Is the increased expression of ubiquitin in CADASIL syndrome a manifestation of aberrant endocytosis in the vascular smooth muscle cells?

Ubiquitin is a highly conserved protein involved in many important cellular processes, such as cell surface receptor signaling, endocytosis and protein degradation. Since ubiquitin plays a key role in the pathomechanisms of many neurodegenerative diseases, we immunohistochemically analyzed its expression in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). CADASIL is a heritable vascular dementia characterized by degeneration of the vascular smooth muscle cells (VSMC) caused by mutations in the notch 3 gene. In CADASIL, there is abnormal accumulation of the Notch 3 extracellular domain on blood vessels, but the molecular pathways linking notch 3 mutations to degeneration of the VSMC are, as yet, poorly understood. We studied human brain and skin biopsy specimens, and observed increased ubiquitin expression on structures primarily affected by the pathological process in CADASIL: the VSMC and vascular lamina media, and also large ballooned macrophages. Ultrastructurally, we noted that pathognomonic CADASIL deposits of granular osmiophilic material were often located inside indentations in the VSMC membrane that resembled endocytic vesicles. We suggest that in CADASIL, damage to the VSMC may be associated with aberrant ubiquitin-dependent endocytosis of the Notch 3 ligand, and increased accumulation of ubiquitin on the vessel wall may be a manifestation of this aberration.