Facial motor neuron migration advances

During development, the migration of specific neuronal subtypes is required for the correct establishment of neural circuits. In mice and zebrafish, facial branchiomotor (FBM) neurons undergo a tangential migration from rhombomere 4 caudally through the hindbrain. Recent advances in the field have capitalized on genetic studies in zebrafish and mouse, and high-resolution time-lapse imaging in zebrafish. Planar cell polarity signaling has emerged as a critical conserved factor in FBM neuron migration, functioning both within the neurons and their environment. In zebrafish, migration depends on specialized 'pioneer' neurons to lead follower FBM neurons through the hindbrain, and on interactions with structural components including pre-laid axon tracts and the basement membrane. Despite fundamental conservation, species-specific differences in migration mechanisms are being uncovered.

Recurrent facial taste neurons of sea catfish Plotosus japonicus: morphology and organization in the ganglion

This study investigated the morphology of the recurrent facial taste neurons and their organization in the recurrent ganglion of the sea catfish Plotosus japonicus. The recurrent ganglion is independent of the anterior ganglion, which consists of trigeminal, facial and anterior lateral line neurons that send peripheral fibres to the head region. The recurrent taste neurons are round or oval and bipolar, with thick peripheral and thin central fibres, and completely wrapped by membranous layers of satellite cells. Two peripheral nerve branches coursing to the trunk or pectoral fin originate from the recurrent ganglion. The results presented here show that the trunk and pectoral-fin neurons are independently distributed to form various sizes of groups, and the groups are intermingled throughout the ganglion. No distinct topographical relationship of the two nerve branches occurs in the ganglion. Centrally, the trunk and pectoral-fin branches project somatotopically in the anterolateral and intermediate medial regions of the trunk tail lobule of the facial lobe, respectively.

Nociceptive afferents to the premotor neurons that send axons simultaneously to the facial and hypoglossal motoneurons by means of axon collaterals

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.

Proteomic analysis of microdissected facial nuclei of the rat following facial nerve injury

Recent studies using molecular and genetic techniques just have started to elucidate the complex process that drives successful peripheral nerve regeneration. Introducing proteomics to this field, we unilaterally performed a facial nerve axotomy in 13 adult Wistar rats. Seven days later, a total of 40 20-microm coronary cryostat sections of the operated and contralateral unoperated nucleus facialis were microdissected. On the one hand, microdissected areas were pooled for each side, lysed and applied to ProteinChip Arrays. On the other hand, one microdissected area from the right and left facial nucleus each was directly placed on the affinity chromatographic array. Facial motoneurons were lysed in situ and released their proteins to spatially defined points. 215 laser addressable distinct positions across the surface of the spot enabled a high spatial resolution of measured protein profiles for the analysed tissue area. Protein profiles of the single positions were plotted over the used tissue section to visualize their distribution. The comparative analysis of the protein lysates from operated and normal nuclei facialis revealed, for both approaches used, differentially expressed proteins. Although by direct application of one cryostat section only a few hundred motoneurons were analysed, results comparable to these using lysates were obtained. Additionally, the applied technique revealed differences in the intensity distribution of several proteins of unknown function in the lesioned in comparison to the contralateral normal facial nucleus. This proteomic analysis with ultra high sensitivity paired with potential for a spatial resolution is a promising methodology for peripheral nerve regeneration studies.

Differentiation of the facio-vestibulocochlear ganglionic complex in human embryos of developmental stages 13-15

A study was made on 18 embryos of developmental stages 13-15 (5(th) week). Serial sections made in horizontal, frontal, and sagittal planes were stained with routine histological methods and some of them were treated with silver. In embryos of stage 13, the otic vesicle is at the rhombomere 5, and close to the vesicle is the facial-vestibulocochlear ganglionic complex in which the geniculate, vestibular, and cochlear ganglion may be discerned. These ganglia are well demarcated in embryos of stage 14. In the last investigated stage (15(th)) the nerve fibres of the ganglia reach the common afferent tract.

Disease-dependent reciprocal phosphorylation of serine and tyrosine residues of c-Met/HGF receptor contributes disease retardation of a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by progressive degeneration of motoneurons. We have demonstrated that hepatocyte growth factor (HGF) attenuates loss of both spinal and brainstem motoneurons of ALS model mice expressing mutated human SOD1(G93A) (G93A). This study was designed to assess disease-dependent regulatory mechanisms of c-Met/HGF receptor (c-Met) activation in the facial motoneurons of G93A mice. Using double transgenic mice expressing HGF and mutated SOD1(G93A) (G93A/HGF), we showed that phosphorylation of c-Met tyrosine residues at positions 1230, 1234 and 1235 (phospho-Tyr), and thereby its activation, was slightly evident in G93A and highly obvious in G93A/HGF mice (but absent in WT and HGF-Tg mice). Phosphorylation of the c-Met serine residue at position 985 (phospho-Ser), a residue involved in the negative regulation of its activation, was evident in WT and HGF-Tg mice. Protein phosphatase 2A (PP2A), which is capable of dephosphorylating c-Met phospho-serine, is upregulated in the facial motoneurons of G93A and G93A/HGF mice compared with WT and HGF-Tg mice. Thus, c-Met activation is reciprocally regulated by phosphorylation between c-Met serine and tyrosine residues through PP2A induction in the presence or absence of mutant SOD1 expression, and HGF functions more efficiently in ALS and ALS-related diseases.

An increase in intracelluar free calcium ions modulated by cholinergic receptors in rat facial nucleus

BACKGROUND

Ca(2+) in the central nervous system plays important roles in brain physiology, including neuronal survival and regeneration in rats with injured facial motoneurons. The present research was to study the modulations of intracellular free Ca(2+) concentrations by cholinergic receptors in rat facial nucleus, and the mechanisms of the modulations.

METHODS

The fluorescence intensity of facial nucleus in Fluo-3 AM loaded acute brainstem slices was detected by applying intracellular free Ca(2+) measurement technique via confocal laser scanning microscope. The changes of fluorescence intensity of facial nucleus indicate the average changes of intracellular free Ca(2+) levels of the neurons.

RESULTS

Acetylcholine was effective at increasing the fluorescence intensity of facial nucleus. Muscarine chloride induced a marked increase of fluorescence intensity in a concentration dependent fashion. The enhancement of fluorescence intensity by muscarine chloride was significantly reduced by thapsigargin (depletor of intracellular Ca(2+) store; P < 0.01), rather than Ca(2+) free artifical cerebrospinal fluid or EGTA (free Ca(2+) chelator; P > 0.05). And the increase of fluorescence intensity was also significantly inhibited by pirenzepine (M(1) subtype selective antagonist; P < 0.01) and 4-DAMP (M(3) subtype selective antagonist; P < 0.01). In addition, fluorescence intensity was markedly increased by nicotine. The enhancement of fluorescence intensity by nicotine was significantly reduced by EGTA, nifedipine (L-type voltage-gated Ca(2+) channel blocker), dihydro-beta-erythroidine (alpha4beta2 subtype selective antagonist), and in Ca(2+) free artificial cerebrospinal fluid (P < 0.01), but not in the presence of mibefradil (M-type voltage-gated Ca(2+) channel blocker) or thapsigargin (P > 0.05).

CONCLUSIONS

The data provide the evidence that muscarinic receptors may induce the increase of intracellular free Ca(2+) levels through the Ca(2+) release of intracellular Ca(2+) stores, in a manner related to M(1) and M(3) subtypes of muscarinic receptors in rat facial nucleus. Nicotine may increase intracellular free Ca(2+) concentrations via the influx of extracellular Ca(2+)+ mainly across L-type voltage-gated Ca(2+) channels, in a manner related to the alpha4beta2 subtype of nicotinic receptors.

