Thesis (M.D.)--Boston University

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The ability of neurons to regulate their activity (homeostatic plasticity) is thought to be responsible for changes in neural responsiveness/gain induced by sensory deprivation, or augmented stimulation. For example, following auditory deprivation, excitatory and inhibitory synaptic transmission is strengthened and weakened, respectively. Abnormally high neural gain results in an 'over amplification' of spontaneous and stimulus-evoked firing rates, and may result in aberrant auditory perceptions including tinnitus and/or hyperacusis, respectively. The first manuscript in the thesis 'Pump up the Volume' (Chapter Three) provides a summary of the neural gain mechanism in the adult auditory system. Aspects of neural gain, including temporal characteristics and frequency specificity, had not been systematically investigated. Therefore, the aim of this thesis was to investigate characteristics of the neural gain mechanism. The thesis comprises three related studies involving normal hearing adult listeners: two studies involved short term sensory deprivation and one study involved short term augmented stimulation. The main outcome measures were the acoustic reflex threshold (ART), auditory brainstem response (ABR) and loudness. In Study One, the time course, frequency specificity and anatomical location of changes in the ART, following 6 days of unilateral earplug use (ca 30 dB attenuation at 2-4 kHz), were investigated. The reduction in ART in the treatment ear was greatest at day 4 and at frequencies most attenuated by the earplug. Ipsilateral and contralateral ARTs were similar when stimuli were presented to the treatment ear. ARTs were not statistically significant from baseline when measured 4 and 24 hours after earplug removal. In Study Two, the ART and ABR were measured at baseline and after 7 days of unilateral and bilateral hearing aid use (13-17 dB real ear insertion gain), to compare the effect of symmetrical and asymmetrical inputs. There was no change in ART and ABR after treatment, suggesting that the augmented stimulation was insufficient to modify neural gain. In Study Three, ARTs, ABRs and loudness were investigated after 4 days of unilateral earplug use (30 dB attenuation at 2-4 kHz). There was a significant reduction in ART (ca 6 dB) in the treatment ear, which returned to baseline within 1-2 hours of earplug removal. There was an unexpected but significant 35 nV decrease in the ABR wave V peak-to-trough amplitude in the treatment ear, and a 12 nV increase in the control ear. The change in ABR was opposite in direction to the change in ART. There was no change in loudness. The thesis has provided information on the threshold of deprivation/stimulation required to elicit a change in neural gain, along with the frequency specificity and temporal characteristics of the gain control mechanism. The anatomical location for changes in neural gain is around the level of the cochlear nucleus. The change in ABR was in the opposite direction to those predicted, but could be due a difference in the compensatory changes of contralateral and ipsilateral inputs at the level of the inferior colliculus.

Two different kinds of stabilising homeostatic behaviour have been observed in neurons. The first type involves the cell-autonomous maintenance of a cell-identity-based level of electrical activity. Neurons continually monitor their own electrical activity and can adjust many intracellular parameters, such as membrane ion channel densities, to keep this activity within a tight physiological range. The second type of homeostatic behaviour shares the same goal, to maintain a fixed level of electrical activity, but instead of adjusting intracellular parameters, the neuron recruits its synaptic partners to assist in maintaining a genetically prescribed activity level. This behaviour is most easily observed when a neuron is either electrically silenced by expressing an inwardly-rectifying potassium channel or rendered less sensitive to neurotransmitter through mutation of its postsynaptic receptors. Both of these perturbations result in increased synaptic drive from the presynaptic cells, either through increasing the number of neurotransmitter release sites or increasing the probability of release from single release sites. Many genes that are instrumental in the second type of homeostatic behaviour have been identified, mainly at the neuromuscular junction in the peripheral nervous system. However, studies on transsynaptic homeostatic compensation in an intact central nervous system have been few and far between. Also, which, if any, of the homeostatic genes are transcriptionally regulated in the nucleus after the onset of transsynaptic homeostatic adjustment, has not been adequately addressed. This thesis has developed a system to measure transcriptome-wide gene expression levels in presynaptic circuit elements after altering the firing properties of the downstream circuit in the CNS. Many transcriptionally regulated genes have been identified and are now being tested for their potential use as reporters for transsynaptic transcriptional regulation. It might be possible to capitalise on endogenous homeostatic signalling pathways to gain genetic access to synaptically connected neurons.

