Neurotrophins: neurotrophic modulation of neurite growth

Enhanced Ras activity in pyramidal neurons induces cellular hypertrophy and changes in afferent and intrinsic connectivity in synRas mice

Neurotrophic actions are critically controlled and transmitted to cellular responses by the small G protein Ras which is therefore essential for normal functioning and plasticity of the nervous system. The present study summarises findings of recent studies on morphological changes in the neocortex of synRas mice expressing Val12-Ha-Ras in vivo under the control of the rat synapsin I promoter. In the here reported model (introduced by Heumann et al. [J. Cell Biol. 151 (2000) 1537]), transgenic Val12-Ha-Ras expression is confined to the pyramidal cell population and starts postnatally at a time, when neurons are postmitotic and their developmental maturation has been basically completed. Expression of Val12-Ha-Ras results in a significant enlargement of pyramidal neurons. Size, complexity and spine density of dendritic trees are increased, which leads, finally, to cortical expansion. However, the main morphological design principles of 'transgenic' pyramidal cells remain preserved. In addition to somato-dendritic changes, expression of Val12-Ha-Ras in pyramidal cells induces augmented axon calibres and upregulates the establishment of efferent boutons. Despite the enlargement of cortical size, the overall density of terminals representing intra- or interhemispheric, specific and non-specific afferents is unchanged or even higher in transgenic mice suggesting a significant increase in the total afferent input to the neocortex. Although interneurons do not express the transgene and are therefore excluded from direct, intrinsic Val12-Ha-Ras effects, they respond with morphological adaptations to structural changes. Thus, dendritic arbours of interneurons are extended to follow the cortical expansion and basket cells establish a denser inhibitory innervation of 'transgenic' pyramidal cells perikarya. It is concluded that expression of Val12-Ha-Ras in pyramidal neurons results in remodelling of neocortical structuring which strongly implicates a crucial involvement of Ras in cortical plasticity.

Fibroblast growth factor-2 gene delivery stimulates axon growth by adult retinal ganglion cells after acute optic nerve injury

Basic fibroblast growth factor (or FGF-2) has been shown to be a potent stimulator of retinal ganglion cell (RGC) axonal growth during development. Here we investigated if FGF-2 upregulation in adult RGCs promoted axon regrowth in vivo after acute optic nerve injury. Recombinant adeno-associated virus (AAV) was used to deliver the FGF-2 gene to adult RGCs providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to a 10-fold increase in the number of axons that extended past 0.5 mm from the lesion site compared to control nerves. Detection of AAV-mediated FGF-2 protein in injured RGC axons correlated with growth into the distal optic nerve. The response to FGF-2 upregulation was supported by our finding that FGF receptor-1 (FGFR-1) and heparan sulfate (HS), known to be essential for FGF-2 signaling, were expressed by adult rat RGCs. FGF-2 transgene expression led to only transient protection of injured RGCs. Thus the effect of this neurotrophic factor on axon extension could not be solely attributed to an increase in neuronal survival. Our data indicate that selective upregulation of FGF-2 in adult RGCs stimulates axon regrowth within the optic nerve, an environment that is highly inhibitory for regeneration. These results support the hypothesis that key factors involved in axon outgrowth during neural development may promote regeneration of adult injured neurons.

TrkB receptors are required for follicular growth and oocyte survival in the mammalian ovary

