Small Retinoprotective Peptides Reveal a Receptor-binding Region on Pigment Epithelium-derived Factor

Rabbit model of oxidative stress-induced retinal degeneration

Retinitis pigmentosa (RP) is a disorder that results in the death of rod photoreceptors in the retina, caused by several different mutations. As rods are 95 % of the photoreceptors in the eye and consume the most oxygen, their death causes tissue hyperoxia resulting in oxidative stress in the retina. Oxidative stress is implicated in the pathogenesis of photoreceptor death in RP. Therefore, oxidative stress models are very relevant to identifying drugs to prevent photoreceptor death in RP. Rabbits are an excellent model to study retinal degeneration due to their large eyes and therefore larger subretinal and vitreous space, makes it easier for surgical and drug interventions techniques. Herein, we have created a rabbit model of oxidative stress-induced retinal degeneration using paraquat (PQ), a known oxidant. PQ causes oxidative stress in the retina, that results in functional and structural loss of photoreceptors. We also show that using an antioxidant PEDF[H105A], we were able to blunt the damage to photoreceptors caused by PQ. Therefore, this rabbit model of oxidative stress-induced retinal degeneration will serve as an excellent model to test a wide array of surgical and drug interventions techniques.

H105A peptide eye drops promote photoreceptor survival in murine and human models of retinal degeneration

BACKGROUND

Photoreceptor death leads to inherited blinding retinal diseases, such as retinitis pigmentosa (RP). As disease progression often outpaces therapeutic advances, developing effective treatments is urgent. This study evaluates the efficacy of small peptides derived from pigment epithelium-derived factor (PEDF), which are known to restrict common cell death pathways associated with retinal diseases.

METHODS

We tested chemically synthesized peptides (17-mer and H105A) with affinity for the PEDF receptor, PEDF-R, delivered as eye drops to two RP mouse models: rd10 (phosphodiesterase 6b mutation) and RhoP23H/+ (rhodopsin P23H mutation). Additionally, we engineered AAV-H105A vectors for intravitreal delivery in RhoP23H/+ mice. To assess peptide effects in human tissue, we used retinal organoids exposed to cigarette smoke extract, a model of oxidative stress. Photoreceptor survival, morphology and function were evaluated.

RESULTS

Here we show that peptides 17-mer and H105A delivered via eye drops successfully reach the retina, promote photoreceptor survival, and improve retinal function in both RP mouse models. Intravitreal delivery of a AAV-H105A vector delays photoreceptor degeneration in RhoP23H/+ mice up to six months. In human retinal organoids, peptide H105A specifically prevents photoreceptor death induced by oxidative stress, a contributing factor to RP progression.

CONCLUSIONS

PEDF peptide-based eye drops offer a promising, minimally invasive therapy to prevent photoreceptor degeneration in retinal disorders, with a favorable safety profile.

Loss of Pigment Epithelium Derived Factor Sensitizes C57BL/6J Mice to Light-Induced Retinal Damage

Purpose

Pigment epithelium-derived factor (PEDF) is a neurotrophic glycoprotein secreted by the retinal pigment epithelium (RPE) that supports retinal photoreceptor health. Deficits in PEDF are associated with increased inflammation and retinal degeneration in aging and diabetic retinopathy. We hypothesized that light-induced stress in C57BL/6J mice deficient in PEDF would lead to increased retinal neuronal and RPE defects, impaired expression of neurotrophic factor Insulin-like growth factor 1 (IGF-1), and overactivation of Galectin-3-mediated inflammatory signaling.

Methods

C57BL/6J mice expressing the RPE65 M450/M450 allele were crossed to PEDF KO/KO and wildtype (PEDF +/+) littermates. Mice were exposed to 50,000 lux light for 5 hours to initiate acute damage. Changes in visual function outcomes were tracked via electroretinogram (ERG), confocal scanning laser ophthalmoscopy(cSLO), and spectral domain optical coherence tomography (SD-OCT) on days 3, 5, and 7 post-light exposure. Gene and protein expression of Galectin-3 were measured by digital drop PCR (ddPCR) and western blot. To further investigate the role of galectin-3 on visual outcomes and PEDF expression after damage, we also used a small-molecule inhibitor to reduce its activity.

Results

Following light damage, PEDF KO/KO mice showed more severe retinal thinning, impaired visual function (reduced a-, b-, and c-wave amplitudes), and increased Galectin-3 expressing immune cell infiltration compared to PEDF +/+. PEDF KO/KO mice had suppressed damage-associated increases in IGF-1 expression. Additionally, baseline Galectin-3 mRNA and protein expression were reduced in PEDF KO/KO mice compared to PEDF +/+. However, after light damage, Galectin-3 expression decreases in PEDF +/+ mice but increases in PEDF KO/KO mice without reaching PEDF +/+ levels. Galectin-3 inhibition worsens retinal degeneration, reduces PEDF expression in PEDF +/+ mice, and mimics the effects seen in PEDF knockouts.

Conclusions

Loss of PEDF alone does not elicit functional defects in C57BL/6J mice. However, under light stress, PEDF deficiency significantly increases severe retinal degeneration, visual deficits, Galectin-3 expression, and suppression of IGF-1 than PEDF +/+. PEDF deficiency reduced baseline expression of Galectin-3, and pharmacological inhibition of Galectin-3 worsens outcomes and suppresses PEDF expression in PEDF +/+, suggesting a novel co-regulatory relationship between the two proteins in mitigating light-induced retinal damage.

Functional Roles of Pigment Epithelium-Derived Factor in Retinal Degenerative and Vascular Disorders: A Scoping Review

Purpose

This review explores the role of pigment epithelium-derived factor (PEDF) in retinal degenerative and vascular disorders and assesses its potential both as an adjunct to established vascular endothelial growth factor inhibiting treatments for retinal vascular diseases and as a neuroprotective therapeutic agent.

