Dysfunction of the mTOR pathway is a risk factor for Alzheimer's disease

Abstract

Background: The development of disease-modifying therapies for Alzheimer's disease is hampered by our lack of understanding of the early pathogenic mechanisms and the lack of early biomarkers and risk factors.We have documented the expression pattern of mTOR regulated genes in the frontal cortex of Alzheimer's disease patients. We have also examined the functional integrity of mTOR signaling in peripheral lymphocytes in Alzheimer's disease patients relative to healthy controls.

Results: In the brain mTOR is seen to control molecular functions related to cell cycle regulation, cell death and several metabolic pathways. These downstream elements of the mTOR signaling cascade are deregulated in the brain of Alzheimer's disease patients well before the development of pathology. This dysregulation of the mTOR downstream signaling cascade is not restricted to the brain but appears to be systemic and can be detected in peripheral lymphocytes as a reduced Rapamycin response.

Conclusions: The dysfunction of the signaling pathways downstream of mTOR may represent a risk factor for Alzheimer's disease and is independent of the ApoE status of the patients.We have also identified the molecular substrates of the beneficial effects of Rapamycin on the nervous system. We believe that these results can further inform the development of clinical predictive tests for the risk of Alzheimer's disease in patients with mild cognitive impairment.

Figure 1

A simplified version of the mTOR signalling pathway. MTORC1 receives and integrates the signals from various upstream pathways, including pathways triggered by nutrients, growth factors, hypoxia and insulin that promote cell growth, division and differentiation. Rapamycin is a powerful inhibitor of mTORC1 activity. Abbreviations: 4E-binding protein (4EBP1); eukaryotic translation initiation factor 4E (eIF4E); Receptor Tyrosine Kinase (RTK); Insulin receptor (INSR); Protein Kinase C (PKC); Extracellular Signal Regulated Kinase (ERK); (Ras); Protein kinase B (AKT).

Figure 2

Molecular and cellular functions associated with genes up-regulated in lymphocytes in response to rapamycin treatment. The chart shows the number of genes, expressed as a percentage of the total number, involved in each of the molecular and cellular functions deemed significant by IPA Ingenuity.

Figure 3

Molecular and cellular functions associated with genes down-regulated in lymphocytes in response to rapamycin treatment. The chart shows the number of genes, expressed as a percentage of the total number, involved in each of the molecular and cellular functions deemed significant by IPA Ingenuity.

Figure 4

Molecular and cellular functions associated with mTOR-regulated genes differentially expressed in the frontal lobe of mild AD patients. Molecular and cellular functions associated with genes differentially expressed in the frontal lobe of mild AD patients (limbic stage) compared to controls (112 genes). The chart shows the number of genes, expressed as a percentage of the total number, involved in each of the molecular and cellular functions deemed significant by IPA Ingenuity.

Figure 5

Molecular and cellular functions associated with mTOR-regulated genes differentially expressed in the frontal lobe of advanced AD patients. Molecular and cellular functions associated with genes differentially expressed in the frontal lobe of advanced AD patients (neocortical stage) compared to controls (176 genes). The chart shows the number of genes, expressed as a percentage of the total number, involved in each of the molecular and cellular functions deemed significant by IPA Ingenuity.

Figure 6

Molecular and cellular functions associated with genes differentially expressed in AD brain irrespective of severity (55 genes). The chart shows the number of genes, expressed as a percentage of the total number, involved in each of the molecular and cellular functions deemed significant by IPA Ingenuity.