Types of taste circuits synaptically linked to a few geniculate ganglion neurons

The present study evaluates the central circuits that are synaptically engaged by very small subsets of the total population of geniculate ganglion cells to test the hypothesis that taste ganglion cells are heterogeneous in terms of their central connections. We used transsynaptic anterograde pseudorabies virus labeling of fungiform taste papillae to infect single or small numbers of geniculate ganglion cells, together with the central neurons with which they connect, to define differential patterns of synaptically linked neurons in the taste pathway. Labeled brain cells were localized within known gustatory regions, including the rostral central subdivision (RC) of the nucleus of the solitary tract (NST), the principal site where geniculate axons synapse, and the site containing most of the cells that project to the parabrachial nucleus (PBN) of the pons. Cells were also located in the rostral lateral NST subdivision (RL), a site of trigeminal and sparse geniculate input, and the ventral NST (V) and medullary reticular formation (RF), a caudal brainstem pathway leading to reflexive oromotor functions. Comparisons among cases, each with a random, very small subset of labeled geniculate neurons, revealed "types" of central neural circuits consistent with a differential engagement of either the ascending or the local, intramedullary pathway by different classes of ganglion cells. We conclude that taste ganglion cells are heterogeneous in terms of their central connectivity, some engaging, predominantly, the ascending "lemniscal," taste pathway, a circuit associated with higher order discriminative and homeostatic functions, others engaging the "local," intramedullary "reflex" circuit that mediates ingestion and rejection oromotor behaviors.

Plastic changes of synapses and excitatory neurotransmitter receptors in facial nucleus following facial-facial anastomosis

The remodeling process of synapses and neurotransmitter receptors of facial nucleus were observed. Models were set up by facial-facial anastomosis in rat. At post-surgery day (PSD) 0, 7, 21 and 60, synaptophysin (p38), NMDA receptor subunit 2A and AMPA receptor subunit 2 (GluR2) were observed by immunohistochemical method and semi-quantitative RT-PCR, respectively. Meanwhile, the synaptic structure of the facial motorneurons was observed under a transmission electron microscope (TEM). The intensity of p38 immunoreactivity was decreased, reaching the lowest value at PSD day 7, and then increased slightly at PSD 21. Ultrastructurally, the number of synapses in nucleus of the operational side decreased, which was consistent with the change in P38 immunoreactivity. NMDAR2A mRNA was down-regulated significantly in facial nucleus after the operation (P<0.05), whereas AMPAR2 mRNA levels remained unchanged (P>0.05). The synapses innervation and the expression of NMDAR2A and AMPAR2 mRNA in facial nucleus might be modified to suit for the new motor tasks following facial-facial anastomosis, and influenced facial nerve regeneration and recovery.

Phenotype of CD4+ T cell subsets that develop following mouse facial nerve axotomy

We have previously shown that CD4(+) T helper (Th) 2 cells, but not Th1 cells, participate in the rescue of mouse facial motoneurons (FMN) from axotomy-induced cell death. Recently, a number of other CD4(+) T cell subsets have been identified in addition to the Th1 and Th2 effector subsets, including Th17, inducible T regulatory type 1 (Tr1), and naturally thymus-born Foxp3(+) regulatory (Foxp3(+) Treg) cells. These subsets regulate the nature of a T cell-mediated immune response. Th1 and Th17 cells are pro-inflammatory subsets, while Th2, Tr1, and Foxp3(+) Treg cells are anti-inflammatory subsets. Pro-inflammatory responses in the central nervous system are thought to be neurodestructive, while anti-inflammatory responses are considered neuroprotective. However, it remains to be determined if another CD4(+) T cell subset, other than the Th2 cell, develops after peripheral nerve injury and participates in FMN survival. In the present study, we used FACS analysis to determine the temporal frequency of Th1, Th17, Th2, Tr1 and Foxp3(+) Treg CD4(+) T cell subset development in C57BL/6 wild type mice after facial nerve transection at the stylomastoid foramen in the mouse. The results indicate that all of the known CD4(+) T cell subsets develop and expand in number within the draining lymph node, with a peak in number primarily at 7 days postoperative (dpo), followed by a decline at 9 dpo. In addition to the increase in subset frequency over time, FACS analysis of individual cells showed that the level of cytokine expressed per cell also increased for interferon-gamma (IFN-gamma), interleukin (IL)-10 and IL-17, but not IL-4. Additional control double-cytokine labeling experiments were done which indicate that, at 7dpo, the majority of cells indeed have committed to a specific phenotype and express only 1 cytokine. Collectively, our findings indicate for the first time that there is no preferential activation and expansion of any single CD4(+) T cell subset after peripheral nerve injury but, rather, that both pro-inflammatory and anti-inflammatory CD4(+) T cells develop.

Impairment of microglial responses to facial nerve axotomy in cathepsin S-deficient mice

Cathepsin S (CS) is a lysosomal/endosomal cysteine protease especially expressed in cells of a mononuclear lineage including microglia. To better understand the role of CS in microglia, we investigated microglial responses after a facial nerve axotomy in CS-deficient (CS-/-) and wild-type mice. Microglia in both groups accumulated in the facial motor nucleus following axotomy. However, the mean number of microglia in CS-/- mice on the axotomized side was significantly smaller than that in wild-type mice. Microglia were found to adhere to injured motoneurons in wild-type mice, whereas microglia abutted on injured motoneurons without spreading on their surface in CS-/- mice. At the same time, the axotomy-induced down-regulation of tenasin-R, an antiadhesive perineuronal net for microglia, was partially abrogated in CS-/- mice. Primary cultured microglia prepared from CS-/- mice showed that CS deficiency caused significant suppression of migration and transmigration of microglia. In CS-/- mice, impaired recruitments of circulating monocytes and T lymphocytes and reduced expression of the class II major compatibility complex on the axotomized side were observed. Interestingly, cathepsin B, a typical lysosomal cysteine protease, was markedly expressed on the axotomized side in CS-/- but not in wild-type microglia. Finally, we compared axotomy-induced neuronal death in the two groups and found that the percentage of motoneurons that survived in CS-/- mice was significantly smaller than that in wild-type mice. The present study strongly suggests that CS plays a role in the migration and activation of microglia to protect facial motoneurons against axotomy-induced injury.

Cellular localization of androgen and estrogen receptors in mouse-derived motoneuron hybrid cells and mouse facial motoneurons

The ability of gonadal steroid hormones to augment axonal regeneration after peripheral nerve injury has been well established in rat and hamster motoneuron systems, and provides a foundation for the use of these agents as neurotherapeutics. With the advent of mouse genetics and the availability of transgenic and knockout mice, the use of mice in studies of neuroprotection is growing. It has recently been demonstrated that both androgens and estrogens rescue motoneurons (MN) from injury in mouse-derived motoneuron hybrid cells in vitro and mouse facial motoneurons (FMN) in vivo (Tetzlaff et al. [2006] J Mol Neurosci 28:53-64). To elucidate the molecular mechanisms of these effects, the present study examined the cellular localization of androgen and estrogen receptors in mouse MN in vitro and in vivo. Immunoblotting and immunocytochemistry studies established the presence of androgen receptor (AR) and estrogen receptor alpha/beta in immortalized mouse motoneuron hybrid cells and AR and estrogen receptor alpha in mouse FMN.

