Abstract
Purpose: :
The Bardet-Biedl Syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder. The ciliopathy is characterized by retinal degeneration, obesity, polydactyly, cognitive impairment and hypogenitalism. To date, defects in 16 genes causing BBS have been identified. Although the exact function of the BBS proteins remain elusive so far, it was demonstrated that a group of BBS proteins assemble into a complex, named BBSome. There is growing evidence that the BBSome plays an essential role in ciliogenesis and cilia function. Since the spatial distribution of individual BBS proteins has not been determined, so far, we accessed its subcellular localization in the retina.
Methods: :
We applied correlative high resolution immunofluorescence and immunoelectron microscopy on rodent and human retina sections. Protein expression was determined in retinal tangential sections and subcellular fractionation by differential sucrose density gradient centrifugation of retina and brain lysates.
Results: :
We show the expression of all BBS proteins investigated in the retina. In photoreceptors, all BBSome proteins are localized in the basal body, the centriole and the connecting cilium. However, in the axonemal part of the outer segment only a subset of BBSome components were identified. In addition, we found all BBSome components in the synapse of the plexiform layers of retina. Correlative immunofluorecent analyses of BBS proteins and molecular markers of synaptic compartments revealed a localization of the BBSome in the post-synaptic terminals of bipolar and horizontal cells. This spatial arrangement was confirmed by Western blots of tangential retinal sections and subcellular fractionations of retinal and brain protein lysates.
Conclusions: :
Our present data indicate functions of the BBSome in photoreceptor cilia and retinal synapses. Our results strengthen the hypothesis of BBSome functions in the assembly of transport complexes at the basis of the photoreceptor cilium and the delivery of cargo through the connecting cilium to the base of the outer segment for disk neogenesis. In addition, we provide first evidence that a BBSome subcomplex may support the transport in the outer segment axoneme. Moreover, localization of the BBSome in post-synaptic density indicates a role in the targeting of neurotransmitter receptors to the post-synaptic membrane. Based on these results retinopathies in BBS are probably based on defects in functions of photoreceptor cilia and/or the post-synaptic terminals of retinal neurons.
Keywords: photoreceptors • proteins encoded by disease genes • synapse