STRUCTURE

CD9 is a type 3 integral protein belonging to the tetraspanin family.¹ It contains 4 transmembrane domains and 2 extracellular loops of different lenghts.^1-2  The large extracellular loop contains a CCG motif. Both the N- and C-terminal domains are located in the cytoplasm.^1-2

1. Horejsí V, Vlcek C. Novel structurally distinct family of leucocyte surface glycoproteins including CD9, CD37, CD53 and CD63. FEBS Lett. 1991;288:1-4.
2. Boucheix C, Benoit P, Frachet P, Billard M, Worthington RE, Gagnon J, Uzan G. Molecular cloning of the CD9 antigen. A new family of cell surface proteins. J Biol Chem. 1991;266:117-22.

LIGANDS

Extracellular

It presumably binds to PSG17.¹ CD9 associates with transmembrane proteins such as claudin-1², EGFR³, β1 and other integrins^3-4, CD44⁵, syndecan⁵, CD42⁶, CD47⁶, CD81⁷, CD82⁷, CD63⁷, HLA-DR⁷, CD3⁸, CD4⁸, CD5⁸, CD2⁸, CD19⁹, EWI-2¹⁰, EWI-F protein¹¹ and MHC class II.¹² CD9 also interacts with the heparin-binding EGF-like growth factor, amphiregulin, 90K, TGF-α and CD1a.^13-16

Intracellular associate molecules

PKC¹⁷ and PI4K.¹⁸

1. Ellerman DA, Ha C, Primakoff P, Myles DG, Dveksler GS. Direct binding of the ligand PSG17 to CD9 requires a CD9 site essential for sperm-egg fusion. Mol Biol  Cell. 2003;14:5098-103.
2. Kovalenko OV, Yang XH, Hemler ME. A novel cysteine cross-linking method reveals a direct association between claudin-1 and tetraspanin CD9. Mol Cell Proteomics. 2007;6:1855-67.
3. Murayama Y, Shinomura Y, Oritani K, Miyagawa J, Yoshida H, Nishida M, Katsube F, Shiraga M, Miyazaki T, Nakamoto T, Tsutsui S, Tamura S, Higashiyama S, Shimomura I, Hayashi N. The tetraspanin CD9 modulates epidermal growth factor receptor signaling in cancer cells. J Cell Physiol. 2008;216:135-43.
4. Boucheix C, Rubinstein E. Tetraspanins. Cell Mol Life Sci. 2001;58:1189-205.
5. Jones PH, Bishop LA, Watt FM. Functional significance of CD9 association with beta 1 integrins in human epidermal keratinocytes. Cell Adhes Commun. 1996;4:297-305.
6. Longhurst CM, White MM, Wilkinson DA, Jennings LK. A CD9, alphaIIbbeta3, integrin-associated protein, and GPIb/V/IX complex on the surface of human platelets is influenced by alphaIIbbeta3 conformational states. Eur J Biochem. 1999;263:104-11.
7. Rubinstein E, Le Naour F, Lagaudrière-Gesbert C, Billard M, Conjeaud H, Boucheix C. CD9, CD63, CD81, and CD82 are components of a surface tetraspan network connected to HLA-DR and VLA integrins. Eur J Immunol. 1996;26:2657-65.
8. Toyo-oka K, Yashiro-Ohtani Y, Park CS, Tai XG, Miyake K, Hamaoka T, Fujiwara H. Association of a tetraspanin CD9 with CD5 on the T cell surface: role of particular transmembrane domains in the association. Int Immunol. 1999;11:2043-52.
9. Horváth G, Serru V, Clay D, Billard M, Boucheix C, Rubinstein E. CD19 is linked to the integrin-associated tetraspans CD9, CD81, and CD82. J Biol Chem. 1998;273:30537-43.
10. Stipp CS, Kolesnikova TV, Hemler ME. EWI-2 is a major CD9 and CD81 partner and member of a novel Ig protein subfamily. J Biol Chem. 2001;276:40545-54.
11. Stipp CS, Orlicky D, Hemler ME. FPRP, a major, highly stoichiometric, highlyspecific CD81- and CD9-associated protein. J Biol Chem. 2001;276:4853-62.
12. Engering A, Pieters J. Association of distinct tetraspanins with MHC class II molecules at different subcellular locations in human immature dendritic cells. Int Immunol. 2001;13:127-34.
13. Inui S, Higashiyama S, Hashimoto K, Higashiyama M, Yoshikawa K, Taniguchi N. Possible role of coexpression of CD9 with membrane-anchored heparin-binding EGF-like growth factor and amphiregulin in cultured human keratinocyte growth. J Cell Physiol. 1997;171:291-8.
14. Shi W, Fan H, Shum L, Derynck R. The tetraspanin CD9 associates with transmembrane TGF-alpha and regulates TGF-alpha-induced EGF receptor activation and cell proliferation. J Cell Biol. 2000;148:591-602.
15. Lee JH, Bae JA, Lee JH, Seo YW, Kho DH, Sun EG, Lee SE, Cho SH, Joo YE, Ahn KY, Chung IJ, Kim KK. Glycoprotein 90K, downregulated in advanced colorectal cancer tissues, interacts with CD9/CD82 and suppresses the Wnt/beta-catenin signal via ISGylation of beta-catenin. Gut. 2010;59:907-17.
16. Sloma I, Zilber MT, Vasselon T, Setterblad N, Cavallari M, Mori L, De Libero G, Charron D, Mooney N, Gelin C. Regulation of CD1a surface expression and antigen presentation by invariant chain and lipid rafts. J Immunol. 2008;180:980-7.
17. Zhang XA, Bontrager AL, Hemler ME. Transmembrane-4 superfamily proteins associate with activated protein kinase C (PKC) and link PKC to specific beta(1)  integrins. J Biol Chem. 2001;276:25005-13.
18. Yauch RL, Hemler ME. Specific interactions among transmembrane 4 superfamily (TM4SF) proteins and phosphoinositide 4-kinase. Biochem J. 2000;351 Pt 3:629-37.

