Measuring amyloid fibril size and length with FIDA: a conversation with Prof. Stefan Rüdiger

Prof. Stefan Rüdiger leads a laboratory at Utrecht University working on protein folding, aggregation and the quality-control systems that control protein aggregates. In a conversation with Fidabio, Prof. Rüdiger described using Flow Induced Dispersion Analysis (FIDA) to measure the length of amyloid fibrils in disease. This is a summary of what he said.

[Video: embed the user-opinion interview here.]

Why Prof. Rüdiger chose FIDA to measure amyloid fibril size and length

The laboratory studies how amyloid fibrils grow and shrink in diseases such as Alzheimer's, where beta-amyloid aggregates outside the cell and tau inside it. The size of a fibril tracks where it sits in that aggregation process, so measuring fibril length matters for understanding disease progression and for developing compounds that act specifically on oligomers or fibrils rather than monomers, yet quantifying fibril size had been a long-standing challenge in amyloid research. The way in was a peptide the group developed, FibrilPaint, which binds specifically to fibrils and not to monomers. The capability that completed it was FIDA's size readout: a small labelled peptide gives a sharp dispersion profile, and when it binds a much larger fibril the profile broadens, so the hydrodynamic radius reports on fibril size.

What FIDA makes possible: a molecular ruler for amyloid fibril size

With FibrilPaint and FIDA, the group measured tau fibrils and then fibrils of beta-amyloid, alpha-synuclein and huntingtin, turning a measured hydrodynamic radius into an actual fibril length using the FIDA software module that predicts the radius of a known structure together with the known 4.7-angstrom spacing between fibril layers. What makes this a genuine molecular ruler is not just the number but where it works: FIDA reads fibril size directly in solution, in minutes and from tiny samples, and, in Prof. Rüdiger's account, is the only technique that also lets the group watch fibrils grow and shrink over time.

"FIDA is actually the only technique right now that allows us to measure the size of protein fibrils, and also the monitoring of the length and increase or shrinkage of fibrils," Prof. Rüdiger said.

Electron microscopy (EM), the traditional way to size fibrils, is accurate but demanding: it needs specialised, costly instrumentation and extensive sample preparation, processes only a few samples at a time, and produces a static, dried snapshot that cannot follow fibrils in solution or work in patient fluids. Here it served purely as a cross-check, and the two agreed closely, a FIDA value of 425 nm against an EM average of 430 nm, confirming the ruler is right. The advantage that actually changes the work is that FIDA delivers that same length in minutes, in solution and at a fraction of the effort, and because FibrilPaint labels fibrils specifically, it measures them in dirty solutions such as blood plasma or lysates and tells them apart from other aggregates, which let the group size patient-derived tau fibrils from Alzheimer's and related diseases directly.

Where the work is heading

The group is now asking whether protein fibrils can be targeted for degradation. Using fluorescently labelled ubiquitin, the ubiquitination cascade can be followed on FIDA, watching the ubiquitin move from one enzyme to the next and onto the fibril by the change in size, a route toward studying fibril degradation.

What we learned from this conversation

• Fibril length as a size readout: FibrilPaint plus FIDA reports fibril length via the hydrodynamic radius, specific to fibrils over monomers.
• A faster, cheaper molecular ruler: reads amyloid fibril size in solution in minutes, matching EM (425 vs 430 nm) without EM's costly instruments, heavy sample prep or static, dried snapshot.
• Works in dirty solutions: the fibril-specific label lets it run in plasma and lysates and on patient-derived samples.
• Monitors change: now following ubiquitination on FIDA as a route to studying fibril degradation.

This is the academic side of the story. The same FibrilPaint approach is being developed into an early-detection biomarker by the spin-out NeurotidalDx, told from the company's side in how a start-up is turning fibril length into a diagnostic.

Curious how FIDA measures size and aggregation in solution? Explore the FIDA technology and the Fida instrument, or browse the peer-reviewed literature. Read more scientists’ experiences with FIDA.