Association of mTOR Pathway and Conformational Alterations in C-Reactive Protein in Neurodegenerative Diseases and Infections

Abstract

Inflammatory biomarkers have been very useful in detecting and monitoring inflammatory processes along with providing helpful information to select appropriate therapeutic strategies. C-reactive protein (CRP) is a nonspecific, but quite useful medical acute inflammatory biomarker and is associated with persistent chronic inflammatory processes. Several studies suggest that different levels of CRP are correlated with neurological disorders such as Alzheimer’s disease (AD). However, dynamics of CRP levels have also been observed in virus/bacterial-related infections leading to inflammatory responses and this triggers mTOR-mediated pathways for neurodegeneration diseases. The biophysical structural transition from CRP to monomeric CRP (mCRP) and the significance of the ratio of CRP levels on the onset of symptoms associated with inflammatory response have been discussed. In addition, mTOR inhibitors act as immunomodulators by downregulating the expression of viral infection and can be explored as a potential therapy for neurological diseases.

Introduction

C-reactive protein (CRP) included in pentraxin group, occurs in two known conformations: native pentameric CRP and monomeric (mCRP). Presently, available literature suggests that CRP has dual properties. It includes pro- and anti-inflammatory traits, whereas mCRP elicits only strong proinflammatory actions in cells: endothelial cells, endothelial progenitor cells, white blood cells, and thrombocytes leading to dilation of the inflammatory responses (Wu et al. 2015). The step which directly links CRP with inflammation is the disjunction of CRP into proinflammatory mCRP. The rise in baseline CRP levels is used to estimate chronic inflammation and tissue impairment which may be the result of a failure of primary inflammatory response or an immoderate inflammatory response. Elevated CRP levels may be linked to cardiovascular diseases (CVDs) and atherosclerosis. Apart from CVDs, CRP is also linked with some neurodegeneration and long-lasting illness, such as severe hemorrhagic stroke, Alzheimer’s disease (AD), and Parkinson’s disease (Luan and Yao 2018).

There is a presence of activated microglia along with certain changes seen in levels of cytokines and neutrophils as a result, neurons become inactive leading to death which is an inflammatory process occurring in the brain. This process highlighted neuro-declinatory diseases, such as Alzheimer’s disease (AD) or Parkinson’s disease (PD). Changes in the immune system become one of the factors for aging and become proneness for diseases in the old-aged (Song et al. 2014). Activated microglia have both neurotoxic and neuroprotective properties. It produces cytokines, such as tumor necrosis factor (TNF) and interleukin-68 (IL-68) that are proficient in causing inflammation and act as a neurotoxic compound and brain-derived neurotrophic factors as neuroprotective compounds. Previous reports given by Sochocka et al. (2017) suggested that in aged brains, the microglia are found in an inflammatory state that responds to even minor stimuli. But this is well controlled in young brains. Although the clinical impact of microglia in patients suffering from AD and its association with aging is poorly understood to date (Xu et al. 2016), not much clinical research has been conducted on the microglia-mediated neuroinflammation and aging in AD.

With this consideration, researchers conducted a study to investigate the neurological inflammatory consequences of microglia linked with the aging process in patients suffering from AD through various estimations and analyses of the serum concentration of CRP as the important inflammatory biomarker and at first diagnosis of neurological disorders (Song et al. 2014). Current explorations that interpret how conformity alterations in CRP are associated with brain-related diseases and certain viral and bacterial infections have been mentioned in later paragraphs.

C-Reactive Protein: Structural, Functional Role and Its Metabolism

CRP has been specified after its capability to precipitate and accompanied by calcium ions and the remnant of phosphorylcholine of C polysaccharide obtained through teichoic acid which is present inside the cellular wall of bacteria Streptococcus pneumoniae (Salazar et al. 2014). CRP is considered to be a member of the pentraxin family made up of 206 amino acids and is conserved phylogenetically. Pentraxins exhibit some structural characteristics: five indistinguishable non-glycosylated globe-shaped subunits—per capita of which is composed of two beta-pleated sheets—non-covalently interlinked and organized in a proportioned isochronous fashion throughout a central vent, forming a pentameric, disciform, and plain arrangement (Agrawal et al. 2009, Du Clos et al. 2013) (Table 1). In immunological terms, the functional role of CRP involves modulation of inflammation, prevention of adhesion of neutrophils to endothelial cells, increased vascular wall inflammation, innate host defense against microbes, non-inflammatory clearing processing via complement activation, and opsonophagocytosis (Fig. 1) (Ansar et al. 2016; Ansar 2020). Production of C-reactive protein takes place mainly inside the liver and is noticed on the long arm of chromosome 1, that is, 1q23.2. In view, no allelic dissimilarity or genetic inadequacy has been observed for this gene, excluding a few heterogeneities (Sproston and Ashworth 2018). The upregulation of IL-6 turns up to be its chief regulator by assisting CRP through de novo synthesis via upregulation of C/EBP, and C/EBP-is the main transcription factor in this process (McFadyen et al. 2018). Furthermore, IL-6 pathway may be strengthened by IL-1 and TNF, both enhanced the transcription rate of CRP. Succeeding production and liberation into circulation, serum CRP amount rises up remarkably 6 to 8 h following inceptive provocation, maximum of 24 to 48 h, their half-life is about 19 h (Salazaar et al. 2014). CRP levels in circulation are mainly decided by the rate of its production. A number of studies suggested that recognizing additional extrahepatic origins of CRP synthesis might determine the lower and additional constant CRP levels that come out to forecast cardiac risk; these discoveries involve coronary smooth muscle cells in CRP production in retaliation to inflaming cytokines. Regionally synthesized CRP plays a requisite part in the process of activating endothelial cells (Sproston and Ashworth 2018) (Table 1).

Table 1.

Functional role of C reactive protein in health and diseases

1	Prevention of adhesion of neutrophils to endothelial cells
2	Modulation of inflammation
3	Increased vascular wall inflammation
4	Innate host defense against microbes
5	Non-inflammatory clearing processing

Fig. 1.

