Potential Role of Extracellular CIRP in Alcohol-induced Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is the sixth leading cause of death in the US and the most common form of neurodegenerative dementia. In AD, microtubule-associated protein tau becomes pathologically phosphorylated and aggregated, leading to neurodegeneration and the cognitive deficits that characterize the disease. Prospective studies have shown that frequent and heavy alcohol drinking is linked to early onset and increased severity of AD. The precise mechanisms of how alcohol leads to AD, however, remain poorly understood. We have shown that extracellular cold inducible RNA binding protein (eCIRP) is a critical mediator of memory impairment induced by exposure to binge-drinking levels of alcohol, leading us to reason that eCIRP may be a key player in the relationship between alcohol and AD. In this review, we first discuss the mechanisms by which alcohol promotes AD. We then review eCIRP’s role as a critical mediator of acute alcohol intoxication-induced neuroinflammation and cognitive impairment. Next, we explore the potential contribution of eCIRP to the development of alcohol-induced AD by targeting tau phosphorylation. We also consider the effects of eCIRP on neuronal death and neurogenesis linking alcohol with AD. Finally, we highlight the importance of further studying eCIRP as a critical molecular mechanism connecting acute alcohol intoxication, neuroinflammation, and tau phosphorylation in AD along with the potential of therapeutically targeting eCIRP as a new strategy to attenuate alcohol-induced AD.

Introduction

Alzheimer’s disease (AD) is the sixth leading cause of death in the US and the most common form of neurodegenerative dementia, currently afflicting 5.8 million Americans [1]. The economic burden for AD and other dementias in the US in 2020 is estimated to be $305 billion [1]. Except for genetic mutations involved in a small number of cases, the etiology of AD is still mostly unknown. As a result, despite tremendous scientific efforts, there are still no effective therapeutic options to prevent or treat AD. The key neuropathological features of AD consist of accumulation of extracellular senile plaques containing aggregates of amyloid-β (Aβ) peptides and neuronal intracellular neurofibrillary tangles (NFTs) containing hyperphosphorylated and aggregated microtubule-associated protein tau, along with glial activation [2]. Neurodegeneration is another feature of AD, which is a combined result of synaptic damage in the circuitry connecting hippocampus and neocortex regions critical in learning and memory and neuronal loss [3]. Alterations in adult hippocampal neurogenesis might also play a role in AD neurodegeneration [3,4]. Interestingly, tau accumulation in AD brain is closely associated with the cognitive impairment and neurodegeneration that characterize the disease [5]. In fact, the density of hyperphosphorylated tau containing NFTs along with their stereotypical spatiotemporal progression correlates better with cognitive decline in AD (Braak staging) than that of Aβ plaques [6]. As such, identifying the mechanisms underlying pathological tau phosphorylation is critical to develop novel effective strategies to prevent or treat AD.

The lifetime alcohol consumption is a major modifiable hazardous risk factor for functional and cognitive outcomes in AD patients [7–10]. Prospective studies have shown that frequent and heavy alcohol drinking is linked to AD with a younger age of the onset and increased severity [11–14], which is consistent with alcohol’s known negative effects on cognitive and motor functions. The loss of cholinergic neurons and hippocampal atrophy observed in AD patients have also been reported in association with heavy alcohol consumption, suggesting common pathways linking the cognitive impairment of alcohol abusers with that of patients with AD [15,16]. Indeed, alcohol consumption correlates with AD-like brain cortical atrophy in individuals at high risk for developing AD [17]. Additionally, alcohol abstinence effectively reduced the cognitive decline of AD patients with a history of habitual drinking, suggesting that heavy alcohol consumption not only increases the risk, but also worsens the progression of AD [18]. Interestingly, in mice transgenic for human tau, Aβ precursor protein, and presenilin-1 (3xTg-AD), chronic alcohol exposure caused neuronal tau phosphorylation in the hippocampus and memory impairment [19]. The precise mechanisms of how alcohol leads to AD, however, remain poorly understood.

