Axonal outgrowth and pathfinding

Building, Breaking, and Repairing Neuromuscular Synapses

A coordinated and complex interplay of signals between motor neurons, skeletal muscle cells, and Schwann cells controls the formation and maintenance of neuromuscular synapses. Deficits in the signaling pathway for building synapses, caused by mutations in critical genes or autoantibodies against key proteins, are responsible for several neuromuscular diseases, which cause muscle weakness and fatigue. Here, we describe the role that four key genes, Agrin, Lrp4, MuSK, and Dok7, play in this signaling pathway, how an understanding of their mechanisms of action has led to an understanding of several neuromuscular diseases, and how this knowledge has contributed to emerging therapies for treating neuromuscular diseases.

Co-firing of levator palpebrae and masseter muscles links the masticatory and oculomotor system in humans

Simultaneous co-firing of the levator palpebrae (LP) and pterygoid muscles were recorded in Marcus Gann Syndrome (MGS) patients in early clinical studies. “Release hypothesis” proposed an intrinsic masticatory oculomotor neural circuit and this kind circuit, which, however, has been observed only in amphibian. On the other hand, congenital miswiring hypothesis has overwhelmed other interpretations. However, the same phenomenon visualized in MGS cases was unveiled in human subjects without any sign of congenital oculomotor disorder. To further study co-firing of the upper eyelid and jaw muscles, we applied non-invasive EMG recording of the upper eyelid and ipsilateral masseter muscle belly in nine healthy volunteers. LP activity was determined initially by looking upward and active retraction of upper eyelid with head fixed. Then, dual channel inputs from upper eyelid and masseter muscle was recorded during tooth occlusion motivated by isometric masseter muscle contraction without jaw and face moving. The EMG recorded from upper eyelid when the subjects retracted eyelid with head fixed exhibited the same pattern as that collected during tooth occlusion, but the pattern was completely different from EMG of active eye closure. This reflects tooth occlusion evoked LP activity. Then, simultaneous co-firing of the LP and masseter muscle was recorded simultaneously during tooth occlusion without jaw movement. Finally, the aforementioned co-firing was recorded when the subjects conducted rhythmic occlusion and synchronous EMG from both muscles was acquired. In conclusions, humans may also have an intrinsic masticatory oculomotor circuit and release hypothesis may apply, at least, to some cases of MGS.

Prenatal smoking and genetic risk: examining the childhood origins of externalizing behavioral problems

An ever-growing body of research has begun to focus closely on the role of prenatal smoke exposure in the development of conduct problems in children. To this point, there appears to be a correlation between prenatal nicotine exposure and behavioral problems. We build on this prior research by examining the coalescence of prenatal smoke exposure and genetic risk factors in the prediction of behavior problems. Specifically, the current study analyzed data from a nationally representative sample of twin pairs collected during early childhood. Our findings suggested that an interaction existed between prenatal smoke exposure and genetic risk factors which corresponded to increased risk of behavior problems. These findings provide evidence of a gene-environment interaction, in that prenatal smoke exposure conditioned the influence of genetic risk factors in the prediction of aggressive behavior. Interestingly, the association between genetic risk and prenatal smoking was sex-specific, and only reached statistical significance in females. Given the nature of our findings, it may shed light on why heterogeneity exists concerning the relationship between prenatal smoke exposure and externalizing behavioral problems in children.

Prenatal tobacco exposure: developmental outcomes in the neonatal period

Smoking during pregnancy is a persistent public health problem that has been linked to later adverse outcomes. The neonatal period--the first month of life--carries substantial developmental change in regulatory skills and is the period when tobacco metabolites are cleared physiologically. Studies to date mostly have used cross-sectional designs that limit characterizing potential impacts of prenatal tobacco exposure on the development of key self-regulatory processes and cannot disentangle short-term withdrawal effects from residual exposure-related impacts. In this study, pregnant participants (N = 304) were recruited prospectively during pregnancy, and smoking was measured at multiple time points, with both self-report and biochemical measures. Neonatal attention, irritable reactivity, and stress dysregulation were examined longitudinally at three time points during the first month of life, and physical growth indices were measured at birth. Tobacco-exposed infants showed significantly poorer attention skills after birth, and the magnitude of the difference between exposed and nonexposed groups attenuated across the neonatal period. In contrast, exposure-related differences in irritable reactivity largely were not evident across the 1st month of life, differing marginally at 4 weeks of age only. Third-trimester smoking was associated with pervasive, deleterious, dose–response impacts on physical growth measured at birth, whereas nearly all smoking indicators throughout pregnancy predicted level and growth rates of early attention. The observed neonatal pattern is consistent with the neurobiology of tobacco on the developing nervous system and fits with developmental vulnerabilities observed later in life.

