Expression of Pannexin3 in human odontoblast-like cells and its hemichannel function in mediating ATP release

Prrx2, the paired-related homeobox transcription factor, functions as a potential regulator of pannexin 3 expression in odontoblast differentiation

OBJECTIVES

This study aimed to elucidate the roles of Prrx1 and Prrx2, homeobox transcription factors, in tooth development and determine whether Prrx2 regulates pannexin 3 (Panx3) expression, which is important in preodontoblasts.

METHODS

Tooth sections were prepared from 13.5-, 15.5-, and 18.5-day-old embryonic ICR mice, and Prrx1- and Prrx2-expressing cells were identified by in situ hybridization. To clarify the direct relationship between Prrx2 and Panx3, dual-luciferase reporter assay and electrophoretic mobility shift assay (EMSA) were performed. The effect of endogenous Prrx2 suppression on Panx3 expression was analyzed using an siRNA assay.

RESULTS

In situ hybridization revealed that in the molars, Prrx1 and Prrx2 were similarly expressed in the bud and cap stages; however, only Prrx2 was expressed in preodontoblasts at the bell stage. In the incisors, Prrx2-expressing cells were observed from dental papilla cells to preodontoblasts. In serial sections, Prrx2-expressing cells in preodontoblasts corresponded to Panx3-expressing cells. Luciferase reporter assay using luciferase reporter plasmids containing Panx3 promoter revealed that Prrx2 overexpression in HEK293 cells significantly increased luciferase activity. EMSA of nuclear extract proteins from Prrx2-overexpressing HEK293 cells or mouse dental papilla-derived cells to the Panx3 promoter showed the protein-probe complex bands. SiRNA assay revealed that Prrx2 knockdown inhibited Panx3 expression.

CONCLUSIONS

Our results suggest that Prrx2 may regulate Panx3 expression during odontoblastic differentiation.

Skin in the game: pannexin channels in healthy and cancerous skin

The skin is a highly organized tissue composed of multiple layers and cell types that require coordinated cell to cell communication to maintain tissue homeostasis. In skin cancer, this organized structure and communication is disrupted, prompting the malignant transformation of healthy cells into melanoma, basal cell carcinoma or squamous cell carcinoma tumours. One such family of channel proteins critical for cellular communication is pannexins (PANX1, PANX2, PANX3), all of which are present in the skin. These heptameric single-membrane channels act as conduits for small molecules and ions like ATP and Ca2+ but have also been shown to have channel-independent functions through their interacting partners or action in signalling pathways. Pannexins have diverse roles in the skin such as in skin development, aging, barrier function, keratinocyte differentiation, inflammation, and wound healing, which were discovered through work with pannexin knockout mice, organotypic epidermis models, primary cells, and immortalized cell lines. In the context of cutaneous cancer, PANX1 is present at high levels in melanoma tumours and functions in melanoma carcinogenesis, and both PANX1 and PANX3 expression is altered in non-melanoma skin cancer. PANX2 has thus far not been implicated in any skin cancer. This review will discuss pannexin isoforms, structure, trafficking, post-translational modifications, interactome, and channel activity. We will also outline the expression, localization, and function of pannexin channels within the diverse cell types of the epidermis, dermis, hypodermis, and adnexal structures of the skin, and how these properties are exploited or abrogated in instances of skin cancer.

Pannexin 3 channels regulate architecture, adhesion, barrier function and inflammation in the skin

The channel-forming glycoprotein Pannexin 3 (PANX3) functions in cutaneous wound healing and keratinocyte differentiation, but its role in skin homeostasis through aging is not yet understood. We found that PANX3 is absent in newborn skin but becomes upregulated with age. We characterized the skin of global Panx3 knockout mice (KO) and found that KO dorsal skin showed sex-differences at different ages, but generally had reduced dermal and hypodermal areas compared to aged-matched controls. Transcriptomic analysis of KO epidermis revealed reduced E-cadherin stabilization and Wnt signaling compared to WT, consistent with the inability of primary KO keratinocytes to adhere in culture, and diminished epidermal barrier function in KO mice. We also observed increased inflammatory signaling in KO epidermis and higher incidence of dermatitis in aged KO mice compared to wildtype controls. These findings suggest that during skin aging, PANX3 is critical in the maintenance of dorsal skin architecture, keratinocyte cell-cell and cell-matrix adhesion and inflammatory skin responses.

