The leucine-rich repeat-containing G protein-coupled receptor 4 (LGR4, also called GPR48) plays a key role in multiple developmental processes, and mice lacking Lgr4 display anterior segment dysgenesis leading to early-onset glaucomatous retinal ganglion cell loss as well as defective eyelid formation. This paper will review Lgr4 signaling and its regulation of the Axenfeld-Rieger syndrome gene Pitx2, a crucial developmental transcription factor. In addition, Wnt signaling plays an important role in eye development, with Norrin functioning to activate the Wnt receptor Frizzled 4 required for proper retinal vascularization. Recent discoveries identifying Lgr4 as a receptor for Norrin highlight the potential for Lgr4 function in retinal vascularization. Finally, several unanswered questions impeding a full understanding of Lgr4 in glaucoma are considered as avenues for further research. 1. Introduction Glaucoma is the second leading cause of blindness worldwide, but its etiology is complex and only partially understood. Frequently associated with elevated intraocular pressure (IOP) leading to a stereotypical pattern of retinal ganglion loss and cup excavation, glaucoma may result from closure of the iridocorneal angle, blockage of the trabecular meshwork responsible for aqueous humor outflow, pupil block of the space between the lens and iris necessary for humor circulation between the anterior and posterior chambers, and several other causes. Loss of the leucine-rich repeat domain-containing G protein-coupled receptor 4 (Lgr4, also called Gpr48) has been implicated in anterior segment dysgenesis including iridocorneal attachment and elevated intraocular pressure leading to early onset retinal ganglion cell loss in mice that is similar to glaucomatous damage in humans [1]. Recently published work identifies Lgr4 as a receptor for Norrin, a secreted protein with established roles in retinal neuron protection and retinal vascularization and therefore suggests an additional mechanism by which Lgr4 functions to prevent glaucoma [2]. This paper will provide an overview of Lgr4 signal transduction and its role in a wide variety of developmental processes, followed by a focus on recent developments in the role of Lgr4 in glaucoma. 2. Lgr4 Signaling The leucine-rich repeat domain-containing G protein-coupled receptors (LGRs) feature a large N-terminal extracellular domain containing multiple leucine-rich repeats and are subdivided into three groups. One group consists of the three glycoprotein hormone receptors: lutenizing hormone receptor, follicle-stimulating
References
[1]
J. Weng, J. Luo, X. Cheng et al., “Deletion of G protein-coupled receptor 48 leads to ocular anterior segment dysgenesis (ASD) through down-regulation of Pitx2,” Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 16, pp. 6081–6086, 2008.
[2]
C. Deng, P. Reddy, Y. Cheng, C. W. Luo, C. L. Hsiao, and A. J. Hsueh, “Multi-functional norrin is a ligand for the LGR4 receptor,” Journal of Cell Science, 2013.
[3]
J. Wang, X. Li, Y. Ke et al., “GPR48 increases mineralocorticoid receptor gene expression,” Journal of the American Society of Nephrology, vol. 23, no. 2, pp. 281–293, 2012.
[4]
X. Y. Li, Y. Lu, H. Sun et al., “G protein-coupled receptor 48 upregulates estrogen receptor α expression via cAMP/PKA signaling in the male reproductive tract,” Development, vol. 137, no. 1, pp. 151–157, 2010.
[5]
J. Luo, W. Zhou, X. Zhou et al., “Regulation of bone formation and remodeling by G-protein-coupled receptor 48,” Development, vol. 136, no. 16, pp. 2747–2756, 2009.
[6]
H. Song, J. Luo, W. Luo et al., “Inactivation of G-protein-coupled receptor 48 (Gpr48/Lgr4) impairs definitive erythropoiesis at midgestation through down-regulation of the ATF4 signaling pathway,” The Journal of Biological Chemistry, vol. 283, no. 52, pp. 36687–36697, 2008.
[7]
Y. Gao, K. Kitagawa, M. Shimada et al., “Generation of a constitutively active mutant of human GPR48/LGR4, a G-protein-coupled receptor,” The Hokkaido Journal of Medical Science, vol. 81, no. 2, pp. 101–109, 2006.
[8]
N. Barker, J. H. van Es, J. Kuipers et al., “Identification of stem cells in small intestine and colon by marker gene Lgr5,” Nature, vol. 449, no. 7165, pp. 1003–1007, 2007.
