Calcium and bicarbonate signaling pathways have pivotal, resonating roles in matching ATP production to demand

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Abstract

Mitochondrial ATP production in cardiac ventricular myocytes must be continually adjusted to rapidly replenish the ATP consumed by the working heart. Two systems are known to be critical in this regulation: mitochondrial matrix Ca²⁺ ([Ca²⁺]_m) and blood flow that is tuned by local ventricular myocyte metabolic signaling. However, these two regulatory systems do not fully account for the physiological range of ATP consumption observed. We report here on the identity, location, and signaling cascade of a third regulatory system -- CO₂/bicarbonate. CO₂ is generated in the mitochondrial matrix as a metabolic waste product of the oxidation of nutrients that powers ATP production. It is a lipid soluble gas that rapidly permeates the inner mitochondrial membrane (IMM) and produces bicarbonate (HCO₃^-) in a reaction accelerated by carbonic anhydrase (CA). The bicarbonate level is tracked physiologically by a bicarbonate-activated adenylyl cyclase, soluble adenylyl cyclase (sAC). Using structural Airyscan super-resolution imaging and functional measurements we find that sAC is primarily inside the mitochondria of ventricular myocytes where it generates cAMP when activated by HCO₃^-. Our data strongly suggest that ATP production in these mitochondria is regulated by this cAMP signaling cascade operating within the inter-membrane space (IMS) by activating local EPAC1 (Exchange Protein directly Activated by cAMP) which turns on Rap1 (Ras-related protein 1). Thus, mitochondrial ATP production is shown to be increased by bicarbonate-triggered sAC signaling through Rap1. Additional evidence is presented indicating that the cAMP signaling itself does not occur directly in the matrix. We also show that this third signaling process involving bicarbonate and sAC activates the cardiac mitochondrial ATP production machinery by working independently of, yet in conjunction with, [Ca²⁺]_m-dependent ATP production to meet the energy needs of cellular activity in both health and disease. We propose that the bicarbonate and calcium signaling arms function in a resonant or complementary manner to match mitochondrial ATP production to the full range of energy consumption in cardiac ventricular myocytes in health and disease.

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The data that support the findings of this study are shown within the figures and their source numeric values are included in this publication as supplementary source data tables. Should additional information be requested it will be available from the corresponding author.

Article and author information

Author details

Maura Greiser

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-3197-0910
Mariusz Karbowski

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
Aaron David Kaplan

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
Andrew Kyle Coleman

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
Nicolas Verhoeven

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
Carmen A Mannella

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
W Jonathan Lederer

Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, Baltimore, United States

Competing interests
The authors declare that no competing interests exist.
Liron Boyman

Department of Physiology, University of Maryland, Baltimore, Baltimore, United States

For correspondence
Lboyman@som.umaryland.edu

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-4485-680X

Funding

American Heart Association (15SDG22100002)

Liron Boyman

National Institutes of Health (7U19 AI090959)

W Jonathan Lederer
Liron Boyman

Frontiers in Anesthesia Research (Award from International Anesthesia Research Society)

W Jonathan Lederer
Liron Boyman

National Institutes of Health (R01 GM129584)

Mariusz Karbowski

University of Maryland Claude D. Pepper Center (P30 AG028747)

Maura Greiser

National Institutes of Health (R01 HL142290)

W Jonathan Lederer

National Institutes of Health (5R35GM140822)

W Jonathan Lederer

National Institutes of Health (U01 HL116321)

Carmen A Mannella
W Jonathan Lederer

National Institutes of Health (T32 AR007592)

Andrew Kyle Coleman

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Ethics

Animal experimentation: All procedures and protocols involving animal use were approved by the Institutional Animal Care and Use Committee of the University of Maryland School of Medicine (IACUC # 0921015).

Copyright

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.