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

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.

 

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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