Differential expression and activity of matrix metalloproteinases 2 and 9 in canine early placenta

The zonary and endotheliochorial dog placenta is the most invasive placenta of carnivores. The importance of matrix metalloproteinases (MMP) in placenta invasiveness has been determined in several mammals including species with haemochorial, epitheliochorial and endotheliochorial placentation. Regarding the latter, the expression of MMP enzymes has been studied in the cat and the mature canine placenta. The aim of this study was to analyse the expression and activity of MMP-2 and MMP-9 in the early dog placenta. Placentae from 18 to 30 days of pregnancy were collected from four bitches. Two placentae from each bitch were analysed. Placental tissue from one uterine horn was fixed in formaldehyde for immunohistochemistry, while marginal haematoma, labyrinth, non-implantative and implantative endometrium from the contralateral horn were immediately frozen in dry ice for the analysis of MMP expression (Western blot [WB]) and activity (zymography). MMP-2 and MMP-9 were evidenced in the labyrinth, maternal glands and marginal haematoma; this finding was directly correlated with levels of MMP expression by WB, and with the activity of MMP-2, mainly in the haematoma (the area of major remodelling of tissues). Thus, although MMP-9 is well expressed in the early canine placenta, it is not active. Given the important role of MMPs for invasiveness, maternal-foetal angiogenesis and the establishment of a correct foetal nutrition, the results are consistent with the findings in other species in which the MMP-2 activation precedes the MMP-9 one in early placentation.