Neuronal cholesterol metabolism increases dendritic outgrowth and synaptic markers via a concerted action of GGTase-I and Trk

Cholesterol 24-hydroxylase (CYP46A1) is responsible for brain cholesterol elimination and therefore plays a crucial role in the control of brain cholesterol homeostasis. Altered CYP46A1 expression has been associated with several neurodegenerative diseases and changes in cognition. Since CYP46A1 activates small guanosine triphosphate-binding proteins (sGTPases), we hypothesized that CYP46A1 might be affecting neuronal development and function by activating tropomyosin-related kinase (Trk) receptors and promoting geranylgeranyl transferase-I (GGTase-I) prenylation activity. Our results show that CYP46A1 triggers an increase in neuronal dendritic outgrowth and dendritic protrusion density, and elicits an increase of synaptic proteins in the crude synaptosomal fraction. Strikingly, all of these effects are abolished by pharmacological inhibition of GGTase-I activity. Furthermore, CYP46A1 increases Trk phosphorylation, its interaction with GGTase-I, and the activity of GGTase-I, which is crucial for the enhanced dendritic outgrowth. Cholesterol supplementation studies indicate that cholesterol reduction by CYP46A1 is the necessary trigger for these effects. These results were confirmed in vivo, with a significant increase of p-Trk, pre- and postsynaptic proteins, Rac1, and decreased cholesterol levels, in crude synaptosomal fractions prepared from CYP46A1 transgenic mouse cortex. This work describes the molecular mechanisms by which neuronal cholesterol metabolism effectively modulates neuronal outgrowth and synaptic markers.