Regulation of the actin cytoskeleton

In 2010 we identified direct interactions between dynamin and actin filaments and we have since shown that the dynamin oligomerization cycle plays a direct role in regulating actin polymerization and crosslinking of actin filaments. These interactions have since been implicated in highly diverse cellular processes including endocytosis, the formation of lamellipodia, filopodia invadopodia, and growth cones in neuronal cells. All these processes are driven by the association of distinct actin structures with the plasma membrane. We now seek to identify the molecular mechanisms by which dynamin establishes such diverse cellular processes. Utilizing biochemical and cell biology assays, and total internal reflection fluorescence (TIRF) single-molecule imaging, we plan to elucidate the mechanism by which dynamin regulates actin and microtubule dynamics.  We use kidney specific cells named podocytes as an experimental model system to study the role of dynamin with regard to endocytosis, the actin cytoskeleton, and microtubule dynamics. Podocytes are terminally differentiated cells that form the filtration barrier in the kidney. Damage or loss of podocytes is an early symptom of many kidney diseases as structural integrity of the podocyte actin and microtubule cytoskeleton is critical for its proper function. Better understanding podocyte pathobiology has lead to the establishment of novel paradigms with regard to role of dynamin in the cell and has the potential to pave the way for developing a cure for kidney diseases. 

This photo shows the structure of podocytes in mice expressing dynamin mutant R725A, which increases dynamin’s propensity to oligomerize into higher-order structures such as rings. Dynamin oligomerization has been implicated in clathrin-mediated endocytosis, but here we show for the first time that its oligomerization plays an essential physiological role by directly regulating the actin cytoskeleton. This, in turn, drives the formation of foot processes that are significantly longer than those in wild-type animals as well as those in animals before Doxycycline treatment (used to drive expression of DynR725A). It is very rare to see such a dramatic effect on the length of the foot processes (FPs) since dominant-negative mutations of diverse proteins expressed in podocytes typically result in the loss of FPs. It should be noted that this particular dynamin mutant (DynR725A) has been published by Dr. Sever back in 1999 in Nature (Sever et al, Nature 1999). This mutant suggested that dynamin is a regulatory GTPase and not a pinchase. As you can see, this mutant has a long history.