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Open Access Publications from the University of California

Scholarly Works (17 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2018)

While it has been known for a while that patients with Alzheimer’s Disease (AD) have increased risk of unprovoked seizures, only recently has hyperexcitability become a major focus of AD research. Increased hippocampal activity in patients with mild cognitive impairment (MCI) was initially thought to be a beneficial compensatory mechanism; however, recent research has shown that patients with MCI treated with antiepileptic drugs have improved cognition. Synaptic zinc, co-released with amyloid-β (Aβ) during neurotransmission, is implicated in both oligomer formation and modulation of excitatory neurotransmission. Synaptic zinc is packaged into vesicles by ZnT3; thus, ZnT3-/- mice lack synaptically-released zinc. These mice exhibit increased seizure susceptibility, synaptic dysfunction, and age-dependent increases in markers of seizure activity, synaptic loss, and neurodegeneration, strikingly similar to AD mouse models. Although ZnT3-/- mice do not exhibit spontaneous convulsive seizures, our data suggest that these mice exhibit epileptiform activity. In addition, our data demonstrate that chronic, but not acute, treatment with the antiepileptic drug Levetiracetam (LEV) prevents age-related cognitive decline in these mice, suggesting a mechanism of action independent of antiseizure activity. In this regard, the mechanism by which LEV prevents cognitive decline is not understood.

The goal of this project was to begin characterizing the mechanism of action of LEV leading to the prevention of cognitive decline in aged ZnT3-/- mice. mRNA was extracted from 6-month-old ZnT3-/- and wildtype (WT) sex-matched mice and analyzed using NanoString. Gene analysis indicated sex-specific changes genes involved in epigenetics, neurogenesis, and hyperexcitability following LEV treatment. To investigate the effects of chronic LEV on neurogenesis important for learning and memory, ZnT3-/- mice were fed bromodeoxyuridine and immunohistochemistry was performed on brain sections. Although there was an increase in neurogenesis in LEV-treated animals, this increase was restricted to females, further suggesting a sex-specific effect of LEV. Lastly, ZnT3-/- mice were implanted with surface electroencephalogram (EEG) electrodes and the results suggest abnormal EEG activity in aged ZnT3-/- mice. Taken together, these results suggest that LEV prevents cognitive impairment in ZnT3-/- mice in a sex-specific manner at multiple layers of regulation, including epigenetics, neurogenesis, and hyperexcitability.

    Cover page: Levetiracetam prevents age-related cognitive impairment in a sex-specific manner in mice lacking synaptic zincCreative Commons 'BY-NC-ND' version 4.0 license
    • Thesis
    • Peer Reviewed
    UC Irvine Electronic Theses and Dissertations (2014)

    In recent years, soluble amyloid beta oligomers (AßO) have emerged as key elements in the cascade leading to synaptic dysfunction and neurodegeneration in Alzheimer's disease (AD). Previous work from our lab has shown that synaptic zinc released during excitatory neurotransmission increases the formation and accumulation of AßO at synaptic sites, and that the AßO-zinc interaction accelerates oligomer formation. Other recent research has shown that sequestration of synaptic zinc by AßO disrupts synaptic function, a significant finding as zinc has been demonstrated to modulate seizure activity and signaling pathways, has a high affinity for AßO and accumulates in Aß plaques in AD brain.

    We sought to investigate markers of seizure activity in zinc transporter ZnT3 knockout (ZnT3KO) mice, which lack synaptic zinc, and the effects of treatment with an anti-seizure drug on the cognitive impairment demonstrated by aged ZnT3KO mice. Hippocampus tissue collected from age-matched cohorts of wild type and ZnT3KO were assayed for markers of seizure activity, finding age-dependent alterations in levels of these markers consistent with seizure activity in ZnT3KO. To study the effects of seizure activity on cognition, memory was assessed after acute treatment with an anti-seizure drug, finding no significant improvement in six month old ZnT3KO.

    We also investigated alterations in neurotrophic signaling pathways in acute hippocampal slices from ZnT3KO mice. Neurotrophic protein expression and phosphorylation were assessed, finding reduction in basal and activity-dependent levels of Erk1/2 and p-Erk1/2, reduction in AKT, age-dependent reduction in BDNF, and reduction in activity-dependent BDNF mRNA expression. Pharmacological treatments of acute hippocampus sections were performed to investigate the effects of Zn2+ on activation of Erk1/2 through NMDA receptors and receptor subunits NR2A and NR2B, finding that Zn2+ inhibits NR2B-mediated activation of Erk1/2. Neurodegeneration was assessed through histochemistry of age-matched hippocampus sections, demonstrating an increase in neurodegeneration in aged ZnT3KO.

    Collectively, the results discussed in this dissertation support the hypothesis that dysregulation of synaptic zinc results in excessive excitatory neuronal activity and neurodegenerative alterations in signaling pathways. Consequently, therapeutics targeting maintenance of zinc homeostasis and reduction of AßO interference with zinc neurotransmission may prove beneficial in managing neuronal hyperactivity and neurodegenerative changes in AD.

