Abstract Traumatic brain injury (TBI) is a chronic and debilitating disease, associated with a high risk of psychiatric and neurodegenerative diseases. Despite significant advancements in improving outcomes, the lack of effective treatments underscore the urgent need for innovative therapeutic strategies. The brain-gut axis has emerged as a crucial bidirectional pathway connecting the brain and the gastrointestinal (GI) system through an intricate network of neuronal, hormonal, and immunological pathways. Four main pathways are primarily implicated in this crosstalk, including the systemic immune system, autonomic and enteric nervous systems, neuroendocrine system, and microbiome. TBI induces profound changes in the gut, initiating an unrestrained vicious cycle that exacerbates brain injury through the brain-gut axis. Alterations in the gut include mucosal damage associated with the malabsorption of nutrients/electrolytes, disintegration of the intestinal barrier, increased infiltration of systemic immune cells, dysmotility, dysbiosis, enteroendocrine cell (EEC) dysfunction and disruption in the enteric nervous system (ENS) and autonomic nervous system (ANS). Collectively, these changes further contribute to brain neuroinflammation and neurodegeneration via the gut-brain axis. In this review article, we elucidate the roles of various anti-inflammatory pharmacotherapies capable of attenuating the dysregulated inflammatory response along the brain-gut axis in TBI. These agents include hormones such as serotonin, ghrelin, and progesterone, ANS regulators such as beta-blockers, lipid-lowering drugs like statins, and intestinal flora modulators such as probiotics and antibiotics. They attenuate neuroinflammation by targeting distinct inflammatory pathways in both the brain and the gut post-TBI. These therapeutic agents exhibit promising potential in mitigating inflammation along the brain-gut axis and enhancing neurocognitive outcomes for TBI patients. 

Dysregulated brain-gut axis in the setting of traumatic brain injury: review of mechanisms and anti-inflammatory pharmacotherapies

The non-neural cholinergic system plays a critical role in regulating immune equilibrium and tissue homeostasis. While the expression of choline acetyltransferase (ChAT), the enzyme catalyzing acetylcholine biosynthesis, has been well documented in lymphocytes, its role in the myeloid compartment is less understood. Here, we identify a significant population of macrophages (Mϕs) expressing ChAT and synthesizing acetylcholine in the resolution phase of acute peritonitis. Using Chat -GFP reporter mice, we observed marked upregulation of ChAT in monocyte-derived small peritoneal Mϕs (SmPMs) in response to Toll-like receptor agonists and bacterial infections. These SmPMs, phenotypically and transcriptionally distinct from tissue-resident large peritoneal macrophages, up-regulated ChAT expression through a MyD88-dependent pathway involving MAPK signaling. Notably, this process was attenuated by the TRIF-dependent TLR signaling pathway, and our tests with a range of neurotransmitters and cytokines failed to induce a similar response. Functionally, Chat deficiency in Mϕs led to significantly decreased peritoneal acetylcholine levels, reduced efferocytosis of apoptotic neutrophils, and a delayed resolution of peritonitis, which were reversible with exogenous ACh supplementation. Intriguingly, despite B lymphocytes being a notable ChAT-expressing population within the peritoneal cavity, Chat deletion in B cells did not significantly alter the resolution process. Collectively, these findings underscore the crucial role of Mϕ-derived acetylcholine in the resolution of inflammation and highlight the importance of the non-neuronal cholinergic system in immune regulation. 

Cholinergic macrophages promote the resolution of peritoneal inflammation

Loewi’s discovery of acetylcholine (ACh) release from the frog vagus nerve and the discovery by Dale and Dudley of ACh in ox spleen led to the demonstration of chemical transmission of nerve impulses. ACh is now well-known to function as a neurotransmitter. However, advances in the techniques for ACh detection have led to its discovery in many lifeforms lacking a nervous system, including eubacteria, archaea, fungi, and plants. Notably, mRNAs encoding choline acetyltransferase and muscarinic and nicotinic ACh receptors (nAChRs) have been found in uninnervated mammalian cells, including immune cells, keratinocytes, vascular endothelial cells, cardiac myocytes, respiratory, and digestive epithelial cells. It thus appears that non-neuronal cholinergic systems are expressed in a variety of mammalian cells, and that ACh should now be recognized not only as a neurotransmitter, but also as a local regulator of non-neuronal cholinergic systems. Here, we discuss the role of non-neuronal cholinergic systems, with a focus on immune cells. A current focus of much research on non-neuronal cholinergic systems in immune cells is α7 nAChRs, as these receptors expressed on macrophages and T cells are involved in regulating inflammatory and immune responses. This makes α7 nAChRs an attractive potential therapeutic target.

