Neuronal alarmin IL-1α evokes astrocyte-mediated protective signals: Effectiveness in chemotherapy-induced neuropathic pain

Neurons as Immunomodulators: From Rapid Neural Activity to Prolonged Regulation of Cytokines and Microglia

Regulation of the brain's neuroimmune system is central to development, normal function, and disease. Neuronal communication to microglia, the primary immune cells of the brain, is well known to involve purinergic signaling mediated via ATP secretion and the cytokine fractalkine. Recent evidence shows that neurons release multiple cytokines beyond fractalkine, yet these are less studied and poorly understood. In contrast to ATP, cytokines are a class of signaling molecule that are much larger, with longer signaling and farther diffusion. We posit that neuron-expressed cytokines are an essential mechanism of neuron-microglia communication that arises as part of both normal learning and memory and in response to tissue pathology. Thus, neurons are underappreciated immunomodulatory cells that express diverse immunomodulatory signals. While neuronally sourced cytokines have been understudied, new technical advances make this a timely topic. The goal of this review is to define what is known about the cytokines expressed from neurons, how they are regulated, and the effects of these cytokines on microglia. We delineate key knowledge gaps and needs for new tools to define and analyze neuronal roles in immunomodulation. Given that cytokines are central regulators of microglial function, a broad new body of work is required to illuminate functional links between these neuronally expressed cytokines and sustained and transient microglial function.

The Power of Movement: How Exercise Influences Chemotherapy-Induced Peripheral Neuropathy

As the number of cancer patients and survivors increases, we face a rising challenge: the long-term impact of the adverse effects of cancer treatment. One of the known adverse effects is chemotherapy-induced peripheral neuropathy (CIPN), which courses with pain complaints. The treatments of CIPN have reduced efficacy. The neurobiological causes of CIPN have been mainly ascribed to peripheral nerve damage, but recent studies show effects in the brain, namely in the descending pain modulatory systems. Physical exercise seems to be associated with better outcomes in CIPN patients, but the mechanisms underlying the effects have not been discussed, namely considering the recent results of the effects of CIPN in brain structures involved in pain modulation. In this critical review, we propose that the beneficial effects of exercise in CIPN also have central mechanisms, namely neuroinflammation and oxidative stress, as well as changes in the actions of neurotransmitters and neurotrophic factors, with a direct effect on optimizing the endogenous pain modulation, namely opioids, monoamines, and endocannabinoids. The effects are multifactorial, as mood improvement and the other psychological benefits of exercise should be considered. The emerging role of the microbiome, which is affected during CIPN, also needs to be considered. This review critically synthesizes the available literature to highlight how the neurobiological effects of physical exercise make it a promising strategy for managing CIPN, both from preventive and treatment perspectives.

Sex Differences in the Regulation of Interleukins in Chronic Pain: A Widely Recognized but Difficult-to-Tackle Factor

Chronic pain is an extremely prevalent healthcare issue that has a profound impact on individuals and society. Sex and sex hormones regulate the pain threshold differently in males and females in pain processing. However, the regulatory mechanisms of sex differences in response to painful stimuli are still unclear, which contributes to the difficulty of analgesic drug development. Interleukins mediate neuroinflammation and are involved in the development of chronic pain. Recent studies have found that sex and sex hormones are involved in the regulation of pain thresholds by interleukins. Most previous studies used male animals to study the analgesic effects of treatments due to the complexity of estrogen. This review summarizes studies that used only female animals or both sexes to examine the impact of sex on interleukin-regulated chronic pain, to provide a theoretical basis for the development of more targeted precision medicines for pain.

Astrocytes in aging

The mammalian nervous system is impacted by aging. Aging alters brain architecture, is associated with molecular damage, and can manifest with cognitive and motor deficits that diminish the quality of life. Astrocytes are glial cells of the CNS that regulate the development, function, and repair of neural circuits during development and adulthood; however, their functions in aging are less understood. Astrocytes change their transcriptome during aging, with astrocytes in areas such as the cerebellum, the hypothalamus, and white matter-rich regions being the most affected. While numerous studies describe astrocyte transcriptional changes in aging, many questions still remain. For example, how is astrocyte function altered by transcriptional changes that occur during aging? What are the mechanisms promoting astrocyte aged states? How do aged astrocytes impact brain function? This review discusses features of aged astrocytes and their potential triggers and proposes ways in which they may impact brain function and health span.

