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A Genetic Clog in the Vitamin A Transport Machinery
Leading Edge
Previews A Genetic Clog in the Vitamin A Transport Machinery Ming Zhong1 and Hui Sun1,* 1Howard Hughes Medical Institute, Department of Physiology and Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, U.S.A *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cell.2015.04.020
Chou et al. discover a new mode of maternal inheritance by analyzing human mutations in plasma retinol binding protein (RBP). Mechanistically, these mutations simultaneously lower RBP’s affinity for vitamin A and greatly increase its affinity for its cell-surface receptor, thus dominantly blocking the transmembrane transport of vitamin A. Vitamin A has been the light sensor for vision, or its equivalent, from life’s beginnings. The diversification of its function to regulating cell growth and differentiation, which occurred about 500 million years ago, coincided with the emergence of a long-range and specific vitamin A transport system consisting of a blood carrier protein called plasma retinol binding protein (RBP) and its cell-surface receptor (STRA6), which mediates vitamin A uptake (Kawaguchi et al., 2007). Many organs depend on vitamin A action; however, the eye is still the organ most sensitive to vitamin A deficiency, the loss of RBP, or the loss of STRA6 (Zhong et al., 2012). Studies of human mutations leading to congenital eye malformation allowed Tom Glaser and colleagues, in this issue of Cell, to uncover a new mode of maternal inheritance and its intricate mechanism involving RBP, STRA6, and vitamin A (Chou et al., 2015). Glaser’s team identified human RBP mutations that cause eye malformation such as anophthalmia, microphthalmia, and coloboma, but the phenotypes are preferentially transmitted if the mutations are inherited from the mother. Through a series of elegant biochemical analyses, they discovered that these mutant RBPs block vitamin A transport by STRA6 in a precise and dominant manner. To understand this mechanism, it is useful to understand the major players involved in specific vitamin A transport. Under physiological conditions, vitamin A/RBP complex (termed holo-RBP) associates with transthyretin (TTR) to increase its molecular weight and thus prevent removal by kidney filtration. Holo-RBP dissociates from the
complex to bind to its cell-surface receptor STRA6 due to its higher affinity for STRA6 than for TTR (Kawaguchi et al., 2011). STRA6 then catalyzes vitamin A release from holo-RBP and its transport into the cell where it is stored (Kawaguchi et al., 2011). After losing its cargo, RBP (apo-RBP) has greatly decreased affinity for TTR and is lost through kidney filtration, which prevents its accumulation in the blood (Figure 1). RBP mutations identified in this study confer several properties that are important for their pathogenicity and the way they affect this process. First, while these mutant RBPs are as well secreted as the wild-type protein, they bind vitamin A weakly and tend to lose their cargo in a receptor-independent manner. Second, like wild-type holo-RBP, the mutant ones can still bind to TTR; however, they have an affinity for STRA6 that is 30- to 40-fold higher than that of wild-type RBP (Figure 1). These properties lead to the sequestration of STRA6 by the mutant RBP, decreasing its ability to transport vitamin A into the cell. Indeed, although the blood of mutation carriers may contain about 66% wild-type RBP and only 33% mutant RBP, the blocking effect is sufficient to cause the malformation phenotypes. A longstanding puzzle in the field was the discrepancy between the way human mutations in RBP and STRA6 affect eye development. While mutations in STRA6 cause anopththalmia and other developmental defects (Pasutto et al., 2007), all previously identified RBP mutations did not. As this study shows, a key to this puzzle is the placenta’s role in vitamin A delivery. Previously identified mutations in RBP were recessive and thus unlikely
