Discovering myosin light chain phosphorylation independent mechanisms of vascular smooth muscle contraction
It is well accepted that the primary mechanism by which smooth muscle contraction is initiated involves calcium/calmodulin dependent activation of myosin light chain kinase and the resultant phosphorylation of the regulatory myosin light chain. However, this thick filament system of activation cannot serve as the only mechanism for contraction as several laboratories have shown a dissociation between force and levels of myosin light chain phosphorylation in response to various stimuli. Thus, attention has been directed to the potential involvement of kinases other than myosin light chain kinase as well as thin filament associated proteins in the regulation of smooth muscle contraction, specifically caldesmon.
Caldesmon binds both actin and myosin and inhibits actin-activated myosin ATPase activity in an unstimulated muscle preventing force production. Previous work investigating caldesmon using in vitro models has shed some light on the structure and function of caldesmon but sparse information has been published regarding caldesmon's precise role(s) in the contracting smooth muscle in vivo. Studies investigating the mechanism(s) by which caldesmon is regulated have mainly been done in vitro and have shown that caldesmon's inhibitory effect can be reversed upon phosphorylation. However, there is ambiguity as to which kinase(s) is involved in an in vivo pathway(s) that modulates caldesmon.
The hypotheses tested in this dissertation investigate the presence of a pathway that is independent of calcium and myosin light chain phosphorylation but still responsible for modulation of a smooth muscle contraction. Furthermore, this dissertation will investigate if this pathway involves caldesmon. Secondly, since it is strongly suggested that disinhibition of caldesmon is important in the maintenance of the normal levels of tone as well as the tonic maintenance of contraction in smooth muscle we hypothesized that a cascade of events involving kinases other than the putative endogenous caldesmon kinase, the mitogen activated protein kinases/extracellular regulated kinases result in phosphorylation of caldesmon reversing its inhibition on actin-activated myosin ATPase activity.