Although it is well established that both follicular assembly and the initiation of follicle growth in the mammalian ovary occur independently of pituitary hormone support, the factors controlling these processes remain poorly understood. We now report that neurotrophins (NTs) signaling via TrkB receptors are required for the growth of newly formed follicles. Both neurotrophin-4/5 (NT-4) and brain-derived neurotrophic factor (BDNF), the preferred TrkB ligands, are expressed in the infantile mouse ovary. Initially, they are present in oocytes, but this site of expression switches to granulosa cells after the newly assembled primordial follicles develop into growing primary follicles. Full-length kinase domain-containing TrkB receptors are expressed at low and seemingly unchanging levels in the oocytes and granulosa cells of both primordial and growing follicles. In contrast, a truncated TrkB isoform lacking the intracellular domain of the receptor is selectively expressed in oocytes, where it is targeted to the cell membrane as primary follicles initiate growth. Using gene-targeted mice lacking all TrkB isoforms, we show that the ovaries of these mice or those lacking both NT-4 and BDNF suffer a stage-selective deficiency in early follicular development that compromises the ability of follicles to grow beyond the primary stage. Proliferation of granulosa cells-required for this transition-and expression of FSH receptors (FSHR), which reflects the degree of biochemical differentiation of growing follicles, are reduced in trkB-null mice. Ovaries from these animals grafted under the kidney capsule of wild-type mice fail to sustain follicular growth and show a striking loss of follicular organization, preceded by massive oocyte death. These results indicate that TrkB receptors are required for the early growth of ovarian follicles and that they exert this function by primarily supporting oocyte development as well as providing granulosa cells with a proliferative signal that requires oocyte-somatic cell bidirectional communication. The predominance of truncated TrkB receptors in oocytes and their developmental pattern of subcellular expression suggest that a significant number of NT-4/BDNF actions in the developing mammalian ovary are mediated by these receptors.

Expression of constitutively active p21H-rasval12 in postmitotic pyramidal neurons results in increased dendritic size and complexity

The small G protein p21Ras is a critical molecular switch for relaying neurotrophic actions and is essential for normal functioning and plasticity of the nervous system. In this study, the morphogenetic effects of p21Ras were investigated on neurons in vivo. Morphological changes of layers II/III and Vb commissural pyramidal neurons of the primary somatosensory cortex were analyzed in transgenic mice expressing permanently active p21H-RasVal12 in postmitotic neurons. Pyramidal cells were retrogradely labelled with biotinylated dextran amine and subsequently traced using Neurolucida. Compared with wild-type mice, transgenic animals showed a significant increase in the surface area and volume of basal dendrites on the proximal and intermediate segments in layers II/III and on further distal segments in layer V. In addition, the surface area and volume of the trunk and of the proximal segments of oblique branches of apical dendrites were enlarged in both layers. Sholl analyses of basal and apical dendrites showed a significant increase in dendritic complexity of layer V neurons. A positive correlation was observed between the size of the basal dendrite and the neuronal soma size in the transgenic population, indicating that growth-promoting effects of p21H-RasVal12 affect both cellular compartments in parallel. However, the dendritic surface correlated with the number of tips and dendritic stem diameter in both wild-type and transgenic populations, demonstrating that these relations represent rather conservative design principles in dendritic morphology. The data presented here suggest an important role of p21Ras-dependent signaling in the final differentiation and maintenance of dendritic morphology.

Constitutive expression of p21H-Rasval12in neurons induces increased axonal size and dendritic microtubule density in vivo

The small G protein p21Ras is a key signal transducer mediating cellular growth and proliferation responses to extracellular stimuli. We investigated by electron microscopy the effects of augmented p21Ras activity on neuronal processes and microtubule arrangement in vivo. We used transgenic mice with a neuron-specific overexpression of p21H-RasVal12, which starts postnatally around Day 15. Axonal and dendritic diameters and the numerical density of dendritic microtubules were analyzed at postnatal Day 12 before the onset of transgene expression and in adult mice. In adult transgenic mice, calibers of both axons (corpus callosum) and dendrites (layers II/III of somatosensory cortex) were enlarged by about 57% and 79%, respectively. The increase in dendritic calibers was associated with an increment in the amount of microtubules. Even in dendrites of equivalent diameters, the number of microtubules was higher in transgenic mice compared to that in wild-type mice suggesting an elevated microtubule density. Changes in process diameters or microtubule density were not observed at postnatal Day 12 before relevant transcription of transgenic p21H-RasVal12. The present results extend previous findings on neuronal hypertrophy as a consequence of p21H-RasVal12 expression and suggest a profound influence on the dendritic microtubule network.