CD9 is expressed in several cell types including platelets^1-2, B cells³, T cells⁴, eosinophils¹, basophils¹, myocardial cells¹, follicular dendritic cells², monocytes², megakariocytes², oocytes⁵, sperm⁵, keratinocytes^2-6, thymocytes⁶, basolateral membrane of immature enterocytes², fibroblasts^2,6, alveolar pneumocytes², Schwann cells², spongiocytes², thyroid cells², macrophages⁶ and melanocytes.⁶

It is also expressed in distal tubules¹and collecting ducts^1-2, the wall of small vessels¹, smooth muscle^2,6, bronchi², bronchioles², tonsil², peripheral nerves², cerebral white matter², liver² and intestine.⁶

1. Boucheix C, Perrot JY, Mirshahi M, Giannoni F, Billard M, Bernadou A, Rosenfeld C. A new set of monoclonal antibodies against acute lymphoblastic leukemia. Leuk Res. 1985;9:597-604.
2. Sincock PM, Mayrhofer G, Ashman LK. Localization of the transmembrane 4 superfamily (TM4SF) member PETA-3 (CD151) in normal human tissues: comparison with CD9, CD63, and alpha5beta1 integrin. J Histochem Cytochem. 1997;45:515-25.
3. Zeleznik-Le NJ, Metzgar RS. Expression of CD9 antigen on normal activated human B cells. Cell Immunol. 1989;123:70-82.
4. Tai XG, Yashiro Y, Abe R, Toyooka K, Wood CR, Morris J, Long A, Ono S, Kobayashi M, Hamaoka T, Neben S, Fujiwara H. A role for CD9 molecules in T cell activation. J Exp Med. 1996;184:753-8.
5. Jankovičová J, Simon M, Antalíková J, Cupperová P, Michalková K. Role of tetraspanin CD9 molecule in fertilization of mammals. Physiol Res. 2015;64:279-93.
6. Huang C, Taki T, Adachi M, Yagita M, Sawada S, Takabayashi A, Inufusa H, Yoshie O, Miyake M. MRP-1/CD9 and KAI1/CD82 expression in normal and  various cancer tissues. Int J Oncol. 1997;11:1045-51.

