SEMA7a regulation of tooth mineralization, root elongation, and innervation

PROJECT SUMMARY/ABSTRACT The development of healthy molar teeth with elongated roots, mineralized dentin, and sensory innervation re- quires coordinated efforts between neuronal and mesenchymal tissue. During the first week of development, mouse molars begin all three developmental processes of secreting a dentin matrix, initiating root elongation,

and secreting chemoattractants to guide sensory afferent penetration into the pulp tissue. Research has shown that disrupting pulp mesenchyme signals during this period can have drastic effects on one or more of these processes. These coordinated processes are conserved in humans and pose clinical challenges for pediatric

dentists and endodontists attempting to ensure the long-term vitality of an infected or damaged immature tooth. The long-term goal of this project is to understand the cellular mechanisms that protect teeth via dental tissue crosstalk. The objective is to determine the mechanisms that regulate the development of a tooth into a miner-

alized, anchored, sensory organ. Our central hypothesis is that Sema7a signaling from differentiating odonto- blasts regulates tooth mineralization, root elongation, and innervation. This hypothesis was formulated because our mouse model of conditional Tgfbr2 deletion in tooth mesenchyme demonstrated reduced mineralization,

root elongation, and innervation coinciding with reduced Sema7a RNA expression in the DP and aberrant lo- calization of SEMA7a in the apical regions of the developing teeth. Guided by our previously generated data, we will test our hypothesis with the following two specific aims: 1) To determine the impact of global and condi-

tional deletion in dental pulp mesenchyme of Sema7a on dentin mineralization, root elongation, and tooth in- nervation; and 2) Isolate the role(s) of SEMA7a signaling to DP neuronal and mesenchymal populations with in vitro assays. Under the first aim, we will analyze tooth morphology, including the dentin volume, density, and

thickness, root length, and tooth innervation using the Sema7a-/- mouse model in addition to a conditional knockout mouse with Sema7a deletion induced by the osterix-promoter driven Cre recombinase induced at postnatal day 3. For the second aim, we will utilize a co-culture model with primary dental pulp cells and tri-

geminal neurons to assess whether SEMA7a regulates the processes of odontoblast differentiation, matrix deposition and neurite outgrowth. The research proposed in this application is innovative because it a) investi- gates a previously unrecognized chemoattractant involved in the process of tooth development during the criti-

cal window of time during the initiation of matrix deposition, root elongation, and tooth innervation; and b) pro- poses to be the first study to examine tooth development in models with global and conditional deletion of Sema7a. The proposed research is significant because it is expected to advance the understanding of how

these three developmental processes are regulated during a critical window of time during postnatal tooth de- velopment. Ultimately, such knowledge has the potential to bolster regenerative endodontics to treat injuries and disease states, and to ensure the proper development and long-term retention of healthy teeth.