Primitive Sponge provide clues to Nerve cell origin
The first sign of neurons and synapses appeared over 600 million years ago in “cnidarians,” (eg: hydra, sea anemones, and jellyfish) and organisms basal to them like sponges have no neurons or synapses.These Sponges are very simple animals with no internal organs.The period between sponges and cnidarians is believed to be the time when nervous system came into existence.Previous work from Kenneth S. Kosik and Bernard Degnan has shown that genome of the demosponge Amphimedon queenslandica possesses a nearly complete set of post-synaptic protein homologs whose conserved interaction motifs suggest assembly into a complex structure.But these sponge lack some important genes ,which are present in Eumetazoa like receptor genes including the entire ionotropic glutamate receptor family.A relatively small number of crucial innovations to this pre-existing structure may represent the founding changes that led to a post-synaptic element.
Bernard Degnan continued from where he left in 2007 ,this time with Michel vervoort in France showed that despite lacking neurons, the sponge Amphimedon queenslandica still has all the basic stuff like the Notch-Delta signaling system and a proneural basic helix loop helix (bHLH) gene in a manner which resembles the conserved molecular mechanisms of primary neurogenesis in bilaterians.
Neurogenic apparatus available in Amphimedon queensladica:
Amphimedon possesses 16 bHLH genes, one of which, AmqbHLH1, appears to be derived from a single ancestral gene that generated, via gene duplications, the atonal-neurogenin-related bHLH gene families (the â€˜â€˜Atonal superfamilyâ€™â€™) that include most bilaterian proneural genes.
Players of the Notch signaling pathway are also present in the Amphimedon genome, which encodes a single Notch receptor, AmqNotch, and five Notch ligands,AmqDelta1â€“ AmqDelta5, as well as a group of related proteins ( Furin, CSL, Fringe,Presenilin, deltex, Presenilin, Adam, mindbomb and neuralised) that direct the initiation, transduction, and deactivation of Notch signaling in bilaterians.But they lack crucial transcriptional coactivator mastermind and the bHLH repressor enhancer of split homologs.
Neurogenic Gene Expression in Amphimedon:
Previous work by Degnan has shown the expression of post synaptic genes in Globular cells of sponge ,providing the cue that it may resemble a Neuron /Nerve cell of bilaterians.so the authors checked for the expression of neurogenic genes in globular cells and they found out that all Notch ,delta and bLHL1 are expressed during the genesis of Amphimedon globular cells.
Phylogenetic analysis suggests that AmqbHLH1 is homologous to all the atonal-related bHLH genes found in bilaterians,including the proneural genes atonal and neurogenin and this means they should also have pro neural activity in sponges ,like controlling the initial determination of globular cell identity.
If this bHLH1 has proneural activity as suggested by phylogenic analysis then they should have some functions like its eumetazoan (Cnidaria and Bilaterians) counterparts.So in order to test whether bHLH1 of sponge works in bilaterians also they made use of Xenopus and Drosophila system.
When bHLH1 was injected in Xenopus at two cell staged induces an ectopic expression of N-tubulin, an early marker of neuronal differentiation (exactly like Xenopus neurogenin (X-ngnr-1)and ectopically activate neurogenic gene X-delta 1,indicating that AmqbHLH1 has a Neurogenin-like proneural activity in Xenopus.But one important thing to note here is sponge gene can perform some aspects like xenopus homolog.It fails to induce ectopic expression of N-CAM, X-NeuroD, and Xath3 like Xenopus neurgenin.Amphimedon bHLH also induces similar effects like drosophila atonal gene when expressed using UAS GAL4 system.AmqbHLH1 thus possesses structural/ functional properties that are found
across a range of bilaterian proneural proteins.
Why Spoges lack a neuron inspite of having basal machinery?
Amphimedon bHLH1 has similar proneural and expression properties like higher organisms ,which justifies its power to initiate initial events that occur during bilaterian neurogenesis. But as they lack important members of Notch pathways like Mastermind and enhancer of split.In addition they also lack diversification in atonal family members unlike higher organisms(one in sponges and more than 10 in higher animals),suggests that the cellular response to an Amphimedon Notch signal differs in sponges. In eumetazoans different members of atonal super family perform various function and with support from other various proteins ensures formation of nerve cell ,which may not be possible in sponges as they lack diversification in atonal super family.
This study indicates that bilaterian neurogenic circuit, comprising proneural atonal-related bHLH genes along with Notch-Delta signaling was functional in sponges and was used to generate ancient sensory cell.
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A Post-Synaptic Scaffold at the Origin of the Animal Kingdom.
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Sponge Genes Provide New Insight into the Evolutionary Origin of the Neurogenic Circuit.
Curr Biol. 2008 Jul 30. (517)