Bertrand P. Tseng

Amyloid deposition precedes tangle formation in a triple transgenic model of Alzheimer’s disease

Amyloid-beta (Abeta) containing plaques and tau-laden neurofibrillary tangles are the defining neuropathological features of Alzheimers disease (AD). To better mimic this neuropathology, we generated a novel triple transgenic model of AD (3xTg-AD) harboring three mutant genes: beta-amyloid precursor protein (betaAPPSwe), presenilin-1 (PS1M146V), and tauP301L. The 3xTg-AD mice progressively develop Abeta and tau pathology, with a temporal- and regional-specific profile that closely mimics their development in the human AD brain. We find that Abeta deposits initiate in the cortex and progress to the hippocampus with aging, whereas tau pathology is first apparent in the hippocampus and then progresses to the cortex. Despite equivalent overexpression of the human betaAPP and human tau transgenes, Abeta deposition develops prior to the tangle pathology, consistent with the amyloid cascade hypothesis. As these 3xTg-AD mice phenocopy critical aspects of AD neuropathology, this model will be useful in pre-clinical intervention trials, particularly because the efficacy of anti-AD compounds in mitigating the neurodegenerative effects mediated by both signature lesions can be evaluated.

Aβ inhibits the proteasome and enhances amyloid and tau accumulation

The accumulation of misfolded protein aggregates is a common feature of numerous neurodegenerative disorders including Alzheimer disease (AD). Here, we examined the effects of different assembly states of amyloid beta (Abeta) on proteasome function. We find that Abeta oligomers, but not monomers, inhibit the proteasome in vitro. In young 3xTg-AD mice, we observed impaired proteasome activity that correlates with the detection of intraneuronal Abeta oligomers. Blocking proteasome function in pre-pathological 3xTg-AD mice with specific inhibitors causes a marked increase in Abeta and tau accumulation, highlighting the adverse consequences of impaired proteasome activity for AD. Lastly, we show that Abeta immunotherapy in the 3xTg-AD mice reduces Abeta oligomers and reverses the deficits in proteasome activity. Taken together, our results indicate that Abeta oligomers impair proteasome activity, contributing to the age-related pathological accumulation of Abeta and tau. These findings provide further evidence that the proteasome represents a viable target for therapeutic intervention in AD.

Consequences of ionizing radiation-induced damage in human neural stem cells.

Cranial irradiation remains a frontline treatment for brain cancer, but also leads to normal tissue damage. Although low-dose irradiation (≤10 Gy) causes minimal histopathologic change, it can elicit variable degrees of cognitive dysfunction that are associated with the depletion of neural stem cells. To decipher the mechanisms underlying radiation-induced stem cell dysfunction, human neural stem cells (hNSCs) subjected to clinically relevant irradiation (0-5 Gy) were analyzed for survival parameters, cell-cycle alterations, DNA damage and repair, and oxidative stress. hNSCs showed a marked sensitivity to low-dose irradiation that was in part due to elevated apoptosis and the inhibition of cell-cycle progression that manifested as a G2/M checkpoint delay. Efficient removal of DNA double-strand breaks was indicated by the disappearance of γ-H2AX nuclear foci. A dose-responsive and persistent increase in oxidative and nitrosative stress was found in irradiated hNSCs, possibly the result of a higher metabolic activity in the fraction of surviving cells. These data highlight the marked sensitivity of hNSCs to low-dose irradiation and suggest that long-lasting perturbations in the CNS microenvironment due to radiation-induced oxidative stress can compromise the functionality of neural stem cells.

Amyloid β-Peptide: The Inside Story

The amyloid β-peptide (Aβ) plays an early and critical role in the pathogenic cascade leading to Alzheimers disease (AD). Aβ is typically found in extracellular amyloid plaques that occur in specific brain regions in the AD and Down syndrome brain. Mounting evidence, however, indicates that intraneuronal accumulation of this peptide may also contribute to the cascade of neurodegenerative events that occur in AD and Down syndrome. A pathogenic role for intracellular Aβ is not without precedent, as it is known to be an early and integral component of the human muscle disorder inclusion body myositis (IBM). Therefore, it is plausible that intracellular Aβ may likewise induce cytopathic effects in the CNS, causing neuronal and synaptic dysfunction and perhaps even neuronal loss. Here we review recent evidence supporting a pathogenic role for intracellular Aβ in AD, Down syndrome, and IBM.

