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Repairing neural damage in a C. elegans chemosensory circuit using genetically engineered synapses

Ithai Rabinowitch, Bishal Upadhyaya, Aaradhya Pant, Jihong Bai

Posted on: 17 May 2020

Preprint posted on 17 April 2020

Article now published in Cell Systems at http://dx.doi.org/10.1016/j.cels.2020.12.003

Unexpected bonds, help to repair and respond: Artificially expressed electrical synapses in C. elegans neurons strengthen weak signals and restore circuit function and behaviour.

Selected by Sahana Sitaraman

Background:

Behind every animal behaviour lies the metaphorical guiding hand of a neural circuit. For many decades, neuroscientists have worked towards understanding how neurons wire together to form functional circuits and how these circuits help execute and modulate behaviours. A widely used approach to studying circuit function involves tinkering with the component neurons and their activity, using techniques such as cell ablation[1] and optogenetics[2]. But recent advances in synthetic neurobiology have allowed researchers to alter circuit wiring by introducing specific synaptic connections and thereby reprogram the associated behavioural output[3].

Remodelling circuit architecture by introducing artificial links can not only help tease apart the roles of different circuit modules, but also repair them in case of synaptic loss or neuronal degeneration. Due to redundancy present in circuit configurations, rerouting information flow by introducing engineered synapses could help restore signal transmission and normal behaviour. This study uses the well described Caenorhabditis elegans (roundworm) nervous system[4], particularly the chemosensory circuit, to test this possibility. The authors use simple experiments to check if expressing engineered electrical synapses in a damaged circuit helps restore information flow and rescue chemotaxis behaviour.

Key Findings:

In 1986, John Graham White described the entire structure of C. elegans nervous system using electron microscopic reconstruction[4]. The high-resolution images allowed White and his colleagues to identify and map out the complete set of 302 neurons, their connections and all the neuronal circuits. Due to the immense level of detail available for the C. elegans nervous system and its amenability to genetic manipulations, it serves as the perfect model system to observe circuit repair by synthetic synapses.

The authors chose the well characterised chemosensory circuit for their study, which comprises of a pair of AWC odor sensory neurons, sending parallel projections to multiple interneuron classes like AIA and AIB, which are also present in pairs (Figure 1). While structurally similar, the left and right AWC are functionally distinct, sensing different odors. But both sense isoamyl alcohol, the chemical used in the experimental assays by the authors.

Figure 1: Simplified C. elegans chemosensory circuit diagram. Thickness of line represents connection strength. Modified from Figure 1, Rabinowitch et al. 2020 under CC-BY-NC-ND 4.0 International license.

 

To determine how altering this circuit would affect its functioning, the researchers removed the AIA neurons and observed the activity AIB neurons. Absence of AIA strongly diminished AIB response, to both isoamyl alcohol presentation and removal. This change also had downstream effects, causing a reduction in chemotaxis of the worms towards the odor source. What was interesting was a further decrease in chemotaxis on removal of both AIA and AIB, suggesting that AIB cells still maintained some level of functioning, possibly due to inputs coming from AWC.

Having established a system where removal of a neuron type affected the proper operation of the circuit, the authors probed the effects of inserting genetically engineered electrical synapses into the system. This was achieved by expressing the mammalian Connexin36 (Cx36) electrical synapse protein in AWC and AIB neurons, with the expectation that the signal transmission between them would be strengthened, compensating for the lack of AIA to AIB connection. As expected, expressing Cx36 in both neuron types led to a rescue in chemotaxis, even exceeding wildtype levels. But surprisingly, expressing Cx36 in either neuron type also rescued the behaviour. This could be explained by lateral electrical coupling between left and right AWC or AIB neuron types, leading to an enhancement of weak signals.

To test this hypothesis, the authors tested the response of AWC neurons to low concentration of isoamyl alcohol, with and without Cx36 expression. Cx36 mediated coupling between AWC neurons caused an upward shift in their response to the same concentration of the odor, essentially strengthening a weak stimulus. This was corroborated by observing the activity of AIB neurons with ectopic Cx36, in the absence of AIA cells. Coupled AIB neurons showed an enhanced response to odor, despite the removal of AIA.

Lastly, to weigh the effects of AWC to AIB coupling versus lateral coupling, the team expressed Cx36 only in one of the AWC neurons along with AIB neurons. They observed
that the recovery in chemotaxis for the above experiment was significantly more than when Cx36 was expressed only in AIB neurons. This suggests a critical role for AWC-AIB coupling in behavioural recovery of the worm.

Using uncomplicated experiments, the authors have been successful in demonstrating how engineered synapses can be used to restore the information transfer within a damaged neuronal circuit. They have also been triumphant in showing the functional recovery of the circuit as well as the behaviour it controls.

What I liked about the preprint:

I found this study to be extremely innovative. While there are many approaches to circuit repair, like brain grafts, stem cells[5], and magnetic stimulation[6], this is one of the few studies targeting the problem at a synaptic level. Their use of electrical coupling to overcome the absence of chemical synapses is clever and easy to translate to other systems.

Electrical synapses are generally overlooked in our pursuit to understand the brain. So, the focus on exploring their benefits and contributions to circuit function was another reason I enjoyed this preprint.

