A_2A receptor signaling drives cisplatin-mediated hippocampal neurotoxicity and cognitive defects in mice

Article summary

Chemotherapy mostly has a huge toll on patients’ quality of life, especially when regimens are prolonged. Among commonly reported side effects, such as fatigue and loss of appetite, defects in mood, memory, and learning affect about 14 million chemo-treated cancer patients. At present, there is no known therapeutic intervention for chemotherapy-induced cognitive impairments (CICI), largely because of its poorly understood molecular drivers. To better elucidate the underlying pathophysiology of CICI and pave the way for novel treatment strategies, Oliveros and colleagues [1] in a recent publication modelled CICI in mice with cisplatin administration and performed RNA sequencing on mouse hippocampus; a structure located in the brain temporal lobes, which is responsible for memory and learning. Unsurprisingly, they identified upregulated levels of A_2A receptor (A_2AR) both by qPCR and Western blot assays. Interestingly, cisplatin increased expressions of the adenosine G-protein-coupled receptor (GPCR) preferentially in hippocampal neurons. Since A_2AR signaling has been implicated in aging and neurodegenerative disease models, the authors reasoned that inhibition of A_2AR would attenuate cisplatin-induced cognitive impairments, further strengthening the causal role of A_2AR in CICI. Indeed, pharmacological antagonism of A_2AR with KW-6002 rescued cisplatin-induced defects in hippocampal neuron proliferation and dendrite length, arguably leading to the observed neuropsychological enhancement in mice mood, memory, and learning. Taken together, Oliveros et al. have set the stage, in part, for a better understanding of the molecular mechanism underlying cisplatin-induced cognitive dysfunction and to further explore the potential of targeting A_2AR as a novel therapeutic approach to treating CICI.

Commentary

Chemotherapy is the most widely used treatment modality for cancers. Unfortunately, up to 75% of cancer patients experience some form of cognitive impairments among several adverse effects during chemotherapy, and nearly 40% do so afterwards. Interestingly, dysfunction in cognition has even been reported in some patients prior to chemotherapy, suggesting a non-chemotherapy-related, cancer or psychological stress-linked form of cognitive defects [2, 3]. Evidently, the onset, duration, and degree of severity of CICI, also known as chemobrain or chemofog, depend on several factors, such as patient demographics, therapy adjuvants, length of treatment, and the type, dose, and administration route of the anticancer drug. Notwithstanding, the drug active agent and its mode of action remain the most explored parameters hypothesized to impair cognitive functions. Observations in animal models have demonstrated the deteriorating effect of alkylating agents, such as cisplatin and cyclophosphamide, on neuronal and myelination integrity, leading to impaired neurogenesis and neurodegeneration [4, 5]. A study using paclitaxel found significant alterations in long-term potentiation, demonstrating off-target effects of drugs on neurotransmission and ultimately on learning and memory [6]. Studies using the antimetabolite-based drugs, methotrexate (MTX) and 5-fluorouracil (5-FU), have greatly contributed to improving our understanding of possible mechanisms leading to CICI. Abnormalities in white matter have been closely followed as a potential tissue degenerative process underlying chemobrain. Clinical studies in breast cancer patients involving the use of MTX and 5-FU have hinted at reduced white matter integrity and its negative impact on cells like microglia and oligodendrocytes [7, 8]. Other proposed mechanisms include immune cell function dysregulation, oxidative stress, and cerebrovascular alterations, as demonstrated by drugs, such as rituximab, doxorubicin, and methotrexate, respectively.

Despite these strongly associated mechanisms hypothesized to cause CICI, a full understanding of the molecular contributors is still lacking and hence its therapeutic options. To give us insight into potential pathophysiological drivers leading to CICI, Oliveros and his team modelled CICI in female adult mice by i.p. administration of the platinum-based alkylating cancer drug, cisplatin, for 5 consecutive days, followed by 5 days of rest, for 3–4 treatment cycles and then searched for differentially expressed genes in the hippocampal tissues. RNA sequencing data revealed significant increments in adenosine A_2AR levels among the top five differentially upregulated genes. This observation was further confirmed by qRT-PCR and Western blot assays. Interestingly, elevated A_2AR expression was highest in hippocampal neurons, suggesting a more direct cisplatin-A_2AR neurotoxicity mechanism. Up to this point, the authors, for the first time, had identified A_2AR signaling as a potential pathway underlying cisplatin-induced cognitive defects, and that therapeutically targeting A_2AR could hold promise in treating CICI.

