Roles for Hedgehog signaling in adult organ homeostasis and repair

Abstract

The hedgehog (HH) pathway is well known for its mitogenic and morphogenic functions during development, and HH signaling continues in discrete populations of cells within many adult mammalian tissues. Growing evidence indicates that HH regulates diverse quiescent stem cell populations, but the exact roles that HH signaling plays in adult organ homeostasis and regeneration remain poorly understood. Here, we review recently identified functions of HH in modulating the behavior of tissue-specific adult stem and progenitor cells during homeostasis, regeneration and disease. We conclude that HH signaling is a key factor in the regulation of adult tissue homeostasis and repair, acting via multiple different routes to regulate distinct cellular outcomes, including maintenance of plasticity, in a context-dependent manner.

Keywords: Adult stem cells; Hedgehog signaling; Homeostasis.

Fig. 1.

Mechanism of canonical HH signal transduction in vertebrates. (A) In the absence of hedgehog (HH) ligand, patched 1 (PTCH) localizes to the primary cilium where it prevents activation of smoothened (SMO), which is sequestered into endocytic vesicles (circle). Microtubule motors within the cilium form the intraflagellar transport (IFT) machinery responsible for shuttling components of the HH signaling pathway, including small amounts of the glioma-associated oncogene proteins (GLIs), in and out of the cilium. At the base of the cilium, the GLI proteins (GLI2 and GLI3) are phosphorylated by protein kinase A (PKA), casein kinase 1α (CK1) and glycogen synthase kinase 3β (GSK3β), which results in their proteolytic cleavage and removal of the C-terminal ‘activator’ domain (green), generating GLI2^R and GLI3^R (red), which then suppress transcription of HH target genes in the nucleus. (B) HH signaling is activated upon binding of the ligand to PTCH proteins, which leads to their exiting the cilium and SMO subsequently entering. With the help of the IFT, the GLIs accumulate in the ciliary tip and then exit the cilium as full-length transcriptional activators (GLI2^A and GLI3^A). GLI^A isoforms translocate to the nucleus, where they induce expression of HH target genes, including the transcriptional activator Gli1. The PTCH-bound HH ligand is internalized and degraded.

Fig. 2.

Neural stem cells in the mouse forebrain SVZ, like other adult stem cells, produce lineage-restricted progenitors and respond to SHH. (A) In the subventricular zone (SVZ) lining the lateral ventricles (LVs), neural stem cells (NSCs; blue) self-renew or divide asymmetrically to generate transit-amplifying cells (TACs, green), progenitors that proliferate and give rise to proliferating neuroblasts (NBs, magenta) that migrate away from the SVZ via the rostral migratory stream (RMS) to the olfactory bulb (OB). The end feet of NSCs and astrocytes (purple) often contact blood vessels (BVs), which are an essential component of the neurogenic niche. Multiciliated ependymal cells (brown) form the immediate boundary between the cerebrospinal fluid-filled ventricle and the SVZ. Cb, cerebellum; DG, dentate gyrus. (B) Mature astrocytes and NSCs share many molecular and morphological characteristics, and both cell types respond to sonic hedgehog (SHH) signaling. Whether one cell type can be transformed into the other (dashed double-headed arrow), as appears to occur during injury (Sirko et al., 2013), and what role HH signaling may play in this process remain to be determined. NSCs also produce a small number of oligodendrocytes (gray), which is augmented by SHH signaling (Loulier et al., 2006). Smoothened (SMO) and patched 1 (PTCH1) are thought to be expressed at all the stages of NSC lineage progression. Activation of the canonical HH signaling pathway, however, occurs only at the NSC stage, where it is important for maintaining the undifferentiated and proliferative state of the NSCs (circular arrow). Expression of the glioma-associated oncogene proteins (GLIs) ends as the lineage progresses from NSCs to TACs. In addition, whereas GLI2 and GLI3 are expressed in mature astrocytes and NSCs throughout the SVZ and the rest of the brain, GLI1 is present in only a subset of NSCs and astrocytes (hatched orange line), possibly in regions where HH signaling is the highest (Balordi and Fishell, 2007a; Garcia et al., 2010; Petrova et al., 2013).

Fig. 3.

Roles of nerve-derived HH signaling in adult organs. (A) (Left) Schematic of the adult rodent forebrain where Shh-expressing neuronal populations (orange) in the medial septal nucleus (MSN) are thought to deliver sonic hedgehog (SHH) to the subventricular zone (SVZ) neural stem cells (NSCs) and adjacent astrocytes (both depicted in blue). (Right) Dopaminergic neurons in the midbrain (orange) also express Shh (Gonzalez-Reyes et al., 2012) and are located in close proximity to a midbrain population of GLI1+ astrocytes (blue). As these neurons project to the forebrain lateral striatum (Str) and SVZ lining the lateral ventricles, it is possible that SHH might travel in an anterograde direction along the axons to be released in the SVZ (orange dashed lines), thus serving as an alternative distant source of ligand. (B) In adult mouse skin, stem cell domains in the upper and the lower bulge are exposed to SHH ligand during telogen phase. An unidentified source induces Gli1 in the lower bulge and dermal papilla (DP), whereas the GLI1+ stem cell domain below the isthmus receives SHH from cutaneous nerves (orange line) originating from dorsal root ganglia (DRG) adjacent to the spinal cord (Brownell et al., 2011). A nerve-derived factor sustains the plasticity of the latter stem cell population, which after injury can transform into interfollicular stem cells and contribute to the regeneration of the interfollicular epidermis (IFE). Upon doing this, these cells downregulate Gli1 expression (dark-blue cells; yellow cells are resident interfollicular cells). (C) An adult mouse prostate duct in which basal epithelial cells (orange) express SHH and signal to the surrounding stroma that consists of GLI1+ (blue) and GLI1– (gray) subepithelial (round) and wrapping (crescent) cells surrounding a smooth muscle layer (circled in gray). Interductal fibroblasts (black ovals) are located between the two ducts and also express Gli1, enabling them to respond to SHH. Nerves from the peripheral nervous system (PNS) innervate the prostate and signal to stromal cells via neurotransmitters (Magnon et al., 2013). Although SHH expression has not been reported in peripheral nervous system (PNS) nerves (dashed orange lines), these nerves might serve as an exogenous source of HH ligand for the interductal prostate stroma.