Generation of a Novel Mouse Model that Constitutively Expresses XBP1s

Objectives: X-box binding protein 1 (XBP1) is a transcription factor that is essential for the differentiation of various types of secretory cells. Under endoplasmic reticulum (ER) stress, inositol-requiring enzyme 1 (IRE1) removes an intron from the unspliced XBP1 (XBP1u) mRNA to yield a spliced XBP1 (XBP1s) mRNA that encodes a potent XBP1s transcription factor. The goals of this study were to generate a mouse model that constitutively expresses XBP1s and to investigate the effects of constitutively active XBP1s on tooth development.
Methods: An Xbp1 knock-in mouse strain on a C57BL/6 genetic background was generated by conventional homologous recombination approach. These mice carried an Xbp1 allele with exons 4 and 5 (encoding XBP1u) flanked by two loxP sites, followed by a modified version of exons 4 and 5 (encoding XBP1s); these mice were referred to as “Xbp1s^IA/+” mice. Xbp1s^IA/+ mice would constitutively expressed XBP1s after Cre-recombinase-mediated recombination event. We bred the Xbp1s^IA/+ mice with Twist2-Cre (Twist2^Cre/+; Jackson Laboratory) mice to generate Xbp1s^IA/+;Twist2^Cre/+ mice. Reverse-transcription polymerase chain reaction (RT-PCR) was performed to determine activation of XBP1s mRNA in various tissues of newborn Xbp1^IA/+;Twist2^Cre/+ mice. Histological analysis and alizarin red/alcian blue staining were employed to analyze the dental phenotype of the mutant mice.
Results: The newborn Xbp1^IA/+;Twist2^Cre/+ mice showed no evidence of sucking milk and died shortly after birth, but they did not have any other gross abnormalities. RT-PCR analysis demonstrated that Cre-mediated XBP1s activation occurred in various tissues. Histologic analysis showed that mandibular first molars of the Xbp1^IA/+;Twist2^Cre/+ mice did not have any obvious developmental defects. Alizarin red/alcian blue staining showed no mineralization defects in the lower jaws of E18.5 Xbp1^IA/+;Twist2^Cre/+ mouse embryos, compared to the Xbp1^IA/+ embryos.
Conclusions: Constitutive expression of XBP1s caused perinatal death in mice, but it did not have any apparent effects on the early tooth development.