Shear-Induced Amyloid Formation in the Brain: II. An Experimental System for Monitoring Amyloid Shear Processes and Investigating Potential Spinal Tap Problems

Artificial intelligence velocimetry reveals in vivo flow rates, pressure gradients, and shear stresses in murine perivascular flows

Quantifying the flow of cerebrospinal fluid (CSF) is crucial for understanding brain waste clearance and nutrient delivery, as well as edema in pathological conditions such as stroke. However, existing in vivo techniques are limited to sparse velocity measurements in pial perivascular spaces (PVSs) or low-resolution measurements from brain-wide imaging. Additionally, volume flow rate, pressure, and shear stress variation in PVSs are essentially impossible to measure in vivo. Here, we show that artificial intelligence velocimetry (AIV) can integrate sparse velocity measurements with physics-informed neural networks to quantify CSF flow in PVSs. With AIV, we infer three-dimensional (3D), high-resolution velocity, pressure, and shear stress. Validation comes from training with 70% of PTV measurements and demonstrating close agreement with the remaining 30%. A sensitivity analysis on the AIV inputs shows that the uncertainty in AIV inferred quantities due to uncertainties in the PVS boundary locations inherent to in vivo imaging is less than 30%, and the uncertainty from the neural net initialization is less than 1%. In PVSs of N = 4 wild-type mice we find mean flow speed 16.33 ± 11.09 µm/s, volume flow rate 2.22 ± 1.983 × 103 µm3/s, axial pressure gradient ( - 2.75 ± 2.01)×10-4 Pa/µm (-2.07 ± 1.51 mmHg/m), and wall shear stress (3.00 ± 1.45)×10-3 Pa (all mean ± SE). Pressure gradients, flow rates, and resistances agree with prior predictions. AIV infers in vivo PVS flows in remarkable detail, which will improve fluid dynamic models and potentially clarify how CSF flow changes with aging, Alzheimer's disease, and small vessel disease.

Kinetics of aggregation of amyloid β under different shearing conditions: Experimental and modelling analyses

Amyloid β (Aβ40) is a class of amyloidogenic proteins known to aggregate into a fibrillar network. The rate of aggregation and fibril yield is sensitive to external energy input, such as shear. In this work, simple shear and shaking experiments are performed on Aβ40 solution using a Couette cell and an orbital shaker, respectively. Experiments show that, under uniform shear, both the mass of fibrils and aggregation rate increase with the shear rate. In the case of orbital shaking, the lag time decreases with the rotational speed of the shaker, but the final fibril mass is the same for all agitation speeds. To explain this contrasting behavior of aggregation kinetics, a population balance model is developed to account for the effect of shear on the aggregation of Aβ. The kinetic model includes primary nucleation, secondary nucleation, elongation, fragmentation, and depolymerization steps. The effect of steady uniform shear is encoded in the depolymerization rate constant (k_d), and it is shown that k_d decreases with shear rate initially and saturates at high shear rates. A competition between elongation and depolymerization rates yields different equilibrium masses of fibril at different shear rates. The model results agree quantitatively well with experimental data on the rate of aggregation and mass of fibrils as a function of shear rate. The modeling framework can be used to explain the shear rate-dependent aggregation of other amyloidogenic proteins.

Shear-Induced Amyloid Aggregation in the Brain: V. Are Alzheimer's and Other Amyloid Diseases Initiated in the Lower Brain and Brainstem by Cerebrospinal Fluid Flow Stresses?

Amyloid-β (Aβ) and tau oligomers have been identified as neurotoxic agents responsible for causing Alzheimer's disease (AD). Clinical trials using Aβ and tau as targets have failed, giving rise to calls for new research approaches to combat AD. This paper provides such an approach. Most basic AD research has involved quiescent Aβ and tau solutions. However, studies involving laminar and extensional flow of proteins have demonstrated that mechanical agitation of proteins induces or accelerates protein aggregation. Recent MRI brain studies have revealed high energy, chaotic motion of cerebrospinal fluid (CSF) in lower brain and brainstem regions. These and studies showing CSF flow within the brain have shown that there are two energetic hot spots. These are within the third and fourth brain ventricles and in the neighborhood of the circle of Willis blood vessel region. These two regions are also the same locations as those of the earliest Aβ and tau AD pathology. In this paper, it is proposed that cardiac systolic pulse waves that emanate from the major brain arteries in the lower brain and brainstem regions and whose pulse waves drive CSF flows within the brain are responsible for initiating AD and possibly other amyloid diseases. It is further proposed that the triggering of these diseases comes about because of the strengthening of systolic pulses due to major artery hardening that generates intense CSF extensional flow stress. Such stress provides the activation energy needed to induce conformational changes of both Aβ and tau within the lower brain and brainstem region, producing unique neurotoxic oligomer molecule conformations that induce AD.

