Expression of Runx2/Cbfa1/Pebp2alphaA during angiogenesis in postnatal rodent and fetal human orofacial tissues

Bone Marrow Microvasculature

The skeleton is highly vascularized due to the various roles blood vessels play in the homeostasis of bone and marrow. For example, blood vessels provide nutrients, remove metabolic by-products, deliver systemic hormones, and circulate precursor cells to bone and marrow. In addition to these roles, bone blood vessels participate in a variety of other functions. This article provides an overview of the afferent, exchange and efferent vessels in bone and marrow and presents the morphological layout of these blood vessels regarding blood flow dynamics. In addition, this article discusses how bone blood vessels participate in bone development, maintenance, and repair. Further, mechanical loading-induced bone adaptation is presented regarding interstitial fluid flow and pressure, as regulated by the vascular system. The role of the sympathetic nervous system is discussed in relation to blood vessels and bone. Finally, vascular participation in bone accrual with intermittent parathyroid hormone administration, a medication prescribed to combat age-related bone loss, is described and age- and disease-related impairments in blood vessels are discussed in relation to bone and marrow dysfunction. © 2020 American Physiological Society. Compr Physiol 10:1009-1046, 2020.

Effects of Oral Anticoagulant Therapy on Gene Expression in Crosstalk between Osteogenic Progenitor Cells and Endothelial Cells

Direct oral anti-coagulants (DOACs) are employed in clinical practice for the prevention and treatment of recurrent venous thromboembolism and for the prevention of stroke in non-valvular atrial fibrillation. DOACs directly and reversibly inhibit activated factor X or thrombin and can interfere with other pathophysiological processes such as inflammation, lipid metabolism, and bone turnover. We aimed to evaluate the possible effects of DOACs on osteogenesis and angiogenesis. We treated 34 patients affected by cardiovascular disorders with DOACs; biochemical and molecular analyses were performed before and after three months of treatment. Circulating progenitors (CPs; CD34⁻, CD45⁻, CD14⁻, CD73⁺, CD105⁺), which share typical bone marrow stem cell (MSCs) features, were harvested from peripheral blood of the study subjects to monitor the expression of osteogenesis-related genes RUNX2 and SPARC. Human umbilical vein endothelial cells (HUVECs) were used to probe angiogenesis-related VEGF, CD31, and CD105 gene expression. We performed co-culture experiments using a commercial human mesenchymal stem cells line (hMSCs) obtained from bone marrow and HUVECs. Clinical parameters related to bone metabolism, coagulation, renal and liver function, and the lipid profile were evaluated. Values of the C-terminal telopeptide type I collagen (CTX) increased after the treatment. We found a significant increase in osteogenesis marker gene expression in CPs after three months of anticoagulant therapy. An increase in the RUNX2 expression determinant alone was detected instead in hMSCs co-cultured with HUVECs in the presence of treated patients’ sera. The VEGF, CD31, and CD105 marker genes appeared to be significantly upregulated in HUVECs co-cultured with hMSCs in the presence of treated patients’ sera. Under these conditions, new vessel formation increased as well. Our results highlight an unexpected influence of DOAC therapy on osteogenic commitment and vascular endothelial function promotion.

Runx2-I is an Early Regulator of Epithelial-Mesenchymal Cell Transition in the Chick Embryo

BACKGROUND

Although normally linked to bone and cartilage development, the Runt-related transcription factor, RUNX2, was reported in the mouse heart during development of the valves. We examined RUNX2 expression and function in the developing avian heart as it related to the epithelial-mesenchymal transition (EMT) in the atrioventricular canal. EMT can be separated into an activation stage involving hypertrophy and cell separation and an invasion stage where cells invade the extracellular matrix. The localization and activity of RUNX2 was explored in relation to these steps in the heart. As RUNX2 was also reported in cancer tissues, we examined its expression in the progression of esophageal cancer in staged tissues.

