Impact of Prenatal Immune System Disturbances on Brain Development

As research into various aging-associated neurodegenerative disorders reveals their immense pathophysiological complexity, the focus is currently shifting from studying changes in an advanced disease state to investigations involving pre-symptomatic periods, possible aberrations in early life, and even abnormalities in brain development. Recent studies on the etiology of schizophrenia and autism spectrum disorders revealed a profound impact of neurodevelopmental disturbances on disease predisposition, onset and progression. Here, we discuss how a prenatal immune challenge can affect the developing brain—with a selective focus on the impact on microglia, the brain's immune cells—and the implications for brain aging and its associated risk of developing Alzheimer's diseas

Chronic clozapine treatment improves prenatal infection-induced working memory deficits without influencing adult hippocampal neurogenesis

Background: Converging evidence indicates that prenatal exposure to immune challenge can induce long-term cognitive deficits relevant to schizophrenia. Such cognitive impairments may be related to deficient hippocampal neurogenesis at adult age. Objectives: In the present study, we sought evidence for the possibility that chronic treatment with the reference atypical antipsychotic drug clozapine may improve prenatal infection-induced cognitive dysfunctions by stimulating adult hippocampal neurogenesis. Methods: This hypothesis was tested in a well-established mouse model of prenatal immune challenge which is based on prenatal administration of the viral mimic, polyriboinosinic-polyribocytidilic acid (PolyI:C). Results: We found that maternal PolyI:C (5mg/kg, i.v.) exposure on gestation day 17 led to significant spatial working memory impairment and reduced hippocampal neurogenesis in the resulting offspring at adult age. The latter effect was apparent in postmortem immunohistochemical analyses of the cell proliferation marker bromodeoxyuridine and the microtubule-associated protein doublecortin, a marker of newborn neuronal cells. Chronic (3weeks) administration of clozapine (5mg/kg/day, i.p.) significantly improved the prenatal PolyI:C-induced working memory deficits, while at the same time, it negatively affected working memory performance in adult offspring born to control mothers. These bidirectional cognitive effects of clozapine were not paralleled by concomitant effects on adult hippocampal neurogenesis. Conclusions: Our findings do not support the hypothesis that the atypical antipsychotic drug clozapine may influence cognitive functions by acting on adult neurogenesis in the hippocampus, regardless of whether the drug is administered to subjects with or without a neurodevelopmental predisposition to adult neuropatholog

Hippocampal pyramidal cells: the reemergence of cortical lamination

The increasing resolution of tract-tracing studies has led to the definition of segments along the transverse axis of the hippocampal pyramidal cell layer, which may represent functionally defined elements. This review will summarize evidence for a morphological and functional differentiation of pyramidal cells along the radial (deep to superficial) axis of the cell layer. In many species, deep and superficial sublayers can be identified histologically throughout large parts of the septotemporal extent of the hippocampus. Neurons in these sublayers are generated during different periods of development. During development, deep and superficial cells express genes (Sox5, SatB2) that also specify the phenotypes of superficial and deep cells in the neocortex. Deep and superficial cells differ neurochemically (e.g. calbindin and zinc) and in their adult gene expression patterns. These markers also distinguish sublayers in the septal hippocampus, where they are not readily apparent histologically in rat or mouse. Deep and superficial pyramidal cells differ in septal, striatal, and neocortical efferent connections. Distributions of deep and superficial pyramidal cell dendrites and studies in reeler or sparsely GFP-expressing mice indicate that this also applies to afferent pathways. Histological, neurochemical, and connective differences between deep and superficial neurons may correlate with (patho-) physiological phenomena specific to pyramidal cells at different radial locations. We feel that an appreciation of radial subdivisions in the pyramidal cell layer reminiscent of lamination in other cortical areas may be critical in the interpretation of studies of hippocampal anatomy and functio

Regulated proteolytic processing of Reelin through interplay of tissue plasminogen activator (tPA), ADAMTS-4, ADAMTS-5, and their modulators

