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Intracellular organelle transport: few motors, many signals
Update
396
TRENDS in Cell Biology
Vol.15 No.8 August 2005
Intracellular organelle transport: few motors, many signals Anna Kashina1 and Vladimir Rodionov2 1
Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce Street, 143 Rosenthal, Philadelphia, PA19104, USA 2 Department of Cell Biology and Center for Cell Analysis and Modeling, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT06032-1507, USA
Bidirectional microtubule-dependent organelle transport in melanophores is regulated by cAMP through organelle-bound protein kinase A (PKA); however, the mechanisms responsible for this regulation are unknown. A recent study by Gelfand and colleagues demonstrates that, in addition to PKA, transport is regulated by the organelle-bound mitogen-activated protein kinase (MAPK) signaling components ERK and MEK, whose activity is required for bidirectional transport along microtubules. This pathway apparently acts downstream of PKA, suggesting that bidirectional organelle transport is regulated by a hierarchical cascade of signaling pathways.
Introduction Intracellular transport of organelles and particles ensures the correct delivery of organelles to various destinations inside the cytoplasm. Transport occurs along cytoskeletal tracks – microtubules (MTs) and actin filaments (AFs) – and is mediated by molecular motors – dyneins and kinesins for MTs, and myosins for AFs (see [1] for review). Cytoskeletal tracks have a distinct polarity. MTs in most cell types form a radial network, with minus (slow growing) ends focused in the cell center, and plus (fast growing) ends at the cell periphery. Accordingly, MT motors move their cargoes either to the minus MT ends, retrogradely to the cell center (dynein and some kinesins) or to the plus end, anterogradely to the cell periphery (the majority of kinesins). Actin-dependent motors, myosins, generally move cargo to the plus (‘barbed’) ends of AFs, although a minus-end-directed variant of myosin has also been identified. AF transport has no distinct cytoplasmic orientation and is required for local redistribution of organelles. It has been shown that AF and MT motors of both polarities are attached to the same organelle at the same time, and therefore delivery of each organelle to its destination requires coordination of the motor action [2,3]. Until recently, the nature of the mechanisms of this coordination remained a mystery. Recent studies of intracellular transport in melanophores, pigment cells of lower vertebrates, have provided some clues about the pathways and mechanisms responsible for Corresponding author: Rodionov, V. ([email protected]). Available online 7 July 2005 www.sciencedirect.com
the selective control over the activities of multiple types of motors bound to cargo organelles (reviewed in [4]). cAMP-regulated signaling events coordinate organelle transport Melanophores redistribute pigment granules to control the color changes of animals in response to the environment. In these cells, membrane-bounded granules undergo MT-dependent pigment aggregation to the cell center, or slower dispersion throughout the cytoplasm along MTs and AFs (Figure 1). The rapid and synchronous movements of granules that can easily be observed in a light microscope make melanophores a unique system to study the mechanisms of coordinated intracellular transport and its regulation. In Xenopus melanophores, movement of pigment granules along MTs and AFs is mediated by three types of molecular motors: kinesin 2, dynein, and myosin V. Pigment aggregation to the cell centre occurs along MT tracks and is driven by dynein, whereas pigment dispersion throughout the cytoplasm is mediated by kinesin 2 and myosin V and involves a switch from MTs onto AF tracks at later stages of dispersion [5,6]. Pigment transport is regulated by changes in cAMP levels that are low during pigment aggregation and exhibit a sharp increase, then gradual decrease, during pigment dispersion [6,7] (Figure 1). These kinetics correlate with the differential activity of motors involved in pigment transport. During pigment aggregation, a drop in cAMP levels apparently induces activation of the MT motor dynein that transports pigment granules to the cell center. During pigment dispersion, cAMP levels first rise sharply, resulting in activation of kinesin 2 and myosin V and inactivation of dynein, and then gradually decrease. This decrease causes downregulation of kinesin 2 activity without affecting the activity of myosin V, resulting in the predominant AF-based transport observed at later stages of dispersion [6]. How can cAMP differentially regulate the activity of motors on the granule surface? cAMP regulates pigment transport through PKA signaling [8,9]. Indeed, inhibition of PKA activity induces pigment aggregation, whereas elevation of cAMP results in pigment dispersion. Moreover, it has been shown recently that components of the PKA signaling pathway are bound to the surface of pigment granules by means of scaffolding molecules, forming two independent complexes, one with dynein
Update
(a) ↑Kinesin
TRENDS in Cell Biology
Dispersion ↑cAMP ↑Myosin V
Vol.15 No.8 August 2005
397
cAMP ↓Dynein PKA ? MAPK
?
