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Lipoprotein Lipase Sorting: Sphingomyelin and a Proteoglycan Show the Way
TICB 1570 No. of Pages 2
Trends in Cell Biology
Spotlight
Lipoprotein Lipase Sorting: Sphingomyelin and a Proteoglycan Show the Way Vytas A. Bankaitis1,2,* and Yaxi Wang2 A mechanistic description for how soluble protein cargos are sorted into distinct vesicle classes at the level of the trans-Golgi network (TGN) has remained elusive. In a recent study in Developmental Cell, Sundberg et al. reveal that sphingomyelin and a proteoglycan mediate lipoprotein lipase sorting in the TGN. Lipoprotein lipase (LPL) hydrolyzes triglyceride-rich lipoprotein particles exported into the bloodstream by the intestine (chylomicrons) or the liver (very low density lipoproteins). In so doing, LPL mobilizes fatty acids from these particles, supplies these liberated fatty acids to various tissues, and promotes metabolic clearance of these particles from circulation. Failure in their clearance results in hypertriglyceridemia with pancreatitis, diabetes, and neurological deficits. LPL traffics to and localizes to the vascular bed. LPL is secreted primarily from parenchymal cells of cardiac, mammary, and adipose tissues into interstitial fluid, where it is taken up by endothelial cells lining the blood vessels, is transcytosed across that endothelial cell lining, and is deposited on the apical surface of these cells where the enzyme can access the circulation [1]. In a recent study in Developmental Cell, Sundberg et al. find that LPL secretion from parenchymal cells into interstitial fluid is more complex than previously appreciated, and dissection of this process provides insights into how soluble proteins
are sorted into distinct trans-Golgi network that mammalian TGN-derived exocytic vesicles are enriched in SM relative to bulk (TGN)-derived vesicular carriers [2]. TGN membrane and this discovery signiThe TGN acts as a station for differential fied that such vesicles are formed from protein sorting into distinct vesicles des- SM-rich microdomains [8]. Recent work tined for delivery of cargo to: (i) endosomal further reinforced this concept by describand/or lysosomal compartments; (ii) the ing a class of TGN-derived vesicles cell surface via regulated (i.e., signal enriched in SM that are selectively loaded induced) secretory pathways; or (iii) the with integral membrane proteins that cell surface via a constitutive route [3]. Of manifest cargo receptor activities and are these trafficking routes, the constitutive trafficked to the cell surface [9]. Those pathway has been considered a default studies set the conceptual stage for pathway. That is, if a protein does not elab- deciphering the coat-independent sorting orate a sorting signal, then it is packaged mechanism for soluble cargo into this speinto vesicles destined for the plasma mem- cific class of vesicles of what is termed the brane by a ‘bulk flow’ mechanism. The evi- SM secretion pathway (SMS). dence to this effect was several-fold. First, inactivation of sorting signal(s) on proteins Sundberg et al. focused on the major destined for the endosomal/lysosomal question of how the soluble SMS cargo system typically result in their secretion [4]. LPL is preferentially sorted into vesicles Second, vesicles destined for endosomal/ of the SMS [2]. They demonstrate that lysosomal systems are differentially coated LPL manifests two properties critical for and these coats have selective capacities sorting: (i) a general membrane binding to bind the cytosolic domains of transmem- activity that shows no selectivity for SM brane cargo receptors and thereby recruit housed in the polycystin-1, lipoxygenase, them (along with any bound cargo) to the alpha toxin (PLAT) domain of LPL; and respective vesicle classes [5]. By contrast, (ii) a cluster of basic residues that binds vesicles destined for the cell surface are un- heparan sulfate chains. By screening the coated and therefore refractory to receptor/ heparan sulfate proteoglycans expressed coat-mediated sorting mechanisms [6]. by their Hela cell model, Sundberg et al. That demixing of specific lipid species identify the integral membrane proteogly[e.g., the major mammalian sphingolipid can Syndecan-1 (SDC1), another cargo sphingomyelin (SM) and the major sterol protein of the SMS, as a specific sorting cholesterol] into discrete microdomains receptor without which preferential loading might form platforms for protein sorting of LPL into vesicles of the SMS is lost nucleated initial ideas for how coat- (Figure 1). How is SDC1 itself selectively independent sorting could occur. Those packaged into exocytic vesicles of the ideas focused