Secretion of shh by a neurovascular bundle niche supports mesenchymal stem cell homeostasis in the adult mouse incisor

Pericytes Are Odontoblast Progenitor Cells Depending on ER Stress

Odontoblasts are terminally differentiated cells that exhibit mechanosensitivity and mineralization capacity. Mechanosensitive ion channels such as Piezo1 are present in odontoblasts and are associated with their physiological functions via Ca2+ signaling. Both Ca2+ signals via Ca2+ influx from mechanosensitive ion channels and Ca2+ release from Ca2+ stores function as secondary messenger systems for various biological phenomena. The endoplasmic reticulum (ER) serves as an intracellular Ca2+ store that mobilizes intracellular Ca2+. Changes in Ca2+ concentration inside the ER are among the factors that cause ER stress. Perivascular cells are located around odontoblasts in the dental pulp. Although such formation indicates that perivascular cells interact with odontoblasts, their detailed profiles under developmental and pathological conditions remain unclear. In this study, we revealed that pericyte marker, neural/glial antigen 2 (NG2)-positive cells, in cell-rich zones (CZs) can differentiate into Piezo1-positive odontoblasts following genetic odontoblast depletion in mice, and modeled as odontoblast death after severe dentin injury and as reparative dentin formation. NG2-positive pericytes differentiated into odontoblasts faster than glial cells. To determine how NG2-positive cells differentiate into Piezo1-positive odontoblasts, we focused on the ER-stress sensor protein, activating transcription factor 6a (ATF6a). After genetic odontoblast depletion, NG2-positive cells regenerated in the odontoblast layer and were capable of acting as functional odontoblasts. In the presence of extracellular Ca2+, the application of a sarco/ER Ca2+-ATPase (SERCA) inhibitor, thapsigargin, known as an ER-stress inducer, increased the intracellular Ca2+ concentration in the odontoblast lineage cells (OLCs). The increase was significantly inhibited by the application of a pharmacologic Piezo1 inhibitor, indicating that ER stress by SERCA inhibition augmented Piezo1-induced responses in odontoblast progenitor cells. However, the physiological activation of Gq-coupled receptors by adenosine diphosphate did not induce Piezo1 activation. Gene silencing of ATF6a and/or NG2 impaired the mineralization of OLCs. Overall, ATF6a orchestrates the differentiation of NG2-positive pericytes into functional odontoblasts that act as sensory receptor cells and dentin-forming cells.

Mesenchymal Meis2 controls whisker development independently from trigeminal sensory innervation

Hair follicle development is initiated by reciprocal molecular interactions between the placode-forming epithelium and the underlying mesenchyme. Cell fate transformation in dermal fibroblasts generates a cell niche for placode induction by activation of signaling pathways WNT, EDA, and FGF in the epithelium. These successive paracrine epithelial signals initiate dermal condensation in the underlying mesenchyme. Although epithelial signaling from the placode to mesenchyme is better described, little is known about primary mesenchymal signals resulting in placode induction. Using genetic approach in mice, we show that Meis2 expression in cells derived from the neural crest is critical for whisker formation and also for branching of trigeminal nerves. While whisker formation is independent of the trigeminal sensory innervation, MEIS2 in mesenchymal dermal cells orchestrates the initial steps of epithelial placode formation and subsequent dermal condensation. MEIS2 regulates the expression of transcription factor Foxd1, which is typical of pre-dermal condensation. However, deletion of Foxd1 does not affect whisker development. Overall, our data suggest an early role of mesenchymal MEIS2 during whisker formation and provide evidence that whiskers can normally develop in the absence of sensory innervation or Foxd1 expression.

Inflammation-triggered Gli1+ stem cells engage with extracellular vesicles to prime aberrant neutrophils to exacerbate periodontal immunopathology

Periodontitis is a prevalent and progressive detrimental disease characterized by chronic inflammation, and the immunopathological mechanisms are not yet fully understood. Mesenchymal stem cells (MSCs) play crucial roles as immunoregulators and maintain tissue homeostasis and regeneration, but their in vivo function in immunopathology and periodontal tissue deterioration is still unclear. Here, we utilized multiple transgenic mouse models to specifically mark, ablate and modulate Gli1+ cells, a critical and representative subset of MSCs in the periodontium, to explore their specific role in periodontal immunopathology. We revealed that Gli1+ cells, upon challenge with an inflammatory microenvironment, significantly induce rapid trafficking and aberrant activation of neutrophils, thus exacerbating alveolar bone destruction. Mechanistically, extracellular vesicles (EVs) released by Gli1+ cells act as crucial immune regulators in periodontal tissue, mediating the recruitment and activation of neutrophils through increased neutrophil generation of reactive oxygen species and stimulation of nuclear factor kappa-B signaling. Furthermore, we discovered that CXC motif chemokine ligand 1 (CXCL1) is exposed on the surface of EVs derived from inflammation-challenged Gli1+ cells to prime aberrant neutrophils via the CXCL1-CXC motif chemokine receptor 2 (CXCR2) axis. Importantly, specific inhibition of EV release from Gli1+ cells or pharmacological therapy with GANT61 ameliorates periodontal inflammation and alveolar bone loss. Collectively, our findings identify previously unrecognized roles of Gli1+ cells in orchestrating infiltration and promoting aberrant activation of neutrophils under inflammation, which provides pathological insights and potential therapeutic targets for periodontitis.

The Role of Gli1+ Mesenchymal Stem Cells in Craniofacial Development and Disease Treatment

OBJECTIVE

This review summarises the role of Gli1+ (Glioma-associated oncogene homologue 1) mesenchymal stem cells in craniofacial growth and development or tissue repair, and their application in the treatment of some diseases.

DESIGN

The search for this narrative review was conducted in PubMed and Web of Science using relevant keywords, including checking reference lists of journal articles by hand searching.

RESULTS

Gli1+ mesenchymal stem cells play an important role in the growth and development of the skull, tooth, periodontium and mandibular condyle. They can be applied to the treatment of pulp and periodontal diseases, temporomandibular joint osteoarthritis and other diseases.

CONCLUSIONS

Gli1+ mesenchymal stem cells are crucial for the development and repair of craniofacial tissue.

Human dental pulp stem cells for spinal cord injury

Spinal cord injury (SCI) is a serious neurological disorder that causes loss of mobility, pain, and autonomic dysfunction, resulting in altered sensation and devastating loss of function. Current treatments for SCI mainly focus on surgery and drug therapy to promote neurological recovery. However, there are virtually no effective remedies for irreversible nerve damage that result in a victim's loss of motor function and sensory changes that occur after an injury. With the continuous development of medical technology, stem-cell-based regenerative medicine provides researchers with new treatment ideas. The effectiveness of mesenchymal stem cells and their derivatives from different sources in treating SCI varies. Recent studies have highlighted that dental pulp stem cells (DPSCs) may contribute to anti-inflammatory regulation, anti-apoptotic regulation, and axonal regeneration in the treatment of SCI patients. In addition, the combination of new biomaterials and dental pulp stem cells is promising in the treatment of SCI. This article reviews the role of DPSCs in SCI treatment in recent years, discusses the advantages of DPSCs, explores potential development directions, and looks forward to providing new insights for future research in this critical field.

Decoding SFRP2 progenitors in sustaining tooth growth at single-cell resolution

BACKGROUND

Single-cell transcriptomics has revolutionized tooth biology by uncovering previously unexplored areas. The mouse is a widely used model for studying human tissues and diseases, including dental pulp tissues. While human and mouse molars share many similarities, mouse incisors differ significantly from human teeth due to their continuous growth throughout their lifespan. The application of findings from mouse teeth to human disease remains insufficiently explored.

METHODS

Leveraging multiple single-cell datasets, we constructed a comprehensive dental pulp cell landscape to delineate tissue similarities and species-specific differences between humans and mice.

RESULTS

We identified a distinct cell population, Sfrp2hi fibroblast progenitors, found exclusively in mouse incisors and the developing tooth root of human molars. These cells play a crucial role in sustaining continuous tissue growth. Mechanistically, we found that the transcription factor Twist1, regulated via MAPK phosphorylation, binds to the Sfrp2 promoter and modulates Wnt signaling activation to maintain stem cell identity.

CONCLUSIONS

Our study reveals a previously unrecognized subset of dental mesenchymal stem cells critical for tooth growth. This distinct subset, evolutionarily conserved between humans and mice, provides valuable insights into translational approaches for dental tissue regeneration and repair.

