In a recent study published in the International Journal of Molecular Sciences, researchers identified that loss of pigment epithelium-derived factor (PEDF) might cause aging in the eye.
The retina is a light-sensitive tissue located on the backside of the eye. Further, age-linked retinal disorders such as age-related macular degeneration (AMD) can cause blindness.
A retinoprotective protein, PEDF, is produced by the serine protease inhibitor family 1 (Serpinf1) gene expressed by the retinal pigment epithelium (RPE) and decreases with retinal degenerations, cell senescence, and aging. PEDF, a serpin superfamily member, has been compared to an ocular guardian since it shields retinal photoreceptors, neurons, and RPE from pathological damages and prevents choroidal and retinal neovascularization.
The current study's authors previously reported that the patatin-like phospholipase domain containing 2 (Pnpla2) decline slows lipid and rhodopsin digestion across the phagocytosis-undergoing RPE. This inference suggests the PEDF receptor (PEDF-R) is necessary for RPE phagocytosis and possibly connects PEDF to the process. Nevertheless, the involvement of PEDF in RPE phagocytosis and the PEDF-R driven activities are unspecified.
About the study
The present work used a Serpinf1 null mice model to determine if PEDF may aid in preventing aging in the RPE.
The authors evaluated the expression of genes linked to senescence across the RPE of three-month-old mice lacking the Serpinf1 gene. They performed a quantitative polymerase chain reaction (qPCR) using complementary deoxyribonucleic acid (cDNA) made from ribonucleic acid (RNA) taken from freshly dissected Serpinf1+/-, Serpinf1+/+, and Serpinf1-/- choroid/RPE eyecups.
The fluorescence intensity of 5-diaminofluorescein (5-DAF) per RPE cell was assessed in both genotypes and plotted to quantitatively examine the impact of PEDF ablation on the senescence-associated galactosidase (SA-β-gal) function. Further, since senescent cells experience nuclear size alterations, the scientists used fluorescent confocal imaging to analyze the RPE's subcellular morphology.
The team examined the Pnpla2 gene expression, which was necessary for the RPE to degrade the photoreceptor's outer segments because this function of the RPE reduces with aging. Moreover, they measured the amounts of phagocytosed rhodopsin and lipids across the RPE of the Serpinf1 deficient mice. The number of particles per RPE cell was computed for each genotype and plotted to quantitatively evaluate the impact of the PEDF deficit on rhodopsin turnover. The intensity of boron-dipyrromethene (BODIPY) per region of interest (ROI) was calculated for each genotype and plotted to quantitatively analyze the influence of PEDF deficiency on the accumulation of lipids in RPE.
The team discovered that Serpinf1 deletion triggered cyclin-dependent kinase inhibitor 1A (Cdkn1a) for p21 protein, H2A histone family member X (H2ax) for histone H2AX protein, and galactosidase beta 1 (Glb1) gene for β-galactosidase. The study results showed that the RPE undergoes senescent-like alterations upon PEDF loss. This conclusion was backed by the identified 1) nuclear enlargement, 2) elevations in SA-β-gal function, 3) triggering of senescence-associated genes Glb1, Cdkn1a, and H2ax, and 4) disorganized F-actin distribution pattern and similar phenotypic alterations across the RPE of Serpinf1−/− (Serpinf1 null) mice versus the wild-type RPE.
The researchers found that Serpinf1 ablation enhanced RPE cell nuclei volume and nucleoli number, suggesting chromatin rearrangement. They observed that upon the deletion of the Serpinf1 gene, the Pnpla2 gene and associated PEDF-R protein decreased.
Besides, rhodopsin and lipids were deposited in the RPE of the Serpinf1 deficient animals relative to littermate controls. The observation that the Serpinf1/ RPE accumulates rhodopsin and lipids and downregulates the Pnpla2 phagocytosis-related gene, indicates RPE phagocytosis malfunction brought on by PEDF ablation.
The study's inferences support earlier findings that aging causes a reduction in RPE phagocytic activity. Together, they demonstrate how PEDF helps to prevent senescence and promote phagocytosis, showing its dual function in assisting RPE activity and its effects.
The scientists noted that the present study was the first one to use Serpinf1 deficient mouse to examine RPE senescence and phagocytosis. The study findings emphasize PEDF as a retinoprotective and a regulating protein of aging-like alterations linked with faulty degradation of the photoreceptor's outer segment across the RPE, establishing PEDF deficiency as a contributor to senescence-like modifications in the RPE.
In summary, the current investigation demonstrates the significance of PEDF signaling in age-associated eye disease development. The team mentioned that using PEDF-null mouse models can assist define novel signaling pathways comprising phagocytosis that affect lipid and visual pigment recycling across ocular diseases and offer mechanistic insight into examinations of senescence and aging.