Abstract
Disturbance in a differentiation program of skeletal stem cells leads to indecorous skeletogenesis. Growing evidence suggests that a fine-tuning of ubiquitin-mediated protein degradation is crucial for skeletal stem cells to maintain their stemness and osteogenic potential. Here, we demonstrate that the deubiquitinating enzyme (DUB) ubiquitin-specific protease 8 (USP8) stabilizes the Wnt receptor frizzled 5 (FZD5) by preventing its lysosomal degradation. This pathway is essential for Wnt/β-catenin signaling and the differentiation of osteoprogenitors to mature osteoblasts. Accordingly, deletion of USP8 in osteoprogenitors (Usp8Osx) resulted in a near-complete blockade in skeletal mineralization, similar to that seen in mice with defective Wnt/β-catenin signaling. Likewise, transplanting USP8-deficient osteoprogenitors under the renal capsule in wild-type secondary hosts did not to induce bone formation. Collectively, this study unveils an essential role for the DUB USP8 in Wnt/β-catenin signaling in osteoprogenitors and osteogenesis during skeletal development.
Highlights
Skeletal stem cells (SSCs) are pluripotent cells that can self-renew and differentiate into osteoblasts, chondrocytes, adipocytes, and stromal cells [1]
Primary osteoblast precursors (COB) were isolated form the calvaria of Usp8fl/fl and Usp8Osx neonates at the age of postnatal day 0, and ubiquitin-specific protease 8 (USP8) expression was assessed by immunoblotting analysis, indicating efficient deletion of Usp8 in Usp8Osx calvarial osteoblasts (COBs) (Figure 1D)
USP8 interacts with frizzled 5 (FZD5), and the deubiquitinating enzyme (DUB) domain of USP8 was sufficient to inhibit the ubiquitination of FZD5 while enhancing Wnt/β-catenin signaling [20]
Summary
Skeletal stem cells (SSCs) are pluripotent cells that can self-renew and differentiate into osteoblasts, chondrocytes, adipocytes, and stromal cells [1]. Recent studies indicate that ubiquitin-mediated protein degradation has a critical role in skeletal formation [7]. The ubiquitin-mediated proteasomal degradation pathway is essential for controlling various cellular processes, such as protein stability, protein interactions, intracellular transport, and transcriptional activity [8]. This pathway is tightly regulated by the balance between enzymatic activities of ubiquitin E3 ligases and deubiquitinating enzymes (DUBs), which play a critical role in maintaining the stemness and osteogenic potential of SSCs [7,9]
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