initially on sorting of integral SMS? Using a familiar mechanism, the membrane proteins to the basolateral and sorting of SDC1 into these vesicles is deapical surfaces of polarized epithelial cells termined by the biophysical properties of and on the biophysical characteristics of its TMD [2]. their transmembrane domains (TMDs) that influenced their lateral partitioning into (or In sum, Sundberg and coworkers conclude exclusion from) such lipid microdomains [7]. that sorting of LPL into the SMS requires bivalent interactions between the LPL PLAT A major breakthrough in this arena came domain and the TGN-membrane and befrom the affinity-purification of mammalian tween LPL and SDC1-linked heparan sulTGN-derived exocytic vesicles coupled to fate chains. Because other heparan sulfate lipidomic analyses of their membrane com- proteoglycans do not elaborate LPL sorting positions. Those studies demonstrated receptor activity, Sundberg et al. also infer Trends in Cell Biology, Month 2020, Vol. xx, No. xx
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Trends in Cell Biology
SM-rich microdomain itself is formed? Is on-site SM synthesis required? Apparently it is not [2], thereby uncoupling the sorting from an additional regulatory network involving interplay between ceramide metabolism and phosphatidylinositol-4phosphate signaling [10]. Interestingly, sorting of other cargo into the SMS pathway requires ongoing SM synthesis, thereby potentially bringing the ceramide/ phosphatidylinositol-4-phosphate signaling circuit into play [9]. Thus, Sundberg and colleagues demonstrate that what first seemed simple is not and their discoveries raise the question of whether a truly ‘bulk flow’ mechanism for protein trafficking even exists. 1
Trends in Cell Biology
Figure 1. Receptor-Mediated Sorting of Soluble Lipoprotein Lipase (LPL) into Exocytic Vesicles of the Sphingomyelin Secretion (SMS) Pathway. LPL engages in multivalent interactions with: (i) the lumenal leaflet of the trans-Golgi network (TGN) membrane via its polycystin-1, lipoxygenase, alpha toxin (PLAT) domain; (ii) with the heparan sulfate chains (black lines) of Syndecan-1 (SDC1) via a cluster of basic amino acids on the LPL protein surface; and (iii) via an inferred protein–protein interaction between LPL and the SDC1 core protein (blue cylinder). The SDC1 transmembrane domain is enriched in residues with short, unbranched side chains that are asymmetrically distributed throughout the domain and this primary sequenceindependent feature favors lateral partitioning of SDC1 from bulk TGN membrane (black) into liquid-ordered membrane regions such as sphingomyelin-enriched microdomains (red) [2]. Vesicular carriers of the SMS pathway bud from these microdomains, whereas carriers of bulk flow cargo do not. The enrichment of LPL/SDC1 complexes into the SMS pathway over bulk-flow is ~threefold.
that LPL must recognize some additional element on the SDC1 core protein. The primary sorting determinant is the TMDdriven partitioning of SDC1 (along with bound LPL) into SM-enriched microdomains from which vesicles of the SMS are derived. In this manner, interactions between the SDC1 TMD and its lipid environment drives lateral partitioning of the cargo receptor into SM-enriched microdomains at the level of the TGN and this lateral partitioning is then leveraged to the sorting of both integral membrane protein and its cognate soluble protein ligand [2] (Figure 1). What is the physiological significance of routing LPL into the SMS using SDC1 as a sorting receptor? That remains unclear.
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Trends in Cell Biology, Month 2020, Vol. xx, No. xx
The levels of LPL and SDC1 expression are not well-correlated in cells that are primary producers of LPL [2]. This begs the question of how many LPL molecules can a single SDC1 bind? If the stoichiometry is ~1:1 then the coexpression data would suggest this SMS sorting mechanism represents a minor pathway. As loss of SMS sorting shunts LPL into the default ‘bulk flow’ pathway without loss of LPL secretion, it also remains an open question of whether the rates of LPL secretion differ between the SMS pathway and alternate ‘bulk flow’ routes to the cell surface. Rate differences could provide one fine-tuning mechanism for regulating LPL levels in the vascular bed. Finally, what is the mechanism by which the
Department of Molecular and Cellular Medicine, Texas A&M Health Science Center, College Station, TX 77843–1114, USA 2 Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843–2128, USA *Correspondence: [email protected] (V.A. Bankaitis). https://doi.org/10.1016/j.tcb.2020.01.001 © 2020 Elsevier Ltd. All rights reserved.
