Towards deciphering the Nt17 code: How the sequence and conformation of the first 17 amino acids in Huntingtin regulate the aggregation, cellular properties, and neurotoxicity of mutant Httex1
Converging evidence points to the N-terminal domain comprising the first 17 amino acids of the Huntingtin protein (Nt17) as a key regulator of its aggregation, cellular properties and toxicity. In this study, we further investigated the interplay between Nt17 and the polyQ domain repeat length in regulating the aggregation and inclusion formation of exon 1 of the Huntingtin protein (Httex1). In addition, we investigated the effect of removing Nt17 or modulating its local structure on the membrane interactions, neuronal uptake, and toxicity of monomeric or fibrillar Httex1. Our results show that the polyQ and Nt17 domains synergistically modulate the aggregation propensity of Httex1 and that the Nt17 domain plays an important role in shaping the surface properties of mutant Httex1 fibrils and regulating their poly-Q-dependent growth, lateral association and neuronal uptake. Removal of Nt17 or disruption of its transient helical conformations slowed the aggregation of monomeric Httex1 in vitro, reduced inclusion formation in cells, enhanced the neuronal uptake and nuclear accumulation of monomeric Httex1 proteins, and was sufficient to prevent cell death induced by Httex1 72Q overexpression. Finally, we demonstrate that the uptake of Httex1 fibrils into primary neurons and the resulting toxicity are strongly influenced by mutations and phosphorylation events that influence the local helical propensity of Nt17. Altogether, our results demonstrate that the Nt17 domain serves as one of the key master regulators of Htt aggregation, internalization, and toxicity and represents an attractive target for inhibiting Htt aggregate formation, inclusion formation, and neuronal toxicity. HighlightsO_LIThe Nt17 and polyQ domains synergistically promote Httex1 aggregation. C_LIO_LIThe Nt17 domain is a key determinant of the lateral association and morphology of fibrils. C_LIO_LIThe Nt17 domain and conformation regulate the nuclear/cytoplasmic distribution and toxicity of Httex1. C_LIO_LINt17 conformation is a key determinant of Httex1 fibril membrane interaction and cellular uptake. C_LIO_LINt17 serves as one of the master regulators of Httex1 aggregation, cellular uptake and toxicity. C_LI Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=147 SRC="FIGDIR/small/431207v2_ufig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@7afd68org.highwire.dtl.DTLVardef@92c67aorg.highwire.dtl.DTLVardef@24823dorg.highwire.dtl.DTLVardef@e2a835_HPS_FORMAT_FIGEXP M_FIG The Nt17 domain: A master switch of Httex1 aggregation, uptake, subcellular localization and neurotoxicity. In this paper, we showed that 1) the Nt17 and polyQ domains synergistically promote Httex1 aggregation; 2) the Nt17 domain is a key determinant of the lateral association and morphology of fibrils in vitro, 3) Nt17 conformation is a key determinant of Httex1 fibril membrane interaction and cellular uptake in primary neurons; 4) the Nt17 domain and conformation regulate the nuclear/cytoplasmic distribution and toxicity of Httex1 in primary neurons. The figure was created with Biorender and https://www.vectorstock.com/royalty-free-vector/icon-on-and-off-toggle-switch-button-white-design-vector-30148026 C_FIG