to inactivate both maternal and fetal RBP because only the fetus could be homozygous for the mutation. In contrast, dominant RBP mutations, like those discovered in this study, can simultaneously inactivate the mother’s and fetal RBP. Similarly, human mutations in RBP and its receptor do not necessarily generate the same phenotypes for the same reason. A human embryo without STRA6 can be directly impacted by the loss of placental absorption of vitamin A from maternal RBP. In contrast, a human embryo without RBP still has STRA6 and functional maternal RBP to ensure vitamin A delivery to the embryo through the placenta. Thus, the dominant human RBP mutations mimic STRA6 mutations in their ability to inactivate both maternal and fetal RBP-mediated transport and therefore can cause anophthalmia. The fact that the dominant RBP mutation can only exert its effect on maternal delivery of vitamin A to the fetus if it is maternal explains this new mode of maternal inheritance. A related question highlighted by this study is the contrast between humans and mice in phenotypic variability even for null mutations in this pathway. For example, human pathologies caused by STRA6 mutations are highly variable, ranging from the ‘‘milder’’ phenotype of anophthalmia to more systemic developmental defects (Pasutto et al., 2007). However, under standard laboratory conditions, STRA6 knockout mice, like RBP knockout mice, have vision-specific phenotypes that lead to blindness due to lack of vitamin A (Amengual et al., 2014; Ruiz et al., 2012). As pointed out here, a key to this puzzle is an RBP-independent pathway mediated by vitamin A esters
Cell 161, April 23, 2015 ª2015 Elsevier Inc. 435
Figure 1. Schematic Diagrams of RBP/Receptor Interaction Schematic diagrams comparing RBP/receptor interaction under physiological conditions and the pathological conditions revealed by Chou et al. (2015). Under physiological conditions (left diagram), holo-RBP dissociates from the RBP/TTR complex to bind to the RBP receptor STRA6 due to its higher affinity for STRA6 than for TTR. STRA6 catalyzes retinol release from holo-RBP and retinol transport into the cell to be stored by binding to CRBP, or by conversion to retinyl esters by LRAT (not shown). After losing its cargo retinol, RBP (apo-RBP) can no longer bind to TTR and is lost through kidney filtration. As depicted in the right diagram, human RBP mutant A55T or A57T (marked in red in RBP structures) tends to lose vitamin A in a receptor-independent manner, but has much higher affinity for STRA6 than wild-type RBP (the mutant RBP/receptor complex is marked by a red circle). These RBP mutants compete with wild-type RBP in binding to STRA6 to impede the transmembrane transport of vitamin A.
(retinyl esters) borrowing the lipoprotein delivery pathway. This pathway can partly compensate for the lack of RBP if sufficient and constant vitamin A is available (Quadro et al., 2005). Retinyl ester bound to lipoprotein depends on immediate vitamin A intake from food because its source is the small intestine. Unlike the RBP system, the retinyl ester pathway is not homeostatically regulated, cannot mobilize the vast amount of vitamin A stored in the liver, and is directly influenced by fluctuating vitamin A intake (Green and Green, 1994). Therefore, pathological phenotype becomes highly variable and ranges from eye-specific pathology to embryonic lethality when only this pathway is depended on for vitamin A delivery in mice (Quadro et al., 2005) or in humans (Pasutto et al., 2007). Another facet of this RBPindependent pathway is that it is associated with toxicity due to its lack of ho-
meostatic control and specificity (Smith and Goodman, 1976). An increase of 10% in retinyl ester in the blood is regarded as a sign of vitamin A overload in humans. This study also illustrates how RPB and its receptor have been finely tuned for each other in evolution. Tighter binding is not necessarily better and can even be pathogenic. Each RBP protein transports only one vitamin A molecule and STRA6 only takes up one vitamin A molecule at a time from RBP. Therefore, the STRA6/RBP interaction must be sufficiently strong and specific to achieve vitamin A uptake, but not too strong or prolonged to prevent RBP from delivering the next vitamin A molecule. The unexpected existence of super-binding RBP mutants in humans revealed a new mode of inheritance depending on the genetic buildup of both mother and fetus.
436 Cell 161, April 23, 2015 ª2015 Elsevier Inc.
ACKNOWLEDGMENTS H.S. is supported by NIH grant R01 EY018144. H.S. is an Early Career Scientist of the Howard Hughes Medical Institute.
