Astro-logics with microRNAs

Abstract

Astrocytes are a subtype of glial cells in the central nervous system that are critical for normal brain activity. In this issue of The EMBO Journal, Shenoy et al provide evidence for the involvement of miRNAs in the molecular mechanism underlying the in vitro differentiation of astrocytes from glial progenitor cells. Let-7 and miR-125 jointly silence multiple mRNA targets that would have potentially disrupted differentiation if expressed, and function in concert with JAK-STAT signaling to promote astrocyte differentiation.

See also: A Shenoy et al (May 2015)

MicroRNAs (miRNAs) are endogenous small RNAs that regulate gene expression at a posttranscriptional level. In recent years, increasing evidence suggests that miRNAs play a crucial role in development (Hornstein & Shomron, 2006), including in cell fate specification, stem cell self-renewal (e.g., Melton et al, 2010), somatic cell reprogramming (e.g., Judson et al, 2013), and maintenance of lineage-specific programs (e.g., Kaspi et al, 2014).

In this issue of The EMBO Journal, Shenoy et al shed light on the involvement of miRNAs in mechanisms underlying the differentiation of astrocytes, important glial cells in the central nervous system that support neurons and are critical for healthy brain function. The authors demonstrate that Let-7 and miR-125 are required for initiating astrocyte differentiation in vitro and exhibit cooperative effects in enabling astrocyte cell fate acquisition.

Let-7 and miR-125 have many shared targets, which is unexpected since each miRNA harbors a different target-recognition motif (the miRNA ‘seed’). This high degree of overlap uncovers evolutionary pressure on the set of mRNAs that should be silenced in astrocyte differentiation, to harbor binding sites for both Let-7 and miR-125. The fact that the three miR-125 gene copies in mammalian genomes share their loci with Let-7s and are processed from the same primary precursor, may further support their partnership.

What are the mRNAs that are preferentially silenced? These are mRNAs that are disallowed in differentiating astrocytes because they disrupt specification. Therefore, miRNAs reinforce astrocyte gene expression by repressing a repertoire of many disallowed mRNAs, whose expression might block differentiation, as seen in Dgcr8-deficient cultures that broadly lack miRNA biogenesis. In many studied examples, miRNAs have been shown to serve similar functions, enabling the execution of new programs by silencing disallowed mRNAs that inhibit differentiation (Giraldez et al, 2005; Stark et al, 2005; Hornstein & Shomron, 2006).

The mutual activity of Let-7 and miR-125 guarantees that the joint set of targets will be effectively repressed. Together with the JAK-STAT signaling pathway, which is crucial for astrocyte differentiation in vivo (Bonni et al, 1997; He et al, 2005), the miRNAs convey robustness to acquisition of the astrocyte fate program (Fig 1A). Shenoy et al (2015) demonstrate that Let-7 and miR-125 interlace with JAK-STAT into a higher-order network, required for regulation of astrocyte differentiation. However, the cooperation between these specific miRNAs and JAK-STAT signaling is not trivial and depends on cellular context. In fact, these molecular mechanisms serve opposing endpoints at earlier stages of development: While Let-7 promotes differentiation and is actively degraded in embryonic stem cells (Melton et al, 2010), JAK-STAT signaling is essential for keeping the pluripotent ground state and preventing differentiation (Fig 1B).

Figure 1. miRNAs enable in vitro differentiation of astrocytes in concert with JAK-STAT signaling.

(A) Diagram describing the mechanism identified by Shenoy et al (2015), for in vitro differentiation of glial progenitor cells into astrocytes; JAK-STAT signaling initiates transcription of astrocyte-specific mRNAs and functions in concert with miR-125 and Let-7. The miRNAs serve to silence the expression of mRNAs that are disallowed, in the sense that they block astrocyte differentiation. (B) JAK-STAT signaling maintains pluripotency of embryonic stem cells and is opposed by Let-7 that promotes differentiation. However, in the context of glial progenitor cells, these two mechanisms act cooperatively to enable astrocyte differentiation. The changing transcriptional landscape that provides the miRNAs with altered set of targets is probably an important element in the ability to provide different context for miRNA activity in different cells.

If the JAK-STAT pathway output varies in different situations, then what enables context-dependent interpretation? The suitable context is provided by the coexpression of miR-125 and Let-7, with the latter miRNA being absent in embryonic stem cells. Along the same lines, Let-7 and miR-125 are also repeatedly utilized in development to regulate cell fate decisions in many different settings, although they are often used in different ways. Alternative outputs are possible because miRNA activity depends on the landscape of targets that are actively expressed in particular cell states. For example, available Let-7 targets in the earliest steps of embryonic stem cell differentiation are different from those expressed in glial progenitor cells, highlighting that miRNA activity can be understood only in the context of its targets. An example that sheds light on this point comes from cancer research, where miR-17 family members often act as oncogenes by targeting tumor suppressors, but in some contexts behave more like tumor suppressors and repress oncogenic targets (Kent & Mendell, 2006).

Let-7 and miR-125 regulate the expression of Plagl2 and Imp2/Igf2bp2 in glial progenitor cells, two genes that are important in controlling astrocyte differentiation. Nonetheless, as Shenoy et al (2015) suggest, the complete effect of the two miRNAs is probably mediated by many additional targets. Given the tendency of miRNAs to silence many disallowed targets simultaneously, it remains an experimental challenge to faithfully describe miRNA activity in developmental transitions. Future work is required to create the models and intellectual framework for assessing the impact of miRNAs that are integrated into a network with transcription factors and signaling cascades in development.