical cells. A major hallmark of neuronal development is the formation of elongating processes from the cell body which subsequently differentiate into dendrites and the axon. The formation and later development of neuronal processes is achieved through the concerted organization of actin filaments and microtubules. Here, we review the literature regarding recent advances in the understanding of cytoskeletal interactions in neurons focusing on the initiation of processes from neuronal cell bodies and the collateral branching of axons. The complex crosstalk between cytoskeletal elements is mediated by a cohort of proteins that either bind both cytoskeletal systems or allow one to regulate the other. Recent studies have highlighted the importance of microtubule plus-tip proteins in the regulation of the dynamics and organization of actin filaments, while also providing a mechanism for the subcellular capture and guidance of microtubule tips by actin filaments. Although the understanding of cytoskeletal crosstalk and interactions in neuronal morphogenesis has advanced significantly in recent years the appreciation of the neuron as an integrated cytoskeletal system remains a frontier.

Coordinating neuronal actin-microtubule dynamics

The growth and migration of neurons require continuous remodelling of the neuronal cytoskeleton, providing a versatile cellular framework for force generation and guided movement, in addition to structural support. Actin filaments and microtubules are central to the dynamic action of the cytoskeleton and rapid advances in imaging technologies are enabling ever more detailed visualisation of the dynamic intracellular networks that they form. However, these filaments do not act individually and an expanding body of evidence emphasises the importance of actin-microtubule crosstalk in orchestrating cytoskeletal dynamics. Here, we summarise our current understanding of the structure and dynamics of actin and microtubules in isolation, before reviewing both the mechanisms and the molecular players involved in mediating actin-microtubule crosstalk in neurons.

A Tale of the Good and Bad: Remodeling of the Microtubule Network in the Brain by Cdk5

Cdk5, a cyclin-dependent kinase family member, is a global orchestrator of neuronal cytoskeletal dynamics. During embryogenesis, Cdk5 is indispensable for brain development. In adults, it is essential for numerous neuronal processes, including higher cognitive functions such as learning and memory formation, drug addiction, pain signaling, and long-term behavior changes through long-term potentiation and long-term depression, all of which rely on rapid alterations in the cytoskeleton. Cdk5 activity becomes deregulated in various brain disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, attention-deficit hyperactivity disorder, epilepsy, schizophrenia, and ischemic stroke; these all result in profound remodeling of the neuronal cytoskeleton. This Commentary specifically focuses on the pleiotropic contribution of Cdk5 in regulating neuronal microtubule remodeling. Because the vast majority of the physiological substrates of Cdk5 are associated with the neuronal cytoskeleton, our emphasis is on the Cdk5 substrates, such as CRMP2, stathmin, drebrin, dixdc1, axin, MAP2, MAP1B, doublecortin, kinesin-5, and tau, that have allowed to unravel the molecular mechanisms through which Cdk5 exerts its divergent roles in regulating neuronal microtubule dynamics, both in healthy and disease states.

A novel role for drebrin in regulating progranulin bioactivity in bladder cancer

We recently established a critical role for the growth factor progranulin in bladder cancer insofar as progranulin promotes urothelial cancer cell motility and contributes, as an autocrine growth factor, to the transformed phenotype by modulating invasion and anchorage-independent growth. In addition, progranulin expression is upregulated in invasive bladder cancer tissues compared to normal controls. However, the molecular mechanisms of progranulin action in bladder cancer have not been fully elucidated. In this study, we searched for novel progranulin-interacting proteins using pull-down assays with recombinant progranulin and proteomics. We discovered that drebrin, an F-actin binding protein, bound progranulin in urothelial cancer cells. We characterized drebrin function in urothelial cancer cell lines and showed that drebrin is critical for progranulin-dependent activation of the Akt and MAPK pathways and modulates motility, invasion and anchorage-independent growth. In addition, drebrin regulates tumor formation in vivo and its expression is upregulated in bladder cancer tissues compared to normal tissue controls. Our data are translationally relevant as indicate that drebrin exerts an essential functional role in the regulation of progranulin action and may constitute a novel target for therapeutic intervention in bladder tumors. In addition, drebrin may serve as novel biomarker for bladder cancer.

