Regeneration of cerebral-buccal interneurons and recovery of ingestion buccal motor programs in Aplysia after CNS lesions

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long‐lived, lineage‐restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ‐restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by ‘stemness’ gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ‐cell markers, but often lack germ‐line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole‐body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the ‘wobbling Penrose’ landscape. Here, totipotent ASCs adopt ascending/descending courses of an ‘Escherian stairwell’, in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

Different microcircuit responses to comparable input from one versus both copies of an identified projection neuron

Neuronal inputs to microcircuits are often present as multiple copies of apparently equivalent neurons. Thus far, however, little is known regarding the relative influence on microcircuit output of activating all or only some copies of such an input. We examine this issue in the crab (Cancer borealis) stomatogastric ganglion, where the gastric mill (chewing) microcircuit is activated by modulatory commissural neuron 1 (MCN1), a bilaterally paired modulatory projection neuron. Both MCN1s contain the same co-transmitters, influence the same gastric mill microcircuit neurons, can drive the biphasic gastric mill rhythm, and are co-activated by all identified MCN1-activating pathways. Here, we determine whether the gastric mill microcircuit response is equivalent when stimulating one or both MCN1s under conditions where the pair are matched to collectively fire at the same overall rate and pattern as single MCN1 stimulation. The dual MCN1 stimulations elicited more consistently coordinated rhythms, and these rhythms exhibited longer phases and cycle periods. These different outcomes from single and dual MCN1 stimulation may have resulted from the relatively modest, and equivalent, firing rate of the gastric mill neuron LG (lateral gastric) during each matched set of stimulations. The LG neuron-mediated, ionotropic inhibition of the MCN1 axon terminals is the trigger for the transition from the retraction to protraction phase. This LG neuron influence on MCN1 was more effective during the dual stimulations, where each MCN1 firing rate was half that occurring during the matched single stimulations. Thus, equivalent individual- and co-activation of a class of modulatory projection neurons does not necessarily drive equivalent microcircuit output.

Neuromodulation of central pattern generators and its role in the functional recovery of central pattern generator activity

Neuromodulators play an important role in how the nervous system organizes activity that results in behavior. Disruption of the normal patterns of neuromodulatory release or production is known to be related to the onset of severe pathologies such as Parkinson's disease, Rett syndrome, Alzheimer's disease, and affective disorders. Some of these pathologies involve neuronal structures that are called central pattern generators (CPGs), which are involved in the production of rhythmic activities throughout the nervous system. Here I discuss the interplay between CPGs and neuromodulatory activity, with particular emphasis on the potential role of neuromodulators in the recovery of disrupted neuronal activity. I refer to invertebrate and vertebrate model systems and some of the lessons we have learned from research on these systems and propose a few avenues for future research. I make one suggestion that may guide future research in the field: neuromodulators restrict the parameter landscape in which CPG components operate, and the removal of neuromodulators may enable a perturbed CPG in finding a new set of parameter values that can allow it to regain normal function.

Regeneration of Aplysia bag cell neurons is synergistically enhanced by substrate-bound hemolymph proteins and laminin

We have investigated Aplysia hemolymph as a source of endogenous factors to promote regeneration of bag cell neurons. We describe a novel synergistic effect between substrate-bound hemolymph proteins and laminin. This combination increased outgrowth and branching relative to either laminin or hemolymph alone. Notably, the addition of hemolymph to laminin substrates accelerated growth cone migration rate over ten-fold. Our results indicate that the active factor is either a high molecular weight protein or protein complex and is not the respiratory protein hemocyanin. Substrate-bound factor(s) from central nervous system-conditioned media also had a synergistic effect with laminin, suggesting a possible cooperation between humoral proteins and nervous system extracellular matrix. Further molecular characterization of active factors and their cellular targets is warranted on account of the magnitude of the effects reported here and their potential relevance for nervous system repair.

Laminar stream of detergents for subcellular neurite damage in a microfluidic device: a simple tool for the study of neuroregeneration

OBJECTIVE

The regeneration and repair of damaged neuronal networks is a difficult process to study in vivo, leading to the development of multiple in vitro models and techniques for studying nerve injury. Here we describe an approach for generating a well-defined subcellular neurite injury in a microfluidic device.

