Secondary nucleation drives polymorph diversity in hIAPP amyloids
Amyloid fibrils are implicated in a myriad of human diseases. A striking observation is that fibrils extracted from diseased tissues are characterized by a restricted set of folds unique to the specific pathology. In contrast, fibrils grown in vitro exhibit extensive structural diversity, suggesting that specific environmental and biochemical mechanisms in vivo enforce structural selectivity. Here, we combine two-dimensional infrared (2D IR) spectroscopy and cryo-electron microscopy (cryo-EM) to investigate the mechanisms governing polymorph formation in the human Islet Amyloid Polypeptide (hIAPP). We demonstrate that 2D IR can resolve populations of distinct polymorphs identified by cryo-EM, enabling rapid label-free screening of conditions prior to labor-intensive microscopy screening. We find that conditions favoring secondary nucleation, such as high protein concentration, increase polymorphic diversity. Crucially, cryo-EM reveals that formed by secondary nucleation do not structurally replicate the parent template. Finally, by selectively inhibiting secondary nucleation using the C-terminal domain of the DNAJB6 chaperone, we steer aggregation toward a monomorphic state. These findings highlight the critical role of molecular chaperones in fibril polymorph selection.