Rebecca Petri

miRNAs in brain development.

MicroRNAs (miRNAs) are small, non-coding RNAs that negatively regulate gene expression at the post-transcriptional level. In the brain, a large number of miRNAs are expressed and there is a growing body of evidence demonstrating that miRNAs are essential for brain development and neuronal function. Conditional knockout studies of the core components in the miRNA biogenesis pathway, such as Dicer and DGCR8, have demonstrated a crucial role for miRNAs during the development of the central nervous system. Furthermore, mice deleted for specific miRNAs and miRNA-clusters demonstrate diverse functional roles for different miRNAs during the development of different brain structures. miRNAs have been proposed to regulate cellular functions such as differentiation, proliferation and fate-determination of neural progenitors. In this review we summarise the findings from recent studies that highlight the importance of miRNAs in brain development with a focus on the mouse model. We also discuss the technical limitations of current miRNA studies that still limit our understanding of this family of non-coding RNAs and propose the use of novel and refined technologies that are needed in order to fully determine the impact of specific miRNAs in brain development.

TRIM28 Represses Transcription of Endogenous Retroviruses in Neural Progenitor Cells

SUMMARY TRIM28 is a corepressor that mediates transcriptional silencing by establishing local heterochromatin. Here, we show that deletion of TRIM28 in neural progenitor cells (NPCs) results in high-level expression of two groups of endogenous retroviruses (ERVs): IAP1 and MMERVK10C. We find that NPCs use TRIM28-mediated histone modifications to dynamically regulate transcription and silencing of ERVs, which is in contrast to other somatic cell types using DNA methylation. We also show that derepression of ERVs influences transcriptional dynamics in NPCs through the activation of nearby genes and the expression of long noncoding RNAs. These findings demonstrate a unique dynamic transcriptional regulation of ERVs in NPCs. Our results warrant future studies on the role of ERVs in the healthy and diseased brain.

let-7 regulates radial migration of new-born neurons through positive regulation of autophagy

During adult neurogenesis, newly formed olfactory bulb (OB) interneurons migrate radially to integrate into specific layers of the OB. Despite the importance of this process, the intracellular mechanisms that regulate radial migration remain poorly understood. Here, we find that microRNA (miRNA) let‐7 regulates radial migration by modulating autophagy in new‐born neurons. Using Argonaute2 immunoprecipitation, we performed global profiling of miRNAs in adult‐born OB neurons and identified let‐7 as a highly abundant miRNA family. Knockdown of let‐7 in migrating neuroblasts prevented radial migration and led to an immature morphology of newly formed interneurons. This phenotype was accompanied by a decrease in autophagic activity. Overexpression of Beclin‐1 or TFEB in new‐born neurons lacking let‐7 resulted in re‐activation of autophagy and restored radial migration. Thus, these results reveal a miRNA‐dependent link between autophagy and adult neurogenesis with implications for neurodegenerative diseases where these processes are impaired.

Distinct cognitive effects and underlying transcriptome changes upon inhibition of individual miRNAs in hippocampal neurons.

MicroRNAs (miRNA) are small, non-coding RNAs mediating post-transcriptional regulation of gene expression. miRNAs have recently been implicated in hippocampus-dependent functions such as learning and memory, although the roles of individual miRNAs in these processes remain largely unknown. Here, we achieved stable inhibition using AAV-delivered miRNA sponges of individual, highly expressed and brain-enriched miRNAs; miR-124, miR-9 and miR-34, in hippocampal neurons. Molecular and cognitive studies revealed a role for miR-124 in learning and memory. Inhibition of miR-124 resulted in an enhanced spatial learning and working memory capacity, potentially through altered levels of genes linked to synaptic plasticity and neuronal transmission. In contrast, inhibition of miR-9 or miR-34 led to a decreased capacity of spatial learning and of reference memory, respectively. On a molecular level, miR-9 inhibition resulted in altered expression of genes related to cell adhesion, endocytosis and cell death, while miR-34 inhibition caused transcriptome changes linked to neuroactive ligand-receptor transduction and cell communication. In summary, this study establishes distinct roles for individual miRNAs in hippocampal function.

Identification of the miRNA targetome in hippocampal neurons using RIP-seq

MicroRNAs (miRNAs) are key players in the regulation of neuronal processes by targeting a large network of target messenger RNAs (mRNAs). However, the identity and function of mRNAs targeted by miRNAs in specific cells of the brain are largely unknown. Here, we established an adeno-associated viral vector (AAV)-based neuron-specific Argonaute2:GFP-RNA immunoprecipitation followed by high-throughput sequencing to analyse the regulatory role of miRNAs in mouse hippocampal neurons. Using this approach, we identified more than two thousand miRNA targets in hippocampal neurons, regulating essential neuronal features such as cell signalling, transcription and axon guidance. Furthermore, we found that stable inhibition of the highly expressed miR-124 and miR-125 in hippocampal neurons led to significant but distinct changes in the AGO2 binding of target mRNAs, resulting in subsequent upregulation of numerous miRNA target genes. These findings greatly enhance our understanding of the miRNA targetome in hippocampal neurons.

microRNA-125 distinguishes developmentally generated and adult-born olfactory bulb interneurons.

