Accumulation of Aβ and chronic inflammation are hallmark neuropathological findings that define Alzheimer’s Disease. Both Aβ and pro-inflammatory cytokines have been linked to poor cognitive function, likely owing to impaired neuronal signaling and attenuation of neurotrophic factor signaling. BDNF endosomal transport is a key signaling system facilitating many aspects of healthy brain function and synaptic plasticity. Furthermore, proper intracellular transport via the endosomal trafficking system is crucial to maintain steady-state BDNF signaling. This dissertation focuses on how Aβ and IL-1β act both synergistically and distinctly to induce a state of abnormal neuronal endosome transport of BDNF. Utilizing a specialized microfluidic isolation chamber for in vitro primary neuronal cultures, we demonstrate that Aβ oligomers compromise BDNF retrograde transport by impairing endosomal transport rate, resulting in impaired downstream signaling driven by BDNF. In a similar fashion, we show that IL-1β also attenuates BDNF endosomal trafficking flux and the dispersion of BDNF signaling endosomes throughout neurite networks in cultures. Distinct from Aβ however, the mechanism for IL-1β-induced deficits to BDNF endosomal signaling did not arise from impaired rate of transport of the BDNF-TrkB receptor complex. Rather, IL-1β may be associated with a Ub-dependent presynaptic sorting deficit. Our data suggests that ubiquitin C-terminal hydrolase L1 (UCH-L1), a deubiquitinating enzyme that functions to regulate cellular ubiquitin, mediates these trafficking deficits, since the irregularity in BDNF trafficking can be reversed by increasing cellular UCH-L1 levels. UCH-L1 is important for regulating neurotrophin receptor sorting and supporting retrograde transport. Thus, this work supports the idea that in AD or other neurological conditions where chronic inflammation is present, down-regulated UCH-L1 may drive BDNF trafficking deficits, compromising synaptic plasticity and neuronal survival. Collectively, the data indicate that both Aβ and IL-1β distinctly contribute to BDNF trafficking deficits, and that Aβ-induced deficits can be rescued in vitro by UCH-L1 overexpression.
Cytokines play crucial roles in the communication between brain cells including neurons and glia, as well as in the brain-periphery interactions. In the brain, cytokines modulate long-term potentiation (LTP), a cellular correlate of memory. Whether cytokines regulate LTP by direct effects on neurons or by indirect mechanisms mediated by non-neuronal cells is poorly understood. Elucidating neuron-specific effects of cytokines has been challenging because most brain cells express cytokine receptors. Moreover, cytokines commonly increase the expression of multiple cytokines in their target cells, thus increasing the complexity of brain cytokine networks even after single-cytokine challenges. Here, we review evidence on both direct and indirect-mediated modulation of LTP by cytokines. We also describe novel approaches based on neuron- and synaptosome-enriched systems to identify cytokines able to directly modulate LTP, by targeting neurons and synapses. These approaches can test multiple samples in parallel, thus allowing the study of multiple cytokines simultaneously. Hence, a cytokine networks perspective coupled with neuron-specific analysis may contribute to delineation of maps of the modulation of LTP by cytokines.
Emerging evidence suggests that histone modifications contribute to age-related cognitive decline. Our lab previously demonstrated that elevated H3K9me3 in aged mice leads to synaptic loss, cognitive impairment and a reduction in brain derived neurotrophic factor (BDNF). Treatment with ETP69, a selective inhibitor of H3K9me3’s catalyzing enzyme (SUV39H1), was shown to restore synapses, BDNF and cognitive performance. However, the mechanism underlying H3K9me3 regulation is poorly understood. In this study, we investigated the role of age-associated stressors such as oxidative stress in H3K9me3 elevation. The oxidative stressor hydrogen peroxide elevated the SUV39H1 regulator SIRT1 but did not increase H3K9me3. The aged brain is also marked by reduced BDNF, and we found that inhibiting BDNF signaling by blocking the BDNF receptor TrkB elevates H3K9me3 in an age-dependent fashion. Antioxidant treatment prevented the H3K9me3 elevation by TrkB-Fc, suggesting that inhibiting BDNF signaling regulates H3K9me3 via an oxidative stress-based mechanism. We further investigated if exercise, which stimulates BDNF production, regulates repressive H3K9me3 at the promoters of neuronal plasticity genes. Exercise decreased H3K9me3 at BDNF promoter VI in aged mice and stimulated BDNF production. Similarly, SUV39H1 inhibition decreased H3K9me3 at BDNF promoter VI in aged mice and showed a corresponding increase in BDNF VI expression. Exercise and SUV39H1 inhibition differentially affected BDNF and GABRA2 expression in young and old mice. H3K9me3 promoter binding at all neuronal plasticity genes except for GABBR1 decreased as mice aged. Overall, our data suggests that H3K9me3 and BDNF are engaged in a negative feedback mechanism that is affected by physical activity, oxidative stress and age.
The pattern separation task has recently emerged as a behavioral model of hippocampus function and has been used in several pharmaceutical trials. The canine is a useful model to evaluate a multitude of hippocampal-dependent cognitive tasks that parallel those in humans. Thus, this study was designed to evaluate the suitability of pattern separation task(s) for detecting age-related changes in canines. We also assessed the dogs' ability to show pattern separation and discrimination reversal, which provides a novel extension of the pattern separation learning literature. Our data show that aged dogs are impaired on a complex pattern separation task (six-well task) relative to easier tasks (four-well or six-well pattern discrimination task), and that the age-related deficits are due to loss of perceptual and inhibitory control in addition to the loss of spatial discrimination and pattern separation ability. Our data also suggest that aged animals show pattern separation deficits when the objects are brought progressively closer together while changing the location of both correct and incorrect objects. However, if the location of any one object is fixed the animals tend to use alternate strategies. Overall, these data provide important insight into age-related pattern separation deficits in a higher animal model and offers additional means for evaluating the impact of lifestyle and pharmaceutical interventions on episodic memory in preclinical trials.
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