View source for Sp7 transcription factor

{{Infobox_gene}}

'''Transcription factor Sp7,''' also called Osterix (Osx),  is a [[protein]] that in humans is encoded by the ''SP7'' [[gene]].<ref name="Nakashima_2002" />  It is a member of the Sp family of [[Zinc finger transcription factor|zinc-finger transcription factors]]<ref name="Nakashima_2002">{{cite journal | vauthors = Nakashima K, Zhou X, Kunkel G, Zhang Z, Deng JM, Behringer RR, de Crombrugghe B | title = The novel zinc finger-containing transcription factor osterix is required for osteoblast differentiation and bone formation | journal = Cell | volume = 108 | issue = 1 | pages = 17–29 | date = January 2002 | pmid = 11792318 }}</ref> It is highly conserved among bone-forming veterbrate species<ref name="Renn_2009">{{cite journal | vauthors = Renn J, Winkler C | title = Osterix-mCherry transgenic medaka for in vivo imaging of bone formation | journal = Developmental Dynamics | volume = 238 | issue = 1 | pages = 241–8 | date = January 2009 | pmid = 19097055 | doi = 10.1002/dvdy.21836 }}</ref><ref>{{cite journal | vauthors = DeLaurier A, Eames BF, Blanco-Sánchez B, Peng G, He X, Swartz ME, Ullmann B, Westerfield M, Kimmel CB | display-authors = 6 | title = Zebrafish sp7:EGFP: a transgenic for studying otic vesicle formation, skeletogenesis, and bone regeneration | journal = Genesis | volume = 48 | issue = 8 | pages = 505–11 | date = August 2010 | pmid = 20506187 | pmc = 2926247 | doi = 10.1002/dvg.20639 }}</ref> It plays a major role, along with [[RUNX2|Runx2]] and [[DLX5|''Dlx5'']] in driving the differentiation of mesenchymal precursor cells into [[osteoblast]]s and eventually [[osteocyte]]s.<ref name="Sinha_2013">{{cite journal | vauthors = Sinha KM, Zhou X | title = Genetic and molecular control of osterix in skeletal formation | journal = Journal of Cellular Biochemistry | volume = 114 | issue = 5 | pages = 975–84 | date = May 2013 | pmid = 23225263 | pmc = 3725781 | doi = 10.1002/jcb.24439 }}</ref> Sp7 also plays a regulatory role by inhibiting [[chondrocyte]] differentiation maintaining the balance between differentiation of mesenchymal precursor cells into ossified bone or cartilage.<ref name="Kaback_2008">{{cite journal | vauthors = Kaback LA, Soung D, Naik A, Smith N, Schwarz EM, O'Keefe RJ, Drissi H | display-authors = 6 | title = Osterix/Sp7 regulates mesenchymal stem cell mediated endochondral ossification | journal = Journal of Cellular Physiology | volume = 214 | issue = 1 | pages = 173–82 | date = January 2008 | pmid = 17579353 | doi = 10.1002/jcp.21176 }}</ref> 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.<ref name="Nakashima_2002" /> Through the use of GWAS studies, the Sp7 locus in humans has been strongly associated with bone mass density.<ref name="Timpson_2009">{{cite journal | vauthors = Timpson NJ, Tobias JH, Richards JB, Soranzo N, Duncan EL, Sims AM, Whittaker P, Kumanduri V, Zhai G, Glaser B, Eisman J, Jones G, Nicholson G, Prince R, Seeman E, Spector TD, Brown MA, Peltonen L, Smith GD, Deloukas P, Evans DM | display-authors = 6 | title = Common variants in the region around Osterix are associated with bone mineral density and growth in childhood | journal = Human Molecular Genetics | volume = 18 | issue = 8 | pages = 1510–7 | date = April 2009 | pmid = 19181680 | pmc = 2664147 | doi = 10.1093/hmg/ddp052 }}</ref> In addition there is significant genetic evidence for its role in diseases such as [[Osteogenesis imperfecta]] (OI).<ref name="Lapunzina_2010">{{cite journal | vauthors = Lapunzina P, Aglan M, Temtamy S, Caparrós-Martín JA, Valencia M, Letón R, Martínez-Glez V, Elhossini R, Amr K, Vilaboa N, Ruiz-Perez VL | display-authors = 6 | title = Identification of a frameshift mutation in Osterix in a patient with recessive osteogenesis imperfecta | journal = American Journal of Human Genetics | volume = 87 | issue = 1 | pages = 110–4 | date = July 2010 | pmid = 20579626 | pmc = 2896769 | doi = 10.1016/j.ajhg.2010.05.016 }}</ref>

