Command neuron for a half-center oscillator

Our latest paper just came out in the Journal of Neuroscience.

Command or obey? Homologous neurons differ in hierarchical position for the generation of homologous behaviors
Akira Sakurai and Paul S. Katz 
J Neurosci 17 June 2019, 3229-18
DOI: https://doi.org/10.1523/JNEUROSCI.3229-18.2019

Click here for the reprint

Nudibranchs have homologous neurons that can be identified across species. Cross-species comparisons of motor system organization provide fundamental insights into their function and origin. This paper shows that an identified cerebral ganglion neuron serves as a command neuron for the swimming behavior in a nudibranch species. The same neuron serves as a member of a central pattern generator (CPG) in another species. We described the synaptic and neuromodulatory mechanisms by which the command neuron initiates and accelerates rhythmic motor patterns.


Two sea slug species show homologous swimming behaviors
In motor systems, higher-level components issue commands that are carried out by lower-level circuits. In this paper, we describe the physiological actions of an identified neuron, which turned out to be a "command" neuron for the swimming behavior of a giant sea slug, Dendronotus iris. We determined which functional components of the swim CPG are modulated by the command inputs to initiate, maintain, and terminate the rhythmic activity of a central pattern generator circuit.

Among the swimming nudibranchs, two species Melibe leonina and Dendronotus iris show the homologous swimming behavior by flexing their bodies from left to right (Sakurai et al., 2011).
 



Homologous behaviors are produced by homologous neurons
Phylogenetic analysis indicates that the most recent common ancestor of these species likely swam in this manner, making the swimming behaviors homologous (Goodheart et al., 2015; Sakurai and Katz, 2017).

The brains of Melibe leonina (left) 
and Dendronotus iris (right)

Homologous behaviors are produced by distinct neural circuit designs
The neural circuits underlying their behaviors have been studied extensively in both species. All neurons in the swim CPGs have been identified, and their synaptic connections have been determined with careful pairwise electrophysiological recordings (Sakurai et al., 2014; Sakurai and Katz 2016). The two swim CPGs employ different network architectures for producing similar rhythmic motor patterns.

The swim CPGs of Melibe (left) and Dendronotus (right)
have distinct synaptic organizations 

Si1 as a command neuron in Dendronotus
In this study we found that Si1 neurons in Dendronotus iris serves as a neuromodulatory "command" neuron for the swim CPG.

We further revealed how such command actions were mediated by performing dynamic clamp experiments and electrophysiological manipulations.

Providing artificial synaptic boost of the Si3-to-Si2 synapse and tonic synaptic excitation of Si3 mimicked the command actions of Si1 neurons.


Synaptic and neuromodulatory actions underlie the command input
It turned out that the organization of the Dendronotus swim CPG closely resembles the model that was originally proposed for a half-center oscillator with excitatory drive arising from a command neuron (Friesen, 1994).

A classical model of the half-center oscillator (left) 
and the Dendronotus swim CPG (right)

The command neuron Si1 provides not only the overall excitatory drive but also the neuromodulation of synaptic potentiation within each half of the oscillator. Our results also suggest that the functional position of neurons in a motor hierarchy can shift from one level (CPG) to another (a command neuron) over evolutionary time.

• Friesen WO (1994) Reciprocal inhibition: a mechanism underlying oscillatory animal movements. Neuroscience and biobehavioral reviews 18:547-553.
• Goodheart JA, Bazinet AL, Collins AG, Cummings MP (2015) Relationships within Cladobranchia (Gastropoda: Nudibranchia) based on RNA-Seq data: an initial investigation. R Soc Open Sci 2:150196. 
• Sakurai A, Katz PS (2016) The central pattern generator underlying swimming in Dendronotus iris: a simple half-center network oscillator with a twist. J Neurophysiol 116:1728-1742.
• Sakurai A, Katz PS (2017) Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors. Curr Biol 27:1721-1734 e1723.
• Sakurai A, Newcomb JM, Lillvis JL, Katz PS (2011) Different roles for homologous interneurons in species exhibiting similar rhythmic behaviors. Curr Biol 21:1036-1043.
• Sakurai A, Gunaratne CA, Katz PS (2014) Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusc Melibe leonina. J Neurophysiol 112:1317-1328. 

Melibe and Dendronotus Dynamic Clamp paper

My latest paper will appear in Current Biology next weekend.

Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors
http://www.cell.com/current-biology/fulltext/S0960-9822(17)30552-3
by Akira Sakurai and Paul S. Katz

Behaviors can be homologous just like any other trait can be. This study directly compared neural circuit mechanisms underlying homologous behaviors in two closely-related species.