APPROACH

A defined laminar stream of sodium dodecyl sulfate (SDS) was used to damage selected portions of neurites of individual neurons. The somata and neurites unaffected by the SDS stream remained viable, thereby enabling the study of neuronal regeneration.

MAIN RESULTS

By using well-characterized neurons from Aplysia californica cultured in vitro, we demonstrate that our approach is useful in creating neurite damage, investigating neurotrophic factors, and monitoring somata migration during regeneration. Supplementing the culture medium with acetylcholinesterase (AChE) or Aplysia hemolymph facilitated the regeneration of the peptidergic Aplysia neurons within 72 h, with longer (p < 0.05) and more branched (p < 0.05) neurites than in the control medium. After the neurons were transected, their somata migrated; intriguingly, for the control cultures, the migration direction was always away from the injury site (7/7). In the supplemented cultures, the number decreased to 6/8 in AChE and 4/8 in hemolymph, with reduced migration distances in both cases.

SIGNIFICANCE

The SDS transection approach is simple and inexpensive, yet provides flexibility in studying neuroregeneration, particularly when it is important to make sure there are no retrograde signals from the distal segments affecting regeneration. Neurons are known to not only be under tension but also balanced in terms of force, and the balance is obviously disrupted by transection. Our experimental platform, verified with Aplysia, can be extended to mammalian systems, and help us gain insight into the role that neurotrophic factors and mechanical tension play during neuronal regeneration.

Feeding behavior of Aplysia: a model system for comparing cellular mechanisms of classical and operant conditioning

Feeding behavior of Aplysia provides an excellent model system for analyzing and comparing mechanisms underlying appetitive classical conditioning and reward operant conditioning. Behavioral protocols have been developed for both forms of associative learning, both of which increase the occurrence of biting following training. Because the neural circuitry that mediates the behavior is well characterized and amenable to detailed cellular analyses, substantial progress has been made toward a comparative analysis of the cellular mechanisms underlying these two forms of associative learning. Both forms of associative learning use the same reinforcement pathway (the esophageal nerve, En) and the same reinforcement transmitter (dopamine, DA). In addition, at least one cellular locus of plasticity (cell B51) is modified by both forms of associative learning. However, the two forms of associative learning have opposite effects on B51. Classical conditioning decreases the excitability of B51, whereas operant conditioning increases the excitability of B51. Thus, the approach of using two forms of associative learning to modify a single behavior, which is mediated by an analytically tractable neural circuit, is revealing similarities and differences in the mechanisms that underlie classical and operant conditioning.

Endogenous neurotrophic factors enhance neurite growth by bag cell neurons of Aplysia

Mechanisms that regulate neurite outgrowth are phylogenetically conserved, including the signaling molecules involved. Here, we describe neurotrophic effects on isolated bag cell neurons (BCNs) of substrate-bound growth factors endogenous to the sea slug Aplysia californica. Sheath cells dissociated from the pleural-visceral connectives of the Aplysia CNS and arterial cells dissociated from the anterior aorta enhance neurite outgrowth when compared to controls, i.e., BCNs grown in defined medium alone. In addition, the substrate remaining after sheath cells or arterial cells are killed significantly enhances growth, relative to all other conditions tested. For instance, primary neurites are more numerous and greater in length for BCNs cultured on substrate produced by arterial cells. These results suggest that sheath and arterial cells produce growth-promoting factors, some of which are found in the substrates produced by these cell types. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), we found that Aplysia collagen-like peptides are produced by dissociated arterial cells, and therefore likely contribute to the observed growth effects. Collagen-like peptides and other factors produced by sheath and arterial cells likely influence neurite growth in the Aplysia CNS during development, learning and memory, and regeneration after injury.

