An elegant miRror: microRNAs in stem cells, developmental timing and cancer

Abstract

MicroRNAs (miRNAs) were first discovered in genetic screens for regulators of developmental timing in the stem-cell-like seam cell lineage in Caenorhabditis elegans. As members of the heterochronic pathway, the lin-4 and let-7 miRNAs are required in the seam cells for the correct progression of stage-specific events and to ensure that cell cycle exit and terminal differentiation occur at the correct time. Other heterochronic genes such as lin-28 and lin-41 are direct targets of the lin-4 and let-7 miRNAs. Recent findings on the functions of the let-7 and lin-4/mir-125 miRNA families and lin-28 and lin-41 orthologs from a variety of organisms suggest that core elements of the heterochronic pathway are retained in mammalian stem cells and development. In particular, these genes appear to form bistable switches via double-negative feedback loops in both nematode and mammalian stem cell development, the functional relevance of which is finally becoming clear. let-7 inhibits stem cell self-renewal in both normal and cancer stem cells of the breast and acts as a tumor suppressor in lung and breast cancer. let-7 also promotes terminal differentiation at the larval to adult transition in both nematode stem cells and fly wing imaginal discs and inhibits proliferation of human lung and liver cancer cells. Conversely, LIN-28 is a highly specific embryonic stem cell marker and is one of four "stemness" factors used to reprogram adult fibroblasts into induced pluripotent stem cells; furthermore, lin-28 is oncogenic in hepatocellular carcinomas. Therefore, a core module of heterochronic genes--lin-28, lin-41, let-7, and lin-4/mir-125-acts as an ancient regulatory switch for differentiation in stem cells (and in some cancers), illustrating that nematode seam cells mirror miRNA regulatory networks in mammalian stem cells during both normal development and cancer.

Fig. 1

The C. elegans seam stem cells. The seam stem cells divide asymmetrically at each larval stage such that they self-renew and produce multiple differentiated ectodermal cell types—epidermal cells and various types of neuronal and glial cells (only the hermaphrodite V lineages are shown for simplicity). At the end of the L4 stage, the seam stem cells undergo the larval to adult (L/A) switch and terminally differentiate by exiting the cell cycle, fusing, and secreting alae (a cuticular structure). A symmetrical proliferative division (shown in bold) occurs early in the L2 stage to expand seam cell number

Fig. 2

The heterochronic genes, including the lin-4 and let-7 family miRNAs, regulate developmental timing in seam stem cells. a The heterochronic pathway is an excellent model for miRNA regulatory networks acting as binary switches to pattern development along the temporal axis. In nematodes, heterochronic genes regulate the transitions between stage-specific patterns of division in the seam stem cells. The early timer consists of lin-4-mediated down-regulation of lin-14 and lin-28 to program the L1/L2 and L2/L3 transitions, respectively. The late timer involves the repression of hbl-1 and lin-41 by miRNAs of the let-7 family in order to program both the L2/L3 transition and the L/A switch. b The seam cell lineages in the lin-28 and lin-41 mutants (both miRNA targets) are termed precocious due to the skipping of L2 and L4 stage events, respectively. This results in the L/A switch occurring in the seam, one stage earlier than normal. The lin-4 and let-7 miRNA mutants, in contrast, have retarded phenotypes in that the L1 and L4 stages, respectively, are reiterated, resulting in a delay of terminal differentiation

Fig. 3

Conserved reciprocal temporal expression of the lin-4/mir-125 and let-7 family miRNAs and their targets, lin-28 and lin-41. lin-28 and lin-41 are expressed specifically in undifferentiated mouse ES and EC cells and early embryos. These genes are down-regulated during differentiation by increasing levels of let-7 and mir-125 miRNAs (b), mirroring their regulation in C. elegans stem cells during development (a)

Fig. 4

Model for conserved functions of the heterochronic genes in stem cells, development and cancer. Bistable switches may regulate stem cell differentiation via double-negative feedback loops between let-7 and its targets, lin-28 and lin-41. The roles of lin-28 and let-7 as an oncogene and tumor suppressor suggests that stem cells and cancer cells share a common strategy for regulating the balance between self-renewal and differentiation