Structural and Functional Phenotyping in the Cone-Specific Photoreceptor Function Loss 1 (cpfl1) Mouse Mutant – A Model of Cone Dystrophies

cGMP Signaling in Photoreceptor Degeneration

Photoreceptors in the retina are highly specialized neurons with photosensitive molecules in the outer segment that transform light into chemical and electrical signals, and these signals are ultimately relayed to the visual cortex in the brain to form vision. Photoreceptors are composed of rods and cones. Rods are responsible for dim light vision, whereas cones are responsible for bright light, color vision, and visual acuity. Photoreceptors undergo progressive degeneration over time in many hereditary and age-related retinal diseases. Despite the remarkable heterogeneity of disease-causing genes, environmental factors, and pathogenesis, the progressive death of rod and cone photoreceptors ultimately leads to loss of vision/blindness. There are currently no treatments available for retinal degeneration. Cyclic guanosine 3', 5'-monophosphate (cGMP) plays a pivotal role in phototransduction. cGMP governs the cyclic nucleotide-gated (CNG) channels on the plasma membrane of the photoreceptor outer segments, thereby regulating membrane potential and signal transmission. By gating the CNG channels, cGMP regulates cellular Ca2+ homeostasis and signal transduction. As a second messenger, cGMP activates the cGMP-dependent protein kinase G (PKG), which regulates numerous targets/cellular events. The dysregulation of cGMP signaling is observed in varieties of photoreceptor/retinal degenerative diseases. Abnormally elevated cGMP signaling interferes with various cellular events, which ultimately leads to photoreceptor degeneration. In line with this, strategies to reduce cellular cGMP signaling result in photoreceptor protection in mouse models of retinal degeneration. The potential mechanisms underlying cGMP signaling-induced photoreceptor degeneration involve the activation of PKG and impaired Ca2+ homeostasis/Ca2+ overload, resulting from overactivation of the CNG channels, as well as the subsequent activation of the downstream cellular stress/death pathways. Thus, targeting the cellular cGMP/PKG signaling and the Ca2+-regulating pathways represents a significant strategy for photoreceptor protection in retinal degenerative diseases.

Validating Fluorescent Chrnb4.EGFP Mouse Models for the Study of Cone Photoreceptor Degeneration

Purpose

To validate the application of a known transgenic mouse line with green fluorescent cones (Chrnb4.EGFP) to study cone photoreceptor biology and function in health and disease.

Methods

Chrnb4.EGFP retinas containing GFP+ cones were compared with retinas without the GFP transgene via immunohistochemistry, quantitative real-time polymerase chain reaction, electroretinograms, and flow cytometry. The Chrnb4.EGFP line was backcrossed to the mouse models of cone degeneration, Pde6ccpfl1 and Gnat2cpfl3 , generating the new lines Gnat2.GFP and Pde6c.GFP, which were also studied as described.

Results

GFP expression spanned the length of the cone cell in the Chrnb4.EGFP line, as well as in the novel Gnat2.GFP and Pde6c.GFP lines. The effect of GFP expression showed no significant changes to outer nuclear layer cell death, cone-specific gene expression, and immune response activation. A temporal decrease in GFP expression over time was observed, but GFP fluorescence was still detected through flow cytometry as late as 6 months. Furthermore, a functional analysis of photopic and scotopic electroretinogram responses of the Chrnb4 mouse showed no significant difference between GFP- and GFP+ mice, whereas electroretinogram recordings for the Pde6c.GFP and Gnat2.GFP lines matched previous reports from the original lines.

Conclusions

This study demonstrates that the Chrnb4.EGFP mouse can be a powerful tool to overcome the limitations of studying cone biology, including the use of this line to study different types of cone degeneration.

Translational Relevance

This work validates research tools that could potentially offer more reliable preclinical data in the development of treatments for cone-mediated vision loss conditions, shortening the gap to clinical translation.