Domestic cat embryos reveal unique transcriptomes of developing incisor, canine, and premolar teeth

Division of the dentition into morphologically distinct classes of teeth (incisors, canines, premolars, and molars) and the acquisition of tribosphenic molars facilitated precise occlusion between the teeth early in mammal evolution. Despite the evolutionary and ecological importance of distinct classes of teeth with unique cusp, crest, and basin morphologies, relatively little is known about the genetic basis for the development of different tooth classes within the embryo. Here we investigated genetic differences between developing deciduous incisor, canine, and premolar teeth in the domestic cat (Felis catus), which we propose to be a new model for tooth development. We examined differences in both developmental timing and crown morphology between the three tooth classes. Using RNA sequencing of early bell stage tooth germs, we showed that each of the three deciduous tooth classes possess a unique transcriptional profile. Three notable groups of genes emerged from our differential expression analysis; genes involved in the extracellular matrix (ECM), Wnt pathway signaling, and members of multiple homeobox gene families (Lhx, Dlx, Alx, and Nkx). Our results suggest that ECM genes may play a previously under-appreciated role in shaping the surface of the tooth crown during development. Differential regulation of these genes likely underlies differences in tooth crown shape and size, although subtle temporal differences in development between the tooth germs could also be responsible. This study provides foundational data for future experiments to examine the function of these candidate genes in tooth development to directly test their potential effects on crown morphology.

Pannexin 3 channels in health and disease

Pannexin 3 (PANX3) is a member of the pannexin family of single membrane channel-forming glycoproteins. Originally thought to have a limited localization in cartilage, bone, and skin, PANX3 has now been detected in a variety of other tissues including skeletal muscle, mammary glands, the male reproductive tract, the cochlea, blood vessels, small intestines, teeth, and the vomeronasal organ. In many cell types of the musculoskeletal system, such as osteoblasts, chondrocytes, and odontoblasts, PANX3 has been shown to regulate the balance of proliferation and differentiation. PANX3 can be induced during progenitor cell differentiation, functioning at the cell surface as a conduit for ATP and/or in the endoplasmic reticulum as a calcium leak channel. Evidence in osteoblasts and monocytes also highlight a role for PANX3 in purinergic signalling through its function as an ATP release channel. PANX3 is critical in the development and ageing of bone and cartilage, with its levels temporally regulated in other tissues such as skeletal muscle, skin, and the cochlea. In diseases such as osteoarthritis and intervertebral disc degeneration, PANX3 can have either protective or detrimental roles depending on if the disease is age-related or injury-induced. This review will discuss PANX3 function in tissue growth and regeneration, its role in cellular differentiation, and how it becomes dysregulated in disease conditions such as obesity, Duchenne's muscular dystrophy, osteosarcoma, and non-melanoma skin cancer, where most of the findings on PANX3 function can be attributed to the characterization of Panx3 KO mouse models.

A Potential Compensatory Role of Panx3 in the VNO of a Panx1 Knock Out Mouse Model

Pannexins (Panx) are integral membrane proteins, with Panx1 being the best-characterized member of the protein family. Panx1 is implicated in sensory processing, and knockout (KO) animal models have become the primary tool to investigate the role(s) of Panx1 in sensory systems. Extending previous work from our group on primary olfaction, the expression patterns of Panxs in the vomeronasal organ (VNO), an auxiliary olfactory sense organ with a role in reproduction and social behavior, were compared. Using qRT-PCR and Immunohistochemistry (IHC), we confirmed the loss of Panx1, found similar Panx2 expression levels in both models, and a significant upregulation of Panx3 in mice with a global ablation of Panx1. Specifically, Panx3 showed upregulated expression in nerve fibers of the non-sensory epithelial layer in juvenile and adult KO mice and in the sensory layer of adults, which overlaps with Panx1 expression areas in WT populations. Since both social behavior and evoked ATP release in the VNO was not compromised in KO animals, we hypothesized that Panx3 could compensate for the loss of Panx1. This led us to compare Panx1 and Panx3 channels in vitro, demonstrating similar dye uptake and ATP release properties. Outcomes of this study strongly suggest that Panx3 may functionally compensate for the loss of Panx1 in the VNO of the olfactory system, ensuring sustained chemosensory processing. This finding extends previous reports on the upregulation of Panx3 in arterial walls and the skin of Panx1 KO mice, suggesting that roles of Panx1 warrant uncharacterized safeguarding mechanisms involving Panx3.

Adenosine receptors enhance the ATP-induced odontoblastic differentiation of human dental pulp cells

Purinergic signaling regulates various biological processes through the activation of adenosine receptors (ARs) and P2 receptors. ATP induces the odontoblastic differentiation of human dental pulp cells (HDPCs) via P2 receptors. However, there is no information available about the roles of ARs in HDPC odontoblastic differentiation induced by ATP. Here, we found that HDPCs treated with ATP showed higher activity of ADORA1 (A₁R), ADORA2B (A_2BR), and ADORA3 (A₃R). Inhibition of A₁R and A_2BR attenuated ATP-induced odontoblastic differentiation of HDPCs, whereas activation of the two receptors enhanced the odontoblastic differentiation induced by ATP. However, activation of ARs by adenosine did not induce the odontoblastic differentiation of HDPCs independently without induction of ATP. Our study indicates a positive role for ARs in ATP-induced odontoblastic differentiation of HDPCs, and demonstrates that ATP-induced odontoblastic differentiation of HDPCs may be due to the combined administration of ARs and P2 receptors. This study provides new insights into the molecular mechanisms of pulpal injury repair induced by ATP.

