AIF1
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Allograft inflammatory factor 1 (AIF-1) also known as ionized calcium-binding adapter molecule 1 (IBA1) is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that in humans is encoded by the ''AIF1''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
.


Gene

The AIF1 gene is located within a segment of the
major histocompatibility complex The major histocompatibility complex (MHC) is a large Locus (genetics), locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for Cell (biology), cell surface proteins essential for the adaptive immune system. The ...
class III region. It has been shown that this gene is highly expressed in testis, spleen, and brain but weakly expressed in lung, and kidney. Among brain cells, the Iba1 gene is strongly and specifically expressed in microglia. Circulating macrophages also express Iba1.


Function

AIF1 is a protein that exists in the
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
, and it is highly evolutionarily conserved. It is also possibly identical to three other proteins, Iba-2, MRF-1 (microglia response factor) and daintain. However complete functional profiles of all three proteins and how they overlap is unknown. IBA1 is a 17-
kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...
EF hand The EF hand is a helix–loop–helix structural domain or ''motif'' found in a large family of calcium-binding proteins. The EF-hand motif contains a helix–loop–helix topology, much like the spread thumb and forefinger of the human hand, in ...
protein that is specifically expressed in
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s /
microglia Microglia are a type of glia, glial cell located throughout the brain and spinal cord of the central nervous system (CNS). Microglia account for about around 5–10% of cells found within the brain. As the resident macrophage cells, they act as t ...
and is upregulated during the activation of these cells. Iba1 expression is up-regulated in microglia following nerve injury, central nervous system ischemia, and several other brain diseases. AIF1 was originally discovered in
atherosclerotic Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by elev ...
lesions in a rat model of chronic
allograft Allotransplant (''allo-'' meaning "other" in Ancient Greek, Greek) is the Organ transplant, transplantation of cell (biology), cells, Biological tissue, tissues, or Organ (anatomy), organs to a recipient from a genetically non-identical donor of ...
cardiac rejection. It was found to be upregulated in
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s and
neutrophil Neutrophils are a type of phagocytic white blood cell and part of innate immunity. More specifically, they form the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. Their functions vary in differe ...
s in response to the cytokine
IFN-γ Interferon gamma (IFNG or IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. ...
. AIF1 expression has been seen to increase in vascular tissue in response to arterial injury, specifically it is found in activated vascular smooth muscle cells in response to IFN-γ,
IL-1β Interleukin-1 beta (IL-1β) also known as leukocytic pyrogen, leukocytic endogenous mediator, mononuclear cell factor, lymphocyte activating factor and other names, is a cytokine protein that in humans is encoded by the ''IL1B'' gene."Catabolin" ...
, and T-cell conditioned media. In
vascular smooth muscle Vascular smooth muscle is the type of smooth muscle that makes up most of the walls of blood vessels. Structure Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood v ...
cells, activation is responsible for arterial thickening in allografts through over proliferation. AIF1 has been found to enhance growth and promote proliferation in vascular smooth muscle cells through deregulation of the cell cycle. It does this by shortening the cell cycle and altering the expression of cyclins. Though histologically different, AIF1 has also been shown to promote the proliferation and activation of
endothelial The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the res ...
cells (EC). EC activation, leads to proliferation and migration of cells, which is involved in multiple normal vascular processes, such as atherosclerosis, angiogenesis, and wound healing. It is currently theorized that AIF1 works to control endothelial cell proliferation and migration through action in
signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...
pathways. It has features of a cytoplasmic signaling protein, including several domains that allow for binding to multiprotein complexes, called
PDZ domain The PDZ domain is a common structural domain of 80-90 Amino acid, amino-acids found in the Signal transduction, signaling proteins of bacteria, yeast, plants, viruses and animals. Proteins containing PDZ domains play a key role in anchoring recept ...
s. In endothelial cells, AIF1 has been specifically shown to regulate
vasculogenesis Vasculogenesis is the process of blood vessel formation, occurring by a ''De novo synthesis, de novo'' production of endothelial cells. It is the first stage of the formation of the vascular network, closely followed by angiogenesis. Process ...
, including the formation of aortic sprouting and tube-like formations. AIF1 been shown to interact with kinase p44/42 and
PAK1 Serine/threonine-protein kinase PAK 1 is an enzyme that in humans is encoded by the ''PAK1'' gene. PAK1 is one of six members of the PAK family of serine/threonine kinases which are broadly divided into group I (PAK1, PAK2 and PAK3) and group II ...
, two previously known signal transduction molecules, in regulating these processes. AIF1 also shows distinct differences in the pathways by which it regulates endothelial cells, macrophages, and vascular smooth muscle cells. Upregulation of AIF-1 is connected with increased migration of mononuclear peripheral blood cell. In the
CD14 CD14 ( cluster of differentiation 14) is a human protein made mostly by macrophages as part of the innate immune system. It helps to detect bacteria in the body by binding lipopolysaccharide (LPS), a pathogen-associated molecular pattern (PAMP). ...
positive cells, AIF-1 support secretion of IL-6 and various
chemokine Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In addit ...
s. AIF-1 may also play a role in the T-cell response. It has been shown that AIF-1 increases expression of IL-2 and IFN-γ in T-cells, while the expression of IL-4 and TGF-β is decreased. The presence of AIF-1 also inhibits polarization into regulatory T cells.


