Find-me signals

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Cells destined for apoptosis release molecules referred to as find-me signals. These signal molecules are used to attract phagocytes which engulf and eliminate damaged cells. [1] Find-me signals are typically released by the apoptotic cells while the cell membrane remains intact. This ensures that the phagocytic cells are able to remove the dying cells before their membranes are compromised. [2] [3] A leaky membrane leads to secondary necrosis which may cause additional inflammation, therefore, it is best to remove dying cells before this occurs. [3] One cell is capable of releasing multiple find-me signals. Should a cell lack the ability to release its find-me signal, other cells may release additional find-me signals to overcome the discrepancy. [1]

Contents

Inflammation can be suppressed by find-me signals during cell clearance. [1] A phagocyte may also be able to engulf more material or enhance its ability to engulf materials when stimulated by find-me signals. [1]

A wide range of molecules, from cellular lipids, proteins, peptides, to nucleotides, act as find-me signals. [3] [4] [5]

History

The correlation between the early stages of cell death and the removal of apoptotic cells was first studied in C. elegans . Mutants that could not carry out normal caspase-mediated apoptosis were used to demonstrate that cells in the beginning stages of death were still efficiently recognized and removed by phagocytes. This occurred because the engulfment machinery of the phagocytes was still functioning normally even though the apoptotic process in the dying cell was disrupted. [6]

A study done in 2003 showed the breast cancer cells release find me signals known as lysophosphatidylcholine. [7] This research brought the concept of find-me signals to the fore front of cell clearance research and introduced the idea that dying cells release signals that flow throughout the body's tissues in order to alert and recruit monocytes to their location. [3]

Chemicals that act as find-me signals

Known types of find-me signals include:

All of these molecules are linked to monocyte or macrophage recruitment towards dying cells. [3] [4] [5] The receptor on the monocyte or other phagocyte for ATP and UTP signals has been shown to be P2Y2 in vivo. The receptor on the monocyte or other phagocyte for the CX3CL1 signal has been shown to be CX3CR1 in vivo. The roles of the S1P and LPC signals remained to be established through a model in vivo. [3]

Lipids

Lysophosphatidylcholine (LPC)

Identified in breast cancer cells, this find-me signals is released by MCF-7 cells to attract the THP-1 monocytes. [7] Other cells and different methods of apoptosis may be able to release LPC, but MCF-7 cells have been the most thoroughly studied.

The enzyme calcium-independent phospholipase A2 (iPLA2) is most likely responsible for the apoptotic cell releasing LPC as it is dying. [7] The amount of LPC released is small, so it is unclear how it is able to set up a concentration gradient in the serum or plasma in order to attract phagocytes to their location. [7] [3] High concentrations of LPC cause lysis of many cells in its vicinity. LPC may be present in a different chemical from rather than its native form when released by an apoptotic cell. It may bind to components of the serum, making it unavailable to be modified or taken into other tissues. LPC may also be able to function with other soluble molecules. [3]

The receptor on the phagocyte that is thought to be linked to LPC is G2A, but it has not been confirmed. [8] The role of LPC as a find-me signal has also not been characterized in vivo. [3]

Sphingosine 1-phosphate (S1P)

It has been suggested that the induction of apoptosis results in increased expression of S1P kinase 1 (SphK1). The increased presence of SphK1 is linked to the creation of S1P, which then recruits macrophages to the immediate area surrounding apoptotic cells. [9] It has also been suggested that S1P kinase 2 (SphK2) is a target of caspase 1, and that a cleaved fragment of SphK2 is what is released from dying cells into the surrounding extracellular space where it is transformed into S1P. [10] All of the studies thus far characterizing S1P have been done in vitro, and the role or S1P in recruiting phagocytes to apoptotic cells in vivo has not been determined. [3] Staurosine-induced cell death has been shown to influence caspase-1 to initiate the cleavage of SphK2. [10] In other forms of apoptosis, caspase-1 is not normally induced, meaning the formation of S1P needs to be further studied.

