Accumulation of mutant proteins is a major cause of many diseases (collectively called proteopathies), and lowering the level of these proteins can be useful for treatment of these diseases. We hypothesized that compounds that interact with both the autophagosome protein microtubule-associated protein 1A/1B light chain 3 (LC3)¹ and the disease-causing protein may target the latter for autophagic clearance. Mutant huntingtin protein (mHTT) contains an expanded polyglutamine (polyQ) tract and causes Huntington’s disease, an incurable neurodegenerative disorder². Here, using small-molecule-microarray-based screening, we identified four compounds that interact with both LC3 and mHTT, but not with the wild-type HTT protein. Some of these compounds targeted mHTT to autophagosomes, reduced mHTT levels in an allele-selective manner, and rescued disease-relevant phenotypes in cells and in vivo in fly and mouse models of Huntington’s disease. We further show that these compounds interact with the expanded polyQ stretch and could lower the level of mutant ataxin-3 (ATXN3), another disease-causing protein with an expanded polyQ tract³. This study presents candidate compounds for lowering mHTT and potentially other disease-causing proteins with polyQ expansions, demonstrating the concept of lowering levels of disease-causing proteins using autophagosome-tethering compounds.

Degrading pathogenic proteins by degrader technologies such as PROTACs (proteolysis-targeting chimeras) provides promising therapeutic strategies, but selective degradation of non-protein pathogenic biomolecules has been challenging. Here, we demonstrate a novel strategy to degrade non-protein biomolecules by autophagy-tethering compounds (ATTECs), using lipid droplets (LDs) as an exemplar target. LDs are ubiquitous cellular structures storing lipids and could be degraded by autophagy. We hypothesized that compounds interacting with both the LDs and the key autophagosome protein LC3 may enhance autophagic degradation of LDs. We designed and synthesized such compounds by connecting LC3-binding molecules to LD-binding probes via a linker. These compounds were capable of clearing LDs almost completely and rescued LD-related phenotypes in cells and in two independent mouse models with hepatic lipidosis. We further confirmed that the mechanism of action of these compounds was mediated through LC3 and autophagic degradation. Our proof-of-concept study demonstrates the capability of degrading LDs by ATTECs. Conceptually, this strategy could be applied to other protein and non-protein targets.

Many diseases are caused by aberrant accumulation of certain proteins that are misfolded and cytotoxic, and lowering the level of these proteins provides promising treatment strategies for these diseases. We hypothesized that compounds that interact with both the disease-causing protein and the phagophore (autophagosome precursor) protein LC3 may tether the former to phagophores for subsequent autophagic degradation. If true, this autophagosome-tethering compound (ATTEC) concept could be applied to many disease-causing proteins to treat diseases. We tested this hypothesis in the scenario of Huntington disease (HD), a neurodegenerative disorder that is caused by the mutant HTT (mHTT) protein with an expanded polyglutamine (polyQ) stretch. In our recent study, we designed a small-molecule microarray-based screening and identified four mHTT-lowering compounds that interact with both mHTT and LC3, but not wild-type (WT) HTT. These compounds target mHTT to phagophores for autophagic degradation without influencing the WT HTT level, and rescue HD-relevant phenotypes in HD cells and in vivo in the fly and mouse HD models. Interestingly, these compounds interact with the expanded polyQ stretch directly and are able to reduce other disease-causing proteins with expanded polyQ. In summary, our study provides the initial validation of lowering mHTT by ATTEC, providing entry points to new treatment strategies of HD and similar diseases.

ATTECs and several other emerging degrader technologies hijacking the lysosomal pathways greatly expand the spectrum of degradable targets and provide new opportunities for targeted drug discovery.