The microtubule destabilizer stathmin mediates the development of dendritic arbors in neuronal cells

Drosophila stathmins bind tubulin heterodimers with high and variable stoichiometries

In vertebrates, stathmins form a family of proteins possessing two tubulin binding repeats (TBRs), which each binds one soluble tubulin heterodimer. The stathmins thus sequester two tubulins in a phosphorylation-dependent manner, providing a link between signal transduction and microtubule dynamics. In Drosophila, we show here that a single stathmin gene (stai) encodes a family of D-stathmin proteins. Two of the D-stathmins are maternally deposited and then restricted to germ cells, and the other two are detected in the nervous system during embryo development. Like in vertebrates, the nervous system-enriched stathmins contain an N-terminal domain involved in subcellular targeting. All the D-stathmins possess a domain containing three or four predicted TBRs, and we demonstrate here, using complementary biochemical and biophysical methods, that all four predicted TBR domains actually bind tubulin. D-stathmins can indeed bind up to four tubulins, the resulting complex being directly visualized by electron microscopy. Phylogenetic analysis shows that the presence of regulated multiple tubulin sites is a conserved characteristic of stathmins in invertebrates and allows us to predict key residues in stathmin for the binding of tubulin. Altogether, our results reveal that the single Drosophila stathmin gene codes for a stathmin family similar to the multigene vertebrate one, but with particular tubulin binding properties.

Increased stathmin1 expression in the dentate gyrus of mice causes abnormal axonal arborizations

Pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in multiple brain functions. To clarify the cause of abnormal behavior in PACAP deficient-mice, we attempted the identification of genes whose expression was altered in the dentate gyrus of PACAP-deficient mice using the differential display method. Expression of stathmin1 was up-regulated in the dentate gyrus at both the mRNA and protein levels. PACAP stimulation inhibited stathmin1 expression in PC12 cells, while increased stathmin1expression in neurons of the subgranular zone and in primary cultured hippocampal neurons induced abnormal arborization of axons. We also investigated the pathways involved in PACAP deficiency. Ascl1 binds to E10 box of the stathmin1 promoter and increases stathmin1 expression. Inhibitory bHLH proteins (Hes1 and Id3) were rapidly up-regulated by PACAP stimulation, and Hes1 could suppress Ascl1 expression and Id3 could inhibit Ascl1 signaling. We also detected an increase of stathmin1 expression in the brains of schizophrenic patients. These results suggest that up-regulation of stathmin1 in the dentate gyrus, secondary to PACAP deficiency, may create abnormal neuronal circuits that cause abnormal behavior.

Kidins220/ARMS modulates the activity of microtubule-regulating proteins and controls neuronal polarity and development

In order for neurons to perform their function, they must establish a highly polarized morphology characterized, in most of the cases, by a single axon and multiple dendrites. Herein we find that the evolutionarily conserved protein Kidins220 (kinase D-interacting substrate of 220-kDa), also known as ARMS (ankyrin repeat-rich membrane spanning), a downstream effector of protein kinase D and neurotrophin and ephrin receptors, regulates the establishment of neuronal polarity and development of dendrites. Kidins220/ARMS gain and loss of function experiments render severe phenotypic changes in the processes extended by hippocampal neurons in culture. Although Kidins220/ARMS early overexpression hinders neuronal development, its down-regulation by RNA interference results in the appearance of multiple longer axon-like extensions as well as aberrant dendritic arbors. We also find that Kidins220/ARMS interacts with tubulin and microtubule-regulating molecules whose role in neuronal morphogenesis is well established (microtubule-associated proteins 1b, 1a, and 2 and two members of the stathmin family). Importantly, neurons where Kidins220/ARMS has been knocked down register changes in the phosphorylation activity of MAP1b and stathmins. Altogether, our results indicate that Kidins220/ARMS is a key modulator of the activity of microtubule-regulating proteins known to actively regulate neuronal morphogenesis and suggest a mechanism by which it contributes to control neuronal development.

Dendrite formation of cerebellar Purkinje cells

During postnatal cerebellar development, Purkinje cells form the most elaborate dendritic trees among neurons in the brain, which have been of great interest to many investigators. This article overviews various examples of cellular and molecular mechanisms of formation of Purkinje cell dendrites as well as the methodological aspects of investigating those mechanisms.

The microtubule network and neuronal morphogenesis: Dynamic and coordinated orchestration through multiple players

Nervous system function and plasticity rely on the complex architecture of neuronal networks elaborated during development, when neurons acquire their specific and complex shape. During neuronal morphogenesis, the formation and outgrowth of functionally and structurally distinct axons and dendrites require a coordinated and dynamic reorganization of the microtubule cytoskeleton involving numerous regulators. While most of these factors act directly on microtubules to stabilize them or promote their assembly, depolymerization or fragmentation, others are now emerging as essential regulators of neuronal differentiation by controlling tubulin availability and modulating microtubule dynamics. In this review, we recapitulate how the microtubule network is actively regulated during the successive phases of neuronal morphogenesis, and what are the specific roles of the various microtubule-regulating proteins in that process. We then describe the specific signaling pathways and inter-regulations that coordinate the different activities of these proteins to sustain neuronal development in response to environmental cues.