[9]
N. Barker, M. Huch, P. Kujala et al., “Lgr5+ve stem cells drive self-renewal in the stomach and build long-lived gastric units in vitro,” Cell Stem Cell, vol. 6, no. 1, pp. 25–36, 2010.
[10]
M. Huch, C. Dorrell, S. F. Boj et al., “In vitro expansion of single Lgr5+ liver stem cells induced by Wnt-driven regeneration,” Nature, vol. 494, pp. 247–250, 2013.
[11]
V. Jaks, N. Barker, M. Kasper et al., “Lgr5 marks cycling, yet long-lived, hair follicle stem cells,” Nature Genetics, vol. 40, no. 11, pp. 1291–1299, 2008.
[12]
V. Plaks, A. Brenot, D. A. Lawson et al., “Lgr5-expressing cells are sufficient and necessary for postnatal mammary gland organogenesis,” Cell Reports, vol. 3, no. 1, pp. 70–78, 2013.
[13]
K. E. de Visser, M. Ciampricotti, E. M. Michalak et al., “Developmental stage-specific contribution of LGR5+ cells to basal and luminal epithelial lineages in the postnatal mammary gland,” The Journal of Pathology, vol. 228, no. 3, pp. 300–309, 2012.
[14]
H. J. Snippert, A. Haegebarth, M. Kasper et al., “Lgr6 marks stem cells in the hair follicle that generate all cell lineages of the skin,” Science, vol. 327, no. 5971, pp. 1385–1389, 2010.
[15]
S. Yui, T. Nakamura, T. Sato et al., “Functional engraftment of colon epithelium expanded in vitro from a single adult Lgr5+ stem cell,” Nature Medicine, vol. 18, no. 4, pp. 618–623, 2012.
[16]
H. X. Hao, Y. Xie, Y. Zhang et al., “ZNRF3 promotes Wnt receptor turnover in an R-spondin-sensitive manner,” Nature, vol. 485, pp. 195–200, 2012.
[17]
K. S. Carmon, X. Gong, Q. Lin, A. Thomas, and Q. Liu, “R-spondins function as ligands of the orphan receptors LGR4 and LGR5 to regulate Wnt/β-catenin signaling,” Proceedings of the National Academy of Sciences of the United States of America, vol. 108, no. 28, pp. 11452–11457, 2011.
[18]
A. Glinka, C. Dolde, N. Kirsch et al., “LGR4 and LGR5 are R-spondin receptors mediating Wnt/β-catenin and Wnt/PCP signalling,” EMBO Reports, vol. 12, no. 10, pp. 1055–1061, 2011.
[19]
E. D. Loh, S. R. Broussard, and L. F. Kolakowski, “Molecular characterization of a novel glycoprotein hormone G-protein-coupled receptor,” Biochemical and Biophysical Research Communications, vol. 282, no. 3, pp. 757–764, 2001.
[20]
S. Mazerbourg, D. M. Bouley, S. Sudo et al., “Leucine-rich repeat-containing, G protein-coupled receptor 4 null mice exhibit intrauterine growth retardation associated with embryonic and perinatal lethality,” Molecular Endocrinology, vol. 18, no. 9, pp. 2241–2254, 2004.
[21]
S. Kato, M. Matsubara, T. Matsuo et al., “Leucine-rich repeat-containing G protein-coupled receptor-4 (LGR4, Gpr48) is essential for renal development in mice,” Nephron, vol. 104, no. 2, pp. e63–e75, 2006.
[22]
T. Hoshii, T. Takeo, N. Nakagata, M. Takeya, K. Araki, and K. Yamamura, “LGR4 regulates the postnatal development and integrity of male reproductive tracts in mice,” Biology of Reproduction, vol. 76, no. 2, pp. 303–313, 2007.
[23]
F. Mendive, P. Laurent, G. van Schoore, W. Skarnes, R. Pochet, and G. Vassart, “Defective postnatal development of the male reproductive tract in LGR4 knockout mice,” Developmental Biology, vol. 290, no. 2, pp. 421–434, 2006.
[24]
Y. Mohri, K. Oyama, A. Akamatsu, S. Kato, and K. Nishimori, “Lgr4-deficient mice showed premature differentiation of ureteric bud with reduced expression of Wnt effector Lef1 and Gata3,” Developmental Dynamics, vol. 240, no. 6, pp. 1626–1634, 2011.
[25]
R. Yamashita, Y. Takegawa, M. Sakumoto et al., “Defective development of the gall bladder and cystic duct in Lgr4- hypomorphic mice,” Developmental Dynamics, vol. 238, no. 4, pp. 993–1000, 2009.
