Modulation of Mouse Rod Photoreceptor Responses by Grb14 Protein

Effect of Knocking Down the Insulin Receptor on Mouse Rod Responses

“…We would therefore have expected some pronounced difference between dark-adapted and light-adapted responses in WT rods that would not have been seen in IR knock-down rods. Moreover our experiments also indicate that the knocking down of IR produces a modest decrease in the time of rod response saturation ( T sat ) and the limiting time constant τ D , similar to effects we previously reported for deletion of the gene of the Grb14 protein 15 . The decrease in T sat may be produced at least in part by the decrease in sensitivity, but model calculations show that an increase in channel affinity can by itself also shorten T sat 15 .…”

“…Although the mean current of IR knock-down rods in darkness was somewhat larger than the mean dark current of WT rods, this difference was not significant. A similar observation was made for Grb14 − / − rods 15 . It is likely that feedback from the very active guanylyl cyclase in mouse rods acts to minimize the rise in circulating current by altering the concentration of cGMP (see for example reference 43 ).…”

“…We also show in Fig. 4B the values of τ REC for rods lacking the gene for the Grb14 protein, which our previous study showed also to exhibit accelerated response decay 15 . The values of τ REC for IR knock-down rods and Grb14 − / − rods were comparable and were both uniformly smaller than for WT rods.…”

“…We discovered that rod responses from IR knock-down mice recover after illumination more rapidly than responses of wild-type (WT) mouse rods, as expected if the channels are more sensitive to cGMP and open at a lower concentration after knocking down the IR gene. Much to our surprise, however, the effect of knocking down the IR gene was similar in dark-adapted and light-adapted animals and seemed to be the result at least in part of an effect on the rod phosphodiesterase, much like our previous results on rods lacking Grb14 15 . Collectively, our studies indicate an effect of the non-canonical IR/Grb14 pathway on rod function but exclude a major role of these proteins in producing the electrical response of the rods.…”

“…They are responsible for generating the light response in photoreceptors and are directly and co-operatively gated by cGMP. The CNG channel sensitivity to cGMP has been studied extensively and has been shown to depend on or to be regulated by several factors, including Ca 2+ /CaM 6 7 , divalent ions 8 9 , diacylglycerol 10 , phospholipids 11 , phosphorylation 12 13 and Grb14 14 15 . The insulin receptor (IR) can also regulate olfactory 16 17 and cone CNG channels 18 through phosphoinositide 3-kinase.…”