Methods

A comprehensive literature review was conducted, focusing on the neuroprotective and anti-angiogenic properties of PEDF. The review evaluated its effects on retinal health, its dysregulation in ocular disorders, and its therapeutic application in preclinical models. Advances in drug delivery, including gene therapy, were also examined.

Results

PEDF, initially identified for promoting neuronal differentiation, is also a potent endogenous angiogenesis inhibitor. Strong anti-angiogenic and neuroprotective effects are observed in preclinical studies. It has pro-apoptotic and antiproliferative effects on endothelial cells thereby reducing neovascularization. Although promising, clinical development is limited with only a single conducted phase I clinical trial for macular neovascularization. Development of PEDF-derived peptides enhances potency and specificity, and emerging gene therapy approaches offer sustained PEDF expression for long-term treatment. However, questions regarding dosage, durability, and efficacy remain, particularly in large animal models.

Conclusions

PEDF shows significant therapeutic potential in preclinical models of retinal degeneration and vascular disorders. Despite inconclusive evidence on PEDF downregulation as a primary disease driver, many studies highlight its therapeutic benefits and favorable safety profile. Advances in gene therapy could enable long-acting PEDF-based treatments, but further research is needed to optimize dosage and durability, potentially leading to clinical trials and expanding treatment options for retinal disorders.

Pigment Epithelium-Derived Factor Binding to VEGFR-1 (Flt-1) Increases the Survival of Retinal Neurons

Purpose

The purpose of this study was to examine possible involvement of vascular endothelial growth factor (VEGF) receptor (VEGFR)-1/Flt-1 in pigment epithelium-derived factor (PEDF)-promoted survival of retinal neurons.

Methods

Survival of growth factor-deprived retinal ganglion cells (RGCs) and R28 cells and activation of ERK-1/-2 MAP kinases were assessed in the presence of PEDF, placental growth factor (PlGF), and VEGF using cell cultures, viability assays and quantitation of ERK-1/-2 phosphorylation. VEGFR-1/Flt-1 expression was determined using quantitative PCR (qPCR) and Western blotting. VEGFR-1/Flt-1 was knocked down in R28 cells by small interfering RNA (siRNA). Binding of a PEDF-IgG Fc fusion protein (PEDF-Fc) to retinal neurons, immobilized VEGFR-1/Flt-1 and VEGFR-1/Flt-1-derived peptides was studied using binding assays and peptide scanning.

Results

PEDF in combination with PlGF stimulated increased cell survival and ERK-1/-2 MAP kinase activation compared to effects of either factor alone. VEGFR-1/Flt-1 expression in RGCs and R28 cells was significantly upregulated by hypoxia, VEGF, and PEDF. VEGFR-1/Flt-1 ligands (VEGF and PlGF) or soluble VEGFR-1 (sflt-1) competed with PEDF-Fc for binding to R28 cells. Depleting R28 cells of VEGFR-1/Flt-1 resulted in reduced PEDF-Fc binding when comparing VEGFR-1/Flt-1 siRNA- and control siRNA-treated cells. PEDF-Fc interacted with immobilized sflt-1, which was specifically blocked by VEGF and PlGF. PEDF-Fc binding sites were mapped to VEGFR-1/Flt-1 extracellular domains D3 and D4. Peptides corresponding to D3 and D4 specifically inhibited PEDF-Fc binding to R28 cells. These peptides and sflt-1 significantly inhibited PEDF-promoted survival of R28 cells.

Conclusions

These results suggest that PEDF can target VEGFR-1/Flt-1 and this interaction plays a significant role in PEDF-mediated neuroprotection in the retina.

H105A peptide eye drops promote photoreceptor survival in murine and human models of retinal degeneration

Photoreceptor death causes blinding inheritable retinal diseases, such as retinitis pigmentosa (RP). As disease progression often outpaces therapeutic advances, finding effective treatments is urgent. This study focuses on developing a targeted approach by evaluating the efficacy of small peptides derived from pigment epithelium-derived factor (PEDF), known to restrict common cell death pathways associated with retinal diseases. Peptides with affinity for the PEDF receptor, PEDF-R, (17-mer and H105A) delivered via eye drops reached the retina, efficiently promoted photoreceptor survival, and improved retinal function in RP mouse models based on both the rd10 mutation and the rhodopsin P23H mutation. Additionally, intravitreal delivery of AAV-H105A vectors delayed photoreceptor degeneration in the latter RP mouse model. Furthermore, peptide H105A specifically prevented photoreceptor death induced by oxidative stress, a contributing factor to RP progression, in human retinal organoids. This promising approach for peptide eye drop delivery holds significant potential as a therapeutic for preventing photoreceptor death in retinal disorders, offering a high safety profile, low invasiveness and multiple delivery options.

PEDF and 34-mer peptide inhibit cardiac microvascular endothelial cell ferroptosis via Nrf2/HO-1 signalling in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion injury (MIRI) represents a critical pathology in acute myocardial infarction (AMI), which is characterized by high mortality and morbidity. Cardiac microvascular dysfunction contributes to MIRI, potentially culminating in heart failure (HF). Pigment epithelium-derived factor (PEDF), which belongs to the non-inhibitory serpin family, exhibits several physiological effects, including anti-angiogenesis, anti-inflammatory and antioxidant properties. Our study aims to explore the impact of PEDF and its functional peptide 34-mer on both cardiac microvascular perfusion in MIRI rats and human cardiac microvascular endothelial cells (HCMECs) injury under hypoxia reoxygenation (HR). It has been shown that MIRI is accompanied by ferroptosis in HCMECs. Furthermore, we investigated the effect of PEDF and its 34-mer, particularly regarding the Nrf2/HO-1 signalling pathway. Our results demonstrated that PEDF 34-mer significantly ameliorated cardiac microvascular dysfunction following MIRI. Additionally, they exhibited a notable suppression of ferroptosis in HCMECs, and these effects were mediated through activation of Nrf2/HO-1 signalling. These findings highlight the therapeutic potential of PEDF and 34-mer in alleviating microvascular dysfunction and MIRI. By enhancing cardiac microvascular perfusion and mitigating endothelial ferroptosis, PEDF and its derivative peptide represent promising candidates for the treatment of AMI.