Adeno-associated viral vector-mediated neurotrophin gene transfer in the injured adult rat spinal cord improves hind-limb function

To foster axonal growth from a Schwann cell bridge into the caudal spinal cord, spinal cells caudal to the implant were transduced with adeno-associated viral (AAV) vectors encoding for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (AAV-NT-3). Control rats received AAV vectors encoding for green fluorescent protein or saline. AAV-BDNF- and AAV-NT-3-transduced 293 human kidney cells produced and secreted BDNF or NT-3, respectively, in vitro. The secreted neurotrophins were biologically active; they both promoted outgrowth of sensory neurites in vitro. In vivo, transgene expression was observed predominantly in neurons for at least 16 weeks after injection. Compared with controls, a modest though significant improvement in hind-limb function was found in rats that received AAV-BDNF and AAV-NT-3. Retrograde tracing demonstrated that twice as many neurons with processes extending toward the Schwann cell graft were present in the second lumbar cord segment of AAV-BDNF- and AAV-NT-3-injected animals compared with controls. We found no evidence, however, for growth of regenerated axons from the Schwann cell implant into the caudal cord. Our results suggest that AAV vector-mediated overexpression of BDNF and NT-3 in the cord caudal to a Schwann cell bridge modified the local lumbar axonal circuitry, which was beneficial for locomotor function.

The p75 receptor acts as a displacement factor that releases Rho from Rho-GDI

The neurotrophin receptor p75(NTR) is involved in the regulation of axonal elongation by neurotrophins as well as several myelin components, including Nogo, myelin-associated glycoprotein (MAG) and myelin oligodendrocyte glycoprotein (OMgp). Neurotrophins stimulate neurite outgrowth by inhibiting Rho activity, whereas myelin-derived proteins activate RhoA and thereby inhibit growth. Here we show that direct interaction of the Rho GDP dissociation inhibitor (Rho-GDI) with p75(NTR) initiates the activation of RhoA, and this interaction between p75(NTR) and Rho-GDI is strengthened by MAG or Nogo. We also found that p75(NTR) facilitates the release of prenylated RhoA from Rho-GDI. The peptide ligand that is associated with the fifth alpha helix of p75(NTR) inhibits the interaction between Rho-GDI and p75(NTR), thus silencing the action mediated by p75(NTR). This peptide has potential as a therapeutic agent against the inhibitory cues that block regeneration in the central nervous system.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Investigating the synergistic effect of combined neurotrophic factor concentration gradients to guide axonal growth

Neurotrophic factors direct axonal growth toward the target tissue by a concentration gradient, which is mediated through different tyrosine kinase cell surface receptors. In this study, well-defined concentration gradients of neurotrophic factors (NFs) allowed us to study the synergistic effect of different NFs (e.g. nerve growth factor [NGF], neurotrophin-3 [NT-3] and brain-derived neurotrophic factor [BDNF]) for axonal guidance of embryonic lumbar dorsal root ganglion cells (DRGs). Effective guidance of DRG axons was achieved with a minimum NGF concentration gradient of 133 ng/ml/mm alone, or combined NGF and NT-3 concentration gradients of 80 ng/ml/mm each. Interestingly, the combined concentration gradients of NGF and BDNF did not show any significant synergism at the concentration gradients studied. The synergism observed between NGF and NT-3 indicates that axons may be guided over a 12.5 mm distance, which is significantly greater than that of 7.5 mm calculated by us for NGF alone or that of 2 mm observed by others.

Current concepts in central nervous system regeneration

A dictum long-held has stated that the adult mammalian brain and spinal cord are not capable of regeneration after injury. Recent discoveries have, however, challenged this dogma. In particular, a more complete understanding of developmental neurobiology has provided an insight into possible ways in which neuronal regeneration in the central nervous system may be encouraged. Knowledge of the role of neurotrophic factors has provided one set of strategies which may be useful in enhancing CNS regeneration. These factors can now even be delivered to injury sites by transplantation of genetically modified cells. Another strategy showing great promise is the discovery and isolation of neural stem cells from adult CNS tissue. It may become possible to grow such cells in the laboratory and use these to replace injured or dead neurons. The biological and cellular basis of neural injury is of special importance to neurosurgery, particularly as therapeutic options to treat a variety of CNS diseases becomes greater.

