Mertk in daily retinal phagocytosis: a history in the making

TAM receptors in phagocytosis: Beyond the mere internalization of particles

TYRO3, AXL, and MERTK constitute the TAM family of receptor tyrosine kinases, activated by their ligands GAS6 and PROS1. TAMs are necessary for adult homeostasis in the immune, nervous, reproductive, skeletal, and vascular systems. Among additional cellular functions employed by TAMs, phagocytosis is central for tissue health. TAM receptors are dominant in providing phagocytes with the molecular machinery necessary to engulf diverse targets, including apoptotic cells, myelin debris, and portions of live cells in a phosphatidylserine-dependent manner. Simultaneously, TAMs drive the release of anti-inflammatory and tissue repair molecules. Disruption of the TAM-driven phagocytic pathway has detrimental consequences, resulting in autoimmunity, male infertility, blindness, and disrupted vascular integrity, and which is thought to contribute to neurodegenerative diseases. Although structurally and functionally redundant, the TAM receptors and ligands underlie complex signaling cascades, of which several key aspects are yet to be elucidated. We discuss similarities and differences between TAMs and other phagocytic pathways, highlight future directions and how TAMs can be harnessed therapeutically to modulate phagocytosis.

High Glucose Impairs Expression and Activation of MerTK in ARPE-19 Cells

MerTK (Mer Tyrosine Kinase) is a cell surface receptor that regulates phagocytosis of photoreceptor outer segments (POS) in retinal pigment epithelial (RPE) cells. POS phagocytosis is impaired in several pathologies, including diabetes. In this study, we investigate whether hyperglycemic conditions may affect MerTK expression and activation in ARPE-19 cells, a retinal pigment epithelial cellular model. ARPE-19 cells were cultured in standard (CTR) or high-glucose (HG) medium for 24 h. Then, we analyzed: mRNA levels and protein expression of MerTK and ADAM9, a protease that cleaves the extracellular region of MerTK; the amount of cleaved Mer (sMer); and the ability of GAS6, a MerTK ligand, to induce MerTK phosphorylation. Since HG reduces miR-126 levels, and ADAM9 is a target of miR-126, ARPE-19 cells were transfected with miR-126 inhibitor or mimic; then, we evaluated ADAM9 expression, sMer, and POS phagocytosis. We found that HG reduced expression and activation of MerTK. Contextually, HG increased expression of ADAM9 and the amount of sMer. Overexpression of miR-126 reduced levels of sMer and improved phagocytosis in ARPE-19 cells cultured with HG. In this study, we demonstrate that HG compromises MerTK expression and activation in ARPE-19 cells. Our results suggest that HG up-regulates ADAM9 expression, leading to increased shedding of MerTK. The consequent rise in sMer coupled to reduced expression of MerTK impairs binding and internalization of POS in ARPE-19 cells.

Study of the Role of the Tyrosine Kinase Receptor MerTK in the Development of Kidney Ischemia-Reperfusion Injury in RCS Rats

Renal ischaemia reperfusion (I/R) triggers a cascade of events including oxidative stress, apoptotic body and microparticle (MP) formation as well as an acute inflammatory process that may contribute to organ failure. Macrophages are recruited to phagocytose cell debris and MPs. The tyrosine kinase receptor MerTK is a major player in the phagocytosis process. Experimental models of renal I/R events are of major importance for identifying I/R key players and for elaborating novel therapeutical approaches. A major aim of our study was to investigate possible involvement of MerTK in renal I/R. We performed our study on both natural mutant rats for MerTK (referred to as RCS) and on wild type rats referred to as WT. I/R was established by of bilateral clamping of the renal pedicles for 30' followed by three days of reperfusion. Plasma samples were analysed for creatinine, aspartate aminotransferase (ASAT), lactate dehydrogenase (LDH), kidney injury molecule -1 (KIM-1), and neutrophil gelatinase-associated lipocalin (NGAL) levels and for MPs. Kidney tissue damage and CD68-positive cell requirement were analysed by histochemistry. monocyte chemoattractant protein-1 (MCP-1), myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS), and histone 3A (H3A) levels in kidney tissue lysates were analysed by western blotting. The phagocytic activity of blood-isolated monocytes collected from RCS or WT towards annexin-V positive bodies derived from cultured renal cell was assessed by fluorescence-activated single cell sorting (FACS) and confocal microscopy analyses. The renal I/R model for RCS rat described for the first time here paves the way for further investigations of MerTK-dependent events in renal tissue injury and repair mechanisms.

