Regulation of neurotrophin-3 expression by epithelial-mesenchymal interactions: the role of Wnt factors

Neurotrophins regulate survival, axonal growth, and target innervation of sensory and other neurons. Neurotrophin-3 (NT-3) is expressed specifically in cells adjacent to extending axons of dorsal root ganglia neurons, and its absence results in loss of most of these neurons before their axons reach their targets. However, axons are not required for NT-3 expression in limbs; instead, local signals from ectoderm induce NT-3 expression in adjacent mesenchyme. Wnt factors expressed in limb ectoderm induce NT-3 in the underlying mesenchyme. Thus, epithelial-mesenchymal interactions mediated by Wnt factors control NT-3 expression and may regulate axonal growth and guidance.     

Astrocytes block axonal regeneration in mammals by activating the physiological stop pathway

Regenerating sensory axons in the dorsal roots of adult mammals are stopped at the junction between the root and spinal cord by reactive astrocytes. Do these cells stop axonal elongation by activating the physiological mechanisms that normally operate to stop axons during development, or do they physically obstruct the elongating axons? In order to distinguish these possibilities, the cytology of the axon tips of regenerating axons that were stopped by astrocytes was compared with the axon tips that were physically obstructed at a cul-de-sac produced by ligating a peripheral nerve. The terminals of the physically obstructed axon tips were distended with neurofilaments and other axonally transported structures that had accumulated when the axons stopped elongating. By contrast, neurofilaments did not accumulate in the tips of regenerating axons that were stopped by spinal cord astrocytes at the dorsal root transitional zone. These axo-glial terminals resembled the terminals that axons make on target neurons during normal development. On the basis of these observations, astrocytes appear to stop axons from regenerating in the mammalian spinal cord by activating the physiological stop pathway that is built into the axon and that normally operates when axons form stable terminals on target cells.     

Transient pioneer neurons are essential for formation of an embryonic peripheral nerve

In developing nervous systems, many peripheral and central pathways are established by early arising populations of pioneer neurons. The growth cones of these pioneer neurons can migrate while embryonic distances are short and while intervening tissue is relatively uncomplicated. Are these pioneers necessary? In grasshopper embryos, a pair of pioneer neurons arise at the tips of limb buds and extend axons through the limb to the central nervous system. Growth cones of later arising sensory neurons migrate along the pioneer axons. After ingrowth of sensory axons, the pioneer neurons die. If the pioneer neurons are prevented from differentiating by heat shock, then the sensory growth cones that would have migrated along them are blocked and fail to reach the central nervous system. Thus, the pioneer axons are necessary for successful migration of these sensory growth cones. By crossing a segment boundary early in embryogenesis, the pioneers circumvent an incompatibility between differentiated segment boundary cells and growth cone migration. Pioneer neurons may resolve similar problems in many systems.     

Gangliosides in isolated neurons and glial cells

Gangliosides occur in much greater amounts in clean isolated neurons and in the neuropil teased from immediately around the neuron cell body and dendrites than in isolated clumps of glial cells. Since the zone of neuropil adjacent to neurons is richest in terminal axons and synaptic endings, these findings indicate a specific concentration of these sialoglycolipids in synaptic membranes.     

Electrophysiologic responses in hamster superior colliculus evoked by regenerating retinal axons

Autologous peripheral nerve grafts were used to permit and direct the regrowth of retinal ganglion cell axons from the eye to the ipsilateral superior colliculus of adult hamsters in which the optic nerves had been transected within the orbit. Extracellular recordings in the superior colliculus 15 to 18 weeks after graft insertion revealed excitatory and inhibitory postsynaptic responses to visual stimulation. The finding of light-induced responses in neurons in the superficial layers of the superior colliculus close to the graft indicates that axons regenerating from axotomized retinal ganglion cells can establish electrophysiologically functional synapses with neurons in the superior colliculus of these adult mammals.     

Fast trap jaws and giant neurons in the ant odontomachus

Ants of the ponerine genus Odontomachus use a trap jaw mechanism when hunting fast prey. When particular trigger hairs, located on the inner edge of the mandibles, are touched by prey, the jaws close extremely rapidly and trap the target. This trap jaw response lasts only 0.33 to 1 millisecond. Electrophysiological recordings demonstrated that the trigger hairs function as mechanoreceptors. Associated with each trigger hair are large sensory cells, the sensory axons of which measure 15 to 20 micrometers in diameter. These are among the largest sensory neurons, and their size implies that these axons conduct information very rapidly.     