Molecular mechanisms underlying inherited photoreceptor degeneration as targets for therapeutic intervention

Retinitis pigmentosa (RP) is a form of retinal degeneration characterized by primary degeneration of rod photoreceptors followed by a secondary cone loss that leads to vision impairment and finally blindness. This is a rare disease with mutations in several genes and high genetic heterogeneity. A challenging effort has been the characterization of the molecular mechanisms underlying photoreceptor cell death during the progression of the disease. Some of the cell death pathways have been identified and comprise stress events found in several neurodegenerative diseases such as oxidative stress, inflammation, calcium imbalance and endoplasmic reticulum stress. Other cell death mechanisms appear more relevant to photoreceptor cells, such as high levels of cGMP and metabolic changes. Here we review some of the cell death pathways characterized in the RP mutant retina and discuss preclinical studies of therapeutic approaches targeting the molecular outcomes that lead to photoreceptor cell demise.

Short peptides derived from pigment epithelium-derived factor attenuate retinal ischemia reperfusion injury through inhibition of apoptosis and inflammatory response in rats

Pigment epithelium-derived factor (PEDF) is widely recognized as a neuroprotective factor expressed in the retina and has shown therapeutic potential in several retinal diseases. Our study aimed to identify the neuroprotective fragment in PEDF and investigate its protective activity in retinas under ischemia-reperfusion (IR) condition. We synthesized a series of shorter synthetic peptides, 6-mer (Ser93-Gln98) and its d-form variant (6 dS) derived from the 44-mer (Val78-Thr121; a PEDF neurotrophic fragment), to determine their cytoprotective activity in IR injury, which was induced in rat retinas by injection of saline into the anterior chamber to increase the intraocular pressure (IOP) followed by reperfusion. We found the cytoprotective effect of 6-mer on glutamate-treated Neuro-2a cells and tert-butyl hydroperoxide (tBHP)-treated 661W cells were 2.6-fold and 1.5-fold higher than the 44-mer, respectively. The cytoprotective effect was blocked by a chemical inhibitor atglistatin and blocking antibody targeting PEDF receptor (PEDF-R). IR induced several impairments in retina, including cell apoptosis, activation of microglia/macroglia, degeneration of retinal capillaries, reduction in electroretinography (ERG) amplitudes, and retinal atrophy. Such IR injuries were ameliorated by treatment with 6-mer and 6 dS eye drops. Also, the neuroprotective activity of 6-mer and 6 dS in ischemic retinas were dramatically reversed by atglistatin preconditioning. Taken together, our data demonstrate smallest neuroprotective fragment of PEDF has potential to treat retinal degeneration-related diseases.

Roles of pigment epithelium-derived factor in cardiomyocytes: implications for use as a cardioprotective therapeutic

OBJECTIVES

Cardiovascular diseases are the leading cause of death worldwide, with patients having limited options for treatment. Pigment epithelium-derived factor (PEDF) is an endogenous multifunctional protein with several mechanisms of action. Recently, PEDF has emerged as a potential cardioprotective agent in response to myocardial infarction. However, PEDF is also associated with pro-apoptotic effects, complicating its role in cardioprotection. This review summarises and compares knowledge of PEDF's activity in cardiomyocytes with other cell types and draws links between them. Following this, the review offers a novel perspective of PEDF's therapeutic potential and recommends future directions to understand the clinical potential of PEDF better.

KEY FINDINGS

PEDF's mechanisms as a pro-apoptotic and pro-survival protein are not well understood, despite PEDF's implication in several physiological and pathological activities. However, recent evidence suggests that PEDF may have significant cardioprotective properties mediated by key regulators dependent on cell type and context.

CONCLUSIONS

While PEDF's cardioprotective activity shares some key regulators with its apoptotic activity, cellular context and molecular features likely allow manipulation of PEDF's cellular activity, highlighting the importance of further investigation into its activities and its potential to be applied as a therapeutic to mitigate damage from a range of cardiac pathologies.

Pigment epithelium-derived factor is an interleukin-6 antagonist in the RPE: Insight of structure-function relationships

Retinal and choroidal inflammatory lesions increase the levels of the pro-inflammatory cytokine interleukin-6 (IL-6). Pigment epithelium-derived factor (PEDF) has anti-inflammatory properties, but it is not known if it can prevent the production of IL-6 by the retinal pigment epithelium. To investigate the anti-inflammatory effects of PEDF in the RPE, we used human ARPE-19 cells stimulated with human recombinant tumor necrosis factor-alpha (TNF-α) to induce overexpression of the IL6 gene. We found that the viability of ARPE-19 cells decreased by 22% with TNF-α at 10 ng/ml, being drastically decreased at ≥50 ng/ml. TNF-α at 5-100 ng/ml elevated the production and secretion of IL-6 protein, as measured by ELISA. To challenge the TNF-α-mediated stimulation of IL-6, we used recombinant human PEDF protein. PEDF at 100 nM recovered the TNF-α-mediated loss of cell viability and repressed IL-6 gene expression as determined by RT-PCR. PEDF at 10-100 nM attenuated the IL-6 protein secretion in a dose dependent fashion (IC50 = 65 nM), being abolished with 100 nM PEDF. To map the region that confers the IL-6 blocking effect to the PEDF polypeptide, we used chemically synthesized peptides designed from its biologically active domains, pro-death 34-mer, and pro-survival 44-mer and 17-mer (H105A), to challenge the IL-6 overproduction. The pro-survival peptides recovered the TNF-α-mediated cell viability loss, and inhibited IL-6 secretion, while the 34-mer did not have an effect, suggesting a role for the pro-survival domain in blocking TNF-α-mediated cell death and IL-6 stimulation. Our findings position PEDF as a novel antagonistic agent of IL-6 production in RPE cells, underscoring its use for the management of retinal disease-related inflammation.

