Role of TNF-α in the Pathogenesis of Migraine

Background Neurogenic neuroinflammation has a wide role in migraine pathogenesis including the transition from episodic migraine to chronic one. The seed molecule of neurogenic neuroinflammation, i.e., the TNF-α proinflammatory molecule, has gathered a lot of attention. This pleiotropic cytokine is a classical component of inflammatory soup, secreted by the microglial cell, and promotes a wide range of inflammatory reactions. Aim In this review, we aimed to provide a culminating and comprehending glimpse into the TNF-α in association with the migraine. Method A systematic literature survey method with a mixture of keywords was utilized to grasp the different elements that represent the association between TNF-α and migraine. Discussion. Highlighted the probable involvement of the TNF-α with migraine, the complexity of the matter such as activation of NF-KB signaling cascade, autoactivation, sensitization, and increased likelihood of transition cannot be neglected. Being TNF-α as a core node, it becomes the factor for linking diseases such as chronic inflammatory disorders, including COVID-19, and also interaction with other genes to develop severe conditions. Conclusion To this end, TNF-α plays a critical role in chronification, and inhibiting its signaling would likely be a crucial strategy for migraine therapy.


Introduction
It is not strange to hear the term "migraine," which has been around since Hippocrates' time.Since then, the defnition has expanded dramatically and is now defned as "a brain condition featured with low neuronal hyperexcitability and vascular dysfunction as a consequence leading to the unwanted severe head pain (the cardinal feature)" [1].Regarding its prevalence and disability, the Global Burden Disorder-2019 (GBD-2019) has enlightened that the general prevalence is about 24% (GBD 2019) with a variation related to the gender diference with the majority of females than males [2,3].Tis gender prevalence diversity is mainly due to hormonal diferences.If we take disability as a concern, the GBD-2019 has provided signifcant data on it, presenting the highest disability-adjusted life years (DALYs) per 100,000 in Italy (775.25)preceded by Germany (729.25), Tailand (724.5), and with the lowest rate in Ethiopia (266), and in India on average of 552.79 with the highest in Sikkim (592.86) and the lowest in Bihar (513.9)(Homepage | Te Institute for Health Metrics and Evaluation (healthdata.org).
Apart from such a high prevalence estimate, migraine is still a matter of debate regarding its pathological mechanism, development of diverse phenotypes, and progression toward chronifcation.Several pathological mechanisms have already been identifed including cortical spreading depression (CSD), activation and sensitization of trigeminovascular foci, and activation of glial cells [4][5][6][7].It is accepted that the hypersensitiveness of neurons, i.e., "lower threshold of neuronal hyperexcitability," is the reason for the migraine attack initiation [8].But still, the matter of fact that cannot be denied is that the seed pathological mechanisms that make migraine so much more complicated disorder and force the researcher to contemplate its cure which is still lacking.
When compared to the other 369 diseases and injuries in the world, headache is ranked as the third most disabling (Global Burden of Disease-2019).It is induced by the activation and sensitization of the trigeminovascular system, which is responsible for the headache phenotype and its sensitization, respectively [6].CSD is responsible for the activation, but various molecules are involved in sensitizations which are collectively called infammatory soup [7].CSD is an abnormal intrinsic neurophysiological phenomenon with a dramatic shift in cortical steady potential [6] and is responsible for the aura.It is responsible for the loss of function of the cerebral cortex mainly due to the increased extracellular potassium, a neurotransmitter (glutamate), as well as infammatory markers such as calcitonin gene-related peptide (CGRP), substance P (SP), bradykinin, histamine, prostaglandin, and TNF-α which are collectively categorized under infammatory soup (IF) [9].
CSD is also responsible for secondary damage in the brain's vital structure including blood-brain barrier (BBB) damage [10] which enhances the sensitivity of neuronal populations near areas of contusive brain injury, as in models of localized cerebral ischemia and trauma [11].Tus, brain immune cells play an important and fascinating part in keeping brain tissue in balance and fxing damage brought on by repeated CSD attacks and the abnormal buildup of "CSD consequences molecules." However, the subjects with a genetic predisposition in the genes of the infammatory system might induce an abnormal infammatory response, i.e., chronic infammation (CI).CI has long been recognized as a signifcant cause of the disease [12].Pieces of evidence have shown that TNF injection can cause headache, and TNF antibody has been shown to reduce pain in humans [13].However, the purpose of attempting to write this review is to better understand migraine as a neuroinfammatory disorder referring to TNFα as an example.Hence, we all know that infammation cannot be covered in a single article.TNF-α is only the tip of the iceberg in the infammatory response, and it cannot be overlooked that it still plays a signifcant role.