References 1. Davies, B.S.J. et al. (2010) GPIHBP1 is responsible for the entry of lipoprotein lipase into capillaries. Cell Metab. 12, 42–52 2. Sundberg, E.L. et al. (2019) Sundecan-1 mediates sorting of soluble lipoprotein lipase with sphingomyelin-rich membrane in the Golgi apparatus. Dev. Cell 51, 387–398 3. Guo, Y. et al. (2014) Protein sorting at the trans-Golgi network. Annu. Rev. Cell Dev. Biol. 30, 169–206 4. Braulke, T. and Bonifacino, J.S. (2009) Sorting of lysosomal proteins. Biochim. Biophys. Acta 1793, 605–614 5. Schekman, R. et al. (1995) Coat proteins and selective protein packaging into transport vesicles. Cold Spring Harb. Symp. Quant. Biol. 60, 11–21 6. Walworth, N.C. and Novick, P.J. (1987) Purification and characterization of constitutive secretory vesicles from yeast. J. Cell Biol. 105, 163–174 7. Sezgin, E. et al. (2017) The mystery of membrane organization, regulation and role of lipid rafts. Nat. Rev. Mol. Cell Biol. 18, 361–374 8. Klemm, R.W. et al. (2009) Segregation of sphingolipids and sterols during formation of secretory vesicles at the trans-Golgi network. J. Cell Biol. 185, 601–612 9. Deng, Y. et al. (2018) Activity of the SPCA1 calcium pump couples sphingomyelin synthesis to sorting of secretory proteins in the trans-Golgi network. Dev. Cell 47, 464–478 10. Wang, Y. et al. (2019) An equal opportunity collaboration between lipid metabolism and proteins in the control of membrane trafficking in the trans-Golgi and endosomal systems. Curr. Opin. Cell Biol. 59, 58–72
Trends in Cell Biology
Spotlight
Lipoprotein Lipase Sorting: Sphingomyelin and a Proteoglycan Show the Way Vytas A. Bankaitis1,2,* and Yaxi Wang2 A mechanistic description for how soluble protein cargos are sorted into distinct vesicle classes at the level of the trans-Golgi network (TGN) has remained elusive. In a recent study in Developmental Cell, Sundberg et al. reveal that sphingomyelin and a proteoglycan mediate lipoprotein lipase sorting in the TGN. Lipoprotein lipase (LPL) hydrolyzes triglyceride-rich lipoprotein particles exported into the bloodstream by the intestine (chylomicrons) or the liver (very low density lipoproteins). In so doing, LPL mobilizes fatty acids from these particles, supplies these liberated fatty acids to various tissues, and promotes metabolic clearance of these particles from circulation. Failure in their clearance results in hypertriglyceridemia with pancreatitis, diabetes, and neurological deficits. LPL traffics to and localizes to the vascular bed. LPL is secreted primarily from parenchymal cells of cardiac, mammary, and adipose tissues into interstitial fluid, where it is taken up by endothelial cells lining the blood vessels, is transcytosed across that endothelial cell lining, and is deposited on the apical surface of these cells where the enzyme can access the circulation [1]. In a recent study in Developmental Cell, Sundberg et al. find that LPL secretion from parenchymal cells into interstitial fluid is more complex than previously appreciated, and dissection of this process provides insights into how soluble proteins
are sorted into distinct trans-Golgi network that mammalian TGN-derived exocytic vesicles are enriched in SM relative to bulk (TGN)-derived vesicular carriers [2]. TGN membrane and this discovery signiThe TGN acts as a station for differential fied that such vesicles are formed from protein sorting into distinct vesicles des- SM-rich microdomains [8]. Recent work tined for delivery of cargo to: (i) endosomal further reinforced this concept by describand/or lysosomal compartments; (ii) the ing a class of TGN-derived vesicles cell surface via regulated (i.e., signal enriched in SM that are selectively loaded induced) secretory pathways; or (iii) the with integral membrane proteins that cell surface via a constitutive route [3]. Of manifest cargo receptor activities and are these trafficking routes, the constitutive trafficked to the cell surface [9]. Those pathway has been considered a default studies set the conceptual stage for pathway. That is, if a protein does not elab- deciphering the coat-independent sorting orate a sorting signal, then it is packaged mechanism for soluble cargo into this speinto vesicles destined for the plasma mem- cific class of vesicles of what is termed the brane by a ‘bulk flow’ mechanism. The evi- SM secretion pathway (SMS). dence to this effect was several-fold. First, inactivation of sorting signal(s) on