We have discovered that extracellular cold-inducible RNA-binding protein (eCIRP) acts as a danger-associated molecular pattern molecule (DAMP) to increase tissue injury and mortality after systemic and organ-specific inflammation [20]. Indeed, eCIRP levels have been associated with increased severity and worse prognosis in patients with sepsis [21], pancreatitis [22], and rheumatoid arthritis [23]. We have also discovered that eCIRP is a novel mediator of neuroinflammation [24,25]. We will review the mechanisms proposed by which alcohol promotes dementias and explore the potential contribution of eCIRP to the development of alcohol-induced AD.

Neuroinflammation in Alcohol and Alzheimer’s Disease

Alcohol and its metabolite acetaldehyde exert a direct neurotoxic effect, causing permanent damage to the brain structure and function [26]. In addition, alcohol has strong immunomodulating effects, which depend on its dose and frequency [27]. Alcohol consumption also impacts the immune system and inflammation in the brain. The neuroimmune factors in the alcoholic brain include Toll-like receptors (TLRs)- in particular TLR4, various cytokines, and high-mobility group protein box 1 (HMGB1), which further amplify neuroinflammation [28]. In the brain, both neurons and supporting glial cells (both astrocytes and microglia) contribute to the release of and responses to these neuroimmune factors. Alcohol induces inflammation in the brain via increasing NF-κB–mediated transcription of proinflammatory factors as well as reducing cAMP response element binding protein (CREB)-mediated transcription of protective neurotrophic-factors [29,30]. Recent studies suggest that neuroinflammation might actually be the mechanism by which alcohol influences the development of AD [31]. Recent genome-wide association studies (GWAS) further support the role of neuroinflammation in the predisposition to AD [32].

Microglial cells are closely associated with Aβ deposits in the AD brain. The interaction of microglia with fibrillar Aβ leads to their activation resulting in the production of chemokines, neurotoxic cytokines and reactive oxygen species (ROS) that are deleterious to the brain [33]. However, microglia can also have a beneficial role due to their ability to phagocytose and clear Aβ [34]. In AD, inflammatory activity of microglia is increased while neuroprotective mechanisms mediated by microglia are defected [33]. In a recent transcriptome analysis study, primary microglia exposed to alcohol showed alterations in phagocytosis-related mRNAs and reduced uptake of Aβ, indicating defective amyloid clearance [31]. Alcohol exposure also increased amyloid precursor protein (APP), beta-site APP cleaving enzyme 1 (BACE1) and Aβ production in vitro, and aggravated Aβ deposition, neuritic plaques and learning and memory impairment in vivo [35].

eCIRP and Mechanisms of Neuroinflammation

CIRP is a constitutively-expressed 172-amino acid RNA chaperone protein which acts as a nuclear regulator of protein translation [36–39]. Stressors such as hypoxia and ischemia cause CIRP upregulation, translocation from the nucleus to stress granules in the cytoplasm, and release to the extracellular space [20,40]. eCIRP binds to the TLR4-MD2 receptor complex on the surface of leukocytes to activate the NF-κB pathway resulting in the release of TNF-α and HMGB1 from macrophages [41], T cell dysregulation [42], dendritic cell activation and enhanced antigen presentation to T cells [43], and the formation of neutrophil extracellular traps (NETs) [44]. In addition, we have recently identified IL-6Rα [45] and triggering receptor expressed on myeloid cells-1 (TREM-1) [46] as novel receptors for eCIRP mediating its differential effects in macrophages. We have now extensively demonstrated that eCIRP activates a number of immune cells including macrophages, lymphocytes, dendritic cells and neutrophils as well as non-immune cells such as endothelial cells [20].

