The PTHrP gene generates low-abundance mRNA and protein products that are not easily localized by in situ hybridization histochemistry or immunohistochemistry. We report here a PTHrP-lacZ knockin mouse in which beta-gal activity seems to provide a simple and sensitive read-out of PTHrP gene expression.
Introduction: PTH-related protein (PTHrP) is widely expressed in fetal and adult tissues, typically as low-abundance mRNA and protein products that maybe difficult to localize by conventional methods. We created a PTHrP-lacZ knockin mouse as a means of surveying PTHrP gene expression in general and of identifying previously unrecognized sites of PTHrP expression.
Materials and methods: We created a lacZ reporter construct under the control of endogenous PTHrP gene regulatory sequences. The AU-rich instability sequences in the PTHrP 3' untranslated region (UTR) were replaced with SV40 sequences, generating products with lacZ/beta gal kinetics rather than those of PTHrP. A nuclear localization sequence was not present in the construct.
Results: We characterized beta-galactosidase (beta-gal) activity in embryonic whole mounts and in the skeleton in young and adult animals. In embryos, we confirmed widespread PTHrP expression in many known sites and in several novel epidermal appendages (nail beds and footpads). In costal cartilage, beta-gal activity localized to the perichondrium but not the underlying chondrocytes. In the cartilaginous molds of forming long bones, beta-gal activity was first evident at the proximal and distal ends. Shortly after birth, the developing secondary ossification center formed in the center of this PTHrP-rich chondrocyte population. As the secondary ossification center developed, it segregated this population into two distinct PTHrP beta-gal+ subpopulations: a subarticular subpopulation immediately subjacent to articular chondrocytes and a proliferative chondrocyte subpopulation proximal to the chondrocyte columns in the growth plate. These discrete populations remained into adulthood. beta-gal activity was not identified in osteoblasts but was present in many periosteal sites. These included simple periosteum as well as fibrous tendon insertion sites of the so-called bony and periosteal types; the beta-gal-expressing cells in these sites were in the outer fibrous layer of the periosteum or its apparent equivalents at tendon insertion sites. Homozygous PTHrP-lacZ knockin mice had the expected chondrodysplastic phenotype and a much expanded region of proximal beta-gal activity in long bones, which appeared to reflect in large part the effects of feedback signaling by Indian hedgehog on proximal cell proliferation and PTHrP gene expression.
Conclusions: The PTHrP-lacZ mouse seems to provide a sensitive reporter system that may prove useful as a means of studying PTHrP gene expression.