Pigment epithelium-derived factor engineered to increase glycosaminoglycan affinity while maintaining bioactivity

Pigment epithelium-derived factor (PEDF) is a secreted protein that is essential in tissue homeostasis and is involved in multiple functions in the eye, such as antiangiogenesis and neuroprotection. However, short retention in the retinal microenvironment can limit its therapeutic effects. In this study, we modified the amino acid sequence of PEDF to increase its affinity for heparin and hyaluronic acid (HA), which are negatively charged extracellular matrix (ECM) molecules. HA is the major component of the vitreous humor. We selectively converted neutral or anionic residues into cationic residues to obtain engineered PEDF (ePEDF). Using in vitro binding assays, we demonstrate that ePEDF had higher affinity for heparin and HA than wild-type PEDF (wtPEDF). ePEDF exhibited antiangiogenic and retinal survival bioactivities. It inhibited endothelial cell proliferation and tube formation in vitro. In an ex vivo model mimicking retinal degeneration, ePEDF protected photoreceptors from cell death. The findings suggest that protein engineering is an approach to develop active PEDF with higher ECM affinity to potentially improve its retention in the retina microenvironment and in turn make it a more efficient therapeutic drug for retinal diseases.

Expression and production of pigment epithelium-derived factor (PEDF) and PEDF receptor variants from mammalian and bacterial cells

Human SERPINF1 gene codes for pigment epithelium-derived factor (PEDF), a secreted glycoprotein and member of the SERPIN superfamily. To obtain large amounts of recombinant PEDF proteins, we subcloned the coding sequence of human SERPINF1 mutated versions into the pCEP4 vector and generated stably transfected HEK.Ebna cells. The cells produced and secreted recombinant PEDF proteins into the culturing media. The recombinant PEDF proteins were purified by ion-exchange column chromatography and milligram amounts of highly purified protein were recovered. PEDF has affinity for PEDF-receptor (PEDF-R), a membrane-linked lipase encoded by the PNPLA2 gene. Recombinant PEDF-R truncated versions were obtained from Escherichia coli containing expression vectors with human PNPLA2 cDNAs with 3'end deletions and by induction with isopropyl β-d-1-thiogalactopyranoside. The bacterially derived PEDF-R proteins in insoluble inclusion bodies were solubilized with urea and purified by cation-exchange column chromatography. C-terminally truncated PEDF-R versions containing the ligand binding region retained the ability to bind PEDF. The data demonstrate that mammalian-derived recombinant PEDF and bacterially derived recombinant PEDF-R can be produced and purified in large amounts for further use in structural and biological studies.

Animal Models of LED-Induced Phototoxicity. Short- and Long-Term In Vivo and Ex Vivo Retinal Alterations

Phototoxicity animal models have been largely studied due to their degenerative communalities with human pathologies, e.g., age-related macular degeneration (AMD). Studies have documented not only the effects of white light exposure, but also other wavelengths using LEDs, such as blue or green light. Recently, a blue LED-induced phototoxicity (LIP) model has been developed that causes focal damage in the outer layers of the superior-temporal region of the retina in rodents. In vivo studies described a progressive reduction in retinal thickness that affected the most extensively the photoreceptor layer. Functionally, a transient reduction in a- and b-wave amplitude of the ERG response was observed. Ex vivo studies showed a progressive reduction of cones and an involvement of retinal pigment epithelium cells in the area of the lesion and, in parallel, an activation of microglial cells that perfectly circumscribe the damage in the outer retinal layer. The use of neuroprotective strategies such as intravitreal administration of trophic factors, e.g., basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) or pigment epithelium-derived factor (PEDF) and topical administration of the selective alpha-2 agonist (Brimonidine) have demonstrated to increase the survival of the cone population after LIP.

Pigment epithelium-derived factor (PEDF) and derived peptides promote survival and differentiation of photoreceptors and induce neurite-outgrowth in amacrine neurons

Pigment epithelium-derived factor (PEDF) is a cytoprotective protein for the retina. We hypothesize that this protein acts on neuronal survival and differentiation of photoreceptor cells in culture. The purpose of the present study was to evaluate the neurotrophic effects of PEDF and its fragments in an in vitro model of cultured primary retinal neurons that die spontaneously in the absence of trophic factors. We used Wistar albino rats. Cell death was assayed by immunofluorescence and flow cytometry through TUNEL assay, propidium iodide, mitotracker, and annexin V. Immunofluorescence of cells for visualizing rhodopsin, CRX, and antisyntaxin under confocal microscopy was performed. Neurite outgrowth was also quantified. Results show that PEDF protected photoreceptor precursors from apoptosis, preserved mitochondrial function and promoted polarization of opsin enhancing their developmental process, as well as induced neurite outgrowth in amacrine neurons. These effects were abolished by an inhibitor of the PEDF receptor or receptor-derived peptides that block ligand/receptor interactions. While all the activities were specifically conferred by short peptide fragments (17 amino acid residues) derived from the PEDF neurotrophic domain, no effects were triggered by peptides from the PEDF antiangiogenic region. The observed effects on retinal neurons imply a specific activation of the PEDF receptor by a small neurotrophic region of PEDF. Our findings support the neurotrophic PEDF peptides as neuronal guardians for the retina, highlighting their potential as promoters of retinal differentiation, and inhibitors of retinal cell death and its blinding consequences.