Neuronal expression of peripherin, a type III intermediate filament protein, in the mouse hindbrain

Peripherin is a 57 kDa Type III intermediate filament protein associated with neurite extension, neuropathies such as amyotrophic lateral sclerosis, and cranial nerve and dorsal root projections. However, knowledge of peripherin expression in the CNS is limited. We have used immunoperoxidase histochemistry to characterise peripherin expression in the mouse hindbrain, including the inferior colliculus, pons, medulla and cerebellum. Peripherin immunolabelling was observed in the nerve fibres and nuclei that are associated with all cranial nerves [(CN) V-XII] in the hindbrain. Peripherin expression was prominent in the cell bodies and axons of the mesenchephalic trigeminal nucleus and the pars compacta region of nucleus ambiguus, and in the fibres that comprise the solitary tract, the descending spinal trigeminal tract and the trigeminal and facial nerves. A small proportion of peripherin positive fibres in CN VIII likely arise from cochlear type II spiral ganglion neurons. Peripherin positive fibres were also observed in the inferior cerebellar peduncle and folia in the intermediate zone of the cerebellum. Antibody specificity was confirmed by absence of labelling in hindbrain tissue from peripherin knockout mice. This study shows that in the adult mouse hindbrain, peripherin is expressed in discrete neuronal subpopulations that have sensory, motor and autonomic functions.

Central nervous system and non-central nervous system antigen vaccines exacerbate neuropathology caused by nerve injury

Previously, we showed that autoimmune (central nervous system myelin-reactive) T cells exacerbate tissue damage and impair neurological recovery after spinal cord injury. Conversely, independent studies have shown T cell-mediated neuroprotection after spinal cord injury or facial nerve axotomy (FNAx). The antigen specificity of the neuroprotective T cells has not been investigated after FNAx. Here, we compared the neuroprotective capacity of autoimmune and non-autoimmune lymphocytes after FNAx. Prior to axotomy, C57BL/6 mice were immunized with myelin basic protein, myelin oligodendrocyte glycoprotein (MOG) or ovalbumin (a non-self antigen) emulsified in complete Freund's adjuvant (CFA). FNAx mice receiving injections of phosphate-buffered saline (PBS) only (unimmunized) or PBS/CFA emulsions served as controls. At 4 weeks after axotomy, bilateral facial motor neuron counts were obtained throughout the facial motor nucleus using unbiased stereology (optical fractionator). The data show that neuroantigen immunizations and 'generic' lymphocyte activation (e.g. PBS/CFA or ovalbumin/CFA immunizations) exacerbated neuron loss above that caused by FNAx alone. We also found that nerve injury potentiated the effector potential of autoimmune lymphocytes. Indeed, prominent forelimb and hindlimb motor deficits were accompanied by disseminated neuroinflammation and demyelination in FNAx mice receiving subencephalitogenic immunization with MOG. FNAx or neuroantigen (MOG or myelin basic protein) immunization alone did not cause these pathological changes. Thus, irrespective of the antigens used to trigger an immune response, neuropathology was enhanced when the immune system was primed in parallel with nerve injury. These data have important implications for therapeutic vaccination in clinical neurotrauma and neurodegeneration.

Exogenous androgen treatment delays the stress response following hamster facial nerve injury

Following injury or stress of any type, cells undergo a stress response, involving the cessation of general protein synthesis and the up-regulation of heat shock proteins (HSP), which have been implicated in promoting cell survival and repair. In a variety of neuronal injury models, including the hamster facial motoneurone (FMN) model, steroid hormones augment regeneration and are neuroprotective. We have previously shown that facial nerve axotomy induces expression of HSP70 (HSP70) and/or up-regulates constitutively expressed HSP70 (HSC70) mRNA in axotomised hamster FMN and that testosterone propionate (TP) treatment reduces this response. These previous studies were unable to differentiate between HSC70 mRNA and HSP70 mRNA. Therefore, an objective of the present study was to determine which HSP (HSC70 or HSP70) was being up-regulated by axotomy and reduced by TP. Axotomy increased HSC70 protein in axotomised and non-axotomised FMN, relative to untreated baseline hamsters. Interestingly, TP transiently delayed the stress-induced up-regulation of HSC70 protein in axotomised FMN compared to axotomised FMN from non-TP treated controls. A potential explanation for this delay may involve the TP-induced liberation of HSP from the androgen receptor, which would provide the injured cell with an immediately available pool of protective HSP. An hypothesis is presented suggesting that this TP-induced delay of stress-induced HSC70 up-regulation might allow for the diversion of cellular energy away from HSP synthesis and towards the synthesis of proteins required for regeneration and survival.

Postnatal switching of NMDA receptor subunits from NR2B to NR2A in rat facial motor neurons

The subunit composition of N-methyl-D-aspartate (NMDA) receptors affects their function under normal and pathological conditions. Functional NMDA receptors are expressed in lower motor neurons, but their subunit composition has not been defined. Here, we employed electrophysiology, quantitative PCR, and immunohistochemistry to investigate the subunit composition of NMDA receptors in postnatal motor neurons of the Wistar rat facial nucleus (FN). Whole-cell patch clamp recordings of acutely dissociated motor neurons from postnatal days 3 and 4 (P3-P4) showed that ifenprodil, a specific antagonist of the NMDA receptor 2B (NR2B) subunit, inhibited 91.62%+/-2.02% of NMDA-induced current, whereas NVP-AAM007, a specific antagonist of the NMDA receptor 2A (NR2A) subunit, inhibited much less of the current (16.69%+/-3.28%). Starting from P5, the inhibitory effects of ifenprodil and NVP-AAM007 gradually decreased and increased, respectively, such that the effect of NVP-AAM007 exceeded that of ifenprodil by P10. At P14, most of the NMDA-induced current was inhibited by NVP-AAM007 (84.59%+/-3.35%). Consistent with this, NR2B mRNA and protein were expressed highly at P3 and then gradually decreased by more than 75% by P14 in FN motor neurons, while NR1 was expressed stably over the same ages. However, NR2A mRNA and protein showed relatively constant levels between P3-P10 and decreased to 45% and 75% of the P3 level, respectively, by P14. Thus, analysis of functional NMDA receptors is critical to revealing subunit switching, which may be an important step in postnatal development of FN motor neurons.

T-Box transcription factor Tbx20 regulates a genetic program for cranial motor neuron cell body migration

Members of the T-box transcription factor family (Tbx) are associated with several human syndromes during embryogenesis. Nevertheless, their functions within the developing CNS remain poorly characterized. Tbx20 is expressed by migrating branchiomotor/visceromotor (BM/VM) neurons within the hindbrain during neuronal circuit formation. We examined Tbx20 function in BM/VM cells using conditional Tbx20-null mutant mice to delete the gene in neurons. Hindbrain rhombomere patterning and the initial generation of post-mitotic BM/VM neurons were normal in Tbx20 mutants. However, Tbx20 was required for the tangential (caudal) migration of facial neurons, the lateral migration of trigeminal cells and the trans-median movement of vestibuloacoustic neurons. Facial cell soma migration defects were associated with the coordinate downregulation of multiple components of the planar cell polarity pathway including Fzd7, Wnt11, Prickle1, Vang1 and Vang2. Our study suggests that Tbx20 programs a variety of hindbrain motor neurons for migration, independent of directionality, and in facial neurons is a positive regulator of the non-canonical Wnt signaling pathway.

Expression of ABCA2 protein in both non-myelin-forming and myelin-forming Schwann cells in the rodent peripheral nerve

We previously reported that ABCA2, of the A subclass of the ATP-binding cassette (ABC) transporter superfamily, is expressed in mature oligodendrocytes and Schwann cells, the cells responsible for myelination in the brain and the peripheral nerve, respectively. However, unidentified cells expressing ABCA2 also were found. Here, we provide evidence for the expression of ABCA2 in the rodent sciatic nerve not only in Schwann cells, which express the Schwann cell marker S100beta and a zinc finger transcription factor Krox20 (a marker for myelin-forming Schwann cells), but also in Krox20-negative cells, which express glial fibrillary acidic protein (GFAP), a cell adhesion molecule L1, and S100beta weekly. We also analyzed developmental changes in ABCA2 expression in Schwann cells. The expression of ABCA2 in Krox20+/S100beta+ Schwann cells was found initially in rat facial nerve at postnatal day (PD) 8, in half (52.4%) of the cells showing myelinization at PD 14, and in all of the cells in the adult stage. These results demonstrate that ABCA2 is expressed in non-myelin-forming as well as in myelin-forming Schwann cells, and that ABCA2 may be involved in transport of a substance associated with cellular maturation rather than initial myelin formation in both types of Schwann cells.

