| GSAP | |||||||||||||||||||||||||||||||||||||||||||||||||||
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| Aliases | GSAP , PION, Protein pigeon homolog, gamma-secretase activating protein | ||||||||||||||||||||||||||||||||||||||||||||||||||
| External IDs | OMIM: 613552 MGI: 2442259 HomoloGene: 45504 GeneCards: GSAP | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Protein pigeon homolog also known as gamma-secretase activating protein (GSAP) is a protein that in humans is encoded by the PION gene. [5]
The human PION gene is located on the long (q) arm of chromosome 7 at band 11.23, from base pair 76,778,007 to base pair 76,883,653. [6] Highly conserved PION orthologs have been identified in most vertebrates for which complete genome data are available. [7] More distantly related orthologs are also expressed in insects including the pigeon gene in Drosophila melanogaster that when mutated produces the "pigeon" phenotype. The name of the human PION gene derives the corresponding Drosophila gene.
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The transcribed human pigeon homolog protein is 854 amino acid residues in length. [8] A 16 kDa fragment (GSAP-16K) derived from 121 residues from the C-terminus region of the full length protein is known as the γ-secretase activating protein (GSAP). [9]
γ-secretase activating protein (GSAP) increases β-amyloid production through a mechanism involving its interactions with both γ-secretase and its substrate, the amyloid precursor protein (APP). [9] By binding to both the γ-secretase enzyme and its APP substrate, GSAP increases the affinity and the selectivity of the enzyme for this particular substrate.
The activating function of GSAP can be inhibited by the anticancer drug imatinib (Gleevec) which in turn prevents γ-secretase from converting APP into plaque forming β-amyloid without affecting the other functions of γ-secretase. Imatinib itself does not get into the brain [10] so imatinib could not be used as an AD therapeutic. However it may be possible to identify imatinib-like drugs that do get into the brain. Hence GSAP represents a potential therapeutic target for the treatment of Alzheimer's disease (AD). [9]
The drug semagacestat in contrast to imatinib, works by directly inhibiting the γ-secretase. While semagacestat reduces β-amyloid plaque formation in AD patients, γ-secretase is also needed to make other important proteins. [11] The failure of semagacestat to improve the cognitive function of AD patients may be due to its non-selective blockade of γ-secretase. The more selective blockade of γ-secretase provided by inhibiting GSAP may make GSAP a more efficacious and safer drug target than γ-secretase. [9]
The PION gene was originally discovered through a large scale genome sequencing effort. [12] However the function of the PION gene product remained a mystery. In the laboratory of Paul Greengard, a screen of compounds that could inhibit the formation of β-amyloid identified imatinib, [13] however it was not immediately known how it accomplished this. Later it was discovered by Greengard's lab that imatinib inhibited the function of GSAP and that GSAP in turn functions as an activator of γ-secretase. [9]
Beta-secretase 2 is an enzyme that cleaves Glu-Val-Asn-Leu!Asp-Ala-Glu-Phe in the Swedish variant of Alzheimer's amyloid precursor protein. BACE2 is a close homolog of BACE1.
Amyloid beta denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid-beta precursor protein (APP), which is cleaved by beta secretase and gamma secretase to yield Aβ in a cholesterol-dependent process and substrate presentation. Aβ molecules can aggregate to form flexible soluble oligomers which may exist in several forms. It is now believed that certain misfolded oligomers can induce other Aβ molecules to also take the misfolded oligomeric form, leading to a chain reaction akin to a prion infection. The oligomers are toxic to nerve cells. The other protein implicated in Alzheimer's disease, tau protein, also forms such prion-like misfolded oligomers, and there is some evidence that misfolded Aβ can induce tau to misfold.
Amyloid-beta precursor protein (APP) is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. It functions as a cell surface receptor and has been implicated as a regulator of synapse formation, neural plasticity, antimicrobial activity, and iron export. It is coded for by the gene APP and regulated by substrate presentation. APP is best known as the precursor molecule whose proteolysis generates amyloid beta (Aβ), a polypeptide containing 37 to 49 amino acid residues, whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.
Beta-secretase 1, also known as beta-site amyloid precursor protein cleaving enzyme 1, beta-site APP cleaving enzyme 1 (BACE1), membrane-associated aspartic protease 2, memapsin-2, aspartyl protease 2, and ASP2, is an enzyme that in humans is encoded by the BACE1 gene. Expression of BACE1 is observed mainly in neurons.
Protein kinase RNA-activated also known as protein kinase R (PKR), interferon-induced, double-stranded RNA-activated protein kinase, or eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2) is an enzyme that in humans is encoded by the EIF2AK2 gene.
Gamma secretase is a multi-subunit protease complex, itself an integral membrane protein, that cleaves single-pass transmembrane proteins at residues within the transmembrane domain. Proteases of this type are known as intramembrane proteases. The most well-known substrate of gamma secretase is amyloid precursor protein, a large integral membrane protein that, when cleaved by both gamma and beta secretase, produces a short 37-43 amino acid peptide called amyloid beta whose abnormally folded fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients. Gamma secretase is also critical in the related processing of several other type I integral membrane proteins, such as Notch, ErbB4, E-cadherin, N-cadherin, ephrin-B2, or CD44.
