Pascal Bielefeld

microRNAs and the regulation of neuronal plasticity under stress conditions

In the brain, the connection between sensory information triggered by the presence of a stressor and the organisms reaction involves limbic areas such as the hippocampus, amygdala and prefrontal cortex. Consequently, these brain regions are the most sensitive to stress-induced changes in neuronal plasticity. However, the specific effects of stress on neuronal plasticity in these regions largely differ. Despite these regional differences, in many cases the steps leading to brain adaptation to stress involve highly coordinated changes in gene expression affecting cell metabolism, neuronal plasticity and synaptic transmission. In adult life the effects of stress on neuronal plasticity are largely reversible but stress in early life induces persistent changes in neuronal plasticity that increases vulnerability to develop psychopathologies and aging-related cognitive decline, suggesting the involvement of epigenetic mechanisms. A growing body of evidence demonstrates that microRNAs (miRs) are key players in epigenetic regulation. In this forefront review we present a critical look on the literature demonstrating the regulation of neuronal plasticity by miRs and the molecular mechanisms of target specificity in neurons. We propose that further progress in the identification of miRs function beyond single target identification would require a combination of developmental expression studies, bioinformatics and a deeper understanding of large networks of targets involved in epigenetic regulation. This will help to extend our understanding of the role miRs play in the regulation of stress-induced neuronal plasticity.

Different subsets of newborn granule cells: a possible role in epileptogenesis?

Several factors, including epileptic seizures, can strongly stimulate ongoing neurogenesis in the adult hippocampus. Although adult‐born granule cells generated after seizure activity have different physiological properties from their normal counterparts, they integrate into the existing, mature network of the adult hippocampal dentate gyrus. However, the exact role of the neurogenic response during epilepsy and its possible involvement in epileptogenesis have remained elusive. Here, we discuss recent studies shedding new light on the interplay between epilepsy and neurogenesis, and try to explain discrepancies in this literature by proposing seizure severity‐dependent induction of two subsets of newborn cells with different properties. We hypothesise that a low seizure intensity would stimulate neurogenesis to a ‘physiological plasticity’ level and have few pathological consequences. In contrast, a high initial seizure intensity may induce a specific subset of altered and/or ectopically located new granule cells with different electrophysiological properties that could initiate hyperexcitatory recurrent networks that could, in turn, contribute to chronic epilepsy. This hypothesis may clarify previously contradictory data in the literature, and could thereby aid in our understanding of the role of neurogenesis in epileptogenesis, and open up promising avenues for therapeutic intervention.

MicroRNA-124 and -137 cooperativity controls caspase-3 activity through BCL2L13 in hippocampal neural stem cells

Adult neurogenesis continuously contributes new neurons to hippocampal circuits and the programmed death of a subset of immature cells provides a primary mechanism controlling this contribution. Epileptic seizures induce strong structural changes in the hippocampus, including the induction of adult neurogenesis, changes in gene expression and mitochondrial dysfunction, which may all contribute to epileptogenesis. However, a possible interplay between this factors remains largely unexplored. Here, we investigated gene expression changes in the hippocampal dentate gyrus shortly after prolonged seizures induced by kainic acid, focusing on mitochondrial functions. Using comparative proteomics, we identified networks of proteins differentially expressed shortly after seizure induction, including members of the BCL2 family and other mitochondrial proteins. Within these networks, we report for the first time that the atypical BCL2 protein BCL2L13 controls caspase-3 activity and cytochrome C release in neural stem/progenitor cells. Furthermore, we identify BCL2L13 as a novel target of the cooperative action of microRNA-124 and microRNA-137, both upregulated shortly after seizure induction. This cooperative microRNA-mediated fine-tuning of BCL2L13 expression controls casp3 activity, favoring non-apoptotic caspase-3 functions in NSPC exposed to KA and thereby may contribute to the early neurogenic response to epileptic seizures in the dentate gyrus.

Multi-omics profile of the mouse dentate gyrus after kainic acid induced status epilepticus

Temporal lobe epilepsy (TLE) can develop from alterations in hippocampal structure and circuit characteristics, and can be modeled in mice by administration of kainic acid (KA). Adult neurogenesis in the dentate gyrus (DG) contributes to hippocampal functions and has been reported to contribute to the development of TLE. Some of the phenotypical changes include neural stem and precursor cells (NPSC) apoptosis, shortly after their birth, before they produce hippocampal neurons. Here we explored these early phenotypical changes in the DG 3 days after a systemic injection of KA inducing status epilepticus (KA-SE), in mice. We performed a multi-omics experimental setup and analyzed DG tissue samples using proteomics, transcriptomics and microRNA profiling techniques, detecting the expression of 2327 proteins, 13401 mRNAs and 311 microRNAs. We here present a description of how these data were obtained and make them available for further analysis and validation. Our data may help to further identify and characterize molecular mechanisms involved in the alterations induced shortly after KA-SE in the mouse DG.

Transcription factor oscillations in neural stem cells: Implications for accurate control of gene expression

ABSTRACT Naturally occurring oscillations in glucocorticoids induce a cyclic activation of the glucocorticoid receptor (GR), a well-characterized ligand-activated transcription factor. These cycles of GR activation/deactivation result in rapid GR exchange at genomic response elements and GR recycling through the chaperone machinery, ultimately generating pulses of GR-mediated transcriptional activity of target genes. In a recent article we have discussed the implications of circadian and high-frequency (ultradian) glucocorticoid oscillations for the dynamic control of gene expression in hippocampal neural stem/progenitor cells (NSPCs) (Fitzsimons et al., Front. Neuroendocrinol., 2016). Interestingly, this oscillatory transcriptional activity is common to other transcription factors, many of which regulate key biological functions in NSPCs, such as NF-kB, p53, Wnt and Notch. Here, we discuss the oscillatory behavior of these transcription factors, their role in a biologically accurate target regulation and the potential importance for a dynamic control of transcription activity and gene expression in NSPCs.

