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One of the earliest events in limb regeneration is the extensive remodeling of the extracellular matrix (ECM). Matrix metalloproteinases (MMPs) are a family of matrix degrading enzymes that have been identified in both normal and disease states. Using RT-PCR and cDNA library screening, we have isolated sequences homologous to four different Mmp genes. The spatial and temporal expression of one of these, Mmp-9, has been analyzed during axolotl limb regeneration. Northern blot analysis identifies a 3.8 kb transcript that is abundantly expressed during regeneration, and whole-mount in situ hybridization has uncovered an unusual bi-phasic expression pattern. The first phase begins at 2 hours after amputation, and expression is confined to the healed wound epithelium. This phase continues for 2 days, showing peak expression at 14 hours after amputation. This early phase may be needed to retard reformation of the basal lamina of the epidermis, and thereby facilitate the epidermal-mesenchymal interactions required for successful regeneration. The second phase begins a few days later when a small blastema has formed. During this phase, expression is in the mesenchyme, localized to cells around the tips of the cut skeletal elements. This expression is maintained through several stages until redifferentiation begins. The timing and position of the second phase of expression is consistent with a role for Mmp-9 in the removal of damaged cartilage matrix. We have also discovered that the time of onset of Mmp-9 expression is sensitive to denervation, which causes a delay of several hours. Finally, retinoids, known for their dramatic effects on the pattern of regenerating limbs, can cause a down regulation of Mmp-9 expression. Dev Dyn 1999;216:2-9. |
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