Scrutinizing the proteostasis network of photoreceptors towards therapeutic intervention of inherited retinal dystrophies

CLINICAL PROBLEM: Inherited retinal dystrophies (IRDs) that cause blindness are still considered incurable diseases, and affect more than two million individuals worldwide. Molecular genetic studies have revealed over ~200 different genes that can be mutated in IRDs, demonstrating the tremendous genetic heterogeneity. RECENT DISCOVERIES: Our large scale systems biology approach to study primary cilia in health and disease (www.syscilia.org) gained important new insights into the pathogenesis of a major subgroup of IRD caused by photoreceptor sensory cilium dysfunction, the retinal ciliopathies. We found that different aspects of the proteostasis network (PN) are essential for correct cilium function. The photoreceptor sensory cilium organizes assembly of the photoreceptor outer segment, the phototransduction site. The PN comprises folding, assembly, and degradation pathways, and quality control mechanisms that maintain a balance of functional proteins within cells. Disturbances in the PN affect photoreceptor cell viability, initiating mechanisms such as the heat shock response (HSR) and the unfolded protein response (UPR), and eventually apoptosis. These downstream events have been observed in several animal models of inherited retinal degeneration (IRD). Importantly, our studies of congenital blindness have recently suggested an important role of the cilium in ubiquitination-based control of glucose metabolism in photoreceptors. This process is of critical importance for the vitality of cone photoreceptors in general, as it generates the energy and lipids required for the extensive regeneration of photoreceptor outer segments that are continuously damaged by daylight. This emphasizes the cilium as a critical photoreceptor PN hub. RESEARCH CHALLENGE/AIM: The PN is regulated by a series of intimately connected, enzymatically regulated pathways that maintain functional protein levels and organize protein quality control. In photoreceptor cells, the details of these pathways that are critical for their vitality, remain largely unstudied. In other medical fields such as oncology, these pathways proved to be valuable targets for treatment with small molecules that either potentiate or inhibit these regulating enzymes. We aim to dissect the cilium-associated PN of photoreceptors, determine the associated defects in IRD, and identify and evaluate pharmacological proteostasis regulators to ameliorate IRD. APPROACH: We will employ an integrated, three-stage systems pharmacology approach to (Stage 1) Establish cell-, organoid-, and animal-based models to study the PN in photoreceptors; (Stage 2) Scrutinize the dynamics of the PN upon disruption of IRD-associated gene activity, and (Stage 3) Evaluate the effect and cell-biological basis of proteostasis modulators on delay or arrest of IRD disease progression. EXPECTED OUTCOMES: (1) Biochemical and cell-based assays that allow testing of pharmaceuticals targeting specific functional modules; (2) Detailed knowledge on the architecture and wiring of functional modules involved in photoreceptor proteostasis, and their disruption in IRD; (3) New active compounds available for clinical testing in specific IRD types. IMPORTANCE: This systems pharmacology approach represents a completely new and non-intuitive approach towards treating IRD. Gene replacement, translational read-through approaches and antisense oligonucleotide technologies offer promising therapeutic avenues, but are only available to a specific subset of patients. The work proposed here has the potential to be transformative for the development of therapeutic approaches to treat many rare and common forms of IRD. In addition, proteostasis regulators could be used in combination with gene replacement or other therapies by reducing the cellular stress response of photoreceptors and prolonging photoreceptor survival, thereby extending the time window-of-opportunity for other treatments that aim to improve visual function. IMPACT ON TARGET GROUP: The therapeutic leads identified in this project may thus ultimately benefit a large group of patients with inherited retinal dystrophies, largely independent of their primary genetic defect, age, gender or ethnicity.