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

Collective migration depends on cell-cell interactions between neighbors that contribute to their overall directionality, yet the mechanisms that control the coordinated migration of neurons remains to be elucidated. During hindbrain development, facial branchiomotor neurons (FBMNs) undergo a stereotypic tangential caudal migration from their place of birth in rhombomere (r)4 to their final location in r6/7. FBMNs engage in collective cell migration that depends on neuron-to-neuron interactions to facilitate caudal directionality. Here, we demonstrate that Cadherin-2-mediated neuron-to-neuron adhesion is necessary for directional and collective migration of FBMNs. We generated stable transgenic zebrafish expressing dominant-negative Cadherin-2 (Cdh2ΔEC) driven by the islet1 promoter. Cell-autonomous inactivation of Cadherin-2 function led to non-directional migration of FBMNs and a defect in caudal tangential migration. Additionally, mosaic analysis revealed that Cdh2ΔEC-expressing FBMNs are not influenced to migrate caudally by neighboring wild-type FBMNs due to a defect in collective cell migration. Taken together, our data suggest that Cadherin-2 plays an essential cell-autonomous role in mediating the collective migration of FBMNs.

The facial branchiomotor neurons (FBMNs) undergo a characteristic tangential migration in the vertebrate hindbrain. We previously used a morpholino knockdown approach to reveal that zebrafish prickle1b (pk1b) is required for this migration. Here we report that FBMN migration is also blocked in a pk1b mutant with a disruption in the consensus farnesylation motif. We confirmed that this lipid modification is required during FBMN migration by disrupting the function of farnesyl biosynthetic enzymes. Furthermore, farnesylation of a tagged Pk1b is required for its nuclear localization. Using a unique rescue approach, we have demonstrated that Pk1b nuclear localization and farnesylation are required during FBMN migration. Our data suggest that Pk1b acts at least partially independently of core planar cell polarity molecules at the plasma membrane, and might instead be acting at the nucleus. We also found that the neuronal transcriptional silencer REST is necessary for FBMN migration, and we provide evidence that interaction between Pk1b and REST is required during this process. Finally, we demonstrate that REST protein, which is normally localized in the nuclei of migrating FBMNs, is depleted from the nuclei of Pk1b-deficient neurons. We conclude that farnesylation-dependent nuclear localization of Pk1b is required to regulate REST localization and thus FBMN migration.

Here, we investigated the intracellular mechanisms that underlie the relative invulnerability of adult versus developing dorsal root ganglion (DRG) sensory neurons. In culture, adult neurons were resistant to stimuli that caused apoptosis of their neonatal counterparts. In both adult and neonatal neurons, death stimuli induced the apoptotic c-Jun N-terminal protein kinase (JNK) pathway, but JNK activation only caused death of neonatal neurons, indicating that adult neurons have a downstream block to apoptosis. Expression of the dominant-inhibitory p53 family member, ΔNp73, rescued JNK-induced apoptosis of neonatal neurons, suggesting that it might participate in the downstream apoptotic block in adult neurons. To test this possibility, we examined adult DRG neurons cultured from p73+/- mice. Adult p73+/- DRG neurons were more vulnerable to apoptotic stimuli than their p73+/+ counterparts, and invulnerability could be restored to the p73+/- neurons by increased expression of ΔNp73. Moreover, although DRG neuron development was normal in p73+/- animalsin vivo, axotomy caused death of adult p73+/- but not p73+/+ DRG neurons. Thus, one way adult neurons become invulnerable is to enhance endogenous survival pathways, and one critical component of these survival pathways is p73.

