Andrew Cho

Dentin sialoprotein and dentin phosphoprotein have distinct roles in dentin mineralization.

Dentin sialophosphoprotein (DSPP), a major non-collagenous matrix protein of odontoblasts, is proteolytically cleaved into dentin sialoprotein (DSP) and dentin phosphoprotein (DPP). Our previous studies revealed that DSPP null mice display a phenotype similar to human autosomal dominant dentinogenesis imperfecta, in which teeth have widened predentin and irregular dentin mineralization resulting in sporadic unmineralized areas in dentin and frequent pulp exposure. Earlier in vitro studies suggested that DPP, but not DSP, plays a significant role in initiation and maturation of dentin mineralization. However, the precise in vivo roles of DSP and DPP are far from clear. Here we report the generation of DPPcKO mice, in which only DSP is expressed in a DSPP null background, resulting in a conditional DPP knockout. DPPcKO teeth show a partial rescue of the DSPP null phenotype with the restored predentin width, an absence of irregular unmineralized areas in dentin, and less frequent pulp exposure. Micro-computed tomography (micro-CT) analysis of DPPcKO molars further confirmed this partial rescue with a significant recovery in the dentin volume, but not in the dentin mineral density. These results indicate distinct roles of DSP and DPP in dentin mineralization, with DSP regulating initiation of dentin mineralization, and DPP being involved in the maturation of mineralized dentin.

Neuronal Cyclin-Dependent Kinase 5 Activity Is Critical for Survival

Cyclin-dependent kinase 5 (Cdk5) null mice exhibit a unique phenotype characterized by perinatal mortality, disrupted cerebral cortical layering attributable to abnormal neuronal migration, lack of cerebellar foliation, and chromatolytic changes of neurons in the brainstem and the spinal cord. Because Cdk5 is expressed in both neurons and astrocytes, it has been unclear whether this phenotype is primarily attributable to defects in neurons or in astrocytes. Herein we report reconstitution of Cdk5 expression in neurons in Cdk5 null mice and its effect on the null phenotype. Unlike the Cdk5 null mice, the reconstituted Cdk5 null mice that express the Cdk5 transgene under the p35 promoter (TgKO mice) were viable and fertile. Because Cdk5 expression is mainly limited to neurons in these mice and rescues the defects in the nervous system of the Cdk5 null phenotype, it clearly demonstrates that Cdk5 activity is necessary for normal development and survival of p35-expressing neurons.

Photoactivation of Endogenous Latent Transforming Growth Factor–β1 Directs Dental Stem Cell Differentiation for Regeneration

Low-power laser–activated endogenous latent transforming growth factor–β1 (LTGF-β1) directs resident dental stem cell differentiation to promote dentin regeneration. Laser Light Encourages Tooth Regeneration A small dose of light may be sufficient to promote new tooth growth, at least in animal models. Arany and colleagues shined low-power laser light on the tooth pulps of rats and saw the formation of tertiary dentin, which is a bone-like substance. Taking this as evidence of tooth regeneration, the authors investigated the mechanism by which light can cause the dental pulp to form bone. Arany et al. discovered that low-power laser activates latent transforming growth factor–β (TGF-β), leading to the generation of reactive oxygen species and the differentiation of dental stem cells into odontoblasts (dentin-forming bone cells). This mechanism was further confirmed in vivo by demonstrating that mice lacking TGF-β or treated with a TGF-β inhibitor were unable to respond to laser therapy. Because lasers are already used in dentistry, it is possible that such light-based treatment could be used in dental regeneration in people. Rapid advancements in the field of stem cell biology have led to many current efforts to exploit stem cells as therapeutic agents in regenerative medicine. However, current ex vivo cell manipulations common to most regenerative approaches create a variety of technical and regulatory hurdles to their clinical translation, and even simpler approaches that use exogenous factors to differentiate tissue-resident stem cells carry significant off-target side effects. We show that non-ionizing, low-power laser (LPL) treatment can instead be used as a minimally invasive tool to activate an endogenous latent growth factor complex, transforming growth factor–β1 (TGF-β1), that subsequently differentiates host stem cells to promote tissue regeneration. LPL treatment induced reactive oxygen species (ROS) in a dose-dependent manner, which, in turn, activated latent TGF-β1 (LTGF-β1) via a specific methionine residue (at position 253 on LAP). Laser-activated TGF-β1 was capable of differentiating human dental stem cells in vitro. Further, an in vivo pulp capping model in rat teeth demonstrated significant increase in dentin regeneration after LPL treatment. These in vivo effects were abrogated in TGF-β receptor II (TGF-βRII) conditional knockout (DSPPCreTGF-βRIIfl/fl) mice or when wild-type mice were given a TGF-βRI inhibitor. These findings indicate a pivotal role for TGF-β in mediating LPL-induced dental tissue regeneration. More broadly, this work outlines a mechanistic basis for harnessing resident stem cells with a light-activated endogenous cue for clinical regenerative applications.

