Single-cell transcriptomic analyses of mouse idh1 mutant growth plate chondrocytes reveal distinct cell populations responsible for longitudinal growth and enchondroma formation.

Enchondromas are a common tumor in bone that can occur as multiple lesions in enchondromatosis, which is associated with deformity of the effected bone. These lesions harbor mutations in IDH and driving expression of a mutant Idh1 in Col2 expressing cells in mice causes an enchondromatosis phenotype. In this study we compared growth plates from E18.5 mice expressing a mutant Idh1 with control littermates using single cell RNA sequencing. Data from Col2 expressing cells were analyzed using UMAP and RNA pseudo-time analyses. A unique cluster of cells was identified in the mutant growth plates that expressed genes known to be upregulated in enchondromas. There was also a cluster of cells that was underrepresented in the mutant growth plates that expressed genes known to be important in longitudinal bone growth. Immunofluorescence showed that the genes from the unique cluster identified in the mutant growth plates were expressed in multiple growth plate anatomic zones, and pseudo-time analysis also suggested these cells could arise from multiple growth plate chondrocyte subpopulations. This data identifies subpopulations of cells in control and mutant growth plates, and supports the notion that a mutant Idh1 alters the subpopulations of growth plate chondrocytes, resulting a subpopulation of cells that become enchondromas at the expense of other populations that contribute to longitudinal growth.

Distinct Roles of Glutamine Metabolism in Benign and Malignant Cartilage Tumors With IDH Mutations.

Enchondromas and chondrosarcomas are common cartilage neoplasms that are either benign or malignant, respectively. The majority of these tumors harbor mutations in either IDH1 or IDH2. Glutamine metabolism has been implicated as a critical regulator of tumors with IDH mutations. Using genetic and pharmacological approaches, we demonstrated that glutaminase-mediated glutamine metabolism played distinct roles in enchondromas and chondrosarcomas with IDH1 or IDH2 mutations. Glutamine affected cell differentiation and viability in these tumors differently through different downstream metabolites. During murine enchondroma-like lesion development, glutamine-derived α-ketoglutarate promoted hypertrophic chondrocyte differentiation and regulated chondrocyte proliferation. Deletion of glutaminase in chondrocytes with Idh1 mutation increased the number and size of enchondroma-like lesions. In contrast, pharmacological inhibition of glutaminase in chondrosarcoma xenografts reduced overall tumor burden partially because glutamine-derived non-essential amino acids played an important role in preventing cell apoptosis. This study demonstrates that glutamine metabolism plays different roles in tumor initiation and cancer maintenance. Supplementation of α-ketoglutarate and inhibiting GLS may provide a therapeutic approach to suppress enchondroma and chondrosarcoma tumor growth, respectively. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Canonical and noncanonical TGF-ß signaling regulate fibrous tissue differentiation in the axial skeleton.

Previously, we showed that embryonic deletion of TGF-ß type 2 receptor in mouse sclerotome resulted in defects in fibrous connective tissues in the spine. Here we investigated how TGF-ß regulates expression of fibrous markers: Scleraxis, Fibromodulin and Adamtsl2. We showed that TGF-ß stimulated expression of Scleraxis mRNA by 2 h and Fibromodulin and Adamtsl2 mRNAs by 8 h of treatment. Regulation of Scleraxis by TGF-ß did not require new protein synthesis; however, protein synthesis was required for expression of Fibromodulin and Adamtsl2 indicating the necessity of an intermediate. We subsequently showed Scleraxis was a potential intermediate for TGF-ß-regulated expression of Fibromodulin and Adamtsl2. The canonical effector Smad3 was not necessary for TGF-ß-mediated regulation of Scleraxis. Smad3 was necessary for regulation of Fibromodulin and Adamtsl2, but not sufficient to super-induce expression with TGF-ß treatment. Next, the role of several noncanonical TGF-ß pathways were tested. We found that ERK1/2 was activated by TGF-ß and required to regulate expression of Scleraxis, Fibromodulin, and Adamtsl2. Based on these results, we propose a model in which TGF-ß regulates Scleraxis via ERK1/2 and then Scleraxis and Smad3 cooperate to regulate Fibromodulin and Adamtsl2. These results define a novel signaling mechanism for TGFß-mediated fibrous differentiation in sclerotome.

Tracing Tumor Evolution in Sarcoma Reveals Clonal Origin of Advanced Metastasis.

Cellular heterogeneity is frequently observed in cancer, but the biological significance of heterogeneous tumor clones is not well defined. Using multicolor reporters and CRISPR-Cas9 barcoding, we trace clonal dynamics in a mouse model of sarcoma. We show that primary tumor growth is associated with a reduction in clonal heterogeneity. Local recurrence of tumors following surgery or radiation therapy is driven by multiple clones. In contrast, advanced metastasis to the lungs is driven by clonal selection of a single metastatic clone (MC). Using RNA sequencing (RNA-seq) and in vivo assays, we identify candidate suppressors of metastasis, namely, Rasd1, Reck, and Aldh1a2. These genes are downregulated in MCs of the primary tumors prior to the formation of metastases. Overexpression of these suppressors of metastasis impair the ability of sarcoma cells to colonize the lungs. Overall, this study reveals clonal dynamics during each step of tumor progression, from initiation to growth, recurrence, and distant metastasis.

