Emerging adenosine receptor agonists: an update

Examining the Role of the Linker in Bitopic N6-Substituted Adenosine Derivatives Acting as Biased Adenosine A1 Receptor Agonists

The adenosine A₁ receptor is a therapeutic target based on its ability to provide cardioprotection during episodes of myocardial ischemia and reperfusion injury. However, the clinical translation of A₁R agonists has been hindered by dose-limiting adverse effects (bradycardia and hypotension). Previously, we demonstrated that the bitopic agonist VCP746 (1), consisting of an adenosine pharmacophore linked to an allosteric moiety, can stimulate cardioprotective A₁R signaling effects in the absence of unwanted bradycardia. This study maps the structure-activity relationships of 1 through modifications to the linker moiety. Derivatives differing in the flexibility, length, and nature of the linker were assessed, which revealed that the linker is tolerant of several modifications including added rigidity. Ligands featuring 1,4-disubstituted 1,2,3-triazoles were the most biased of the novel analogues but also displayed sub-nanomolar potency in a cAMP accumulation assay at the A_2BR. To our knowledge, 10 is the most potent A_2BR agonist published to date.

Design, synthesis and evaluation of amino-3,5-dicyanopyridines and thieno[2,3-b]pyridines as ligands of adenosine A1 receptors for the potential treatment of epilepsy

Due to the implication of adenosine in seizure suppression, adenosine-based therapies such as adenosine receptor (AR) agonists have been investigated. This study aimed at investigating thieno[2,3-b]pyridine derivatives as non-nucleoside A₁ agonists that could be used in pharmaco-resistant epilepsy (PRE). Compound 7c (thieno[2,3-b]pyridine derivative), displayed good binding affinity to the rA₁ AR (K_i = 61.9 nM). This could be a breakthrough for further investigation of this heterocyclic scaffold as potential ligand. In silico evaluation of this compound raised bioavailability concerns but performed well on drug-likeness tests. The effect of intramolecular cyclisation that occurs during synthesis of thieno[2,3-b]pyridines from the lead compounds, amino-3,5-dicyanopyridine derivatives (6a-s) in relation to AR binding was also evaluated. A significant loss of activity against rA₁/rA_2A ARs with cyclisation was revealed. Amino-3,5-dicyanopyridines exhibited greater affinity towards rA₁ ARs (K_i < 10 nM) than rA_2A. Compound 6c had the best rA₁ affinity (K_i = 0.076 nM). Novel compounds (6d, 6k, 6l, 6m, 6n, 6o, 6p) were highly selective towards rA₁ AR (K_i between 0.179 and 21.0 nM). Based on their high selectivity for A₁ ARs, amino-3,5-dicyanopyridines may be investigated further as AR ligands in PRE with the right structural optimisations and formulations.

Toward a Molecular Basis of Cellular Nucleoside Transport in Humans

Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.

Targeted Regulation of Blood-Brain Barrier for Enhanced Therapeutic Efficiency of Hypoxia-Modifier Nanoparticles and Immune Checkpoint Blockade Antibodies for Glioblastoma

Glioblastoma is the most destructive type of brain cancer. The blood-brain barrier (BBB) is a tremendous obstacle that hinders therapeutic agents, such as chemical drugs and antibodies, from reaching glioblastoma tissues. Meanwhile, the abnormal microenvironment of glioblastoma extremely restricts the expected therapeutic effects of accumulated drugs. Therefore, in the present study, BBB-regulating nanovesicles (BRN) are developed to achieve targeted and controlled BBB regulation, carrying adenosine 2A receptor (A_2AR) agonists and perfluorocarbon (PF). The red-blood-cell membrane (RBCM) is included on the outside to avoid the premature release of therapeutic agents. In the presence of ultrasonication (US), A_2AR agonists are released and induce effects on both F-actin and tight junctions of endothelial cells. Subsequently, BBB permeability is temporarily increased and enables small molecules and nanoparticles to enter brain parenchymal tissues. The high affinity between manganese dioxide and temozolomide (TMZ) is utilized to form multifunctional nanoparticles to ameliorate the hypoxic microenvironment, which yields improved glioblastoma inhibition combined with radiotherapy. Moreover, with the aid of targeted BBB regulation, programmed death ligand-1 (PD-L1) antibody induces a tumor-specific immune response. Taken together, the findings suggest that synergistic combination may have the potential in amplifying the therapeutic efficacies of clinical drugs and immune checkpoint blockade antibodies to overcome the therapeutic resistance of glioblastoma.