Release of various cytokines in viral infection

C-Reactive Protein: Transition from Pentameric to Monomeric Form

Despite the fact CRP is the chief configuration identified in serum and comes out to be an extremely stable particle, ongoing study reports suggest that the arrangement of subunits from CRP can be separated, both in vitro and in vivo, into mCRP components (Lv and Wang 2018). In contrast, independent or free mCRP synthesis can be a prime source of this configuration. Processes that are involved in mCRP generation in vivo are as follows.

Local Expression of C-Reactive Protein

Several analyses suggested the existence of mCRP in tissues other than hepatic cells, such as lipocytes, myocytes, and provocative cells or inflammatory cells, enclosed by atherosclerotic plaques. Nevertheless, the process for the formation of fragments and their congregation into CRP has not been clearly defined till now. In vitro, it encourages localized expression, along with the spotting of monomeric CRP mRNA in U937 macrophages of atherosclerotic lesions. Furthermore, elevated mCRP expression was found to be in atherosclerotic lesions mutually with extensive inflammation (Kaplan et al. 2014).

Local Dissociation of C-Reactive Protein

Disengagement of CRP into mCRP located in the membranes of mitotic catastrophic cells and activated platelets in arteriosclerosis plaques appears to be an important coupling between innate and adaptive immunity, thrombosis, and atherogenesis. Phosphatidylcholine molecules illustrating per cell membrane of triggered thrombocytes seem to be dominant in this phenomenon, as the phospholipid is capable of binding the disseminated CRP and inducing its detachment (Eisenhardt et al. 2009). In the initial step of this procedure, mCRP is produced as a hybrid intervening molecule depicting CRP subunit immunogenicity. However, the remaining native CRP configuration mCRP is linked with intensified complement fixation and speedily dissociates from the cell membrane, after which, it is eventually disassembled in solution into the concluding and chief form of mCRP. The next continuative step is related to even more effective atheromatous traits. mCRP is barely observed in a movable form via regular estimation processes, indicating the supremacy of regional expression. Nevertheless, innovative approaches, such as the implementation of monoclonal antibodies and ribonucleic acid aptamers for mCRP, authorize the perseverance of nano-molar levels of this conformation and display a major step for upcoming comprehension impact of mCRP in succeeding research (McFadyen et al. 2018).

Ligands and Receptors for C-Reactive Protein

It is found that phosphatidylcholine, which belongs to the family of phospholipids present in prokaryotes such as bacterial, fungal, and eukaryotic cells if it is necrotic or apoptotic; also, in indigenous and altered lipoproteins and remains the most important CRP ligand so far. The binding sites of CRP for phosphatidylcholine are situated on each subunit but at their lateral surfaces. This process requires the sticking of two calcium ions at the aquaphobic region clustered on Phe66. Thus, the process is designated as calcium-dependent ligand binding. On mingling with C1q, the commencement of the classical complement pathway (Chang et al. 2002) was seen as cited here. CRP has the potential to associate with other related molecules, such as altered and unaltered plasma lipoproteins, histones, chromatin, and small ribonucleoproteins and also some extrinsic molecules, usually somatic constituents of prokaryotes’ cell walls and membranes. In addition, introduction to low pH environments, mainly observed at inflammation sites, activates conformational modifications in CRP resulting in exposure of two sites that are involved in attaching within the inter-subunit regions of CRP in the loop having residues 115–123. However, the related fixed amino acids will remain unexposed (Juan Salazar et al. 2014). Besides phosphatidylcholine, CRP has the ability to bind with a protein, that is, human complement factor H with phosphocholine before its disengagement into mCRP (Thompson et al. 1999). When individual subunits are released, it allows the exposure of specific hidden epitopes which require well-defined immune triggering properties (antigenic) and are efficient in turning on various components in vitro, such as thrombocytes, polymorphonucleate WBCs, monocytes, lipoproteins, and complement system. Moving on to its receptors, the CFHR4 protein is acknowledged by immunoglobulin G and FcR receptors, which are displayed on macrophages, mast cells, platelets, and leukocytes of both lymphoid and myeloid lineages. The FcR receptors are categorized as high-affinity receptors, such as Fc gamma RI (CD64) and low-affinity receptors, such as Fc gamma RII (CD32) and Fc gamma RIII (CD16), based on the potential of its binding between immunoglobulins to its receptors. (Devraj et al. 2005). Lectin viz. oxidized LDL receptor-1, that is, LOX-1 capable to bind with CRP has been verified through various methods, together with immunofluorescence (Stein et al. 2000). CRP also elucidates to produce soluble (sLOX-1) which is similar to LOX-1, typically activated macrophages and those obtained from outermost blood mononucleate cells of affected individuals with dreadful coronary dysfunction. This process involves FcRIIa, TNF and ROS production (Hein et al. 2014). Furthermore, similar observations are seen in existing snuffers and individuals affected with strong coronary disease. Recent findings outline probable clinical importance as a prognostic factor for LOX-1, in severe coronary syndrome (Marnell et al. 2005).

Transition in the CRP Structure Results in the Disclosure of Proinflammatory Binding Sites