Prenatal and perinatal morbidity in children with Tourette syndrome and attention-deficit hyperactivity disorder

OBJECTIVE

Tourette syndrome (TS) and attention-deficit hyperactivity disorder (ADHD) are frequently seen in combination, though the cause of comorbidity is uncertain. Low birth weight is a known risk factor for ADHD. The objective of the study was to assess the association between pre- and perinatal morbidity and the comorbid diagnosis of ADHD in children with TS.

METHOD

A nested case-control study of children evaluated for TS at a subspecialty clinic was performed. Cases were defined as children with TS and ADHD; controls had TS without ADHD. Exposure to pre- and perinatal morbidity was assessed using demographic information booklets completed by parents before the diagnostic interview.

RESULTS

Three hundred fifty-three children were included, 181 cases and 172 controls. Children with TS and ADHD had a greater odds of exposure to low birth weight status, prematurity, breathing problems, and maternal smoking compared with children with TS only. A multivariable logistic regression model found adjusted odds ratios for the comorbid diagnosis of TS and ADHD of 2.74 (95% CI 1.03-7.29, p = .04) in children born low birth weight, and of 2.43 (95% CI 1.23-4.82, p = .01) for children exposed to maternal smoking.

CONCLUSION

In children with TS, there is a greater odds of comorbid ADHD in children born with low birth weight or with exposure to maternal smoking. The commonality of risk factors for ADHD only and tic-related ADHD supports a common underlying neurobiology. Women with fetuses at risk for TS should avoid smoking and preventable causes of low birth weight to minimize the risk of comorbid ADHD.

Early motor neuron pool identity and muscle nerve trajectory defined by postmitotic restrictions in Nkx6.1 activity

The fidelity with which spinal motor neurons innervate their limb target muscles helps to coordinate motor behavior, but the mechanisms that determine precise patterns of nerve-muscle connectivity remain obscure. We show that Nkx6 proteins, a set of Hox-regulated homeodomain transcription factors, are expressed by motor pools soon after motor neurons leave the cell cycle, before the formation of muscle nerve side branches in the limb. Using mouse genetics, we show that the status of Nkx6.1 expression in certain motor neuron pools regulates muscle nerve formation, and the pattern of innervation of individual muscles. Our findings provide genetic evidence that neurons within motor pools possess an early transcriptional identity that controls target muscle specificity.

A role for neurotrophin-3 in targeting developing cholinergic axon projections to cerebral cortex

This study examined the relationship between expression of neurotrophin-3 (NT-3) and the ingrowth of cholinergic axonal projections in cerebral cortex. Patterns of expression of NT-3 (defined by beta-galactosidase reporter expression in heterozygous offspring of transgenic NT-3(lacZneo/+) mice) revealed that limbic cortical regions (including frontal, cingulate, and insular cortex, as well as the dentate gyrus) express NT-3 and that these cortical regions receive early and relatively dense cholinergic axons (stained for acetylcholinesterase, AChE). Using the dentate gyrus as a model system, studies revealed that expression of the NT-3 reporter parallels, and precedes by approximately 2 days, the ingrowth of AChE positive cholinergic axons. Studies of forebrain organotypic slice cultures demonstrate that basal forebrain-derived cholinergic axons extend into cortical regions in a pattern that mimics the pattern of expression of the NT-3 reporter. Similarly, chimeric co-cultures, combining wild type septum with a slice of hippocampus from heterozygous NT-3(lacZneo/+) mice, demonstrate that cholinergic axons grow into regions of the dentate gyrus that express the NT-3 reporter. Hemisphere slice cultures made from NT-3 knockout mice reveal cholinergic axonal growth into cortex, but these axons do not form the regional pattern characteristic of slice cultures made from wild type or heterozygous NT-3(lacZneo/+) mice. Further, chimeric co-cultures made using slices of wild type septum combined with slices of hippocampus from NT-3 knockout mice demonstrate robust cholinergic axonal growth into the hippocampus, but the cholinergic axons do not form the characteristic preterminal pattern associated with the dentate gyrus. Slice cultures from limbic cortical tissue from the NT-3 null mice do not display exaggerated levels of cell death. In aggregate, these data support the hypothesis that expression of NT-3 by cortical neurons serves to attract basal forebrain cholinergic projections to their target cells in cerebral cortex.