      Cover page: The Role of Zinc in NeurotransmissionCreative Commons 'BY-NC-ND' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2012)
      Genome-wide gene deregulation and oxidative stress appear to be critical factors determining the high variability of phenotypes in Down's syndrome (DS). Even though individuals with trisomy 21 exhibit a higher survival rate compared to other aneuploidies, most of them die in utero or early during postnatal life. While the survivors are currently predicted to live past 60 years, they suffer higher incidence of age-related conditions including Alzheimer's disease (AD). This paper is centered on the mechanisms by which mitochondrial factors and oxidative stress may orchestrate an adaptive response directed to maintain basic cellular functions and survival in DS. In this context, the timing of therapeutic interventions should be carefully considered for the successful treatment of chronic disorders in the DS population.
        Cover page: Oxidative Stress and Mitochondrial Dysfunction in Down’s Syndrome: Relevance to Aging and DementiaCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2010)
        Background

        Down's syndrome (DS) is the most common genetic cause of mental retardation. Reduced number and aberrant architecture of dendritic spines are common features of DS neuropathology. However, the mechanisms involved in DS spine alterations are not known. In addition to a relevant role in synapse formation and maintenance, astrocytes can regulate spine dynamics by releasing soluble factors or by physical contact with neurons. We have previously shown impaired mitochondrial function in DS astrocytes leading to metabolic alterations in protein processing and secretion. In this study, we investigated whether deficits in astrocyte function contribute to DS spine pathology.

        Methodology/Principal Findings

        Using a human astrocyte/rat hippocampal neuron coculture, we found that DS astrocytes are directly involved in the development of spine malformations and reduced synaptic density. We also show that thrombospondin 1 (TSP-1), an astrocyte-secreted protein, possesses a potent modulatory effect on spine number and morphology, and that both DS brains and DS astrocytes exhibit marked deficits in TSP-1 protein expression. Depletion of TSP-1 from normal astrocytes resulted in dramatic changes in spine morphology, while restoration of TSP-1 levels prevented DS astrocyte-mediated spine and synaptic alterations. Astrocyte cultures derived from TSP-1 KO mice exhibited similar deficits to support spine formation and structure than DS astrocytes.

        Conclusions/Significance

        These results indicate that human astrocytes promote spine and synapse formation, identify astrocyte dysfunction as a significant factor of spine and synaptic pathology in the DS brain, and provide a mechanistic rationale for the exploration of TSP-1-based therapies to treat spine and synaptic pathology in DS and other neurological conditions.

          Cover page: A Role for Thrombospondin-1 Deficits in Astrocyte-Mediated Spine and Synaptic Pathology in Down's SyndromeCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2018)
          Abnormal dendritic spine structure and function is one of the most prominent features associated with neurodevelopmental disorders including Down syndrome (DS). Defects in both spine morphology and spine density may underlie alterations in neuronal and synaptic plasticity, ultimately affecting cognitive ability. Here we briefly examine the role of astrocytes in spine alterations and more specifically the involvement of astrocyte-secreted thrombospondin 1 (TSP-1) deficits in spine and synaptic pathology in DS.
            Cover page: Dendritic spine pathology and thrombospondin-1 deficits in Down syndrome
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2010)

            Background

            Down's syndrome (DS) is the most common genetic cause of mental retardation. Reduced number and aberrant architecture of dendritic spines are common features of DS neuropathology. However, the mechanisms involved in DS spine alterations are not known. In addition to a relevant role in synapse formation and maintenance, astrocytes can regulate spine dynamics by releasing soluble factors or by physical contact with neurons. We have previously shown impaired mitochondrial function in DS astrocytes leading to metabolic alterations in protein processing and secretion. In this study, we investigated whether deficits in astrocyte function contribute to DS spine pathology.

            Methodology/principal findings

            Using a human astrocyte/rat hippocampal neuron coculture, we found that DS astrocytes are directly involved in the development of spine malformations and reduced synaptic density. We also show that thrombospondin 1 (TSP-1), an astrocyte-secreted protein, possesses a potent modulatory effect on spine number and morphology, and that both DS brains and DS astrocytes exhibit marked deficits in TSP-1 protein expression. Depletion of TSP-1 from normal astrocytes resulted in dramatic changes in spine morphology, while restoration of TSP-1 levels prevented DS astrocyte-mediated spine and synaptic alterations. Astrocyte cultures derived from TSP-1 KO mice exhibited similar deficits to support spine formation and structure than DS astrocytes.