It is seen that C-reactive protein amount increases concerning tissue trauma or severe infection. The moving CRP forms are restricted to impaired tissue which persuades complement activation and promotes tissue injury. It was a thorough understanding of the CRP stimulating procedure which is necessary to expand therapeutic levels to reduce tissue bruises. It has been demonstrated by several studies that CRP when bound to microvesicles, which are obtained from cells, goes through a structural reformation without interrupting the symmetry—CRP, which is known to compose the utmost CRP type in human-infected tissue enabling the association of complement factor C1q and triggering the classical complement pathway. CRP–macrovesicle complexes result in the hiring of white blood cells to inflamed tissue (Braig et al. 2017), a type of cytokinesis. The interactions between subunits are non-covalent and attached via ionic and non-polar interactions. The disclosed faces of CRP constitute the A/effector face and B/binding face. The B face/binding face binds to the disrupted cell membranes of apoptotic cells and walls of the bacterial cells. A face/effector face of CRP correlated with complement factor C1q and the Fc gamma receptors and linked to inflammation responses (Chang et al. 2002, McFayen et al. 2018). CRP restricts impaired tissues where it allows some changes, that is, generation of mCRP leading to new epitope introduction (residues—199–206 become available to immunoglobins 9C9 or 3H12 and are specific) stimulating the complement system. The relation between CRP deposition in tissues, complement activation and increased level of WBCs infiltration is well secured, very little is known about the pattern of molecular interactions that take place in damaged tissue. CRP is known to bind to the lipo-polysaccharides of the activated monocytes, and later is allowed to leave on microvesicles where it goes through some conformational alterations (Braig et al. 2017). A suppressor 1,6-bis(phosphocholine)-hexane (1,6-bis-PC), interrelates with the binding site of CRP phosphocholine and blocks the CRP–microvesicle interplay and consequently inhibit the process of tissue injury via CRP pathway. At alkaline conditions, CRP slowly separates into monomeric CRP (mCRP) in calcium-free buffers (Williams et al. 2020). The dissociation of CRP is improbable in vivo as the body fluids hold increased levels of calcium. However, there have been the results of observation by negative-stained electron microscopy that CRP is linked to lipid monolayer. The arrangements on lipid monolayers imitate the disrupted cell membranes and evolve the formation of mCRP (Strang et al. 2012). The process of disassembly can also occur on activated platelets, necrotic cell membranes, acidic pH, oxidative stress, microparticle, amyloid plaques, neutrophil extracellular traps, and impaired cell membranes, necessary for the generation of lysophosphatidylcholine (McFadyen et al. 2018). At the location of the injured endothelial, moving thrombocytes boost the surface of the activated platelets, and CRP dissociates to mCRP, which employs proinflammatory outcomes by activating hematocytes and promoting inflammatory response. mCRP originated in human inflamed tissue and commences along with the tethering of first or native CRP to activated monocytes, which get free on microvesicles. Upon binding, CRP gets separated into mCRP resulting in modification without disturbing the symmetry which is involved in the triggering of complement pathways and induces an inflammatory response by appointing the endothelial cells and leukocytes. The above data highlight the post-translational modifications of the CRP molecule in the regulation of its proinflammatory activity. Glycosylation is a protein modification and post-translational mechanism associated with disease processes and provides protein stability (Boncler et al. 2019). Protein functions get affected when there is an alteration in the glycosylation of serum proteins. However, with respect to CRP, it is non-glycosylated. It was reported by Das et al. that under some pathophysiological conditions, CRP can go through this process. CRP purified from patients suffering from diseases, such as leukemia, tuberculosis, Cushing’s syndrome, and os on, contains sugars, disclosing the contrast in CRP biomolecules, such as carbohydrate and amino acid mixture. It was suggested by molecular modeling that the existence of glycosylation sites on the pit of the C-reactive protein, gets switched on after slight alterations in the sequence of the protein (Das et al. 2014; Braig et al. 2017).

Association Between CRP and Brain Changes Linked with Neurodegenerative Disorders

Neurodegeneration occurs due to moderate and continuous defects in neurons, and depletion of the components of the central nervous system. It is the prime curative characteristic of severe neural ailments, such as Alzheimer’s disease and Parkinson’s disease, neurotropic viral infections, stroke, paraneoplastic disorders, traumatic brain injury and multiple sclerosis (Table 2). Alzheimer’s disease (AD) is neuro-declinatory disarray specified by moderately ongoing empirical decrease and practical deterioration. Many enzymes such as β- secretase, γ-secretase, butyrylcholinesterase (BChE), monoamine oxidase, acetylcholinesterase (AChE), and glycogen synthase kinase-3β (GSK-3β) are involved in the metabolism of amyloid precursor protein (APP), amyloid-beta (Aβ) deposition and hyperphosphorylation of tau protein contributed for the development of AD. Therefore, therapeutic compounds are recently investigated against these enzymes for the management of AD (Jabir et al. 2021a; b, c).

Table 2.

CRP levels related to chronic inflammatory diseases (Yao et al. 2019)

Diseases	Serum CRP levels	Clinical importance of CRP	Reference
Hemorrhagic stroke	High CRP concentration	Inflammatory biomarker, risk prognosticator	Towfighi et al. (2017)
Alzheimer’s disease	Both low/high CRP concentrations	Inflammatory biomarker, occasional role	Gong et al. (2016)
Parkinson’s disease	High CRP concentration	Inflammatory biomarker, risk prognosticator	Prins et al. (2016)
Cardiovascular disease	High mCRP concentration	Inflammatory biomarker, risk prognosticator, competitor	Anchah et al. (2017)

CRP is acknowledged in the senile plaques clearly, from patients with AD using immunostaining, indicating that the procedure of senile plaques emergence may involve an acute-stage inflammatory condition and rising of CRP (Iwamoto et al. 1994). The alliance among mCRP with patients of Alzheimer’s and beta-amyloid plaques is indicated, which can resultantly persuade the detachment of CRP into solitary monomeric forms. Furthermore, cardinal analysis of humans suffering from AD/stroke disclosed that elevated serum levels of mCRP from infracted interior/central regions were correlated with a decreased formulation of A-β/Tau protein, indicating the job of mCRP in encouraging softening of the brain after ischemia. Analyzed together, a straight practical effect obtained by CRP may at their minimum demonstrate the pathophysiology of AD (Duong et al. 1997). CRP genes with few single nucleotide polymorphisms (SNPs) are linked with increased serum levels of CRP. Among these polymorphisms, the most dominating is rs3091244 (T and A alleles) and some others such as rs1205 (G allele) and rs1130864. The increased rate of transcription of the CRP allele is mediated by SNP rs2794521 (T allele). It is assumed that people with the CRP genotypes that are linked with the production of CRP levels are more prone to inhibit SNP formation when present in the prodromal phase before the actual AD formation. Upregulation of CRP levels is seen in areas of brains affected with AD. Increased mortality has been observed due to polymorphism in the CRP gene associated with raised CRP amount (Marchesi 2005; Moura et al. 2019; Wang et al. 2009).

