Animal Models to Study AMPK

Fine-tuning AMPK in physiology and disease using point-mutant mouse models

AMP-activated protein kinase (AMPK) is an evolutionarily conserved serine/threonine kinase that monitors the cellular energy status to adapt it to the fluctuating nutritional and environmental conditions in an organism. AMPK plays an integral part in a wide array of physiological processes, such as cell growth, autophagy and mitochondrial function, and is implicated in diverse diseases, including cancer, metabolic disorders, cardiovascular diseases and neurodegenerative diseases. AMPK orchestrates many different physiological outcomes by phosphorylating a broad range of downstream substrates. However, the importance of AMPK-mediated regulation of these substrates in vivo remains an ongoing area of investigation to better understand its precise role in cellular and metabolic homeostasis. Here, we provide a comprehensive overview of our understanding of the kinase function of AMPK in vivo, as uncovered from mouse models that harbor phosphorylation mutations in AMPK substrates. We discuss some of the inherent limitations of these mouse models, highlight the broader implications of these studies for understanding human health and disease, and explore the valuable insights gained that could inform future therapeutic strategies for the treatment of metabolic and non-metabolic disorders.

Allosteric regulation of C. elegans AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is a key metabolic regulator which responds to changes in the AMP:ATP ratio within cells. In response to high AMP levels, AMPK promotes a metabolic shift towards increased catabolism and autophagy to restore cellular energy and maintain homeostasis. In C. elegans, AMPK is important for controlling a multitude of functions including metabolism, reproductive health, and lifespan. AMPK is a heterotrimeric protein consisting of α catalytic, β linker, and γ regulatory subunits. Active AMPK is characterised by phosphorylation of the α subunit. It is also regulated allosterically by the nucleotide AMP binding within the γ subunit. C. elegans have five different AMPKγ subunits and their primary amino acid sequence implies two different modes of AMP-binding. Modifying the ability of AMPKγ to bind adenine nucleotides could directly impact how effectively AMPK manages energy homeostasis. Despite the importance of the γ subunit, most C. elegans AMPK research has focused on the catalytic α subunit. Here, we genetically dissect the functional role of the different γ subunits in relation to physiology and lifespan. We show that in normal animals, three of these γ subunits ( aakg-1, aakg-2, and aakg-3 ) are required for normal responses to AMP, and contribute to normal fecundity and lifespan.

Spatiotemporal AMPKα2 deletion in mice induces cardiac dysfunction, fibrosis and cardiolipin remodeling associated with mitochondrial dysfunction in males only

Background

The AMP-activated protein kinase (AMPK) is a major regulator of cellular energetics which plays key role in acute metabolic response and in long-term adaptation to stress. Recent works have also suggested non-metabolic effects.

Methods

To decipher AMPK roles in the heart, we generated a cardio-specific inducible model of gene deletion of the main cardiac catalytic subunit of AMPK (Ampkα2) in mice. This allowed us to avoid the eventual impact of AMPK-KO in peripheral organs.

Results

Cardio-specific Ampkα2 deficiency led to a progressive left ventricular systolic dysfunction and the development of cardiac fibrosis in males. We observed a reduction in complex I-driven respiration without change in mitochondrial mass or in vitro complex I activity, associated with a rearrangement of the cardiolipins and reduced integration of complex I into the electron transport chain supercomplexes. Strikingly, none of these defects were present in females. Interestingly, suppression of estradiol signaling by ovariectomy partially mimicked the male sensitivity to AMPK loss, notably the cardiac fibrosis and the rearrangement of cardiolipins, but not the cardiac function that remained protected.

Conclusion

Our results confirm the close link between AMPK and cardiac mitochondrial function, but also highlight links with cardiac fibrosis. Importantly, we show that AMPK is differently involved in these processes in males and females, which may have clinical implications for the use of AMPK activators in the treatment of heart failure.

AMPK is a metabolic sensor of cellular energy which regulates energy homeostasis.

We generated a cardiac-specific inducible deletion of Ampkα2 and demonstrated that this deletion induces mild cardiac dysfunction in male only.

Cardiac dysfunction observed in males was associated with cardiac fibrosis and cardiac cardiolipin remodeling that are not seen in females.

