Secretagogin in the Central Nervous System of the Gecko, Hemidactylus leschenaultii

Secretagogin (scgn), a novel calcium-binding protein (CBP), regulates Ca²⁺ homeostasis in neurons, controls exocytosis, and serves as a Ca²⁺-sensor. Scgn is widely expressed in the mammalian brain and has emerged as a novel neuroendocrine regulator. Although scgn is considered as a phylogenetically conserved CBP, little is known about its significance in the brain of nonmammalian vertebrates. Herein, we characterized the scgn-encoding cDNA and mapped scgn-equipped elements in the brain and spinal cord of a gecko, Hemidactylus leschenaultii (hl). In the brain, a 1303-bp hlscgn transcript containing 822-bp cds and 481-bp 3' untranslated region (UTR) was identified. The hlscgn transcript predicted to encode a hexa EF-hand 273-aa hlscgn showed 70%-90% similarity with scgn from other vertebrates. The EF2-motif in hlscgn showed highest Ca²⁺-binding affinity as compared to that in scgn from other vertebrates. The anti-scgn antiserum detected a ∼32-kDa band in the brain, and cell bodies/fibers in the cortex, dorsal ventricular ridge, amygdalae, striatum/septum, hypothalamic/thalamic nuclei, substantia nigra, interpeduncular, solitary tract, and raphe nuclei, and spinal cord. The scgn neurons in the paraventricular (PVN) and supraoptic nuclei co-expressed either thyrotropin-releasing hormone or vasotocin, but not mesotocin. As insulin regulates the activity of PVN neurons in mammals, and scgn expression in the mammalian/teleost brains, the superfused hypothalamic slices of H. leschenaultii were treated with insulin, and the changes in scgn-ir/hlscgn-mRNA in PVN were analyzed. Compared to control, insulin treatment significantly increased scgn expression. Thus, scgn seems to be a highly conserved protein in the vertebrate hierarchy with a variety of attributes, inclusive of neuroanatomy, neuroendocrine control, and regulation by insulin.