RESULTS

A specific isoform, RUNX2-I, is present and required for EMT by endothelia of the atrioventricular canal. Knockdown of RUNX2-I inhibits the cell-cell separation that is characteristic of initial activation of EMT. Loss of RUNX2-I altered expression of EMT markers to a greater extent during activation than during subsequent cell invasion. Transforming growth factor beta 2 (TGFβ2) mediates activation during cardiac endothelial EMT. Consistent with a role in activation, RUNX2-I is regulated by TGFβ2 and its activity is independent of similarly expressed Snai2 in regulation of EMT. Examination of RUNX2 expression in esophageal cancer showed its upregulation concomitant with the development of dysplasia and continued expression in adenocarcinoma.

CONCLUSIONS

These data introduce the RUNX2-I isoform as a critical early transcription factor mediating EMT in the developing heart after induction by TGFβ2. Its expression in tumor tissue suggests a similar role for RUNX2 in the EMT of metastasis. Developmental Dynamics 247:542-554, 2018. © 2017 Wiley Periodicals, Inc.

Roles of RUNX in Hypoxia-Induced Responses and Angiogenesis

During the past two decades, Runt domain transcription factors (RUNX1, 2, and 3) have been investigated in regard to their function, structural elements, genetic variants, and roles in normal development and pathological conditions. The Runt family proteins are evolutionarily conserved from Drosophila to mammals, emphasizing their physiological importance. A hypoxic microenvironment caused by insufficient blood supply is frequently observed in developing organs, growing tumors, and tissues that become ischemic due to impairment or blockage of blood vessels. During embryonic development and tumor growth, hypoxia triggers a stress response that overcomes low-oxygen conditions by increasing erythropoiesis and angiogenesis and triggering metabolic changes. This review briefly introduces hypoxic conditions and cellular responses, as well as angiogenesis and its related signaling pathways, and then describes our current knowledge on the functions and molecular mechanisms of Runx family proteins in hypoxic responses, especially in angiogenesis.

Hyperglycemia and redox status regulate RUNX2 DNA-binding and an angiogenic phenotype in endothelial cells

Angiogenesis is regulated by hyperglycemic conditions, which can induce cellular stress responses, reactive oxygen species (ROS), and anti-oxidant defenses that modulate intracellular signaling to prevent oxidative damage. The RUNX2 DNA-binding transcription factor is activated by a glucose-mediated intracellular pathway, plays an important role in endothelial cell (EC) function and angiogenesis, and is a target of oxidative stress. RUNX2 DNA-binding and EC differentiation in response to glucose were conserved in ECs from different tissues and inhibited by hyperglycemia, which stimulated ROS production through the aldose reductase glucose-utilization pathway. Furthermore, the redox status of cysteine and methionine residues regulated RUNX2 DNA-binding and reversal of oxidative inhibition was consistent with an endogenous Methionine sulfoxide reductase-A (MsrA) activity. Low molecular weight MsrA substrates and sulfoxide scavengers were potent inhibitors of RUNX2 DNA binding in the absence of oxidative stress, but acted as antioxidants to increase DNA binding in the presence of oxidants. MsrA was associated with RUNX2:DNA complexes, as measured by a sensitive, quantitative DNA-binding ELISA. The related RUNX2 protein family member, RUNX1, which contains an identical DNA-binding domain, was a catalytic substrate of recombinant MsrA. These findings define novel redox pathways involving aldose reductase and MsrA that regulate RUNX2 transcription factor activity and biological function in ECs. Targeting of these pathways could result in more effective strategies to alleviate the vascular dysfunction associated with diabetes or cancer.