Identifying Viral Infections by the Level of Acute-Phase Proteins (APPs) and Circulating Cytokines

Recognition of the causative agent of the disease, that is, a viral or bacterial infection is not only necessary for medical care but also in the reduction of the mortality rate. The reaction of the host’s immune system is examined to several infectious diseases through increased cytokine levels and acute-phase proteins (APPs) can function as a quick and effective tool to determine the infectious disease. Proinflammatory cytokine IL-18 is present in plasma in greater amounts, along with enhanced serum levels of circulating ferritin distinguish viral infections from others (Jain et al. 2011) (Table 3). Therefore, these levels of APPs and cytokines can be used as an important concept to forecast disease succession. In response to various inflammatory cytokines, hepatocytes start producing acute phase proteins mainly IL-1β and IL-6, although IL-18 is involved in the release of proteins (Aziz et al. 2020; Slaats et al. 2016) (Fig. 1). CRP is a type example of human acute phase protein. In unaffected/normal individuals, the levels of CRP found are very low with an average plasma level of 0.8 mg/L, but after an inflammatory impulse, CRP values quickly rose up to 1,000-fold (Bansal et al. 2014; Weatherall et al. 1996).

Table 3.

IL-18 and ferritin serum levels during viral infection (Slaats et al. 2016)

Type of viral infection	IL-18 (pg/mL)	Ferritin (μg/L)
Normal levels (no infection)	> 200	120
Epstein Barr virus (EBV)	< 1000	431
HIV	< 1000	487
Acute dengue infections	Increases in correlation with ferritin	1264

Assembly of APPs as C-reactive protein during Inflammation and Viral Infections

During a pathogen interruption, there is a rising in acute-phase proteins that participate in initiating the inflammatory reaction. The extremely sustained family of APP which plays a crucial part in the defense of the host against infection is pentraxin (PTX). CRP and serum amyloid proteins (SAP) are designated as small pentraxins and are included in the pentraxins family. Even though elevated APP turned out to be predominantly related to bacterial infections, there is evidence that states that enhanced PTX, C-reactive protein, and serum amyloid protein expression throughout viral infection (Perez et al. 2020) (Table 4).

Table 4.

Variations occurring in the level of acute phase proteins upon viral infection (Slaats et al. 2016)

APPs	Viral and bacterial infection	References
C-reactive protein, PCT (procalcitonin)	Triggered both bacterial and viral infections but gain elevated values during bacterial infections	Lannergård et al. (2003), Gendrel et al. (1999)
Ferritin	Elevated in viral infections	van de Weg et al. (2014), van de Veerdonk et al. (2012)
LPS-linked protein, sTREM-1 myeloid cells expressing soluble activated receptor	Higher during bacterial infections vs viral infections	ten Oever et al. (2012)
Neutrophil lipocalin	Higher during bacterial infections	Venge et al. (2015)
Retinol	Decreased during infections	Thurnham et al. (2003)

Association of C-reactive protein with Influenza and Other Deadly Respiratory Viruses

Interminable associations between a succession of infections and increased serum levels of APP have been stated in infections caused by viruses that severely affect the respiratory tract. When compared with serum APP concentrations of healthy individuals, the infected patients suffering respiratory illness were found to have serum APP levels elevated to a hundred thousand-fold. Patients with virulent type influenza A virus infection have been reported with high CRP levels. Therefore, the human influenza sickness consequence is correlated with the raised CRP amount associated with serious symptoms and even fatality (Jeon et al. 2017; Vollmer et al. 2016). CRP induction can be dissimilar in other airborne influenza A virus types: H7N9 is a more potent organizer of CRP and cytokines formation than the H1N1 subtype. Actually, high serum levels of CRP are induced by pathogenic influenza A H7N9 and are associated with lethal clinical results (Zimmerman et al. 2010).

Persistent Viral Infections and C-Reactive Protein

Hepatitis is the most frequent long-term viral infection that occurs in humans. CRP and amyloid proteins are produced by the liver and are targeted by hepatitis B and hepatitis C viruses. If a patient is suffering from a hepatitis C infection, the level of CRP must be higher as compared to healthy individuals, and this is not surprising (Ma et al. 2015). It was reported that for any dysfunction in the liver due to hepatitis or any chronic infections, higher CRP levels were found. For patients troubled with cirrhosis, CRP is regarded as a useful determinant of fatality in hepatitis B-infected individuals. Similarly, individuals affected with the hepatitis C virus have quite high PTX3 plasma levels and are viewed as an intermediary factor for liver cancer development (Hao et al. 2017). Retroviruses have an eminent capacity to develop life-long infections in the body. Infants that are exposed to this retrovirus show increased CRP concentration and other useful markers like IL-6. A continually elevated amount of CRP in patients goes through antiretroviral treatment is correlated with higher illness progression and the possibility of clinical consequences (Zhu et al. 2017; Perez et al. 2020).

Higher Level of CRP can be a Preliminary Marker to Anticipate the Threat for the Severity of COVID-19

The breakout of the deadliest COVID-19 has become a world-life threat to mankind along with healthcare systems. One of the major challenges that have been acknowledged is that an insignificant subgroup of patients with tender or non-severe COVID-19 infections thereafter evolves into critical disease flow (Sharma et al. 2022). Hence, it is necessary to recognize and provide therapeutics to this subgroup of patients at an early stage to decrease the intensity and ameliorate the consequence of COVID-19 (Luan et al. 2021). Although coronavirus is extremely transmissible and lethal, there is no reliable and advantageous biomarker to foresee the austerity of the ailment (Sharifpour et al. 2020). Medical studies suggested that diversified or modified amounts of a few blood markers are associated with the level of seriousness and fatality of COVID-19-affected individuals. Among all these clinical criteria, serum CRP turns up as a prime marker that shows a remarkable transition in critical COVID-19 patients. The idea of the recent work is to evaluate whether CRP level is used to function as an acceptable marker in assigning the degree of the severity of this disease (Luo et al. 2020). Results have highlighted that tolerance was resolved into low, moderate, and critical pneumonia classified only after employing qRT-PCR followed by a chest computed tomography (CT) scan. The association between serum CRP levels and the severity of COVID-19 pneumonia can be analyzed by applying linear regression models (Ali 2020; Chien et al. 2006). The CRP biomarker has a half-life of about 19 hrs. CRP levels fall at the end of the inflammatory stages when the patient is healing (Pepys and Hirschfield 2003).