== Genetics ==
In humans Sp7 has been mapped to 12q13.13. It has 78% homology to another Sp family member, [[Sp1 transcription factor|Sp1]], especially in the regions which code for the three Cys-2 His-2 type DNA-binding [[zinc finger]]s.<ref>{{cite journal | vauthors = Gao Y, Jheon A, Nourkeyhani H, Kobayashi H, Ganss B | title = Molecular cloning, structure, expression, and chromosomal localization of the human Osterix (SP7) gene | journal = Gene | volume = 341 | pages = 101–10 | date = October 2004 | pmid = 15474293 | doi = 10.1016/j.gene.2004.05.026 }}</ref> Sp7 consists of three exons the first two of which are alternatively spliced, encoding a 431-residue isoform and an amino-terminus truncated 413-residue short protein isoform<ref>{{cite journal | vauthors = Milona MA, Gough JE, Edgar AJ | title = Expression of alternatively spliced isoforms of human Sp7 in osteoblast-like cells | language = En | journal = BMC Genomics | volume = 4 | issue = 1 | pages = 43 | date = November 2003 | pmid = 14604442 | pmc = 280673 | doi = 10.1186/1471-2164-4-43 }}</ref>

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 [[Single-nucleotide polymorphism|SNPs]] within the 12q13 region were in an area of linkage disequillibrium.<ref name="Timpson_2009" />

== Transcriptional Pathway ==
There are two main pathways which cause in the induction of Sp7/Osx gene expression, indirectly or directly. [[Msh homeobox 2|Msx2]] has the ability to directly induce Sp7. Whereas [[Bone morphogenetic protein 2|BMP2]] indirectly induces Sp7 through either [[DLX5|Dlx5]] or [[RUNX2|Runx2]].<ref name="Sinha_2013" /> Once Sp7 expression is triggered it then induces the expression of a slew of mature osteoblast genes such as [[Collagen, type I, alpha 1|Col1a1]], [[osteonectin]], [[osteopontin]] and [[bone sialoprotein]] which are all necessary for productive osteoblasts during the creation of ossified bone.<ref name="Renn_2009" />

Negative regulation of this pathway comes in the form of [[p53]], [[microRNA]]s and the [[Tumor necrosis factor superfamily|TNF]] inflammatory pathway.<ref name="Sinha_2013" /> 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<ref name="Gilbert_2000">{{cite journal | vauthors = Gilbert L, He X, Farmer P, Boden S, Kozlowski M, Rubin J, Nanes MS | title = Inhibition of osteoblast differentiation by tumor necrosis factor-alpha | journal = Endocrinology | volume = 141 | issue = 11 | pages = 3956–64 | date = November 2000 | pmid = 11089525 | doi = 10.1210/en.141.11.3956 }}</ref>
[[File:Regulatory_pathway_for_Sp7-Osx.png|alt=|none|thumb|500x500px|Diagram detailing the positive regulatory transcriptional pathway between genes expressed early in development such as BMP2 and the eventual expression of bone-specific genes.<ref name="Sinha_2013" />]]