Through its association with other transmembrane proteins such as integrins or proteins of the EWI family, CD9 forms functional complexes that regulate cell adhesion and motility.^1-2 n order to do it, CD9 modulates the conformational state of integrins, their signalling, their clustering and the formation of stress fibers.^3-6

Apart from its role in adhesion and migration, CD9 inhibits cell proliferation and promotes apoptosis of cancer cells.⁷ On oocytes, CD9 is hypothesized to play a role in fertilization.8 On immune cells, CD9 participates in TCR and BCR signaling and in antigen presentation through MHC-II regulation.^9-10

Last, CD9 is involved in osteoclastogenesis, multinucleated giant cell formation, fusion of myotubes, platelet aggregation and cell contraction.^11-15

1. Powner D, Kopp PM, Monkley SJ, Critchley DR, Berditchevski F. Tetraspanin CD9 in cell migration. Biochem Soc Trans. 2011;39:563-7.
2. Murayama Y, Oritani K, Tsutsui S. Novel CD9-targeted therapies in gastric cancer. World J Gastroenterol. 2015;21:3206-13.
3. Reyes R, Monjas A, Yánez-Mó M, Cardeñes B, Morlino G, Gilsanz A, Machado-Pineda Y, Lafuente E, Monk P, Sánchez-Madrid F, Cabañas C. Different states of integrin LFA-1 aggregation are controlled through its association with tetraspanin CD9. Biochim Biophys Acta. 2015;1853:2464-80.
4. Rocha-Perugini V, González-Granado JM, Tejera E, López-Martín S, Yañez-Mó M, Sánchez-Madrid F. Tetraspanins CD9 and CD151 at the immune synapse support T-cell integrin signaling. Eur J Immunol. 2014;44:1967-75.
5. Kotha J, Longhurst C, Appling W, Jennings LK. Tetraspanin CD9 regulates beta 1 integrin activation and enhances cell motility to fibronectin via a PI-3 kinase-dependent pathway. Exp Cell Res. 2008;314:1811-22.
6. Scherberich A, Giannone G, Perennou E, Takeda K, Boucheix C, Rubinstein E, Lanza F, Beretz A. FAK-mediated inhibition of vascular smooth muscle cell migration by the tetraspanin CD9. Thromb Haemost. 2002;87:1043-50.
7. Murayama Y, Miyagawa J, Oritani K, Yoshida H, Yamamoto K, Kishida O, Miyazaki T, Tsutsui S, Kiyohara T, Miyazaki Y, Higashiyama S, Matsuzawa Y, Shinomura Y. CD9-mediated activation of the p46 Shc isoform leads to apoptosis in cancer cells. J Cell Sci. 2004;117:3379-88.
8. Jankovičová J, Simon M, Antalíková J, Cupperová P, Michalková K. Role of tetraspanin CD9 molecule in fertilization of mammals. Physiol Res. 2015;64:279-93.
9. Rocha-Perugini V, Martínez Del Hoyo G, González-Granado JM, Ramírez-Huesca M, Zorita V, Rubinstein E, Boucheix C, Sánchez-Madrid F. CD9 Regulates Major Histocompatibility Complex Class II Trafficking in Monocyte-Derived Dendritic Cells. Mol Cell Biol. 2017 14;37.
10. Ovalle S, Gutiérrez-López MD, Monjas A, Cabañas C. Implication of the tetraspanin CD9 in the immune system and cancer. Inmunología. 2007;26:65-72.
11. Charrin S, Latil M, Soave S, Polesskaya A, Chrétien F, Boucheix C, Rubinstein E. Normal muscle regeneration requires tight control of muscle cell fusion bytetraspanins CD9 and CD81. Nat Commun. 2013;4:1674.
12. Yi T, Kim HJ, Cho JY, Woo KM, Ryoo HM, Kim GS, Baek JH. Tetraspanin CD9 regulates osteoclastogenesis via regulation of p44/42 MAPK activity. Biochem Biophys Res Commun. 2006;347:178-84. Epub 2006 Jun 21.
13. Parthasarathy V, Martin F, Higginbottom A, Murray H, Moseley GW, Read RC, Mal G, Hulme R, Monk PN, Partridge LJ. Distinct roles for tetraspanins CD9, CD63 and CD81 in the formation of multinucleated giant cells. Immunology. 2009;127:237-48.
14. Nakazawa Y, Sato S, Naito M, Kato Y, Mishima K, Arai H, Tsuruo T, Fujita N. Tetraspanin family member CD9 inhibits Aggrus/podoplanin-induced platelet aggregation and suppresses pulmonary metastasis. Blood. 2008;112:1730-9.
15. Herr MJ, Mabry SE, Jennings LK. Tetraspanin CD9 regulates cell contraction and actin arrangement via RhoA in human vascular smooth muscle cells. PLoS One. 2014 3;9:e106999.