Individuals affected with critical infections face much more levels of CRP than low or normal individuals. A report suggested that individuals affected with serious illness have symptoms with an average CRP concentration of 39.4 mg/L and 18.8 mg/L in patients with mild symptoms (Gao et al. 2020). In further studies, researchers concluded that the near concentration of CRP was found to be a rise in patients having critical symptoms—46 mg/L than non-critical patients—23 mg/L (Mo et al. 2020). The tenfold sharp levels (median range-100 mg/L vs 9.6 mg/L) of CRP were found in the people who became COVID-19 victims, causing their life expectancy to become very low (Wang et al. 2020).

Identifying the interrelation between CRP and inflamed non-severe patients with COVID-19, researchers state that CRP is a satisfactory marker predicting the worse possibility of non-critical COVID-19 patients, followed by a rise of 5% in critical conditions Moreover, CRP concentrations with different oxygen saturation were also observed as follows (Baldi et al. 2008).

In patients of COVID-19 patients, various inflammatory cytokines induce CRP production, and this occurs through tissue destruction. The link between the CRP amount and size of the major lung lesion observed by a lung CT scan helps us to determine the severity of lung damage or COVID pneumonia (Pan et al. 2020). After comparing the variations in the diameter or size of the lung lesion and CRP levels in the low, moderate, and critical groups, it is found that sharp CRP levels have raised in the moderate group than in the mild group and excessive in the severe group of patients. In the critical group, CRP levels and lung lesion size, both are on the higher side (Luo et al. 2020; Warusevitane et al. 2016). Hence the CRP amount and lesion size are proportional to the disease progression. Thus, it was suggested that preliminary phase of COVID-19, CRP levels give back lung lesions and disease severity. In conclusion, high levels of CRP are a major early marker in specifying the probability of disease worsening in the moderate group of COVID-19 patients, which in turn encourages practitioners to recognize those individuals who are at a high chance of severe infection at an early stage for proper treatment (Bohmwald et al. 2019; Wang 2020).

Elevated Cytokines/C-reactive protein during Viral Infection and Their Role in Differentiating Between Viral and Bacterial Diseases

The geographical location of a person with respect to his age, along with the bacterial pathogen and patient’s comorbidities spike up the CRP level. CRP which is found to be very stable under physiological conditions is observed to increase sufficiently during an infection or tissue damage and generate an immune response over a 24 hrs to 72 hrs period. mCRP has the potential to stimulate the production of cytokines and also alter tissue factor expression. Several studies suggest that the disassembly of CRP via calcium-dependent binding results in mCRP conformation, only when it binds to cell membranes. Pathogen-associated molecular patterns (PAMPs) or endogenous stress signals (damage-associated molecular patterns [DAMPs]) are conserved patterns on pathogens sensed by inherent immune cells and inflammation is triggered. Pattern recognition receptors (PRRs) are mostly indicated by immune and non-immune cells (Slaats et al. 2016). During infection, stressed cells release DAMPs that cooperate with PAMPs to trigger PRRs. The ability of DAMPs in PRR activation is seen in viral infections because the fate of infected cells is decided by the viral spread. This activates major inflammatory cytokines and stimulates various chronic-stage proteins, such as CRP and APPs, which are important in the inflammatory cascade (Table 3, Fig. 2). Viral infections can infect via many modes. It is already known that a virus has a complex and advanced strategy once perceived by the host. They initiate by entering the human body from the mucosa like in HPV, blood in HIV, vagus nerve, and olfactory as seen in COVID-19. The property of the Trojan horse of viruses facilitates it to pass through the BBB and be released in the synaptic cleft into the CNS. Another feature of viruses is that they can remain latent or dormant in the human cell and get activated later on. Also, on infecting the cell, immune cells of primary response get activated which gives a false alarm to the secondary immune cells causing them to get hijacked by the virus. Normal cellular functioning is taken over by the virus and ultimately the bloodstream is septic. Beta-amyloid is a major protein responsible for AD. On viral infections, synapsis and neuron-to-neuron transmission has altered the homeostasis of the CNS gets affected when this protein is altered. Viral infections impersonate tropism and result in the production of toxic enzymes that can cause CNS issues leading to neuro dysfunction. The inflammatory stimuli release CRP (Xiang et al. 2020; De-Paula et al. 2012; Salinas et al. 2010; McGavern and Kang 2011). On microbial etiology diagnostics, it has been seen how gram-negative bacteria can vary in infection potential. To differ from a viral and bacterial infection, it is stated that the alteration from primary to secondary CRP rate can be measured (Levinson et al. 2020; Lobo et al. 2012; Paran et al. 2009).

Fig. 2.

Comparison between bacterial and viral infections and the role of CRP, IP-10 in response to both kinds of infections

Patients with Severe Viral Infections May be More Prone to Generate Neurodegenerative Diseases Such as AD Through mTOR Pathways

Translational studies have shown that inflammatory cytokine levels built up in the Alzheimer’s affected cerebrospinal fluid that could aid to decode the basic procedure that influences several other kinds of cells. Phosphorylated forms of the mTOR, p70S6K, eIF2, and PKR may constitute putative biomarkers of rational drop in AD and analysis of their concentration in lymphocytes can enhance remaining biological tests, such as the amount of Aβ42 and tau protein resides in the cerebrospinal fluid (Pei and Hugon 2008; Serrano-Pozzo et al. 2010; Crino 2016).

mTOR-dependent pathway is involved in neurodegenerative diseases, for example, Alzheimer’s, amyotrophic lateral sclerosis, Parkinson’s, and Huntington’s disease (Mueed et al. 2019; Agarwal et al. 2021; Poddar et al. 2022). Researchers suggested that bioengineered mice with a G93A variant SOD1 gene, a version for amyotrophic lateral sclerosis, have increased autophagy. mTOR plays a key role in reducing autophagy. In the SOD1G93A genetically modified mice compared to normal mice, the fraction of p-mTOR immune-effective motor neurons and complete final motor neurons were found to be decreased and revealed that mTOR confers autophagy progression (Bunton-Stasyshyn et al. 2015) (Fig. 3). It has been found that in molecular diseases like Parkinson’s disease, RTP801 regulates the mTOR function which leads to cell mortality (Lipton and Sahin 2014; Francois et al. 2016; Pei and Hugon 2008) (Fig. 3).

Fig. 3.