Control of Dopamine Signal in High-Order Receptor Complex on Striatal Astrocytes

The receptor-receptor interaction (RRI) of G protein-coupled receptors (GPCRs) leads to new functional entities that are conceptually distinct from the simple addition of signals mediated by the activation of the receptors that form the heteromers. Focusing on astrocytes, there is evidence for the existence of inhibitory and facilitatory RRIs, including the heteromers formed by the adenosine A2A and the dopamine D2 receptors, by A2A and the oxytocin receptor (OTR), and the D2-OTR heteromers. The possible involvement of these receptors in mosaicism has never been investigated in striatal astrocytes. By biophysical and functional approaches, we focused our attention on the existence of an A2A-D2-OTR high-order receptor complex and its role in modulating cytosolic calcium levels and endogenous glutamate release, when striatal astrocyte processes were stimulated with 4-aminopyridine. Functional data indicate a permissive role of OTR on dopamine signaling in the regulation of the glutamatergic transmission, and an inhibitory control mediated by A2A on both the D2-mediated signaling and on the OTR-facilitating effect on D2. Imaging biochemical and bioinformatic evidence confirmed the existence of the A2A-D2-OTR complex and its ternary structure in the membrane. In conclusion, the D2 receptor appears to be a hotspot in the control of the glutamate release from the astrocytic processes and may contribute to the regulation and integration of different neurotransmitter-mediated signaling in the striatum by the A2A-D2-OTR heterotrimers. Considering the possible selectivity of allosteric interventions on GPCRs organized as receptor mosaics, A2A-D2-OTR heterotrimers may offer selective pharmacological targets in neuropsychiatric disorders and neurodegenerative diseases.

Profiling the neuroimmune cascade in 3xTg-AD mice exposed to successive mild traumatic brain injuries

Repetitive mild traumatic brain injuries (rmTBI) sustained within a window of vulnerability can result in long term cognitive deficits, depression, and eventual neurodegeneration associated with tau pathology, amyloid beta (Aβ) plaques, gliosis, and neuronal and functional loss. However, a comprehensive study relating acute changes in immune signaling and glial reactivity to neuronal changes and pathological markers after single and repetitive mTBIs is currently lacking. In the current study, we addressed the question of how repeated injuries affect the brain neuroimmune response in the acute phase of injury (< 24 h) by exposing the 3xTg-AD mouse model of tau and Aβ pathology to successive (1x-5x) once-daily weight drop closed-head injuries and quantifying immune markers, pathological markers, and transcriptional profiles at 30 min, 4 h, and 24 h after each injury. We used young adult 2-4 month old 3xTg-AD mice to model the effects of rmTBI in the absence of significant tau and Aβ pathology. We identified pronounced sexual dimorphism in this model, with females eliciting more diverse changes after injury compared to males. Specifically, females showed: (1) a single injury caused a decrease in neuron-enriched genes inversely correlated with inflammatory protein expression and an increase in AD-related genes within 24 h, (2) each injury significantly increased a group of cortical cytokines (IL-1α, IL-1β, IL-2, IL-9, IL-13, IL-17, KC) and MAPK phospho-proteins (phospho-Atf2, phospho-Mek1), several of which co-labeled with neurons and correlated with phospho-tau, and (3) repetitive injury caused increased expression of genes associated with astrocyte reactivity and macrophage-associated immune function. Collectively our data suggest that neurons respond to a single injury within 24 h, while other cell types, including astrocytes, transition to inflammatory phenotypes within days of repetitive injury.

Current understanding of the molecular mechanisms of chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is the most common off-target adverse effects caused by various chemotherapeutic agents, such as cisplatin, oxaliplatin, paclitaxel, vincristine and bortezomib. CIPN is characterized by a substantial loss of primary afferent sensory axonal fibers leading to sensory disturbances in patients. An estimated of 19-85% of patients developed CIPN during the course of chemotherapy. The lack of preventive measures and limited treatment options often require a dose reduction or even early termination of life-saving chemotherapy, impacting treatment efficacy and patient survival. In this Review, we summarized the current understanding on the pathogenesis of CIPN. One prominent change induced by chemotherapeutic agents involves the disruption of neuronal cytoskeletal architecture and axonal transport dynamics largely influenced by the interference of microtubule stability in peripheral neurons. Due to an ineffective blood-nerve barrier in our peripheral nervous system, exposure to some chemotherapeutic agents causes mitochondrial swelling in peripheral nerves, which lead to the opening of mitochondrial permeability transition pore and cytochrome c release resulting in degeneration of primary afferent sensory fibers. The exacerbated nociceptive signaling and pain transmission in CIPN patients is often linked the increased neuronal excitability largely due to the elevated expression of various ion channels in the dorsal root ganglion neurons. Another important contributing factor of CIPN is the neuroinflammation caused by an increased infiltration of immune cells and production of inflammatory cytokines. In the central nervous system, chemotherapeutic agents also induce neuronal hyperexcitability in the spinal dorsal horn and anterior cingulate cortex leading to the development of central sensitization that causes CIPN. Emerging evidence suggests that the change in the composition and diversity of gut microbiota (dysbiosis) could have direct impact on the development and progression of CIPN. Collectively, all these aspects contribute to the pathogenesis of CIPN. Recent advances in RNA-sequencing offer solid platform for in silico drug screening which enable the identification of novel therapeutic agents or repurpose existing drugs to alleviate CIPN, holding immense promises for enhancing the quality of life for cancer patients who undergo chemotherapy and improve their overall treatment outcomes.