Co-grafts of Human Embryonic Stem Cell Derived Retina Organoids and Retinal Pigment Epithelium for Retinal Reconstruction in Immunodeficient Retinal Degenerate Royal College of Surgeons Rats

End-stage age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are two major retinal degenerative (RD) conditions that result in irreversible vision loss. Permanent eye damage can also occur in battlefields or due to accidents. This suggests there is an unmet need for developing effective strategies for treating permanent retinal damages. In previous studies, co-grafted sheets of fetal retina with its retinal pigment epithelium (RPE) have demonstrated vision improvement in rat retinal disease models and in patients, but this has not yet been attempted with stem-cell derived tissue. Here we demonstrate a cellular therapy for irreversible retinal eye injuries using a "total retina patch" consisting of retinal photoreceptor progenitor sheets and healthy RPE cells on an artificial Bruch's membrane (BM). For this, retina organoids (ROs) (cultured in suspension) and polarized RPE sheets (cultured on an ultrathin parylene substrate) were made into a co-graft using bio-adhesives [gelatin, growth factor-reduced matrigel, and medium viscosity (MVG) alginate]. In vivo transplantation experiments were conducted in immunodeficient Royal College of Surgeons (RCS) rats at advanced stages of retinal degeneration. Structural reconstruction of the severely damaged retina was observed based on histological assessments and optical coherence tomography (OCT) imaging. Visual functional assessments were conducted by optokinetic behavioral testing and superior colliculus electrophysiology. Long-term survival of the co-graft in the rat subretinal space and improvement in visual function were observed. Immunohistochemistry showed that co-grafts grew, generated new photoreceptors and developed neuronal processes that were integrated into the host retina. This novel approach can be considered as a new therapy for complete replacement of a degenerated retina.

MERTK-Mediated LC3-Associated Phagocytosis (LAP) of Apoptotic Substrates in Blood-Separated Tissues: Retina, Testis, Ovarian Follicles

Timely and efficient elimination of apoptotic substrates, continuously produced during one’s lifespan, is a vital need for all tissues of the body. This task is achieved by cells endowed with phagocytic activity. In blood-separated tissues such as the retina, the testis and the ovaries, the resident cells of epithelial origin as retinal pigmented epithelial cells (RPE), testis Sertoli cells and ovarian granulosa cells (GC) provide phagocytic cleaning of apoptotic cells and cell membranes. Disruption of this process leads to functional ablation as blindness in the retina and compromised fertility in males and females. To ensure the efficient elimination of apoptotic substrates, RPE, Sertoli cells and GC combine various mechanisms allowing maintenance of tissue homeostasis and avoiding acute inflammation, tissue disorganization and functional ablation. In tight cooperation with other phagocytosis receptors, MERTK—a member of the TAM family of receptor tyrosine kinases (RTK)—plays a pivotal role in apoptotic substrate cleaning from the retina, the testis and the ovaries through unconventional autophagy-assisted phagocytosis process LAP (LC3-associated phagocytosis). In this review, we focus on the interplay between TAM RTKs, autophagy-related proteins, LAP, and Toll-like receptors (TLR), as well as the regulatory mechanisms allowing these components to sustain tissue homeostasis and prevent functional ablation of the retina, the testis and the ovaries.

Pathogenic variants of AIPL1, MERTK, GUCY2D, and FOXE3 in Pakistani families with clinically heterogeneous eye diseases

Significant number out of 2.2 billion vision impairments in the world can be attributed to genetics. The current study is aimed to decipher the genetic basis of Leber congenital Amaurosis (LCA), Anterior Segment dysgenesis (ASD), and Retinitis Pigmentosa (RP), segregating in four large consanguineous Pakistani families. The exome sequencing followed by segregation analysis via Sanger sequencing revealed the LCA phenotypes segregating in families GCUF01 and GCUF04 can be attributed to c.465G>T (p.(Gln155His)) missense and novel c.139_140delinsA p.(Pro47Trhfster38) frameshift variant of AIPL1 and GUCY2D, respectively. The c.1843A>T (p.(Lys615*) truncating allele of MERTK is homozygous in all the affected individuals, presumably suffering with RP, of the GCUF02 family. Meanwhile, co-segregation of the ASD phenotype and the c.289A>G (p.(Ile97Val)) variant of FOXE3 was found in the GCUF06 family. All the identified variants were either absent or present in very low frequencies in the control databases. Our in-silico analyses and 3D molecular modeling support the deleterious impact of these variants on the encoded proteins. Variants identified in MERTK, GUCY2D, and FOXE3 were categorized as “pathogenic” or “likely pathogenic”, while the missense variant found in AIPL1 was deemed to have “uncertain significance” based upon the variant pathogenicity guidelines from the American College of Medical Genetics and Genomics (ACMG). This paper highlights the genetic diversity of vision disorders in the Pakistani population and reports the identification of four novel mutations in families who segregate clinically heterogeneous eye diseases. Our results give insight into the genotype-phenotype correlations of AIPL1, FOXE3, MERTK, and GUCY2D variants.