proteins Sundberg et al. focused on the major destined for the endosomal/lysosomal question of how the soluble SMS cargo system typically result in their secretion [4]. LPL is preferentially sorted into vesicles Second, vesicles destined for endosomal/ of the SMS [2]. They demonstrate that lysosomal systems are differentially coated LPL manifests two properties critical for and these coats have selective capacities sorting: (i) a general membrane binding to bind the cytosolic domains of transmem- activity that shows no selectivity for SM brane cargo receptors and thereby recruit housed in the polycystin-1, lipoxygenase, them (along with any bound cargo) to the alpha toxin (PLAT) domain of LPL; and respective vesicle classes [5]. By contrast, (ii) a cluster of basic residues that binds vesicles destined for the cell surface are un- heparan sulfate chains. By screening the coated and therefore refractory to receptor/ heparan sulfate proteoglycans expressed coat-mediated sorting mechanisms [6]. by their Hela cell model, Sundberg et al. That demixing of specific lipid species identify the integral membrane proteogly[e.g., the major mammalian sphingolipid can Syndecan-1 (SDC1), another cargo sphingomyelin (SM) and the major sterol protein of the SMS, as a specific sorting cholesterol] into discrete microdomains receptor without which preferential loading might form platforms for protein sorting of LPL into vesicles of the SMS is lost nucleated initial ideas for how coat- (Figure 1). How is SDC1 itself selectively independent sorting could occur. Those packaged into exocytic vesicles of the ideas focused initially on sorting of integral SMS? Using a familiar mechanism, the membrane proteins to the basolateral and sorting of SDC1 into these vesicles is deapical surfaces of polarized epithelial cells termined by the biophysical properties of and on the biophysical characteristics of its TMD [2]. their transmembrane domains (TMDs) that influenced their lateral partitioning into (or In sum, Sundberg and coworkers conclude exclusion from) such lipid microdomains [7]. that sorting of LPL into the SMS requires bivalent interactions between the LPL PLAT A major breakthrough in this arena came domain and the TGN-membrane and befrom the affinity-purification of mammalian tween LPL and SDC1-linked heparan sulTGN-derived exocytic vesicles coupled to fate chains. Because other heparan sulfate lipidomic analyses of their membrane com- proteoglycans do not elaborate LPL sorting positions. Those studies demonstrated receptor activity, Sundberg et al. also infer Trends in Cell Biology, Month 2020, Vol. xx, No. xx
1
Trends in Cell Biology
SM-rich microdomain itself is formed? Is on-site SM synthesis required? Apparently it is not [2], thereby uncoupling the sorting from an additional regulatory network involving interplay between ceramide metabolism and phosphatidylinositol-4phosphate signaling [10]. Interestingly, sorting of other cargo into the SMS pathway requires ongoing SM synthesis, thereby potentially bringing the ceramide/ phosphatidylinositol-4-phosphate signaling circuit into play [9]. Thus, Sundberg and colleagues demonstrate that what first seemed simple is not and their discoveries raise the question of whether a truly ‘bulk flow’ mechanism for protein trafficking even exists. 1
Trends in Cell Biology
Figure 1. Receptor-Mediated Sorting of Soluble Lipoprotein Lipase (LPL) into Exocytic Vesicles of the Sphingomyelin Secretion (SMS) Pathway. LPL engages in multivalent interactions with: (i) the lumenal leaflet of the trans-Golgi network (TGN) membrane via its polycystin-1, lipoxygenase, alpha toxin (PLAT) domain; (ii) with the heparan sulfate chains (black lines) of Syndecan-1 (SDC1) via a cluster of basic amino acids on the LPL protein surface; and (iii) via an inferred protein–protein interaction between LPL and the SDC1 core protein (blue cylinder). The SDC1 transmembrane domain is enriched in residues with short, unbranched side chains that are asymmetrically distributed throughout the domain and this primary sequenceindependent feature favors lateral partitioning of SDC1 from bulk TGN membrane (black) into liquid-ordered membrane regions such as sphingomyelin-enriched microdomains (red) [2]. Vesicular carriers of the SMS pathway bud from these microdomains, whereas carriers of bulk flow cargo do not. The enrichment of LPL/SDC1 complexes into the SMS pathway over bulk-flow is ~threefold.