Patterns of afferent projections to the dentate gyrus studied in organotypic co-cultures

Cholinergic axons originating from the septum form a characteristic layer of preterminal axons and apparent termination in the molecular layer of the hippocampal dentate gyrus. The present study explored the specificity of this characteristic axonal pattern, through the use of organotypic slice co-cultures. Slices of hippocampus were co-cultured with a slice from one of a variety of other potential sources of afferents, and the afferent axons were labeled histochemically or immunocytochemically to determine which afferents distribute within the dentate molecular layer in a pattern similar to that formed by septal cholinergic projections. Acetylcholinesterase (AChE) histochemistry demonstrated that cholinergic axons from septum, substantia innominata, and striatum all consistently targeted the inner molecular layer of the dentate gyrus. AChE-labeled cholinergic axons from dorsal lateral pontine tegmentum and from spinal cord sometimes formed this pattern, while axons from the habenula failed to extend into the dentate gyrus. Immunocytochemically identified monoaminergic axons from the substantia nigra, locus coeruleus, and raphe extended into co-cultured hippocampus; each of these afferent systems displayed a prominent axonal plexus within the hilus of the dentate, but only the raphe axons projected prominently to the molecular layer. These data demonstrate that the molecular layer of the dentate gyrus provides an attractive target zone for some cholinergic and monoaminergic afferents, but not all. Commonalities between neuronal populations that preferentially project to the molecular layer in vitro may offer clues regarding the axon guidance mechanisms that normally direct cholinergic axons to target sites in the dentate gyrus molecular layer.

Basal forebrain cholinergic cell attachment and neurite outgrowth on organotypic slice cultures of hippocampal formation

Distributions of somata and neurites of cholinergic neurons were studied after seeding dissociated cells onto organotypic slice cultures. Slice cultures were made from hippocampal formation and adjacent cortical regions from rats or mice. Dissociated cell suspensions of basal forebrain tissue from rat or mouse fetuses were seeded onto the slice cultures. Combined cultures were maintained for 1-21 days in vitro. Cultures processed for acetylcholinesterase (AChE) histochemistry demonstrated non-random patterns of cholinergic cells and their neurites. Labeled cells appeared most frequently in the molecular layer of the dentate gyrus, and in the deeper layers of cortical regions adjacent to the hippocampus. Neurites extending from these labeled cells appeared to target the dentate molecular layer and the cortical subplate layer. By 4 days in vitro, AChE-positive basal forebrain cells display several short and thick neurites that appear to be dendrites, and one long process that appears to be an axon. By 5 days in vitro, dendrites are well developed; by 7 days the presumed axon has extended widely over the cortical target zone. These neurites are maintained through 3 weeks in culture. Distributions of cells varied with the age of the slice. AChE-labeled cells were not seen overlying hippocampal tissue when dissociated cells were seeded on slice cultures made from day 0 rats, but a few labeled cells were seen when seeded on slices from day 2 rats. Clear non-random patterns of labeled cells and neurite outgrowth were seen on slice cultures from day 5 or older pups. The non-random distribution seen with AChE-positive neurons was not seen using other techniques that labeled all cells (non-selective fluorescent labels) or all neurons; these techniques resulted in labeled cells scattered apparently homogenously across the slice culture.These studies demonstrate a non-random pattern of attachment or differentiation of basal forebrain cholinergic neurons when these cells are seeded onto cultured cortical slices; this pattern mimics the normal patterns of basal forebrain cholinergic projections to these cortical regions. These data suggest that the factors that normally guide basal forebrain-derived cholinergic axons to their target cells in vivo are present and detectable in this model system.

