Persistent low level of osterix accelerates interleukin-6 production and impairs regeneration after tissue injury

SLC38A2 provides proline to fulfil unique synthetic demands arising during osteoblast differentiation and bone formation

Cellular differentiation is associated with the acquisition of a unique protein signature which is essential to attain the ultimate cellular function and activity of the differentiated cell. This is predicted to result in unique biosynthetic demands that arise during differentiation. Using a bioinformatic approach, we discovered osteoblast differentiation is associated with increased demand for the amino acid proline. When compared to other differentiated cells, osteoblast-associated proteins including RUNX2, OSX, OCN and COL1A1 are significantly enriched in proline. Using a genetic and metabolomic approach, we demonstrate that the neutral amino acid transporter SLC38A2 acts cell autonomously to provide proline to facilitate the efficient synthesis of proline-rich osteoblast proteins. Genetic ablation of SLC38A2 in osteoblasts limits both osteoblast differentiation and bone formation in mice. Mechanistically, proline is primarily incorporated into nascent protein with little metabolism observed. Collectively, these data highlight a requirement for proline in fulfilling the unique biosynthetic requirements that arise during osteoblast differentiation and bone formation.

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

Tsc1 ablation in Prx1 and Osterix lineages causes renal cystogenesis in mouse

Tuberous Sclerosis Complex (TSC) is caused by mutations in TSC1 or TSC2, which encode negative regulators of the mTOR signaling pathway. The renal abnormalities associated with TSC include angiomyolipoma, cysts, and renal cell carcinoma. Here we report that specific ablation of Tsc1 using the mesenchymal stem cell-osteoblast lineage markers induced cystogenesis in mice. Using Rosa-tdTomato mice, we found that Prx1- or Dermo1-labeled cells were present in the nephron including glomerulus but they were not stained by markers for podocytes, mesangial cells, endothelial cells, or proximal or loop of Henle tubular cells, while Osx is known to label tubular cells. Tsc1 deficiency in Prx1 lineage cells caused development of mild cysts that were positive only for Tamm-Horsfall protein (THP), a loop of Henle marker, while Tsc1 deficiency in Osx lineage cells caused development of cysts that were positive for Villin, a proximal tubular cell marker. On the other hand, Tsc1 deficiency in the Dermo1 lineage did not produce detectable phenotypical changes in the kidney. Cyst formation in Prx1-Cre; Tsc1^f/f and Osx-Cre; Tsc1^f/f mice were associated with increase in both proliferative and apoptotic cells in the affected tissue and were largely suppressed by rapamycin. These results suggest that Prx1 and Osx lineages cells may contribute to renal cystogenesis in TSC patients.