Boosting Endogenous Resistance of Brain to Ischemia

Interactions between the Autonomic Nervous System and the Immune System after Stroke

Acute stroke is one of the leading causes of morbidity and mortality worldwide. Stroke-induced immune-inflammatory response occurs in the perilesion areas and the periphery. Although stroke-induced immunosuppression may alleviate brain injury, it hinders brain repair as the immune-inflammatory response plays a bidirectional role after acute stroke. Furthermore, suppression of the systemic immune-inflammatory response increases the risk of life-threatening systemic bacterial infections after acute stroke. Therefore, it is essential to explore the mechanisms that underlie the stroke-induced immune-inflammatory response. Autonomic nervous system (ANS) activation is critical for regulating the local and systemic immune-inflammatory responses and may influence the prognosis of acute stroke. We review the changes in the sympathetic and parasympathetic nervous systems and their influence on the immune-inflammatory response after stroke. Importantly, this article summarizes the mechanisms on how ANS regulates the immune-inflammatory response through neurotransmitters and their receptors in immunocytes and immune organs after stroke. To facilitate translational research, we also discuss the promising therapeutic approaches modulating the activation of the ANS or the immune-inflammatory response to promote neurologic recovery after stroke. © 2022 American Physiological Society. Compr Physiol 12:3665-3704, 2022.

Changes in Cyclin D1, cdk4, and Their Associated Molecules in Ischemic Pyramidal Neurons in Gerbil Hippocampus after Transient Ischemia and Neuroprotective Effects of Ischemic Preconditioning by Keeping the Molecules in the Ischemic Neurons

Simple Summary

Cyclin D1 and cyclin-dependent kinase 4 (cdk4) is implicated in neuronal death induced by various pathological conditions. Ischemic preconditioning (IPC) confers neuroprotective effect, but underlying mechanisms have been poorly addressed. In this study, IPC protected pyramidal neurons (cells) in gerbil hippocampus after transient ischemia. Additionally, IPC controlled expressions of cyclin D1, cdk4, phosphorylated retinoblastoma (p-Rb), and E2 promoter binding factor 1 (E2F1). In particular, the expression of p16INK4a was not different by IPC. These findings indicate that cyclin D1/cdk4-related signals may play important roles in events in neurons related to damage/death following ischemic insults. Especially, the preservation of p16INK4a by IPC may be crucial in attenuating neuronal death/damage or protecting neurons after brain ischemic insults.

Abstract

Inadequate activation of cell cycle proteins including cyclin D1 and cdk4 is involved in neuronal cell death induced by diverse pathological stresses, including transient global brain ischemia. The neuroprotective effect of ischemic preconditioning is well-established, but the underlying mechanism is still unknown. In this study, we examined changes in cyclin D1, cdk4, and related molecules in cells or neurons located in Cornu Ammonis 1 (CA1) of gerbil hippocampus after transient ischemia for 5 min (ischemia and reperfusion) and investigated the effects of IPC on these molecules after ischemia. Four groups were used in this study as follows: sham group, ischemia group, IPC plus (+) sham group, and IPC+ischemia group. IPC was developed by inducing 2-min ischemia at 24 h before 5-min ischemia (real ischemia). Most pyramidal cells located in CA1 of the ischemia group died five days after ischemia. CA1 pyramidal cells in the IPC+ischemia group were protected. In the ischemia group, the expressions of cyclin D1, cdk4, phosphorylated retinoblastoma (p-Rb), and E2F1 (a transcription factor regulated by p-Rb) were significantly altered in the pyramidal cells with time after ischemia; in the IPC+ischemia group, they were controlled at the level shown in the sham group. In particular, the expression of p16^INK4a (an endogenous cdk inhibitor) in the ischemia group was reversely altered in the pyramidal cells; in the IPC+TI group, the expression of p16^INK4a was not different from that shown in the sham group. Our current results indicate that cyclin D1/cdk4-related signals may have important roles in events in neurons related to damage/death following ischemia and reperfusion. In particular, the preservation of p16^INK4a by IPC may be crucial in attenuating neuronal death/damage or protecting neurons after brain ischemic insults.

Activation of α7 nicotinic acetylcholine receptor ameliorates HIV-associated neurology and neuropathology

HIV-associated neurocognitive disorders (HAND) in the era of combination antiretroviral therapy are primarily manifested as impaired behaviours, glial activation/neuroinflammation and compromised neuronal integrity, for which there are no effective treatments currently available.

In the current study, we used doxycycline-inducible astrocyte-specific HIV Tat transgenic mice (iTat), a surrogate HAND model, and determined effects of PNU-125096, a positive allosteric modulator of α7 nicotinic acetylcholine receptor (α7 nAChR) on Tat-induced behavioural impairments and neuropathologies.

