KRIT1 haploinsufficiency increases ROCK activity in human cerebral lesion endothelium. (A) Human CCM cerebral lesions were compared with nonlesion control brain tissue. Slides were processed as described in Materials and methods, stained with anti-pMLC as an index of ROCK activation, and counterstained with hematoxylin. Control antibody stains are with rabbit IgG. The left and center columns are stained for pMLC, and the right column is stained with control antibody. Bars, 500 µm. The CCM lesions are clusters of distended capillaries lacking SMC underlayer. Arrows indicate cavern endothelial cells; asterisks mark lumen of a typical expanded lesion vessel. The control non-CCM brain tissue panel shows an arteriole in cross section, exhibiting pMLC in SMC subendothelial layer, but little in endothelial cells (arrow) facing vessel lumen. Sporadic lesion #1 is KRIT1 protein deficient (Fig. S3). Familial lesion #2 is CCM2 deficient (Fig. S3) and has equally robust endothelial pMLC staining. (B) Magnified view of familial lesion #2 endothelium.