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Abstract :
[en] Etmopterus spinax (Linnaeus 1758), is a bioluminescent deep-sea shark able to emit a blue-green
light thanks to thousands of tiny epidermal light organs called photophores spread mainly over the
body ventral side. Photophores are composed of photogenic cell clusters (photocytes) enclosed in a
pigmented sheath and topped by a shutter-like structure and a lens. Counter-illumination is
suspected to be the shark bioluminescence main function. According to recent pharmacological
studies, this camouflage function is achieved thanks to a complex photophore control mechanism,
which involves hormones (melatonin, prolactin, -MSH) and neural agents (NO, GABA). In addition,
new evidences suggest that extraocular photoreception could play a role in the bioluminescence
control mechanism.
Here, we investigated, using transcriptomic analyses and immunodetection labelling, (i) the E. spinax
opsin-based extraocular photoreception; (ii) two hormonal G-protein coupled receptor: melatonin
(mel1b) and -MSH receptor (MC4R); and (iii) the different G-protein implicated in the transduction
cascade events between photoreception and photoemission. cAMP concentration assays were
performed on photogenic tissues stimulated with melatonin (for light induction), with -MSH (for light
inhibition) and with blue light (to mimic natural light emission).
Transcriptomic analyses allowed us to identify, in the ventral skin, the expression of an
encephalopsin (non visual ciliary opsin), a mel1b receptor, a MC4R receptor, and proteins Gi / Go /
Gs / Gt. Immunodetection results allow us to visualized the distribution of all these proteins near or
inside the shark photophores. cAMP concentration assay demonstrated that the cAMP levels vary in
function of treatments (melatonin, -MSH, blue-green light illumination). These results support the
implication of intracellular cAMP in the transduction cascade controling the light emission within the
photogenic cells.
The hypothesis of a deep interaction between light emission and reception is strongly supported. The
implication of an extraocular photoreception cascade is suggested in the Etmopteridae bioluminescence control mechanism.