Although no significant cardiac function alteration was noticed in ovariectomized female Ampkα2ciKO mice, these latter exhibited cardiac fibrosis and mild cardiolipins remodeling.

Our results show a higher dependence on AMPK signaling fibrosis and cardiolipin biosynthesis/maturation in males, either due to the absence of female hormones protection or/and to the action of male hormones. This may contribute to the known difference in cardiovascular risk and outcome between sexes.

Transient phases of OXPHOS inhibitor resistance reveal underlying metabolic heterogeneity in single cells

Cell-to-cell heterogeneity in metabolism plays an unknown role in physiology and pharmacology. To functionally characterize cellular variability in metabolism, we treated cells with inhibitors of oxidative phosphorylation (OXPHOS) and monitored their responses with live-cell reporters for ATP, ADP/ATP, or activity of the energy-sensing kinase AMPK. Across multiple OXPHOS inhibitors and cell types, we identified a subpopulation of cells resistant to activation of AMPK and reduction of ADP/ATP ratio. This resistant state persists transiently for at least several hours and can be inherited during cell divisions. OXPHOS inhibition suppresses the mTORC1 and ERK growth signaling pathways in sensitive cells, but not in resistant cells. Resistance is linked to a multi-factorial combination of increased glucose uptake, reduced protein biosynthesis, and G0/G1 cell-cycle status. Our results reveal dynamic fluctuations in cellular energetic balance and provide a basis for measuring and predicting the distribution of cellular responses to OXPHOS inhibition.

A functional role for AMPK in female fertility and endometrial regeneration

Adenosine monophosphate-activated protein kinase (AMPK) is a highly conserved heterotrimeric complex that acts as an intracellular energy sensor. Based on recent observations of AMPK expression in all structures of the female reproductive system, we hypothesized that AMPK is functionally required for maintaining fertility in the female. This hypothesis was tested by conditionally ablating the two catalytic alpha subunits of AMPK, Prkaa1 and Prkaa2, using Pgr-cre mice. After confirming the presence of PRKAA1, PRKAA2 and the active phospho-PRKAA1/2 in the gravid uterus by immunohistochemistry, control (Prkaa1/2 fl/fl ) and double conditional knockout mice (Prkaa1/2 d/d ) were placed into a six-month breeding trial. While the first litter size was comparable between Prkaa1/2 fl/fl and Prkaa1/2 d/d female mice (P = 0.8619), the size of all subsequent litters was dramatically reduced in Prkaa1/2 d/d female mice (P = 0.0015). All Prkaa1/2 d/d female mice experienced premature reproductive senescence or dystocia by the fourth parity. This phenotype manifested despite no difference in estrous cycle length, ovarian histology in young and old nulliparous or multiparous animals, mid-gestation serum progesterone levels or uterine expression of Esr1 or Pgr between Prkaa1/2 fl/fl and Prkaa1/2 d/d female mice suggesting that the hypothalamic-pituitary-ovary axis remained unaffected by PRKAA1/2 deficiency. However, an evaluation of uterine histology from multiparous animals identified extensive endometrial fibrosis and disorganized stromal-glandular architecture indicative of endometritis, a condition that causes subfertility or infertility in most mammals. Interestingly, Prkaa1/2 d/d female mice failed to undergo artificial decidualization. Collectively, these findings suggest that AMPK plays an essential role in endometrial regeneration following parturition and tissue remodeling that accompanies decidualization.

Compound C/Dorsomorphin: Its Use and Misuse as an AMPK Inhibitor

The evolutionary conserved energy sensor AMPK plays crucial roles in many biological processes-both during normal development and pathology. Loss-of-function genetic studies in mice as well as in lower organisms underscore its importance in embryonic development, stress physiology in the adult, and in key metabolic disorders including cardiovascular disease, diabetes, cancer, and metabolic syndrome. In contrast to several other kinases important in human health and medicine where specific/selective inhibitors are available, no AMPK-specific inhibitors are available. The only reagent called dorsomorphin or compound C that is occasionally used as an AMPK inhibitor unfortunately inhibits several other kinases much more potently than AMPK and is therefore highly non-specific. In this chapter, we discuss the pros and cons of using this reagent to study AMPK functions.