On chemical and synaptic brains and the evolution of nervous systems

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
some neurons may be sensory, tuned to environmental cues
neurons have projections
connections by synapses
connections are specific (wiring specificity)
organised into a complex network (connectome)
is there a different way to wire complex networks?

A synaptic connectivity matrix

Origin of the nervous system as a reflex arc?

George Howard Parker (1864 - 1955)

Origin of the nervous system as a reflex arc?

independent effectors evolved first
then receptor-effector systems
then organisation into nerve nets
diffuseness of transmission (no centralisation)

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

Chemical signalling between cells

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

Neuropeptides are produced from precursor peptides

mass spectrometry (LC-MS/MS) can identify the processed neuropeptides
MetOH/acetic acid extraction of small peptides
search also for modifications

Global view of the evolution of neuropeptides

Tree of main animal lineages

Emerging marine model organisms

Image: Patrick Steinmetz

Nematostella vectensis

Platynereis dumerilii

Trichoplax adhaerens

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 – fission

Placozoa – external digestion

adhearance to substrate
secretion of digestive enzymes

external digestion

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

Placozoa – peptide effects

SIFGamide peptide

WPPF peptide

How can we identify and characterise neuropeptide receptors?

Identification of placozoan neuropeptide GPCRs

Neuropeptides in Cnidaria

Peptidergic nerve nets

LWamide transgene

Large-scale GPCR-peptide screen in Nematostella

Mapping to single-cell data

A peptidergic connectome

Neuropeptides can induce behaviour in cnidaria

Hydra vulgaris

Hym-248 neuropeptide induces somersaulting

A peptidergic model for Hydra somersaulting

Peptide-controlled spawning in Clytia

an opsin coexpresses with the maturation-inducing neuropeptide

CRISPR knockout animals for the opsin do not spawn in light

Peptide-controlled spawning in Clytia

The connectome is multilayered

Platynereis dumerilii

breeding culture, full life-cycle
embryos daily, year round
genome sequence
microinjection, transgenesis
neuron-specific promoters and antibodies
knock-out lines
neuronal connectome
whole-body neuronal activity imaging
whole-animal pharmacology by bath application 😎

Platynereis dumerilii

Spawning
movie by Albrecht Fischer

Synchronously developing larvae

Whole-body volume EM of an entire three-day-old larva

Whole-body connectome of a segmented annelid larva

The nervous system of the larva

~2,000 neurons

Synaptic connectome

Many peptides and modulators in the circuit

Multiplex immunogold for neuropeptides in the EM volume

Mapping of (pro)neuropeptides

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

Strong cPRC activation after UV exposure

Single-cell data for cPRC cells and their interneurons

::: aside Kei Jokura :::

Nitric-oxyde synthase in postsynaptic interneurons

HCR Transgenic labelling

NO is produced in the neuropil after UV stimulation

NOS mutants have altered cPRC response

NOS mutants have altered INRGW and motoneuron response

NOS mutants show defective UV avoidance

Two unusual guanylyl cyclases in the cPRCs

NIT-GC1 RNA

NIT-GC1 protein

NIT-GC2 RNA

NIT-GC2 protein

Two unusual guanylyl cyclases 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.

Integration and memory of UV exposure

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
Ira Maegele
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