Identifying circRNA-miRNA-mRNA Regulatory Networks in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a frequent and severe side effect of first-line chemotherapeutic agents. The association between circular RNAs (circRNAs) and CIPN remains unclear. In this study, CIPN models were constructed with Taxol, while 134 differentially expressed circRNAs, 353 differentially expressed long non-coding RNAs, and 86 differentially expressed messenger RNAs (mRNAs) were identified utilizing RNA sequencing. CircRNA-targeted microRNAs (miRNAs) were predicted using miRanda, and miRNA-targeted mRNAs were predicted using TargetScan and miRDB. The intersection of sequencing and mRNA prediction results was selected to establish the circRNA-miRNA-mRNA networks, which include 15 circRNAs, 18 miRNAs, and 11 mRNAs. Functional enrichment pathway analyses and immune infiltration analyses revealed that differentially expressed mRNAs were enriched in the immune system, especially in T cells, monocytes, and macrophages. Cdh1, Satb2, Fas, P2ry2, and Zfhx2 were further identified as hub genes and validated by RT-qPCR, correlating with macrophages, plasmacytoid dendritic cells, and central memory CD4 T cells in CIPN. Additionally, we predicted the associated diseases, 36 potential transcription factors (TFs), and 30 putative drugs for hub genes using the DisGeNET, TRRUST, and DGIdb databases, respectively. Our results indicated the crucial role of circRNAs, and the immune microenvironment played in CIPN, providing novel insights for further research.

Profiling the neuroimmune cascade in 3xTg mice exposed to successive mild traumatic brain injuries

Repetitive mild traumatic brain injuries (rmTBI) sustained within a window of vulnerability can result in long term cognitive deficits, depression, and eventual neurodegeneration associated with tau pathology, amyloid beta (Aβ) plaques, gliosis, and neuronal and functional loss. However, we have limited understanding of how successive injuries acutely affect the brain to result in these devastating long-term consequences. In the current study, we addressed the question of how repeated injuries affect the brain in the acute phase of injury (<24hr) by exposing the 3xTg-AD mouse model of tau and Aβ pathology to successive (1x, 3x, 5x) once-daily weight drop closed-head injuries and quantifying immune markers, pathological markers, and transcriptional profiles at 30min, 4hr, and 24hr after each injury. We used young adult mice (2-4 months old) to model the effects of rmTBI relevant to young adult athletes, and in the absence of significant tau and Aβ pathology. Importantly, we identified pronounced sexual dimorphism, with females eliciting more differentially expressed proteins after injury compared to males. Specifically, females showed: 1) a single injury caused a decrease in neuron-enriched genes inversely correlated with inflammatory protein expression as well as an increase in AD-related genes within 24hr, 2) each injury significantly increased expression of a group of cortical cytokines (IL-1α, IL-1β, IL-2, IL-9, IL-13, IL-17, KC) and MAPK phospho-proteins (phospho-Atf2, phospho-Mek1), several of which were co-labeled with neurons and correlated with phospho-tau, and 3) repetitive injury caused increased expression of genes associated with astrocyte reactivity and immune function. Collectively our data suggest that neurons respond to a single injury within 24h, while other cell types including astrocytes transition to inflammatory phenotypes within days of repetitive injury.