Gustatory terminal field organization and developmental plasticity in the nucleus of the solitary tract revealed through triple-fluorescence labeling

Early dietary sodium restriction has profound influences on the organization of the gustatory brainstem. However, the anatomical relationships among multiple gustatory nerve inputs have not been examined. Through the use of triple-fluorescence labeling and confocal laser microscopy, terminal fields of the greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves were visualized concurrently in the nucleus of the solitary tract (NTS) of developmentally sodium-restricted and control rats. Dietary sodium restriction during pre- and postnatal development resulted in a twofold increase in the volume of both the CT and the IX nerve terminal fields but did not affect the volume of the GSP terminal field. In controls, these nerve terminal fields overlapped considerably. The dietary manipulation significantly increased the overlapping zones among terminal fields, resulting in an extension of CT and IX fields past their normal boundaries. The differences in terminal field volumes were exaggerated when expressed relative to the respective NTS volumes. Furthermore, increased terminal field volumes could not be attributed to an increase in the number of afferents because ganglion cell counts did not differ between groups. Taken together, selective increases in terminal field volume and ensuing overlap among terminal fields suggest an increased convergence of these gustatory nerve terminals onto neurons in the NTS. The genesis of such convergence is likely related to disruption of cellular and molecular mechanisms during the development of individual terminal fields, the consequences of which have implications for corresponding functional and behavioral alterations.

Long-term adeno-associated viral vector-mediated expression of truncated TrkB in the adult rat facial nucleus results in motor neuron degeneration

Adult facial motor neurons continue to express full-length TrkB tyrosine kinase receptor (TrkB.FL), the high-affinity receptor for the neurotrophins BDNF and neurotrophic factor-4/5 (NT-4/5), suggesting that they remain dependent on target-derived and locally produced neurotrophins in adulthood. Studies on the role of TrkB signaling in the adult CNS have been hampered by the early lethality of bdnf, nt-4/5, and trkB knock-out mice. We disrupted TrkB.FL signaling in adult facial motor neurons using adeno-associated viral vector-mediated overexpression of a naturally occurring dominant-negative TrkB receptor, TrkB.T1. Expression of TrkB.T1 resulted in neuronal atrophy and downregulation of NeuN (neuronal-specific nuclear protein) and ChAT expression in facial motor neurons. A subset of transduced neurons displayed signs of motor neuron degeneration that included dendritic beading and rounding of the soma at 2 months of TrkB.T1 expression. Cell counts revealed a significant reduction in motor neuron number in the facial nucleus at 4 months after onset of expression of TrkB.T1, suggesting that a proportion of TrkB.T1-expressing motor neurons became undetectable as a result of severe atrophy or was lost because of cell death. In contrast, overexpression of TrkB.FL did not result in a decrease in facial motor neuron number. Our results indicate that a subset of facial motor neurons remains dependent on TrkB ligands for the maintenance of structural and molecular characteristics in adulthood.

Neuronal FasL induces cell death of encephalitogenic T lymphocytes

Apoptosis of inflammatory cells plays a crucial role in the recovery from autoimmune CNS disease. However, the underlying mechanisms of apoptosis induction are as yet ill-defined. Here we report on the neuronal expression of FasL and its potential function in inducing T-cell apoptosis. Using a combination of facial nerve axotomy and passive transfer encephalomyelitis, the fate of CD4+ encephalitogenic T cells engineered to express the gene for green fluorescent protein was followed. FasL gene transcripts and FasL protein were detected in neurons by in sit-hybridization and immunohistochemistry. T cells infiltrating preferentially the injured brain parenchyma were found in the immediate vicinity of FasL expressing neurons and even inside their perikarya. In contrast to neurons, T cells rapidly underwent apoptosis. In co-cultures of hippocampal nerve cells and CD4 T lymphocytes, we confirmed expression of FasL in neurons and concomitant induction of T-cell death. Antibodies blocking neuronal FasL were shown to have a protective effect on T-cell survival. Thus, FasL expression by neurons in neuroinflammatory diseases may constitute a pivotal mechanism underlying apoptosis of encephalitogenic T cells.

Practical nerve morphometry

Histopathological examination of peripheral nerves is often complemented by morphometric analysis in both clinical and research settings. However, existing manual or semi-automated methods are highly tedious, labour intensive and time-consuming, whereas fully automated morphometry is prone to error from the conversion of maldetected particles to spurious data. Both fully and interactive-automated morphometry have significant hardware requirements and may be difficult to implement. A new method for nerve morphometry is described aiming to combine the speed of automated morphometry with the accuracy of manual or semi-automated methods, and requiring only a digital image of the nerve section and two widely available software packages. Comparison with a standard digitizer pen method of nerve morphometry without sampling yielded statistically similar axon counts, mean area assessments and axonal area frequency distribution histograms, with assessment times of the new method between 35% and 45% of those of the standard method. This has widespread potential experimental and clinical applications and offers a means of relieving much of the tedium currently associated with nerve morphometry.

Effects of axotomy on telomere length, telomerase activity, and protein in activated microglia

The adult central nervous system (CNS) is generally thought of as a postmitotic organ. However, DNA labeling studies have shown that one major population of nonneuronal cells, called microglia, retain significant mitotic potential. Microglial cell division is prominent during acute CNS injury involving neuronal damage or death. Prior work from this laboratory has shown that purified microglia maintained in vitro with continual mitogenic stimulation exhibit telomere shortening before entering senescence. In the current study, we sought to investigate whether telomere shortening occurs in dividing microglia in vivo. For this purpose, we used a nerve injury model that is known to trigger localized microglial proliferation in a well-defined CNS region, the facial motor nucleus. Adult Sprague-Dawley rats underwent facial nerve axotomy, and facial motor nuclei were microdissected after 1, 4, 7, and 10 days. Whole tissue samples were subjected to measurements of telomere length, telomerase activity, and telomerase protein. Results revealed a tendency for all of these parameters to be increased in lesioned samples. In addition, microglial cells isolated directly from axotomized facial nuclei with fluorescence-activated cell sorting (FACS) showed increased telomerase activity relative to unoperated controls, suggesting that microglia are the primary cell type responsible for the increases observed in whole tissue samples. Overall, the results show that microglia activated by injury are capable of maintaining telomere length via telomerase during periods of high proliferation in vivo. We conclude that molecular mechanisms pertaining to telomere maintenance are active in the injured CNS.

The facial motor nucleus transcriptional program in response to peripheral nerve injury identifies Hn1 as a regeneration-associated gene

Facial nerve axotomy (FNA) is a well-established experimental model of motoneuron regeneration. After peripheral nerve axotomy, a sequence of events including glial activation and axonal regrowth leads to functional recovery of the afflicted pool of motoneurons. Using microarray analysis we identified an increase in the expression of 60 genes (at a false discovery rate of 0.1, genes were significant P < 0.004) within the facial nucleus as a consequence of nerve injury. In situ hybridization analysis validated the increased expression of many of these axotomy-induced genes. One specific gene, encoding a unique primary amino acid sequence, termed hemopoietic- and neurologic-expressed sequence-1 (Hn1), was evaluated more extensively using several additional nerve injury paradigms. Hn1 mRNA was upregulated in injured facial motoneurons in both rats and mice. Sustained upregulation of Hn1 mRNA was evident after nerve resection whereas levels of Hn1 mRNA returned to baseline in animals subjected to nerve crush or nerve transection. Hn1 was also increased in the dorsal motor nucleus and the nucleus ambiguous after vagus nerve axotomy, another regeneration model. No upregulation of Hn1 expression was observed, however, in two nonregeneration models: FNA in newborn rats and rubrospinal tractotomy. Hn1 mRNA was ubiquitous in the developing central nervous system whereas its expression in adult brain was confined to neurons of the hippocampus, cortex and cerebellum. These findings identify Hn1 as a gene associated with nervous system development and nerve regeneration.

