Signaling circuitries controlling stem cell fate: to be or not to be

Abstract

The integration of extrinsic and intrinsic signals is required to preserve the self-renewal and tissue regenerative capacity of adult stem cells, while protecting them from malignant conversion or loss of proliferative potential by death, differentiation or senescence. Here we review emerging signaling circuitries regulating stem cell fate, with emphasis on epithelial stem cells. Wnt, mTOR, GPCRs, Notch, Rho GTPases, YAP and DNA and histone methylases are some of the mechanisms that allow stem cells to balance their regenerative potential and the initiation of terminal differentiation programs, guaranteeing appropriate tissue homeostasis. Understanding the signaling circuitries regulating stem cell fate decisions might provide important insights into cancer initiation and numerous human pathologies that involve the progressive loss of tissue-specific adult stem cells.

Figure 1. Stem cell fate

An exquisite balance between microenvironmental cues from the niche and cell autonomous signals is required to preserve the self-renewal and tissue regenerative capacity of stem cells. Under physiological situations, epithelial stem cells undergo asymmetric cell division, thus self-renewing within their niche and generating daughter cells that proliferate rapidly, in a process known as transient amplification, followed by terminal differentiation. Environmental and intrinsic processes, including oncogenic stress, can cause stem cells to undergo cell death or senescence. The activation of cell differentiation, death or senescence programs -- DDS response -- results in the loss of proliferative potential of adult stem cells. The demise of stem cells by DDS activation may represent a natural protective barrier, preventing the incorporation of genetic and epigenetic alterations into self-renewing multipotent cells, and thus its devastating consequences, including tumor formation. On the other hand, stem cell depletion resulting from the activation of the DDS response may contribute to reduced tissue regenerative capacity and accelerated aging.

Figure 2. Signaling circuitries controlling stem cell fate decisions

A) Wnt family members initiate signaling by interacting with Frizzled (FZ) receptors and LRP5 or LRP6. Wnt stimulates a variety of intracellular signaling routes, including the canonical β-catenin pathway wherein β-catenin is stabilized due to GSK3 inhibition and translocates to the nucleus, where it promotes stem cell self-renewal. However, GSK3 inhibition may also result in the activation of mTOR due to inactivation of the TSC2, which inhibits RHEB, thereby initiating mTOR-driven stem cell senescence programs. B) The GPCR-like LGR5 and 6 are expressed in stem cells, where they potentiate Wnt signaling when bound to R-spondins (RSp). LGR5/6 may be able to activate G proteins in response to R-spondins or other ligands. C) The release of the neurotransmitter acetylcholine promotes the growth of stem cells in the salivary gland by acting on Gαq-coupled muscarinic 1 (M1) receptors though Ca⁺² release and activation of PKC. D) Notch signaling is dispensable for stem cell self-renewal but necessary to induce differentiation of epithelial stem cells. LGN, NuMA and dynactin (Dctn1) control asymmetric cell division and differentiation by activating Notch signaling. The distribution of asymmetric cell division components is polarized in mitotic basal keratinocytes, where they form an apical crescent of LGN and an interacting partner, NuMA. NuMA in turn binds microtubules and interacts with cytoplasmic Dctn1. LGN is thought to be recruited to the cell membrane through Gα_i3.

Figure 3. Other signaling pathways controlling stem cell fate

A) MAL is normally retained in the cytosol in an inactive state while bound to monomeric actin (G-actin). Signaling pathways that activate Rho (also due to inactivation of Rac) lead to ROCK and LIMK activation, resulting in the phosphorylation of the actin severing protein cofilin and accumulation of fibrillar actin (F-actin). This change relieves MAL from the inhibitory activity of G-actin, and free MAL can then bind SRF in the nucleus, enhancing the expression of multiple SRF regulated genes, including key components of the AP-1 transcription factor which controls the expression of proteins involved in multiple differentiated epithelial cell function. Rho GTPase stimulation may result also in activation of cell contraction or mitogen-activated protein kinase (MAPK) cascades, such as the JNK and p38 pathways, that can lead to the activation of differentiation programs. B) Yes-associated protein (YAP) activity appears to be regulated by α-catenin (α-cat). In differentiating keratinocytes, phosphorylated (inactive) Yap binds 14-3-3 proteins and is sequestered by α-catenin, preventing its dephosphorylation and activation. The dissolution of cell-cell contacts causes dissociation and depletion of α-catenin and, concomitantly, dissociation of YAP from 14-3-3 proteins. YAP can be then dephosphorylated and translocates to the nucleus, where it binds the TEA domain (TEAD) transcription factor and induces stem cell proliferation while repressing differentiation gene programs.

Figure 4. Epigenetic regulation of stem cell fate

Members of the polycomb repressive complex (PRC) 2 (Ezh1/2) and PRC1 (Bmi1) and DNA methyltransferase 1 (DNMT1) are expressed in basal epithelial progenitor cells and promote the methylation of histones and DNA, preventing the recruitment of transcriptional activators to genes involved in differentiation and senescence (such as the Ink4a/p16 and Ink4b/Arf locus), promoting stem cell self-renewal. The opposite function is performed by demethylases, including the histone demethylase JMJD3, which hence regulates the switch between stem cell self renewal and differentiation/senescence resulting in the loss of proliferative potential of adult stem cells.