Retinaldehyde-binding protein 1 (RLBP1) also known as cellular retinaldehyde-binding protein (CRALBP) is a 36-kD water-soluble protein that in humans is encoded by the RLBP1gene.[5][6][7]
Discovery
Cellular retinol binding protein (CRBP) was first discovered in 1973 from lung tissues by Bashor et al.[8] There have been three cellular retinol binding protein categories discovered; Cellular retinol-binding protein, cellular retinoic acid-binding protein and cellular retinaldehyde-binding protein(CRALBP).[8][9][10] CRALBP was first discovered in 1977, after it was purified from retina and retinal pigment epithelial cells.[10][11]
Function
The cellular retinaldehyde-binding protein transports 11-cis-retinal (also known as 11-cis-retinaldehyde) as its physiological ligands. It plays a critical role as an 11-cis-retinal acceptor which facilitates the enzymaticisomerization of all 11-trans-retinal to 11-cis-retinal, in the isomerization of the rod and cones of the visual cycle.[12][13]
Tissue distribution
CRALBP is not just found in retina and retinal pigment epithelial cells, but also expressed in other cell types. It is majorly found in the iris, cornea, ciliary epithelium, Muller cells, the pineal gland and oligodendrocytes of the optic nerve and brain. This protein is also found in other tissues than the aforementioned ones, however its function in cells not related to the eyes are not yet known [14]
Clinical significance
When a visual pigment molecule in photoreceptors of mammalian rod and cone cells are triggered by photons of light, the pigment molecule is unable to detect an ensuing photon of light. All the retinal molecules in the chromophore of the visual pigment molecule, exist in the 11-trans-retinal state after stimulation by photons. RLBP1 helps in converting the 11-trans-retinal to the light sensitive 11-cis retinal. This process is a part of the visual cycle, which involves the expulsion of all 11-trans-retinal containing chromophores out of photoreceptors, and subsequent conversion to the 11-cis-retinal state in retinal pigment epithelial cells, for both rod and cone cells. The 11-cis chromophore is then signalled back into photoreceptor cells, where it undergoes fusion with a free opsin molecule to regenerate the visual pigment.[15][16][13]
Gene location
The RLBP1 gene is located on human chromosome 15, specifically on 15q26. This gene was formerly believed to have 8 exons and 7 introns.[5][17] However, Vogel et al. demonstrated that there are actually 8 introns on that chromosome.[18] A gene element, upstream of the previously thought exon 1 was originally thought to be an enhancer. In reality, this assumed enhancer is the main promoter for this gene. The newly discovered intron 1 lies within and just near the end of the promoter region of RLBP1 gene.[18]
Mutations and associated diseases
Mutations of RLBP1 include several diseases associated with vision. All of these are autosomalrecessive including Bothnia dystrophy, retinitis punctata albescens, retinitis pigmentosa, Newfoundland rod-cone dystrophy and fundus albipunctatus. The characteristics of the associated diseases vary with age, severity and rate of progression. These all have similar qualities such as, photoreceptor deterioration and slower dark adaptation, ultimately leading to visual impairment, often leading to complete blindness.[14]
Bothnia dystrophy
People suffering from Bothnia dystrophy have a homozygous C to T base pair substitution in exon 7 of the RLBP1 gene. This leads to a missense mutation from Arginine to Tryptophan at the 234 position of the RLBP1.[19]
Retinitis punctata albescens and fundus albipunctatus
Katsanis et al. showed that a homozygous alteration from Arginine to Glutamine amino acid expression, at the 150 position on RLBP1, brings about the onset of retinitis punctata albescens and or fundus albipunctatus.[20]
^"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.
^ abSparkes RS, Heinzmann C, Goldflam S, Kojis T, Saari JC, Mohandas T, Klisak I, Bateman JB, Crabb JW (January 1992). "Assignment of the gene (RLBP1) for cellular retinaldehyde-binding protein (CRALBP) to human chromosome 15q26 and mouse chromosome 7". Genomics. 12 (1): 58–62. doi:10.1016/0888-7543(92)90406-I. PMID1733864.
^Maw MA, Kennedy B, Knight A, Bridges R, Roth KE, Mani EJ, Mukkadan JK, Nancarrow D, Crabb JW, Denton MJ (October 1997). "Mutation of the gene encoding cellular retinaldehyde-binding protein in autosomal recessive retinitis pigmentosa". Nature Genetics. 17 (2): 198–200. doi:10.1038/ng1097-198. PMID9326942. S2CID37723395.