The clearance of dead cells by efferocytosis

Multiple modes of cell death have been identified, each with a unique function and each induced in a setting-dependent manner. As billions of cells die during mammalian embryogenesis and daily in adult organisms, clearing dead cells and associated cellular debris is important in physiology. In this Review, we present an overview of the phagocytosis of dead and dying cells, a process known as efferocytosis. Efferocytosis is performed by macrophages and to a lesser extent by other 'professional' phagocytes (such as monocytes and dendritic cells) and 'non-professional' phagocytes, such as epithelial cells. Recent discoveries have shed light on this process and how it functions to maintain tissue homeostasis, tissue repair and organismal health. Here, we outline the mechanisms of efferocytosis, from the recognition of dying cells through to phagocytic engulfment and homeostatic resolution, and highlight the pathophysiological consequences that can arise when this process is abrogated.

Retinal Laser Therapy Preserves Photoreceptors in a Rodent Model of MERTK-Related Retinitis Pigmentosa

Purpose

We investigated the effects of various retinal laser therapies on preservation of the photoreceptors in an animal model of Mer tyrosine kinase receptor (MERTK)-related retinitis pigmentosa (RP). These modalities included photocoagulation with various pattern densities, selective RPE therapy (SRT), and nondamaging retinal therapy (NRT).

Methods

Laser treatments were performed on right eyes of RCS rats, using one of three laser modalities. For photocoagulation, six pattern densities (spot spacings of 0.5, 1, 1.5, 3, 4, and 5 spot diameters) were delivered in 19-day-old animals, prior to the onset of photoreceptor degeneration, to determine the optimal treatment density for the best preservation of photoreceptors. The left eye was used as control. Rats were monitored for 6 months after treatment using electroretinography, optical coherence tomography, and histology.

Results

Photocoagulation resulted in long-term preservation of photoreceptors, manifested morphologically and functionally, with the extent of the benefit dependent on the laser pattern density. Eyes treated with a 1.5 spot size spacing showed the best morphologic and functional preservation during the 6-month follow-up. SRT-treated eyes exhibited short-term morphologic preservation, but no functional benefit. NRT-treated eyes did not show any observable preservation benefit from the treatment.

Conclusions

In a rodent model of MERTK-related RP, pattern photocoagulation of about 15% of the photoreceptors (1.5 spot diameter spacing) provides long-term preservation of photoreceptors in the treatment area.

Translational Relevance

Availability of retinal lasers in ophthalmic practice enables rapid translation of our study to clinical testing and may help preserve the sight in patients with photoreceptor degeneration.

Cell Therapy for Retinal Dystrophies: From Cell Suspension Formulation to Complex Retinal Tissue Bioengineering

Retinal degeneration is an irreversible phenomenon caused by various disease conditions including age-related macular degeneration (AMD) and retinitis pigmentosa (RP). During the course of these diseases, photoreceptors (PRs) are susceptible to degeneration due to their malfunctions or to a primary dysfunction of the retinal pigment epithelium (RPE). Once lost, these cells could not be endogenously regenerated in humans, and cell therapy to replace the lost cells is one of the promising strategies to recover vision. Depending on the nature of the primary defect and the stage of the disease, RPE cells, PRs, or both might be transplanted to achieve therapeutic effects. We describe in this review the current knowledge and recent progress to develop such approaches. The different cell sources proposed for cell therapy including human pluripotent stem cells are presented with their advantages and limits. Another critical aspect described herein is the pharmaceutical formulation of the end product to be delivered into the eye of patients. Finally, we also outline the future research directions in order to develop a complex multilayered retinal tissue for end-stage patients.

Identification of PROS1 as a Novel Candidate Gene for Juvenile Retinitis Pigmentosa

Homozygous mutations of PROS1, encoding vitamin K-dependent protein S (PS), have been reported so far to be associated with purpura fulminans, a characteristic fatal venous thromboembolic disorder. The current work for the first time reports the clinical phenotype in patients with juvenile retinitis pigmentosa harboring a novel likely pathogenic variant in thePROS1 gene. Whole-exome sequencing was performed on probands of a cohort with inherited retinal disease. Detailed phenotyping was performed, including clinical evaluation, electroretinography, fundus photography and spectral-domain optical coherence tomography. Analysis of whole-exome and Sanger sequencing led to the identification of a homozygous missense substitution (c.G122C:p.R41P) in PROS1 in affected individuals from two unrelated consanguineous families of Persian origin which had classic retinitis pigmentosa with no history of venous thromboembolic disorder. This variant was segregated, fully congruous with the phenotype in all family members. Consistently, none of 1000 unrelated healthy individuals from the same population carried the mentioned variant, according to Iranian national genome database (Iranome) and additional in-house exome control data. This study provides inaugural clinical traces for different role of PS as a ligand for TAM receptor-mediated efferocytosis at the retinal pigmented epithelium; the R41P variant may affect proper folding of PS needed for γ-carboxylation and extra-cellular secretion. That conformational change may also lead to defective apoptotic cell phagocytosis resulting in postnatal degeneration of photoreceptors.

A new immunodeficient retinal dystrophic rat model for transplantation studies using human-derived cells

PURPOSE

To create new immunodeficient Royal College of Surgeons (RCS) rats by introducing the defective MerTK gene into athymic nude rats.