== 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 [[Sp1 transcription factor|Sp1]] seems to indicate that it might act in a similar fashion during gene regulatory processes. Previous studies done on Sp1 have shown that Sp1 utilizes the zinc-finger DNA binding domains in its structure to bind directly to a GC-rich region of the genome known as the [[GC box]].<ref>{{Cite journal| vauthors = Kadonaga JT, Jones KA, Tjian R |date= January 1986 |title=Promoter-specific activation of RNA polymerase II transcription by Sp1 |journal=Trends in Biochemical Sciences|volume=11|issue=1|pages=20–23|doi=10.1016/0968-0004(86)90226-4|issn=0968-0004}}</ref>  creating downstream regulatory effects. There are a number of studies which support this mechanism as also applicable for Sp7,<ref>{{cite journal | vauthors = Zhang C, Tang W, Li Y | title = Matrix metalloproteinase 13 (MMP13) is a direct target of osteoblast-specific transcription factor osterix (Osx) in osteoblasts | journal = PLOS One | volume = 7 | issue = 11 | pages = e50525 | date = 2012-11-21 | pmid = 23185634 | doi = 10.1371/journal.pone.0050525 }}</ref> however other researchers were unable to replicate the GC box binding seen in Sp1 when looking at Sp7.<ref name="Hojo_2016">{{cite journal | vauthors = Hojo H, Ohba S, He X, Lai LP, McMahon AP | title = Sp7/Osterix Is Restricted to Bone-Forming Vertebrates where It Acts as a Dlx Co-factor in Osteoblast Specification | journal = Developmental Cell | volume = 37 | issue = 3 | pages = 238–53 | date = May 2016 | pmid = 27134141 | pmc = 4964983 | doi = 10.1016/j.devcel.2016.04.002 }}</ref><ref>{{cite journal | vauthors = Hekmatnejad B, Gauthier C, St-Arnaud R | title = Control of Fiat (factor inhibiting ATF4-mediated transcription) expression by Sp family transcription factors in osteoblasts | journal = Journal of Cellular Biochemistry | volume = 114 | issue = 8 | pages = 1863–70 | date = August 2013 | pmid = 23463631 | doi = 10.1002/jcb.24528 }}</ref> Another proposed mechanism of action is indirect gene regulation through the protein known as homeobox transcription factor [[DLX5|Dlx5]]. This is plausible because Dlx5 has much higher affinity to AT-rich gene regulatory regions than Sp7 has been shown to have to the GC box<ref name="Hojo_2016" /> thus providing an alternate methodology through which regulation can occur.

Mass spectrometry and proteomics methods have shown that Sp7 also interacts with [[RNA Helicase A|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 ===
[[File:Sp7diagram2.png|alt=|left|thumb|475x475px|Diagram detailing the role of Sp7 in the differentiation pathway of osteoblasts and the decision pathway between cartilage and ossified bone<ref name="Nakashima_2002" /><ref name="Sinha_2013" />]]
[[File:Frontal_Parietal_Cranial_Bone_Sp7_mutant.png|thumb|Zebrafish skull, 34 days post-fertilication, imaged using a [[calcein]] stain showing normal frontal and parietal bones. Overlayed using fuschia are irregular initiation sites of bone formation seen in an Sp7 mutant fish indicating abnormal pseudo-sutures.<ref name="Kague_2016">{{cite journal | vauthors = Kague E, Roy P, Asselin G, Hu G, Simonet J, Stanley A, Albertson C, Fisher S | title = Osterix/Sp7 limits cranial bone initiation sites and is required for formation of sutures | journal = Developmental Biology | volume = 413 | issue = 2 | pages = 160–72 | date = May 2016 | pmid = 26992365 | pmc = 5469377 | doi = 10.1016/j.ydbio.2016.03.011 }}</ref>]]
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 cell]]s 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.<ref name="Kronenberg_2003">{{cite journal | vauthors = Kronenberg HM | title = Developmental regulation of the growth plate | journal = Nature | volume = 423 | issue = 6937 | pages = 332–6 | date = May 2003 | pmid = 12748651 | doi = 10.1038/nature01657 }}</ref>

Sp7/Osx null mouse embryos displayed a severe phenotype in which there were unaffected chondrocytes and cartilage but absolutely no formation of bone tissue.<ref name="Nakashima_2002" /> Ablation of Sp7 genes also led to decreased expression of various other osteocyte-specific markers such as: Sost, [[DKK1|Dkk1]], [[DMP1|Dmp1]], and Phe.<ref name="Zhou_2010">{{cite journal | vauthors = Zhou X, Zhang Z, Feng JQ, Dusevich VM, Sinha K, Zhang H, Darnay BG, de Crombrugghe B | display-authors = 6 | title = Multiple functions of Osterix are required for bone growth and homeostasis in postnatal mice | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 29 | pages = 12919–24 | date = July 2010 | pmid = 20615976 | pmc = 2919908 | doi = 10.1073/pnas.0912855107 }}</ref> 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.<ref name="Sinha_2013" /> 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|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.<ref name="Zhou_2010" /> Other studies observed that a conditional knockout of Sp7 in adult mice osteoblasts resulted in [[osteopenia]] in the vertebrae of the animals, issues with bone turnover and more porosity in cortical outer surface of the long bones of the body.<ref>{{cite journal | vauthors = Baek WY, Lee MA, Jung JW, Kim SY, Akiyama H, de Crombrugghe B, Kim JE | title = Positive regulation of adult bone formation by osteoblast-specific transcription factor osterix | journal = Journal of Bone and Mineral Research | volume = 24 | issue = 6 | pages = 1055–65 | date = June 2009 | pmid = 19113927 | pmc = 4020416 | doi = 10.1359/jbmr.081248 }}</ref> Observation of an opposite effect, overproliferation of Sp7+ osteoblasts, further supports the important regulatory effects of Sp7 in vertebrates. A mutation in the zebrafish homologue of Sp7 caused severe craniofacial irregularities in maturing organisms while leaving the rest of the skeleton largely unaffected. Instead of normal suture patterning along the developing skull, the affected organisms displayed a mosaic of sites where bone formation was being initiated but not completed. This caused the appearance of many small irregular bones instead of the normal smooth [[Frontal bone|frontal]] and [[Parietal bone|parietal]] bones. These phenotypic shifts corresponded to an overproliferation of Runx2+ osteoblast progenitors indicating that the phenotype observed was related to an abundance of initiation sites for bone proliferation creating many pseudo-[[Sutures of skull|sutures]].<ref name="Kague_2016" />

