Retinal disease in ciliopathies: Recent advances with a focus on stem cell-based therapies

Abstract

Ciliopathies display extensive genetic and clinical heterogeneity, varying in severity, age of onset, disease progression and organ systems affected. Retinal involvement, as demonstrated by photoreceptor dysfunction or death, is a highly penetrant phenotype among a vast majority of ciliopathies. Photoreceptor cells possess a specialized and modified sensory cilium with membrane discs where efficient photon capture and ensuing signaling cascade initiate the visual process. Disruptions of cilia biogenesis and protein transport lead to impairment of photoreceptor function and eventually degeneration. Despite advances in elucidation of ciliogenesis and photoreceptor cilia defects, we have limited understanding of pathogenic mechanisms underlying retinal phenotype(s) observed in human ciliopathies. Patient-derived induced pluripotent stem cell (iPSC)-based approaches offer a unique opportunity to complement studies with model organisms and examine cilia disease relevant to humans. Three-dimensional retinal organoids from iPSC lines feature laminated cytoarchitecture, apical-basal polarity and emergence of a ciliary structure, thereby permitting pathogenic modeling of human photoreceptors in vitro. Here, we review the biology of photoreceptor cilia and associated defects and discuss recent progress in evolving treatment modalities, especially using patient-derived iPSCs, for retinal ciliopathies.

Keywords: Photoreceptor cilium; cell replacement; drug discovery; gene therapy; iPSC; organoid culture; retinal neurodegeneration; translational therapeutics.

Fig.1

Schematic of the rod photoreceptor primary cilium. (A) The rod primary cilium is highly adapted to efficiently capture photons of light that initiate the phototransduction cascade through rhodopsin GPCR activation (in the photoreceptor outer segment) and facilitate the transportation of proteins involved cilia maintenance, structure and function from the inner segment to the outer segment (photoreceptor connecting cilium). The photoreceptor inner segment is crucial for photoreceptor proteins and lipid synthesis. (B) Within the inner segment, vesicles containing cargo proteins are transported from the Golgi apparatus to the basal body via small Rab GTPases together with proteins involved in trafficking cargo to the cilia (e.g., TULP1). The basal body is formed of the mother centriole and daughter centriole, the mother centriole docks onto the apical photoreceptor cell membrane to supports ciliogenesis and axoneme development. The basal body also contains additional structural elements, including distal appendages, with subdistal appendages and associated proteins (e.g., CC2D2A) to support vesicular transportation.The BBSome complex, including a number of BBS proteins, and multiprotein complexes in the pericentriolar material play critical functions in ciliogenesis and ciliary trafficking. Cargo transport to the outer segment is further regulated by proteins within the photoreceptor connecting cilium, such as CEP290 and NPHP proteins. Other proteins, such as prominin-1 (PROM1), peripherin (PRPH2), RP1 and RP1L1 regulate more structural components of the cilia, such as outer segment disk stacking and organization. Proteins are transported along the axoneme within the photoreceptor outer segment through anterograde and retrograde transportation, involving kinesin and dynein motor proteins, respectively. IFT particles help to support the protein transportation along the axoneme.

Fig.2

Modeling human retinal ciliopathies using model organisms (such as mice), human-derived somatic cells, and human iPSC-derived retinal organoids.

Fig.3

Translational therapeutics using patient iPSC-derived retinal organoids. Patient fibroblasts are obtained from skin biopsy samples and subsequently reprogramed to generate induced pluripotent stem cells (iPSCs). Retinal organoids can be differentiated from iPSCs and applied as a cell source to replace dysfunctional or dead photoreceptors in retinal ciliopathy patients, or as an in vitro platform to evaluate gene-based treatments or drug candidates.