We have reported improved outcomes in CIRP-deficient mice in murine models of binge alcohol-induced cognitive dysfunction [24], cerebral ischemia [25], renal ischemic/reperfusion (I/R) [47], intestinal I/R [48], acute lung injury [49] and wound-healing [50]. Treatment with polyclonal anti-CIRP antibody reduced inflammation, injury and prolonged survival in animal models of hemorrhage, sepsis and renal I/R injury [51,41]. Moreover, treatment with compound 23 (C23), an eCIRP competitive antagonist peptide [41], showed protective effect and attenuated inflammation and injury in rodent models of hemorrhage, renal ischemia/reperfusion and sepsis [52–55]. Also, we have identified extracellular miR-130b-3p as a novel endogenous inhibitor of eCIRP. Injection of a miR-130b-3p mimic attenuated systemic inflammation and acute lung injury in mouse model of sepsis [56].

We have shown that both alcohol and cerebral ischemia increase microglial cell expression of CIRP and release of eCIRP which lead to local inflammation and neuronal cell injury [24,25]. Thus, we have revealed eCIRP as a novel mediator of neuroinflammation. Another recent study also showed that eCIRP aggravated neuroinflammation in intracerebral hemorrhage-induced brain injury via TLR4 signaling [57]. CIRP’s involvement in hypoxic-ischemic brain injury in neonatal rats supported our findings, where CIRP worked through downregulation of HIF-1α expression [58]. However, a few studies have reported total CIRP to be neuroprotective [59–61], highlighting the juxtaposition between the protective role of intracellular CIRP and the deleterious pro-inflammatory deleterious effects of eCIRP. We have recently shown that brain eCIRP is a critical mediator of regional brain hypoactivity and cognitive impairment caused by exposure to binge-drinking alcohol levels [62,63]. Control mice showed hypoactivity in the temporal (secondary visual) and limbic (entorhinal/perirhinal) cortices using fluorodeoxyglucose (¹⁸FDG) and positron emission tomography (PET), which was significantly less suppressed in the CIRP-deficient mice after alcohol exposure [63]. Alcohol-treated control mice had impaired recognition of a repositioned object in the object-place memory task, and were more anxious in the open field task, whereas CIRP-deficient mice were not impaired in these tasks [63]. Thus, eCIRP is a neuroinflammatory mediator released in response to triggers such as alcohol with the ability to activate various receptors triggering pro-inflammatory responses on numerous cell types. Based on above findings, we propose that eCIRP could be a key mediator explaining the association between alcohol and AD.

Inflammasomes function as intracellular sensors of danger signals, inducing an innate immune response upon activation by secreting IL-1β and IL-18 and inducing pyroptosis [64]. NLRP3 inflammasome activation has been documented in vivo in the transgenic APP/PS1 mouse model of AD, and deficiency in NLRP3 significantly ameliorated spatial memory deficits and hyperactive behavior in these mice, which was associated with reduced hippocampal and cortical Aβ deposition, smaller plaque volumes, decreased levels of proinflammatory cytokines, and improved microglial phagocytic ability [65]. Indirect inhibition of NLRP3 inflammasome activation by fenamate has suppressed microglia-mediated neuroinflammation and memory loss in 3×TgAD mice [66]. Also, pharmacological inhibition of caspase-1 activity by VX-765 reduced Aβ accumulation, brain inflammation, and cognitive impairment [67]. Alcohol is known to up-regulate and activate the NLRP3 inflammasome, leading to caspase-1 activation and IL-1β increase in the brain. IL-1β amplifies neuroinflammation, as well as impaired hippocampal neurogenesis, and disruption of IL-1/IL-1R signaling has been shown to prevent alcohol-induced inflammasome activation and neuroinflammation and neurogenesis inhibition [68,69]. Alcohol induced HMGB1 release in neuronal cells has also been reported to be mediated by NOX2/NLRP1 inflammasome [70]. Moreover, chronic alcohol exposure induced mitochondrial ROS can also promote NLRP3 inflammasome activation [71]. Interestingly, we have reported activation of NLRP3 inflammasome by eCIRP leading to induction of pyroptosis in lung endothelial cells [72].