Influence of scaffold composition over in vitro osteogenic differentiation of hBMSCs and in vivo inflammatory response

To understand the role of chitosan in chitosan-poly(butylene succinate) scaffolds (50% wt), 50%, 25%, and 0% of chitosan were used to produce different scaffolds. These scaffolds were in vitro seeded and cultured with human bone marrow stromal cells in osteogenic conditions, revealing that higher percentage of chitosan showed enhanced cell viability over time, adhesion, proliferation, and osteogenic differentiation. Scaffolds were also implanted in cranial defects and iliac submuscular region in Wistar rats, and the results evidenced that chitosan-containing scaffolds displayed mild inflammatory response and good integration with surrounding tissues, showed by connective tissue colonization and the presence of new blood vessels. Scaffolds without chitosan-evidenced necrotic tissue in scaffolds' interior, proving that chitosan exerts a positive effect over cell behavior and displays a milder host inflammatory response in vivo.

Physical and functional interactions between Runx2 and HIF-1α induce vascular endothelial growth factor gene expression

Angiogenesis and bone formation are intimately related processes. Hypoxia during early bone development stabilizes hypoxia-inducible factor-1α (HIF-1α) and increases angiogenic signals including vascular endothelial growth factor (VEGF). Furthermore, stabilization of HIF-1α by genetic or chemical means stimulates bone formation. On the other hand, deficiency of Runx2, a key osteogenic transcription factor, prevents vascular invasion of bone and VEGF expression. This study explores the possibility that HIF-1α and Runx2 interact to activate angiogenic signals. Runx2 over-expression in mesenchymal cells increased VEGF mRNA and protein under both normoxic and hypoxic conditions. In normoxia, Runx2 also dramatically increased HIF-1α protein. In all cases, the Runx2 response was inhibited by siRNA-mediated suppression of HIF-1α and completely blocked by the HIF-1α inhibitor, echinomycin. Similarly, treatment of preosteoblast cells with Runx2 siRNA reduced VEGF mRNA in normoxia or hypoxia. However, Runx2 is not essential for the HIF-1α response since VEGF is induced by hypoxia even in Runx2-null cells. Endogenous Runx2 and HIF-1α were colocalized to the nuclei of MC3T3-E1 preosteoblast cells. Moreover, HIF-1α and Runx2 physically interact using sites within the Runx2 RUNT domain. Chromatin immunoprecipitation also provided evidence for colocalization of Runx2 and HIF-1α on the VEGF promoter. In addition, Runx2 stimulated HIF-1α-dependent activation of an HRE-luciferase reporter gene without requiring a separate Runx2-binding enhancer. These studies indicate that Runx2 functions together with HIF-1α to stimulate angiogenic gene expression in bone cells and may in part explain the known requirement for Runx2 in bone vascularization.

Induction of vascular progenitor cells from endothelial cells stimulates coronary collateral growth

RATIONALE

A well-developed coronary collateral circulation improves the morbidity and mortality of patients following an acute coronary occlusion. Although regenerative medicine has great potential in stimulating vascular growth in the heart, to date there have been mixed results, and the ideal cell type for this therapy has not been resolved.

OBJECTIVE

To generate induced vascular progenitor cells (iVPCs) from endothelial cells, which can differentiate into vascular smooth muscle cells (VSMCs) or endothelial cells (ECs), and test their capability to stimulate coronary collateral growth.

METHODS AND RESULTS

We reprogrammed rat ECs with the transcription factors Oct4, Klf4, Sox2, and c-Myc. A population of reprogrammed cells was derived that expressed pluripotent markers Oct4, SSEA-1, Rex1, and AP and hemangioblast markers CD133, Flk1, and c-kit. These cells were designated iVPCs because they remained committed to vascular lineage and could differentiate into vascular ECs and VSMCs in vitro. The iVPCs demonstrated better in vitro angiogenic potential (tube network on 2-dimensional culture, tube formation in growth factor reduced Matrigel) than native ECs. The risk of teratoma formation in iVPCs is also reduced in comparison with fully reprogrammed induced pluripotent stem cells (iPSCs). When iVPCs were implanted into myocardium, they engrafted into blood vessels and increased coronary collateral flow (microspheres) and improved cardiac function (echocardiography) better than iPSCs, mesenchymal stem cells, native ECs, and sham treatments.

CONCLUSIONS

We conclude that iVPCs, generated by partially reprogramming ECs, are an ideal cell type for cell-based therapy designed to stimulate coronary collateral growth.