Enhancer-driven Shh signaling promotes glia-to-mesenchyme transition during bone repair

Plp1-lineage Schwann cells (SCs) of peripheral nerve play a critical role in vascular remodeling and osteogenic differentiation during the early stage of bone healing, and the abnormal plasticity of SCs would jeopardize the bone regeneration. However, how Plp1-lineage cells respond to injury and initiate the vascularized osteogenesis remains incompletely understood. Here, by employing single-cell transcriptional profiling combined with lineage-specific tracing models, we uncover that Plp1-lineage cells undergoing injury-induced glia-to-MSCs transition contributed to osteogenesis and revascularization in the initial stage of bone injury. Importantly, our data demonstrated that the Sonic hedgehog (Shh) signaling was responsible for the transition process initiation, which was strongly activated by c-Jun/SIRT6/BAF170 complex-driven Shh enhancers. Collectively, these findings depict an injury-specific niche signal-mediated Plp1-lineage cells transition towards Gli1+ MSCs and may be instructive for approaches to promote bone regeneration during aging or other bone diseases.

In vivo dynamics of hard tissue-forming cell origins: Insights from Cre/loxP-based cell lineage tracing studies

Bone tissue provides structural support for our bodies, with the inner bone marrow (BM) acting as a hematopoietic organ. Within the BM tissue, two types of stem cells play crucial roles: mesenchymal stem cells (MSCs) (or skeletal stem cells) and hematopoietic stem cells (HSCs). These stem cells are intricately connected, where BM-MSCs give rise to bone-forming osteoblasts and serve as essential components in the BM microenvironment for sustaining HSCs. Despite the mid-20th century proposal of BM-MSCs, their in vivo identification remained elusive owing to a lack of tools for analyzing stemness, specifically self-renewal and multipotency. To address this challenge, Cre/loxP-based cell lineage tracing analyses are being employed. This technology facilitated the in vivo labeling of specific cells, enabling the tracking of their lineage, determining their stemness, and providing a deeper understanding of the in vivo dynamics governing stem cell populations responsible for maintaining hard tissues. This review delves into cell lineage tracing studies conducted using commonly employed genetically modified mice expressing Cre under the influence of LepR, Gli1, and Axin2 genes. These studies focus on research fields spanning long bones and oral/maxillofacial hard tissues, offering insights into the in vivo dynamics of stem cell populations crucial for hard tissue homeostasis.

Comprehensive analysis of transcription factors involved in odontoblast differentiation mechanism

Primary cultured odontoblasts rapidly lose their tissue-specific phenotype. To identify transcription factors (TF) that are important for the maintenance of the odontoblast phenotype, primary cultures of C57BL/6 J mouse dental mesenchymal cells (DMC) were isolated, and expression of TF and odontoblast marker genes in cells immediately after isolation and 2 days after culture were comprehensively evaluated and compared using RNA-sequencing (RNA-seq). The expression of odontoblast markers in mouse dental mesenchymal cells decreased rapidly after isolation. In addition, the expression of Hedgehog-related, Notch-related, and immediate- early gene (IEG)-related transcription factors significantly decreased. Forced expression of these genes in lentiviral vectors, together with fibroblast growth factor 4 (FGF4), fibroblast growth factor 9 (FGF9), and the Wnt pathway activator CHIR99021, significantly induced the expression of odontogenic marker genes. These results indicate, for the first time, that Notch signaling and early genes may be important for maintaining odontoblast cultures. Furthermore, simultaneous stimulation of FGF, Wnt, Hedgehog, Notch pathways, and IEG transcription factors cooperatively promoted the maintenance of the odontoblast phenotype. These results suggest that the Hedgehog and Notch signaling pathways may play an important role in maintaining odontoblast phenotypes, in addition to FGF and Wnt signaling.

Unraveling the hidden complexity: Exploring dental tissues through single-cell transcriptional profiling

Understanding the composition and function of cells constituting tissues and organs is vital for unraveling biological processes. Single-cell analysis has allowed us to move beyond traditional methods of categorizing cell types. This innovative technology allows the transcriptional and epigenetic profiling of numerous individual cells, leading to significant insights into the development, homeostasis, and pathology of various organs and tissues in both animal models and human samples. In this review, we delve into the outcomes of major investigations using single-cell transcriptomics to decipher the cellular composition of mammalian teeth and periodontal tissues. The recent single-cell transcriptome-based studies have traced in detail the dental epithelium-ameloblast lineage and dental mesenchyme lineages in the mouse incisors and the tooth germ of both mice and humans; unraveled the microenvironment, the identity of niche cells, and cellular intricacies in the dental pulp; shed light on the molecular mechanisms orchestrating root formation; and characterized cellular dynamics of the periodontal ligament. Additionally, cellular components in dental pulps were compared between healthy and carious teeth at a single-cell level. Each section of this review contributes to a comprehensive understanding of tooth biology, offering valuable insights into developmental processes, niche cell identification, and the molecular secrets of the dental environment.

Cathepsin K-Positive Cell Lineage Promotes In Situ Dentin Formation Controlled by Nociceptive Sonic Hedgehog

Oral diseases affect nearly half of the global population throughout their lifetime causing pain, as estimated by the World Health Organization. Preservation of vital pulp is the therapeutic core as well as a challenge to protect natural teeth. Current bottleneck lies in that the regenerative capacity of injured pulp is undetermined. In this study, we identified a lifelong lineage that is labelled by cathepsin K (Ctsk) contributing to the physiological, reactionary and reparative odontogenesis of mouse molars. Ctsk+ cell-mediated dentin formation is regulated by nociceptive nerve-derived Sonic Hedgehog (Shh), especially rapidly responsive to acute injury. Notably, exogenous Shh protein to the injury pulp can preserve Ctsk+ cell capacity of odontogenesis for the nearby crown pulp and even remote root apex growth, alleviating conventionally developmental arrest in youth pulpitis. Exposed to chronical attrition, aged pulp Ctsk+ cells still hold the capacity to respond to acute stimuli and promote reparative odontogenesis, also enhanced by exogenous Shh capping. Therefore, Ctsk+ cells may be one of the lineages for accelerating precision medicine for efficient pulp treatment across ages. Shh application can be a candidate for vital pulp preservation and pulp injury repair by promoting regenerative odontogenesis to a certain extent from young adults to older individuals.

Deciphering the spatial distribution of Gli1-lineage cells in dental, oral, and craniofacial regions

The craniofacial bone, crucial for protecting brain tissue and supporting facial structure, undergoes continuous remodeling through mesenchymal (MSCs) or skeletal stem cells in their niches. Gli1 is an ideal marker for labeling MSCs and osteoprogenitors in this region, and Gli1-lineage cells are identified as pivotal for bone growth, development, repair, and regeneration. Despite its significance, the distribution of Gli1-lineage cells across the dental, oral, and craniofacial (DOC) regions remains to be systematically explored. Utilizing tissue-clearing and light sheet fluorescence microscopy with a Gli1CreER; tdTomatoAi14 mouse model, we mapped the spatial distribution of Gli1-lineage cells throughout the skull, focusing on calvarial bones, sutures, bone marrow, teeth, periodontium, jaw bones, and the temporomandibular joint. We found Gli1-lineage cells widespread in these areas, underscoring their significance in DOC regions. Additionally, we observed their role in repairing calvarial bone defects, providing novel insights into craniofacial biology and stem cell niches and enhancing our understanding of stem cells and their progeny's behavior in vivo.

The sensory nerve regulates stem cell homeostasis through Wnt5a signaling

Increasing evidence indicates that nerves play a significant role in regulating stem cell homeostasis and developmental processes. To explore the impact of nerves on epithelial stem cell homeostasis during tooth development, the regulation of sensory nerves on stem cell homeostasis was investigated using a rat model of incisor development. Impaired mineralization, decreased enamel thickness, and fractured enamel rods of the incisor were observed after denervation. qPCR and histological staining revealed that the expression of enamel-related factors ameloblastin (AMBN), kallikrein-4, amelogenin (Amelx), collagen type XVII (col17a), and enamelin were decreased in the incisor enamel of rats with sensory nerve injure. The decreased expression of Wnt5a in ameloblasts was coupled with the downregulation of calcium ion-related calmodulin kinase II. These results implicate that the sensory nerves are essential in stem cell homeostasis for enamel mineralization and development.