METHODS

Female homozygous RCS (RCS-p+/RCS-p+) and male nude rats (Hsd:RH-Foxn1^mu, mutation in the foxn1 gene; no T cells) were crossed to produce heterozygous F1 progeny. Double homozygous F2 progeny obtained by crossing the F1 heterozygotes was identified phenotypically (hair loss) and genotypically (RCS-p+ gene determined by PCR). Retinal degenerative status was confirmed by optical coherence tomography (OCT) imaging, electroretinography (ERG), optokinetic (OKN) testing, superior colliculus (SC) electrophysiology, and by histology. The effect of xenografts was assessed by transplantation of human embryonic stem cell-derived retinal pigment epithelium (hESC-RPE) and human-induced pluripotent stem cell-derived RPE (iPS-RPE) into the eye. Morphological analysis was conducted based on hematoxylin and eosin (H&E) and immunostaining. Age-matched pigmented athymic nude rats were used as control.

RESULTS

Approximately 6% of the F2 pups (11/172) were homozygous for RCS-p+ gene and Foxn1^mu gene. Homozygous males crossed with heterozygous females resulted in 50% homozygous progeny for experimentation. OCT imaging demonstrated significant loss of retinal thickness in homozygous rats. H&E staining showed photoreceptor thickness reduced to 1-3 layers at 12 weeks of age. Progressive loss of visual function was evidenced by OKN testing, ERG, and SC electrophysiology. Transplantation experiments demonstrated survival of human-derived cells and absence of apparent immune rejection.

CONCLUSIONS

This new rat animal model developed by crossing RCS rats and athymic nude rats is suitable for conducting retinal transplantation experiments involving xenografts.

Human ESC-derived retinal epithelial cell sheets potentiate rescue of photoreceptor cell loss in rats with retinal degeneration

Replacing defective retinal pigment epithelial (RPE) cells with those derived from human embryonic stem cells (hESCs) or human-induced pluripotent stem cells (hiPSCs) is a potential strategy for treating retinal degenerative diseases. Early clinical trials have demonstrated that hESC-derived or hiPSC-derived RPE cells can be delivered safely as a suspension to the human eye. The next step is transplantation of hESC/hiPSC-derived RPE cells as cell sheets that are more physiological. We have developed a tissue-engineered product consisting of hESC-derived RPE cells grown as sheets on human amniotic membrane as a biocompatible substrate. We established a surgical approach to engraft this tissue-engineered product into the subretinal space of the eyes of rats with photoreceptor cell loss. We show that transplantation of the hESC-RPE cell sheets grown on a human amniotic membrane scaffold resulted in rescue of photoreceptor cell death and improved visual acuity in rats with retinal degeneration compared to hESC-RPE cells injected as a cell suspension. These results suggest that tissue-engineered hESC-RPE cell sheets produced under good manufacturing practice conditions may be a useful approach for treating diseases of retinal degeneration.

Assessment of Safety and Functional Efficacy of Stem Cell-Based Therapeutic Approaches Using Retinal Degenerative Animal Models

Dysfunction and death of retinal pigment epithelium (RPE) and or photoreceptors can lead to irreversible vision loss. The eye represents an ideal microenvironment for stem cell-based therapy. It is considered an “immune privileged” site, and the number of cells needed for therapy is relatively low for the area of focused vision (macula). Further, surgical placement of stem cell-derived grafts (RPE, retinal progenitors, and photoreceptor precursors) into the vitreous cavity or subretinal space has been well established. For preclinical tests, assessments of stem cell-derived graft survival and functionality are conducted in animal models by various noninvasive approaches and imaging modalities. In vivo experiments conducted in animal models based on replacing photoreceptors and/or RPE cells have shown survival and functionality of the transplanted cells, rescue of the host retina, and improvement of visual function. Based on the positive results obtained from these animal experiments, human clinical trials are being initiated. Despite such progress in stem cell research, ethical, regulatory, safety, and technical difficulties still remain a challenge for the transformation of this technique into a standard clinical approach. In this review, the current status of preclinical safety and efficacy studies for retinal cell replacement therapies conducted in animal models will be discussed.

Reengineering Human Bruch's Membrane Increases Rod Outer Segment Phagocytosis by Human Retinal Pigment Epithelium

PURPOSE

We have shown previously that Bruch's membrane (BM) aging decreases retinal pigment epithelium (RPE) phagocytosis. Herein, we determine the effects of BM reengineering on RPE phagocytosis.

METHODS

BM explants were dissected from young and old donor eyes. Some old BM explants were reengineered by cleaning with Triton X-100 and/or coating with extracellular matrix (ECM) ligands. ARPE-19 cell-derived ECM (ARPE-ECM) modified ("aged") by sodium nitrite was subjected to similar treatments. ARPE-19 cells were then cultured to confluence onto the different surfaces. Fluorescently-labeled bovine rod outer segments (ROS) were fed to cells with or without αVβ5 integrin antibody. Image acquisition and phagocytosis quantification was performed by fluorescence microscopy and ImageJ analysis.

