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.

Arid1a regulates cell cycle exit of transit-amplifying cells by inhibiting the Aurka-Cdk1 axis in mouse incisor.

Stem cells self-renew or give rise to transit-amplifying cells (TACs) that differentiate into specific functional cell types. The fate determination of stem cells to TACs and their transition to fully differentiated progeny is precisely regulated to maintain tissue homeostasis. Arid1a, a core component of the switch/sucrose nonfermentable complex, performs epigenetic regulation of stage- and tissue-specific genes that is indispensable for stem cell homeostasis and differentiation. However, the functional mechanism of Arid1a in the fate commitment of mesenchymal stem cells (MSCs) and their progeny is not clear. Using the continuously growing adult mouse incisor model, we show that Arid1a maintains tissue homeostasis through limiting proliferation, promoting cell cycle exit and differentiation of TACs by inhibiting the Aurka-Cdk1 axis. Loss of Arid1a overactivates the Aurka-Cdk1 axis, leading to expansion of the mitotic TAC population but compromising their differentiation ability. Furthermore, the defective homeostasis after loss of Arid1a ultimately leads to reduction of the MSC population. These findings reveal the functional significance of Arid1a in regulating the fate of TACs and their interaction with MSCs to maintain tissue homeostasis.

Ror2-mediated non-canonical Wnt signaling regulates Cdc42 and cell proliferation during tooth root development.

The control of size and shape is an important part of regulatory process during organogenesis. Tooth formation is a highly complex process that fine-tunes the size and shape of the tooth, which are crucial for its physiological functions. Each tooth consists of a crown and one or more roots. Despite comprehensive knowledge of the mechanism that regulates early tooth crown development, we have limited understanding of the mechanism regulating root patterning and size during development. Here, we show that Ror2-mediated non-canonical Wnt signaling in the dental mesenchyme plays a crucial role in cell proliferation, and thereby regulates root development size in mouse molars. Furthermore, Cdc42 acts as a potential downstream mediator of Ror2 signaling in root formation. Importantly, activation of Cdc42 can restore cell proliferation and partially rescue the root development size defects in Ror2 mutant mice. Collectively, our findings provide novel insights into the function of Ror2-mediated non-canonical Wnt signaling in regulating tooth morphogenesis, and suggest potential avenues for dental tissue engineering.

Lhx6 regulates canonical Wnt signaling to control the fate of mesenchymal progenitor cells during mouse molar root patterning.

Mammalian tooth crown formation has long served as a model for investigating how patterning and morphogenesis are orchestrated during development. However, the mechanism underlying root patterning and morphogenesis remains poorly understood. In this study, we find that Lhx6 labels a subpopulation of root progenitor cells in the apical dental mesenchyme, which is closely associated with furcation development. Loss of Lhx6 leads to furcation and root number defects, indicating that Lhx6 is a key root patterning regulator. Among the multiple cellular events regulated by Lhx6 is the odontoblast fate commitment of progenitor cells, which it controls in a cell-autonomous manner. Specifically, Lhx6 loss leads to elevated expression of the Wnt antagonist Sfrp2 and down-regulation of Wnt signaling in the furcation region, while overactivation of Wnt signaling in Lhx6+ progenitor cells partially restore the furcation defects in Lhx6-/- mice. Collectively, our findings have important implications for understanding organ morphogenesis and future strategies for tooth root regeneration.

Ubiquitin-specific protease USP34 controls osteogenic differentiation and bone formation by regulating BMP2 signaling.

The osteogenic differentiation of mesenchymal stem cells (MSCs) is governed by multiple mechanisms. Growing evidence indicates that ubiquitin-dependent protein degradation is critical for the differentiation of MSCs and bone formation; however, the function of ubiquitin-specific proteases, the largest subfamily of deubiquitylases, remains unclear. Here, we identify USP34 as a previously unknown regulator of osteogenesis. The expression of USP34 in human MSCs increases after osteogenic induction while depletion of USP34 inhibits osteogenic differentiation. Conditional knockout of Usp34 from MSCs or pre-osteoblasts leads to low bone mass in mice. Deletion of Usp34 also blunts BMP2-induced responses and impairs bone regeneration. Mechanically, we demonstrate that USP34 stabilizes both Smad1 and RUNX2 and that depletion of Smurf1 restores the osteogenic potential of Usp34-deficient MSCs in vitro Taken together, our data indicate that USP34 is required for osteogenic differentiation and bone formation.

BMP signaling orchestrates a transcriptional network to control the fate of mesenchymal stem cells in mice.

