Hypomyelination and oligodendroglial alterations in a mouse model of autism spectrum disorder

Autism spectrum disorders (ASDs) are neuropsychiatric diseases characterized by impaired social interaction, communication deficits, and repetitive and stereotyped behaviors. ASD etiology is unknown, and both genetic and environmental causes have been proposed. Different brain structures are believed to play a role in ASD-related behaviors, including medial prefrontal cortex (mPFC), hippocampus, piriform cortex (Pir), basolateral amygdala (BLA) and Cerebellum. Compelling evidence suggests a link between white matter modifications and ASD symptoms in patients. Besides, an hypomyelination of the mPFC has been associated in rodents to social behavior impairment, one of the main symptoms of ASD. However, a comparative analysis of myelination as well as oligodendroglial (OL)-lineage cells in brain regions associated to social behaviors in animal models of ASD has not been performed so far. Here, we investigated whether OL-lineage cells and myelination are altered in a murine model of ASD induced by the prenatal exposure to valproic acid (VPA). We showed an hypomyelination in the BLA and Pir of adult VPA-exposed mice. These results were accompanied by a decrease in the number of OL-lineage cells and of mature OLs in the Pir, in addition to the mPFC, where myelination presented no alterations. In these regions the number of oligodendrocyte progenitors (OPCs) remained unaltered. Likewise, activation of histone deacetylases (HDACs) on OL-lineage cells in adulthood showed no differences. Overall, our results reveal OL-lineage cell alterations and hypomyelination as neuropathological hallmarks of ASD that have been overlooked so far.Fil: Graciarena, Mariana. Inserm; Francia. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Fisiología, Biología Molecular y Celular; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; ArgentinaFil: Seiffe, Araceli. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Fisiología, Biología Molecular y Celular; ArgentinaFil: Nait Oumesmar, Brahim. Inserm; FranciaFil: Depino, Amaicha Mara. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Fisiología, Biología Molecular y Celular; Argentin

Alteration of synaptic connectivity of oligodendrocyte precursor cells following demyelination

International audienceOligodendrocyte precursor cells (OPCs) are a major source of remyelinating oligodendrocytes in demyelinating diseases such as Multiple Sclerosis (MS). While OPCs are innervated by unmyelinated axons in the normal brain, the fate of such synaptic contacts after demyelination is still unclear. By combining electrophysiology and immunostainings in different transgenic mice expressing fluorescent reporters, we studied the synaptic innervation of OPCs in the model of lysolecithin (LPC)-induced demyelination of corpus callosum. Synaptic innervation of reactivated OPCs in the lesion was revealed by the presence of AMPA receptor-mediated synaptic currents, VGluT1+ axon-OPC contacts in 3D confocal reconstructions and synaptic junctions observed by electron microscopy. Moreover, 3D confocal reconstructions of VGluT1 and NG2 immunolabeling showed the existence of glutamatergic axon-OPC contacts in post-mortem MS lesions. Interestingly, patch-clamp recordings in LPC-induced lesions demonstrated a drastic decrease in spontaneous synaptic activity of OPCs early after demyelination that was not caused by an impaired conduction of compound action potentials. A reduction in synaptic connectivity was confirmed by the lack of VGluT1+ axon-OPC contacts in virtually all rapidly proliferating OPCs stained with EdU (50-ethynyl-20-deoxyuridine). At the end of the massive proliferation phase in lesions, the proportion of innervated OPCs rapidly recovers, although the frequency of spontaneous synaptic currents did not reach control levels. In conclusion, our results demonstrate that newly-generated OPCs do not receive synaptic inputs during their active proliferation after demyelination, but gain synapses during the remyelination process. Hence, glutamatergic synaptic inputs may contribute to inhibit OPC proliferation and might have a physiopathological relevance in demyelinating disorders

Timing Specific Requirement of microRNA Function is Essential for Embryonic and Postnatal Hippocampal Development