Overview: Engineering Transgenic Constructs and Mice

Cell biology research encompasses everything from single cells to whole animals. Recent discoveries concerning particular gene functions can be applied to the whole animal for understanding genotype‐phenotype relationships underlying disease mechanisms. For this reason, genetically manipulated mouse models are now considered essential to correctly understand disease processes in whole animals. This unit reviews the basic mouse technologies used to generate conventional transgenic mice, which represent a gain‐of‐function approach. First, an overview of transgenic construct design is presented. This unit then explains basic strategies for the identification and establishment of independent transgenic mouse lines, followed by comments on historical and emerging techniques. It then describes typical problems that are encountered when researchers start to generate transgenic mice. Curr. Protoc. Cell Biol. 42:19.10.1‐19.10.9.

Conditional overexpression of TGF-β1 disrupts mouse salivary gland development and function

Transforming growth factor-β (TGF-β) signaling is known to affect salivary gland physiology by influencing branching morphogenesis, regulating ECM deposition, and controlling immune homeostasis. To study the role of TGF-β1 in the salivary gland, we created a transgenic mouse (β1glo) that conditionally overexpresses active TGF-β1 upon genomic recombination by Cre recombinase. β1glo mice were bred with an MMTV (mouse mammary tumor virus)-Cre (MC) transgenic line that expresses the Cre recombinase predominantly in the secretory cells of both the mammary and salivary glands. Although most of the double positive (β1glo/MC) pups die either in utero or just after birth, clear defects in salivary gland morphogenesis such as reduced branching and increased mesenchyme could be seen. Those β1glo/MC mice that survived into adulthood, however, had hyposalivation due to salivary gland fibrosis and acinar atrophy. Increased TGF-β signaling was observed in the salivary gland with elevated phosphorylation of Smad2 and concomitant increase in ECM deposition. In particular, aberrant TGF-β1 overexpression caused salivary gland hypofunction in this mouse model because of the replacement of normal glandular parenchyma with interstitial fibrous tissue. These results further implicate TGF-β in pathological cases of salivary gland inflammation and fibrosis that occur with chronic infections in the glands or with the autoimmune disease, Sjögrens syndrome, or with radiation therapy given to head-and-neck cancer patients.

Generation of Transgenic Mice

This unit describes detailed step‐by‐step protocols, reagents, and equipment required for successful generation of transgenic mice using pronuclear injection. The experimental methods and practical tips given here will help guide beginners in understanding what is required and what to avoid in these standard protocols for efficiently generating transgenic mice. Curr. Protoc. Cell Biol. 42:19.11.1‐19.11.22.

Neuron-specific expression of Cre recombinase during the late phase of brain development.

Gene targeting to disrupt gene expression in a temporal and spatial manner in a specific tissue using Cre recombinase-mediated gene inactivation has been proven to be useful to study in vivo gene function. To delete genes specifically in neurons during the late phase of brain development, we have generated transgenic mouse lines that express Cre recombinase under the control of the murine neurofilament-H (mNF-H) gene promoter. In this study, we report that one of these mouse lines expresses Cre recombinase specifically in the neurons of the brain and spinal cord during the late stage of their development. The transgenic line displays specific excision of the loxP-flanked gene in the neurons just after embryonic day 18.5 (E.18.5), which coincides with the later phase of brain maturation including spinal cord and olfactory bulb area. This mNF-H-cre transgenic mouse line will be valuable for studying in vivo functions of neuron-specific genes, particularly, defining their precise roles in the mature nervous system using conditional gene targeting strategies.

