Repository logo

Browsing by Author "Tsunoda, Susan, advisor"

Now showing 1 - 4 of 4
  • Thumbnail Image
    ItemOpen Access
    Age-dependent decline in Kv4 channels, underlying molecular mechanisms, and potential consequences for coordinated motor function
    (Colorado State University. Libraries, 2019) Vallejos, Maximiliano Jose, author; Tsunoda, Susan, advisor; Amberg, Gregory C., committee member; Bouma, Gerrit, committee member; Mykles, Donald, committee member; Tamkun, Michael, committee member
    The voltage-gated potassium channel, Kv4, is widely expressed in the central nervous system and it is responsible for a highly conserved rapidly inactivating A-type K+ current. Kv4 channels play a role in the regulation of membrane excitability, contributing to learning/memory and coordinated motor function. Indeed, recent genetic and electrophysiological studies in Drosophila have linked Kv4 A-type currents to repetitive rhythmic behaviors. Because a deterioration in locomotor performance is a hallmark of aging in all organisms, we were interested in examining the effects of age on Kv4/Shal channel protein. In this dissertation, I use Drosophila as a model organism to characterize an age-dependent decline in Kv4/Shal protein levels that contributes to the decline in coordinated motor performance in aging flies. Our findings suggest that accumulation of hydrogen peroxide (H2O2) is amongst the molecular mechanisms that contribute to the age-dependent decline of Kv4/Shal. We show that an acute in vivo H2O2 exposure to young flies leads to a decline of Kv4/Shal protein levels, and that expression of Catalase in older flies results in an increase in levels of Kv4/Shal and improved locomotor performance. We also found that the scaffolding protein SIDL plays a role in maintaining Kv4/Shal protein levels and that SIDL mRNA declines with age, suggesting that an age-dependent loss of SIDL may also lead to Kv4/Shal loss. In behavioral studies, we found that a knockdown of SIDL resulted in a lethal phenotype, leading to a large decline in Drosophila eclosion rates, an event that requires coordinated peristaltic motions. Expression of SIDL or Kv4/Shal in this SIDL knockdown genetic background resulted in a partial rescue; these results are consistent with a model in which SIDL and Kv4/Shal play a role in coordinated peristaltic motions and are required for successful eclosion. The results presented in this dissertation provide new insight into the possible molecular mechanisms that underlie an age-dependent decline in Kv4/Shal protein. We identify two contributing factors: 1) ROS accumulation, and 2) the interacting protein SIDL. Our data also suggests that this age-dependent decline in Kv4/Shal levels is likely to be conserved across species, at least in some brain regions. Because Kv4/Shal channels have been implicated in the regulation of long-term potentiation and in repetitive rhythmic behaviors, the loss of Kv4/Shal may contribute to the age-related decline in learning/memory and motor function.
  • Thumbnail Image
    ItemOpen Access
    Cholinergic synaptic homeostasis is regulated by Drosophila α7 nicotinic acetylcholine receptors and Kv4 potassium channels
    (Colorado State University. Libraries, 2021) Eadaim, Abdunaser Omar, author; Tsunoda, Susan, advisor; Tamkun, Michael, committee member; Amberg, Gergory, committee member; Bouma, Gerrit, committee member; Clay, Colin, committee member; DeLuca, Jennifer, committee member
    Homeostatic synaptic plasticity (HSP) is an important mechanism that stabilizes neural activity during changes that occur during development and learning and memory formation, and some pathological conditions. HSP in cholinergic neurons has been implicated in pathological conditions, such as Alzheimer's disease and nicotine addiction. In a previous study in primary Drosophila neuron culture, cholinergic activity was blocked using pharmacological tools and this induced a homeostatic response that was mediated by an increase in the Drosophila α7 (Dα7) nAChR, which was subsequently tuned by an increase in the voltage-dependent potassium channel, Kv4/Shal. In this study, we inhibit cholinergic activity in live flies using temperature-sensitive mutant alleles of the choline acetyltransferase gene (Chats2 mutants). We show that this in vivo activity inhibition induces HSP similarly mediated by Dα7 nAChRs followed by an up-regulation of Kv4/Shal. We show that the up-regulation of Dα7 nAChRs alone is sufficient to induce an increase in Kv4/Shal protein, as well as mRNA. Finally, we test the involvement of transcription factors, dCREB2 and nuclear factor of activated T cells (NFAT) in the up-regulation of Kv4/Shal. In particular, we find that NFAT is required for the inactivity-induced up-regulation of Kv4/Shal channels. Our studies reveal a novel receptor-ion channel system transcriptionally coupled to prevent over-excitation.