Material and Method
For the literature search, we used the connected paper tool (https://www.connectedpapers.com/)initially using the following search words: TNF-α and migraine, which revealed the network of related research.Following the analysis of the article in the network, a selection was made based on TNF-α genotyping research, serum level research, protein expression, etc.We also used the standard approach of literature surfng for confrmation and increased accuracy, i.e., surfng in electronic databases such as Web of Science, Google Scholar, PubMed, Springer, and Elsevier until Feb. 2022.Te relevant writers (AS, SS IS, HK, and PK) independently verifed the data's validity.Keywords that were used include "migraine as a neurological disorder," "polymorphism in TNF-α gene associated with migraine," "TNF-α-308 promoter polymorphism associated with migraine," "G-308A polymorphism of TNF-α in migraine," "rs1800629 association with migraine," "neuroinfammation role in migraine," "microglial and migraine," and TNFA expression in migraine model."Only articles published in the English language were evaluated using the linguistic flters as a factor for publication selection.Primary and secondary research sources including scientifc work, meta-analysis, and systematic reviews were critically reviewed.Te study excluded incomplete data or only partially available.Because partial and missing data are not included in the study, there is no such detrimental efect, and we did our utmost to eliminate any undesirable characteristics.Tis narration includes a signifcant quantity of current material and follows PRISMA guidelines (PRISMA (prisma-statement.org)(Figure 1)).

Background
Chronic CNS (central nervous system) infammation is characterized as a reaction that occurs as a result of deliberate injury, such as the breakdown of the BBB, and results in the activation of immune cells in the brain, such as microglia and astrocytes [14].Such activation releases nitric oxide, glutamate, and proinfammatory cytokines [15].When it comes to proinfammatory cytokines, TNF-α is one of the best examples.It starts infammation and helps keep the body's balance.It is an undeniable truth that an excess of TNF-α may pose a major threat to the system, resulting in a variety of disorders (Figure 2).As a result of its core activity in infammation [16], it is regarded as a hotspot molecule that has been investigated for decades.
TNF-α is secreted by the microglial cell upon activation by the release of histamine and serotonin, and changes in the level of TNF-α [7] across subjects can be a key risk factor for migraine susceptibility and severity.Te researchers have been captivated by its role in neurogenic neuroinfammation [17].Neurogenic neuroinfammation has a wide role in migraine pathogenesis which has been thought to have a critical role in the pathogenesis [18].
3.1.TNF-α.TNF-α (NCBI Entrez Gene: 7124) encodes for a pleiotropic cytokine that promotes a wide range of proinfammatory reactions and is responsible for various conditions including migraine (Figure 2) (Mala-Cards-human disease database; OMIM-Online Mendelian Inheritance in Man).Studying the expression of TNF-α from diferent expression databases (Brain tissue expression of TNF-Summary-Te Human Protein Atlas), it is found that higher expression of the same is in the white matter (Figure 3(a)) which is characterized by the presence of myelinated nerve fbers.Using Allen Human Brain Atlas (Microarray Data-Allen Brain Atlas: human brain (brainmap.org),shows the increased expression of the TNF-α (probe: A_23_P376488) in diferent subcortical regions of the brain (Figure 3(b)) but shows a signifcantly higher expression in the medullary reticular formation (donor: H0351.2002:medullary reticular formation Z-score: 14.67, log2-level:12.71).Te reticular formation is a net-like 2 Pain Research and Management structure made up of various brainstem nuclei and neurons that cover a large portion of the brainstem.Te network is distributed throughout the brainstem as an interconnected network of neurons with many projections rostrally to subcortical and cortical brain structures as well as caudally to the spinal cord.Interneurons in the lateral and medial tegmental felds, such as trigeminal, facial, vagal, and hypoglossal, have many functions, including the transmission of proprioceptive stimuli via descending axonal tracts [19].At position −308 of the TNF-α promoter/enhancer region, a G to A transition polymorphism has been identifed which shows diferent allele frequencies (rs1800629 (SNP): Explore this variant; Homo_sapiens: Ensembl genome browser 110).TNF1 or 308.1 nomenclatures are used to denote the more frequent 308G allele whereas the less common 308A allele is known as TNF2 or 308.2 and shows the allelic diversity within the diferent populations (Figure 3(c)).Te rare 308A allele has been related to greater baseline/constitutive and inducible TNF-α expression both in vivo and in vitro [20] and is associated with elevated plasma levels [21].
For identifying the risk variants that cause the disease of interest, the "candidate gene studies approach" has been at the forefront in uncovering the correlation between genetic variants with complex diseases [22].Multiple meta-analyses and other researchers have found a relationship between TNF-α 308GandA polymorphism with variations in susceptibility to several diseases between diferent people and, on a wider scale, across diferent populations [23][24][25][26].

