Augustine Blood Group System and Equilibrative Nucleoside Transporter 1

Augustine (AUG) is a blood group system comprising four antigens: AUG1, AUG2 (At^a), and AUG4 are of very high frequency; AUG3 is of very low frequency. These antigens are located on ENT1, an equilibrative nucleoside transporter encoded by <i>SLC19A1</i>. AUG antibodies are of clinical relevance in blood transfusion and pregnancy: anti-AUG2 have caused haemolytic transfusion reactions; the only anti-AUG3 was associated with severe haemolytic disease of the fetus and newborn. ENT1 is present in almost all human tissues. It facilitates the transfer of purine and pyrimidine nucleosides and is responsible for the majority of adenosine transport across plasma membranes. Adenosine transport appears to be an important factor in the regulation of bone metabolism. The AUG_null phenotype (AUG:–1,–2,–3,–4) has been found in three siblings, who are homozygous for an inactivating splice-site mutation in <i>SLC29A1</i>. Although ENT1 is very likely to be absent from all cells in these three individuals, they were apparently healthy with normal lifestyles. However, they suffered frequent attacks of pseudogout, a form of arthritis, in various joints with multiple calcifications around their hand joints. Ectopic calcification in the hips, pubic symphysis, and lumbar discs was present in the propositus. The three AUG_null individuals had misshapen red cells with deregulated protein phosphorylation, but no anaemia or shortening of red cell lifespan. Defective in vitro erythropoiesis in the absence of ENT1 was confirmed by shRNA-mediated knockdown of ENT1 during in vitro erythropoiesis of CD34⁺ progenitor cells from individuals with normal ENT1. Nucleoside transporters, such as ENT1, are vital in the uptake of synthetic nucleoside analogue drugs, used in cancer and viral chemotherapy. It is feasible that the efficacy of these drugs would be compromised in patients with the extremely rare AUG_null phenotype.

Effects of dipyridamole and its use in neurology

Dipyridamole has been on the pharmaceutical market since 1959 and, as a pyrimidyl-pyrimidine compound, has a variety of mechanisms of action. The very first action of dipyridamole was its antianginal effect. In subsequent years, attention was drawn to the antiplatelet properties of dipyridamole, which are related to inhibition of platelet phosphodiesterase as well as to blocking adenosine transport. Another important property of dipyridamole is its effect on the deformability of red blood cells, thereby improving microcirculation. Dipyridamole affects changes in the dynamics of platelet activity and vascular reactivity and causes improvement of cerebral perfusion. Due to its pronounced antiplatelet properties, the drug has been widely studied for the prevention of ischemic strokes and transient ischemic attacks, both as monotherapy and in combination with other drugs. Unlike other platelet antiaggregants, dipyridamole does not have a damaging effect on mucous membranes. Its antiplatelet effect is not accompanied with inhibition of cyclooxygenase activity and reduction of prostacyclin synthesis. In the treatment of cerebral circulation disorders, dipyridamole can be used to control the antithrombotic effect by selecting the optimal dose of the drug. Dipyridamole has antioxidant properties, enhances NO-mediated pathways, has indirect anti-inflammatory effects via adenosine and prostaglandin-2 as well as direct anti-inflammatory effects and several other effects. Dipyridamole is considered a safe drug based on decades of clinical experience. Its side effects are usually limited and transient. Given the diverse effects of dipyridamole, it can be used for a wide range of pathologies other than thrombosis prevention. Data on the efficacy and safety of dipyridamole in various diseases of the neurological spectrum are presented.

A Circadian Clock in the Retina Regulates Rod-Cone Gap Junction Coupling and Neuronal Light Responses via Activation of Adenosine A2A Receptors

Adenosine, a major neuromodulator in the central nervous system (CNS), is involved in a variety of regulatory functions such as the sleep/wake cycle. Because exogenous adenosine displays dark- and night-mimicking effects in the vertebrate retina, we tested the hypothesis that a circadian (24 h) clock in the retina uses adenosine to control neuronal light responses and information processing. Using a variety of techniques in the intact goldfish retina including measurements of adenosine overflow and content, tracer labeling, and electrical recording of the light responses of cone photoreceptor cells and cone horizontal cells (cHCs), which are post-synaptic to cones, we demonstrate that a circadian clock in the retina itself—but not activation of melatonin or dopamine receptors—controls extracellular and intracellular adenosine levels so that they are highest during the subjective night. Moreover, the results show that the clock increases extracellular adenosine at night by enhancing adenosine content so that inward adenosine transport ceases. Also, we report that circadian clock control of endogenous cone adenosine A2A receptor activation increases rod-cone gap junction coupling and rod input to cones and cHCs at night. These results demonstrate that adenosine and A2A receptor activity are controlled by a circadian clock in the retina, and are used by the clock to modulate rod-cone electrical synapses and the sensitivity of cones and cHCs to very dim light stimuli. Moreover, the adenosine system represents a separate circadian-controlled pathway in the retina that is independent of the melatonin/dopamine pathway but which nevertheless acts in concert to enhance the day/night difference in rod-cone coupling.

Adenosine-Related Mechanisms in Non-Adenosine Receptor Drugs

Many ligands directly target adenosine receptors (ARs). Here we review the effects of noncanonical AR drugs on adenosinergic signaling. Non-AR mechanisms include raising adenosine levels by inhibiting adenosine transport (e.g., ticagrelor, ethanol, and cannabidiol), affecting intracellular metabolic pathways (e.g., methotrexate, nicotinamide riboside, salicylate, and 5-aminoimidazole-4-carboxamide riboside), or undetermined means (e.g., acupuncture). However, other compounds bind ARs in addition to their canonical ‘on-target’ activity (e.g., mefloquine). The strength of experimental support for an adenosine-related role in a drug’s effects varies widely. AR knockout mice are the ‘gold standard’ method for investigating an AR role, but few drugs have been tested on these mice. Given the interest in AR modulation for treatment of cancer, CNS, immune, metabolic, cardiovascular, and musculoskeletal conditions, it is informative to consider AR and non-AR adenosinergic effects of approved drugs and conventional treatments.

Adenosine-Related Mechanisms in Non-Adenosine Receptor Drugs

Many ligands directly target adenosine receptors (ARs). Here we review the effects on adenosinergic signaling of other drugs that are not typically identified as binding ARs. Non-AR mechanisms include raising adenosine levels by inhibiting adenosine transport (e.g. ticagrelor, ethanol, cannabidiol), affecting intracellular metabolic pathways (e.g. methotrexate, nicotinamide riboside, salicylate, AICA riboside), or undetermined means (e.g. acupuncture). Yet other compounds bind ARs, in addition to their canonical &lsquo;on-target&rsquo; activity (e.g. mefloquine). The strength of experimental support varies widely. AR knockout mice are the &lsquo;gold standard&rsquo; method for investigating an AR role, but few drugs have been tested in these mice. Given the interest in AR modulation for treatment of cancer, CNS, immune, metabolic, cardiovascular, and musculoskeletal conditions, it is informative to consider AR and non-AR adenosinergic effects of approved drugs and conventional treatments.