Insulin-like growth factor l (IGF-l) is a peptide that is structurally homologous and has overlapping functions with insulin. IGF-l is produced in tissues throughout the body including the brain and retina, where it acts as a paracrine signal from pigment epithelial to rod photoreceptors. IGF-l plays important roles in cell differentiation, growth, and survival, and recent studies show that IGF-l also has acute physiological effects on ion channels in the brain. The levels of IGF-1 and IGF-1 receptors are changed dramatically in diabetes, and have been proposed to play an integral part in diabetic neuropathies. The goal of this project is to understand more about the acute effects of IGF-l on the retina, with a specific focus on rod photoreceptors. Our preliminary experiments show that IGF-l alters the sensitivity of rods to light. Moreover, IGF-1 modulates the activity of the ion channels responsible for generating the light response. Thus, IGF-1 alters the sensitivity of cyclic nucleotide-gated (CNG) channels to cyclic GMP, the crucial """"""""internal transmitter"""""""" of phototransduction. The effect of IGF-l is mediated by a tyrosine kinase cascade, perhaps resulting in changes in the tyrosine phosphorylation state of the CNG channels.
The aims of our project are to understand more about how IGF-l affects CNG channels in rods, and how changes in the activity of the channels might underlie changes in the light response. First, we will investigate the biochemical cascade that couples the activated IGF-1 receptor to modulation of the channels. Second, we will investigate how changes in tyrosine phosphorylation modulate the channels, and we will use site-directed mutagenesis of exogenously expressed CNG channels to locate the crucial phosphorylation site(s) on the CNG channel protein. Finally, we will examine the effect of IGF-l on single human photoreceptors from diabetic and non-diabetic donors, to understand how CNG channel modulation may alter the light response. These studies will lead to a molecular-level understanding of the effects of IGF-l on the retina, and will provide important insights into cellular and molecular processes that may be related to normal retinal physiology and diabetic retinopathy.
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