Objective To investigate the roles of Huntingtin associated protein 1 (HAP1) in the endocytosis of mature brain derived neurotrophic factor (mBDNF) and the underlying mechanism.Methods PC12 cells were differentiated by NGF and co-transfected with plasmid of HAP1A-CFP and (or) mBDNF-ds-red.The cells were incubated with either recombinant ds-red-labeled mBDNF, or in combination of sheep anti-BDNF antibodies or rabbit anti-p75NTR.The expression of fluorescence and its intercellular location were determined by laser scanning confocal microscope.Cortical neurons from HAP1^+/+ and HAP1^-/- mice at postnatal day 1 were cultured and treated with biotin labeled mBDNF to trigger endocytosis.After 60 minutes, the cells were washed and fixed, followed by immunostaining and confocal imaging.Results Co-transfected PC12 cells expressed partly co-localization of HAP1 with mBDNF.Cells transfected with HAP1A-CFP which were incubated with recombinant ds-red-labeled mBDNF showed almost complete co-localization of HAP1 with mBDNF.The antibodies to BDNF and p75NTR abolished co-internalization of HAP1 with mBDNF.In addition, the labeled mBDNF was detected in almost all HAP1^+/+, but not in HAP1^-/- cortical neurons.Conclusions HAP1 plays an important role in the endocytosis of mBDNF.