Abstract
Since the initial characterization of the cardioexcitatory peptide FMRFamide in the bivalve mollusk Macrocallista nimbosa, a great number of FMRFamide-like peptides (FLPs) have been identified in mollusks. FLPs were initially isolated and molecularly characterized in model mollusks using biochemical methods. The development of recombinant technologies and, more recently, of genomics has boosted knowledge on their diversity in various mollusk classes. Today, mollusk FLPs represent approximately 75 distinct RFamide peptides that appear to result from the expression of only five genes: the FMRFamide-related peptide gene, the LFRFamide gene, the luqin gene, the neuropeptide F gene, and the cholecystokinin/sulfakinin gene. FLPs display a complex spatiotemporal pattern of expression in the central and peripheral nervous system. Working as neurotransmitters, neuromodulators, or neurohormones, FLPs are involved in the control of a great variety of biological and physiological processes including cardiovascular regulation, osmoregulation, reproduction, digestion, and feeding behavior. From an evolutionary viewpoint, the major challenge will then logically concern the elucidation of the FLP repertoire of orphan mollusk classes and the way they are functionally related. In this respect, deciphering FLP signaling pathways by characterizing the specific receptors these peptides bind remains another exciting objective.
Highlights
Most of the functional assignments of this family of neuropeptides were obtained from the key model species A. californica and L. stagnalis because they have a central nervous system (CNS) with large and reproducibly identified neurons, readily accessible and amenable to in vivo or in vitro handling
Since the activity of these neurons can be correlated with physiological states or behaviors, establishing functional circuitries has become feasible in these species [61, 109, 135]
Sequence information is still lacking about so far completely understudied mollusk classes, such as monoplacophora, placophora, aplacophora, and scaphopoda, substantial genomic data are becoming available in economically important classes of mollusks such as bivalves [91, 92] and cephalopods [136, 137]
Summary
These mRNAs share the first exon coding for the signal peptide and appear to be splice variants of a single gene, but in contrast to Lymnaea alternative transcripts they both encode copies of FMRFamide and FLRFamide along with a small number of new peptides These two FaRP transcripts are differentially expressed in the gastropod nervous system. In Lymnaea, different FaRPs and APGWamide have distinct actions on the penis retractor muscle; this demonstrates a complex peptidergic regulation of the male copulation behavior [72, 73] Together, these two tetrapeptides modulate the feeding behavior in the gastropod mollusks H. trivolvis and L. stagnalis in association with other factors like biogenic amines (serotonin, dopamine, octopamine) [74]. A biochemical investigation showed that the processing of this precursor generates Luqin, a mature amidated decapeptide, together with a
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