Sp7 transcription factor

Transcription factor Sp7, also called osterix (Osx), is a protein that in humans is encoded by the SP7 gene.^[5] It is a member of the Sp family of zinc-finger transcription factors^[5] It is highly conserved among bone-forming vertebrate species^[6][7] It plays a major role, along with Runx2 and Dlx5 in driving the differentiation of mesenchymal precursor cells into osteoblasts and eventually osteocytes.^[8] Sp7 also plays a regulatory role by inhibiting chondrocyte differentiation maintaining the balance between differentiation of mesenchymal precursor cells into ossified bone or cartilage.^[9] Mutations of this gene have been associated with multiple dysfunctional bone phenotypes in vertebrates. During development, a mouse embryo model with Sp7 expression knocked out had no formation of bone tissue.^[5] Through the use of GWAS studies, the Sp7 locus in humans has been strongly associated with bone mass density.^[10] In addition there is significant genetic evidence for its role in diseases such as Osteogenesis imperfecta (OI).^[11]

Genetics

In humans Sp7 has been mapped to 12q13.13. It has 78% homology to another Sp family member,

A GWAS study has found that bone mass density (BMD) is associated with the Sp7 locus, adults and children with either low or high BMD were analyzed showing that several common variant SNPs within the 12q13 region were in an area of linkage disequilibrium.^[10]

Transcriptional pathway

There are two main pathways which cause in the induction of Sp7/Osx gene expression. Msx2 induces Sp7 directly, whereas bone morphogenetic protein 2 (BMP2) induces it indirectly through either Dlx5 or Runx2.^[8] Once Sp7 expression is triggered, it then induces the expression of a slew of mature osteoblast genes such as Col1a1, osteonectin, osteopontin and bone sialoprotein which are all necessary for productive osteoblasts during the creation of ossified bone.^[6]

Negative regulation of this pathway comes in the form of p53, microRNAs and the TNF inflammatory pathway.^[8] Disregulation of the TNF pathway blocking appropriate bone growth by osteoblasts is a partial cause of the abnormal degradation of bone seen in osteoporosis or rheumatoid arthritis^[14]

Mechanism of action

The exact mechanisms of action for Sp7/Osterix are currently in contention and the full protein structure has yet to be solved. As a zinc-finger transcription factor, its relatively high homology with

Mass spectrometry and proteomics methods have shown that Sp7 also interacts with RNA helicase A and is possibly negatively regulated by RIOX1 both of which provide evidence for regulatory mechanisms outside of the GC box paradigm.

Function

Sp7 acts as a master regulator of bone formation during both embryonic development and during the homeostatic maintenance of bone in adulthood.

During development

In a developing organism, Sp7 serves as one of the most important regulatory shepherds for bone formation. The creation of ossified bone is preceded by the differentiation of mesenchymal stem cells into chondrocytes and the conversion of some of those chondrocytes into cartilage. Certain populations of that initial cartilage serves as a template for bone cells as skeletogenesis proceeds.^[20]

Sp7/Osx null mouse embryos displayed a severe phenotype in which there were unaffected chondrocytes and cartilage but absolutely no formation of bone tissue.^[5] Ablation of Sp7 genes also led to decreased expression of various other osteocyte-specific markers such as: Sost, Dkk1, Dmp1, and Phe.^[21] The close relationship between Sp7/Osx and Runx2 was also demonstrated through this particular experiment because the Sp7 knockout bone phenotype greatly resembled that of the Runx2 knockout, and further experiments proved that Sp7 is downstream of and very closely associated with Runx2.^[8] The important conclusion of this particular series of experiments was the clear regulatory role of Sp7 in the decision process made by mesenchymal stem cells to progress from their original highly Sox9 positive osteoprogenitors into either bone or cartilage. Without sustained Sp7 expression the progenitor cells take the pathway into becoming chondrocytes and eventually cartilage rather than creating ossified bone.

In adult organisms

Outside of the context of development, in adult mice ablation of Sp7 led to a lack of new bone formation, highly irregular cartilage accumulation beneath the growth plate and defects in osteocyte maturation and functionality.

Osteoporosis

GWAS studies have shown associations between adult and juvenile bone mass density (BMD) and the Sp7 locus in humans. Though low BMD is a good indicator of susceptibility for osteoporosis in adults, the amount of information currently available from these studies does not allow for a direct correlation to be made between osteoporosis and Sp7.

Accelerated bone fracture healing was found when researchers implanted Sp7 overexpressing bone marrow stroma cells at a site of bone fracture. It was found that the mechanism by which Sp7 expression accelerated bone healing was through triggering new bone formation by inducing neighboring cells to express genes characteristic of bone progenitors.[25] Along similar mechanistic lines as bone repair is the integration of dental implants into alveolar bone, since the insertion of these implants causes bone damage that must be healed before the implant is successfully integrated.^[26] Researchers have shown that when bone marrow stromal cells are exposed to artificially elevated levels of Sp7/Osx, mice with dental implants were shown to have better outcomes through the promotion of healthy bone regeneration.^[27]

Treatment of osteosarcomas

Overall Sp7 expression is decreased in mouse and human osteosarcoma cell lines when compared to endogenous osteoblasts and this decrease in expression correlates with metastatic potential. Transfection of the SP7 gene into a mouse osteosarcoma cell line to create higher levels of expression reduced overall malignancy in-vitro and reduced tumor incidence, tumor volume, and lung metastasis when the cells were injected into mice. Sp7 expression was also found to decrease bone destruction by the sarcoma likely through supplementing the normal regulatory pathways controlling osteoblasts and osteocytes.^[28]

References