R-Spondin 1 (RSPO1), also known as cysteine-rich and single thrombospondin domain containing protein 3 (Cristin 3), is a 27 kDa secreted protein that shares ~40% amino acid (aa) identity with three other R-Spondin family members [1, 2]. All R-Spondins regulate Wnt/ beta-Catenin signaling but have distinct expression patterns [1-3]. Human R-Spondin 1 (aa21-263) shares 89%, 87%, 92%, 91%, 91% and 89% aa identity with mouse, rat, horse, dog, goat, and cow RSPO-1, respectively. R-Spondin 1 competes with the Wnt antagonist DKK-1 for binding to the Wnt co-receptors, Kremen and LRP-6, reducing their DKK-1-mediated internalization [4]. However, reports are mixed on whether R-Spondin 1 binds LRP-6 directly [4-6]. R-Spondin 1 is expressed in early development at the roof plate boundary and is thought to contribute to dorsal neural tube development [3, 7]. Interest in R-Spondin 1 as a cell culture supplement has grown with the expansion of the organoid field. R-Spondin 1 is widely used in organoid cell culture workflows as a vital component that promotes both growth and survival of 3D organoids [8]. Structurally similar to other R-Spondins, R-Spondin 1 contains two adjacent cysteine-rich furin-like domains (aa 34-135) with one potential N-glycosylation site, followed by a thrombospondin (TSP-1) motif (aa 147-207) and a region rich in basic residues (aa 211-263). Only the furin-like domains are needed for beta-catenin stabilization [2, 9]. A putative nuclear localization signal at the C-terminus may allow some expression in the nucleus [10]. Potential isoforms of 200 and 236 aa have an alternate, shorter N-terminus or are missing aa 146-208, respectively [11].
Reference
[1]. Chen, J-Z. et al. (2002) Mol. Biol. Rep. 29:287.
[2]. Kim, K.-A. et al. (2006) Cell Cycle 5:23.
[3]. Nam, J.-S. et al. (2007) Gene Expr. Patterns 7:306.
[4]. Binnerts, M.E. et al. (2007) Proc. Natl. Acad. Sci. USA 104:14700.
[5]. Nam, J.-S. et al. (2006) J. Biol. Chem. 281:13247.
[6]. Wei, Q. et al. (2007) J. Biol. Chem. 282:15903.
[7]. Kamata, T. et al. (2004) Biochim. Biophys. Acta 1676:51.
[8]. Drost and Clevers. (2018) Nature Reviews Cancer 18:407.
[9]. Kazanskaya, O. et al. (2004) Dev. Cell 7:525.
[10]. Tomaselli, S. et al. (2008) Hum. Mutat. 29:220.