LIM homeobox transcription factor Isl1 is required for melatonin synthesis in the pig pineal gland

Melatonin is a key hormone that regulates circadian rhythms, metabolism, and reproduction. However, the mechanisms of melatonin synthesis and secretion have not been fully defined. The purpose of this study was to investigate the functions of the LIM homeobox transcription factor Isl1 in regulating melatonin synthesis and secretion in porcine pineal gland. We found that Isl1 is highly expressed in the melatonin-producing cells in the porcine pineal gland. Further functional studies demonstrate that Isl1 knockdown in cultured primary porcine pinealocytes results in the decline of melatonin and arylalkylamine N-acetyltransferase (AANAT) mRNA levels by 29.2% and 72.2%, respectively, whereas Isl1 overexpression raised by 1.3-fold and 2.7-fold. In addition, the enhancing effect of norepinephrine (NE) on melatonin synthesis was abolished by Isl1 knockdown. The in vivo intracerebroventricular NE injections upregulate Isl1 mRNA and protein levels by about threefold and 4.5-fold in the porcine pineal gland. We then examined the changes in Isl1 expression in the pineal gland and global melatonin levels throughout the day. The results show that Isl1 protein level at 24:00 is 2.5-fold higher than that at 12:00, which is parallel to melatonin levels. We further found that Isl1 increases the activity of AANAT promoter, and the effect of NE on Isl1 expression was blocked by an ERK inhibitor. Collectively, the results presented here demonstrate that Isl1 positively modulates melatonin synthesis by targeting AANAT, via the ERK signaling pathway of NE. These suggest that Isl1 plays important roles in maintaining the daily circadian rhythm.