[26]
M. A. Lambot, F. Mendive, P. Laurent et al., “Three-dimensional reconstruction of efferent ducts in wild-type and Lgr4 knock-out mice,” Anatomical Record, vol. 292, no. 4, pp. 595–603, 2009.
[27]
Y. Mohri, T. Umezu, S. Hidema et al., “Reduced fertility with impairment of early-stage embryos observed in mice lacking Lgr4 in epithelial tissues,” Fertility and Sterility, vol. 94, no. 7, pp. 2878–2881, 2010.
[28]
R. C. Mustata, T. van Loy, A. Lefort et al., “Lgr4 is required for Paneth cell differentiation and maintenance of intestinal stem cells ex vivo,” EMBO Reports, vol. 12, no. 6, pp. 558–564, 2011.
[29]
S. Liu, Y. Qian, L. Li et al., “Lgr4 gene deficiency increases susceptibility and severity of dextran sodium sulfate-induced inflammatory bowel disease in mice,” The Journal of Biological Chemistry, vol. 288, no. 13, pp. 8794–8803, 2013.
[30]
Y. Mohri, S. Kato, A. Umezawa, R. Okuyama, and K. Nishimori, “Impaired hair placode formation with reduced expression of hair follicle-related genes in mice lacking Lgr4,” Developmental Dynamics, vol. 237, no. 8, pp. 2235–2242, 2008.
[31]
K. Oyama, Y. Mohri, M. Sone, A. Nawa, and K. Nishimori, “Conditional knockout of Lgr4 leads to impaired ductal elongation and branching morphogenesis in mouse mammary glands,” Sexual Development, vol. 5, no. 4, pp. 205–212, 2011.
[32]
C. Jin, F. Yin, M. Lin et al., “GPR48 regulates epithelial cell proliferation and migration by activating EGFR during eyelid development,” Investigative Ophthalmology & Visual Science, vol. 49, no. 10, pp. 4245–4253, 2008.
[33]
S. Kato, Y. Mohri, T. Matsuo et al., “Eye-open at birth phenotype with reduced keratinocyte motility in LGR4 null mice,” FEBS Letters, vol. 581, no. 24, pp. 4685–4690, 2007.
[34]
Z. Wang, C. Jin, H. Li et al., “GPR48-induced keratinocyte proliferation occurs through HB-EGF mediated EGFR transactivation,” FEBS Letters, vol. 584, no. 18, pp. 4057–4062, 2010.
[35]
M. H. Strungaru, I. Dinu, and M. A. Walter, “Genotype-phenotype correlations in Axenfeld-Rieger malformation and glaucoma patients with FOXC1 and PITX2 mutations,” Investigative Ophthalmology & Visual Science, vol. 48, no. 1, pp. 228–237, 2007.
[36]
E. T. Cunningham Jr., D. Eliott, N. R. Miller, I. H. Maumenee, and W. R. Green, “Familial Axenfeld-Rieger anomaly, atrial septal defect, and sensorineural hearing loss: a possible new genetic syndrome,” Archives of Ophthalmology, vol. 116, no. 1, pp. 78–82, 1998.
[37]
J. Antevil, R. Umakanthan, M. Leacche et al., “Idiopathic mitral valve disease in a patient presenting with Axenfeld-Rieger syndrome,” The Journal of Heart Valve Disease, vol. 18, no. 3, pp. 349–351, 2009.
[38]
N. A. Bekir and K. Güng？r, “Atrial septal defect with interatrial aneurysm and Axenfeld-Rieger syndrome,” Acta Ophthalmologica Scandinavica, vol. 78, no. 1, pp. 101–103, 2000.
[39]
J. C. Tsai and A. L. Grajewski, “Cardiac valvular disease and Axenfeld-Rieger syndrome,” American Journal of Ophthalmology, vol. 118, no. 2, pp. 255–256, 1994.
[40]
S. Grosso, M. A. Farnetani, R. Berardi et al., “Familial Axenfeld-Rieger anomaly, cardiac malformations, and sensorineural hearing loss: a provisionally unique genetic syndrome?” American Journal of Medical Genetics, vol. 111, no. 2, pp. 182–186, 2002.
[41]
E. V. Semina, R. Reiter, N. J. Leysens et al., “Cloning and characterization of a novel bicoid-related homeobox transcription factor gene, RIEG, involved in Rieger syndrome,” Nature Genetics, vol. 14, no. 4, pp. 392–399, 1996.