The extracellular signaling protein Reelin, indispensable for proper neuronal migration and cortical layering during development, is also expressed in the adult brain where it modulates synaptic functions. It has been shown that proteolytic processing of Reelin decreases its signaling activity and promotes Reelin aggregation in vitro, and that proteolytic processing is affected in various neurological disorders, including Alzheimer's disease (AD). However, neither the pathophysiological significance of dysregulated Reelin cleavage, nor the involved proteases and their modulators are known. Here we identified the serine protease tissue plasminogen activator (tPA) and two matrix metalloproteinases, ADAMTS-4 and ADAMTS-5, as Reelin cleaving enzymes. Moreover, we assessed the influence of several endogenous protease inhibitors, including tissue inhibitors of metalloproteinases (TIMPs), α-2-Macroglobulin, and multiple serpins, as well as matrix metalloproteinase 9 (MMP-9) on Reelin cleavage, and described their complex interplay in the regulation of this process. Finally, we could demonstrate that in the murine hippocampus, the expression levels and localization of Reelin proteases largely overlap with that of Reelin. While this pattern remained stable during normal aging, changes in their protein levels coincided with accelerated Reelin aggregation in a mouse model of AD

Characterization and Turnover of CD73/IP3R3-positive Microvillar Cells in the Adult Mouse Olfactory Epithelium

The main olfactory epithelium consists of 4 major cell types: sensory neurons, supporting cells, microvillar cells, and basal progenitor cells. Several populations of microvillar olfactory cells have been described, whose properties are not yet fully understood. In this study, we aimed to clarify the classification of microvillar cells by introducing a specific marker, CD73. Furthermore, we investigated the turnover of CD73-microvillar cells during adult life. Using direct and indirect immunofluorescence in adult main olfactory epithelium, we first demonstrate that ecto-5′-nucleotidase (CD73) is a reliable marker for microvillar cells reported previously to express phospholipase C β2 (PLC β2) along with type 3 IP3 receptors (IP3R3) and transient receptor potential channels 6 (TRPC6), as well as for cells labeled by transgenic expression of tauGFP driven by the IP3R3 promoter. The ubiquitous CD73 immunoreactivity in the microvilli of these 2 cell populations indicates that they correspond to the same cell type (CD73-microvillar cell), endowed with a signal transduction cascade mobilizing Ca++ from intracellular stores. These microvillar cells respond to odors, possess a basal process, and do not degenerate after bulbectomy, suggesting that they contribute to cellular homeostasis in the olfactory epithelium. Next, we examined whether CD73-microvillar cells undergo turnover in the adult olfactory epithelium. By combining CD73 immunofluorescence and BrdU pulse labeling, we show delayed BrdU incorporation in a small fraction of CD73-positive microvillar cells, which persists for several weeks after BrdU administration. These findings indicate that CD73-microvillar cells likely differentiate from proliferating progenitor cells and have a slow turnover despite their apical position in the olfactory epithelium. These combined properties are unique among olfactory cells, in line with the possibility that they might regulate cellular homeostasis driven by extracellular ATP and adenosin

Molecular and behavioral changes associated with adult hippocampus-specific SynGAP1 knockout

The synaptic Ras/Rap-GTPase-activating protein (SynGAP1) plays a unique role in regulating specific downstream intracellular events in response to N-methyl-D-aspartate receptor (NMDAR) activation. Constitutive heterozygous loss of SynGAP1 disrupts NMDAR-mediated physiological and behavioral processes, but the disruptions might be of developmental origin. Therefore, the precise role of SynGAP1 in the adult brain, including its relative functional significance within specific brain regions, remains unexplored. The present study constitutes the first attempt in achieving adult hippocampal-specific SynGAP1 knockout using the Cre/loxP approach. Here, we report that this manipulation led to a significant numerical increase in both small and large GluA1 and NR1 immunoreactive clusters, many of which were non-opposed to presynaptic terminals. In parallel, the observed marked decline in the amplitude of spontaneous excitatory currents (sEPSCs) and inter-event intervals supported the impression that SynGAP1 loss might facilitate the accumulation of extrasynaptic glutamatergic receptors. In addition, SynGAP1-mediated signaling appears to be critical for the proper integration and survival of newborn neurons. The manipulation impaired reversal learning in the probe test of the water maze and induced a delay-dependent impairment in spatial recognition memory. It did not significantly affect anxiety or reference memory acquisition but induced a substantial elevation in spontaneous locomotor activity in the open field test. Thus, the present study demonstrates the functional significance of SynGAP1 signaling in the adult brain by capturing several changes that are dependent on NMDAR and hippocampal integrity