TNF-α in Association with
Migraine.Continuous pieces of evidence of an association between the TNF gene and migraine have been collected, with the most noteworthy fnding coming from Franceschini and colleagues.Tey showed that LPS injection signifcantly increased TNF and macrophage production in Cav2.1R192Q (Familial Hemiplegic Migraine) KI (knock-in) ganglia when compared to WT (wild-type) ganglia.According to their fndings, TNF-α may have a role in neuronal sensitization via altering crosstalk between sensory neurons and resident glial [27].Te addition of CGRP (Figure 4(a)) to primary trigeminal ganglion cells boosted the levels of many cytokines, including TNF-α [28,29].Concerning the expression analysis, Taheri and colleagues, after utilizing the expression assays for fnding the expression level of cytokines in migraine suferers compared to healthy controls, observed increased TNF-α expression inpatients than controls [30].Similarly, Sarchielli and his team found a temporary increase in TNF-α levels in the blood from the internal jugular in migraine patients who did not have an aura in the two hours following the start of their migraine [31].Also, Tang and group have shown that genetic deletion of TNF-α or intraspinal trigeminal nucleus caudalis (Sp5C) injection of TNFα receptor antagonist completely blocks pain prolongation [32].
Te candidate gene study approach has been at the forefront of uncovering the correlation of genetic variants with complex diseases [22].Using the same, multiple research studies have been conducted over the previous two decades; however, contradictory results have been discovered (Table 1), which might be related to demographic diversifcation, ethnic diversity, and varied sample size.But, in the recent polled analysis, a nonsignifcant association was observed with overall migraine but critical increases in the Asian ethnic group [50].We cannot ignore the fndings from the genomewide association study (GWAS), which identifed several infammatory genes such as TSPAN2, MEF2D, NLRP1, JAM3, and NOTCH4, but the TNF-α was not replicated [51].Tis inconsistency is mainly due to several reasons and one such important is the study design diference between the candidate gene approach and GWAS [52].
Plasma level of the same was found to be signifcantly higher in the patients compared to controls [53][54][55][56] which is also supported by the recent meta-analysis [30,57] with contradiction [58][59][60].It has been also seen that TNF-a levels fuctuate more among children than in adults, and this difference might be due to a long medical history of migraine in adult patients and frequent intake of analgesic drugs or prophylactic treatment [61].Te CSF level of TNF alpha was found to be signifcantly elevated in chronic migraine patients [62], but with it, there is a contradiction [63].
Te fact that a greater level of TNF-α showed a positive connection between TNF-α levels in CSF and serum and the extent of developing brain infarction cannot be overlooked [64].Tis can be easily correlated to the cause of dementia including Parkinson's and Alzheimer's disease (AD) [65,66], where the shreds of evidence from the model organism also support it [67].Te story does not end here, the interaction of TNF-α with other genes is the culprit for the various diseases (Figure 2).Pain Research and Management

