The Regulation of Troponins I, C and ANP by GATA4 and Nkx2-5 in Heart of Hibernating Thirteen-Lined Ground Squirrels, Ictidomys tridecemlineatus
February
The Regulation of Troponins I, C and ANP by GATA4 and Nkx2-5 in Heart of Hibernating Thirteen-Lined Ground Squirrels, Ictidomys tridecemlineatus
Data Availability Statement: All relevant data are within the paper. 0 1
Bryan E. Luu 0 1
Shannon N. Tessier 0 1
Dianna L Duford 0 1
Kenneth B. Storey 0 1
0 Institute of Biochemistry & Department of Biology, Carleton University , 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6 , Canada
1 Academic Editor: Marcelo Hermes-Lima, Universidade de Brasilia , BRAZIL
Hibernation is an adaptive strategy used by various mammals to survive the winter under situations of low ambient temperatures and limited or no food availability. The heart of hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus) has the remarkable ability to descend to low, near 0C temperatures without falling into cardiac arrest. We hypothesized that the transcription factors GATA4 and Nkx2-5 may play a role in cardioprotection by facilitating the expression of key downstream targets such as troponin I, troponin C, and ANP (atrial natriuretic peptide). This study measured relative changes in transcript levels, protein levels, protein post-translational modifications, and transcription factor binding over six stages: euthermic control (EC), entrance into torpor (EN), early torpor (ET), late torpor (LT), early arousal (EA), and interbout arousal (IA). We found differential regulation of GATA4 whereby transcript/protein expression, post-translational modification (phosphorylation of serine 261), and DNA binding were enhanced during the transitory phases (entrance and arousal) of hibernation. Activation of GATA4 was paired with increases in cardiac troponin I, troponin C and ANP protein levels during entrance, while increases in p-GATA4 DNA binding during early arousal was paired with decreases in troponin I and no changes in troponin C and ANP protein levels. Unlike its binding partner, the relative mRNA/protein expression and DNA binding of Nkx2-5 did not change during hibernation. This suggests that either Nkx2-5 does not play a substantial role or other regulatory mechanisms not presently studied (e.g. posttranslational modifications) are important during hibernation. The data suggest a significant role for GATA4-mediated gene transcription in the differential regulation of genes which aid cardiac-specific challenges associated with torporarousal.
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Funding: This work was supported by a Natural
Sciences and Engineering Research Council of
Canada (NSERC) Discovery grant (#6793) (www.
nserc-crsng.gc.ca) and a grant-in-aid from the Heart
and Stroke Foundation of Canada (#G-14-0005874)
(http://www.hsf.ca/research/en/grants-aid) to KBS.
BEL and SNT were funded by undergraduate and
postgraduate NSERC scholarships. The funders had
no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
Through evolution, animals have developed mechanisms to cope with environmental stressors
(e.g. heat, cold, drought, anoxia, lack of food) encountered in their natural habitats. One
wellknown mammalian survival response is hibernation that allows animals to survive the cold
winter months when there is little or no access to food. By abandoning homeothermy, strongly
suppressing metabolic rate, and sustaining only processes vital to survival, many small
mammals can survive the whole winter using only endogenous body fuel reserves (mainly lipids) to
generate energy. During the hibernation season, animals transition through prolonged periods
of torpor which are interrupted by brief periods of arousal. During torpor, basal metabolic rate
may be depressed by 9698% compared to euthermia, and core body temperature (Tb) falls to
near ambient (often as low as 05C) [15]. For overwintering ground squirrels, studies have
shown that the use of hibernation can conserve up to 88% of the energy that would otherwise
be needed to remain euthermic over the winter [1]. Ground squirrels prepare for hibernation
in autumn by entering a phase of hyperphagia with documented weight gains of up to 40%,
prior to switching their metabolism towards a preference for the oxidation of body lipid depots
over the long winter months without food [67]. Additionally, periods of deep torpor are
characterized by reduced organ perfusion (<10% of euthermia), respiration rates (~2.5% of
euthermia) and neuronal firing [5,7,8]. The present study analyzes the thirteen-lined ground squirrel
(Spermophilus tridecemlineatus) to study how a mammalian heart adapts to the stresses
encountered during cycles of torpor-arousal.
In order for a mammalian heart to survive under the low temperature and reduced
perfusion conditions (which would be normally considered ischemic) of hibernation, heart
metabolism is biochemically suppressed in order to restore balance between ATP supply and
demand [2,9]. On the physiological level (...truncated)