Genes that encode neuropeptides can be recognized in the human genome because their products are (1) highly conserved across species, (2) trafficked though cells by secretory signal peptides and (3) susceptible to proteolytic cleavage at consensus sequences for processing to neuroactive products. One candidate gene is oesophageal cancer-related gene (ECRG4) which was first identified by screening for up- or down-regulated genes in oesophageal cancer. The product of ECRG4, which we call Argillin, has the features of a neuroendocrine precursor. Accordingly, we evaluated immunoreactive Argillin distribution in the rat brain.
In situ hybridisation shows the choroid plexus (CP) to be a major site of gene expression in development (www.genepaint.org) and immunostaining of rat brain shows Argillin localising to CP epithelial and ventricular ependymal cells. Surprisingly, additional loci of immunostaining included neurones of the paraventricular (PVN) and supraoptic nucleus (SON) that are commonly recognised as the source of arginine vasopressin (AVP) and oxytocin (OT) in the CNS. Accordingly, axonal projections from the PVN and SON to the neurohypophysis and median eminence are also Argillin immunoreactive. Confocal microscopy of sections from combinations of single, double and triple staining of Argillin, AVP and/or OT revealed that, while some neurones contain both Argillin and AVP, the large majority of OT positive neurones contain Argillin immunoreactivity. Furthermore, in some sections this staining is associated with the synaptic-capillary interface and, in the posterior pituitary, the herring bodies that contain secretory granules that release neuropeptides into the pituitary circulation.
Taken together, this immunohistochemical evidence suggests that Argillin and/or its proteolytically processed product(s), is a neuropeptide. While its activity and the exact structural identity of the active fragment remain to be determined, the findings point to physiological functions normally associated with OT neurones of the PVN, SON and neurohypophysis: fluid homeostasis and the control of emotion and/or behaviour.