Expression of HSP70 mRNA in the embryonic chicken lens: association with differentiation

The heat shock protein, HSP70 is constitutively expressed in the embryonic chicken lens. The present study investigates the localization of HSP70 mRNA in the lens as a function of developmental age to determine whether the state of proliferation or differentiation of the cells is correlated with expression of this gene. Using reverse transcription and quantitative PCR for HSP70 mRNA in RNA samples derived from microdissected regions of the lens, we have found that the concentration of HSP70 mRNA is highest in the annular pad cells at 6 days of development, and in the annular pad and fiber cells at 14 and 19 days of development, suggesting that expression is associated with fiber cell formation. This association was confirmed by studies of HSP70 transcription and HSP70 mRNA levels in cultured explants of 6 day embryonic chicken lens epithelia differentiating in culture. Nuclear run-on transcription experiments indicated that transcription of HSP70 is elevated after 5 and 24 hr of differentiation in vitro. Other genes whose transcription was monitored, c-myc and delta-crystallin, showed distinct patterns of transcriptional regulation, indicating that the changes in HSP70 transcription are specific. RNA extracted from cultured explants at these same times showed increased levels of a 3.4 kb mRNA which hybridized at high stringency to a chicken HSP70 cDNA probe. These results demonstrate that HSP70 mRNA in the embryonic chicken lens is associated primarily with cells in the early stages of fiber formation, and that increased transcription of this gene is part of the differentiation process.