The blood-retinal barrier and carnosine

The retina is a neural tissue that is associated with vision, and the transport of various compounds between the retina and the circulating blood takes place at the blood-retinal barrier (BRB). The BRB consists of two different barrier structures, namely the inner BRB formed by the retinal capillary endothelial cells and the outer BRB formed by the retinal pigment epithelial (RPE) cells. The BRB protects the neural retina from xenobiotics by restricting the nonspecific paracellular transport of low-weight molecules, and promoting the efficient supply of nutrients, including bioactive compounds, to the retina from the circulating blood that involves membrane transporters expressed at the BRB. Previously, l-carnosine was detected in the retina of vertebrates, and it was suggested to have physiological and neuroprotective effects were suggested in the retina. However, it has been unclear how l-carnosine is supplied to the retina from the circulating blood although the involvement of a carrier-mediated process has been reported for l-carnosine transport in several tissues. This chapter focuses on the mechanisms for l-carnosine supply to the retina across the BRB, and suggests the involvement of taurine transporter (TAUT) and l-type amino acid transporter (LAT1) in the retinal biosynthesis of l-carnosine from β-alanine and l-histidine.