Odontogenesis-Empowered Extracellular Vesicles Safeguard Donor-Recipient Stem Cell Interplay to Support Tooth Regeneration

Harnessing the developmental events of mesenchymal condensation to direct postnatal dental stem cell aggregation represents a cutting-edge and promising approach to tooth regeneration. Tooth avulsion is among the most prevalent and serious dental injuries, and odontogenic aggregates assembled by stem cells from human exfoliated deciduous teeth (SHED) have proven effective in revitalizing avulsed teeth after replantation in the clinical trial. However, whether and how SHED aggregates (SA) communicate with recipient components and promote synergistic tissue regeneration to support replanted teeth remains elusive. Here, it is shown that SA-mediated avulsed tooth regeneration involves periodontal restoration and recovery of recipient Gli1+ stem cells, which are mobilized and necessarily contribute to the reestablishment of the tooth-periodontal ligament-bone interface. Mechanistically, the release of extracellular vesicles (EVs) is revealed indispensable for the implanted SA to mobilize recipient Gli1+ cells and regenerate avulsed teeth. Furthermore, SHED aggregates-released EVs (SA-EVs) are featured with odontogenic properties linked to tissue regeneration, which enhance migration, proliferation, and differentiation of Gli1+ cells. Importantly, local application of SA-EVs per se empowers recipient Gli1+ cells and safeguards regeneration of avulsed teeth. Collectively, the findings establish a paradigm in which odontogenesis-featured EVs govern donor-recipient stem cell interplay to achieve tooth regeneration, inspiring cell-free translational regenerative strategies.

Neural regulation of mesenchymal stem cells in craniofacial bone: development, homeostasis and repair

Mesenchymal stem cells endow various functions, including proliferation, multipotency, migration, etc. Craniofacial bones originate from the cranial neural crest and are developed mainly through intramembranous ossification, which are different from long bones. There are varied mesenchymal stem cells existing in the craniofacial bone, including Gli1 + cells, Axin2 + cells, Prx1 + cells, etc. Nerves distributed in craniofacial area are also derived from the neural crest, and the trigeminal nerve is the major sensory nerve in craniofacial area. The nerves and the skeleton are tightly linked spatially, and the skeleton is broadly innervated by sensory and sympathetic nerves, which also participate in bone development, homeostasis and healing process. In this review, we summarize mesenchymal stem cells located in craniofacial bone or, to be more specific, in jaws, temporomandibular joint and cranial sutures. Then we discuss the research advance concerning neural regulation of mesenchymal stem cells in craniofacial bone, mainly focused on development, homeostasis and repair. Discovery of neural regulation of mesenchymal stem cells may assist in treatment in the craniofacial bone diseases or injuries.

Distraction force promotes the osteogenic differentiation of Gli1+ cells in facial sutures via primary cilia-mediated Hedgehog signaling pathway

BACKGROUND

Trans-sutural distraction osteogenesis (TSDO) involves the application of distraction force to facial sutures to stimulate osteogenesis. Gli1+ cells in the cranial sutures play an important role in bone growth. However, whether Gli1+ cells in facial sutures differentiate into bone under distraction force is unknown.

METHODS

4-week-old Gli1ER/Td and C57BL/6 mice were used to establish a TSDO model to explore osteogenesis of zygomaticomaxillary sutures. A Gli1+ cell lineage tracing model was used to observe the distribution of Gli1+ cells and explore the role of Gli1+ cells in facial bone remodeling.

RESULTS

Distraction force promoted bone remodeling during TSDO. Fluorescence and two-photon scanning images revealed the distribution of Gli1+ cells. Under distraction force, Gli1-lineage cells proliferated significantly and co-localized with Runx2+ cells. Hedgehog signaling was upregulated in Gli1+ cells. Inhibition of Hedgehog signaling suppresses the proliferation and osteogenesis of Gli1+ cells induced by distraction force. Subsequently, the stem cell characteristics of Gli1+ cells were identified. Cell-stretching experiments verified that mechanical force promoted the osteogenic differentiation of Gli1+ cells through Hh signaling. Furthermore, immunofluorescence staining and RT-qPCR experiments demonstrated that the primary cilia in Gli1+ cells exhibit Hedgehog-independent mechanosensitivity, which was required for the osteogenic differentiation induced by mechanical force.

CONCLUSIONS

Our study indicates that the primary cilia of Gli1+ cells sense mechanical stimuli, mediate Hedgehog signaling activation, and promote the osteogenic differentiation of Gli1+ cells in zygomaticomaxillary sutures.

Bone or Tooth dentin: The TGF-β signaling is the key

To investigate the cell linkage between tooth dentin and bones, we studied TGF-β roles during postnatal dentin development using TGF-β receptor 2 (Tgfβr2) cKO models and cell lineage tracing approaches. Micro-CT showed that the early Tgfβr2 cKO exhibit short roots and thin root dentin (n = 4; p<0.01), a switch from multilayer pre-odontoblasts/odontoblasts to a single-layer of bone-like cells with a significant loss of ~85% of dentinal tubules (n = 4; p<0.01), and a matrix shift from dentin to bone. Mechanistic studies revealed a statistically significant decrease in odontogenic markers, and a sharp increase in bone markers. The late Tgfβr2 cKO teeth displayed losses of odontoblast polarity, a significant reduction in crown dentin volume, and the onset of massive bone-like structures in the crown pulp with high expression levels of bone markers and low levels of dentin markers. We thus concluded that bones and tooth dentin are in the same evolutionary linkage in which TGF-β signaling defines the odontogenic fate of dental mesenchymal cells and odontoblasts. This finding also raises the possibility of switching the pulp odontogenic to the osteogenic feature of pulp cells via a local manipulation of gene programs in future treatment of tooth fractures.

Mapping Somatosensory Afferent Circuitry to Bone Identifies Neurotrophic Signals Required for Fracture Healing

The profound pain accompanying bone fracture is mediated by somatosensory neurons, which also appear to be required to initiate bone regeneration following fracture. Surprisingly, the precise neuroanatomical circuitry mediating skeletal nociception and regeneration remains incompletely understood. Here, we characterized somatosensory dorsal root ganglia (DRG) afferent neurons innervating murine long bones before and after experimental long bone fracture in mice. Retrograde labeling of DRG neurons by an adeno-associated virus with peripheral nerve tropism showed AAV-tdT signal. Single cell transcriptomic profiling of 6,648 DRG neurons showed highest labeling across CGRP+ neuron clusters (6.9-17.2%) belonging to unmyelinated C fibers, thinly myelinated Aδ fibers and Aβ-Field LTMR (9.2%). Gene expression profiles of retrograde labeled DRG neurons over multiple timepoints following experimental stress fracture revealed dynamic changes in gene expression corresponding to the acute inflammatory ( S100a8 , S100a9 ) and mechanical force ( Piezo2 ). Reparative phase after fracture included morphogens such as Tgfb1, Fgf9 and Fgf18 . Two methods to surgically or genetically denervate fractured bones were used in combination with scRNA-seq to implicate defective mesenchymal cell proliferation and osteodifferentiation as underlying the poor bone repair capacity in the presence of attenuated innervation. Finally, multi-tissue scRNA-seq and interactome analyses implicated neuron-derived FGF9 as a potent regulator of fracture repair, a finding compatible with in vitro assessments of neuron-to-skeletal mesenchyme interactions.

Vascular architecture regulates mesenchymal stromal cell heterogeneity via P53-PDGF signaling in the mouse incisor

Mesenchymal stem cells (MSCs) reside in niches to maintain tissue homeostasis and contribute to repair and regeneration. Although the physiological functions of blood and lymphatic vasculature are well studied, their regulation of MSCs as niche components remains largely unknown. Using adult mouse incisors as a model, we uncover the role of Trp53 in regulating vascular composition through THBS2 to maintain mesenchymal tissue homeostasis. Loss of Trp53 in GLI1+ progeny increases arteries and decreases other vessel types. Platelet-derived growth factors from arteries deposit in the MSC region and interact with PDGFRA and PDGFRB. Significantly, PDGFRA+ and PDGFRB+ cells differentially contribute to defined cell lineages in the adult mouse incisor. Collectively, our results highlight Trp53's importance in regulating the vascular niche for MSCs. They also shed light on how different arterial cells provide unique cues to regulate MSC subpopulations and maintain their heterogeneity. Furthermore, they provide mechanistic insight into MSC-vasculature crosstalk.

