Rapid induction of dendritic spine morphogenesis by trans-synaptic ephrinB-EphB receptor activation of the Rho-GEF kalirin

Kalirin: a dual Rho guanine nucleotide exchange factor that is so much more than the sum of its many parts

A large number of Rho guanine nucleotide exchange factors (GEFs) and Rho GTPase activating proteins (GAPs) are used in the CNS to activate specific Rho GTPase family members, thereby inducing various signaling mechanisms that regulate neuronal shape, growth, and plasticity, in part through their effects on the actin cytoskeleton. Kalirin is a large neuronal dual Rho GEF that activates Rac1, RhoA, and RhoG via its two Rho GEF domains. This activation, which is spatially and temporally regulated, allows Kalirin to influence neurite initiation, axonal growth, and dendritic morphogenesis. In addition, this alternatively spliced gene generates developmentally regulated transcripts that yield proteins localized to the postsynaptic density (PSD). Kalirin-7, which interacts with PSD-95, is necessary for dendritic spine formation. In addition, Kalirins have the ability to regulate and influence other aspects of neuronal morphogenesis via protein-protein interactions with their other domains, including many spectrins, other protein and lipid interaction domains, and a potential kinase. These interactions have implications not only for neuronal morphogenesis but also for vesicle trafficking, secretion, neuronal maintenance, and neurodegenerative disease.

Calcium Signaling and the Control of Dendritic Development

Dendrites serve a critical role in neuronal information processing as sites of synaptic integration. The morphological diversity of dendritic architecture reflects specialized strategies that neurons have evolved to detect and process incoming information. Recent observations suggest that calcium signals exert an important influence on neuronal morphology by regulating the growth and branching of dendrites and the formation of dendritic spines. Calcium signals appear to influence branch dynamics by affecting the cytoskeleton near the site of calcium entry, whereas calcium-dependent dendritic growth involves activation of a transcriptional program.

Induction of ephrin-B1 and EphB receptors during denervation-induced plasticity in the adult mouse hippocampus

Abstract It has been widely demonstrated that Eph receptors and their ephrin ligands play multiple pivotal roles in the development of the nervous system. However, less is known about their roles in the adult brain. Here we reported the expression of ephrin-B1 and its cognate EphB receptors in the adult mouse hippocampus at 3, 7, 15, 30 and 60 days after transections of the entorhinal afferents. In situ hybridization and immunohistochemistry showed the time-dependent up-regulation of ephrin-B1 in the denervated areas of the hippocampus, which initiated at 3 days postlesion (dpl), reached maximal levels at 7-15 dpl, remained slightly elevated at 30 dpl and recovered to normal levels by 60 dpl. Double labeling of ephrin-B1 and glial fibrillary acidic protein revealed that ephrin-B1-expressing cells in the denervated areas were reactive astrocytes. Furthermore, a ligand-binding assay using ephrin-B1/Fc chimera protein also displayed the up-regulation of EphB receptors in the denervated areas of the hippocampus in a similar manner to that of ephrin-B1. Within the first week postlesion, the EphB receptors were expressed by reactive astrocytes. After 7 dpl, however, EphB receptors were expressed not only by reactive astrocytes but also first by sprouting axons and later by regrowing dendrites. These results suggest that the ephrin-B1/EphB system may participate in the lesion-induced plasticity processes in the adult mouse hippocampus.

EphrinB-EphB signalling regulates clathrin-mediated endocytosis through tyrosine phosphorylation of synaptojanin 1

Recent studies show that Eph receptors act mainly through the regulation of actin reorganization. Here, we show a novel mode of action for EphB receptors. We identify synaptojanin 1 - a phosphatidylinositol 5'-phosphatase that is involved in clathrin-mediated endocytosis - as a physiological substrate for EphB2. EphB2 causes tyrosine phosphorylation in the proline-rich domain of synaptojanin 1, and inhibits both the interaction with endophilin and the 5'-phosphatase activity of synaptojanin 1. Treatment with the EphB ligand, ephrinB2, elevates the cellular level of phosphatidylinositol 4,5-bisphosphate and promotes transferrin uptake. A kinase inactive mutant of EphB2 and a phosphorylation site mutant of synaptojanin 1 both neutralize the increase of transferrin uptake after ephrinB2 treatment. These mutants also inhibit AMPA glutamate receptor endocytosis in hippocampal neurons. Interestingly, incorporated transferrin does not reach endosomes, suggesting dual effects of EphB signalling on the early and late phases of clathrin-mediated endocytosis. Our results indicate that ephrinB-EphB signalling regulates clathrin-mediated endocytosis in various cellular contexts by influencing protein interactions and phosphoinositide turnover through tyrosine phosphorylation of synaptojanin 1.

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.

