Bioluminescence in lanternsharks: Insight from hormone receptor localization

Highlights

• Localization of hormonal receptors involved in lanternshark light emission.

• Melatonin/melanocortin receptors; Gαs/Gαi proteins are localized in photophores.

• Melatonin and melanocortin receptors are conserved during Etmopterid evolution.

• Prolactin receptors absence challenges prolactin role in the luminescence control.

• Luminescence and ILS pigment motion are under control of proteins and modulators.

Abstract

As part of the study of their bioluminescence, the deep-sea lanternshark Etmopterus spinax and Etmopterus molleri (Chondrichthyes, Etmopteridae) received growing interest over the past ten years. These mesopelagic sharks produce light thanks to a finely tuned hormonal control involving melatonin, adrenocorticotropic hormone and α-melanocyte-stimulating hormone. Receptors of these hormones, respectively the melatonin receptors and the melanocortin receptors, are all members of the G-protein coupled receptor family i.e. coupled with specific G proteins involved in the preliminary steps of their transduction pathways. The present study highlights the specific localization of the hormonal receptors, as well as of their associated G-proteins within the light organs, the so-called photophores, in E. spinax and E. molleri through immunohistofluorescence technic. Our results allow gaining insight into the molecular actors and mechanisms involved in the control of the light emission in Etmopterid sharks.

Introduction

At present, Etmopteridae and Dalatiidae are the only two families of sharks that unambiguously contain luminous species (Claes and Mallefet, 2009a, Straube et al., 2015). Among bioluminescent Etmopteridae, Etmopterus spinax and Etmopterus molleri have been studied over the past ten years to better understand their luminescence and the associated biological functions (Claes and Mallefet for review). These deep-sea lanternsharks display intrinsic light organs, called photophores, spread over the ventral epidermis (Claes and Mallefet, 2009a, Claes and Mallefet, 2009b, Duchatelet et al., 2019b; Fig. 1a, b). Etmopterid photophores, ultrastructurally depicted by Renwart et al., 2014, Renwart et al., 2015, consist of emitting cells, the photocytes, enclosed in a multicellular cup-shaped pigmented sheath and surmounted by one or several lens cells. A multilayer cell zone, the so-called iris-like structure (ILS), is composed of a complex pigmented melanophore-like cell network present between the lens cells and the photocytes and is used as a shutter of the light organ (Claes and Mallefet, 2009a, Renwart et al., 2014; Fig. 1c, d). Counterillumination appears to be the main ecological function of bioluminescence for lanternsharks but aposematism and intraspecific communication have also been suggested (Claes and Mallefet, 2008, Claes and Mallefet, 2009a, Claes et al., 2010a, Claes et al., 2013, Duchatelet et al., 2019d). Conversely to the majority of bioluminescent bony fishes presenting a nervous light emission control mainly through adrenaline, noradrenaline or nitric oxide (NO) (Anctil, 1972, Krönström and Mallefet, 2009, Zaccone et al., 2011, Mallefet et al., 2019), luminescent lanternsharks are characterized by a hormonal control of the light output: melatonin (MT) and prolactin (PRL) triggering the light emission while α-melanocyte-stimulating (α-MSH) or adrenocorticotropic (ACTH) hormones inhibit luminescence (Claes et al., 2011b, Claes and Mallefet, 2009c, Duchatelet et al., 2020). Typical neurotransmitters, NO and γ-aminobutyric acid (GABA) also modulate the hormonally-triggered E. spinax light emission (Claes et al., 2010b, Claes et al., 2011a). Although Claes et al. (2010b) showed that acetylated tubulin positive nerve processes innervated the surrounding of the photophores, there is no pharmacological evidence of a direct nervous control of the light emission in lanternsharks (Claes et al., 2010b, Claes et al., 2011a). The ultrastructural characterization of the E. spinax photophore did not highlight any nervous structures within the photophore itself (Renwart et al., 2014, Renwart et al., 2015). The role of the terminal nerves, reaching the surrounding of photophores, then remain to be investigated.

Interestingly, some hormones involved in the light emission control in Etmopteridae (i.e. PRL, α-MSH) are also involved in pigment motion regulation in shallow water shark species (Visconti et al., 1999). Despite the increasing collection of pharmacological data on the luminous control, data is lacking concerning the expression and localization of the hormone receptors involved in the light emission control (i.e. stimulation or inhibition). According to the recently available skin transcriptome of E. spinax (Delroisse et al., 2018), the presence of predicted mRNA coding for such hormonal receptors (i.e. MT and α-MSH/ACTH receptors) was pinpointed.

To date, in vertebrates: (i) four different melatonin receptors (Mel1a/MTNR1A, Mel1b/MTNR1B, Mel1c/MTNR1C, and Mel1D/MTNR1D) are known to be expressed (Shiu et al., 1996, Witt-Enderby et al., 2003, Falcon et al., 2010, Li et al., 2013); (ii) melanocortin system comprises five G protein-coupled receptors (MC1-5R) that could link melanotropic peptides (α-/ß-/γ-MSH and ACTH) (Chhajlani and Wikberg, 1992, Chakrabothy et al., 1995); and (iii) PRL receptors (PRLRa and PRLRb), closely related to growth hormone receptors (GHRa and b), appear to be expressed from gnathostomes (Ellens et al., 2013). Recently, Daza and Larhammar (2018) demonstrated a secondary loss of PRLR in cartilaginous fishes during the GHR/PRLR family evolution.

To gain insights on the molecular pathways involved in the light emission control in Etmopteridae, immunolocalizations of the hormone G-protein coupled receptors, MTNR and MCR as well as of their associated G proteins, were performed within the Etmopterid photogenic organs of the two investigated species. Based on transcriptome data, phylogenetic analyses were performed in parallel to assign new predicted receptors to known vertebrate receptors subclasses.