Myosin Light Chain Kinase and the Onset of Labour in Humans

Myosin light chain kinase (MLCK) is essential for myometrial contractions induced by calcium‐mobilizing agonists. From the gene of vertebrate smooth muscle/non‐muscle MLCK there are at least four proteins expressed. We have found that both a > 200 and a 137 kDa MLCK are equally expressed in human non‐pregnant (NP) and term pregnant (P) uterine smooth muscle and confirmed that 19 kDa telokin (TK) is only expressed in P myometrium. In addition, we have observed that a MLCK immunogen at ∼ 60 kDa is only expressed in NP myometrium, suggesting that its expression is inhibited during normal pregnancy in a hormonally dependent manner. However, when we compared pregnant myometrium from patients delivered preterm (PT) (< 34 weeks gestation), but not in labour (NIL), with PT patients in labour (IL) we found that PT(IL) samples expressed the ∼ 60 kDa MLCK immunogen and thus displayed a NP phenotype whereas PT(NIL) samples did not express the protein and retained a pregnant phenotype. We hypothesize that the novel ∼ 60 kDa MLCK immunogen contributes to the aberrent contractility associated with preterm labour.