The significance of RUNX2 in postnatal development of the mandibular condyle

OBJECTIVE

RUNX2, in the Runt gene family, is one of the most important transcription factors in the development of the skeletal system. Research in recent decades has shown that this factor plays a major role in the development, growth and maturation of bone and cartilage. It is also important in tooth development, mechanotransduction and angiogenesis, and plays a significant role in various pathological processes, i.e. tumor metastasization. Mutations in the RUNX2 gene correlate with the cleidocranial dysplasia (CCD) syndrome, important to dentistry, particularly orthodontics because of its dental and orofacial symptoms. Current research on experimentally-induced mouse mutants enables us to study the etiology and pathogenesis of these malformations at the cellular and molecular biological level. This study's aim is to provide an overview of the RUNX2 gene's function especially in skeletal development, and to summarize our research efforts to date, which has focused on investigating the influence of RUNX2 on mandibular growth, which is slightly or not at all altered in many CCD patients.

MATERIALS AND METHODS

Immunohistochemical analyses were conducted to reveal RUNX2 in the condylar cartilage of normal mice and of heterozygous RUNX2 knockout mice in early and late growth phases; we also performed radiographic and cephalometric analyses.

RESULTS

We observed that RUNX2 is involved in normal condylar growth in the mouse and probably plays a significant role in osteogenesis and angiogenesis. The RUNX2 also has a biomechanical correlation in relation to cartilage compartmentalization. At the protein level, we noted no differences in the occurrence and distribution of RUNX2 in the condyle, except for a short phase during the 4th and 6th postnatal weeks, so that one allele might suffice for largely normal growth; other biological factors may have compensatory effects. However, we did observe small changes in a few cephalometric parameters concerning the mandibles of heterozygous knockout animals. We discuss potential correlations to our findings by relating them to the most current knowledge about the RUNX2 biology.

Low-intensity pulsed ultrasound affects RUNX2 immunopositive osteogenic cells in delayed clinical fracture healing

INTRODUCTION

Osteogenic cell proliferation and differentiation play an important role in adequate fracture healing, and is target for osteoinductive therapies in delayed fracture healing. The aim of this study was to investigate whether low-intensity pulsed ultrasound enhances fracture healing at the tissue level in patients with a delayed union of the osteotomized fibula through an effect on the presence of RUNX2 immunopositive osteogenic cells. The effect was studied in both atrophic and hypertrophic delayed unions.

MATERIALS AND METHODS

Biopsies were obtained from 6 female and 1 male patient (age 43-63) with a delayed union of the osteotomized fibula after a high tibial osteotomy treated for 2-4 months with or without low-intensity pulsed ultrasound in a randomized prospective double-blind placebo-controlled trial. Immunolocalization of RUNX2 protein was performed to identify osteogenic cells. Histomorphometrical analysis was performed to determine the number of cells expressing RUNX2 located within and around the newly formed woven bone at the fracture end (area of new bone formation), and up to 3 mm distant from the fracture end.

RESULTS

Cells expressing RUNX2 were present in all histological sections of control and low-intensity pulsed ultrasound-treated bone evaluated. Within the area of new bone formation, RUNX2 immunopositive cells were found in the undifferentiated soft connective tissue, at the bone surface (presumably osteoblasts), and within the newly formed woven bone. Low-intensity pulsed ultrasound treatment of fibula delayed unions significantly reduced the number of RUNX2 immunopositive cells within the soft connective tissue at the fracture ends, whereas the number of RUNX2 immunopositive cells at the bone surface was not affected. The number of RUNX2 immunopositive cells was similar for the atrophic and hypertrophic delayed unions.

