Ependymal cells of the mouse brain express urate transporter 1 (URAT1)

Abstract

BackgroundElevated uric acid (UA) is commonly associated with gout and it is also a known cardiovascular disease risk factor. In contrast to such deleterious effects, UA possesses neuroprotective properties in the brain and elucidating the molecular mechanisms involved may have significant value regarding the therapeutic treatment of neurodegenerative disease. However, it is not yet fully established how UA levels are regulated in the brain. In this study, we investigated the distribution of mouse urate transporter 1 (URAT1) in the brain. URAT1 is a major reabsorptive urate transporter predominantly found in the kidney.MethodsImmunohistochemistry of wild type and URAT1 knockout mouse brain using paraffin or frozen sections and a rabbit polyclonal anti-mouse URAT1 antibody were employed.ResultsAntibody specificity was confirmed by the lack of immunostaining in brain tissue from URAT1 knockout mice. URAT1 was distributed throughout the ventricular walls of the lateral ventricle, dorsal third ventricle, ventral third ventricle, aqueduct, and fourth ventricle, but not in the non-ciliated tanycytes in the lower part of the ventral third ventricle. URAT1 was localized to the apical membrane, including the cilia, of ependymal cells lining the wall of the ventricles that separates cerebrospinal fluid (CSF) and brain tissue.ConclusionIn this study, we report that URAT1 is expressed on cilia and the apical surface of ventricular ependymal cells. This is the first report to demonstrate expression of the urate transporter in ventricular ependymal cells and thus raises the possibility of a novel urate transport system involving CSF.

Immunohistochemical and in situ hybridization study of urate transporters GLUT9/URATv1, ABCG2, and URAT1 in the murine brain

“…In accordance with our previous URAT1 immunostaining results, where URAT1 was distributed in ependymal cells [13], Slc22a12 mRNA was expressed in the ependymal cells (Fig. 5a).…”

“…In contrast, mouse GLUT9 is reportedly expressed both in the apical and basolateral membranes of distal convoluted tubules of the murine kidney and enterocytes of the murine jejunum, albeit with no information about its isoform-specific localization [22, 28, 29]. Since the current study did not reveal the exclusive plasma membrane localization compared to our previous finding of apical localization of URAT1 [13], further work including electron microscopy analysis is required to determine if GLUT9 is specifically localized in the apical and/or basolateral membrane of ependymal cells and neuronal somatic membranes. Unknown mechanisms, such as a stimulus-dependent translocation to the plasma membrane like the insulin-dependent GLUT4 translocation may exist [30].…”

“…Immunoblotting analysis in earlier studies has reported that URAT1 was detected in the murine choroid plexus, brain capillaries, cultured dopaminergic neurons, and astrocytes [23, 33]. Our previous study exhibited weak immunostaining in the choroid plexus and the brain parenchyma, both in wild type and URAT1 KO mice, indicating the possibility of non-specific staining [13]. The discrepancy between the immunostaining and in situ hybridization data of GLUT9 ( Slc2a9 ) and URAT1 ( Slc22a12 ) could be considered that the mRNA is not translated to protein or that the translated protein is unstable.…”

“…CSF flows through the ventricles, enters into the subarachnoid space, and is reabsorbed into the blood or reaches lymphatic drainage [36]. During the ventricular circulation, UA in the CSF could be transported into ependymal cells by URAT1 located at the apical membrane of ependymal cells [13]. Since it has been reported that ependymal cells express xanthine oxidase, which converts hypoxanthine to xanthine and xanthine to uric acid [37], metabolized UA in the ependymal cells could also be the transport target.…”

“…We previously demonstrated that URAT1, which is a urate transporter responsible for urate reabsorption in the kidney [12], is localized to cilia and the apical surface of ventricular ependymal cells in the murine brain [13]. Ependymal cells form a single-layer of epithelial cells which line the surface of the cerebral ventricles.…”