Effect on neurons during cellular stresses in Alzheimer’s disease (AD). mTOR and tau protein phosphorylation are reduced in sensitive neurons whereas these pathways are maintained in resistant neurons

mTOR is an extremely supervised serine/threonine kinase that inhabits two viably definite varied-protein complexes mTORC1 and mTORC2, that are explained by alliance related with proteins Raptor or Rictor (Montero et al. 2012). mTORC1 manages the production of cellular protein biogenesis, growth and multiplication in return for supplements, such as amino acids and growth factors. The transportation of amino acids causes mTORC1 to enroll in lysosomes, which resultantly leads to the stimulation of mTORC1 (Rabanal-Ruiz and Korolchuk 2018). Growth factors trigger mTORC1 through different pathway axis which includes phosphoinositide 3-kinase and Akt. The role of mTORC2 is not well understood, however, the difference lies on the basis of how mTORC1 controls the protein that promotes cell natality which is one of its recently recognized roles (Crino 2016).

Certain viruses, such as Japanese encephalitis (JE) virus and Influenza virus, are associated with PD, herpes simplex virus type 1 (HSV-1) and Chlamydia pneumoniae with AD (Balin et al. 1998) and so on. Specific pathogens directly infect the CNS and cause disorders through the host’s response to the infection. Direct damage occurs when pathogens cross the intact blood–brain barrier (BBB), causing acute infections that can be fatal to chronic diseases (McGavern and Kang 2011). Indirect damages are associated with various factors and may be involved in the accumulation of protein aggregates, oxidative stress, variation in autophagic mechanisms, synaptopathy, and neuronal destruction (Fig. 4) (De Chiara et al. 2012). Neurotropic viruses may enter the CNS through retrograde axonal transport. These pathogens infect the peripheral nerve get infected by pathogens and create a link between the skin and the mucosa to the sensory, motor, and olfactory neurons. Viruses can replicate in neuronal cells and infect the adjacent cells. Some viruses re-infect the peripheral tissue through anterograde transport and are released into the synaptic cleft (Salinas et al. 2010).

Fig. 4.

Pathogens infect through various pathways and trigger inflammatory responses (cytokines) damage the blood–brain barrier and activate microglia that associate and form clusters around neuronal cells and hence neuronal damage

A viral infection undergoes its replication process followed by transcription. A viral nucleocapsid is formed which inhibits rapamycin and blocks mTOR1. Fusion of viral nucleocapsid with F1 protein activates Akt and further mTOR2 which is responsible for virion release. Knockdown of Rap/Ric can also initiate viral replication indirectly. Viruses like HSV and cold’s viruses are also known to stay in a dormant stage and renew their mechanism later on and cross the BBB (blood–brain barrier). Viruses modulate the mTOR pathway by activation of phosphoinositide 3-kinase (PI3K), Akt, or mTOR itself. There is ample evidence to suggest that mTOR inhibition suppresses viral protein synthesis by triggering downstream activators of protein synthesis in addition to interfering with virus-mediated transcription events. (Le Sage et al. 2016) (Fig. 5).

Fig. 5.

Schematic representation of viruses that activate the Akt/mTOR pathways leading to a possibility for Alzheimer’s disease and inhibition of mTORC1 (Karam et al. 2021; Kuss-Duerkop et al. 2017; Le Sage et al. 2016)

Bacterial infection releases its toxin and triggers the immune system. Cytokines are released followed by vasodilation and by chemotaxis macrophages linked B cells and T cells. This causes inflammation and an increase in CRP levels. The monomeric form of CRP then binds to C-polysaccharide on the bacterial cell wall and activates the complement system I and helps in opsonizing them for phagocytosis (Fig. 6). Thus, it shows that CRP not only acts as a biomarker, but it has a protective role in bacterial infections. On the other hand, it has been observed that excessive host response through the immune system plays a critical role in the pathogenesis of viral infections including influenza disease. The monomeric form of CRP is considered an inflammatory derivative of pentameric CRP and it is evident that an inhibitor of monomeric CRP can be a potential target for the immunopathological lesions caused by infection (Gao et al. 2017).

Fig. 6.

Bacterial strategies and mechanisms to eliminate disease after toxic release and replication followed by fusion with CRP protein leading to elimination of infection (Tattoli et al. 2012)

Neuroinflammation is a characteristic seen in Alzheimer’s pathogenesis. Although several studies revealed that there is no remarkable contrast in the serum CRP of Alzheimer’s patients and controls, there is no proven case to show a direct relationship between Alzheimer’s, CRP, and viral infections; however, it can be estimated that there is a possibility of CRP protein to rise in serum after mTORC2 is activated to release a greater number of virions. In another case, some latent viruses such as herpes simplex virus (HSV) are activated under favorable conditions and enter the BBB (blood–brain barrier) and cause the deposition of β amyloid in the brain cells and which might lead to AD (Fig. 5) (Strazza et al. 2011). Low levels of CRP were found in patients having moderate symptoms of Alzheimer’s disease. Patients with moderate symptoms of AD had lower CRP levels (Serrano-Pozzo et al. 2011). This indicates that the CRP can act as a representative systemic inflammatory marker to diagnose AD. Alzheimer’s disease is found in the senile plaques (SP) and during the formation of plaques, it arose an inflammatory state that leads to the generation of CRP. It was suggested recently that the disruption of CRP into monomers is due to the association of mCRP in Alzheimer’s patients and beta-amyloid plaques. Through some experiments, it was concluded incubation together with plaques, mCRP gets dissociated with CRP. Substantially more mCRP prevails in the frontal peridium of affected individuals (Francois et al. 2016). Higher concentrations of serum CRP are connected with Parkinson’s disease and baseline CRP concentration is related to the chance of death and anticipated life foresight of patients with Parkinson’s, although the action of CRP and its procedure in the etiology of Parkinson’s disease is currently lacking (Luan and Yao 2018; Sierra et al. 2004).