[42]
K. Kozlowski and M. A. Walter, “Variation in residual PITX2 activity underlies the phenotypic spectrum of anterior segment developmental disorders,” Human Molecular Genetics, vol. 9, no. 14, pp. 2131–2139, 2000.
[43]
A. S. Borges, R. Susanna Jr., J. C. E. Carani et al., “Genetic analysis of PITX2 and FOXC1 in Rieger syndrome patients from Brazil,” Journal of Glaucoma, vol. 11, no. 1, pp. 51–56, 2002.
[44]
N. Weisschuh, E. de Baere, B. Wissinger, and Z. Tümer, “Clinical utility gene card for: Axenfeld-Rieger syndrome,” European Journal of Human Genetics, vol. 19, no. 3, 2011.
[45]
B. D'haene, F. Meire, I. Claerhout et al., “Expanding the spectrum of FOXC1 and PITX2 mutations and copy number changes in patients with anterior segment malformations,” Investigative Ophthalmology & Visual Science, vol. 52, no. 1, pp. 324–333, 2011.
[46]
P. J. Gage, H. Suh, and S. A. Camper, “Dosage requirement of Pitx2 for development of multiple organs,” Development, vol. 126, no. 20, pp. 4643–4651, 1999.
[47]
K. Kitamura, H. Miura, S. Miyagawa-Tomita et al., “Mouse Pitx2 deficiency leads to anomalies of the ventral body wall, heart, extra- and periocular mesoderm and right pulmonary isomerism,” Development, vol. 126, no. 24, pp. 5749–5758, 1999.
[48]
A. G. Diehl, S. Zareparsi, M. Qian, R. Khanna, R. Angeles, and P. J. Gage, “Extraocular muscle morphogenesis and gene expression are regulated by Pitx2 gene dose,” Investigative Ophthalmology & Visual Science, vol. 47, no. 5, pp. 1785–1793, 2006.
[49]
A. L. Evans and P. J. Gage, “Expression of the homeobox gene Pitx2 in neural crest is required for optic stalk and ocular anterior segment development,” Human Molecular Genetics, vol. 14, no. 22, pp. 3347–3359, 2005.
[50]
T. A. Hjalt, E. V. Semina, B. A. Amendt, and J. C. Murray, “The Pitx2 protein in mouse development,” Developmental Dynamics, vol. 218, no. 1, pp. 195–200, 2000.
[51]
J. Holmberg, C. Y. Liu, and T. A. Hjalt, “PITX2 gain-of-function in Rieger syndrome eye model,” The American Journal of Pathology, vol. 165, no. 5, pp. 1633–1641, 2004.
[52]
P. J. Gage, M. Qian, D. Wu, and K. I. Rosenberg, “The canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during normal eye development,” Developmental Biology, vol. 317, no. 1, pp. 310–324, 2008.
[53]
A. N. Smith, L. D. Miller, N. Song, M. M. Taketo, and R. A. Lang, “The duality of β-catenin function: a requirement in lens morphogenesis and signaling suppression of lens fate in periocular ectoderm,” Developmental Biology, vol. 285, no. 2, pp. 477–489, 2005.
[54]
L. A. Miller, A. N. Smith, M. M. Taketo, and R. A. Lang, “Optic cup and facial patterning defects in ocular ectoderm β-catenin gain-of-function mice,” BMC Developmental Biology, vol. 6, article 14, 2006.
[55]
M. Mukhopadhyay, M. Gorivodsky, S. Shtrom et al., “Dkk2 plays an essential role in the corneal fate of the ocular surface epithelium,” Development, vol. 133, pp. 2149–2154, 2006.
[56]
W. H. Wang, L. G. McNatt, I. H. Pang et al., “Increased expression of the WNT antagonist sFRP-1 in glaucoma elevates intraocular pressure,” The Journal of Clinical Investigation, vol. 118, no. 3, pp. 1056–1064, 2008.
[57]
P. Briata, C. Ilengo, G. Corte et al., “The Wnt/β-catenin→Pitx2 pathway controls the turnover of Pitx2 and other unstable mRNAs,” Molecular Cell, vol. 12, no. 5, pp. 1201–1211, 2003.
[58]
C. Kioussi, P. Briata, S. H. Baek et al., “Identification of a Wnt/Dvl/β-catenin → Pitx2 pathway mediating cell-type-specific proliferation during development,” Cell, vol. 111, no. 5, pp. 673–685, 2002.
