The stage-specific regulation of ameloblastin and enamelin by the distinct nuclear factors

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Project Summary/Abstract Cell identity is largely determined by specific epigenetic landscapes and transcriptional networks. Ameloblast is the only epithelial cell that can generate calcified tissue during development, where preameloblasts (PABs) first differentiate to the secretory ameloblasts (SABs) that synthesize and deposit enamel matrix proteins (EMPs) to scaffold organic matrix, and then to the maturation ameloblasts (MABs) that hydrolyze, endocytose EMPs, and transport ions to mineralize enamel. To bioengineer enamel, a nonregenerative tissue, we must understand the transcriptional regulation of ameloblasts that has been limited due to a loss of ameloblasts after the tooth eruption and a lack of cell line fully recapitulating the characteristics of ameloblasts. Previous fundings allow us to establish a novel and comprehensive list of genes significant to each developmental stage of ameloblasts across species and to explore the functions of chromatin organizer SATB1, and enamel matrix modeling regulators -peptidase KLK4 and the major calcium transporter NCKX4- in the context of ameloblast differentiation. These efforts resulted in a discovery that SATB1, KLK4, and NCKX4 all contribute to the transcriptional regulation of ameloblastin (Ambn) and enamelin (Enam), encoding the major EMPs co- upregulated in SABs and then co-downregulated in MABs. We found that ablation of SATB1, highly expressed in PABs, repressed Ambn & Enam transcription and H3K27ac level. Our organ culture showed that elevated histone acetylation upregulated Ambn & Enam. An enhancer and base unpairing region (BUR, selective SATB1 DNA binding site) have been predicted in the vicinity of Ambn & Enam. These data suggest that SATB1 organizes chromatin conformation and poises a transcriptional complex to upregulate Ambn & Enam to advance PABs to SABs. In the case of mice lacking Klk4 and Nckx4—the causative genes for amelogenesis imperfecta—we found a retention of proline/glutamine (P/G)-rich EMPs resulting from defective hydrolysis. These Nckx4-/- and Klk4-/- MABs had upregulated Ambn & Enam and downregulated Hif1a. In vitro studies showed that P/G-rich peptides downregulated Ambn & Enam and upregulated Hif1a. Our RNA-seq analyses revealed that HIF1A, a transcription factor regulating cell responses to oxidative stress, had a 6-fold upregulation in MABs vs SABs, reflecting MAB’s robust anti-oxidative capacity to continuously provide energy for ion transport and protein degradation. These data suggest that retake of P/G-rich peptides upregulate Hif1a, which in turn downregulate Ambn & Enam. Our in vivo and in vitro studies allow us to hypothesize that the dynamic expression of Ambn & Enam in the two major functional stages of ameloblasts is coordinately regulated by distinct factors chromatin organizer SATB1 and transcription factor HIF1A. This hypothesis will be addressed by specific aim 1: To determine the roles of SATB1 as a pioneer factor in PABs to poise the enhancer establishment for activating the transcription of Ambn & Enam gene in SABs; and specific aim 2: To determine the regulatory roles of HIF1A on Ambn & Enam expression and enamel formation.