Activated Platelets Express GPI-Anchored Fibrocystin-L (PKHD1L1), from Granules, Absent or Deficient in Paroxysmal Nocturnal Hemoglobinuria.

Platelets express several glycophosphatidylinositol-(GPI-) anchored receptors, mainly involved in protection against lysis by activated complements. Most of these are well-characterised and are expressed on other blood cells. More recently, CD109 with a mass of 175 kDa was also shown to be a GPI-anchored receptor, surface expressed only on activated platelets, with a role as a co-receptor for transforming growth factor-β (TGF-β). CD109 is expressed on a wide range of cells and is a marker for various types of tumors. Activated platelets express an even larger GPI-anchored receptor at about 500 kDa. We have now isolated this and identified it as fibrocystin L, also known as polycystic kidney hepatic disease L1 (PKHD1L1). Fibrocystin L, like other platelet GPI-anchored receptors, is missing or deficient in paroxysmal nocturnal hemoglobinuria. Fibrocystin L is also expressed in activated T-cells and may be involved in immune responses. Recently, there have been additional reports of PKHD1L1 expression and roles as a coat protein of hair-cell stereocilia essential for normal hearing, as well as reports of them in the dentate gyrus in mice involved in susceptibility to seizure. In all these cases, GPI anchors were not reported, but neither were they tested for. During the fluorescence microscopy studies, we used CD109 as a control and observed that it is also a granule protein that had not been previously reported.