Elucidating the mechanisms that regulate the life versus death of mammalian neurons is important not only for our understanding of the normal biology of the nervous system but also for our efforts to devise approaches to maintain neuronal survival in the face of traumatic injury or neurodegenerative disorders. Here, we review the emerging evidence that a key survival/death checkpoint in both peripheral and central neurons involves the p53 tumor suppressor and its newly discovered family members, p73 and p63. The full-length isoforms of these proteins function as proapoptotic proteins, whereas naturally occurring N-terminal truncated variants of p73 and p63 act as prosurvival proteins, at least partially by antagonizing the full-length family members. The authors propose that together, these isoforms comprise an upstream rheostat that sums different environmental cues to ultimately determine neuronal survival during development, during neuronal maintenance in adult animals, and even following traumatic injury.

Nerve growth factor (NGF) mediates the survival and differentiation of neurons by stimulating the tyrosine kinase activity of the TrkA/NGF receptor. Here, we identify SHP-1 as a phosphotyrosine phosphatase that negatively regulates TrkA. SHP-1 formed complexes with TrkA at Y490, and dephosphorylated it at Y674/675. Expression of SHP-1 in sympathetic neurons induced apoptosis and TrkA dephosphorylation. Conversely, inhibition of endogenous SHP-1 with a dominant-inhibitory mutant stimulated basal tyrosine phosphorylation of TrkA, thereby promoting NGF-independent survival and causing sustained and elevated TrkA activation in the presence of NGF. Mice lacking SHP-1 had increased numbers of sympathetic neurons during the period of naturally occurring neuronal cell death, and when cultured, these neurons survived better than wild-type neurons in the absence of NGF. These data indicate that SHP-1 can function as a TrkA phosphatase, controlling both the basal and NGF-regulated level of TrkA activity in neurons, and suggest that SHP-1 regulates neuron number during the developmental cell death period by directly regulating TrkA activity.

Neuronal activity and neurotrophins play a central role in the formation, maintenance, and plasticity of dendritic arbors. Here, we show that neuronal activity, mediated by electrical stimulation, KCl depolarization, or cholinergic receptor activation, promotes reversible dendrite formation in sympathetic neurons and that this effect is enhanced by NGF. Activity-dependent dendrite formation is accompanied by increased association of HMW MAP2 with microtubules and increased microtubule stability. Inhibition of either CaMKII or the MEK-ERK pathway, both of which phosphorylate MAP2, inhibits dendrite formation, but inhibition of both pathways simultaneously is required for dendrites to retract. These data indicate that neuronal activity signals via CamKII and the ERKs to regulate MAP2:microtubule interactions and hence reversible dendrite stability, and to provide a mechanism whereby activity and neurotrophins converge intracellularly to dynamically regulate dendritic morphology.

Elevating target-derived levels of nerve growth factor (NGF) in peripheral organs of postnatal mammals is known to enhance the survival of postganglionic sympathetic neurons and to promote the terminal arborization of sympathetic axons within such NGF-rich target tissues. Although increasing levels of NGF in the central nervous system can ameliorate cholinergic function of damaged and aged neurons of the medial septum, it remains undetermined whether the postnatal development of this neuronal population and their projections that innervate the hippocampus are likewise affected by elevated levels of target-derived NGF. To address this question, the cholinergic septohippocampal pathway was examined in adult transgenic mice which display elevated levels of NGF protein production in the dorsal hippocampus during postnatal development. Adult transgenic mice possessed a cholinergic population of septal neurons approximately 15% larger than that seen in age-matched control animals. Despite increased numbers of cholinergic septal neurons, as well as elevated levels of hippocampal NGF, the density of cholinergic septal axons in the outer molecular layer of the hippocampal dentate gyrus of adult transgenic animals was comparable with that found in wild-type controls. These results reveal that elevating levels of target-derived NGF during postnatal development can increase the population size of the cholinergic septal neurons but does not alter their pattern of afferent innervation in the hippocampus of adult mice.

