This month we turn the spotlight to retinol binding protein 2 (gene: RBP2), which plays an important role in the uptake of vitamin A in the intestines. Now that winter has arrived, we increasingly need good night vision and a strong immune system - two physiological processes that we have vitamin A to thank for.

Retinol binding protein 2 (RBP2) is found in the enterocytes of the proximal intestine where it binds two forms of dietary vitamin A, namely retinol and retinaldehyde, to facilitate their intracellular metabolism and transport (1). Vitamin A is then distributed to other organs via the bloodstream, where it plays a key role in numerous different physiological processes, including vision, immune system, reproduction and cellular growth (2). Interestingly, based on studies in mice, RBP2's role seems to be to optimize vitamin A uptake when this vitamin is scarce in the diet to make sure the body gets enough. In contrast, RBP2 is not essential for uptake when vitamin A is abundant (3,4).

On the RNA level, RBP2 is expressed only in duodenum and small intestine, more specifically in proximal enterocytes. This is supported by immunostaining, which shows that the RBP2 protein is localized mainly in the cytoplasm of enterocytes (brown). It is only present in the villi, where absorption of dietary molecules takes place, and is absent in the bottom of the crypts that mainly contains stem cells.

1. Blaner W.S. et al., Retinol-binding protein 2 (RBP2): biology and pathobiology. Crit Rev Biochem Mol Biol (2020)
2. Carazo A. et al., Vitamin A Update: Forms, Sources, Kinetics, Detection, Function, Deficiency, Therapeutic Use and Toxicity. Nutrients (2021)
3. Xueping E. et al., Increased neonatal mortality in mice lacking cellular retinol-binding protein II. J Biol Chem (2002)
4. Wongsiriroj N. et al., The molecular basis of retinoid absorption: a genetic dissection. J Biol Chem (2008)