Mammalian SAD kinases are required for neuronal polarization

Electrical activity in neurons is generally initiated in dendritic processes then propagated along axons to synapses, where it is passed to other neurons. Major structural features of neurons-their dendrites and axons-are thus related to their fundamental functions: the receipt and transmission of information. The acquisition of these distinct properties by dendrites and axons, called polarization, is a critical step in neuronal differentiation. We show here that SAD-A and SAD-B, mammalian orthologs of a kinase needed for presynaptic differentiation in Caenorhabditis elegans, are required for neuronal polarization. These kinases will provide entry points for unraveling signaling mechanisms that polarize neurons.     

Rate of movement and redistribution of stainable neurosecretory granules in hypothalamic neurons

Electrical stimulation of the olfactory tract of goldfish for one minute can deplete completely the stainable neurosecretory granules from cells of the preoptic nucleus as well as from their axons. Thus, in stimulated neurons secretory granules appear to move toward the neurohemal point of discharge at a rate of about 2 millimeters per minuite. Reaccumutlation of neurosecretory granules in depleted neurons to approximately normal numbers requires about 1 to 1.5 hours. Histological evidence indicates that, during the period of reaccumulation, granules move out of the perikaryon until normal granulation in the axons is achieved; finally, granulation of the perikaryon is restored.     

Geometrical differences among homologous neurons in mammals

The dendritic arbors of sympathetic neurons in different species of mammals vary systematically: the superior cervical ganglion cells of smaller mammals have fewer and less extensive dendrites than the homologous neurons in larger animals. This difference in dendritic complexity according to body size is reflected in the convergence of ganglionic innervation; the ganglion cells of progressively larger mammals are innervated by progressively more axons. These relations have implications both for the function of homologous neural systems in animals of different sizes and for the regulation of neuronal geometry during development.     

Early forebrain wiring: genetic dissection using conditional Celsr3 mutant mice

Development of axonal tracts requires interactions between growth cones and the environment. Tracts such as the anterior commissure and internal capsule are defective in mice with null mutation of Celsr3. We generated a conditional Celsr3 allele, allowing regional inactivation. Inactivation in telencephalon, ventral forebrain, or cortex demonstrated essential roles for Celsr3 in neurons that project axons to the anterior commissure and subcerebral targets, as well as in cells that guide axons through the internal capsule. When Celsr3 was inactivated in cortex, subcerebral projections failed to grow, yet corticothalamic axons developed normally, indicating that besides guidepost cells, additional Celsr3-independent cues can assist their progression. These observations provide in vivo evidence that Celsr3-mediated interactions between axons and guidepost cells govern axonal tract formation in mammals.     

Retrograde axonal transport in the central nervous system

When horseradish peroxidase is injected into the optic tectum of a chick, axons of ganglion cells transport it centripetally to their cell bodies in the retina at a rate of about 72 millimeters per day. After intraocular injections in the young chick, the peroxidase is transported centripetally along efferent axons, and is concentrated in cell bodies within the isthmo-optic nucleus. This retrograde movement of protein from axon terminal to cell body suggests a possible mechanism by which neurons respond to their target areas.     

Cell recognition by neuronal growth cones in a simple vertebrate embryo

The mechanism that guides neuronal growth cones to their targets in vertebrate embryos has been difficult to study primarily because of the complexity and large number of neurons found in many vertebrate nervous systems. The spinal cord of a simple vertebrate, the fish embryo, is used to analyze pathfinding mechanisms. The early embryonic spinal cord consists of a relatively small number of identifiable neurons. From the beginning of axonal outgrowth the growth cones of these identified neurons extend along stereotyped and precise pathways in the spinal cord. Laser ablation experiments (i) support the hypothesis that early growth cones that pioneer specific spinal tracts appear to recognize cues on subsets of longitudinally arrayed neuroepithelial cells and (ii) demonstrate that later growth cones that selectively fasciculate in these spinal tracts appear to recognize cues on specific subsets of axons.     

Subplate neurons pioneer the first axon pathway from the cerebral cortex

During the development of the nervous system, growing axons must traverse considerable distances to find their targets. In insects, this problem is solved in part by pioneer neurons, which lay down the first axonal pathways when distances are at a minimum. Here the existence of a similar kind of neuron in the developing mammalian telencephalon is described. These are the subplate cells, the first postmitotic neurons of the cerebral cortex. Axons from subplate neurons traverse the internal capsule and invade the thalamus early in fetal life, even before the neurons of cortical layers 5 and 6, which will form the adult subcortical projections, are generated. During postnatal life, after the adult pattern of axonal projections is firmly established, most subplate neurons disappear. These observations raise the possibility that the early axonal scaffold formed by subplate cells may prove essential for the establishment of permanent subcortical projections.     

