Myosin II mediates Shh signals to shape dental epithelia via control of cell adhesion and movement.

The development of ectodermal organs begins with the formation of a stratified epithelial placode that progressively invaginates into the underlying mesenchyme as the organ takes its shape. Signaling by secreted molecules is critical for epithelial morphogenesis, but how that information leads to cell rearrangement and tissue shape changes remains an open question. Using the mouse dentition as a model, we first establish that non-muscle myosin II is essential for dental epithelial invagination and show that it functions by promoting cell-cell adhesion and persistent convergent cell movements in the suprabasal layer. Shh signaling controls these processes by inducing myosin II activation via AKT. Pharmacological induction of AKT and myosin II can also rescue defects caused by the inhibition of Shh. Together, our results support a model in which the Shh signal is transmitted through myosin II to power effective cellular rearrangement for proper dental epithelial invagination.

Regulation of Chromatin Modifications through Coordination of Nucleus Size and Epithelial Cell Morphology Heterogeneity.

Cell morphology heterogeneity within epithelial collectives is a pervasive phenomenon intertwined with tissue mechanical properties. Despite its widespread occurrence, the underlying mechanisms driving cell morphology heterogeneity and its consequential biological ramifications remain elusive. Here, we investigate the dynamic evolution of epithelial cell morphology and nucleus morphology during crowding, unveiling a consistent correlation between the two. Our investigation reveals a persistent log-normal probability distribution characterizing both cell and nucleus areas across diverse crowding stages and epithelial model systems. We showed that this morphological diversity arises from asymmetric partitioning during cell division and is perpetuated through actomyosin-mediated regulation of cell-nucleus size coordination. Moreover, we provide insights into the impact of nucleus morphology on chromatin dynamics, demonstrating that constraining nucleus area leads to downregulation of the euchromatic mark H3K9ac and upregulation of the heterochromatic mark H3K27me3 through modulation of histone demethylase UTX expression. These findings under-score the significance of cell morphology heterogeneity as a driver of chromatin state diversity, shaping functional variability within epithelial tissues.

Proliferation-driven mechanical compression induces signalling centre formation during mammalian organ development.

Localized sources of morphogens, called signalling centres, play a fundamental role in coordinating tissue growth and cell fate specification during organogenesis. However, how these signalling centres are established in tissues during embryonic development is still unclear. Here we show that the main signalling centre orchestrating development of rodent incisors, the enamel knot (EK), is specified by a cell proliferation-driven buildup in compressive stresses (mechanical pressure) in the tissue. Direct mechanical measurements indicate that the stresses generated by cell proliferation are resisted by the surrounding tissue, creating a circular pattern of mechanical anisotropy with a region of high compressive stress at its centre that becomes the EK. Pharmacological inhibition of proliferation reduces stresses and suppresses EK formation, and application of external pressure in proliferation-inhibited conditions rescues the formation of the EK. Mechanical information is relayed intracellularly through YAP protein localization, which is cytoplasmic in the region of compressive stress that establishes the EK and nuclear in the stretched anisotropic cells that resist the pressure buildup around the EK. Together, our data identify a new role for proliferation-driven mechanical compression in the specification of a model signalling centre during mammalian organ development.

Using Ex Vivo Live Imaging to Investigate Cell Divisions and Movements During Mouse Dental Renewal.

The continuously growing mouse incisor is emerging as a highly tractable model system to investigate the regulation of adult epithelial and mesenchymal stem cells and tooth regeneration. These progenitor populations actively divide, move, and differentiate to maintain tissue homeostasis and regenerate lost cells in a responsive manner. However, traditional analyses using fixed tissue sections could not capture the dynamic processes of cellular movements and interactions, limiting our ability to study their regulations. This paper describes a protocol to maintain whole mouse incisors in an explant culture system and live-track dental epithelial cells using multiphoton timelapse microscopy. This technique adds to our existing toolbox for dental research and allows investigators to acquire spatiotemporal information on cell behaviors and organizations in a living tissue. We anticipate that this methodology will help researchers further explore mechanisms that control the dynamic cellular processes taking place during both dental renewal and regeneration.

Nail mesenchyme: Tipping the hand on regeneration.

Digit tip regeneration rebuilds amputated structures in some mammals if the nail organ is preserved. In recently published Cell Reports papers, Castilla-Ibeas et al., Johnson et al., and Mahmud et al. define the patterning function and regenerative capacity of the dorsal nail mesenchyme in this process.

Understanding the development of oral epithelial organs through single cell transcriptomic analysis.