Kinesin 2
?
Dynein Aggregation complex
Dispersion complex
Myosin Va
Pigment granule
(a)
TRENDS in Cell Biology
(b) ↓Kinesin
Aggregation ↓cAMP ↓Myosin V
↑Dynein
Figure 1. Melanophores as a model for coordinated bidirectional transport. (a) During pigment dispersion, high cAMP levels correlate with high activity of the microtubule motor kinesin and the actin filament motor myosin V that mediate coordinated transport events, resulting in a uniform distribution of pigment granules throughout the cytoplasm. (b) During pigment aggregation, low cAMP levels correlate with high dynein activity, resulting in aggregation of pigment granules at the cell center. Bars, 20 mm.
(‘aggregation complex’) and one with kinesin 2 and myosin V (‘dispersion complex’) [10] (Figure 2). A simple explanation is that PKA regulation of transport is achieved through direct phosphorylation of the corresponding motors. While such direct phosphorylation possibly takes place, it cannot explain the complexity of differential motor regulation. Therefore, it seems likely that another mechanism exists that involves differential regulation of the motors by additional signaling cascades acting downstream of PKA. www.sciencedirect.com
Figure 2. cAMP-regulated signaling events coordinate organelle transport by differentially regulating the activity and scaffolding of motors involved in aggregation and dispersion. Dynein (blue) is regulated directly or indirectly by protein kinase A (PKA), possibly through inhibition of dynein, and by a mitogenactivated protein kinase (MAPK) cascade, involved in dynein activation. Kinesin (red) and myosin (brown) form a separate complex, where their activities are differentially regulated by PKA (a direct effect on kinesin and possibly an indirect effect on myosin) and by a MAPK cascade, which exerts a direct or indirect effect on kinesin.
MAPK components regulate pigment transport Recent papers by Deacon et al. and Andersson et al. [11,12] demonstrate that an additional signaling cascade is indeed involved in the regulation of bidirectional organelle transport. The authors show that inhibition of the major components of the mitogen-activated protein kinase (MAPK) cascade – ERK and MEK – inhibits pigment transport. Deacon et al. show that ERK and MEK are bound directly to the granule surface and that inhibition of ERK and MEK inhibits bidirectional transport along MTs without affecting actin-dependent transport during pigment dispersion, suggesting that the components of the MAPK cascade are involved in the regulation of kinesin and dynein activity but not the activity of myosin V. The authors also show that ERK and MEK activity, monitored by their phosphorylation state, is constitutively high in dispersed cells and is differentially regulated during pigment aggregation, where ERK and MEK phosphorylation first rises sharply and then gradually declines, apparently to stop granule movement at the end of aggregation. These kinetics are apparently independent of the changes in cAMP levels; however, Deacon et al. suggest that the MAPK cascade in the regulation of pigment transport acts downstream of PKA, as evidenced by the fact that the MAPK inhibitor U0126 prevents the pigment aggregation induced by PKA inhibition. Thus, Deacon et al. demonstrate that, in addition to PKA, the MAPK cascade is also involved in the regulation of pigment transport (Figure 2). This discovery adds complexity to the existing model for regulation of pigment transport; however, it still does not explain the
398
Update
TRENDS in Cell Biology
mechanisms of the differential regulation of motor activity during pigment aggregation and dispersion. We now know that molecular motors on organelles form complexes with the components of the PKA pathway [10] and that ERK and MEK are also found on the granule surface. It will be interesting to discover whether ERK and MEK, like PKA, also form a complex with the motors or at least are scaffolded into close proximity with the motor-regulatory complexes. Such scaffolding appears to be even more important for transport regulation in other cell types, where switching of organelle direction and track is not a cell-wide event but occurs locally for each organelle. Very little is known about the role of signaling molecules in the regulation of motor activity and organelle transport in other systems, but it has been shown that PKA scaffolding is crucial for many processes controlled through PKA phosphorylation [13]. It seems likely that local regulation by second messengers plays an important part in coordination of intracellular transport in cells where no predominant direction of organelle movement exists at a given point in time. Concluding remarks As regulation by PKA and MAPKs cannot explain the complexity of pigment transport events, it seems likely that other signaling cascades exist that are involved in the regulation of transport. The questions of which other signaling cascades take part in the regulation of the intracellular transport and what are the exact molecular mechanisms of such regulation are among the complex mysteries yet to be unravelled.