[Protecting effect of human-derived neurotrophin-6 on retrogradely degenerated motoneurons of facial nucleus of rats]

AIM

The protecting effect of human-derived neurotrophin-6 (NT-6) on injured neurons was investigated to test the neurobiological characteristics of human-derived NT-6.

METHODS

Adult SD rats were used and divided into two groups, normal control group and experiment group. The right facial nerve of the experimental rats was sectioned. The experimental animals were subdivided into three groups, blank control group, NT-6 group and saline control group. The rats were raised for two weeks. Brain stem of the rats was removed and transversely sliced. Nissle stain and ChAT immunohistochemical stain of the slices were carried out to observe the role of NT-6 in protecting facial motoneurons.

RESULTS

The number of the ChAT positive neurons and the intensity of Nissle stain in the facial nucleus of NT-6 group were significantly augmented compared with that of the blank and saline control groups.

CONCLUSION

The human-derived NT-6 could partially protect the facial motoneurons from retrograde degeneration induced by their axon damage.

Trigemino-solitarii-facial pathway in rats

This study was undertaken to identify premotor neurons in the nucleus tractus solitarii (NTS) serving as relay neurons between the sensory trigeminal complex (STC) and the facial motor nucleus in rats. Trigemino-solitarii connections were first investigated following injections of anterograde and/or retrograde (biotinylated dextran amine, biocytin, or gold-HRP) tracers in STC or NTS. Trigemino-solitarii neurons were abundant in the ventral and dorsal parts of the STC and of moderate density in its intermediate part. They project throughout the entire rostrocaudal extent of the NTS with a strong lateral preponderance. Solitarii-trigeminal neurons were observed mostly in the rostral and rostrolateral NTS. They mainly project to the ventral and dorsal parts of the spinal trigeminal nucleus but not to the principal nucleus. Additional neurons located in the middle NTS were found to project exclusively to the spinal trigeminal nucleus pars caudalis. No solitarii-trigeminal cells were observed in the caudal NTS. In addition, evidence was obtained of NTS retrogradely labeled neurons contacted by anterogradely labeled trigeminal terminals. Second, solitarii-facial projections were analyzed following injections of anterograde and retrograde tracers into the NTS and the facial nucleus, respectively. NTS neurons, except those of the rostrolateral part, reached the dorsal aspect of the facial nucleus. Finally, simultaneous injections of anterograde tracer in the STC and retrograde tracer in the facial nucleus gave retrogradely labeled neurons in the NTS receiving contacts from anterogradely labeled trigeminal boutons. Thus, the present data demonstrate for the first time the existence of a trigemino-solitarii-facial pathway. This could account for the involvement of the NTS in the control of orofacial motor behaviors.

Modulation of GABA receptor subunits in rat facial motoneurons after axotomy

Facial nerve axotomy is a good model for studying neuronal plasticity and regeneration in the peripheral nervous system. In the present study, we investigated the effect of axotomy on the different subunits of GABA(A) and GABA(B) receptors of facial motoneurons. The facial nerve trunk was unilaterally sectioned and operated rats were sacrificed at 1, 3, 8, 30, and 60 days later. mRNAs coding for alpha1, beta2, and gamma2 of GABA(A) receptors and for GABA(1B) and GABA(B2) receptors were down-regulated by axotomy. This decrease began as soon as 1 or 3 days after axotomy, and the minimum was 8 days post-lesion; the mRNA levels remained lower than normal at day post-lesion 60. The abundance of mRNAs coding for the three other alpha2, beta1, and beta3 facial subunits of GABA(A) receptors and for the pre-synaptic GABA(B1A) subunit remained unchanged during the period 1-8 days post-lesion. Immunohistochemistry using specific antibodies against alpha1, gamma2 subunits of GABA(A) and against GABA(B2) subunits confirmed this down-regulation. Colchicine treatment and blockade of action potential by tetrodotoxin significantly decreased GABA(A)alpha1 immunoreactivity in the axotomized facial nucleus after 7 days. Finally, muscle destruction by cardiotoxin or facial palsy induced by botulinum toxin failed to change GABA(A)alpha1 subunit expression. Our data demonstrate that axotomy strongly reduced the amounts of alpha1, beta2, and gamma2 subunits of GABA(A) receptors and B(1B) and B(2) subunits of GABA(B) receptors in the axotomized facial motoneurons. The loss of GABA(A)alpha1 subunit was most probably induced by both the loss of trophic factors transported from the periphery and a positive injury signal. It also seems to be dependent on activity disruption.

Brain-derived neurotrophic factor supports facial motoneuron survival after facial nerve transection in immunodeficient mice

Numerous studies have shown that motoneuron survival can be facilitated by neurotrophic factors (NTF) after injury. However, the ability of specific NTF to rescue facial motoneurons (FMN) from axotomy-induced death in immunodeficient mice has not been tested. Therefore, one goal of this study was to determine if brain-derived neurotrophic factor (BDNF), an NTF with a known ability to rescue FMN from axotomy-induced death, supports FMN from axotomy-induced death in recombinase activating gene-2 knockout (RAG-2 KO) mice that lack functional T and B lymphocytes. Nerve growth factor, which has been shown not to play a role in motoneuron survival, was used as a negative control. Brain derived neurotrophic factor treatment restored FMN survival to wild-type (WT) control levels 4 weeks post-operative (wpo) (80% +/- 1.9, 83% +/- 2.4, respectively). The second goal of this study was to begin to elucidate if CD4+ T cells produce NTF after facial nerve axotomy. Cervical lymph nodes were collected from WT mice 9 days post-operative, re-activated with anti-CD3 and supernatant collected 24 h later. Immediately after injury, the supernatant was administered to RAG-2 KO mice leading to an increase in FMN survival equivalent to WT controls (80% +/- 1.4, 84% +/- 2.1, respectively, 4 wpo). In addition, cervical lymph node supernatant treated with anti-BDNF attenuated FMN rescue in RAG-2 KO mice (62% +/- 3.3) 4 wpo. These data lend support to the hypothesis that CD4+ T cells produce NTF that support motoneuron survival before target reconnection occurs.

Distribution of facial motor neurons in the pond turtle Pseudemys scripta elegans

A tract tracing study was performed to examine the localization of the facial nucleus in the brain stem of the pond turtle, Pseudemys scripta elegans. Neurobiotin and the fluorescent tracers alexa fluor 488 and 594 were used to retrogradely label neurons of the abducens or facial nerves. The results showed that the facial nucleus has two subnuclei, a medial group and a lateral group. Measurements of cell size revealed no significant differences between these populations. Double labeling studies showed that the medial cell group of the facial nucleus lies between the principal and accessory abducens nuclei in the pons, whereas the lateral group lies adjacent to the accessory abducens nucleus. The facial nucleus of pond turtles largely overlaps the rostrocaudal extent of the accessory abducens nucleus, but extends well beyond it into the medulla. These data elucidate the position and distribution of the facial nucleus in the brain stem of pond turtles and contribute to the body of comparative neuroanatomical literature on the distribution of the cranial nerve nuclei of reptiles.

