On chemical and synaptic brains and the evolution of nervous systems
Universität Bonn, Institut für Physiologie
Evolution of nervous systems
mollusk 
Kilias, 1985
planarian 
Kellogg, 1903
vertebrate 
Jefferys, 1763
When did the first nervous systems evolve and how did they work?
Budd & Jékely
Detlev Arendt
Dickinsonia costata
What is a nervous system?
- consists of cells (neurons, glia etc.)
- neurons are excitable
- neurons can influence each others activity
- and the activity of effectors
- connections are specific
- organised into a complex network (connectome)
- some neurons may be sensory
Origin of the nervous system as a reflex arc?
George Howard Parker (1864 - 1955) 
- independent effectors evolved first
- then receptor-effector systems
- then organisation into nerve nets
Neuronal communication by chemical signals?
- argued against the “connectionist view”
- proposed instead that neurons could communicate by specific chemical signals
- like a radio broadcast
- chemicals diffuse through the nervous system
- detected by specific chemical receptors in the target cells
- communications could occur in the absence of synaptic transmission
- discovery of pituitary hormones (1950ies)
Paul Alfred Weiss (1898 – 1989)
Chemical signalling between cells
- some cells release a chemical (e.g. short peptide)
- other cells express a specific receptor for the chemical
- receptor activation -> change in cell state
- a specific cell-to-cell signalling is possible
- no synaptic connection
The chemical brain hypothesis
- several cell types, each expressing a different chemical signal
- several specific receptors expressed in target cells
- chemical ‘wiring diagram’
- no synaptic connection
Chemical connectomes
- sending and receiving cells
- a chemical connectivity matrix
- can be arbitrarily complex
Chemical connectomes
- co-expression of chemical signals or receptors
- combinatorics, cascades
- synergism, antagonism
What was in there?
Dickinsonia costata
Marine animal models
Nematostella
Hydra
Macrostomum
Platynereis
Trichoplax
Placozoa – no synapses, many peptides
Trichoplax adhaerens
- placozoans - simplest animals
- no neurons, no muscles
- upper and lower ciliated epithelium
- many neuropeptide-like molecules
- few morphological cell types
Placozoa – no synapses, many peptides
- neuropeptides are expressed in specific cell populations
- non-overlapping expression
- over 30 proneuropeptides
- peptidergic cells tile the epithelium
Placozoa – peptide effects
- neuropeptides influence placozoan behaviour
LF peptide
Identification of placozoan neuropeptide GPCRs
Large-scale GPCR-peptide screen in Nematostella
- mass spec for small neuropeptides
- 64 peptides derived from 33 neuropeptide precursors
Neuropeptide transmitter systems in cnidarians
- over 1000 GPCRs in the Nematostella genome
- screened 161 Nematostella GPCRs
- 31 receptors specifically activated by one of 14 peptides
Independent diversification of receptor-peptide systems
- at least 8 ancestral cnidarian families of peptide GPCRs
- independent divergence from bilaterian receptors
- no one-to-one orthologs to bilaterians
- prediction of receptor-ligand pairs across many cnidarians
The multilayer peptidergic connectome of Nematostella
- every node is a cell type from scRNAseq data
- links connect peptide-expressing with receptor-expressing cells
- different colours represent different peptide-receptor pairs
- highly peptidergic nervous systems
A peptidergic model for Hydra somersaulting
What about bilaterians?
Caenorhabditis elegans
- discovery of 461 peptide-GPCR pairs
The connectome is multilayered
Ok, but the cortex…
- 37 cognate NPP/NP-GPCR pairs (at least 37 layers)
Platynereis dumerilii
- breeding culture
- embryos daily
- genome sequence
- microinjection, transgenesis
- neuron-specific promoters and antibodies
- knock-out lines
- neuronal connectome
- neuronal activity imaging
Whole-body volume EM of an entire three-day-old larva
Synaptic connectome
Many peptides and modulators in the circuit
Multiplex immunogold for neuropeptides in the EM volume
Large-scale mapping of neuromodulatory networks
The connectome is multilayered
UV response in Platynereis larvae
UV-responding brain ciliary photoreceptors (cPRCs)
No UV avoidance in c-opsin1 knockouts
No cPRC response in c-opsin1 knockouts
Circuitry of ciliary photoreceptors
Circuitry of ciliary photoreceptors
Strong cPRC activation after UV exposure
Nitric-oxyde synthase in postsynaptic interneurons
HCR in situ
Transgenic labelling
Immunostaining
NO is produced in the neuropil after UV stimulation
NOS mutants have altered cPRC response
NOS mutants have altered INRGW response
NOS mutants show defective UV avoidance
An unusual guanylyl cyclase in the cPRCs
NIT-GC1 RNA
NIT-GC1 protein
An unusual guanylyl cyclase in the cPRCs
NIT-GC morphants have altered circuit activity
Mathematical modelling of the circuit
Model fitting
wild type
NOS-11
NOS-23
NIT-GC2 mo.
Synaptic and chemical signalling work together
Acknowledgements
- Sanja Jasek
- Alexandra Kerbl
- Emily Savage
- Simone Wolters
- Lara Keweloh
- Kevin Urbansky
- Karel Mocaer
- David Hug
- Benedikt Dürr
- Emelie Brodrick (Exeter)
Former lab members
- Kei Jokura, Luis A. Bezares-Calderón, Luis A. Yanez-Guerra, Victoria Moris, Daniel Thiel, Albina Asadulina, Cameron Hird, Adam Johnstone, Markus Conzelmann, Nadine Randel, Philipp Bauknecht, Martin Gühmann, Cristina Pineiro-Lopez, Nobuo Ueda, Aurora Panzera, Csaba Verasztó, Elizabeth Williams