Vol.15 No.8 August 2005
References 1 Vale, R.D. (2003) The molecular motor toolbox for intracellular transport. Cell 112, 467–480 2 Welte, M.A. (2004) Bidirectional transport along microtubules. Curr. Biol. 14, R525–R537 3 Mallik, R. and Gross, S.P. (2004) Molecular motors: strategies to get along. Curr. Biol. 14, R971–R982 4 Nascimento, A.A. et al. (2003) Pigment cells: a model for the study of organelle transport. Annu. Rev. Cell Dev. Biol. 19, 469–491 5 Gross, S.P. et al. (2002) Interactions and regulation of molecular motors in Xenopus melanophores. J. Cell Biol. 156, 855–865 6 Rodionov, V. et al. (2003) Switching between microtubule- and actinbased transport systems in melanophores is controlled by cAMP levels. Curr. Biol. 13, 1837–1847 7 Daniolos, A. et al. (1990) Action of light on frog pigment cells in culture. Pigment Cell Res. 3, 38–43 8 Rozdzial, M.M. and Haimo, L.T. (1986) Bidirectional pigment granule movements of melanophores are regulated by protein phosphorylation and dephosphorylation. Cell 47, 1061–1070 9 Reilein, A.R. et al. (1998) Regulation of organelle movement in melanophores by protein kinase A (PKA), protein kinase C (PKC), and protein phosphatase 2A (PP2A). J. Cell Biol. 142, 803–813 10 Kashina, A.S. et al. (2004) Protein kinase A, which regulates intracellular transport, forms complexes with molecular motors on organelles. Curr. Biol. 14, 1877–1881 11 Deacon, S.W. et al. (2005) Regulation of bidirectional melanosome transport by organelle bound MAP kinase. Curr. Biol. 15, 459–463 12 Andersson, T.P. et al. (2003) Regulation of melanosome movement by MAP kinase. Pigment Cell Res. 16, 215–221 13 Wong, W. and Scott, J.D. (2004) AKAP signalling complexes: focal points in space and time. Nat. Rev. Mol. Cell Biol. 5, 959–970
0962-8924/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2005.06.002
ScienceDirect collection reaches six million full-text articles Elsevier recently announced that six million articles are now available on its premier electronic platform, ScienceDirect. This milestone in electronic scientific, technical and medical publishing means that researchers around the globe will be able to access an unsurpassed volume of information from the convenience of their desktop. ScienceDirect’s extensive and unique full-text collection covers over 1900 journals, including titles such as The Lancet, Cell, Tetrahedron and the full suite of Trends and Current Opinion journals. With ScienceDirect, the research process is enhanced with unsurpassed searching and linking functionality, all on a single, intuitive interface. The rapid growth of the ScienceDirect collection is due to the integration of several prestigious publications as well as ongoing addition to the Backfiles – heritage collections in a number of disciplines. The latest step in this ambitious project to digitize all of Elsevier’s journals back to volume one, issue one, is the addition of the highly cited Cell Press journal collection on ScienceDirect. Also available online for the first time are six Cell titles’ long-awaited Backfiles, containing more than 12,000 articles highlighting important historic developments in the field of life sciences. The six-millionth article loaded onto ScienceDirect entitled "Gene Switching and the Stability of Odorant Receptor Gene Choice" was authored by Benjamin M. Shykind and colleagues from the Dept. of Biochemistry and Molecular Biophysics and Howard Hughes Medical Institute, College of Physicians and Surgeons at Columbia University. The article appears in the 11 June issue of Elsevier’s leading journal Cell, Volume 117, Issue 6, pages 801–815.