We showed that PNU-125096 treatment significantly improved locomotor, learning and memory deficits of iTat mice while inhibited glial activation and increased PSD-95 expression in the cortex and hippocampus of iTat mice. Using α7 nAChR knockout mice, we showed that α7 nAChR knockout eliminated the protective effects of PNU-125096 on iTat mice. In addition, we showed that inhibition of p38 phosphorylation by SB239063, a p38 MAPK-specific inhibitor exacerbated Tat neurotoxicity in iTat mice. Last, we used primary mouse cortical individual cultures and neuron-astrocytes co-cultures and in vivo staining of iTat mouse brain tissues and showed that glial activation was directly involved in the interplay among Tat neurotoxicity, α7 nAChR activation and the p38 MAPK signalling pathway.

Taken together, these findings demonstrated for the first time that α7 nAChR activation led to protection against HAND and suggested that α7 nAChR modulator PNU-125096 holds significant promise for development of therapeutics for HAND.

Immunosuppression and Neuroinflammation in Stroke Pathobiology

Over the preceding decades, there have been substantial advances in our knowledge of the pathophysiology of stroke. One such advance has been an increased understanding of the multifarious crosstalk in which the nervous and immune systems engage in order to maintain homeostasis. By interrupting the immune-nervous nexus, it is thought that stroke induces change in both systems. Additionally, it has been found that both innate and adaptive immunosuppression play protective roles against the effects of stroke. The release of danger-/damage-associated molecular patterns (DAMPs) activates Toll-like receptors (TLRs), contributing to the harmful inflammatory effects of ischemia/reperfusion injury after stroke; the Tyro3, Axl, and MerTK (TAM)/Gas6 system, however, has been shown to suppress inflammation via downstream signaling molecules that inhibit TLR signaling. Anti-inflammatory cytokines have also been found to promote neuroprotection following stroke. Additionally, adaptive immunosuppression merits further consideration as a potential endogenous protective mechanism. In this review, we highlight recent studies regarding the effects and mechanism of immunosuppression on the pathophysiology of stroke, with the hope that a better understanding of the function of both of innate and adaptive immunity in this setting will facilitate the development of effective therapies for post-stroke inflammation.

Systemic Administration of α7-Nicotinic Acetylcholine Receptor Ligands Does Not Improve Renal Injury or Behavior in Mice With Advanced Systemic Lupus Erythematosus

There is a critical need for safe treatment options to control inflammation in patients with systemic lupus erythematosus (SLE) since the inflammation contributes to morbidity and mortality in advanced disease. Endogenous neuroimmune mechanisms like the cholinergic anti-inflammatory pathway can be targeted to modulate inflammation, but the ability to manipulate such pathways and reduce inflammation and end organ damage has not been fully explored in SLE. Positive allosteric modulators (PAM) are pharmacological agents that inhibit desensitization of the nicotinic acetylcholine receptor (α7-nAChR), the main anti-inflammatory feature within the cholinergic anti-inflammatory pathway, and may augment α7-dependent cholinergic tone to generate therapeutic benefits in SLE. In the current study, we hypothesize that activating the cholinergic anti-inflammatory pathway at the level of the α7-nAChR with systemic administration of a partial agonist, GTS-21, and a PAM, PNU-120596, would reduce inflammation, eliminating the associated end organ damage in a mouse model of SLE with advanced disease. Further, we hypothesize that systemic α7 ligands will have central effects and improve behavioral deficits in SLE mice. Female control (NZW) and SLE mice (NZBWF1) were administered GTS-21 or PNU-120596 subcutaneously via minipumps for 2 weeks. We found that the increased plasma dsDNA autoantibodies, splenic and renal inflammation, renal injury and hypertension usually observed in SLE mice with advanced disease at 35 weeks of age were not altered by GTS-21 or PNU-120596. The anxiety-like behavior presented in SLE mice was also not improved by GTS-21 or PNU-120596. Although no significant beneficial effects of α7 ligands were observed in SLE mice at this advanced stage, we predict that targeting this receptor earlier in the pathogenesis of the disease may prove to be efficacious and should be addressed in future studies.

Therapeutic efficacy of α7 ligands after acute ischaemic stroke is linked to conductive states of α7 nicotinic ACh receptors

BACKGROUND AND PURPOSE

Targeting α7 nicotinic ACh receptors (nAChRs) in neuroinflammatory disorders including acute ischaemic stroke holds significant therapeutic promise. However, therapeutically relevant signalling mechanisms remain unidentified. Activation of neuronal α7 nAChRs triggers ionotropic signalling, but there is limited evidence for it in immunoglial tissues. The α7 ligands which are effective in reducing acute ischaemic stroke damage promote α7 ionotropic activity, suggesting a link between their therapeutic effects for treating acute ischaemic stroke and activation of α7 conductive states.

