Heparin promotes rapid fibrillation of the basic parathyroid hormone at physiological pH

Abstract

In acidic secretory granules of mammalian cells, peptide hormones including the parathyroid hormone are presumably stored in the form of functional amyloid fibrils. Mature PTH, however, is considerably positively charged in acidic environments, a condition known to impede unassisted self-aggregation into fibrils. Here, we studied the role of the polyanion heparin on promoting fibril formation of PTH. Employing ITC, CD spectroscopy, NMR, SAXS, and fluorescencebased assays, we could demonstrate that heparin binds PTH with submicromolar affinity and facilitates its conversion into fibrillar seeds, enabling rapid formation of amyloid fibrils under acidic conditions. In the absence of heparin, PTH remained in a soluble monomeric state. We suspect that heparin-like surfaces are required in vivo to convert PTH efficiently into fibrillar deposits.

A Competition of Secondary and Primary Nucleation Controls Amyloid Fibril Formation of the Parathyroid Hormone

“…The micrographs show curvilinear fibrils characteristic for PTH 84 as previously reported. [ 22,24,25 ] The fibrils are up to several µm in length, only showing few shorter individual fibrils. Notably, PTH 84 fibrils exhibit a strong polymorphism.…”

“…In order to gain a deeper understanding of the underlying mechanism of amyloid formation by PTH 84 we performed individual curve fittings to the increasing part of the ThT monitored fibrillation kinetics depicted in Figure 2A (Equation (6)) as described previously, thus revealing information about the effective rates of primary ( λ ) and secondary ( κ ) processes. [ 25 ] The fits reproduce the data well, only at c 0 = 600 µ m (red curve) it is obvious that the applied simple model is not sufficient to describe the full curve ( Figure 5 A). Still, the parameters provide a solid trend.…”

“…[ 22,24 ] Under physiological context, on the other hand, no spontaneous fibril formation is observed. [ 25 ] Instead, glycose‐amino‐glycans (GAG), such as heparin, can act as a trigger molecule initializing amyloid fibril formation. The role of GAGs in assisting fibril formation is also known for other amyloidogenic peptides.…”

The pro‐sequence of parathyroid hormone prevents premature amyloid fibril formation

“…During secretion and maturation of PTH the physiological pH drops from neutral values in the ER to values of about 5.5 in the trans‐Golgi apparatus and secretory granules. For PTH, we found in a previous study that no fibrils are formed under these acidic conditions, but the presence of heparin could facilitate fibril formation [9]. Polyanionic heparin belongs to the family of glycosaminoglycans abundant in secretory granules and amyloid fibril deposits of peptide hormones [6].…”

“…One such example of functional amyloids is the storage of peptide hormones at a high concentration in membrane‐coated secretory granules in the cytosol before release into the bloodstream [6]. PTH belongs to the latter class of peptides, which readily showed amyloid fibril formation also under in vitro conditions exhibiting a cross‐β sheet rich conformation [7–9].…”