REFERENCES Amengual, J., Zhang, N., Kemerer, M., Maeda, T., Palczewski, K., and Von Lintig, J. (2014). Hum. Mol. Genet. 23, 5402–5417. Chou, C.M., Nelson, C., Tarle, S.A., Pribila, J.T., Bardakjian, T., Woods, S., Schneider, A., and Glaser, T. (2015). Cell 161, this issue, 634–646. Green, M.H., and Green, J.B. (1994). Dynamics and Control of Plasma Retinol. In Vitamin A in Health and Disease, R. Blomhoff, ed. (Marcel Dekker, Inc.). Kawaguchi, R., Yu, J., Honda, J., Hu, J., Whitelegge, J., Ping, P., Wiita, P., Bok, D., and Sun, H. (2007). Science 315, 820–825. Kawaguchi, R., Yu, J., Ter-Stepanian, M., Zhong, M., Cheng, G., Yuan, Q., Jin, M., Travis, G.H.,
Ong, D., and Sun, H. (2011). ACS Chem. Biol. 6, 1041–1051. Pasutto, F., Sticht, H., Hammersen, G., GillessenKaesbach, G., Fitzpatrick, D.R., Nu¨rnberg, G., Brasch, F., Schirmer-Zimmermann, H., Tolmie, J.L., Chitayat, D., et al. (2007). Am. J. Hum. Genet. 80, 550–560.
Quadro, L., Hamberger, L., Gottesman, M.E., Wang, F., Colantuoni, V., Blaner, W.S., and Mendelsohn, C.L. (2005). Endocrinology 146, 4479– 4490.
R.A., Ghyselinck, N.B., et al. (2012). Invest. Ophthalmol. Vis. Sci. 53, 3027–3039.
Ruiz, A., Mark, M., Jacobs, H., Klopfenstein, M., Hu, J., Lloyd, M., Habib, S., Tosha, C., Radu,
Zhong, M., Kawaguchi, R., Kassai, M., and Sun, H. (2012). Nutrients 4, 2069–2096.
Smith, F.R., and Goodman, D.S. (1976). N. Engl. J. Med. 294, 805–808.
Cell 161, April 23, 2015 ª2015 Elsevier Inc. 437
Previews A Genetic Clog in the Vitamin A Transport Machinery Ming Zhong1 and Hui Sun1,* 1Howard Hughes Medical Institute, Department of Physiology and Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, U.S.A *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cell.2015.04.020
Chou et al. discover a new mode of maternal inheritance by analyzing human mutations in plasma retinol binding protein (RBP). Mechanistically, these mutations simultaneously lower RBP’s affinity for vitamin A and greatly increase its affinity for its cell-surface receptor, thus dominantly blocking the transmembrane transport of vitamin A. Vitamin A has been the light sensor for vision, or its equivalent, from life’s beginnings. The diversification of its function to regulating cell growth and differentiation, which occurred about 500 million years ago, coincided with the emergence of a long-range and specific vitamin A transport system consisting of a blood carrier protein called plasma retinol binding protein (RBP) and its cell-surface receptor (STRA6), which mediates vitamin A uptake (Kawaguchi et al., 2007). Many organs depend on vitamin A action; however, the eye is still the organ most sensitive to vitamin A deficiency, the loss of RBP, or the loss of STRA6 (Zhong et al., 2012). Studies of human mutations leading to congenital eye malformation allowed Tom Glaser and colleagues, in this issue of Cell, to uncover a new mode of maternal inheritance and its intricate mechanism involving RBP, STRA6, and vitamin A (Chou et al., 2015). Glaser’s team identified human RBP mutations that cause eye malformation such as anophthalmia, microphthalmia, and coloboma, but the phenotypes are preferentially transmitted if the mutations are inherited from the mother. Through a series of elegant biochemical analyses, they discovered that these mutant RBPs block vitamin A transport by STRA6 in a precise and dominant manner. To understand this mechanism, it is useful to understand the major players involved in specific vitamin A transport. Under physiological conditions, vitamin A/RBP complex (termed holo-RBP) associates with transthyretin (TTR) to increase its molecular weight and thus prevent removal by kidney filtration. Holo-RBP dissociates from the
