Localized signals that regulate transendothelial migration

Transendothelial migration (TEM) of leukocytes is the step in leukocyte emigration in which the leukocyte actually leaves the blood vessel to carry out its role in the inflammatory response. It is therefore, arguably the most critical step in emigration. This review focuses on two of the many aspects of this process that have seen important recent developments. The adhesion molecules, PECAM (CD31) and CD99 that regulate two major steps in TEM, do so by regulating specific signals. PECAM initiates the signaling pathway responsible for the calcium flux that is required for TEM. Calcium enters through the cation channel TRPC6 and recruits the first wave of trafficking of membrane from the lateral border recycling compartment (LBRC). CD99 signals through soluble adenylate cyclase to activate protein kinase A to recruit a second wave of LBRC trafficking. Another process that is critical for TEM is transient removal of VE-cadherin from the site of TEM. However, the local signaling pathways that are responsible for this appear to be different from those that open the junctions to increase vascular permeability.