The aggregation mechanisms in amyloid systems are prone to multiple pathways, depending on the ensemble of co-existing amyloidogenic conformations and environmental factors. Thus, several aggregation mechanisms and multiple pathways have been described depending on protein sequence, conformational states adopted by the amyloidogenic monomer and experimental conditions (for instance, temperature, pH, protein concentration, and solvent effects). The aggregation processes take place over a wide time range, spanning several orders of magnitude, with conformational changes occurring in the milliseconds and formation of particles observable with the naked eye in days, weeks or months. Elucidation of the mechanisms of amyloid formation and characterization of the most relevant molecular species involved are crucial to devise new rational therapeutic strategies against amyloid diseases. A brief overview of the most common amyloid formation mechanisms is presented in Figure 4, and a summary of the events and conditions that may trigger protein aggregation is shown in Table 4.
5.1. Aggregation Via a Nucleation-Dependent Mechanism
The nucleation-dependent mechanism of amyloid formation (
Figure 4), also known as nucleation–elongation polymerization, displays a typical sigmoidal shape curve as a function of time and consists of three consecutive steps: (1) initial lag or nucleation phase; (2) elongation, growth, polymerization, or fibrillation phase; (3) equilibrium, stationary, or saturation phase
[211][212].
The nucleation phase corresponds to the assembly of transient, critical nuclei that next will act as seeding intermediates where additional monomeric subunits can latch on, driving the assembly of oligomers with cross-β structure. At this stage, the rate constants for monomer addition and dissociation are similar, making the global process of nucleation slow and the rate limiting step in fibril formation. The nucleation phase can be shortened or eliminated by the addition of pre-formed aggregates or fibrillar species, a process known as seeding
[213][214][215]. In the elongation phase, monomers, nucleus and oligomers continue to interact, assembling into prefibrillar structures that rapidly grow to form ordered fibrillar structures known as protofibrils. Because this phase gives rise to more stable protofibrils, this is a faster and thermodynamically favorable process. Lastly, the saturation phase, where monomer concentration is low and approximately constant, involves the assembly of protofibrils into mature amyloid fibrils with different morphological structures and different levels of polymorphism.
The Finke–Watzky aggregation model (
Scheme 1) is one of the numerous models proposed for nucleation–elongation polymerization and has been applied to more than 40 different aggregating proteins
[216][217]. As shown in
Scheme 1, the Finke–Watzky model consists of two simple steps: (1) nucleation and (2) growth. Due to its simplicity, this model does have some limitations, including: (1) a vast number of aggregation steps is condensed into two elementary steps; (2) the rate constants,
knucleation and
kgrowth, are average rate constants and independent of the size of aggregating species; (3) a higher kinetic order in [
M] may be kinetically hidden in the nucleation step in particular; (4) all growing polydisperse aggregates are hidden behind the descriptor “
An”; (5) the descriptor “
An” can also hide processes such as fragmentation. However, the simplicity and quality of the fits obtained in many practical examples suggest that the Finke–Watzky two-step mechanism encompasses the main characteristics and it is a good general kinetic model for nucleation-growth aggregation.
Scheme 1. The minimalistic Finke–Watzky mechanism for protein aggregation via a nucleation-dependent pathway, where
M is the monomer, and
A is the transient nucleus aggregate. The rate constants for the nucleation and growth steps are
knucleation and
kgrowth, respectively
[216][217][218].
5.1.1. Primary Nucleation Mechanisms
In the amyloidogenesis cascade, primary nucleation is a critical step in a variety of oligomerization mechanisms and includes the initial formation of amyloidogenic nuclei without contributions from pre-formed oligomers. Two types of primary nucleation mechanisms have been described: homogeneous and heterogeneous. Homogeneous primary nucleation comprises monomers aggregation in bulk solution, while heterogeneous primary nucleation involves association of monomeric subunits on the surface of a different object, such as the wall of a reaction container
[191][192][193][194], other proteins [
202], phospholipid bilayers
[203][204][205][206][207][208][209][210], or the air–water interface
[175][176][177][178][179][180].
From a structural point of view, the simplest manifestation of a primary nucleation mechanism is the nucleated polymerization (NP) mechanism. In this case, amyloidogenic monomers aggregate and originate the nucleus, which further grows into amyloid protofilaments and protofibrils through an elongation process involving mostly monomer addition
[219][220][221]. This is the preferential mechanism at relatively low protein concentrations favoring the presence of monomeric species in solution.
