**3. Membrane receptors participation in the inflammation and edema pathophysiology modulation**

Scientific advances provide new discoveries about plasma membrane receptors function and identity. Molecules impermeable to the membrane can selectivity cross to the intracellular environment through these receptors. Many receptors characteristics are investigated in the physicochemical field, including biophysical properties and structure. Membrane receptors generally have three classifications: receptors coupled to enzymes such as tyrosine kinase (RTKs), G protein-coupled receptors (GPCRs), and ion channels [46]. Interestingly, there is a group of membrane proteins that are widely addressed in scientific research for modulating inflammatory mediators release and search for new anti-inflammatory drugs. Based on this, the following topics exhibit some of studied plasma membrane receptors related to the inflammatory response.

#### **3.1 Toll-like receptors**

The host defense against infections and tissue damage is a complex mechanism. In this process, the cells must recognize PAMPs and DAMPs to initiate a specific intracellular response against infectious agents, such as viruses and bacteria or dangerous signs, such as burn injuries [47].

The Toll-Like Receptors (TLRs) are a group of membrane proteins involved in inflammation and immunity. They act on PRRs expressed in macrophages, neutrophils, and dendritic cells [47, 48]. TLRs compose the interleukin 1 receptors superfamily (IL-1Rs) with slight structural differences. Ten TLRs subtypes were described in humans (TLR1–10), although other species may exhibit variations.

TLRs are located in different compartments in the cell. For instance, the subtypes 1, 2, 4, 5, and 6 are located at the cell plasma membrane, whereas subtypes 3, 7, 8, 9, and 10 are in the intracellular compartment, located in endosomes. [RF1] TLR2 and TLR4 are the best-studied receptors of this family [49, 50].

TLRs, when activated, are essential for the host response to harmful agents, since these receptors modulate the inflammatory mediators release. The factor nuclear kappa β (NF-kβ) and mitogen-activated protein kinase (MAPKs) are classical pathways activated by Toll-Like Receptors (**Figure 2**) [48]. When stimulated

**Figure 2.** *Plasma membrane TLR signaling pathway. TLR receptor activation triggers AP-1 and NF-k*β *transcription factors.*

by a ligand, such as lipopolysaccharides (LPS), TLRs transduces the signal through adaptor molecules in the intracellular environment. Myeloid differentiation primary response 88 (MyD88) is an adaptor molecule of the interleukin- 1 receptorassociated kinases (IRKs) signaling with subsequent TNF receptor-associated factor 6 (TRAF6) activation. TRAF6 activates the growth factor β-activated kinase 1 (TAK1), which triggers an enzymatic complex associated with NF-kβ translocation to the cell nucleus. TAK1 signaling also activates the MAPKs pathway with activator protein 1 (AP-1) nuclear factor translocation. This pathway leads to various pro-inflammatory mediators transcription, such as cytokines (IL-1 family, IL-6, TNF-α), **COX-2** stimulation (prostaglandin E2), and interferons [50, 51].

TLR activation may exhibit a crucial role in edema formation through inflammatory mediator production (**Table 1**). In a recent paper, Okada and colleagues [55] described brain edema reduction in a subarachnoid hemorrhage model (SAH) mouse after treatment with TAK-242, a TLR4 receptor inhibitor. The molecular mechanism by which this occur was not evaluated. However, the pathophysiology development of brain edema shows association with TLR4 function. In liver diseases, such as acute liver failure, astrocyte swelling is a notable characteristic that promotes brain edema formation. Interestingly, NF-kβ and MAPKs-induced cytokine release are crucial mechanisms for astrocyte swelling development [56, 57]. Jayakumar et al. [58] have demonstrated LPS and cytokines-induced astrocyte swelling increase. These data suggest TLR4 may be a target in the brain edema pathophysiology**. Table 1** represent more data about TLR receptors in the inflammatory context.

#### **3.2 Histamine receptors**

Histamine constitutes an essential molecule in cell biology, edema pathophysiology, and the inflammatory process. The histamine synthesis occurs with the amino acid L-histidine decarboxylation through the histidine decarboxylase enzyme (HDC). Other inflammatory mediators can lead to increase HDC activity, such as IL-1 cytokines [60]. Histamine synthesis occurs in different body cells, although this production primordially occurs in mast cells and basophils [61]. In these cells, histamine is


*Inflammatory Mediators Leading to Edema Formation through Plasma Membrane Receptors DOI: http://dx.doi.org/10.5772/intechopen.99230*

#### **Table 1.**

*Plasma membrane TLRs modulate inflammatory mediators.*

stored in cytoplasmatic granules and released according to the stimulus presented. Histamine interacts with GPCRs membrane receptors classified as histamine receptors (HRs) and divided into four subtypes: HR1, HR2, HR3, and HR4 (**Table 2**) [61].

The histamine action is remarkable in the vascular modulation mechanism, including vascular permeability increase. HRs actuate as a second messenger, leading to intracellular signal and cytokine synthesis [68]. A study by Delaunois and co-authors [69] showed a protective HR3 agonist role in pulmonary edema stimulated by inflammation-promoting molecules. In addition, HR3 stimulation appears to play a significant role in perfusion in post-burn tissues [70]. HRs also participate in the mechanisms related to antinociception [61].

Among HRs, HR4 has become a new antihistamines studies target. The HR4 activation triggers MAPK, which leads to pro-inflammatory mediators synthesis [60]. Coruzzi and collaborates [66] showed promising results in inhibiting paw edema by HR4 in acute inflammation. After carrageenan-induced edema, two selective HR4 inhibitors, JNJ7777120, and VUF6002, respectively, were evaluated. Inhibition by JNJ7777120 after two hours of carrageenan induction has shown notable values compared to VUF6002. Another study using JNJ7777120 described the anti-nociceptive role in a pain inflammation model through HR4 antagonism. Additionally, HR4 inhibition decreases neutrophilic influx to stimulated area pretreated with JNJ7777120 [67]. These findings suggest HR4 with a crucial role in edema and pain mechanism.

