Disturbed neurotransmitter transporter expression in Alzheimer's disease brain

Abstract

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss and behavioral and psychological symptoms of dementia. An imbalance of different neurotransmitters--glutamate, acetylcholine, dopamine, and serotonin--has been proposed as the neurobiological basis of behavioral symptoms in AD. The molecular changes associated with neurotransmission imbalance in AD are not clear. We hypothesized that altered reuptake of neurotransmitters by vesicular glutamate transporters (VGLUTs), excitatory amino acid transporters (EAATs), the vesicular acetylcholine transporter (VAChT), the serotonin reuptake transporter (SERT), or the dopamine reuptake transporter (DAT) are involved in the neurotransmission imbalance in AD. We tested this hypothesis by examining protein and mRNA levels of these transporters in postmortem prefrontal cortex from 10 AD patients and 10 matched non-AD controls. Compared with controls, protein and mRNA levels of VGLUTs, EAAT1-3, VAChT, and SERT were reduced significantly in AD. Expression of DAT and catechol O-methyltransferase was unchanged. Reduced VGLUTs and EAATs may contribute to an alteration in glutamatergic recycling, and reduced SERT could exacerbate depressive symptoms in AD. The reduced VAChT expression could contribute to the recognized cholinergic deficit in AD. Altered neurotransmitter transporters could contribute to the pathophysiology of AD and are potential targets for therapy.

Conflict of interest statement

CONFLICT OF INTEREST

No author has a conflict of interest.

Figures

Figure 1

Schematic representation of neurotransmitter transporters – EAAT(1–4), VAChT, SERT, DAT – in the brain. EAAT1 and EAAT2 are present on glia cells as well as at the presynaptic site of neurons. EAAT3 and EAAT4 are found on the postsynaptic site. Glutamate released from the pre-synaptic site is taken up by EAATs and converted into glutamine. At the presynaptic site, glutamine is converted back to glutamate. VAChT is present at the presynaptic site on the membrane of synaptic vesicles. SERT and DAT are both located at the presynaptic terminal.

Figure 2

Mean VGLUT1, VGLUT2, EAAT1 and EAAT2 protein levels (A – D) (with representative immunoblots) in control and AD frontal cortex. Data are ratios of optical densities of EAAT1 and EAAT2 protein to β-actin, expressed as percent of control. mRNA levels of VGLUT1, VGLUT2, EAAT1 and EAAT2 (E – H) in control and AD frontal cortex, measured using real time RT-PCR. Data are mRNA levels of VGLUT1, VGLUT2, EAAT1 and EAAT2 in AD brains normalized to the endogenous control (β-2 microglobulin) and relative to a standard level (calibrator), using the ΔΔCT method. Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 3

Mean EAAT3, VAChT and NSE protein levels (A, C and E) (with representative immunoblots) in control and AD frontal cortex. Data are ratios of optical densities of EAAT3, VAChT and NSE protein to β-actin, expressed as percent of control. mRNA level of EAAT3, VAChT and NSE (B, D and F) in postmortem control and AD frontal cortex, measured using real time RT-PCR. Data are mRNA levels of EAAT3, VAChT and NSE in AD brains normalized to the endogenous control (β-2 microglobulin) and relative to a standard level (calibrator), using the ΔΔCT method. Mean ± SEM, *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 4

Mean SERT and DAT protein levels (A and C) (with representative immunoblot) in control and AD frontal cortex. Data are ratios of optical densities of SERT and DAT protein to β-actin, expressed as percent of control. mRNA levels of SERT (B) and DAT (D) in postmortem control and AD frontal cortex, measured using real time RT-PCR. Data are mRNA levels of SERT and DAT in AD brains normalized to the endogenous control (β-2 microglobulin) and relative to a standard level (calibrator), using the ΔΔCT method. Mean ± SEM, *p < 0.05, **p < 0.01.

Figure 5

Mean GFAP and synaptophysin (A and C) (with representative immunoblot) in control and AD frontal cortex. Data are ratios of optical densities of DAT and GFAP protein to β-actin, expressed as percent of control. mRNA levels of GFAP and synaptophysin (B and D) in postmortem control and AD frontal cortex, measured using real time RT-PCR. Data are mRNA levels of GFAP and synaptophysin in AD brains normalized to the endogenous control (β-2 microglobulin) and relative to a standard level (calibrator), using the ΔΔCT method. Mean ± SEM, *p < 0.05.