However, in several instances, it has been observed the presence of multiple conformational heterogeneous oligomers and transient intermediate species during fibril formation which NP mechanisms cannot explain. In these cases, a nucleated conformational conversion (NCC) mechanism has been proposed. NCC comprises structurally organized oligomers as intermediates which are able to subsequently conformationally transition into cross-β dominated fibrillar species. The formation of these conformationally dynamic oligomers may be favored at higher protein concentrations and they undergo a rate-limiting conformational change to form protofibrils and then amyloid fibrils
[222]. This type of nucleation was observed, in particular, in the yeast prion protein (Sup35)
[223], among variants of the amyloid-β peptide
[224][225][226], SH3 domain
[227][228][229], Ure2p yeast prion
[230], polyglutamine (polyGln) peptides
[231], and lysozyme
[232][233].
5.1.2. Secondary Nucleation Mechanisms
Although conceptually appealing and observed in several instances, a simple homogeneous primary nucleation mechanism is not always observed
[149][223][234]. Several studies have pointed out that simple homogeneous nucleation could not fit certain experimental aggregation kinetics data
[235][236]. Simple homogeneous primary nucleation does not take into account other nucleation mechanisms and events, such as fibril-catalyzed secondary nucleation (a monomer-dependent process) and fibril fragmentation (a monomer-independent process) (
Figure 4), both contributing to the formation of new aggregation nuclei
[237][238][239][240][241]. In fibril-catalyzed secondary nucleation nucleus formation occurs on the surface of an already existing oligomer (
Figure 4). No foreign surface is involved in this type of nucleation as in the case of heterogeneous primary nucleation. This nucleation mechanism appears to be highly dependent on the structural compatibility of the amyloid precursor protein
[241].
Secondary nucleation has been inferred for several proteins, including amyloid-β peptides
[235][242], tau protein
[243], α-synuclein
[244][245], islet amyloid polypeptide (IAPP)
[246], insulin
[247], and bovine carbonic anhydrase
[248].
Amyloid fibril formation may also be seeded by the presence of pre-formed aggregates. In this case, the primary nucleation event is negligible, leading directly to the growth phase, the absence of secondary mechanisms, and the polymerization process is expected to follow a single exponential function
[249]. This is a consequence of the slower rate of primary nucleation when compared with the rate of addition of monomers onto an existing fibril (growth). This seeding process has been proposed to be an important factor in the propagation of the pathogenesis in most, if not all, amyloidoses
[250][251][252][253][254][255][256].
5.2. Aggregation Via a Nucleation-Independent Mechanism
The nucleation-independent mechanism of protein aggregation (
Figure 4 and
Scheme 2) is an isodesmic or linear polymerization mechanism and may be exemplified by the simplest possible model for the formation of spherical oligomers or linear multimers [
[152]]. This model is characterized by an infinite number of steps with identical rate constants (
k) independent of the size of the aggregate (
Scheme 2), resulting in an exponential polymerization curve with the absence of a lag phase. Once aggregation starts, the process undergoes downhill-polymerization. In this case, aggregation proceeds through a sequence of multiple energetically favorable steps, where the successive addition of amyloidogenic monomers to the growing aggregate is energetically favorable without the need of a multimeric nucleus.
Scheme 2. The sequential monomer (
M) addition mechanism for protein aggregation with identical equilibrium constants (
k) via a nucleation-independent pathway
[257][258].
Generally, seeding does not increase the aggregation rate in a downhill-polymerization process. However, this model disregards other aggregation processes that can change the number, size and shape of oligomeric species. For these reasons, this model sometimes predicts incorrect length distributions of amyloid fibrils at equilibrium. Nevertheless, this kinetic model has been used to investigate the effect of mutations on the rate of amyloid fibril formation
[259][260][261].
This aggregation mechanism was observed in transthyretin
[262][263], among variants of the amyloid-β peptide
[219][264][265], and also in the four-repeat domain of tau (Tau4RD)
[266], β2-microglobulin
[152], human and bovine serum albumins
[267][268], HypF-N
[269], FF domain
[270], human muscle acylphosphatase
[271], apolipoprotein C-II
[272][273], and several SH3 domains
[228][259][274][275][276].
5.3. The Energy Landscape View of Protein Aggregation
The protein folding energy landscape available to each polypeptide chain includes a wide range of different conformational states and a multitude of pathways en route to the folded state. The energy landscape in the case of short polypeptide sequences tends to be a smooth funnel-shaped surface where the polypeptide chain folds quickly towards a single folded state
[277]. On the other hand, larger proteins have rougher energy landscapes, with local minima and a population of intermediate states that eventually interconvert to the low energy folded state (
Figure 5)
[278][279].