Osteoblast differentiation of Gli1⁺ cells via Wnt and BMP signaling pathways during orthodontic tooth movement

OBJECTIVES

Factors that induce bone formation during orthodontic tooth movement (OTM) remain unclear. Gli1 was recently identified as a stem cell marker in the periodontal ligament (PDL). Therefore, we evaluated the mechanism of differentiation of Cre/LoxP-mediated Gli1/Tomato+ cells into osteoblasts during OTM.

METHODS

After the final administration of tamoxifen to 8-week-old Gli1-CreERT2/ROSA26-loxP-stop-loxP-tdTomato mice for 2 days, nickel-titanium closed coil springs were attached between the upper anterior alveolar bone and the first molar. Immunohistochemical localizations of β-catenin, Smad4, and Runx2 were observed in the PDL on 2, 5, and 10 days after OTM initiation.

RESULTS

In the untreated tooth, few Gli1/Tomato+ cells were detected in the PDL. Two days after OTM initiation, the number of Gli1/Tomato+ cells increased in the PDL on the tension side. On this side, 49.3 ± 7.0% of β-catenin+ and 48.7 ± 5.7% of Smad4+ cells were found in the PDL, and Runx2 expression was detected in some Gli1/Tomato+ cells apart from the alveolar bone. The number of positive cells in the PDL reached a maximum on day 5. In contrast, on the compression side, β-catenin and Smad4 exhibited less immunoreactivity. On day 10, Gli1/Tomato+ cells were aligned on the alveolar bone on the tension side, with some expressing Runx2.

CONCLUSIONS

Gli1+ cells in the PDL differentiated into osteoblasts during OTM. Wnt and bone morphogenetic proteins signaling pathways may be involved in this differentiation.

ARID1B maintains mesenchymal stem cell quiescence via inhibition of BCL11B-mediated non-canonical Activin signaling

ARID1B haploinsufficiency in humans causes Coffin-Siris syndrome, associated with developmental delay, facial dysmorphism, and intellectual disability. The role of ARID1B has been widely studied in neuronal development, but whether it also regulates stem cells remains unknown. Here, we employ scRNA-seq and scATAC-seq to dissect the regulatory functions and mechanisms of ARID1B within mesenchymal stem cells (MSCs) using the mouse incisor model. We reveal that loss of Arid1b in the GLI1+ MSC lineage disturbs MSCs' quiescence and leads to their proliferation due to the ectopic activation of non-canonical Activin signaling via p-ERK. Furthermore, loss of Arid1b upregulates Bcl11b, which encodes a BAF complex subunit that modulates non-canonical Activin signaling by directly regulating the expression of activin A subunit, Inhba. Reduction of Bcl11b or non-canonical Activin signaling restores the MSC population in Arid1b mutant mice. Notably, we have identified that ARID1B suppresses Bcl11b expression via specific binding to its third intron, unveiling the direct inter-regulatory interactions among BAF subunits in MSCs. Our results demonstrate the vital role of ARID1B as an epigenetic modifier in maintaining MSC homeostasis and reveal its intricate mechanistic regulatory network in vivo, providing novel insights into the linkage between chromatin remodeling and stem cell fate determination.

GLI1+ Cells Contribute to Vascular Remodeling in Pulmonary Hypertension

BACKGROUND

The precise origin of newly formed ACTA2+ (alpha smooth muscle actin-positive) cells appearing in nonmuscularized vessels in the context of pulmonary hypertension is still debatable although it is believed that they predominantly derive from preexisting vascular smooth muscle cells (VSMCs).

METHODS

Gli1Cre-ERT2; tdTomatoflox mice were used to lineage trace GLI1+ (glioma-associated oncogene homolog 1-positive) cells in the context of pulmonary hypertension using 2 independent models of vascular remodeling and reverse remodeling: hypoxia and cigarette smoke exposure. Hemodynamic measurements, right ventricular hypertrophy assessment, flow cytometry, and histological analysis of thick lung sections followed by state-of-the-art 3-dimensional reconstruction and quantification using Imaris software were used to investigate the contribution of GLI1+ cells to neomuscularization of the pulmonary vasculature.

RESULTS

The data show that GLI1+ cells are abundant around distal, nonmuscularized vessels during steady state, and this lineage contributes to around 50% of newly formed ACTA2+ cells around these normally nonmuscularized vessels. During reverse remodeling, cells derived from the GLI1+ lineage are largely cleared in parallel to the reversal of muscularization. Partial ablation of GLI1+ cells greatly prevented vascular remodeling in response to hypoxia and attenuated the increase in right ventricular systolic pressure and right heart hypertrophy. Single-cell RNA sequencing on sorted lineage-labeled GLI1+ cells revealed an Acta2high fraction of cells with pathways in cancer and MAPK (mitogen-activated protein kinase) signaling as potential players in reprogramming these cells during vascular remodeling. Analysis of human lung-derived material suggests that GLI1 signaling is overactivated in both group 1 and group 3 pulmonary hypertension and can promote proliferation and myogenic differentiation.

CONCLUSIONS

Our data highlight GLI1+ cells as an alternative cellular source of VSMCs in pulmonary hypertension and suggest that these cells and the associated signaling pathways represent an important therapeutic target for further studies.

Unveiling heterogeneity in MSCs: exploring marker-based strategies for defining MSC subpopulations

Mesenchymal stem/stromal cells (MSCs) represent a heterogeneous cell population distributed throughout various tissues, demonstrating remarkable adaptability to microenvironmental cues and holding immense promise for disease treatment. However, the inherent diversity within MSCs often leads to variability in therapeutic outcomes, posing challenges for clinical applications. To address this heterogeneity, purification of MSC subpopulations through marker-based isolation has emerged as a promising approach to ensure consistent therapeutic efficacy. In this review, we discussed the reported markers of MSCs, encompassing those developed through candidate marker strategies and high-throughput approaches, with the aim of explore viable strategies for addressing the heterogeneity of MSCs and illuminate prospective research directions in this field.

A role for TGFβ signaling in Gli1+ tendon and enthesis cells

The development of musculoskeletal tissues such as tendon, enthesis, and bone relies on proliferation and differentiation of mesenchymal progenitor cells. Gli1+ cells have been described as putative stem cells in several tissues and are presumed to play critical roles in tissue formation and maintenance. For example, the enthesis, a fibrocartilage tissue that connects tendon to bone, is mineralized postnatally by a pool of Gli1+ progenitor cells. These cells are regulated by hedgehog signaling, but it is unclear if TGFβ signaling, necessary for tenogenesis, also plays a role in their behavior. To examine the role of TGFβ signaling in Gli1+ cell function, the receptor for TGFβ, TbR2, was deleted in Gli1-lineage cells in mice at P5. Decreased TGFβ signaling in these cells led to defects in tendon enthesis formation by P56, including defective bone morphometry underlying the enthesis and decreased mechanical properties. Immunohistochemical staining of these Gli1+ cells showed that loss of TGFβ signaling reduced proliferation and increased apoptosis. In vitro experiments using Gli1+ cells isolated from mouse tail tendons demonstrated that TGFβ controls cell proliferation and differentiation through canonical and non-canonical pathways and that TGFβ directly controls the tendon transcription factor scleraxis by binding to its distant enhancer. These results have implications in the development of treatments for tendon and enthesis pathologies.

Mx1 -labeled pulp progenitor cells are main contributors to postnatal odontoblasts and pulp cells in murine molars

Regeneration of dentin and odontoblasts from dental pulp stem cells (DPSCs) is essential for permanent tooth maintenance. However, the in vivo identity and role of endogenous DPSCs in reparative dentinogenesis are elusive. Here, using pulp single-cell analysis before and after molar eruption, we revealed that endogenous DPSCs are enriched in Cxcl12- GFP + coronal papilla-like cells with Mx1- Cre labeling. These Mx1 + Cxcl12- GFP + cells are long-term repopulating cells that contribute to the majority of pulp cells and new odontoblasts after eruption. Upon molar injury, Mx1 + DPSCs localize into the injury site and differentiate into new odontoblasts, forming scleraxis -GFP + and osteocalcin -GFP + dentinal tubules and reparative dentin. Single-cell and FACS analysis showed that Mx1 + Cxcl12- GFP + DPSCs are the most primitive cells with stem cell marker expression and odontoblast differentiation. Taken together, our findings demonstrate that Mx1 labels postnatal DSPCs, which are the main source of pulp cells and new odontoblasts with reparative dentinogenesis in vivo .