Questions for the authors:

  1. How do you propose to repair the chemosensory circuit to respond to odors like butanone and 2,3-pentanedione which activate either AWC-ON or AWC-OFF neuron?
  2. How translatable is this method of circuit recovery towards therapeutic advances for neurodegenerative diseases like Alzheimer’s disease?
  3. Keeping in mind the massive role glia play in synapse formation and pruning, have you considered coupling glial cells to reprogram circuits?

References:

1. Marquart GD, Tabor KM, Bergeron SA, Briggman KL, Burgess HA (2019) Prepontine non giant neurons drive flexible escape behavior in zebrafish. PLoS Biol

2. Zhu P, Narita Y, Bundschuh S, Fajardo O, Schärer Y-PZ, Chattopadhyaya B, Bouldoires EA, Stepien AE, Deisseroth K, Arber S, Sprengel R, Rijli FM and Friedrich RW (2009). Optogenetic dissection of neuronal circuits in zebrafish using viral gene transfer and the Tet system

3. Rabinowitch, I. et al (2014) Rewiring neural circuits by the insertion of ectopic electrical synapses in transgenic C. elegans. Nat. Commun

4. White, J., Southgate, E., Thomson, J. & Brenner, S (1986) The structure of the nervous system of the nematode Caenorhabditis elegans. Phil. Trans. R. Soc. Lond. (Biol.)

5. Jessberger, Sebastian (2016) “Neural repair in the adult brain.” F1000Research

6. T. Dufor, S. Grehl, A. D. Tang, M. Doulazmi, M. Traoré, N. Debray, C. Dubacq, Z.-D. Deng, J. Mariani, A. M. Lohof, R. M. Sherrard (2019) Neural circuit repair by low-intensity magnetic stimulation requires cellular magnetoreceptors and specific stimulation patterns. Sci. Adv.

Tags: c. elegans, chemotaxis, circuit repair, connexin, electrical synapse

doi: https://doi.org/10.1242/prelights.20727

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Author's response

Dr. Ithai Rabinowitch and Dr. Jihong Bai shared

1. How do you propose to repair the chemosensory circuit to respond to odors like butanone and 2,3-pentanedione which activate either AWC-ON or AWC-OFF neuron?

First, it is likely that the repair of AWC-ON or AWC-OFF circuits depends on the position of the synthetic electrical coupling. For example, we can infer from our findings that the same engineered left-right synthetic electrical synapses inserted between AIBL and AIBR would be sufficient to overcome deficient butanone or 2,3-pentanedione responses, just like they mitigate reduced isoamyl alcohol sensitivity. We generally speculate that L-R connections between interneurons may have a broad impact on multiple sensory activities, as these interneurons receive and process information from several sensory neurons. Indeed, we are currently examine a hypothesis that an AIBL-AIBR artificial coupling as well as other lateral couplings (e.g., RIML-RIMR), may improve performance in the damaged circuit for any sensory input that is associated with these neurons (e.g., AWB-sensed aversive odors, ASE-sensed salt, etc.).

The situation becomes more complex when synthetic electrical coupling brings in both lateral amplification and direct strengthening into a damaged circuit, as in the case of AWC-AIB connections. On the one hand, synthetic electrical coupling could facilitate the transfer of information from the relevant AWC neuron to the AIB neurons. On the other hand, it could undermine the functional distinctions between the two AWC neurons and dilute their unique sensory signals. Thus, it is hard to predict whether such synthetic connections, would help, or do more damage in the case of two different AWC neurons. An alternative way to address this question could perhaps consist of developing a duplexing approach. Install an independent connection between each AWC and the AIB neurons using, for example, different, incompatible connexin proteins, and thus keep the two pathways segregated. In fact, we will examine this option in future work.

2. How translatable is this method of circuit recovery towards therapeutic advances for neurodegenerative diseases like Alzheimer’s disease?

Second, we resonate with your thoughts regarding the potential generalizability of using genetically engineered synthetic electrical coupling for neural circuit repair. We would love to see this approach mature into new therapeutic strategies. At present, it is early to say whether and how this could be done. However, we believe that a deep understanding of engineered synapses and their impact on damaged neural circuits could pave the way for the rational design of circuit-level approaches for repairing brain function in the future. In the short term, it could be possible to target particular mammalian neuronal populations (scaling up from individual neuron pairs in the compact C. elegans nervous system) and ectopically express in them new electrical synapses, perhaps based on innexin proteins, generating an electrical network that could amplify circuit activity in conditions such as Alzheimer’s. This is of course just a first glance at this key problem.

3. Keeping in mind the massive role glia play in synapse formation and pruning, have you considered coupling glial cells to reprogram circuits?

Third, your question about the contribution of glia is very intriguing. Currently, we don’t have an answer, as we have only engineered electrical coupling between neurons. It is reasonable to postulate that if glial cells, which are electrically active and intimately involved in neuronal dynamics, were artificially coupled, this would have an effect on circuit function and perhaps even on its formation and final configuration. This is an area that is definitely worth exploring.

Thanks for your interest in our work and your questions! We hope that our study will stimulate more research activities and discussions in this exciting new area.

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