Regeneration of afferent axons into discrete tracts within peripheral nerves in the leech

Atypical embryonic synapses fail to regenerate in adulthood

Functional recovery following central nervous system (CNS) injury in adult animals may depend on the reestablishment of the precise pattern of connections made during development. When the nervous system is injured during embryonic development, functional recovery may involve the formation of atypical connections. Can such atypical synapses regenerate in adults, particularly in a nervous system known for its capacity for repair? When the S interneuron in one segmental ganglion of the leech Hirudo is killed during development, two neighboring S cells extend their axons into the ganglion and restore function by making electrical synapses with the usual synaptic targets of the killed S cell. Although adult S-cell axons reliably regenerated their usual synaptic connections, the novel synapses induced following embryonic injury failed to regenerate in adults. In these preparations severed S-cell axons did not reach the denervated ganglion but grew close to it, independent of the distance required to grow. Thus, the developmental changes that permit aberrant but functional connections in embryos do not lead to a similar change in the capacity for axon growth and subsequent synapse regeneration in adults.

Pathfinding, target recognition, and synapse formation of single regenerating fibers in the adult grasshopper Schistocerca gregaria

After lesion of the peripheral tympanal nerve of the adult locust (Schistocerca gregaria), sensory axons regenerate into their original target areas. We examined the individual behavior of single regenerating auditory afferents during pathway and target selection by intracellularly recording and labeling them at different times postlesion. During axotomy, spontaneous activity is not increased in either the distal or proximal part of the cells. Stimulus response properties of lesioned cells with or without regenerating axons are not influenced. Surprisingly, only 55% of sensory neurons regenerate through the lesion site and often give rise to more than one axonal fiber. Within the central nervous system, 70% of regenerated axons consistently follow an incorrect pathway to reach the correct target region. Often, one of two processes formed by a cell chooses the correct pathway, and the other the incorrect one. In the target region, regenerated axons reconstitute somatotopically ordered projections and form synapses that resemble those of intact fibers in number and structure. The regeneration process does not induce a detectable expression of antigens that are known to be expressed during neural development in these neurons. Our study clearly demonstrates that precise synaptic regeneration is possible in adult animals within a completely differentiated central nervous system, although pathfinding and formation of arborizations are disturbed in a particular and probably system-related manner. The results strongly suggest that accurate pathfinding is unlikely to be a decisive factor in target area recognition and synaptogenesis.

Accurate synapse regeneration despite ablation of the distal axon segment

In each body ganglion of the leech Hirudo medicinalis there is a single S-cell. After an S-cell axon is severed, it regenerates along its surviving distal segment and reconnects with its synaptic target, the axon of the neighbouring S-cell. In approximately half the cases the regenerating axon forms a temporary electrical synapse specifically with the distal segment, which remains active and connected to the target, thereby functioning as a splice until regeneration is complete. To determine whether the distal axon segment is required for successful regeneration, distal segments of severed S-cell axons were ablated by intracellular injection of bacterial protease. Fifty-seven preparations were examined from 2 to 212 days after injection of the axon segment. The extent of S-cell axon regeneration was assessed electrophysiologically by intracellular and extracellular recording, and anatomically by intracellular injection of markers followed by light microscopy and electron microscopy. The S-cell axons regenerated successfully in almost 90% of animals examined after 2 weeks or more. In a further four animals the target S-cell was ablated in addition to the distal axon segment, permanently disrupting conduction along the S-cell pathway. Nevertheless, the regenerating axon grew along its usual pathway and there was no evidence that alternative connections were formed. It is concluded that, although the distal axon segment can provide a means for rapid functional repair, the segment is not required for reliable regeneration of the axon along its usual pathway and accurate formation of an electrical synapse.

Multiple strategies for directed growth cone extension and navigation of peripheral neurons

Leeches have a diverse constellation of peripheral neural elements that are challenged to extend growth cones in highly specific ways in a constantly changing embryonic environment. Two major systems are reviewed here. In one, peripheral afferents extend growth cones toward the central nervous system (CNS), forming common pathways, and then segregate into particular tracts within the CNS. A majority of these afferents depend on CNS-derived guidance cues and projections from the CNS to guide their way. However, not all of the nerves are established this way and at least one of the peripheral nerves is likely to be pioneered by sensillar sensory afferents. The distribution of particular antigens (such as the lan3-2 antigen) suggests the identity of molecules involved in homophilic adhesion along common pathways, whereas others (such as the lan4-2 and 3-6 antigens) are candidates for mediating specific pathway choices. In the second system, the myo-organizing Comb cell (C cell) projects multiple growth cones simultaneously along oblique trajectories not influenced by segmental or midline boundaries. Its parallel growth cones exhibit space-filling as well as directional growth and are guided by local cues to extend in discrete phases that are coordinated with the development of the environment. Both systems exhibit highly directed outgrowth orchestrated by a hierarchy of cues, establish patterns of neurites used to direct later migrating cells, and seem to be regulated temporally and spatially by interactions with the embryonic environment. These systems illustrate the strengths of examining neural development in vivo across several levels of analysis.

