NT5E - 5'-nucleotidase ecto | Elisa - Clia - Antibody - Protein

Background

The NT5E gene encodes 5'-nucleotidase ecto, also known as CD73, a glycosylphosphatidylinositol (GPI)-anchored enzyme that is crucial in purinergic signaling. It catalyzes the hydrolysis of extracellular nucleotides into nucleosides, with a primary function of converting AMP (adenosine monophosphate) to adenosine. CD73 is expressed on a wide range of cell types, including endothelial cells, epithelial cells, fibroblasts, T cells, and tumor cells. It plays a fundamental role in regulating inflammation, vascular permeability, immune response, and tissue protection under stress conditions such as ischemia.

Adenosine generated by CD73 is a potent signaling molecule that binds to adenosine receptors (A1, A2A, A2B, and A3) to mediate various physiological processes, including anti-inflammatory effects, vasodilation, and immunosuppression. In the immune system, CD73 suppresses pro-inflammatory responses and helps in the resolution of inflammation. This dual role makes NT5E a key molecule in both health and disease, especially in pathologies such as cancer, cardiovascular diseases, and chronic inflammatory disorders.

Protein Structure

CD73 is a glycosylated, homodimeric enzyme consisting of two identical subunits, each with a molecular weight of about 70 kDa. The structure of CD73 is essential for its enzymatic activity and its interaction with extracellular matrix components and membrane proteins. Its structure can be described in terms of the following domains:

Extracellular Domain:

• The majority of CD73 is located extracellularly, where its 5'-nucleotidase activity occurs. This domain includes the active site responsible for the dephosphorylation of AMP to adenosine.
• The active site contains zinc-binding domains, essential for its catalytic activity. Two zinc ions are coordinated in the catalytic site to facilitate the cleavage of the phosphate group from AMP, forming adenosine.
• Several N-linked glycosylation sites exist on the extracellular domain, contributing to protein stability, folding, and resistance to proteolysis.

GPI Anchor:

• CD73 is attached to the cell membrane via a GPI anchor, which anchors the protein to the outer leaflet of the plasma membrane. This GPI anchor is critical for its proper localization on the cell surface and for its interactions with other membrane-bound proteins.
• GPI anchoring facilitates lateral mobility within the membrane, allowing CD73 to cluster in lipid rafts, where it participates in signaling complexes.

Dimerization:

• CD73 functions as a homodimer, with each subunit contributing to the formation of the active site. The dimeric configuration enhances its enzymatic activity and allows for efficient catalysis of AMP dephosphorylation.
• Dimerization occurs through non-covalent interactions between the subunits, and this quaternary structure is important for its stability and function.

Catalytic Domain:

• The catalytic domain is located within the extracellular portion of CD73. This domain is highly conserved and facilitates the dephosphorylation of nucleotides. In particular, the catalytic domain hydrolyzes AMP into adenosine and inorganic phosphate in a zinc-dependent manner.
• Zinc ions in the active site act as cofactors that stabilize the transition state of the substrate and assist in the nucleophilic attack on the phosphate group.

Classification and Subtypes

CD73 belongs to the family of ecto-nucleotidases, which are enzymes that catalyze the hydrolysis of extracellular nucleotides. Specifically, CD73 is classified as a 5'-nucleotidase, which means it preferentially hydrolyzes nucleotides at the 5' position, such as AMP. CD73 is part of the NTPDase (nucleoside triphosphate diphosphohydrolase) family but is unique in that it acts specifically on AMP to generate adenosine, unlike other ecto-enzymes that may hydrolyze a broader range of nucleotides.

There are no specific subtypes of the NT5E gene that encode CD73, but different splice variants of CD73 have been identified, which could influence its function and expression in various tissues. These splice variants, while not fully characterized, suggest some tissue-specific expression patterns and may contribute to differential regulation in various physiological and pathological conditions.

