Role of soluble adenylyl cyclase in mitochondria

Abstract

The soluble adenylyl cyclase (sAC) catalyzes the conversion of ATP into cyclic AMP (cAMP). Recent studies have shed new light on the role of sAC localized in mitochondria and its product cAMP, which drives mitochondrial protein phosphorylation and regulation of the oxidative phosphorylation system and other metabolic enzymes, presumably through the activation of intra-mitochondrial PKA. In this review article, we summarize recent findings on mitochondrial sAC activation by bicarbonate (HCO(3)(-)) and calcium (Ca²⁺) and the effects on mitochondrial metabolism. We also discuss putative mechanisms whereby sAC-mediated mitochondrial protein phosphorylation regulates mitochondrial metabolism. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Keywords: Mitochondria; PKA; Protein phosphorylation; cAMP; sAC.

Figure 1. sAC-cAMP-PKA pathway in mitochondria

sAC converts ATP in cAMP, the ubiquitous second messenger, which is degraded by phosphodiesterase (PDE). sAC is activated by Ca²⁺ (e.g. overexpression of the mitochondrial calcium uniporter, MCU) and HCO₃⁻ originated from the CO₂ derived from the krebs cycle spontaneously or by the action of carbonic anhydrase. It was proposed that cAMP activates PKA, leading to phosphorylation of the NDUFS4 subunit of complex I (CI, green line), thereby regulating CI activity. sAC also induces phosphorylation of subunit COXIV-1 of complex IV (blue line), leading to an increase in COX activity, membrane potential (Δψ), O₂ consumption and ATP production, without altering ROS level. It needs to be noted that the evidence that PKA is the sAC effector in mitochondria is contradictory and other kinases may be involved. Other OXPHOS complexes are phosphorylated, but whether this involves the mitochondrial sAC-signaling pathway is still unknown. Furthermore, proteins related to TCA cycle, calcium homeostasis, apoptosis, transporters, fatty acid oxidation and β-oxidation are also phosphorylated.

Figure 2. Genetically-encoded tools to study the mitochondrial sAC-cAMP pathway

Recently, new tools were developed to investigate the mitochondrial sAC-cAMP pathway in living cells. The diagram schematically depicts the genetically encoded probes. To detect cAMP in the matrix H30 and H90 were targeted to the mitochondria matrix (4mtH30 and mito-EpacH90) by adding 4 copies of the mitochondrial targeting sequence of subunit VIII of the human cytochrome oxidase (COX8) at the N-terminus of the Epac1-cAMP sensor. The same strategy was used to target AKAR3 and AKAR4, the FRET-based sensor to measure PKA activity, and PKI-mCherry (mt-PKI) and PKA-Cat-mCherry to the matrix [12, 18]. To target PKA, AKAR3 and AKAR4 to the OMM the targeting peptide used was yTom70 [18].