Functional consequences of radiation-induced oxidative stress in cultured neural stem cells and the brain exposed to charged particle irradiation

AIMS Redox homeostasis is critical in regulating the fate and function of multipotent cells in the central nervous system (CNS). Here, we investigated whether low dose charged particle irradiation could elicit oxidative stress in neural stem and precursor cells and whether radiation-induced changes in redox metabolism would coincide with cognitive impairment. RESULTS Low doses (<1 Gy) of charged particles caused an acute and persistent oxidative stress. Early after (<1 week) irradiation, increased levels of reactive oxygen and nitrogen species were generally dose responsive, but were less dependent on dose weeks to months thereafter. Exposure to ion fluences resulting in less than one ion traversal per cell was sufficient to elicit radiation-induced oxidative stress. Whole body irradiation triggered a compensatory response in the rodent brain that led to a significant increase in antioxidant capacity 2 weeks following exposure, before returning to background levels at week 4. Low dose irradiation was also found to significantly impair novel object recognition in mice 2 and 12 weeks following irradiation. INNOVATION Data provide evidence that acute exposure of neural stem cells and the CNS to very low doses and fluences of charged particles can elicit a persisting oxidative stress lasting weeks to months that is associated with impaired cognition. CONCLUSIONS Exposure to low doses of charged particles causes a persistent oxidative stress and cognitive impairment over protracted times. Data suggest that astronauts subjected to space radiation may develop a heightened risk for mission critical performance decrements in space, along with a risk of developing long-term neurocognitive sequelae.

Mitochondrial-Targeted Human Catalase Affords Neuroprotection From Proton Irradiation

Significant past work has linked radiation exposure of the CNS to elevated levels of oxidative stress and inflammation. These secondary reactive processes are both dynamic and persistent and are believed to compromise the functionality of the CNS, in part, by disrupting endogenous neurogenesis in the hippocampus. While evidence has shown neurogenesis to be sensitive to irradiation and redox state, the mechanistic basis underlying these effects is incompletely understood. To clarify the role of reactive oxygen species (ROS) in mediating radiation-induced changes in neurogenesis we have analyzed transgenic mice that overexpress human catalase localized to the mitochondria. With this model, we investigated the consequences of low dose and clinically relevant proton irradiation on neurogenesis, and how that process is modified in response to genetic disruption of mitochondrial ROS levels. In unirradiated animals, basal neurogenesis was improved significantly by reductions in mitochondrial ROS. In animals subjected to proton exposure, hippocampal progenitor cell proliferation was attenuated significantly by overexpression of human catalase in the mitochondria. Furthermore, expression of the MCAT transgene significantly improved neurogenesis in WT animals after low-dose proton exposure (0.5 Gy), with similar trends observed at higher dose (2 Gy). Our report documents for the first time the impact of proton irradiation on hippocampal neurogenesis, and the neuroprotective properties of reducing mitochondrial ROS through the targeted overexpression of catalase.

The Radiosensitivity of Satellite Cells: Cell Cycle Regulation, Apoptosis and Oxidative Stress

Abstract Skeletal muscles are the organ of movement, and their growth, regeneration and maintenance are dependent in large part on a population of myogenic stem cells known as satellite cells. Skeletal muscles and these resident myogenic stem cells (i.e., satellite cells) are commonly exposed to significant doses of radiation during diagnostic procedures and/or during the radiotherapeutic management of cancer. The main objective of this study was to examine the effects of clinically relevant doses of &ggr; radiation on satellite cell survival and proliferation, cell cycle regulation, apoptosis, DNA double-strand break repair, oxidative stress and NO production. Overall, our findings demonstrate that doses of &ggr; radiation ≥5 Gy reduced satellite cell numbers by at least 70% due in part to elevated apoptosis and the inhibition of cell cycle progression. Radiation was also found to cause a significant and persistent increase in the level of reactive oxygen and nitrogen species. Interestingly, and within this backdrop of elevated oxidative stress, similar doses were found to produce substantial reductions in the levels of nitric oxide (NO). Proliferation of satellite cells has been shown to depend in part on the production of NO, and our findings give rise to the possibility that radiation-induced reductions in NO levels may provide a mechanism for the inhibition of satellite cell proliferation in vitro and possibly the regrowth of skeletal muscle exposed during clinical irradiation procedures.

