Cell Types Promoting Goosebumps Form a Niche to Regulate Hair Follicle Stem Cells.

Piloerection (goosebumps) requires concerted actions of the hair follicle, the arrector pili muscle (APM), and the sympathetic nerve, providing a model to study interactions across epithelium, mesenchyme, and nerves. Here, we show that APMs and sympathetic nerves form a dual-component niche to modulate hair follicle stem cell (HFSC) activity. Sympathetic nerves form synapse-like structures with HFSCs and regulate HFSCs through norepinephrine, whereas APMs maintain sympathetic innervation to HFSCs. Without norepinephrine signaling, HFSCs enter deep quiescence by down-regulating the cell cycle and metabolism while up-regulating quiescence regulators Foxp1 and Fgf18. During development, HFSC progeny secretes Sonic Hedgehog (SHH) to direct the formation of this APM-sympathetic nerve niche, which in turn controls hair follicle regeneration in adults. Our results reveal a reciprocal interdependence between a regenerative tissue and its niche at different stages and demonstrate sympathetic nerves can modulate stem cells through synapse-like connections and neurotransmitters to couple tissue production with demands.

Corticosterone inhibits GAS6 to govern hair follicle stem-cell quiescence.

Chronic, sustained exposure to stressors can profoundly affect tissue homeostasis, although the mechanisms by which these changes occur are largely unknown. Here we report that the stress hormone corticosterone-which is derived from the adrenal gland and is the rodent equivalent of cortisol in humans-regulates hair follicle stem cell (HFSC) quiescence and hair growth in mice. In the absence of systemic corticosterone, HFSCs enter substantially more rounds of the regeneration cycle throughout life. Conversely, under chronic stress, increased levels of corticosterone prolong HFSC quiescence and maintain hair follicles in an extended resting phase. Mechanistically, corticosterone acts on the dermal papillae to suppress the expression of Gas6, a gene that encodes the secreted factor growth arrest specific 6. Restoring Gas6 expression overcomes the stress-induced inhibition of HFSC activation and hair growth. Our work identifies corticosterone as a systemic inhibitor of HFSC activity through its effect on the niche, and demonstrates that the removal of such inhibition drives HFSCs into frequent regeneration cycles, with no observable defects in the long-term.

Hair follicles' transit-amplifying cells govern concurrent dermal adipocyte production through Sonic Hedgehog.

Growth and regeneration of one tissue within an organ compels accommodative changes in the surrounding tissues. However, the molecular nature and operating logic governing these concurrent changes remain poorly defined. The dermal adipose layer expands concomitantly with hair follicle downgrowth, providing a paradigm for studying coordinated changes of surrounding lineages with a regenerating tissue. Here, we discover that hair follicle transit-amplifying cells (HF-TACs) play an essential role in orchestrating dermal adipogenesis through secreting Sonic Hedgehog (SHH). Depletion of Shh from HF-TACs abrogates both dermal adipogenesis and hair follicle growth. Using cell type-specific deletion of Smo, a gene required in SHH-receiving cells, we found that SHH does not act on hair follicles, adipocytes, endothelial cells, and hematopoietic cells for adipogenesis. Instead, SHH acts directly on adipocyte precursors, promoting their proliferation and their expression of a key adipogenic gene, peroxisome proliferator-activated receptor γ (Pparg), to induce dermal adipogenesis. Our study therefore uncovers a critical role for TACs in orchestrating the generation of both their own progeny and a neighboring lineage to achieve concomitant tissue production across lineages.

Spatiotemporal expression and transcriptional perturbations by long noncoding RNAs in the mouse brain.

Long noncoding RNAs (lncRNAs) have been implicated in numerous cellular processes including brain development. However, the in vivo expression dynamics and molecular pathways regulated by these loci are not well understood. Here, we leveraged a cohort of 13 lncRNAnull mutant mouse models to investigate the spatiotemporal expression of lncRNAs in the developing and adult brain and the transcriptome alterations resulting from the loss of these lncRNA loci. We show that several lncRNAs are differentially expressed both in time and space, with some presenting highly restricted expression in only selected brain regions. We further demonstrate altered regulation of genes for a large variety of cellular pathways and processes upon deletion of the lncRNA loci. Finally, we found that 4 of the 13 lncRNAs significantly affect the expression of several neighboring proteincoding genes in a cis-like manner. By providing insight into the endogenous expression patterns and the transcriptional perturbations caused by deletion of the lncRNA locus in the developing and postnatal mammalian brain, these data provide a resource to facilitate future examination of the specific functional relevance of these genes in neural development, brain function, and disease.

Airway basal stem cells are necessary for the maintenance of functional intraepithelial airway macrophages.

