Two neuronal retinal components of the electroretinogram c-wave

Microglia preserve visual function loss in the aging retina by supporting retinal pigment epithelial health

BACKGROUND

Increased age is a risk factor for the development and progression of retinal diseases including age-related macular degeneration (AMD). Understanding the changes that occur in the eye due to aging is important in enhancing our understanding of AMD pathogenesis and the development of novel AMD therapies. Microglia, the resident brain and retinal immune cells are associated with both maintaining homeostasis and protection of neurons and loss of microglia homeostasis could be a significant player in age related neurodegeneration. One important characteristic of retinal aging is the migration of microglia from the inner to outer retina where they reside in the subretinal space (SRS) in contact with the retinal pigment epithelial (RPE) cells. The role of aged subretinal microglia is unknown. Here, we depleted microglia in aged C57/BL6 mice fed for 6 weeks with a chow containing PLX5622, a small molecule inhibitor of colony-stimulating factor-1 receptor (Csf1r) required for microglial survival.

RESULTS

The subretinal P2RY12 + microglia in aged mice displayed a highly amoeboid and activated morphology and were filled with autofluorescence droplets reminiscent of lipofuscin. TEM indicates that subretinal microglia actively phagocytize shed photoreceptor outer segments, one of the main functions of retinal pigmented epithelial cells. PLX5622 treatment depleted up to 90% of the retinal microglia and was associated with significant loss in visual function. Mice on the microglia depletion diet showed reduced contrast sensitivity and significantly lower electroretinogram for the c-wave, a measurement of RPE functionality, compared to age-matched controls. The loss of c-wave coincided with a loss of RPE cells and increased RPE swelling in the absence of microglia.

CONCLUSIONS

We conclude that microglia preserve visual function in aged mice and support RPE cell function, by phagocytosing shed photoreceptor outer segments and lipids, therefore compensating for the known age-related decline of RPE phagocytosis.

Using Micro-Electrode-Array Recordings and Retinal Disease Models to Elucidate Visual Functions: Simultaneous Recording of Local Electroretinograms and Ganglion Cell Action Potentials Reveals the Origin of Retinal Oscillatory Potentials

BACKGROUND

The electroretinogram (ERG) is an essential diagnostic tool for visual function, both in clinical and research settings. Here, we establish an advanced in vitro approach to assess cell-type-specific ERG signal components.

METHODS

Retinal explant cultures, maintained under entirely controlled conditions, were derived from wild-type mice and rd10 rod- and cpfl1 cone-degeneration mouse models. Local micro-ERG (µERG) and simultaneous ganglion cell (GC) recordings were obtained from the retinal explants using multi-electrode arrays. Band-pass filtering was employed to distinguish photoreceptor, bipolar cell, amacrine cell (AC), and GC responses.

RESULTS

Scotopic and photopic stimulation discriminated between rod and cone responses in wild-type and mutant retina. The 25 kHz sampling rate allowed the visualization of oscillatory potentials (OPs) in extraordinary detail, revealing temporal correlations between OPs and GC responses. Pharmacological isolation of different retinal circuits found that OPs are generated by inner retinal AC electrical synapses. Importantly, this AC activity helped synchronise GC activity.

CONCLUSION

Our µERG protocol simultaneously records the light-dependent activities of the first-, second-, and third-order neurons within the native neuronal circuitry, providing unprecedented insights into retinal physiology and pathophysiology. This method now also enables complete in vitro retinal function testing of therapeutic interventions, providing critical guidance for later in vivo investigations.

Use of an alternating current amplifier when recording the ERG c-wave to evaluate the function of retinal pigment epithelial cells in rats

PURPOSE

We studied the conditions under which c-waves of the electroretinogram (ERG), that represent retinal pigment epithelium (RPE) function, were detectable using an alternating current (AC) amplifier and whether the c-wave recorded using an AC amplifier was useful for evaluating RPE function.

METHODS

We recorded ERG responses in rats to 5 s stimuli under the conditions in which the low-cut frequency and the stimulus luminance were varied. In addition, changes in ERGs were studied after intravenous injection of sodium iodate (SI) to induce RPE degeneration.

RESULTS

The c-wave was detected clearly when the frequency of the low-cut filter was set at 0.01 Hz and light stimulus luminances were ≥ - 1.0 log cd/m². The c-wave was attenuated earlier than other waves (e.g., a-wave and b-wave) after SI administration.

CONCLUSIONS

The c-wave was easily detectable using an AC amplifier with the low-cut filter set at 0.01 Hz. Using the AC amplifier may allow easier c-wave recording, compared with the conventional use of a direct current (DC) amplifier, and could be useful for evaluating RPE function.