A Method for Rapid Demineralization of Teeth and Bones

Tooth and bone specimen require extensive demineralization for careful analysis of cell morphology, as well as gene and protein expression levels. The LacZ gene, which encodes the ß-galactosidase enzyme, is often used as a reporter gene to study gene-structure function, tissue-specific expression by a promoter, cell lineage and fate. This reporter gene is particularly useful for analyzing the spatial and temporal gene expression pattern, by expressing the LacZ gene under the control of a promoter of interest. To analyze LacZ activity, and the expression of other genes and their protein products in teeth and bones, it is necessary to carry out a complete demineralization of the specimen before cutting sections. However, strong acids, such as formic acid used for tooth demineralization, destroy the activities of enzymes including those of ß-galactosidase. Therefore, most protocols currently use mild acids such as 0.1 M ethylene diamine tetra-acetic acid (EDTA) for demineralization of tooth and bone specimen, which require a longer period of treatment for complete demineralization. A method by which hard tissue specimens such as teeth and bones can be rapidly, but gently, decalcified is necessary to save time and effort. Here, we report a suitable method for rapid demineralization of mouse teeth in 0.1M EDTA at 42˚C without any loss of ß-galactosidase activity.

TGF-ß Regulates Enamel Mineralization and Maturation through KLK4 Expression

Transforming growth factor-ß (TGF-ß) signaling plays an important role in regulating crucial biological processes such as cell proliferation, differentiation, apoptosis, and extracellular matrix remodeling. Many of these processes are also an integral part of amelogenesis. In order to delineate a precise role of TGF-ß signaling during amelogenesis, we developed a transgenic mouse line that harbors bovine amelogenin promoter-driven Cre recombinase, and bred this line with TGF-ß receptor II floxed mice to generate ameloblast-specific TGF-ß receptor II conditional knockout (cKO) mice. Histological analysis of the teeth at postnatal day 7 (P7) showed altered enamel matrix composition in the cKO mice as compared to the floxed mice that had enamel similar to the wild-type mice. The µCT and SEM analyses revealed decreased mineral content in the cKO enamel concomitant with increased attrition and thinner enamel crystallites. Although the mRNA levels remained unaltered, immunostaining revealed increased amelogenin, ameloblastin, and enamelin localization in the cKO enamel at the maturation stage. Interestingly, KLK4 mRNA levels were significantly reduced in the cKO teeth along with a slight increase in MMP-20 levels, suggesting that normal enamel maturation is regulated by TGF-ß signaling through the expression of KLK4. Thus, our study indicates that TGF-ß signaling plays an important role in ameloblast functions and enamel maturation.

Targeted overexpression of tumor necrosis factor-α increases cyclin-dependent kinase 5 activity and TRPV1-dependent Ca2+ influx in trigeminal neurons.

Abstract We reported earlier that TNF-&agr;, a proinflammatory cytokine implicated in many inflammatory disorders causing orofacial pain, increases the activity of Cdk5, a key kinase involved in brain development and function and recently found to be involved in pain signaling. To investigate a potential mechanism underlying inflammatory pain in trigeminal ganglia (TGs), we engineered a transgenic mouse model (TNFglo) that can conditionally overexpresses TNF-&agr; upon genomic recombination by Cre recombinase. TNFglo mice were bred with Nav1.8-Cre mouse line that expresses the Cre recombinase in sensory neurons to obtain TNF-&agr;:Nav1.8-Cre (TNF-&agr; cTg) mice. Although TNF-&agr; cTg mice appeared normal without any gross phenotype, they displayed a significant increase in TNF-&agr; levels after activation of NF&kgr;B signaling in the TG. IL-6 and MCP-1 levels were also increased along with intense immunostaining for Iba1 and GFAP in TG, indicating the presence of infiltrating macrophages and the activation of satellite glial cells. TNF-&agr; cTg mice displayed increased trigeminal Cdk5 activity, and this increase was associated with elevated levels of phospho-T407-TRPV1 and capsaicin-evocated Ca2+ influx in cultured trigeminal neurons. Remarkably, this effect was prevented by roscovitine, an inhibitor of Cdk5, which suggests that TNF-&agr; overexpression induced sensitization of the TRPV1 channel. Furthermore, TNF-&agr; cTg mice displayed more aversive behavior to noxious thermal stimulation (45°C) of the face in an operant pain assessment device as compared with control mice. In summary, TNF-&agr; overexpression in the sensory neurons of TNF-&agr; cTg mice results in inflammatory sensitization and increased Cdk5 activity; therefore, this mouse model would be valuable for investigating the mechanism of TNF-&agr; involved in orofacial pain.