Transplantation of olfactory mucosa minimizes axonal branching and promotes the recovery of vibrissae motor performance after facial nerve repair in rats

The occurrence of abnormally associated movements is inevitable after facial nerve transection. The reason for this post-paralytic syndrome is poor guidance of regrowing axons, whereby a given muscle group is reinnervated by misrouted axonal branches. Olfactory ensheathing glia have been shown to reduce axonal sprouting and stimulate axonal regeneration after transplantation into the spinal cord. In the present study, we asked whether transplantation of olfactory mucosa (OM) would also reduce sprouting of a damaged peripheral pure motor nerve. The adult facial nerve was transected, and the effect of the OM placed at the lesion site was analyzed with regard to the accuracy of target reinnervation, axonal sprouting of motoneurons, and vibrissal motor performance. Accuracy of target reinnervation and axonal sprouting were studied using preoperative/postoperative labeling and triple retrograde labeling of facial motoneurons, respectively. The vibrissal motor performance was monitored using a video-based motion analysis. We show here that implantation of OM, compared with simple facial-facial anastomosis, (1) improved the protraction, amplitude, angular velocity, and acceleration of vibrissal movements up to 80% of the control values, (2) reduced the percentage of branching motoneurons from 76 to 39%, and (3) improved the accuracy of reinnervation from 22 to 49%. Moreover, we present evidence, that transplanted OM but not buccal mucous membrane induced a sustained upregulation of trophic factors at the lesion site. It is concluded that transplantation of OM to the transected facial nerve significantly improves nerve regeneration.

Patterns of Nogo mRNA and protein expression in the developing and adult rat and after CNS lesions

Nogo-A is a neurite growth inhibitor involved in regenerative failure and restriction of structural plasticity in the adult CNS. Three major protein products (Nogo-A, -B, and -C) are derived from the nogo gene. Here we describe the embryonic and postnatal expression of the three Nogo isoforms in the rat by in situ hybridization and immunohistochemistry. Northern and Western blot analysis indicated that Nogo-A is predominantly expressed in the nervous system with lower levels also present in testis and heart. In CNS myelin, confocal and immunoelectron microscopy revealed that Nogo-A is expressed in oligodendrocyte cell bodies and processes and localized in the innermost adaxonal and outermost myelin membranes. Additionally, we find Nogo-A to be expressed by projection neurons, in particular during development, and by postmitotic cells in the developing cortex, spinal cord, and cerebellum. The expression levels of Nogo-A/B were not changed significantly after traumatic lesions to the cortex or spinal cord. Nogo-B showed widespread expression in the central and peripheral nervous systems and other peripheral tissues. Nogo-C was mainly found in skeletal muscle, but brain and heart were also found to express this isoform. The localization of Nogo-A in oligodendrocytes fits well with its role as a myelin-associated inhibitor of regenerative fiber growth and structural plasticity. However, expression of Nogo-A in other tissues and, in particular, in neurons and the widespread expression of the two shorter isoforms, Nogo-B and -C, suggest that the Nogo family of proteins might have function(s) additional to the neurite growth-inhibitory activity.

A novel role for p75NTR in subplate growth cone complexity and visual thalamocortical innervation

In cortical development, subplate axons pioneer the pathway from neocortex to the internal capsule, leading to the proposal that they are required for subsequent area-specific innervation of cortex by thalamic axons. A role for p75 neutrophin receptor (NTR) in area-specific thalamic innervation of cortex is suggested by the observation that p75NTR expression is restricted to subplate neurons in a low-rostral to high-caudal gradient throughout the period of thalamocortical innervation. In vitro, neurotrophin 3 binding to p75NTR increases neurite length and filopodial formation of immunopurified subplate neurons, suggesting a role for p75NTR in subplate growth cone morphology and function in vivo. Consistent with this idea, subplate growth cones have markedly fewer filopodia in mice lacking p75NTR than in wild type mice. Despite this gross morphologic defect, many subplate axons in knock-out mice pioneer the projection to the internal capsule as they do in wild-type mice. However a few subplate axons in the knock-out mice make ectopic projections rostral in the intermediate zone and frontal cortex. Concomitant with the altered morphology of subplate growth cones, mice lacking p75NTR have diminished innervation of visual cortex from the lateral geniculate nucleus, with markedly reduced or absent connections in 48% of knock-out mice. Thalamic projections to auditory and somatosensory cortex are normal, consistent with the gradient of p75NTR expression. Our present results are unusual in that they argue that p75NTR functions in a novel way in subplate neurons, that is, in growth cone morphology and function rather than in axon extension or neuronal survival.