Evidence for target preferences by cholinergic axons originating from different subdivisions of the basal forebrain

Possible target preferences of basal forebrain cholinergic neurons were studied in organotypic slice cultures. Cholinergic neurons in slices of medial septum or substantia innominata send axons into both hippocampus and neocortex when co-cultured together. However, septal cholinergic axons course through adjacent slices of neocortex to reach and branch densely in slices of hippocampus, but septal axons seldom grow beyond adjacent hippocampal tissue to reach neocortex. In contrast, cholinergic axons from substantia innominata commonly grow through hippocampus to reach neocortex, and also grow through neocortex to reach hippocampus, with similar branching densities in each target. The greater density of septal axonal branches in hippocampus than in neocortex suggests a preference of septal axons for the hippocampal target.

Arrival of afferents and the differentiation of target neurons: studies of developing cholinergic projections to the dentate gyrus

This study examined the relationship between the development of cholinergic axons originating from the septum and a group of their target cells, the granule cells of the dentate gyrus of the rat. Acetylcholinesterase histochemistry was used to identify septal cholinergic afferents to the dentate gyrus; parallel studies used anterograde movement of a carbocyanine dye to label the septal projections. Septal cholinergic axons are present in the molecular layer of the internal blade of the dentate gyrus shortly after birth, but these axons do not reach the external blade until several days later. Results demonstrate that acetylcholinesterase positive septal axons grow into the external blade of the dentate gyrus only after the recently generated granule cells have coalesced to form a clearly defined layer. Results from studies using in situ hybridization techniques demonstrate that dentate gyrus granule cells express messenger RNAs for brain derived neurotrophic factor and for neurotrophic factor 3 shortly after formation of the granule cell layer. Ingrowth of septal cholinergic axons follows two days after the formation of the external blade of the dentate gyrus and the expression of neurotrophin messenger RNAs by the dentate granule cells. These data support the hypothesis that target cell development is a prerequisite for attracting the ingrowth of septal afferent axons.

Behavioral and neural consequences of prenatal exposure to nicotine

OBJECTIVE

To review evidence for the neurodevelopmental effects of in utero exposure to nicotine. Concerns about long-term cognitive and behavioral effects of prenatal exposure to nicotine arise from reports of increased rates of disruptive behavioral disorders in children whose mothers smoked during pregnancy. The relatively high rate of tobacco smoking among pregnant women (25% of all pregnancies in the U.S.) underlines the seriousness of these concerns.

METHOD

This review examines the largest and most recent epidemiological and clinical studies that investigated the association of prenatal nicotine exposure with health, behavioral, and cognitive problems. Because of the numerous potential confounding variables in human research, findings from animal studies, in which environmental factors are strictly controlled, are also discussed. Finally, neural and molecular mechanisms that are likely to underlie neurodevelopmental disruptions produced by prenatal nicotine exposure are outlined.

RESULTS

A dose-response relationship between maternal smoking rates and low birth weight (potentially associated with lower cognitive ability) and spontaneous abortion is consistently found, whereas long-term developmental and behavioral effects in the offspring are still controversial, perhaps because of the difficulty of separating them from other genetic and environmental factors. Despite the wide variability of experimental paradigms used in animal studies, common physical and behavioral effects of prenatal exposure to nicotine have been observed, including low birth weight, enhanced locomotor activity, and cognitive impairment. Finally, disturbances in neuronal pathfinding, abnormalities in cell proliferation and differentiation, and disruptions in the development of the cholinergic and catecholaminergic systems all have been reported in molecular animal studies of in utero exposure to nicotine.