Eph-Dependent Tyrosine Phosphorylation of Ephexin1 Modulates Growth Cone Collapse

Ephs regulate growth cone repulsion, a process controlled by the actin cytoskeleton. The guanine nucleotide exchange factor (GEF) ephexin1 interacts with EphA4 and has been suggested to mediate the effect of EphA on the activity of Rho GTPases, key regulators of the cytoskeleton and axon guidance. Using cultured ephexin1-/- mouse neurons and RNA interference in the chick, we report that ephexin1 is required for normal axon outgrowth and ephrin-dependent axon repulsion. Ephexin1 becomes tyrosine phosphorylated in response to EphA signaling in neurons, and this phosphorylation event is required for growth cone collapse. Tyrosine phosphorylation of ephexin1 enhances ephexin1's GEF activity toward RhoA while not altering its activity toward Rac1 or Cdc42, thus changing the balance of GTPase activities. These findings reveal that ephexin1 plays a role in axon guidance and is regulated by a switch mechanism that is specifically tailored to control Eph-mediated growth cone collapse.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

The genetics of Pak

p21-activated kinases (Paks) are a highly conserved family of enzymes that bind to and are activated by small GTPases of the Cdc42 and Rac families. With the notable exception of plants, nearly all eukaryotes encode one or more Pak genes, indicating an ancient origin and important function for this family of enzymes. Genetic approaches in many different experimental systems, ranging from yeast to mice, are beginning to decipher the different functions of Paks. Although some of these functions are unique to a given organism, certain common themes have emerged, such as the activation of mitogen-activated protein kinase (MAPK) cascades and the regulation of cytoskeletal structure through effects on the actin and tubulin cytoskeletons.

The role of the Rho GTPases in neuronal development

Our brain serves as a center for cognitive function and neurons within the brain relay and store information about our surroundings and experiences. Modulation of this complex neuronal circuitry allows us to process that information and respond appropriately. Proper development of neurons is therefore vital to the mental health of an individual, and perturbations in their signaling or morphology are likely to result in cognitive impairment. The development of a neuron requires a series of steps that begins with migration from its birth place and initiation of process outgrowth, and ultimately leads to differentiation and the formation of connections that allow it to communicate with appropriate targets. Over the past several years, it has become clear that the Rho family of GTPases and related molecules play an important role in various aspects of neuronal development, including neurite outgrowth and differentiation, axon pathfinding, and dendritic spine formation and maintenance. Given the importance of these molecules in these processes, it is therefore not surprising that mutations in genes encoding a number of regulators and effectors of the Rho GTPases have been associated with human neurological diseases. This review will focus on the role of the Rho GTPases and their associated signaling molecules throughout neuronal development and discuss how perturbations in Rho GTPase signaling may lead to cognitive disorders.

Phosphorylation of spinophilin by ERK and cyclin-dependent PK 5 (Cdk5)

Spinophilin is a protein that binds to protein phosphatase-1 and actin and modulates excitatory synaptic transmission and dendritic spine morphology. We have identified three sites phosphorylated by ERK2 (Ser-15 and Ser-205) and cyclin-dependent PK 5 (Cdk5) (Ser-17), within the actin-binding domain of spinophilin. Cdk5 and ERK2 both phosphorylated spinophilin in intact cells. However, in vitro, phosphorylation by ERK2, but not by Cdk5, was able to modulate the ability of spinophilin to bind to and bundle actin filaments. In neurons and HEK293 cells expressing GFP-tagged variants of spinophilin, imaging studies demonstrated that introduction of a phospho-site mimic (Ser-15 to glutamate) was associated with increased filopodial density. These results support a role for spinophilin phosphorylation by ERK2 in the regulation of spine morphogenesis.

Making protein interactions druggable: targeting PDZ domains

Modulating protein-protein interactions involved in disease pathways is an attractive strategy for developing drugs, but remains a challenge to achieve. One approach is to target certain domains within proteins that mediate these interactions. One example of such a domain is the PDZ domain, which is involved in interactions between many different proteins in a variety of cellular contexts. Because PDZ domains have well-defined binding sites, they are promising targets for drug discovery. However, there is still much to learn about the function of these domains before drugs targeting PDZ interactions can become a reality.

The organization of recent and remote memories

A fundamental question in memory research is how our brains can form enduring memories. In humans, memories of everyday life depend initially on the medial temporal lobe system, including the hippocampus. As these memories mature, they are thought to become increasingly dependent on other brain regions such as the cortex. Little is understood about how new memories in the hippocampus are transformed into remote memories in cortical networks. However, recent studies have begun to shed light on how remote memories are organized in the cortex, and the molecular and cellular events that underlie their consolidation.

Rho proteins, mental retardation and the neurobiological basis of intelligence

For several decades it has been known that mental retardation is associated with abnormalities in dendrites and dendritic spines. The recent cloning of eight genes which cause nonspecific mental retardation when mutated, provides an important insight into the cellular mechanisms that result in the dendritic abnormalities underlying mental retardation. Three of the encoded proteins, oligophrenin1, PAK3 and alphaPix, interact directly with Rho GTPases. Rho GTPases are key signaling proteins which integrate extracellular and intracellular signals to orchestrate coordinated changes in the actin cytoskeleton, essential for directed neurite outgrowth and the generation/rearrangement of synaptic connectivity. Although many details of the cell biology of Rho signaling in the CNS are as yet unclear, a picture is unfolding showing how mutations that cause abnormal Rho signaling result in abnormal neuronal connectivity which gives rise to deficient cognitive functioning in humans.