Cytoskeletal social networking in the growth cone: How +TIPs mediate microtubule-actin cross-linking to drive axon outgrowth and guidance

The growth cone is a unique structure capable of guiding axons to their proper destinations. Within the growth cone, extracellular guidance cues are interpreted and then transduced into physical changes in the actin filament (F-actin) and microtubule cytoskeletons, providing direction and movement. While both cytoskeletal networks individually possess important growth cone-specific functions, recent data over the past several years point towards a more cooperative role between the two systems. Facilitating this interaction between F-actin and microtubules, microtubule plus-end tracking proteins (+TIPs) have been shown to link the two cytoskeletons together. Evidence suggests that many +TIPs can couple microtubules to F-actin dynamics, supporting both microtubule advance and retraction in the growth cone periphery. In addition, growing in vitro and in vivo data support a secondary role for +TIPs in which they may participate as F-actin nucleators, thus directly influencing F-actin dynamics and organization. This review focuses on how +TIPs may link F-actin and microtubules together in the growth cone, and how these interactions may influence axon guidance. © 2016 Wiley Periodicals, Inc.

Drebrin coordinates the actin and microtubule cytoskeleton during the initiation of axon collateral branches

Drebrin is a cytoskeleton-associated protein which can interact with both actin filaments and the tips of microtubules. Its roles have been studied mostly in dendrites, and the functions of drebrin in axons are less well understood. In this study, we analyzed the role of drebrin, through shRNA-mediated depletion and overexpression, in the collateral branching of chicken embryonic sensory axons. We report that drebrin promotes the formation of axonal filopodia and collateral branches in vivo and in vitro. Live imaging of cytoskeletal dynamics revealed that drebrin promotes the formation of filopodia from precursor structures termed axonal actin patches. Endogenous drebrin localizes to actin patches and depletion studies indicate that drebrin contributes to the development of patches. In filopodia, endogenous drebrin localizes to the proximal portion of the filopodium. Drebrin was found to promote the stability of axonal filopodia and the entry of microtubule plus tips into axonal filopodia. The effects of drebrin on the stabilization of filopodia are independent of its effects on promoting microtubule targeting to filopodia. Inhibition of myosin II induces a redistribution of endogenous drebrin distally into filopodia, and further increases branching in drebrin overexpressing neurons. Finally, a 30 min treatment with the branch-inducing signal nerve growth factor increases the levels of axonal drebrin. This study determines the specific roles of drebrin in the regulation of the axonal cytoskeleton, and provides evidence that drebrin contributes to the coordination of the actin and microtubule cytoskeleton during the initial stages of axon branching. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1092-1110, 2016.

Building Blocks of Functioning Brain: Cytoskeletal Dynamics in Neuronal Development

Neural connectivity requires proper polarization of neurons, guidance to appropriate target locations, and establishment of synaptic connections. From when neurons are born to when they finally reach their synaptic partners, neurons undergo constant rearrangment of the cytoskeleton to achieve appropriate shape and polarity. Of particular importance to neuronal guidance to target locations is the growth cone at the tip of the axon. Growth-cone steering is also dictated by the underlying cytoskeleton. All these changes require spatiotemporal control of the cytoskeletal machinery. This review summarizes the proteins that are involved in modulating the actin and microtubule cytoskeleton during the various stages of neuronal development.

Nerve growth factor promotes reorganization of the axonal microtubule array at sites of axon collateral branching

The localized debundling of the axonal microtubule array and the entry of microtubules into axonal filopodia are two defining features of collateral branching. We report that nerve growth factor (NGF), a branch-inducing signal, increases the frequency of microtubule debundling along the axon shaft of chicken embryonic sensory neurons. Sites of debundling correlate strongly with the localized targeting of microtubules into filopodia. Platinum replica electron microscopy suggests physical interactions between debundled microtubules and axonal actin filaments. However, as evidenced by depolymerization of actin filaments and inhibition of myosin II, actomyosin force generation does not promote debundling. In contrast, loss of actin filaments or inhibition of myosin II activity promotes debundling, indicating that axonal actomyosin forces suppress debundling. MAP1B is a microtubule associated protein that represses axon branching. Following treatment with NGF, microtubules penetrating filopodia during the early stages of branching exhibited lower levels of associated MAP1B. NGF increased and decreased the levels of MAP1B phosphorylated at a GSK-3β site (pMAP1B) along the axon shaft and within axonal filopodia, respectively. The levels of MAP1B and pMAP1B were not altered at sites of debundling, relative to the rest of the axon. Unlike the previously determined effects of NGF on the axonal actin cytoskeleton, the effects of NGF on microtubule debundling were not affected by inhibition of protein synthesis. Collectively, these data indicate that NGF promotes localized axonal microtubule debundling, that actomyosin forces antagonize microtubule debundling, and that NGF regulates pMAP1B in axonal filopodia during the early stages of collateral branch formation.

