An international research team headed by scientists at the Neurobiology Division of MRC Laboratory of Molecular Biology, has built the first ever map showing how every single neuron in the nervous system of the tiny nematode worm Caenorhabditis elegans communicates wirelessly via neuropeptide signaling.

Neuropeptides allow communication between neurons that are not immediately next to each other, so their networks can be thought of as a wireless connectome. A connectome is a map of the neurons that make up an organism’s brain and the detailed circuitry of neural pathways within it. The new map, which details 31,479 neuropeptide interactions between the worm’s 302 neurons, shows where each neuropeptide, as well as each receptor for those peptides, acts in the animal’s nervous system.

Understanding how neurons communicate via neuropeptides will help scientists understand how emotions and mental states are controlled, as well as provide new insights into neuropsychiatric conditions such as eating disorders, obsessive compulsive disorder (OCD), and post-traumatic stress disorder (PSTD).

“The idea of mapping these wireless networks has been one of our goals for a long time, but only now have the right combination of people and resources come together to make this actually possible,” said research co-lead William Schafer, PhD.

The team reported on its work and results in Neuron, in a paper titled, “The neuropeptidergic connectome of C. elegans,” in which they concluded, “The novel structure and topology of this neuropeptide connectome serves as a prototype for understanding the brain-wide organization of peptidergic signaling in a whole organism.”

Understanding how behavior arises from neuronal interactions in the brain is one of the great challenges of modern neuroscience, the authors wrote. “Efforts are ongoing to map synaptic wiring diagrams, or connectomes, to understand the neural basis of brain function.” However, the team continued, while most connectomics research has focused on synaptic connectivity, chemical synapses are not the only means by which neurons communicate. “ … extrasynaptic, ‘wireless’ signaling by neuropeptides is widespread and plays essential roles in all nervous systems … Neuropeptides are ancient and conserved signaling molecules that mediate important functions in the brains of all organisms.”

Neuropeptides play a critical role in lasting biological responses. “Neuropeptide systems play conserved roles in the control of behavioral states, including those involved in feeding, sleep, arousal, reproduction, and learning,” the authors further commented. They function throughout the nervous system, but can also act on other types of tissue as hormones. Oxytocin is one example: it acts on various circuits in the brain that affect bonding between parents and children, but it also causes contraction of the muscles of the uterus during childbirth. Even when neuropeptides act in the brain, they can allow communication between neurons that are not connected by the synapse junctions that are used by classical neurotransmitters.

Schafer added, “Neuropeptides and their receptors are among the hottest new targets for neuroactive drugs. For example, the diabetes and obesity drug Wegovy targets the receptor for the peptide GLP-1.” As the authors also noted, “In humans, neuropeptide receptors have become sought-after targets for new neuropsychiatric treatments; 50 drugs targeting peptidergic GPCRs have been approved by the FDA.” Nevertheless, Schafer pointed out, “… the way these drugs act in the brain at the network level is not well-understood.”

Conceptual graphic illustrating the diffusion of neuropeptides from neuron cell bodies within the worm nervous system
Conceptual graphic illustrating the diffusion of neuropeptides from neuron cell bodies within the worm nervous system. [MRC Laboratory of Molecular Biology]
Since most neurons seem to make both neuropeptides and neuropeptide receptors, the communication pathways formed by neuropeptides make up large neural networks. These networks are extensive, complex, and critical to the functioning of the brain. As such, they are important for understanding the neuronal basis of behavior. Researchers are making rapid progress in building connectomes for simple organisms, but until now, no one had managed to build a map of a neuropeptide network in any animal.

For their study, Schafer, together with co-lead Lidia Ripoll-Sánchez, also at the MRC Laboratory of Molecular Biology, Petra Vértes, PhD, of Cambridge University, and Isabel Beets, PhD, from KU Leuven, focused their studies on C. elegans. This soil-living, 1 mm-long nematode has a very simple anatomy, but shares many of the essential biological characteristics that are central problems of human biology.