Dendritic spine morphogenesis and plasticity

Dendritic spines are small protrusions off the dendrite that receive excitatory synaptic input. Spines vary in size, likely correlating with the strength of the synapses they form. In the developing brain, spines show highly dynamic behavior thought to facilitate the formation of new synaptic contacts. Recent studies have illuminated the numerous molecules regulating spine development, many of which converge on the regulation of actin filaments. In addition, interactions with glial cells are emerging as important regulators of spine morphology. In many cases, spine morphogenesis, plasticity, and maintenance also depend on synaptic activity, as shown by recent studies demonstrating changes in spine dynamics and maintenance with altered sensory experience.

Dynamics and pathology of dendritic spines

Dendritic spines are key players in information processing in the brain. Changes in spine shape and wholesale spine turnover provide mechanisms for modifying existing synaptic connections and altering neuronal connectivity. Although neuronal cell death in acute and chronic neurodegenerative diseases is clearly an important factor in decline of cognitive or motor function, loss of dendritic spines, in the absence of cell death, may also contribute to impaired brain function in these diseases, as well as in psychiatric disorders and aging. Because spines can function in neuroprotection in vitro, advances toward a molecular understanding of spine maintenance might one day aid in the design of therapies to minimize neurological damage following excitotoxic injury. In addition, progress in defining the biochemical basis of spine development and stabilization may yield insights into mental retardation and psychiatric disorders.

ABI2-deficient mice exhibit defective cell migration, aberrant dendritic spine morphogenesis, and deficits in learning and memory

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short- and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

Induction of spine growth and synapse formation by regulation of the spine actin cytoskeleton

We explored the relationship between regulation of the spine actin cytoskeleton, spine morphogenesis, and synapse formation by manipulating expression of the actin binding protein NrbI and its deletion mutants. In pyramidal neurons of cultured rat hippocampal slices, NrbI is concentrated in dendritic spines by binding to the actin cytoskeleton. Expression of one NrbI deletion mutant, containing the actin binding domain, dramatically increased the density and length of dendritic spines with synapses. This hyperspinogenesis was accompanied by enhanced actin polymerization and spine motility. Synaptic strengths were reduced to compensate for extra synapses, keeping total synaptic input per neuron constant. Our data support a model in which synapse formation is promoted by actin-powered motility.

Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly

Eph receptor tyrosine kinases and ephrins have key roles in regulation of the migration and adhesion of cells required to form and stabilize patterns of cell organization during development. Activation of Eph receptors or ephrins can lead either to cell repulsion or to cell adhesion and invasion, and recent work has found that cells can switch between these distinct responses. This review will discuss biochemical mechanisms and developmental roles of the diverse cell responses controlled by Eph receptors and ephrins.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Eph/ephrin signaling in morphogenesis, neural development and plasticity

Ephrins are cell-surface-tethered ligands for Eph receptors, the largest family of receptor tyrosine kinases. During development, the Eph/ephrin cell communication system appears to influence cell behavior such as attraction/repulsion, adhesion/de-adhesion and migration, thereby influencing cell fate, morphogenesis and organogenesis. During adulthood, the Eph/ephrin system continues to play roles in tissue plasticity, for example in shaping dendritic spines during neuronal plasticity. Mechanistically, Eph-ephrin repulsive behavior appears to require ligand-receptor internalization and signaling to Rho GTPases.

PDZ domain proteins of synapses

PDZ domains are protein-interaction domains that are often found in multi-domain scaffolding proteins. PDZ-containing scaffolds assemble specific proteins into large molecular complexes at defined locations in the cell. In the postsynaptic density of neuronal excitatory synapses, PDZ proteins such as PSD-95 organize glutamate receptors and their associated signalling proteins and determine the size and strength of synapses. PDZ scaffolds also function in the dynamic trafficking of synaptic proteins by assembling cargo complexes for transport by molecular motors. As key organizers that control synaptic protein composition and structure, PDZ scaffolds are themselves highly regulated by synthesis and degradation, subcellular distribution and post-translational modification.

Rho GTPases and activity-dependent dendrite development

Dendritic morphology has an important influence on neuronal information processing. Multiple environmental cues, including neuronal activity, the neurotrophin family of growth factors, and extracellular guidance molecules have been shown to influence dendritic size, shape, and development. The Rho GTPases have emerged as key integrators of these environmental cues to regulate the underlying dendritic cytoskeleton.