The adult hippocampus consists of the dentate gyrus (DG) and the CA1, CA2 and CA3 regions and is essential for learning and memory functions. During embryonic development, hippocampal neurons are derived from hippocampal neuroepithelial cells and dentate granular progenitors. The molecular mechanisms that control hippocampal progenitor proliferation and differentiation are not well understood. Here we show that noncoding microRNAs (miRNAs) are essential for early hippocampal development in mice. Conditionally ablating the RNAase III enzyme Dicer at different embryonic time points utilizing three Cre mouse lines causes abnormal hippocampal morphology and affects the number of hippocampal progenitors due to altered proliferation and increased apoptosis. Lack of miRNAs at earlier stages causes early differentiation of hippocampal neurons, in particular in the CA1 and DG regions. Lack of miRNAs at a later stage specifically affects neuronal production in the CA3 region. Our results reveal a timing requirement of miRNAs for the formation of specific hippocampal regions, with the CA1 and DG developmentally hindered by an early loss of miRNAs and the CA3 region to a late loss of miRNAs. Collectively, our studies indicate the importance of the Dicer-mediated miRNA pathway in hippocampal development and functions

Reelin Controls Progenitor Cell Migration in the Healthy and Pathological Adult Mouse Brain

Understanding the signals that control migration of neural progenitor cells in the adult brain may provide new therapeutic opportunities. Reelin is best known for its role in regulating cell migration during brain development, but we now demonstrate a novel function for reelin in the injured adult brain. First, we show that Reelin is upregulated around lesions. Second, experimentally increasing Reelin expression levels in healthy mouse brain leads to a change in the migratory behavior of subventricular zone-derived progenitors, triggering them to leave the rostral migratory stream (RMS) to which they are normally restricted during their migration to the olfactory bulb. Third, we reveal that Reelin increases endogenous progenitor cell dispersal in periventricular structures independently of any chemoattraction but via cell detachment and chemokinetic action, and thereby potentiates spontaneous cell recruitment to demyelination lesions in the corpus callosum. Conversely, animals lacking Reelin signaling exhibit reduced endogenous progenitor recruitment at the lesion site. Altogether, these results demonstrate that beyond its known role during brain development, Reelin is a key player in post-lesional cell migration in the adult brain. Finally our findings provide proof of concept that allowing progenitors to escape from the RMS is a potential therapeutic approach to promote myelin repair

Neurogenesis in the chronic lesions of multiple sclerosis

Subcortical white matter in the adult human brain contains a population of interneurons that helps regulate cerebral blood flow. We investigated the fate of these neurons following subcortical white matter demyelination. Immunohistochemistry was used to examine neurons in normal-appearing subcortical white matter and seven acute and 59 chronic demyelinated lesions in brains from nine patients with multiple sclerosis and four controls. Seven acute and 44 of 59 chronic multiple sclerosis lesions had marked neuronal loss. Compared to surrounding normal-appearing white matter, the remaining 15 chronic multiple sclerosis lesions contained a 72% increase in mature interneuron density, increased synaptic densities and cells with phenotypic characteristics of immature neurons. Lesion areas with increased neuron densities contained a morphologically distinct population of activated microglia. Subventricular zones contiguous with demyelinated lesions also contained an increase in cells with phenotypes of neuronal precursors. These results support neurogenesis in a subpopulation of demyelinated subcortical white matter lesions in multiple sclerosis brains

Time-Lapse Imaging of the Dynamics of CNS Glial-Axonal Interactions In Vitro and Ex Vivo

Myelination is an exquisite and dynamic example of heterologous cell-cell interaction, which consists of the concentric wrapping of multiple layers of oligodendrocyte membrane around neuronal axons. Understanding the mechanism by which oligodendrocytes ensheath axons may bring us closer to designing strategies to promote remyelination in demyelinating diseases. The main aim of this study was to follow glial-axonal interactions over time both in vitro and ex vivo to visualize the various stages of myelination.We took two approaches to follow myelination over time: i) time-lapse imaging of mixed CNS myelinating cultures generated from mouse spinal cord to which exogenous GFP-labelled murine cells were added, and ii) ex vivo imaging of the spinal cord of shiverer (Mbp mutant) mice, transplanted with GFP-labelled murine neurospheres. We demonstrate that oligodendrocyte-axonal interactions are dynamic events with continuous retraction and extension of oligodendroglial processes. Using cytoplasmic and membrane-GFP labelled cells to examine different components of the myelin-like sheath, we provide evidence from time-lapse fluorescence microscopy and confocal microscopy that the oligodendrocytes' cytoplasm-filled processes initially spiral around the axon in a corkscrew-like manner. This is followed subsequently by focal expansion of the corkscrew process to form short cuffs, which then extend longitudinally along the axons. We predict from this model that these spiral cuffs must extend over each other first before extending to form internodes of myelin.These experiments show the feasibility of visualizing the dynamics of glial-axonal interaction during myelination over time. Moreover, these approaches complement each other with the in vitro approach allowing visualization of an entire internodal length of myelin and the ex vivo approach validating the in vitro data