eCIRP and Tau Phosphorylation

Neuroinflammation has been linked to tau phosphorylation in AD [73]. In P301S mice, microglial activation is an early event which happens prior to NFT formation, and has been suggested to contribute to the disease progression [74]. In addition, LPS-induced activation of inflammation in the brain exacerbated tau phosphorylation in tau-transgenic mice, which has been shown to be mediated by tau kinases such as cyclin-dependent kinase 5 (cdk5), and glycogen synthase kinase-3β (GSK3β) [75–77]. IL-1R blockade has been shown to alleviate cognitive deficits and attenuated tau pathology by reducing the activity of cdk5/p25, GSK-3β, and p38-MAPK tau kinases in 3xTg-AD mice [78]. Thus, activated microglia/astrocytes release cytokines (e.g., IL-1β or TNF-α) or other neurotoxic inflammatory molecules which modulate tau kinases augmenting pathologically phosphorylated tau. Also, microglia may contribute to tau propagation by releasing phagocytosed pathological tau via exosomes [73,79].

Cyclin-dependent kinase 5 (cdk5) is a neuronal-specific proline-directed serine/threonine kinase that is involved in the normal physiological activities of the brain and plays a critical role in neurotransmission, synaptic plasticity, and cognitive functions [80]. However, aberrant activation of cdk5 is a major contributor to pathological tau phosphorylation [81,82]. In neurons, cdk5 is activated through direct binding of the proteins p35/p39 and their cleavage products p25/p29. The deregulation of cdk5 occurs mainly due to its association with p25, a C-terminal fragment of p35 [83]. Being more stable, truncated p25 causes prolonged activation of cdk5, which leads to increased tau phosphorylation and ultimately results in neurodegeneration [84]. Additionally, inhibition of the cdk5/p25 complex has reduced neurodegeneration and improved cognitive function [85,86].

Interestingly, the neuroprotective effect of cdk5 inhibition was enhanced when combined with inhibition of GSK3β [87]. GSK3β is known to be involved in alcohol induced neurotoxicity [88]. Binge alcohol exposure caused GSK3β activation, suggesting its involvement in the neuronal damage to the hippocampus [89]. However, the role of cdk5 in alcohol-induced neurotoxicity is not well defined. A recent study showed involvement of cdk5 pathway in tau hyperphosphorylation and neuropathological abnormalities after postnatal alcohol exposure [90]. GSK-3β and caspase-3 were shown to be activated by alcohol injection resulting in increase in phosphorylated tau as well as tau cleaved at Asp421/422 during alcohol-induced neurodegeneration in P7 rodent model of fetal alcohol spectrum [91].

Recently, we showed that eCIRP directly binds to IL-6Rα and activates STAT3 [92]. IL-6R is a receptor for IL-6 which has been linked to AD [93,94]. IL-6R signaling has been shown to induce tau hyperphosphorylation in rat hippocampal neurons by deregulating the cdk5/p35 kinase pathway [95]. We have shown earlier that alcohol increased brain levels of eCIRP in a model of binge alcohol drinking, and that microglial cells were the probable source of eCIRP in the brain after alcohol exposure [24]. Moreover, chronic alcohol exposure has been reported to cause neuronal tau phosphorylation and memory impairment 3xTg-AD mice [19]. Recently, activation of NLRP3 inflammasome has also been shown to drive tau pathology. Tau has been shown to trigger NLRP3 inflammasome activation, with loss of NLRP3 function regulating tau kinases and reducing hyperphosphorylation and aggregation of tau [96]. Moreover, tau pathology was induced in an NLRP3-dependent manner with intracerebral injection of brain homogenates containing fibrillar Aβ [96]. Taken as a whole, these indications suggest that microglial eCIRP released in response to alcohol may act as a key molecular mediator of neuronal tau phosphorylation, thus connecting alcohol with AD.

