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

Molecular identification and expression of the NMDA receptor NR1 subunit in the leech

The N-methyl-D-aspartate receptor (NMDAR) is involved in a number of physiological and pathophysiological processes in vertebrates, but there have been few studies examining the role of invertebrate NMDA receptors. In the leech, pharmacological evidence suggests that NMDARs contribute to synaptic plasticity, but there has been no molecular identification of NMDA receptors. In this report, a partial cDNA encoding the leech NR1 subunit of the NMDA receptor (HirNR1) is presented. Reverse transcriptase-polymerase chain reaction from single neurons of the leech central nervous system confirms HirNR1 expression in the Retzius (R), Anterior Pagoda (AP), Pressure (P), and Touch (T) neurons. Immunoblotting with an anti-NR1 antibody yielded a approximately 110 kDa protein, similar to the expected weight of the NR1 subunit (approximately 116 kDa). Finally, pairing pre- and postsynaptic activity elicited long-term potentiation in synapses between neurons expressing NR1 mRNA (P-to-AP synapse) and this potentiation was blocked by the NMDAR antagonist AP5.

Leech filamin and Tractin: markers for muscle development and nerve formation

The Lan3-14 and Laz10-1 monoclonal antibodies recognize a 400 kDa antigen that is specifically expressed by all muscle cells in leech. We show that the antigen recognized by both antibodies is a member of the filamin family of actin binding proteins. Leech filamin has two calponin homology domains and 35 filamin/ABP-repeat domains. In addition, we used the Laz10-1 antibody to characterize the development of the segmentally iterated dorsoventral flattener muscles. We demonstrate that the dorsoventral flattener muscle develops as three discrete bundles of myofibers and that CNS axons pioneering the DP nerve extend only along the middle bundle. Interestingly, the middle dorsoventral muscle anlage is associated with only non-neuronal expression of the L1-family cell adhesion molecule Tractin. This expression is transient and occurs at the precise developmental stages when DP nerve formation takes place. Based on these findings we propose that the middle dorsoventral muscle anlagen provides a substrate for early axonal outgrowth and nerve formation and that this function may be associated with differential expression of distinct cell adhesion molecules.

Extracellular matrix glycoproteins inhibit neurite production by cultured neurons

We have analyzed the role of extracellular matrix glycoproteins in the formation of a bipolar outgrowth pattern of identified leech neurons in culture. Adult anterior pagoda (AP) neurons cultured on the inner surface of the ganglion capsules that surround central nervous system, generate two processes oriented in opposite directions. This pattern differs from those produced by these neurons cultured on other substrates, and is similar to the pattern of developing AP neurons at embryonic day 10. We used different lectins to identify subsets of glycoproteins in the extracellular matrix (ECM) of the capsules and to study their contribution to the formation of the bipolar outgrowth pattern. ECM glycoproteins binding to peanut agglutinin (PNA) or Galanthus nivalis aglutinin (GNA) lectins were detected in ganglion capsules and in ganglion extracts that had been separated by electrophoresis and blotted to nitrocellulose membranes. Four protein bands bound to PNA lectin and six other bands, including laminin subunits, bound to GNA lectin. Other lectins failed to recognize any of the proteins. For AP neurons cultured on capsules, addition of PNA lectin to the culture medium produced a dose-dependent increase in the number of primary neurites without affecting their shape, length or number of branch points. However, PNA lectin used as substrate did not affect sprouting of AP neurons. Our results suggest that PNA-binding extracellular matrix glycoproteins regulate the formation of the bipolar pattern of AP neurons by inhibiting the formation of neurites.

Neuronal growth and target recognition: lessons from the leech

The nervous system of the leech has been the subject of numerous studies since its "rediscovery" in the 1960s as a unique system for the study of the properties of glial cells. Subsequently, anatomical, physiological, and embryological studies of identified neurons have yielded a wealth of information about the differentiation of neuronal structure and function. In recent years, cellular approaches to the development of identified central and peripheral neurons have been complemented by molecular studies that promise to reveal the mechanisms by which neurons form their complex arbors and innervate specific targets.

Development of neuronal circuits and behaviors in the medicinal leech

We are studying the neuronal mechanisms responsible for establishing circuitry underlying the local bending response in the medicinal leech. Local bending replaces an embryonic behavior, circumferential indentation, during the time of initial chemical synaptogenesis in leech embryos. We found that the electrical connections among the motor neurons are established first, about 5% of embryonic time (almost 2 full days) before chemical connections form. The inhibitory connections from muscle inhibitors to muscle excitors are, we hypothesize, responsible for the emergence of local bending. We have also found that the central processes of the excitors--but not the inhibitors--have much longer central processes when their peripheral processes are kept from contacting their target muscles. This system should allow us to test ideas about how individual neurons find their appropriate targets to form functional neuronal circuits.

