Sensory ganglia from tadpoles but not adult bullfrogs synthesize heat shock‐like proteins in vitro at non‐heat shock temperature

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Abstract
Lumbar dorsal root ganglia (DRG) from premetamorphic bullfrog tadpoles showed a selectively enhanced accumulation of two radiolabeled proteins when incubated in vitro at 22°C for longer than 1 hr. During the first hour in vitro or after in vivo labeling, these polypeptides–of 25 and 68 kD molecular mass and slightly acidic isoelectric point–were detected in the DRG only at relatively low levels. Accumulation of the 25 kD species increased in lumbar DRG from tadpoles of all metamorphic stages but waned at stage XXV (completion of metamorphosis) and was absent from DRG of adult bullfrogs. In contrast, increased levels of the 68 kD species were detected only at premetamorphic stages. Tail DRG from premetamorphic tadpoles did not increase their synthesis of either polypeptide in vitro, which is consistent with the above findings because sensory neurons in tail DRG, but not in lumbar DRG, are mature at this developmental stage. Although the molecular sizes of the two polypeptides led us initially to consider them as heat shock‐like proteins, only the 68 kD species was further increased (by approximately 100‐fold) when premetamorphic lumbar DRG were incubated at 37°C. The 25 kD species, which was the predominant protein induced at 22°C, showed no further increase in response to hyperthermia. The enhanced in vitro synthesis of these polypeptides at control temperature (22°C) is an example of cellular stress responses that may occur not as a consequence of elevated temperature or exogenous chemical agents, but as a result of transplantation‐associated trauma. This phenomenon also represents an example of a cellular stress response that is “outgrown” during development.