Heat shock proteins in retinal neurogenesis: identification of the PM1 antigen as the chick Hsc70 and its expression in comparison to that of other chaperones

While the role of heat shock proteins under experimental stress conditions is clearly characterized, their expression in unstressed cells and tissues and their functions in normal cell physiology, besides their chaperone action, remain largely undetermined. We report here the identification in chicken of the antigen recognized by the monoclonal antibody PM1 [Hernández‐Sánchez et al . (1994) Eur. J. Neurosci. 6, 1801–1810) as the non‐inducible chaperone heat‐shock cognate 70 (Hsc70). Its identity was determined by partial peptide sequencing, immuno‐crossreactivity & two‐dimensional gel‐electrophoresis. In addition, we examined its expression during chick embryo retinal neurogenesis. The early widespread Hsc70 immunostaining corresponding to most, if not all, of the neuroepithelial cells becomes restricted to a subpopulation of these cells in the peripheral retina as development proceeds. On the other hand, retinal ganglion cells, differentiating in the opposite central‐to‐peripheral gradient, retained Hsc70 immunostaining. Other molecular chaperones, the heat‐shock proteins Hsp40, Hsp60 & Hsp90, did not seem to compensate the loss of Hsc70. They also showed decreasing immunostaining patterns as neurogenesis proceeds, although distinctive from that of Hsc70, whereas Hsp70 was not detected in the embryonic retina. This precise cellular & developmental regulation of Hsc70, a generally considered constitutive molecular chaperone, in unstressed embryos, together with the expression of other chaperones, provides new tools & a further insight on neural precursor heterogeneity & suggests possible specific cellular roles of chaperone function during vertebrate neurogenesis.