Mutant Huntingtin Promotes the Fibrillogenesis of Wild-type Huntingtin

Abstract

Aggregation of huntingtin (htt) in neuronal inclusionsis associated with the development of Huntington's disease (HD). Previously, we have shown that mutant htt fragments with polyglutamine (polyQ) tracts in the pathological range (>37 glutamines) form SDS-resistant aggregates with a fibrillar morphology, whereas wildtype htt fragments with normal polyQ domains do not aggregate. In this study we have investigated the coaggregation of mutant and wild-type htt fragments. We found that mutant htt promotes the aggregation of wildtype htt, causing the formation of SDS-resistant co-aggregates with a fibrillar morphology. Conversely, mutant htt does not promote the fibrillogenesis of the polyQ-containing protein NOCT3 or the polyQ-binding protein PQBP1, although these proteins are recruited into inclusions containing mutant htt aggregates in mammalian cells. The formation of mixed htt fibrils is a highly selective process that not only depends on polyQ tract length but also on the surrounding amino acid sequence. Our data suggest that mutant and wild-type htt fragments may also co-aggregate in neurons of HD patients and that a loss of wild-type htt function may contribute to HD pathogenesis. Huntington's disease (HD)1 is an autosomal dominant neurodegenerative disorder characterized by uncontrolled movements, motor impairment, and dementia (1). The disease is associated with selective neuronal cell death occurring mainly in the cerebral cortex and the striatum (2). The mutation causing HD is an expanded CAG repeat located within exon 1 of the IT-15 gene encoding huntingtin (htt), a 350-kDa protein of unknown function (3). The CAG repeat is translated into a polyglutamine (polyQ) sequence, and the disease appears when the critical length of ϳ37 glutamines is exceeded (4).It is generally assumed that the expanded polyQ stretch confers a toxic gain of function to the htt protein. However, the current data do not exclude the possibility that loss of wild-type htt function may also contribute to the disease phenotype in post-mitotic neurons. htt function is essential for neurogenesis (5) and also important for neuron survival in adult brains (6, 7). Strikingly, deletion of the gene encoding the htt mouse homologue hdh resulted in a progressive neurological phenotype (8). Therefore, it is intriguing to speculate that the toxic gain of function of mutant htt may cause a loss of the essential function of wild-type htt and thereby induce selective neurotoxicity (9). However, the molecular basis for such a disease mechanism remains unclear. A characteristic feature of mutant htt proteins with elongated polyQ sequences is that they spontaneously self-assemble into SDS-resistant protein aggregates with a fibrillar morphology in vitro and in vivo (10,11). This suggests that HD is due to a toxic gain of function that leads to the formation of abnormal protein aggregates in a nucleation-dependent process.Indeed, the formation of neuronal inclusions (NIs) in affected brain regions of patients is a characteristic feature of HD (11,12),...

Polyglutamine-Rich Suppressors of Huntingtin Toxicity Act Upstream of Hsp70 and Sti1 in Spatial Quality Control of Amyloid-Like Proteins

“…Some of these polyQ stretch proteins are capable of interacting with the expanded polyQ domains of disease proteins [6], [28], [45], [48], [61], [69], [70], and in most cases such interactions seem to have negative outcomes on protein function and cell viability [6], [7], [48], [69], [70]. In one instance, interaction between Htt103Q and cellular proteins appears to seed formation of toxic Htt103Q [13], [17], [34], [58], [71].…”

Interaction with Polyglutamine Aggregates Reveals a Q/N-rich Domain in TDP-43

“…Based on this model, proteins with nonpathogenic polyglutamine stretches are predicted to incorporate into polyglutamine aggregates (20), and this has been demonstrated for several nonpathogenic length polyglutamine proteins, including cAMP-response element-binding protein-binding protein, and TATA box-binding protein (40,41,52,53). In these cases, the unexpanded polyglutamine proteins were depleted from their normal cellular localization and incorporated into detergent-insoluble polyglutamine aggre- gates, leading to loss of their normal function, similar to TDP-43.…”

Monomeric, Oligomeric and Polymeric Proteins in Huntington Disease and Other Diseases of Polyglutamine Expansion

“…Sequestration of heterologous proteins within the aggregates may render these proteins unable to exert their function. Proteins rich in Q/N residues, such as the TATA-box binding protein, the cAMP-response element-binding protein, TIA-1 or normal huntingtin are trapped into inclusions [66,144,168,169,170]. The structure of the polyQ aggregates may condition their affinity for heterologous proteins as well as the nature of the sequestered proteins.…”