Biology subjects

Ravassard, P.

Publications and source records attributed to Ravassard, P..

11 recordsLinked to original sources

bioRxiv2026

Differential endothelial cell remodeling in Down syndrome and APP duplication drives distinct blood brain barrier dysfunction

An extra copy of the amyloid precursor protein gene (APP) is present in Down syndrome (DS) and APP duplication (APPdup), increasing the risk of Alzheimer disease (AD) and cerebral amyloid angiopathy (CAA). However, we reported greater vascular A{beta} deposition in APPdup than in DS with AD, suggesting that other triplicated chromosome 21 genes may mitigate CAA severity. We investigated endothelial cell (EC) function and blood brain barrier (BBB) integrity in DS and APPdup using human induced pluripotent stem cell-derived ECs (iECs), assessing their morphology, junctional proteins, A{beta} peptide secretion, barrier function, and transcriptomic profiles. Both DS and APPdup iECs secreted more A{beta}40 A{beta}42 compared with their respective isogenic controls but exhibited distinct A{beta} peptide profiles. DS iECs showed increased junctional protein expression and reduced permeability, whereas APPdup iECs displayed increased permeability. Transcriptomic analysis revealed extensive dysregulation in DS iECs, with enrichment of junctional, adhesion, angiogenic, immune, and apoptosis pathways, and reduced cell-cycle, proliferative, and membrane-targeting signatures. APPdup iECs showed milder changes, including impaired protein targeting and reduced adhesion-related pathways. Despite comparable APP dosage, DS iECs maintained greater junctional integrity and lower permeability than APPdup iECs, potentially contributing to differential CAA vulnerability. However, impaired proliferation and altered angiogenesis may reduce BBB resilience during ageing.

neuroscience↗

bioRxiv2024

Mutations in PSEN1 predispose inflammation in an astrocyte model of familial Alzheimer's disease through disrupted regulated intramembrane proteolysis

Mutations in PSEN1 cause familial Alzheimers disease with almost complete penetrance. Age at onset is highly variable between different PSEN1 mutations and even within families with the same mutation. Current research into late onset Alzheimers disease implicates inflammation in both disease onset and progression. PSEN1 is the catalytic subunit of {gamma}-secretase, responsible for regulated intramembrane proteolysis of numerous substrates that include cytokine receptors. For this reason, we tested the hypothesis that mutations in PSEN1 impact inflammatory responses in astrocytes, thereby contributing to disease progression. Here, using iPSC-astrocytes, we show that PSEN1 is upregulated in response to inflammatory stimuli, and this upregulation is disrupted by pathological PSEN1 mutations. Using transcriptomic analyses, we demonstrate that PSEN1 mutant astrocytes have an augmented inflammatory profile in their basal state, concomitant with an upregulation of genes coding for regulated intramembrane proteolytic and robust activation of JAK-STAT signalling. Using JAK-STAT2 as an example signalling pathway, we show altered phosphorylation cascades in PSEN1 mutant astrocytes, reinforcing the notion of altered cytokine signalling cascades. Finally, we use small molecule modulators of {gamma}-secretase to confirm a role for PSEN1/{gamma}-secretase in regulating the astrocytic response to inflammatory stimuli. Together, these data suggest that mutations in PSEN1 enhance cytokine signalling via impaired regulated intramembrane proteolysis, thereby predisposing astrocytic inflammatory profiles. These findings support a two-hit contribution of PSEN1 mutations to fAD pathogenesis, not only impacting APP and A{beta} processing but also altering the cellular response to inflammation.

neuroscience↗

bioRxiv2024

The UPRER governs the cell-specific response of human dopaminergic neurons to mitochondrial stress

Mitochondrial dysfunction is thought to be central to the pathophysiology of Parkinsons disease. The preferential vulnerability of dopaminergic (DA) neurons of the substantia nigra pars compacta to mitochondrial stress may underlie their massive degeneration and the occurrence of motor symptoms. Using LUHMES-derived DA neurons, we demonstrated that inhibition of the mitochondrial electron transport chain resulted in a severe alteration of mitochondrial turnover, pushing the balance towards mitochondrial loss, a reduction of the maturation status of the DA population and an increased proportion of apoptotic cells. PERK-mediated Unfolded Protein Response of the Endoplasmic Reticulum (UPRER) emerged as the key coordinator of the stress response, governing the inactivation of the mitochondrial UPR (UPRmt), the initiation of mitophagy and the cell-specific expression of long non-coding RNAs (lncRNAs). Importantly, we discovered novel lncRNAs specifically expressed in human DA neurons upon stress. Among them, we showed that lnc-SLC6A15-5 contributes to the resumption of translation after mitochondrial stress. SummaryThe Unfolded Protein Response of the Endoplasmic Reticulum is induced upon stress in human dopaminergic neurons and modulates mitochondrial homeostasis and transcriptional programs including expression of long non-coding RNAs (lncRNAs). We discovered a lncRNA involved in translation resumption after stress.

