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BBS4 and BBS5 show functional redundancy in the BBSome to regulate the degradative sorting of ciliary sensory receptors.

Xu Q, Zhang Y, Wei Q, Huang Y, Li Y, Ling K, Hu J - Sci Rep (2015)

Bottom Line: However, the mechanisms underlying the ciliary homeostasis of sensory receptors remain elusive.Here, we demonstrate that BBS-4 and BBS-5, two distinct BBSome components, show unexpected functional redundancy in the context of cilia in C. elegans.Further analyses indicate that co-depletion of BBS-4 and BBS-5 disrupts the lysosome-targeted degradative sorting of ciliary sensory receptors.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.

ABSTRACT
Cilia harbor sensory receptors for various signaling cascades critical for vertebrate development. However, the mechanisms underlying the ciliary homeostasis of sensory receptors remain elusive. Here, we demonstrate that BBS-4 and BBS-5, two distinct BBSome components, show unexpected functional redundancy in the context of cilia in C. elegans. BBS-4 directly interacts with BBS-5 and the interaction can be disrupted by a conserved mutation identified in human BBS4. Surprisingly, we found that BBS-4 and BBS-5 act redundantly in the BBSome to regulate the ciliary removal, rather than the ciliary entry or retrograde IFT transport, of various sensory receptors. Further analyses indicate that co-depletion of BBS-4 and BBS-5 disrupts the lysosome-targeted degradative sorting of ciliary sensory receptors. Moreover, mammalian BBS4 and BBS5 also interact directly and coordinate the ciliary removal of polycystin 2. Hence, we reveal a novel and highly conserved role for the BBSome in fine-tuning ciliary signaling by regulating the ciliary removal of sensory receptors for lysosomal degradation.

No MeSH data available.


Related in: MedlinePlus

BBS-4 and BBS-5 downregulate PKD-2 through lysosomal degradation pathway.(a) Enhancing lysosomal degradation by overexpressing RAB-5 significantly reduces PKD-2 accumulation in bbs-4; bbs-5 cilia. (b) The results shown in (a) were quantified. Data represent three or more experiments. (c) Disruption of lysosomal degradation in stam-1 mutants leads to strong accumulation of WT PKD-2, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP comparable to that observed in bbs-4; bbs-5 double mutants (see Fig. 4). Arrows and arrowheads indicate the cilia base and tip, respectively. Brackets indicate distal dendrites. In each experiment, n > 40 were counted in each group. Results represented as mean ± SEM. ***p < 0.001. Scale bars, 5 μm.
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f5: BBS-4 and BBS-5 downregulate PKD-2 through lysosomal degradation pathway.(a) Enhancing lysosomal degradation by overexpressing RAB-5 significantly reduces PKD-2 accumulation in bbs-4; bbs-5 cilia. (b) The results shown in (a) were quantified. Data represent three or more experiments. (c) Disruption of lysosomal degradation in stam-1 mutants leads to strong accumulation of WT PKD-2, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP comparable to that observed in bbs-4; bbs-5 double mutants (see Fig. 4). Arrows and arrowheads indicate the cilia base and tip, respectively. Brackets indicate distal dendrites. In each experiment, n > 40 were counted in each group. Results represented as mean ± SEM. ***p < 0.001. Scale bars, 5 μm.

Mentions: Overexpression of early endosome protein Rab5 can enhance endocytic traffic to lysosomes where polyubiquinated protein is efficiently degraded44. We reasoned that if the BBSome acts upstream of lysosomal degradation, overexpression of RAB-5 may restore the degradation defect in bbs mutants. As expected, we observed significant reduced ciliary accumulation of Ubi-PKD-2-GFP signal in bbs-4; bbs-5 mutants upon the overexpression of RAB-5-RFP, indicative of restored degradative sorting (Fig. 5a,b). In contrary, depletion of early endosomal protein STAM-1 leads to strong accumulation of PKD-2-GFP, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP (Fig. 5c). Taken together, these observations suggest that the BBSome acts upstream of the early endosome, probably in endocytic stage at cilia base, to regulate the lysosomal sorting of ciliary receptors.