Structural analysis, virtual screening and molecular simulation to identify potential inhibitors targeting 2'-O-ribose methyltransferase of SARS-CoV-2 coronavirus

SARS-CoV-2, an emerging coronavirus, has spread rapidly around the world, resulting in over ten million cases and more than half a million deaths as of July 1, 2020. Effective treatments and vaccines for SARS-CoV-2 infection do not currently exist. Previous studies demonstrated that nonstructural protein 16 (nsp16) of coronavirus is an S-adenosyl methionine (SAM)-dependent 2'-O-methyltransferase (2'-O-MTase) that has an important role in viral replication and prevents recognition by the host innate immune system. In the present study, we employed structural analysis, virtual screening, and molecular simulation approaches to identify clinically investigated and approved drugs which can act as promising inhibitors against nsp16 2'-O-MTase of SARS-CoV-2. Comparative analysis of primary amino acid sequences and crystal structures of seven human CoVs defined the key residues for nsp16 2-O'-MTase functions. Virtual screening and docking analysis ranked the potential inhibitors of nsp16 from more than 4,500 clinically investigated and approved drugs. Furthermore, molecular dynamics simulations were carried out on eight top candidates, including Hesperidin, Rimegepant, Gs-9667, and Sonedenoson, to calculate various structural parameters and understand the dynamic behavior of the drug-protein complexes. Our studies provided the foundation to further test and repurpose these candidate drugs experimentally and/or clinically for COVID-19 treatment.Communicated by Ramaswamy H. Sarma.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Effect and Mechanism of Phosphodiesterase Inhibitors on Trabecular Outflow

PURPOSE

Phosphodiesterase (PDE) inhibitors increase matrix metalloproteinase (MMP) production by inhibiting re-uptake of adenosine and may potentiate nitric oxide (NO) activity. This study was performed to investigate the effects and mechanisms of PDE inhibitors on trabecular outflow in cultured human trabecular meshwork cells (HTMCs).

METHODS

Primary HTMC cultures were exposed to 0, 20, and 50 μM dipyridamole (DPD) or theophylline (TPN). Permeability through the HTMC monolayer was assessed using carboxyfluorescein. The production of NO was assessed using the Griess assay and MMP-2 levels were measured via Western blotting.

RESULTS

DPD significantly increased permeability accompanied with increased nitrite concentration and MMP-2 levels (all p < 0.05). TPN increased nitrite but did not affect permeability or MMP-2 levels significantly (p > 0.05). When treated with DPD and TPN together, both permeability and nitrite production were increased; however, MMP-2 levels showed no difference compared to DPD exposure alone (p > 0.05).

CONCLUSIONS

DPD increased trabecular permeability accompanied with increased nitrite production and MMP-2 levels. PDE inhibitors may increase trabecular outflow by increasing MMP-2 levels and by potentiating NO activity through cyclic GMP in HTMC.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

Novel Irreversible Agonists Acting at the A1 Adenosine Receptor

The A₁ adenosine receptor (A₁AR) is an important G protein-coupled receptor that regulates a range of physiological functions. Herein we report the discovery of novel irreversible agonists acting at the A₁AR, which have the potential to serve as useful research tools for studying receptor structure and function. A series of novel adenosine derivatives bearing electrophilic substituents was synthesized, and four compounds, 8b, 15a, 15b, and 15d, were shown to possess similar potency and efficacy to the reference high efficacy agonist, NECA, in an assay of ERK1/2 phosphorylation assay. Insensitivity to antagonist addition in a real-time, label-free, xCELLigence assay was subsequently used to identify compounds that likely mediated their agonism through an irreversible interaction with the A₁AR. Of these compounds, 15b and 15d were more directly validated as irreversible agonists of the A₁AR using membrane-based [³H]DPCPX and [³⁵S]GTPγS binding experiments.

