Deletion of the Igf1 gene: Suppressive effects on adult Leydig cell development

Deletion of the insulin-like growth factor 1 (Igf1) gene was shown in previous studies to result in reduced numbers of Leydig cells in the testes of 35-day-old mice, and in reduced circulating testosterone levels. In the current study, we asked whether deletion of the Igf1 gene affects the number, proliferation, and/or steroidogenic function of some or all of the precursor cell types in the developmental sequence that leads to the establishment of adult Leydig cells (ALCs). Decreased numbers of cells in the Leydig cell lineage (ie, 3β-hydroxysteroid dehydrogenase-positive cells) were seen in testes of postnatal day (PND) 14-90 Igf1^-/- mice compared with age-matched Igf1^+/+ controls. The development of ALCs proceeds from stem Leydig cells (SLCs) through progenitor Leydig cells (PLCs) and immature Leydig cells (ILCs). The bromodeoxyuridine labeling index of putative SLCs was similar in the Igf1^-/- and Igf1^+/+ mice. In contrast, the labeling index of PLCs was reduced in the Igf1^-/- mice on each day of PND 14 through PND 35, and that of more mature Leydig cells (referred to herein as LCs, a combination of ILCs plus ALCs) was reduced from PND 21 through PND 56. In Igf1^-/- mice that received recombinant IGF-I, the labeling indices of PLCs and LCs were similar to those of age-matched Igf1^+/+ mice, indicating that the reductions in the labeling indices seen in the PLCs and LCs of the Igf1^-/- mice were a consequence of reduced IGF-I. On each day of PND 21 through PND 90, testicular testosterone concentrations were significantly reduced in the Igf1^-/- mice, as were the expressions of testis-specific mRNAs involved in steroidogenesis, including Star, Cyp11a1, and Cyp17a1. The increased expression of the gene for 5α-reductase (Srd5a1) in adult Igf1^-/- testes suggests that the depletion of Igf1 might suppress or delay Leydig cell maturation. These observations, taken together, indicate that the reduced numbers of Leydig cells in the adult testes of Igf1^-/- mice result at least in part from altered proliferation and differentiation of ALC precursor cells, but not of the stem cells that give rise to these cells.