New In Vitro Cellular Model for Molecular Studies of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is an inherited form of retinal degeneration characterized by primary rod photoreceptor cell death followed by cone loss. Mutations in several genes linked to the disease cause increased levels of cyclic guanosine monophosphate (cGMP) and calcium ion influxes. The purpose of this project was to develop a new in vitro photoreceptor degeneration model for molecular studies of RP. 661W cells were genetically modified to stably express the neural retina leucine zipper (NRL) transcription factor. One clone (661W-A11) was selected based on the expression of Nrl target genes. 661W-A11 showed a significant increase in expression of rod-specific genes but not of cone-specific genes, compared with 661W cells. Zaprinast was used to inhibit phosphodiesterase 6 (PDE6) activity to mimic photoreceptor degeneration in vitro. The activation of cell death pathways resulting from PDE6 inhibition was confirmed by detection of decreased viability and increased intracellular cGMP and calcium, as well as activation of protein kinase G (PKG) and calpains. In this new in vitro system, we validated the effects of previously published neuroprotective drugs. The 661W-A11 cells may serve as a new model for molecular studies of RP and for high-throughput drug screening.

Delivery Systems of Retinoprotective Proteins in the Retina

Retinoprotective proteins play important roles for retinal tissue integrity. They can directly affect the function and the survival of photoreceptors, and/or indirectly target the retinal pigment epithelium (RPE) and endothelial cells that support these tissues. Retinoprotective proteins are used in basic, translational and in clinical studies to prevent and treat human retinal degenerative disorders. In this review, we provide an overview of proteins that protect the retina and focus on pigment epithelium-derived factor (PEDF), and its effects on photoreceptors, RPE cells, and endothelial cells. We also discuss delivery systems such as pharmacologic and genetic administration of proteins to achieve photoreceptor survival and retinal tissue integrity.

Degradation of Photoreceptor Outer Segments by the Retinal Pigment Epithelium Requires Pigment Epithelium-Derived Factor Receptor (PEDF-R)

Purpose

To examine the contribution of pigment epithelium-derived factor receptor (PEDF-R) to the phagocytosis process. Previously, we identified PEDF-R, the protein encoded by the PNPLA2 gene, as a phospholipase A2 in the retinal pigment epithelium (RPE). During phagocytosis, RPE cells ingest abundant phospholipids and protein in the form of photoreceptor outer segment (POS) tips, which are then hydrolyzed. The role of PEDF-R in RPE phagocytosis is not known.

Methods

Mice in which PNPLA2 was conditionally knocked out (cKO) in the RPE were generated. Mouse RPE/choroid explants were cultured. Human ARPE-19 cells were transfected with siPNPLA2 silencing duplexes. POSs were isolated from bovine retinas. The phospholipase A2 inhibitor bromoenol lactone was used. Transmission electron microscopy, immunofluorescence, lipid labeling, pulse-chase experiments, western blots, and free fatty acid and β-hydroxybutyrate assays were performed.

Results

The RPE of the cKO mice accumulated lipids, as well as more abundant and larger rhodopsin particles, compared to littermate controls. Upon POS exposure, RPE explants from cKO mice released less β-hydroxybutyrate compared to controls. After POS ingestion during phagocytosis, rhodopsin degradation was stalled both in cells treated with bromoenol lactone and in PNPLA2-knocked-down cells relative to their corresponding controls. Phospholipase A2 inhibition lowered β-hydroxybutyrate release from phagocytic RPE cells. PNPLA2 knockdown also resulted in a decline in fatty acids and β-hydroxybutyrate release from phagocytic RPE cells.

Conclusions

PEDF-R downregulation delayed POS digestion during phagocytosis. The findings imply that the efficiency of RPE phagocytosis depends on PEDF-R, thus identifying a novel contribution of this protein to POS degradation in the RPE.

Pigment Epithelium-Derived Factor (PEDF) Receptors Are Involved in Survival of Retinal Neurons

The demise of retinal ganglion cells (RGCs) is characteristic of diseases of the retina such as glaucoma and diabetic or ischemic retinopathies. Pigment epithelium-derived factor (PEDF) is a multifunctional secreted protein that mediates neuroprotection and inhibition of angiogenesis in the retina. We have studied expression and regulation of two of several receptors for PEDF, patatin-like phospholipase 2 gene product/PEDF-R and laminin receptor (LR), in serum-starved RGC under normoxia and hypoxia and investigated their involvement in the survival of retinal neuronal cells. We show that PEDF-R and LR are co-expressed in RGC and R28 retinal precursor cells. Expression of both receptors was enhanced in the presence of complex secretions from retinal glial (Müller) cells and upregulated by VEGF and under hypoxic conditions. PEDF-R- and LR-knocked-down cells demonstrated a markedly attenuated expression of anti-apoptotic Bcl-2 family members (Bcl-2, Bcl-xL) and neuroprotective mediators (PEDF, VEGF, BDNF) suggesting that both PEDF-R and LR mediate pro-survival effects of PEDF on RGC. While this study does not provide evidence for a differential survival-promoting influence of either PEDF-R or LR, it nevertheless highlights the importance of both PEDF receptors for the viability of retinal neurons.