Contributions of Non-Neuronal Cholinergic Systems to the Regulation of Immune Cell Function, Highlighting the Role of α7 Nicotinic Acetylcholine Receptors

High Mobility Group Box 1 (HMGB1) is a nuclear protein that upon extracellular release acts as an alarmin to initiate and amplify inflammation. HMGB1 release from nociceptors contributes to both inflammation and pain; however, the mechanisms for its regulation remain incompletely understood. The cholinergic anti-inflammatory pathway, mediated by α7 nicotinic acetylcholine receptor (α7nAChR) activation, inhibits HMGB1 release from immune cells and reduces inflammation. This study investigates whether α7nAChR signaling similarly inhibits HMGB1 release from nociceptors, thereby affecting pain and inflammation. Dorsal root ganglia (DRG) neurons were isolated from C57BL/6 or VGlut2-Cre/ChR2-YFP mice (expressing ChR2 in sensory neurons for optogenetic stimulation at 470 nm). HMGB1 release in vitro was triggered by optogenetic stimulation or exposure to capsaicin (5 µM), in the presence or absence of cholinergic agonists (acetylcholine, GTS-21, PNU-282987), and subsequently measured by ELISA. Immunohistochemistry was used to visualize cellular HMGB1 localization. In vivo models, including optogenetic stimulation and formalin-induced pain-like behavior, were used to evaluate the effects of cholinergic agonists on pain-like behavior, mechanical allodynia and inflammation. α7nAChR knockout (KO) mice served to determine receptor-specific effects. Levels of proinflammatory mediators calcitonin gene-related peptide (CGRP), substance P, HMGB1, and IL-6 were also measured. Optogenetic stimulation of cultured DRG neurons significantly increased HMGB1 release, which was markedly inhibited by cholinergic agonists. Similarly, capsaicin-induced HMGB1 release was suppressed by acetylcholine, GTS-21, and PNU-282987, promoting HMGB1 retention within the nucleus; this effect was abolished in α7nAChR KO neurons. In contrast, the release of CGRP and substance P following optogenetic or capsaicin stimulation of DRG neurons from wild-type mice was not influenced by cholinergic agonists. In vivo, GTS-21 reduced pain-like behaviors and mechanical allodynia in both the formalin-induced and optogenetically-stimulated nociceptive behavior models, as demonstrated by reduced mechanical allodynia and extracellular HMGB1 levels. These effects were absent in α7nAChR KO mice, confirming the critical role of α7nAChR in mediating these responses. This study reveals a novel α7nAChR-dependent cholinergic mechanism that reduces nociceptive behavior and inflammation by retaining nuclear HMGB1 in nociceptors. Cholinergic agonists may serve as promising therapeutic agents to mitigate nociceptive behavior and inflammation by targeting α7nAChR in sensory neurons.

pdf/nociceptor-a7nachr-activation-blunts-neuronal-hmgb1-release-bnrrelah04n8.pdf

Nociceptor α7nAChR activation blunts neuronal HMGB1 release and attenuates inflammation and nociceptive behavior

Despite the advent of new treatment strategies, cholinesterase inhibitors (ChEIs) are still the go-to treatment for dementia disorders. ChEIs act by inhibiting the main acetylcholine-degrading enzyme, acetylcholinesterase (AChE). Nonetheless, accumulating evidence indicates that the impact of inhibition of the sister enzyme, butyrylcholinesterase (BChE), could be even broader in older adults due to the multifaceted role of BChE in several biological functional pathways. Therefore, we employed an in silico modeling-based drug repurposing strategy to identify novel potent BChE inhibitors from the FDA drug database. This was followed by in vitro screening and ex vivo enzyme kinetic validation using human plasma samples as the source of BChE. The analysis revealed that the antidepressant drug, duloxetine, inhibited BChE with high selectivity in comparison to AChE. In contrast, two other antidepressants, namely, citalopram and escitalopram exhibited a weak to moderate activity. Ex vivo enzyme inhibition kinetic analyses indicated that duloxetine acted as a competitive inhibitor of BChE with an inhibition constant (Ki) of 210 nM. This Ki value is comparable with 100–400 nM concentration of duloxetine following normal dosages in humans, thereby indicating that duloxetine should be able to induce a pharmacologically and biologically relevant in vivo inhibition of BChE. Additionally, we performed the enzyme inhibition kinetic assessment in parallel for ethopropazine, a known potent selective BChE inhibitor, and physostigmine, a dual inhibitor of AChE and BChE. These analyses indicated that duloxetine should be considered a potent BChE inhibitor since its Ki was comparable with ethopropazine (Ki = 150 nM) but was 4 times smaller than that of physostigmine (Ki = 840 nM). In conclusion, this study reports the discovery of duloxetine being a highly potent selective competitive BChE inhibitor. This, in turn, indicates that duloxetine could be the choice of antidepressive treatment in older adults with both depressive and dementia symptoms since it may offer additional clinically beneficial effects via this secondary mode of cholinergic enhancing action. 

Repurposing Duloxetine as a Potent Butyrylcholinesterase Inhibitor: Potential Cholinergic Enhancing Benefits for Elderly Individuals with Depression and Cognitive Impairment

Choline acetyltransferase (ChAT) is required for the biosynthesis of acetylcholine, the molecular mediator that inhibits cytokine production in the cholinergic anti‐inflammatory pathway of the vagus nerve inflammatory reflex. Abundant work has established the biology of cytoplasmic ChAT in neurons, but much less is known about the potential presence and function of ChAT in the extracellular milieu.