EXPERIMENTAL APPROACH

This hypothesis was tested using a transient middle cerebral artery occlusion (MCAO) model of acute ischaemic stroke, NS6740, a known selective non-ionotropic agonist of α7 nAChRs and 4OH-GTS-21, a partial α7 agonist. NS6740-like ligands exhibiting low efficacy/potency for ionotropic activity will be referred to as non-ionotropic agonists or "metagonists".

KEY RESULTS

4OH-GTS-21, used as a positive control, significantly reduced neurological deficits and brain injury after MCAO as compared to vehicle and NS6740. By contrast, NS6740 was ineffective in identical assays and reversed the effects of 4OH-GTS-21 when these compounds were co-applied. Electrophysiological recordings from acute hippocampal slices obtained from NS6740-injected animals demonstrated its remarkable brain availability and protracted effects on α7 nAChRs as evidenced by sustained (>8 h) alterations in α7 ionotropic responsiveness.

CONCLUSION AND IMPLICATIONS

These results suggest that α7 ionotropic activity may be obligatory for therapeutic efficacy of α7 ligands after acute ischaemic stroke yet, highlight the potential for selective application of α7 ligands to disease states based on their mode of receptor activation.

Opposite Pathways of Cholinergic Mechanisms of Hypoxic Preconditioning in the Hippocampus: Participation of Nicotinic α7 Receptors and Their Association with the Baseline Level of Startle Prepulse Inhibition

(1) Background. A one-time moderate hypobaric hypoxia (HBH) has a preconditioning effect whose neuronal mechanisms are not studied well. Previously, we found a stable correlation between the HBH efficiency and acoustic startle prepulse inhibition (PPI). This makes it possible to predict the individual efficiency of HBH in animals and to study its potential adaptive mechanisms. We revealed a bi-directional action of nicotinic α7 receptor agonist PNU-282987 and its solvent dimethyl sulfoxide on HBH efficiency with the level of PPI > or < 40%. (2) The aim of the present study was to estimate cholinergic mechanisms of HBH effects in different brain regions. (3) Methods: in rats pretested for PPI, we evaluated the activity of synaptic membrane-bound and water-soluble choline acetyltransferase (ChAT) in the sub-fractions of ‘light’ and ‘heavy’ synaptosomes of the neocortex, hippocampus and caudal brainstem in the intact brain and after HBH. We tested the dose-dependent influence of PNU-282987 on the HBH efficiency. (4) Results: PPI level and ChAT activity correlated negatively in all brain structures of the intact animals, so that the values of the latter were higher in rats with PPI < 40% compared to those with PPI > 40%. After HBH, this ChAT activity difference was leveled in the neocortex and caudal brainstem, while for membrane-bound ChAT in the ‘light’ synaptosomal fraction of hippocampus, it was reversed to the opposite. In addition, a pharmacological study revealed that PNU-282987 in all used doses and its solvent displayed corresponding opposite effects on HBH efficiency in rats with different levels of PPI. (5) Conclusion: We substantiate that in rats with low and high PPI two opposite hippocampal cholinergic mechanisms are involved in hypoxic preconditioning, and both are implemented by forebrain projections via nicotinic α7 receptors. Possible causes of association between general protective adaptation, HBH, PPI, forebrain cholinergic system and hippocampus are discussed.

Spleen is not required for therapeutic effects of 4OH-GTS-21, a selective α7 nAChR agonist, in the sub-acute phase of ischemic stroke in rats

Acute ischemic stroke (AIS) causes both central and peripheral inflammation, while activation of α7 nicotinic acetylcholine receptors (nAChRs) provides both central and peripheral anti-inflammatory and anti-apoptotic effects. Here, we provide evidence that 4OH-GTS-21, a selective α7 agonist, produces its therapeutic effects via primarily central sites of action because 4OH-GTS-21 was found equally effective in splenectomized and non-spenectomized rats in the sub-acute phase of ischemic stroke (≤1 week). However, the spleen may boost the therapeutic efficacy of 4OH-GTS-21 in certain behavioral tasks as our data also indicated. In our tests, AIS was modeled by transient middle cerebral artery occlusion (tMCAO). Splenectomy was done 2 weeks before tMCAO. We determined that: 1) Daily 4OH-GTS-21 treatments for 7 days after tMCAO significantly reduced neurological deficits and brain injury in both splenectomized and non-spelenectomized rats demonstrating that the spleen is not required for therapeutic benefits of 4OH-GTS-21; 2) The effects of 4OH-GTS-21 in the adhesive sticker removal test were significantly weaker in splenectomized animals suggesting that the spleen boosts the efficacy of 4OH-GTS-21 in the first week after tMCAO; and 3) Ischemic brain injury was not significantly affected by splenectomy in both vehicle-treated and 4OH-GTS-21-treated animals. These data support the hypothesis that the therapeutic efficacy of sub-chronic (≤1 week) 4OH-GTS-21 primarily originates from central sites of action. These results validate brain availability as a critical factor for developing novel α7 ligands for AIS.

