The brain has a limited ability to synthesize the essential polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA) from its omega-3 fatty acid precursors. Therefore, to maintain brain concentrations of this PUFA at physiological levels, plasma-derived DHA must be transported across the blood–brain barrier (BBB). While DHA is able to partition into the luminal membrane of brain endothelial cells, its low aqueous solubility likely limits its cytosolic transfer to the abluminal membrane, necessitating the requirement of an intracellular carrier protein to facilitate trafficking of this PUFA across the BBB. As the intracellular carrier protein fatty acid-binding protein 5 (FABP5) is expressed at the human BBB, the current study assessed the putative role of FABP5 in the brain endothelial cell uptake and BBB transport of DHA in vitro and in vivo, respectively. hFAPB5 was recombinantly expressed and purified from Escherichia coli C41(DE3) cells and the binding affinity of DHA to hFABP5 assessed using isothermal titration calorimetry. The impact of FABP5 siRNA on uptake of ¹⁴C-DHA into immortalized human brain microvascular endothelial (hCMEC/D3) cells was assessed. An in situ transcardiac perfusion method was optimized in C57BL/6 mice and subsequently used to compare the BBB influx rate (K_in) of ¹⁴C-DHA between FABP5-deficient (FABP5^–/–) and wild-type (FABP5^+/+) C57BL/6 mice. DHA bound to hFABP5 with an equilibrium dissociation constant of 155 ± 8 nM (mean ± SEM). FABP5 siRNA transfection decreased hCMEC/D3 mRNA and protein expression of FABP5 by 53.2 ± 5.5% and 44.8 ± 13.7%, respectively, which was associated with a 14.1 ± 2.7% reduction in ¹⁴C-DHA cellular uptake. By using optimized conditions for the in situ transcardiac perfusion (a 1 min preperfusion (10 mL/min) followed by perfusion of ¹⁴C-DHA (1 min)), the K_in of ¹⁴C-DHA was 0.04 ± 0.01 mL/g/s. Relative to FABP5^+/+ mice, the K_in of ¹⁴C-DHA decreased 36.7 ± 12.4% in FABP5^–/– mice. This study demonstrates that FABP5 binds to DHA and is involved in the brain endothelial cell uptake and subsequent BBB transport of DHA, confirming the importance of this cytoplasmic carrier protein in the CNS exposure of this PUFA essential for neuronal function.

Fatty acid-binding protein 5 (FABP5) at the blood–brain barrier contributes to the brain uptake of docosahexaenoic acid (DHA), a blood-derived polyunsaturated fatty acid essential for maintenance of cognitive function. Given the importance of DHA in cognition, the aim of this study was to investigate whether deletion of FABP5 results in cognitive dysfunction and whether this is associated with reduced brain endothelial cell uptake of exogenous DHA and subsequent attenuation in the brain levels of endogenous DHA. Cognitive function was assessed in male and female FABP5^+/+and FABP5^−/−mice using a battery of memory paradigms. FABP5^−/−mice exhibited impaired working memory and short-term memory, and these cognitive deficits were associated with a 14.7 ± 5.7% reduction in endogenous brain DHA levels. The role of FABP5 in the blood–brain barrier transport of DHA was assessed by measuring¹⁴C-DHA uptake into brain endothelial cells and capillaries isolated from FABP5^+/+and FABP5^−/−mice. In line with a crucial role of FABP5 in the brain uptake of DHA,¹⁴C-DHA uptake into brain endothelial cells and brain capillaries of FABP5^−/−mice was reduced by 48.4 ± 14.5% and 14.0 ± 4.2%, respectively, relative to those of FABP5^+/+mice. These results strongly support the hypothesis that FABP5 is essential for maintaining brain endothelial cell uptake of DHA, and that cognitive deficits observed in FABP5^−/−mice are associated with reduced CNS access of DHA.SIGNIFICANCE STATEMENTGenetic deletion of fatty acid-binding protein 5 (FABP5) in mice reduces uptake of exogenous docosahexaenoic acid (DHA) into brain endothelial cells and brain capillaries and reduces brain parenchymal levels of endogenous DHA. Therefore, FABP5 in the brain endothelial cell is a crucial contributor to the brain levels of DHA. Critically, lowered brain DHA levels in FABP5^−/−mice occurred in tandem with cognitive deficits in a battery of memory paradigms. This study provides evidence of a critical role for FABP5 in the maintenance of cognitive function via regulating the brain uptake of DHA, and suggests that upregulation of FABP5 in neurodegenerative diseases, where brain DHA levels are possibly diminished (e.g., Alzheimer's disease), may provide a novel therapeutic approach for restoring cognitive function.