www.sciencedirect.com www.sciencedirect.com
396
TRENDS in Cell Biology
Vol.15 No.8 August 2005
Intracellular organelle transport: few motors, many signals Anna Kashina1 and Vladimir Rodionov2 1
Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce Street, 143 Rosenthal, Philadelphia, PA19104, USA 2 Department of Cell Biology and Center for Cell Analysis and Modeling, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT06032-1507, USA
Bidirectional microtubule-dependent organelle transport in melanophores is regulated by cAMP through organelle-bound protein kinase A (PKA); however, the mechanisms responsible for this regulation are unknown. A recent study by Gelfand and colleagues demonstrates that, in addition to PKA, transport is regulated by the organelle-bound mitogen-activated protein kinase (MAPK) signaling components ERK and MEK, whose activity is required for bidirectional transport along microtubules. This pathway apparently acts downstream of PKA, suggesting that bidirectional organelle transport is regulated by a hierarchical cascade of signaling pathways.
Introduction Intracellular transport of organelles and particles ensures the correct delivery of organelles to various destinations inside the cytoplasm. Transport occurs along cytoskeletal tracks – microtubules (MTs) and actin filaments (AFs) – and is mediated by molecular motors – dyneins and kinesins for MTs, and myosins for AFs (see [1] for review). Cytoskeletal tracks have a distinct polarity. MTs in most cell types form a radial network, with minus (slow growing) ends focused in the cell center, and plus (fast growing) ends at the cell periphery. Accordingly, MT motors move their cargoes either to the minus MT ends, retrogradely to the cell center (dynein and some kinesins) or to the plus end, anterogradely to the cell periphery (the majority of kinesins). Actin-dependent motors, myosins, generally move cargo to the plus (‘barbed’) ends of AFs, although a minus-end-directed variant of myosin has also been identified. AF transport has no distinct cytoplasmic orientation and is required for local redistribution of organelles. It has been shown that AF and MT motors of both polarities are attached to the same organelle at the same time, and therefore delivery of each organelle to its destination requires coordination of the motor action [2,3]. Until recently, the nature of the mechanisms of this coordination remained a mystery. Recent studies of intracellular transport in melanophores, pigment cells of lower vertebrates, have provided some clues about the pathways and mechanisms responsible for Corresponding author: Rodionov, V. ([email protected]). Available online 7 July 2005 www.sciencedirect.com
the selective control over the activities of multiple types of motors bound to cargo organelles (reviewed in [4]). cAMP-regulated signaling events coordinate organelle transport Melanophores redistribute pigment granules to control the color changes of animals in response to the environment. In these cells, membrane-bounded granules undergo MT-dependent pigment aggregation to the cell center, or slower dispersion throughout the cytoplasm along MTs and AFs (Figure 1). The rapid and synchronous movements of granules that can easily be observed in a light microscope make melanophores a unique system to study the mechanisms of coordinated intracellular transport and its regulation. In Xenopus melanophores, movement of pigment granules along MTs and AFs is mediated by three types of molecular motors: kinesin 2, dynein, and myosin V. Pigment aggregation to the cell centre occurs along MT tracks and is driven by dynein, whereas pigment dispersion throughout the cytoplasm is mediated by kinesin 2 and myosin V and involves a switch from MTs onto AF tracks at later stages of dispersion [5,6]. Pigment transport is regulated by changes in cAMP levels that are low during pigment aggregation and exhibit a sharp increase, then gradual decrease, during pigment dispersion [6,7] (Figure 1). These kinetics correlate with the differential activity of motors involved in pigment transport. During pigment aggregation, a drop in cAMP levels apparently induces activation of the MT motor dynein that transports pigment granules to the cell center. During pigment dispersion, cAMP levels first rise sharply, resulting in activation of kinesin 2 and myosin V and inactivation of dynein, and then gradually decrease. This decrease causes downregulation of kinesin 2 activity without affecting the activity of myosin V, resulting in the predominant AF-based transport observed at later stages of dispersion [6]. How can cAMP differentially regulate the activity of motors on the granule surface? cAMP regulates pigment transport through PKA signaling [8,9]. Indeed, inhibition of PKA activity induces pigment aggregation, whereas elevation of cAMP results in pigment dispersion. Moreover, it has been shown recently that components of the PKA signaling pathway are bound to the surface of pigment granules by means of scaffolding molecules, forming two independent complexes, one with dynein
Update
(a) ↑Kinesin
TRENDS in Cell Biology
Dispersion ↑cAMP ↑Myosin V
Vol.15 No.8 August 2005
397
cAMP ↓Dynein PKA ? MAPK
?