Sequential steps in axonal targeting are mediated by carbohydrate markers

Mannose and hybrid/complex-type oligosaccharides serve as markers for both the full set of peripheral sensory afferent neurons in the leech and also for disjoint subsets of these neurons. We have shown that these various surface carbohydrates play crucial roles in the multistep process by which afferents meet their synaptic partners in the central nervous system (CNS). The carbohydrate marker common to all these afferents allows their projections (which are fasciculated as they enter the CNS) to disperse and search out target regions. Carbohydrate markers specific for subsets of these afferents subsequently allow each subset to consolidate the position of its projections in appropriate regions of the CNS where it contacts its synaptic partners.

Repair of the central nervous system: lessons from lesions in leeches

In contrast to the limited repair observed in the mammalian central nervous system (CNS), injured neurons in the leech reliably regenerate synapses and restore function with remarkable accuracy at the level of individual neurons. New and recent results reveal important roles for microglial cells and extracellular matrix components, including laminin, in repair. Tissue culture experiments have permitted isolation of neurons and manipulation of their environment, providing insights into the influence of substrate, electrical activity, and other cells, including microglia, on axon growth and synapse formation. The results account for distinctive features of successful repair in the adult leech, where axonal sprouting and target selection can be influenced by unequal competition between neurons. Differences between the formation of connections during embryonic development and repair in the adult include dissimilarities in the roles of glia and microglia in adults and embryos, suggesting that axon growth during regeneration in the CNS is not simply a recapitulation of processes observed during embryonic development. It may be possible in the future to improve mammalian CNS regeneration by recruiting cells whose counterparts in the leech have been identified as instrumental in repair.

Specific pathway selection by the early projections of individual peripheral sensory neurons in the embryonic medicinal leech

In leech, the central annulus of each midbody segment possesses seven pairs of sensilla, which are mixed clusters of primary peripheral sensory neurons that extend their axons into the CNS where they segregate into distinct fascicles. Pathway selection by individual afferent growth cones of sensillar neurons was examined by double labeling using intracellular dye-filling with antibody labeling in early Hirudo medicinalis embryos. The monoclonal antibody Lan3-2 was used because sensillar neuronal tracts are specifically labeled by this antibody. Examining 68 individually filled neurons we found that sensillar neuron growth cones bifurcate within the CNS, that they project long filopodia capable of sampling the local environment, and that all of them appeared to choose a single particular CNS fascicle without apparent retraction or realignment of growth cones. Furthermore, each side of the bifurcating afferent growth cones always chose the same fascicle, implying a specific choice of a distinct labeled pathway. By dye-filling individual central neurons (P-cells), we show that there are centrally projecting axons present at the time sensillar afferents enter the ganglionic primordia and select a particular fascicle, and we confirm that at least the dorsal peripheral nerve is likely to be pioneered by central neurons, not by the peripheral afferents. In the sensillum studied here, we found examples of sensory neurons extending axons into one of all the available fascicles. Thus, an individual embryonic sensillum possesses a heterogeneous population of afferents with respect to the central fascicle chosen. This is consistent with the idea that segregation into distinct axon fascicles may be based upon functional differences between individual afferent neurons. Our findings argue strongly in favor of specific pathway selection by afferents in this system and are consistent with previous suggestions that there exists a hierarchy of cues, including surface glycoconjugates that mediate navigation of the sensillar growth cones and the fasciculation of their axons.

Tract formation and axon fasciculation of molecularly distinct peripheral neuron subpopulations during leech embryogenesis

In leech, the central projections of peripheral sensory neurons segregate into specific axonal tracts, which are distinguished by differential expression of surface antigens recognized by the monoclonal antibodies Lan3-2 and Lan4-2. Lan3-2 recognizes an epitope expressed on axons that segregate into three distinct axon fascicles. In contrast, the Lan4-2-positive axons selectively project into only one of the Lan3-2-positive axon tracts. These observations provide evidence for a hierarchy of guidance cues mediating specific pathway formation in this system. Since the Lan3-2 antibody has been shown to perturb this process and since, as shown here, the Lan3-2 and Lan4-2 antigens are closely molecularly interrelated, these antibodies may help define molecules and epitopes mediating neuronal recognition and axonal guidance.