S1P can be recognized by the G protein-coupled receptors S1P1 through S1P5. Which one of these receptors is relevant in the recruitment of phagocytes to apoptotic cells is not yet known. [3]

Sphingosine kinase 1 and sphingosine kinase 2 have been linked to S1P generation during apoptosis through different pathways. [10] The level of SphK1 is increased during apoptosis while caspases cleave SphK2. [3]

CX3CL1

CX3CL1 is a soluble fragment of fractalkine protein that serves as a find-me signal for monocytes. [11] A soluble fragment of fractalkine that is usually on the plasma membrane as an intercellular adhesion molecule is sent out as a 60 kDa fragment during apoptosis as a find me signal. CX3CL1 release is dependent upon caspase indirectly. [11] CX3CL1 could also be released as part of microparticles from the beginning stages of apoptotic death of Burkitt Lymphoma cells. [11] [3]

The receptors on monocytes that are able to detect the presence of CX3CL1 are CX3R1 receptors, as shown in both in vivo and in vitro studies. [3]

Nucleotides: ATP and UTP

These were the most recent find me signals to be characterized as components of the supernatant of apoptotic cells. [12] Studies were able to show that the controlled release of the nucleotides ATP and UTP from cells in the beginning stages of apoptosis can potentially attract monocytes in vivo and in vitro. This has been observed in Jurkat cells (primary thymocytes), MCF-7 cells, and lung epithelial cells. Release is dependent upon caspase activity. [3]

Less than 2% of ATP released from the beginning stages of cell death is released when the dying cell's plasma membrane is still intact. The released ATP preferentially attracts phagocytes through chemotaxis, rather than random migration through chemokineses. [3]

The receptors on monocytes that are able to sense the release of nucleotides are in the P2Y family of nucleotide receptors. Monocytic P2Y2 has been shown to be able to recognize nucleotides in vitro and in genetically modified mice. [12]

Nucleotides are often degraded by nucleotide triphosphatases (NTPases) when they are in the extracellular space. [13] Only a small amount of ATP is released during find me signaling, so it is unclear how the nucleotide avoids degradation by NTPases in order to establish a gradient used to signal clearing by monocytes. NTPases may serve as regulators in various tissues in order to control how far the nucleotide signal can travel. [3] [12]

The signaling pathway within the monocyte downstream of P2Y receptor activation is still unknown. [3]

Others

The ribosomal protein S19 has been suggested as a possible find me signal. Apoptosis causes a dimerization of S19, inducing a conformation change that allows it to bind to the C5a receptor on monocytes. [14] Research suggests that S19 is released during the late to final stages of apoptosis. [3]

EMAPII, a fragment of tyrosyl tRNA synthetase, has also been shown to attract monocytes. [15] This molecule has inflammatory properties, meaning it is capable of attracting and activating neutrophils.

In apoptosis

Background

Humans turn over billions of cells as a part of normal bodily processes every day, which correlates with about 1 million cells being replaced per second. [16] The ultimate goal of the body's intrinsic cell death mechanisms is to efficiently and asymptomatically clear dying cells. [3] There are many reasons as to why the body needs to get rid of non diseased and diseased cells.

As a part of the cell's natural division process, excess cells may be generated during normal growth, development, or tissue repair after illness or an injury. Only a fraction of these new cells will stay and become mature, while the rest will die and be cleared by the body's immune system. [3]

Cells may also need to be removed because they are too old or become damaged overtime. Cell damage can occur through environmental factors such as air pollution, UV radiation from the sun, or physical injury. [3]

In most cases, the cells that are dying are recognized by phagocytes through find-me signals and removed. Quick and efficient clearing of apoptotic cells is crucial to prevent secondary necrosis of dying cells and to avoid autoantigens causing immune responses. Find-me signals alert the presence of apoptotic cells to phagocytes when they are in the beginning states of dying. The phagocytes are able to use the find-me signals to locate the dying cell. [3]

Find-me signals set up a gradient within the tissue they are in to attract phagocytes to their location. The phagocytes migrate to the dying cell through the use of their receptors responding to the find-me signals initiating a signaling pathway within, causing them to move to the proximity of the cell emitting those signals. [17]

If the body's immune system, or more specifically phagocytes, fail to clear dying cells in the body, symptoms such as chronic inflammation, autoimmune disorders, and developmental abnormalities have been shown to occur. [18] As long as the engulfment process is functioning and efficient, uncleared apoptotic cells go unnoticed in the body and do not cause any long-term symptoms. If this process is disrupted in any way, the accumulation of secondary necrotic cells in tissues of the body can occur. This is associated with autoimmune disorders, causing the immune system to attack self-antigens on the uncleared cells. [19]

Release from dying cells

The main function of a find-me signal is to be released while a cell undergoing apoptosis is still intact in order to attract phagocytes to come and clear the dying cell before secondary necrosis can occur. [3] This suggests that the initiation of apoptosis may be coupled with the release of find me signals from the dying cells.