Pigment Epithelium-Derived Factor (PEDF) Fragments Prevent Mouse Cone Photoreceptor Cell Loss Induced by Focal Phototoxicity In Vivo

Here, we evaluated the effects of PEDF (pigment epithelium-derived factor) and PEDF peptides on cone-photoreceptor cell damage in a mouse model of focal LED-induced phototoxicity (LIP) in vivo. Swiss mice were dark-adapted overnight, anesthetized, and their left eyes were exposed to a blue LED placed over the cornea. Immediately after, intravitreal injection of PEDF, PEDF-peptide fragments 17-mer, 17-mer[H105A] or 17-mer[R99A] (all at 10 pmol) were administered into the left eye of each animal. BDNF (92 pmol) and bFGF (27 pmol) injections were positive controls, and vehicle negative control. After 7 days, LIP resulted in a consistent circular lesion located in the supratemporal quadrant and the number of S-cones were counted within an area centered on the lesion. Retinas treated with effectors had significantly greater S-cone numbers (PEDF (60%), 17-mer (56%), 17-mer [H105A] (57%), BDNF (64%) or bFGF (60%)) relative to their corresponding vehicle groups (≈42%). The 17-mer[R99A] with no PEDF receptor binding and no neurotrophic activity, PEDF combined with a molar excess of the PEDF receptor blocker P1 peptide, or with a PEDF-R enzymatic inhibitor had undetectable effects in S-cone survival. The findings demonstrated that the cone survival effects were mediated via interactions between the 17-mer region of the PEDF molecule and its PEDF-R receptor.

Cell-based assays to identify novel retinoprotective agents

Degeneration of the retina can lead ultimately to devastating irreversible vision loss, such as in inherited retinitis pigmentosa and age-related macular degeneration. Currently there is no cure to prevent retinal degeneration. Quantitative cell-based assays can be used to test potential drugs that prevent the death of retinal cells. Here, we describe in detail three semi-automated cell-based protocols to identify retinoprotective factors with two retinal cell lines, rat R28 cells and mouse 661W cells. In these protocols, cells are induced to undergo death by photo-oxidation stress, growth factor depletion or cytotoxicity with sodium iodate. Pigment epithelium-derived factor, an established neurotrophic factor for retinal cells, was used as a positive control. We discuss how these protocols will prove useful in high-throughput quantitative screening to identify novel therapeutics for retinal disorders.

PEDF deficiency increases the susceptibility of rd10 mice to retinal degeneration

The SERPINF1 gene encodes pigment epithelium-derived factor (PEDF), a member of the serpin superfamily with neurotrophic and antiangiogenic properties in the retina. We hypothesized that absence of PEDF would lead to increased stress-associated retinal degeneration in Serpinf1 null mice. Accordingly, using a Serpinf1 null mouse model, we investigated the impact of PEDF absence on retinal morphology, and susceptibility to induced and inherited retinal degeneration. We studied the pattern of Serpinf1 expression in the mouse retina layers. PEDF protein was detected by western blotting. Transmission electron microscopy was performed on mouse retina. Serpinf1 null mice and wild type littermates were injected with NaIO₃ (30 mg/kg body weight) intraperitonially. At post-injection day 1, 3, 4, 6 and 8 mice were euthanized, and eyes were enucleated. Serpinf1 null and rd10 double mutant mice were generated and their eyes enucleated at different time points from post-natal day 15 to post-natal day 28. Enucleated eyes were processed for hematoxylin and eosin staining and histopathological evaluations. We found that Serpinf1 was expressed in the retinal pigment epithelium, in the inner nuclear layer and in the ganglion cell layer, but undetectable in the outer nuclear layer of wild type mice. Plasma PEDF protein levels were undetectable in Serpinf1 null animals. RPE atrophy and retinal thinning were observed in NaIO₃-treated wild type mice that progressed with time post-injection. NaIO₃-treated Serpinf1 null mice showed comparatively better retinal morphology than wild type mice at day 4 post-injection. However, the absence of PEDF in Serpinf1 null x rd10 mice increased the susceptibility to retinal degeneration relative to that of rd10 mice. We concluded that histopathological evaluation of retinas lacking PEDF showed that removal of the Serpinf1 gene may activate PEDF-independent compensatory mechanisms to protect the retina against oxidative stress, while it increases the susceptibility to degenerate the retina in inherited retinal degeneration models.

Potential of Müller Glia for Retina Neuroprotection

Müller glia constitute the main glial cells of the retina. They are spatially distributed along this tissue, facilitating their close membrane interactions with all retinal neurons. Müller glia are characterized by their active metabolic functions, which are neuroprotective in nature. Although they can become reactive under pathological conditions, leading to their production of inflammatory and neurotoxic factors, their main metabolic functions confer neuroprotection to the retina, resulting in the promotion of neural cell repair and survival. In addition to their protective metabolic features, Müller glia release several neurotrophic factors and antioxidants into the retinal microenvironment, which are taken up by retinal neurons for their survival. This review summarizes the Müller glial neuroprotective mechanisms and describes advances made on the clinical application of these factors for the treatment of retinal degenerative diseases. It also discusses prospects for the use of these cells as a vehicle to deliver neuroprotective factors into the retina.

Signaling Mechanisms Involved in PEDF-Mediated Retinoprotection

Pigment epithelium-derived factor (PEDF) is involved in signal transduction cascades necessary for protection of the retina. The interaction between PEDF and retinal cells elicits neuroprotective effects in vitro and in vivo. The direct substrates and signaling mechanisms involved in the survival response derived from such interaction are beginning to be revealed. It is of interest to elucidate cell death pathways that are crucial for the retinoprotective response of PEDF for the identification of targets that interfere with retina degeneration with potential therapeutic value. Here we review the molecular pathways triggered by PEDF that are involved in retinal survival activity.

Targeting molecular pathways for the treatment of inherited retinal degeneration

Inherited retinal degeneration is a major cause of incurable blindness characterized by loss of retinal photoreceptor cells. Inherited retinal degeneration is characterized by high genetic and phenotypic heterogeneity with several genes mutated in patients affected by these genetic diseases. The high genetic heterogeneity of these diseases hampers the development of effective therapeutic interventions for the cure of a large cohort of patients. Common cell demise mechanisms can be envisioned as targets to treat patients regardless the specific mutation. One of these targets is the increase of intracellular calcium ions, that has been detected in several murine models of inherited retinal degeneration. Recently, neurotrophic factors that favor the efflux of calcium ions to concentrations below toxic levels have been identified as promising molecules that should be evaluated as new treatments for retinal degeneration. Here, we discuss therapeutic options for inherited retinal degeneration and we will focus on neuroprotective approaches, such as the neuroprotective activity of the Pigment epithelium-derived factor. The characterization of specific targets for neuroprotection opens new perspectives together with many questions that require deep analyses to take advantage of this knowledge and develop new therapeutic approaches. We believe that minimizing cell demise by neuroprotection may represent a promising treatment strategy for retinal degeneration.