The Role of Sensory Nerves in Dental Pulp Homeostasis: Histological Changes and Cellular Consequences after Sensory Denervation

Homeostatic maintenance is essential for pulp function. Disrupting pulp homeostasis may lead to pulp degeneration, such as fibrosis and calcifications. Sensory nerves constitute a crucial component of the dental pulp. However, the precise involvement of sensory nerves in pulp homeostasis remains uncertain. In this study, we observed the short-term and long-term histological changes in the dental pulp after inferior alveolar nerve transection. Additionally, we cultured primary dental pulp cells (DPCs) from the innervated and denervated groups and compared indicators of cellular senescence and cellular function. The results revealed that pulp fibrosis occurred at 2 w after the operation. Furthermore, the pulp area, as well as the height and width of the pulp cavity, showed accelerated reductions after sensory denervation. Notably, the pulp area at 16 w after the operation was comparable to that of 56 w old rats. Sensory denervation induced excessive extracellular matrix (ECM) deposition and increased predisposition to mineralization. Furthermore, sensory denervation promoted the senescence of DPCs. Denervated DPCs exhibited decelerated cell proliferation, arrest in the G2/M phase of the cell cycle, imbalance in the synthesis and degradation of ECM, and enhanced mineralization. These findings indicate that sensory nerves play an essential role in pulp homeostasis maintenance and dental pulp cell fate decisions, which may provide novel insights into the prevention of pulp degeneration.

Sensory nerve regulates progenitor cells via FGF-SHH axis in tooth root morphogenesis

Nerves play important roles in organ development and tissue homeostasis. Stem/progenitor cells differentiate into different cell lineages responsible for building the craniofacial organs. The mechanism by which nerves regulate stem/progenitor cell behavior in organ morphogenesis has not yet been comprehensively explored. Here, we use tooth root development in mouse as a model to investigate how sensory nerves regulate organogenesis. We show that sensory nerve fibers are enriched in the dental papilla at the initiation of tooth root development. Through single cell RNA-sequencing analysis of the trigeminal ganglion and developing molar, we reveal several signaling pathways that connect the sensory nerve with the developing molar, of which FGF signaling appears to be one of the important regulators. Fgfr2 is expressed in the progenitor cells during tooth root development. Loss of FGF signaling leads to shortened roots with compromised proliferation and differentiation of progenitor cells. Furthermore, Hh signaling is impaired in Gli1-CreER;Fgfr2fl/fl mice. Modulation of Hh signaling rescues the tooth root defects in these mice. Collectively, our findings elucidate the nerve-progenitor crosstalk and reveal the molecular mechanism of the FGF-SHH signaling cascade during tooth root morphogenesis.

Interactions of Neuronally Induced Stem Cells from Apical Papilla Spheres, Stems Cells from Apical Papilla, and Human Umbilical Vascular Endothelial Cells on Vasculogenesis and Neurogenesis

INTRODUCTION

Stem cell-based dental pulp regeneration has been extensively studied, mainly focusing on exploiting dental stem cells' osteogenic and angiogenic potentials. Dental stem cells' neurogenic role is often overlooked. Stem cells from apical papilla (SCAPs), originating from the neural crest and capable of sphere formation, display potent neurogenic capacity. This study aimed to investigate the interactions of neuronally induced stem cells from apical papilla (iSCAP) spheres, SCAPs, and human umbilical vascular endothelial cells (HUVECs) on vasculogenesis and neurogenesis.

METHODS

SCAPs were isolated and characterized using flow cytometry and multilineage differentiation assays. SCAP monolayer culture and spheres were neuronally induced by a small molecule neural induction medium, and the neural gene expression and neurite formation at days 0, 3, and 7 were evaluated by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and using phase-contrast light and fluorescence microscopy. Direct coculture or pulp-on-chip was used to investigate iSCAP sphere interaction with SCAPs and HUVECs. RT-qPCR, fluorescence microscopy, and immunostaining with β-tubulin III, alpha-smooth muscle actin, and CD31 were used to study neural gene expression, neurite formation, and neurovascular cell interactions.

RESULTS

Neural induction medium with small molecules rapidly induced SCAP differentiation toward neural-like cells. Gene expression of Nestin, β-tubulin III, microtubule-associated protein 2, neuron-specific enolase, and NeuN was higher in iSCAP spheres than in iSCAPs. iSCAP spheres formed more and longer neurites compared with iSCAPs. iSCAP sphere, HUVEC, and SCAP direct coculture significantly enhanced vessel formation along with up-regulated VEGF (P < .001) and multiple neural markers, such as Nestin (P < .01), microtubule-associated protein 2 (P < .001), S100 (P < .001), and NG2 (P < .001). iSCAP spheres, SCAPs, and HUVECs cultured in a pulp-on-chip system promoted endothelial and neural cell migration toward each other and alpha-smooth muscle actin-positive and CD31-positive cells assembling for the vascular constitution.

CONCLUSIONS

iSCAP-formed spheres interact with SCAPs and HUVECs, promoting vasculogenesis and neurogenesis.

Identifying Dental Pulp Stem Cell as a Novel Therapeutic trategy for Digestive Diseases

Mesenchymal stem cells (MSCs) have been identified as potential therapeutics for various diseases. In contrast to other sources of MSCs, dental stem cells (DSCs) have received increased attention due to their high activity and easy accessibility. Among them, dental pulp stem cells (DPSCs) exhibit superior self-renewal, multipotency, immunomodulatory, and regenerative capacities. Following their inspiring performance in animal models and clinical trials, DPSCs show pharmacological potential in regenerative medicine. In this review, we have generalized the sources, heterogeneity, and biological characteristics of DPSCs, as well as compared them with other types of dental stem cells. In addition, we summarized the application of DPSCs in digestive diseases (such as liver, esophageal, and intestinal diseases), highlighting their regenerative and pharmacological potential based on the existing preclinical and clinical evidence. Specifically, DPSCs can be home to injured or inflamed tissues and exert repair and regeneration functions by facilitating immune regulation, anti-inflammation, and directional differentiation. Although DPSCs have a rosy prospect, future studies should handle the underlying drawbacks and pave the way for the identification of DPSCs as novel regenerative medicine.

Using tissue clearing and light sheet fluorescence microscopy for the three-dimensional analysis of sensory and sympathetic nerve endings that innervate bone and dental tissue of mice

Bone and dental tissues are richly innervated by sensory and sympathetic neurons. However, the characterization of the morphology, molecular phenotype, and distribution of nerves that innervate hard tissue has so far mostly been limited to thin histological sections. This approach does not adequately capture dispersed neuronal projections due to the loss of important structural information during three-dimensional (3D) reconstruction. In this study, we modified the immunolabeling-enabled imaging of solvent-cleared organs (iDISCO/iDISCO+) clearing protocol to image high-resolution neuronal structures in whole femurs and mandibles collected from perfused C57Bl/6 mice. Axons and their nerve terminal endings were immunolabeled with antibodies directed against protein gene product 9.5 (pan-neuronal marker), calcitonin gene-related peptide (peptidergic nociceptor marker), or tyrosine hydroxylase (sympathetic neuron marker). Volume imaging was performed using light sheet fluorescence microscopy. We report high-quality immunolabeling of the axons and nerve terminal endings for both sensory and sympathetic neurons that innervate the mouse femur and mandible. Importantly, we are able to follow their projections through full 3D volumes, highlight how extensive their distribution is, and show regional differences in innervation patterns for different parts of each bone (and surrounding tissues). Mapping the distribution of sensory and sympathetic axons, and their nerve terminal endings, in different bony compartments may be important in further elucidating their roles in health and disease.

Human Dental Pulp Stem Cells Are Subjected to Metabolic Reprogramming and Repressed Proliferation and Migration by the Sympathetic Nervous System via α1B-Adrenergic Receptor

INTRODUCTION

Human dental pulp stem cells (hDPSCs) reside in specialized microenvironments in the dental pulp, termed "niches," which are composed of diverse cellular components including nerves. Sensory nerves can positively regulate the expansion and differentiation of pulp cells, while the biological effects of the sympathetic nervous system (SNS) on hDPSCs remain elusive. This study is devoted to investigating the effects and underlying mechanisms of the SNS on the proliferation and migration of hDPSCs.