RESULTS

Cleaning old donor-derived BM with detergent does not increase the uptake of ROS, but a combination of cleaning and coating with ECM ligands significantly increases RPE phagocytosis (54.9 ± 6.2 vs. 83.5 ± 6.5 arbitrary units; P < 0.05) to levels closer to young donor BM (123.6 ± 9.9 arbitrary units). Similar effects were observed on nitrite-modified ARPE-ECM subjected to the same treatments. Incubation of αVβ5 blocking antibody with ROS significantly decreased RPE phagocytosis.

CONCLUSIONS

The detrimental effects of aging BM on RPE phagocytosis can be reversed by reengineering the BM surface with detergent cleaning and recoating with ECM ligands.

TRANSLATION RELEVANCE

These results demonstrate that the therapeutic success of transplanted RPE cells may require, at least in part, reengineering of diseased BM to make it a more suitable environment for attachment, survival and proper functioning of the RPE.

Expression pattern of Ccr2 and Cx3cr1 in inherited retinal degeneration

Background

Though accumulating evidence suggests that microglia, resident macrophages in the retina, and bone marrow-derived macrophages can cause retinal inflammation which accelerates photoreceptor cell death, the details of how these cells are activated during retinal degeneration (RD) remain uncertain. Therefore, it is important to clarify which cells play a dominant role in fueling retinal inflammation. However, distinguishing between microglia and macrophages is difficult using conventional techniques such as cell markers (e.g., Iba-1). Recently, two mouse models for visualizing chemokine receptors were established, Cx3cr1^GFP/GFP and Ccr2^RFP/RFP mice. As Cx3cr1 is expressed in microglia and Ccr2 is reportedly expressed in activated macrophages, these mice have the potential to distinguish microglia and macrophages, yielding novel information about the activation of these inflammatory cells and their individual roles in retinal inflammation.

Methods

In this study, c-mer proto-oncogene tyrosine kinase (Mertk)^−/− mice, which show photoreceptor cell death due to defective retinal pigment epithelium phagocytosis, were employed as an animal model of RD. Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were established by breeding Mertk^−/−, Cx3cr1^GFP/GFP, and Ccr2^RFP/RFP mice. The retinal morphology and pattern of inflammatory cell activation and invasion of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice were evaluated using retina and retinal pigment epithelium (RPE) flat mounts, retinal sections, and flow cytometry.

Results

Four-week-old Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice showed Cx3cr1-GFP-positive microglia in the inner retina. Cx3cr1-GFP and Ccr2-RFP dual positive activated microglia were observed in the outer retina and subretinal space of 6- and 8-week-old animals. Ccr2-RFP single positive bone marrow-derived macrophages were observed to migrate into the retina of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. These invading cells were still observed in the subretinal space in 18-week-old animals.

Conclusions

Cx3cr1-GFP-positive microglia and Ccr2-RFP-positive macrophages were distinguishable in the retinas of Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice. In addition, Ccr2 expression in Cx3cr1 positive microglia is a feature of microglial activation in RD. Mertk^−/−Cx3cr1^GFP/+Ccr2^RFP/+ mice enabled observation of microglial activation over time during RD and may be useful for developing inflammation-targeted treatment strategies for RD in the future.

NHE8 is essential for RPE cell polarity and photoreceptor survival

A new N-ethyl-N-nitrosourea (ENU)-induced mouse recessive mutation, identified by fundus examination of the eye, develops depigmented patches, indicating retinal disorder. Histology data show aberrant retinal pigment epithelium (RPE) and late-onset photoreceptor cell loss in the mutant retina. Chromosomal mapping and DNA sequencing reveal a point mutation (T to A) of the Slc9a8 gene, resulting in mutant sodium/proton exchanger 8 (NHE8)-M120K protein. The lysine substitution decreases the probability of forming the 3(rd) transmembrane helix, which impairs the pore structure of the Na(+)/H(+) exchanger. Various RPE defects, including mislocalization of the apical marker ezrin, and disrupted apical microvilli and basal infoldings are observed in mutant mice. We have further generated NHE8 knockout mice and confirmed similar phenotypes, including abnormal RPE cells and late-onset photoreceptor cell loss. Both in vivo and in vitro data indicate that NHE8 co-localizes with ER, Golgi and intracellular vesicles in RPE cells. Thus, NHE8 function is necessary for the survival of photoreceptor cells and NHE8 is important for RPE cell polarity and function. Dysfunctional RPE may ultimately lead to photoreceptor cell death in the NHE8 mutants. Further studies will be needed to elucidate whether or not NHE8 regulates pH homeostasis in the protein secretory pathways of RPE.

