Matrix metalloproteinase-20 over-expression is detrimental to enamel development: a Mus musculus model.
BACKGROUND
Matrix metalloproteinase-20 (Mmp20) ablated mice have enamel that is thin and soft with an abnormal rod pattern that abrades from the underlying dentin. We asked if introduction of transgenes expressing Mmp20 would revert this Mmp20 null phenotype back to normal. Unexpectedly, for transgenes expressing medium or high levels of Mmp20, we found opposite enamel phenotypes depending on the genetic background (Mmp20(-/-) or Mmp20(+/+) ) in which the transgenes were expressed.


RESULTS
Amelx-promoter-Mmp20 transgenic founder mouse lines were assessed for transgene expression and those expressing low, medium or high levels of Mmp20 were selected for breeding into the Mmp20 null background. Regardless of expression level, each transgene brought the null enamel back to full thickness. However, the high and medium expressing Mmp20 transgenes in the Mmp20 null background had significantly harder more mineralized enamel than did the low transgene expresser. Strikingly, when the high and medium expressing Mmp20 transgenes were present in the wild-type background, the enamel was significantly less well mineralized than normal. Protein gel analysis of enamel matrix proteins from the high and medium expressing transgenes present in the wild-type background demonstrated that greater than normal amounts of cleavage products and smaller quantities of higher molecular weight proteins were present within their enamel matrices.


CONCLUSIONS
Mmp20 expression levels must be within a specific range for normal enamel development to occur. Creation of a normally thick enamel layer may occur over a wider range of Mmp20 expression levels, but acquisition of normal enamel hardness has a narrower range. Since over-expression of Mmp20 results in decreased enamel hardness, this suggests that a balance exists between cleaved and full-length enamel matrix proteins that are essential for formation of a properly hardened enamel layer. It also suggests that few feedback controls are present in the enamel matrix to prevent excessive MMP20 activity.
Department of Mineralized Tissue Biology and Harvard School of Dental Medicine, The Forsyth Institute, Cambridge Massachusetts, United States of America.
Masashi
M
Shin
Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, United States of America.
Yuanyuan
Y
Hu
Department of Mineralized Tissue Biology and Harvard School of Dental Medicine, The Forsyth Institute, Cambridge Massachusetts, United States of America.
Coralee E
CE
Tye
Department of Mineralized Tissue Biology and Harvard School of Dental Medicine, The Forsyth Institute, Cambridge Massachusetts, United States of America.
Xiaomu
X
Guan
Program in Endodontics, Harvard School of Dental Medicine, Boston Massachusetts, United States of America.
Craig C
CC
Deagle
Department of Mineralized Tissue Biology and Harvard School of Dental Medicine, The Forsyth Institute, Cambridge Massachusetts, United States of America.
Jerry V
JV
Antone
Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, United States of America ; Facility for Electron Microscopy Research, Department of Anatomy & Cell Biology, and Faculty of Dentistry, McGill University, Montreal, QC, Canada.
Charles E
CE
Smith
Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, United States of America.
James P
JP
Simmer
Department of Mineralized Tissue Biology and Harvard School of Dental Medicine, The Forsyth Institute, Cambridge Massachusetts, United States of America.
John D
JD
Bartlett
D053523:Amelogenin / Q000235:genetics / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D003743:Dental Enamel / Q000002:abnormalities / Q000254:growth & development* / Q000648:ultrastructure; D003746:Dental Enamel Proteins / Q000235:genetics / Q000378:metabolism; D005260:Female; D006244:Hardness / Q000502:physiology*; D006801:Humans; D008297:Male; D053519:Matrix Metalloproteinase 20 / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008855:Microscopy, Electron, Scanning; D023421:Models, Animal; D010641:Phenotype; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014074:Tooth Calcification / Q000502:physiology*; D055114:X-Ray Microtomography
D053523:Amelogenin; C507200:Amelx protein, mouse; D003746:Dental Enamel Proteins; D012333:RNA, Messenger; C109069:enamel matrix proteins; D053519:Matrix Metalloproteinase 20; C507113:Mmp20 protein, mouse