Clinical significance

Allograft Inflammatory Factor 1 is found in activated macrophages. Activated macrophages are found in tissues with inflammation. AIF1 levels in healthy humans have been found to positively correlate with metabolic indicators, such as body mass index,
triglyceride A triglyceride (from '' tri-'' and '' glyceride''; also TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates ...
s, and fasting plasma glucose levels. The excess of adipose tissue found in obese patients is found to cause chronic inflammation with an increase in the number of activated macrophages. Subsequently, AIF1 may be an accurate indicator of macrophage activation in the body. There is also evidence that AIF1 could be a marker for
diabetic nephropathy Diabetic nephropathy, also known as diabetic kidney disease, is the chronic loss of kidney function occurring in those with diabetes mellitus. Diabetic nephropathy is the leading cause of chronic kidney disease (CKD) and end-stage renal disease ...
when detected in serum. Since diabetic nephropathy is a consequence of long-term type 1 and type 2 diabetes, this consistent with evidence that AIF1 may be associated with other aspects of diabetes. It is found in activated macrophages in the
pancreatic islets The pancreatic islets or islets of Langerhans are the regions of the pancreas that contain its endocrine (hormone-producing) cells, discovered in 1869 by German pathological anatomist Paul Langerhans. The pancreatic islets constitute 1–2% o ...
, and has been shown to decrease insulin secretion, while simultaneously impairing glucose elimination.


Role in cancer progression

In recent years, the possibility of a role for AIF-1 in cancer development has also been considered. Significantly higher levels of ''AIF-1'' expression were found in hepatocarcinoma cell lines and in tissue compared to healthy samples. One option in which AIF-1 may contribute to the development of pathology is involvement in the proliferation and migration of tumor cells. It was also shown that AIF-1 promote cell proliferation in the breast cancer cells line. This effect was dependent on time and level of AIF-1 protein. Upregulation of ''AIF-1'' enhanced activity of NF-ΚB and increased expression of
cyclin D1 Cyclin D1 is a protein that in humans is encoded by the ''CCND1'' gene. Gene expression The CCND1 gene encodes the cyclin D1 protein. The human CCND1 gene is located on the long arm of chromosome 11 (band 11q13). It is 13,388 base pairs lo ...
. Cyclin D1 contribute to cell proliferation and mutation in this gene has been connected with variety of tumors. It has also been shown that ''AIF-1'' expression can contribute to progression of cancer by inhibition of
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
in cells.


Role in Rheumatoid arthritis

The role of up-regulation of expression of ''AIF-1'' was demonstrated in
rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects synovial joint, joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and h ...
. Presence of AIF-1 was confirmed in synovial tissue of patient with this pathology. ''AIF-1'' was strongly expressed in several cell types of synovial tissue, such as
fibroblast A fibroblast is a type of cell (biology), biological cell typically with a spindle shape that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and ...
and synovial cells, but also in infiltrated immune cells. It was also shown that upregulation of ''AIF-1'' contribute to induction of enhances the production of IL-6 . Another factor by which the expression of ''AIF-1'' affects the course of the disease is increased proliferation of synovial cells.


Role in kidney diseases

AIF-1 was also considered as a player in the diseases connected with
fibrosis Fibrosis, also known as fibrotic scarring, is the development of fibrous connective tissue in response to an injury. Fibrosis can be a normal connective tissue deposition or excessive tissue deposition caused by a disease. Repeated injuries, ch ...
. For example, in kidney diseases overexpression of AIF-1 in the
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s contributes to signaling through
AKT Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, tra ...
and
mTOR The mammalian target of rapamycin (mTOR), also referred to as the mechanistic target of rapamycin, and sometimes called FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a kinase that in humans is encoded by the ''MTOR'' gene. ...
. Another way in which AIF-1 contribute to the pathology of kidney is upregulation of enzyme NADPH oxidase 2. This upregulation leads to
oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...
in the cells and progression of renal injury. Expression of AIF-1 is considered to be linked with calcification in hemodyalisis patients. Higher presence of AIF-1/NF-''κ''B/MCP-1/CCR-2-pathway was detected in calcifaied vascular smooth muscle cells. AIF-1 was also detected as a potential factor which contribue to apoptosis and inflammation.


Role in retinal diseases

Since the immune response in the retina is tightly regulated under physiological conditions, microglia may play a role in retinal diseases.
Retinitis pigmentosa Retinitis pigmentosa (RP) is a member of a group of genetic disorders called inherited retinal dystrophy (IRD) that cause loss of vision. Symptoms include trouble seeing at night and decreasing peripheral vision (side and upper or lower visua ...
is an inherited disease in which photoreceptors are gradually degenerated. That condition gradually leads to reduced of dark vision and eventually complete blindness. In an experimental model RCS (The Royal College of Surgeons) rats with progression loss of photoreceptors, level of AIF-1 was elevated in retina in contrast to wild type


References


Further reading

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External links

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PDBe-KB
provides an overview of all the structure information available in the PDB for Human Allograft inflammatory factor 1 {{PDB Gallery, geneid=199 EF-hand-containing proteins