In vivo regulation of cell death by embryonic (pro)insulin and the insulinreceptor during early retinal neurogenesis

Programmed cell death is an established developmental process in the nervou s system. Whereas the regulation and the developmental role of neuronal cel l death have been widely demonstrated, the relevance of cell death during e arly neurogenesis, the cells affected and the identity of regulatory local growth factors remain poorly characterized, We have previously described sp ecific in vivo patterns of apoptosis during early retinal neurogenesis, and that exogenous insulin acts as survival factor (Diaz, B., Pimentel, B., De Pablo, F. and de la Rosa, E. J. (1999) Eur. J. Neurosci. 11, 1624-1632). P roinsulin mRNA was found to be expressed broadly in the early embryonic chi ck retina, and decreased later between days 6 and 8 of embryonic developmen t, when there was increased expression of insulin-like growth factor I mRNA , absent or very scarce at earlier stages. Consequently, we studied whether proinsulin and/or insulin ((pro)insulin) action in prevention of cell deat h has physiological relevance during early neural development. In ovo treat ment at day 2 of embryonic development with specific antibodies against (pr o)insulin or the insulin receptor induced apoptosis in the neuroretina.The distribution of apoptotic cells two days after the blockade was similar to naturally occurring cell death, as visualized by TdT-mediated dUTP nick end labeling, The apoptosis induced by the insulin receptor blockade preferent ially affected to the Islet1/2 positive cells, that is, the differentiated retinal ganglion cells. In parallel, the insulin survival effect on culture d retinas correlated with the activation of Akt to a greater extent than wi th the activation of MAP kinase. These results suggest that the physiologic al cell death occurring in early stages of retinal development is regulated by locally produced (pro)insulin through the activation of the Akt surviva l pathway.