During craniofacial development, the oral epithelium begins as a morphologically homogeneous tissue that gives rise to locally complex structures, including the teeth, salivary glands and taste buds. How the epithelium is initially patterned and specified to generate diverse cell types remains largely unknown. To elucidate the genetic programs that direct the formation of distinct oral epithelial populations, we mapped the transcriptional landscape of embryonic day 12 mouse mandibular epithelia at single cell resolution. Our analysis identified key transcription factors and gene regulatory networks that define different epithelial cell types. By examining the spatiotemporal patterning process along the oral-aboral axis, our results propose a model in which the dental field is progressively confined to its position by the formation of the aboral epithelium anteriorly and the non-dental oral epithelium posteriorly. Using our data, we also identified Ntrk2 as a proliferation driver in the forming incisor, contributing to its invagination. Together, our results provide a detailed transcriptional atlas of the embryonic mandibular epithelium, and unveil new genetic markers and regulators that are present during the specification of various oral epithelial structures.

FACEts of mechanical regulation in the morphogenesis of craniofacial structures.

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Targeting acid ceramidase inhibits YAP/TAZ signaling to reduce fibrosis in mice.

Hepatic stellate cells (HSCs) drive hepatic fibrosis. Therapies that inactivate HSCs have clinical potential as antifibrotic agents. We previously identified acid ceramidase (aCDase) as an antifibrotic target. We showed that tricyclic antidepressants (TCAs) reduce hepatic fibrosis by inhibiting aCDase and increasing the bioactive sphingolipid ceramide. We now demonstrate that targeting aCDase inhibits YAP/TAZ activity by potentiating its phosphorylation-mediated proteasomal degradation via the ubiquitin ligase adaptor protein ß-TrCP. In mouse models of fibrosis, pharmacologic inhibition of aCDase or genetic knockout of aCDase in HSCs reduces fibrosis, stromal stiffness, and YAP/TAZ activity. In patients with advanced fibrosis, aCDase expression in HSCs is increased. Consistently, a signature of the genes most down-regulated by ceramide identifies patients with advanced fibrosis who could benefit from aCDase targeting. The findings implicate ceramide as a critical regulator of YAP/TAZ signaling and HSC activation and highlight aCDase as a therapeutic target for the treatment of fibrosis.

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis.

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

A large pool of actively cycling progenitors orchestrates self-renewal and injury repair of an ectodermal appendage.

The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function.

Efforts from diverse disciplines, including evolutionary studies and biomechanical experiments, have yielded new insights into the genetic, signaling, and mechanical control of tooth formation and functions. Evidence from fossils and non-model organisms has revealed that a common set of genes underlie tooth-forming potential of epithelia, and changes in signaling environments subsequently result in specialized dentitions, maintenance of dental stem cells, and other phenotypic adaptations. In addition to chemical signaling, tissue forces generated through epithelial contraction, differential growth, and skeletal constraints act in parallel to shape the tooth throughout development. Here recent advances in understanding dental development from these studies are reviewed and important gaps that can be filled through continued application of evolutionary and biomechanical approaches are discussed.

Non-arteritic ischemic optic neuropathy and supplemental nitric oxide usage.

PURPOSE: To report a case of Non-Arteritic Ischemic Optic Neuropathy (NAION) in a middle-aged bodybuilder in excellent physiological condition without any signs or symptoms of vasculopathy and a history of nitric oxide supplement usage. OBSERVATIONS: The patient had visual acuity of 20/25 in the right eye, and 20/30 in the left eye, with a relative afferent pupillary defect and dyschromatopsia in the right eye. Visual field testing with Humphrey perimetry demonstrated an inferior altitudinal field defect OD. Fundus examination showed a small cupless disc OD with mild pallor, and a small cupless disc OS. He denied usage of sildenafil or other phosphodiesterase (PDE) inhibitor medications but frequently ingested megadoses of nitric oxide (NO) as part of his bodybuilding regimen. CONCLUSIONS: Nitric oxide supplements act through the same pharmacologic pathway as PDE inhibitors, and this case is suggestive that other vasodilating agents may be similarly associated with NAION.

Effect of Diabetes Mellitus and Metformin on Central Corneal Endothelial Cell Density in Eye Bank Eyes.

