Differential Akt activation in the photoreceptors of normal and rd1 mice

Retinitis pigmentosa is a blinding disease in which unknown mechanisms cause the degeneration of retinal photoreceptors. The retinal degeneration (rd1) mouse is a relevant model for this condition, since it carries a mutation also found in some forms of retinitis pigmentosa. To understand the degenerative process in the rd1 mouse, we must identify the survival and apoptosis-related signaling pathways in its photoreceptors and determine whether signaling differs from that in normal mice. The phosphatidylinositol 3-kinase/Akt kinase pathway promotes survival in several different cell types. The purpose of the present study has been to compare Akt activity in retinal cells of normal and rd1 mice. We have found that, in normal mice, Akt becomes activated in the retina in a developmentally regulated and cell-type-specific fashion, encompassing essentially all retinal cells. In most cell types, once Akt activation has begun, it remains in this state throughout life. An exception is seen in the rod photoreceptors, in which Akt is activated only transiently during their development. The rd1 retina behaves identically in all but one respect, namely that the activation of Akt in rod photoreceptors persists until these cells undergo apoptosis. Thus, Akt may participate in constitutive survival processes in retinal neurons, except in rod photoreceptors in which the role of this pathway may be restricted to the developmental period. However, Akt activation in the rods may be part of a defense mechanism initiated in response to insults, such as the retinal degeneration seen in the rd1 mouse.