Estudio de los procesos de plasticidad a largo plazo en el hipocampo dorsal: papel del Canal Girk en modelos amiloidosis

La hiperexcitabilidad hipocampal, acontecimiento que se observa de manera temprana en la enfermedad de Alzheimer (EA), puede afectar a la generación de procesos de plasticidad a largo plazo fundamentales para la formación de memorias y el aprendizaje, capacidades cognitivas que se ven gravemente deterioradas en esta patología. En este escenario, los canales de K+ rectificadores de entrada acoplados a proteína-G (GirK), esenciales para el control de la excitabilidad neuronal en el hipocampo dorsal, podrían verse afectados durante la patogénesis de la EA. Por esta razón, estrategias dirigidas hacia el aumento de la actividad inhibitoria de los canales GirK, con el fin de restaurar el balance excitación/inhibición hipocampal necesario para el desarrollo adecuado de los procesos de plasticidad sináptica, podrían suponer un abordaje prometedor para la prevención de los déficits cognitivos asociados a esta enfermedad. En la presente tesis doctoral, hemos examinado el papel de la señalización dependiente de los canales GirK en los procesos de plasticidad a largo plazo en hipocampo dorsal de ratón. Asimismo, hemos estudiado la relación existente entre los canales GirK y la disfunción hipocampal originada por la patología amiloide, así como la posibilidad de modular su actividad con el fin de revertir las anomalías sinápticas y cognitivas que se observan en la EA. Para alcanzar este objetivo, hemos realizado un abordaje multidisciplinar utilizando modelos de amiloidosis generados por la administración de formas oligoméricas solubles del péptido A 1-42. Tras la modulación farmacológica de la actividad inhibitoria mediada por el canal GirK, hemos estudiado in vitro, por registros electrofisiológicos en rodajas, e in vivo, mediante pruebas de comportamiento, y posteriormente técnicas inmunohistoquímicas, la participación de los canales GirK en las alteraciones producidas por A 1-42 en la función hipocampal. Nuestros datos revelan que, en condiciones normales, la disrupción del balance excitación/inhibición en el hipocampo dorsal por el aumento o la disminución de la transmisión inhibitoria mediada por GirK provoca alteraciones en la inducción de procesos de plasticidad a largo plazo, lo que se traduce en déficits en la memoria y el aprendizaje. Sin embargo, en una situación de hiperexcitabilidad ocasionada por A 1-42, el aumento de la señalización dependiente de GirK restaura la excitabilidad neuronal hipocampal, y con ello la generación de los procesos de plasticidad sináptica necesarios para un funcionamiento cognitivo normal. En conjunto, estos resultados evidencian la necesidad de un nivel adecuado de actividad de los canales GirK en el hipocampo dorsal para la generación de los procesos de plasticidad que subyacen a la formación de memorias y el aprendizaje, y arronjan luz sobre el empleo de estos canales como una herramienta de control de la hiperexcitabilidad que se da en las fases más tempranas de la EA a fin de restablecer la función cognitiva

Systematic characterization of a non-transgenic Aβ 1–42 amyloidosis model: synaptic plasticity and memory deficits in female and male mice

Abstract Background The amyloid-β (Aβ) cascade is one of the most studied theories linked to AD. In multiple models, Aβ accumulation and dyshomeostasis have shown a key role in AD onset, leading to excitatory/inhibitory imbalance, the impairments of synaptic plasticity and oscillatory activity, and memory deficits. Despite the higher prevalence of Alzheimer’s disease (AD) in women compared to men, the possible sex difference is scarcely explored and the information from amyloidosis transgenic mice models is contradictory. Thus, given the lack of data regarding the early stages of amyloidosis in female mice, the aim of this study was to systematically characterize the effect of an intracerebroventricular (icv.) injection of Aβ 1–42 on hippocampal-dependent memory, and on associated activity-dependent synaptic plasticity in the hippocampal CA1–CA3 synapse, in both male and female mice. Methods To do so, we evaluated long term potentiation (LTP) with ex vivo electrophysiological recordings as well as encoding and retrieval of spatial (working, short- and long-term) and exploratory habituation memories using Barnes maze and object location, or open field habituation tasks, respectively. Results Aβ 1–42 administration impaired all forms of memory evaluated in this work, regardless of sex. This effect was displayed in a long-lasting manner (up to 17 days post-injection). LTP was inhibited at a postsynaptic level, both in males and females, and a long-term depression (LTD) was induced for the same prolonged period, which could underlie memory deficits. Conclusions In conclusion, our results provide further evidence on the shifting of LTP/LTD threshold due to a single icv. Aβ 1–42 injection, which underly cognitive deficits in the early stages of AD. These long-lasting cognitive and functional alterations in males and females validate this model for the study of early amyloidosis in both sexes, thus offering a solid alternative to the inconsistence of amyloidosis transgenic mice models