Connectivity of identified central synapses in the cricket is normal following regeneration and blockade of presynaptic activity

Cercal sensory neurons in the cricket innervate interneurons in the central nervous system (CNS) and provide a model system for studying the formation of central synapses. When axons of the sensory neurons were transected during larval development, the cell bodies and the soma-bearing portion of axons, which are located within the cercus, survived but lost their excitability for 9-10 days. During this period, the sensory neurons grew new axons and reinnervated the terminal abdominal ganglion. Physiological recordings showed that sensory neurons of known identity reestablished monosynaptic contacts with their normal postsynaptic interneuron. Moreover, each synapse exhibited a characteristic strength indistinguishable from the intact synapse in an unoperated cricket. Since this selective connectivity was apparent immediately after the excitability of the axotomized sensory neurons was restored, action potentials in the sensory neurons appear to be unnecessary for normal synaptic regeneration to occur. Consistent with this, the reinnervation process was unaffected even when action potentials in the sensory neurons were blocked by tetrodotoxin (TTX) immediately following axotomy until just before testing. During the normal course of development, the characteristic strength of individual synapses changes systematically, resulting in the developmental rearrangement of these synapses (Chiba et al., 1988). This synaptic rearrangement was also unaffected when action potentials in the sensory neurons were blocked by TTX for the last 30% of larval development. Therefore, in the cricket cercal sensory system, both regeneration of the central synapses following axotomy of the presynaptic sensory neurons and the normal rearrangement of connectivity during larval development appear not to require axonal action potentials.

Segregation of afferent projections in the central nervous system of the leech Hirudo medicinalis

Sensory axons originating in peripheral tissues converge onto each segmental ganglion in the central nervous system (CNS) of the leech, where they segregate into well-defined regions of the synaptic neuropil. Here we report on several aspects of the molecular and anatomical organizations of these afferent projections that bear upon the hypothesis that surface markers are involved in organizing these axons as they grow into the CNS. First, we show that the distribution of some surface markers in the adult is restricted to axons of peripheral origin and is not present on the neighboring axons of central neurons. Second, we demonstrate that the number of afferents increases postembryonically as the leech increases in size, suggesting that at least some of the cues employed by afferent axons to grow to appropriate central targets must be present throughout the life of the animal. We then show, using anterograde axonal tracing and immunohistochemistry, that there is both convergence and divergence of afferent axons into highly specific regions of the neuropil. Lastly, we examine the distribution of surface markers present on different subsets of afferents and show that axons having one type of marker segregate from those having the second type. Our results, considered together with previous observations in this system, provide new clues about the organization of afferent projections in the nervous system of the leech. They also suggest how a relatively small number of molecular markers might mediate fiber-fiber interactions to organize afferent axons as they grow into the CNS.

Expression of surface glycoproteins early in leech neural development

Cell migration and axon growth during neural development rely upon cell-cell and cell-matrix interactions mediated by surface glycoproteins. The surface glycoprotein recognized on leech neurons by monoclonal antibody Lan3-2 has previously been implicated in the process of axon fasciculation during regeneration in adults. In adult leeches, Lan3-2 binds to a carbohydrate epitope of a 130 kD protein. The present study demonstrates that in embryos the antibody binds to the same carbohydrate epitope of glycoproteins with molecular weights of 130 kD and higher. As a first step in evaluating a possible role of the Lan3-2 glycoprotein or the cells that express it during neural development, we determined its distribution in the developing nervous system of the leech Hirudo medicinalis. In embryos, Lan3-2 epitope is expressed on fasciculated sensory afferents and it appears on the cell bodies before neurite outgrowth. The sensory fibers appear rostrally by embryonic day 10, less than halfway through development. Earlier, by 7 days of development at 20 degrees C, Lan3-2 binds to previously undocumented cell types: (1) cells appearing along the embryonic midline and (2) a cluster of cells located at the rostral edge of the germinal plate. These cells only transiently express this antigen and are present at critical left-right and rostrocaudal boundaries during a period of cell proliferation, movement, and migration that produces the nervous system. Thus the Lan3-2 surface glycoprotein or the cells expressing it are candidates for involvement in axon fasciculation, cell migration, and directed axonal growth.

Defasciculation as a neuronal pathfinding strategy: involvement of a specific glycoprotein

Leech sensory afferents change their growth behavior as they enter the CNS. Arriving from the periphery in fasciculated tracts, they abruptly defasciculate and expand into diffuse trees before reassembling into four distinct central tracts. In the organ-cultured germinal plate, growing sensory afferents were incubated with monovalent Fab fragments of the Lan3-2 antibody, which recognizes a 130 kd sensory neuron protein by its mannose epitope. Very low concentrations of Lan3-2 (6 and 12 nM) specifically inhibited the central defasciculation of sensory afferents, which then continued growing as a single tract. In contrast, monoclonal antibody Lan3-6, which binds to an internal sensory antigen, failed to yield the same effect. These observations suggest that this sensory neuron 130 kd surface glycoprotein participates in a developmentally significant heterophilic interaction specific for the CNS.

A group of related surface glycoproteins distinguish sets and subsets of sensory afferents in the leech nervous system

The distribution of 4 surface glycoproteins on axons of peripheral neurons was studied in the leech Hirudo medicinalis through monoclonal antibodies. All 4 glycoproteins have a similar molecular weight of 130 kDa. Immunohistochemical localization of these glycoproteins on tissue sections of nerves and neuropil reveals tracts of afferent axons organized as nested sets. Their distribution suggests a possible role for these molecules in mediating axon fasciculation.