Ripoll-Sánchez explained, “Basic mechanisms of neuropeptide signaling are shared in all animals: neuropeptides are released from dense core vesicles in cells and diffuse to neurons unconnected to the releasing cell by wired synapses. The worm’s nervous system is anatomically small, but at the molecular level its neuropeptide systems are highly complex, showing significant parallels to larger animals, and its synaptic connectome shows many features that are conserved in bigger brains.”  The scientists further commented, “… neuropeptide signaling in nematodes shows surprising conservation and similar diversity to neuropeptide signaling in the human brain, despite vast differences in neuron number and anatomical complexity.”

Researchers have previously mapped the C. elegans synaptic neuronal connectome, the authors further pointed out. “C. elegans was the first organism with a completely mapped synaptic neuronal connectome, with each of its 302 neurons and approximately 2,300 synaptic connections identified through EM reconstructions.”

To build the C. elegans neuropeptide map the investigators combined biochemical, anatomical, and gene expression datasets, using them to determine which neurons can communicate with each other using specific neuropeptide signals. Once the network was constructed, they used graph theory to analyze its structure and identify key topological features as well as neurons with important roles in linking different parts of the network.

Conceptual graphic illustrating the diffusion of neuropeptides between neurons in the worm nervous system
Conceptual graphic illustrating the diffusion of neuropeptides between neurons in the worm nervous system. [MRC Laboratory of Molecular Biology]
As well as generating the first comprehensive map of neuropeptide signaling in a whole animal, the researchers found that the wireless neuropeptide network in C. elegans has a different structure from wired connectomes. They are denser, more decentralized, and have different key neurons, or hubs. As they noted, the draft connectome generated through the reported study appears … “remarkably dense; even with highly conservative assumptions about peptidergic diffusion the network is over 10-fold denser than the C. elegans synaptic connectome.” Moreover, the team pointed out, given factors including the conservative assumptions of their models, it’s likely that the “actual” neuropeptide connectome will be even more dense than the draft network.

“A salient feature of the neuropeptide connectome is its decentralized topology, which contrasts sharply with the more centralized structure of wired neural connectomes,” the scientists also noted. This “contrasts sharply with the more centralized structure of the wired neural connectomes … The decentralized structure of the neuropeptidergic connectome implies that it may employ different strategies for computation and information flow than more centrally organized synaptic networks.” Added Schafer, “The structure of neuropeptide networks suggests that they may process information in a different way to synaptic networks. Understanding how this works will not only help us understand how drugs work but also how our emotions and mental states are controlled.”

Graphic illustrating the wide range of wireless neuropeptide network patterns identified in the worm nervous system
Graphic illustrating the wide range of wireless neuropeptide network patterns identified in the worm nervous system. [MRC Laboratory of Molecular Biology]
The network was also found to connect parts of the nervous system that are isolated from the wired synaptic connectome. Ripoll-Sánchez commented, “We expect the neuropeptide connectome of C. elegans will serve as a prototype to understand wireless signaling in larger nervous systems.” As the authors noted, “Basic mechanisms of neuropeptide signaling are shared in all animals, from nematodes to mammals, and although the C. elegans nervous system is anatomically small, at the molecular level its neuropeptide systems are highly complex and show significant homology to other animals.”

Jo Latimer, head of neurosciences and mental health at the Medical Research Council, added, “This is another exciting and significant body of work by colleagues at the MRC Laboratory of Molecular Biology and others, adding to the connectome work of LMB researchers earlier this year. Not only have they worked out which neuropeptides act where in the animal’s nervous system, they have discovered that the network is complex, but clearly organized, with an information processing circuit within it. This is a further important step forward in understanding how brains and nervous systems work, and this increased understanding may have the potential to lead to the future development of targeted therapies for a range of conditions.”

The next step will be to see whether the principles by which neuropeptide networks in worms are organized also apply in bigger brains. “In principle, the approaches described here should also be applicable to mapping the peptidergic networks of animals with larger brains,” the authors pointed out. They are currently working with other collaborators to map wireless neuropeptide networks in animals such as fish, octopuses, mice, and even humans.

Previous articleFoxp3 Essential for Creating Regulatory T Cell Chromatin Architecture
Next articleInaccurate Genetic Association Studies Pose Issues for Genomic Medicine