Effects of 1-naphthyl acetyl spermine on dendrite formation by cultured cerebellar Purkinje cells

Cerebellar Purkinje cells have the most elaborate dendritic trees among neurons in the brain. To date, the contributions of calcium-permeable AMPA receptors (CP-AMPARs) in calcium signaling and dendrite formation of Purkinje cells remain to be elucidated. In the present study, therefore, we examined the effects of 1-naphthyl acetyl spermine (NAS), a blocker of CP-AMPARs, on dendrite formation by cultured Purkinje cells. NAS markedly inhibited elongation and branching of Purkinje cell dendrites. Calcium imaging experiments using caged glutamate demonstrated that NAS inhibits the increase of intracellular calcium concentration in Purkinje cells after glutamate release. These results suggest that calcium signaling mediated through CP-AMPARs plays an important role in Purkinje cell dendrite formation.

Disruption of normal cytoskeletal dynamics may play a key role in the pathogenesis of epilepsy

Epilepsy, a common disease affecting 1% to 2% of the population, is characterized by seizures, hyperexcitability at synapses, and aberrant extension of neurons following seizures. Much work has been done on the role of synaptic components in the pathogenesis of epilepsy, but relatively little attention has been given to the potential role of the cytoskeleton. The neuronal cytoskeleton consists of microtubules, actin filaments, intermediate filaments, and associated proteins. A number of mutations in both microtubule-associated proteins (MAPs) and actin-binding proteins, as well as altered expression levels of several cytoskeletal proteins, are known to be involved in epilepsy. These changes will affect the dynamics of the neuronal cytoskeleton and therefore are likely to contribute to the pathogenesis of epilepsy through mechanisms such as increased neurotrophic support to neurons and increased sprouting of mossy fibers. These changes may also contribute to hyperexcitability of neurons through an as yet unidentified mechanism.

Microtubule assembly, organization and dynamics in axons and dendrites

During the past decade enormous advances have been made in our understanding of the basic molecular machinery that is involved in the development of neuronal polarity. Far from being mere structural elements, microtubules are emerging as key determinants of neuronal polarity. Here we review the current understanding of the regulation of microtubule assembly, organization and dynamics in axons and dendrites. These studies provide new insight into microtubules' function in neuronal development and their potential contribution to plasticity.

N-acetyltransferase ARD1-NAT1 regulates neuronal dendritic development

ARD1 and NAT1 constitute an N-acetyltransferase complex where ARD1 holds the enzymatic activity of the complex. The ARD1-NAT1 complex mediates N-terminal acetylation of nascent polypeptides that emerge from ribosomes after translation. ARD1 may also acetylate the internal lysine residues of proteins. Although ARD1 and NAT1 have been found in the brain, the physiological role and substrates of the ARD1-NAT1 complex in neurons remain unclear. Here we investigated role of N-acetyltransferase activity in the process of neuronal development. Expression of ARD1 and NAT1 increased during dendritic development, and both proteins colocalized with microtubules in dendrites. The ARD1-NAT1 complex displayed acetyltransferase activity against a purified microtubule fraction in vitro. Inhibition of the complex limited the dendritic extension of cultured neurons. These findings suggest that the ARD1-NAT1 complex has acetyltransferase activity against microtubules in dendrites. Regulation by acetyltransferase activity is a novel mechanism that is required for dendritic arborization during neuronal development.

Rnd1 regulates axon extension by enhancing the microtubule destabilizing activity of SCG10

The GTPase Rnd1 affects actin dynamics antagonistically to Rho and has been implicated in the regulation of neurite outgrowth, dendrite development, and axon guidance. Here we show that Rnd1 interacts with the microtubule regulator SCG10. This interaction requires a central domain of SCG10 comprising about 40 amino acids located within the N-terminal-half of a putative alpha-helical domain and is independent of phosphorylation at the four identified phosphorylation sites that regulate SCG10 activity. Rnd1 enhances the microtubule destabilizing activity of SCG10 and both proteins colocalize in neurons. Knockdown of Rnd1 or SCG10 by RNAi suppressed axon extension, indicating a critical role for both proteins during neuronal differentiation. Overexpression of Rnd1 in neurons induces the formation of multiple axons. The effect of Rnd1 on axon extension depends on SCG10. These results indicate that SCG10 acts as an effector downstream of Rnd1 to regulate axon extensions by modulating microtubule organization.