BBS4 and BBS5 show functional redundancy in the BBSome to regulate the degradative sorting of ciliary sensory receptors.

Xu Q, Zhang Y, Wei Q, Huang Y, Li Y, Ling K, Hu J - Sci Rep (2015)

BBS-4 and BBS-5 downregulate PKD-2 through lysosomal degradation pathway.(a) Enhancing lysosomal degradation by overexpressing RAB-5 significantly reduces PKD-2 accumulation in bbs-4; bbs-5 cilia. (b) The results shown in (a) were quantified. Data represent three or more experiments. (c) Disruption of lysosomal degradation in stam-1 mutants leads to strong accumulation of WT PKD-2, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP comparable to that observed in bbs-4; bbs-5 double mutants (see Fig. 4). Arrows and arrowheads indicate the cilia base and tip, respectively. Brackets indicate distal dendrites. In each experiment, n > 40 were counted in each group. Results represented as mean ± SEM. ***p < 0.001. Scale bars, 5 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4493597&req=5

f5: BBS-4 and BBS-5 downregulate PKD-2 through lysosomal degradation pathway.(a) Enhancing lysosomal degradation by overexpressing RAB-5 significantly reduces PKD-2 accumulation in bbs-4; bbs-5 cilia. (b) The results shown in (a) were quantified. Data represent three or more experiments. (c) Disruption of lysosomal degradation in stam-1 mutants leads to strong accumulation of WT PKD-2, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP comparable to that observed in bbs-4; bbs-5 double mutants (see Fig. 4). Arrows and arrowheads indicate the cilia base and tip, respectively. Brackets indicate distal dendrites. In each experiment, n > 40 were counted in each group. Results represented as mean ± SEM. ***p < 0.001. Scale bars, 5 μm.
Mentions: Overexpression of early endosome protein Rab5 can enhance endocytic traffic to lysosomes where polyubiquinated protein is efficiently degraded44. We reasoned that if the BBSome acts upstream of lysosomal degradation, overexpression of RAB-5 may restore the degradation defect in bbs mutants. As expected, we observed significant reduced ciliary accumulation of Ubi-PKD-2-GFP signal in bbs-4; bbs-5 mutants upon the overexpression of RAB-5-RFP, indicative of restored degradative sorting (Fig. 5a,b). In contrary, depletion of early endosomal protein STAM-1 leads to strong accumulation of PKD-2-GFP, Ubi-PKD-2-GFP, and UbiK48R-PKD-2-GFP (Fig. 5c). Taken together, these observations suggest that the BBSome acts upstream of the early endosome, probably in endocytic stage at cilia base, to regulate the lysosomal sorting of ciliary receptors.

Bottom Line: However, the mechanisms underlying the ciliary homeostasis of sensory receptors remain elusive.Here, we demonstrate that BBS-4 and BBS-5, two distinct BBSome components, show unexpected functional redundancy in the context of cilia in C. elegans.Further analyses indicate that co-depletion of BBS-4 and BBS-5 disrupts the lysosome-targeted degradative sorting of ciliary sensory receptors.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota, USA.

ABSTRACT
Cilia harbor sensory receptors for various signaling cascades critical for vertebrate development. However, the mechanisms underlying the ciliary homeostasis of sensory receptors remain elusive. Here, we demonstrate that BBS-4 and BBS-5, two distinct BBSome components, show unexpected functional redundancy in the context of cilia in C. elegans. BBS-4 directly interacts with BBS-5 and the interaction can be disrupted by a conserved mutation identified in human BBS4. Surprisingly, we found that BBS-4 and BBS-5 act redundantly in the BBSome to regulate the ciliary removal, rather than the ciliary entry or retrograde IFT transport, of various sensory receptors. Further analyses indicate that co-depletion of BBS-4 and BBS-5 disrupts the lysosome-targeted degradative sorting of ciliary sensory receptors. Moreover, mammalian BBS4 and BBS5 also interact directly and coordinate the ciliary removal of polycystin 2. Hence, we reveal a novel and highly conserved role for the BBSome in fine-tuning ciliary signaling by regulating the ciliary removal of sensory receptors for lysosomal degradation.

No MeSH data available.


Related in: MedlinePlus