Periventricular Expression of Megalin in Adult Male Rat Brains and In-Vitro SHBG Binding to Cells Through Interaction with a Megalin Receptor
Sex Hormone Binding Globulin (SHBG) carries androgens and estrogens throughout the body to specific target cells. It was previously believed that SHBG transported its cargo outside of target cells, and the hormones then diffused through the membrane into the target cell. However, recent studies have suggested SHBG binds to a cell surface receptor, and then SHBG and its associated hormone are internalized by the cell. This paper studies the in vitro interaction of fluorescently-Iabeled SHBG with fetal mouse hippochampal cells and fibroblast cells to determine how blocking one ofSHBG's known receptors, megalin, affects SHBG. Immunohistochemistry in adult male rat brain sections was performed to localize megalin using an antibody raised against itself. This identified megalin in ependymal cells lining the lateral and third ventricles. In vitro results showed fluorescently labeled SHBG (SHBG-Alexa-555™) bound to the outside of HT22 mouse hippocampal cells and fibroblast NIH3 T3 cells after short treatment periods; longer treatment times resulted in the appearance of granular fluorescence inside these cells. Binding of SHBG was decreased when cells were treated with megalin antibody before the addition of fluorescent SHBG; this suggests megalin bound by antibody was unable to interact with SHBG as strongly. These studies suggest SHBG binds to brain cells through the interaction with its cell surface receptor megalin, which subsequently internalizes SHBG and its associated steroid hormone.