Bacterial infections are less severe due to their opsonization behavior in comparison with viral infections, where there is a dynamic nature that has been observed with respect to CRP level in the serum. It has been found that the level of CRP is higher at the initial period of bacterial infection as compared to virus infection and in contrast, the CRP levels have shown vice-versa at the onset of symptoms. This dynamic behavior of CRP levels reflects a new term velocity CRP that distinguishes viral and bacterial infection and has shown a better way to diagnose a specific infection (Sait et al. 2021; Largman Chalamish et al. 2022).

mTOR Pathway Activated During Viral Infections: A Mediator of Progression of Neurodegenerative Diseases can be Targeted for Viral Therapies

The high amounts of CRP in COVID-19 patients impair the lungs due to the deficient supply of blood (ischemic), that is, more than 30–50 mg/L. This can cause intra-alveolar edema and hemorrhage, therefore, it is presumed that C-reactive protein plays a major part in the chlorosis condition of COVID-19 patients (Sheriff et al. 2021). In the lungs of COVID patients, accumulation of mainly C1q is seen. C1q first binds with CRP and after this, it binds to the lysophosphatidylcholine of those cells that are deficient in blood or ischemic cells (Awogbindin et al. 2021). CD8+ effector T cell response gets inhibited by the C1q complement. If the recurrence of CD8+, PD1+ and T cells is higher, it leads to T-cell depletion. In spite of the exhaustion of T cells, massive damage in the respiratory organ is observed along with the increased concentration of C-reactive protein in the affected individuals of COVID-19 (Strauss et al. 2007; Kulinski et al. 2013; Araki et al. 2009). CRP specifically binds to receptors FcץRll (CD32/CD64) and activates various mechanisms of inflammation, fibrosis, etc. CRP is not restricted to binding to FcץRll (CD32/CD64) and activates various mechanisms of inflammation but is also concerned with the activation of rapamycin mTOR mediated pathway (Fig. 7). CRP is not restricted to binding to FcץRll (CD32/CD64) and activates various mechanisms of inflammation but is also concerned with the activation of the rapamycin mTOR pathway (Fig. 7).

Fig. 7.

Association between Alzheimer’s disease and the infection burden. Proinflammatory mediators are highly associated with Aβ and NFTs production in the brain

mTOR pathway is fundamental for cell metabolism and its continuity, proliferation and growth. It plays an important role in lipid metabolism, protein synthesis, transcription and autophagy (Awogbindin et al. 2021; Khan 2021). SARS-CoV-2 stimulates this pathway and activates phosphatidylinositol 3-kinase (PI3K) and its downstream molecule AKT, or even by activating mTOR (Le Sage et al. 2016; Karam et al. 2021). A protein kinase family phosphatidylinositol 3‐kinase, mTOR belongs to this kinase family and is organized into two main multiprotein combinations, mTOR complex 1 and mTOR complex 2, which sense and control different signals and various functions of the cells (Terrazzano et al. 2020). This leads to the generation of IL-1β as a mediator of lung inflammation, fibrosis, and fever. Gene expression of myeloid immune cells is regulated by mTOR which controls their cytokine expression and migration. Metformin, sirolimus, and everolimus are important mTOR inhibitor drugs and can moderate the infection by blocking the viral genome transcription and protein synthesis (Mashayekhi-Sardoo and Hosseinjani 2022). Such inhibitors inhibit cell-cycle progression and suppress the proliferation of T cells because of the cross-link of antigenic peptides and cytokines like IL-2 with the T-cell receptors (Corradetti and Guan 2006; Park et al. 2010). Furthermore, sirolimus could reverse the induction of IL-1β production caused by the binding of SARS-CoV-2 to toll-like receptors (TLR) (Zhou et al. 2016; Mashayekhi-Sardoo and Hosseinjani 2022). It is very necessary to understand the functional duty of mTOR in stimulating immune reciprocation and the role of inhibitors in COVID-19 infection (Radbel et al. 2020; Rai et al. 2022; Wijayasinghe et al. 2021)  (Fig. 8).

Fig. 8.

A mTORC1 inhibited by mTOR inhibitors promotes autophagy and suppresses the translation of SARS-CoV2 viral polymerase (Mashayekhi-Sardoo and Hosseinjani 2022)

In myeloid cells, gene expression is regulated by mTOR which controls their cytokine expression and migration. Hence, it is necessary to understand the functional duty of mTOR in stimulating immune reciprocation and the role of inhibitors in COVID-19 infection (Radbel et al. 2020). Numerous FDA-approved mTOR inhibitors are found including metformin, rapamycin, and everolimus (Fig. 8). The mTORC1 is rapamycin-sensitive and manages the biosynthesis of organelle (Kuss-Duerkop et al. 2017; Terrazzano et al. 2020; Maiese 2020; Gatti et al. 2020).

mTORC1 Inhibited by mTOR Inhibitors Promotes Autophagy and Suppresses the Translation of Viral Polymerase

The mTORC2 is unresponsive to rapamycin and governs actin cytoskeleton, metabolism, and cell survival. mTOR Complex 1 formed when mTOR along with raptor, GBL, proline-rich Akt substrate molecular weight of 40 kDa, and Deptor. mTORC1 gets activated in the presence of elevated ATP, growth factors, and hormones and gets inhibited by low levels of ATP and in the absence of growth factors (Hara et al. 2002). Activated mTORC1, phosphorylate p70 S6 kinase, induces mRNA translation, ribosome biosynthesis, protein formation via translation, metabolism in mitochondria, and lipocyte biogenesis. mTOR complex 2 or mTORC2 is formed by the association of different proteins involved in the formation of complexes such as mTOR, Rictor, GBL, Sin1, PRR5/Protor-1, and Deptor. It helps in cell to maintain its durability through Akt, cytoskeleton through protein kinase C, ion transport, and cellular growth via serum and glucocorticoid-inducible protein kinases (SGK1). DNA in/of adenovirus, HSV, etc., and RNA in/of MERS-CoV, HIV, influenza, etc., control the mTOR expression by stimulation of phosphoinositide 3-kinase, Akt, or mTOR itself (Karam et al. 2021). Changes in the mTOR pathway can also induce various diseases, such as cancer, cardiovascular disease, inflammation, autoimmunity, and metabolic disorders.