A novel function of dcf1 during the differentiation of neural stem cells in vitro

The study of neural dendrite formation is of great significance both in theory and applications. However, the molecular mechanisms of regulation remain unclear. We previously described a novel EST, which has high homology with dentritic cell factors (DCF1), expressed differentially between undifferentiated and differentiated neural stem cells (NSCs). In this study, we cloned, expressed, and silenced the dcf1 gene and offered insight into its function in regulating dendrite formation during the differentiation of NSCs. The results indicated that dcf1 encoded a 42 kD protein and could be successfully expressed both in Escherichia coli and NSCs. In order to silence dcf1 gene, three different kinds of siRNA vectors were constructed and transformed into the NSC line C17.2 and primary NSCs, resulting in down regulation of the dcf1 mRNA. Analysis of immunofluorescence or GFP illuminated that with overexpression of the dcf1 gene, the NSCs were maintained in undifferentiated status. After the dcf1 gene was silenced, cells tended to differentiate into neurons and astrocytes.

SCLIP is crucial for the formation and development of the Purkinje cell dendritic arbor

Cerebellar Purkinje cells elaborate one of the most complex dendritic arbors among neurons to integrate the numerous signals they receive from the cerebellum circuitry. Their dendritic differentiation undergoes successive, tightly regulated phases of development involving both regressive and growth events. Although many players regulating the late phases of Purkinje cell dendritogenesis have been identified, intracellular factors controlling earlier phases of dendritic development remain mostly unknown. In this study, we explored the biological properties and functions of SCLIP, a protein of the stathmin family, in Purkinje cell dendritic differentiation and cerebellum development. Unlike the other stathmins, SCLIP is strongly expressed in Purkinje cells during cerebellar development and accumulates in their dendritic processes at a critical period of their formation and outgrowth. To reveal SCLIP functions, we developed a lentiviral-mediated approach on cerebellar organotypic cultures to inhibit or increase its expression in Purkinje cells in their tissue environment. Depletion of SCLIP promoted retraction of the Purkinje cell primitive process and then prevented the formation of new dendrites at early stages of postnatal development. It also prevented their elongation and branching at later phases of differentiation. Conversely, SCLIP overexpression promoted dendritic branching and development. Together, our results demonstrate for the first time that SCLIP is crucial for both the formation and proper development of Purkinje cell dendritic arbors. SCLIP appears thus as a novel and specific factor that controls the early phases of Purkinje cell dendritic differentiation during cerebellum development.

Rapid regulation of microtubule-associated protein 2 in dendrites of nucleus laminaris of the chick following deprivation of afferent activity

Differential innervation of segregated dendritic domains in the chick nucleus laminaris (NL), composed of third-order auditory neurons, provides a unique model to study synaptic regulation of dendritic structure. Altering the synaptic input to one dendritic domain affects the structure and length of the manipulated dendrites while leaving the other set of unmanipulated dendrites largely unchanged. Little is known about the effects of neuronal input on the cytoskeletal structure of NL dendrites and whether changes in the cytoskeleton are responsible for dendritic remodeling following manipulations of synaptic input. In this study, we investigate changes in the immunoreactivity of high-molecular weight microtubule associated protein 2 (MAP2) in NL dendrites following two different manipulations of their afferent input. Unilateral cochlea removal eliminates excitatory synaptic input to the ventral dendrites of the contralateral NL and the dorsal dendrites of the ipsilateral NL. This manipulation produced a dramatic decrease in MAP2 immunoreactivity in the deafferented dendrites. This decrease was detected as early as 3 h following the surgery, well before any degeneration of afferent axons. A similar decrease in MAP2 immunoreactivity in deafferented NL dendrites was detected following a midline transection that silences the excitatory synaptic input to the ventral dendrites on both sides of the brain. These changes were most distinct in the caudal portion of the nucleus where individual deafferented dendritic branches contained less immunoreactivity than intact dendrites. Our results suggest that the cytoskeletal protein MAP2, which is distributed in dendrites, perikarya, and postsynaptic densities, may play a role in deafferentation-induced dendritic remodeling.

Motor discoordination of transgenic mice overexpressing a microtubule destabilizer, stathmin, specifically in Purkinje cells

The proper regulation of microtubule (MT) structure is important for dendritic and neural circuit development. However, the relationship between the regulation of the MTs in dendrites and the formation of neural function is still unclear. Stathmin is a MT destabilizer, and we have previously reported that the expression and the activity of stathmin is downregulated during cerebellar Purkinje cell (PC) development. In this study, we generated transgenic mice that specifically overexpress the constitutively active form of stathmin in the PCs. These mutant mice did not show any obvious morphological or excitatory transmission abnormalities in the cerebellum. In contrast, we observed a decline in the expression of MAP2 and KIF5 signal in the PC dendrites and a discoordination of motor function in the mutant mice, although they displayed normal general behavior. These data indicate that the overexpression of stathmin disrupts dendritic MT organization, motor protein distribution, and neural function in PCs.