Differential expression and seasonal modulation of VGF peptides in sheep pituitary

Abstract

The inducible gene vgf and its peptide products are relevant to the neuroendocrine regulation of homeostasis and reproduction in rodents. We show here that in the anterior pituitary of female sheep the somatotrope, gonadotrope, and lactotrope/thyrotrope cell populations each expressed vgf mRNA, but displayed a distinct profile of VGF immunoreactive peptides. ProVGF C-terminus and VGF(443-588) immunoreactivities were found in lactotropes and thyrotropes, often in a subcellular location restricted to the Golgi area and suggestive of rapid peptide (or proVGF) release upon biosynthesis, while high molecular weight bands consistent with proVGF were shown in pituitary extracts. Distinct seasonal changes were revealed, proVGF C-terminus immunoreactive cells being largely identified as lactotropes during the summer (83.7 +/- 2.1% (mean +/-s.e.m.) versus 27.0 +/- 1.9% during the winter), as opposed to thyrotropes during the winter (73.0 +/- 1.9% versus 16.3 +/- 2.1% during the summer). Conversely, antisera to peptides adjacent to the 'Arg-Pro-Arg' cleavage site, and to the VGF(553-555) N-terminus of the proVGF-derived peptide V, selectively labeled gonadotropes, indicating processing to small peptides not retaining the proVGF C-terminus in such cells. Finally, a peptide related to the VGF(4-240) region was immunostained in somatotropes, shown in a Western blot as a band of relative molecular mass of approximately 16,000. In conclusion, a complex, endocrine cell-type-specific processing of proVGF was revealed. Further to the known inducibility of vgf mRNA upon a range of stimuli, discreet, selective modulations of VGF-peptide profile/s are suggested, possibly involved in specific neuro/endocrine or modulatory mechanisms.