Cholesterol excess in the brain is mainly disposed via
cholesterol 24-hydroxylation catalysed by
cytochrome P450 46A1, a CNS-specific
enzyme.
Cytochrome P450 46A1 is emerging as a promising therapeutic target for various
brain diseases with both enzyme activation and inhibition having therapeutic potential. The rate of
cholesterol 24-hydroxylation determines the rate of brain
cholesterol turnover and the rate of
sterol flux through the plasma membranes. The latter was shown to affect membrane properties and thereby
membrane proteins and membrane-dependent processes. Previously we found that treatment of 5XFAD mice, an
Alzheimer's disease model, with a small dose of
anti-HIV drug efavirenz allosterically activated
cytochrome P450 46A1 in the brain and mitigated several disease manifestations. Herein, we generated
Cyp46a1-/- 5XFAD mice and treated them, along with 5XFAD animals, with
efavirenz to ascertain
cytochrome P450 46A1-dependent and independent
drug effects.
Efavirenz-treated versus control
Cyp46a1-/- 5XFAD and 5XFAD mice were compared for the brain
sterol and
steroid hormone content,
amyloid β burden,
protein and
mRNA expression as well as synaptic ultrastructure. We found that the
cytochrome P450 46A1-dependent
efavirenz effects included changes in the levels of brain
sterols,
steroid hormones, and such
proteins as
glial fibrillary acidic protein, Iba1, Munc13-1, post-synaptic density-95,
gephyrin,
synaptophysin and synapsin-1. Changes in the expression of genes involved in neuroprotection, neurogenesis, synaptic function,
inflammation, oxidative stress and apoptosis were also
cytochrome P450 46A1-dependent. The total
amyloid β load was the same in all groups of animals, except lack of
cytochrome P450 46A1 decreased the production of the
amyloid β40 species independent of treatment. In contrast, altered transcription of genes from
cholinergic, monoaminergic, and peptidergic neurotransmission,
steroid sulfation and production as well as
vitamin D3 activation was the main CYP46A1-independent
efavirenz effect. Collectively, the data obtained reveal that
CYP46A1 controls
cholesterol availability for the production of
steroid hormones in the brain and the levels of biologically active
neurosteroids. In addition,
cytochrome P450 46A1 activity also seems to affect the levels of post-synaptic density-95, the main
postsynaptic density protein, possibly by altering the
calcium/calmodulin-dependent protein kinase II inhibitor 1 expression and activity of
glycogen synthase kinase 3β. Even at a small dose,
efavirenz likely acts as a transcriptional regulator, yet this regulation may not necessarily lead to functional effects. This study further confirmed that
cytochrome P450 46A1 is a key
enzyme for
cholesterol homeostasis in the brain and that the therapeutic
efavirenz effects on 5XFAD mice are likely realized via
cytochrome P450 46A1 activation.