METHODS

The distribution of sympathetic nerve fibers in human dental pulp was examined by immunofluorescence staining of tyrosine hydroxylase. The concentration of norepinephrine in healthy and carious human dental pulp tissues was detected using enzyme-linked immunosorbent assay. RNA-sequencing was applied to identify the dominant sympathetic neurotransmitter receptor in hDPSCs. Seahorse metabolic assay, adenosine triphosphate assay, lactate assay, and mitochondrial DNA copy number were performed to determine the level of glycometabolism. Transwell assay, wound healing assay, 5-ethynyl-2'-deoxyuridine staining assay, cell cycle assay, and Cell Counting Kit-8 assay were conducted to analyze the migratory and proliferative capacities of hDPSCs.

RESULTS

Sprouting of sympathetic nerve fibers and an increased concentration of norepinephrine were observed in inflammatory pulp tissues. Sympathetic nerve fibers were mainly distributed along blood vessels, and aldehyde dehydrogenase 1-positive hDPSCs resided in close proximity to neurovascular bundles. ADRA1B was identified as the major sympathetic neurotransmitter receptor expressed in hDPSCs, and its expression was enhanced in inflammatory pulp tissues. In addition, the SNS inhibited the proliferation and migration of hDPSCs through metabolic reprogramming via ADRA1B and its crosstalk with serine-threonine kinase and p38 mitogen-activated protein kinase signaling pathways.

CONCLUSIONS

This study demonstrates that the SNS can shift the metabolism of hDPSCs from oxidative phosphorylation to anaerobic glycolysis via ADRA1B and its crosstalk with serine-threonine kinase and p38 mitogen-activated protein kinase signaling pathways, thereby inhibiting the proliferative and migratory abilities of hDPSCs. This metabolic shift may facilitate the maintenance of the quiescent state of hDPSCs.

RvE1 Promotes Axin2+ Cell Regeneration and Reduces Bacterial Invasion

Vital pulp therapy and root canal therapy (RCT) are the dominant treatment for irreversible pulpitis. While the success rate of these procedures is favorable, they have some limitations. For instance, RCT leads to removing significant dentin in the coronal third of the tooth that increases root-fracture risk, which forces tooth removal. The ideal therapeutic goal is dental pulp regeneration, which is not achievable with RCT. Specialized proresolving mediators (SPMs) are well known for inflammatory resolution. The resolution of inflammation and tissue restoration or regeneration is a dynamic and continuous process. SPMs not only have potent immune-modulating functions but also effectively promote tissue homeostasis and regeneration. Resolvins have been shown to promote dental pulp regeneration. The purpose of this study was to explore further the cellular target of Resolvin E1 (RvE1) therapy in dental pulp regeneration and the impact of RvE1 in infected pulps. We investigated the actions of RvE1 on experimentally exposed pulps with or without microbial infection in an Axin2Cre-Dox;Ai14 genetically defined mouse model. Our results showed RvE1 promoted Axin2-tdTomato+ cell expansion and odontoblastic differentiation after direct pulp capping in the mouse, which we used to mimic reversible pulpitis cases in the clinic. In cultured mouse dental pulp stem cells (mDPSCs), RvE1 facilitated Axin2-tdTomato+ cell proliferation and odontoblastic differentiation and also rescued impaired functions after lipopolysaccharide stimulation. In infected pulps exposed to the oral environment for 24 h, RvE1 suppressed inflammatory infiltration, reduced bacterial invasion in root canals, and prevented the development of apical periodontitis, while its proregenerative impact was limited. Collectively, topical treatment with RvE1 facilitated dental pulp regenerative properties by promoting Axin2-expressing cell proliferation and differentiation. It also modulated the resolution of inflammation, reduced infection severity, and prevented apical periodontitis, presenting RvE1 as a novel therapeutic for treating endodontic diseases.

Coexpression analysis of angiogenesis, proliferation, apoptosis, autophagy and SHH pathway genes involved in skin expansion

Skin and soft tissue expansion is a widely used technique in plastic surgery. However, the regulatory mechanisms associated with cellular processes involved in skin expansion are not well elucidated. In the present study, we aimed at exploring the transcriptome changes associated with skin expansion and profiling the difference in gene expression between the skin tissue in the top of the dilator and the skin tissue in the side of the dilator. A mouse model of skin expansion was established and RNA sequencing (RNA-Seq) was performed on samples collected at different time points. Differential expression analysis was performed using the DESeq2 package while STEM was used for time series clustering profiling. The regulatory networks were established and the functions of sets of genes were analyzed. The mRNA expression levels of candidate genes were validated by the quantitative RT-PCR. Among the skin tissue in the top of the dilator and normal samples at days 1, 3, 7, 14 and 28, 53 commonly upregulated and 7 commonly downregulated genes were identified while among the skin tissue in the side of the dilator and normal samples, 98 downregulated and 255 upregulated genes were identified. Genes differentially expressed among the skin tissue in the top of the dilator and normal samples were involved in coagulation and proliferation-associated pathways while those among the skin tissue in the side of the dilator and normal samples were involved in the inflammation, immune response, and defense response. Among the skin tissue in the top of the dilator and the skin tissue in the side of the dilator samples, 161 were constantly upregulated while 27 were constantly downregulated; these genes were enriched in the biological processes of cell adhesion and regulation of cell proliferation (n = 11). Furthermore, we identified that SHH signaling genes formed a coexpression regulatory network with cellular proliferation, apoptosis, autophagy and angiogenesis-related genes in the expanded skin. In conclusion, our findings can promote research and understanding of the mechanism of skin expansion and will find application in plastic surgery.

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.

The Role of Gli1+ Mesenchymal Stem Cells in Osteogenesis of Craniofacial Bone

Glioma-associated oncogene homolog 1 (Gli1) is a transcriptional activator of hedgehog (Hh) signaling that regulates target gene expression and several cellular biological processes. Cell lineage tracing techniques have highlighted Gli1 as an ideal marker for mesenchymal stem cells (MSCs) in vivo. Gli1+ MSCs are critical for the osteogenesis of the craniofacial bone; however, the regulatory mechanism by which Gli1+ MSCs mediate the bone development and tissue regeneration of craniofacial bone has not been systematically outlined. This review comprehensively elucidates the specific roles of Gli1+ MSCs in craniofacial bone osteogenesis. In addition to governing craniofacial bone development, Gli1+ MSCs are associated with the tissue repair of craniofacial bone under pathological conditions. Gli1+ MSCs promote intramembranous and endochondral ossification of the craniofacial bones, and assist the osteogenesis of the craniofacial bone by improving angiopoiesis. This review summarizes the novel role of Gli1+ MSCs in bone development and tissue repair in craniofacial bones, which offers new insights into bone regeneration therapy.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

Hedgehog signaling in tissue homeostasis, cancers, and targeted therapies

The past decade has seen significant advances in our understanding of Hedgehog (HH) signaling pathway in various biological events. HH signaling pathway exerts its biological effects through a complex signaling cascade involved with primary cilium. HH signaling pathway has important functions in embryonic development and tissue homeostasis. It plays a central role in the regulation of the proliferation and differentiation of adult stem cells. Importantly, it has become increasingly clear that HH signaling pathway is associated with increased cancer prevalence, malignant progression, poor prognosis and even increased mortality. Understanding the integrative nature of HH signaling pathway has opened up the potential for new therapeutic targets for cancer. A variety of drugs have been developed, including small molecule inhibitors, natural compounds, and long non-coding RNA (LncRNA), some of which are approved for clinical use. This review outlines recent discoveries of HH signaling in tissue homeostasis and cancer and discusses how these advances are paving the way for the development of new biologically based therapies for cancer. Furthermore, we address status quo and limitations of targeted therapies of HH signaling pathway. Insights from this review will help readers understand the function of HH signaling in homeostasis and cancer, as well as opportunities and challenges of therapeutic targets for cancer.

Dental Pulp Stem Cells and Current in vivo Approaches to Study Dental Pulp Stem Cells in Pulp Injury and Regeneration

Dental pulp stem cells (DPSCs) have garnered significant interest in dental research for their unique characteristics and potential in tooth development and regeneration. While there were many studies to define their stem cell-like characteristics and osteogenic differentiation functions that are considered ideal candidates for regenerating damaged dental pulp tissue, how endogenous DPSCs respond to dental pulp injury and supply new dentin-forming cells has not been extensively investigated in vivo. Here, we review the recent progress in identity, function, and regulation of endogenous DPSCs and their clinical potential for pulp injury and regeneration. In addition, we discuss current advances in new mouse models, imaging techniques, and its practical uses and limitations in the analysis of DPSCs in pulp injury and regeneration in vivo.

