Estudio del papel de la proteína Tau en la modulación de la neurogénesis hipocampal adulta

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura: 17-07-2015Alterations in tau protein metabolism are found in a subset of neurodegenerative disorders known as tauopathies, the most common of which is Alzheimer´s disease. As several tauopathies are accompanied by memory deficits, and adult hippocampal neurogenesis is crucial for learning and memory, tau protein might be involved in this process. In fact, Tau isoforms with three microtubule‐binding domains (Tau3R) and high phosphorylation levels are found in neurons during embryonic development and also in murine adult newborn neurons, but not in mature neurons in the adult brain. Therefore, the aim of this work was to study the role of Tau protein on adult hippocampal neurogenesis. Besides, since adult neurogenesis is regulated by different environmental factors, we were aimed to investigate the role of Tau in the regulation of adult neurogenesis that either pro‐ or anti‐ neurogenic stimuli exert. We have studied Tau knockout mice (Dawson et al., 2001) to analyze the generation and maturation of newborn neurons in the dentate gyrus. We have used three different approaches to study the effect of lacking tau protein on this region: thymidine analogs (in order to study cell survival at different time points), PSD95‐GFP retrovirus infection (to analyze the morphology and connectivity of newborn neurons), and specific markers (to analyze the different stages of newborn cells maturation). These methodologies were combined with the use of several behavioral tests aimed to analyze the effect of lacking tau protein on hippocampal‐dependent memory (Novel Object Location Preference test) and mood‐related behaviors (Elevated Plus Maze and Porsolt tests). As a pro‐neurogenic stimulus we have used environmental enrichment, and as an anti‐neurogenic stimulus we have used the acute stressor induced by the Porsolt test. The results showed, first, that lacking Tau protein has no effects on adult hippocampal neurogenesis rate under basal conditions. However, it impairs dendritic development and results in postsynaptic density (PSD) alterations. Nevertheless, these alterations are not accompanied by impairments of behavioral pattern separation tasks nor by the appearance of depression‐like behavior, although an increase in anxiety‐like behavior was detected in these mice. On the other hand, lacking Tau protein weakened the pro‐neurogenic effects of environmental enrichment on adult hippocampal neurogenesis. It resulted in an attenuated increase in adult neurogenesis rate. Besides, nor dendritogenesis nor synaptic adaptations to the higher activity demand took place on mature newborn neurons in absence of Tau as they did in the wild type mice. Finally, lacking Tau protein exerted neuroprotective effects on adult hippocampal neurogenesis after acute stress. It partially protected immature newborn neurons against Porsolt test induced cell death. Moreover, after newborn neurons completed their maturation, the lack of Tau protein was still neuroprotective with regard to stress induced damage in their dendrites and synaptic connections. Summarizing, lack of Tau protein does not seem to produce marked alterations in the basal rate of adult hippocampal neurogenesis, although Tau seems to be involved in newborn neuron maturation. In addition, it plays a crucial role in the modulation of adult hippocampal neurogenesis exerted by either positive or negative stimuli

Maturation dynamics of the axon initial segment (AIS) of newborn dentate granule cells in young adult C57BL/6J mice

This published version will be available under a Creative Commons Attribution 4.0 International License (CC-BY) after a 6 month periodNewborn dentate granule cells (DGCs) are generated in the hippocampal dentate gyrus (DG) of rodents through a process called adult hippocampal neurogenesis, which is subjected to tight intrinsic and extrinsic regulation. The use of retroviruses encoding fluorescent proteins has allowed the characterization of the maturation dynamics of newborn DGCs, including their morphological development and the establishment and maturation of their afferent and efferent synaptic connections. However, the study of a crucial cellular compartment of these cells, namely, the axon initial segment (AIS), has remained unexplored to date. The AIS is not only the site of action potential initiation, but it also has a unique molecular identity that makes it one of the master regulators of neural plasticity and excitability. Here we examined the dynamics of AIS formation in newborn DGCs of young female adult C57BL/6J mice in vivo. Our data reveal notable changes in AIS length and thickness throughout cell maturation under physiological conditions and show that the most remarkable structural changes coincide with periods of intense morphological and functional remodeling. Moreover, we demonstrate that AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuliThis work was supported by the Spanish Ministry of Economy and Competitiveness SAF-2017-82185-R and RYC-2015-171899 to M.L.-M. and SAF-2014-53040-P to J.Á., Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas to J.Á., Alzheimer's Association 2015-NIRG-340709 and AARG-17-528125 to M.L.-M., Association for Frontotemporal Degeneration 2016 Basic Science Pilot Grant Award to M.L.-M., and Fundación Ramón Areces and Banco de Santander to the Centro de Biología Molecular Severo Ochoa Institutional Grant

