Tracking the deep evolutionary origins of neurons

Project Information

ORIGINEURO

Grant agreement ID: 101044989

DOI

10.3030/101044989

EC signature date 25 May 2022

Start date 1 January 2023

End date 31 December 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 000 000,00

EU contribution

€ 2 000 000,00

Coordinated by

UNIVERSITETET I BERGEN
Norway

Periodic Reporting for period 1 - ORIGINEURO (Tracking the deep evolutionary origins of neurons)

Reporting period: 2023-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

When and how did the first neurons evolve? How did this key event lead to the vast biodiversity we see today? The evolutionary origin of first neurons remains poorly understood. Strikingly, early branching animals, as well as their closest unicellular relatives, contain key components of the molecular toolkits for neuronal functions. This indicates that neurons and nervous systems may have evolved early in animal evolution. Ctenophores, strong candidates for one of the first animal lineages, are ideal model organisms to study the evolutionary origin of neurons. In the project “Tracking the deep evolutionary origins of neurons” we aim to uncover the ultrastructure and connectivity of ctenophore neurons forming the subepithelial nerve net, to reveal ctenophore neuron development and to find out how ctenophore neurons work and what they do. This will be approached by techniques such as correlative volume electron microscopy, super-resolution imaging, quantitative live cell imaging, state-of-the-art molecular biology and single cell RNA sequencing. At the end, we hope to understand how ctenophore neurons work, uncovering key aspect of nervous system evolution and help clarify the current dispute regarding the origin of neurons and nervous systems.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Objective 1 - Ctenophore nerve net connectivity and molecular composition.

Using 3D electron microscopy, we found that nerve-net neurons are not separate entities, but rather are interconnected through continuous neurite plasma membranes without evidence of synapses (syncytial nervous system). Our findings suggest fundamental differences of nerve net architectures between ctenophores and cnidarians/bilaterians and offer an alternative perspective on neural network organization and neurotransmission. We are currently working on a complete connectome of a 5-day old larva. 2500 of ca. 6000 sections have been prepared and imaging of the first 300 sections has started as well. This work is done in collaboration with the EM facility at the University of Bergen.

Objective 2 - Development of ctenophore nerve net neurons.

We found that the ctenophore Mnemiopsis leidyi is capable of reversal from mature lobate to early cydippid when fed following a period of stress. Our findings illuminate central aspects of ctenophore development, ecology, and evolution and show the high potential of Mnemiopsis as a unique model system to study reverse development and rejuvenation. Moreover, we described an easy and affordable setup to maintain a stable culture of the ctenophore Mnemiopsis which allows for stable laboratory lines, a continuous supply of embryos for molecular biological studies, and independence from population responses to environmental fluctuations. We are currently working towards understanding the development of the ctenophore nervous system at a single cell level.

Objective 3 - Functional characterisation of ctenophore neurons.

We are now able to perform imaging of the nerve net with fluorophores in live animals, allowing visualization of neurites and cell bodies, over short (second) to medium (min-hours) time scales. Apart from being able to identify the cells in living tissue, this has identified the dynamic nature of structures within the neurites, which are likely functionally relevant.
We have generated codon harmonized version of GCaMP6 for Mnemiopsis and cloned into an in vitro transcription vector followed by generation and purification of GCaMP6 mRNA for injection. Moreover, we generated a Mnemiopsis expression plasmid for injection. Both, mRNA and the plasmid generation were injected into 1, 2, and 4 cell stage embryos. To better allow ctenophore monitoring we developed mounting and imaging methods for visualizing the SNN (with cellular resolution) in living animals. For perturbations of the nerve net – splice blocking morpholinos targeting a neuropeptide have been designed and ordered, as well as gRNA targeting a neuropeptide has been designed and ordered for injection.

Outcome:

Soto Angel JJ, Burkhardt P (2024) Reverse development in the ctenophore Mnemiopsis leidyi. Proceedings of the National Academy of Sciences 121 (45): e2411499121.
Soto Angel JJ, Nordmann EL, Sturm D, Sachkova M, Pang K, Burkhardt P (2024) Stable laboratory culture system for the ctenophore Mnemiopsis leidyi. Methods in Molecular Biology 2757: 123–145.
Burkhardt P, Colgren J, Medhus A, Digel L, Naumann B, Soto Angel JJ, Nordmann EL, Sachkova MY, Kittelmann M (2023) Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems. Science 376 (6642): 293-297.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

It is widely accepted that neurons communicate through synapses—either chemical or electrical—in all studied animals. However, our findings reveal that ctenophore neurons within the subepithelial nerve net share a continuous plasma membrane, forming a syncytium. This discovery provides a fresh perspective on the evolution of neuronal networks and mechanisms of neurotransmission.

We report the occurrence of ontogeny reversal in Ctenophora. This discovery of an alternative, upstream developmental pathway—allowing escape from the typical ontogenetic trajectory toward senescence—has broad implications for the plasticity of developmental programs in animals. In this context, reverse development in a ctenophore raises intriguing questions about its phylogenetic distribution, evolutionary origins, and the broader evolution of life history strategies both within and beyond Ctenophora.

Until the end of the project we hope to shed light on the deep evolutionary origins of nervous systems in animals by revealing the entire connectome of a ctenophore and by a detailed description of how ctenophore neurons form and function.

Unique nervous system of the ctenophore Mnemiopsi leidyi