Ciliopathies are a group of heterogeneous inherited disorders associated with dysfunction of the cilium, a ubiquitous microtubule-based organelle involved in a broad range of cellular functions. Most ciliopathies are syndromic, since several organs whose cells produce a cilium, such as the retina, cochlea or kidney, are affected by mutations in ciliary-related genes. In the retina, photoreceptor cells present a highly specialized neurosensory cilium, the outer segment, stacked with membranous disks where photoreception and phototransduction occurs. The daily renewal of the more distal disks is a unique characteristic of photoreceptor outer segments, resulting in an elevated protein demand. All components necessary for outer segment formation, maintenance and function have to be transported from the photoreceptor inner segment, where synthesis occurs, to the cilium. Therefore, efficient transport of selected proteins is critical for photoreceptor ciliogenesis and function, and any alteration in either cargo delivery to the cilium or intraciliary trafficking compromises photoreceptor survival and leads to retinal degeneration. To date, mutations in more than 100 ciliary genes have been associated with retinal dystrophies, accounting for almost 25% of these inherited rare diseases. Interestingly, not all mutations in ciliary genes that cause retinal degeneration are also involved in pleiotropic pathologies in other ciliated organs. Depending on the mutation, the same gene can cause syndromic or non-syndromic retinopathies, thus emphasizing the highly refined specialization of the photoreceptor neurosensory cilia, and raising the possibility of photoreceptor-specific molecular mechanisms underlying common ciliary functions such as ciliary transport. In this review, we will focus on ciliary transport in photoreceptor cells and discuss the molecular complexity underpinning retinal ciliopathies, with a special emphasis on ciliary genes that, when mutated, cause either syndromic or non-syndromic retinal ciliopathies.

Activation of NF-κB transcription factor is strictly regulated to accurately direct cellular processes including inflammation, immunity, and cell survival. In the retina, the modulation of the NF-κB pathway is essential to prevent excessive inflammatory responses, which plays a pivotal role in many retinal neurodegenerative diseases, such as age-related macular degeneration (AMD), diabetic retinopathy (DR), and inherited retinal dystrophies (IRDs). A critical cytokine mediating inflammatory responses in retinal cells is tumor necrosis factor-alpha (TNFα), leading to the activation of several transductional pathways, including NF-κB. However, the multiple factors orchestrating the appropriate regulation of NF-κB in retinal cells still remain unclear. The present study explores how the ubiquitin-specific protease 48 (USP48) downregulation impacts the stability and transcriptional activity of NF-κB/p65 in retinal pigment epithelium (RPE), at both basal conditions and following TNFα stimulation. We described that USP48 downregulation stabilizes p65. Notably, the accumulation of p65 is mainly detectable in the nuclear compartment and it is accompanied by an increased NF-κB transcriptional activity. These results delineate a novel role of USP48 in negatively regulating NF-κB in retinal cells, providing new opportunities for therapeutic intervention in retinal pathologies.

Proteins related to the ubiquitin-proteasome system play an important role during the differentiation and ciliogenesis of photoreceptor cells. Mutations in several genes involved in ubiquitination and proteostasis have been identified as causative of inherited retinal dystrophies (IRDs) and ciliopathies. USP48 is a deubiquitinating enzyme whose role in the retina is still unexplored although previous studies indicate its relevance for neurosensory organs. In this work, we describe that a pool of endogenous USP48 localises to the basal body in retinal cells and provide data that supports the function of USP48 in the photoreceptor cilium. We also demonstrate that USP48 interacts with the IRD-associated proteins ARL3 and UNC119a, and stabilise their protein levels using different mechanisms. Our results suggest that USP48 may act in the regulation/stabilisation of key ciliary proteins for photoreceptor function, in the modulation of intracellular protein transport, and in ciliary trafficking to the photoreceptor outer segment.