Ectopic CNS projections guide peripheral neuron axons along novel pathways in leech embryos

Previous studies have indicated that the formation of stereotyped segmental nerves in leech embryos depends on the interactions between CNS projections and ingrowing afferents from peripheral neurons. Especially, CNS-ablation experiments have suggested that CNS-derived guidance cues are required for the correct navigation of several groups of peripheral sensory neurons. In order to directly test this hypothesis we have performed transplantations of CNS ganglia into ectopic sites in segments from which the resident ganglia have been removed. We find that the transplanted ganglia extend numerous axons distributed roughly equally in all directions. When these CNS projections reach and make contact with peripheral sensory axons they are used as guides for peripheral neurons to grow toward and into the ectopic ganglia even when this means following novel pathways that cross the midline and/or segmental boundaries. The peripheral sensory axons turn and grow toward the ectopic ganglia only when in physical contact with CNS axons, suggesting that diffusible chemoattractants are not a factor. These results demonstrate that the guidance cues provided by ectopic CNS projections are both necessary and sufficient to steer peripheral sensory neuron axons into the CNS.

The leech receptor protein tyrosine phosphatase HmLAR2 is concentrated in growth cones and is involved in process outgrowth

Developing neurons extend long processes to specific distal targets using extracellular molecules as guidance cues to navigate through the embryo. Growth cones, specialized structures at the tip of the extending processes, are thought to accomplish this navigation through receptors that recognize guidance cues and modulate growth accordingly. In Drosophila, several receptor tyrosine phosphatases (rPTPs), including DLAR, have been shown to participate in directing neurite outgrowth. As yet, however, it is not known how rPTPs act to affect navigation. To gain insight into the mechanisms of rPTP-mediated outgrowth guidance, we have investigated the role of HmLAR2, a Hirudo medicinalis homologue of DLAR, in process outgrowth. HmLAR2 is expressed by, among other cells, a transient neuron-like template cell, the Comb cell. Here we present evidence that HmLAR2 protein becomes concentrated within their growth cones at a stage when C cell processes undergo rapid outgrowth. When antibodies raised against the extracellular domain of HmLAR2 were injected into intact embryos, they bound specifically to the C cell surface at growth cones and along processes and caused the partial internalization of HmLAR2 receptors. Moreover, the C cell processes were found to project aberrantly, to deviate from their normally highly regular trajectories and to extend shorter distances in the presence of the antibodies. We propose that HmLAR2 is required by the C cell for guidance and extension and suggest that it functions via its ectodomain to transduce extracellular guidance cues.

Outgrowth patterns and directed growth of identified neurons induced by native substrates in culture

In this study, we have tested how various identified leech neurons in culture grow on surfaces that they normally contact in situ. Neurons were cultured either on ganglion capsules from which neurons had been removed or on skin. On these substrates, outgrowth patterns were characteristic for each cell type. Retzius cells plated on capsules extended bundles of thick, fasciculated processes with few branching points and in the opposite direction a tangle of fine neurites. Anterior pagoda (AP) neurons plated on capsules extended two single processes in opposite directions but failed to grow on skin. Sensory P and N neurons on capsules extended multiple processes. On skin, P neurons extended only two long branches in opposite directions over the superficial body wall. N neurons on skin extended multiple processes. Varicosities were common in the processes of P and N neurons on capsules or skin. The branching patterns described here bore closer resemblance to those in the developing or adult nervous system than to those on Concanavalin A or laminin-enriched extract. Pairs of Retzius or AP neurons plated at a distance on the same capsule extended neurites from one neuron toward the other and formed contacts. Such directed growth failed in hybrid pairs of Retzius and AP neurons or in pairs plated on laminin-enriched extract or Concanavalin A. Our results suggest that multiple growth-promoting molecules anchored to the extracellular matrix may cooperate in regulating the branching pattern of neurons, fasciculation, and direction of growth.

Different determinants on growth and synapse formation in cultured neurons

The influence of substrate and target on growth and synapse formation was investigated in identified leech neurons in culture. Nociceptive neurons and annulus erector motoneurons were cultured on ganglion capsules, a substrate they encounter in the leech. Within a few hours, single nociceptive cells sprouted profusely, whereas annulus erector cells failed to grow. When annulus erector neurons were plated near a nociceptive cell on the same capsule both neurons grew, made contact and formed an electrical synapse, different from the chemical synapse they form in the ganglion, but identical to that formed when plated on Concanavalin A. These results suggest that substrate and target have complementary effects regulating growth, but fail to define the type of synapse formed.

Initial formation and secondary condensation of nerve pathways in the medicinal leech

Invertebrates have proved to be important experimental systems for examining questions related to growth cone navigation and nerve formation, in large part because of their simpler nervous systems. However, such apparent simplicity can be deceiving because the final stereotyped patterns may be the result of multiple developmental mechanisms and not necessarily the sole consequence of the pathway choices of individual growth cones. We have examined the normal sequence of events that are involved in the formation of the major peripheral nerves in leech embryos by employing (1) an antibody directed against acetylated tubulin to label neurons growing out from the central nervous system, (2) the Lan3-2 antibody to label a specific population of peripheral neurons growing into the central nervous system, and (3) intracellular dye filling of single cells. We found that the mature pattern of nerves was characterized by a pair of large nerve roots, each of which branched into two major tracts. The earliest axonal projections did not, however, establish this pattern definitively. Rather, each of the four nerves initially formed as discrete, roughly parallel tracts without bifurcation, with the final branching pattern of the nerve roots being generated by a secondary condensation. In addition, we found that some of the nerves were pioneered in different ways and by different groups of neurons. One of the nerves was established by central neurons growing peripherally, another by peripheral neurons growing centrally. These results suggest that the formation of common nerves and neuronal pathfinding in the leech involves multiple sets of growth cone guidance strategies and morphogenetic mechanisms that belie its apparent simplicity.