Source and Mechanism of TNF-α Action in Migraine.
Microglia, which has macrophage-like properties, is thought to be signifcant immune cells and make up about 5-10% of the parenchyma of the brain and spinal cord [68].It continuously searches for any infection or alteration in the brain and thus aids in the regulation of brain hemostasis.Notably, when microglial cells are activated, they diferentiate into "M1" and "M2" microglia types [69].Te "M1" cell type is found to be responsible for releasing cytokines like IL1, TNF-α, and NO, while the "M2" phenotype helps tissue repair and regeneration [70].But long, excessive, and persistent stimulation of microglial cells can cause harmful infammations that help neurodegenerative and neoplastic diseases develop [71][72][73].It has also been found that microglial activation is the main cause of migraine pain and that this activation happens through cortical spreading depression [74,75].
Concerning the production of TNF-α, which is known to cause infammation and is known for its keen role in neuroinfammation [76,77], it is made as a transmembrane protein (tmTNF-α) and then changed into soluble TNF-α (sTNF-α) by a matrix metalloprotease TNF-α-converting Phenotype Gene Phenotypic series (PS) Figure 2: OMIM radial PheneGene graphs depicts the complex interaction of the TNF-α with diferent gene and are responsible for the diverse number of diseases and also with the migraine (highlighted with star) (OMIM-Online Mendelian Inheritance in Man (https://www.omim.org/)).Pain Research and Management enzyme [78].TNF-α exerts its efects via binding to the extracellular domain of the TNFR1 receptor, triggering receptor trimerization and thus initiating the cascade of events.After receptors are trimerized, the adaptor protein TRADD is brought in which then brings in other adaptor proteins such as receptor-interacting protein (RIP) and TNF receptor-associated factor 2 (TRAF2) [79].TRAF2 is presented in a complex interaction with E3 ligase (cIAP1 and cIAP2) in the cytoplasm.Due to the activity of E3 ligase, it can tag RIP protein with ubiquitin molecule to create a binding site for another E3 ligase "linked ubiquitin chain assembly complex" (LUBAC) [80].To facilitate the recruitment of a diferent kinase, "transforming growth factoractivated kinase-1" (TAK1), LUBAC further tags RIP [81].
TAK1 is bound to its docking site via an adaptor protein known as TAK1 binding protein-2 (TBP-2) and a kinase called Inhibitor of the kappa B kinase (IKK) complex.IKK is responsible for tagging IKB (an inhibitor of NF-kB) and promoting its proteasomal degradation [82,83].Phosphorylation of the NF-kB inhibitor (IKB) triggers nuclear translocation of NF-kB dimers, where it controls the production of proinfammatory cytokines such as TNF-α and IL-1β [84] (Figure 4(b)).Another fnding shows that epigenetics, like miR-342 activating NF-kB/p65 by TNF-α, causes more TNFα and IL-1β to be released in an autocrine way by encouraging the breakdown of BAG-1, which is a negative inhibitor of proinfammatory NF-kB in microglia [85].

Comorbidities with Migraine in Association with TNF-α.
Comorbidity, or the occurrence of two or more chronic illnesses at the same time, is another major factor of concern that should be taken into account [86].People who have more than one common disease at the same time are more likely to get it because they share risk factors which can be genetic or environmental.

Pain Research and Management
Taking TNF-α as a reference point, various disorders have been discovered to be comorbid with migraine.It is not only that the amount of the same has risen, which can be linked to the illness; there's also evidence from other chronic conditions, such as up-regulation of TNF levels in the trigeminal nociceptive pathway, that gut microbiota dysbiosis contributes to the chronicity of migraine-like pain [32].Patients with psoriasis [87,88], infammatory bowel diseases (IBD) [89,90], and rheumatoid arthritis [91] have a higher risk of migraines.
Tere are several lines of shreds of evidence which directs that COVID-19 has been linked with migraine headaches [92][93][94].Tus, this can't be neglected due to the enhanced rise of proinfammatory molecules including TNF-α [95].Te major consequence of the immune system to the infection is the release of proinfammatory molecules which are collectively called cytokines storm.Te cytokine storm featured the presence of IL-1, IL-2, IL-6, IL-8, and TNF-α [95].Enhanced production of TNF-α during COVID-19 infection causes the server activation of the trigeminovascular system [96].