== Clinical relevance ==

=== Osteogenesis imperfecta ===
The most direct example of the role of Sp7 in human disease has been in recessive [[osteogenesis imperfecta]](OI), which is a type-I collagen related disease that causes a heterogeneous set of bone-related symptoms which can range from mild to very severe. Generally this disease is caused by mutations in [[COL1A1|Col1a1]] or [[COL1A2|Col1a2]] which are regulators of collagen growth. OI-causing mutations in these collagen genes are generally heritable in an [[Dominance (genetics)|autosomal-dominant]] fashion. However there has been a recent case of a patient with recessive OI with a documented frameshift mutation in Sp7/Osx as the etiological origin of the disease.<ref name="Lapunzina_2010" /> This patient displayed abnormal fracturing of the bones after relatively minor injuries and markedly delayed motor milestones, requiring assistance to stand at age 6 and was unable to walk at age 8 due to pronounced bowing of the arms and legs. This provides a direct link between the Sp7 gene and the OI disease phenotype.
[[File:Osteogenesis_Imperfecta_Type_V.jpg|thumb|Bowed bones of the legs as seen in an adult osteogenesis imperfecta patient.]]

=== 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.<ref name="Timpson_2009" /> Abnormal expression of inflammatory cytokines such as [[Tumor necrosis factor alpha|TNF-α]] is present in osteoporosis can have detrimental effects on the expression of Sp7.<ref name="Gilbert_2000" />

=== Rheumatoid Arthritis ===
[[Adiponectin]] is a protein hormone that has been shown to be upregulated in rheumatoid arthritis disease pathology, causing the release of inflammatory cytokines and enhancing the breakdown of the bone matrix. In primary human cell cultures Sp7 was shown to be inhibited by adiponectin thus contributing downregulation of the creation of ossified bone.<ref>{{cite journal | vauthors = Krumbholz G, Junker S, Meier FM, Rickert M, Steinmeyer J, Rehart S, Lange U, Frommer KW, Schett G, Müller-Ladner U, Neumann E | display-authors = 6 | title = Response of human rheumatoid arthritis osteoblasts and osteoclasts to adiponectin | journal = Clinical and Experimental Rheumatology | volume = 35 | issue = 3 | pages = 406–414 | date = May 2017 | pmid = 28079506 }}</ref> This data is further backed up by another study in which inflammatory cytokines such as [[Tumor necrosis factor alpha|TNF-α]] and [[Interleukin 1 beta|IL-1β]] were shown to downregulate gene expression of Sp7 in mouse primary mesenchymal stem cells in culture.<ref>{{cite journal | vauthors = Lacey DC, Simmons PJ, Graves SE, Hamilton JA | title = Proinflammatory cytokines inhibit osteogenic differentiation from stem cells: implications for bone repair during inflammation | journal = Osteoarthritis and Cartilage | volume = 17 | issue = 6 | pages = 735–42 | date = June 2009 | pmid = 19136283 | doi = 10.1016/j.joca.2008.11.011 }}</ref> These studies seem to indicate that an inflammatory environment is detrimental to the creation of ossified bone.<ref name="Gilbert_2000" />