Signaling pathways are used reiteratively in different developmental processes yet produce distinct cell fates through specific downstream transcription factors. In this study, we used tooth root development as a model with which to investigate how the BMP signaling pathway regulates transcriptional complexes to direct the fate determination of multipotent mesenchymal stem cells (MSCs). We first identified the MSC population supporting mouse molar root growth as Gli1+ cells. Using a Gli1-driven Cre-mediated recombination system, our results provide the first in vivo evidence that BMP signaling activity is required for the odontogenic differentiation of MSCs. Specifically, we identified the transcription factors Pax9, Klf4, Satb2 and Lhx8 as being downstream of BMP signaling and expressed in a spatially restricted pattern that is potentially involved in determining distinct cellular identities within the dental mesenchyme. Finally, we found that overactivation of one key transcription factor, Klf4, which is associated with the odontogenic region, promotes odontogenic differentiation of MSCs. Collectively, our results demonstrate the functional significance of BMP signaling in regulating MSC fate during root development and shed light on how BMP signaling can achieve functional specificity in regulating diverse organ development.

The Dlx5-FGF10 signaling cascade controls cranial neural crest and myoblast interaction during oropharyngeal patterning and development.

Craniofacial development depends on cell-cell interactions, coordinated cellular movement and differentiation under the control of regulatory gene networks, which include the distal-less (Dlx) gene family. However, the functional significance of Dlx5 in patterning the oropharyngeal region has remained unknown. Here, we show that loss of Dlx5 leads to a shortened soft palate and an absence of the levator veli palatini, palatopharyngeus and palatoglossus muscles that are derived from the 4th pharyngeal arch (PA); however, the tensor veli palatini, derived from the 1st PA, is unaffected. Dlx5-positive cranial neural crest (CNC) cells are in direct contact with myoblasts derived from the pharyngeal mesoderm, and Dlx5 disruption leads to altered proliferation and apoptosis of CNC and muscle progenitor cells. Moreover, the FGF10 pathway is downregulated in Dlx5-/- mice, and activation of FGF10 signaling rescues CNC cell proliferation and myogenic differentiation in these mutant mice. Collectively, our results indicate that Dlx5 plays crucial roles in the patterning of the oropharyngeal region and development of muscles derived from the 4th PA mesoderm in the soft palate, likely via interactions between CNC-derived and myogenic progenitor cells.

Constitutive activation of hedgehog signaling adversely affects epithelial cell fate during palatal fusion.

Cleft palate is one of the most common craniofacial congenital defects in humans. It is associated with multiple genetic and environmental risk factors, including mutations in the genes encoding signaling molecules in the sonic hedgehog (Shh) pathway, which are risk factors for cleft palate in both humans and mice. However, the function of Shh signaling in the palatal epithelium during palatal fusion remains largely unknown. Although components of the Shh pathway are localized in the palatal epithelium, specific inhibition of Shh signaling in palatal epithelium does not affect palatogenesis. We therefore utilized a hedgehog (Hh) signaling gain-of-function mouse model, K14-Cre;R26SmoM2, to uncover the role of Shh signaling in the palatal epithelium during palatal fusion. In this study, we discovered that constitutive activation of Hh signaling in the palatal epithelium results in submucous cleft palate and persistence of the medial edge epithelium (MEE). Further investigation revealed that precise downregulation of Shh signaling is required at a specific time point in the MEE during palatal fusion. Upregulation of Hh signaling in the palatal epithelium maintains the proliferation of MEE cells. This may be due to a dysfunctional p63/Irf6 regulatory loop. The resistance of MEE cells to apoptosis is likely conferred by enhancement of a cell adhesion network through the maintenance of p63 expression. Collectively, our data illustrate that persistent Hh signaling in the palatal epithelium contributes to the etiology and pathogenesis of submucous cleft palate through its interaction with a p63/Irf6-dependent biological regulatory loop and through a p63-induced cell adhesion network.

Role of Notch Signaling in the Physiological Patterning of Posterofrontal and Sagittal Cranial Sutures.