Evaluation of mouse experimental autoimmune uveoretinitis by spectral domain optical coherence tomography

AIMS

To evaluate the efficacy of spectral domain optical coherence tomography (SD-OCT) in monitoring the development of mouse experimental autoimmune uveoretinitis (EAU) as an animal model of endogenous uveitis, and to develop an OCT-based grading system for EAU severity.

METHODS

C57BL/6 mice were immunised with human interphotoreceptor retinoid-binding protein (amino acid sequence 1-20) peptide and complete Freund's adjuvant to induce EAU. The development of EAU was monitored by SD-OCT serially throughout the disease course, and the images were graded from 1 to 4 and compared with the clinical and histopathological grades.

RESULTS

SD-OCT images depicted retinal lamella structures including the inner segment/outer segment (IS/OS) line in normal mice. Retinal structural changes were observed on SD-OCT images in mice that developed EAU clinically scored as grade 1 or higher, which precisely corresponded to the pathological findings. The SD-OCT images of EAU were graded as follows: grade 1, a few infiltrating cells in the vitreous and retina; grade 2, increased vitreous cells, retinal vasculitis, and granulomatous lesion; grade 3, cell infiltration into the whole retina, disappearance of IS/OS line, and destruction of the retinal layer structure; and grade 4, disappearance of the outer retina. The SD-OCT grade of EAU based on these criteria correlated significantly with both the clinical grade (R(2)=0.282, p<0.005) and histopathological grade (R(2)=0.846, p<0.0001).

CONCLUSIONS

SD-OCT is useful for evaluating the development and severity of mouse EAU. The SD-OCT scoring system we developed accurately reflects clinical and histopathological changes.

Long-term characterization of retinal degeneration in rd1 and rd10 mice using spectral domain optical coherence tomography

PURPOSE

We characterize the in vivo changes over time in the retinal structure of wild-type mice alongside two lines of mice deficient in the β-subunit of phosphodiesterase (rd1 and rd10 mice) using spectral domain optical coherence tomography (SD-OCT).

METHODS

SD-OCT images were obtained using the Bioptigen spectral domain ophthalmic imaging system (SDOIS). Wild-type C57BL/6J, rd1 and rd10 mice ranging in age from P14 to P206 were sedated with 1% isoflurane. Horizontal and vertical linear scans through the optic nerve, and annular scans around the optic nerve were obtained.

RESULTS

SD-OCT imaging of wild-type mice demonstrated visibility of the inner segment/outer segment (IS/OS) junction, external limiting membrane (ELM), outer nuclear layer (ONL), and outer plexiform layer (OPL). At P14, most rd10 mice exhibited normal SD-OCT profiles, but some displayed changes in the IS/OS junction. At the same time point, rd1 mice had severe outer retinal degeneration. In rd10 mice, imaging revealed loss of the IS/OS junction by P18, hyperreflective changes in the ONL at P20, hyperreflective vitreous opacities, and shallow separation of the neural retina from the RPE. Retinal separations were not observed in rd1 mice. Segmentation analysis in wild-type mice demonstrated relatively little variability between animals, while in rd10 and rd1 mice there was a steady decline in outer retinal thickness. Histologic studies demonstrated correlation of retinal features with those seen on SD-OCT scans. Segmentation analysis provides a quantitative and reproducible method for measuring in vivo retinal changes in mice.

CONCLUSIONS

SD-OCT provides a non-invasive method of following long-term retinal changes in mice in vivo. Although rd10 and rd1 mice have mutations in the same gene, they demonstrate significantly different features on SD-OCT.

