Bardet–Biedl syndrome-8 (BBS8) protein is crucial for the development of outer segments in photoreceptor neurons

Abstract

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy characterized by developmental abnormalities and vision loss. To date, mutations in 21 genes have been linked to BBS. The products of eight of these BBS genes form a stable octameric complex termed the BBSome. Mutations in BBS8, a component of the BBSome, cause early vision loss, but the role of BBS8 in supporting vision is not known. To understand the mechanisms by which BBS8 supports rod and cone photoreceptor function, we generated animal models lacking BBS8. The loss of BBS8 protein led to concomitant decrease in the levels of BBSome subunits, BBS2 and BBS5 and increase in the levels of the BBS1 and BBS4 subunits. BBS8 ablation was associated with severe reduction of rod and cone photoreceptor function and progressive degeneration of each photoreceptor subtype. We observed disorganized and shortened photoreceptor outer segments (OS) at post-natal day 10 as the OS elaborates. Interestingly, loss of BBS8 led to changes in the distribution of photoreceptor axonemal proteins and hyper-acetylation of ciliary microtubules. In contrast to properly localized phototransduction machinery, we observed OS accumulation of syntaxin3, a protein normally found in the cytoplasm and the synaptic termini. In conclusion, our studies demonstrate the requirement for BBS8 in early development and elaboration of ciliated photoreceptor OS, explaining the need for BBS8 in normal vision. The findings from our study also imply that early targeting of both rods and cones in BBS8 patients is crucial for successful restoration of vision.

The route of the visual receptor rhodopsin along the cilium

“…Mutations in genes encoding the anterograde motor, kinesin-2 (e.g. Marszalek et al, 2000;Lopes et al, 2010), IFT20 (Keady et al, 2011), IFT88 (Pazour et al, 2002), IFT140 (Crouse et al, 2014), IFT122 (Boubakri et al, 2016), IFT57 (Krock and Perkins, 2008), and the BBS proteins, BBS2 (Nishimura et al, 2004), BBS3 (Zhang et al, 2011), BBS4 (Abd-El- Barr et al, 2007), BBS6/MKKS (Ross et al, 2005) and BBS8 (Hsu et al, 2017;Dilan et al, 2018) have all been shown to affect RHO delivery, outer segment formation and maintenance, and typically result in photoreceptor degeneration.…”

Limited time window for retinal gene therapy in a preclinical model of ciliopathy

“…Accumulation of IS proteins STX3 and STXBP1 in the OS has been described in all BBS mouse models examined and in Cep290 mutant mice (30)(31)(32)40). Mislocalization of these proteins was evident as early as at P13 (the youngest age examined) in Lztfl1 gt/gt mice and observed throughout the length of the OS (Figure 2D), indicating that the pathogenic process begins before or when the OS begins to elongate.…”

“…Ablation of BBS gene function causes disc morphogenesis defects and disorganization of the OS ultrastructure (30)(31)(32). We examined whether restoring Lztfl1 expression at P5-12 could preserve the OS ultrastructure, using transmission electron microscopy (TEM) (Figures 3H and S4).…”

“…primarily a disease in rods), deficits in cone functions are also evident in young patients. Recent studies in BBS mouse models showed that aberrant accumulation of inner segment (IS) proteins in the OS is a primary cause of photoreceptor degeneration and is accompanied by disorganization of the OS structure (30)(31)(32).…”

CEP290 myosin-tail homology domain is essential for protein confinement between inner and outer segments in photoreceptors

“…Accumulation of STX3 and STXBP1 in the outer segment is a phenotype commonly observed in BBS mutant photoreceptors [45][46][47]. While CEP290 is a stationary protein constituting the ciliary gate, BBSomes (a protein complex composed of 8 BBS proteins [56,57]) are adapter complexes that link ciliary cargos to IFT particles and transport them in and out of the ciliary compartment [58][59][60][61][62][63][64][65].…”