Minding the Gaps—and the Junctions, Too

Platelets may be small, but their behavior in vivo is governed by molecular mechanisms every bit as complex as those in larger cells. New molecules that provide unexpected insights turn up with surprising regularity. One recent example is the connexin family member Cx37 or GJA4. The role of Cx37 in platelets is examined in this issue of Circulation by Vaiyapuri et al,1 but a related study by Angelillo-Scherrer et al2 appeared here a year ago, and both will be considered together.Article see p 2479Cx37 is a member of a family of gap junction proteins that can assemble a comparatively nonselective channel when cells come into close contact with each other. These channels are large enough to allow free passage of soluble molecules such as cAMP, Ca2+, and IP3 (inositol 1,4,5-trisphosphate) between cells. They are formed by the interaction of 2 Cx37 hemichannels on the surface of opposing cells. Hemichannels themselves can also pass small molecules, but with different specificities than the complete channel (see Kar et al3 for a recent review).The identification of a previously unsuspected molecule in platelets inevitably leads to questions about its role in either platelets or megakaryocytes. Platelets have limited synthetic capability. According to current models, they are formed by carefully orchestrated events in which proteins synthesized by megakaryocytes are transshipped to the tip of a developing proplatelet projection.4 Presumably anything found in mature platelets is there for a reason. What, then, is the role of Cx37? Based on studies in other cells, it is possible to make some reasoned guesses. However, as will be discussed below, published evidence leaves the answer in platelets somewhat uncertain.First, consider that a major design goal of platelets is to be able to form a stable hemostatic plug in …