Shear-induced aggregation of amyloid β (1-40) in a parallel plate geometry

Protein aggregation is induced by various environmental or external factors and associated with various neurodegenerative diseases. Among various external factors, shear stress is inevitable for both in vivo and in vitro applications of proteins. In this study, Aβ (1-40) peptide, a derivative of the amyloid precursor protein, was subjected to constant (300, 500, 700 s^-1) and varying (ramp) shear in a parallel plate geometry to explore the implications of shear in terms of macro (viscosity) and micro (secondary structure, morphology) characteristics. Aβ (1-40) solution followed a shear thickening flow behaviour with performance index value 'n' of 2.12. The fibrillation process resulting from the shear force was evaluated in terms of dissipation energy, which was found to exceed the free energy of unfolding. This resulted in the formation of β-sheet rich structures, which were confirmed by CD and FTIR analyses and enhanced Th-T fluorescence. The apparent rate of aggregation (k) was found to increase with the shear rate, and inversely related to the solution viscosity. The maximum k value was 0.21 ± 0.3 min^-1 at 700 s^-1. The molecular weights of aggregates were determined using gel filtration, which were proportionally related to the solution viscosity. The average molecular weights were estimated to be 70, 62 and 52 KDa for samples sheared at 300, 500 and 700 s^-1, respectively. The present study has deciphered the interplay of viscosity, a fluid property, with the aggregation process and its corresponding change in the secondary structures of the peptide. These findings provide useful insights for understanding various proteopathies under shear force.Communicated by Ramaswamy H. Sarma.

Shear-Induced Amyloid Formation in the Brain: III. The Roles of Shear Energy and Seeding in a Proposed Shear Model

If cerebrospinal and interstitial fluids move through very narrow brain flow channels, these restrictive surroundings generate varying levels of fluid shear and different shear rates, and dissolved amyloid monomers absorb different shear energies. It is proposed that dissolved amyloid-β protein (Aβ) and other amyloid monomers undergo shear-induced conformational changes that ultimately lead to amyloid monomer aggregation even at very low brain flow and shear rates. Soluble Aβ oligomers taken from diseased brains initiate in vivo amyloid formation in non-diseased brains. The brain environment is apparently responsible for this result. A mechanism involving extensional shear is proposed for the formation of a seed Aβ monomer molecule that ultimately promotes templated conformational change of other Aβ molecules. Under non-quiescent, non-equilibrium conditions, gentle extensional shear within the brain parenchyma, and perhaps even during laboratory preparation of Aβ samples, may be sufficient to cause subtle conformational changes in these monomers. These result from brain processes that significantly lower the high activation energy predicted for the quiescent Aβ dimerization process. It is further suggested that changes in brain location and changes brought about by aging expose Aβ molecules to different shear rates, total shear, and types of shear, resulting in different conformational changes in these molecules. The consequences of such changes caused by variable shear energy are proposed to underlie formation of amyloid strains causing different amyloid diseases. Amyloid researchers are urged to undertake studies with amyloids, anti-amyloid drugs, and antibodies while all of these are under shear conditions similar to those in the brain.

Preliminary Capillary Flow Experiments with Amyloid-β, Possible Needle and Capillary Aβ Adsorption, and a Proposal for Drug Evaluation Under Shear Conditions

Amyloid-β (Aβ) solution injections into an aqueous mobile phase moving through narrow bore stainless-steel capillary tubing results in adsorption of at least 99% Aβ within the tubing or injection valve. However, if flow is stopped for a period of 5-10 minutes, then started, wall desorption yields Aβ-containing molecules in the new effluent. The amount of desorbed Aβ-containing effluent depends on flow rate, period of flow cessation, and number of successive Aβ injections into the same tube without cleaning between injections. Unexpected multiple chromatographic peaks in these experiments seem to imply "separation" of released, previously adsorbed Aβ-containing products in the empty capillary tubing. These preliminary experiments raise questions about possible errors in Alzheimer's disease (AD) spinal tap analyses, which use stainless-steel needles of approximately the same inner diameter and encounter similar flow rates as those in our capillary experiments. Microliter syringes and HPLC connectors also contain stainless-steel tubing that have similar inner diameter dimensions and similar flow rates. The capillary system involved in these experiments has previously been proposed as a model system for studying the effects of shear on Aβ within the brain because it offers a research environment that provides highly restrictive flow through very small dimension channels. A suggestion is made for the use of this system in exploratory anti-amyloid drug studies in which both the drug and Aβ are injected in the same solution so that both drug and Aβ are subjected to the same shear environment. Reduction in adsorbed Aβ is suggested as an indicator of effective anti-Aβ drugs.

Shear-Induced Amyloid Formation in the Brain: IV. Effects on Synapses Surrounding Senile Plaque and in Plaque-Free Regions

Amyloid-β oligomers (AβO) have been proposed as neurotoxins in the synaptic dysfunction that precedes Alzheimer's disease symptoms. Human and animal model studies report that senile plaques contain a halo of AβO molecules surrounding these plaques. A far smaller number of oligomers are distributed widely in plaque-free regions. It has been suggested that oligomers migrate from halos to nearby synapses and are incorporated into both pre- and postsynaptic terminals. These two types of oligomers have two different toxicities when extracted and injected in animal models. This paper proposes a shear-energy based explanation for the data in these studies. Shear hypotheses in the preceding three papers in this series are applied to suggest how the hydrodynamics and resulting shear patterns explain the spatial distribution of both AβO types, the apparent synapse loss in the vicinity of plaque particles, and possible reasons for the differing toxicities. A shear-based mechanism is proposed for the preferential migration of locally shear-excited Aβ molecules into the synaptic cleft. It is proposed that high energy laminar shear generated by the forced diversion of interstitial fluid around the flow-impeding plaque particle is responsible for the formation of AβOs around the plaque. It is suggested that in plaque-free regions, a different type of AβO with different toxicity is generated by lower energy shear flow around synapses, depositing AβO within the synapse from either the neuron membrane surface or by prion-like seeding within the synaptic cleft by locally-sheared Aβ molecules near the synapse entry.