Experimental autoimmune encephalomyelitis repressed by microglial paralysis

Although microglial activation occurs in inflammatory, degenerative and neoplastic central nervous system (CNS) disorders, its role in pathogenesis is unclear. We studied this question by generating CD11b-HSVTK transgenic mice, which express herpes simplex thymidine kinase in macrophages and microglia. Ganciclovir treatment of organotypic brain slice cultures derived from CD11b-HSVTK mice abolished microglial release of nitrite, proinflammatory cytokines and chemokines. Systemic ganciclovir administration to CD11b-HSVTK mice elicited hematopoietic toxicity, which was prevented by transfer of wild-type bone marrow. In bone marrow chimeras, ganciclovir blocked microglial activation in the facial nucleus upon axotomy and repressed the development of experimental autoimmune encephalomyelitis. We conclude that microglial paralysis inhibits the development and maintenance of inflammatory CNS lesions. The microglial compartment thus provides a potential therapeutic target in inflammatory CNS disorders. These results validate CD11b-HSVTK mice as a tool to study the impact of microglial activation on CNS diseases in vivo.

Vascular endothelial growth factor controls neuronal migration and cooperates with Sema3A to pattern distinct compartments of the facial nerve

Developing neurons accurately position their somata within the neural tube to make contact with appropriate neighbors and project axons to their preferred targets. Taking advantage of a collection of genetically engineered mouse mutants, we now demonstrate that the behavior of somata and axons of the facial nerve is regulated independently by two secreted ligands for the transmembrane receptor neuropilin 1 (Nrp1), the semaphorin Sema3A and the VEGF164 isoform of Vascular Endothelial Growth Factor. Although Sema3A is known to control the guidance of facial nerve axons, we now show that it is not required for the pathfinding of their somata. Vice versa, we find that VEGF164 is not required for axon guidance of facial motor neurons, but is essential for the correct migration of their somata. These observations demonstrate, for the first time, that VEGF contributes to neuronal patterning in vivo, and that different compartments of one cell can be co-ordinately patterned by structurally distinct ligands for a shared receptor.

Restoration of axotomy-induced PACAP gene induction in SCID mice with CD4+ T-lymphocytes

PACAP is a neuropeptide with putative neuroprotective, regenerative, and immunomodulatory actions. PACAP mRNA is up-regulated in motor neurons following facial nerve axotomy in wild type, but not immunodeficient SCID mice. Because CD4+ lymphocytes appear to be neuroprotective in facial nerve and other injury models, we studied PACAP gene expression in SCID mice preinfused with CD4+ enriched splenocytes. Whereas the mean number of PACAP hybridizing neurons after axotomy was reduced by 75% in uninfused SCID mice, infusion of CD4+ enriched splenocytes restored the number to a value not significantly different than controls. The CD4+ cell-dependent induction of PACAP in motor neurons may thus be a factor in the cascade of events triggered by immune cells that ultimately lead to nerve regeneration.

CD4-positive T cell-mediated neuroprotection requires dual compartment antigen presentation

Our laboratory discovered that CD4-positive (CD4+) T cells of the immune system convey transitory neuroprotection to injured mouse facial motoneurons (FMNs) (Serpe et al., 1999, 2000, 2003). A fundamental question in the mechanisms responsible for neuroprotection concerns the identity of the cell(s) that serves as the antigen-presenting cell (APC) to activate the CD4+ T cells. Here, we first establish that CD4+ T cells reactive to non-CNS antigen fail to support FMN survival and, second, demonstrate a two-compartment model of CD4+ T cell activation. Mouse bone marrow (BM) chimeras were developed that discriminate between resident antigen-presenting host cell and BM-derived antigen-presenting donor cell expression of major histocompatibility complex II within central and peripheral compartments, respectively. After facial nerve transection, neither compartment alone is sufficient to result in activated CD4+ T cell-mediated FMN survival. Rather, CD4+ T cell-mediated neuroprotection appears to depend on both resident microglial cells in the central compartment and a BM-derived APC in the peripheral compartment. This is the first in vivo report demonstrating a neuroprotective mechanism requiring APC functions by resident (i.e., parenchymal) microglial cells.

The AP-1 transcription factor c-Jun is required for efficient axonal regeneration

Nerve injury triggers numerous changes in the injured neurons and surrounding nonneuronal cells that ultimately result in successful target reinnervation or cell death. c-Jun is a component of the heterodimeric AP-1 transcription factor, and c-Jun is highly expressed in response to neuronal trauma. Here we have investigated the role of c-jun during axonal regeneration using mice lacking c-jun in the central nervous system. After transection of the facial nerve, the absence of c-Jun caused severe defects in several aspects of the axonal response, including perineuronal sprouting, lymphocyte recruitment, and microglial activation. c-Jun-deficient motorneurons were atrophic, resistant to axotomy-induced cell death, and showed reduced target muscle reinnervation. Expression of CD44, galanin, and alpha7beta1 integrin, molecules known to be involved in regeneration, was greatly impaired, suggesting a mechanism for c-Jun-mediated axonal growth. Taken together, our results identify c-Jun as an important regulator of axonal regeneration in the injured central nervous system.

Medullary efferent and afferent neurons of the facial nerve of the pit viper Gloydius brevicaudus

For the purposes of comparative anatomy, we used tracer techniques and immunohistochemistry to study the facial nerve in the pit viper Gloydius brevicaudus and obtained much new data applicable to the function of this nerve in snakes and, in particular, pit vipers. We were able to identify the superior salivatory nucleus in these snakes. Preganglionic fibers from this nucleus pass along the palatine nerve and an anterior communicating branch to reach the pterygopalatine ganglion attached to the deep branch of the trigeminal maxillary nerve. The palatine nerve also contains general somatic afferents and a very few special visceral afferents from some taste buds on the palate. In the mandibular direction, preganglionic fibers from the superior salivatory nucleus join special visceral efferents from the motor nucleus in the hyomandibular nerve, from which they pass into the chorda tympani to course together for a short distance. The special visceral efferents branch off outside the cranium, and the preganglionic fibers continue on to join the trigeminal mandibular nerve to project to small ganglia within the mandible. The chorda tympani also contains general somatic afferents from the mandibular region but no special visceral afferents. This is the first time that the superior salivatory nucleus and its adjuncts have been identified in a snake. The chorda tympani of these snakes is also distinguished from the mammalian condition by lacking any special visceral afferents and by branching outside the cranium.

Orbital passage of pterygopalatine ganglion efferents to paranasal sinuses and nasal mucosa in man

Parasympathetic nerves of pterygopalatine ganglion origin are considered to enter the orbit and distribute to the nasal mucosa with the anterior ethmoidal nerve. As their distribution has never been demonstrated the present study was undertaken to seek evidence of their passage and to identify their relationship with the ethmoidal nerves. The soft tissues of the pterygopalatine fossa and orbit from sixteen sides of twelve cadavers were removed in one piece and either dissected or cut coronally into slabs and prepared histologically using montages of thin resin-embedded sections at intervals suitable for nerve path tracing. Several of the rami orbitales passing mediodorsally from the ganglion enter the orbit apically, branch and enter the posterior ethmoidal foramen terminating in the lining of the paranasal sinuses and others advance to enter the anterior ethmoidal canal to reach the nasal mucosa. No junctions were made with ethmoidal nerves within the orbit or the canal. Failure of surgical lesions of the anterior ethmoidal nerve as a treatment for vasomotor rhinitis may be attributed to the sparing of the separate parasympathetic nerves. Appropriate chemical lesions, on the other hand, could ensure destruction of isolated parasympathetic nerves while limiting damage to the larger anterior ethmoidal nerve.

Changes in mRNA of Slit–Robo GTPase-activating protein 2 following facial nerve transection

Complex processes following peripheral nerve injury integrate a number of various external cues and their intracellular responses resulting in the cytoskeletal remodeling. One of these cues, Slit protein, plays an important role in neuronal migration and axonal guidance through the interaction with Roundabout (Robo) receptor. It was reported that the signal from Robo is transmitted to a specific family of GTPase-activating proteins (GAPs) named Slit-Robo GAPs. The Slit-Robo GAPs (srGAPs) further transmit the signal to the actin cytoskeleton controlling Rho GTPases and thus provide a direct link between Slit-Robo signaling and actin cytoskeleton. We examined the effects of facial nerve transection on srGAP2 mRNA expression in the facial nerve nuclei by in situ hybridization. SrGAP2 mRNA was initially expressed, and its expression increased from 3 to 28 days after transection, with the peak at the seventh day after axotomy. The upregulation was found mostly in the neuronal cells and only to a small extent in the glial cells. Our results suggest that srGAP2, as a part of Slit-Robo pathway, plays an important role in the axonal regeneration after axotomy.