AMPK is a direct tau Ser/Thr kinase which on activation phosphorylates tau proteins at several sites in the microtubule-binding domains and mediates the detrimental effects of Aβ [97]. Phosphorylated AMPK is normally nuclear, but accumulates in the cytoplasm of AD neurons [98]. Aβ exposure induces rapid activation of AMPK which acts as a tau kinase [99]. Additionally, in vivo tau phosphorylation in tauP301S mice has been shown to be mediated by AMPKα2 [100]. In contrast, another recent study showed isoform-specific roles for AMPKα in AD pathophysiology with repression of AMPKα1, not AMPKα2, improving the cognitive impairment in two different amyloid-based AD model mice [101]. However, alcohol exposure has been shown to decrease AMPK activity in neonatal mouse brain [102] and the role of AMPK in alcohol induced cognitive dysfunction has not been studied.

eCIRP, Neuronal Death and Neurogenesis

Alcohol exposure has been shown to induce endoplasmic reticulum (ER) stress and neuronal death and inhibition of ER stress to alleviate alcohol-induced ER stress and neuronal apoptosis [103]. Simultaneously, recent studies with the unfolded protein response (UPR) signaling manipulations have indicated a key role for ER stress in AD neurodegeneration [104]. Interestingly, we have shown that eCIRP induces ER stress in sepsis-associated acute lung injury [49]. Oxidative stress is another important mechanism resulting in brain damage due to neurotoxicity of alcohol [105,106]. A recent study showed that apoE4 and high-concentration alcohol augment neurotoxicity in a synergistic manner via increasing oxidative stress in N2a cells stably transfected with human APP695 (N2a-APP), implicating this finding in AD especially in APOE ε4 carriers [107]. Alcohol also induces mitochondrial dysfunction, which plays a critical role in neurodegeneration [108,109]. Joshi et al. found that impairment in the metabolism of alcohol-derived acetaldehyde, mediated by the mitochondrial enzyme aldehyde dehydrogenase 2 (ALDH2), contributed to increased mitochondrial dysfunction and AD associated pathology in the alcohol-treated AD patient-derived fibroblasts and in mice deficient in functional ALDH2 [110]. Interestingly, eCIRP also induces mitochondrial DNA damage through TLR4 signaling causing necroptosis in macrophages [111]. Long-term epigenetic modifications in mice exposed to alcohol dysregulate the epigenetic landscape of normal aging [112,113] which is also seen in AD. Alcohol exposure alters the expression of neurotrophins and their receptors affecting many downstream signaling pathways [114]. Prenatal alcohol exposure has been suggested to adversely affect learning and memory via alcohol-neurotrophin interactions [115]. In addition, alcohol use leads to altered hypothalamic-pituitary-adrenal (HPA) axis, which is seen in AD as well [116,117]. Alterations in adult neurogenesis in the hippocampal formation have also been linked to Alzheimer’s Disease [4]. Interestingly, alcohol exposure has been reported to decrease most of the stages of adult neurogenesis in the hippocampal formation [118]. In addition, interneuronal accumulation of phosphorylated tau has been reported to disrupt adult hippocampal neurogenesis [119].