CONCLUSIONS

Immunolocalization of RUNX2 positive cells in delayed unions of the fibula reveals that delayed clinical fracture healing does not result in impairment of osteogenic cell proliferation and/or differentiation at the tissue level, even if delayed unions are clinically regarded as atrophic. Reduced number of osteogenic RUNX2 immunopositive cells within the soft connective tissue, and unchanged number of RUNX2 immunopositive cells at the bone surface, implicate that low-intensity pulsed ultrasound does not increase osteogenic cell presence, but likely affects osteogenic cell differentiation.

Osterix overexpression in mesenchymal stem cells stimulates healing of critical-sized defects in murine calvarial bone

Osterix (Osx) is a zinc-finger-containing transcription factor that is expressed in osteoblasts of all endochondral and membranous bones. In Osx null mice, osteoblast differentiation is impaired, and bone formation is absent. We hypothesized that overexpression of Osx in bone marrow-derived mesenchymal stem cells (BMSCs) would enhance osteogenic differentiation during bone regeneration in vivo. Overexpression of Osx in mouse BMSCs was achieved using retroviral infection together with a green fluorescent protein (GFP) vector to monitor transduction efficiency and determine the source of regenerative cells in implantation studies. Bone regeneration in vivo was evaluated by implanting BMSCs overexpressing Osx into 4-mm calvarial bone defects in adult mice using type I collagen sponge as a carrier. New bone formation in the defects was quantified using radiological and histological procedures 5 weeks after implantation. The results showed that implantation of Osx-transduced BMSCs resulted in 85% healing of calvarial bone defects as detected using radiological analyses. Histological examination of the implants demonstrated that the Osx-transduced group exhibited amounts of newly formed bone that was five times as high as in a group transduced with the empty vector. Immunohistochemistry for GFP showed positive immunoreaction localized to areas of newly engineered bone in the Osx-transduced group. Immunohistochemistry with antibodies against the extracellular matrix protein bone sialoprotein resulted in strong staining in areas of new bone formation. In addition, the clonal BMSCs showed an osteogenic potential similar to that of primary cultures of BMSCs, suggesting the usefulness of this model in bone tissue engineering. These results indicate that ex vivo gene therapy of Osx is a useful therapeutic approach in regenerating adult bone tissue.

Establishment of cell lines that exhibit pluripotency from miniature swine periodontal ligaments

OBJECTIVE

The periodontal ligament (PDL) is a fibrous connective tissue composed of heterogeneous cell types, including PDL fibroblasts. It is not clear whether cells within the PDL fibroblast population retain the potency to differentiate into other cell types.

DESIGN

In the present study, clonal cell lines, derived from Clawn miniature swine PDLs, were established by gene transfection for a human telomerase reverse transcriptase, and characterized.

RESULTS

These cell lines, denoted TesPDL1-4, had PDL fibroblasts that showed fibroblastic morphology and expressed procollagen alpha1(I), osteopontin, periostin and alkaline phosphatase mRNA. Under the specific culture conditions, TesPDL3 cells also have the ability to express CD31, vascular endothelial cadherin, von Willebrand factor, osteocalcin, and to form extracellular mineralized nodules.

CONCLUSIONS

Our data indicate that TesPDL3 cells have unique properties of expressing several phenotype of fibroblasts, vascular endothelial cells and osteoblasts in cultures.

Effect of thrombin peptide 508 (TP508) on bone healing during distraction osteogenesis in rabbit tibia

Thrombin-related peptide 508 (TP508) accelerates bone regeneration during distraction osteogenesis (DO). We have examined the effect of TP508 on bone regeneration during DO by immunolocalization of Runx2 protein, a marker of osteoblast differentiation, and of osteopontin (OPN) and bone sialoprotein (BSP), two late markers of the osteoblast lineage. Distraction was performed in tibiae of rabbits over a period of 6 days. TP508 (30 or 300 μg) or vehicle was injected into the distraction gap at the beginning and end of the distraction period. Two weeks after active distraction, tissue samples were harvested and processed for immunohistochemical analysis. We also tested the in vitro effect of TP508 on Runx2 mRNA expression in osteoblast-like (MC3T3-E1) cells by polymerase chain reaction analysis. Runx2 and OPN protein were observed in preosteoblasts, osteoblasts, osteocytes of newly formed bone, blood vessel cells and many fibroblast-like cells of the soft connective tissue. Immunostaining for BSP was more restricted to osteoblasts and osteocytes. Significantly more Runx2- and OPN-expressing cells were seen in the group treated with 300 μg TP508 than in the control group injected with saline or with 30 μg TP508. However, TP508 failed to increase Runx2 mRNA levels significantly in MC3T3-E1 cells after 2–3 days of exposure. Our data suggest that TP508 enhances bone regeneration during DO by increasing the proportion of cells of the osteoblastic lineage. Clinically, TP508 may shorten the healing time during DO; this might be of benefit when bone regeneration is slow.