Cell marker

CD9 is a useful marker for diagnosis and classification of megakaryoblastic leukemia and B-cell precursor acute lymphoblastic leukemia.¹ CD9 expression is a prognostic marker of several cancers including colorectal carcinoma and follicular lymphoma.^2-3

1. Van Dongen JJ, Lhermitte L, Böttcher S, Almeida J, van der Velden VH, Flores-Montero J, Rawstron A, Asnafi V, Lécrevisse Q, Lucio P, Mejstrikova E, Szczepański T, Kalina T, de Tute R, Brüggemann M, Sedek L, Cullen M, Langerak AW, Mendonça A, Macintyre E, Martin-Ayuso M, Hrusak O, Vidriales MB, Orfao A; EuroFlow Consortium (EU-FP6, LSHB-CT-2006-018708). EuroFlow antibody panels for standardized n-dimensional flow cytometry immunophenotyping of normal, reactive and malignant leukocytes. Leukemia. 2012;26:1908-75.
2. Kim KJ, Kwon HJ, Kim MC, Bae YK. CD9 Expression in Colorectal Carcinomas and Its Prognostic Significance. J Pathol Transl Med. 2016;50:459-468.
3. Dong T, Liu Z, Zhao S, Hu C, Liu Y, Ma W, Zhang Q. The Expression of CD9 and PIK3CD is Associated with Prognosis of Follicular Lymphoma. J Cancer. 2015;6:1222-9.

Therapeutic

To our knowledge, there are no monoclonal antibodies targeting CD9 used in the clinical practice. However, CD9 is a potential candidate in cancer treatment due to its role in cell motility, apoptosis and metastasis. In particular, positive results have been reported when administering anti-CD9 monoclonal antibodies in in vivo models of gastric and colon cancer.^1-2 Furthermore, CD9 antibodies are also useful for bone marrow purging.^3-4

1. Nakamoto T, Murayama Y, Oritani K, Boucheix C, Rubinstein E, Nishida M, Katsube F, Watabe K, Kiso S, Tsutsui S, Tamura S, Shinomura Y, Hayashi N. A novel therapeutic strategy with anti-CD9 antibody in gastric cancers. J  Gastroenterol. 2009;44:889-96.
2. Ovalle S, Gutiérrez-López MD, Olmo N, Turnay J, Lizarbe MA, Majano P, Molina-Jiménez F, López-Cabrera M, Yáñez-Mó M, Sánchez-Madrid F,   Cabañas C. The tetraspanin CD9 inhibits the proliferation and tumorigenicity of human colon carcinoma cells. Int J Cancer. 2007;121:2140-52.
3. Soiffer RJ, Roy DC, Gonin R, Murray C, Anderson KC, Freedman AS, Rabinowe SN, Robertson MJ, Spector N, Pesek K, et al. Monoclonal antibody-purged autologous bone marrow transplantation in adults with acute lymphoblastic leukemia at high risk of relapse. Bone Marrow Transplant. 1993;12:243-51.

1. Vervoordeldonk SF, van den Berg H, von dem Borne AE, van Leeuwen EF, Slaper-Cortenbach IC. Optimization of purging of autologous bone marrow grafts for children with precursor B acute lymphoblastic leukemia. J Hematother. 1997;6:495-500.