Comparison of intrabody-mediated degradation of mHTT through ubiquitin independent vs. dependent pathways

Abstract

Huntington’s disease (HD) is a progressive brain disorder that causes various symptoms, such as uncontrolled movements, emotional problems, and cognitive deficits. In US there are more than 40,000 patients with HD disease and the global prevalence is between 1/10,000 to 1/20,000 based on the information from Huntington’s Disease Society of America. HD is caused by a mutation in Huntingtin gene (HTT) coding for the mutant Huntington (mHtt) protein. The mHtt protein causes neural cell death, leading to symptoms in HD patients. Degrons are degradation signals located in some proteins that direct misfolded or aggregated proteins to Ubiquitin–Proteasome System (UPS) for degradation. Intrabody is a small recombinant antibody that targets antigens intracellularly. Anti-mHtt intrabodies were found to be able to promote mHtt degradation and degrons can enhance the intrabody-mediated mHtt degradation. By involving various degrons, intrabodies can work through either ubiquitin-dependent or ubiquitin-independent pathway to degrade mutant proteins. However, the mechanism underlying the mHtt degradation mediated by intrabody is not fully understood and the efficiency of each degradation pathway involved is unclear. To investigate which of the above pathways is more efficient, we compared the intrabody-mediated degradation pathways with CL1 vs. PEST degrons fused to the intrabody. Results showed that the ubiquitin-dependent proteasome pathway involving CL1 degron is more efficient in mHtt degradation than the ubiquitin-independent proteasome pathway involving PEST degron. These results provide the scientific foundation of developing a potential intrabody-mediated treatment for HD disease via the ubiquitin-dependent pathway.