CONCLUSIONS

Prenatal exposure to nicotine may lead to dysregulation in neurodevelopment and can indicate higher risk for psychiatric problems, including substance abuse. Knowledge of prenatal exposure to nicotine should prompt child psychiatrists to closely monitor at-risk patients.

Motor axon pathfinding in the peripheral nervous system

Functional motor performance is dependent upon the correct assemblage of neural circuitry, a process initiated during embryonic development. How is the complicated neural circuitry that underlies functional behavior formed? During early stages of development, motor neurons extend their axons in a precise manner to their target destinations where they form fine synaptic connections. This process is not random but rather, highly stereotyped and specific. Results of recent studies indicate that positive and negative molecules influence particular steps in the navigation of motor axons to their targets. These molecules include, but are not limited to, members of the Semaphorin family and their receptors, Neuropilins and Plexins, Slits and their Robo receptors, members of the Eph family, extracellular matrix molecules, Hepatocyte Growth Factor/Scatter Factor, peanut agglutinin-binding glycoproteins, and neural cell adhesion molecule. The developing avian peripheral nervous system has served as an excellent model system for many years for studies of the basic cellular interactions that underlie motor axon pathfinding. The principal advantage for the experimental use of the avian embryo is the ease of access to early developmental events. Fine microsurgical manipulations, difficult at best in mouse embryonic development, are readily accomplished in avian embryos and have provided a powerful approach to unraveling the cellular interactions that govern motor axon pathfinding. These approaches, combined in recent years with molecular biology, have begun to produce critical insights into the mechanisms that sculpt cellular architecture during neural development.

The "waiting period" of sensory and motor axons in early chick hindlimb: its role in axon pathfinding and neuronal maturation

During embryonic development motor axons in the chick hindlimb grow out slightly before sensory axons and wait in the plexus region at the base of the limb for approximately 24 hr before invading the limb itself (Tosney and Landmesser, 1985a). We have investigated the role of this waiting period by asking, Is the arrest of growth cones in the plexus region a general property of both sensory and motor axons? Why do axons wait? Does eliminating the waiting period affect the further development of motor and sensory neurons? Here we show that sensory axons, like motor axons, pause in the plexus region and that neither sensory nor motor axons require cues from the other population to wait in or exit from the plexus region. By transplanting older or younger donor limbs to host embryos, we show that host axons innervate donor limbs on a schedule consistent with the age of the grafted limbs. Thus, axons wait in the plexus region for maturational changes to occur in the limb rather than in the neurons themselves. Both sensory and motor axons innervate their appropriate peripheral targets when the waiting period is eliminated by grafting older donor limbs. Therefore, axons do not require a prolonged period in the plexus region to sort out and project appropriately. Eliminating the waiting period does, however, accelerate the onset of naturally occurring cell death, but it does not enhance the development of central projections or the biochemical maturation of sensory neurons.

Coordinate roles for LIM homeobox genes in directing the dorsoventral trajectory of motor axons in the vertebrate limb

Motor neurons extend axons along specific trajectories, but the molecules that control their pathfinding remain poorly defined. We show that two LIM homeodomain transcription factors, Lim1 and Lmx1b, control the initial trajectory of motor axons in the developing mammalian limb. The expression of Lim1 by a lateral set of lateral motor column (LMC) neurons ensures that their axons select a dorsal trajectory in the limb. In a complementary manner, the expression of Lmx1b by dorsal limb mesenchymal cells controls the dorsal and ventral axonal trajectories of medial and lateral LMC neurons. In the absence of these two proteins, motor axons appear to select dorsal and ventral trajectories at random. Thus, LIM homeodomain proteins act within motor neurons and cells that guide motor axons to establish the fidelity of a binary choice in axonal trajectory.