Cdk5 and Trio modulate endocrine cell exocytosis

Hormone secretion by pituitary cells is decreased by roscovitine, an inhibitor of cyclin-dependent kinase 5 (Cdk5). Roscovitine treatment reorganizes cortical actin and ultrastructural analysis demonstrates that roscovitine limits the ability of secretory granules to approach the plasma membrane or one another. Trio, a multifunctional RhoGEF expressed in pituitary cells, interacts with peptidylglycine α-amidating monooxygenase, a secretory granule membrane protein known to affect the actin cytoskeleton. Roscovitine inhibits the ability of Trio to activate Rac, and peptides corresponding to the Cdk5 consensus sites in Trio are phosphorylated by Cdk5. Together, these data suggest that control of the cortical actin cytoskeleton, long known to modulate hormone exocytosis and subsequent endocytosis, involves Cdk5-mediated activation of Trio.

BTB/POZ-Zinc Finger Protein Abrupt Suppresses Dendritic Branching in a Neuronal Subtype-Specific and Dosage-Dependent Manner

How dendrites of different neuronal subtypes exhibit distinct branching patterns during development remains largely unknown. Here we report the mapping and identification of loss-of-function mutations in the abrupt (ab) gene that increased the number of dendritic branches of multiple dendritic (MD) sensory neurons in Drosophila embryos. Ab encodes an evolutionarily conserved transcription factor that contains a BTB/POZ domain and C2H2 zinc finger motifs. We show that ab has a cell-autonomous function in postmitotic neurons to limit dendritic branching. Ab and the homeodomain protein Cut are expressed in distinct but complementary subsets of MD neurons, and Ab functions in a transcriptional program that does not require Cut. Deleting one copy of ab or overexpressing ab had opposite effects on the formation of higher-order dendritic branches, suggesting that the Ab level in a specific neuron directly regulates dendritic complexity. These results demonstrate that dendritic branching can be suppressed by neuronal subtype-specific transcription factors in a cell-autonomous and dosage-dependent manner.

GEFT, a Rho family guanine nucleotide exchange factor, regulates neurite outgrowth and dendritic spine formation

The Rho family of small GTPases controls a wide range of cellular processes in eukaryotic cells, such as normal cell growth, proliferation, differentiation, gene regulation, actin cytoskeletal organization, cell fate determination, and neurite outgrowth. The activation of Rho-GTPases requires the exchange of GDP for GTP, a process catalyzed by the Dbl family of guanine nucleotide exchange factors. We demonstrate that a newly identified guanine nucleotide exchange factor, GEFT, is widely expressed in the brain and highly concentrated in the hippocampus, and the Purkinje and granular cells of the cerebellum. Exogenous expression of GEFT promotes dendrite outgrowth in hippocampal neurons, resulting in spines with larger size as compared with control spines. In neuroblastoma cells, GEFT promotes the active GTP-bound state of Rac1, Cdc42, and RhoA and increases neurite outgrowth primarily via Rac1. Furthermore, we demonstrated that PAK1 and PAK5, both downstream effectors of Rac1/Cdc42, are necessary for GEFT-induced neurite outgrowth. AP-1 and NF-kappaB, two transcriptional factors involved in neurite outgrowth and survival, were up-regulated in GEFT-expressing cells. Together, our data suggest that GEFT enhances dendritic spine formation and neurite outgrowth in primary neurons and neuroblastoma cells, respectively, through the activation of Rac/Cdc42-PAK signaling pathways.

Altered cortical synaptic morphology and impaired memory consolidation in forebrain- specific dominant-negative PAK transgenic mice

Molecular and cellular mechanisms for memory consolidation in the cortex are poorly known. To study the relationships between synaptic structure and function in the cortex and consolidation of long-term memory, we have generated transgenic mice in which catalytic activity of PAK, a critical regulator of actin remodeling, is inhibited in the postnatal forebrain. Cortical neurons in these mice displayed fewer dendritic spines and an increased proportion of larger synapses compared to wild-type controls. These alterations in basal synaptic morphology correlated with enhanced mean synaptic strength and impaired bidirectional synaptic modifiability (enhanced LTP and reduced LTD) in the cortex. By contrast, spine morphology and synaptic plasticity were normal in the hippocampus of these mice. Importantly, these mice exhibited specific deficits in the consolidation phase of hippocampus-dependent memory. Thus, our results provide evidence for critical relationships between synaptic morphology and bidirectional modifiability of synaptic strength in the cortex and consolidation of long-term memory.

Overexpression of glial cell line-derived neurotrophic factor induces genes regulating migration and differentiation of neuronal progenitor cells