As of now, it is unknown how LPC is released from apoptotic cells. [3]

S1P generation involved caspase-1-dependent release of sphingosine kinase 2 (SphK2) fragments. [10]

CX3CL1 release is mediated through the release of a 60 kDa microparticle fragment of fractalkine from the beginning stages of Burkitt Lymphoma cell apoptosis. [11]

Nucleotide release is one of the better defined find me signal release mechanisms. [20] They are released through a pannexin family channel known as PANX1. PANX1 is a four pass transmembrane protein that forms large pores in the plasma membrane of a cell, allowing molecules up to 1 kDa in size to pass through. [21] The nucleotides are detected by P2Y2 on monocytes, which causes them to migrate to the location of the apoptotic cell. [3]

Engulfment and clearance of apoptotic cells by phagocytes

Phagocytes are able to sense the find-me signals presented by an apoptotic cell during the beginning stages of cell death. They sense the find-me signal gradient and migrate to the vicinity of the signaling cell. Using the presented find-me signal along with the "eat-me" signal also exposed by the apoptotic cell, the phagocyte is able to recognize the dying cell and engulf it. [3]

Phagocytes contribute to the "final stages" of cell death by apoptosis. [3] They are often already nearby a dying cell and do not have to travel far in order to engulf and clear it. In most mammalian systems, however, this is not the case. In the human thymus, for example, a dying thymocyte is likely to be engulfed by a healthy neighboring thymocyte, and a macrophage or dendritic cell that resides in the thymus is likely to carry out clearance of the corpse. [3] In this case, a dying cell needs to be able to send out an advertisement of sorts to declare its state of death in order to recruit phagocytes to its location. Phagocytic cells use the soluble find-me signals released by the apoptotic signals to do this. [3] Phagocytes detect the gradient set up by the find-me signals presented by the dying cell in order to navigate to their location.

Steps in the engulfment and clearance of apoptotic cells by phagocytes:

  1. Phagocytes need to be in the vicinity of the cells presenting find-me signals. The phagocytes use the find-me signals to locate these cells and move to their location. [22]
  2. The phagocytes interact with the dying cells through the presenting eat-me signals through specific eat-me signal receptors on the phagocytic cell. [23]
  3. The phagocyte will engulf the eat-me signal presenting cell through induced signaling of engulfment receptors and by the reorganization of the phagocytic cell's cytoskeleton. [24]
  4. The components of the dying cell are processed by the phagocytes within their lysosomes. [25]

Non-apoptotic roles

Find me signals may also play a role in phagocytic activity of cell in the direct vicinity of cells undergoing apoptosis. [3] This phenomenon allows neighboring cells adjacent to the apoptotic cell sending out the find me signal to be engulfed without going through the trouble of releasing find me signals of their own. [12]

Find me signals could possibly play a role in priming phagocytes to enhance their phagocytic capacity. [26] In addition, they may also be able to enhance production of certain bridging molecules created by macrophages. [27]

See also

Related Research Articles

<span class="mw-page-title-main">Apoptosis</span> Programmed cell death in multicellular organisms

Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses between 50 and 70 billion cells each day due to apoptosis. For an average human child between eight and fourteen years old, each day the approximate loss is 20 to 30 billion cells.

<span class="mw-page-title-main">Phagocytosis</span> Process by which a cell uses its plasma membrane to engulf a large particle

Phagocytosis is the process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is called a phagocyte.

<span class="mw-page-title-main">Caspase</span> Family of cysteine proteases

Caspases are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cysteine protease activity – a cysteine in its active site nucleophilically attacks and cleaves a target protein only after an aspartic acid residue. As of 2009, there are 12 confirmed caspases in humans and 10 in mice, carrying out a variety of cellular functions.