Use of a reverse transcriptase-polymerase chain reaction assay to analyze allele-specific expression in individual hippocampal neurons

We report here a single-cell RT-PCR assay for allele-specific gene expression that can be used to probe for somatic variability within the CNS. Such variability, arising from epigenetic (nonmutational) events or somatic mutation early in development, may give clues as to clonal origin and may also affect the inheritance pattern of some CNS disorders. As a model system, we used reciprocal F1 hybrids of the cross Mus musculus C57BL/6J x Mus musculus castaneus. RNA was isolated from individual dissociated pyramidal neurons from hippocampi of F1 pups. For each gene of interest, single base polymorphisms were identified between the two parental strains by automated sequencing of RT-PCR products. Allele-specific expression was then analyzed by means of the previously described quantitative RT-PCR single nucleotide primer extension (SNuPE) assay (Singer-Sam et al., PCR Methods Appl. 1:160-163, 1992). Individual neurons showed monoallelic expression of the two control genes, X-linked Rps4, and the imprinted gene Snrpn; in contrast expression of Ncam and F3cam, coding for neural cell adhesion molecules, was found to be biallelic.

New observations on the development of the gonadotropin-releasing hormone system in the mouse

In ongoing efforts to study the ontogeny of gonadotropin-releasing hormone (GnRH) neurons, we serendipitously observed that increasing times of incubation in antibodies enhanced signal detection. Here, we describe significant differences in the early migration pattern, population dynamics, and growth cone morphology from published reports. The first immunoreactive GnRH cells were detected in the mouse at E10.75 (7.6 +/- 2.8 cells; morning after mating = E0.5), prior to the closure of the olfactory placode. Although half of these cells were in the medial wall of the olfactory pit, the other half had already initiated their migration, and approximately one quarter had reached the telencephalic vesicle. Although the migratory pattern of the GnRH cells after E11.00 was identical to that described previously, these earliest migrating cells traveled singly rather than in cords, with some reaching the presumptive preoptic area (posterior to the ganglionic eminence) by E11.75. The number of GnRH cells increased significantly (p < 0.05) to 777 +/- 183 at E11.75 and peaked at 1949.6 +/- 161.6 (p < 0.05) at E12.75. The adult population was approximately 800 cells distributed between the central nervous system (CNS) and the nasal region. Hence, the population of GnRH neurons during early development is much larger than previously appreciated; mechanisms for its decline are discussed. Neuritic extensions on the earliest GnRH neurons are short (30-50 microm) and blunt and may represent the leading edge of the moving cell. By E12.75, GnRH axons in the CNS had a ribboned or beaded morphology and increasingly more complex growth cones were noted from this time until the day of birth. The most complex growth cones were associated with apparent choice points along the axons' trajectory. By E13.75, GnRH axons were seen at the presumptive median eminence in all animals, and it was at this stage that the axons began to branch profusely. Branching, as well as the presence of growth cones, continued post-natally. These results provide further insights into the pathfinding mechanisms of GnRH cells and axons.

Specificity of attachment and neurite outgrowth of dissociated basal forebrain cholinergic neurons seeded on to organotypic slice cultures of forebrain

Development and differentiation of basal forebrain-derived cholinergic neurons were studied using a new technique that combines dissociated cell cultures with organotypic slice cultures. Slices of cerebral cortex or entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic cultures on membranes. Dissociated cell suspensions of basal forebrain tissue, taken from rat or mouse fetuses at gestational day 15-17, were seeded on to the slice cultures. Combined cultures were maintained for two to 14 days in vitro. Cultures processed for acetylcholinesterase histochemical staining demonstrated that stained neurons display regional variation in attachment to the slice, with most attachment occurring on cortex and with no detectable attachment on the caudate-putamen. Regional differences in attachment occur between cortical areas, with medial (cingulate) cortex showing much denser cell attachment than lateral (parietal) cortex, and across cortical layers, with layer I and deep layers showing more attachment than middle cortical layers. Similar patterns were observed on slices from rat brain irrespective of whether rat or mouse dissociated cells were used. Tyrosine hydroxylase-stained dissociated cells from ventral midbrain displayed a different pattern of attachment, with prominent attachment to the caudate putamen and less apparent specificity of regional and cortical laminar attachment. Little evidence of neurite outgrowth occurred during the first two days in vitro, but by four days, acetylcholinesterase-positive basal forebrain cells displayed several short and thick neurites that appeared to be dendrites, and one long process that appeared to be an axon. By seven days in vitro, dendrites are well developed and the presumed axon has extended branches over wide areas of cortex. These studies revealed several different types of cell-tissue interaction. The degree of cell growth and differentiation ranged from robust growth when dissociated cells were seeded on to slice cultures of normal target tissue, to apparently no attachment or growth when cells were seeded on to non-target tissue. This combined technique appears to be a useful method for studies of specificity of cell attachment and patterns of neurite outgrowth.