Engineered exosomes reprogram Gli1+ cells in vivo to prevent calcification of vascular grafts and autologous pathological vessels

Calcification of autologous pathological vessels and tissue engineering blood vessels (TEBVs) is a thorny problem in clinic. However, there is no effective and noninvasive treatment that is available against the calcification of TEBVs and autologous pathological vessels. Gli1+ cells are progenitors of smooth muscle cells (SMCs) and can differentiate into osteoblast-like cells, leading to vascular calcification. Our results showed that the spatiotemporal distribution of Gli1+ cells in TEBVs was positively correlated with the degree of TEBV calcification. An anticalcification approach was designed consisting of exosomes derived from mesenchymal stem cells delivering lncRNA-ANCR to construct the engineered exosome-Ancr/E7-EXO. The results showed that Ancr/E7-EXO effectively targeted Gli1+ cells, promoting rapid SMC reconstruction and markedly inhibiting Gli1+ cell differentiation into osteoblast-like cells. Moreover, Ancr/E7-EXO significantly inhibited vascular calcification caused by chronic kidney disease. Therefore, Ancr/E7-EXO reprogrammed Gli1+ cells to prevent calcification of vascular graft and autologous pathological vessel, providing unique insights for an effective anticalcification.

Stem Cells from the Apical Papilla (SCAPs): Past, Present, Prospects, and Challenges

Dental diseases occurring on young permanent teeth usually lead to the premature arrest of tooth root development. Sustained tooth root elongation is necessary to achieve the goal of long-term preservation of affected teeth. To this end, stem cell-based regenerative endodontic treatment has been regarded as one of the most promising strategies for treating young permanent teeth with pulp and periapical infections. Endogenous stem cells residing in the apical papilla, named stem cells from the apical papilla (SCAPs), have been intensively investigated due to their critical roles in pulp regeneration and root redevelopment. The present review summarizes advances in the field of SCAPs studies and discusses the challenges that need to be further addressed.

Icariin promotes the proliferation and osteogenic differentiation of bone-derived mesenchymal stem cells in patients with osteoporosis and T2DM by upregulating GLI-1

BACKGROUND

The function of mesenchymal stem cells (MSCs) from patients with osteoporosis (OP) is impaired and worsens in patients with type 2 diabetes mellitus (T2DM). Icariin (ICA) is the major active flavonoid glucoside isolated from traditional Chinese herbal Epimedium pubescens, and confirmed able to improve bone mass of OP patients.

OBJECTIVE

To investigate the effect of ICA on the proliferation and osteogenic differentiation of bone-derived MSCs (BMSCs) from patients with OP and T2DM and uncover the potential mechanism.

METHODS

BMSCs were treated with ICA, and proliferation and osteogenic potency were evaluated using the 2,5-diphenyl-2H-tetrazolium bromide (MTT) assay and detection of osteogenic markers (ALP, RUNX2, SPP1, COL1A1, and mineralized nodules) was performed. RNA sequencing and bioinformatic analysis were performed to identify differentially expressed genes (DEGs) after ICA treatment and screen proliferation- and osteogenic differentiation-related processes. Gene gain and loss were performed to confirm the role of the key candidate gene.

RESULTS

ICA significantly promoted the proliferation and osteogenic differentiation of BMSCs. A total of 173 DEGs were identified after ICA treatment. Six DEGs (GLI-1, IGF2, BMP6, WNT5A, PTHLH, and MAPK14) enriched in both proliferation- and osteogenic differentiation-related processes were screened; GLI-1 had the highest validated |log2FC| value. Overexpression of GLI-1 enhanced the proliferation and osteogenic differentiation of BMSCs, and knockdown of GLI-1 weakened the positive effect of ICA on BMSCs.

CONCLUSION

ICA promoted the proliferation and osteogenic differentiation of impaired BMSCs by upregulating GLI-1.

Aldehyde Dehydrogenase, a Marker of Normal and Malignant Stem Cells, Typifies Mesenchymal Progenitors in Perivascular Niches

Innate mesenchymal stem cells exhibiting multilineage differentiation and tissue (re)generative-or pathogenic-properties reside in perivascular niches. Subsets of these progenitors are committed to either osteo-, adipo-, or fibrogenesis, suggesting the existence of a developmental organization in blood vessel walls. We evaluated herein the activity of aldehyde dehydrogenase, a family of enzymes catalyzing the oxidation of aldehydes into carboxylic acids and a reported biomarker of normal and malignant stem cells, within human adipose tissue perivascular areas. A progression of ALDHLow to ALDHHigh CD34+ cells was identified in the tunica adventitia. Mesenchymal stem cell potential was confined to ALDHHigh cells, as assessed by proliferation and multilineage differentiation in vitro of cells sorted by flow cytometry with a fluorescent ALDH substrate. RNA sequencing confirmed and validated that ALDHHigh cells have a progenitor cell phenotype and provided evidence that the main isoform in this fraction is ALDH1A1, which was confirmed by immunohistochemistry. This demonstrates that ALDH activity, which marks hematopoietic progenitors and stem cells in diverse malignant tumors, also typifies native, blood vessel resident mesenchymal stem cells.

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ regeneration.

Deciphering the Heterogeneity Landscape of Mesenchymal Stem/Stromal Cell-Derived Extracellular Vesicles for Precise Selection in Translational Medicine

Mesenchymal stem/stromal cell-derived extracellular vesicles (MSC-EVs) have been considered promising therapeutics for disease treatments. However, MSC-EVs harvested from different tissues present unique biological features reflective of their origins. The heterogeneity of MSC-EVs constitutes an important barrier to their precise application in clinical translation that may probably lead to uncertain therapeutic effects. To give hints for future clinical translation, five MSCs are employed, whose derived EVs are most intensively utilized, namely bone marrow mesenchymal stem/stromal cells (BMMSCs), umbilical cord stem/stromal cells (UCSCs), adipose-derived stem/stromal cells (ASCs), dermal stem/stromal cells (DSCs) and dental pulp stem/stromal cells (DPSCs) and the heterogeneity landscape of the corresponding MSC-EVs are documented. Overall, the basic parameters, stability, and biosafety of different MSC-EVs are indiscriminate. Strikingly, UCSC-EVs exhibit distinguishing productivity. UCSC-EVs as well as DPSC-EVs present better drug loading/delivery capacity. In addition, the heterogeneity of different MSC-EVs in cargo diversity, cellular affinity, organ biodistribution, and therapeutic effects may cue the rational selection in different disease treatments. Through a combined assessment, a rational strategy is combined for selecting MSC-EVs in future clinics. Offering a panoramic view of MSC-EVs harvested from different tissues, the current study may provide guidelines for the precise selection of MSC-EVs in next-generation therapeutics.

Identifying fibrogenic cells following salivary gland obstructive injury

Fibrosis results from excess extracellular matrix accumulation, which alters normal tissue architecture and impedes function. In the salivary gland, fibrosis can be induced by irradiation treatment for cancer therapy, Sjögren's Disease, and other causes; however, it is unclear which stromal cells and signals participate in injury responses and disease progression. As hedgehog signaling has been implicated in fibrosis of the salivary gland and other organs, we examined contributions of the hedgehog effector, Gli1, to fibrotic responses in salivary glands. To experimentally induce a fibrotic response in female murine submandibular salivary glands, we performed ductal ligation surgery. We detected a progressive fibrotic response where both extracellular matrix accumulation and actively remodeled collagen significantly increased at 14 days post-ligation. Macrophages, which participate in extracellular matrix remodeling, and Gli1+ and PDGFRα+ stromal cells, which may deposit extracellular matrix, both increased with injury. Using single-cell RNA-sequencing, Gli1 + cells were not found in discrete clusters at embryonic day 16 but were found in clusters expressing the stromal genes Pdgfra and/or Pdgfrb. In adult mice, Gli1+ cells were similarly heterogeneous but more cells co-expressed PDGFRα and PDGFRβ. Using Gli1-CreERT2; ROSA26tdTomato lineage-tracing mice, we found that Gli1-derived cells expand with ductal ligation injury. Although some of the Gli1 lineage-traced tdTomato+ cells expressed vimentin and PDGFRβ following injury, there was no increase in the classic myofibroblast marker, smooth muscle alpha-actin. Additionally, there was little change in extracellular matrix area, remodeled collagen area, PDGFRα, PDGFRβ, endothelial cells, neurons, or macrophages in Gli1 null salivary glands following injury when compared with controls, suggesting that Gli1 signaling and Gli1+ cells have only a minor contribution to mechanical injury-induced fibrotic changes in the salivary gland. We used scRNA-seq to examine cell populations that expand with ligation and/or showed increased expression of matrisome genes. Some Pdgfra + /Pdgfrb + stromal cell subpopulations expanded in response to ligation, with two stromal cell subpopulations showing increased expression of Col1a1 and a greater diversity of matrisome genes, consistent with these cells being fibrogenic. However, only a few cells in these subpopulations expressed Gli1, consistent with a minor contribution of these cells to extracellular matrix production. Defining the signaling pathways driving fibrotic responses in stromal cell sub-types could reveal future therapeutic targets.

