activity assays from both rat neonatal cardiomyocytes acutely trans-
duced with Arg302Gln γ2-expressing adenovirus and cardiac
extracts from 1-week-old TGR302Q mice—experimental settings
expected to be devoid of significant glycogen accumulation—
revealed increased AMPK activity, contrasting with adult mice
[49]. Verifying the role of enhanced basal AMPK activity as the
driver of cardiomyopathy, double transgenic TGN488Imice over-
expressing a dominant negative α2-mutant (TGα 2 DN) exhibit
marked amelioration in cardiac hypertrophy, glycogen levels, and
PR interval shortening, together with normalization of contractile
function, compared to TGN488Imice alone [15].
In addition to the dynamic relationship between cardiac glyco-
genosis and AMPK activity, ex vivo cardiac perfusion studies using
the TGN488Imodel highlight the importance of ambient nucleotide
concentrations on AMPK activity [54]. Determination of AMPK
activity from excised cardiac tissue has potential limitations in
determining theγ2-mutation’s true in situ effect, largely due to
the inherent organ anoxia and attendant ATP depletion accompa-
nying postmortem tissue harvesting. Perfusion of isolated TGN488I,
TGWT, and non-TG WT hearts followed by assessment of myocar-
dial energetic status after a period of stabilization revealed no
differences in phosphocreatine, ATP, or free AMP levels deter-
mined by^31 P NMR spectroscopy, suggesting normal energetic
status under these conditions [54]. Assays of freeze-clamped car-
diac tissue in this metabolically equilibrated state revealed increased
α2-associated AMPK activity—and a corresponding ~4-fold
increase in panα-Thr172 phosphorylation, reflecting total AMPK
activity—in TGN488Imice over both TGWTand non-TG controls.
In contrast, assessment of AMPK activity after a period of no-flow
global cardiac ischemia, which resulted in a ~15-fold rise in the
AMP/ATP ratio, revealed a sevenfold increase in panα-Thr172
phosphorylation in WT but no corresponding increase in TGN488I
hearts (Fig.7). These findings, in conjunction with other studies
[26, 50], point to a more nuanced view of the activity consequences
ofγ2-mutations beyond a simple “gain of function.” Thus, under
conditions of replete cellular ATP, lack of (appropriate) inhibition
of mutantγ2-AMPK complexes leads to enhanced activity. In con-
trast, mutantγ2-AMPK is less responsive to activation under con-
ditions of cellular stress where ATP is depleted and AMP levels rise.5 Clinical Phenotype Associated withPRKAG2Mutations
5.1 Typical
Presentation
The cardiac phenotype associated with PRKAG2 mutations
includes left ventricular hypertrophy (LVH, varying from absent
to massive), ventricular pre-excitation, sinus node dysfunction
including chronotropic incompetence, atrial tachyarrhythmia,PRKAG2 syndrome 599