SynGAP regulates spine formation

SynGAP is a brain-specific ras GTPase-activating protein that is an abundant component of the signaling complex associated with the NMDA-type glutamate receptor. We generated mutant mice lacking synGAP to study its physiological role. Homozygous mutant mice die in the first few days after birth; however, neurons from mutant embryos can be maintained in culture. Here, we report that spine and synapse formation are accelerated in cultured mutant neurons, and the spines of mature mutant neurons are significantly larger than those of wild type. Clusters of PSD-95 and subunits of AMPA-type and NMDA-type glutamate receptors accumulate in spines of mutant neurons by day 10 in vitro, whereas in wild-type neurons they are still mostly located in dendritic shafts. The frequency and amplitude of miniature EPSCs are larger in mutant neurons at day 10 in vitro, confirming that they have more functional synapses. At day 21 in vitro, the spines of mutant neurons remain significantly larger than those of wild type. The mutant phenotype at day 10 in vitro can be rescued by introduction of recombinant wild-type synGAP on day 9. In contrast, introduction of mutant synGAP with a mutated GAP domain or lacking the terminal domain that binds to PSD-95 does not rescue the mutant phenotype, indicating that both domains play a role in control of spine formation. Thus, the GAP activity of synGAP and its association with PSD-95 are important for normal regulation of spine and synapse formation in hippocampal neurons

Altered human oligodendrocyte heterogeneity in multiple sclerosis

Oligodendrocyte pathology is increasingly implicated in neurodegenerative diseases as oligodendrocytes both myelinate and provide metabolic support to axons. In multiple sclerosis (MS), demyelination in the central nervous system thus leads to neurodegeneration, but the severity of MS between patients is very variable. Disability does not correlate well with the extent of demyelination1, which suggests that other factors contribute to this variability. One such factor may be oligodendrocyte heterogeneity. Not all oligodendrocytes are the same—those from the mouse spinal cord inherently produce longer myelin sheaths than those from the cortex2, and single-cell analysis of the mouse central nervous system identified further differences3,4. However, the extent of human oligodendrocyte heterogeneity and its possible contribution to MS pathology remain unknown. Here we performed single-nucleus RNA sequencing from white matter areas of post-mortem human brain from patients with MS and from unaffected controls. We identified subclusters of oligodendroglia in control human white matter, some with similarities to mouse, and defined new markers for these cell states. Notably, some subclusters were underrepresented in MS tissue, whereas others were more prevalent. These differences in mature oligodendrocyte subclusters may indicate different functional states of oligodendrocytes in MS lesions. We found similar changes in normal-appearing white matter, showing that MS is a more diffuse disease than its focal demyelination suggests. Our findings of an altered oligodendroglial heterogeneity in MS may be important for understanding disease progression and developing therapeutic approaches

Hippocampal pyramidal cells: the reemergence of cortical lamination

The increasing resolution of tract-tracing studies has led to the definition of segments along the transverse axis of the hippocampal pyramidal cell layer, which may represent functionally defined elements. This review will summarize evidence for a morphological and functional differentiation of pyramidal cells along the radial (deep to superficial) axis of the cell layer. In many species, deep and superficial sublayers can be identified histologically throughout large parts of the septotemporal extent of the hippocampus. Neurons in these sublayers are generated during different periods of development. During development, deep and superficial cells express genes (Sox5, SatB2) that also specify the phenotypes of superficial and deep cells in the neocortex. Deep and superficial cells differ neurochemically (e.g. calbindin and zinc) and in their adult gene expression patterns. These markers also distinguish sublayers in the septal hippocampus, where they are not readily apparent histologically in rat or mouse. Deep and superficial pyramidal cells differ in septal, striatal, and neocortical efferent connections. Distributions of deep and superficial pyramidal cell dendrites and studies in reeler or sparsely GFP-expressing mice indicate that this also applies to afferent pathways. Histological, neurochemical, and connective differences between deep and superficial neurons may correlate with (patho-) physiological phenomena specific to pyramidal cells at different radial locations. We feel that an appreciation of radial subdivisions in the pyramidal cell layer reminiscent of lamination in other cortical areas may be critical in the interpretation of studies of hippocampal anatomy and function