Haploinsufficiency of RanBP2 Causes Deficits in the Electrophysiological Output of Receptoral and Postreceptoral Retinal Neurons
In light of the prominent expression of RanBP2 and HKI in retinal neurons [1,19], the vital dependence of the neuronal retina (and brain) on glucose as the main substrate source for energy production, and the determinant impact of metabolic disorders, such as diabetes, in retinal function (e.g., diabetic retinopathy) [37], we probed the impact of deficits in RanBP2, HKI, and ATP, on the electrophysiological responses of subclasses (rod and cone) photoreceptor and postreceptor retinal neurons of RanBP2+/− and in RanBP2+/+ mice. The scotopic (dark-adapted) responses mediated by the rod photoreceptor pathway at low-stimulus intensities and mixed rod and cone pathways at high-stimulus intensities were substantially reduced in RanBP2+/− mice (Figure 8A). The differences in the photopic (light-adapted) responses, initiated by cone photoreceptors, which make up 3% of the photosensory neurons in the mouse retina [38], were less obvious but still exhibited a trend toward reduced amplitudes across a range of increasing light stimulus intensities (Figure 8B). The reduction in the scotopic responses included decreases in both b-wave (Figure 8C) and a-wave (Figure 8D) amplitudes mediated by postreceptoral and receptoral neurons, respectively. Postreceptoral second-order neuron responses, represented by the b-wave, tended to be more consistently and substantially reduced than the a-waves, which directly reflect photoreceptor activity. Since second-order neuron responses depend on input from photoreceptors, this suggests that reduced b-wave amplitudes are the result of the accumulation of decreases in the light response of both photoreceptors and postreceptoral neurons. Anesthetics, particularly ketamine, can cause sustained elevation of glucose in mice, which in turn affects electroretinogram responses [39]. Thus, we were concerned that differences in electroretinogram amplitudes between RanBP2+/− and RanBP2+/+ may reflect differences in glucose level changes in response to anesthesia. However, we found no significant differences in glucose levels measured before and every 15 min during 75 min of anesthesia (n = 4–5). Glucose rose at the same rate and reached a maximum of approximately 3.3 times the pre-anesthesia level in both genotypes (unpublished data).
Figure 8  Electroretinograms from 6-Mo-Old RanBP2+/−and RanBP2+/+ Inbred Mice Showing Photoreceptor and Postreceptor Neuron Electrophysiological Response Phenotypes
(A) Scotopic (dark-adapted) responses from RanBP2+/− mice to light stimuli of increasing intensity, beginning at threshold, have reduced amplitudes compared to those observed in RanBP2+/+ mice. The three lower intensities represent responses generated in the rod photoreceptor neuronal pathway. The upper intensities are comprised of responses generated in both the rod and cone pathways.
(B) Photopic (light-adapted, cone photoreceptor pathway) responses of RanBP2+/− mice to increasing light stimulus intensities also exhibited reduced amplitudes compared to those observed in RanBP2+/+ mice.
(C) Average ± SE (n = 9) scotopic b-wave amplitudes from RanBP2+/− (open circles) and RanBP2+/+ (filled squares) mice representing postreceptoral neuron function. (Note: log amplitude scale.)
(D) Average ± SE (n = 5) scotopic a-wave amplitudes, representing photoreceptor function, for RanBP2+/− and RanBP2+/+ mice in response to bright flashes. Amplitudes of responses from RanBP2+/− mice were lower over the entire range of stimulus intensities for both b- and a-waves. Asterisks represent significant differences between the groups (Student's t test, p < 0.05). Statistical significance was found across all intensities for b-wave amplitudes (2-way ANOVA, p < 0.0001), but not for the a-wave.