Stimulation of skeletal stem cells in the growth plate promotes linear bone growth.

Recently, skeletal stem cells were shown to be present in the epiphyseal growth plate (epiphyseal skeletal stem cells, epSSCs), but their function in connection with linear bone growth remains unknown. Here, we explore the possibility that modulating the number of epSSCs can correct differences in leg length. First, we examined regulation of the number and activity of epSSCs by Hedgehog (Hh) signaling. Both systemic activation of Hh pathway with Smoothened agonist (SAG) and genetic activation of Hh pathway by Patched1 (Ptch1) ablation in Pthrp-creER Ptch1fl/fl tdTomato mice promoted proliferation of epSSCs and clonal enlargement. Transient intra-articular administration of SAG also elevated the number of epSSCs. When SAG-containing beads were implanted into the femoral secondary ossification center of 1 leg of rats, this leg was significantly longer 1 month later than the contralateral leg implanted with vehicle-containing beads, an effect that was even more pronounced 2 and 6 months after implantation. We conclude that Hh signaling activates growth plate epSSCs, which effectively leads to increased longitudinal growth of bones. This opens therapeutic possibilities for the treatment of differences in leg length.

Coordinated hedgehog signaling induces new hair follicles in adult skin.

Hair follicle (HF) development is orchestrated by coordinated signals from adjacent epithelial and mesenchymal cells. In humans this process only occurs during embryogenesis and viable strategies to induce new HFs in adult skin are lacking. Here, we reveal that activation of Hedgehog (Hh) signaling in adjacent epithelial and stromal cells induces new HFs in adult, unwounded dorsal mouse skin. Formation of de novo HFs recapitulated embryonic HF development, and mature follicles produced hair co-occurring with epithelial tumors. In contrast, Hh-pathway activation in epithelial or stromal cells alone resulted in tumor formation or stromal cell condensation respectively, without induction of new HFs. Provocatively, adjacent epithelial-stromal Hh-pathway activation induced de novo HFs also in hairless paw skin, divorced from confounding effects of pre-existing niche signals in haired skin. Altogether, cell-type-specific modulation of a single pathway is sufficient to reactivate embryonic programs in adult tissues, thereby inducing complex epithelial structures even without wounding.

We are born with all the hair follicles that we will ever have in our life. These structures are maintained by different types of cells (such as keratinocytes and fibroblasts) that work together to create hair. Follicles form in the embryo thanks to complex molecular signals, which include a molecular cascade known as the Hedgehog signaling pathway. After birth however, these molecular signals are shut down to avoid conflicting messages ­ inappropriate activation of Hedgehog signaling in adult skin, for instance, leads to tumors. This means that our skin loses the ability to make new hair follicles, and if skin is severely damaged it cannot regrow hair or produce the associated sebaceous glands that keep skin moisturized. Being able to create new hair follicles in adult skin would be both functionally and aesthetically beneficial for patients in need, for example, burn victims. Overall, it would also help to understand if and how it is possible to reactivate developmental programs after birth. To investigate this question, Sun, Are et al. triggered Hedgehog signaling in the skin cells of genetically modified mice; this was done either in keratinocytes, in fibroblasts, or in both types of cells. The experiments showed that Hedgehog signaling could produce new hair follicles, but only when activated in keratinocytes and fibroblasts together. The process took several weeks, mirrored normal hair follicle development and resulted in new hair shafts. The follicles grew on both the back of mice, where hair normally occurs, and even in paw areas that are usually hairless. Not unexpectedly the new hair follicles were accompanied with skin tumors. But, promisingly, treatment with Hedgehog-pathway inhibitor Vismodegib restricted tumor growth while keeping the new follicles intact. This suggests that future work on improving "when and where" Hedgehog signaling is activated may allow the formation of new follicles in adult skin with fewer adverse effects.

Dynamics of Lgr6⁺ Progenitor Cells in the Hair Follicle, Sebaceous Gland, and Interfollicular Epidermis.

