Overview Of The Interleukin-1 Beta (IL-1 Beta) Protein And Its life cycle

Introduction

This review illustrates the life cycle of the interleukin-1 beta (IL-1 beta) protein. IL-1 beta is a proinflammatory cytokine that plays an important role in immune response. Immune system is the body’s response to the pathological change or the antigen. It can be divided into two subsystems, an adaptive and an innate immune system. Adaptive immune system is acquired through one’s life, whereas one’s innate immune system starts developing before birth. Innate immune system is one’s first response to the antigen and if it is unsuccessful in eliminating the threat, more specific adaptive immune response sets in. IL-1 beta is part of the innate immune system and therefore involved in inflammatory response and defence against pathogens. Since it is a proinflammatory cytokine it promotes inflammatory response and can contribute to autoinflammatory disease, if excessively produced. Its function, gene expression, synthesis and secretion have been vastly studied. As a consequence of many studies there is some confusion in terms of categorizing IL-1 beta and IL-1 family in general. This review uses the most recent and widely excepted terms and abbreviations.

An important aspect of IL-1 beta’s function is its ability to cross the blood brain barrier and therefore influencing the neural activity. With a growing literature on involvement of neuroinflammation in many psychiatric disorders, understanding the IL-1 beta’s life cycle is even more important.The purpose of this narrative review is to provide an overview of IL-1 beta’s genetic background, expression, structure, synthesis and secretion and a brief description of its function.

The Interleukin-1 beta family

Interleukin-1 beta (IL-1 beta) is an immune system protein that is a part of protein family called cytokines. Although cytokines have many different roles, they are all involved in inflammatory response and defence against pathogens and therefore released as an immune system response to an antigen. Cytokines can be further divided into several families. IL-1 beta was at first part of the interleukin-1 family (IL-1). Although that has not changed, further scientific discoveries also placed IL-1 beta in an immunoglobulin (Ig) superfamily. There is no unified agreement on how to categorise cytokines. Some of the categories arose at the early stage of cytokine research but were not renamed after the additional discoveries. IL-1 family consist ligands and receptors. Its receptors are part of Toll-like receptors (TLRs) group. When it comes to naming the receptors, we can also see some discrepancies, for example Interleukin-1 receptor accessory protein can be abbreviated to IL-1RAcP, IL-1RAP or IL1-R3. This review will use the abbreviation IL-1RAP, consistent with NCBI’s gene name.

IL-1 was first described in the 1940s as a polypeptide hormone. In an extended review of IL-1, Dinarello (1996) described three ligands, two receptors and a receptor accessory protein. Three ligands mentioned were IL-1 alfa, IL-1 beta and IL-1Ra. The first two are receptor agonists and IL-1Ra is a receptor antagonist. Receptors described were IL-1RI and IL-1RII. While IL-1RI transduces a signal, IL-1RII does not and therefore acts as a decoy receptor. A decoy receptor decoys IL-1 beta, so it does not bind to IL-1RI. Moreover, it binds IL-1 beta with greater affinity than IL-1RI and therefore regulates the inflammatory response. Interleukin 1 receptor accessory protein (IL-1RAP) joins IL-1RI to form a high-affinity complex but does not bind to the ligands.

Since these initial descriptions of IL-1, the researchers have found new members of IL-1. The family has vastly expanded, now containing eleven ligands and ten receptors reported. In a more recent review, Dinarello (2011) reported properties of IL-1 ligands and receptors. Most ligands have a proinflammatory purpose, but some of them work as a receptor antagonist or an anti-inflammatory protein. IL-1 receptors include coreceptors, binding proteins, decoy and inhibitory receptors. For IL-1 beta functioning only three receptors are important (IL-1RI, IL-1RII and IL-1RAP), therefore this review focuses on them.