Drebrin regulates neuroblast migration in the postnatal mammalian brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). This migration is crucial for the proper integration of newborn neurons in a pre-existing synaptic network and is believed to play a key role in infant human brain development. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. Here, we have investigated the function of drebrin, an actin-binding protein highly expressed in the RMS of the postnatal mammalian brain. Neuroblast migration was monitored both in culture and in brain slices obtained from electroporated mice by time-lapse spinning disk confocal microscopy. Depletion of drebrin using distinct RNAi approaches in early postnatal mice affects neuroblast morphology and impairs neuroblast migration and orientation in vitro and in vivo. Overexpression of drebrin also impairs migration along the RMS and affects the distribution of neuroblasts at their final destination, the OB. Drebrin phosphorylation on Ser142 by Cyclin-dependent kinase 5 (Cdk5) has been recently shown to regulate F-actin-microtubule coupling in neuronal growth cones. We also investigated the functional significance of this phosphorylation in RMS neuroblasts using in vivo postnatal electroporation of phosphomimetic (S142D) or non-phosphorylatable (S142A) drebrin in the SVZ of mouse pups. Preventing or mimicking phosphorylation of S142 in vivo caused similar effects on neuroblast dynamics, leading to aberrant neuroblast branching. We conclude that drebrin is necessary for efficient migration of SVZ-derived neuroblasts and propose that regulated phosphorylation of drebrin on S142 maintains leading process stability for polarized migration along the RMS, thus ensuring proper neurogenesis.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

Phosphorylation of drebrin by cyclin-dependent kinase 5 and its role in neuronal migration

Cyclin-dependent kinase 5 (Cdk5)-p35 is a proline-directed Ser/Thr kinase which plays a key role in neuronal migration, neurite outgrowth, and spine formation during brain development. Dynamic remodeling of cytoskeletons is required for all of these processes. Cdk5-p35 phosphorylates many cytoskeletal proteins, but it is not fully understood how Cdk5-p35 regulates cytoskeletal reorganization associated with neuronal migration. Since actin filaments are critical for the neuronal movement and process formation, we aimed to find Cdk5 substrates among actin-binding proteins. In this study, we isolated actin gels from mouse brain extracts, which contain many actin-binding proteins, and phosphorylated them by Cdk5-p35 in vitro. Drebrin, a side binding protein of actin filaments and well known for spine formation, was identified as a phosphorylated protein. Drebrin has two isoforms, an embryonic form drebrin E and an adult type long isoform drebrin A. Ser142 was identified as a common phosphorylation site to drebrin E and A and Ser342 as a drebrin A-specific site. Phosphorylated drebrin is localized at the distal area of total drebrin in the growth cone of cultured primary neurons. By expressing nonphosphorylatable or phosphorylation mimicking mutants in developing neurons in utero, the reversible phosphorylation/dephosphorylation reaction of drebrin was shown to be involved in radial migration of cortical neurons. These results suggest that Cdk5-p35 regulates neuronal migration through phosphorylation of drebrin in growth cone processes.