The spleen may be an important target of stem cell therapy for stroke

Stroke is the most common cerebrovascular disease, the second leading cause of death behind heart disease and is a major cause of long-term disability worldwide. Currently, systemic immunomodulatory therapy based on intravenous cells is attracting attention. The immune response to acute stroke is a major factor in cerebral ischaemia (CI) pathobiology and outcomes. Over the past decade, the significant contribution of the spleen to ischaemic stroke has gained considerable attention in stroke research. The changes in the spleen after stroke are mainly reflected in morphology, immune cells and cytokines, and these changes are closely related to the stroke outcomes. Autonomic nervous system (ANS) activation, release of central nervous system (CNS) antigens and chemokine/chemokine receptor interactions have been documented to be essential for efficient brain-spleen cross-talk after stroke. In various experimental models, human umbilical cord blood cells (hUCBs), haematopoietic stem cells (HSCs), bone marrow stem cells (BMSCs), human amnion epithelial cells (hAECs), neural stem cells (NSCs) and multipotent adult progenitor cells (MAPCs) have been shown to reduce the neurological damage caused by stroke. The different effects of these cell types on the interleukin (IL)-10, interferon (IFN), and cholinergic anti-inflammatory pathways in the spleen after stroke may promote the development of new cell therapy targets and strategies. The spleen will become a potential target of various stem cell therapies for stroke represented by MAPC treatment.

Treatment duration affects cytoprotective efficacy of positive allosteric modulation of α7 nAChRs after focal ischemia in rats

To minimize irreversible brain injury after acute ischemic stroke (AIS), the time to treatment (i.e., treatment delay) should be minimized. However, thus far, all cytoprotective clinical trials have failed. Analysis of literature identified short treatment durations (≤72 h) as a common motif among completed cytoprotective clinical trials. Here, we argue that short cytoprotective regimens even if given early after AIS may only slow down the evolution of ischemic brain injury and fail to deliver sustained long-term solutions leading to relapses that may be misinterpreted for conceptual failure of cytoprotection. In this randomized blinded study, we used young adult male rats subjected to transient 90 min suture middle cerebral artery occlusion (MCAO) and treated with acute vs. sub-chronic regimens of PNU120596, a prototypical positive allosteric modulator of α7 nicotinic acetylcholine receptors with anti-inflammatory cytoprotective properties to test the hypothesis that insufficient treatment durations may reduce therapeutic benefits of otherwise efficacious cytoprotectants after AIS. A single acute treatment 90 min after MCAO significantly reduced brain injury and neurological deficits 24 h later, but these effects vanished 72 h after MCAO. These relapses were avoided by utilizing sub-chronic treatments. Thus, extending treatment duration augments therapeutic efficacy of PNU120596 after MCAO. Furthermore, sub-chronic treatments could offset the negative effects of prolonged treatment delays in cases where the acute treatment window after MCAO was left unexploited. We conclude that a combination of short treatment delays and prolonged treatment durations may be required to maximize therapeutic effects of PNU120596, reduce relapses and ensure sustained therapeutic efficacy after AIS. Similar concepts may hold for other cytoprotectants including those that failed in clinical trials.

Cholinergic modulation of the immune system presents new approaches for treating inflammation

The nervous system and immune system have broad and overlapping distributions in the body, and interactions of these ubiquitous systems are central to the field of neuroimmunology. Over the past two decades, there has been explosive growth in our understanding of neuroanatomical, cellular, and molecular mechanisms that mediate central modulation of immune functions through the autonomic nervous system. A major catalyst for growth in this field was the discovery that vagal nerve stimulation (VNS) caused a prominent attenuation of the systemic inflammatory response evoked by endotoxin in experimental animals. This effect was mediated by acetylcholine (ACh) stimulation of nicotinic receptors on splenic macrophages. Hence, the circuit was dubbed the "cholinergic anti-inflammatory pathway". Subsequent work identified the α7 nicotinic ACh receptor (α7nAChR) as the crucial target for attenuation of pro-inflammatory cytokine release from macrophages and dendritic cells. Further investigation made the important discovery that cholinergic T cells within the spleen and not cholinergic nerve cells were the source of ACh that stimulated α7 receptors on splenic macrophages. Given the important role that inflammation plays in numerous disease processes, cholinergic anti-inflammatory mechanisms are under intensive investigation from a basic science perspective and in translational studies of animal models of diseases such as inflammatory bowel disease and rheumatoid arthritis. This basic work has already fostered several clinical trials examining the efficacy of VNS and cholinergic therapeutics in human inflammatory diseases. This review provides an overview of basic and translational aspects of the cholinergic anti-inflammatory response and relevant pharmacology of drugs acting at the α7nAChR.