Nerve growth factor (NGF) regulates the expression of the glycoprotein L1 among neural cell populations. The purpose of this investigation was to determine whether NGF equally affects the immunolocalization of L1 on both sympathetic and sensory axons, and whether the functional expression of the p75 neurotrophin receptor (p75NTR) is required for the immunodetection of this glycoprotein on peripheral axons. Two lines of transgenic mice overexpressing NGF among glial cells were used in this study: (1) one line of mice possessing two normal alleles for p75NTR, and (2) another line of mice possessing two mutated alleles for p75NTR. In both types of animals, sensory axons stained immunohistochemically for calcitonin gene-related peptide and sympathetic axons stained immunohistochemically for tyrosine hydroxylase invaded the deep white matter portions of the cerebellum (a central structure containing high levels of transgene expression and synthesis); the cerebella of wild type (C57Bl/6) and p75NTR-deficient mice lacked these sensory and sympathetic fibers. Both lines of transgenic animals also possessed a dense plexus of L1-immunoreactive axons in their cerebella; the spatial distribution of these L1-immunostained axons paralleled that seen for the sensory and sympathetic axons. A unilateral removal of the superior cervical ganglion in both lines of transgenic animals caused a complete reduction of tyrosine hydroxylase-immunopositive axons in the cerebellum but did not affect the density of L1-immunopositive axons. From these in vivo data, we conclude that collateral branches of sensory axons which invade a NGF-rich target area display L1 immunoreactivity, and that such immunodetection does not require the functional expression of p75NTR.

Recent evidence indicates that the p53 tumor suppressor protein, and its related family member, p73, play an essential role in regulating neuronal apoptosis in both the developing and injured, mature nervous system. In the developing nervous system, they do so by regulating naturally-occurring cell death in neural progenitor cells and in postmitotic neurons, acting to ensure the apoptosis of cells that either do not appropriately undergo the progenitor to postmitotic neuron transition, or that fail to compete for sufficient quantities of trophic support. Somewhat surprisingly, in developing postmitotic neurons, p53 plays a proapoptotic role, while a naturally-occurring, truncated form of p73, DeltaNp73, antagonizes p53 and plays an anti-apoptotic role. In the mature nervous system, numerous studies indicate that p53 is essential for the neuronal death in response to a variety of insults, including DNA damage, ischemia and excitotoxicity. It is likely that all of these insults culminate in DNA damage, which may well be a common trigger for neuronal apoptosis. In this regard, the signaling pathways that are responsible for triggering p53-dependent neuronal apoptosis are starting to be elucidated, and involve cell cycle deregulation and activation of the JNK pathway. Finally, accumulating evidence indicates that p53 is perturbed in the CNS in a number of neurodegenerative disorders, leading to the hypothesis that longterm oxidative damage and/or excitotoxicity ultimately trigger p53-dependent apoptosis in the chronically degenerating nervous system.

Axonal growth in the adult mammalian CNS is limited because of inhibitory influences of the glial environment and/or a lack of growth-promoting molecules. Here, we investigate whether supplementation of nerve growth factor (NGF) to the CNS during postnatal development and into adulthood can support the growth of sympathetic axons within myelinated portions of the maturing brain. We have also asked whether p75NTRplays a role in this NGF-induced axon growth. To address these questions we used two lines of transgenic mice overexpressing NGF centrally, with or without functional expression of p75NTR(NGF/p75+/+and NGF/p75−/−mice, respectively). Sympathetic axons invade the myelinated portions of the cerebellum, beginning shortly before the second week of postnatal life, in both lines of NGF transgenic mice. Despite the presence of central myelin, these sympathetic axons continue to sprout and increase in density between postnatal days 14 and 100, resulting in a dense plexus of sympathetic fibers within this myelinated environment. Surprisingly, the growth response of sympathetic fibers into the cerebellar white matter of NGF/p75−/−mice is enhanced, such that both the density and extent of axon ingrowth are increased, compared with age-matched NGF/p75+/+mice. These dissimilar growth responses cannot be attributed to differences in cerebellar levels of NGF protein or sympathetic neuron numbers between NGF/p75+/+and NGF/p75−/−mice. Our data provide evidence demonstrating that growth factors are capable of overcoming the inhibitory influences of central myelin in the adult CNS and that neutralization of the p75NTRmay further enhance this growth response.

