Activation of mTORC1 is sufficient for long-term cone survival in Retinits Pigmentosa

Abstract

Purpose: The mechanism of cone death in Retinitis Pigmentosa (RP) remains elusive. We previously proposed that cones are nutrient deprived in RP based on gene expression changes seen during disease in metabolic genes and genes of the insulin/mTOR pathway. These findings led us to treat RP mice with insulin, which significantly improved cone survival although only for a 4 week period. The short-term effect was attributed to the feedback loop in the pathway. To determine if insulin acted directly on cones, and to test the long-term therapeutic potential of the pathway, we have now genetically activated the pathway directly in cones at two different junctions downstream of the insulin receptor. This has been achieved by conditional deletion of the phosphatase and tensin homolog (PTEN), and separately by conditional ablation of the tuberous sclerosis complex 1 (TSC1) in cones. In contrast to insulin, loss of PTEN or TSC1 leads to constitutive activation of the pathway in a feedback loop independent manner. Loss of PTEN activates the entire pathway including both mTOR complexes and AKT thus promoting pro-growth and pro-survival mechanisms, while loss of TSC1 activates only mTOR complex 1 thereby promoting mainly pro-growth mechanisms.

Methods: Mice carrying conditional knockout alleles for PTEN, TSC1, raptor (loss of mTOR complex 1) and rictor (loss of mTOR complex 2) were crossed to a cone-specific Cre recombinase line in a retinal degeneration-1 (rd¹) mutant background. Additionally, double conditional alleles for PTEN & raptor, and TSC1 & raptor were used to delineate the contribution of pro-growth and pro-survival mechanisms. Retinal flat mounts were used to quantify surviving cones.

Results: Activation of the pathway via removal of PTEN or TSC1 significantly improved cone survival for up to 8 months. The combinatorial genetics showed that activation of mTORC1 is both required and sufficient for long-term cone survival in RP. In fact, upon loss of TSC1 at 2 months of age, the cone distribution in the mutant background was similar to that of a wild-type retina.

Conclusions: Our results show that cell-autonomous activation of the pathway is sufficient for long-term protection of cones in RP and suggest that altering pro-growth mechanisms alone can promote cone survival. The data suggests that mTORC1 targets are ideal candidates to develop therapeutic strategies.

Keywords: 648 photoreceptors • 696 retinal degenerations: hereditary • 702 retinitis