Structure-Activity Relationships of the Sustained Effects of Adenosine A2A Receptor Agonists Driven by Slow Dissociation Kinetics

The duration of action of adenosine A_2A receptor (A2A) agonists is critical for their clinical efficacy, and we sought to better understand how this can be optimized. The in vitro temporal response profiles of a panel of A2A agonists were studied using cAMP assays in recombinantly (CHO) and endogenously (SH-SY5Y) expressing cells. Some agonists (e.g., 3cd; UK-432,097) but not others (e.g., 3ac; CGS-21680) demonstrated sustained wash-resistant agonism, where residual receptor activation continued after washout. The ability of an antagonist to reverse pre-established agonist responses was used as a surrogate read-out for agonist dissociation kinetics, and together with radioligand binding studies suggested a role for slow off-rate in driving sustained effects. One compound, 3ch, showed particularly marked sustained effects, with a reversal t_1/2 > 6 hours and close to maximal effects that remained for at least 5 hours after washing. Based on the structure-activity relationship of these compounds, we suggest that lipophilic N6 and bulky C2 substituents can promote stable and long-lived binding events leading to sustained agonist responses, although a high compound logD is not necessary. This provides new insight into the binding interactions of these ligands and we anticipate that this information could facilitate the rational design of novel long-acting A2A agonists with improved clinical efficacy.

Analysis of Extracellular Nucleotide Metabolism in Adult Zebrafish After Embryological Exposure to Valproic Acid

Autism is a neurodevelopmental disorder characterized by symptoms related to stereotyped movements, deficits in social interaction, impaired communication, anxiety, hyperactivity, and the presence of restricted interests. Evidence indicates an important role of extracellular ATP and adenosine as signaling molecules in autism. ATP hydrolysis by ectonucleotidases is an important source of adenosine, and adenosine deaminase (ADA) contributes to the control of the nucleoside concentrations. Considering zebrafish is an animal model that may contribute towards to understanding the mechanisms that underlie social behavior, we investigated the purinergic signaling in a model of embryological exposure to valproic acid (VPA) that induces social interaction deficit in adult zebrafish. We demonstrated embryological exposure to VPA did not change ATP and ADP hydrolysis in zebrafish at 120 dpf, and the cytosolic (soluble) ADA activity was not altered. However, we observed an increase of AMP hydrolysis (12.5 %) whereas the ecto-ADA activity was decreased (19.2 %) in adult zebrafish submitted to embryological exposure to VPA. Quantitative reverse transcription PCR (RT-PCR) analysis showed changes on ntpd8, ADA 2.1, and A2a1 mRNA transcript levels. Brain ATP metabolism showed a rapid catabolism of ATP and ADP, whereas the extracellular metabolism of AMP and adenosine (ADO) occurred slowly. We demonstrated that embryological exposure to VPA altered biochemical and molecular parameters related to purinergic system in adult zebrafish. These findings indicate that the enzyme activities involved in the control of ATP and adenosine levels may be involved in the pathophysiological mechanisms of diseases related to the impairment of social interaction, such as autism.

Identification of A3 adenosine receptor agonists as novel non-narcotic analgesics

Chronic pain negatively impacts the quality of life in a variety of patient populations. The current therapeutic repertoire is inadequate in managing patient pain and warrants the development of new therapeutics. Adenosine and its four cognate receptors (A1 , A2A , A2B and A3 ) have important roles in physiological and pathophysiological states, including chronic pain. Preclinical and clinical studies have revealed that while adenosine and agonists of the A1 and A2A receptors have antinociceptive properties, their therapeutic utility is limited by adverse cardiovascular side effects. In contrast, our understanding of the A3 receptor is only in its infancy, but exciting preclinical observations of A3 receptor antinociception, which have been bolstered by clinical trials of A3 receptor agonists in other disease states, suggest pain relief without cardiovascular side effects and with sufficient tolerability. Our goal herein is to briefly discuss adenosine and its receptors in the context of pathological pain and to consider the current data regarding A3 receptor-mediated antinociception. We will highlight recent findings regarding the impact of the A3 receptor on pain pathways and examine the current state of selective A3 receptor agonists used for these studies. The adenosine-to-A3 receptor pathway represents an important endogenous system that can be targeted to provide safe, effective pain relief from chronic pain.

Novel homologated-apio adenosine derivatives as A3 adenosine receptor agonists: design, synthesis and molecular docking studies

A series of homologated-apio adenosine derivatives including homologated-apio IB-MECA and Cl-IB-MECA have been designed and synthesized successfully. The molecular modelling and docking studies of the compounds have been explored as A₃AR agonists.

Expression of adenosine receptor subclasses in malignant and adjacent normal human prostate tissues

BACKGROUND

Adenosine, a purine nucleoside plays important roles in the pathogenesis of cancer initiation and promotion via interaction with four adenosine receptors. In the present study we examined the differential expression pattern of adenosine receptors in the malignant and adjacent normal human prostate tissues.