^ abFutterman S, Saari JC (August 1977). "Occurrence of 11-cis-retinal-binding protein restricted to the retina". Investigative Ophthalmology & Visual Science. 16 (8): 768–71. PMID560359.
^ abVogel JS, Bullen EC, Teygong CL, Howard EW (August 2007). "Identification of the RLBP1 gene promoter". Investigative Ophthalmology & Visual Science. 48 (8): 3872–7. doi:10.1167/iovs.06-1523. PMID17652763.
^Burstedt MS, Sandgren O, Holmgren G, Forsman-Semb K (April 1999). "Bothnia dystrophy caused by mutations in the cellular retinaldehyde-binding protein gene (RLBP1) on chromosome 15q26". Investigative Ophthalmology & Visual Science. 40 (5): 995–1000. PMID10102298.
^Katsanis N, Shroyer NF, Lewis RA, Cavender JC, Al-Rajhi AA, Jabak M, Lupski JR (June 2001). "Fundus albipunctatus and retinitis punctata albescens in a pedigree with an R150Q mutation in RLBP1". Clinical Genetics. 59 (6): 424–9. doi:10.1034/j.1399-0004.2001.590607.x. PMID11453974. S2CID9196078.
Burstedt MS, Sandgren O, Holmgren G, Forsman-Semb K (April 1999). "Bothnia dystrophy caused by mutations in the cellular retinaldehyde-binding protein gene (RLBP1) on chromosome 15q26". Investigative Ophthalmology & Visual Science. 40 (5): 995–1000. PMID10102298.
Morimura H, Berson EL, Dryja TP (April 1999). "Recessive mutations in the RLBP1 gene encoding cellular retinaldehyde-binding protein in a form of retinitis punctata albescens". Investigative Ophthalmology & Visual Science. 40 (5): 1000–4. PMID10102299.
Thumann G, Kociok N, Bartz-Schmidt KU, Esser P, Schraermeyer U, Heimann K (December 1999). "Detection of mRNA for proteins involved in retinol metabolism in iris pigment epithelium". Graefe's Archive for Clinical and Experimental Ophthalmology. 237 (12): 1046–51. doi:10.1007/s004170050343. PMID10654176. S2CID26118763.
Burstedt MS, Forsman-Semb K, Golovleva I, Janunger T, Wachtmeister L, Sandgren O (February 2001). "Ocular phenotype of bothnia dystrophy, an autosomal recessive retinitis pigmentosa associated with an R234W mutation in the RLBP1 gene". Archives of Ophthalmology. 119 (2): 260–7. PMID11176989.
Harrington JJ, Sherf B, Rundlett S, Jackson PD, Perry R, Cain S, Leventhal C, Thornton M, Ramachandran R, Whittington J, Lerner L, Costanzo D, McElligott K, Boozer S, Mays R, Smith E, Veloso N, Klika A, Hess J, Cothren K, Lo K, Offenbacher J, Danzig J, Ducar M (May 2001). "Creation of genome-wide protein expression libraries using random activation of gene expression". Nature Biotechnology. 19 (5): 440–5. doi:10.1038/88107. PMID11329013. S2CID25064683.
Katsanis N, Shroyer NF, Lewis RA, Cavender JC, Al-Rajhi AA, Jabak M, Lupski JR (June 2001). "Fundus albipunctatus and retinitis punctata albescens in a pedigree with an R150Q mutation in RLBP1". Clinical Genetics. 59 (6): 424–9. doi:10.1034/j.1399-0004.2001.590607.x. PMID11453974. S2CID9196078.
Fishman GA, Roberts MF, Derlacki DJ, Grimsby JL, Yamamoto H, Sharon D, Nishiguchi KM, Dryja TP (January 2004). "Novel mutations in the cellular retinaldehyde-binding protein gene (RLBP1) associated with retinitis punctata albescens: evidence of interfamilial genetic heterogeneity and fundus changes in heterozygotes". Archives of Ophthalmology. 122 (1): 70–5. doi:10.1001/archopht.122.1.70. PMID14718298.
Nawrot M, West K, Huang J, Possin DE, Bretscher A, Crabb JW, Saari JC (February 2004). "Cellular retinaldehyde-binding protein interacts with ERM-binding phosphoprotein 50 in retinal pigment epithelium". Investigative Ophthalmology & Visual Science. 45 (2): 393–401. doi:10.1167/iovs.03-0989. PMID14744877.
