Heat Shock May Relieve a Posttranscriptional Block on sigma Crystallin Synthesis in Cultures of Chick Embryo Neuroretinal Cells. (neural retina/transdifferentiation/posttranscriptional control/delta crystallin/heat shock)

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Abstract
The lens protein δ crystallin is expressed at high levels in lentoids fromed by the transdifferentiation of chick embryo neuroretinal cells in long-term culture under permissive conditions. Previous work has shown that one non-permissive medium (F199) allows transcription of δ crystallin RNA in late neural retinal cultures, but that these δ transcripts remain confined to the nuclei and are neither translated nor indeed detectable in the cytoplasm. We show here that F199 cultures maintained at 43°c throughout transdifferentiate extensively into lentoids, in contrast to similar cultures at 37°c. Following transfer of late (30 day) F199 cultures from 37°c to 43°c, δ crystallin synthesis becomes increasingly prominent over a period of at least 5 days, suggesting that recovery from heat shock (rather than heat shock per se) may be important in this phenomenon. A similar response can be elicited by chemical agents (sodium arsenite, azetidine 2-carboxylic acid) which induce heat-shock (stress) proteins at 37°c. Treatment of late F199 cultures with actinomycin D does not prevent this increase in δ crystallin synthesis, provided the drug is not added until after the initiation of a heat-shock response (6 hr after temperature shift). This suggests that the observed increase in δ synthesis following heat shock involves primarily the export and/or stabilisation of δ mRNAs already present in the nuclei in late F199 cultures. Possible wider implications of this posttranscriptional level of control over δ crystallin synthesis are discussed.