Discussion
Migraine is a complex disorder and is still a matter of concern despite large treatment modalities.Discussing its various aspects including an abnormal spreading depression of neuronal and glial cells gave a glimpse into the imitation of the attack cycle, which further leads to the activation of the trigeminovascular system [97].Te activation of the same generates head pain, which has been extensively researched.Te chronifcation, which is more than just "one cup of tea" might be the product of a slew of molecules and is something that must be understood.It has been shown that several genes are responsible for defning one's susceptibility to migraine conditions, and these genes also indicated that they are population-specifc [98,99].: Te cycle begins with the binding of CGRP to its receptor, the CGRPR, which activates PKA (protein kinase A), which then phosphorylates the MAPK (mitogen activating protein kinase).When MAPK is phosphorylated, it causes p38 expression to increase as a result of the phosphorylation.Te phosphorylated form of p38 subsequently enters the nucleus, where it stimulates the production of infammatory genes such as TNF-α, protease.(b) After enhanced expression of protease, cleavage of mTNF-α is cleaved which after then binds with the TNFR1.Cascade of signal protein activation occurs after binding of sTNF-α to its TNFR1 which leads to the activation of IKK.IKK is responsible for the activation of NF-KB and activated form of protein enters into the nucleus and cause enhanced infammation reaction.6 Pain Research and Management Repeated CSD increases the chance of secondary damage which hinders the sensitivity of neuronal populations [11].Terefore, in maintaining the homeostasis of brain tissue and responding to injuries microglial cells play a signifcant role.However, genetic predisposition in the genes of the infammatory system might be the cause for CI and TNF-α is the prime candidate which is seen to be higher in the subjects of migraine.Furthermore, the −308G > A polymorphism causes higher protein production and makes the individual more susceptible to persistent reactions (Table 1).Increased release of TNF-α by the microglial can enhance the activation of the entire population in a positive feed mechanism (Figure 5).Evidence has shown that increased TNF-α expression spontaneously develops chronic infammatory demyelination with 100% penetrance [100].Tis negative impact on demyelination and degeneration ultimately leads to neuronal damage.However, the irrefutable reality is that damaged neuronal populations release a substantial quantity of ATP, which binds to the P2X7 receptor on microglial cells and causes TNF-α to be produced and secreted in a de-novo manner [101].Tere is additional evidence that TNF-α mediates activated microglial production of NO (nitric oxide), which inhibits neuronal respiration, resulting in glutamate release and consequent excitotoxicity [102].TNF-α-induced glutamate cytotoxicity was achieved through two mechanisms: the autocrine microglial release and inhibition of the glutamate transporter on astrocytes.
Also, some data suggests that TNF-α raises the level of brain-derived neurotrophic factor (BDNF) in trigeminal ganglion neurons in a way that depends on their activity.Tese neurons are thought to be involved in how BDNF afects the plasticity of neurons [103].Migraine suferers have been found to have signifcantly higher amounts of BDNF compared to the healthy population, linking this protein not just to brain plasticity but also to the regulation and central sensitization of pain [104].Te BDNF variant rs2049046, located on the 5′ end of the same transcript, has been shown to have a strong correlation with migraine [105].
Terefore, the higher level of TNF-α and binding to TNFR1 initiate a series of downstream signaling proteins and thus transduce the signal.Furthermore, activation leads to the activation of kinases such as TAK1 and the IKK complex.IKK is responsible for the phosphorylation of IkBα (an inhibitor of NF-kB) and promotes its proteasomal degradation [82].NF-κB dimmers are the transcription factor which is the prime signaling protein that is being activated by the signaling pathway.Activated NF-B dimers are then translocated to the nucleus, where they regulate the production of proinfammatory cytokines such as tumor necrosis factor-alpha and interleukin-1 [84].Autocrine signaling maintains the level of TNF-α, but this chronic level further decreases the threshold and increases the sensitivity.
To this end, polymorphism in the TNF-α would result in increased protein production, and persistent infammation would most likely be caused by TNF-positive loop activation.TNF-α just one molecule among many and represents just the "tip of the iceberg" but it still presents a signifcant role in neurogenic neuroinfammation.Accordingly, neurogenic neuroinfammation is becoming acknowledged as a potential pathogenic process that may have the potential to control the severity of the disease and categorize the condition as an infammatory disorder.

