Cytoplasmic localization of mPER1 clock protein isoforms in the mouse retina.

The mammalian Period1 gene is rhythmically expressed and its proteins are found within the nucleus of the cells of the suprachiasmatic nuclei (SCN), the central circadian pacemaker in mammals; however, whether the target of the PER1 proteins is also the nucleus in the retinal peripheral clock cells is yet to be determined. Using an anti-PER1 protein antibody in Western blot analyses, we found three isoforms (75, 110 and 140kDa) in extracts of the SCN, as well as in other different parts of the brain, whereas just two isoforms (75 and 110kDa) were detected in the retinal extracts. We have observed that PER1 immunolabelling has a cytoplasmic location in many cells of the ganglion cell layer and in a few cells in the inner nuclear layer of the mouse retina. This cellular location was seen in any of the tissue samples taken at 4h intervals, either in the day/night cycle or in constant darkness, of both wild type and rd mice. Unlike this situation, PER1 isoforms were nuclear proteins in the SCN cells as well as in other parts of the brain of the same animals. No circadian changes were found for these clock proteins in the neural retina. These findings suggest that PER1 proteins play roles in the retina different from those established in the SCN.