Dual role for p75(NTR) signaling in survival and cell death: can intracellular mediators provide an explanation?

Several recent reports support a dual role of p75(NTR) in cell death, as well as survival, depending on the physiological or developmental stage of the cells. Coexpression of the TrkA receptor with p75(NTR) further enhances the complexity of nerve growth factor (NGF) signaling. Recent identification of serine/threonine kinases that interact with the p75(NTR) provides an explanation for the lack of an apparent kinase domain needed for signaling. In this report, we review the possible roles of the intracellular proteins that directly interact with the p75(NTR), atypical protein kinase C (PKC) binding protein, p62 and second messengers in the functional antagonism exhibited by TrkA and p75(NTR) with an emphasis on the nuclear factor-kappa B activation pathway.

The p75 receptor transduces the signal from myelin-associated glycoprotein to Rho

Myelin-associated glycoprotein (MAG) is a potent inhibitor of neurite outgrowth from a variety of neurons. The receptor for MAG or signals that elicit morphological changes in neurons remained to be established. Here we show that the neurotrophin receptor p75 (p75(NTR)) is the signal transducing element for MAG. Adult dorsal root ganglion neurons or postnatal cerebellar neurons from mice carrying a mutation in the p75(NTR) gene are insensitive to MAG with regard to neurite outgrowth. MAG activates small GTPase RhoA, leading to retarded outgrowth when p75(NTR)) is present. Colocalization of p75(NTR) and MAG binding is seen in neurons. Ganglioside GT1b, which is one of the binding partners of MAG, specifically associates with p75(NTR). Thus, p75(NTR) and GT1b may form a receptor complex for MAG to transmit the inhibitory signals in neurons.

The brain-derived neurotrophic factor enhances synthesis of Arc in synaptoneurosomes

Protein synthesis in neurons is essential for the consolidation of memory and for the stabilization of activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP). Activity-dependent translation of dendritically localized mRNAs has been proposed to be a critical source of new proteins necessary for synaptic change. mRNA for the activity-regulated cytoskeletal protein, Arc, is transcribed during LTP and learning, and disruption of its translation gives rise to deficits in both. We have found that selective translation of Arc in a synaptoneurosomal preparation is induced by the brain-derived neurotrophic factor, a neurotrophin that is released during high-frequency stimulation patterns used to elicit LTP. This effect involves signaling through the TrkB receptor and is blocked by the N-methyl-d-aspartate-type glutamate receptor antagonist, MK801. The results suggest there is a synergy between neurotrophic and ionotropic mechanisms that may influence the specificity and duration of changes in synaptic efficacy at glutamatergic synapses.

Transplantation of olfactory ensheathing cells stimulates the collateral sprouting from axotomized adult rat facial motoneurons

Axon regrowth after CNS and PNS injury is only the first step toward complete functional recovery which depends largely on the specificity of the newly formed nerve-target projections. Since most of the studies involving the application of glial cells to the lesioned nervous system have focused primarily on the extent of neurite outgrowth, little is known regarding their effects on the accompanying processes of axonal sprouting and pathfinding. In this study, we analyzed the effects of transplanted olfactory ensheathing cells (OECs) on axonal sprouting of adult facial neurons by using triple fluorescent retrograde tracing and biometrical analysis of whisking behavior. We found that 2 months after facial nerve axotomy and immediate implantation of OECs in between both nerve stumps fixed in a silicon tube, the total number of labeled neurons was increased by about 100%, compared to animals with simple facial nerve suture or entubulation in an empty conduit. This change in the number of axon sprouts was not random. The highest increase in axon number was observed in the marginal mandibular branch, whereas no changes were detected in the zygomatic branch. This increased sprouting did not improve the whisking behavior as measured by biometric video analysis. Our results demonstrate that OECs are potent inducers of axonal sprouting in vivo. Hence OEC-filled nerve conduits may be a powerful tool to enforce regeneration of a peripheral nerve under adverse conditions, e.g., after long delay between injury and surgical repair. In mixed nerves, increased axonal sprouting will improve specificity since inappropriate nerve-target connections are pruned off during preferential motor innervation. In pure motor nerves, however, OEC-mediated axonal sprouting may result in polyneuronal innveration of target muscles.