neuroscience↗

bioRxiv2024

Link between respiratory pauses and vigilance states in freely moving mice

Brain activity and breathing rate influence each other but it remains unclear how fine respiratory features vary across vigilance states. Using simultaneous nasal pressure and hippocampal local field potential recordings in freely-moving mice, we show that the position of respiratory pauses within breathing cycles distinguish Wake, Rapid Eye Movement (REM) and non-REM (NREM) sleep states. Model-based predictions of vigilance states based on respiratory features perform well even on animals outside of the training set, suggesting the rules are generalized. Respiratory features underwent specific changes at state transitions, such as progressive elimination of pauses after inhalation foreshadowing REM. During NREM, respiratory changes predicted moment-to moment sigma power variations beyond movement-defined packets delineated by micro-arousals, as pauses after inhalation fragmented NREM sleep into [~]30s windows of high sigma power. Overall, our findings reveal that respiratory features structure the macro- and micro-architecture of sleep, opening new windows into brain states through respiration.

neuroscience↗

bioRxiv2022

Transient regulation of focal adhesion via Tensin3 is required for nascent oligodendrocyte differentiation

The differentiation of oligodendroglia from oligodendrocyte precursor cells (OPCs) to complex and extensive myelinating oligodendrocytes (OLs) is a multistep process that involves largescale morphological changes with significant strain on the cytoskeleton. While key chromatin and transcriptional regulators of differentiation have been identified, their target genes responsible for the morphological changes occurring during OL myelination are still largely unknown. Here, we show that the regulator of focal adhesion, Tensin3 (Tns3), is a direct target gene of Olig2, Chd7, and Chd8, transcriptional regulators of OL differentiation. Tns3 is transiently upregulated and localized to cell processes of immature OLs, together with integrin-{beta}1, a key mediator of survival at this transient stage. Constitutive Tns3 loss-of-function leads to reduced viability in mouse and humans, with surviving knockout mice still expressing Tns3 in oligodendroglia. Acute deletion of Tns3 in vivo, either in postnatal neural stem cells (NSCs) or in OPCs, leads to a two-fold reduction in OL numbers. We find that the transient upregulation of Tns3 is required to protect differentiating OPCs and immature OLs from cell death by preventing the upregulation of p53, a key regulator of apoptosis. Altogether, our findings reveal a specific time window during which transcriptional upregulation of Tns3 in immature OLs is required for OL differentiation likely by mediating integrin-{beta}1 survival signaling to the actin cytoskeleton as OL undergo the large morphological changes required for their terminal differentiation.

neuroscience↗

bioRxiv2021

HASTER is a transcriptional stabilizer of HNF1A

The biological purpose and disease relevance of long noncoding RNAs (lncRNAs) is poorly understood. We examined HASTER, a lncRNA antisense to HNF1A. Haploinsufficient mutations in HNF1A, encoding a homeodomain transcription factor, cause diabetes mellitus. Using mouse and human models, we show that HASTER maintains HNF1A at cell-specific physiological concentrations through positive and negative feedback loops. Haster mutant pancreatic {beta} cells thus showed variegated HNF1A overexpression or silencing, causing insulin-deficiency and diabetes. We demonstrate that the HASTER promoter acts in cis to prevent HNF1A overexpression and silencing, and link HASTER-dependent inhibition to local remodelling of 3D chromatin architecture. We further show that HASTER negative feedback ensures that HNF1A creates open chromatin at appropriate cell-specific genome regions. Our studies expose a cis-regulatory element that is unlike enhancers or silencers, and instead stabilizes expression levels of a pioneer transcription factor. They also show that disruption of a mammalian lncRNA can cause diabetes mellitus.

genomics↗

bioRxiv2020

Neonatal diabetes mutations disrupt a chromatin pioneering function that activates the human insulin gene

Despite the central role of chromosomal context in gene transcription, human noncoding DNA variants are generally studied outside of their endogenous genomic location. This poses major limitations to understand the true consequences of causal regulatory variants. We focused on a cis-regulatory mutation (c.-331C>G) in the INS gene promoter that is recurrently mutated in unrelated patients with recessive neonatal diabetes. We created mice in which a ~3.1 kb human INS upstream region carrying -331C or -331G alleles replaced the orthologous mouse Ins2 region. This human sequence drove cell-specific transcription in mice. It also recapitulated poised chromatin during pancreas development and active chromatin in differentiated {beta}-cells. The c.-331C>G mutation, however, blocked active chromatin formation in differentiated b-cells. We further show that another neonatal diabetes gene product, GLIS3, had a singular pioneer-like ability to activate INS chromatin in non-pancreatic cells, which was hampered by the c.-331C>G mutation. This in vivo analysis of human regulatory defects, therefore, uncovered cis and trans components of a mechanism that is essential to activate the endogenous INS gene.