<span class="mw-page-title-main">Phagocyte</span> Cells that ingest harmful matter within the body

Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek phagein, "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek kutos, "hollow vessel". They are essential for fighting infections and for subsequent immunity. Phagocytes are important throughout the animal kingdom and are highly developed within vertebrates. One litre of human blood contains about six billion phagocytes. They were discovered in 1882 by Ilya Ilyich Mechnikov while he was studying starfish larvae. Mechnikov was awarded the 1908 Nobel Prize in Physiology or Medicine for his discovery. Phagocytes occur in many species; some amoebae behave like macrophage phagocytes, which suggests that phagocytes appeared early in the evolution of life.

<span class="mw-page-title-main">Fas ligand</span> Protein-coding gene in the species Homo sapiens

Fas ligand is a type-II transmembrane protein expressed on various types of cells, including cytotoxic T lymphocytes, monocytes, neutrophils, breast epithelial cells, vascular endothelial cells and natural killer (NK) cells. It binds with its receptor, called FAS receptor and plays a crucial role in the regulation of the immune system and in induction of apoptosis, a programmed cell death.

<span class="mw-page-title-main">Sphingosine kinase</span> Class of enzymes

Sphingosine kinase (SphK) is a conserved lipid kinase that catalyzes formation sphingosine-1-phosphate (S1P) from the precursor sphingolipid sphingosine. Sphingolipid metabolites, such as ceramide, sphingosine and sphingosine-1-phosphate, are lipid second messengers involved in diverse cellular processes. There are two forms of SphK, SphK1 and SphK2. SphK1 is found in the cytosol of eukaryotic cells, and migrates to the plasma membrane upon activation. SphK2 is localized to the nucleus.

<span class="mw-page-title-main">Lipid signaling</span> Biological signaling using lipid molecules

Lipid signaling, broadly defined, refers to any biological cell signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses. Lipid signaling is thought to be qualitatively different from other classical signaling paradigms because lipids can freely diffuse through membranes. One consequence of this is that lipid messengers cannot be stored in vesicles prior to release and so are often biosynthesized "on demand" at their intended site of action. As such, many lipid signaling molecules cannot circulate freely in solution but, rather, exist bound to special carrier proteins in serum.

Chemokine ligand 1 (CCL1) is also known as small inducible cytokine A1 and I-309 in humans. CCL1 is a small glycoprotein that belongs to the CC chemokine family.

<span class="mw-page-title-main">CX3CL1</span> Protein-coding gene in the species Homo sapiens

Fractalkine, also known as chemokine ligand 1, is a protein that in humans is encoded by the CX3CL1 gene.

Sphingosine-1-phosphate (S1P) is a signaling sphingolipid, also known as lysosphingolipid. It is also referred to as a bioactive lipid mediator. Sphingolipids at large form a class of lipids characterized by a particular aliphatic aminoalcohol, which is sphingosine.

<span class="mw-page-title-main">CX3C motif chemokine receptor 1</span> Protein-coding gene in the species Homo sapiens

CX3C motif chemokine receptor 1 (CX3CR1), also known as the fractalkine receptor or G-protein coupled receptor 13 (GPR13), is a transmembrane protein of the G protein-coupled receptor 1 (GPCR1) family and the only known member of the CX3C chemokine receptor subfamily.

Ceramidase is an enzyme which cleaves fatty acids from ceramide, producing sphingosine (SPH) which in turn is phosphorylated by a sphingosine kinase to form sphingosine-1-phosphate (S1P).

<span class="mw-page-title-main">S1PR1</span> Protein and coding gene in humans

Sphingosine-1-phosphate receptor 1, also known as endothelial differentiation gene 1 (EDG1) is a protein that in humans is encoded by the S1PR1 gene. S1PR1 is a G-protein-coupled receptor which binds the bioactive signaling molecule sphingosine 1-phosphate (S1P). S1PR1 belongs to a sphingosine-1-phosphate receptor subfamily comprising five members (S1PR1-5). S1PR1 was originally identified as an abundant transcript in endothelial cells and it has an important role in regulating endothelial cell cytoskeletal structure, migration, capillary-like network formation and vascular maturation. In addition, S1PR1 signaling is important in the regulation of lymphocyte maturation, migration and trafficking.

<span class="mw-page-title-main">Lysophosphatidylcholine</span> Class of compounds

Lysophosphatidylcholines, also called lysolecithins, are a class of chemical compounds which are derived from phosphatidylcholines.