Background: Liver is a vital organ that plays a major role in glucose production and regulationthroughout the body (Musso et al., 2012). Liver disease has long been linked with insulin resistance (IR), dating back to 1906 (Megyesi et al., 1967). IR has been found to be prevalent in a range of liver diseases, including chronic Hepatitis C Virus (HCV), hemochromatosis, and alcoholic liver disease (Goswami et al., 2014). Liver disease is highly prevalent in the United States population with 30 million people (or one out of ten Americans) suffering from some type of liver disease (Peery et al., 2015). Although research demonstrates a significant relationship between liver disease and IR, the University of Arizona (UA) hepatology clinic does not currently screen liver disease patients for IR. Homeostatic model assessment for insulin resistance (HOMA-IR) score is used to study IR in non-insulin resistant population. HOMA-IR score is calculated using formula fasting plasma glucose (mmol/l) times fasting serum insulin (mU/l) divided by 22.5 (Bonora et al., 2002). Low HOMA-IR (HOMA< 2.0) values indicate high insulin sensitivity, whereas high HOMA-IR (HOMA> 2.0) values indicate low insulin sensitivity (insulin resistance) (Bonora et al., 2002). Objective: The purpose of this quality improvement (QI) project is to show the prevalence of IR in euglycemic liver disease patients at the UA hepatology clinic by using their HOMA-IR scores as a screening tool. By screening euglycemic liver disease patients for IR based on their HOMA-IR score, providers at the UA hepatology clinic can prevent liver disease progression and complications associated with IR early on. By doing so, the providers can improve the quality of care for liver disease patients. An essential part of calculating HOMA-IR is the availability of labs (serum glucose and serum insulin). A part of this QI project is to determine if the UA hepatology clinic has necessary labs to calculate HOMA-IR for euglycemic liver disease patients. A related matter is whether there is a correlation between liver disease patients' HOMA-IR score and Model for End-stage Liver Disease (MELD) score. If there is a direct correlation between HOMA-IR and MELD scores, providers can identify severity and progression of liver disease in euglycemic liver disease patients. Design: A case control retrospective study. Study Questions: 1) Do UA Hepatology clinic providers order sufficient labs (fasting plasma glucose and fasting plasma insulin) to calculate HOMA-IR in euglycemic patients? 2) What is the prevalence of IR in euglycemic liver patients indicated by HOMA-IR score? 3) Is there any correlation between HOMA-IR score and MELD score in euglycemic liver disease patients? Participants: Data will be collected from 1000 liver disease patients' at the UA hepatologyclinic, a tertiary level referral center. Settings: Banner University Medical Center (UMC) in Tucson, Arizona from January 1, 2011 until December 31, 2014. Measurements: HOMA-IR score using serum fasting glucose and serum fasting insulin levels laboratory values. MELD score to identify the severity of liver disease in euglycemic liver disease patients. Results: Among 1000 patients, 506 (60.5%) were found to have a previous diagnosis of T2DMand 395 (39.5 %) were euglycemic liver disease patients (Figure 1). Out of the 395 euglycemic liver disease patients, 217 (55%) participants were found to have both insulin level and glucose11level in their charts; 178 (45%) euglycemic liver disease patients were missing either insulin level or glucose level needed to calculate HOMA-IR score (Figure 2). Of the 217 euglycemic liver disease patients, 54.8% of had HOMA-IR> 2 and 45.2% patients had HOMA-IR<2 (Figure 3). The Pearson Correlation between HOMA-R>2 and MELD scores was 0.092 and the significance value using 2-tailed was 0.321 (Table 4). Conclusion: The results showed a significant high prevalence of IR in euglycemic patients with HOMA-IR score> 2 (54.8%) compare to those patients with HOMA-IR score<2 (45.2%). Furthermore, about 178 (45%) euglycemic liver disease patients were missing either insulin level or glucose level needed to calculate HOMA-IR score. This is a significant number of patients missing important labs to identify them as high risk for IR. This QI project identified HOMA-IRas an important screening tool that should be used both in hepatology clinics and primary healthcare settings. Use of such tool will lead to improved quality of care for euglycemic liver disease patients.

