Claudins are a family of proteins that are important components of tight junctions, which are specialized structures found in epithelial and endothelial cells that regulate the permeability of cellular barriers. Tight junctions are crucial for maintaining the integrity and function of various tissues and organs, such as the blood-brain barrier, kidney tubules, and digestive tract.

Claudins are transmembrane proteins that span the cell membrane and interact with other claudins on adjacent cells to form tight junctions. They play a key role in regulating the movement of ions and molecules across cellular barriers, which is critical for various physiological processes such as nutrient absorption, waste elimination, and immune surveillance.

There are at least 27 different claudin genes that have been identified in humans, and different tissues express different combinations of claudins to create barriers with specific permeability properties. Dysregulation of claudin expression or function has been implicated in various diseases, including inflammatory bowel disease, cancer, and neurological disorders.

Claudin 1

Claudin 1 is a protein that is part of the tight junctions between cells in various tissues of the body. It plays a critical role in maintaining the integrity and barrier function of these tissues. Dysregulation of Claudin 1 has been implicated in various diseases, including cancer, viral infections, and inflammatory bowel disease

Claudin 5

At the blood–brain barrier, Claudin-5 is the most enrichedtight junction protein and its dysfunction has been implicated in neurodegenerative disorders such as Alzheimer's disease, neuroinflammatory disorders such as multiple sclerosis as well as psychiatric disorders including depression and schizophrenia.

Claudin 7

Claudin-7 plays a significant role in maintaining the physiological functions and pathological conditions of the TJ barrier. The dysregulation of claudin-7 plays a tumor suppressor role or conversely has carcinogenic effects in different target tissues or cells, but the exact underlying mechanism is still unclear. In this review, we will summarize the expression pattern of claudin-7 in tumors, focusing on the expression and regulation of claudin-7 in colorectal cancer and discussing the correlation between claudin-7 and invasion, metastasis and epithelial–mesenchymal transition (EMT) in colorectal cancer.

Claudin 18

Claudin-18 is a tight-junction protein uniquely expressed in gastric epithelial cellsand has been shown to be expressed in gastric and pancreatic adenocarcinoma. There are two isotypes:

Claudin 18.2 is reported only limitedly expressed in differentiated epithelial cells of the gastric mucosa whereas significantly expressed in a variety of malignant tumor tissues such as gastric cancer, pancreatic cancer, cholangiocarcinoma, ovarian cancer and lung cancer etc. Thus Claudin 18.2 has emerged as an ideal target for immunotherapy including monoclonal antibody, bispecific antibody, ADC and CAR-T therapy.

Claudin 18. is highly homologous to Claudin 18.2. Taking the human Claudin 18 protein as an example, the two isoforms only differ by 7 amino acids in the first extracellular domain (the mouse origin contains 8 amino acid differences).