Adenosine receptor signaling is implicated in both physiological and pathological settings. Regarding the latter, signaling via A_2AR is almost always the culprit compared to its three sisters of the GPCR member family, playing significant roles in the pathogenesis of cancers and disorders of the immune, cardiovascular, and central nervous systems. Genetic deletion or pharmacological inhibition of A_2AR has been demonstrated to significantly improve tumor growth inhibition and abrogate tumor metastases in tumor-bearing mice [9]. Though small molecule blockade of A_2AR alone in aggressive tumors mostly yields minimal to no inhibition of tumor growth, its effects on infiltrating immune cells have always been substantial [10]. It is, therefore, no surprise that KW-6002-mediated inhibition of A_2AR in MCF-7 breast cancer-bearing immunodeficient mice showed no tumor growth enhancement or interrupted the action of cisplatin. Cisplatin is an alkylating agent with no known cross talk with immune cell-antitumor activity. However, cancer patients receiving NK- or CD8 + T-cell-based therapies, for example, alongside CICI treatment with A_2AR blockers could potentially enjoy synergistic benefits, as A_2AR blockade has been shown to markedly enhanced CD8 + T cells’ tumor infiltration and cytotoxicity and the maturation, granzyme B expression, and cytotoxic activities of NK cells [11, 12]. Data on immune cell regulation in immunocompetent mice would, therefore, be a good addition to the authors’ already excellent contribution.

Adenosine is a key homeostatic modulator in all cell types. In the central nervous system (CNS), however, adenosine and its signaling via A₁ and A_2A receptors offer additional neuromodulatory and -transmission roles [13]. Recent preclinical studies have revealed adenosine-mediated blood–brain barrier (BBB) permeability regulation in mouse models and with human brain primary endothelial cells [14, 15]. In fact, activation of A₁ and A_2A receptors expressed on BBB cells markedly promoted the entry of macromolecules and inflammatory cells into the CNS and vice versa. Could the relative deposit of cisplatin in brain tissues, and for that matter the hippocampus, leading to CICI be regulated by cisplatin-induced A_2AR expressed on BBB cells? I bet to imagine this could be. Consequently, an assay of adenosine receptor expression levels on BBB cells would give us better insight into this phenomenon and potential therapeutic strategies.

Brain adenosine homeostasis disturbances have been shown to negatively impact cognition and neuron health, leading to age-related and neurodegenerative disorders [16]. Significantly improved synaptic plasticity, learning and memory after pharmacological blockade, or genetic deletion of A_2AR in models of neurodegenerative diseases, such as PD and AD [17, 18], givesupport to the suggestion that cisplatin-mediated upregulation of A_2AR may impair cognition. To test this hypothesis, Oliveros and colleagues therapeutically antagonized A_2AR with KW-6002 after cisplatin administration and analyzed mice behavioral cognitive activities and protein modulation downstream of the A_2AR signaling pathway. The authors clearly demonstrated that blocking A_2AR reversed cisplatin-induced cognitive defects in mood, memory, and learning activities in mice, as well as impairments in hippocampal neurogenesis, strengthening a causal role for A_2AR in CICI. Adenosine A_2AR signaling increases intracellular cAMP levels leading to downstream activation of PKA, phosphorylation of CREB, and ultimate modulation of key cellular pathways. Increments in cAMP and pCREB levels suggest that cisplatin impairs cognitive function via receptor upregulation and increased activity downstream of the cAMP/pCREB axis, and that A_2AR upregulation may be epigenetically regulated by cisplatin [19].

Overall, the current study identifies A_2AR signaling as a key molecular pathway that can be therapeutically targeted to prevent CICI. In brief detail, the authors proved that cisplatin impairs hippocampal neurogenesis and dendrite morphogenesis, as well as major cognition functions, such as learning and memory, and that by antagonizing A_2AR signaling, these impairments could be markedly rescued. Further studies involving different chemotherapies with different mechanism of actions would help to elucidate other pathophysiological mechanisms driving CICI. Moreover, global and site-specific A_2AR KO mice models would be very instrumental in solidifying A_2AR role in CICI and its mechanistic regulation of specific cognitive activities.

Benjamin B. Gyau

graduated with a BSc (Hons) in Biochemistry from the Kwame Nkrumah University of Science and Technology in Ghana and MSc in Biomedical Sciences and in Medical Pharmaceutical Drug Innovation from the University of Chester, UK, and University of Groningen in the Netherlands, respectively. He recently completed a Marie-Curie funded Ph.D. program at the University of Turin, Italy, aimed at targeting the CD73-dependent nucleotide-metabolizing pathway in the tumor immune microenvironment (TIME). Benjamin is passionate about cancer immunotherapies and is keen on exploiting the TIME for innovative cancer treatment options.

Author contribution

BBG wrote the main manuscript text. Both authors reviewed the manuscript.

Funding

B. B. G. received funding from EU H2020 ITN INTEGRATA program (GA 813284).

Data availability

No data was generated for this article.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

This article does not contain any studies with human participants or animals performed by the authors.

Informed consent

Not applicable.

Conflict of interest

The authors declare no competing interests.