The glial cell line-derived neurotrophic factor (GDNF) is involved in the development and maintenance of neural tissues. Mutations in components of its signaling pathway lead to severe migration deficits of neuronal crest stem cells, tumor formation, or ablation of the urinary system. In animal models of Parkinson's disease, GDNF has been recognized to be neuroprotective and to improve motor function when delivered into the cerebral ventricles or into the substantia nigra. Here, we characterize the network of 43 genes induced by GDNF overproduction of neuronal progenitor cells (ST14A), which mainly regulate migration and differentiation of neuronal progenitor cells. GDNF down-regulates doublecortin, Paf-ah1b (Lis1), dynamin, and alpha-tubulin, which are involved in neocortical lamination and cytoskeletal reorganization. Axonal guidance depends on cell-surface molecules and extracellular matrix proteins. Laminin, Mpl3, Alcam, Bin1, Id1, Id2, Id3, neuregulin1, the ephrinB2-receptor, neuritin, focal adhesion kinase (FAK), Tc10, Pdpk1, clusterin, GTP-cyclooxygenase1, and follistatin are genes up-regulated by GDNF overexpression. Moreover, we found four key enzymes of the cholesterol-synthesis pathway to be down-regulated leading to decreased farnesyl-pyrophospate production. Many proteins are anchored by farnesyl-derivates at the cell membrane. The identification of these GDNF-regulated genes may open new opportunities for directly influencing differentiation and developmental processes of neurons.

Regulation of dendritic spine motility and stability by Rac1 and Rho kinase: evidence for two forms of spine motility

Dendritic spines are major sites of excitatory synapses in the brain and display rapid motility, which is believed to be important for synapse formation and plasticity. Spine morphology was previously shown to be regulated by the Rho GTPases Rac1 and RhoA. Here, we analyzed the roles of Rac1 and a downstream effector of RhoA, Rho kinase, in controlling spine morphogenesis and their effects on spine motility and stability. Blockade of Rac1 induced long, thin spines and inhibited spine head growth, morphing, and stability. Spine head growth was more severely affected in mature spines. On the other hand, inhibition of Rho kinase induced new, long spines and protrusive motility. These data demonstrate that Rac1 and RhoA/Rho kinase pathways regulate different aspects of spine morphology, motility, and stability and presumably also different aspects of synaptic functions. Moreover, our data show that there are two different types of spine motility: protrusive motility and head morphing, which are differentially regulated by Rac1 and Rho kinase. We propose that these two different types of spine motility serve different functions in synaptogenesis and synapse maturation.

Microanatomy of dendritic spines: emerging principles of synaptic pathology in psychiatric and neurological disease

Psychiatric and neurologic disorders ranging from mental retardation to addiction are accompanied by structural and functional alterations of synaptic connections in the brain. Such alterations include abnormal density and morphology of dendritic spines, synapse loss, and aberrant synaptic signaling and plasticity. Recent work is revealing an unexpectedly complex biochemical and subcellular organization of dendritic spines. In this review, we highlight the molecular interplay between functional domains of the spine, including the postsynaptic density, the actin cytoskeleton, and membrane trafficking domains. This research points to an emerging level of analysis--a microanatomical understanding of synaptic physiology--that will be critical for discerning how synapses operate in normal physiologic states and for identifying and reversing microscopic changes in psychiatric and neurologic disease.

Eph receptor–ephrin bidirectional signals that target Ras and Rho proteins

The ability of cells to respond to their surrounding environment and relay signals to the cell interior is essential for numerous processes during the development and maintenance of tissues. Eph receptors and their membrane-bound ligands, the ephrins, are unique in the receptor tyrosine kinase family in that their signaling is bidirectional, through both the receptor and the ligand. Eph receptors and ephrins are essential for a variety of biological processes, and play a particularly important role in regulating cell shape and cell movement. Recent data have linked Eph receptor-ephrin signaling complexes to the Ras and Rho families of small molecular weight GTPases and also to heterotrimeric G proteins. Understanding the signaling networks involved is an important step to understand the molecular basis for normal and defective cell-cell communication through Eph receptors and ephrins.

Eph receptors in the adult brain

The Eph receptors are a large family of receptor tyrosine kinases with important roles in the establishment of neuronal and vascular networks during embryonic development. The functions of Eph receptors in the adult brain have only recently been investigated, and the results are forcing us to amend the conventional view that these molecules function predominantly in a developmental context. This review summarizes this rapidly expanding new area of research, which has shown that the Eph receptors regulate the structure and physiological function of excitatory synapses through multiple mechanisms, and might thus play a significant role in higher brain functions.

The Phosphorylation of EphB2 Receptor Regulates Migration and Invasion of Human Glioma Cells

Eph receptor tyrosine kinases and their ligands, ephrins, mediate neurodevelopmental processes such as boundary formation, axon guidance, vasculogenesis, and cell migration. We determined the expression profiles of the Eph family members in five glioma cell lines under migrating and nonmigrating conditions. EphB2 mRNA was overexpressed in all five during migration (1.2-2.8-fold). We found abundant EphB2 protein as well as strong phosphorylation of EphB2 in migrating U87 cells. Confocal imaging showed EphB2 localized in lamellipodia of motile U87 cells. Treatment with ephrin-B1/Fc chimera stimulated migration and invasion of U87, whereas treatment with a blocking EphB2 antibody significantly inhibited migration and invasion. Forced expression of EphB2 in U251 cells stimulated cell migration and invasion and diminished adhesion concomitant with the tyrosine phosphorylation of EphB2. U251 stably transfected with EphB2 showed more scattered and more pronounced invasive growth in an ex vivo rat brain slice. In human brain tumor specimens, EphB2 expression was higher in glioblastomas than in low-grade astrocytomas or normal brain; patterns of phosphorylated EphB2 matched the expression levels. Laser capture microdissection of invading glioblastoma cells revealed elevated EphB2 mRNA (1.5-3.5-fold) in 7 of 7 biopsy specimens. Immunohistochemistry demonstrated EphB2 localization primarily in glioblastoma cells (56 of 62 cases) and not in normal brain. This is the first demonstration that migrating glioblastoma cells overexpress EphB2 in vitro and in vivo; glioma migration and invasion are promoted by activation of EphB2 or inhibited by blocking EphB2. Dysregulation of EphB2 expression or function may underlie glioma invasion.

