miR-9: a versatile regulator of neurogenesis

Abstract

Soon after its discovery, microRNA-9 (miR-9) attracted the attention of neurobiologists, since it is one of the most highly expressed microRNAs in the developing and adult vertebrate brain. Functional analyses in different vertebrate species have revealed a prominent role of this microRNA in balancing proliferation in embryonic neural progenitor populations. Key transcriptional regulators such as FoxG1, Hes1 or Tlx, were identified as direct targets of miR-9, placing it at the core of the gene network controlling the progenitor state. Recent data also suggest that this function could extend to adult neural stem cells. Other studies point to a role of miR-9 in differentiated neurons. Moreover miR-9 has been implicated in human brain pathologies, either displaying a protective role, such as in Progeria, or participating in disease progression in brain cancers. Altogether functional studies highlight a prominent feature of this highly conserved microRNA, its functional versatility, both along its evolutionary history and across cellular contexts.

Keywords: embryonic progenitors; microRNA-9; neural stem cells; neurogenesis; proliferation.

Figure 1

History of the miR-9 gene family. (A) Phylogenetic tree showing the evolutionary relationships between different model species and the composition of the miR-9 gene family in their respective genomes. The preferred microRNA strand is represented in red, while the non-preferred (or “star”) is represented in black. No miR-9 gene has been recovered so far from genomes of cnidarian species, such as the sea anemone or hydra, suggesting that the emergence of a miR-9 gene occurred at the transition towards triploblasty. At the origin of jawed vertebrates, two rounds of whole genome duplications (WGD) have occurred. An additional WGD occurred in the teleost lineage. These duplication events likely account for the presence of multiple miR-9 genes in vertebrates. Eutherian mammals lost one class of miR-9 genes (corresponding to miR-9-4, also called miR-9b). (B) Alignment of pre-miR-9 sequences from Drosophila and human. Sequences, names and reference numbers were retrieved from miRBase. Nucleotides highlighted in bold correspond to the two microRNA strands, with the preferred strand in red, and the non-preferred one in black.

Figure 2

Role of miR-9 in the development of drosophila sensory organs. (A) Different steps of the formation of sensory organs. Among the ectodermal tissue, groups of cells, called proneural clusters (green), acquire neural competence via the induction of proneural genes. The dLMO protein participates in the acquisition of this competence. Among competent cells, one will maintain high levels of proneural genes expression, notably of the sense gene, and become a SOP cell (SOP, yellow). Concomitantly the neural fate is inhibited in the neighboring cells (non-SOP, blue). The SOP cell later divides to give rise to a sensory organ. miR-9a is present in all ectodermal cells except the SOP cell, and inhibits dLMO and Sens protein expression. (B) Role of miR-9a in SOP cell specification. The SOP cell expresses high levels of the ligand Delta, which interacts with Notch receptors located at the surface of neighboring cells. This interaction leads to Notch cleavage, which releases Notch intracellular domain (N^intra) in non-SOP cells. N^intra interacts with the transcription factor Suppressor of Hairless Su(H), which induces E(spl) gene expression. E(spl) genes encode transcriptional repressors inhibiting the expression of proneural genes and in particular sens. In the SOP cell, in the absence of N^intra, Su(H) has an inhibitory effect on the transcription of E(spl) genes which allows for the expression of sens. Sens activates the expression of proneural genes of the ac/scute complex (ac/sc) which specify the SOP cell fate. miR-9a, expressed in non-SOP cells, prevents ectopic expression of sens, thereby conferring robustness to the developmental program. Other genes of the miR-9 family might also play a role here, as miR-4 and miR-79 have been shown to regulate the expression Notch target genes, such as E(spl) or Brd.

Figure 3

miR-9 regulates progenitor states in Vertebrates. (A) miR-9 is expressed in active neurogenic zones. Sagittal section through a zebrafish embryo at 48 h post fertilization, showing the expression of miR-9 as revealed by in situ hybridization (blue). miR-9 is expressed at the ventricular zone, and excluded from differentiated neurons expressing the protein HuC (magenta). Its expression is induced in neurogenic areas, where the expression of proneural genes such as neurogenin1 (neurog1) is detected (green). In contrast, miR-9 is excluded specifically from boundary regions, containing long-lasting neural progenitors, such as the MHB (big arrowhead) or rhombomere boundaries (small arrowheads). (B) Functional data suggest that miR-9 promotes the transition from a non-neurogenic progenitor, expressing high levels of Hes1, to a neurogenic progenitor, in which Hes1 levels oscillate. The miR-9 expressing neurogenic progenitor is in an ambivalent state, poised to respond to proliferation or differentiation cues. (C) Scheme representing negative feedback loops between miR-9 and its targets, some of which promoting proliferation (purple) and others promoting differentiation (green).

Figure 4

miR-9 protects the brain from Progerin. Two alternative transcripts are generated from the the Lamin A/C gene, encoding Lamin C and Lamin A proteins. HGPS is caused by a mutation in exon 11, which is specific to Lamin A encoding transcripts. The mutation generates an additional splice site, which leads to the generation of a new transcript encoding a truncated form of Lamin A. The truncated protein is referred to as Progerin, and is toxic to cells. The presence of miR-9 in the central nervous system can explain, at least in part, the low levels of Lamin A detected in this tissue compared to Lamin C. In HGPS patients, miR-9 repression of Progerin expression protects the CNS from this harmful protein.