complex to bind to its cell-surface receptor STRA6 due to its higher affinity for STRA6 than for TTR (Kawaguchi et al., 2011). STRA6 then catalyzes vitamin A release from holo-RBP and its transport into the cell where it is stored (Kawaguchi et al., 2011). After losing its cargo, RBP (apo-RBP) has greatly decreased affinity for TTR and is lost through kidney filtration, which prevents its accumulation in the blood (Figure 1). RBP mutations identified in this study confer several properties that are important for their pathogenicity and the way they affect this process. First, while these mutant RBPs are as well secreted as the wild-type protein, they bind vitamin A weakly and tend to lose their cargo in a receptor-independent manner. Second, like wild-type holo-RBP, the mutant ones can still bind to TTR; however, they have an affinity for STRA6 that is 30- to 40-fold higher than that of wild-type RBP (Figure 1). These properties lead to the sequestration of STRA6 by the mutant RBP, decreasing its ability to transport vitamin A into the cell. Indeed, although the blood of mutation carriers may contain about 66% wild-type RBP and only 33% mutant RBP, the blocking effect is sufficient to cause the malformation phenotypes. A longstanding puzzle in the field was the discrepancy between the way human mutations in RBP and STRA6 affect eye development. While mutations in STRA6 cause anopththalmia and other developmental defects (Pasutto et al., 2007), all previously identified RBP mutations did not. As this study shows, a key to this puzzle is the placenta’s role in vitamin A delivery. Previously identified mutations in RBP were recessive and thus unlikely
to inactivate both maternal and fetal RBP because only the fetus could be homozygous for the mutation. In contrast, dominant RBP mutations, like those discovered in this study, can simultaneously inactivate the mother’s and fetal RBP. Similarly, human mutations in RBP and its receptor do not necessarily generate the same phenotypes for the same reason. A human embryo without STRA6 can be directly impacted by the loss of placental absorption of vitamin A from maternal RBP. In contrast, a human embryo without RBP still has STRA6 and functional maternal RBP to ensure vitamin A delivery to the embryo through the placenta. Thus, the dominant human RBP mutations mimic STRA6 mutations in their ability to inactivate both maternal and fetal RBP-mediated transport and therefore can cause anophthalmia. The fact that the dominant RBP mutation can only exert its effect on maternal delivery of vitamin A to the fetus if it is maternal explains this new mode of maternal inheritance. A related question highlighted by this study is the contrast between humans and mice in phenotypic variability even for null mutations in this pathway. For example, human pathologies caused by STRA6 mutations are highly variable, ranging from the ‘‘milder’’ phenotype of anophthalmia to more systemic developmental defects (Pasutto et al., 2007). However, under standard laboratory conditions, STRA6 knockout mice, like RBP knockout mice, have vision-specific phenotypes that lead to blindness due to lack of vitamin A (Amengual et al., 2014; Ruiz et al., 2012). As pointed out here, a key to this puzzle is an RBP-independent pathway mediated by vitamin A esters
Cell 161, April 23, 2015 ª2015 Elsevier Inc. 435
Figure 1. Schematic Diagrams of RBP/Receptor Interaction Schematic diagrams comparing RBP/receptor interaction under physiological conditions and the pathological conditions revealed by Chou et al. (2015). Under physiological conditions (left diagram), holo-RBP dissociates from the RBP/TTR complex to bind to the RBP receptor STRA6 due to its higher affinity for STRA6 than for TTR. STRA6 catalyzes retinol release from holo-RBP and retinol transport into the cell to be stored by binding to CRBP, or by conversion to retinyl esters by LRAT (not shown). After losing its cargo retinol, RBP (apo-RBP) can no longer bind to TTR and is lost through kidney filtration. As depicted in the right diagram, human RBP mutant A55T or A57T (marked in red in RBP structures) tends to lose vitamin A in a receptor-independent manner, but has much higher affinity for STRA6 than wild-type RBP (the mutant RBP/receptor complex is marked by a red circle). These RBP mutants compete with wild-type RBP in binding to STRA6 to impede the transmembrane transport of vitamin A.