Hair cycle-dependent plasticity of skin and hair follicle innervation in normal murine skin

The innervation of normal, mature mammalian skin is widely thought to be constant. However, the extensive skin remodeling accompanying the transformation of hair follicles from resting stage through growth and regression back to resting (telogen-anagen-catagen-telogen) may also be associated with alteration of skin innervation. We, therefore, have investigated the innervation of the back skin of adolescent C57BL/6 mice at various stages of the depilation-induced hair cycle. By using antisera against neuronal (protein gene product 9.5 [PGP 9.5], neurofilament 150) and Schwann cell (S-100, myelin basic protein) markers, as well as against neural cell adhesion molecule (NCAM) and growth-associated protein-43 (GAP-43), we found a dramatic increase of single fibers within the dermis and subcutis during early anagen. This was paralleled by an increase in the number of anastomoses between the cutaneous nerve plexuses and by distinct changes in the nerve fiber supply of anagen vs. telogen hair follicles. The follicular isthmus, including the bulge, the seat of epithelial follicle stem cells, was found to be the most densely innervated skin area. Here, a defined subpopulation of nerve fibers increased in number during anagen and declined during catagen, accompanied by dynamic alterations in the expression of NCAM and GAP-43. Thus, our study provides evidence for a surprising degree of plasticity of murine skin innervation. Because hair cycle-associated tissue remodeling evidently is associated with tightly regulated sprouting and regression of nerve fibers, hair cycle-dependent alterations in murine skin and hair follicle innervation offer an intriguing model for studying the controlled rearrangement of neuronal networks in peripheral tissues under physiological conditions.

Activation of alpha7 nicotinic acetylcholine receptor promotes survival of spinal cord motoneurons

Spinal cord motoneurons (MNs) undergo a process of cell death during embryonic development and are the target of lethal acquired or inherited disorders, such as the amyotrophic lateral sclerosis. Therefore, the identification of mechanisms leading to MN survival is of crucial importance. Elevations in intracellular Ca2+ promote chicken MN survival during the embryonic period of naturally occurring cell death. We have recently demonstrated that the alpha7 nicotinic acetylcholine receptor (nAChR) mediates significant increases in free Ca2+ concentration at membrane potentials at which other pathways for Ca2+ influx are inactive. Although it is possible that Ca2+ influx through alpha7 nAChR promotes cell survival, the relation between alpha7 nAChR activation, cytosolic free Ca2+ and mammalian spinal cord MN survival has not been established. In the present study we have now demonstrated that Ca2+ influx through the alpha7-subunit is sufficient to rescue a significant number of cultured spinal cord MNs from programmed cell death induced by trophic factor deprivation. This is the first demonstration that neuronal nAChRs are involved in the regulation of MN survival.

Laser ablation of Drosophila embryonic motoneurons causes ectopic innervation of target muscle fibers

We have tested the effects of neuromuscular denervation in Drosophila by laser-ablating the RP motoneurons in intact embryos before synaptogenesis. We examined the consequences of this ablation on local synaptic connectivity in both 1st and 3rd instar larvae. We find that the partial or complete loss of native innervation correlates with the appearance of alternate inputs from neighboring motor endings and axons. These collateral inputs are found at ectopic sites on the denervated target muscle fibers. The foreign motor endings are electrophysiologically functional and are observed on the denervated muscle fibers by the 1st instar larval stage. Our data are consistent with the existence of a local signal from the target environment, which is regulated by innervation and influences synaptic connectivity. Our results show that, despite the stereotypy of Drosophila neuromuscular connections, denervation can induce local changes in connectivity in wild-type Drosophila, suggesting that mechanisms of synaptic plasticity may also be involved in normal Drosophila neuromuscular development.