Immunolocalization of sibling and RUNX2 proteins during vertical distraction osteogenesis in the human mandible

We tested the hypothesis that mechanical loading of human bone increases expression of the transcription factor RUNX2 and bone matrix proteins osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein-1 (DMP1), and matrix extracellular phosphoglycoprotein (MEPE). We examined this in tissue sections of atrophic mandibular bone taken from edentulous patients who had undergone distraction osteogenesis. In undistracted bone, weak to moderate staining for OPN and BSP was found in osteoblasts and bone matrix of immature woven bone. RUNX2 was also detectable in osteoblasts and in cells of the periosteum. In woven bone, but not in lamellar bone, a small number of osteocytes stained for all proteins tested. After distraction, staining intensity had increased in the existing old bone and staining was seen in more bone cells than before distraction. We also found a high expression of DMP1 and MEPE in many osteocytes embedded in woven bone and in some osteocytes of lamellar bone not seen before distraction. New bone trabeculae were forming in the fibrous tissue of the distraction gap containing all stages of intramembranous bone formation. Moderate to strong staining was seen for all five proteins tested in osteocytes located in woven bone of these trabeculae and for RUNX2, OPN, and BSP in osteoblasts lining the trabecular surfaces. We conclude that loading of atrophic human jawbone by distraction activates matrix synthesis of bone cells in and around existing bone. Increased staining of DMP1 and MEPE in osteocytes after loading is in line with the concept that these proteins may be involved in signaling the effector cells to adapt the bone structure to its mechanical demands.

Microarray analysis on Runx2-deficient mouse embryos reveals novel Runx2 functions and target genes during intramembranous and endochondral bone formation

A major challenge in developmental biology is to correlate genome-wide gene expression modulations with developmental processes in vivo. In this study, we analyzed the role of Runx2 during intramembranous and endochondral bone development, by comparing gene expression profiles in 14.5 dpc wild-type and Runx2 (-/-) mice. A total of 1277, 606 and 492 transcripts were found to be significantly modulated by Runx2 in calvaria, forelimbs and hindlimbs, respectively. Bioinformatics analysis indicated that Runx2 not only controls the processes of osteoblast differentiation and chondrocyte maturation, but may also play a role in axon formation and hematopoietic cell commitment during bone development. A total of 41 genes are affected by the Runx2 deletion in both intramembranous and endochondral bone, indicating common pathways between these two developmental modes of bone formation. In addition, we identified genes that are specifically involved in endochondral ossification. In conclusion, our data show that a comparative genome-wide expression analysis of wild-type and mutant mouse models allows the examination of mutant phenotypes in complex tissues.

Formation of new bone during vertical distraction osteogenesis of the human mandible is related to the presence of blood vessels