Differential TAM receptor-ligand-phospholipid interactions delimit differential TAM bioactivities

The TAM receptor tyrosine kinases Tyro3, Axl, and Mer regulate key features of cellular physiology, yet the differential activities of the TAM ligands Gas6 and Protein S are poorly understood. We have used biochemical and genetic analyses to delineate the rules for TAM receptor–ligand engagement and find that the TAMs segregate into two groups based on ligand specificity, regulation by phosphatidylserine, and function. Tyro3 and Mer are activated by both ligands but only Gas6 activates Axl. Optimal TAM signaling requires coincident TAM ligand engagement of both its receptor and the phospholipid phosphatidylserine (PtdSer): Gas6 lacking its PtdSer-binding ‘Gla domain’ is significantly weakened as a Tyro3/Mer agonist and is inert as an Axl agonist, even though it binds to Axl with wild-type affinity. In two settings of TAM-dependent homeostatic phagocytosis, Mer plays a predominant role while Axl is dispensable, and activation of Mer by Protein S is sufficient to drive phagocytosis.

DOI:http://dx.doi.org/10.7554/eLife.03385.001

Cells send out and receive signals to communicate with other cells. Detecting these signals is largely carried out by proteins called receptors that span the cell surface membrane. These proteins typically have extracellular domains outside of the cell that can bind to specific signaling molecules and an intracellular domain inside the cell that relays the information inwards to trigger a response.

Three such receptor proteins are collectively known as the TAM receptors. Each day, many billions of cells in the human body die and are engulfed by other cells and broken down so that their building blocks can be reused. TAM receptors are required for this process; and the TAM receptors also help prevent the immune system from going out of control, which would damage the body's own tissues.

Two different signaling proteins, called Gas6 and Protein S, can bind to and activate TAM receptors. Both of the signaling proteins can also bind to a phospholipid molecule that is found on the surface membrane of dead cells. However, it is not known if all three TAM receptors bind to both signaling proteins equally, and the importance of the phospholipid-binding domain in the signaling proteins remains unclear.

To shed light on the workings of these receptors, Lew et al. created mouse cells that each only express one out of the three TAM receptors. These cells were then exposed to intact Gas6 and Protein S, or shortened versions that lacked the phospholipid-binding domain. Lew et al. found that Gas6 could trigger a response through all three TAM receptors but that Protein S was specific for only two out of the three receptors. Signaling proteins with or without their phospholipid-binding domains bound equally well to the receptors, but the maximum level of response was only triggered when both signaling proteins were intact and the phospholipid molecule was present. This is important since the phospholipid can be thought of as an ‘eat-me’ signal by which the dead cells are recognized by the TAM receptor-expressing cells that will engulf them.

Using mice that only produce a TAM receptor called Mer, Lew et al. show that Protein S alone can trigger the process that engulfs and breaks down cells in a living organism. These data and previous work suggest that two TAM receptors—including Mer—are involved in the daily engulfment of dying cells, whereas the third mediates this process during infection and tissue damage.

Molecules that inhibit or activate the function of TAM receptors are currently being developed to treat cancer and other diseases. By revealing which receptors respond to which signaling molecules, the findings of Lew et al. will serve to guide these efforts.

DOI:http://dx.doi.org/10.7554/eLife.03385.002

Effects of aflibercept on primary RPE cells: toxicity, wound healing, uptake and phagocytosis

BACKGROUND/AIM

Anti-VEGF treatment is the therapy of choice in age-related macular degeneration, and is also applied in diabetic macular oedema or retinal vein occlusion. Recently, the fusion protein, aflibercept, has been approved for therapeutic use. In this study, we investigate the effects of aflibercept on primary RPE cells.

METHODS

Primary RPE cells were prepared from freshly slaughtered pigs' eyes. The impact of aflibercept on cell viability was investigated with MTT and trypan blue exclusion assay. The influence of aflibercept on wound healing was assessed with a scratch assay. Intracellular uptake of aflibercept was investigated in immunohistochemistry and its influence on phagocytosis with a phagocytosis assay using opsonised latex beads.

RESULTS

Aflibercept displays no cytotoxicity on RPE cells but impairs its wound healing ability. It is taken up into RPE cells and can be intracellularly detected for at least 7 days. Intracellular aflibercept impairs the phagocytic capacity of RPE cells.

CONCLUSIONS

Aflibercept interferes with the physiology of RPE cells, as it is taken up into RPE cells, which is accompanied by a reduction of the phagocytic ability. Additionally, it impairs the wound healing capacity of RPE cells. These effects on the physiology of RPE cells may indicate possible side effects.

