Endothelial Responses to Aneurysm Initiating Hemodynamics

The ability of the vessel wall to respond to changes in hemodynamic stimuli is in large part mediated by the endothelial cells (ECs), which form the inner lining of blood vessel walls. The ECs, which are in direct contact with blood flow and so are acted upon by hemodynamic forces, are known sensors of WSS [8-12]. However, considerably little is known about EC responses to the very high WSS values that exist chronically adjacent to the apices of bifurcations and along the outer curvatures of vessels- the locations where IAs preferentially localize.

In addition to high WSS, our lab has implicated the spatial WSS gradient (WSSG) as an additional hemodynamic parameter involved in the initiation of intracranial aneurysms. In our studies of surgically created bifurcation in dogs and in the basilar terminus of rabbits exposed to increased flow we have shown matrix degradation and cell loss specifically localize to regions of accelerating flow where WSS is high and WSSG is positive. However, the adjacent regions experiencing equally high WSS but decelerating flow, i.e., a negative WSSG, remain undamaged, suggesting that the endothelium responds differently to positive and negative WSSG. To elucidate the effects of high WSS and WSSG on ECs we have designed and constructed several in vitro flow loop systems. These include a “T-chamber” which mimics impinging flow seen at bifurcations, a “tapered chamber” which produces multiple WSS values in a given experiment and most recently a “constant gradient chamber” which allows for the simultaneous study of high WSS, very high WSS and positive and negative WSSG.

The constant gradient chamber consists of four distinct flow regions: constant-height channels to create regions of uniform WSS and converging and diverging channels to create positive and negative WSSG respectively. Using this device, we examined how fundamental EC responses-cell morphology, alignment, DNA replication and programmed cell death- are affected by high WSS (28.4 Pa) as well as positive and negative WSSG.  We found that high WSS inhibited EC alignment to flow, increased EC proliferation and increased apoptosis. These responses to high WSS were either accentuated or ameliorated by WSSG: Positive WSSG (+980 Pa/m) inhibited alignment and stimulated proliferation and apoptosis, whereas negative WSSG (-1120Pa/m) promoted alignment and suppressed proliferation and apoptosis. These results demonstrate that ECs discriminate between positive and negative WSSG under high WSS conditions.

Recognizing that EC responses to high WSS and positive WSSG may contribute to pathogenic remodeling that occurs at bifurcations preceding aneurysm formation we are also interested in identifying how ECs, under these flow conditions, alter their gene and protein expressions to signal the surrounding vessel wall to undergo mal-adaptive remodeling.  This question is currently under investigation in the lab.