New neurons, originating from the subventricular zone, are continuously integrating into neuronal circuitry in the olfactory bulb (OB). Using a transgenic sensor mouse, we found that adult-born OB interneurons express microRNA-125 (miR-125), whereas the pre-existing developmentally generated OB interneurons represent a unique population of cells in the adult brain, without miR-125 activity. Stable inhibition of miR-125 in newborn OB neurons resulted in enhanced dendritic morphogenesis, as well as in increased synaptic activation in response to odour sensory stimuli. These data demonstrate that miR-125 controls functional synaptic integration of adult-born OB interneurons. Our results also suggest that absence of an otherwise broadly expressed miRNA is a novel mechanism with which to achieve neuronal subtype specification.

Identifying miRNA Targets Using AGO-RIPseq

microRNAs (miRNA) are small, noncoding RNAs that bind to messenger RNAs (mRNAs) and regulate their activity. They are, therefore, important posttranscriptional regulators. In recent years it has become clear that miRNAs regulate large genetic networks, rather than single genes, and that one gene can be targeted by several miRNAs. To understand the role of miRNAs in cells or tissues, it is therefore important to analyze the targetome of miRNAs. Here, we present a technique called Argonaute-RNA Immunoprecipitation (AGO-RIP) which takes advantages of the fact that miRNAs and their targets are directly bound by the Argonaute protein family. With this approach quantitative, genome-wide analysis of miRNA targets is possible. In this chapter we describe the RIP-methodology and provide advice for RNA sequencing and bioinformatic analyses.

Combined Experimental and System-Level Analyses Reveal the Complex Regulatory Network of miR-124 during Human Neurogenesis

Summary Non-coding RNAs regulate many biological processes including neurogenesis. The brain-enriched miR-124 has been assigned as a key player of neuronal differentiation via its complex but little understood regulation of thousands of annotated targets. To systematically chart its regulatory functions, we used CRISPR/Cas9 gene editing to disrupt all six miR-124 alleles in human induced pluripotent stem cells. Upon neuronal induction, miR-124-deleted cells underwent neurogenesis and became functional neurons, albeit with altered morphology and neurotransmitter specification. Using RNA-induced-silencing-complex precipitation, we identified 98 high-confidence miR-124 targets, of which some directly led to decreased viability. By performing advanced transcription-factor-network analysis, we identified indirect miR-124 effects on apoptosis, neuronal subtype differentiation, and the regulation of previously uncharacterized zinc finger transcription factors. Our data emphasize the need for combined experimental- and system-level analyses to comprehensively disentangle and reveal miRNA functions, including their involvement in the neurogenesis of diverse neuronal cell types found in the human brain.

Huntingtin Aggregation Impairs Autophagy Leading to Argonaute-2 Accumulation and Global MicroRNA Dysregulation

Many neurodegenerative diseases are characterized by the presence of intracellular protein aggregates, resulting in alterations in autophagy. However, the consequences of impaired autophagy for neuronal function remain poorly understood. In this study, we used cell culture and mouse models of huntingtin protein aggregation as well as post-mortem material from patients with Huntingtons disease to demonstrate that Argonaute-2 (AGO2) accumulates in the presence of neuronal protein aggregates and that this is due to impaired autophagy. Accumulation of AGO2, a key factor of the RNA-induced silencing complex that executes microRNA functions, results in global alterations of microRNA levels and activity. Together, these results demonstrate that impaired autophagy found in neurodegenerative diseases not only influences protein aggregation but also directly contributes to global alterations of intracellular post-transcriptional networks.

LINE-2 transposable elements shape post-transcriptional gene regulation in the human brain

Transposable elements (TEs) are dynamically expressed at high levels in multiple human tissues including the brain, but the function of TE-derived transcripts remains largely unknown. In this study we identify numerous miRNAs that are derived from TEs and expressed in the human brain by conducting AGO2-RIP, followed by small RNA sequencing on human brain tissue. Many of these miRNAs originated from L2 elements, which entered the human genome around 100-300 million years ago. We found that L2-miRNAs derive from the 3′ end of the L2 consensus sequence and that they share very similar sequences, indicating that they could target transcripts with L2s in their 3′UTR. In line with this, we found that many protein-coding genes expressed in the brain carry fragments of L2-derived sequences in the 3′UTR, which serve as target sites for L2-derived miRNAs. Our findings uncover a TE-based post-transcriptional network that shapes transcriptional regulation in the human brain.Transposable elements (TEs) are dynamically expressed at high levels in multiple human tissues, but the function of TE-derived transcripts remains largely unknown. In this study, we identify numerous TE-derived microRNAs (miRNAs) by conducting Argonaute2 RNA Immunoprecipitation followed by small RNA sequencing (AGO2 RIP-seq) on human brain tissue. Many of these miRNAs originated from LINE-2 (L2) elements, which entered the human genome around 100-300 million years ago. We found that L2-miRNAs derive from the 3’ end of the L2 consensus sequence and thus share very similar sequences, indicating that they could target transcripts with L2s in their 3’UTR. In line with this, we found that many protein-coding genes carry fragments of L2-derived sequences in their 3’UTR, which serve as target sites for L2-miRNAs. L2-miRNAs and targets were generally ubiquitously expressed at low levels in multiple human tissues, suggesting a role for this network in buffering transcriptional levels of housekeeping genes. Interestingly, we also found evidence that this network is perturbed in glioblastoma. In summary, our findings uncover a TE-based post-transcriptional network that shapes transcriptional regulation in human cells.