Binding structures of SERF1a with NT17-polyQ peptides of huntingtin exon 1 revealed by SEC-SWAXS, NMR and molecular simulation.

The aberrant fibrillization of huntingtin exon 1 (Httex1) characterized by an expanded polyglutamine (polyQ) tract is a defining feature of Huntington's disease, a neurodegenerative disorder. Recent investigations underscore the involvement of a small EDRK-rich factor 1a (SERF1a) in promoting Httex1 fibrillization through interactions with its N terminus. By establishing an integrated approach with size-exclusion-column-based small- and wide-angle X-ray scattering (SEC-SWAXS), NMR, and molecular simulations using Rosetta, the analysis here reveals a tight binding of two NT17 fragments of Httex1 (comprising the initial 17 amino acids at the N terminus) to the N-terminal region of SERF1a. In contrast, examination of the complex structure of SERF1a with a coiled NT17-polyQ peptide (33 amino acids in total) indicates sparse contacts of the NT17 and polyQ segments with the N-terminal side of SERF1a. Furthermore, the integrated SEC-SWAXS and molecular-simulation analysis suggests that the coiled NT17 segment can transform into a helical conformation when associated with a polyQ segment exhibiting high helical content. Intriguingly, NT17-polyQ peptides with enhanced secondary structures display diminished interactions with SERF1a. This insight into the conformation-dependent binding of NT17 provides clues to a catalytic association mechanism underlying SERF1a's facilitation of Httext1 fibrillization.

Amyloid Modifier SERF1a Accelerates Alzheimer's Amyloid-ß Fibrillization and Exacerbates the Cytotoxicity.

Alzheimer's disease (AD) is a devastating, progressive neurodegenerative disease affecting the elderly in the world. The pathological hallmark senile plaques are mainly composed of amyloid-ß (Aß), in which the main isoforms are Aß40 and Aß42. Aß is prone to aggregate and ultimately forms amyloid fibrils in the brains of AD patients. Factors that alter the Aß aggregation process have been considered to be potential targets for treatments of AD. Modifier of aggregation 4 (MOAG-4)/small EDRK-rich factor (SERF) was previously selected from a chemical mutagenesis screen and identified as an amyloid modifier that promotes amyloid aggregation for α-synuclein, huntingtin, and Aß40. The interaction and effect of yeast ScSERF on Aß40 were previously described. Here, we examined the human SERF1a effect on Aß40 and Aß42 fibrillization by the Thioflavin T assay and found that SERF1a accelerated Aß fibrillization in a dose-dependent manner without changing the fibril amount and without incorporation. By Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM), we found that SERF1a altered the secondary structures and the morphology of Aß fibrils. The electrospray ionization mass spectrometry (ESI-MS) and analytical ultracentrifugation (AUC) results showed that SERF1a binds to Aß in a 1:1 stoichiometry. Moreover, the NMR study showed that SERF1a interacts with Aß via its N-terminal region. Cytotoxicity assay demonstrated that SERF1a enhanced toxicity of Aß intermediates, and the effect can be rescued by SERF1a antibody. Overall, our study provides the underlying molecular mechanism for the SERF1a effect on Aß fibrillization and facilitates the therapeutic development of AD.

Amyloid modifier SERF1a interacts with polyQ-expanded huntingtin-exon 1 via helical interactions and exacerbates polyQ-induced toxicity.

Abnormal polyglutamine (polyQ) expansion and fibrillization occur in Huntington's disease (HD). Amyloid modifier SERF enhances amyloid formation, but the underlying mechanism is not revealed. Here, the fibrillization and toxicity effect of SERF1a on Htt-exon1 are examined. SERF1a enhances the fibrillization of and interacts with mutant thioredoxin (Trx)-fused Httex1. NMR studies with Htt peptides show that TrxHttex1-39Q interacts with the helical regions in SERF1a and SERF1a preferentially interacts with the N-terminal 17 residues of Htt. Time-course analysis shows that SERF1a induces mutant TrxHttex1 to a single conformation enriched of ß-sheet. Co-expression of SERF1a and Httex1-polyQ in neuroblastoma and lentiviral infection of SERF1a in HD-induced polypotent stem cell (iPSC)-derived neurons demonstrates the detrimental effect of SERF1a in HD. Higher level of SERF1a transcript or protein is detected in HD iPSC, transgenic mice, and HD plasma. Overall, this study provides molecular mechanism for SERF1a and mutant Httex1 to facilitate therapeutic development for HD.