Kinesin 2
?
Dynein Aggregation complex
Dispersion complex
Myosin Va
Pigment granule
(a)
TRENDS in Cell Biology
(b) ↓Kinesin
Aggregation ↓cAMP ↓Myosin V
↑Dynein
Figure 1. Melanophores as a model for coordinated bidirectional transport. (a) During pigment dispersion, high cAMP levels correlate with high activity of the microtubule motor kinesin and the actin filament motor myosin V that mediate coordinated transport events, resulting in a uniform distribution of pigment granules throughout the cytoplasm. (b) During pigment aggregation, low cAMP levels correlate with high dynein activity, resulting in aggregation of pigment granules at the cell center. Bars, 20 mm.
(‘aggregation complex’) and one with kinesin 2 and myosin V (‘dispersion complex’) [10] (Figure 2). A simple explanation is that PKA regulation of transport is achieved through direct phosphorylation of the corresponding motors. While such direct phosphorylation possibly takes place, it cannot explain the complexity of differential motor regulation. Therefore, it seems likely that another mechanism exists that involves differential regulation of the motors by additional signaling cascades acting downstream of PKA. www.sciencedirect.com
Figure 2. cAMP-regulated signaling events coordinate organelle transport by differentially regulating the activity and scaffolding of motors involved in aggregation and dispersion. Dynein (blue) is regulated directly or indirectly by protein kinase A (PKA), possibly through inhibition of dynein, and by a mitogenactivated protein kinase (MAPK) cascade, involved in dynein activation. Kinesin (red) and myosin (brown) form a separate complex, where their activities are differentially regulated by PKA (a direct effect on kinesin and possibly an indirect effect on myosin) and by a MAPK cascade, which exerts a direct or indirect effect on kinesin.
MAPK components regulate pigment transport Recent papers by Deacon et al. and Andersson et al. [11,12] demonstrate that an additional signaling cascade is indeed involved in the regulation of bidirectional organelle transport. The authors show that inhibition of the major components of the mitogen-activated protein kinase (MAPK) cascade – ERK and MEK – inhibits pigment transport. Deacon et al. show that ERK and MEK are bound directly to the granule surface and that inhibition of ERK and MEK inhibits bidirectional transport along MTs without affecting actin-dependent transport during pigment dispersion, suggesting that the components of the MAPK cascade are involved in the regulation of kinesin and dynein activity but not the activity of myosin V. The authors also show that ERK and MEK activity, monitored by their phosphorylation state, is constitutively high in dispersed cells and is differentially regulated during pigment aggregation, where ERK and MEK phosphorylation first rises sharply and then gradually declines, apparently to stop granule movement at the end of aggregation. These kinetics are apparently independent of the changes in cAMP levels; however, Deacon et al. suggest that the MAPK cascade in the regulation of pigment transport acts downstream of PKA, as evidenced by the fact that the MAPK inhibitor U0126 prevents the pigment aggregation induced by PKA inhibition. Thus, Deacon et al. demonstrate that, in addition to PKA, the MAPK cascade is also involved in the regulation of pigment transport (Figure 2). This discovery adds complexity to the existing model for regulation of pigment transport; however, it still does not explain the
398
Update
TRENDS in Cell Biology
mechanisms of the differential regulation of motor activity during pigment aggregation and dispersion. We now know that molecular motors on organelles form complexes with the components of the PKA pathway [10] and that ERK and MEK are also found on the granule surface. It will be interesting to discover whether ERK and MEK, like PKA, also form a complex with the motors or at least are scaffolded into close proximity with the motor-regulatory complexes. Such scaffolding appears to be even more important for transport regulation in other cell types, where switching of organelle direction and track is not a cell-wide event but occurs locally for each organelle. Very little is known about the role of signaling molecules in the regulation of motor activity and organelle transport in other systems, but it has been shown that PKA scaffolding is crucial for many processes controlled through PKA phosphorylation [13]. It seems likely that local regulation by second messengers plays an important part in coordination of intracellular transport in cells where no predominant direction of organelle movement exists at a given point in time. Concluding remarks As regulation by PKA and MAPKs cannot explain the complexity of pigment transport events, it seems likely that other signaling cascades exist that are involved in the regulation of transport. The questions of which other signaling cascades take part in the regulation of the intracellular transport and what are the exact molecular mechanisms of such regulation are among the complex mysteries yet to be unravelled.