   This work originates from two questions. First, we wanted to grasp a clue to figure out how species-specific behaviors have evolved. Mollusks are good models to study this because of their wide variety of speciation and the simplicity of the nervous system. In other animal models, functional elements of a neural circuit often consist of a population of neurons having the same function. This makes it difficult to manipulate because there are so many. In contrast, mollusks have one large neuron playing a key role in generating motor output for behavior. Their behaviors are also simple and reliable. The neuronal activity can be precisely manipulated so that one can easily relate one neuron to one behavior. By looking into molluscan species, we hoped we might be able to witness how a species-specific behavior has evolved.
   Secondly, we have been wondering what would happen if we swap a neural circuit of one species with that of the other species. There is a technique called "dynamic clamping", by which one can modify the strength of synapses or membrane conductances by injecting electrical current into neurons. The amount of the injected current is calculated in real-time by a computer based on the membrane potential that is being recorded. With this technique, we hoped we could reveal a crucial element that provides a neural circuit function.

   In this paper, we compared two neural circuits underlying swimming behaviors of two nudibranchs, Melibe leonina and Dendronotus iris. These sea slugs swim by flexing their body from left to right. These behaviors are likely to be homologous because both species belong to a clade that consists only of families that contain species that swim in the same way.
   We found that their swimming behaviors are produced by distinct neural circuit mechanisms. In Melibe, one of the neurons called Si3 makes an inhibitory synapse to fine-tune the rhythm made by other neurons.  In Dendoronotus, Si3 provides excitatory drive to other neurons to induce their rhythmic activities. When the Si3 synapses were blocked by curare, the swim rhythm slowed down in Melibe; whereas in Dendronotus, curare abolished the motor pattern. Replacing these synapses by artificial computer-generated synapses using "dynamic clamping" immediately restored the motor pattern. Using the dynamic clamp, we also rewired the Dendronotus circuit to the Melibe circuit. Then the Dendronotus neurons started to burst like the Melibe neurons in curare.

   From the results, we discussed that the neural mechanisms underlying homologous behaviors appear to have diverged but are still interchangeable in the other species. This has important significance for making inferences into neural mechanisms based on behavior. It also provides a real life instantiation for a prediction based on models, that there are multiple circuit architectures that can produce the same pattern of activity.

Molluscan Memory of Injury: Evolutionary Insights into Chronic Pain

 Brain Behav Evol 2009; 74: 206-218

DOI: 10.1159/000258667

Edgar T. Walters and Leonid L. Moroz

In this paper, the authors raise an interesting hypothesis that the plasticity mechanisms underlying learning and memory in higher organisms may have evolved from adaptive responses that repair damaged neuronal processes or body parts. Persistent nociceptive sensitization in Aplysia nervous system displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. The original responses induced regrowth of damaged axon, increased excitability, enhanced release of transmitter, and reorganization of cellular network. Such compensatory responses would be critical for survival in the early age.

Before discussing about the origin of chronic pain and synaptic plasticity, the authors also described how great Aplysia is. They compared molluscs with other model systems like arthropod and nematode by comparing the number of gene homologues that are shared by mammals, and their evolutionary distances from mammals. Mollucs are closer to mammals because they split earlier. Slower rate of gene evolution provided molluscs more homologous genes associated with human disease than Drosophila or C. elegans. DNA methylation can also be seen in molluscs. Thus, integration of genomics and physiological studies in molluscan neurons offers a powerful comparative approach to address molecular and cellular aspects of selected neurological problems.

Altogether, molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.

Review: New genes from old: asymmetric divergence of gene duplicates and the evolution of development

Holland PW, Marlétaz F, Maeso I, Dunwell TL, Paps J. 2017

Phil. Trans. R. Soc. B 372: 20150480.

http://dx.doi.org/10.1098/rstb.2015.0480

Genome or genes sometimes get duplicated just partially within a single cell even when it is not dividing. The gene duplication and divergence provide a critical source of genetic novelty during evolution. This review paper discusses the fates of duplicated homeobox genes, focusing on asymmetric divergence after gene duplication. Duplicated genes differentiate independently (or asymmetrically) to different degrees. They often become key genes of new functions.
   One of the copied genes can mutate and sometimes acquires a new function, while the others may lose their function. Overall number of genes changes as a result of total duplication-modification and deletions. Repetition of this process over millions of years result in a whole family of genes in a single genome. Duplicate copies often change asymmetrically to have different functions. Their expression becomes different in time and loci. This is how orthologs and paralogs were made.

