CD90
Thy-1 or CD90 is a 25–37 kDa heavily N-glycosylated, glycophosphatidylinositol anchored conserved cell surface protein with a single V-like immunoglobulin domain, originally discovered as a thymocyte antigen. Thy-1 can be used as a marker for a variety of stem cells and for the axonal processes of mature neurons. Structural study of Thy-1 led to the foundation of the Immunoglobulin superfamily, of which it is the smallest member, and led to some of the initial biochemical description and characterization of a vertebrate GPI anchor and also the first demonstration of tissue specific differential glycosylation.
Discovery and nomenclature
The antigen Thy-1 was the first T cell marker to be identified. Thy-1 was discovered by Reif and Allen in 1964 during a search for heterologous antisera against mouse leukemia cells, and was demonstrated by them to be present on murine thymocytes, on T lymphocytes, and on neuronal cells. It was originally named theta antigen, then Thy-1 due to its prior identification in thymocytes. The human homolog was isolated in 1980 as a 25kDa protein of T-lymphoblastoid cell line MOLT-3 binding with anti-monkey-thymocyte antisera. The discovery of Thy-1 in mice and humans led to the subsequent discovery of many other T cell markers, which is very significant to the field of immunology since T cells are the major cellular components of the adaptive immune response.The conserved gene and its alleles
Thy-1 has been conserved throughout vertebrate evolution and even in some invertebrates, with homologs described in many species like squid, frogs, chickens, mice, rats, dogs, and humans.The Thy-1 gene is located at human chromosome 11q22.3. In , it covers 6.82 kb, from 119294854 to 119288036, on the reverse strand. This locus is very close to CD3 & CD56/NCAM genes. Some believe that there may be a functional significance of both this gene and CD3 delta subunit mapping to chromosome 11q in man and chromosome 9 in mouse, though there is no homology. In mice, there are two alleles: Thy-1.1 and Thy-1.2. They differ by only one amino acid at position 108; an arginine in Thy-1.1 and a glutamine in Thy-1.2. Thy-1.2 is expressed by most strains of mice, whereas Thy-1.1 is expressed by others such as AKR/J and PL mouse strains.
The Protein
The 25-kDa core protein of rodent Thy-1 is 111 or 112 amino acids in length, and is N-glycosylated at three sites. The 162aa Thy1 precursor has 19 amino acid signal sequence and 31 amino acid C-terminal transmembrane domain that is present in pro form but removed when transferring the 112 amino acid mature peptide to GPI anchor which would attach through the aa 131.Some of the common monoclonal antibodies used to detect this protein are clones OX7, 5E10, K117 and L127.
There have been some reports of Thy1 monoclonal antibodies cross reacting with some cytoskeletal elements: anti Thy-1.2 with actin in marsupial, murine, and human cells and anti Thy-1.1 with vimentin, and were suggested to be due to sequence homology by studies done more than 20 years back.
Thy-1, like many other GPI anchored proteins can be shed by special types of Phospholipase C e.g. PI-PLC. it can also be involved in cell to cell transfer of GPI anchored proteins like CD55 and CD59.
Glycosylation
Thy-1 is one of the most heavily glycosylated membrane proteins with a carbohydrate content up to 30% of its molecular mass. Thy-1 in most species has 3 N-glycosylation sites but no O-glycosylation. The composition of Thy-1 carbohydrate moieties varies considerably between different tissues or even among cells of the same lineage at different stages of differentiation: e.g., galactosamine only in brain Thy-1, sialic acid in thymic Thy-1 in far excess than brain Thy-1, that too increasing in parallel with T cell maturation. In this regard it has yet another historic association: Thy-1 happens to be the first glycoprotein in which cell type specificity of variant glycosylation on an invariant protein was demonstrated. Analysis of Differencial glycosylation of Thy-1 from brain and thymus showed that all the complex N-linked structures differed between the two forms, superimposed upon a site specific common core. In case of Thy-1 this core pattern was constituted by Asn23 carrying mostly oligomannose structures, Asn74 carrying the most extended complex structures, and Asn98 carrying smaller complex structure. The structure of the sugar residues in the GPI anchor and their associated esterified structures also can be cell type and species specific.Expression
Thy-1 expression varies between species. Amongst the cells reported to generally express Thy-1 are thymocytes & CD34 prothymocytes; neurons, mesenchymal stem cells, hematopoietic stem cells, NK cells, murine T-cells, endothelium, renal glomerular mesangial cells, circulating metastatic melanoma cells, follicular dendritic cells, a fraction of fibroblasts and myofibroblasts.Detailed expression of Thy-1
- In mice, Thy-1 is also found on thymocytes, peripheral T cells, myoblasts, epidermal cells, and keratinocytes. It is one of the "pan T cell markers" like CD2, CD5 and CD28.
- In humans, Thy-1 is also expressed by endothelial cells, smooth muscle cells, a subset of CD34+ bone marrow cells, and umbilical cord blood-, cardiac fibroblasts, and fetal liver-derived hemopoietic cells.
- Thy-1 is present on a fraction of brain cells and a fraction of fibroblasts of most vertebrate species studied.
- Nervous tissue: Thy-1 expression in the nervous system is predominantly neuronal, but some glial cells also express Thy-1 especially at later stages of their differentiation. One study compared Thy-1 expression in four human neuronal cell lines, two neuroglial cell lines, and fresh tumor cells of neuronal origin and found three of the four neuronal cell lines, all of the neuroglial cell lines, and 80% of the tumors to be strongly positive for Thy-1. Brain part specific ELISA reports are available which show highest concentrations of Thy-1 protein in the striatum and hippocampus, followed by the neocortex, cerebellum, spinal cord, and the retina and optic nerve. Thy-1 promoter has often been assumed to be "brain specific". "Neuron specific" mouse Thy-1 promoter has been used to drive "brain specific" forced expression of proteins e.g. mutated Amyloid precursor protein as transgenic animal models of Alzheimer's disease. Thy-1 expression in the brain is developmentally regulated. Thy-1 levels in the neonatal rat brain, as well as the developing human brain, are low compared to adult brain. During the first few weeks of postnatal development, Thy-1 levels increase exponentially as the brain matures.
- Lymphoid tissue Thy-1 expression is highly variable between species. In humans, Thy-1 expression is restricted to only a small population of cortical thymocytes and not expressed in mature human T cells. It is probably the most abundant glycoprotein of murine thymocytes, with about One million copies per cell covering up to 10–20% of the cell surface. Mouse cortical thymocytes express higher levels of Thy-1 than medullary thymocytes which in turn express more than lymph node cells. A similar inverse developmental temporal expression profile is seen in rats T cells, although rat Thy-1 is lost at an earlier stage of T cell maturation. Thy-1 is only expressed on thymocytes in rats. The third intron of the mouse Thy-1 gene has a 36 base pair region that recruits nuclear transcription factors, such as Ets-1-like NF, expressed in thymocytes and splenocytes. The homologous region of the rat gene lacks the Ets-1-like NF binding site, but instead binds another NF expressed in rat thymocytes but not splenocytes.
Induction of Thy-1 expression
- Agents shown to induce Thy-1 expression include: Thymopoietin, thymosin, prostaglandins, nerve growth factor, IL-1, TNF, PMA, Ca2+ ionophore, and diacylglycerol.
Localization