Sympathetic axons invade the trigeminal ganglia of mice overexpressing nerve growth factor (NGF) (NGF/p75+/+mice) and surround sensory neurons having intense NGF immunolabeling; the growth of these axons appears to be directional and specific (Walsh and Kawaja, 1998). In this investigation, we provide new insight into the neurochemical features and receptor requirements of this sympathosensory sprouting. Using double-antigen immunohistochemistry, we demonstrate that virtually all (98%) trigeminal neurons that exhibit a sympathetic plexus aretrktyrosine kinase receptor (trkA)-positive. In addition, the majority (86%) of those neurons enveloped by sympathetic fibers is also calcitonin gene-related peptide (CGRP)-positive; a smaller number of plexuses (14%) surrounded other somata lacking this neuropeptide. Our results show that sympathosensory interactions form primarily between noradrenergic sympathetic efferents and the trkA/CGRP-expressing sensory somata. To assess the contribution of the p75 neurotrophin receptor (p75NTR) in sympathosensory sprouting, a hybrid strain of mice was used that overexpresses NGF but lacks p75NTRexpression (NGF/p75−/−mice). The trigeminal ganglia of NGF/p75−/−mice, like those of NGF/p75+/+mice, have increased levels of NGF protein and display a concomitant ingrowth of sympathetic axons. In contrast to the precise pattern of sprouting seen in the ganglia of NGF/p75+/+mice, sympathetic axons course randomly throughout the ganglionic neuropil of NGF/p75−/−mice, forming few perineuronal plexuses. Our results indicate that p75NTRis not required to initiate or sustain the growth of sympathetic axons into the NGF-rich trigeminal ganglia but rather plays a role in regulating the directional patterns of axon growth.

Transgenic mice possessing elevated levels of mRNA expression and synthesis for the neurotrophin nerve growth factor among astrocytes display a robust ingrowth of new sympathetic fibers to the cerebellum. In this investigation, we report that the cerebellum of these mice also possesses a dense plexus of aberrant axons of sensory origin. Axons stained immunohistochemically for calcitonin gene-related peptide were seen in the transgenic cerebellum as early as one week after birth. The density of these axons dramatically increased with age. Immunopositive axons were confined predominantly to the deep white matter of the cerebellum in the adult transgenic mice, with a smaller number of axons seen coursing along blood vessels in the gray matter. Axons stained immunohistochemically for the neurotrophin receptor, p75NTR, displayed a similar pattern of distribution and density as those immunostained for calcitonin gene-related peptide. Wild-type post-natal and adult animals lacked such calcitonin gene-related peptide- and p75NTR-immunoreactive axons in the cerebellum. Retrograde labelling revealed that these axons within the transgenic cerebellum originated from neurons in the sensory trigeminal and dorsal root ganglia (upper cervical levels). This investigation demonstrates that overexpression of nerve growth factor is capable of inducing the directional growth of collateral axons of sensory neurons into the undamaged mammalian central nervous system.

BackgroundThe evaluation and treatment of Jefferson fractures, a burst fracture of the ring of CI, has been well documented in the medical literature. Vertebral artery injury associated with a Jefferson fracture is very rare.MethodsThe case study technique was used to summarize the case. Review of the literature was performed to discuss the case. Retrospective chart review of the 174 patients with cervical fractures admitted to St. Michael’s Hospital from 1989–1994 was also performed.ResultsThe case of a patient with a Jefferson fracture, with bilateral lateral displacement of the lateral masses causing bilateral vertebral artery occlusions resulting in a lateral medullary and cerebellar infarction is reported. A review of the literature is provided.ConclusionA high index of suspicion for this injury is paramount, especially in patients with multiple trauma, where the diagnosis of Jefferson fractures can be delayed.