Abstract
Purpose: :
Our BrdU pulse-labeling and birthdating experiments showed that GLE increased retinal progenitor cell (RPC) proliferation and differentiation of late-born rods and bipolar cells, but not Müller glial cells [Env Hlth Perspect 2011]. Our present goals were to utilize confocal immunohistochemistry and immunoblots to individually and simultaneously assess age-dependent RPC cell cycle progression and cell cycle exit, and the production of late-born neuronal precursors.
Methods: :
Female C57BL/6 mice were exposed to tap water or 55 ppm lead solution throughout gestation and until postnatal day 10 (PND10): equivalent to the human gestation period. On PND1-7 pups were injected with BrdU (ip) and sacrificed 24 hours later. PND2-10 pups were used for RT-qPCR, Westerns and confocal immunohistochemistry.
Results: :
Single-label confocal studies showed that the age-dependent spatial patterns of BrdU, MCM6 and Otx2 were similar in control and GLE retinas. However, in GLE retinas, the number of BrdU positive(+ve), MCM6+ve and Otx2+ve cells was increased at each age relative to controls. RT-qPCR and Western blots were consistent with these confocal results. Triple-label confocal experiments revealed that the proportion of RPCs remaining in cell cycle [BrdU+ve and MCM6+ve/Total BrdU+ve] and those that exited cell cycle [BrdU+ve and MCM6 negative/Total BrdU+ve] was higher in GLE retinas, compared to controls. The age-dependent pattern and decrease of these two processes were similar in control and GLE retinas, although RPC proliferation was prolonged in GLE retinas. Moreover, the proportion of RPCs that exited as late-born neuronal precursors [BrdU+ve and Otx2+ve/Total BrdU+ve] was higher in GLE retinas, compared to controls.
Conclusions: :
In GLE retinas, compared to controls, an increased number and proportion of cells progressed through and exited from the cell cycle. Together, this selectively increased the number of late-born rods and bipolar cells. In addition to discerning the novel proliferative effects of low-level GLE on RPCs and neurons, our findings present an experimental model that will help decipher the complex and intricate molecular mechanisms underlying RPC cell cycle and cell fate. Moreover, these findings have relevance for neurotoxicology, ophthalmology, pediatrics and public health.
Keywords: retinal development • retina: distal (photoreceptors, horizontal cells, bipolar cells) • microscopy: confocal/tunneling