Apoptotic-cell associated molecular patterns (ACAMPs) are molecular markers present on cells which are going through apoptosis, i.e. programmed cell death. The term was used for the first time by C. D. Gregory in 2000. Recognition of these patterns by the pattern recognition receptors (PRRs) of phagocytes then leads to phagocytosis of the apoptotic cell. These patterns include eat-me signals on the apoptotic cells, loss of don’t-eat-me signals on viable cells and come-get-me signals ) secreted by the apoptotic cells in order to attract phagocytes. Thanks to these markers, apoptotic cells, unlike necrotic cells, do not trigger the unwanted immune response.

<span class="mw-page-title-main">Necroptosis</span> Programmed form of necrosis, or inflammatory cell death

Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.

Immunogenic cell death is any type of cell death eliciting an immune response. Both accidental cell death and regulated cell death can result in immune response. Immunogenic cell death contrasts to forms of cell death that do not elicit any response or even mediate immune tolerance.

The KX Blood-group Antigen (KXA) Family (TC# 2.A.112) consists of transport proteins that are part of the TOG superfamily. The KX gene codes for a novel protein with characteristics of membrane transporters that has been proposed to be a Na+ -dependent neutral amine and/or oligopeptide transporter. It is predicted to be 444 amino acyl residues in length and exhibits 10 putative transmembrane α-helical segments. The KX blood group antigen mRNA expression pattern correlates with McLeod syndrome.

<span class="mw-page-title-main">Eat-me signals</span>

Eat-me signals are molecules exposed on the surface of a cell to induce phagocytes to phagocytose (eat) that cell. Currently known eat-me signals include: phosphatidylserine, oxidized phospholipids, sugar residues, deoxyribonucleic acid (DNA), calreticulin, annexin A1, histones and pentraxin-3 (PTX3).

<span class="mw-page-title-main">Cell extrusion</span> Process in cell biology

Cell extrusion, discovered in 2001, is a process conserved in epithelial from humans to sea sponge to seamlessly remove unwanted or dying cells while maintaining the integrity of the epithelial barrier. If cells were to die without extrusion, gaps would be created, compromising the epithelia's function. While cell targeted to die by apoptotic stimuli extrude to prevent gaps from forming, most cells die as a result of extruding live cells. To maintain epithelial cell number homeostasis, live cells extrude when they become too crowded.

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  23. ↑ Gardai, Shyra J.; Bratton, Donna L.; Ogden, Carole Anne; Henson, Peter M. (May 2006). "Recognition ligands on apoptotic cells: a perspective". Journal of Leukocyte Biology. 79 (5): 896–903. doi:10.1189/jlb.1005550. PMID   16641135. S2CID   20869810.
  24. ↑ Ravichandran, Kodi S.; Lorenz, Ulrike (December 2007). "Engulfment of apoptotic cells: signals for a good meal". Nature Reviews Immunology. 7 (12): 964–974. doi:10.1038/nri2214. ISSN   1474-1741. PMID   18037898. S2CID   10670430.
  25. ↑ Kinchen, Jason M.; Doukoumetzidis, Kimon; Almendinger, Johann; Stergiou, Lilli; Tosello-Trampont, Annie; Sifri, Costi D.; Hengartner, Michael O.; Ravichandran, Kodi S. (May 2008). "A pathway for phagosome maturation during engulfment of apoptotic cells". Nature Cell Biology. 10 (5): 556–566. doi:10.1038/ncb1718. ISSN   1476-4679. PMC   2851549 . PMID   18425118.
  26. ↑ MacDonald, Jennifer M.; Beach, Margaret G.; Porpiglia, Ermelinda; Sheehan, Amy E.; Watts, Ryan J.; Freeman, Marc R. (2006-06-15). "The Drosophila Cell Corpse Engulfment Receptor Draper Mediates Glial Clearance of Severed Axons". Neuron. 50 (6): 869–881. doi: 10.1016/j.neuron.2006.04.028 . ISSN   0896-6273. PMID   16772169. S2CID   6442528.
  27. ↑ Miksa, Michael; Amin, Dhruv; Wu, Rongqian; Dong, Weifeng; Ravikumar, Thanjavur S.; Wang, Ping (November 2007). "Fractalkine-Induced MFG-E8 Leads to Enhanced Apoptotic Cell Clearance by Macrophages". Molecular Medicine. 13 (11): 553–560. doi:10.2119/2007-00019.Miksa. ISSN   1528-3658. PMC   1936982 . PMID   17673941.
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