Erk5 in Bone Marrow Mesenchymal Stem Cells Regulates Bone Homeostasis by Preventing Osteogenesis in Adulthood

Extracellular signal-regulated kinase 5 (Erk5) belongs to the mitogen-activated protein kinase (MAPK) family. Previously, we demonstrated that Erk5 directly phosphorylates Smad-specific E3 ubiquitin protein ligase 2 (Smurf2) at Thr(249) (Smurf2(Thr249)) to activate its E3 ubiquitin ligase activity. Although we have clarified the importance of Erk5 in embryonic mesenchymal stem cells (MSCs) on skeletogenesis, its role in adult bone marrow (BM)-MSCs on bone homeostasis remains unknown. Leptin receptor-positive (LepR(+)) BM-MSCs represent a major source of bone in adult bone marrow and are critical regulators of postnatal bone homeostasis. Here, we identified Erk5 in BM-MSCs as an important regulator of bone homeostasis in adulthood. Bone marrow tissue was progressively osteosclerotic in mice lacking Erk5 in LepR(+) BM-MSCs with age, accompanied by increased bone formation and normal bone resorption in vivo. Erk5 deficiency increased the osteogenic differentiation of BM-MSCs along with a higher expression of Runx2 and Osterix, essential transcription factors for osteogenic differentiation, without affecting their stemness in vitro. Erk5 deficiency decreased Smurf2(Thr249) phosphorylation and subsequently increased Smad1/5/8-dependent signaling in BM-MSCs. The genetic introduction of the Smurf2(T249E) mutant (a phosphomimetic mutant) suppressed the osteosclerotic phenotype in Erk5-deficient mice. These findings suggest that the Erk5-Smurf2(Thr249) axis in BM-MSCs plays a critical role in the maintenance of proper bone homeostasis by preventing excessive osteogenesis in adult bone marrow.

细胞外信号调节激酶5（Erk5）属于丝裂原活化蛋白激酶（MAPK）家族。此前，我们已经证明Erk5在苏氨酸249位（Smurf2（Thr249））直接磷酸化Smad特异性E3泛素蛋白连接酶2（Smurf2），以激活其E3泛素连接酶活性。尽管我们已经阐明了Erk5在胚胎间充质干细胞（MSCs）骨骼发生中的重要性，但其在成年骨髓（BM）-MSCs对骨稳态的作用仍不清楚。瘦素受体阳性（LepR（+））BM - MSCs是成年骨髓中骨的主要来源，是出生后骨稳态的关键调节因子。在此，我们确定BM - MSCs中的Erk5是成年期骨稳态的重要调节因子。随着年龄增长，在LepR（+）BM - MSCs中缺乏Erk5的小鼠骨髓组织逐渐出现骨硬化，同时体内骨形成增加且骨吸收正常。Erk5缺失增加了BM - MSCs的成骨分化，同时成骨分化的关键转录因子Runx2和Osterix表达升高，且在体外不影响其干性。Erk5缺失降低了Smurf2（Thr249）的磷酸化，随后增加了BM - MSCs中Smad1/5/8依赖性信号传导。Smurf2（T249E）突变体（一种磷酸模拟突变体）的基因导入抑制了Erk5缺失小鼠的骨硬化表型。这些发现表明，BM - MSCs中的Erk5 - Smurf2（Thr249）轴通过防止成年骨髓中过度成骨，在维持适当的骨稳态中起关键作用。