somatolactin

SL belongs to the class I helical cytokine family. Two types of SLs, SLα and SLβ, are encoded by separate genes. SLα is widely present in fish, whereas SLβ has been found in a limited number of species. Cod SL (SLα) consists of 235 aa residues containing eight cysteine residues in the mature hormone, six of which are involved in disulfide bonds. Asparagine at position 147 is N-glycosylated. The SLβ of zebrafish consists of 230 aa residues and has six cysteine residues in the mature hormone.  Mr 23,000~26,000. pI 5.7~5.9 (Cod SL), 5.2 (zebrafish SLβ). SL is soluble in alkaline buffers.

The zebrafish SLα gene, smtla, location 18, consists of five exons. The zebrafish SLα mRNA has 778 bases that encode a signal peptide of 26 aa residues and a mature protein of 209 aa residues. The zebrafish SLβ gene, smtlb, location 10, consists of five exons. The zebrafish SLβ mRNA has 854 bases that encode a signal peptide of 24 aa residues and a mature protein of 206 aa residues. The SL gene is expressed in the pars intermedia in the pituitary. In the zebrafish, SLα and SLβ mRNAs are distributed in different cells in the pars intermedia.

The single type of SL receptor (SLR), belonging to the type I cytokine receptor family, is reported in teleosts, but not in sturgeon and lungfish. In some teleost species, SLR was originally identified as a type 1 GH receptor, having binding activity to GH but lower than to SL. Salmon SLR consists 657 aa residues that contain a signal peptide of 20 aa residues, the FGEFS motif, seven cysteine residues, a single transmembrane region, and Box 1 and 2 regions in the intracellular domain. Carp SLα and SLβ activate Janus kinase 2 (JAK2)/signal transducers and activators of transcription 5 (STAT5), phosphoinositide 3-kinase (PI3K)/Akt, and mitogenactivated protein kinase (MAPK) signaling pathways.

SL synthesis and release are affected by various neurotransmitters and neuropeptides, such as dopamine, adrenaline, serotonin, corticotropin-releasing hormone, gonadotropin-releasing hormone, melanin-concentrating hormone, adenylate cyclase-activating polypeptide,leptin, neuropeptide Y, kisspeptin, and SL itself. The synthesis and plasma concentration of SL are also influenced by sexual maturation, low environmental calcium, salinity and pH, acute stress, seasonal rhythms, background color, plasma acidosis, and bicarbonate concentration.

In various fish species, the expression of the SLR gene is observed in the liver, visceral fat, muscle skin, pituitary, brain, heart, spleen, testis, ovary, gill, stomach, gall bladder, intestine, and kidney. SL is involved in sexual maturation, plasma ion and pH regulation, lipid metabolism, and body-color regulation. Although changes in the plasma concentration and expression level of SL are reported in a number of studies, the effects of administration and/or deficiency of SL have been demonstrated in few.

This is secreted by the pars intermedia of the pituitary of fish. SL is involved in a wide range of physiological functions, including sexual maturation, plasma ion metabolism, acidbase regulation, and body-color regulation. SL was first isolated and characterized from cod in figure early 1990s during the course of characterizing growth hormone (GH). In the 2000s, two distinct SL forms encoded by separate genes were reported from several teleosts. As far, SL is found only in fish (teleosts, sturgeon, and lungfish), and there are no reports from other vertebrates.