METHODS

Prostate cancer tissue samples and adjacent normal tissues were obtained from 20 patients undergoing radical prostatectomy and histopathological diagnosis was confirmed for each sample. Total RNA was extracted and reverse transcribed into cDNA and the mRNA expression levels of adenosine receptors were investigated by Taq-man real-time RT-PCR experiment. Quantitative protein analysis was done by Western blotting experiment. Moreover, the mRNA and protein expression levels of adenosine receptors were measured after androgen treatment.

RESULT

Taq-man real-time RT-PCR measurements show different expression levels of adenosine receptor transcripts. A2B adenosine receptor was predominantly expressed in tumor tissues (2.4-fold) followed by significantly expression of A3 (1.6-fold) and A2A adenosine receptors (1.5-fold) compared to adjacent normal tissues. The presence of adenosine receptors at protein levels in prostate cancer tissues compared with normal tissues was shown the following rank order: A2B  > A3  > A2A  > A1 . Androgen receptor regulates adenosine receptors mRNA and protein expression in AR-positive LNCaP cells, which was not seen in AR-negative PC-3 cells.

CONCLUSION

These results indicated for the first time, the differential mRNA expression profile and protein levels of adenosine receptors in the human prostate cancer. Interestingly, the A2B adenosine receptor followed by A3 is highly expressed in prostate tumor samples in comparison with the adjacent normal tissues. The findings support the possible key role of A2B adenosine receptor in promoting cancer cell growth and suggest that A2B may be a novel target for prostate cancer treatment.

An adenosine receptor-Krüppel-like factor 4 protein axis inhibits adipogenesis

Adipogenesis represents a key process in adipose tissue development and remodeling, including during obesity. Exploring the regulation of adipogenesis by extracellular ligands is fundamental to our understanding of this process. Adenosine, an extracellular nucleoside signaling molecule found in adipose tissue depots, acts on adenosine receptors. Here we report that, among these receptors, the A2b adenosine receptor (A2bAR) is highly expressed in adipocyte progenitors. Activation of the A2bAR potently inhibits differentiation of mouse stromal vascular cells into adipocytes, whereas A2bAR knockdown stimulates adipogenesis. The A2bAR inhibits differentiation through a novel signaling cascade involving sustained expression of Krüppel-like factor 4 (KLF4), a regulator of stem cell maintenance. Knockdown of KLF4 ablates the ability of the A2bAR to inhibit differentiation. A2bAR activation also inhibits adipogenesis in a human primary preadipocyte culture system. We analyzed the A2bARKLF4 axis in adipose tissue of obese subjects and, intriguingly, found a strong correlation between A2bAR and KLF4 expression in both subcutaneous and visceral human fat. Hence, our study implicates the A2bAR as a regulator of adipocyte differentiation and the A2bAR-KLF4 axis as a potentially significant modulator of adipose biology.

Anti-inflammatory effects of adenosine N1-oxide

BACKGROUND

Adenosine is a potent endogenous anti-inflammatory and immunoregulatory molecule. Despite its promise, adenosine's extremely short half-life in blood limits its clinical application. Here, we examined adenosine N1-oxide (ANO), which is found in royal jelly. ANO is an oxidized product of adenosine at the N1 position of the adenine base moiety. We found that it is refractory to adenosine deaminase-mediated conversion to inosine. We further examined the anti-inflammatory activities of ANO in vitro and in vivo.

METHODS

The effect of ANO on pro-inflammatory cytokine secretion was examined in mouse peritoneal macrophages and the human monocytic cell line THP-1, and compared with that of adenosine, synthetic adenosine receptor (AR)-selective agonists and dipotassium glycyrrhizate (GK2). The anti-inflammatory activity of ANO in vivo was examined in an LPS-induced endotoxin shock model in mice.

RESULTS

ANO inhibited secretion of inflammatory mediators at much lower concentrations than adenosine and GK2 when used with peritoneal macrophages and THP-1 cells that were stimulated by LPS plus IFN-γ. The potent anti-inflammatory activity of ANO could not be solely accounted for by its refractoriness to adenosine deaminase. ANO was superior to the synthetic A1 AR-selective agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA), A2A AR-selective agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamideadenosine hydrochloride (CGS21680), and A3 AR-selective agonist, N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA), in suppressing the secretion of a broad spectrum of pro-inflammatory cytokines by peritoneal macrophages. The capacities of ANO to inhibit pro-inflammatory cytokine production by THP-1 cells were comparable with those of CCPA and IB-MECA. Reflecting its potent anti-inflammatory effects in vitro, intravenous administration of ANO significantly reduced lethality of LPS-induced endotoxin shock. A significant increase in survival rate was also observed by oral administration of ANO. Mechanistic analysis suggested that the up-regulation of the anti-inflammatory transcription factor c-Fos was, at least in part, involved in the ANO-induced suppression of pro-inflammatory cytokine secretion.

