Preclinical jockeying on the translational track of adenosine A2A receptors

Life style and Parkinson's disease

The question whether life style may impair the advent or course of the disease in patients with Parkinsonism is of great importance for patients and physicians alike. We present here comprehensive information on the influence of the environment, diet (especially caffeine, nicotine, alcohol, chocolate and dairy products), physical activity and sleep on risk and course of Parkinson’s disease.

The belated US FDA approval of the adenosine A2A receptor antagonist istradefylline for treatment of Parkinson's disease

After more than two decades of preclinical and clinical studies, on August 27, 2019, the US Food and Drug Administration (FDA) approved the adenosine A_2A receptor antagonist Nourianz® (istradefylline) developed by Kyowa Hakko Kirin Inc., Japan, as an add-on treatment to levodopa in Parkinson's disease (PD) with "OFF" episodes. This milestone achievement is the culmination of the decade-long clinical studies of the effects of istradefylline in more than 4000 PD patients. Istradefylline is the first non-dopaminergic drug approved by FDA for PD in the last two decades. This approval also provides some important lessons to be remembered, namely, concerning disease-specific adenosine signaling and targeting subpopulation of PD patients. Importantly, this approval paves the way to foster entirely novel therapeutic opportunities for adenosine A_2A receptor antagonists, such as neuroprotection or reversal of mood and cognitive deficits in PD and other neuropsychiatric diseases.

Systemic inflammation regulates microglial responses to tissue damage in vivo

Microglia, the resident immune cells of the central nervous system, exist in either a "resting" state associated with physiological tissue surveillance or an "activated" state in neuroinflammation. We recently showed that ATP is the primary chemoattractor to tissue damage in vivo and elicits opposite effects on the motility of activated microglia in vitro through activation of adenosine A2A receptors. However, whether systemic inflammation affects microglial responses to tissue damage in vivo remains largely unknown. Using in vivo two-photon imaging of mice, we show that injection of lipopolysaccharide (LPS) at levels that can produce both clear neuroinflammation and some features of sepsis significantly reduced the rate of microglial response to laser-induced ablation injury in vivo. Under proinflammatory conditions, microglial processes initially retracted from the ablation site, but subsequently moved toward and engulfed the damaged area. Analyzing the process dynamics in 3D cultures of primary microglia indicated that only A2A , but not A1 or A3 receptors, mediate process retraction in LPS-activated microglia. The A2A receptor antagonists caffeine and preladenant reduced adenosine-mediated process retraction in activated microglia in vitro. Finally, administration of preladenant before induction of laser ablation in vivo accelerated the microglial response to injury following systemic inflammation. The regulation of rapid microglial responses to sites of injury by A2A receptors could have implications for their ability to respond to the neuronal death occurring under conditions of neuroinflammation in neurodegenerative disorders.

Adenosine A2A receptor antagonism reverses inflammation-induced impairment of microglial process extension in a model of Parkinson's disease

Microglia, the immune cells of the central nervous system, constantly survey the parenchyma in the healthy brain to maintain homeostasis. When a disturbance, such as cell death, results in ATP release in vivo, microglial processes respond by utilizing P2Y12 purinergic receptors to trigger extension toward the site of damage. Processes ultimately surround the injury site, preventing the spread of harmful cellular constituents and assisting with tissue repair. In contrast to the healthy brain, many neurodegenerative diseases, including Parkinson's disease, are characterized by the presence of neuroinflammation. Yet, the ability of microglia to respond to tissue damage under pro-inflammatory conditions has not been well studied. To assess the ability of microglia to respond to tissue injury and localized cell death in the context of Parkinson's disease, we performed confocal imaging of acute brain slices from mice with microglia-specific green fluorescent protein expression. Microglia in coronal slices containing the substantia nigra extend processes toward a mechanical injury in a P2Y12 receptor-dependent manner. However, microglia in mice treated for 5days with 20mg/kg/day 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) show significantly reduced process displacement toward the injury compared to microglia in control animals. Pre-treatment of slices from MPTP-injected mice with the A2A receptor-selective antagonist preladenant restores the ability of activated microglia to respond to tissue damage. These data support the hypothesis that chronic inflammation impedes microglial motility in response to further injury, such as cell death, and suggest that some aspects of the neuroprotection observed with adenosine A2A receptor antagonists may involve direct or indirect actions at microglia.

The emerging role of nutrition in Parkinson's disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease in ageing individuals. It is now clear that genetic susceptibility and environmental factors play a role in disease etiology and progression. Because environmental factors are involved with the majority of the cases of PD, it is important to understand the role nutrition plays in both neuroprotection and neurodegeneration. Recent epidemiological studies have revealed the promise of some nutrients in reducing the risk of PD. In contrast, other nutrients may be involved with the etiology of neurodegeneration or exacerbate disease progression. This review summarizes the studies that have addressed these issues and describes in detail the nutrients and their putative mechanisms of action in PD.

Adenosine A2A receptor gene disruption protects in an α-synuclein model of Parkinson's disease

To investigate the putative interaction between chronic exposure to adenosine receptor antagonist caffeine and genetic influences on Parkinson's disease (PD), we determined whether deletion of the adenosine A(2A) receptor in knockout (KO) mice protects against dopaminergic neuron degeneration induced by a mutant human α-synuclein (hm(2)-αSYN) transgene containing both A53T and A30P. The A(2A) KO completely prevented loss of dopamine and dopaminergic neurons caused by the mutant α-synuclein transgene without altering levels of its expression. The adenosine A(2A) receptor appears required for neurotoxicity in a mutant α-synuclein model of PD. Together with prior studies the present findings indirectly support the neuroprotective potential of caffeine and more specific A(2A) antagonists.