(retinyl esters) borrowing the lipoprotein delivery pathway. This pathway can partly compensate for the lack of RBP if sufficient and constant vitamin A is available (Quadro et al., 2005). Retinyl ester bound to lipoprotein depends on immediate vitamin A intake from food because its source is the small intestine. Unlike the RBP system, the retinyl ester pathway is not homeostatically regulated, cannot mobilize the vast amount of vitamin A stored in the liver, and is directly influenced by fluctuating vitamin A intake (Green and Green, 1994). Therefore, pathological phenotype becomes highly variable and ranges from eye-specific pathology to embryonic lethality when only this pathway is depended on for vitamin A delivery in mice (Quadro et al., 2005) or in humans (Pasutto et al., 2007). Another facet of this RBPindependent pathway is that it is associated with toxicity due to its lack of ho-
meostatic control and specificity (Smith and Goodman, 1976). An increase of 10% in retinyl ester in the blood is regarded as a sign of vitamin A overload in humans. This study also illustrates how RPB and its receptor have been finely tuned for each other in evolution. Tighter binding is not necessarily better and can even be pathogenic. Each RBP protein transports only one vitamin A molecule and STRA6 only takes up one vitamin A molecule at a time from RBP. Therefore, the STRA6/RBP interaction must be sufficiently strong and specific to achieve vitamin A uptake, but not too strong or prolonged to prevent RBP from delivering the next vitamin A molecule. The unexpected existence of super-binding RBP mutants in humans revealed a new mode of inheritance depending on the genetic buildup of both mother and fetus.
436 Cell 161, April 23, 2015 ª2015 Elsevier Inc.
ACKNOWLEDGMENTS H.S. is supported by NIH grant R01 EY018144. H.S. is an Early Career Scientist of the Howard Hughes Medical Institute.
REFERENCES Amengual, J., Zhang, N., Kemerer, M., Maeda, T., Palczewski, K., and Von Lintig, J. (2014). Hum. Mol. Genet. 23, 5402–5417. Chou, C.M., Nelson, C., Tarle, S.A., Pribila, J.T., Bardakjian, T., Woods, S., Schneider, A., and Glaser, T. (2015). Cell 161, this issue, 634–646. Green, M.H., and Green, J.B. (1994). Dynamics and Control of Plasma Retinol. In Vitamin A in Health and Disease, R. Blomhoff, ed. (Marcel Dekker, Inc.). Kawaguchi, R., Yu, J., Honda, J., Hu, J., Whitelegge, J., Ping, P., Wiita, P., Bok, D., and Sun, H. (2007). Science 315, 820–825. Kawaguchi, R., Yu, J., Ter-Stepanian, M., Zhong, M., Cheng, G., Yuan, Q., Jin, M., Travis, G.H.,
Ong, D., and Sun, H. (2011). ACS Chem. Biol. 6, 1041–1051. Pasutto, F., Sticht, H., Hammersen, G., GillessenKaesbach, G., Fitzpatrick, D.R., Nu¨rnberg, G., Brasch, F., Schirmer-Zimmermann, H., Tolmie, J.L., Chitayat, D., et al. (2007). Am. J. Hum. Genet. 80, 550–560.
Quadro, L., Hamberger, L., Gottesman, M.E., Wang, F., Colantuoni, V., Blaner, W.S., and Mendelsohn, C.L. (2005). Endocrinology 146, 4479– 4490.
R.A., Ghyselinck, N.B., et al. (2012). Invest. Ophthalmol. Vis. Sci. 53, 3027–3039.
Ruiz, A., Mark, M., Jacobs, H., Klopfenstein, M., Hu, J., Lloyd, M., Habib, S., Tosha, C., Radu,
Zhong, M., Kawaguchi, R., Kassai, M., and Sun, H. (2012). Nutrients 4, 2069–2096.
Smith, F.R., and Goodman, D.S. (1976). N. Engl. J. Med. 294, 805–808.
Cell 161, April 23, 2015 ª2015 Elsevier Inc. 437