Pax6 and engrailed 1 regulate two distinct aspects of renshaw cell development

Many of the interneuron cell types present in the adult spinal cord contribute to the circuits that control locomotion and posture. Little is known, however, about the embryonic origin of these cell types or the molecular mechanisms that control their differentiation. Here we provide evidence that V1 interneurons (INs), an embryonic class of interneurons that transiently express the En1 transcription factor, differentiate as local circuit inhibitory interneurons and form synapses with motor neurons. Furthermore, we show that a subset of V1 INs differentiates as Renshaw cells, the interneuronal cell type that mediates recurrent inhibition of motor neurons. We analyze the role that two V1 IN-related transcription factor genes play in Renshaw cell development. Pax6 (paired box gene 6) is necessary for an early step in Renshaw cell development, whereas Engrailed 1 (En1), which is genetically downstream of Pax6, regulates the formation of inhibitory synapses between Renshaw cells and motor neurons. Together, these results show that Pax6 and En1 have essential roles in establishing the recurrent inhibitory circuit between motor neurons and Renshaw cells.

Identification of the Jak/Stat Proteins as Novel Downstream Targets of EphA4 Signaling in Muscle

Eph receptors and their cognate ligands ephrins are important players in axon guidance and neural patterning during development of the nervous system. Much of our knowledge about the signal transduction pathways triggered by Eph receptors has been related to the modulation of actin cytoskeleton, which is fundamental in mediating the cellular responses in growth cone navigation, cell adhesion, and cell migration. In contrast, little was known about whether long term activation of Eph receptor would regulate gene expression. Here we report a novel signaling pathway of EphA4, which involves activation of the tyrosine kinase Jak2 and the transcriptional activator Stat3. Transfection of COS7 cells with EphA4, but not the kinase-dead mutant, induced tyrosine phosphorylation of Jak2, Stat1, and Stat3. Treatment of cultured C2C12 myotubes with ephrin-A1 also induced tyrosine phosphorylation of Stat3, which was abolished by the Jak2 inhibitor AG490. Moreover, Jak2 was co-immunoprecipitated with EphA4 in muscle, and both proteins were concentrated at the neuromuscular junction (NMJ) of adult muscle. By using microarray analysis, we have identified acetylcholinesterase, the critical enzyme that hydrolyzed the neurotransmitter acetylcholine at the NMJ, as a downstream target gene of the Jak/Stat pathway in muscle. More importantly, ephrin-A1 increased the expression of acetylcholinesterase protein in C2C12 myotubes, which was abolished by AG490. In contrast, ephrin-A1 reduced the expression of fibronectin mRNA in C2C12 myotubes independently of Jak2. Finally, the expression level of acetylcholinesterase in limb muscle of EphA4 null mice was significantly reduced compared with the wild-type control. Taken together, these results have identified Jak/Stat proteins as the novel downstream targets of EphA4 signaling. In addition, the present study provides the first demonstration of a potential function of Eph receptors and Jak/Stat proteins at the NMJ.

Stability of dendritic spines and synaptic contacts is controlled by alpha N-catenin

Morphological plasticity of dendritic spines and synapses is thought to be crucial for their physiological functions. Here we show that alpha N-catenin, a linker between cadherin adhesion receptors and the actin cytoskeleton, is essential for stabilizing dendritic spines in rodent hippocampal neurons in culture. In the absence of alpha N-catenin, spine heads were abnormally motile, actively protruding filopodia from their synaptic contact sites. Conversely, alpha N-catenin overexpression in dendrites reduced spine turnover, causing an increase in spine and synapse density. Tetrodotoxin (TTX), a neural activity blocker, suppressed the synaptic accumulation of alpha N-catenin, whereas bicuculline, a GABA antagonist, promoted it. Furthermore, excess alpha N-catenin rendered spines resistant to the TTX treatment. These results suggest that alpha N-catenin is a key regulator for the stability of synaptic contacts.

