Michael T. Lin

Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases

Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. Mitochondria are critical regulators of cell death, a key feature of neurodegeneration. Mutations in mitochondrial DNA and oxidative stress both contribute to ageing, which is the greatest risk factor for neurodegenerative diseases. In all major examples of these diseases there is strong evidence that mitochondrial dysfunction occurs early and acts causally in disease pathogenesis. Moreover, an impressive number of disease-specific proteins interact with mitochondria. Thus, therapies targeting basic mitochondrial processes, such as energy metabolism or free-radical generation, or specific interactions of disease-related proteins with mitochondria, hold great promise.

Oligomerization of Alzheimer's β-Amyloid within Processes and Synapses of Cultured Neurons and Brain

Multiple lines of evidence implicate β-amyloid (Aβ) in the pathogenesis of Alzheimers disease (AD), but the mechanisms whereby Aβ is involved remain unclear. Addition of Aβ to the extracellular space can be neurotoxic. Intraneuronal Aβ42 accumulation is also associated with neurodegeneration. We reported previously that in Tg2576 amyloid precursor protein mutant transgenic mice, brain Aβ42 localized by immunoelectron microscopy to, and accumulated with aging in, the outer membranes of multivesicular bodies, especially in neuronal processes and synaptic compartments. We now demonstrate that primary neurons from Tg2576 mice recapitulate the in vivo localization and accumulation of Aβ42 with time in culture. Furthermore, we demonstrate that Aβ42 aggregates into oligomers within endosomal vesicles and along microtubules of neuronal processes, both in Tg2576 neurons with time in culture and in Tg2576 and human AD brain. These Aβ42 oligomer accumulations are associated with pathological alterations within processes and synaptic compartments in Tg2576 mouse and human AD brains.

SK channels and NMDA receptors form a Ca2+-mediated feedback loop in dendritic spines.

Small-conductance Ca2+-activated K+ channels (SK channels) influence the induction of synaptic plasticity at hippocampal CA3–CA1 synapses. We find that in mice, SK channels are localized to dendritic spines, and their activity reduces the amplitude of evoked synaptic potentials in an NMDA receptor (NMDAR)-dependent manner. Using combined two-photon laser scanning microscopy and two-photon laser uncaging of glutamate, we show that SK channels regulate NMDAR-dependent Ca2+ influx within individual spines. SK channels are tightly coupled to synaptically activated Ca2+ sources, and their activity reduces the amplitude of NMDAR-dependent Ca2+ transients. These effects are mediated by a feedback loop within the spine head; during an excitatory postsynaptic potential (EPSP), Ca2+ influx opens SK channels that provide a local shunting current to reduce the EPSP and promote rapid Mg2+ block of the NMDAR. Thus, blocking SK channels facilitates the induction of long-term potentiation by enhancing NMDAR-dependent Ca2+ signals within dendritic spines.

Increased plaque burden in brains of APP mutant MnSOD heterozygous knockout mice

A growing body of evidence suggests a relationship between oxidative stress and β‐amyloid (Aβ) peptide accumulation, a hallmark in the pathogenesis of Alzheimers disease (AD). However, a direct causal relationship between oxidative stress and Aβ pathology has not been established in vivo. Therefore, we crossed mice with a knockout of one allele of manganese superoxide dismutase (MnSOD), a critical antioxidant enzyme, with Tg19959 mice, which overexpress a doubly mutated human β‐amyloid precursor protein (APP). Partial deficiency of MnSOD, which is well established to cause elevated oxidative stress, significantly increased brain Aβ levels and Aβ plaque burden in Tg19959 mice. These results indicate that oxidative stress can promote the pathogenesis of AD and further support the feasibility of antioxidant approaches for AD therapy.

Reduction of oxidative stress, amyloid deposition, and memory deficit by manganese superoxide dismutase overexpression in a transgenic mouse model of Alzheimer’s disease

In Alzheimers disease (AD), oxidative stress is present early and contributes to disease pathogenesis. We previously reported that in Tg19959 transgenic AD mice, partial deficiency of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) exacerbated amyloid pathology. We therefore asked whether MnSOD overexpression would prove beneficial against AD pathogenesis, by studying the offspring of Tg19959 mice crossed with MnSOD‐ overexpressing mice. At 4 mo of age, there was a 2‐ to 3‐fold increase in MnSOD protein levels in Tg19959‐ MnSOD mice compared to Tg19959 littermates. Tg19959‐MnSOD mice also had a 50% increase in catalase protein levels, a 50% decrease in levels of oxidized protein, and a 33% reduction in cortical plaque burden compared to Tg19959 littermates. Spatial memory was impaired and synaptophysin levels were decreased in Tg19959 mice compared to wild‐type littermates, but memory and synaptophysin levels were restored to wild‐type levels in Tg19959‐MnSOD littermates. These benefits occurred without changes in sodium dodecyl sulfate‐soluble or formic acid‐soluble Aβ pools or Aβ oligomers in Tg19959‐MnSOD mice compared to Tg19959 littermates. These data demonstrate that facilitation of the mitochondrial antioxidant response improves resistance to Aβ, slows plaque formation or increases plaque degradation, and markedly attenuates the phenotype in a transgenic AD mouse model.— Dumont, M.,Wille, E., Stack, C., Calingasan, N. Y., Beal, M. F., Lin, M. T. Reduction of oxidative stress, amyloid deposition, and memory deficit by manganese superoxide dismutase overexpression in a transgenic mouse model of Alzheimers disease. FASEB J. 23, 2459–2466 (2009)

