Suzuki Dai, Bush Jason R, Bryce Dawn-Marie, Kamijo Ryutaro, Beier Frank
Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario N6A 5C1, Canada.
Department of Biochemistry, School of Dentistry, Showa University, Shinagawa, Tokyo 142-8555, Japan.
Endocrinology. 2017 Oct 1;158(10):3386-3398. doi: 10.1210/en.2016-1691.
Rac1, a member of the small Rho GTPase family, plays multiple cellular roles. Studies of mice conditionally lacking Rac1 have revealed essential roles for Rac1 in various tissues, including cartilage and limb mesenchyme, where Rac1 loss produces dwarfism and long bone shortening. To gain further insight into the role of Rac1 in skeletal development, we have used transgenic mouse lines to express a constitutively active (ca) Rac1 mutant protein in a Cre recombinase-dependent manner. Overexpression of caRac1 in limb bud mesenchyme or chondrocytes leads to reduced body weight and shorter bones compared with control mice. Histological analysis of growth plates showed that caRac1;Col2-Cre mice displayed ectopic hypertrophic chondrocytes in the proliferative zone and enlarged hypertrophic zones. These mice also displayed a reduced proportion of proliferating cell nuclear antigen-positive cells in the proliferative zone and nuclear β-catenin localization in the ectopic hypertrophic chondrocytes. Importantly, overexpression of caRac1 partially rescued the phenotypes of Rac1fl/fl;Col2-Cre and Rac1fl/fl;Prx1-Cre conditional knockout mice, including body weight, bone length, and growth plate disorganization. These results suggest that tight regulation of Rac1 activity is necessary for normal cartilage development.
Rac1是小Rho GTPase家族的成员之一，发挥着多种细胞功能。对条件性缺失Rac1的小鼠的研究揭示了Rac1在包括软骨和肢体间充质在内的各种组织中的重要作用，在这些组织中，Rac1的缺失会导致侏儒症和长骨缩短。为了进一步深入了解Rac1在骨骼发育中的作用，我们使用转基因小鼠品系以Cre重组酶依赖性方式表达组成型活性（ca）Rac1突变蛋白。与对照小鼠相比，在肢芽间充质或软骨细胞中过表达caRac1会导致体重减轻和骨骼变短。生长板的组织学分析表明，caRac1;Col2-Cre小鼠在增殖区出现异位肥大软骨细胞，肥大区扩大。这些小鼠在增殖区增殖细胞核抗原阳性细胞的比例也降低，并且在异位肥大软骨细胞中出现核β-连环蛋白定位。重要的是，caRac1的过表达部分挽救了Rac1fl/fl;Col2-Cre和Rac1fl/fl;Prx1-Cre条件性敲除小鼠的表型，包括体重、骨长度和生长板紊乱。这些结果表明，Rac1活性的严格调控对于正常软骨发育是必要的。