Figure 5. Schematic representation of funnel-shaped energy landscapes for protein folding (non-amyloidogenic pathway, green) and aggregation (amyloidogenic pathway, red). The surface exhibits the roughness of the protein energy landscape showing the possible conformational states adopted by the polypeptide chain. Unfolded, partially unfolded and folded species may be implicated in the aggregation landscape, as long as they are prone to establish intermolecular interactions and aggregate, thereby resulting in the formation of amorphous aggregates, amyloid fibrils, and native-like aggregates, respectively. Adapted from references
[1][279][280].
At the top of the funnel, as depicted in
Figure 5, the unfolded state of the polypeptide chain has high Gibbs free energy and high conformational entropy. Upon polypeptide chain folding, the number of conformational states and thus the conformational entropy decreases. Concurrently, the hydrophobic collapse and the increase in the number of intramolecular contacts leads to a decrease in free energy toward the native state occupying the global free energy minimum, yielding the necessary conformational stability of the folded state. However, changes in amino acid sequence, and/or chemical or biological environment, including changes in pH, temperature, ionic strength, pressure, agitation, shear forces, interaction with surfaces and many other factors, may tip the energetic balance towards a different free energy minimum. This is highlighted in
Figure 5, where in parallel with the common folding funnel for a protein it is also depicted an aggregation funnel
[280][281][282]. Regarding the protein aggregation process, the funnel-shaped free energy surface is potentially rougher and more complex, since the energy landscape encodes not only the relative stability of unfolded states, partially unfolded states, and folded states, but also the relative stability of amorphous aggregates, β-sheet-rich amyloid fibrils, and native-like aggregates (
Figure 1 and
Figure 5).
Protein aggregation involves several processes that are interconnected, such as folding, unfolding and partial folding/unfolding, conformational changes, formation of intermolecular interactions, and fibril nucleation, elongation and stationary phases (
Figure 1 and
Figure 4)
[237][238][283][284][285]. There is well documented evidence that protein aggregation states may be formed not only by amyloidogenic intermediates but also by denatured and native states, with polypeptide chains establishing critical contacts with neighboring molecules through intermolecular interactions. Aggregated states are in general thermodynamically and kinetically favorable and it is a fine balance of forces that tip the processes towards a native soluble state or any type of aggregated state. It seems that most polypeptide chains under the right “stress” conditions tend to form extended β-sheet structures and thus amyloid aggregates
[14][284][286][287]. The energy landscape of protein systems forming large aggregates is described by numerous peaks corresponding to different conformational states, which is the case of amyloids due to the heterogeneity of fibrillar morphology. Even under the same experimental conditions, a large number of polymorphic fibrils with distinct morphologies might be formed at the same time, emphasizing the complexity and multiplicity of the aggregation pathways
[67][288]. The energy minimum of mature fibrils is deeper and sharper than the native state of a given protein (
Figure 5), as suggested by the high stability of the fibrillar state
[286][289][290].
Under specified conditions, a given polypeptide chain has its own folding and aggregation surface funnel. Each point on this surface expresses a specific and unique conformation of the protein. The profile of the energy landscape is affected by enthalpic contributions due to interactions between amino acids residues, and enthalpic and entropic contributions due to the interaction with the aqueous environment, as well as entropic contributions due to changes in conformational freedom of the polypeptide chain. The driving forces towards the low free energy state for both protein folding and aggregation are mainly hydrophobic in nature, with additional contributions from electrostatic and polar interactions, as well as hydrogen bonds
[14][280][281][287]. In the case of amyloid fibrils, the cross-β motif conformation is stabilized essentially by polar interactions due to intermolecular hydrogen bonds, and intermediate aggregated species are formed by intermolecular hydrophobic and electrostatic interactions
[284][291].
In the late 1990s
[292] amyloid-like aggregates or fibrils were found to be formed in vitro under specific experimental conditions by proteins entirely unrelated to well-established amyloid diseases
[293]. As we previously mentioned, under the right conditions, many if not most polypeptide chains may form amyloid. The term “amylome” was thus coined to describe all the proteins that can form amyloid-like fibrils
[294]. There are authors that consider that the fibrillar amyloid state represents a standard state that every polypeptide chain can adopt under appropriate conditions, and that this state is the thermodynamic ground state. In this sense, the amyloid fibrillar conformation would be the universal global free-energy minimum of any polypeptide chain
[295].