Biology subjects

Burns, M. E.

Publications and source records attributed to Burns, M. E..

3 recordsLinked to original sources

bioRxiv2025

RPE pseudopods maintain photoreceptor outer segment renewal despite subretinal space expansion in Adam9 knockout mice

Vision begins in the outer segment compartment of photoreceptor cells, which is constantly renewed through the addition of membrane material at its base and ingestion of mature membranes at its tip by the retinal pigment epithelium (RPE). The close apposition of outer segments to the RPE is believed to be critical for maintaining this renewal process. Yet, in several retinal diseases, expansion of the subretinal space separating photoreceptors from the RPE does not immediately impact photoreceptor functionality. Here, we analyzed outer segment function and renewal in the Adam9 knockout mouse characterized by a major expansion of the subretinal space. Surprisingly, photoreceptor-RPE separation affected neither the sensitivity of photoreceptor light-responses nor the normal rate of outer segment renewal in this mouse prior to the onset of photoreceptor degeneration. The latter is achieved through the formation of elongated RPE "pseudopods" extending across the enlarged subretinal space to ingest outer segment tips. This work suggests that pseudopod formation may underlie the persistence of photoreceptor function in human diseases accompanied by photoreceptor-RPE separation, such as vitelliform macular dystrophy or age-related macular degeneration associated with subretinal drusenoid deposits.

cell biology↗

bioRxiv2025

Biophysical Basis of the in vivo ERG of the Mouse: Rod, RPE and Müller glial Cell Contributions To Dark and to Light-driven Changes in Sub-Retinal Space o

The subretinal space (SRS) is a contiguous extracellular volume bounded by the external limiting membrane (ELM) and the retinal pigment epithelium (RPE). Approximately 90% of mouse rod photoreceptor dark current circulates in the SRS, and rod photoresponses materially alter SRS concentrations of permeant ions, especially K+. Following intravitreal injection of drugs that block glutamatergic synapses, and drugs that inhibit Kir4.1 and Kir7.1 channels expressed in Muller and RPE cells respectively, we measured electroretinograms (ERGs) in response to light flashes that completely suppress rod dark current, and compared ERG c-waves with predictions of a biophysical model of extracellular current sources generated by rod, Muller and RPE cells. SRS K+ fluxes in the dark steady-state were dominated by rods - NKX (-6.6 mM s-1), Kv2.1 channels (+5.4 mM s-1), NCKX (0.3 mM s-1) and HCN1 channel fluxes (1.0 mM s-1) - with much smaller contributions from Kir4.1 and Kir7.1 channels and trans-ELM and trans-RPE fluxes combining to zero net flux at resting SRS Ko of 5 mM (re 3.5 mM [K+] in blood plasma). In response to rod-saturating stimulation, Ko was predicted to decline to [~]3.5 mM with a half-time of [~] 0.5 s, driven mainly by a hyperpolarization-dependent decline in Kv2.1 current. The kinetics and amplitude of the c-wave are explained by the decline in SRS Ko, which negatively shifts the Nernst potential of apical RPE Kir7.1 channels, increasing outward current that sinks through the paracellular resistance to basal Bestrophin and CFTR chloride channels causing a trans-epithelial potential of [~] 2 mV.

biophysics↗

bioRxiv2025

Biophysical Basis of the in vivo Electroretinogram of the Mouse: Current Source Density Analysis of Genetically and Pharmacologically Isolated Rod photoreceptor-driven Currents

To better understand the molecular basis of the mouse electroretinogram (ERG) we have developed a biophysical model of the rod photoreceptor layers ionic mechanisms and applied current source-density (CSD) analysis to predict the genetically and pharmacologically isolated rod ERG a-wave. The saturating a-wave is characterized by a rapid relaxation ({tau} ~ 6 ms) from its maximum that has been hypothesized to be caused by HCN1 channel opening consequent to light-triggered sustained hyperpolarization, or by extracellular flow of capacitive current during hyperpolarization. To test these hypotheses, the CSD model included an ensemble of 11 rods with cell body locations spanning the outer nuclear layer, and with ionic mechanisms -- including CNG channels, NCKX in the outer segment, NKX, Kv2.1 channels in the inner segment, and HCN1 throughout the "non-outer segment" -- fully specified as to axial distributions, voltage dependencies, and the extracellular conductivity of the photoreceptor layer extracellular space. Predicting with CSD the steady-state axial distributions of rod dark current and transretinal potential, and the spatio-temporal response of the activation phase of the rods light response to intense stimuli, the analysis confirmed the Robson-Frishman hypothesis that extracellular capacitive current flowing towards the inner segment upon the rapid closure of CNG current plays a major role in shaping the early a-wave. The CSD analysis also reveals that extracellular current from HCN1 channels opened by the hyperpolarization contributes a net extracellular current flowing toward the ONL that contributes materially to the saturated a-wave relaxation and to setting its plateau level.

biophysics↗