Pannexins in vision, hearing, olfaction and taste

In mammals, the pannexin gene family consists of three members (Panx1, 2, 3), which represent a class of integral membrane channel proteins sharing some structural features with chordate gap junction proteins, the connexins. Since their discovery in the early 21st century, pannexin expression has been detected throughout the vertebrate body including eye, ear, nose and tongue, making the investigation of the roles of this new class of channel protein in health and disease very appealing. The localization in sensory organs, coupled with unique channel properties and associations with major signaling pathways make Panx1, and its relative's, significant contributors for fundamental functions in sensory perception. Until recently, cell-based studies were at the forefront of pannexin research. Lately, the availability of mice with genetic ablation of pannexins opened new avenues for testing pannexin functions and behavioural phenotyping. Although we are only at the beginning of understanding the roles of pannexins in health and disease, this review summarizes recent advances in elucidating the various emerging roles pannexins play in sensory systems, with an emphasis on unresolved conflicts.

The Role of Pannexin 3 in Bone Biology

Cell-cell and cell-matrix communications play important roles in both cell proliferation and differentiation. Gap junction proteins mediate signaling communication by exchanging small molecules and dramatically stimulating intracellular signaling pathways to determine cell fate. Vertebrates have 2 gap junction families: pannexins (Panxs) and connexins (Cxs). Unlike Cxs, the functions of Panxs are not fully understood. In skeletal formation, Panx3 and Cx43 are the most abundantly expressed gap junction proteins from each family. Panx3 is induced in the transient stage from the proliferation and differentiation of chondrocytes and osteoprogenitor cells. Panx3 regulates both chondrocyte and osteoblast differentiation via the activation of intracellular Ca²⁺ signaling pathways through multiple channel activities: hemichannels, endoplasmic reticulum (ER) Ca²⁺ channels, and gap junctions. Moreover, Panx3 also inhibits osteoprogenitor cell proliferation and promotes cell cycle exit through the inactivation of Wnt/β-catenin signaling and the activation of p21. Panx3-knockout (KO) mice have more severe skeletal abnormalities than those of Cx43-KO mice. A phenotypic analysis of Panx3-KO mice indicates that Panx3 regulates the terminal differentiation of chondrocytes by promoting vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP) 13. Based on the generation of Panx3^-/-; Cx43^-/- mice, Panx3 is upstream of Cx43 in osteogenesis. Panx3 promotes Cx43 expression by regulating Wnt/β-catenin signaling and osterix expression. Further, although Panx3 can function in 3 ways, Cx43 cannot function through the ER Ca²⁺ channel, only via the hemichannels and gap junction routes. In this review, we discuss the current knowledge regarding the roles of Panx3 in skeletal formation and address the potential for new therapies in the treatment of diseases and pathologies associated with Panx3, such as osteoarthritis (OA).

Pannexin3 inhibits TNF-α-induced inflammatory response by suppressing NF-κB signalling pathway in human dental pulp cells

Human dental pulp cells (HDPCs) play a crucial role in dental pulp inflammation. Pannexin 3 (Panx3), a member of Panxs (Pannexins), has been recently found to be involved in inflammation. However, the mechanism of Panx3 in human dental pulp inflammation remains unclear. In this study, the role of Panx3 in inflammatory response was firstly explored, and its potential mechanism was proposed. Immunohistochemical staining showed that Panx3 levels were diminished in inflamed human and rat dental pulp tissues. In vitro, Panx3 expression was significantly down‐regulated in HDPCs following a TNF‐α challenge in a concentration‐dependent way, which reached the lowest level at 10 ng/ml of TNF‐α. Such decrease could be reversed by MG132, a proteasome inhibitor. Unlike MG132, BAY 11‐7082, a NF‐κB inhibitor, even reinforced the inhibitory effect of TNF‐α. Quantitative real‐time PCR (qRT‐PCR) and enzyme‐linked immunosorbent assay (ELISA) were used to investigate the role of Panx3 in inflammatory response of HDPCs. TNF‐α‐induced pro‐inflammatory cytokines, interleukin (IL)‐1β and IL‐6, were significantly lessened when Panx3 was overexpressed in HDPCs. Conversely, Panx3 knockdown exacerbated the expression of pro‐inflammatory cytokines. Moreover, Western blot, dual‐luciferase reporter assay, immunofluorescence staining, qRT‐PCR and ELISA results showed that Panx3 participated in dental pulp inflammation in a NF‐κB‐dependent manner. These findings suggested that Panx3 has a defensive role in dental pulp inflammation, serving as a potential target to be exploited for the intervention of human dental pulp inflammation.