Heteromerization of Dopamine D2 and Oxytocin Receptor in Adult Striatal Astrocytes

The ability of oxytocin (OT) to interact with the dopaminergic system through facilitatory D2-OT receptor (OTR) receptor-receptor interaction in the limbic system is increasingly considered to play roles in social or emotional behavior, and suggested to serve as a potential therapeutic target. Although roles of astrocytes in the modulatory effects of OT and dopamine in the central nervous system are well recognized, the possibility of D2-OTR receptor-receptor interaction in astrocytes has been neglected. In purified astrocyte processes from adult rat striatum, we assessed OTR and dopamine D2 receptor expression by confocal analysis. The effects of activation of these receptors were evaluated in the processes through a neurochemical study of glutamate release evoked by 4-aminopyridine; D2-OTR heteromerization was assessed by co-immunoprecipitation and proximity ligation assay (PLA). The structure of the possible D2-OTR heterodimer was estimated by a bioinformatic approach. We found that both D2 and OTR were expressed on the same astrocyte processes and controlled the release of glutamate, showing a facilitatory receptor-receptor interaction in the D2-OTR heteromers. Biochemical and biophysical evidence confirmed D2-OTR heterodimers on striatal astrocytes. The residues in the transmembrane domains four and five of both receptors are predicted to be mainly involved in the heteromerization. In conclusion, roles for astrocytic D2-OTR in the control of glutamatergic synapse functioning through modulation of astrocytic glutamate release should be taken into consideration when considering interactions between oxytocinergic and dopaminergic systems in striatum.

Melatonin and metformin counteract cognitive dysfunction equally in male rats with doxorubicin-induced chemobrain

Melatonin (Mel) and metformin (Met) show beneficial effects in various brain pathologies. However, the effects of Mel and Met on doxorubicin (DOX)-induced chemobrain remain in need of elucidation. We aimed to investigate whether Mel and Met provide neuroprotective effects on glial dysmorphologies, brain inflammation, oxidative stress, brain mitochondrial dysfunction, apoptosis, necroptosis, neurogenesis, hippocampal dysplasticity, and cognitive dysfunction in rats with DOX-induced chemobrain. Thirty-two male Wistar rats were divided into 2 groups and received normal saline (NSS, as control, n = 8) or DOX (3 mg/kg/day; n = 24) by intraperitoneal (i.p.) injection on days 0, 4, 8, 15, 22, and 29. The DOX-treated group was divided into 3 subgroups receiving either vehicle (NSS; n = 8), Mel (10 mg/kg/day; n = 8), or Met (250 mg/kg/day; n = 8) by gavage for 30 consecutive days. Following this, cognitive function was assessed in all rats. The number of glial cells and their fluorescence intensity had decreased, while the glial morphology in DOX-treated rats showed a lower process complexity. Brain mitochondrial dysfunction, an increase in brain inflammation, oxidative stress, apoptosis and necroptosis, a decrease in the number of hippocampal dendritic spines and neurogenesis, and cognitive decline were also observed in DOX-treated rats. Mel and Met equally improved those brain pathologies, resulting in cognitive improvement in DOX-treated rats. In conclusion, concomitant treatment with either Mel or Met counteract DOX-induced chemobrain by preservation of glial morphology, brain inflammation, brain oxidative stress, brain mitochondrial function, hippocampal plasticity, and brain apoptosis. This study highlighted the role of the glia as key mediators in DOX-induced chemobrain.

Characterization of the Dahl salt-sensitive rat as a rodent model of inherited, widespread, persistent pain

Animal models are essential for studying the pathophysiology of chronic pain disorders and as screening tools for new therapies. However, most models available do not reproduce key characteristics of clinical persistent pain. This has limited their ability to accurately predict which new medicines will be clinically effective. Here, we characterize the Dahl salt-sensitive (SS) rat strain as the first rodent model of inherited widespread hyperalgesia. We show that this strain exhibits physiological phenotypes known to contribute to chronic pain, such as neuroinflammation, defective endogenous pain modulation, dysfunctional hypothalamic-pituitary-adrenal axis, increased oxidative stress and immune cell activation. When compared with Sprague Dawley and Brown Norway rats, SS rats have lower nociceptive thresholds due to increased inflammatory mediator concentrations, lower corticosterone levels, and high oxidative stress. Treatment with dexamethasone, the reactive oxygen species scavenger tempol, or the glial inhibitor minocycline attenuated the pain sensitivity in SS rats without affecting the other strains while indomethacin and gabapentin provided less robust pain relief. Moreover, SS rats presented impaired diffuse noxious inhibitory controls and an exacerbated response to the proalgesic mediator PGE₂, features of generalized pain conditions. These data establish this strain as a novel model of spontaneous, widespread hyperalgesia that can be used to identify biomarkers for chronic pain diagnosis and treatment.