Temporal and Tissue-Specific Expression of Kallikrein (Klk) Genes and Identification of a Novel Klk Messenger Ribonucleic Acid Transcript during Early Development in the Mouse1

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Abstract
The kallikreins are a multigene family of serine proteases that act on a diverse number of substrates, including several growth factors and extracellular matrix (ECM) glycoproteins and proteinases. Recently, this family has been implicated in the process of early development and embryo implantation. In this study, we used reverse transcription-polymerase chain reaction with gene-specific primers and Southern hybridization to elucidate the temporal and tissue-specific expression patterns of the mouse kallikreins Klk1, Klk3, Klk5, Klk9, and Klk21 during early development in the embryo, uterus, and decidua. We observed the expression of Klk1 (tissue kallikrein), Klk3 (γ-nerve growth factor), Klk9 (epidermal growth factor-binding protein), and Klk21 in the early conceptus (until the 2-cell stage). Only Klk21 continued to be expressed in the blastocyst until Day 7.5 of pregnancy. Expression of Klk9 reappeared at Day 7.5 and was consistently detected until Day 11, the last day studied; Klk1 was again expressed in the embryo from Day 9.5, with decreased levels by Day 11. In contrast, in the uterus or decidua, there was no expression of Klk1 until Day 7.5, when mRNA transcripts were abundant; transcripts then decreased in the Day 9.5 and Day 11 uterus. Expression of Klk21 in the uterus and decidua displayed a similar pattern but was detected at much lower levels. Interestingly, a novel Klk21-like mRNA was also detected in uterine tissue samples but not in embryonic samples; Klk3, Klk5, and Klk9 were not consistently expressed in the uterus or decidua over this time. This is the first report of the expression of specific kallikreins during early development. The distinct gene- and tissue-specific expression patterns presented in this study, in conjunction with the well-characterized roles of kallikreins in regulation of protein activation, ECM degradation, and proliferative events, suggests the involvement of the kallikrein gene family during early development.