CCL3 production by microglial cells modulates disease severity in murine models of retinal degeneration

Many degenerative retinal diseases illustrate retinal inflammatory changes that include infiltration of microglia and macrophages into the subretinal space. In this study, we examined the role of chemokines in the Abca4(-/-)Rdh8(-/-) mouse model of Stargardt disease and the Mertk(-/-) mouse model of retinitis pigmentosa. PCR array analysis of 84 chemokines and related molecules revealed 84.6-fold elevated expression of Ccl3 (MIP-1a) 24 h after light exposure in Abca4(-/-)Rdh8(-/-) mice. Only MIP-1 chemokines, including Ccl3 and Ccl4, displayed peak expression 24 h after light exposure, and peaked earlier than the other chemokines. Secretion of Ccl3 was documented only in microglia, whereas both microglia and retinal pigment epithelium cells produced Ccl2. Exposure of Cx3Cr1(gfp/Δ)Abca4(-/-)Rdh8(-/-) mice to intense light resulted in the appearance of Cx3Cr1GFP(+) monocytes in the subretinal space. To address the in vivo role of CCL3 in retinal degeneration, Ccl3(-/-)Abca4(-/-)Rdh8(-/-) mice and Ccl3(-/-)Mertk(-/-) mice were generated. Following intense light exposure, Ccl3(-/-)Abca4(-/-)Rdh8(-/-) mice displayed persistent retinal inflammation with appearance of Iba-1(+) cells in the subretinal space, severe photoreceptor cell death, and increased Ccl4 expression compared with Abca4(-/-)Rdh8(-/-) mice. In contrast, Ccl3(-/-)Abca4(-/-)Rdh8(-/-) mice exhibited a milder retinal inflammation and degeneration than Abca4(-/-)Rdh8(-/-) mice did in age-related chronic retinal degeneration under room light conditions. The deficiency of Ccl3 also attenuated the severity of retinal degeneration in Mertk(-/-) mice. Taken together, our results indicate that Ccl3 has an essential role in regulating the severity of retinal inflammation and degeneration in these mouse models.

Biology of the TAM receptors

The TAM receptors--Tyro3, Axl, and Mer--comprise a unique family of receptor tyrosine kinases, in that as a group they play no essential role in embryonic development. Instead, they function as homeostatic regulators in adult tissues and organ systems that are subject to continuous challenge and renewal throughout life. Their regulatory roles are prominent in the mature immune, reproductive, hematopoietic, vascular, and nervous systems. The TAMs and their ligands--Gas6 and Protein S--are essential for the efficient phagocytosis of apoptotic cells and membranes in these tissues; and in the immune system, they act as pleiotropic inhibitors of the innate inflammatory response to pathogens. Deficiencies in TAM signaling are thought to contribute to chronic inflammatory and autoimmune disease in humans, and aberrantly elevated TAM signaling is strongly associated with cancer progression, metastasis, and resistance to targeted therapies.

Photoreceptor proteins initiate microglial activation via Toll-like receptor 4 in retinal degeneration mediated by all-trans-retinal

Although several genetic and biochemical factors are associated with the pathogenesis of retinal degeneration, it has yet to be determined how these different impairments can cause similar degenerative phenotypes. Here, we report microglial/macrophage activation in both a Stargardt disease and age-related macular degeneration mouse model caused by delayed clearance of all-trans-retinal from the retina, and in a retinitis pigmentosa mouse model with impaired retinal pigment epithelium (RPE) phagocytosis. Mouse microglia displayed RPE cytotoxicity and increased production of inflammatory chemokines/cytokines, Ccl2, Il1b, and Tnf, after coincubation with ligands that activate innate immunity. Notably, phagocytosis of photoreceptor proteins increased the activation of microglia/macrophages and RPE cells isolated from model mice as well as wild-type mice. The mRNA levels of Tlr2 and Tlr4, which can recognize proteins as their ligands, were elevated in mice with retinal degeneration. Bone marrow-derived macrophages from Tlr4-deficient mice did not increase Ccl2 after coincubation with photoreceptor proteins. Tlr4(-/-)Abca4(-/-)Rdh8(-/-) mice displayed milder retinal degenerative phenotypes than Abca4(-/-)Rdh8(-/-) mice. Additionally, inactivation of microglia/macrophages by pharmacological approaches attenuated mouse retinal degeneration. This study demonstrates an important contribution of TLR4-mediated microglial activation by endogenous photoreceptor proteins in retinal inflammation that aggravates retinal cell death. This pathway is likely to represent an underlying common pathology in degenerative retinal disorders.

Genetic dissection of TAM receptor-ligand interaction in retinal pigment epithelial cell phagocytosis

Although TAM receptor tyrosine kinases play key roles in immune regulation, cancer metastasis, and viral infection, the relative importance of the two TAM ligands-Gas6 and Protein S-has yet to be resolved in any setting in vivo. We have now performed a genetic dissection of ligand function in the retina, where the TAM receptor Mer is required for the circadian phagocytosis of photoreceptor outer segments by retinal pigment epithelial cells. This process is severely attenuated in Mer mutant mice, which leads to photoreceptor death. We find that retinal deletion of either Gas6 or Protein S alone yields retinae with a normal number of photoreceptors. However, concerted deletion of both ligands fully reproduces the photoreceptor death seen in Mer mutants. These results demonstrate that Protein S and Gas6 function as independent, bona fide Mer ligands, and are, to a first approximation, interchangeable with respect to Mer-driven phagocytosis in the retina.