The SHH-GLI1 pathway is required in skin expansion and angiogenesis

To investigate the role of GLI1 on skin proliferation and neovascularization during skin expansion in mice. We constructed GLI1-cre/R26-Tdtomato and GLI1-cre/R26-mtmg gene-tagged skin expansion mouse models. Using a two-photon in vivo imaging instrument to observe the changes in the number and distribution of GLI1(+) cells during the expansion process and to clarify the spatial relationship between GLI1(+) cells and blood vessels during the expansion process. In vitro proliferation assays were performed to further validate the effects of SHH (sonic hedgehog) and its downstream component GLI1 on cell proliferation viability. Finally, qRT-PCR was used to verify the changes in proliferation, angiogenesis-related factors, SHH signalling pathway-related factors, and the role of GLI1 cells in the process of skin expansion in mice. The number of GLI1(+) cells increased during dilation and were attached to the outer membrane of the vessel. The epidermis was thickened and the dermis thinned after the dilated skin was taken, while the epidermal thickening was suppressed and the dermis became thinner after the GLI1 cells were inhibited. The non-inhibited group showed a significant increase in PCNA positivity with prolonged dilation compared to the GANT61(GLI specificity inhibitor) inhibited group; CD31 immunofluorescence showed a significant increase in the number of dilated skin vessels and a significant decrease in the number of vessels after treatment with GANT61 inhibitor. In vitro proliferation results showed that SHH signalling activator significantly increased the proliferation viability of GLI1(+) hair follicle mesenchymal stem cells, while GNAT61 significantly inhibited the proliferation viability of GLI1(+) hair follicle mesenchymal stem cells. GLI1 is necessary for proliferation and neovascularization in expansion skin of mice through activation of the SHH signalling pathway.

Clinical-grade human umbilical cord-derived mesenchymal stem cells improved skeletal muscle dysfunction in age-associated sarcopenia mice

With the expansion of the aging population, age-associated sarcopenia (AAS) has become a severe clinical disease of the elderly and a key challenge for healthy aging. Regrettably, no approved therapies currently exist for treating AAS. In this study, clinical-grade human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) were administrated to two classic mouse models (SAMP8 mice and D-galactose-induced aging mice), and their effects on skeletal muscle mass and function were investigated by behavioral tests, immunostaining, and western blotting. Core data results showed that hUC-MSCs significantly restored skeletal muscle strength and performance in both mouse models via mechanisms including raising the expression of crucial extracellular matrix proteins, activating satellite cells, enhancing autophagy, and impeding cellular aging. For the first time, the study comprehensively evaluates and demonstrates the preclinical efficacy of clinical-grade hUC-MSCs for AAS in two mouse models, which not only provides a novel model for AAS, but also highlights a promising strategy to improve and treat AAS and other age-associated muscle diseases. This study comprehensively evaluates the preclinical efficacy of clinical-grade hUC-MSCs in treating age-associated sarcopenia (AAS), and demonstrates that hUC-MSCs restore skeletal muscle strength and performance in two AAS mouse models via raising the expression of extracellular matrix proteins, activating satellite cells, enhancing autophagy, and impeding cellular aging, which highlights a promising strategy for AAS and other age-associated muscle diseases.

Transit Amplifying Cells (TACs): a still not fully understood cell population

Maintenance of tissue homeostasis and tissue regeneration after an insult are essential functions of adult stem cells (SCs). In adult tissues, SCs proliferate at a very slow rate within "stem cell niches", but, during tissue development and regeneration, before giving rise to differentiated cells, they give rise to multipotent and highly proliferative cells, known as transit-amplifying cells (TACs). Although differences exist in diverse tissues, TACs are not only a transitory phase from SCs to post-mitotic cells, but they also actively control proliferation and number of their ancestor SCs and proliferation and differentiation of their progeny toward tissue specific functional cells. Autocrine signals and negative and positive feedback and feedforward paracrine signals play a major role in these controls. In the present review we will consider the generation and the role played by TACs during development and regeneration of lining epithelia characterized by a high turnover including epidermis and hair follicles, ocular epithelial surfaces, and intestinal mucosa. A comparison between these different tissues will be made. There are some genes and molecular pathways whose expression and activation are common to most TACs regardless their tissue of origin. These include, among others, Wnt, Notch, Hedgehog and BMP pathways. However, the response to these molecular signals can vary in TACs of different tissues. Secondly, we will consider cultured cells derived from tissues of mesodermal origin and widely adopted for cell therapy treatments. These include mesenchymal stem cells and dedifferentiated chondrocytes. The possible correlation between cell dedifferentiation and reversion to a transit amplifying cell stage will be discussed.

Bone formation ability of Gli1+ cells in the periodontal ligament after tooth extraction

During the process of socket healing after tooth extraction, osteoblasts appear in the tooth socket and form alveolar bone; however, the source of these osteoblasts is still uncertain. Recently, it has been demonstrated that cells expressing Gli1, a downstream factor of sonic hedgehog signaling, exhibit stem cell properties in the periodontal ligament (PDL). Therefore, in the present study, the differentiation ability of Gli1⁺-PDL cells after tooth extraction was analyzed using Gli1-Cre^ERT2/ROSA26-loxP-stop-loxP-tdTomato (iGli1/Tomato) mice. After the final administration of tamoxifen to iGli1/Tomato mice, Gli1/Tomato⁺ cells were rarely detected in the PDL. One day after the tooth extraction, although inflammatory cells appeared in the tooth socket, Periostin⁺ PDL-like tissues having a few Gli1/Tomato⁺ cells remained near the alveolar bone. Three days after the extraction, the number of Gli1/Tomato⁺ cells increased as evidenced by numerous PCNA⁺ cells in the socket. Some of these Gli1/Tomato⁺ cells expressed BMP4 and Phosphorylated (P)-Smad1/5/8. After seven days, the Osteopontin⁺ bone matrix was formed in the tooth socket apart from the alveolar bone. Many Gli1/Tomato⁺ osteoblasts that were positive for Runx2⁺ were arranged on the surface of the newly formed bone matrix. In the absence of Gli1⁺-PDL cells in Gli1-Cre^ERT2/Rosa26-loxP-stop-loxP-tdDTA (iGli1/DTA) mice, the amount of newly formed bone matrix was significantly reduced in the tooth socket. Therefore, these results collectively suggest that Gli1⁺-PDL cells differentiate into osteoblasts to form the bone matrix in the tooth socket; thus, this differentiation might be regulated, at least in part, by bone morphogenetic protein (BMP) signaling.

Intercellular crosstalk in adult dental pulp is mediated by heparin-binding growth factors Pleiotrophin and Midkine

BACKGROUND

In-depth knowledge of the cellular and molecular composition of dental pulp (DP) and the crosstalk between DP cells that drive tissue homeostasis are not well understood. To address these questions, we performed a comparative analysis of publicly available single-cell transcriptomes of healthy adult human DP to 5 other reference tissues: peripheral blood mononuclear cells, bone marrow, adipose tissue, lung, and skin.

RESULTS

Our analysis revealed that DP resident cells have a unique gene expression profile when compared to the reference tissues, and that DP fibroblasts are the main cell type contributing to this expression profile. Genes coding for pleiotrophin (PTN) and midkine (MDK), homologous heparin-binding growth-factors, possessed the highest differential expression levels in DP fibroblasts. In addition, we identified extensive crosstalk between DP fibroblasts and several other DP resident cells, including Schwann cells, mesenchymal stem cells and odontoblasts, mediated by PTN and MDK.

CONCLUSIONS

DP fibroblasts emerge as unappreciated players in DP homeostasis, mainly through their crosstalk with glial cells. These findings suggest that fibroblast-derived growth factors possess major regulatory functions and thus have a potential role as dental therapeutic targets.