  • Thumbnail Image
    ItemEmbargo
    miR-137 regulates PTP61F, affecting insulin signaling, metabolic homeostasis, and starvation resistance in Drosophila melanogaster
    (Colorado State University. Libraries, 2023) Saedi, Hana Ibrahim, author; Tsunoda, Susan, advisor; Hoerndli, Frederic, committee member; Amberg, Gregory, committee member; Di Pietro, Santiago, committee member
    miR-137 is a highly conserved brain-enriched microRNA (miRNA) that has been associated with neuronal function and proliferation. Here, we show that Drosophila miR-137 null mutants display increased body weight with enhanced triglyceride and glucose levels and decreased locomotor activity. When challenged by nutrient deprivation, miR-137 mutants exhibit reduced motivation to feed and significantly prolonged survival. Together, these phenotypes suggest a new role for miR-137 in energy homeostasis. Genetic epistasis experiments show that the starvation resistance of miR-137 mutants involves the insulin signaling pathway, and that loss of miR-137 results in drastically reduced phosphorylation/activation of the single insulin receptor, InR, in Drosophila. We explore the possibility that the protein tyrosine phosphatase61F (PTP61F), ortholog of TC-PTP/PTP1B, known to dephosphorylate InR across species, is a potential in vivo target of miR-137. We show that loss of miR-137 results in upregulation of an endogenously tagged PTP61F protein, and that genetically increasing levels of PTP61F mimics the loss of phosphorylated InR and increased starvation resistance seen in miR-137 mutants. Finally, we show that the enhanced starvation resistance of miR-137 mutants is normalized by activation of the insulin signaling pathway in the nervous system. Our study introduces miR-137 as a new player in the regulation of central insulin signaling and metabolic homeostasis.
  • Thumbnail Image
    ItemOpen Access
    Na+ -activated K+ channels protect against overexcitation and seizure-like behavior in Drosophila
    (Colorado State University. Libraries, 2021) Byers, Nathan S., author; Tsunoda, Susan, advisor; Garrity, Deborah, committee member; Hentges, Shane, committee member; Hoerndli, Frederic, committee member; Tamkun, Michael, committee member
    Na+-activated K+ channels (KNa) encode K+ channels that are activated by internal Na+ and are widely expressed throughout the mammalian central nervous system. Based on the biophysical properties of the channels, it has long been postulated that they act as a reserve mechanism to combat neuronal overexcitation. Specifically, early electrophysiological recordings suggested that only when intracellular Na+ levels rise significantly, for instance in neuropathological conditions, do KNa channels become active. More recent evidence suggests that they may function under normal physiological circumstances by means of binding cytoplasmic factors and via the persistent Na+ current. However, to date it is unclear if KNa channels function to prevent overexcitation in vivo. Therefore, research in my dissertation sets out to test the hypothesis that KNa channels protect against overexcitation in Drosophila models of epilepsy. Drosophila contain one gene encoding a KNa channel, dSlo2. In the third chapter of this dissertation, I examine expression of dSlo2 channels throughout the nervous system. Findings from this chapter show that dSlo2 channels are expressed in cholinergic neurons, the main excitatory neuron of the Drosophila brain. Furthermore, dSlo2 channels were excluded from GABAergic neurons. I additionally found that dSlo2 channels are localized to axonal regions of multiple neuronal subtypes in the nervous system. Thus, these results suggest that as K+ channels widely and preferentially expressed in excitatory neurons in the brain, dSlo2 channels may function to dampen neuronal, and perhaps behavioral, excitability. In Chapter 4, I test the hypothesis that dSlo2 channels protect against behavioral abnormalities caused by cholinergic overexcitation. I first show that the loss of dSlo2 exacerbates behavioral deficits and death associated with prolonged exposure to a cholinergic agonist, Imidacloprid. Furthermore, I found that adult flies lacking dSlo2 exhibit mechanically induced seizure-like behavior following feeding of Imidacloprid, which does not occur in wild-type flies. Combined, these results suggest that dSlo2 channels do indeed protect against cholinergic overexcitation. It has previously been shown that mammalian KNa channels are activated by a persistent Na+ current (INaP) in neurons, suggesting that these channels may ameliorate behavioral consequences of an increased INaP in vivo. In Chapter 5, I test the hypothesis that dSlo2 channels protect against Drosophila seizure-like behavior induced by an increased INaP. I find that the loss of dSlo2 significantly exacerbates seizure-like behavior in multiple Drosophila epileptic models, including a model for human generalized epilepsy with febrile seizures plus (GEFS+). Additionally, the absence of dSlo2 worsens seizure-like behavior when flies are exposed to Veratridine, a pharmacological agent known to increase INaP. Interestingly, the loss of dSlo2 also revealed a spontaneous seizure phenotype in INaP-affected seizure models that was otherwise absent. Altogether, these results are consistent with the model that KNa channels are activated by INaP, and protect against seizure-like behavior actuated by increased INaP. Overall, the work in my dissertation expands our understanding of the role of KNa channels. These findings suggest that KNa channels may play a protective role for many neuropathological diseases associated with an increased INaP, such as epilepsy, amyotrophic lateral sclerosis, neuropathic pain, and ischemia.