Facial nerve injury induces facilitation of responses in both trigeminal and facial nuclei of rat

A study was made of the effects of facial nerve transection on trigeminal stimulation- evoked field potentials in the principal trigeminal (Pr5) and facial nuclei (7) in rats. Although the transected branch of the facial nerve contains pure motoric efferents, it resulted in enhanced responses in both Pr5 and 7. These electrophysiological results suggest a functional circuitry involving the whiskers, trigeminal nerve, Pr5 and 7 and the facial nerve as efferent. The disconnection (opening) of this loop results in enhanced responsiveness of the neurons in both Pr5 and 7.

A point mutation in the motor domain of nonmuscle myosin II-B impairs migration of distinct groups of neurons

We generated mice harboring a single amino acid mutation in the motor domain of nonmuscle myosin heavy chain II-B (NMHC II-B). Homozygous mutant mice had an abnormal gait and difficulties in maintaining balance. Consistent with their motor defects, the mutant mice displayed an abnormal pattern of cerebellar foliation. Analysis of the brains of homozygous mutant mice showed significant defects in neuronal migration involving granule cells in the cerebellum, the facial neurons, and the anterior extramural precerebellar migratory stream, including the pontine neurons. A high level of NMHC II-B expression in these neurons suggests an important role for this particular isoform during neuronal migration in the developing brain. Increased phosphorylation of the myosin II regulatory light chain in migrating, compared with stationary pontine neurons, supports an active role for myosin II in regulating their migration. These studies demonstrate that NMHC II-B is particularly important for normal migration of distinct groups of neurons during mouse brain development.

Modulation of the beta1-3 voltage-gated sodium channels in rat vestibular and facial nuclei after unilateral labyrinthectomy and facial nerve section: an in situ hybridization study

We investigated whether the production of the mRNAs for the auxiliary beta subunits of the Na channels are modulated in deafferented medial vestibular nucleus (MVN) and in axotomized facial motoneurons. No beta1-3 mRNAs modulation was detected at any time following unilateral labyrinthectomy in the deafferented and intact medial vestibular nucleus. In contrast, beta1 gene expression in the axotomized facial nucleus decreased compared to controls as soon as day post-lesion 3.

Selective expression of Gi/o-coupled ATP receptor P2Y12 in microglia in rat brain

Extracellular nucleotides, including ATP, have been demonstrated to transmit important physiological signals in the brain through either G-protein-coupled P2Y receptors or P2X receptors, which are ligand-gated ion channels. In this study, we performed a detailed analysis of the expression of the Gi/o-coupled receptor P2Y12 in the brain. Northern blot analysis demonstrated that P2Y12 is expressed predominantly in the brain, and to a lesser extent in the spleen. The cellular localization of P2Y12 was investigated by in situ hybridization, and P2Y12 mRNA was detected in small cells distributed throughout the brain, including the hippocampus. Expression of P2Y12 was also observed in naive and axotomized facial nuclei, and the number of P2Y12-expressing cells increased following facial nerve axotomy. Selective expression of P2Y12 mRNA in microglia was confirmed by double-label in situ hybridization and immunohistochemistry with antibodies against NeuN and Iba1 as an immunohistochemical marker for neurons and microglia, respectively. Hardly any P2Y12 mRNA was detected in macrophages obtained from the spleen and abdominal cavity, which share many surface molecules with microglia.

Natural killer cells do not mediate facial motoneuron survival after facial nerve transection

The goal of the current study was to determine if natural killer (NK) cells mediate facial motoneuron (FMN) survival following injury. Wild-type (WT), perforin/recombinase activating gene-2 knockout (pfp/RAG-2 KO), and common gamma-chain (gammac)/RAG-2 KO mice received a right facial nerve axotomy. In WT mice, FMN survival was 86+/-1.0% relative to the contralateral control side. In contrast, pfp/RAG-2 and gammac/RAG-2 KO mice exhibited significant decreases in FMN survival ( approximately 20% and approximately 30%, respectively), relative to WT. Reconstitution of pfp/RAG-2 and gammac/RAG-2 KO mice with normal NK cells alone, failed to restore FMN survival levels to those of WT, but did restore functional lytic activity against YAC-1 cells. Reconstitution of pfp/RAG-2 and gammac/RAG-2 KO mice with splenocytes, and pfp/RAG-2 KO mice with CD4+ T-lymphocytes alone or in combination with NK cells, restored FMN survival levels to those of WT. Thus, NK cells appear to not be a component of immune cell-mediated rescue of motoneurons from axotomy induced cell death.

CD4+ T, but not CD8+ or B, lymphocytes mediate facial motoneuron survival after facial nerve transection

The capacity of facial motor neurons (FMN) to survive injury and successfully regenerate is substantially compromised in immunodeficient mice, which lack T and B lymphocytes (). The goal of the present study was to determine which T cell subset (CD4+ and/or CD8+), and whether the B lymphocyte, is involved in FMN survival after nerve injury. All mice were subjected to a right facial nerve axotomy, with the left (uncut) side serving as an internal control. FMN survival, of the right (cut) side, was measured 4 weeks post-operative, and expressed as a percentage of the left (uncut) control side. FMN survival in wild-type mice was 86%+/-1.5. In contrast, FMN survival in CD4 KO mice was 60%+/-2.0. Reconstitution of either CD4 KO mice, or recombinase activating gene-2 knockout (RAG-2 KO) mice (which lack functional T and B cells) with CD4+ T cells alone restored FMN survival to wild-type levels (85%+/-1.2 and 84%+/-2.5, respectively). There was no difference in FMN survival between wild-type, CD8 KO and MmuMT (B cell deficient) mice. Reconstitution of RAG-2 KO mice with CD8+ T cells alone, or B cells alone, failed to restore FMN survival levels (65%+/-1.5 and 63%+/-1.0, respectively). It is concluded that, of the population of FMN that do not survive injury, CD4+ T lymphocytes, but not CD8+ T lymphocytes or B cells, mediate FMN survival after peripheral nerve injury.

Ciliary neurotrophic factor is an early lesion-induced retrograde signal for axotomized facial motoneurons

To investigate the involvement of ciliary neurotropic factor (CNTF) in the postlesional response of motoneurons, we studied the activation of STAT3 signaling, the main signal transduction pathway of CNTF-like cytokines, in the facial nucleus of wildtype and CNTF-deficient mice following peripheral nerve transection. As shown by immunocytochemistry and immunoblot analysis, phosphorylation and nuclear translocation of STAT3 was maximally induced within 12 h postlesion in motoneurons of the ipsilateral facial nucleus of wildtype mice and is maintained for at least 3 days. In CNTF(-/-) mouse mutants, activation of STAT3 signaling was delayed by 10-12 h. Application of CNTF to the transected nerve restored rapid STAT3 activation in CNTF-deficient animals, whereas application of colchicine suppressed STAT3 signaling in wildtype mice for at least 24 h. These results identify CNTF as an early retrograde signal in axotomized facial motoneurons by showing that CNTF released at the lesion site is responsible for the initial induction of STAT3 signaling. Other cytokines like leukemia inhibitory factor obviously become active at later time points.

