CD63 - CD63 molecule |Elisa - Clia - Antibody - Protein

Background

CD63, also known as LAMP-3 (lysosome-associated membrane protein 3), granulophysin, or tetraspanin-30, is a member of the tetraspanin family of proteins, which are characterized by four transmembrane domains. CD63 is primarily localized in intracellular vesicles, such as lysosomes and late endosomes, but can also be expressed on the plasma membrane following cellular activation. CD63 is widely expressed across various cell types, including platelets, leukocytes, endothelial cells, fibroblasts, and epithelial cells, and it plays critical roles in cell adhesion, intracellular trafficking, immune response, and exosome biology.

One of the most notable features of CD63 is its role in exosome biogenesis and secretion. Exosomes are small extracellular vesicles that participate in cell-to-cell communication by transferring proteins, lipids, and nucleic acids between cells. CD63 is often used as a marker for exosomes due to its abundant expression on their surface. CD63 is also involved in immune system regulation and cellular activation processes, particularly in platelets and immune cells. Furthermore, it interacts with other tetraspanins and various partner proteins, including integrins, which further implicates CD63 in cell migration, immune synapse formation, and tumor metastasis.

Protein Structure

CD63 is a tetraspanin protein consisting of four transmembrane domains (TM1-TM4), which create its unique structure, enabling interactions with other proteins in the cell membrane and within vesicles. The structural features of CD63 are:

Transmembrane Domains:

• CD63 has four alpha-helical transmembrane domains (TM1-TM4), which allow it to integrate into membranes, including lysosomes, endosomes, and the plasma membrane. These transmembrane helices enable CD63 to form complexes with other tetraspanins, such as CD9, CD81, and CD82, as well as with other membrane proteins like integrins and immunoglobulin superfamily members.

Extracellular Domains:

• Small Extracellular Loop (SEL): Between the first and second transmembrane domains (TM1 and TM2), there is a small extracellular loop (SEL) composed of a few amino acids. The SEL is thought to participate in protein-protein interactions, but its specific function remains less defined.
• Large Extracellular Loop (LEL): Between the third and fourth transmembrane domains (TM3 and TM4), CD63 has a large extracellular loop (LEL), which is critical for binding to ligands and interacting with other tetraspanins and signaling molecules. The LEL is also subject to glycosylation, a post-translational modification that can regulate its function. This glycosylation also contributes to the structural stability of CD63.

Cytoplasmic N-terminal and C-terminal Domains:

• CD63 has short cytoplasmic tails at both the N-terminal and C-terminal ends. These tails play a role in the intracellular trafficking of CD63 to different membrane compartments. Importantly, these tails contain sorting signals that direct CD63 to lysosomes and endosomes, as well as to the plasma membrane during activation events, such as platelet activation or immune cell stimulation.

Glycosylation:

• CD63 is highly glycosylated, with N-glycosylation sites located on its large extracellular loop. Glycosylation is critical for maintaining the protein’s structure, stability, and interactions with other molecules. It also affects the function of CD63 in cellular processes such as immune signaling and vesicle trafficking.

Classification and Subtypes

CD63 belongs to the tetraspanin family, a large group of proteins that share a common structural organization with four transmembrane domains and extracellular loops. Tetraspanins are often referred to as molecular organizers because they interact with a wide variety of proteins, including integrins, immunoglobulin superfamily members, and growth factor receptors, to form complexes in specialized membrane microdomains known as tetraspanin-enriched microdomains (TEMs).

There are no specific subtypes of CD63, but it is closely related to other tetraspanins such as CD9, CD81, and CD82, with which it frequently interacts to regulate cell adhesion, signaling, and vesicle trafficking.

Function and Biological Significance

CD63 plays a diverse array of roles in cellular processes, including:

Exosome Biogenesis and Secretion:

• One of the primary roles of CD63 is in the formation and release of exosomes. Exosomes are extracellular vesicles that facilitate cell-to-cell communication by transferring proteins, lipids, and nucleic acids between cells. CD63 is highly enriched in exosomal membranes, and it is commonly used as a marker for exosome identification and isolation. In addition to serving as a marker, CD63 actively participates in the process of exosome formation by interacting with proteins involved in vesicle budding and trafficking.

Cell Adhesion and Migration:

• CD63 is involved in cell adhesion by interacting with integrins, particularly integrin alphaIIb-beta3 (ITGA2B-ITGB3) in platelets. It also contributes to cell migration by regulating the dynamics of integrin-mediated cell attachment and detachment. CD63’s role in these processes is crucial during wound healing, immune responses, and cancer metastasis, where cells must migrate to specific sites in the body.

Platelet Activation:

• CD63 is highly expressed in platelets, where it is stored in alpha-granules and transported to the plasma membrane during platelet activation. Upon activation, CD63 facilitates the formation of platelet aggregates and their interaction with leukocytes and endothelial cells. This role is essential for the regulation of thrombosis and hemostasis.

Immune Response:

• In immune cells, such as macrophages, neutrophils, and T cells, CD63 is involved in antigen presentation and immune synapse formation. It is also involved in phagocytosis, where it regulates the engulfment of pathogens by immune cells. Additionally, CD63 interacts with proteins that mediate inflammatory responses, thereby playing a role in the regulation of innate immunity.

Lysosomal and Endosomal Function:

• CD63 is predominantly located in lysosomal and endosomal membranes and plays a role in the sorting and trafficking of proteins to these compartments. It helps regulate the degradation of cellular waste products and damaged proteins by facilitating their transport to lysosomes for breakdown.

Clinical Issues

Cancer:

• CD63 has been implicated in the progression and metastasis of various cancers, including melanoma, breast cancer, and prostate cancer. It regulates cancer cell adhesion, migration, and invasion by interacting with integrins and other cell surface receptors. CD63 expression is often elevated in tumor tissues and is associated with increased metastatic potential. CD63-positive exosomes also contribute to the pre-metastatic niche formation, where they prepare distant tissues for tumor cell colonization.

Immune Disorders:

• Given its role in immune cell activation and antigen presentation, dysregulation of CD63 can contribute to immune-related disorders. For example, abnormal CD63 expression or function has been observed in autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis, where improper immune cell activation and trafficking exacerbate tissue damage.

Thrombosis:

• CD63 is upregulated on the surface of activated platelets and is involved in the regulation of thrombus formation. Elevated levels of CD63 have been linked to hypercoagulable states, including venous thromboembolism (VTE) and myocardial infarction. CD63 is also used as a biomarker for platelet activation, and its expression is increased in individuals with cardiovascular disease.

Allergic Responses:

• CD63 is a marker of mast cell degranulation and is involved in allergic reactions. Upon allergen exposure, CD63 is translocated to the plasma membrane of activated mast cells, where it facilitates the release of histamine and other pro-inflammatory mediators. Thus, CD63 is used as a marker in allergy testing, particularly in assessing basophil activation.

Summary

CD63 is a versatile tetraspanin molecule with a wide range of functions, including cell adhesion, immune cell activation, exosome biogenesis, and lysosomal trafficking. Structurally, it features four transmembrane domains, a small extracellular loop (SEL), and a large extracellular loop (LEL), which are crucial for its interactions with integrins, other tetraspanins, and proteins involved in vesicle trafficking. CD63 plays significant roles in the regulation of immune responses, platelet activation, and cell migration, and is involved in pathological conditions such as cancer metastasis, immune disorders, and thrombosis. As such, CD63 is an important therapeutic target and biomarker for various diseases, including cardiovascular disease, autoimmune disorders, and cancer.