Search for cerebral G cluster neurons responding to taste stimulation with seaweed in Aplysia kurodai by the use of calcium imaging

The calcium imaging method can detect the spike activities of many neurons simultaneously. In the present experiments, this method was used to search for unique neurons contributing to feeding behavior in the cerebral ganglia of Aplysia kurodai. We mainly explored the neurons whose cell bodies were located in the G cluster and the neuropile region posterior to this cluster on the ventral surface of the cerebral ganglia. When the extract of the food seaweed Ulva was applied to the tentacle-lip region, many neurons stained with a calcium-sensitive dye, Calcium Green-1, showed changes in fluorescence. Some neurons showed rhythmic responses and others showed transient responses, suggesting that these neurons may be partly involved in the feeding circuits. We also identified three motor neurons among these neurons that showed rhythmic fluorescence responses to the taste stimulation. One of them was a motor neuron shortening the anterior tentacle (ATS), and the other two were motor neurons producing lip opening-like (LO(G)) and closing-like (LC(G)) movements, respectively. Application of the Ulva extract to the tentacle-lip region induced phase-locked rhythmic firing activity in these motor neurons, suggesting that these neurons may contribute to the rhythmic patterned movements of the anterior tentacles and lips during the ingestion of seaweed.

Egestive feeding responses in Aplysia persist after sectioning of the cerebral-buccal connectives: evidence for multiple sites of control of motor programs

Ingestive and egestive behaviors in Aplysia are generated by motor neurons and interneurons chiefly located in the buccal ganglion, but cerebral ganglion neurons appear to contribute to both ingestive and egestive motor programs. We investigated if the cerebral ganglion input to the buccal ganglion is necessary for the generation of buccal ingestive and egestive behaviors in free-moving animals. We confirmed a prior study that showed that animals with lesions of the cerebro-buccal connectives (CBCs) do not exhibit rhythmic biting following seaweed stimulation of the lips, but do show swallowing of seaweed inserted into the buccal cavity. We found that CBC-lesioned animals also exhibited rejection of a tube inserted into the buccal cavity and esophagus. The programs for swallowing and rejection behaviors were similar to those observed before lesioning the CBCs, although the rate of swallowing was slower. These results suggest that the cerebral input to the buccal ganglion is necessary for generating biting responses, but is not required for producing swallowing or rejection responses.

Colocalization of gamma-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica

Functional consequences of neurotransmitter coexistence and cotransmission can be readily studied in certain experimentally favorable invertebrate motor systems. In this study, whole-mount histochemical methods were used to identify neurons in which gamma-aminobutyric acid (GABA)-like immunoreactivity (GABAli) was colocalized with catecholamine histofluorescence (CAh; FaGlu method) and tyrosine hydroxylase (TH)-like immunoreactivity (THli) in the feeding motor circuitry (buccal and cerebral ganglia) of the marine mollusc Aplysia californica. In agreement with previous reports, five neurons in the buccal ganglia were found to exhibit CAh. These included the paired B20 buccal-cerebral interneurons (BCIs), the paired B65 buccal interneurons, and an unpaired cell with projections to both cerebral-buccal connectives (CBCs). Experiments in which the FaGlu method was combined with the immunohistochemical detection of GABA revealed double labeling of all five of these neurons. An antibody generated against TH, the rate-limiting enzyme in the biosynthesis of catecholamines, was used to obtain an independent determination of GABA-CA colocalization. Biocytin backfills of the CBC performed in conjunction with TH immunohistochemistry revealed labeling of the rostral B20 cell pair and the unpaired CBI near the caudal surface of the right hemiganglion. THli was also present in a prominent bilateral pair of caudal neurons that were not stained with CBC backfills. On the basis of their position, size, shape, and lack of CBC projections, the lateral THli neurons were identified as B65. Double-labeling immunohistochemical experiments revealed GABAli in all five buccal THli neurons. Finally, GABAli was observed in individual B20 and B65 neurons that were identified using electrophysiological criteria and injected with a marker (neurobiotin). Similar methods were used to demonstrate that a previously identified catecholaminergic cerebral-buccal interneuron (CBI) designated CBI-1 contained THli but did not contain GABAli. Although numerous THli and GABAli neurons and fibers were present in the cerebral and buccal ganglia, additional instances of their colocalization were not observed. These findings indicate that GABA and a catecholamine (probably dopamine) are colocalized in a limited number of interneurons within the central pattern generator circuits that control feeding-related behaviors in Aplysia.