Receptor protein tyrosine phosphatase sigma inhibits axonal regeneration and the rate of axon extension

Transgenic mice lacking receptor protein tyrosine phophatase-sigma (RPTPsigma), a type IIa receptor protein tyrosine phosphatase, exhibit severe neural developmental deficits. Continued expression of RPTPsigma in the adult suggests that it plays a functional role in the mature nervous system. To determine if RPTPsigma might influence axonal regeneration, the time course of regeneration following facial nerve crush in wild-type and RPTPsigma (-/-) mice was compared. Mice lacking RPTPsigma exhibited an accelerated rate of functional recovery. Immunocytochemical examination of wild-type neurons in cell culture showed RPTPsigma protein in the growth cone. To determine if RPTPsigma affects the ability of a neuron to extend an axon, the rate of axon growth in neuronal cultures derived from wild-type and RPTPsigma (-/-) embryonic mice was compared. RPTPsigma did not affect the rate of axon initiation, but the rate of axon extension is enhanced in neurons obtained from RPTPsigma (-/-) mice. These findings indicate that RPTPsigma slows axon growth via a mechanism intrinsic to the neuron and identify a role for RPTPsigma regulating axonal regeneration by motoneurons.

Induction of NMDA and GABAA receptor-mediated Ca2+ oscillations with KCC2 mRNA downregulation in injured facial motoneurons

To clarify the changes that occur in gamma-aminobutyric acid type A (GABA(A)) receptor-mediated effects and contribute to alterations in the network activities after neuronal injury, we studied intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics in a rat facial-nerve-transection model. In facial motoneurons, an elevation of the resting [Ca(2+)](i), GABA-mediated [Ca(2+)](i) transients, enhancement of the glutamate-evoked [Ca(2+)](i) increases, and spontaneous [Ca(2+)](i) oscillations were induced by axotomy. All these axotomy-induced modifications were abolished by the GABA(A)-receptor antagonist bicuculline and N-methyl-d-aspartate (NMDA)-receptor antagonist d(-)-2-amino-5-phosphonopentanoic acid. A downregulation of K(+)-Cl(-) cotransporter (KCC2) mRNA, an increase in intracellular Cl(-) concentration ([Cl(-)](i)), and transformation of GABAergic hyperpolarization to depolarization were also induced by axotomy. We suggest that in axotomized neurons KCC2 downregulation impairs Cl(-) homeostasis and makes GABA act depolarizing, resulting in endogenous GABA inducing [Ca(2+)](i) oscillations via facilitation of NMDA-receptor activation. Such GABA(A)-receptor-mediated [Ca(2+)](i) oscillations may play a role in neural survival and regeneration.

Role of androgens in the regulation of urotensin II precursor mRNA expression in the rat brainstem and spinal cord

It has been reported that both urotensin II precursor (pro UII) mRNA and androgen receptors (ARs) are highly expressed in rat brainstem motor nuclei and ventral horn of the spinal cord. In order to determine the possible involvement of androgens in regulation of pro UII mRNA expression, we have studied the co-localization of pro UII mRNA and AR immunoreactivity and the effect of castration and dihydrotestosterone (DHT) replacement therapy on pro UII mRNA in the rat facial nucleus and ventral horn of the spinal cord. By in situ hybridization, pro UII mRNA was only detected in motoneurons in both the facial nucleus and ventral horn of the spinal cord. Double-labelling studies revealed that the vast majority (over 95%) of motoneurons immunostained for AR also expressed pro UII mRNA in both areas examined. Three weeks after castration, pro UII mRNA expression, as measured by semi-quantitative in situ hybridization, was increased by 17% and 58% in the ventral horn of the spinal cord and the facial nucleus, respectively. The administration of DHT completely prevented the stimulating effect of castration. These results indicate that circulating androgens are exerting a down-regulation of pro UII expression possibly by a direct action at the level of motoneurons. The physiological relevance of these new findings remains to be fully explored.

IL-2 gene knockout affects T lymphocyte trafficking and the microglial response to regenerating facial motor neurons

Following facial nerve axotomy in mice, T cells cross the intact blood-brain barrier (BBB), home to nerve cell bodies in the facial motor nucleus (FMN), and augment neuroregenerative processes. The pivotal T cell immunoregulatory cytokine, IL-2, appears to have bidirectional effects on neuronal and microglial cell function, suggesting rival hypotheses that IL-2 could either enhance or disrupt processes associated with regeneration of axotomized facial motor neurons. We tested these competing hypotheses by comparing the effect of facial nerve axotomy on C57BL/6-IL-2(-/-) knockout and C57BL/6-IL-2(+/+) wild-type littermates. Since IL-2 may also be produced endogenously in the brain, we also sought to determine whether differences between the knockout and wild-type mice were attributable to loss of IL-2 gene expression in the CNS, loss of peripheral sources of IL-2 and the associated effects on T cell function, or a combination of these factors. To address this question, we bred novel congenic mice with the SCID mutation (mice lacking T cell derived IL-2) that were homozygous for either the IL-2 knockout or wild-type gene alleles (C57BL/6scid-IL-2(-/-) and C57BL/6scid-IL-2(+/+) littermates, respectively). Groups were assessed for differences in (1) T lymphocytes entering the axotomized FMN; (2) perineuronal CD11b(+) microglial phagocytic clusters, a measure of motor neuron death; and (3) activated microglial cells as measured by MHC-II positivity. C57BL/6-IL-2(-/-) knockout mice had significantly higher numbers of T cells and lower numbers of activated MHC-II-positive microglial cells in the regenerating FMN than wild-type littermates, although the number of CD11b(+) phagocytic microglia clusters did not differ. Thus, despite the significant impairment of T cell function known to be associated with loss of peripheral IL-2, the increased number of T cells entering the axotomized FMN appears to have sufficient activity to support neuroregenerative processes. Congenic C57BL/6scid-IL-2(-/-) knockout mice had lower numbers of CD11b(+) microglial phagocytic clusters than congenic C57BL/6scid-IL-2(+/+) wild-type littermates, suggesting that loss of the IL-2 gene in the CNS (and possibly the loss of other unknown sources of the gene) enhanced neuronal regeneration. Further study of IL-2's complex actions in neuronal injury may provide greater understanding of key variables that determine whether or not immunological processes in the brain are proregenerative.

Neuregulin-1 Isoforms Are Differentially Expressed in the Intact and Regenerating Adult Rat Nervous System

Our knowledge on Neuregulin-1 (Nrg-1) during development of the nervous system is increasing rapidly, but little is known about Nrg-1-ErbB signaling in the adult brain. Nrg-1 is involved in determination, proliferation, differentiation, and migration of neurons and glial cells in the developing brain. In the peripheral nervous system, Nrg-1 signaling is required for Schwann cell differentiation and myelination, and establishment of neuromuscular junctions (NMJs). Multiple alternative splicing of Nrg-1 was shown, but correlation of its structural and functional diversity was rarely addressed. Therefore, we investigated the expression of Nrg-1 isoforms in the rat brain and brain-derived cell types, and their involvement in regeneration of the adult brain, using immunohistochemistry, in situ hybridization, and semiquantitative RT-PCR. We found expression of at least 12 distinct Nrg-1 isoforms in the brain and altered expression of several isoforms in the facial motor nucleus after peripheral transection of the seventh cranial nerve. An upregulation of Nrg-1 type-I mRNA, probably type- I-alpha, was observed in reactive astrocytes of the facial nucleus 1 d postaxotomy. Nrg-1 type-III and the splice variants beta1 and beta5 are dramatically downregulated in axotomized motoneurons, which lack contact to their target tissue. Baseline expression levels were reestablished when the first axons reached the facial muscles and reformed NMJs. Nrg-1-beta1 and -beta5 might act in maintenance of NMJs. The splice variants beta2 and beta4 display an initial downregulation of mRNA levels, followed by an increase during the period of axon remyelination. Thus, Nrg- 1-beta2 and -beta4 might be involved in myelination.