Discovery of conventional prolactin from the holocephalan elephant fish, Callorhinchus milii

The conventional <a href="#200015597">prolactin</a> (<a href="#200015597">PRL</a>), also known as PRL1, is an adenohypophysial hormone that critically regulates various physiological events in reproduction, metabolism, growth, <a href="#200023200">osmoregulation</a>, among others. PRL1 shares its evolutionary origin with PRL2, <a href="#200013375">growth hormone</a> (<a href="#200013375">GH</a>), somatolactin and <a href="#200012560">placental lactogen</a>, which together form the GH/PRL hormone family. Previously, several bioassays implied the existence of PRL1 in elasmobranch <a href="#200023851">pituitaries</a>. However, to date, all attempts to isolate PRL1 from chondrichthyans have been unsuccessful. Here, we cloned PRL1 from the <a href="#200023851">pituitary</a> of the holocephalan elephant fish, <em>Callorhinchus milii</em>, as the first report of chondrichthyan PRL1. The putative <a href="#200004272">mature protein</a> of elephant fish PRL1 (cmPRL1) consists of 198 amino acids, containing two conserved disulfide bonds. The orthologous relationship of cmPRL1 to known vertebrate PRL1s was confirmed by the analyses of molecular <a href="#200019980">phylogeny</a> and gene synteny. The <em>cmPRL1</em> gene was similar to teleost <em>PRL1</em> genes in gene synteny, but was distinct from amniote <em>PRL1</em> genes, which most likely arose in an early amphibian by duplication of the ancestral <em>PRL1</em> gene. The mRNA of cmPRL1 was predominantly expressed in the pituitary, but was considerably less abundant than has been previously reported for bony fish and tetrapod PRL1s; the copy number of cmPRL1 mRNA in the pituitary was less than 1% and 0.1% of that of GH and <a href="#200017995">pro-opiomelanocortin</a> mRNAs, respectively. The cells expressing cmPRL1 mRNA were sparsely distributed in the <em>rostral <a href="#200001959">pars distalis</a></em>. Our findings provide a new insight into the studies on molecular and functional evolution of PRL1 in vertebrates.