Peripheral myelin protein 22
Peripheral myelin protein 22 (PMP22), also called Growth arrest-specific protein 3 (GAS-3), is a protein which in humans is encoded by the PMP22 gene. Mutations in PMP22 cause changes in the expression of peripheral myelin protein 22 which can result in several neuropathies.
PMP22 is a 22 kDa transmembrane glycoprotein made up of 160 amino acids, and is mainly expressed in the Schwann cells of the peripheral nervous system. Schwann cells show high expression of PMP22, where it can constitute 2-5% of total protein content in compact myelin. Compact myelin is the bulk of the peripheral neuron's myelin sheath, a protective fatty layer that provides electrical insulation for the neuronal axon.[5] The level of PMP22 expression is relatively low in the central nervous system of adults.[6]
Like other membrane proteins, newly translated PMP22 protein is temporarily sequestered to the endoplasmic reticulum (ER) and Golgi apparatus for post-translational modifications. PMP22 protein is glycosylated with an N terminus-linked sugar and co-localized with the chaperone protein calnexin in the ER.[7] After the protein is transported to the Golgi apparatus it can then become incorporated in the plasma membrane of the cell.[5]
Structure and function
In humans, the PMP22 gene is located on chromosome 17p12 and spans approximately 40kb. The gene contains six exons conserved in both humans and rodents, two of which are 5’ untranslated exons (1a and 1b) and result in two different RNA transcripts with identical coding sequences. The two transcripts differ in their 5' untranslated regions and have their own promoter regulating expression. Exon 1a corresponds to protein transcription in the peripheral myelin sheath, while exon 1b corresponds to tissue outside of the nervous system.[8] The remaining exons (2 to 5) include the coding region of the PMP22 gene, and are joined together after post-transcriptional modification (i.e. alternative splicing).[6] The PMP22 protein is characterized by four transmembrane domains, two extracellular loops (ECL1 and ECL2), and one intracellular loop.[9] Exon 2 codes for the first transmembrane domain, located on the N-terminus of the PMP22 protein. Exon 3 codes for the first extracellular loop. Exon 4 corresponds to the second transmembrane domain and half of the third. Exon 5 is responsible for the rest of the third and the fourth transmembrane domain, the second extracellular loop, and the 3' UTR.[8] ECL1 has been suggested to mediate a homophilic interaction between two PMP22 proteins, whereas ECL2 has been shown to mediate a heterophilic interaction between PMP22 protein and Myelin protein zero (MPZ).[6]
Although the PMP22 mechanism of action in myelinating Schwann cells is not fully known, it plays an essential role in the formation and maintenance of compact myelin.[5] When Schwann cells come into contact with a neuronal axon, expression of PMP22 is significantly up-regulated,[6] whereas PMP22 is down-regulated during axonal degeneration or transection.[5] PMP22 has shown association with zonula-occludens 1 and occludin, proteins that are involved in adhesion with other cells and the extracellular matrix, and also support functioning of myelin.[5] Along with cell adhesion function, PMP22 is also up-regulated during Schwann cell proliferation, suggesting a role in cell-cycle regulation. PMP22 is detectable in non-neural tissues, where its expression has been shown to serve as growth-arrest-specific (gas-3) function.[5]
Gene-dosage
Improper gene dosage of the PMP22 gene can cause aberrant protein synthesis and function of myelin sheath. Since the components of myelin are stoichiometrically set, any irregular expression of a component can cause destabilization of myelin and neuropathic disorders.[5] Alterations of PMP22 gene expression are associated with a variety of neuropathies, such as Charcot–Marie–Tooth type 1A (CMT1A), Dejerine–Sottas disease, Hereditary Neuropathy with Liability to Pressure Palsy (HNPP), and Charcot-Marie-Tooth type 1E (CMT1E).[6] Too much PMP22 (e.g. caused by gene duplication) results in CMT1A, and too little PMP22 (e.g. caused by gene deletion) results in HNPP.[10] Point mutations in PMP22 can result in CMT1E.[6] Gene duplication of PMP22 is the most common genetic cause of CMT;[11][12] up to half of all cases confirmed by a genetic diagnosis are caused by a 1.4 Mb duplication on chromosome 17, which contains the PMP22 gene.[13] Overproduction of PMP22 results in defects in multiple signaling pathways and dysfunction of transcriptional factors like KNOX20, SOX10 and EGR2.[5]
Interactions and Regulation
PMP22 has been found to interact with several different factors, some of which regulate expression. Peripheral myelin protein 22 has been shown to interact with myelin protein zero, with the proteins forming complexes in myelin.[14] Transcription factors SOX10 and EGR2 have been found to increase the expression of PMP22 through a super-enhancer upstream of the gene.[13] TEAD1 and YAP/TAZ (of the hippo signaling pathway) have been found to bind at the enhancers, with studies showing a decrease in PMP22 expression with the knockdown of these factors. Additionally, PKC activators and HDAC inhibitors have been characterized as regulators of PMP22, as well as microRNAs such as miR-29a and miR-381.[13]
References
- ^ a b c GRCh38: Ensembl release 89: ENSG00000109099 – Ensembl, May 2017
- ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000018217 – Ensembl, May 2017
- ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ a b c d e f g h Watila MM, Balarabe SA (August 2015). "Molecular and clinical features of inherited neuropathies due to PMP22 duplication". Journal of the Neurological Sciences. 355 (1–2): 18–24. doi:10.1016/j.jns.2015.05.037. PMID 26076881. S2CID 40080925.