Function and Biological Significance

CD73 plays a crucial role in purinergic signaling by catalyzing the final step in the conversion of extracellular nucleotides to nucleosides. This conversion is biologically significant for a number of reasons:

Adenosine Production:

• The primary function of CD73 is the production of adenosine from AMP. Adenosine is a key immunomodulatory molecule that exerts various biological effects through binding to specific adenosine receptors (A1, A2A, A2B, A3) on target cells.
• Adenosine has potent anti-inflammatory and immunosuppressive effects, which are critical in maintaining tissue homeostasis, preventing excessive immune responses, and aiding in the resolution of inflammation.
• CD73-generated adenosine is also involved in vasodilation, neuroprotection, and metabolic regulation, reflecting its broad physiological significance.

Immune Modulation:

• CD73 plays a significant role in immune regulation by modulating the balance between pro-inflammatory and anti-inflammatory responses. On immune cells, such as regulatory T cells (Tregs) and myeloid cells, CD73-generated adenosine suppresses pro-inflammatory cytokine production and promotes an immunosuppressive microenvironment.
• CD73 is essential in maintaining immune tolerance and preventing autoimmune diseases by limiting the activity of effector immune cells, such as T cells and macrophages.
• In cancer, CD73-generated adenosine contributes to immune evasion by tumors, enabling cancer cells to escape destruction by suppressing anti-tumor immune responses.

Tissue Protection:

• Under conditions of hypoxia or ischemia, CD73 is upregulated as part of a protective response to generate adenosine, which protects tissues from damage. Adenosine promotes vasodilation, reduces inflammation, and enhances tissue perfusion, minimizing ischemic injury.
• In the cardiovascular system, CD73 has a protective role in ischemia-reperfusion injury, reducing damage to heart tissue during episodes of restricted blood flow.

Vascular Integrity:

• CD73 is highly expressed on endothelial cells and is critical for maintaining vascular permeability and integrity. By generating adenosine, CD73 controls vascular tone and inhibits leukocyte adhesion to the endothelium, preventing excessive infiltration of immune cells into tissues during inflammation.

Clinical Issues

Cancer:

• CD73 is overexpressed in several types of cancer, including breast, lung, and colon cancers. Tumor cells exploit the immunosuppressive effects of CD73-generated adenosine to evade detection and destruction by the immune system. High CD73 expression is often associated with poor prognosis due to enhanced tumor growth, metastasis, and immune evasion.
• Targeting CD73 has emerged as a therapeutic strategy in cancer immunotherapy. Inhibitors of CD73, as well as adenosine receptor antagonists, are being investigated to enhance the efficacy of immune checkpoint inhibitors by reducing adenosine-mediated immunosuppression.

Autoimmune Diseases:

• CD73 has been implicated in the pathogenesis of several autoimmune diseases, including multiple sclerosis (MS), rheumatoid arthritis (RA), and inflammatory bowel disease (IBD). In these conditions, dysregulation of CD73 expression or activity may lead to impaired adenosine production, contributing to unchecked inflammation and tissue damage.
• Therapeutic strategies aimed at enhancing CD73 activity or increasing adenosine levels could offer a way to modulate excessive immune responses in autoimmune diseases.

Cardiovascular Diseases:

• In the cardiovascular system, CD73 plays a protective role during ischemia-reperfusion injury, where adenosine generated by CD73 helps to reduce inflammation and promote tissue recovery. However, dysregulation of CD73 can contribute to pathological conditions such as atherosclerosis, where reduced CD73 expression is associated with enhanced inflammation and plaque formation.

Metabolic Disorders:

• CD73 and adenosine signaling also play roles in metabolic regulation, particularly in glucose and lipid metabolism. Altered CD73 activity has been linked to metabolic diseases such as type 2 diabetes and obesity, where impaired adenosine production may exacerbate insulin resistance and inflammation.

Summary

NT5E encodes CD73, a crucial enzyme in the purinergic signaling pathway that converts AMP to adenosine, a molecule with broad immunomodulatory, vasodilatory, and tissue-protective roles. CD73 is a glycosylated GPI-anchored homodimeric enzyme, with its active site located in the extracellular domain and zinc ions functioning as essential cofactors. It has key roles in immune regulation, inflammation resolution, and vascular homeostasis. Dysregulation of CD73 is implicated in cancer, autoimmune diseases, and cardiovascular disorders, making it a significant therapeutic target in these conditions.