=== Bone fracture repair ===
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.<ref name="Tu_2007">{{cite journal | vauthors = Tu Q, Valverde P, Li S, Zhang J, Yang P, Chen J | title = Osterix overexpression in mesenchymal stem cells stimulates healing of critical-sized defects in murine calvarial bone | journal = Tissue Engineering | volume = 13 | issue = 10 | pages = 2431–40 | date = October 2007 | pmid = 17630878 | pmc = 2835465 | doi = 10.1089/ten.2006.0406 }}</ref> Along similar mechanistic lines as bone repair is the integration of dental implants into [[Alveolar process|alveolar bone]], since the insertion of these implants causes bone damage that must be healed before the implant is successfully integrated.<ref name="pmid164666992">{{cite journal | vauthors = Tu Q, Valverde P, Chen J | title = Osterix enhances proliferation and osteogenic potential of bone marrow stromal cells | journal = Biochemical and Biophysical Research Communications | volume = 341 | issue = 4 | pages = 1257–65 | date = March 2006 | pmid = 16466699 | pmc = 2831616 | doi = 10.1016/j.bbrc.2006.01.092 }}</ref> Researchers have shown that when [[Bone marrow|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.<ref name="pmid198288872">{{cite journal | vauthors = Xu B, Zhang J, Brewer E, Tu Q, Yu L, Tang J, Krebsbach P, Wieland M, Chen J | display-authors = 6 | title = Osterix enhances BMSC-associated osseointegration of implants | journal = Journal of Dental Research | volume = 88 | issue = 11 | pages = 1003–7 | date = November 2009 | pmid = 19828887 | pmc = 2831612 | doi = 10.1177/0022034509346928 }}</ref>

=== 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.<ref name="Cao_2006">{{cite journal | vauthors = Cao Y, Zhou Z, de Crombrugghe B, Nakashima K, Guan H, Duan X, Jia SF, Kleinerman ES | display-authors = 6 | title = Osterix, a transcription factor for osteoblast differentiation, mediates antitumor activity in murine osteosarcoma | journal = Cancer Research | volume = 65 | issue = 4 | pages = 1124–8 | date = February 2005 | pmid = 15734992 | doi = 10.1158/0008-5472.CAN-04-2128 }}</ref>

== References ==
{{reflist|32em}}

== Further reading ==
{{refbegin|32em}}
* {{cite journal | vauthors = Gronthos S, Chen S, Wang CY, Robey PG, Shi S | title = Telomerase accelerates osteogenesis of bone marrow stromal stem cells by upregulation of CBFA1, osterix, and osteocalcin | journal = Journal of Bone and Mineral Research | volume = 18 | issue = 4 | pages = 716–22 | date = April 2003 | pmid = 12674332 | doi = 10.1359/jbmr.2003.18.4.716 }}
* {{cite journal | vauthors = Morsczeck C | title = Gene expression of runx2, Osterix, c-fos, DLX-3, DLX-5, and MSX-2 in dental follicle cells during osteogenic differentiation in vitro | journal = Calcified Tissue International | volume = 78 | issue = 2 | pages = 98–102 | date = February 2006 | pmid = 16467978 | doi = 10.1007/s00223-005-0146-0 }}
* {{cite journal | vauthors = Wu L, Wu Y, Lin Y, Jing W, Nie X, Qiao J, Liu L, Tang W, Tian W | title = Osteogenic differentiation of adipose derived stem cells promoted by overexpression of osterix | journal = Molecular and Cellular Biochemistry | volume = 301 | issue = 1-2 | pages = 83–92 | date = July 2007 | pmid = 17206379 | doi = 10.1007/s11010-006-9399-9 }}
* {{cite journal | vauthors = Fan D, Chen Z, Wang D, Guo Z, Qiang Q, Shang Y | title = Osterix is a key target for mechanical signals in human thoracic ligament flavum cells | journal = Journal of Cellular Physiology | volume = 211 | issue = 3 | pages = 577–84 | date = June 2007 | pmid = 17311298 | doi = 10.1002/jcp.21016 }}
* {{cite journal | vauthors = Zheng L, Iohara K, Ishikawa M, Into T, Takano-Yamamoto T, Matsushita K, Nakashima M | title = Runx3 negatively regulates Osterix expression in dental pulp cells | journal = The Biochemical Journal | volume = 405 | issue = 1 | pages = 69–75 | date = July 2007 | pmid = 17352693 | pmc = 1925241 | doi = 10.1042/BJ20070104 }}
{{refend}}

== External links ==
* {{MeshName|Sp12+Transcription+Factor}}

{{Transcription factors|g2}}

[[Category:Transcription factors]]