Hippocampal long-term synaptic depression and memory deficits induced in early amyloidopathy are prevented by enhancing G-protein-gated inwardly-rectifying potassium channel activity

Hippocampal synaptic plasticity disruption by amyloid-β (Aβ) peptides + thought to be responsible for learning and memory impairments in Alzheimer's disease (AD) early stage. Failures in neuronal excitability maintenance seems to be an underlying mechanism. G-protein-gated inwardly rectifying potassium (GirK) channels control neural excitability by hyperpolarization in response to many G-protein-coupled receptors activation. Here, in early in vitro and in vivo amyloidosis mouse models, we study whether GirK channels take part of the hippocampal synaptic plasticity impairments generated by Aβ1-42 . In vitro electrophysiological recordings from slices showed that Aβ1-42 alters synaptic plasticity by switching high-frequency stimulation (HFS) induced long-term potentiation (LTP) to long-term depression (LTD), which led to in vivo hippocampal-dependent memory deficits. Remarkably, selective pharmacological activation of GirK channels with ML297 rescued both HFS-induced LTP and habituation memory from Aβ1-42 action. Moreover, when GirK channels were specifically blocked by Tertiapin-Q, their activation with ML297 failed to rescue LTP from the HFS-dependent LTD induced by Aβ1-42 . On the other hand, the molecular analysis of the recorded slices by western blot showed that the expression of GIRK1/2 subunits, which form the prototypical GirK channel in the hippocampus, was not significantly regulated by Aβ1-42 . However, immunohistochemical examination of our in vivo amyloidosis model showed Aβ1-42 to down-regulate hippocampal GIRK1 subunit expression. Together, our results describe an Aβ-mediated deleterious synaptic mechanism that modifies the induction threshold for hippocampal LTP/LTD and underlies memory alterations observed in amyloidosis models. In this scenario, GirK activation assures memory formation by preventing the transformation of HFS-induced LTP into LTD.Spanish Ministry of Economy and Competitivity MINECO-FEDER, Grant (BFU2014-56164-P and BFU2017-82494-P)Fundación Tatiana Pérez de Guzmán el BuenoUniversity of Castilla la Mancha5.372 JCR (2020) Q1, 78/295 Biochemistry & Molecular Biology1.75 SJR (2020) Q1, 69/438 BiochemistryNo data IDR 2020UE

Impairments of Synaptic Plasticity Induction Threshold and Network Oscillatory Activity in the Hippocampus Underlie Memory Deficits in a Non-Transgenic Mouse Model of Amyloidosis

In early Alzheimer disease (AD) models synaptic failures and upstreaming aberrant patterns of network synchronous activity result in hippocampal-dependent memory deficits. In such initial stage, soluble forms of Amyloid-β (Aβ) peptides have been shown to play a causal role. Among different Aβ species, Aβ25–35 has been identified as the biologically active fragment, as induces major neuropathological signs related to early AD stages. Consequently, it has been extensively used to acutely explore the pathophysiological events related with neuronal dysfunction induced by soluble Aβ forms. However, the synaptic mechanisms underlying its toxic effects on hippocampal-dependent memory remain unresolved. Here, in an in vivo model of amyloidosis generated by intracerebroventricular injections of Aβ25–35 we studied the synaptic dysfunction mechanisms underlying hippocampal cognitive deficits. At the synaptic level, long-term potentiation (LTP) of synaptic excitation and inhibition was induced in CA1 region by high frequency simulation (HFS) applied to Schaffer collaterals. Aβ25–35 was found to alter metaplastic mechanisms of plasticity, facilitating long-term depression (LTD) of both types of LTP. In addition, aberrant synchronization of hippocampal network activity was found while at the behavioral level, deficits in hippocampal-dependent habituation and recognition memories emerged. Together, our results provide a substrate for synaptic disruption mechanism underlying hippocampal cognitive deficits present in Aβ25–35 amyloidosis model