Functional analysis of optineurin and some of its disease-associated mutants

Optineurin is a multifunctional protein involved in a variety of cellular functions such as protein trafficking by vesicles, autophagy, and signal transduction. Certain mutations in optineurin (gene OPTN) are associated with neurodegenerative diseases like glaucoma and amyotrophic lateral sclerosis (ALS). Optineurin is also seen in pathological structures present in several other neurodegenerative diseases. In glaucoma, loss of vision occurs due to progressive degeneration of retinal ganglion cells, and perhaps loss of photoreceptor cone cells as well. Most of the glaucoma-associated mutations of optineurin are heterozygous missense mutations, whereas the ALS-associated mutations include deletion, truncation, and missense mutations. Optineurin mediates its functions by interacting with various proteins, often acting as an adaptor to provide a link between two or more proteins. Disease-causing mutations alter these interactions leading to functional defects in membrane vesicle trafficking, autophagy, signaling, aggregate formation, and other processes. Some of these functional defects, caused by glaucoma-associated mutants of optineurin, led to retinal cell death mediated by apoptosis and therefore may contribute to pathogenesis directly. Other mutations are likely to cause glaucoma by indirect mechanisms involving other cell types. Mechanisms of ALS pathogenesis by optineurin mutations are yet to be investigated in detail; however, some ALS-associated mutants cause defects in signaling, autophagy, and ubiquitin binding, which might contribute to pathogenesis.