We examined the effect of distraction rate on blood vessel growth in intramembraneous ossification after vertical distraction osteogenesis in the human mandible. Six edentulous patients (aged 60+/-9 years) with a severely atrophic mandible underwent bone augmentation with distraction osteogenesis. Two distraction rates (0.5 and 1 mm/day) were compared and for each group three patients were analyzed. Vascular histomorphometry was carried out in two different areas in the distraction gap: (1) in the first and (2) in the second 1 mm area from the osteotomy line, representing the oldest and younger new-bone area, respectively. Correlation analysis was performed between blood vessel parameters and the amount of new bone formed during distraction. Histological analysis demonstrated the presence of blood vessels throughout the soft connective tissue in the distraction gap. The volume density of blood vessels between the two investigated areas was significantly lower in the 1 mm/day groups, suggesting a delay in angiogenesis in this group of patients. A positive correlation between blood vessel volume and bone volume density was found in the younger new-bone area but not in the oldest new-bone area. This correlation was due to a higher number of blood vessels rather than to a larger size of the blood vessels. Our data suggest that the lower blood vessel density found in the patients with 1 mm/day distraction rate may be related to disruption of angiogenesis in the soft connective tissue of the gap or to a less optimal mechanical stimulation of cells involved in angiogenesis. This probably results in the slower rate of osteogenesis seen at the 1 mm/day distraction rate compared with the 0.5 mm/day distraction rate. The data support the concept that a positive relationship exists between the density of blood vessels and the formation of bone. For distraction of the human mandible in elderly patients, a distraction rate of 0.5 mm/day seems beneficial.

Osterix enhances proliferation and osteogenic potential of bone marrow stromal cells

Osterix (Osx) is a zinc-finger-containing transcription factor that is expressed in osteoblasts of all endochondral and membranous bones. In Osx null mice osteoblast differentiation is impaired and bone formation is absent. In this study, we hypothesized that overexpression of Osx in murine bone marrow stromal cells (BMSC) would be able to enhance their osteoblastic differentiation and mineralization in vitro. Retroviral transduction of Osx in BMSC cultured in non-differentiating medium did not affect expression of Runx2/Cbfa1, another key transcription factor of osteoblast differentiation, but induced an increase in the expression of other markers associated with the osteoblastic lineage including alkaline phosphatase, bone sialoprotein, osteocalcin, and osteopontin. Retroviral transduction of Osx in BMSC also increased their proliferation, alkaline phosphatase activity, and ability to form bone nodules. These events occurred without significant changes in the expression of alpha1(II) procollagen or lipoprotein lipase, which are markers of chondrogenic and adipogenic differentiation, respectively.

Protein–Protein Interactions of the Developing Enamel Matrix

Extracellular matrix proteins control the formation of the inorganic component of hard tissues including bone, dentin, and enamel. The structural proteins expressed primarily in the enamel matrix are amelogenin, ameloblastin, enamelin, and amelotin. Other proteins, like biglycan, are also present in the enamel matrix as well as in other mineralizing and nonmineralizing tissues of mammals. In addition, the presence of sulfated enamel proteins, and "tuft" proteins has been examined and discussed in relation to enamel formation. The structural proteins of the enamel matrix must have specific protein-protein interactions to produce a matrix capable of directing the highly ordered structure of the enamel crystallites. Protein-protein interactions are also likely to occur between the secreted enamel proteins and the plasma membrane of the enamel producing cells, the ameloblasts. Such protein-protein interactions are hypothesized to influence the secretion of enamel proteins, establish short-term order of the forming matrix, and to mediate feedback signals to the transcriptional machinery of these cells. Membrane-bound proteins identified in ameloblasts, and which interact with the structural enamel proteins, include Cd63 (cluster of differentiation 63 antigen), annexin A2 (Anxa2), and lysosomal-associated glycoprotein 1 (Lamp1). These and related data help explain the molecular and cellular mechanisms responsible for the removal of the organic enamel matrix during the events of enamel mineralization, and how the enamel matrix influences its own fate through signaling initiated at the cell surface. The knowledge gained from enamel developmental studies may lead to better dental and nondental materials, or materials inspired by Nature. These data will be critical to scientists, engineers, and dentists in their pursuits to regenerate an entire tooth. For tooth regeneration to become a reality, the protein-protein interactions involving the key dental proteins must be identified and understood. The scope of this review is to discuss the current understanding of protein-protein interactions of the developing enamel matrix, and relate this knowledge to enamel biomineralization.