Multiple EphB receptor tyrosine kinases shape dendritic spines in the hippocampus

Here, using a genetic approach, we dissect the roles of EphB receptor tyrosine kinases in dendritic spine development. Analysis of EphB1, EphB2, and EphB3 double and triple mutant mice lacking these receptors in different combinations indicates that all three, although to varying degrees, are involved in dendritic spine morphogenesis and synapse formation in the hippocampus. Hippocampal neurons lacking EphB expression fail to form dendritic spines in vitro and they develop abnormal spines in vivo. Defective spine formation in the mutants is associated with a drastic reduction in excitatory glutamatergic synapses and the clustering of NMDA and AMPA receptors. We show further that a kinase-defective, truncating mutation in EphB2 also results in abnormal spine development and that ephrin-B2–mediated activation of the EphB receptors accelerates dendritic spine development. These results indicate EphB receptor cell autonomous forward signaling is responsible for dendritic spine formation and synaptic maturation in hippocampal neurons.

Signalling mechanisms mediating neuronal responses to guidance cues

Several families of extracellular guidance cues have been implicated in guiding neurons and axons to their appropriate destinations in the nervous system. Their receptors include single- and seven-transmembrane receptors, and their signal transduction pathways converge onto the Rho family of small GTPases, which control the cytoskeleton. A single guidance protein can use different mechanisms to regulate different kinds of motility or the motilities of different cell types. There is crosstalk between the signalling pathways initiated by distinct guidance cues. Studies of neuronal guidance mechanisms have shed light not only on neural development, but also on other processes that involve the extracellular regulation of the cytoskeleton.

A role of Rnd1 GTPase in dendritic spine formation in hippocampal neurons

Rho family of small GTPases are key regulators for morphological changes of neurons on the basis of reorganization of the actin cytoskeleton. Rnd1, a novel member of this family, is predominantly expressed in neurons in brain; however, the neuronal functions of Rnd1 are not known. Here we investigated the effect of Rnd1 on neuronal morphology. Northern blot analysis of Rnd1 expression in rat brain showed that Rnd1 mRNA was highly expressed during early postnatal period, the synaptogenic stage. In situ hybridization analysis at this period revealed that Rnd1 mRNA was strongly expressed in neurons, including the hippocampal pyramidal neurons. Furthermore, immunoblot analysis showed that Rnd1 protein was localized in synaptosomal membrane fraction. Ectopical overexpression of Rnd1 in cultured rat hippocampal neurons promoted the elongation of dendritic spines. On the other hand, suppression of endogenous Rnd1 level by antisense oligonucleotide of Rnd1 caused the increase in the percentage of headless protrusions accompanied by the reduction in the spine number and spine width and shortened the length of the headless protrusions. These results indicate that Rnd1 plays a role in spine formation in the developmental synaptogenic stage.

Hippocampal plasticity requires postsynaptic ephrinBs

Chemical synapses contain specialized pre- and postsynaptic structures that regulate synaptic transmission and plasticity. EphB receptor tyrosine kinases are important molecular components in this process. Previously, EphB receptors were shown to act postsynaptically, whereas their transmembrane ligands, the ephrinBs, were presumed to act presynaptically. Here we show that in mouse hippocampal CA1 neurons, the Eph/ephrin system is used in an inverted manner: ephrinBs are predominantly localized postsynaptically and are required for synaptic plasticity. We further demonstrate that EphA4, a candidate receptor, is also critically involved in long-term plasticity independent of its cytoplasmic domain, suggesting that ephrinBs are the active signaling partner. This work raises the intriguing possibility that depending on the type of synapse, Eph/ephrins can be involved in activity-dependent plasticity in converse ways, with ephrinBs on the pre- or the postsynaptic side.

Calcium/calmodulin-dependent protein kinase II contributes to activity-dependent filopodia growth and spine formation

Remodeling of synaptic networks through an activity-dependent formation or elimination of synaptic connections is believed to contribute to information processing and long-term memory. Recent work showed that enhanced synaptic activation, including induction of long-term potentiation and sensory stimulation, promote a rapid growth of dendritic filopodia and the formation of new spines or new types of synapses. Here, we investigated whether calcium/calmodulin-dependent protein kinase II (CaMKII), an enzyme implicated in the control of synaptic efficacy, also participated in these mechanisms. We show that the intracellular application of autophosphorylated CaMKII reproduced these morphological changes and triggered filopodia growth and spine formation. In addition, we find that activation of endogenous kinase through the inhibition of phosphatases or the application of calmodulin in the cell produced similar effects. Conversely, blockade of CaMKII activity prevented the synaptic enhancement, the growth of filopodia and formation of new spines triggered by LTP induction, and a short anoxia/hypoglycemia. Together, these results support the interpretation that CaMKII contributes to the control of activity-dependent structural plasticity.

Kalirin, a multifunctional Rho guanine nucleotide exchange factor, is necessary for maintenance of hippocampal pyramidal neuron dendrites and dendritic spines

The structures of dendritic spines and the dendritic tree, key determinants of neuronal function, are regulated by diverse inputs that affect many scaffolding and signaling molecules. Nevertheless, here we show that reduced expression of a single gene results in loss of dendritic spines and a decrease in dendritic complexity. Kalirin, a dual Rho GDP-GTP exchange factor, causes spine formation when overexpressed. Reduced expression of Kalirin in CA1 hippocampal neurons resulted in a reduction in linear spine density, with dispersion of postsynaptic density markers and elimination of presynaptic endings. Simplification of the apical dendritic tree preceded simplification of basal dendrites. Pyramidal cell axons were not dramatically altered. Although many factors determine dendrite shape and spine formation, expression of Kalirin is necessary for the normal function of these many regulatory elements.

