Cdc42 is critical for postnatal cartilage development

Objectives: Cdc42, a member of the small Rho GTPase family, is known to be a regulator of multiple cellular functions in vitro, including actin cytoskeletal reorganization, cell migration, proliferation, and gene expression. However, its tissue-specific roles remain largely unknown, especially in postnatal cartilage development, as cartilage-specific Cdc42 inactivated mice die within a few days after birth (Suzuki W. et al., Endocrinology, 156; 314-322, 2015). The purpose of this study was to investigate the physiological functions of Cdc42 during cartilage development after birth using tamoxifen-induced cartilage specific inactivated Cdc42 conditional knockout mice (Cdc42^fl/fl; Col2-CreERT; Cdc42 cKO).
Methods: Cdc42 cKO mice were generated by crossing Cdc42 flox mice with tamoxifen-induced type II collagen Cre transgenic mice using a Cre-loxp system. Mice received 0.1 mg of tamoxifen in an intraperitoneal manner on 3 different days (postnatal day 4, 7, 10) and were euthanized on postnatal day 30. To confirm the influence of the tamoxifen-induced Cre recombinase transgene, Col2-CreERT mice were bred with a strain possessing the targeted gene trap ROSA26 reporter (R26R mice). Cre-mediated recombination in R26R mice induces LacZ expression, which can easily be monitored by LacZ staining.
Results: Cdc42 gene expression was successfully reduced in cartilage of Cdc42 cKO mice by tamoxifen treatment. The gross morphology of Cdc42 cKO mice revealed shorter limbs and body as compared with the control mice, as well as reduced body weight (16.6±0.41 vs. 14.4±1.83 g, n=9, P<0.01). In addition, severe defects were found in growth plate chondrocytes of the long bones, which were characterized by a shorter proliferating zone, wider hypertrophic zone, and loss of columnar organization of proliferating chondrocytes, resulting in delayed endochondral bone formation associated with abnormal bone growth.
Conclusions: In conclusion, our findings demonstrate that Cdc42 is important for cartilage development during both the embryonic and postnatal stages.