Maintaining the intrinsic excitability of neurons is crucial for stable brain activity. This can be achieved by the homeostatic regulation of membrane ion channel conductances, although it is not well understood how these processes influence broader aspects of neuron and network function. One of the many mechanisms which contribute towards this task is the modulation of potassium channel conductances by activity-dependent nitric oxide signalling. Here, we first investigate this mechanism in a conductance-based neuron model. By fitting the model to experimental data we find that nitric oxide signalling improves synaptic transmission fidelity at high firing rates, but that there is an increase in the metabolic cost of action potentials associated with this improvement. Although the improvement in function had been observed previously in experiment, the metabolic constraint was unknown. This additional constraint provides a plausible explanation for the selective activation of nitric oxide signalling only at high firing rates. In addition to mediating homeostatic control of intrinsic excitability, nitric oxide can diffuse freely across cell membranes, providing a unique mechanism for neurons to communicate within a network, independent of synaptic connectivity. We next conduct a theoretical investigation of the distinguishing roles of diffusive homeostasis mediated by nitric oxide in comparison with canonical non-diffusive homeostasis in cortical networks. We find that both forms of homeostasis robustly maintain stable activity. However, the resulting networks differ, with diffusive homeostasis maintaining substantial heterogeneity in activity levels of individual neurons, a feature disrupted in networks with non-diffusive homeostasis. This results in networks capable of representing input heterogeneity, and linearly responding over a broader range of inputs than those undergoing non-diffusive homeostasis. We further show that diffusive homeostasis interferes less than non-diffusive homeostasis in the synaptic weight dynamics of networks undergoing Hebbian plasticity. Overall, these results suggest a novel homeostatic mechanism for maintaining stable network activity while simultaneously minimising metabolic cost and conserving network functionality.

Stroke is the third leading cause of death and the primary cause of adult long-term disability in Canada. Despite advances in rehabilitation research, stroke survivors experience an unusually high incidence of depressive symptoms which undermine recovery outcomes by reducing patient motivation levels. Human and animal studies have linked the incidence of post-stroke depression and the extent of prefrontal cortex (PFC) damage. The PFC and the lateral habenula (LHb) are limbic structures that are strongly connected to the serotonergic dorsal raphe nucleus (DRN), a key neuronal hub for mood regulation. We hypothesized that PFC stroke produces a depressive phenotype by triggering maladaptive reorganization in mood-related networks. We used viral and optogenetic strategies to functionally characterize PFC and LHb projections to DRN. Moreover, we found that PFC stroke causes a time-dependent remodeling of LHb inputs to DRN 5-HT neurons which results in altered postsynaptic glutamate receptor number and subunit composition. This remodeling likely reflects a homeostatic upregulation of LHb-DRN synapses in response to stroke-induced challenge to network activity. Since these synapses encode stress and aversion, potentiation of this pathway could contribute to depressive symptoms following stroke. However, more work will be needed to identify the behavioral and network-level consequences of altered LHb-DRN dynamics. Thus, a deeper understanding of circuit mechanisms implicated in post-stroke depression will provide insights into this disease and open new treatment avenues to improve recovery.

Accumulation of amyloid beta (Aβ) in the brain is a pathological hallmark of Alzheimer's disease (AD) and has been shown to lead to synaptic dysfunction and cognitive decline. Recent studies have indicated synapse dysfunction as an early pathology in AD, but how synaptic function is altered by Aβ remains unclear. We hypothesize that neuronal functional stability may be altered by Aβ via dysregulation of homeostatic synaptic plasticity (HSP), a negative-feedback-based regulation that serves to restrain neuronal activity within a physiological range. Here, I show that Aβ can regulate HSP in response to activity deprivation with an over scaling up of postsynaptic AMPAR expression and excitatory synaptic currents. Aβ treatment during activity deprivation increases the surface expression of both calcium-permeable (Cp), GluA2-lacking (CpAMPARs) and regular, GluA2-containing AMPARs. This in turn may make neurons more vulnerable to neuronal injury after a toxic glutamatergic challenge. Homeostatic synaptic scaling requires the PI3K/Akt signaling pathway and expression of CpAMPARs. Consistent with this, I found that blockade of either PI3K or CpAMPARs occludes over-scaling in the presence of Aβ, suggesting that the enhancement of HSP is mediated through homeostatic mechanisms. Furthermore, challenging neurons with glutamate after Aβ-mediated enhancement of HSP shows increased neuronal death. These findings provide a novel mechanism by which Aβ alters neuronal plasticity and calcium homeostasis in the brain, suggesting that the HSP pathway may be a target in clinical treatment of Alzheimer's disease.

京都大学 / 0048 / 新制・論文博士 / 博士(医学) / 乙第13160号 / 論医博第2147号 / 新制||医||1029(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 生田 宏一, 教授 杉田 昌彦, 教授 椛島 健治 / 学位規則第4条第2項該当 / Doctor of Medical Science / Kyoto University / DFAM

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