The 2 miRs were upregulated by adrenergic activation, being
induced by combined β1- and α1-AR stimulation with phenylephrine
and isoproterenol in neonatal rat ventricular myocytes
(NRVMs) in vitro and in the mouse heart in vivo, and in NRVMs
by toxins that activate Gq and Gs. The same adrenergic stimuli
that induced miR-212/132 concomitantly repressed MeCP2.
MiR-132 repressed MeCP2 directly, in vitro and in vivo, by
interacting with the MeCP2 3′untranslated region. Overall, the
evidence was strong for a direct link between norepinephrine
elevation, myocyte β1- and α1-AR stimulation, miR-212/132
induction, and MeCP2 repression.
To test the functional consequences of MeCP2 downregulation
in HF, Mayer et al1
used genetic gain and loss of function.
The results were different from what might have been
expected.
MeCP2 overexpression in a standard α-MyHC transgenic
caused a severe cardiomyopathy with early mortality, so the
team developed a cardiac inducible transgenic model (iTG),
controlled by withdrawing doxycycline at weaning. This iTG
model eliminated MeCP2 repression when TAC was done
at the age of 8 to 12 weeks. Failure to repress MeCP2 with
TAC was maladaptive. When compared with wild-type (WT)
mice with TAC, failure to repress MeCP2 after TAC caused
more hypertrophy, apoptosis, and fibrosis and worse cardiac
function, with reductions in dp/dt and dobutamine response.
Removal of the aortic constriction after 4 weeks in WT mice,
in the rTAC model, caused a rapid reduction of hypertrophy
and improvement in fractional shortening (FS), whereas in
the MeCP2 iTG, reversal of hypertrophy and FS was delayed,
indicating impaired reverse remodeling. An MeCP2 cardiac
knockout (cKO) was established using MLC2a-Cre. MeCP2
cKO enhanced FS recovery in the rTAC model, without
changing hypertrophy. Overall, the data suggested that failure
to repress MeCP2 after TAC is maladaptive, and repression
could be adaptive.
The team did extensive studies of gene expression and metabolism
to investigate the molecular underpinnings of MeCP2
effects. Metabolic genes were downregulated in the MeCP2
iTG with TAC. Even in the absence of TAC, mitochondria in
the MeCP2 iTG were abnormally located around the nucleus,
rather than between myofibrils, and this abnormal location in
the iTG was accompanied by reduced oxygen consumption
and ATP synthesis in myocardial fibers and mitochondria in
vitro. Peroxisome proliferator-activated receptor γ coactivator
1α (PGC1α), a major mitochondrial regulator, was a direct
target of MeCP2 in NRVMs, being downregulated by adenoviral
MeCP2 expression and upregulated by MeCP2 knockdown;
PGC1α was also downregulated by MeCP2 in the 2
transgenic models. Other genes of fatty acid metabolism had
similar regulation. In short, MeCP2-mediated downregulation