that LPL must recognize some additional element on the SDC1 core protein. The primary sorting determinant is the TMDdriven partitioning of SDC1 (along with bound LPL) into SM-enriched microdomains from which vesicles of the SMS are derived. In this manner, interactions between the SDC1 TMD and its lipid environment drives lateral partitioning of the cargo receptor into SM-enriched microdomains at the level of the TGN and this lateral partitioning is then leveraged to the sorting of both integral membrane protein and its cognate soluble protein ligand [2] (Figure 1). What is the physiological significance of routing LPL into the SMS using SDC1 as a sorting receptor? That remains unclear.
2
Trends in Cell Biology, Month 2020, Vol. xx, No. xx
The levels of LPL and SDC1 expression are not well-correlated in cells that are primary producers of LPL [2]. This begs the question of how many LPL molecules can a single SDC1 bind? If the stoichiometry is ~1:1 then the coexpression data would suggest this SMS sorting mechanism represents a minor pathway. As loss of SMS sorting shunts LPL into the default ‘bulk flow’ pathway without loss of LPL secretion, it also remains an open question of whether the rates of LPL secretion differ between the SMS pathway and alternate ‘bulk flow’ routes to the cell surface. Rate differences could provide one fine-tuning mechanism for regulating LPL levels in the vascular bed. Finally, what is the mechanism by which the
Department of Molecular and Cellular Medicine, Texas A&M Health Science Center, College Station, TX 77843–1114, USA 2 Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843–2128, USA *Correspondence: [email protected] (V.A. Bankaitis). https://doi.org/10.1016/j.tcb.2020.01.001 © 2020 Elsevier Ltd. All rights reserved.
References 1. Davies, B.S.J. et al. (2010) GPIHBP1 is responsible for the entry of lipoprotein lipase into capillaries. Cell Metab. 12, 42–52 2. Sundberg, E.L. et al. (2019) Sundecan-1 mediates sorting of soluble lipoprotein lipase with sphingomyelin-rich membrane in the Golgi apparatus. Dev. Cell 51, 387–398 3. Guo, Y. et al. (2014) Protein sorting at the trans-Golgi network. Annu. Rev. Cell Dev. Biol. 30, 169–206 4. Braulke, T. and Bonifacino, J.S. (2009) Sorting of lysosomal proteins. Biochim. Biophys. Acta 1793, 605–614 5. Schekman, R. et al. (1995) Coat proteins and selective protein packaging into transport vesicles. Cold Spring Harb. Symp. Quant. Biol. 60, 11–21 6. Walworth, N.C. and Novick, P.J. (1987) Purification and characterization of constitutive secretory vesicles from yeast. J. Cell Biol. 105, 163–174 7. Sezgin, E. et al. (2017) The mystery of membrane organization, regulation and role of lipid rafts. Nat. Rev. Mol. Cell Biol. 18, 361–374 8. Klemm, R.W. et al. (2009) Segregation of sphingolipids and sterols during formation of secretory vesicles at the trans-Golgi network. J. Cell Biol. 185, 601–612 9. Deng, Y. et al. (2018) Activity of the SPCA1 calcium pump couples sphingomyelin synthesis to sorting of secretory proteins in the trans-Golgi network. Dev. Cell 47, 464–478 10. Wang, Y. et al. (2019) An equal opportunity collaboration between lipid metabolism and proteins in the control of membrane trafficking in the trans-Golgi and endosomal systems. Curr. Opin. Cell Biol. 59, 58–72