PURPOSE: To determine whether metformin use and diabetes mellitus (DM) affect central corneal endothelial cell density (ECD) by examining an eye bank corneal donor database. METHODS: The Lions Eye Institute corneal donor database, which consists of 38,318 corneal samples, was examined. Associations of ECD with metformin use and DM were tested by mixed effects linear models that account for correlations of outcomes between eyes within subjects adjusting for age, intraocular lens status, and glaucoma. Subjects (N = 17,056) with observed ECD counts for both eyes are included for analysis. RESULTS: Average donor age was 56.3 (SD = 15.0). ECD was not associated with metformin use (mean ± SE = 2592 ± 11.9 (N = 1014) versus nonuse [2592 ± 3.0 (N = 16,042), P = 0.302]; further analysis showed that ECD was not significantly associated with metformin use in patients with diabetes. However, metformin use was significantly associated with lower ECD among patients with glaucoma: [2658 ± 50.7 (N = 27) for use versus 2789 ± 19.0 (N = 164) for nonuse, P = 0.018]. The presence of DM was significantly associated with lower ECD 2581 ± 5.6 (N = 4766) for DM versus 2595 ± 3.4 (N = 12,290) for non-DM, P = 0.031). CONCLUSIONS: Lower ECD was associated with DM. Lower ECD was not associated with metformin use except in a subgroup of patients with glaucoma, in which subgroup analysis showed lower ECD. The differences in ECD observed were small and unlikely to affect the suitability for transplantation of donor corneas.

Comprehensive Biological Monitoring to Assess Isocyanates and Solvents Exposure in the NSW Australia Motor Vehicle Repair Industry.

Urethane products that contain isocyanates are extensively used in the motor vehicle repair (MVR) industry and other industries such as furniture and cabinet-making as two-pack spray paints, clears, and adhesives. Attention has recently been refocussed on isocyanate-containing chemicals, particularly in paints. The spray painters in the MVR industry had a propensity to develop industrial asthma at a rate 80 times higher than the general public, which was previously reported in the UK. To track workers exposure to isocyanates, urine samples were collected from 196 spray painters who worked mainly in 78 MVR shops across 54 New South Wales (NSW) towns and suburbs. The biological monitoring also covered exposure testing to a wide variety of solvents including aromatic hydrocarbons, ketones, and alcohols. The main finding of the study was that 2.6% of the spray painters surveyed in the MVR industry in NSW that handled isocyanate-containing paints showed exposure to isocyanates; with 1.0% being moderately exposed, which is more than twice the current UK's Health and Safety Executive (HSE) Biological Monitoring Guidance Value (BMGV) of 1 µmol mol-1 creatinine. Potential exposures to toluene (a solvent often found in paint thinners) was monitored via hippuric acid (HA) urine levels and showed 2.6% of the spray painters surveyed to be over the US' American Conference of Government Industrial Hygienists (ACGIH) Biological Exposure Index (BEI) of 1010 mmol/mole creatinine for HA. The other solvents or their metabolites were all below their respective BEI; these comprised benzene, xylene, ethyl benzene, methyl ethyl ketone, acetone, methanol, and ethanol. These findings indicate that isocyanates and certain solvents exposure were occurring in the NSW Australia vehicle repair industry, albeit at lower levels than previous occupational biological monitoring studies that showed higher exposure levels, particularly for isocyanates. One reason for this could be the increasing use of water-based paints in the industry, resulting in lower than expected isocyanate and solvent metabolite levels detected in this more recent study. Further, the completion of sample context form, along with spot urine collection in relation to the isocyanate exposure monitoring work details will provide crucial information to interpret the biological analysis results. The development of new biomarkers of isocyanate oligomer-derived triamines should be incorporated in the assessment of isocyanate exposure in the MVR industry to provide a more complete picture of isocyanate exposure.

Lineage tracing of epithelial cells in developing teeth reveals two strategies for building signaling centers.

An important event in organogenesis is the formation of signaling centers, which are clusters of growth factor-secreting cells. In the case of tooth development, sequentially formed signaling centers known as the initiation knot (IK) and the enamel knot (EK) regulate morphogenesis. However, despite the importance of signaling centers, their origin, as well as the fate of the cells composing them, remain open questions. Here, using lineage tracing of distinct epithelial populations, we found that the EK of the mouse incisor is derived de novo from a group of SRY-box 2 (Sox2)-expressing cells in the posterior half of the tooth germ. Specifically, EK progenitors are located in the posterior ventral basal layer, as demonstrated by DiI labeling of cells. Lineage tracing the formed EK with ShhCreER , which encodes an inducible Cre recombinase under the control of the Sonic hedgehog promoter, at subsequent developmental stages showed that, once formed, some EK cells in the incisor give rise to differentiated cells, whereas in the molar, EK cells give rise to the buccal secondary EK. This work thus establishes the developmental origin as well as the fate of the EK and reveals two strategies for the emergence of serially formed signaling centers: one through de novo establishment and the other by incorporation of progeny from previously formed signaling centers.

An FAK-YAP-mTOR Signaling Axis Regulates Stem Cell-Based Tissue Renewal in Mice.