Properties of Flicker ERGs in Rat Models with Retinal Degeneration

Purpose. To describe the characteristics of rod and cone functions in rat models for congenital stationary night blindness (CSNB) and retinal cone dysfunction (RCD). Methods. Rod and cone function were isolated by recording the rod-/cone-driven flicker and blue light flicker electroretinograms (ERGs). Results. During dark adaptation, the amplitudes of flicker ERGs in CSNB rats were lower than those in control rats; the responses of RCD rats were similar to control rats. During light adaptation, the amplitudes of flicker ERGs in CSNB rats were reduced; whereas the responses of RCD rats were not detected. Blue flicker ERGs were not observed in CSNB rats at lower frequencies. The cone driven critical flicker frequencies (CFFs) in control rats were 62 Hz. The rod driven CFF of RCD rats was 20 Hz; whereas the rod-/cone-driven CFF of CSNB rats both were about 25 Hz. Conclusions. The function of the rod system was damaged completely, the cones were the source of vision in CSNB rats. Rod system function is excellent in RCD rat. The rods of albinism rats are sensitive to frequencies less than 20 Hz; whereas the cones are sensitive to frequencies up to 62 Hz.

Early-onset, slow progression of cone photoreceptor dysfunction and degeneration in CNG channel subunit CNGB3 deficiency

PURPOSE

To investigate the progression of cone dysfunction and degeneration in CNG channel subunit CNGB3 deficiency.

METHODS

Retinal structure and function in CNGB3(-/-) and wild-type (WT) mice were evaluated by electroretinography (ERG), lectin cytochemistry, and correlative Western blot analysis of cone-specific proteins. Cone and rod terminal integrity was assessed by electron microscopy and synaptic protein immunohistochemical distribution.

RESULTS

Cone ERG amplitudes (photopic b-wave) in CNGB3(-/-) mice were reduced to approximately 50% of WT levels by postnatal day 15, decreasing further to approximately 30% of WT levels by 1 month and to approximately 20% by 12 months of age. Rod ERG responses (scotopic a-wave) were not affected in CNGB3(-/-) mice. Average CNGB3(-/-) cone densities were approximately 80% of WT levels at 1 month and declined slowly thereafter to only approximately 50% of WT levels by 12 months. Expression levels of M-opsin, cone transducin α-subunit, and cone arrestin in CNGB3(-/-) mice were reduced by 50% to 60% by 1 month and declined to 35% to 45% of WT levels by 9 months. In addition, cone opsin mislocalized to the outer nuclear layer and the outer plexiform layer in the CNGB3(-/-) retina. Cone and rod synaptic marker expression and terminal ultrastructure were normal in the CNGB3(-/-) retina.

CONCLUSIONS

These findings are consistent with an early-onset, slow progression of cone functional defects and cone loss in CNGB3(-/-) mice, with the cone signaling deficits arising from disrupted phototransduction and cone loss rather than from synaptic defects.

Optic nerve crush mice followed longitudinally with spectral domain optical coherence tomography

PURPOSE

To investigate the longitudinal effect of optic nerve crush injury in mice by measuring retinal thickness with spectral-domain optical coherence tomography (SD-OCT).

METHODS

Optic nerves of one eye from each C57Bl/6 mouse were crushed under direct visualization for 3 seconds, 1 mm posterior to the globe. The optic nerve head (ONH) was imaged with SD-OCT (1.5 × 1.5 × 2.0 mm scan) before the surgical intervention and repeated subsequently for up to 32 days postinjury. A cohort of mice not exposed to the nerve crush procedure served as control. En face SD-OCT images were used to manually align subsequent scans to the baseline en face image. Total retinal thickness (TRT) (along a sampling band with radii 0.33-0.42 mm centered on the ONH) from each follow-up day was automatically quantified for global and sectoral measurements using custom software. Linear mixed-effects models with quadratic terms were fitted to compare TRT of nerve-crushed and control eyes over time.

RESULTS

Eleven eyes from 11 nerve crush mice (baseline age 76 ± 11.8 days) and eight eyes from four healthy mice (baseline age 64 ± 0 days) were included. The control eyes showed a small, gradual, and consistent TRT increase throughout follow-up. Nerve-crushed eyes showed an initial period of thickening, followed by thinning and slight rebound after day 21. The decrease in thickness observed after the early thickening resolved was statistically significantly different from the control eyes (P < 0.05 for global and sectoral measurements).

CONCLUSIONS

SD-OCT can be used to quantitatively monitor changes in retinal thickness in mice over time.