PEDF peptides promote photoreceptor survival in rd10 retina models

The purpose of the study is to evaluate the protective properties of PEDF peptide fragments on rd10 mouse models of retinal degeneration ex vivo. Human recombinant PEDF and synthetic peptides were used. Rd10 retinal explants as well as wild-type retinal explants treated with zaprinast to mimic the rd10 photoreceptor cell death were employed. PEDF protein was intravitreally administered into rd10 mice. Outer nuclear layer thickness measurements in retinal sections, TUNEL labeling in retinal explants, western blots and immunofluorescence with retinal samples were performed. PEDF protein levels in the RPE of rd10 mice decreased with age (P15 - P25). Levels of PEDF receptor PEDF-R declined in the photoreceptor inner segments from rd10 relative to wild-type mice at P25. PEDF administration increased the outer nuclear layer thickness of rd10 retinas in vivo and decreased the number of TUNEL⁺ nuclei of photoreceptors in rd10 retinal explant cultures, both relative to untreated controls. Peptides containing the PEDF neurotrophic region decreased the number of TUNEL⁺ photoreceptors in both rd10 and zaprinast-induced cell death ex vivo models, while peptides without the neurotrophic region and/or lacking affinity for PEDF-R were ineffective in protecting photoreceptors. Thus, retinal explants are a valuable system to evaluate PEDF activity. Short peptides with the photoreceptor-protective property of PEDF may prove useful for the development of therapeutic agents for photoreceptor protection in retinal degenerations.

Photoreceptor degeneration in microphthalmia (Mitf) mice: partial rescue by pigment epithelium-derived factor

Dysfunction and loss of the retinal pigment epithelium (RPE) are hallmarks of retinal degeneration, but the underlying pathogenetic processes are only partially understood. Using mice with a null mutation in the transcription factor gene Mitf, in which RPE deficiencies are associated with retinal degeneration, we evaluated the role of trophic factors secreted by the RPE in retinal homeostasis. In such mice, the thickness of the outer nuclear layer (ONL) is as in wild type up to postnatal day 10, but then is progressively reduced, associated with a marked increase in the number of apoptotic cells and a decline in staining for rhodopsin. We show that retinal degeneration and decrease in rhodopsin staining can be prevented partially in three different ways: first, by recombining mutant-derived postnatal retina with postnatal wild-type RPE in tissue explant cultures; second, by adding to cultured mutant retina the trophic factor pigment epithelium-derived factor (PEDF; also known as SERPINF1), which is normally produced in RPE under the control of Mitf; and third, by treating the eyes of Mitf mutant mice in vivo with drops containing a bioactive PEDF 17-mer peptide. This latter treatment also led to marked increases in a number of rod and cone genes. The results indicate that RPE-derived trophic factors, in particular PEDF, are instrumental in retinal homeostasis, and suggest that PEDF or its bioactive fragments may have therapeutic potential in RPE deficiency-associated retinal degeneration.

Pigment epithelium-derived factor hinders photoreceptor cell death by reducing intracellular calcium in the degenerating retina

Calcium ions play a critical role in neuronal cell death. Pigment epithelium-derived factor (PEDF) is a promising neuroprotective protein for photoreceptor cells but the mechanisms mediating its effects against retinal degeneration are still not well characterized. We addressed this question in the rd1 degenerating mouse retina that bears a mutation in the Pde6b gene encoding one subunit of the phosphodiesterase enzyme. Loss of phosphodiesterase activity in rod photoreceptor cells increases cyclic guanosine monophosphate (cGMP) levels leading to a rise in intracellular calcium. Short-term treatments with recombinant human PEDF protein decreased intracellular calcium in photoreceptors in vivo. Taking advantage of calcium pump blockers, we defined that PEDF signaling acts on PMCA calcium pumps to lower intracellular calcium. PEDF restrained cell death pathways activated by high calcium levels and engaging calpains, BAX and AIF. The neurotrophic effects were mediated by the PEDF receptor (PEDF-R), encoded by the PNPLA2 gene. Finally, peptides containing the neurotrophic domain of PEDF targeted these same cell death pathways in vivo. The findings reveal rescue from death of degenerating photoreceptor cells by a PEDF-mediated preservation of intracellular calcium homeostasis.

PEDF expression affects retinal endothelial cell proangiogenic properties through alterations in cell adhesive mechanisms

Pigment epithelium-derived factor (PEDF) is an endogenous inhibitor of angiogenesis. Although various ocular cell types including retinal endothelial cells (EC) produce PEDF, we know very little about cell autonomous effects of PEDF in these cell types. Here we determined how PEDF expression affects retinal EC proangiogenic properties. Retinal EC were prepared from wild-type (PEDF+/+) and PEDF-deficient (PEDF-/-) mice. The identity of EC was confirmed by staining for specific markers including vascular endothelial cadherin, CD31, and B4-lectin. Retinal EC also expressed VEGF receptor 1 and endoglin, as well as ICAM-1, ICAM-2, and VCAM-1. PEDF-/- retinal EC were more proliferative, less apoptotic when challenged with H2O2, less migratory, and less adherent compared with PEDF+/+ EC. These changes could be associated, at least in part, with increased levels of tenascin-C, fibronectin, thrombospondin-1 and collagen IV, and lower amounts of osteopontin. PEDF-/- EC also exhibited alterations in expression of a number of integrins including α2, αv, β1, β8, and αvβ3, and cell-cell adhesion molecules including CD31, zonula occluden-1, and occludin. These observations correlated with attenuation of capillary morphogenesis and increased levels of oxidative stress in PEDF-/- EC. PEDF-/- EC also produced lower levels of VEGF compared with PEDF+/+ cells. Thus, PEDF deficiency has a significant impact on retinal EC adhesion and migration, perhaps through altered production of extracellular matrix and junctional proteins in response to increased oxidative stress affecting their proangiogenic activity.