Future Research and Perspectives

eCIRP’s precise role on how alcohol consumption predisposes to AD has not been completely elucidated. Considering the variety of pathways shared between alcohol, AD and eCIRP (Table 1), it is plausible that eCIRP released from microglia due to alcohol exposure could act directly on the neurons. A potential mechanism of eCIRP’s effect on the brain leading to tau hyperphosphorylation and impaired cognition is proposed (Figure 1). Being a stress response protein and DAMP [41], alcohol-mediated stress and TLR activation could release eCIRP from the immune cells in the brain such as microglia. We have earlier shown that eCIRP was released from the BV2 mouse microglia cells exposed to high doses of alcohol, which suggests that microglia in the brain could be a source of eCIRP release [24]. CIRP is expressed in various brain cells including neurons and microglia [39,20]. Intracellular CIRP (iCIRP) may protect neurons from apoptosis; but eCIRP released from microglia in response to ischemia and alcohol mediates neuroinflammation by reactivating microglia, and it also induces neuronal damage [24,25,39,20]. Further, studies are needed to show the mechanism by which alcohol causes eCIRP release and which cell types are involved. It would be interesting to find whether additional risk factors for AD other than alcohol could also cause eCIRP release. We recommend that future mechanistic studies should be performed keeping in mind that the eCIRP binds to variety of receptors. It would be crucial to dissect out which receptors alcohol-induced eCIRP activates and if there is crosstalk or cell-type specificity among these receptors. iCIRP is known to mainly exert its effect via its binding to RNA, while eCIRP has been shown to mediate inflammation via its binding to various receptors [40]. Effect of RNA-binding on eCIRP’s activity is not yet well explored. Considering our recent finding that extracellular miR-130b-3p can bind to eCIRP and inhibit its inflammatory activity [56], it would be interesting to further consider the potential impact of RNA binding on the function of eCIRP in the context of alcohol-induced AD. Moreover, eCIRP’s activation of ER stress and of the NLRP3 inflammasome may further contribute to the effects of alcohol on AD development. The best understood mechanism of alcohol associated cognitive impairment leading to AD is neuroinflammation [27]. However, neuroinflammation has been shown to promote both Aβ pathology as well as tauopathy. Future studies would be needed to understand if eCIRP also has any role in promoting Aβ formation and deposition. Since, immune cells, such as microglia, may also play a role in spreading tau [79], we need to further consider eCIRP’s contribution to the tauopathy spreading. Interestingly, eCIRP skews macrophages and microglia to the pro-inflammatory M1-like phenotype in TLR4- and TREM1-dependent manner [46,41]. However, eCIRP was also shown to induce endotoxin tolerance in macrophages via IL-6R-STAT3 pathway, with chronic eCIRP stimulation skewing macrophages to M2 phenotype [45]. Further studies are needed to warrant understanding the role of eCIRP in complex microglial polarization, phagocytic activity and amyloid clearance in the setting of alcohol-induced Alzheimer’s disease.

Table 1.

Common activated pathways and players in alcohol, AD and eCIRP.