Subretinal injection of amyloid-β peptide accelerates RPE cell senescence and retinal degeneration

Drusen are considered a hallmark characteristic of age-related macular degeneration (AMD). In our previous study, we found that amyloid-β (Aβ) peptide, a component of drusen, induced the cells of the retinal pigment epithelium (RPE; RPE cells) to enter senescence; however, its effects in vivo remain unknown. Thus, the present study was carried out to explore the in vivo effects of Aβ peptide on RPE cell senescence and senescence-associated inflammation in C57BL/6 mice. C57BL/6 mice received a subretinal injection of Aβ(1-42) peptide; on day 7 post-injection, the mice were anesthetized and subjected to whole-body perfusion with 4% paraformaldehyde (PFA) in PBS and the whole eyes were then enucleated. Retinal function was assessed by electroretinography (ERG), and the morphological characteristics of the retina were examined by light and electron microscopy. Fundus autofluorescence (FAF) was examined by confocal scanning laser ophthalmoscopy (cSLO). The expression of p16INK4a, a marker of cellular senescence, was examined by immunofluorescence staining and western blot analysis. The RPE-choroid was analyzed for cytokine expression by RT-PCR. In Aβ(1-42)-injected mice, scotopic ERG responses declined. Degenerative alterations, including the disruption of the inner segment (IS)/outer segment (OS) junction and extensive vacuolation and thickness of Bruch's membrane (BrM) were observed under a a light microscope. The accumulation of vacuoles and the loss of basal infoldings in the RPE were identified using an electron microscope. FAF and p16INK4a expression increased in Aβ(1-42)-injected mice. In addition, Aβ(1-42) upregulated interleukin (IL)-6 and IL-8 gene expression in the RPE-choroid. In conclusion, our results confirm the effects of Aβ(1-42) peptide on RPE senescence in vivo. The Aβ-injected mice developed AMD-like ocular pathology. It is thus suggested that RPE cell senescence is a potential mechanistic link between inflammation and retinal degeneration.

Loss of Otx2 in the adult retina disrupts retinal pigment epithelium function, causing photoreceptor degeneration

Photoreceptors are specialized neurons of the retina that receive nursing from the adjacent retinal pigment epithelium (RPE). Frequent in the elderly, photoreceptor loss can originate from primary dysfunction of either cell type. Despite intense interest in the etiology of these diseases, early molecular actors of late-onset photoreceptor degeneration remain elusive, mostly because of the lack of dedicated models. Conditional Otx2 ablation in the adult mouse retina elicits photoreceptor degeneration, providing a new model of late-onset neuronal disease. Here, we use this model to identify the earliest events after Otx2 ablation. Electroretinography and gene expression analyses suggest a nonautonomous, RPE-dependent origin for photoreceptor degeneration. This is confirmed by RPE-specific ablation of Otx2, which results in similar photoreceptor degeneration. In contrast, constitutive Otx2 expression in RPE cells prevents degeneration of photoreceptors in Otx2-ablated retinas. We use chromatin immunoprecipitation followed by massive sequencing (ChIP-seq) analysis to identify the molecular network controlled in vivo by Otx2 in RPE cells. We uncover four RPE-specific functions coordinated by Otx2 that underpin the cognate photoreceptor degeneration. Many direct Otx2 target genes are associated with human retinopathies, emphasizing the significance of the model. Importantly, we report a secondary genetic response after Otx2 ablation, which largely precedes apoptosis of photoreceptors, involving inflammation and stress genes. These findings thus provide novel general markers for clinical detection and prevention of neuronal cell death.

Interpretation of the mouse electroretinogram

The mouse electroretinogram (ERG) consists of a complex set of signals or "waves" generated by multiple types of retinal cell. The origins of these waves are reviewed briefly for the C57BL/6J mouse. The differences in the properties of these waves are described for 34 strains of mice and 11 F1 hybrid mice, as is the way that inter-strain genetic polymorphisms can be exploited in order to help pin-point the genes responsible for ERG differences. There are certain technical difficulties, some subtle, that can arise in recording the ERG and these are classified and illustrated in order to facilitate their diagnosis. Forward genetic screens are described, along with abnormal mice that have been generated in a large screen. Several means are suggested for determining if a mouse having an abnormal ERG is a mutant.

Results from screening over 9000 mutation-bearing mice for defects in the electroretinogram and appearance of the fundus

Random mutagenesis combined with phenotypic screening using carefully crafted functional tests has successfully led to the discovery of genes that are essential for a number of functions. This approach does not require prior knowledge of the identity of the genes that are involved and is a way to ascribe function to the nearly 6000 genes for which knowledge of the DNA sequence has been inadequate to determine the function of the gene product. In an effort to identify genes involved in the visual system via this approach, we have tested over 9000 first and third generation offspring of mice treated with the mutagen N-ethyl-N-nitrosourea (ENU) for visual defects, as evidenced by abnormalities in the electroretinogram and appearance of the fundus. We identified 61 putative mutations with this procedure and outline the steps needed to identify the affected genes.

Do horizontal cells contribute to the rabbit ERG?

An increase in the amplitude of horizontal cell (HC) potentials, a decrease in the extracellular potassium around the photoreceptors and an increase in the PIII component of the ERG that occur with increasing light stimulus intensity are shown to be concomitant occurrences, which could, therefore, be causally related. With high intensity light stimuli the PIII component is prominent and the HC potentials exhibit an afterpotential. Within the range of intensities tested the peak times of the HC afterpotentials coincide with the peak times of PIII. This indicates the likelihood that the HC potential contributes to the PIII, although these results do not allow a quantitative assessment of the contribution of HC potentials to the PIII-component.