Axonal elongation into peripheral nervous system "bridges" after central nervous system injury in adult rats

The origin, termination, and length of axonal growth after focal central nervous system injury was examined in adult rats by means of a new experimental model. When peripheral nerve segments were used as "bridges" between the medulla and spinal cord, axons from neurons at both these levels grew approximately 30 millimeters. The regenerative potential of these central neurons seems to be expressed when the central nervous system glial environment is changed to that of the peripheral nervous system.     

Thalamocortical relay neurons: antidromic invasion of spikes from a cortical epileptogenic focus

Thalamocortical relay neurons whose axons project into a penicillininduced cortical epileptogenic focus generate bursts of action potentials during spontaneous interictal epileptiform discharges. These bursts originate in intracortical axons and propagate antidromically into thalamic neurons. Repetitive spike generation in cortical axons and presynaptic terminals could produce a potent excitatory drive and contribute to the generation of the large depolarization shifts which are seen in cortical elements during focal epileptogenesis.     

Nerve growth factor treatment after brain injury prevents neuronal death

Cholinergic neuronal degeneration after axotomy has been proposed to be due to the loss of a retrogradely transported neurotrophic factor, possibly nerve growth factor (NGF). To test this hypothesis, NGF was continuously infused into the lateral ventricles of adult rats that had received bilateral lesions of all cholinergic axons projecting from the medial septum to the dorsal hippocampus. After 2 weeks of NGF treatment, identification of cholinergic neurons by the presence of the biosynthetic enzyme choline acetyltransferase revealed a dramatic increase (350%) in the survival of the axotomized septal cholinergic neurons. Thus, NGF or an NGF-like molecule can act as a neurotrophic factor for these neurons.     

Grafting genetically modified cells to the damaged brain: restorative effects of NGF expression

Fibroblasts were genetically modified to secrete nerve growth factor (NGF) by infection with a retroviral vector and then implanted into the brains of rats that had surgical lesions of the fimbria-fornix. The grafted cells survived and produced sufficient NGF to prevent the degeneration of cholinergic neurons that would die without treatment. In addition, the protected cholinergic cells sprouted axons that projected in the direction of the cellular source of NGF. These results indicate that a combination of gene transfer and intracerebral grafting may provide an effective treatment for some disorders of the central nervous system.     

Human CHN1 mutations hyperactivate alpha2-chimaerin and cause Duane's retraction syndrome

Duane's retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes alpha2-chimaerin, a Rac guanosine triphosphatase-activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase alpha2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance alpha2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant alpha2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that alpha2-chimaerin has a critical developmental function in ocular motor axon pathfinding.     

Cortex is driven by weak but synchronously active thalamocortical synapses

Sensory stimuli reach the brain via the thalamocortical projection, a group of axons thought to be among the most powerful in the neocortex. Surprisingly, these axons account for only approximately 15% of synapses onto cortical neurons. The thalamocortical pathway might thus achieve its effectiveness via high-efficacy thalamocortical synapses or via amplification within cortical layer 4. In rat somatosensory cortex, we measured in vivo the excitatory postsynaptic potential evoked by a single synaptic connection and found that thalamocortical synapses have low efficacy. Convergent inputs, however, are both numerous and synchronous, and intracortical amplification is not required. Our results suggest a mechanism of cortical activation by which thalamic input alone can drive cortex.     

Cell recognition during neuronal development

Insect embryos, with their relatively simple nervous systems, provide a model system with which to study the cellular and molecular mechanisms underlying cell recognition during neuronal development. Such an approach can take advantage of the accessible cells of the grasshopper embryo and the accessible genes of Drosophila. The growth cones of identified neurons express selective affinities for specific axonal surfaces; such specificities give rise to the stereotyped patterns of selective fasciculation common to both species. These and other results suggest that early in development cell lineage and cell interactions lead to the differential expression of cell recognition molecules on the surfaces of small subsets of embryonic neurons whose axons selectively fasciculate with one another. Monoclonal antibodies reveal surface molecules in the Drosophila embryo whose expression correlates with this prediction. It should now be possible to isolate the genes encoding these potential cell recognition molecules and to test their function through the use of molecular genetic approaches in Drosophila.