Function of IL-1 beta

The main function of all members of the IL-1 family is the initiation of an innate immune response, a nonspecific response to infection, and its receptors are also involved in defence against stress. Immune response must be tightly regulated otherwise it can cause autoimmune and autoinflammatory diseases, in which over secretion of IL-1 beta plays an important role. Autoinflammatory and autoimmune disease are characterized by pathological processes of immune system where the body is attacking itself. Interestingly, genetic research shows that one’s inflammatory response depends on the IL-1 genetic patterns. IL-1 beta was primary the point of interested because of its involvement in pyroptosis, but current research shifted into another direction. IL-1 beta can cross the blood-brain barrier and therefore influence neuronal activity – in such a case it can produce classical sickness behaviour, influence autonomic and neuroendocrine regulation, and influence cognitive and affective functions. A recent review of IL-1 beta polymorphisms described its involvement in major depressive disorder (MDD), bipolar disorder (BD), schizophrenia, cognitive functions and dementia. Moreover, IL-1 beta was used to investigate MRI traits of patients with MDD, BD, schizophrenia and Alzheimer’s disease. Another review and meta-analysis reported the role of cytokines, including IL-1 beta, in autism spectrum disorder.

It all starts with genesIL-1 beta gene is located on chromosome 2, its exact location being 2q14 (IL1B interleukin 1 beta. Promoter region of IL-1 beta contains a clear TATA box and regulatory regions that are found upstream and downstream from the gene. A regulator region is part of genome that controls expression of other genes. They usually occur in proximity of the gene but in the IL-1 beta case, they can be found several base pairs upstream and a few base pairs downstream from the transcription start site. The gene expression is triggered by the inflammation. Process includes several kinases and transmitting the functional nuclear factor kappa-light-chain-enhancer of activated B-cells (NF-kB) to the nucleus.

Bensi et al. (1987) were the first one to report a complete nucleotide sequence of a gene encoding for IL-1 beta protein in humans. The gene has seven exons and it occupies the region of 7.5 kb. Among other things, they conclude that its sequence was conserved through evolution. They compared human and murine genes and discovered that the lengths of both exons are almost identical. Moreover, Hughes (1994) suggests that IL-1Ra evolved from an IL-1 beta gene duplication. Interestingly, the first exons of both proteins are homologous. In IL-1Ra this exon encodes the signal peptide, which is in fact a non-coding exon in IL-1 beta. The current translation of IL-1 beta without the signal peptide therefore probably occurred about 120 million years ago. Hughes (1994) pointed out that the possible common ancestor of both molecules could be an alternatively spliced gene, which would explain alternative splicing in this family. IL-1 beta as moleculeIL-1 beta is synthesized as precursor with a molecular mass of 31-kD and then cleaved to 17-kD mature active molecule. The mature protein has a sequence of 153 amino acids. It is folded in an antiparallel beta-sheet conformation with 12 beta strands. The strands are arranged in a three-fold symmetric pattern forming a beta-barrel core motif. Veerapandian (1992) described the structure as a barrel, where one side is shallow and open and the other one is closed. At the open end we can find an amino and a carboxyl group. It also possesses five beta-loops, two of them can be found on the open end and three on the closed one. The conformation is held together by hydrogen bonds. Craig et al., (1987) reported that IL-1 beta is a globular protein. Globular proteins can be further categorized into structural classes in which case IL-1 beta is, according to Levitt and Chothia (1976), a part of all-beta class. Globular proteins have long polypeptide chains, folded in a way that hydrophobic residues are predominantly on the inside and hydrophilic ones on the outside. Indeed, the inner side of IL-1 beta barrel has 24 hydrophobic residues, mainly leucines and phenylalanines. On the surface we can find residues and loops between beta-strands.

IL-1 receptors are part of toll-like receptors (TLRs) with a functional toll-IL1-recptor (TIR) cytoplasmic domain. IL-1RI and IL-1RII also contain an extracellular Ig-like domain. IL-1RII does not have a TIR domain but is nonetheless considered to be part of this subgroup due to its high resemblance to IL-1RI. The lack of TIR domain is the reason why IL-1RII cannot transduce a signal. From synthesis to secretionThe secretion of IL-1 beta is tightly regulated, since it can quickly become toxic resulting in an autoinflammatory disease. During the activation of innate immune system, levels of IL-1 beta are regulated through gene expression, synthesis, secretion, and receptors. The secretion of the protein is mainly carried out by blood monocytes and macrophages. Monocytes and macrophages start producing IL-1 after various proinflammatory stimuli. It is first synthesized as a non-active precursor and it is lacking a signal peptide. The precursor needs cleavage to yield the active IL-1 beta protein. Cleavage is provided by interleukin-1 converting enzyme (ICE), of which caspase-1 is most extensively studied. Caspase-1 is an intracellular cysteine protease found in tissue macrophages and dendritic cells. Although it was believed that caspase-1 is indispensable for IL-1 beta maturation, recent studies have proposed other potential enzymes as ICEs. Probably most cases of caspase-1 independent processing’s, involve other ICEs because of tissue and function specific factors. Various joint inflammations involve neutrophils, where IL-1 beta also plays a role. When there are neutrophils involved, maturation of IL-1 beta is caspase-1 independent. Bode et al. (2015) reported a caspase-8 dependant IL-1 beta maturation, and Becherer et al. (1995) reported a granzyme A cleaved maturation of IL-1 beta.

