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Amyloidogenic processing of the Alzheimer beta-amyloid precursor protein depends on lipid rafts.

Ehehalt R, Keller P, Haass C, Thiele C, Simons K - J. Cell Biol. (2003)

Bottom Line: A beta generation was dependent on endocytosis and was reduced after expression of the dynamin mutant K44A and the Rab5 GTPase-activating protein, RN-tre.Although APP inside raft clusters seems to be cleaved by beta-secretase, APP outside rafts undergoes cleavage by alpha-secretase.Thus, access of alpha- and beta-secretase to APP, and therefore A beta generation, may be determined by dynamic interactions of APP with lipid rafts.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, D-01307 Dresden, Germany.

ABSTRACT
Formation of senile plaques containing the beta-amyloid peptide (A beta) derived from the amyloid precursor protein (APP) is an invariant feature of Alzheimer's disease (AD). APP is cleaved either by beta-secretase or by alpha-secretase to initiate amyloidogenic (release of A beta) or nonamyloidogenic processing of APP, respectively. A key to understanding AD is to unravel how access of these enzymes to APP is regulated. Here, we demonstrate that lipid rafts are critically involved in regulating A beta generation. Reducing cholesterol levels in N2a cells decreased A beta production. APP and the beta-site APP cleavage enzyme (BACE1) could be induced to copatch at the plasma membrane upon cross-linking with antibodies and to segregate away from nonraft markers. Antibody cross-linking dramatically increased production of A beta in a cholesterol-dependent manner. A beta generation was dependent on endocytosis and was reduced after expression of the dynamin mutant K44A and the Rab5 GTPase-activating protein, RN-tre. This inhibition could be overcome by antibody cross-linking. These observations suggest the existence of two APP pools. Although APP inside raft clusters seems to be cleaved by beta-secretase, APP outside rafts undergoes cleavage by alpha-secretase. Thus, access of alpha- and beta-secretase to APP, and therefore A beta generation, may be determined by dynamic interactions of APP with lipid rafts.

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Model for raft clustering and β-cleavage of APP. (A) Under normal (steady state) conditions, two cellular pools of APP and presumably also of BACE1 exist at the plasma membrane. One is associated with rafts (black), and another is outside of rafts. Rafts are small and highly dispersed at the cell surface. Because they contain only a few proteins, APP and BACE1 are mainly localized in separate rafts at the plasma membrane. Endocytosis is necessary for APP- and BACE1-containing rafts to cluster for consecutive β-cleavage. We assume that raft APP is cleaved by raft BACE1. (B) If endocytic clustering is inhibited, e.g., by the expression of dynamin K44A, the block can be relieved by copatching with antibodies against BACE1 and APP. This leads to coalescence of rafts at the plasma membrane and results in β-cleavage. These conditions are postulated to mimic the process normally taking place after internalization.
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fig8: Model for raft clustering and β-cleavage of APP. (A) Under normal (steady state) conditions, two cellular pools of APP and presumably also of BACE1 exist at the plasma membrane. One is associated with rafts (black), and another is outside of rafts. Rafts are small and highly dispersed at the cell surface. Because they contain only a few proteins, APP and BACE1 are mainly localized in separate rafts at the plasma membrane. Endocytosis is necessary for APP- and BACE1-containing rafts to cluster for consecutive β-cleavage. We assume that raft APP is cleaved by raft BACE1. (B) If endocytic clustering is inhibited, e.g., by the expression of dynamin K44A, the block can be relieved by copatching with antibodies against BACE1 and APP. This leads to coalescence of rafts at the plasma membrane and results in β-cleavage. These conditions are postulated to mimic the process normally taking place after internalization.

Mentions: To account for these results, we envisage that β-cleavage would normally not take place at the cell surface because surface APP and BACE1 are most likely present in separate rafts (Fig. 8). Rafts are small and highly dispersed at the cell surface and are suggested to contain only a subset of ∼10–30 protein molecules (Pralle et al., 2000). Therefore, the likelihood is low that APP and BACE1 are in the same individual raft. For β-cleavage to occur rafts would have to be clustered to get APP and BACE1 into the same raft platform. Thus, we hypothesize that APP and BACE1 meet after endocytosis by clustering and coalescence of APP- or BACE1-containing rafts within endosomes (Fig. 8 A). How and where clustering is accomplished during internalization from the plasma membrane is not known. However, raft clustering can be artificially induced at the cell surface by cross-linking with antibodies (Fig. 8 B). This could lead to the increased β-cleavage in clusters/patches containing both APP and BACE1 that we had observed. Remarkably, we did not detect a dramatic increase in α-secretase processing of APP after inhibition of endocytosis. We assume that this is due to a continued raft association of a fraction of cell surface APP, which would not be accessible to α-cleavage. On the other hand, cholesterol depletion would shift the partitioning of APP from lipid rafts to the surrounding lipid bilayer and lead to the observed increase of α-cleavage.


