BBSome Component BBS5 Is Required for Cone Photoreceptor Protein Trafficking and Outer Segment Maintenance

Abstract

To identify the role of the BBSome protein Bardet-Biedl syndrome 5 (BBS5) in photoreceptor function, protein trafficking, and structure using a congenital mutant mouse model. METHODS. Bbs5-/mice (2 and 9 months old) were used to assess retinal function and morphology. Hematoxylin and eosin staining of retinal sections was performed to visualize histology. Electroretinography was used to analyze rod and cone photoreceptor function. Retinal protein localization was visualized using immunofluorescence (IF) within retinal cryosections. TUNEL staining was used to quantify cell death. Transmission electron microscopy (TEM) was used to examine retinal ultrastructure. RESULTS. In the Bbs5-/retina, there was a significant loss of nuclei in the outer nuclear layer accompanied by an increase in cell death. Through electroretinography, Bbs5-/mice showed complete loss of cone photoreceptor function. IF revealed mislocalization of the cone-specific proteins M-and S-opsins, arrestin-4, CNGA3, and GNAT2, as well as a light-dependent arrestin-1 mislocalization, although perpherin-2 was properly localized. TEM revealed abnormal outer segment disk orientation in Bbs5-/-. CONCLUSIONS. Collectively, these data suggest that, although BBS5 is a core BBSome component expressed in all ciliated cells, its role within the retina mediates specific photoreceptor protein cargo transport. In the absence of BBS5, cone-specific protein mislocalization and a loss of cone photoreceptor function occur.

Evolutionarily conserved genetic interactions between nphp-4 and bbs-5 mutations exacerbate ciliopathy phenotypes

“…It has previously been reported that male Nphp4 -/mice are sterile due to sperm motility defects (WON et al 2011). Similarly, we previously described sterility defects and sub-mendelian ratios at birth and weaning in congenital Bbs5 -/animals (BALES et al 2020;BENTLEY-FORD et al 2021). For this reason, double heterozygous Bbs5 -/+ ;Nphp4 -/+ female by Bbs5 -/+ ;Nphp4 -/+ male matings were established.…”

Some thoughts about intraflagellar transport in reproduction

“…However, mutations of core human BBSome components will not cause early mortality. This is also supported by mouse models, which showed that no embryonic lethality was found in the homozygous mutant mice when BBSome genes were disrupted globally (Bales et al, 2020; Mykytyn et al, 2004; Q. Zhang et al, 2011, 2013). Spermatogenesis was affected in these global knockout mice, but it is not known which cells contributed to the failure of sperm formation.…”

The Bardet-Biedl protein Bbs1 controls photoreceptor outer segment protein and lipid composition

“…To further investigate the role of the BBSome in photoreceptor function, we turned to the zebrafish, an established animal model for ciliopathies [42][43][44] . The cone-rich retina of zebrafish larvae represents a particular asset to study this photoreceptor subtype compared to the rod-dominated mouse retina, as diverging effects on cone and rod photoreceptors have been described 38,41,45 . In this work, we generated a new zebrafish bbs1 knock-out model and found that despite normal retinal development and photoreceptor differentiation, visual functional deficits were present prior to the appearance of any morphological anomaly in mutant fish.…”

“…Numerous BBSome mutant studies highlight the crucial role of the BBSome complex for photoreceptor homeostasis and morphogenesis 40,41,45 . The bbs1 mutant zebrafish presented here recapitulates the progressive retinal dysfunction and degeneration seen in humans and in all published models.…”