Characterizing low dose and dose rate effects in rodent and human neural stem cells exposed to proton and gamma irradiation

Past work has shown that exposure to gamma rays and protons elicit a persistent oxidative stress in rodent and human neural stem cells (hNSCs). We have now adapted these studies to more realistic exposure scenarios in space, using lower doses and dose rates of these radiation modalities, to further elucidate the role of radiation-induced oxidative stress in these cells. Rodent neural stem and precursor cells grown as neurospheres and human neural stem cells grown as monolayers were subjected to acute and multi-dosing paradigms at differing dose rates and analyzed for changes in reactive oxygen species (ROS), reactive nitrogen species (RNS), nitric oxide and superoxide for 2 days after irradiation. While acute exposures led to significant changes in both cell types, hNSCs in particular, exhibited marked and significant elevations in radiation-induced oxidative stress. Elevated oxidative stress was more significant in hNSCs as opposed to their rodent counterparts, and hNSCs were significantly more sensitive to low dose exposures in terms of survival. Combinations of protons and γ-rays delivered as lower priming or higher challenge doses elicited radioadaptive changes that were associated with improved survival, but in general, only under conditions where the levels of reactive species were suppressed compared to cells irradiated acutely. Protective radioadaptive effects on survival were eliminated in the presence of the antioxidant N-acetylcysteine, suggesting further that radiation-induced oxidative stress could activate pro-survival signaling pathways that were sensitive to redox state. Data corroborates much of our past work and shows that low dose and dose rate exposures elicit significant changes in oxidative stress that have functional consequences on survival.

Consequences of low dose ionizing radiation exposure on the hippocampal microenvironment.

The response of the brain to irradiation is complex, involving a multitude of stress inducible pathways that regulate neurotransmission within a dynamic microenvironment. While significant past work has detailed the consequences of CNS radiotherapy following relatively high doses (≥ 45 Gy), few studies have been conducted at much lower doses (≤ 2 Gy), where the response of the CNS (like many other tissues) may differ substantially from that expected from linear extrapolations of high dose data. Low dose exposure could elicit radioadaptive modulation of critical CNS processes such as neurogenesis, that provide cellular input into hippocampal circuits known to impact learning and memory. Here we show that mice deficient for chemokine signaling through genetic disruption of the CCR2 receptor exhibit a neuroprotective phenotype. Compared to wild type (WT) animals, CCR2 deficiency spared reductions in hippocampal neural progenitor cell survival and stabilized neurogenesis following exposure to low dose irradiation. While radiation-induced changes in microglia levels were not found in WT or CCR2 deficient animals, the number of Iba1+ cells did differ between each genotype at the higher dosing paradigms, suggesting that blockade of this signaling axis could moderate the neuroinflammatory response. Interestingly, changes in proinflammatory gene expression were limited in WT animals, while irradiation caused significant elevations in these markers that were attenuated significantly after radioadaptive dosing paradigms in CCR2 deficient mice. These data point to the importance of chemokine signaling under low dose paradigms, findings of potential significance to those exposed to ionizing radiation under a variety of occupational and/or medical scenarios.

Satellite Cells Say NO to Radiation

Abstract Skeletal muscles are commonly exposed to radiation for diagnostic procedures and the treatment of cancers and heterotopic bone formation. Few studies have considered the impact of clinical doses of radiation on the ability of satellite cells (myogenic stem cells) to proliferate, differentiate and contribute to recovering/maintaining muscle mass. The primary objective of this study was to determine whether the proliferation of irradiated satellite cells could be rescued by manipulating NO levels via pharmacological approaches and mechanical stretch (which is known to increase NO levels). We used both SNP (NO donor) and PTIO (NO scavenger) to manipulate NO levels in satellite cells. We observed that SNP was highly effective in rescuing the proliferation of irradiated satellite cells, especially at doses less than 5 Gy. The potential importance of NO was further illustrated by the effects of PTIO, which completely inhibited the rescue effect of SNP. Mechanical cyclic stretch was found to produce significant increases in NO levels of irradiated satellite cells, and this was associated with a robust increase in satellite cell proliferation. The effects of both radiation and NO on two key myogenic regulatory factors (MyoD and myogenin) were also explored. Irradiation of satellite cells produced a significant increase in both MyoD and myogenin, effects that were mitigated by manipulating NO levels via SNP. Given the central role of myogenic regulatory factors in the proliferation and differentiation of satellite cells, the findings of the current study underscore the need to more fully understand the relationship between radiation, NO and the functionality of satellite cells.