Topographical analysis of the subependymal zone neurogenic niche

The emerging model for the adult subependymal zone (SEZ) cell population indicates that neuronal diversity is not generated from a uniform pool of stem cells but rather from diverse and spatially confined stem cell populations. Hence, when analysing SEZ proliferation, the topography along the anterior-posterior and dorsal-ventral axes must be taken into account. However, to date, no studies have assessed SEZ proliferation according to topographical specificities and, additionally, SEZ studies in animal models of neurological/psychiatric disorders often fail to clearly specify the SEZ coordinates. This may render difficult the comparison between studies and yield contradictory results. More so, by focusing in a single spatial dimension of the SEZ, relevant findings might pass unnoticed. In this study we characterized the neural stem cell/progenitor population and its proliferation rates throughout the rat SEZ anterior-posterior and dorsal-ventral axes. We found that SEZ proliferation decreases along the anterior-posterior axis and that proliferative rates vary considerably according to the position in the dorsal-ventral axis. These were associated with relevant gradients in the neuroblasts and in the neural stem cell populations throughout the dorsal-ventral axis. In addition, we observed spatially dependent differences in BrdU/Ki67 ratios that suggest a high variability in the proliferation rate and cell cycle length throughout the SEZ; in accordance, estimation of the cell cycle length of the neuroblasts revealed shorter cell cycles at the dorsolateral SEZ. These findings highlight the need to establish standardized procedures of SEZ analysis. Herein we propose an anatomical division of the SEZ that should be considered in future studies addressing proliferation in this neural stem cell niche.Fundação para a Ciência e a Tecnologia (FCT

Convergence of Cells from the Progenitor Fraction of Adult Olfactory Bulb Tissue to Remyelinating Glia in Demyelinating Spinal Cord Lesions

Progenitor cells isolated from adult brain tissue are important tools for experimental studies of remyelination. Cells harvested from neurogenic regions in the adult brain such as the subependymal zone have demonstrated remyelination potential. Multipotent cells from the progenitor fraction have been isolated from the adult olfactory bulb (OB) but their potential to remyelinate has not been studied. cell bodies adjacent to and surrounding peripheral-type myelin rings.We report that neural cells from the progenitor fraction of the adult rat OB grown in monolayers can be expanded for several passages in culture and that upon transplantation into a demyelinated spinal cord lesion provide extensive remyelination without ectopic neuronal differentiation

Rat forebrain neurogenesis and striatal neuron replacement after focal stroke

The persistence of neurogenesis in the forebrain subventricular zone (SVZ) of adult mammals suggests that the mature brain maintains the potential for neuronal replacement after injury. We examined whether focal ischemic injury in adult rat would increase SVZ neurogenesis and direct migration and neuronal differentiation of endogenous precursors in damaged regions. Focal stroke was induced in adult rats by 90-minute right middle cerebral artery occlusion (tMCAO). Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (BrdU) labeling and immunostaining for cell type-specific markers. Brains examined 10–21 days after stroke showed markedly increased SVZ neurogenesis and chains of neuroblasts extending from the SVZ to the peri-infarct striatum. Many BrdU-labeled cells persisted in the striatum and cortex adjacent to infarcts, but at 35 days after tMCAO only BrdU-labeled cells in the neostriatum expressed neuronal markers. Newly generated cells in the injured neostriatum expressed markers of medium spiny neurons, which characterize most neostriatal neurons lost after tMCAO. These findings indicate that focal ischemic injury increases SVZ neurogenesis and directs neuroblast migration to sites of damage. Moreover, neuroblasts in the injured neostriatum appear to differentiate into a region-appropriate phenotype, which suggests that the mature brain is capable of replacing some neurons lost after ischemic injury.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34890/1/10393_ftp.pd