            Conclusions/significance

            These results indicate that human astrocytes promote spine and synapse formation, identify astrocyte dysfunction as a significant factor of spine and synaptic pathology in the DS brain, and provide a mechanistic rationale for the exploration of TSP-1-based therapies to treat spine and synaptic pathology in DS and other neurological conditions.
              Cover page: A Role for Thrombospondin-1 Deficits in Astrocyte-Mediated Spine and Synaptic Pathology in Down's SyndromeCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2013)
              Mitochondrial dysfunction and oxidative stress are common features of Down syndrome (DS). However, the underlying mechanisms are not known. We investigated the relationship between abnormal energy metabolism and oxidative stress with transcriptional and functional changes in DS cells. Impaired mitochondrial activity correlated with altered mitochondrial morphology. Increasing fusion capacity prevented morphological but not functional alterations in DS mitochondria. Sustained stimulation restored mitochondrial functional parameters but increased reactive oxygen species production and cell damage, suggesting that reduced DS mitochondrial activity is an adaptive response for avoiding injury and preserving basic cellular functions. Network analysis of genes overexpressed in DS cells demonstrated functional integration in pathways involved in energy metabolism and oxidative stress. Thus, although preventing extensive oxidative damage, mitochondrial downregulation may contribute to increased susceptibility of individuals with DS to clinical conditions in which altered energy metabolism may play a role, such as Alzheimer's disease, diabetes, and some types of autistic spectrum disorders.
                Cover page: Adaptive Downregulation of Mitochondrial Function in Down SyndromeCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2017)
                Wireless electroencephalography (EEG) of small animal subjects typically utilizes miniaturized EEG devices which require a robust recording and electrode assembly that remains in place while also being well-tolerated by the animal so as not to impair the ability of the animal to perform normal living activities or experimental tasks. We developed simple and fast electrode assembly and method of electrode implantation using electrode wires and wire-wrap technology that provides both higher survival and success rates in obtaining recordings from the electrodes than methods using screws as electrodes. The new wire method results in a 51% improvement in the number of electrodes that successfully record EEG signal. Also, the electrode assembly remains affixed and provides EEG signal for at least a month after implantation. Screws often serve as recording electrodes, which require either drilling holes into the skull to insert screws or affixing screws to the surface of the skull with adhesive. Drilling holes large enough to insert screws can be invasive and damaging to brain tissue, using adhesives may interfere with conductance and result in a poor signal, and soldering screws to wire leads results in fragile connections. The methods presented in this article provide a robust implant that is minimally invasive and has a significantly higher success rate of electrode implantation. In addition, the implant remains affixed and produces good recordings for over a month, while using economical, easily obtained materials and skills readily available in most animal research laboratories.
                  Cover page: Low Cost Electrode Assembly for EEG Recordings in MiceCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2014)
                  Amyloid precursor protein (APP), encoded on Hsa21, functions as a cell adhesion molecule (CAM) in axonal growth cones (GCs) of the developing brain. We show here that axonal GCs of human fetal Down syndrome (DS) neurons (and of a DS mouse model) overexpress APP protein relative to euploid controls. We investigated whether DS neurons generate an abnormal, APP-dependent GC phenotype in vitro. On laminin, which binds APP and β1 integrins (Itgb1), DS neurons formed enlarged and faster-advancing GCs compared to controls. On peptide matrices that bind APP only, but not on those binding exclusively Itgb1 or L1CAM, DS GCs were significantly enlarged (2.0-fold), formed increased close adhesions (1.8-fold), and advanced faster (1.4-fold). In assays involving alternating stripes of monospecific matrices, human control GCs exhibited no preference for any of the substrates, whereas DS GCs preferred the APP-binding matrix (cross-over decreased significantly from 48.2 to 27.2%). Reducing APP expression in DS GCs with siRNA normalized most measures of the phenotype, including substrate choice. These experiments show that human DS neurons exhibit an APP-dependent, abnormal GC phenotype characterized by increased adhesion and altered contact guidance. The results suggest that APP overexpression may perturb axonal pathfinding and circuit formation in developing DS brain.
                    Cover page: Dosage of amyloid precursor protein affects axonal contact guidance in Down syndromeCreative Commons 'BY' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2018)
                    Mounting evidence implicates chronic oxidative stress as a critical driver of the aging process. Down syndrome (DS) is characterized by a complex phenotype, including early senescence. DS cells display increased levels of reactive oxygen species (ROS) and mitochondrial structural and metabolic dysfunction, which are counterbalanced by sustained Nrf2-mediated transcription of cellular antioxidant response elements (ARE). Here, we show that caspase 3/PKCδdependent activation of the Nrf2 pathway in DS and Dp16 (a mouse model of DS) cells is necessary to protect against chronic oxidative damage and to preserve cellular functionality. Mitochondria-targeted catalase (mCAT) significantly reduced oxidative stress, restored mitochondrial structure and function, normalized replicative and wound healing capacity, and rendered the Nrf2-mediated antioxidant response dispensable. These results highlight the critical role of Nrf2/ARE in the maintenance of DS cell homeostasis and validate mitochondrial-specific interventions as a key aspect of antioxidant and antiaging therapies.
                      Cover page: Nrf2 stabilization prevents critical oxidative damage in Down syndrome cellsCreative Commons 'BY' version 4.0 license
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