Activity-Dependent Reconnection of Adult-Born Dentate Granule Cells in a Mouse Model of Frontotemporal Dementia

Frontotemporal dementia (FTD) is characterized by neuronal loss in the frontal and temporal lobes of the brain. Here, we provide the first evidence of striking morphological alterations in dentate granule cells (DGCs) of FTD patients and in a mouse model of the disease, Tau VLW mice. Taking advantage of the fact that the hippocampal dentate gyrus (DG) gives rise to newborn DGCs throughout the lifetime in rodents, we used RGB retroviruses to study the temporary course of these alterations in newborn DGCs of female Tau VLW mice. In addition, retroviruses that encode either PSD95:GFP or Syn:GFP revealed striking alterations in the afferent and efferent connectivity of newborn Tau VLW DGCs, and monosynaptic retrograde rabies virus tracing showed that these cells are disconnected from distal brain regions and local sources of excitatory innervation. However, the same cells exhibited a predominance of local inhibitory innervation. Accordingly, the expression of presynaptic and postsynaptic markers of inhibitory synapses was markedly increased in the DG of Tau VLW mice and FTD patients. Moreover, an increased number of neuropeptide Y-positive interneurons in the DG correlated with a reduced number of activated egr-1 + DGCs in Tau VLW mice. Finally, we tested the therapeutic potential of environmental enrichment and chemoactivation to reverse these alterations in mice. Both strategies reversed the morphological alterations of newborn DGCs and partially restored their connectivity in a mouse model of the disease. Moreover, our data point to remarkable morphological similarities between the DGCs of Tau VLW mice and FTD patients.Fil: Terreros Roncal, Julia. Universidad Autónoma de Madrid. Facultad de Ciencias; EspañaFil: Flor García, Miguel. Universidad Autónoma de Madrid. Facultad de Ciencias; EspañaFil: Moreno Jiménez, Elena P.. Universidad Autónoma de Madrid. Facultad de Ciencias; EspañaFil: Pallas Bazarra, Noemí. Universidad Autónoma de Madrid. Facultad de Ciencias; EspañaFil: Rábano, Alberto. No especifíca;Fil: Sah, Nirnath. National Institutes of Health; Estados UnidosFil: Van Praag, Henriette. National Institutes of Health; Estados UnidosFil: Giacomini, Damiana Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Schinder, Alejandro Fabián. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquímicas de Buenos Aires. Fundación Instituto Leloir. Instituto de Investigaciones Bioquímicas de Buenos Aires; ArgentinaFil: Ávila, Jesús. Universidad Autónoma de Madrid. Facultad de Ciencias; EspañaFil: Llorens Martín, Maria. Universidad Autónoma de Madrid. Facultad de Ciencias; Españ

Tau is required for the function of extrasynaptic NMDA receptors

Tau is a microtubule-associated neuronal protein found mainly in axons. However, increasing evidence indicates that it is also present in dendrites, where it serves as an essential mediator of synaptic NMDA (N-methyl-D-aspartate) receptor-dependent excitotoxicity. Of note, NMDA receptors can also be found outside synapses in the plasma membrane, and activation of extrasynaptic NMDA receptors has been shown to be more linked to excitotoxicity than the activation of synaptic ones. Little is known about the role of Tau in the activity of extrasynaptic NMDA receptors. Thus, we have used a Tau knockout mouse model (Tau mice) to analyze the consequences of Tau absence in extrasynaptic NMDA receptor activity. We demonstrate that absence of Tau leads to a decrease in functional extrasynaptic NMDA receptors in the hippocampus. We propose that this impairment in extrasynaptic NMDA receptor activity may contribute to the well-known neuroprotective effect associated with Tau deficiency under pathological conditions.Spanish Ministry of Economy and Competitiveness (SAF-2014-53040-P to Jesús Ávila and CSD-2010-00045, SAF-2011-24730 and SAF2014-57233-R to José A. Esteban) and the Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED, ISCIII) (Jesús Ávila). Institutional grants from the Fundación Ramón Areces and Banco Santande

