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D. Krizaj, J.L. Duncan, H. Yang, T. Doan, R. Silverstein, G.J. Murphy, B.L. Tempel; Expression and Function of High–Affinity Calcium Transport at the Photoreceptor Synapse . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3716.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify the expression of plasma membrane Ca ATPase (PMCA) transporters in the mouse retina and determine the functional role of PMCA2, the isoform with the highest affinity for Ca, in scotopic and photopic visual signaling.
Retinas from deafwaddler (dfw2J) mice, in which the PMCA2 Ca transporter is not expressed, and wild type (wt) mice were investigated with quantitative PCR, ERG recording, suction recording and recording from retinal slices. qPCR was used with PMCA gene expression normalized with respect to actin and YWHAZ reference genes. ERGs were recorded from anesthetized dfw2J and wt littermate mice at 1 month –2 years of age. Rod bipolar cell responses were recorded in voltage clamp in the whole cell configuration.
PMCA1 accounted for ∼70% of the PMCA transcript in the mouse retina, and PMCA2 for ∼20%; PMCA3 and 4 were expressed at much lower levels. Minimal compensatory upregulation for PMCA1, 3 & 4 was detected in dfw2J retinas. PMCA2 was expressed in inner segments and terminals of photoreceptors and in dendrites and cell bodies of rod bipolar cells. In response to flashes producing ∼103 Rh*/rod, b–waves from dfw2J mice were reduced by more than 45% compared to control (P < 0.0001). The reduction in b–wave amplitude was observed across a wide range of intensities, including intensities where the rod a–wave responses were little affected. The sensitivity and kinetics of dfw2J photocurrents was not significantly different from controls. Responses of dfw2J rod bipolar cells, however, were about 50% less sensitive than control rod bipolar cells; the half–saturating flash strength was 3.7 + 0.2 Rh*/rod for dfw2J rod bipolars and 2.4 + 0.1 Rh*/rod for control rod bipolars (n=61). No difference in the amplitude or kinetics of photopic ERG responses was observed.
PMCA2 is the second most abundant PMCA isoform in the mouse retina. Loss of PMCA2 impaired the sensitivity and kinetics of rod–mediated, but not cone–mediated signals. These effects are post–phototransduction, as they are not observed in isolated rod outer segments. Thus, high affinity calcium extrusion acts to selectively increase the sensitivity of the rod synapse.
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