Presenilins are a family of related multi-pass transmembrane proteins which constitute the catalytic subunits of the gamma-secretase intramembrane protease protein complex. They were first identified in screens for mutations causing early onset forms of familial Alzheimer's disease by Peter St George-Hyslop. Vertebrates have two presenilin genes, called PSEN1 that codes for presenilin 1 (PS-1) and PSEN2 that codes for presenilin 2 (PS-2). Both genes show conservation between species, with little difference between rat and human presenilins. The nematode worm C. elegans has two genes that resemble the presenilins and appear to be functionally similar, sel-12 and hop-1.
Cathepsin B belongs to a family of lysosomal cysteine proteases known as the cysteine cathepsins and plays an important role in intracellular proteolysis. In humans, cathepsin B is encoded by the CTSB gene. Cathepsin B is upregulated in certain cancers, in pre-malignant lesions, and in various other pathological conditions.
APH-1 is a protein gene product originally identified in the Notch signaling pathway in Caenorhabditis elegans as a regulator of the cell-surface localization of nicastrin. APH-1 homologs in other organisms, including humans, have since been identified as components of the gamma secretase complex along with the catalytic subunit presenilin and the regulatory subunits nicastrin and PEN-2. The gamma-secretase complex is a multimeric protease responsible for the intramembrane proteolysis of transmembrane proteins such as the Notch protein and amyloid precursor protein (APP). Gamma-secretase cleavage of APP is one of two proteolytic steps required to generate the peptide known as amyloid beta, whose misfolded form is implicated in the causation of Alzheimer's disease. All of the components of the gamma-secretase complex undergo extensive post-translational modification, especially proteolytic activation; APH-1 and PEN-2 are regarded as regulators of the maturation process of the catalytic component presenilin. APH-1 contains a conserved alpha helix interaction motif glycine-X-X-X-glycine (GXXXG) that is essential to both assembly of the gamma secretase complex and to the maturation of the components.
Presenilin-1(PS-1) is a presenilin protein that in humans is encoded by the PSEN1 gene. Presenilin-1 is one of the four core proteins in the gamma secretase complex, which is considered to play an important role in generation of amyloid beta (Aβ) from amyloid-beta precursor protein (APP). Accumulation of amyloid beta is associated with the onset of Alzheimer's disease.
Protein numb homolog is a protein that in humans is encoded by the NUMB gene. The protein encoded by this gene plays a role in the determination of cell fates during development. The encoded protein, whose degradation is induced in a proteasome-dependent manner by MDM2, is a membrane-bound protein that has been shown to associate with EPS15, LNX1, and NOTCH1. Four transcript variants encoding different isoforms have been found for this gene.
Amyloid beta A4 precursor protein-binding family B member 1 is a protein that in humans is encoded by the APBB1 gene.
Amyloid-like protein 1, also known as APLP1, is a protein that in humans is encoded by the APLP1 gene. APLP1 along with APLP2 are important modulators of glucose and insulin homeostasis.
HIG1 domain family member 1A (HIGD1A), also known as hypoglycemia/hypoxia inducible mitochondrial protein1-a (HIMP1-a) and hypoxia induced gene 1 (HIG1), is a protein that in humans is encoded by the HIGD1A gene on chromosome 3. This protein promotes mitochondrial homeostasis and survival of cells under stress and is involved in inflammatory and hypoxia-related diseases, including atherosclerosis, ischemic heart disease, and Alzheimer’s disease, as well as cancer.
Numb-like protein is a protein that in humans is encoded by the NUMBL gene.
Transmembrane protein 59 is a protein that in humans is encoded by the TMEM59 gene.
Amyloid beta A4 precursor protein-binding family B member 3 is a protein that in humans is encoded by the APBB3 gene.
Protein tyrosine phosphatase non-receptor type 5 is an enzyme that in humans is encoded by the PTPN5 gene.
Semagacestat (LY-450139) was a candidate drug for a causal therapy against Alzheimer's disease. It was originally developed by Eli Lilly and Elan, and clinical trials were conducted by Eli Lilly. Phase III trials included over 3000 patients, but in August 2010, a disappointing interim analysis, in which semagacestat performed worse than the placebo, led to the trials being stopped.
Leukocyte immunoglobulin-like receptor subfamily A member 5 (LILR-A5) also known as CD85 antigen-like family member F (CD85f), immunoglobulin-like transcript 7 (ILT-7), and leukocyte immunoglobulin-like receptor 9 (LIR-9) is a protein that in humans is encoded by the LILRA5 gene. This gene is one of the leukocyte receptor genes that form a gene cluster on the chromosomal region 19q13.4. Four alternatively spliced transcript variants encoding distinct isoforms have been described.
