Notch/Hes signaling and miR-9 engage in complex feedback interactions controlling neural progenitor cell proliferation and differentiation.

Canonical Notch signaling has diverse functions during nervous system development and is critical for neural progenitor self-renewal, timing of differentiation and specification of various cell fates. A key feature of Notch-mediated self-renewal is its fluctuating activity within the neural progenitor cell population and the oscillatory expression pattern of the Notch effector Hes1 and its target genes. A negative feedback loop between Hes1 and neurogenic microRNA miR-9 was found to be part of this oscillatory clock. In a recent study we discovered that miR-9 expression is further modulated by direct binding of the Notch intracellular domain/RBPj transcriptional complex to the miR-9_2 promoter. In turn, miR-9 not only targets Hes1 but also Notch2 to attenuate Notch signaling and promote neuronal differentiation. Here, we discuss how the two interwoven feedback loops may provide an additional fail-save mechanism to control proliferation and differentiation within the neural progenitor cell population. Furthermore, we explore potential implications of miR-9-mediated regulation of Notch/Hes1 signaling with regard to neural progenitor homeostasis, patterning, timing of differentiation and tumor formation.

Reciprocal Regulation between Bifunctional miR-9/9(∗) and its Transcriptional Modulator Notch in Human Neural Stem Cell Self-Renewal and Differentiation.

Tight regulation of the balance between self-renewal and differentiation of neural stem cells is crucial to assure proper neural development. In this context, Notch signaling is a well-known promoter of stemness. In contrast, the bifunctional brain-enriched microRNA miR-9/9(∗) has been implicated in promoting neuronal differentiation. Therefore, we set out to explore the role of both regulators in human neural stem cells. We found that miR-9/9(∗) decreases Notch activity by targeting NOTCH2 and HES1, resulting in an enhanced differentiation. Vice versa, expression levels of miR-9/9(∗) depend on the activation status of Notch signaling. While Notch inhibits differentiation of neural stem cells, it also induces miR-9/9(∗) via recruitment of the Notch intracellular domain (NICD)/RBPj transcriptional complex to the miR-9/9(∗)_2 genomic locus. Thus, our data reveal a mutual interaction between bifunctional miR-9/9(∗) and the Notch signaling cascade, calibrating the delicate balance between self-renewal and differentiation of human neural stem cells.

The role of microRNAs in human neural stem cells, neuronal differentiation and subtype specification.

The impressive neuronal diversity found within the nervous system emerges from a limited pool of neural progenitor cells that proceed through different gene expression programs to acquire distinct cell fates. Here, we review recent evidence indicating that microRNAs (miRNAs) are critically involved in conferring neural cell identities during neural induction, neuronal differentiation and subtype specification. Several studies have shown that miRNAs act in concert with other gene regulatory factors and genetic switches to regulate the spatial and temporal expression profiles of important cell fate determinants. So far, most studies addressing the role of miRNAs during neurogenesis were conducted using animal models. With the advent of human pluripotent stem cells and the possibility to differentiate these into neural stem cells, we now have the opportunity to study miRNAs in a human context. More insight into the impact of miRNA-based regulation during neural fate choice could in the end be exploited to develop new strategies for the generation of distinct human neuronal cell types.

MicroRNA-based promotion of human neuronal differentiation and subtype specification.

MicroRNAs are key regulators of neural cell proliferation, differentiation and fate choice. Due to the limited access to human primary neural tissue, the role of microRNAs in human neuronal differentiation remains largely unknown. Here, we use a population of long-term self-renewing neuroepithelial-like stem cells (lt-NES cells) derived from human embryonic stem cells to study the expression and function of microRNAs at early stages of human neural stem cell differentiation and neuronal lineage decision. Based on microRNA expression profiling followed by gain- and loss-of-function analyses in lt-NES cells and their neuronal progeny, we demonstrate that miR-153, miR-324-5p/3p and miR-181a/a contribute to the shift of lt-NES cells from self-renewal to neuronal differentiation. We further show that miR-125b and miR-181a specifically promote the generation of neurons of dopaminergic fate, whereas miR-181a inhibits the development of this neurotransmitter subtype. Our data demonstrate that time-controlled modulation of specific microRNA activities not only regulates human neural stem cell self-renewal and differentiation but also contributes to the development of defined neuronal subtypes.

Correction: MicroRNA-Based Promotion of Human Neuronal Differentiation and Subtype Specification.

[This corrects the article DOI: 10.1371/journal.pone.0059011.].