Adult stem cells play a crucial role in tissue homeostasis and repair through multiple mechanisms. In addition to being able to replace aged or damaged cells, stem cells provide signals that contribute to the maintenance and function of neighboring cells. In the lung, airway basal stem cells also produce cytokines and chemokines in response to inhaled irritants, allergens, and pathogens, which affect specific immune cell populations and shape the nature of the immune response. However, direct cell-to-cell signaling through contact between airway basal stem cells and immune cells has not been demonstrated. Recently, a unique population of intraepithelial airway macrophages (IAMs) has been identified in the murine trachea. Here, we demonstrate that IAMs require Notch signaling from airway basal stem cells for maintenance of their differentiated state and function. Furthermore, we demonstrate that Notch signaling between airway basal stem cells and IAMs is required for antigen-induced allergic inflammation only in the trachea where the basal stem cells are located whereas allergic responses in distal lung tissues are preserved consistent with a local circuit linking stem cells to proximate immune cells. Finally, we demonstrate that IAM-like cells are present in human conducting airways and that these cells display Notch activation, mirroring their murine counterparts. Since diverse lung stem cells have recently been identified and localized to specific anatomic niches along the proximodistal axis of the respiratory tree, we hypothesize that the direct functional coupling of local stem cell-mediated regeneration and immune responses permits a compartmentalized inflammatory response.

Ciliated cells of pseudostratified airway epithelium do not become mucous cells after ovalbumin challenge.

Mucous cell metaplasia is a hallmark of airway diseases, such as asthma and chronic obstructive pulmonary disease. The majority of human airway epithelium is pseudostratified, but the cell of origin of mucous cells has not been definitively established in this type of airway epithelium. There is evidence that ciliated, club cell (Clara), and basal cells can all give rise to mucus-producing cells in different contexts. Because pseudostratified airway epithelium contains distinct progenitor cells from simple columnar airway epithelium, the lineage relationships of progenitor cells to mucous cells may be different in these two epithelial types. We therefore performed lineage tracing of the ciliated cells of the murine basal cell-containing airway epithelium in conjunction with the ovalbumin (OVA)-induced murine model of allergic lung disease. We genetically labeled ciliated cells with enhanced Yellow Fluorescent Protein (eYFP) before the allergen challenge, and followed the fate of these cells to determine whether they gave rise to newly formed mucous cells. Although ciliated cells increased in number after the OVA challenge, the newly formed mucous cells were not labeled with the eYFP lineage tag. Even small numbers of labeled mucous cells could not be detected, implying that ciliated cells make virtually no contribution to the new goblet cell pool. This demonstrates that, after OVA challenge, new mucous cells do not originate from ciliated cells in a pseudostratified basal cell-containing airway epithelium.

In Situ Modification of Tissue Stem and Progenitor Cell Genomes.

In vivo delivery of genome-modifying enzymes holds significant promise for therapeutic applications and functional genetic screening. Delivery to endogenous tissue stem cells, which provide an enduring source of cell replacement during homeostasis and regeneration, is of particular interest. Here, we use a sensitive Cre/lox fluorescent reporter system to test the efficiency of genome modification following in vivo transduction by adeno-associated viruses (AAVs) in tissue stem and progenitor cells. We combine immunophenotypic analyses with in vitro and in vivo assays of stem cell function to reveal effective targeting of skeletal muscle satellite cells, mesenchymal progenitors, hematopoietic stem cells, and dermal cell subsets using multiple AAV serotypes. Genome modification rates achieved through this system reached >60%, and modified cells retained key functional properties. This study establishes a powerful platform to genetically alter tissue progenitors within their physiological niche while preserving their native stem cell properties and regulatory interactions.

Fate by Chance, not by Choice: Epidermal Stem Cells Go Live.

The skin epidermis is constantly renewed by epidermal stem cells. In a recent Science paper, Rompolas et al. utilize live imaging to track epidermal stem cells over their lifetimes. Their findings provide new insights into epidermal stem cell behaviors and unravel how newly generated cells are integrated into pre-existing tissues.

Yap tunes airway epithelial size and architecture by regulating the identity, maintenance, and self-renewal of stem cells.

Our understanding of how stem cells are regulated to maintain appropriate tissue size and architecture is incomplete. We show that Yap (Yes-associated protein 1) is required for the actual maintenance of an adult mammalian stem cell. Without Yap, adult airway basal stem cells are lost through their unrestrained differentiation, resulting in the simplification of a pseudostratified epithelium into a columnar one. Conversely, Yap overexpression increases stem cell self-renewal and blocks terminal differentiation, resulting in epithelial hyperplasia and stratification. Yap overexpression in differentiated secretory cells causes them to partially reprogram and adopt a stem cell-like identity. In contrast, Yap knockdown prevents the dedifferentiation of secretory cells into stem cells. We then show that Yap functionally interacts with p63, the cardinal transcription factor associated with myriad epithelial basal stem cells. In aggregate, we show that Yap regulates all of the cardinal behaviors of airway epithelial stem cells and determines epithelial architecture.