Vol.15 No.8 August 2005
References 1 Vale, R.D. (2003) The molecular motor toolbox for intracellular transport. Cell 112, 467–480 2 Welte, M.A. (2004) Bidirectional transport along microtubules. Curr. Biol. 14, R525–R537 3 Mallik, R. and Gross, S.P. (2004) Molecular motors: strategies to get along. Curr. Biol. 14, R971–R982 4 Nascimento, A.A. et al. (2003) Pigment cells: a model for the study of organelle transport. Annu. Rev. Cell Dev. Biol. 19, 469–491 5 Gross, S.P. et al. (2002) Interactions and regulation of molecular motors in Xenopus melanophores. J. Cell Biol. 156, 855–865 6 Rodionov, V. et al. (2003) Switching between microtubule- and actinbased transport systems in melanophores is controlled by cAMP levels. Curr. Biol. 13, 1837–1847 7 Daniolos, A. et al. (1990) Action of light on frog pigment cells in culture. Pigment Cell Res. 3, 38–43 8 Rozdzial, M.M. and Haimo, L.T. (1986) Bidirectional pigment granule movements of melanophores are regulated by protein phosphorylation and dephosphorylation. Cell 47, 1061–1070 9 Reilein, A.R. et al. (1998) Regulation of organelle movement in melanophores by protein kinase A (PKA), protein kinase C (PKC), and protein phosphatase 2A (PP2A). J. Cell Biol. 142, 803–813 10 Kashina, A.S. et al. (2004) Protein kinase A, which regulates intracellular transport, forms complexes with molecular motors on organelles. Curr. Biol. 14, 1877–1881 11 Deacon, S.W. et al. (2005) Regulation of bidirectional melanosome transport by organelle bound MAP kinase. Curr. Biol. 15, 459–463 12 Andersson, T.P. et al. (2003) Regulation of melanosome movement by MAP kinase. Pigment Cell Res. 16, 215–221 13 Wong, W. and Scott, J.D. (2004) AKAP signalling complexes: focal points in space and time. Nat. Rev. Mol. Cell Biol. 5, 959–970
0962-8924/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2005.06.002
ScienceDirect collection reaches six million full-text articles Elsevier recently announced that six million articles are now available on its premier electronic platform, ScienceDirect. This milestone in electronic scientific, technical and medical publishing means that researchers around the globe will be able to access an unsurpassed volume of information from the convenience of their desktop. ScienceDirect’s extensive and unique full-text collection covers over 1900 journals, including titles such as The Lancet, Cell, Tetrahedron and the full suite of Trends and Current Opinion journals. With ScienceDirect, the research process is enhanced with unsurpassed searching and linking functionality, all on a single, intuitive interface. The rapid growth of the ScienceDirect collection is due to the integration of several prestigious publications as well as ongoing addition to the Backfiles – heritage collections in a number of disciplines. The latest step in this ambitious project to digitize all of Elsevier’s journals back to volume one, issue one, is the addition of the highly cited Cell Press journal collection on ScienceDirect. Also available online for the first time are six Cell titles’ long-awaited Backfiles, containing more than 12,000 articles highlighting important historic developments in the field of life sciences. The six-millionth article loaded onto ScienceDirect entitled "Gene Switching and the Stability of Odorant Receptor Gene Choice" was authored by Benjamin M. Shykind and colleagues from the Dept. of Biochemistry and Molecular Biophysics and Howard Hughes Medical Institute, College of Physicians and Surgeons at Columbia University. The article appears in the 11 June issue of Elsevier’s leading journal Cell, Volume 117, Issue 6, pages 801–815.
www.sciencedirect.com www.sciencedirect.com