Future Perspective
Praise to the research that has revealed various treatment strategies, but still all relate to the symptomatic treatment.Various new treatment strategies have been now discovered which probably increases the chance of Pain Research and Management treatment.Anti-TNF therapy has been used for treating various diseases for a long period [66,[106][107][108].Te usage of Venlafaxine in headaches, which was dosageand time-dependent, resulted in pain relief [109].Furthermore, Abdolahi and his colleagues reported that taking extra β-3 fatty acids and curcumin could be a new, promising way to treat migraines because the two together greatly decreased TNF-α messenger RNA (mRNA) in a way that worked well with each other [110].As a result, there is a window of opportunity for developing novel migraine treatment options related to the TNF-α signaling pathway.Te signifcant involvement in maintaining the autocrine loop mediated by the degradation of BAG-1 and miR-342 might be a possibility for a therapeutic approach in the TNF-α signaling pathway.Tis may aid in preventing the progression of episodic migraine to chronic migraine.Enclosing the section, continued research for the discovery of genetic, epigenetic, and molecular biomarkers, is needed which help for identifying the diseases and also motivate personalized medicine to treat migraine.

Limitation
Tere is an urgent need for a meta-analysis for the −308G > A, which would likely enhance statistical power and shed light on the strong association between the variant and migraine.Also, the facts remain unanswered, including whether indiscriminate use of anti-TNF medicines might alter the disease's natural course and whether anti-TNF therapy should be used alone or in conjunction with immunomodulators over time.Although biomarker research and validation have signifcantly improved diagnostic precision and measures of therapeutic efcacy, there are currently no biomarkers for chronic or episodic migraine.

Conclusion
For a long period, TNF-α is the primary candidate when linking neurogenic neuroinfammation and migraine.Activation of the microglial and auto-activation loops of TNF-α signifcantly infuences the chronicity and disease severity.As a result, it is obvious that neurogenic neuroinfammation plays a substantial role in migraine pathogenesis and that microglial activation plays a vital role in disease development in terms of TNF-α signaling.) of all cortical neuronal and astrocyte depolarization leads to release of vasoactive product such as CGRP and neurotransmitter (glutamate).Microglial activation is caused by both CGRP and glutamate and stimulates the release of TNF-α which is the prime focus of infammation.TNF-α also autostimulates and causes positive loop activation and is responsible for the sensitization of nociceptors, and this continuous stimulation will become a risk factor for the transition of episodic to chronic headache.

Figure 4
Figure4: Te cycle begins with the binding of CGRP to its receptor, the CGRPR, which activates PKA (protein kinase A), which then phosphorylates the MAPK (mitogen activating protein kinase).When MAPK is phosphorylated, it causes p38 expression to increase as a result of the phosphorylation.Te phosphorylated form of p38 subsequently enters the nucleus, where it stimulates the production of infammatory genes such as TNF-α, protease.(b) After enhanced expression of protease, cleavage of mTNF-α is cleaved which after then binds with the TNFR1.Cascade of signal protein activation occurs after binding of sTNF-α to its TNFR1 which leads to the activation of IKK.IKK is responsible for the activation of NF-KB and activated form of protein enters into the nucleus and cause enhanced infammation reaction.

Figure 5 :
Figure 5: Flow diagram depicts how migrainous brain is triggered by slow propagating wave (2-5 mm•min −1) of all cortical neuronal and astrocyte depolarization leads to release of vasoactive product such as CGRP and neurotransmitter (glutamate).Microglial activation is caused by both CGRP and glutamate and stimulates the release of TNF-α which is the prime focus of infammation.TNF-α also autostimulates and causes positive loop activation and is responsible for the sensitization of nociceptors, and this continuous stimulation will become a risk factor for the transition of episodic to chronic headache.

Table 1 :
Features of diferent association study utilizing case-control and cohort study design.