Molecular regulation of odontoblast activity under dentin injury

Pulp tissue responds to dentin damage by laying down a tertiary dentin matrix (reactionary or reparative) beneath the site of injury. Reactionary dentin is secreted by surviving odontoblasts in response to environmental stimuli, leading to an increase in metabolic activities of the cells. The inductive molecules that determine the success of the pulp healing may be released from the damaged dentin as well as from the pulp tissue subjacent to the injury. This paper will schematically consider two major growth factors probably implicated in the control of odontoblast activity: TGF beta-1 released from demineralized dentin and NGF from pulp. To analyze their role with an in vitro system that mimics the in vivo situation, we have used thick-sliced teeth cultured as described previously. The supply of factors was accomplished by means of a small tube glued onto the dentin. The tube was filled with TGF beta-1 (20 ng/mL) or NGF (50 ng/mL), and slices were cultured for 4 or 7 days. Results showed that TGF beta-1 binding sites are strongly detected on odontoblasts in the factor-rich zone. A strong expression of alpha 1(I) collagen transcripts was also detected. In the NGF-rich environment, p75NTR was re-expressed on odontoblasts and the transcription factor NF-kappa B activated. Modifications in the odontoblast morphology were observed with an atypical extension of the cell processes filled with actin filaments. These results suggest that odontoblasts respond to influences from both dentin and pulp tissue during pulp repair.

Lack of neurotrophin-4 causes selective structural and chemical deficits in sympathetic ganglia and their preganglionic innervation

Neurotrophin-4 (NT-4) is perhaps the still most enigmatic member of the neurotrophin family. We show here that NT-4 is expressed in neurons of paravertebral and prevertebral sympathetic ganglia, i.e., the superior cervical (SCG), stellate (SG), and celiac (CG) ganglion. Mice deficient for NT-4 showed a significant reduction (20-30%) of preganglionic sympathetic neurons in the intermediolateral column (IML) of the thoracic spinal cord. In contrast, neuron numbers in the SCG, SG, and CG were unchanged. Numbers of axons in the thoracic sympathetic trunk (TST) connecting the SG with lower paravertebral ganglia were also reduced, whereas axon numbers in the cervical sympathetic trunk (CST) were unaltered. Axon losses in the TST were paralleled by losses of synaptic terminals on SG neurons visualized by electron microscopy. Furthermore, immunoreactivity for the synaptic vesicle antigen SV2 was clearly reduced in the SG and CG. Levels of catecholamines and tyrosine hydroxylase immunoreactivity were dramatically reduced in the SG and the CG but not in the SCG. Despite this severe phenotype in the sympathetic system, blood pressure levels were not reduced and displayed a pattern more typical of deficits in baroreceptor afferents. Numbers of IML neurons were unaltered at postnatal day 4, suggesting a postnatal requirement for their maintenance. In light of these and previous data, we hypothesize that NT-4 provided by postganglionic sympathetic neurons is required for establishing and/or maintaining synapses of IML neurons on postganglionic cells. Impairment of synaptic connectivity may consequently reduce impulse flow, causing a reduction in transmitter synthesis in postganglionic neurons.

How do dendrites take their shape?

Recent technical advances have made possible the visualization and genetic manipulation of individual dendritic trees. These studies have led to the identification and characterization of molecules that are important for different aspects of dendritic development. Although much remains to be learned, the existing knowledge has allowed us to take initial steps toward a comprehensive understanding of how complex dendritic trees are built. In this review, we describe recent advances in our understanding of the molecular mechanisms underlying dendritic morphogenesis, and discuss their cell-biological implications.