Because of their importance in cell signaling and regulation,<unk> s have been implicated in a number of diseases and disorders.<unk>  knockout mice are often unable to develop and such knockouts generally result in embryonic or perinatal lethality. In immunology in particular, the term<unk> " often describes a secondary receptor used by a pathogen to gain access to the cell, or a receptor that works alongside T cell receptors such as CD4, CD8, or CD28 to bind antigens or regulate T cell activity in some way. Inherited<unk>  autosomal disorders Many<unk> -related disorders occur due to mutations in the receptor's coding gene. LRP5 (low-density lipoprotein receptor-related protein 5) acts as a<unk>  for the Wnt-family of glycoproteins which regulate bone mass. Malfunctions in this<unk>  lead to lower bone density and strength which contribute to osteoporosis. Loss of function mutations in LRP5 have been implicated in Osteoporosis-pseudoglioma syndrome, Familial exudative vitreoretinopathy, and a specific missense mutation in the first β-propeller region of LRP5 can lead to abnormally high bone density or osteopetrosis. Mutations in LRP1 have also been found in cases of Familial Alzheimer's disease Loss of function mutations in the Cryptic<unk>  can lead to random organ positioning due to developmental left-right orientation defects. Gigantism is believed to be caused, in some cases, by a loss of function of the Glypican 3<unk> . Cancer Carcinoembryonic antigen cell adhesion molecule-1 (Caecam1) is an immunoglobulin-like<unk>  that aids in cell adhesion in epithelial, endothelial and hematopoietic cells, and plays a vital role during vascularization and angiogenesis by binding vascular endothelial growth factor (VEGF). Angiogenesis is important in embryonic development but it is also a fundamental process of tumor growth. Deletion of the gene in Caecam1-/- mice results in a reduction of the abnormal vascularization seen in cancer and lowered nitric oxide production, suggesting a therapeutic possibility through targeting of this gene. The neuropilin<unk>  family mediates binding of VEGF in conjunction with the VEGFR1/VEGFR2 and Plexin signaling receptors, and therefore also plays a role in tumor vascular development. CD109 acts as a negative regulator of the tumor growth factor β (TGF-β) receptor. Upon binding TGF-β, the receptor is internalized via endocytosis through CD109's action which lowers signal transmission into the cell. In this case, the<unk>  is functioning in a critical regulatory manner to reduce signals that instruct the cell to grow and migrate – the hallmarks of cancer. In conjunction, the LRP<unk>  family also mediates binding of TGF-β with a variety of membrane receptors. Interleukins 1, 2, and 5 all rely on interleukin<unk> s to bind to the primary interleukin receptors. Syndecans 1 and 4 have been implicated in a variety of cancer types including cervical, breast, lung, and colon cancer, and abnormal expression levels have been associated with poorer prognosis. HIV In order to infect a cell, the envelope glycoprotein GP120 of the HIV virus interacts with CD4 (acting as the primary receptor) and a<unk> : either CCR5 or CXCR4. This binding results in membrane fusion and the subsequent intracellular signaling that facilitates viral invasion. In approximately half of all HIV cases, the viruses using the CCR5<unk>  seem to favor immediate infection and transmission while those using the CXCR4 receptor do not present until later in the immunologically suppressed stage of the disease. The virus will often switch from using CCR5 to CXCR4 during the course of the infection, which serves as an indicator for the progression of the disease. Recent evidence suggests that some forms of HIV also use the large integrin a4b7 receptor to facilitate increased binding efficiency in mucosal tissues. Hepatitis C The Hepatitis C virus requires the CD81<unk>  for infection. Studies suggest that the tight junction protein Claudin-1 (CLDN1) may also play a part in HCV entry. Claudin family abnormalities are also common in hepatocellular carcinoma, which can result from HPV infection. Blockade as a treatment for autoimmunity It is possible to perform a CD4<unk>  blockade, using antibodies, in order to lower T cell activation and counteract autoimmune disorders. This blockade appears to elicit a "dominant" effect, that is to say, once blocked, the T cells do not regain their ability to become active. This effect then spreads to native T cells which then switch to a CD4+CD25+GITR+FoxP3+ T regulatory phenotype.
Co-receptor
Preceptor
Lectin
K-complex
Fc receptor
Baroreceptor
Thromboxane receptor
Serpin