Amyloidogenic processing of the Alzheimer beta-amyloid precursor protein depends on lipid rafts.

Ehehalt R, Keller P, Haass C, Thiele C, Simons K - J. Cell Biol. (2003)

Model for raft clustering and β-cleavage of APP. (A) Under normal (steady state) conditions, two cellular pools of APP and presumably also of BACE1 exist at the plasma membrane. One is associated with rafts (black), and another is outside of rafts. Rafts are small and highly dispersed at the cell surface. Because they contain only a few proteins, APP and BACE1 are mainly localized in separate rafts at the plasma membrane. Endocytosis is necessary for APP- and BACE1-containing rafts to cluster for consecutive β-cleavage. We assume that raft APP is cleaved by raft BACE1. (B) If endocytic clustering is inhibited, e.g., by the expression of dynamin K44A, the block can be relieved by copatching with antibodies against BACE1 and APP. This leads to coalescence of rafts at the plasma membrane and results in β-cleavage. These conditions are postulated to mimic the process normally taking place after internalization.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172747&req=5

fig8: Model for raft clustering and β-cleavage of APP. (A) Under normal (steady state) conditions, two cellular pools of APP and presumably also of BACE1 exist at the plasma membrane. One is associated with rafts (black), and another is outside of rafts. Rafts are small and highly dispersed at the cell surface. Because they contain only a few proteins, APP and BACE1 are mainly localized in separate rafts at the plasma membrane. Endocytosis is necessary for APP- and BACE1-containing rafts to cluster for consecutive β-cleavage. We assume that raft APP is cleaved by raft BACE1. (B) If endocytic clustering is inhibited, e.g., by the expression of dynamin K44A, the block can be relieved by copatching with antibodies against BACE1 and APP. This leads to coalescence of rafts at the plasma membrane and results in β-cleavage. These conditions are postulated to mimic the process normally taking place after internalization.
Mentions: To account for these results, we envisage that β-cleavage would normally not take place at the cell surface because surface APP and BACE1 are most likely present in separate rafts (Fig. 8). Rafts are small and highly dispersed at the cell surface and are suggested to contain only a subset of ∼10–30 protein molecules (Pralle et al., 2000). Therefore, the likelihood is low that APP and BACE1 are in the same individual raft. For β-cleavage to occur rafts would have to be clustered to get APP and BACE1 into the same raft platform. Thus, we hypothesize that APP and BACE1 meet after endocytosis by clustering and coalescence of APP- or BACE1-containing rafts within endosomes (Fig. 8 A). How and where clustering is accomplished during internalization from the plasma membrane is not known. However, raft clustering can be artificially induced at the cell surface by cross-linking with antibodies (Fig. 8 B). This could lead to the increased β-cleavage in clusters/patches containing both APP and BACE1 that we had observed. Remarkably, we did not detect a dramatic increase in α-secretase processing of APP after inhibition of endocytosis. We assume that this is due to a continued raft association of a fraction of cell surface APP, which would not be accessible to α-cleavage. On the other hand, cholesterol depletion would shift the partitioning of APP from lipid rafts to the surrounding lipid bilayer and lead to the observed increase of α-cleavage.

Bottom Line: A beta generation was dependent on endocytosis and was reduced after expression of the dynamin mutant K44A and the Rab5 GTPase-activating protein, RN-tre.Although APP inside raft clusters seems to be cleaved by beta-secretase, APP outside rafts undergoes cleavage by alpha-secretase.Thus, access of alpha- and beta-secretase to APP, and therefore A beta generation, may be determined by dynamic interactions of APP with lipid rafts.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, D-01307 Dresden, Germany.

ABSTRACT
Formation of senile plaques containing the beta-amyloid peptide (A beta) derived from the amyloid precursor protein (APP) is an invariant feature of Alzheimer's disease (AD). APP is cleaved either by beta-secretase or by alpha-secretase to initiate amyloidogenic (release of A beta) or nonamyloidogenic processing of APP, respectively. A key to understanding AD is to unravel how access of these enzymes to APP is regulated. Here, we demonstrate that lipid rafts are critically involved in regulating A beta generation. Reducing cholesterol levels in N2a cells decreased A beta production. APP and the beta-site APP cleavage enzyme (BACE1) could be induced to copatch at the plasma membrane upon cross-linking with antibodies and to segregate away from nonraft markers. Antibody cross-linking dramatically increased production of A beta in a cholesterol-dependent manner. A beta generation was dependent on endocytosis and was reduced after expression of the dynamin mutant K44A and the Rab5 GTPase-activating protein, RN-tre. This inhibition could be overcome by antibody cross-linking. These observations suggest the existence of two APP pools. Although APP inside raft clusters seems to be cleaved by beta-secretase, APP outside rafts undergoes cleavage by alpha-secretase. Thus, access of alpha- and beta-secretase to APP, and therefore A beta generation, may be determined by dynamic interactions of APP with lipid rafts.

Show MeSH
Related in: MedlinePlus