The dynamics and interactions between stem cell pools in the hair follicle (HF), sebaceous gland (SG), and interfollicular epidermis (IFE) of murine skin are still poorly understood. In this study, we used multicolor lineage tracing to mark Lgr6⁺ -expressing basal cells in the HF isthmus, SG, and IFE.We show that these Lgr6⁺ cells constitute long-term self-renewing populations within each compartment in adult skin. Quantitative analysis of clonal dynamics revealed that the Lgr6⁺ progenitor cells compete neutrally in the IFE, isthmus, and SG, indicating population asymmetry as the underlying mode of tissue renewal. Transcriptional profiling of Lgr6⁺ and Lgr6⁺ cells did not reveal a distinct Lgr6⁺ -associated gene expression signature, raising the question of whether Lgr6⁺ expression requires extrinsic niche signals. Our results elucidate the interrelation and behavior of Lgr6⁺ populations in the IFE, HF, and SG and suggest population asymmetry as a common mechanism for homeostasis in several epithelial skin compartments.

Wounding enhances epidermal tumorigenesis by recruiting hair follicle keratinocytes.

Chronic wounds and acute trauma constitute well-established risk factors for development of epithelial-derived skin tumors, although the underlying mechanisms are largely unknown. Basal cell carcinomas (BCCs) are the most common skin cancers displaying a number of features reminiscent of hair follicle (HF)-derived cells and are dependent on deregulated Hedgehog (Hh)/GLI signaling. Here we show, in a mouse model conditionally expressing GLI1 and in a model with homozygous inactivation of Ptch1, mimicking the situation in human BCCs, that the wound environment accelerates the initiation frequency and growth of BCC-like lesions. Lineage tracing reveals that both oncogene activation and wounding induce emigration of keratinocytes residing in the lower bulge and the nonpermanent part of the HFs toward the interfollicular epidermis (IFE). However, only oncogene activation in combination with a wound environment enables the participation of such cells in the initiation of BCC-like lesions at the HF openings and in the IFE. We conclude that, in addition to the direct enhancement of BCC growth, the tumor-promoting effect of the wound environment is due to recruitment of tumor-initiating cells originating from the neighboring HFs, establishing a link between epidermal wounds and skin cancer risk.

Enterococcus faecalis from newborn babies regulate endogenous PPARgamma activity and IL-10 levels in colonic epithelial cells.

The postembryonic development of the gastrointestinal tract is subject to regulation by the colonizing microbiota. This maturation process requires the commensal bacteria to cross-talk with host cells by way of recognizing receptors and inducing signaling pathways to activate transcription factors such as the nuclear receptors. Here, we show that in colonic cell lines and in primary colonic cells, Enterococcus faecalis isolated from newborn babies possess the ability to regulate peroxisome proliferator-activated receptor-gamma1 (PPARgamma1) activity through phosphorylation. This results in elevated DNA binding and transcriptional activation of downstream target genes, including IL-10, a cytokine known to modulate innate immune function. Furthermore, phosphorylation appears tightly regulated as phospho-PPARgamma1 becomes an immediate substrate for degradation possibly to curtail any extended transactivation. The involvement of PPARgamma1 in a myriad of physiological processes further confirms that microflora-driven regulation might be important for a number of homeostatic strategies in the gut.

The Wnt/beta-catenin signaling pathway targets PPARgamma activity in colon cancer cells.

Control of colon cell fate in adenocarcinomas is disrupted, in part, due to aberrant Wnt/beta-catenin signaling. The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) has been implicated in the development of colon cancers. In the adenomatous polyposis coli multiple intestinal neoplasia (APCMin) mouse cancer model, PPARgamma expression in the colonic mucosa is markedly altered. In addition, PPARgamma protein levels are elevated, possibly through sequestration by activated beta-catenin in colon cancer cell lines. Induction of the Wnt/beta-catenin pathway by LiCl also elevated PPARgamma levels and induced PPARgamma-dependent reporter and endogenous target genes. Mechanistically, PPARgamma, through interactions with beta-catenin and T cell transcription factor (Tcf)-4, may be a determinant of cell fate and is likely a target of the Wnt pathway in cancer cells.