p53 Selectively Regulates Developmental Apoptosis of Rod Photoreceptors

Retinal cells become post-mitotic early during post-natal development. It is likely that p53, a well-known cell cycle regulator, is involved in regulating the genesis, differentiation and death of retinal cells. Furthermore, retinal cells are under constant oxidative stress that can result in DNA damage, due to the extremely high level of metabolic activity associated with phototransduction. If not repaired, this damage may result in p53-dependent cell death and ensuing vision loss. In this study, the role of p53 during retinal development and in the post-mitotic retina is investigated. A previously described super p53 transgenic mouse that expresses an extra copy of the mouse p53 gene driven by its endogenous promoter is utilized. Another transgenic mouse (HIP) that expresses the p53 gene in rod and cone photoreceptors driven by the human interphotoreceptor retinoid binding protein promoter was generated. The electroretinogram (ERG) of the super p53 mouse exhibited reduced rod-driven scotopic a and b wave and cone-driven photopic b wave responses. This deficit resulted from a reduced number of rod photoreceptors and inner nuclear layer cells. However, the reduced photopic signal arose only from lost inner retinal neurons, as cone numbers did not change. Furthermore, cell loss was non-progressive and resulted from increased apoptosis during retinal developmental as determined by TUNEL staining. In contrast, the continuous and specific expression of p53 in rod and cone photoreceptors in the mature retinas of HIP mice led to the selective loss of both rods and cones. These findings strongly support a role for p53 in regulating developmental apoptosis in the retina and suggest a potential role, either direct or indirect, for p53 in the degenerative photoreceptor loss associated with human blinding disorders.视网膜细胞在出生后发育早期即退出细胞周期，成为有丝分裂后细胞。作为经典的细胞周期调控因子，p53（p53）极有可能参与调控视网膜细胞的发生、分化与死亡过程。此外，视网膜细胞伴随光转导过程存在极高水平的代谢活性，因此持续处于氧化应激状态，可引发DNA损伤；若此类损伤未得到修复，则可能引发p53依赖的细胞死亡，进而导致视力丧失。本研究旨在探究p53在视网膜发育阶段及有丝分裂后视网膜中的功能。本研究采用了此前已报道的超级p53（super p53）转基因小鼠，该小鼠在内源性p53启动子驱动下额外表达一份小鼠p53基因；同时构建了另一种转基因小鼠（HIP），该小鼠在人类感光细胞间视黄醇结合蛋白（interphotoreceptor retinoid binding protein, IRBP）启动子的驱动下，于视杆细胞与视锥细胞中特异性表达p53基因。对超级p53小鼠的视网膜电图（electroretinogram, ERG）检测显示，其视杆细胞介导的暗适应a波、b波以及视锥细胞介导的明适应b波反应均出现减弱，该功能缺陷源于视杆感光细胞与内核层细胞的数量减少。但明适应信号的减弱仅由视网膜内层神经元丢失所致，视锥细胞的数量并未发生改变。此外，通过末端脱氧核苷酸转移酶介导的dUTP缺口末端标记（TUNEL）染色检测可知，细胞丢失呈非进行性，且源于视网膜发育阶段的凋亡水平升高。与之相反，HIP小鼠成熟视网膜中p53在视杆与视锥感光细胞内的持续性特异性表达，导致视杆与视锥细胞均发生选择性丢失。上述研究结果充分证实p53可调控视网膜发育过程中的细胞凋亡，并提示p53或通过直接或间接途径，参与人类致盲性疾病相关的感光细胞退行性丢失过程。