Cholinergic innervation of cerebral cortex in organotypic slice cultures: sustained basal forebrain and transient striatal cholinergic projections

Slices of entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic slice cultures. Basal forebrain cholinergic neurons, identified by histochemical staining for acetylcholinesterase, develop axons that grow rapidly into cerebral cortex. Ingrowth occurs by two routes: some axons course laterally from the basal forebrain region to reach lateral neocortex; others course dorsally from the septum to reach medial cortex. By one to two weeks in vitro, acetylcholinesterase-positive axons have extended throughout most of the cortical territory. In addition to basal forebrain cholinergic axons, the normally local circuit cholinergic neurons of the striatum also send axons into cerebral cortex. These striatum-derived axons can be distinguished from basal forebrain axons by their distinct morphological characteristics and by their different response to excision of the striatum or basal forebrain. Further, acetylcholinesterase-positive axons in cortex that originate from striatum appear to retract or degenerate after about one week in culture, while those from basal forebrain remain present and apparently healthy beyond two weeks. These data document the basal forebrain cholinergic ingrowth into cerebral cortex using this whole hemisphere slice culture system and also demonstrate different degrees of maintenance of cortical afferents that are derived from different subcortical sources.

Axons of Xenopus neural tube respond to reversals of neural tube orientation

Axonal trajectories of the Kolmer-Agduhr (KA) neurons of Xenopus embryos, were observed after anterior-posterior (A-P) inversions of neural tube grafts to determine whether KA axons follow cell-inherent directional cues, cues from their immediate environment, or rostrocaudal signals from the embryo. KA axons form one of the earliest ascending spinal pathways in Xenopus and are visible in the lateral marginal zone of whole mounts processed for GABA immunoreactivity. Grafts were made at trunk levels at stages 22-24, 3-5 h before the first KA neurons were detectable and prior to axonal out-growth. Embryos were fixed and immunostained 6-36 h later. KA trajectories within and adjacent to reversed grafts were compared to those of nonrotated control grafts and to neural tube lengths comparable in position and in length in unoperated embryos. Most KA axons within rotated grafts followed the graft's orientation. However, others changed direction, taking novel routes, including turning to conform to the orientation of the host embryo. Reorientations were most common near the posterior host/graft interface. Some host KA cells also reoriented, always within a few hundred microns of the graft interface. Taken together, these growth patterns show that most KA axons within the grafts grow normally with respect to the original polarity of the graft neural tube and maintain that direction even into tissue of opposite polarity, suggesting that their routes are mainly determined by cell-intrinsic and/or local tissue factors. However, the reorientation of many other axons, particularly near graft seams, implies that KA axons can respond to local fluctuations in directional or segment identity signals generated in both host and graft after this perturbation.

Unilateral containment of retinal axons by tectal glia: a possible role for sulfated proteoglycans

(1) A distinct group of radial glia resides along the roofplate of the mesencephalon. Results of experiments, in which the neonatal tectum is manipulated surgically, point to the involvement of these glia in compartmentalizing retinotectal axons to one side of the midbrain. (2) Immunohistochemical studies document that the GAGs CS and KS are expressed along these midline glia during development: their expression occurs after the intertectal axons grow across the midline, but is coincident with the time of ingrowth of retinotectal axons, which fail to cross the midline. Together with results of in vitro experiments from other laboratories, these observations suggest that CS and KS are involved in the barrier function of the midline cells. (3) Preliminary data on biochemical characterization of PGs in developing tectum indicate that similar PG core proteins are found in the midline region as well as in the lateral tectum, whereas metabolic labeling shows a significantly higher uptake of radioactive sulfates along the midline. Thus differential glycosylation of proteins along the midline is likely, along with the possibility that it is the sugar chains which contribute to the barrier function of the raphe glia. Taken in the context of what we currently know about the biochemical heterogeneity of PGs, their developmental expression, and their functions in relation to the growth of axons from a variety of different neuronal cell types, it is clear that the analyses of interactions between PGs and growing axons must occur at several different levels, not the least of which involves a detailed understanding of the milieu in vivo within which these interactions take place.