BACKGROUND: The mutations in a Notch signaling ligand, jagged 1, are associated with unilateral coronal craniosynostosis in humans. However, the underlying mechanisms of Notch signaling in cranial suture biology still remain unclear. METHODS: The temporal and spatial patterns of Notch signaling expression were examined in the posterofrontal and sagittal sutures of Sprague-Dawley rats by real-time quantitative reverse-transcription polymerase chain reaction at postnatal ages of 2, 15, and 25 days. The role of Notch signaling in the proliferation and differentiation of osteoblasts isolated from calvarial was examined in vitro by EdU incorporation assays and real-time quantitative reverse-transcription polymerase chain reaction after activating and inhibiting Notch signaling. RESULTS: The mRNA levels of Notch family members (including Jagged 1, Delta 1, 3, 4, Notch 1-4, Hes 1, and Hes 5) decreased during the posterofrontal cranial suture fusion in rat. However, in the patent sagittal sutures, the mRNA levels of Notch family members (Jagged 2, Delta 1, Notch 1, Notch 3, Hes 5, and Hey 1) increased during suture development. The EdU incorporation assays revealed that the induction of Notch signaling in calvaria osteobalsts using Jagged 1 promoted the proliferation rates in those cells in vitro. Further studies showed that activation of Notch signaling calvaria osteobalsts using Jagged 1 led to the suppression of late osteogenetic markers such as type I collagen and osteocalcin. CONCLUSIONS: The regulation of Notch signaling is of crucial importance during the physiological patterning of posterofrontal and sagittal cranial sutures. Thus, targeting this pathway may prove significant for the development of future therapeutic applications in craniosynostosis.

Generation and characterization of tamoxifen-inducible Pax9-CreER knock-in mice using CrispR/Cas9.

Pax9 encodes a paired-box homeodomain (Pax) transcription factor and is critical for the development of multiple organs. Using CrispR/Cas9-mediated homologous directed repair (HDR), we generated a new Pax9-CreER knock-in mouse line in which the CreER(T2) fusion protein is produced after synthesis of endogenous Pax9 protein. We found that tdTomato reporter expression in Pax9-CreER;tdTomato reporter mice is detectable in a similar pattern to the endogenous Pax9 expression, faithfully recapitulating the Pax9 expression domains throughout the embryo and in the adult mouse. At early embryonic stages, the tdTomato reporter is expressed first in the pharyngeal pouch region and later in the craniofacial mesenchyme, somites, limbs, and lingual papillae in the adult tongue. These results demonstrate that this new Pax9-CreER knock-in mouse line can be used for lineage tracing and genetic targeting of Pax9-expressing cells and their progeny in a temporally and spatially controlled manner during development and organogenesis.

Critical role of Bmpr1a in mandibular condyle growth.

The importance of Bone Morphogenetic Proteins (BMPs) in the regulation of cell fate, differentiation and proliferation in the growth plate is well-known. However, in secondary cartilages (such as that in the temporomandibular joint) that grow by proliferation of prechondrocytes and differ in their pattern of growth, the role of BMPs is largely unexplored. To examine this question, we ablated Bmpr1a in the condylar cartilage of neonatal mice and assessed the consequences for mandibular condyle growth and organization at intervals over the ensuing 4 weeks. Bmpr1a deficiency caused significant chondrodysplasia and almost eliminated the chondrocytic phenotype in the TMJ. Expression of Sox9, collagen II, proteoglycan were all greatly reduced, and cell proliferation as detected by BrdU was almost non-existent in the knockout mice. Primary bone spongiosa formation was also disturbed and was accompanied by reduced Osterix expression. These findings strongly suggest that Bmpr1a is critical for the development and growth of the mandibular condyle via its effect on proliferation of prechondroblasts and chondrocyte differentiation.

Emerging roles of exosomes in oral diseases progression.

Oral diseases, such as periodontitis, salivary gland diseases, and oral cancers, significantly challenge health conditions due to their detrimental effects on patient's digestive functions, pronunciation, and esthetic demands. Delayed diagnosis and non-targeted treatment profoundly influence patients' prognosis and quality of life. The exploration of innovative approaches for early detection and precise treatment represents a promising frontier in oral medicine. Exosomes, which are characterized as nanometer-sized extracellular vesicles, are secreted by virtually all types of cells. As the research continues, the complex roles of these intracellular-derived extracellular vesicles in biological processes have gradually unfolded. Exosomes have attracted attention as valuable diagnostic and therapeutic tools for their ability to transfer abundant biological cargos and their intricate involvement in multiple cellular functions. In this review, we provide an overview of the recent applications of exosomes within the field of oral diseases, focusing on inflammation-related bone diseases and oral squamous cell carcinomas. We characterize the exosome alterations and demonstrate their potential applications as biomarkers for early diagnosis, highlighting their roles as indicators in multiple oral diseases. We also summarize the promising applications of exosomes in targeted therapy and proposed future directions for the use of exosomes in clinical treatment.