- ^ a b c d e f Li J, Parker B, Martyn C, Natarajan C, Guo J (April 2013). "The PMP22 gene and its related diseases". Molecular Neurobiology. 47 (2): 673–698. doi:10.1007/s12035-012-8370-x. PMC 3594637. PMID 23224996.
- ^ Dickson KM, Bergeron JJ, Shames I, Colby J, Nguyen DT, Chevet E, et al. (July 2002). "Association of calnexin with mutant peripheral myelin protein-22 ex vivo: a basis for "gain-of-function" ER diseases". Proceedings of the National Academy of Sciences of the United States of America. 99 (15): 9852–9857. Bibcode:2002PNAS...99.9852D. doi:10.1073/pnas.152621799. PMC 125041. PMID 12119418.
- ^ a b Zhang N, Zhu HP, Huang W, Wen X, Xie X, Jiang X, et al. (October 2022). "Unraveling the structures, functions and mechanisms of epithelial membrane protein family in human cancers". Experimental Hematology & Oncology. 11 (1): 69. doi:10.1186/s40164-022-00321-x. PMC 9552464. PMID 36217151.
- ^ Nelis E, Haites N, Van Broeckhoven C (1999). "Mutations in the peripheral myelin genes and associated genes in inherited peripheral neuropathies". Human Mutation. 13 (1): 11–28. doi:10.1002/(SICI)1098-1004(1999)13:1<11::AID-HUMU2>3.0.CO;2-A. PMID 9888385. S2CID 31130790.
- ^ Brennan KM, Bai Y, Shy ME (June 2015). "Demyelinating CMT--what's known, what's new and what's in store?". Neuroscience Letters. 596: 14–26. doi:10.1016/j.neulet.2015.01.059. PMID 25625223. S2CID 23911870.
- ^ Al-Thihli K, Rudkin T, Carson N, Poulin C, Melançon S, Der Kaloustian VM (September 2008). "Compound heterozygous deletions of PMP22 causing severe Charcot-Marie-Tooth disease of the Dejerine-Sottas disease phenotype". American Journal of Medical Genetics. Part A. 146A (18): 2412–2416. doi:10.1002/ajmg.a.32456. PMID 18698610. S2CID 205309846.
- ^ Berger P, Young P, Suter U (March 2002). "Molecular cell biology of Charcot-Marie-Tooth disease". Neurogenetics. 4 (1): 1–15. doi:10.1007/s10048-002-0130-z. PMID 12030326. S2CID 25129077.
- ^ a b c Pantera H, Shy ME, Svaren J (January 2020). "Regulating PMP22 expression as a dosage sensitive neuropathy gene". Brain Research. 1726: 146491. doi:10.1016/j.brainres.2019.146491. PMC 7006452. PMID 31586623.
- ^ D'Urso D, Ehrhardt P, Müller HW (May 1999). "Peripheral myelin protein 22 and protein zero: a novel association in peripheral nervous system myelin". The Journal of Neuroscience. 19 (9). Society for Neuroscience: 3396–3403. doi:10.1523/JNEUROSCI.19-09-03396.1999. PMC 6782240. PMID 10212299.
Further reading
- Patel PI, Lupski JR (April 1994). "Charcot-Marie-Tooth disease: a new paradigm for the mechanism of inherited disease". Trends in Genetics. 10 (4): 128–133. doi:10.1016/0168-9525(94)90214-3. PMID 7518101.
- Roa BB, Lupski JR (1994). "Molecular Genetics of Charcot-Marie-Tooth Neuropathy". Advances in Human Genetics. Vol. 22. pp. 117–52. doi:10.1007/978-1-4757-9062-7_3. ISBN 978-1-4757-9064-1. PMID 7762451.
- Nelis E, Haites N, Van Broeckhoven C (1999). "Mutations in the peripheral myelin genes and associated genes in inherited peripheral neuropathies". Human Mutation. 13 (1): 11–28. doi:10.1002/(SICI)1098-1004(1999)13:1<11::AID-HUMU2>3.0.CO;2-A. PMID 9888385. S2CID 31130790.
- Jetten AM, Suter U (2000). "The peripheral myelin protein 22 and epithelial membrane protein family". Progress in Nucleic Acid Research and Molecular Biology. 64: 97–129. doi:10.1016/S0079-6603(00)64003-5. ISBN 9780125400640. PMID 10697408.