Antagonism of BMP signaling is insufficient to induce fibrous differentiation in primary sclerotome.

Sclerotome is the embryonic progenitor of the axial skeleton. It was previously shown that Tgfbr2 is required in sclerotome for differentiation of fibrous skeletal tissues including the annulus fibrosus of the intervertebral disc. Alternatively, BMP signaling is required to form the vertebral body through chondrogenesis. In addition, TGFß added to sclerotome cultures induces expression of markers for fibrous tissue differentiation but not cartilage or bone. The mechanism of how TGFß signaling regulates this lineage decision in sclerotome is not known and could be due to the production of instructive or inhibitory signals or a combination of the two. Here we show that TGFß antagonizes BMP/ Smad1/5 signaling in primary sclerotome likely through regulation of Noggin, an extracellular BMP antagonist, to prevent chondrogenesis. We then tested whether inhibition of BMP signaling, and inhibition of chondrogenesis, is sufficient to push cells toward the fibrous cell fate. While Noggin inhibited BMP/ Smad1/5 signaling and the formation of chondrogenic nodules in sclerotome cultures; Noggin and inhibition of BMP signaling through Gremlin or DMH2 were insufficient to induce fibrous tissue differentiation. The results suggest inhibition of BMP signaling is not sufficient to stimulate fibrous tissue differentiation and additional signals are likely required. We propose that TGFß has a dual role in regulating sclerotome fate. First, it inhibits BMP signaling potentially through Noggin to prevent chondrogenesis and, second, it provides an unknown instructive signal to promote fibrous tissue differentiation in sclerotome. The results have implications for the design of stem cell-based therapies for skeletal diseases.

IVD Development: Nucleus pulposus development and sclerotome specification.

PURPOSE OF REVIEW: Intervertebral discs (IVD) are derived from embryonic notochord and sclerotome. The nucleus pulposus is derived from notochord while other connective tissues of the spine are derived from sclerotome. This manuscript will review the past 5 years of research into IVD development. RECENT FINDINGS: Over the past several years, advances in understanding the step-wise process that govern development of the nucleus pulposus and the annulus fibrosus have been made. Generation of tissues from induced or embryonic stem cells into nucleus pulposus and paraxial mesoderm derived tissues has been accomplished in vitro using pathways identified in normal development. A balance between BMP and TGF-ß signaling as well as transcription factors including Pax1/Pax9, Mkx and Nkx3.2 appear to be very important for cell fate decisions generating tissues of the IVD. SUMMARY: Understanding how the IVD develops will provide the foundation for future repair, regeneration, and tissue engineering strategies for IVD disease.

Erg cooperates with TGF-ß to control mesenchymal differentiation.

Transforming growth factor ß (TGF-ß) signaling plays an integral role in skeletal development. Conditional deletion of the TGF-ß type II receptor (Tgfbr2) from type II Collagen (Col2a) expressing cells results in defects in development of the annulus fibrosus (AF) of the intervertebral disc (IVD). We previously used microarray analysis to search for marker genes of AF as well as transcription factors regulated by TGF-ß during AF development. The transcription factor avian erythroblastosis virus E-26 (v-ets) oncogene related (Erg) was identified in the microarray screen as a candidate regulator of AF development. To study the effects of TGF-ß on AF differentiation and the role of Erg in this process, we used mouse sclerotome grown in micromass cultures. At 0.5ng TGF-ß/ml, sclerotome cells started to express markers of AF. Regulation of Erg by TGF-ß was confirmed in these cells. In addition, TGF-ß soaked Affi-gel beads implanted into the axial skeleton of stage HH 25 chick embryos showed that TGF-ß could induce expression of Erg mRNA in vivo. Next, an adenovirus to over-express Erg in primary sclerotome micromass cultures was generated. Over-expression of Erg led to a change in cell morphology and inhibition of differentiation into hyaline cartilage as seen by reduced Alcian blue staining and decreased Sox9 and c-Maf expression. Erg was not sufficient to induce expression of AF markers and expression of Sca1, a marker of pluripotent progenitor cells, was up-regulated in Erg expressing cells. When cells that ectopically expressed Erg were treated with TGF-ß, enhanced expression of specific differentiation markers was observed suggesting Erg can cooperate with TGF-ß to regulate differentiation of the sclerotome. Furthermore, we showed using co-immunopreciptiation that Erg and Smad3 bind to each other suggesting a mechanism for their functional interaction.