Adenylosuccinic Acid: An Orphan Drug with Untapped Potential

Abstract

Adenylosuccinic acid (ASA) is an orphan drug that was once investigated for clinical application in Duchenne muscular dystrophy (DMD). Endogenous ASA participates in purine recycling and energy homeostasis but might also be crucial for averting inflammation and other forms of cellular stress during intense energy demand and maintaining tissue biomass and glucose disposal. This article documents the known biological functions of ASA and explores its potential application for the treatment of neuromuscular and other chronic diseases.

Keywords: ADSSL1 myopathy; Duchenne muscular dystrophy; Nrf2 activation; adenylosuccinate; adenylosuccinic acid; immunometabolism; metabolic disease; purine metabolism; skeletal muscle; succinyl-AMP.

Figure 1

Adenylosuccinic acid (ASA) is an aromatic small molecule that functions within the purine nucleotide cycle (PNC) metabolon. (A) Chemical structure of ASA according to [18]; (B) ASA is endogenously synthesized by adenylosuccinate synthetase (ADSS) using inosine monophosphate (IMP), aspartate and guanosine triphosphate (GTP). Guanosine diphosphate (GDP) and inorganic phosphate (Pi) are generated as by-products of this reaction. ASA is catalyzed by adenylosuccinate lyase (ADSL) generating adenosine monophosphate (AMP) and fumarate. Under low energy conditions, AMP exits the PNC and enters the adenylate kinase (AK) reaction to generate two molecules of ADP that are transported across the mitochondrial membranes via adenine nucleotide translocase for re-phosphorylation by ATP synthase (Complex V). When cellular energy status is high, AMP is catalyzed to IMP, which is further degraded and eliminated from cells producing ammonia (NH₄). The PNC facilitates energy balance, mitochondrial coupling via fumarate-aspartate exchange and excretion of nitrogenous waste. Thick solid lines = reactions; thin solid lines = cofactors and products; broken lines = PNC exit pathways. Figure created using Biorender.com.

Figure 2

Inflammatory gene signatures of murine Duchenne muscular dystrophy (DMD) skeletal muscle treated with and without hydroxycarboxylic acid receptor 2 (HCAR2) agonist immunomodulatory metabolites. Juvenile (14-day-old) wild-type (WT) and mdx mice were treated with either a methyl cellulose (MC) vehicle, or 325 mg/kg/day ASA, 100 mg/kg/day DMF or 380 mg/kg/day BHB suspended in MC via oral gavage for 2 weeks. Quadriceps muscles were harvested under non-recovery anesthesia and the inflammatory gene signature was profiled using qPCR array (for methodology see Supplemental information). Data shown are log2 fold change expression from WT.

Figure 3

Schema of the critical role adenylosuccinic acid (ASA) plays in coupling cellular metabolic stress signals with mitochondrial function. During intense energy expenditure (exercise, cell damage, tissue repair) or metabolic disease, adenosine triphosphate (ATP) is rapidly degraded to inosine monophosphate (IMP) which, if not sequestered into the purine nucleotide cycle (PNC), leads to further degradation to hypoxanthine and xanthine (yellow box). The PNC sequesters IMP into a reaction with GDP catalyzed by adenylosuccinate synthetase (ADSS) generating ASA. ASA is subsequently catalyzed to AMP and fumarate (FUM), a key step in ensuring cytosolic energy demand is coupled to mitochondrial ATP synthesis via oxidative phosphorylation (OXPHOS; green box). FUM is converted to malate by cytosolic fumarate hydratase (FHc), which is exchanged across the double mitochondrial membrane for aspartate via the malate–aspartate shuttle. Malate drives the mitochondrial tricarboxylic acid cycle, a key generator of NADH that supports OXPHOS-mediated conversion of ADP derived from the linked PNC-adenylate–kinase system. Aspartate fuels ASA synthesis within the PNC at a matched rate. When energy expenditure exceeds the capacity of this coupled system (which can be compromised by metabolic disorders), IMP is diverted away from the PNC and rapidly degraded to hypoxanthine and xanthine, which is extruded from cells via xanthine oxidoreductase (XOR) metabolism as uric acid (UA). XOR is a rampant producer of reactive oxygen species (ROS), which drives oxidative stress and inflammation (pink box). ASA-generated FUM can suppress oxidative stress and inflammation via activation of cytoprotective transcription factor, Nrf2. During critical ATP shortage, purines can be de novo biosynthesized in a slow, and bioenergetically expensive, process yielding IMP. ASA is still required to convert IMP to usable bioenergy (AMP > ADP > ATP) and is therefore staple in both purine salvage and biosynthesis pathways.

Figure 4

Adenylosuccinic acid (ASA) modulates the Duchenne muscular dystrophy (DMD) disease program. (A) Simplified schematic of the DMD disease program generated by Lombardo et al. using a network medicines approach, which is driven by dysregulated expression of matrix metalloproteinase 2 (MMP2), secreted phosphoprotein 1 (SPP1/osteopontin) and tissue inhibitor of matrix metalloproteinase 1 (TIMP1) [70]. (B) Heatmap of DMD seed gene expression in mdx gastrocnemius muscle. Juvenile (14-day-old) wild-type (WT) and mdx mice were treated with either a methyl cellulose (MC) vehicle (mdx), or 325 mg/kg/day ASA suspended in MC via oral gavage for 2 weeks. Quadriceps muscles were harvested under non-recovery anesthesia and extracellular matrix gene signature was profiled using qPCR array (for methodology see Supplemental information). (C) Heatmap of the ECM genes most modulated ASA. Data shown are log2 fold change expression from WT (for mdx) or mdx (for mdx ASA).