genomics↗

bioRxiv2019

G-protein coupled receptor 88 knock-down in the associative striatum reduces the psychiatric symptoms in a translational model of Parkinson’s disease

Beyond the motor disability, Parkinsons disease (PD) is also characterized by an early appearance of psychiatric symptoms such as apathy, depression, anxiety and cognitive deficits, which can entail dementia and psychosis in later stages. While current treatments may provide some level of symptomatic relief, their use is limited by the development of adverse effects such as impulse-control disorders. There is thus a medical need for targets with novel modes of action to treat these aspects of PD. In this context, we investigated GPR88, an orphan G-protein coupled receptor that is associated with psychiatric disorders and highly enriched in the striatum, where it exerts an inhibitory control over neurotransmitter systems that are compromised in PD. To evaluate the potential of GPR88 as a target for the treatment of the psychiatric symptoms of PD, we knocked-down (KD) its expression in sensorimotor (dorsolateral, DLS) or associative (dorsomedial, DMS) striatal areas in a translational rat model of early PD. Our findings indicate that Gpr88-KD in the DMS, but not DLS, reduced the alterations in mood, motivation and cognition that characterized the model, through modulation of the expression of regulator of G-protein signaling 4 (Rgs4) and of transcription factor {Delta}FosB. Furthermore, the rat model of PD exhibited allostatic changes in striatal activity markers that may be related to patterns observed in patients, and which were reduced by Gpr88-KD. Taken together, these results thus highlight the relevance of GPR88 as a therapeutic target for the psychiatric symptoms of PD.

neuroscience↗

bioRxiv2018

Human pancreatic islet 3D chromatin architecture provides insights into the genetics of type 2 diabetes

Genetic studies promise to provide insight into the molecular mechanisms underlying type 2 diabetes (T2D). Variants associated with T2D are often located in tissue-specific enhancer regions (enhancer clusters, stretch enhancers or super-enhancers). So far, such domains have been defined through clustering of enhancers in linear genome maps rather than in 3D-space. Furthermore, their target genes are generally unknown. We have now created promoter capture Hi-C maps in human pancreatic islets. This linked diabetes-associated enhancers with their target genes, often located hundreds of kilobases away. It further revealed sets of islet enhancers, super-enhancers and active promoters that form 3D higher-order hubs, some of which show coordinated glucose-dependent activity. Hub genetic variants impact the heritability of insulin secretion, and help identify individuals in whom genetic variation of islet function is important for T2D. Human islet 3D chromatin architecture thus provides a framework for interpretation of T2D GWAS signals.

genomics↗

bioRxiv2016

Dynamics of Cortical Dendritic Membrane Potential and Spikes in Freely Behaving Rats

Neural activity in vivo is primarily measured using extracellular somatic spikes, which provide limited information about neural computation. Hence, it is necessary to record from neuronal dendrites, which generate dendritic action potentials (DAP) and profoundly influence neural computation and plasticity. We measured neocortical sub- and supra-threshold dendritic membrane potential (DMP) from putative distal-most dendrites using tetrodes in freely behaving rats over multiple days with a high degree of stability and sub-millisecond temporal resolution. DAP firing rates were several fold larger than somatic rates. DAP rates were modulated by subthreshold DMP fluctuations which were far larger than DAP amplitude, indicting hybrid, analog-digital coding in the dendrites. Parietal DAP and DMP exhibited egocentric spatial maps comparable to pyramidal neurons. These results have important implications for neural coding and plasticity.\n\nOne Sentence SummaryMeasurement of cortical dendritic membrane potential for several days in freely behaving rats reveals disproportionate dendritic spiking and analog and digital coding.

neuroscience↗

bioRxiv2016

Human pancreatic β cell lncRNAs control cell-specific regulatory networks

Recent studies have uncovered thousands of long non-coding RNAs (IncRNAs) in human pancreatic {beta} cells. {beta} cell lncRNAs are often cell type-specific, and exhibit dynamic regulation during differentiation or upon changing glucose concentrations. Although these features hint at a role of lncRNAs in {beta} cell gene regulation and diabetes, the function of {beta} cell lncRNAs remains largely unknown. In this study, we investigated the function of {beta} cell-specific lncRNAs and transcription factors using transcript knockdowns and co-expression network analysis. This revealed lncRNAs that function in concert with transcription factors to regulate {beta} cell-specific transcriptional networks. We further demonstrate that lncRNA PLUTO affects local three-dimensional chromatin structure and transcription of PDX1, encoding a key {beta} cell transcription factor, and that both PLUTO and PDX1 are downregulated in islets from donors with type 2 diabetes or impaired glucose tolerance. These results implicate lncRNAs in the regulation of {beta} cell-specific transcription factor networks.

genomics↗