Pathway	Alcohol	AD	eCIRP
Neuroinflammation	Alcohol induces proinflammatory signaling, glutamate excitotoxicity, myelin damage, neuronal death, and inhibits neurogenesis [29,31,27].	In AD, immune cells release inflammatory mediators and are involved in amyloid clearance, synapse loss and tau spreading [34,33,32].	eCIRP causes release of inflammatory mediators and neuronal damage in cerebral ischemia and binge alcohol intake [24,20,25].
Cognitive dysfunction	Alcohol causes hippocampal atrophy and loss of forebrain cholinergic neurons leading to functional cognitive decline and dementia [15,14,11,9].	In AD, senile/neuritic plaques, NFTs and loss of cholinergic neurons in the neocortex and hippocampus cause cognitive impairment and dementia [3,2].	eCIRP mediates hypometabolism in the neocortex, cortical amygdala and hippocampus during binge alcohol, leading to spatial cognitive impairment [63].
Microglial activation	Increased expression of microglial activation marker Iba-1 in the alcoholic brains, increases inflammatory mediators and reduces phagocytosis [31,29].	Microglial activation by β-amyloid increases inflammatory mediators, reduces amyloid clearance, and causes synapse loss and tau spreading [34,33].	Increased expression of microglial activation marker Iba-1 in the ischemic brains, increased TNF-α release in BV2 microglial cells [24].
TLR4 activation	Alcohol increases TLR4 expression and activates TLR4 complex signaling, causing induction of proinflammatory cytokines, ROS and other inflammatory mediators [29].	AD associated amyloid activates TLR4 complex signaling, causing induction of proinflammatory cytokines, amyloid deposition and oxidative stress [125].	eCIRP activates TLR4 complex signaling, causing induction of proinflammatory cytokines, chemokines, and ROS in macrophages, T cell dysregulation, dendritic cell activation, and NETs formation [20].
HMGB1 release	Increased HMGB1 release in the alcoholic brains mediates neuroinflammation [29,28].	HMGB1 released from neurons and microglia mediates inflammation, inhibits microglial phagocytosis reducing amyloid clearance and participates in the process of AD neurodegeneration [125–127].	Increased HMGB1 release in the eCIRP treated RAW 264.7 macrophages and in serum of eCIRP injected rats [41].
IL-6Rα activation	Alcohol increases liver IL-6Rα expression and activation, indicated by increased STAT3 phosphorylation [128].	AD risk is linked to an IL-6Rα variant associated with increased IL-6 gene signature. IL-6R signaling induces tau hyperphosphorylation [95,93].	eCIRP directly binds to IL-6Rα and activates it inducing STAT3 phosphorylation in macrophages [92] and neuronal cells (Sharma A, unpublished).
Inflammasome activation	Alcohol activates NLRP3 inflammasome causing increased caspase-1 activity, neuroinflammation and reduced hippocampal neurogenesis [68,71,69].	NLRP3 inflammasome activation in AD induces brain inflammation, Aβ plaque deposition, and cognitive impairment [67,66,65,64].	eCIRP activates NLRP3 inflammasome leading to induction of pyroptosis in lung endothelial cells [72].
ER stress	Alcohol induces ER stress in brain, which is mediated by mTOR signaling [103].	In AD, protein misfolding causes ER stress, triggering synaptic failure and neurodegeneration [104].	eCIRP induces ER stress mediated by TLR4 causing acute lung injury associated with sepsis [49].

Figure 1. Potential mechanisms of alcohol-induced microglial eCIRP in promoting AD pathology.

Alcohol induces microglial cells to release eCIRP, which binds to IL-6Rα activating neuronal STAT3/Cdk5 pathway (shown in inset) and causes AD-associated pathological tau phosphorylation and aggregation in neurons. Apart from IL-6Rα signaling, eCIRP can also potentially activate TLR4 signaling in neurons causing ER stress, NLRP3 activation, or mitochondrial dysfunction to cause neuronal damage and death. The oligopeptide C23 can potentially attenuate alcohol-induced AD tauopathy by targeting eCIRP’s activation. The released eCIRP also reactivates microglia via TLR4-complex and mediates neuroinflammation and potential indirect neuronal damage. eCIRP can also potentially mediate HMGB1 release or impact phagocytic activity of microglia regulating amyloid clearance or influence tau spreading.

Abnormal forms of phosphorylated and aggregated tau protein are directly involved in the initiation of neurodegenerative processes, which has rendered them important therapeutic targets [120,121,85]. It would be interesting to explore whether CIRP-targeting strategies can also inhibit eCIRP-mediated cognitive impairment. This would further answer if targeting eCIRP could help attenuate the development of AD. Anti-CIRP antibodies could be one way to target eCIRP. However, antibodies, being large molecules, would need to be modified and engineered for safe, effective and targeted delivery across blood-brain-barrier (BBB), possibly using methods such as adeno-associated viral-vector delivery. C23, a much smaller 15-mer oligopeptide that acts as an eCIRP competitive antagonist, may be a more suitable alternative, but C23 delivered peripherally is still unlikely to be able to spontaneously cross the blood-brain-barrier (BBB). However, novel techniques such as focused ultrasound could be utilized to transiently open the BBB and deliver C23 directly to the hippocampus and other brain regions [122–124]. In summary, eCIRP is a potential critical mediator connecting alcohol consumption and AD development. Future studies are needed to elucidate the precise mechanisms by which eCIRP causes AD-associated impaired cognition, to improve our understanding of how alcohol predisposes to AD.