IL-1 beta is produced by macrophages and secreted into the extracellular milieu (Eder, 2009). Secretion is partly depended on the signal peptide, which IL-1 beta does not have. Eder (2009) mentions different pathways for this non-classical secretion. Some of the suggested pathways include secretion via exocytosis of secretory lysosomes or exosomes, by shedding of plasma membrane microvesicles, by export through the plasma membrane, and as leakage of cell contents from lysed cells. Eder (2009) further states, that it is unclear why different ways of secretion exist and which is the most important one. On the other hand, Brough & Lopez-Castejon (2011) suggested that all secretion pathways are part of one continuum. The specific secretion pathway is therefore dependent on the strength of the inflammatory stimulus.

The circle of life: Back to the genes

Dinarello (1996) explains regulation of IL-1 beta after secretion. One of the mechanisms includes a decoy receptor and the other a receptor antagonist. Decoy receptor IL-1RII tightly binds the mature protein and therefore prevents IL-1 beta to transduce a signal. Receptor antagonist sIL-1Ra (a structural variant of IL-1Ra) competes with IL-1 beta for binding to cell surface receptors. After the ligand binding, IL-1RI undergoes conformational changes and thus can bind to IL-1RAP to form a functional complex. To do that they also need to recruit adaptor protein MyD88. Adaptor proteins assist protein binding in signalling complexes. IL-1 family has four adaptors – TRAF6, TOLLIP, IRAK3 and MyD88, which are involved in activation of kinases (Dembic, 2015). The signalling cascade of IL-1 beta futher involved IL-1R associated kinase (IRAK) family, E3 ubiquitin ligases (PELI-1 and -2) and Toll-interacting protein (Tollip) among others. The main goal of IL-1 beta cascade is to activate NF-kB, which then influences the transcription of IL-1 genes in the target cell.

Conclusion

As we saw, IL-1 beta plays an important and a diverse role in one’s immune response. It is therefore tightly regulated by various mechanisms including IL-1RII and IL-1Ra. From the initial descriptions of IL-1 ligands and receptors, the family has vastly expanded. IL-1 beta gene is located at 2q14, where its promoter regain includes a clear TATA box. Interestingly, regulator regions are spread up to thousand base pairs upstream the transcription start site, which is quite unusual. Another interesting insight from genetic research is the fact that IL-1Ra has probably evolved from IL-1 beta gene. Furthermore, the first exon in IL-1 beta is non-coding, whereas the first exon in IL-1Ra codes for a signal peptide. The mutation of signal peptide exon expression in IL-1 beta probably already occurred about 120 million years ago. Upon secretion precursor IL-1 beta must be cleaved to turn into active protein. The cleavage was thought to be caspase-1 dependent, but recent studies have proved that many other pathways are possible. The secretion of IL-1 beta must be regulated since excessive levels of it can lead to autoimmune disease. Part of this regulation happens after secretion, when IL-1 competes for receptor binding with IL-1Ra. Furthermore, it is decoyed by the IL-1RII so that its binding does not transduce a signal. Since IL-1 beta is part of the innate immune system, part of its role is to further influence the immune response. After it binds to IL-1RI the structure undergoes conformational changes, allowing to bind IL-1RAP and therefore form a functional complex. This complex further recruits MyD88 activating several kinases. The finish line of an IL-1 beta signalling cascade is the transferring of NF-kB to the nucleus, which then influences further transcription of IL-1 beta.