Various analogs of rapamycin, which is a prototypic mTOR inhibitor have been manufactured for remedial use (Das et al., 2017). Everolimus which is a derivative of rapamycin derivative acts as a successful suppressor of mTORC1, which forbid the translation of mRNA, biosynthesis of the ribosome, mitochondrial metabolism, and differentiation of the adipose cell. mTORC1 inhibition by everolimus also has a differential effect in COVID-19 that resultantly causes the decrease in a multiplication of standard T lymphocytes, which could lessen the release of different cytokines, as well as preserved regulatory T cells, its growth and activity, which could result in the decrement of uncontrollable reactivity in the crucial stage of the illness (Fig. 9). The suppression of mTOR may result in the prevention of the overactive immune signal via the STAT3 pathway to elevate the power of sensory receptors for Interleukin-6 and also its production by itself (Terrazzano et al. 2020; Crino 2016; Radbel et al. 2020) (Fig. 9). Nevertheless, everolimus can be provided with the present therapeutic approaches, mainly in the acute stages of the SARS-Cov2 infection. Abnormal high sensitivity is the major critical cause of COVID-19 in the severe phase, so inhibitor everolimus can be used wisely. It has been observed that viral replication depends on the host cell translation machinery. The viral infection will lead to the activation of mTOR which in turn facilitates the phosphorylation of 4E-BP and eIF4G and thus stimulate efficient translation. This is the reason that everolimus delays viral synthesis via the mTOR-dependent translation mechanism. Thus, sensitization of the cell with mTOR inhibitors can not only be the potential therapy for organ transplant recipients but also for AD associated with the activation of autophagy and clearing of misfolded proteins (Tan et al. 2019; Subramanian et al. 2022). Moreover, the combinatorial role of two mTOR inhibitors, curcumin and plumbagin, has shown more effective in regulating PI3K/Akt/mTOR pathway and thus may have a significant role against cancer cells (Ahmad et al. 2023). On the other hand, nitric oxide is a neurotransmitter that activates the glial cells to overproduce NO which further leads to inflammation and neuro deficits. Thus, natural anti-inflammatory compounds such as alpha-lipoic acid (α-LA) can be explored for the management of neurological disorders (Behl et al. 2022; Kaur et. al. 2021). Interestingly, it has been observed that elevated levels of CRP have been associated with lower levels of Aβ in APOE ε4 patients, who carry the genetic risk factors of AD. The increased concentration of CRP may activate microglia that reflect the neuroprotective role by attenuating the production of nitric oxide, a toxic compound for neuroinflammation. This shows that increased CRP levels provide a neuroprotective role at different stages of neuroinflammation (Zhang et al. 2023).

Fig. 9.

The remedial treatment makes use of mTOR inhibitors specifically (everolimus) in COVID-19. This inhibits SARS COV-2 replication and blocks the cytokines rush relying upon the super activated STAT3 pathway, inhibition of Treg cell activity, and increased overactive T cells in COVID-19

Conclusion and Perspectives

CRP is a nonspecific biological indicator of inflammatory responses to any type of infection and has an important part in monitoring infection caused by bacteria, inflammation, neuronal impairment, tissue injury, and retrieval. It is seen that elevated CRP levels are found during the critical stage of inflammation and chronic inflammatory illnesses (Sproston and Ashworth 2018). The conclusion is derived from both the investigational and the clinical data that increment in serum levels of CRP is linked with the patient’s high possibility of cardiovascular diseases, T2DM, Alzheimer’s disease, hemorrhagic stroke, and Parkinson’s disease.

CRP is found to be stable in the physiological condition and its dissociation will happen during the infection via a calcium-dependent manner into monomeric CRP (mCRP) conformation when it binds to cell membranes. To differentiate between viral and bacterial infection, it is suggested that the ratio of primary to secondary CRP values should be measured from symptoms onset to the respective period of incubation in different clinical aspects.

Although there is no correlation between the activation of mTOR and CRP levels in the blood that has been established, it was evident that during the proinflammatory infection, the CRP has been dissociated into monomeric CRP (mCRP) and it is associated with beta-amyloid, promoting Tau phosphorylation and Aβ (42) and further promoting the AD pathobiology. This shows that aggravating neuroinflammation by infection results in the decrease of native CRP and increase of monomeric CRP and in turn leads to mTOR activation via proinflammatory agents. Thus, the levels of mCRP may be the potential biomarker for the pathology of neurological disorders. Given this importance, search for pharmacological inhibitors that could potentially block the dissociation of native CRP to mCRP and thus protect the pathology caused by the proinflammatory response associated with mTOR-induced neurological diseases. Thus, mCRP can be a potential biomarker for inflammatory-related neurological disorders.

Medications such as everolimus, an inhibitor of mTOR, have been used as an immunomodulator of the inflammatory response of neutrophils. The suppression of mTOR may result not only in controlling the hyperactivation of inflammatory response but also in controlling the viral infection, particularly the acute stages of COVID-19 infection. This shows that mTOR inhibitors may indirectly participate to regulate the CRP levels with respect to inflammatory responses.

Various reports have concluded and support the job of mTOR function in controlling the immune response and antiviral activity (Maiese 2020). Yet, a number of analysis and discussions exist for the mTOR pathway to develop effective antiviral therapy along with early-stage prevention of neurological disorders that may be the future outcome of serious virus infection. mTOR regulation may require exact biological oversight. Investigations are required and should go on to gain in-depth knowledge about the mTOR-related pathways for interpretation and to develop productive clinical antiviral strategies for numerous pathogens. Also, its inhibitory target may lead to the rehabilitation of various neurodegenerative diseases (Maiese 2016).

The limitations of the current study imply that various inflammatory markers such as IL-6, Ferritin, and CRP are stimulated during the infections and interlinked with the cascades of proinflammatory responses which could not be directly associated with the pathological conditions of specific diseases; however, further prospects may include experiments on the same.

However, modulation of mTOR and mCRP might protect from  neurological disorders mediated by a chronic inflammatory/proinflammatory responses. In the coming years, more understanding would be anticipated in terms of infection with mCRP deposition and mTOR activation will be identified and characterized. Hence, approaches for therapeutic intervention against mCRP and mTOR that block the aggravating inflammatory response and activation of pathological forms of various neurodegenerative diseases should be needed with urgency.

Author Contributions

AK, FF, GG, and NKP wrote and edited the manuscript; AS edited and contributed all the figures of the manuscript; SK and NKP assisted in editing the final version of the manuscript. The author(s) read and approved the final manuscript.

Funding

No funding is available.

Data Availability

Not applicable.

Declarations

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical Approval

Not applicable.