microRNA Regulation of Skeletal Development

Abstract

Purpose of review: Osteogenesis is a complex process involving the specification of multiple progenitor cells and their maturation and differentiation into matrix-secreting osteoblasts. Osteogenesis occurs not only during embryogenesis but also during growth, after an injury, and in normal homeostatic maintenance. While much is known about osteogenesis-associated regulatory genes, the role of microRNAs (miRNAs), which are epigenetic regulators of protein expression, is just beginning to be explored. While miRNAs do not abrogate all protein expression, their purpose is to finely tune it, allowing for a timely and temporary protein down-regulation.

Recent findings: The last decade has unveiled a multitude of miRNAs that regulate key proteins within the osteogenic lineage, thus qualifying them as "ostemiRs." These miRNAs may endogenously target an activator or inhibitor of differentiation, and depending on the target, may either lead to the prolongation of a progenitor maintenance state or to early differentiation. Interestingly, cellular identity seems intimately coupled to the expression of miRNAs, which participate in the suppression of previous and subsequent differentiation steps. In such cases where key osteogenic proteins were identified as direct targets of miRNAs in non-bone cell types, or through bioinformatic prediction, future research illuminating the activity of these miRNAs during osteogenesis will be extremely valuable. Many bone-related diseases involve the dysregulation of transcription factors or other proteins found within osteoblasts and their progenitors, and the dysregulation of miRNAs, which target such factors, may play a pivotal role in disease etiology, or even as a possible therapy.

Keywords: Neural crest; Osteoblast; Osteogenesis; Runx2; Skeletal defect; microRNA.

Figure 1. Osteogenesis in the long bones and from cranial neural crest cells

In the lateral mesoderm-derived long bones, osteoblasts are created through three different processes: i) in the perichondrium, they are directly differentiated from mesenchymal cells, ii) in trabecular bone, they are also created from mesenchymal cells, but with influence from hypertrophic chondrocytes, iii) trabecular osteoblasts, according to recent lineage tracing evidence, may also derive from transdifferentiating hypertrophic chondrocytes [2,3,4,5]. In contrast, neural crest-derived craniofacial bone is differentiated from cranial neural crest cells that specify at the neural plate border, which later delaminate and become migratory. Through intramembranous ossification, these crest-derived progenitors first differentiate into mesenchyme which directly converts into pre-osteoblasts without a cartilage intermediate. Developmental trajectory is denoted by encircled numbers.

Figure 2. microRNA biogenesis and target recognition

miRNAs encoded in intergenic regions can be transcribed between exons or as polycistronic miRNAs by RNA polymerase II/III [37]. Single transcribed miRNAs are spliced out of the exons by the spliceosome into a hairpin structure known as pre-miRNAs, while polycistronic miRNAs are known as pri-miRNAs before being processed to single pre-miRNAs by DROSHA and DGCR8 proteins [41,42]. The pre-miRNAs are then exported from the nucleus via Exportin-5 [40]. The hairpin loop of the pre-miRNA is then cleaved off by Dicer, forming a miRNA duplex [43], and loaded onto the RNA induced silencing complex (RISC) via Argonaut proteins [44,45]. The miRNA along with RISC is then loaded onto the target mRNA transcript and can halt transcription [45], or cause degradation by exonuclease activity [–48].

Figure 3. MicroRNAs potentially implicated in neural crest osteogenesis

Candidate miRNAs predicted to target neural crest GRN genes categorized according to the potential stages of neural crest development they may regulate. Candidate microRNAs that are predicted to target genes in the neural crest gene regulatory network (adopted from [127•]) were assembled with the miRanda algorithm available at microRNA.org [153] and categorized according to the expression of their targets during neural crest development. Based on mouse miRNAs, only conserved microRNAs with a good miRSVR score were taken into account.