Huntingtin and Its Partner Huntingtin-Associated Protein 40: Structural and Functional Considerations in Health and Disease

Abstract

Since the discovery of the mutation causing Huntington’s disease (HD) in 1993, it has been debated whether an expanded polyglutamine (polyQ) stretch affects the properties of the huntingtin (HTT) protein and thus contributes to the pathological mechanisms responsible for HD. Here we review the current knowledge about the structure of HTT, alone (apo-HTT) or in a complex with Huntingtin-Associated Protein 40 (HAP40), the influence of polyQ-length variation on apo-HTT and the HTT-HAP40 complex, and the biology of HAP40. Phylogenetic analyses suggest that HAP40 performs essential functions. Highlighting the relevance of its interaction with HTT, HAP40 is one of the most abundant partners copurifying with HTT and is rapidly degraded, when HTT levels are reduced. As the levels of both proteins decrease during disease progression, HAP40 could also be a biomarker for HD. Whether declining HAP40 levels contribute to disease etiology is an open question. Structural studies have shown that the conformation of apo-HTT is less constrained but resembles that adopted in the HTT-HAP40 complex, which is exceptionally stable because of extensive interactions between HAP40 and the three domains of HTT. The complex— and to some extent apo-HTT— resists fragmentation after limited proteolysis. Unresolved regions of apo-HTT, constituting about 25% of the protein, are the main sites of post-translational modifications and likely have major regulatory functions. PolyQ elongation does not substantially alter the structure of HTT, alone or when associated with HAP40. Particularly, polyQ above the disease length threshold does not induce drastic conformational changes in full-length HTT. Therefore, models of HD pathogenesis stating that polyQ expansion drastically alters HTT properties should be reconsidered.

ProteinCoLoc streamlines Bayesian analysis of colocalization in microscopic images

“…The applicability and utility of ProteinCoLoc were demonstrated through two exemplary analyses focusing on the colocalization of HAP40 with different antibodies and its colocalization with HTT. Studying the colocalization of HAP40 with HTT is of interest to understand the pathophysiology of Huntington disease 8 , a neurodegenerative disorder characterised by the abnormal aggregation of the HTT protein 24 . While these studies showcase the software’s capabilities, it is crucial to note that the quality of any colocalization analysis is inherently dependent on the quality of the input data 2 , 3 , 25 .…”

“…As both antibodies detect the same protein, a high colocalization, especially in the cell nucleus 6 , 7 , is expected. In the second analysis, we co-stained HAP40 and huntingtin (HTT), two proteins, whose interaction is well documented 8 and which are routinely studied in our lab 8 , 10 , 12 , 15 , 16 . HAP40 is an abundant interactor of HTT 10 , 11 , the protein that is pathologically altered in Huntington disease (HD) due to a mutation in the HTT gene.…”

“…Moreover, the interaction of the two proteins is likely essential for their function, as both proteins have co-evolved and their interaction is conserved in various species 15 . Interestingly, the protein levels and stability of both HAP40 and HTT are reduced in tissues from HD patients and mouse models 9 , 16 – 18 , suggesting that the reduction of their levels in cells may contribute to the pathophysiology of HD 8 , 18 . However, although the HTT-HAP40 complex is well characterised in vitro 10 , 12 , 15 , 18 , little is known regarding the extent of their interaction in vivo.…”

Huntingtin polyglutamine expansions misdirect axonal transport by perturbing motor and adaptor recruitment

“…HAP1 interacts with kinesin-1's C-terminal domain, kinesin-1 adaptor kinesin-light chain (KLC), and dynein adaptor dynactin's p150 Glued subunit 6,8,9 . HTT/HTT-associated protein 40 (HAP40) associates with Ras-related protein 5 (RAB5) to recruit kinesin-1 and -3 and is implicated in switching endocytic and secretory cargoes from microtubule to actin-based transport 10,11 . HTT/optineurin (OPTN) recruits myosin-VI to secretory and golgi-derived cargoes via Ras-related protein 8 (RAB8) to facilitate short-range transport along actin filaments 12,13 .…”

Identification and Optimization of RNA-Splicing Modulators as Huntingtin Protein-Lowering Agents for the Treatment of Huntington’s Disease

“…Small molecules are ideal for CNS targets if suitable binding sites can be identified. The intrinsically disordered nature of the N-terminus that still evades structural characterization in the full-length HTT construct, plus its enormous size3144 amino acids for wild-type human HTT, spanning 67 exonshas posed challenges for structure-based design of HTT binders/degraders . In addition, the polyQ-containing HTT exon-1 fragment readily forms protein aggregates which, although amenable to the design of mHTT aggregate-specific ligands for positron emitting tomography (PET) imaging, hindered the screening for compounds that target the nonaggregated N-terminal fragment.…”