CONCLUSIONS

Our data suggest that ANO, a naturally occurring molecule that is structurally close to adenosine but is functionally more potent, presents potential strategies for the treatment of inflammatory disorders.

Design, synthesis and evaluation of N6-substituted 2-aminoadenosine-5′-N-methylcarboxamides as A3 adenosine receptor agonists

A series of N⁶-substituted 2-aminoadenosine-5′-N-methylcarboxamides were synthesized from the versatile intermediate, O⁶-(benzotriazol-1-yl)-2-amino-2′,3′-O-isopropylideneinosine-5′-N-methylcarboxamide (1) and evaluated as A₃ adenosine receptor agonists.

Dysregulation of leukocyte gene expression in women with medication-refractory depression versus healthy non-depressed controls

BACKGROUND

Depressive Disorders (DD) are a great financial and social burden. Females display 70% higher rate of depression than males and more than 30% of these patients do not respond to conventional medications. Thus medication-refractory female patients are a large, under-served, group where new biological targets for intervention are greatly needed.

METHODS

We used real-time quantitative polymerase chain reaction (qPCR) to evaluate mRNA gene expression from peripheral blood leukocytes for 27 genes, including immune, HPA-axis, ion channels, and growth and transcription factors. Our sample included 23 females with medication refractory DD: 13 with major depressive disorder (MDD), 10 with bipolar disorder (BPD). Our comparison group was 19 healthy, non-depressed female controls. We examined differences in mRNA expression in DD vs. controls, in MDD vs. BPD, and in patients with greater vs. lesser depression severity.

RESULTS

DD patients showed increased expression for IL-10, IL-6, OXTR, P2RX7, P2RY1, and TRPV1. BPD patients showed increased APP, CREB1, NFKB1, NR3C1, and SPARC and decreased TNF expression. Depression severity was related to increased IL-10, P2RY1, P2RX1, and TRPV4 expression.

CONCLUSIONS

These results support prior findings of dysregulation in immune genes, and provide preliminary evidence of dysregulation in purinergic and other ion channels in females with medication-refractory depression, and in transcription and growth factors in those with BPD. If replicated in future research examining protein levels as well as mRNA, these pathways could potentially be used to explore biological mechanisms of depression and to develop new drug targets.

Inflammation, pain, and pressure--purinergic signaling in oral tissues

Signaling by extracellular purines such as ATP and adenosine has implications for dental research on multiple levels, with the association of purinergic signaling with inflammation, mechanical strain, and pain making the system particularly relevant for the specific challenges in the oral cavity. Oral tissues express a variety of G-protein-coupled P2Y receptors for ATP and P1 receptors for adenosine in addition to ionotropic P2X receptors for ATP. When these receptors are combined with the plethora of extracellular enzymes capable of manipulating extracellular agonist levels, a complex system for regulating oral health emerges, and recent findings have begun to identify a key role for purinergic signaling in oral pathophysiology. For example, the manipulation of extracellular ATP levels by P. gingivalis reduces inflammasome activation and apoptosis linked to P2X(7) receptor activation. Release of ATP by periodontal ligaments may link mechanical strain to bone remodeling. Activation of P2X receptors is implicated in dental pain, and receptor antagonists represent important targets for new analgesics. Altered levels of adenosine receptors in periodontal disease also suggest a role for nucleosides in dental signaling. The intricacies of the purinergic signaling system make it well-suited for the unique concerns of dental research, and future findings will doubtless confirm this importance.

Adenosine and autism: a spectrum of opportunities

In rodents, insufficient adenosine produces behavioral and physiological symptoms consistent with several comorbidities of autism. In rodents and humans, stimuli postulated to increase adenosine can ameliorate these comorbidities. Because adenosine is a broad homeostatic regulator of cell function and nervous system activity, increasing adenosine's influence might be a new therapeutic target for autism with multiple beneficial effects. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.