Circulating extracellular choline acetyltransferase regulates inflammation.

Inflammatory bowel disease (IBD), including Crohn’s disease and ulcerative colitis, affects up to 3 million people in the United States alone, significantly worsens a patient’s quality of life, and is associated with a high financial burden. Current treatments for IBD are mainly biologicals that have significant side effects. No efficient pharmacological treatments are currently available for IBD. Previous studies established that activation of the vagus nerve‐based inflammatory reflex inhibits cytokine production and regulates immune responses through cholinergic signaling. Acetylcholine released by a T cell subset expressing choline acetyltransferase (ChAT), the enzyme that catalyzes the biosynthesis of acetylcholine, inhibits cytokine production via signaling through α7 nicotinic acetylcholine receptors (α7nAChR). However, the possibility of controlling inflammation by systemic administration of ChAT is unexplored. Here we examined the efficacy of ChAT in improving disease activity in a preclinical model of colitis. To prolong the half‐life of circulating ChAT, the protein was modified by covalently attaching repeating units of polyethylene glycol (PEG), resulting in enzymatically active PEG‐ChAT. We utilized a well‐established model of colitis ‐ dextran sulfate sodium induced colitis. Wild type C57/black 6 mice were exposed to 4% DSS (M.W 36 000‐50 000) in drinking water for 7 days prior to administration of PEG‐ChAT. Starting on day 8, animals received PEG‐ChAT or vehicle intraperitoneally twice per day for 7 days. PEG‐ ChAT treated mice showed a significantly lower disease activity index as compared with vehicle treated controls with the disease (d11 p=0.006 ChAT mice: 0.17DAI±0.05, n=9 versus vehicle mice: 0.66DAI±0.13, n=10). The decreased severity of colitis in PEG‐ChAT‐treated mice as compared with vehicle‐treated controls is further demonstrated by a significant improvement in body weight gain (d14 p=0.01 ChAT mice: ‐5.1%±1.1, n=10 versus vehicle mice: ‐15.5%±2.9, n=10) and colon length (p=0.006 ChAT mice: 7.15cm±0.2, n=10 versus vehicle mice: 6.2cm±0.2, n=10). In conclusion, PEG‐ChAT reduces disease activity in a preclinical model of colitis. Together these studies provide new insights into the role of PEG‐ChAT as an experimental anti‐inflammatory therapeutic modality for the treatment of IBD.

Protective Effects of Pegylated Choline Acetyltransferase in a Murine Model of DSS Colitis


 Excessive immune cell activation and cytokine release leading to inflammatory conditions are associated with bidirectional immune system-brain communication and other physiological responses. The vagus nerve conveys sensory information to the brain and brain derived immunoregulatory signals suppressing peripheral cytokine levels and inflammation. Acetylcholine (A mediated cholinergic signaling has been implicated in this regulation. However, the possibility of controlling inflammation by peripheral administration of choline acetyl transferase (ChAT), enzyme that catalyzes biosynthesis of acetylcholine, is unexplored. We studied the administration of ChAT in endotoxemia, sepsis and DSS colitis models. Intraperitoneal administration of ChAT significantly (p=0.002) suppresses serum levels of TNF (ChAT 1062.0pg/mL ± 113.0 vs vehicle 1711.0pg/mL ± 210.2) during murine endotoxemia. In a murine CLP sepsis model, administration of pegylated ChAT (PEG-ChAT) significantly improves survival (p=0.015), with a 55.6% survival in PEG-ChAT treated mice compared to a 22.2% survival in vehicle mice. In a preclinical model of inflammatory bowel disease, ChAT administration significantly improves body weight gain (d14 p=0.01 ChAT mice -5.1%±1.1 vs vehicle mice -15.5%±2.9), disease score (d11 p=0.006 ChAT mice 0.17DAI±0.05, vs vehicle mice 0.66DAI±0.13), and colon length (p=0.006 ChAT mice 7.15cm±0.2 vs vehicle mice 6.2cm±0.2) compared to vehicle. These results indicate that administration of ChAT inhibits TNF levels in acute endotoxemia and attenuates disease severity in murine models of sepsis and DSS-induced colitis, suggesting that further study of ChAT as an experimental anti-inflammatory therapeutic is warranted.
 Supported by grant from NIH to KJT and SSC.  

Choline acetyltransferase attenuates inflammation in a murine model of DSS colitis and sepsis

Correction: Galantamine ameliorates experimental pancreatitis

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Circulating extracellular choline acetyltransferase regulates inflammation.

Protective Effects of Pegylated Choline Acetyltransferase in a Murine Model of DSS Colitis

Choline acetyltransferase attenuates inflammation in a murine model of DSS colitis and sepsis

Correction: Galantamine ameliorates experimental pancreatitis