1.A.79 The Cholesterol Uptake Protein (ChUP) or Double Stranded RNA Uptake Family
When dsRNA is injected into C. elegans, it spreads to silence gene expression throughout the animal and in its progeny. This phenomenon is termed RNA interference (RNAi) and has been observed in plants and nematodes. SID-1 is a 776 aa residue integral membrane C. elegans protein with a 400 aa extracelular N-terminal domain and a C-terminal domain of 11 putative TMSs that mediates passive dsRNA transport into cells. However, export of RNA silencing from C. elegans tissues does not require SID-1 (Jose et al., 2009). A 9 TMS model with two regions that dip into the membrane from the external side has been proposed (Feinberg and Hunter, 2003). Several distant but probable paralogues of SID-1 are found in C. elegans, and mammals contain SID-1 homologues. It has been shown that a human SID-1 homologue enhances siRNA uptake and gene silencing (Duxbury et al., 2005). A homologue could not be identified encoded within the genome of Drosophila melanogaster or in other organisms.
The human SID-1 homologue FLJ20174 localizes to the cell plasma membrane and enhances uptake of small interfering RNA (siRNA). This results in increased siRNA-mediated gene silencing efficacy. Thus, overexpression enhances siRNA internalization in mammalian cells. The N-terminal extracellular domain of human SID-1 has been characterized (Pratt et al., 2012). It is glycosylated and forms a compact, globular tetramer. It may control access of dsRNA to the transmembrane pore. SID-1 is a dsRNA-selective dsRNA-gated channel (Shih and Hunter, 2011). Both single- and double-stranded dsRNA, such as hairpin RNA and pre-microRNA, can be transported by SID-1.
Survival of C. elegans depends on the dietary absorption of sterols present in the environment. Valdes et al. (2012) provided evidence that Cholesterol Uptake Protein-1 (ChUP-1) (ZK721; tag-130) is involved in dietary cholesterol uptake in C. elegans. Animals lacking ChUP-1 showed hypersensitivity to cholesterol limitation and were unable to uptake cholesterol. A ChUP-1-GFP fusion protein colocalized with cholesterol-rich vesicles, endosomes and lysosomes as well as the plasma membrane. A direct interaction was found between the cholesterol analog DHE and the transmembrane 'cholesterol recognition/interaction amino acid consensus' (CRAC) motif present in C. elegans ChUP-1. In-silico analysis identified two mammalian homologues of ChUP-1. CRAC motifs are conserved in mammalian ChUP-1 homologues (Valdes et al., 2012).
Single-stranded oligonucleotides (ssOligos) are efficiently taken up by living cells without the use of transfection reagents. This phenomenon, called 'gymnosis', enables the sequence-specific silencing of target genes. Several antisense ssOligos are used for the treatment of human diseases. Systemic RNA interference deficient-1 (SID-1) transmembrane family 2 (SIDT2), a mammalian ortholog of the Caenorhabditis elegans double-stranded RNA channel SID-1, mediates gymnosis. Takahashi et al. 2017 showed that the uptake of naked ssOligos into cells is downregulated by knockdown of SIDT2, and it inhibited the effect of antisense RNA mediated by gymnosis. Overexpression of SIDT2 enhanced the uptake of naked ssOligos into cells, while a single amino acid mutation in SIDT2 abolished this effect. Thus, SIDT2 mediates extra- and intracellular RNA transport.
Transport reactions believed to be catalyzed by SID-1 and ChUP1 are:
dsRNA (out) ⇌ dsRNA (in)
Cholesterol (out) ⇌ Cholesterol (in)
A transport reaction believed to be catalyzed by SIDT2 of mammals is:
ssRNAout (oligonucleoties) ⇌ ssRNAin (oligonucleotides)