SK2 channel plasticity contributes to LTP at Schaffer collateral–CA1 synapses

Long-term potentiation (LTP) of synaptic strength at Schaffer collateral synapses has largely been attributed to changes in the number and biophysical properties of AMPA receptors (AMPARs). Small-conductance Ca2+-activated K+ channels (SK2 channels) are functionally coupled with NMDA receptors (NMDARs) in CA1 spines such that their activity modulates the shape of excitatory postsynaptic potentials (EPSPs) and increases the threshold for induction of LTP. Here we show that LTP induction in mouse hippocampus abolishes SK2 channel activity in the potentiated synapses. This effect is due to SK2 channel internalization from the postsynaptic density (PSD) into the spine. Blocking PKA or cell dialysis with a peptide representing the C-terminal domain of SK2 that contains three known PKA phosphorylation sites blocks the internalization of SK2 channels after LTP induction. Thus the increase in AMPARs and the decrease in SK2 channels combine to produce the increased EPSP underlying LTP.

Dysregulation of the mTOR Pathway Mediates Impairment of Synaptic Plasticity in a Mouse Model of Alzheimer's Disease

Background The mammalian target of rapamycin (mTOR) is an evolutionarily conserved Ser/Thr protein kinase that plays a pivotal role in multiple fundamental biological processes, including synaptic plasticity. We explored the relationship between the mTOR pathway and β-amyloid (Aβ)-induced synaptic dysfunction, which is considered to be critical in the pathogenesis of Alzheimers disease (AD). Methodology/Principal Findings We provide evidence that inhibition of mTOR signaling correlates with impairment in synaptic plasticity in hippocampal slices from an AD mouse model and in wild-type slices exposed to exogenous Aβ1-42. Importantly, by up-regulating mTOR signaling, glycogen synthase kinase 3 (GSK3) inhibitors rescued LTP in the AD mouse model, and genetic deletion of FK506-binding protein 12 (FKBP12) prevented Aβ-induced impairment in long-term potentiation (LTP). In addition, confocal microscopy demonstrated co-localization of intraneuronal Aβ42 with mTOR. Conclusions/Significance These data support the notion that the mTOR pathway modulates Aβ-related synaptic dysfunction in AD.

Protection from Alzheimer's-like disease in the mouse by genetic ablation of inducible nitric oxide synthase

Brains from subjects who have Alzheimers disease (AD) express inducible nitric oxide synthase (iNOS). We tested the hypothesis that iNOS contributes to AD pathogenesis. Immunoreactive iNOS was detected in brains of mice with AD-like disease resulting from transgenic expression of mutant human β-amyloid precursor protein (hAPP) and presenilin-1 (hPS1). We bred hAPP-, hPS1-double transgenic mice to be iNOS+/+ or iNOS−/−, and compared them with a congenic WT strain. Deficiency of iNOS substantially protected the AD-like mice from premature mortality, cerebral plaque formation, increased β-amyloid levels, protein tyrosine nitration, astrocytosis, and microgliosis. Thus, iNOS seems to be a major instigator of β-amyloid deposition and disease progression. Inhibition of iNOS may be a therapeutic option in AD.

Behavioral deficits and progressive neuropathology in progranulin-deficient mice: a mouse model of frontotemporal dementia

Progranulin haploinsufficiency causes frontotemporal dementia with tau-negative, ubiquitin-positive neuronal inclusion pathology. In this study, we showed that progranulin-deficient mice displayed increased depression- and disinhibition-like behavior, as well as deficits in social recognition from a relatively young age. These mice did not have any deficit in locomotion or exploration. Eighteen-month-old progranulin-deficient mice demonstrated impaired spatial learning and memory in the Morris water maze. In addition to behavioral deficits, progranulin-deficient mice showed a progressive development of neuropathology from 12 mo of age, including enhanced activation of microglia and astrocytes and ubiquitination and cytoplasmic accumulation of phosphorylated TDP-43. Thus, progranulin deficiency induced FTD-like behavioral and neuropathological deficits. These mice may serve as an important tool for deciphering underlying mechanisms in frontotemporal dementia.

Alzheimer's APP mangles mitochondria.

New findings in humans examine how mitochondrial function declines during Alzheimer disease.