Tau and neuron aging

Tau protein could appear like a family of multiple isoforms rising by alternative splicing of its nuclear RNA or by different posttranslational modifications. The levels (or proportion) of these different tau isoforms could change in different neurons during development, aging or disease (tauopathies) in mammals. It is discussed that in some disorders there is a gain of toxic function of modified tau, due to the phosphorylation or aggregation of tau protein. These phenotypic changes are mainly found in aging organisms. On the other hand, loss of tau function could facilitate the appearance of some defects (related to iron toxicity) in aging animals lacking tau.stry of Health (SAF 2011-24841), Comunidad de Madrid (S2010/BMD2331), Fundación M. Botín and an institutional grant from Fundación R. Areces

Maturation Dynamics of the Axon Initial Segment (AIS) of Newborn Dentate Granule Cells in Young Adult C57BL/6J Mice

Newborn dentate granule cells (DGCs) are generated in the hippocampal dentate gyrus (DG) of rodents through a process called adult hippocampal neurogenesis, which is subjected to tight intrinsic and extrinsic regulation. The use of retroviruses encoding fluorescent proteins has allowed the characterization of the maturation dynamics of newborn DGCs, including their morphological development and the establishment and maturation of their afferent and efferent synaptic connections. However, the study of a crucial cellular compartment of these cells, namely, the axon initial segment (AIS), has remained unexplored to date. The AIS is not only the site of action potential initiation, but it also has a unique molecular identity that makes it one of the master regulators of neural plasticity and excitability. Here we examined the dynamics of AIS formation in newborn DGCs of young female adult C57BL/6J mice in vivo Our data reveal notable changes in AIS length and thickness throughout cell maturation under physiological conditions and show that the most remarkable structural changes coincide with periods of intense morphological and functional remodeling. Moreover, we demonstrate that AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.SIGNIFICANCE STATEMENT The hippocampal dentate gyrus (DG) of rodents generates newborn dentate granule cells (DGCs) throughout life. This process, named adult hippocampal neurogenesis, confers a unique degree of plasticity to the hippocampal circuit, and it is crucial for learning and memory. Here we studied, for the first time, the formation of a key cellular compartment of newborn DGCs, namely, the axon initial segment (AIS) in vivo Our data reveal remarkable AIS structural remodeling throughout the maturation of these cells under physiological conditions. Moreover, AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.Spanish Ministry of Economy and Competitiveness SAF-2017-82185-R and RYC-2015-171899 to M.L.-M. and SAF-2014-53040-P to J.A´., Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas to J.A´., Alzheimer’s Association 2015-NIRG-340709 and AARG-17-528125 to M.L.-M., Association for Frontotemporal Degeneration 2016 Basic Science Pilot Grant Award to M.L.-M., and Fundación Ramón Areces and Banco de Santander to the Centro de Biología Molecular SeveroOcho

Forced swimming sabotages the morphological and synaptic maturation of newborn granule neurons and triggers a unique pro-inflammatory milieu in the hippocampus

Recent experimental data suggest that mood disorders are related to inflammatory phenomena and have led to the "inflammatory hypothesis of depression". Given that the hippocampus is one of the most affected areas in these disorders, we used a model of acute stress (the Porsolt test) to evaluate the consequences of forced swimming on two crucial events related to the pathophysiology of major depression: the functional maturation of newborn granule neurons; and the hippocampal inflammatory milieu. Using PSD95:GFP-expressing retroviruses, we found that forced swimming selectively alters the dendritic morphology of newborn neurons and impairs their connectivity by reducing the number and volume of their postsynaptic densities. In addition, acute stress triggered a series of morphological changes in microglial cells, together with an increase in microglial CD68 expression, thus suggesting the functional and morphological activation of this cell population. Furthermore, we observed an intriguing change in the hippocampal inflammatory milieu in response to forced swimming. Importantly, the levels of several molecules affected by acute stress (such as Interleukin-6 and eotaxin) have been described to also be altered in patients with depression and other mood disorders.Spanish Ministry of Health (SAF-2014-5040-P), and the Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED, ISCIII) (J. Ávila); and the Alzheimer’s Association (2015-NIRG-340709) (M. Llorens-Martín)

