Abstract
Increasing evidence suggests that gonadotropin-releasing hormone (GnRH), corazonin, adipokinetic hormone (AKH), and red pigment-concentrating hormone all share common ancestry to form a GnRH superfamily. Despite the wide presence of these peptides in protostomes, their biological effects remain poorly characterized in many taxa. This study had three goals. First, we cloned the full-length sequence of a novel AKH, termed Aplysia-AKH, and examined its distribution in an opisthobranch mollusk, Aplysia californica. Second, we investigated in vivo biological effects of Aplysia-AKH. Lastly, we compared the effects of Aplysia-AKH to a related A. californica peptide, Aplysia-GnRH. Results suggest that Aplysia-AKH mRNA and peptide are localized exclusively in central tissues, with abdominal, cerebral, and pleural ganglia being the primary sites of Aplysia-AKH production. However, Aplysia-AKH-positive fibers were found in all central ganglia, suggesting diverse neuromodulatory roles. Injections of A. californica with Aplysia-AKH significantly inhibited feeding, reduced body mass, increased excretion of feces, and reduced gonadal mass and oocyte diameter. The in vivo effects of Aplysia-AKH differed substantially from Aplysia-GnRH. Overall, the distribution and biological effects of Aplysia-AKH suggest it has diverged functionally from Aplysia-GnRH over the course of evolution. Further, that both Aplysia-AKH and Aplysia-GnRH failed to activate reproduction suggest the critical role of GnRH as a reproductive activator may be a phenomenon unique to vertebrates.
Highlights
Adipokinetic hormone and red pigment-concentrating hormones (AKH/RPCHs) are structurally related peptides with wellcharacterized functions in arthropods [1]
The deduced prepro ap-adipokinetic hormone (AKH) consists of a 22-amino acid signal peptide, a 10-amino acid ap-AKH, a dibasic processing site with an a-amidation signal (GKR), and a 45-amino acid Aplysia AKH (apAKH)-associated peptide (Figure 1; GenBank Accession# JQ929303.2)
RT-PCR RT-PCR revealed that the ap-AKH is expressed in the bag cell neurons (BCN), abdominal ganglion, cerebral ganglion and pedal/pleural ganglia (Figure 2)
Summary
Adipokinetic hormone and red pigment-concentrating hormones (AKH/RPCHs) are structurally related peptides with wellcharacterized functions in arthropods [1]. RPCHs, a group of multifunctional crustacean neuropeptides, are synthesized in neurons of the X-organ (XO) and transported to the sinus gland (SG) to be stored and released [7]. Both the protocerebrum/CC and XOSG are neuroendocrine complexes considered to be analogous to the vertebrate hypothalamo-hypophysial system [8,9,10]. These peptides are structurally and evolutionarily related [2], their functions vary somewhat across and within phyla
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