Spine-Tingling Excitement from Glutamate Receptors

Ionotropic glutamate receptors excite nerve cells by forming cation-selective pores in the membrane. Recent work, however, provides evidence that atypical signaling by glutamate receptors regulates the production and maintenance of dendritic spines, the short outgrowths that receive most central excitatory synapses. The control of spine formation involves the amino-terminal extracellular domain of the GluR2 subunit of AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptors. How interactions with this domain elicit signals to downstream effectors remains to be elucidated, but ion flux through the channel may not be required. This Perspective discusses the possibility that regulation of spines by GluR2 may be one of a growing collection of cases in which ionotropic glutamate receptors can elicit biochemical changes that are conventionally attributed to metabotropic receptors. It is suggested that proteins in contact with specific glutamate receptor subunits may directly sense the conformational changes produced by agonist binding.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Rac-dependent trans-endocytosis of ephrinBs regulates Eph-ephrin contact repulsion

Eph receptor-ephrin signals are important for controlling repulsive and attractive cell movements during tissue patterning in embryonic development. However, the dynamic cellular responses to these signals at cell-cell contact sites are poorly understood. To examine these events we have used cell microinjection to express EphB4 and ephrinB2 in adjacent Swiss 3T3 fibroblasts and have studied the interaction of the injected cells using time-lapse microscopy. We show that Eph receptors are locally activated wherever neighbouring cells make contact. This triggers dynamic, Rac-regulated membrane ruffles at the Eph-ephrin contact sites. Subsequently, the receptor and ligand cells retract from one another, concomitantly with the endocytosis of the activated Eph receptors and their bound, full-length ephrinB ligands. Both the internalization of the receptor-ligand complexes and the subsequent cell retraction events are dependent on actin polymerization, which in turn is dependent on Rac signalling within the receptor-expressing cells. Similar events occur in primary human endothelial cells. Our findings suggest a novel mechanism for cell repulsion, in which the contact between Eph-expressing and ephrin-expressing cells is destabilized by the localized phagocytosis of the ligand-expressing cell plasma membrane by the receptor-expressing cell.

Ephrin-B2 and EphB2 regulation of astrocyte-meningeal fibroblast interactions in response to spinal cord lesions in adult rats

The present study provides the first evidence that signaling occurs between B-ephrins and EphB receptors in the adult CNS in response to injury. Specifically, our combined histological and biochemical data indicate that two members of the B-class of ephrins and Eph receptors, ephrin-B2 and EphB2, are expressed by astrocytes and meningeal fibroblasts, respectively, in the adult spinal cord. In response to thoracic spinal cord transection lesions, ephrin-B2 and EphB2 protein levels exhibit an initial decrease (1 d after lesion), followed by a significant increase by day 14. Immunohistochemical data indicate that ephrin-B2 is expressed by reactive CNS astrocytes, and EphB2 is present on fibroblasts invading the lesion site from the adjacent meninges. During the first 3 d after injury, there is intermingling of ephrin-B2-expressing reactive astrocytes at the lesion surface with EphB2-containing fibroblasts that is concurrent with bidirectional activation (phosphorylation) of ephrin-B2 and EphB2. By 7 d, both cell types are establishing restricted cellular domains containing dense networks of cells and interweaving processes. This astroglial-meningeal fibroblast scar is fully developed by day 14 when there is strict segregation of ephrin-B2-expressing astrocytes from EphB2-positive meningeal fibroblasts. These morphological changes are concomitant with a simultaneous decrease in ephrin-B2 and EphB2 activation. These observations provide strong evidence that cell contact-mediated bidirectional signaling between ephrin-B2 on reactive astrocytes and EphB2 on meningeal fibroblasts is an early event in the cellular cascades that result in the development of the glial scar and the exclusion of meningeal fibroblasts from the injured spinal cord.

'Eph'ective signaling: forward, reverse and crosstalk

The Eph receptors comprise the largest group of receptor tyrosine kinases and are found in a wide variety of cell types in developing and mature tissues. Their ligands are the ephrins, a family of membrane-bound proteins found in lipid rafts. In the past decade, Eph receptors and ephrins have been implicated in a vast array of cellular processes. Unlike other receptor tyrosine kinases, however, the Eph receptors seem to be geared towards regulating cell shape and movement rather than proliferation. Studies have uncovered intricate signaling networks that center around the ligand-receptor complex, and this may account for the broad repertoire of functions of Eph proteins. Deciphering the bi-directional pathways emanating from an Eph receptor-ephrin complex will not only help us to understand basic biological processes, but may also provide important insight into disease.