Figure 7

Differentially expressed mTOR genes in the frontal lobe of AD patients. Heat map of differentially expressed mTOR genes in the frontal lobe of AD patients compared to control. Differential expression was determined by multiclass analysis in SAM, with the subjects split into three groups according to Braak staging: control, mild AD (limbic stage) and advanced AD (neocortical stage). The cut-off point for differential expression was <10% FDR. The heat map was generated with Cluster and Treeview, with complete hierarchical clustering by gene and array. The expression level of each gene relative to the median expression of the gene across all samples is represented on a red-green colour scale. The colours represent the log ratio of gene expression in an individual patient. Downregulated genes are represented in green (saturated green: log ratios −3.0 and below) while upregulated genes are represented in red (saturated red: log ratios 3.0 and above). Cells with log ratios of 0 (gene expression unchanged) are black. The arrays are identified by the patient codes that are described in Additional file 2: Table S1 (C: Control, L: Limbic stage AD, N: Neocortical stage AD) while the genes are identified by the gene name.

Figure 8

Rapamycin-regulated genes differentially expressed in both mild and advanced AD. Heat map of the 65 rapamycin-regulated genes that were shown to be differentially expressed in both mild and advanced AD compared to control, in separate two-class unpaired SAM analyses. The cut-off point for differential expression was <10% FDR. The heat map was generated with Cluster and Treeview, with complete hierarchical clustering by gene and array. The expression level of each gene relative to the median expression of the gene across all samples is represented on a red-green colour scale. The colours represent the log ratio of gene expression in an individual patient. Downregulated genes are represented in green (saturated green: log ratios −3.0 and below) while upregulated genes are represented in red (saturated red: log ratios 3.0 and above). Cells with log ratios of 0 (gene expression unchanged) are black. The arrays are identified by the patient codes that are described in Additional file 2: Table S1 (C: Control, L: Limbic stage AD, N: Neocortical stage AD) while the genes are identified by the gene name.

Figure 9

Differentially expressed mTOR-related genes in either mild or advanced AD. Heat map visualising the expression level of the 98 genes that were either differentially expressed in mild AD (17 genes) or in advanced AD (81 genes) only. The cut-off point for differential expression was <10% FDR. The heat map was generated with Cluster and Treeview, with complete hierarchical clustering by gene and array. The expression level of each gene relative to the median expression of the gene across all samples is represented on a red-green colour scale. The colours represent the log ratio of gene expression in an individual patient. Downregulated genes are represented in green (saturated green: log ratios −3.0 and below) while upregulated genes are represented in red (saturated red: log ratios 3.0 and above). Cells with log ratios of 0 (gene expression unchanged) are black. The arrays are identified by the patient codes that are described in Additional file 2: Table S1 (C: Control, L: Limbic stage AD, N: Neocortical stage AD) while the genes are identified by the gene name.

Figure 10

ROC curve analysis of the performance of the lymphocyte test. a). Receiver operating characteristic (ROC) curve of the lymphocyte test for differentiation between Alzheimer’s disease patients (probable AD, NINCDS-ARDRA criteria) and healthy control individual. b). Receiver operating characteristic (ROC) curve of the combination of the lymphocyte test and ApoE4 status for differentiation between Alzheimer’s disease patients (probable AD, NINCDS-ARDRA criteria) and healthy control individual.

Figure 11

Comparison of ROC curves. ROC curve comparison. Green curve: probability of AD due to age; Blue curve: probability of AD based on ApoE4 status; Red line: probability of AD predicted by the Rapamycin response in lymphocytes; Black line: probability of AD as predicted by the combination of the Rapamycin response and ApoE4 status.

Figure 12

Causes and consequences of mTOR activation in the brain. Vascular disease, diabetes, elevated plasma homocysteine levels, trauma and ageing (all risk factor for AD) can elicit the various extracellular signals that can activate mTORC1. Normally, the activation of downstream signals would ensure cell survival and damage repair. However, the dysfunction of the downstream effector pathways, du to genetic variation, will lead to aberrant responses to mTOR activation in the brain, leading to AD-type pathology, neuronal death and glial activation. Abbreviations: Homocysteine (Hcy); Receptor Tyrosine Kinase (RTK); Insulin receptor (INSR); Protein Kinase C (PKC); Extracellular Signal Regulated Kinase (ERK); (Ras); Protein kinase B (AKT); Neurofibrillary tangle (NFT; Amyloid Precursor Protein (APP).