Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Abstract

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

Keywords: AMP-activated protein kinase; ATP-sensitive potassium channel; Phosphotransfer enzymes; adenosine 5’-monophosphate; adenosine 5’-triphosphate; energy transfer; nucleotide metabolism; system bioenergetics.

Figure 1.

Adenylate kinase shuttle facilitates transfer of ATP β- and γ-phosphoryls from generation to utilization sites. Adenylate kinase (AK), present in mitochondrial and myofibrillar compartments, enables the transfer and makes available the energy of two high-energy phosphoryls, the β- and the γ-phosphoryls of a single ATP molecule. In this case, AMP signals feedback to mitochondrial respiration amplified by the generation of two molecules of ADP at the mitochondrial intermembrane site. Within the intracellular environment of a cardiomyocyte, the transfer of ATP and AMP between ATP-production and ATP-consumption sites may involve multiple, sequential, phosphotransfer relays that result in a flux wave propagation along clusters of adenylate kinase molecules (lower panel). Handling of substrates by “bucket-brigade” or a ligand conduction mechanism facilitates metabolic flux without apparent changes in metabolite concentrations. AK1 and AK2 – cytosolic and mitochondrial AK isoforms, respectively. i.m. and o.m. – inner and outer membranes, respectively. Modified from [5] with permission.

Figure 2.

Adenylate kinase isoform network and intracellular localization. Adenylate kinase isoforms are coded by separate genes, KAD1 – KAD7, localized to different chromosomes. Corresponding proteins AK1 – AK7 define separate adenylate kinase isoforms with different molecular weights, kinetic properties and intracellular localization. The AK1 isoform mostly consists of the ADK domain which is characteristic for the whole protein family. The AK1β splice variant has an additional myristoylation domain that targets the protein to the plasma membrane. The proteins Rad50 and C9orf98 with ADK domains and activity have specific cellular functions. The AK2, AK3 and AK4 isoforms have a flexible lid domain which closes over the site of phosphoryl transfer upon ATP binding to prevent water accessibility. A short form of the lid domain exists also in AK1, AK5 and AK6. Within the network adenylate kinase proteins distribute high-energy phosphoryls (~ P) and communicate AMP signals.

Figure 3.

Adenylate kinase metabolic monitoring system. Adenylate kinase reads the cellular energy state, generates, tunes and communicates AMP signals to metabolic sensors. In such way adenylate kinase conveys information about the adenine nucleotide pool status and, thus, the overall energy balance. In response to AMP signals metabolic sensors reduce ATP-consuming and activate ATP-generating pathways to adjust energy metabolism, functional activity and increase fuel and oxygen supply.

Figure 4.

AMPing up and down — integration cellular AMP signals by adenylate kinase. Adenylate kinase integrates AMP metabolic signals produced or downregulated during exercise, stress response, food consumption and during changes in hormonal balance or mitochondrial coupling state. Adenylate kinase relays deliver AMP signals to metabolic sensors and by catalyzing nucleotide exchange in the intimate “sensing zone” of metabolic sensors facilitate decoding of cellular information.

Figure 5.

Regulation of intracellular AMP levels. Cytosolic adenylate kinase (AK1) is the major AMP generator while mitochondrial AK2 isoform, due to low Km(AMP), is the major AMP sequestration and tune-up mechanism. AMP is also generated during free fatty and amino acid activation, during adenosine rephosphorylation by adenosine kinase (ADK), during IMP reamination and by cyclic nucleotide phosphodiseterase (PDE). Oxygen deprivation and intense muscle contraction increase AMP removal through adenosine and IMP pathways catalyzed by 5’-nucleotidase (5’-NT) and AMP-deaminase (AMPD). Defects in mitochondria metabolism would reduce AMP tuning capacity of AK2 and, in fact, can reverse reaction towards AMP generation. The metabolically active AMP pool, estimated about 10–20%, is in dynamic equilibrium with bound and/or compartmentalized AMP [1,5].

Figure 6.

Myocardial-vascular metabolic signaling as a paradigm of global energy sensing. Metabolic signal transduction cascades initiated by phosphotransfer redistribution between adenylate kinase (AK) and creatine kinase (CK) govern AMP/adenosine (Ado) cycle and the response of metabolic sensors (ATP-sensitive potassium channel, K-ATP and AMP-activated protein kinase, AMPK). Hypoxia or metabolic stress diminishes CK and increases AK flux inducing AMP generation and subsequent AMP/adenosine signaling events. Adenosine/AMP signals delivered to vascular tissue through intercellular and paracellular pathways induce signaling through A(2A) adenosine receptors, AMPK and K-ATP channels. AMPK activates eNOS inducing NO/cGMP signaling and could regulate K-ATP channels. Collectively, A(2A)AR, AMPK, eNOS and K-ATP signaling converge on contractile protein, Ca²⁺ and membrane potential regulation, critical determinants of vascular tone. Dipyridamole, an adenosine uptake inhibitor, disrupts Ado- AMP cycle and tuning of adenosine signals, thus potentiating vascular response. Modified from [4] with permission.