Configurations of the Re-scan Confocal Microscope (RCM) for biomedical applications

The new high‐sensitive and high‐resolution technique, Re‐scan Confocal Microscopy (RCM), is based on a standard confocal microscope extended with a re‐scan detection unit. The re‐scan unit includes a pair of re‐scanning mirrors that project the emission light onto a camera in a scanning manner. The signal‐to‐noise ratio of Re‐scan Confocal Microscopy is improved by a factor of 4 compared to standard confocal microscopy and the lateral resolution of Re‐scan Confocal Microscopy is 170 nm (compared to 240 nm for diffraction limited resolution, 488 nm excitation, 1.49 NA). Apart from improved sensitivity and resolution, the optical setup of Re‐scan Confocal Microscopy is flexible in its configuration in terms of control of the mirrors, lasers and filters. Because of this flexibility, the Re‐scan Confocal Microscopy can be configured to address specific biological applications. In this paper, we explore a number of possible configurations of Re‐scan Confocal Microscopy for specific biomedical applications such as multicolour, FRET, ratio‐metric (e.g. pH and intracellular Ca2+ measurements) and FRAP imaging.

A Standardized Protocol for Stereotaxic Intrahippocampal Administration of Kainic Acid Combined with Electroencephalographic Seizure Monitoring in Mice

Lack of scientific reproducibility is a growing concern and weak experimental practices may contribute to irreproducibility. Here, we describe an optimized and versatile protocol for stereotaxic intrahippocampal administration of Kainic Acid (KA) in mice with a C57Bl6 background. In this protocol, KA administration is combined with in vivo recording of neuronal activity with wired and wireless setups. Following our protocol, KA administration results in a robust dose-dependent induction of low-level epileptiform activity or Status Epilepticus (SE) and induces previously characterized hallmarks of seizure-associated pathology. The procedure consists of three main steps: Craniotomy, stereotaxic administration of KA, and placement of recording electrodes in intrahippocampal, and subdural locations. This protocol offers extended possibilities compared to the systemic administration of KA, as it allows the researcher to accurately regulate the local dose of KA and resulting seizure activity, and permits the use and study of convulsive and non-convulsive KA doses, resulting in higher reproducibility and lower inter-individual variability and mortality rates. Caution should be taken when translating this procedure to different strains of mice as inter-strain sensitivity to KA has been described before. The procedure can be performed in ~1 h by a trained researcher, while intrahippocampal administration of KA without placing recording electrodes can be done in 25 min, and can be easily adapted to the titrated intrahippocampal administration of other drugs.

miRNA-Mediated Regulation of Adult Hippocampal Neurogenesis; Implications for Epilepsy

Hippocampal neural stem/progenitor cells (NSPCs) proliferate and differentiate to generate new neurons across the life span of most mammals, including humans. This process takes place within a characteristic local microenvironment where NSPCs interact with a variety of other cell types and encounter systemic regulatory factors. Within this microenvironment, cell intrinsic gene expression programs are modulated by cell extrinsic signals through complex interactions, in many cases involving short non-coding RNA molecules, such as miRNAs. Here we review the regulation of gene expression in NSPCs by miRNAs and its possible implications for epilepsy, which has been linked to alterations in adult hippocampal neurogenesis.

Epigenetic Mechanisms Regulating the Transition from Embryonic Stem Cells Towards a Differentiated Neural Progeny

Adult tissues preserve characteristic populations of self-renewing cells, which can give rise to various specialized cell types, and the brain is not an exception to this rule. The identification of neural stem cells (NSC) present in several areas of the adult brain has challenged conservative ideas regarding the applicability of regenerative medicine to the brain, creating a research field dedicated to unraveling the mechanisms of adult NSC self-renewal and differentiation, particularly within well defined tissue microenvironments termed neurogenic niches. Research over the past 50 years has revealed that NSC can give rise to different types of neural cells: neurons; astrocytes and oligodendrocytes; and recent observations have demonstrated that epigenetic mechanisms play a central role in the regulation of NSC self-renewal and differentiation under physiological and pathological conditions. In this chapter we review the literature describing these epigenetic mechanisms and discuss their possible implications for regenerative therapies for neurodegenerative disorders, which have been linked to alterations in the generation of new neurons from resident neural stem cells in the brain.

Glucocorticoid-mediated modulation of morphological changes associated with aging in microglia

Microglia dynamically adapt their morphology and function during increasing age. However, the mechanisms behind these changes are to date poorly understood. Glucocorticoids (GCs) are long known and utilized for their immunomodulatory actions and endogenous GC levels are described to alter with advancing age. We here tested the hypothesis that age‐associated elevations in GC levels implicate microglia function and morphology. Our data indicate a decrease in microglial complexity and a concomitant increase in GC levels during aging. Interestingly, enhancing GC levels in young mice enhanced microglial ramifications, while the knockdown of the glucocorticoid receptor expression in old mice aggravated age‐associated microglial amoebification. These data suggest that GCs increase ramification of hippocampal microglia and may modulate age‐associated changes in microglial morphology.