Untold New Beginnings: Adult Hippocampal Neurogenesis and Alzheimer's Disease

Neurogenesis occurs in a limited number of brain regions during adulthood. Of these, the hippocampus has attracted great interest due to its involvement in memory processing. Moreover, both the hippocampus and the main area that innervates this structure, namely the entorhinal cortex, show remarkable atrophy in patients with Alzheimer's disease (AD). Adult hippocampal neurogenesis is a process that continuously gives rise to newborn granule neurons in the dentate gyrus. These cells coexist with developmentally generated granule neurons in this structure, and both cooperative and competition phenomena regulate the communication between these two types of cells. Importantly, it has been revealed that GSK-3β and tau proteins, which are two of the main players driving AD pathology, are cornerstones of adult hippocampal neurogenesis regulation. We have shown that alterations either promoting or impeding the actions of these two proteins have detrimental effects on the structural plasticity of granule neurons. Of note, these impairments occur both under basal conditions and in response to detrimental and neuroprotective stimuli. Thus, in order to achieve the full effectiveness of future therapies for AD, we propose that attention be turned toward identifying the pathological and physiological actions of the proteins involved in the pathogenesis of this condition.Spanish Ministry of Economy and Competitiveness (SAF-2014-53040-P (Jesu´s A´ vila) and RYC-2015-17189 (María Llorens-Mart´ın)); the Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED, Spain) (Jesu´s Avila); the Alzheimer’s Association (2015-NIRG-340709 (Mar´ıa Llorens-Mart´ın)); and the Association for Frontotemporal Degeneration (2016 Basic Science Pilot Grant Award (María Llorens-Martín))

GSK-3β overexpression alters the dendritic spines of developmentally generated granule neurons in the mouse hippocampal dentate gyrus

The dentate gyrus (DG) plays a crucial role in hippocampal-related memory. The most abundant cellular type in the DG, namely granule neurons, are developmentally generated around postnatal day P6 in mice. Moreover, a unique feature of the DG is the occurrence of adult hippocampal neurogenesis, a process that gives rise to newborn granule neurons throughout life. Adult-born and developmentally generated granule neurons share some maturational aspects but differ in others, such as in their positioning within the granule cell layer. Adult hippocampal neurogenesis encompasses a series of plastic changes that modify the function of the hippocampal trisynaptic network. In this regard, it is known that glycogen synthase kinase 3β (GSK-3β) regulates both synaptic plasticity and memory. By using a transgenic mouse overexpressing GSK-3β in hippocampal neurons, we previously demonstrated that the overexpression of this kinase has deleterious effects on the maturation of newborn granule neurons. In the present study, we addressed the effects of GSK-3β overexpression on the morphology and number of dendritic spines of developmentally generated granule neurons. To this end, we performed intracellular injections of Lucifer Yellow in developmentally generated granule neurons of wild-type and GSK-3β-overexpressing mice and analyzed the number and morphologies of dendritic spines (namely, stubby, thin and mushroom). GSK-3β overexpression led to a general reduction in the number of dendritic spines. In addition, it caused a slight reduction in the percentage, head diameter and length of thin spines, whereas the head diameter of mushroom spines was increased

Cognitive Decline in Neuronal Aging and Alzheimer's Disease: Role of NMDA Receptors and Associated Proteins

Molecular changes associated with neuronal aging lead to a decrease in cognitive capacity. Here we discuss these alterations at the level of brain regions, brain cells, and brain membrane and cytoskeletal proteins with an special focus in NMDA molecular changes through aging and its effect in cognitive decline and Alzheimer disease. Here, we propose that some neurodegenerative disorders, like Alzheimer's disease (AD), are characterized by an increase and acceleration of some of these changes