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.

The Role of Mitochondrial Permeability Transition in Bone Metabolism, Bone Healing, and Bone Diseases.

Bone is a dynamic organ with an active metabolism and high sensitivity to mitochondrial dysfunction. The mitochondrial permeability transition pore (mPTP) is a low-selectivity channel situated in the inner mitochondrial membrane (IMM), permitting the exchange of molecules of up to 1.5 kDa in and out of the IMM. Recent studies have highlighted the critical role of the mPTP in bone tissue, but there is currently a lack of reviews concerning this topic. This review discusses the structure and function of the mPTP and its impact on bone-related cells and bone-related pathological states. The mPTP activity is reduced during the osteogenic differentiation of mesenchymal stem cells (MSCs), while its desensitisation may underlie the mechanism of enhanced resistance to apoptosis in neoplastic osteoblastic cells. mPTP over-opening triggers mitochondrial swelling, regulated cell death, and inflammatory response. In particular, mPTP over-opening is involved in dexamethasone-induced osteoblast dysfunction and bisphosphonate-induced osteoclast apoptosis. In vivo, the mPTP plays a significant role in maintaining bone homeostasis, with many bone disorders linked to its excessive opening. Genetic deletion or pharmacological inhibition of the over-opening of mPTP has shown potential in enhancing bone injury recovery and alleviating bone diseases. Here, we review the findings on the relationship of the mPTP and bone at both the cellular and disease levels, highlighting novel avenues for pharmacological approaches targeting mitochondrial function to promote bone healing and manage bone-related disorders.

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.

Crosstalk between ALK3(BMPR1A) deficiency and autophagy signaling mitigates pathological bone loss in osteoporosis.

Postmenopausal osteoporosis is recognized to be one of the major skeleton diseases strongly associated with impaired bone formation. Previous reports have indicated that the importance of bone morphogenetic protein (BMP) signaling of osteoblast lineage in bone development via classical Smad signaling, however, its critical role in osteoporosis is still not well understood. In the current study, we aim to investigate the pathological role of BMPR1A, a key receptor of BMPs, in osteoporosis and its underlying mechanism. We first found that knockdown of BMPR1A by using Col1a1-creER in osteoblasts mitigated early bone loss of osteoporosis in mice, yet along with late bone maturation defects by reducing mineral adherence rate and bone formation rate in vivo. At the cellular level, we then observed that BMPR1A deficiency promoted the proliferation of pre-osteoblasts under osteoporotic conditions but hindered their late-stage mineralization. We finally elucidated that BMPR1A deficiency compensatorily triggered mTOR-autophagy perturbation by a higher level in early osteoporotic pre-osteoblasts thus resulting in the enhancement of transient cell proliferation but impairment of final mineralization. Taken together, this study indicated the significance of BMPR1A-mTOR/autophagy axis, as a double-edged sword, in osteoporotic bone formation and provided new cues for therapeutic strategies in osteoporosis.

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.

FGF signaling modulates mechanotransduction/WNT signaling in progenitors during tooth root development.

Stem/progenitor cells differentiate into different cell lineages during organ development and morphogenesis. Signaling pathway networks and mechanotransduction are important factors to guide the lineage commitment of stem/progenitor cells during craniofacial tissue morphogenesis. Here, we used tooth root development as a model to explore the roles of FGF signaling and mechanotransduction as well as their interaction in regulating the progenitor cell fate decision. We show that Fgfr1 is expressed in the mesenchymal progenitor cells and their progeny during tooth root development. Loss of Fgfr1 in Gli1+ progenitors leads to hyperproliferation and differentiation, which causes narrowed periodontal ligament (PDL) space with abnormal cementum/bone formation leading to ankylosis. We further show that aberrant activation of WNT signaling and mechanosensitive channel Piezo2 occurs after loss of FGF signaling in Gli1-CreER;Fgfr1fl/fl mice. Overexpression of Piezo2 leads to increased osteoblastic differentiation and decreased Piezo2 leads to downregulation of WNT signaling. Mechanistically, an FGF/PIEZO2/WNT signaling cascade plays a crucial role in modulating the fate of progenitors during root morphogenesis. Downregulation of WNT signaling rescues tooth ankylosis in Fgfr1 mutant mice. Collectively, our findings uncover the mechanism by which FGF signaling regulates the fate decisions of stem/progenitor cells, and the interactions among signaling pathways and mechanotransduction during tooth root development, providing insights for future tooth root regeneration.

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.