This review provides examples are Hox genes in Lepidoptera, TALE-class genes in molluscs, extra PRD-class genes in placental mammas.  Tapeworms have lost around one-third of all homeobox genes generally present in bilaterian animals. The evolution of the globulin gene family shows how DNA duplications contribute to the evolution of organisms (Molecular Biology of the Cell, pp461).

Production of redundant copies and repurposing them for other functions sounds similar to the neural circuit evolution.  It seems like evolution is all about repurposing of given resources to create new functions. 

Journal club: Development of the nervous system in Solenogastres (Mollusca) reveals putative ancestral spiralian features

Redl et al. EvoDevo 2014 5:48
DOI: 10.1186/2041-9139-5-48

Background: The evolutionary emergence of the Mollusca is unclear.  Some have proposed that molluscs stem from unsegmented organisms, while others say they stem from a segmented annelid-like ancestor. 

   In this study, the authors investigated the development of the nervous system in two species of solenogasters to describe the larval nervous system and also to test the hypotheses on segmented or unsegmented ancestry of molluscs. 

Observations: During the embryonic development, first neurons appear at the apical and abapical pole; the flask-shaped cells of the apical organ and the large cells associated with the suprarectal commissure are lost.

   The neuropile beneath the apical organ develops into the cerebral commissure. The cellular posterior connection of the lateral neurite bundles becomes the suprarectal commissure.

   Interestingly early nervous system development in the polychaetes shows strong similarity to the mode of neural development described here for solenogasters. They both develop apical organ with flask-shaped cells, and a single pair of longitudinal neurite bundles. Similarity in the pattern of serotonin-like immunoreactivity, and formation of the CNS from anterior and posterior ends. 

Conclusions: This study supports a nonsegmented ancestry of molluscs, but there is similarities between solenogasters and polychaetes during early nervous system development, such as the formation of the nervous system from an apical and abapical neurogenic domain.
   The authors suggest that they share neural features descent from the last common ancestor, which had no segmentation.  Segmentation may have evolved only along the line leading to the annelids.

Journal club: Evolution of highly diverse forms of behavior in molluscs

Current Biology 26, R965-71 (2016)
Binyamin Hochner and David L. Glanzman
DOI: http://dx.doi.org/10.1016/j.cub.2016.08.047

This short review paper starts off with the comparative anatomy of the nervous system.  The authors discuss the diversity of the nervous system and its co-evolution with body plan by showing a variety of nervous systems from Solenogastres to cephalopods. Then, cellular mechanisms of synaptic plasticity underlying learning in the gastropod Aplysia and the cephalopod Octopus were discussed.

The first part was fun to read.
Comparative anatomy of the nervous system is a good reminder that the molluscan nervous system, or the medullary cord, is organized in a ladder-like fashion. The loss of collinear pattern of gene expression may explain their simple body plans. The supremacy of Octopus in the motor and cognitive abilities can be due to the high expansion of two developmentally important gene families, extensive transposable element activity, and genome rearrangements.

The second part was somewhat boring.
The title says the diversity of behavior, but this part actually covers just synaptic plasticity in Aplysia (serotonin-mediated long-term facilitation) and Octopus (long-term potentiation). The mechanisms underlying the serotonergic enhancement of synaptic strength has already been described five hundred times elsewhere.  Plus, I don't think this is a valid comparison to discuss about the evolutional process, because the gill-withdrawal reflex and the higher-order learning are completely different brain functions. Such comparison merely shows different types of learning regardless of species, not actually explains the species-dependent differences or the evolution. This is like comparing the spinal reflex and motor learning in two different vertebrate species. No wonder they are different; synaptic plasticity has little to do with the diversity of behavioral expressions.

The Dendronotus and Melibe paper in Current Biology

My latest paper appeared in Current Biology!

Different Roles for Homologous Interneurons in Species Exhibiting Similar Rhythmic Behaviors

This paper describes the differences in synaptic properties and organization of the swim CPGs in closely-related nudibranch species, Melibe leonina and Dendronotus iris. These animals show very similar swimming behaviors with left-right body flexions. However, their CPGs have quite different network organization; they have different synaptic connectivity and responded differently when perturbed by current injections. Thus, similarity in species-typical behavior is not necessarily predictive of common neural mechanisms.



We also discuss about how species-specific behaviors have developed through the animal evolution. We showed that, even though closely-related animals with similar neuronal architectures exhibit similar behaviors, some degree of divergence can be found in the underlying neural circuitry. A pair of homologous neurons in one species have regulatory role to modify the motor rhythm, whereas in the other species they are part of the CPG and reinforce the rhythm regularity. We don't know what is the functional significance; maybe such difference will be hidden until some time when it becomes more critical in the face of some environmental perturbations.