#### **3.3 Serotonin receptors**

Diseases involving the psychiatric area have been widely addressed in scientific research, such as depression. [RF2] Factors involving mood and mental disorders,


#### **Table 2.**

*Histamine receptors.*

include serotonin, a critical functional amine in this disease. Interestingly, serotonin regulates inflammatory signaling, playing a role in vascular permeability. Therefore, serotonin becomes a multifunctional molecule modulating many body processes [71–73].

5-hydroxytryptamine (5-HT), serotonin is synthesized from the amino acid tryptophan. The enzymes tryptophan hydroxylase and tryptophan decarboxylase are responsible for 5-HT production. Serotonin may be found in various body tissues, such as enterochromaffin, platelets, brain, and lung [71]. 5-HT interacts with membrane receptors (5-HT receptors), divided into seven families (5-HT1–7), where these receptors are GPCRs, except for 5HT3, which belongs to ion channels. These receptors possess fourteen subtypes: 5-HT1 (A, B, D, E, and F), 5-HT2 (A, B, and C), 5-HT3 (A, B), 5-HT4, 5-HT5 (A), 5-HT6, and 5-HT7 [74, 75].

The 5-HT role in other systems has been studied over the years. During inflammation, 5-HT plays an essential role in vascular permeability, as well as histamine, in addition to participating in pro-inflammatory mediator production [72]. In this context, serotonergic receptor subtypes act on inflammation process biochemistry. 5-HT7 is influential in peripheral inflammatory modulation, according to Albayrak and co-authors [76]. The 5-HT7 participates in the nociception mechanism with other 5-HT receptors, such as 5-HT1 and 5-HT2 [77, 78]. The 5-HT2 subtype (A) subtype also modulates the inflammatory process. Nishiyama studies [79] have demonstrated a role for 5-HT2A in cytokines synthesis during an inflammation model induced by endotoxin shock. The 5-HT2A inhibition reduced TNF-α, IL-1β, IL-8, and IL-6 levels. Interestingly, IL-10 levels (cytokine with anti-inflammatory function) increased due to 5-HT2A inhibition. Additionally, 5-HT2A shows to play a function in body temperature control [80]. These data demonstrate a relevant role for 5-HT2A receptors in inflammation pathophysiology (**Table 3**).

#### **3.4 Purinergic receptors**

The purinergic system is a group of transmembrane proteins activated by extracellular purine ligands, such as adenosine and other derivatives, adenosine triphosphate and diphosphate (ATP and ADP). Interestingly, when the ATP molecule is found in elevated concentration in the extracellular environment (eATP), this nucleotide may become a DAMP and regulates the inflammatory process. Purinergic receptors are formed by two groups (P1 and P2) differing in structure and activation ligands on mammalian cells [93, 94].

*Inflammatory Mediators Leading to Edema Formation through Plasma Membrane Receptors DOI: http://dx.doi.org/10.5772/intechopen.99230*


#### **Table 3.**

*Serotonin receptors.*

The adenosine molecule activates the P1 group and possesses four subtypes (A1, A2a, A2b, and A3). The P1 group comprises GPCRs receptors, and the P2 group is extensive and divided into two families, P2X and P2Y. The P2X receptors form ATPactivated ion channel receptors with seven subtypes (P2X1–7). P2Y receptors are GPCRs, like the P1 group. Interestingly, ATP and their derivatives activate the P2Y receptors, although, pyrimidine molecules, such as uridine diphosphate (UDP and UDP-glucose), also modulate some subtypes activation. This family consists in eight subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P213, and P2Y14) in mammals. The purinergic receptors participate in inflammation and immune response and are expressed in several tissues [14].

In the purinergic group, the receptor of great scientific notoriety is the P2X7 receptor (P2X7R), addressed in several mechanisms, such as cell death and inflammatory cytokines release [14]. P2X7R have the capacity to increase membrane permeability for large solutes after prolonged ATP activation. The prolonged P2X7R stimulation induces a pore opening that allows the molecules of up to 900 Da. This mechanism highlights the P2X7R as a pore-forming protein, similar to other membrane receptors, such as some TRP channels [95].

However, a striking P2X7R feature is the participation in the maturation of IL-1 cytokine family (IL-1β and IL-18) release. The IL-1β and IL-18 production and maturation require two signaling mechanisms, one mediated by pattern recognition receptors (via TLRs family activation) and a second by a danger signal, such as eATP. The activation of TLRs induces nuclear transcription through NF-kβ of the

immature forms of these cytokines (ProIL-1β and ProIL-18), concluding the first stage.The eATP activates the P2X7R, beginning the cascade signaling that compose the Nod-like receptor protein-3 (NLRP3) inflammasome complex with subsequent IL-1β and IL-18 maturation and release [48, 96]. The following figure illustrates this mechanism more clearly (**Figure 3**).

The IL-1β inhibition in inflammation and pain has been addressed in several inflammation studies. Experiments in vivo using P2X7R antagonist have demonstrated improvements in the swelling caused by inflammation in a model of paw edema [97, 98]. The pain sensibility mechanism is linked to vascular permeability, causing edema [2]. Furthermore, P2X7R inhibition reduces pro-inflammatory cytokines, such as IL-1β and other mediators, since the P2X7R is responsible for these mechanisms [96]. Additionally, the P2X4 receptor has participated in IL-1β and IL-18 signaling based on Chen et al. [99]. Further, other purinergic receptors data in edema and inflammation have already been approached in the literature (**Table 4**).
