Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura: 12-02-2016Esta tesis tiene embargado el acceso al texto completo hasta el 12-08-2017Transcription of immediate early memory genes is an essential process in brain
function, regulating, among other processes, synaptic plasticity. It is well established the
necessity of gene transcription in order to maintain the late phases of the long-term
potentiation (LTP) and the long-term depression (LTD). Several works performed in
animals subjected to different memory paradigms have shown that there are epigenetic
mechanisms involved in memory genes regulation. However, little is known about the
contribution of these epigenetics mechanisms in response to a single stimulus, in the adult
and in the old brain. The aim of this thesis was to characterise such mechanisms in
response to LTD, in order to better understand the regulation of Bdnf gene expression,
and its possible relation with the aged associated learning and memory deficits. In this
thesis we present that LTD stimulation triggered by low NMDA dose in young adult
animals, induces the transcription of Bdnf gene from promoters I, II, IV and VI by
H3K27Me3 demethylation and H3K27Me3 phosphorylation at Serine 28, leading to
displacement of EZH2, the catalytic subunit of Polycomb Repressor Complex 2. LTD not
only does induce EZH2 repressor detachment, but also the dissociation of another
transcriptionally repressive enzyme such as histone deacetylase 4 (HDAC4). We also
show that LTD enhances acetylation of histone H3K27 via pCREB/CBP. Differently from
the described situation, typical of the mature brain, we present data showing that the
normal singling transduction of the young upon LTD is impaired in the aged hippocampus,
leading to a different basal chromatin state at Bdnf promoters in the old. The
consequence of this impairment is the loss of Bdnf induction in the old when exposed to
LTD, as a result of impaired HDAC4 dissociation, CBP recruitment and Histone H3K27
acetylation at Bdnf promoters. We also have observed that the loss of cholesterol at the
neuronal plasma membrane, a physiological feature of the old, plays a role in these
epigenetic deficits. In fact, cholesterol addition to old hippocampal slices rescued Bdnf
epigenetic regulation and expression in response to LTD. Furthermore, cholesterol
reduction in young adult hippocampal slices led to similar deficits to the ones found in the
old animals. In further support of the cholesterol loss-epigenetic dysregulation in the old,
oral administration of Voriconazole, an inhibitor of the enzyme responsible for cerebral
cholesterol loss (Cyp46A1), rescued hippocampal cholesterol loss and enhanced
cognitive abilities in the old animals, improving Bdnf epigenetic regulation and expression
in response to LTD. These results unveil one of the mechanisms involved in the cognitive
decline of the old and propose Cyp46A1 as valuable therapeutic possibility