Tissue homeostasis requires the production of newly differentiated cells from resident adult stem cells. Central to this process is the expansion of undifferentiated intermediates known as transit-amplifying (TA) cells, but how stem cells are triggered to enter this proliferative TA state remains an important open question. Using the continuously growing mouse incisor as a model of stem cell-based tissue renewal, we found that the transcriptional cofactors YAP and TAZ are required both to maintain TA cell proliferation and to inhibit differentiation. Specifically, we identified a pathway involving activation of integrin α3 in TA cells that signals through an LATS-independent FAK/CDC42/PP1A cascade to control YAP-S397 phosphorylation and nuclear localization. This leads to Rheb expression and potentiates mTOR signaling to drive the proliferation of TA cells. These findings thus reveal a YAP/TAZ signaling mechanism that coordinates stem cell expansion and differentiation during organ renewal.

αE-catenin inhibits YAP/TAZ activity to regulate signalling centre formation during tooth development.

Embryonic signalling centres are specialized clusters of non-proliferating cells that direct the development of many organs. However, the mechanisms that establish these essential structures in mammals are not well understood. Here we report, using the murine incisor as a model, that αE-catenin is essential for inhibiting nuclear YAP localization and cell proliferation. This function of αE-catenin is required for formation of the tooth signalling centre, the enamel knot (EK), which maintains dental mesenchymal condensation and epithelial invagination. EK formation depends primarily on the signalling function of αE-catenin through YAP and its homologue TAZ, as opposed to its adhesive function, and combined deletion of Yap and Taz rescues the EK defects caused by loss of αE-catenin. These findings point to a developmental mechanism by which αE-catenin restricts YAP/TAZ activity to establish a group of non-dividing and specialized cells that constitute a signalling centre.

A relative shift in cloacal location repositions external genitalia in amniote evolution.

The move of vertebrates to a terrestrial lifestyle required major adaptations in their locomotory apparatus and reproductive organs. While the fin-to-limb transition has received considerable attention, little is known about the developmental and evolutionary origins of external genitalia. Similarities in gene expression have been interpreted as a potential evolutionary link between the limb and genitals; however, no underlying developmental mechanism has been identified. We re-examined this question using micro-computed tomography, lineage tracing in three amniote clades, and RNA-sequencing-based transcriptional profiling. Here we show that the developmental origin of external genitalia has shifted through evolution, and in some taxa limbs and genitals share a common primordium. In squamates, the genitalia develop directly from the budding hindlimbs, or the remnants thereof, whereas in mice the genital tubercle originates from the ventral and tail bud mesenchyme. The recruitment of different cell populations for genital outgrowth follows a change in the relative position of the cloaca, the genitalia organizing centre. Ectopic grafting of the cloaca demonstrates the conserved ability of different mesenchymal cells to respond to these genitalia-inducing signals. Our results support a limb-like developmental origin of external genitalia as the ancestral condition. Moreover, they suggest that a change in the relative position of the cloacal signalling centre during evolution has led to an altered developmental route for external genitalia in mammals, while preserving parts of the ancestral limb molecular circuitry owing to a common evolutionary origin.

Isolation and culture of dental epithelial stem cells from the adult mouse incisor.

Understanding the cellular and molecular mechanisms that underlie tooth regeneration and renewal has become a topic of great interest(1-4), and the mouse incisor provides a model for these processes. This remarkable organ grows continuously throughout the animal's life and generates all the necessary cell types from active pools of adult stem cells housed in the labial (toward the lip) and lingual (toward the tongue) cervical loop (CL) regions. Only the dental stem cells from the labial CL give rise to ameloblasts that generate enamel, the outer covering of teeth, on the labial surface. This asymmetric enamel formation allows abrasion at the incisor tip, and progenitors and stem cells in the proximal incisor ensure that the dental tissues are constantly replenished. The ability to isolate and grow these progenitor or stem cells in vitro allows their expansion and opens doors to numerous experiments not achievable in vivo, such as high throughput testing of potential stem cell regulatory factors. Here, we describe and demonstrate a reliable and consistent method to culture cells from the labial CL of the mouse incisor.

On the cutting edge of organ renewal: Identification, regulation, and evolution of incisor stem cells.

The rodent incisor is one of a number of organs that grow continuously throughout the life of an animal. Continuous growth of the incisor arose as an evolutionary adaptation to compensate for abrasion at the distal end of the tooth. The sustained turnover of cells that deposit the mineralized dental tissues is made possible by epithelial and mesenchymal stem cells residing at the proximal end of the incisor. A complex network of signaling pathways and transcription factors regulates the formation, maintenance, and differentiation of these stem cells during development and throughout adulthood. Research over the past 15 years has led to significant progress in our understanding of this network, which includes FGF, BMP, Notch, and Hh signaling, as well as cell adhesion molecules and micro-RNAs. This review surveys key historical experiments that laid the foundation of the field and discusses more recent findings that definitively identified the stem cell population, elucidated the regulatory network, and demonstrated possible genetic mechanisms for the evolution of continuously growing teeth.