Abstract
Purpose: :
We have previously proposed that a novel light-dependent phosphoinositide 3-kinase (PI3K) neuron survival pathway in rod photoreceptors is regulated through three key proteins: the insulin receptor (IR), protein tyrosine phosphatase 1B (PTP1B), and growth factor receptor-bound protein 14 (Grb14). Our hypothesis is that in dark-adapted rod outer segments (ROS), the IR is maintained in an inactive state by rapid dephosphorylation by PTP1B. Light activation of rhodopsin initiates the localization of Grb14 to photoreceptor outer segment membranes, where it protects the IR from dephosphorylation by PTP1B. Light-activated IR subsequently associates with PI3K and thus regulates the downstream neuronal survival pathway. We tested this hypothesis using a genetic approach.
Methods: :
Rod-specific IR, global PTP1B, and Rpe65 knockout mice were used to study IR signaling in rod photoreceptors. We measured the retinal PTP1B activity, IR phosphorylation, Akt activation, and Grb14 localization in dark- and light-adapted (30 minutes at ~200 lux) retinas of wild type, IR-/-, PTP1B-/-, and Rpe65-/- mice. The neuroprotective potential of the IR and PTP1B knockout mouse retinas was examined in response to bright light stress, and photoreceptor structural and functional integrity was measured.
Results: :
Increased activation of the IR and Akt, increased binding of Grb14 to ROS, and decreased PTP1B activity were observed in light-adapted retinas. In the Rpe65-/- retinas, we observed the absence of IR activation, mislocalization of Grb14 in photoreceptors, and increased PTP1B activity. These observations suggest that IR activation is due to rhodopsin-controlled PTP1B activity through the proper localization of Grb14 to ROS. Disruption of IR expression in rod photoreceptors significantly decreased ERG amplitudes and ONL thickness in mice exposed to bright light stress, whereas mice lacking PTP1B were protected from bright light-induced photoreceptor degeneration. Photoreceptor structural and functional integrity was well-preserved in PTP1B knockout mice compared to wild type controls.
Conclusions: :
Our studies clearly demonstrate that rhodopsin regulates the PTP1B activity through the translocation of Grb14 to ROS membranes, which prevents IR dephosphorylation and thus regulates the IR/PI3K/Akt survival pathway. These findings demonstrate that rhodopsin may have additional signaling functions in rod photoreceptors.
Keywords: growth factors/growth factor receptors • retinal degenerations: cell biology • neuroprotection