[59]
Q. Xu, Y. Wang, A. Dabdoub et al., “Vascular development in the retina and inner ear: control by Norrin and Frizzled-4, a high-affinity ligand-receptor pair,” Cell, vol. 116, no. 6, pp. 883–895, 2004.
[60]
K. Ruether, D. van de Pol, G. Jaissle, W. Berger, R. Tornow, and E. Zrenner, “Retinoschisislike alterations in the mouse eye caused by gene targeting of the Norrie disease gene,” Investigative Ophthalmology & Visual Science, vol. 38, no. 3, pp. 710–718, 1997.
[61]
M. Richter, J. Gottanka, C. A. May, U. Welge-Lü？en, W. Berger, and E. Lütjen-Drecoll, “Retinal vasculature changes in Norrie disease mice,” Investigative Ophthalmology & Visual Science, vol. 39, no. 12, pp. 2450–2457, 1998.
[62]
H. L. Rehm, D. S. Zhang, M. C. Brown et al., “Vascular defects and sensorineural deafness in a mouse model of norrie disease,” The Journal of Neuroscience, vol. 22, no. 11, pp. 4286–4292, 2002.
[63]
U. F. O. Luhmann, D. Meunier, W. Shi et al., “Fetal loss in homozygous mutant Norrie disease mice: a new role of Norrin in reproduction,” Genesis, vol. 42, no. 4, pp. 253–262, 2005.
[64]
J. Chen, A. Stahl, N. M. Krah et al., “Wnt signaling mediates pathological vascular growth in proliferative retinopathy,” Circulation, vol. 124, no. 17, pp. 1871–1881, 2011.
[65]
X. Ye, Y. Wang, H. Cahill et al., “Norrin, frizzled-4, and Lrp5 signaling in endothelial cells controls a genetic program for retinal vascularization,” Cell, vol. 139, no. 2, pp. 285–298, 2009.
[66]
A. Ohlmann, M. Scholz, A. Goldwich et al., “Ectopic Norrin induces growth of ocular capillaries and restores normal retinal angiogenesis in Norrie disease mutant mice,” The Journal of Neuroscience, vol. 25, no. 7, pp. 1701–1710, 2005.
[67]
A. Ohlmann, R. Seitz, B. Braunger, D. Seitz, M. R. B？sl, and E. R. Tamm, “Norrin promotes vascular regrowth after oxygen-induced retinal vessel loss and suppresses retinopathy in mice,” The Journal of Neuroscience, vol. 30, no. 1, pp. 183–193, 2010.
[68]
K. T. Paes, E. Wang, K. Henze et al., “Frizzled 4 is required for retinal angiogenesis and maintenance of the blood-retina barrier,” Investigative Ophthalmology & Visual Science, vol. 52, no. 9, pp. 6452–6461, 2011.
[69]
N. W. Gale, G. Thurston, S. F. Hackett et al., “Angiopoietin-2 is required for postnatal angiogenesis and lymphatic patterning, and only the latter role is rescued by Angiopoietin-1,” Developmental Cell, vol. 3, no. 3, pp. 411–423, 2002.
[70]
S. F. Hackett, S. Wiegand, G. Yancopoulos, and P. A. Campochiaro, “Angiopoietin-2 plays an important role in retinal angiogenesis,” Journal of Cellular Physiology, vol. 192, no. 2, pp. 182–187, 2002.
[71]
R. Seitz, S. Hackl, T. Seibuchner, E. R. Tamm, and A. Ohlmann, “Norrin mediates neuroprotective effects on retinal ganglion cells via activation of the Wnt/β-catenin signaling pathway and the induction of neuroprotective growth factors in Müller cells,” The Journal of Neuroscience, vol. 30, no. 17, pp. 5998–6010, 2010.
[72]
S. Lin, M. Cheng, W. Dailey, K. Drenser, and S. Chintala, “Norrin attenuates protease-mediated death of transformed retinal ganglion cells,” Molecular Vision, vol. 15, pp. 26–37, 2009.
[73]
W. Y. Yu, C. Sheridan, I. Grierson et al., “Progenitors for the corneal endothelium and trabecular meshwork: a potential source for personalized stem cell therapy in corneal endothelial diseases and glaucoma,” Journal of Biomedicine and Biotechnology, vol. 2011, Article ID 412743, 13 pages, 2011.