Induction of endoplasmic reticulum stress genes, BiP and chop, in genetic and environmental models of retinal degeneration

PURPOSE

Endoplasmic reticulum (ER) stress has been observed in animal models of retinitis pigmentosa expressing P23H rhodopsin. We compared levels of tightly induced ER stress genes, Binding of immunoglobulin protein (BiP) and CCAAT/enhancer-binding protein homologous protein (Chop), in seven additional models of retinal degeneration arising from genetic or environmental causes.

METHODS

Retinas from transgenic S334ter rhodopsin (lines 3, 4, and 5) and Royal College of Surgeons (RCS and RCS-p+) rats from postnatal (P) days 10 to 120 were analyzed. In a constant light (CL) model of retinal degeneration, BALB/c mice were exposed to 15,000 lux of CL for 0 to 8 hours. Retinal tissues from three to eight animals per experimental condition were collected for histologic and molecular analyses.

RESULTS

S334ter animals revealed significant increases in BiP, S334ter-3 (3.3× at P15), S334ter-4 (4× at P60), and S334ter-5 (2.2× at P90), and Chop, S334ter-3 (1.3× at P15), S334ter-4 (1.5× at P30), and S334ter-5 (no change), compared with controls. P23H-3 rats showed significant increase of BiP at P60 (2.3×) and Chop (1.6×). RCS and RCS-p+ rats showed significant increases in BiP at P60 (2.4×) and P20 (1.8×), respectively, but no statistically significant changes in Chop. BALB/c mice showed increases in BiP (1.5×) and Chop (1.3×) after 4 hours of CL. Increased levels of these ER stress markers correlated with photoreceptor cell loss.

CONCLUSIONS

Our study reveals surprising increases in BiP and to a lesser degree Chop in retinal degenerations arising from diverse causes. We propose that manipulation of ER stress responses may be helpful in treating many environmental and heritable forms of retinal degeneration.

Homozygous mutation in MERTK causes severe autosomal recessive retinitis pigmentosa

PURPOSE

Gene identification in retinitis pigmentosa is a prerequisite to future therapies. Accordingly, autosomal recessive retinitis pigmentosa families were genotyped to search for causative mutations.

METHODS

Members of a consanguineous Moroccan family had standard ophthalmologic examination, optical coherence tomography-3 scan, autofluorescence testing, and electroretinogram. Their DNA was genotyped with the 250K SNP microchip (Affymetrix) and homozygosity mapping was done. MERTK exons were polymerase chain reaction amplified and sequenced.

RESULTS

Two sisters and one brother out of 6 siblings had rod cone dystrophy type of retinitis pigmentosa. Salient features were night blindness starting in early infancy, dot-like whitish deposits in fovea and macula with corresponding autofluorescent dots in youngest patients, decreased visual acuity, and cone responses higher than rod responses at electroretinogram. The patients were homozygous in regions from chromosomes 2 and 8, but only that of chromosome 2 was inherited from a common ancestor. Sequencing of the MERTK gene belonging to the chromosome 2 region showed that the 3 affected patients carried a novel homozygous mutation in exon 17, c.2323C>T, leading to p.Arg775X, while their unaffected brothers and sister, parents, and paternal grandfather were heterozygous.

CONCLUSIONS

MERTK mutations lead to severe retinitis pigmentosa with discrete dot-like autofluorescent deposits at early stages, which are a hallmark of this MERTK-specific dystrophy.

Increased plasma levels of the soluble Mer tyrosine kinase receptor in systemic lupus erythematosus relate to disease activity and nephritis

Introduction

Mer and Tyro3 are receptor tyrosine kinases important for the phagocytosis of apoptotic cells. Together with Axl, they constitute the TAM receptor family. These receptors can be shed from the cell membrane and their soluble extracellular regions can be found in plasma. The objective of this study was to elucidate whether the plasma levels of soluble Mer (sMer) and Tyro3 (sTyro3) were increased in systemic lupus erythematosis (SLE), rheumatoid arthritis (RA), or critical limb ischemia (CLI).

Methods

ELISA kits were used to test plasma concentrations in controls and in patients with SLE, RA or CLI.

Results

Increased levels of, in particular, sMer and, to some extent, sTyro3, were found in patients with SLE or RA, but not in patients with CLI. Patients with SLE demonstrated the highest sMer levels and there was a strong correlation to higher SLE disease activity score (SLEDAI). In contrast, in patients with RA, the sMer levels did not correlate with the disease activity score (DAS). In SLE, sMer levels were particularly high in those with lupus nephritis, patients who also had decreased C1q levels and increased titers of anti-DNA antibodies. After therapy, the plasma concentrations of sMer decreased in parallel to the decrease in SLEDAI score.

Conclusions

The plasma concentrations of sMer and sTyro3 were significantly increased in patients with active SLE and RA, suggesting the TAM receptor shedding was affected by these autoimmune diseases. In particular, sMer was increased in SLE, the plasma levels of sMer reflecting disease activity.