Comparison of two neurotrophic serpins reveals a small fragment with cell survival activity

PURPOSE

Protease nexin-1 (PN-1), a serpin encoded by the SERPINE2 gene, has serine protease inhibitory activity and neurotrophic properties in the brain. PN-1 inhibits retinal angiogenesis; however, PN-1's neurotrophic capacities in the retina have not yet been evaluated. Pigment epithelium-derived factor (PEDF) is a serpin that exhibits neurotrophic and antiangiogenic activities but lacks protease inhibitory properties. The aim of this study is to compare PN-1 and PEDF.

METHODS

Sequence comparisons were performed using computer bioinformatics programs. Mouse and bovine eyes, human retina tissue, and ARPE-19 cells were used to prepare RNA and protein samples. Interphotoreceptor matrix lavage was obtained from bovine eyes. Gene expression and protein levels were evaluated with reverse-transcription PCR (RT-PCR) and western blotting, respectively. Recombinant human PN-1, a version of PN-1 referred to as PN-1[R346A] lacking serine protease inhibitory activity, and PEDF proteins were used, as well as synthetic peptides designed from PEDF and PN-1 sequences. Survival activity in serum-starved, rat-derived retinal precursor (R28) cells was assessed with terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL) cell death assays. Bcl2 levels were measured with RT-PCR.

RESULTS

PN-1 is analogous in primary and tertiary structure to PEDF. A region in PN-1 shares homology with the neurotrophic active region of PEDF, a 17-residue region within alpha helix C. The native human retina, ARPE-19 cells, and murine RPE and retina expressed the gene for PN-1 (SERPINE2 and Serpine2 mRNA). The retina, ARPE-19 cell lysates, and bovine interphotoreceptor matrix contained PN-1 protein. The addition of PN-1, PN-1[R346A], or the 17mer peptide of PN-1 to serum-starved retina cells decreased the number of TUNEL-positive nuclei relative to the untreated cells, such as PEDF. PN-1, PN-1[R346A], and PN-1-17mer treatments increased the Bcl2 transcript levels in serum-starved cells, as seen with PEDF.

CONCLUSIONS

PN-1 and PEDF share structural and functional features, and expression patterns in the retina. These serpins' mechanisms of action as cell survival factors are independent of serine protease inhibition. We have identified PN-1 as a novel factor for the retina that may play a neuroprotective role in vivo, and small peptides as relevant candidates for preventing retinal degeneration.

PEDF plus DHA modulate inflammation and stimulate nerve regeneration after HSV-1 infection

Herpes simplex virus type-1 (HSV-1) infection leads to impaired corneal sensation and, in severe cases, to corneal ulceration, melting and perforation. Here, we explore the potential therapeutic action of pigment epithelial-derived factor (PEDF) plus docosahexaenoic acid (DHA) on corneal inflammation and nerve regeneration following HSV-1 infection. Rabbits inoculated with 100,000 PFU/eye of HSV-1 strain 17Syn+ were treated with PEDF + DHA or vehicle. PEDF + DHA treatment resulted in a biphasic immune response with stronger infiltration of CD4+T cells, neutrophils and macrophages at 7-days post-treatment (p.t.) that was significantly decreased by 14 days, compared to the vehicle-treated group. Screening of 14 immune-related genes by q-PCR showed that treatment induced higher expression of IFN-γ and CCL20 and inhibition of IL-18 by 7 days in the cornea. PEDF + DHA-treated animals developed less dendritic corneal lesions, opacity and neovascularization. Corneal nerve density increased at 12-weeks p.t. with functional recovery of corneal sensation. Treatment with PEDF + DHA that was postponed by 3 weeks also showed increased nerve density when compared to vehicle. Our data demonstrate that PEDF + DHA promotes resolution of the inflammatory response to the virus and, most importantly, induces regeneration of damaged corneal nerves vital for maintaining ocular surface homeostasis.

Pigment Epithelium-Derived Factor (PEDF) Protects Osteoblastic Cell Line from Glucocorticoid-Induced Apoptosis via PEDF-R

Pigment epithelial-derived factor (PEDF) is known as a widely expressed multifunctional secreted glycoprotein whose biological actions are cell-type dependent. Recent studies demonstrated that PEDF displays cytoprotective activity in several cell types. However, it remains unknown whether PEDF is involved in glucocorticoid-induced osteoblast death. The aim of this study was to examine the role of PEDF in osteoblast survival in response to dexamethasone, an active glucocorticoid analogue, and explore the underlying mechanism. In the present study, dexamethasone (DEX) was used to induce MC3T3-E1 pre-osteoblast apoptosis. PEDF mRNA and protein levels and cell apoptosis were determined respectively. Then PEDF receptor (PEDF-R)- and lysophosphatidic acid (LPA)-related signal transductions were assessed. Here we show that DEX down-regulates PEDF expression, which contributes to osteoblast apoptosis. As a result, exogenous recombinant PEDF (rPEDF) inhibited DEX-induced cell apoptosis. We confirmed that PEDF-R was expressed on MC3T3-E1 pre-osteoblast membrane and could bind to PEDF which increased the level of LPA and activated the phosphorylation of Akt. Our results suggest that PEDF attenuated DEX-induced apoptosis in MC3T3-E1 pre-osteoblasts through LPA-dependent Akt activation via PEDF-R.