Resveratrol increases cerebral glycogen synthase kinase phosphorylation as well as protein levels of drebrin and transthyretin in mice: an exploratory study

Alzheimer's disease (AD) is characterized by intraneuronal β-amyloid plaques and hyperphosphorylated tau, leading to neuronal cell death and progressive memory losses. This exploratory work investigates if dietary resveratrol, previously shown to have broad anti-aging effects and improve AD pathology in vivo, leads to neuroprotective changes in specific protein targets in the mouse brain. Both wild-type and APP/PS1 mice, a transgenic AD mouse model, received control AIN-93G diet or AIN-93G supplemented with resveratrol. Pathology parameters and AD risk were assessed via measurements on plaque burden, levels of phosphorylated glycogen synthase kinase 3-β (GSK3-β), tau, transthyretin and drebrin. Dietary resveratrol treatment did not decrease plaque burden in APP/PS1 mice. However, resveratrol-fed mice demonstrated increases in GSK3-β phosphorylation, a 3.8-fold increase in protein levels of transthyretin, and a 2.2-fold increase in drebrin. This study broadens our understanding of specific mechanisms and targets whereby resveratrol provides neuroprotection.

Drebrin contains a cryptic F-actin-bundling activity regulated by Cdk5 phosphorylation

Drebrin activity in F-actin bundling and filopodia induction relies on two adjacent F-actin binding sites and a Cdk5 phosphorylation-regulated intramolecular inhibitory interaction.

Drebrin is an actin filament (F-actin)–binding protein with crucial roles in neuritogenesis and synaptic plasticity. Drebrin couples dynamic microtubules to F-actin in growth cone filopodia via binding to the microtubule-binding +TIP protein EB3 and organizes F-actin in dendritic spines. Precisely how drebrin interacts with F-actin and how this is regulated is unknown. We used cellular and in vitro assays with a library of drebrin deletion constructs to map F-actin binding sites. We discovered two domains in the N-terminal half of drebrin—a coiled-coil domain and a helical domain—that independently bound to F-actin and cooperatively bundled F-actin. However, this activity was repressed by an intramolecular interaction relieved by Cdk5 phosphorylation of serine 142 located in the coiled-coil domain. Phospho-mimetic and phospho-dead mutants of serine 142 interfered with neuritogenesis and coupling of microtubules to F-actin in growth cone filopodia. These findings show that drebrin contains a cryptic F-actin–bundling activity regulated by phosphorylation and provide a mechanistic model for microtubule–F-actin coupling.

Mechanisms underlying the initiation and dynamics of neuronal filopodia: from neurite formation to synaptogenesis

Filopodia are finger-like cellular protrusions found throughout the metazoan kingdom and perform fundamental cellular functions during development and cell migration. Neurons exhibit a wide variety of extremely complex morphologies. In the nervous system, filopodia underlie many major morphogenetic events. Filopodia have roles spanning the initiation and guidance of neuronal processes, axons and dendrites to the formation of synaptic connections. This chapter addresses the mechanisms of the formation and dynamics of neuronal filopodia. Some of the major lessons learned from the study of neuronal filopodia are (1) there are multiple mechanisms that can regulate filopodia in a context-dependent manner, (2) that filopodia are specialized subcellular domains, (3) that filopodia exhibit dynamic membrane recycling which also controls aspects of filopodial dynamics, (4) that neuronal filopodia contain machinery for the orchestration of the actin and microtubule cytoskeleton, and (5) localized protein synthesis contributes to neuronal filopodial dynamics.

The role of drebrin in glioma migration and invasion

Glioblastoma (GBM) is the most common primary brain tumor in adults. Despite current advances in therapy consisting of surgery followed by chemotherapy and radiation, the overall survival rate still remains poor. Therapeutic failures are partly attributable to the highly infiltrative nature of tumor adjacent to normal brain parenchyma. Recently, evidence is mounting to suggest that actin cytoskeleton dynamics are critical components of the cell invasion process. Drebrin is an actin-binding protein involved in the regulation of actin filament organization, and plays a significant role in cell motility; however, the role of drebrin in glioma cell invasiveness has not yet been fully elucidated. Therefore, this study was aimed to clarify the role of drebrin in glioma cell morphology and cell motility. Here we show that drebrin is expressed in glioma cell lines and in operative specimens of GBM. We demonstrate that stable overexpression of drebrin in U87 cells leads to alterations in cell morphology, and induces increased invasiveness in vitro while knockdown of drebrin in U87 cells by small interfering RNA (siRNA) decreases invasion and migration. In addition, we show that depletion of drebrin by siRNA alters glioma cell morphology in A172 GBM cell line. Our results suggest that drebrin contributes to the maintenance of cell shape, and may play an important role in glioma cell motility.