Differential organ phenotypes after postnatal Igf1r gene conditional deletion induced by tamoxifen in UBC-CreERT2; Igf1r fl/fl double transgenic mice

Abstract

Insulin-like growth factor type 1 receptor (IGF1R) is a ubiquitously expressed tyrosine kinase that regulates cell proliferation, differentiation and survival. It controls body growth and organ homeostasis, but with specific functions depending on developmental time and cell type. Human deficiency in IGF1R is involved in growth failure, microcephaly, mental retardation and deafness, and its overactivation is implicated in oncogenesis. Igf1r-deficient mice die at birth due to growth retardation and respiratory failure. Although multiple Igf1r tissue-specific mutant lines have been analyzed postnatally, using Igf1r-floxed (Igf1r (fl/fl) ) mice mated with diverse cell-type recombinase Cre-expressing transgenics, no mouse models for the study of generalized Igf1r deficiency in adults have been reported. To this end we generated UBC-CreERT2; Igf1r (fl/fl) transgenic mice with an inducible deletion of Igf1r activated by tamoxifen. Tamoxifen administration to 4 week-old prepuberal male mice delayed their growth, producing a distinct impact on organ size 4 weeks later. Whereas testes were smaller, spleen and heart showed an increased organ to body weight ratio. Mosaic Igf1r genomic deletions caused a significant reduction in Igf1r mRNA in all organs analyzed, resulting in diverse phenotypes. While kidneys, spleen and cochlea had unaltered gross morphology, testes revealed halted spermatogenesis, and liver and alveolar lung parenchyma showed increased cell proliferation rates without affecting apoptosis. We demonstrate that UBC-CreERT2 transgenic mice efficiently delete Igf1r upon postnatal tamoxifen treatment in multiple mouse organs, and corroborate that IGF1R function is highly dependent on cell, tissue and organ type.

Involvement of Igf1r in Bronchiolar Epithelial Regeneration: Role during Repair Kinetics after Selective Club Cell Ablation

“…Actually, apoptotic rates in the entire lung were low, and it is difficult to assume that apoptosis is physiologically relevant in this context. Accordingly, similar results were previously described in the lung and liver of Igf1r postnatal conditional mutants [31]. Considering that alveolar macrophages could be involved in removing cells or cellular debris by efferocytosis in the lungs of Nkx2-1-Cre; Igf1r fl/fl mutant mice, the presence of these macrophages was also evaluated by immunostaining using the F4/80 macrophage marker in NT and 7dN lungs of both genotypes.…”

“…Since phenotypic changes in lungs of these Igf1r mutant mice are noticed even before the naphthalene treatment, and considering that some of these histopathological changes in the bronchiolar epithelium, including lower height and cell density, elongated nuclei, interruptions in epithelial continuity and increased proliferation rates, were recently reported by our group in a different mutant mouse line with a generalized postnatal conditional deletion of Igf1r [31], a role of Igf1r in airway epithelial homeostasis is definitively demonstrated. Interestingly, among the different airway cell types, club cells emerge as the most dependent on Igf1r.…”

“…Mutant mouse models with Igf1r reduced expression or signaling in adulthood display histopathological alterations in the airway epithelium. Also, increased proliferation rates in the alveolar parenchyma demonstrate better recovery after lung epithelial injury induced by hyperoxia or bleomycin [31–33]. All this scientific evidenceis consistent with IGF signaling being an essential mediator during pulmonary development and homeostasis.…”

“…In addition, absence of the phenotype could be due to low and/or variable efficiency of floxed sequencesin the Igf1r locus by Cre recombinase depending on the cell types and mouse developmental stage. Accordingly, inconstant lung epithelial phenotypes were common in both mutant lines, and different degrees of Cre efficiency, depending on tissues and cell types, were previously reported in a different Igf1r fl/fl conditional mutant mouse [31]. …”

“…Accordingly, lack of Igf1 and Igf1r was reported to promote cell proliferation and/or to alter epithelial differentiation not only in the lung [25, 26, 28, 29, 31], but also in the prostate, thyroid gland, liver and nervous system[31, 44, 72, 73]. Furthermore, limited activity of Igf1r accelerates tumorigenesis and promotes more aggressive phenotypes in prostate and breast cancer in mice [72, 74].…”