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

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.

Transcriptome analysis in prenatal IGF1-deficient mice identifies molecular pathways and target genes involved in distal lung differentiation.

BACKGROUND: Insulin-like Growth Factor 1 (IGF1) is a multifunctional regulator of somatic growth and development throughout evolution. IGF1 signaling through IGF type 1 receptor (IGF1R) controls cell proliferation, survival and differentiation in multiple cell types. IGF1 deficiency in mice disrupts lung morphogenesis, causing altered prenatal pulmonary alveologenesis. Nevertheless, little is known about the cellular and molecular basis of IGF1 activity during lung development. METHODS/PRINCIPAL FINDINGS: Prenatal Igf1(-/-) mutant mice with a C57Bl/6J genetic background displayed severe disproportional lung hypoplasia, leading to lethal neonatal respiratory distress. Immuno-histological analysis of their lungs showed a thickened mesenchyme, alterations in extracellular matrix deposition, thinner smooth muscles and dilated blood vessels, which indicated immature and delayed distal pulmonary organogenesis. Transcriptomic analysis of Igf1(-/-) E18.5 lungs using RNA microarrays identified deregulated genes related to vascularization, morphogenesis and cellular growth, and to MAP-kinase, Wnt and cell-adhesion pathways. Up-regulation of immunity-related genes was verified by an increase in inflammatory markers. Increased expression of Nfib and reduced expression of Klf2, Egr1 and Ctgf regulatory proteins as well as activation of ERK2 MAP-kinase were corroborated by Western blot. Among IGF-system genes only IGFBP2 revealed a reduction in mRNA expression in mutant lungs. Immuno-staining patterns for IGF1R and IGF2, similar in both genotypes, correlated to alterations found in specific cell compartments of Igf1(-/-) lungs. IGF1 addition to Igf1(-/-) embryonic lungs cultured ex vivo increased airway septa remodeling and distal epithelium maturation, processes accompanied by up-regulation of Nfib and Klf2 transcription factors and Cyr61 matricellular protein. CONCLUSIONS/SIGNIFICANCE: We demonstrated the functional tissue specific implication of IGF1 on fetal lung development in mice. Results revealed novel target genes and gene networks mediators of IGF1 action on pulmonary cellular proliferation, differentiation, adhesion and immunity, and on vascular and distal epithelium maturation during prenatal lung development.