The Pathobiology of Age-related Macular Degeneration

Research conducted in our laboratory is dedicated to understanding the molecular and cellular mechanisms that contribute to age-related macular degeneration (AMD) pathogenesis and pathobiology such that novel targets can be identified and used for the development of therapeutics to treat people with AMD.

AMD is a late-onset, progressive, neurodegenerative disease with devastating impact on the elderly. This disease occurs primarily in people over the age of 65 years and accounts for approximately 50% of registered blindness in Western Europe and North America. AMD develops as either dry (atrophic) or wet (exudative). AMD is characterized by the accumulation of extracellular lipid- and protein-rich deposits between the retinal pigment epithelium (RPE) and Bruch’s membrane (BrM). These sub-RPE deposits may be focal (drusen) or diffuse and likely contribute to disease pathogenesis and progression similar to intercellular deposits characteristic of other diseases like Alzheimer’s disease, atherosclerosis, and glomerulonephritis. Although the molecular bases of these diseases may be diverse, their pathogenic deposits contain many shared constituents that are attributable, in part, to local inflammation and activation of the complement cascade.

Support for a role for complement activation in AMD pathogenesis comes from studies implicating variations in the complement factor H (CFH) gene as the strongest genetic factor associated with risk for AMD. The precise mechanisms of complement system dysregulation in AMD are unknown, although there are several candidate molecules. Among these is amyloid beta (Abeta), a constituent of drusen, and known activator of the complement system. Abeta deposits in drusen are associated with activated complement proteins and cell injury.

A major focus of our research program is to develop, characterize and use murine models of AMD that are based on multiple known risk factors of human disease and that faithfully recapitulate many aspects of human AMD. We have been using these models to elucidate the mechanisms of pathology and progression for AMD as well as identify therapeutic targets and test emerging therapies. We are using AAV-based gene delivery/therapy, immunotherapy and small molecules to establish disease pathways and preclinical proof of efficacy in our AMD models.

Our models of age-related macular degeneration:

1. Human APOE isoform knock-in mice: We developed a murine model of AMD by combining three of the risk factors for AMD: advanced age, apolipoprotein E isoform expression and exposure to a high-fat, high-cholesterol (HFC) diet that develop pathological features similar to the morphologic hallmarks observed in both dry and wet human AMD. The phenotype mimics several of the important phenotypic characteristics of AMD in a temporal, non-fully penetrant and non-invasive manner that is analogous to human AMD progression. We are using this model to study the pathobiology of AMD and to test therapies. Investigation of this model has revealed that lipid transport dysregulation, inflammation and Abeta deposition contribute to the pathogenesis of the retinal changes observed. This led us to studies that showed therapies targeting Abeta can preserve retinal function in these mice. Validation of these therapeutic targets is currently in clinical trials.
2. Cfh heterozygous mice: Based on the substantial evidence implicating complement factor H (CFH) in the pathogenesis of AMD we tested the effect of advanced age, exposure to a HFC diet and CFH deficiency (Cfh knock out) or Cfh haploinsufficiency (Cfh hemizygous). Characterization of these mice established a link between the complement system and lipid pathways by demonstrating that (i) CFH and lipoproteins compete for binding in the sub-RPE extracellular matrix such that decreasing CFH leads to lipoprotein accumulation and sub-RPE deposit formation; and (ii) detrimental complement activation within sub-RPE deposits leads to RPE damage and vision loss. This new understanding of the complicated interactions of CFH in development of AMD-like pathology represents a paradigm shift in our understanding of AMD and paves the way for identifying more targeted therapeutic strategies for AMD.
3. Human CFH transgenic Mice: We developed a model of AMD susceptibility by generating transgenic mice carrying the full length CFH gene encoding the normal (Tyr402) and risk-associated (His402) human forms of factor H. We are using these animals and functional studies of the human factor H protein to determine the functional consequence of the AMD risk-associated change. We showed that only the aged His402 expressing mice on HFC diet (CFH-HH~HFC) developed AMD-like pathologies consisting of attenuated rod-mediated visual function, RPE damage and basal laminar deposit formation compared to the Tyr402 expressing mice on HFC diet (CFH-Y~HFC). Analysis of these models establish a causal link between CFH and lipoprotein dysregulation and development of an AMD phenotype. We are the first to establish a functional consequence of the AMD-risk associated His402 form in vivo.

RPE exosomes in AMD

Another research program in our laboratory focuses on the study of RPE-derived exosomes. Exosomes are cell-derived nanovesicles that carry protein, lipid, and genetic material from their cell of origin, enabling cells to communicate in a non-contact manner and modulate immune-regulatory and other cellular processes. They are released from various cell types under normal and pathological conditions, influencing the activity of recipient cells by carrying active signals or modifying the extracellular milieu. We are specifically interested in their role in the RPE.

The RPE functions to maintain the outer blood-retinal barrier and to support photoreceptor function, including regeneration of visual pigment and turnover of outer segments. Dysfunction of the RPE underlies pathology leading to development of AMD. Multiple lines of evidence indicate that one of the major culprits observed in RPE dysfunction is dysregulation in the endosomal pathway. It is thought that in AMD this dysregulation in RPE cells is at least in part responsible for the formation of the protein- and lipid-rich extracellular deposits between the basal lamina of the RPE and the pentalaminar collagen- and elastin-rich Bruch’s membrane. At present, the exact mechanisms for drusen formation are unknown. Since RPE-released exosomes and other nano-sized lipid bilayer extracellular vesicles (EVs) are essential parts of the endosomal pathway, we hypothesize that exosomes released from stressed RPE cells are distinct from those released from unstressed RPE cells, and that these exosomes are involved in the pathognomonic deposit formation and extracellular matrix (ECM) changes that underlie the early and late stages of AMD. Accordingly, approaches to characterize these vesicles and modulate their release have high potential to give important insight to disease mechanisms and new treatment targets. Significantly, very little is known about RPE-released exosomes and other EVs. By evaluating complementary in vitro and ex vivo AMD models, our overall goal for these studies is to determine the role of exosome secretion in sub-RPE deposit formation and in ECM changes under conditions relevant to AMD, and whether pharmacological or gene therapy/biological therapeutic interventions are possible.

In a related project we are investigating the potential of isolating RPE-derived exosomes from the systemic circulation. Successful isolation and characterization of these blood-borne RPE-derived exosomes could yield specific protein and nucleic acid cargo that give real-time biological information of RPE health in retinal diseases such as AMD. Thus, they may serve as biomarkers to evaluate efficacy of novel treatments in clinical trials or as prognostic biomarkers to predict disease progression.