Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 114, Issue 14, Pages E2937-E2946Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1620572114
Keywords
osmotic stress; phototransduction; optical coherence tomography; intrinsic optical signals; photoreceptor waveguiding
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Funding
- National Institutes of Health [EY02660]
- National Science Foundation Industry-University Cooperative Research Centers Program
- Center for Biophotonic Sensors and Systems grant
- National Eye Institute UC Davis core grant [EY012576]
- UC Davis Research Investments in Science and Engineering (RISE) grant
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The light responses of rod and cone photoreceptors have been studied electrophysiologically for decades, largely with ex vivo approaches that disrupt the photoreceptors' subretinal microenvironment. Here we report the use of optical coherence tomography (OCT) to measure light-driven signals of rod photoreceptors in vivo. Visible light stimulation over a 200-fold intensity range caused correlated rod outer segment (OS) elongation and increased light scattering in wildtype mice, but not in mice lacking the rod G-protein alpha subunit, transducin (Gat), revealing these responses to be triggered by photo-transduction. For stimuli that photoactivated one rhodopsin per Gat the rod OS swelling response reached a saturated elongation of 10.0 +/- 2.1%, at a maximum rate of 0.11% s(-1). Analyzing swelling as osmotically driven water influx, we find the H2O membrane permeability of the rodOS to be (2.6 +/- 0.4) x 10(-5) cm.s(-1), comparable to that of other cells lacking aquaporin expression. Application of Van't Hoff's law reveals that complete activation of phototransduction generates a potentially harmful 20% increase in OS osmotic pressure. The increased backscattering from the base of the OS is explained by a model combining cytoplasmic swelling, translocation of dissociated G-protein subunits from the disc membranes into the cytoplasm, and a relatively higher H2O permeability of nascent discs in the basal rod OS. Translocation of phototransduction components out of the OS may protect rods from osmotic stress, which could be especially harmful in disease conditions that affect rod OS structural integrity.
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