Identification and function of conformational dynamics in the multidomain GTPase dynamin.

Vesicle release upon endocytosis requires membrane fission, catalyzed by the large GTPase dynamin. Dynamin contains five domains that together orchestrate its mechanochemical activity. Hydrogen-deuterium exchange coupled with mass spectrometry revealed global nucleotide- and membrane-binding-dependent conformational changes, as well as the existence of an allosteric relay element in the α2(S) helix of the dynamin stalk domain. As predicted from structural studies, FRET analyses detect large movements of the pleckstrin homology domain (PHD) from a 'closed' conformation docked near the stalk to an 'open' conformation able to interact with membranes. We engineered dynamin constructs locked in either the closed or open state by chemical cross-linking or deletion mutagenesis and showed that PHD movements function as a conformational switch to regulate dynamin self-assembly, membrane binding, and fission. This PHD conformational switch is impaired by a centronuclear myopathy-causing disease mutation, S619L, highlighting the physiological significance of its role in regulating dynamin function. Together, these data provide new insight into coordinated conformational changes that regulate dynamin function and couple membrane binding, oligomerization, and GTPase activity during dynamin-catalyzed membrane fission.

Wbox2: A clathrin terminal domain-derived peptide inhibitor of clathrin-mediated endocytosis.

Clathrin-mediated endocytosis (CME) occurs via the formation of clathrin-coated vesicles from clathrin-coated pits (CCPs). Clathrin is recruited to CCPs through interactions between the AP2 complex and its N-terminal domain, which in turn recruits endocytic accessory proteins. Inhibitors of CME that interfere with clathrin function have been described, but their specificity and mechanisms of action are unclear. Here we show that overexpression of the N-terminal domain with (TDD) or without (TD) the distal leg inhibits CME and CCP dynamics by perturbing clathrin interactions with AP2 and SNX9. TDD overexpression does not affect clathrin-independent endocytosis or, surprisingly, AP1-dependent lysosomal trafficking from the Golgi. We designed small membrane-permeant peptides that encode key functional residues within the four known binding sites on the TD. One peptide, Wbox2, encoding residues along the W-box motif binding surface, binds to SNX9 and AP2 and potently and acutely inhibits CME.

Insights into the mechanisms of action of anti-Abeta antibodies in Alzheimer's disease mouse models.

A number of hypotheses regarding how anti-Abeta antibodies alter amyloid deposition have been postulated, yet there is no consensus as to how Abeta immunotherapy works. We have examined the in vivo binding properties, pharmacokinetics, brain penetrance, and alterations in Abeta levels after a single peripheral dose of anti-Abeta antibodies to both wild-type (WT) and young non-Abeta depositing APP and BRI-Abeta42 mice. The rapid rise in plasma Abeta observed after antibody (Ab) administration is attributable to prolongation of the half-life of Abeta bound to the Ab. Only a miniscule fraction of Ab enters the brain, and despite dramatic increases in plasma Abeta, we find no evidence that total brain Abeta levels are significantly altered. Surprisingly, cerebral spinal fluid Abeta levels transiently rise, and when Ab:Abeta complex is directly injected into the lateral ventricles of mice, it is rapidly cleared from the brain into the plasma where it remains stable. When viewed in context of daily turnover of Abeta, these data provide a framework to evaluate proposed mechanisms of Abeta attenuation mediated by peripheral administration of an anti-Abeta monoclonal antibody (mAb) effective in passive immunization paradigm. Such quantitative data suggest that the mAbs are either indirectly enhancing clearance of Abeta or targeting a low abundance aggregation intermediate.

Regulation of clathrin-mediated endocytosis by hierarchical allosteric activation of AP2.

The critical initiation phase of clathrin-mediated endocytosis (CME) determines where and when endocytosis occurs. Heterotetrameric adaptor protein 2 (AP2) complexes, which initiate clathrin-coated pit (CCP) assembly, are activated by conformational changes in response to phosphatidylinositol-4,5-bisphosphate (PIP2) and cargo binding at multiple sites. However, the functional hierarchy of interactions and how these conformational changes relate to distinct steps in CCP formation in living cells remains unknown. We used quantitative live-cell analyses to measure discrete early stages of CME and show how sequential, allosterically regulated conformational changes activate AP2 to drive both nucleation and subsequent stabilization of nascent CCPs. Our data establish that cargoes containing Yxxφ motif, but not dileucine motif, play a critical role in the earliest stages of AP2 activation and CCP nucleation. Interestingly, these cargo and PIP2 interactions are not conserved in yeast. Thus, we speculate that AP2 has evolved as a key regulatory node to coordinate CCP formation and cargo sorting and ensure high spatial and temporal regulation of CME.

Amyloid-beta aggregates formed at polar-nonpolar interfaces differ from amyloid-beta protofibrils produced in aqueous buffers.

The deposition of aggregated amyloid-beta (Abeta) peptides in the brain as senile plaques is a pathological hallmark of Alzheimer's disease (AD). Several lines of evidence indicate that fibrillar and, in particular, soluble aggregates of these 40- and 42-residue peptides are important in the etiology of AD. Recent studies also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we review our recent reports that Abeta(1-40) in vitro can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta(1-40) in low ionic strength buffers. These aggregates were quite stable and disaggregated to only a limited extent on dilution. A second class of soluble Abeta aggregates was generated at polar-nonpolar interfaces. Aggregation in a two-phase system of buffer over chloroform occurred more rapidly than in buffer alone. In buffered 2% hexafluoroisopropanol (HFIP), microdroplets of HFIP were formed and the half-time for aggregation was less than 10 minutes. Like Abeta protofibrils, these interfacial aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. However, electron microscopy and atomic force microscopy revealed very different morphologies. The HFIP aggregates formed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP these aggregates initially were very unstable and disaggregated completely within 2 minutes. However, their stability increased as they progressed to fibers. It is important to determine whether similar interfacial Abeta aggregates are produced in vivo.

Amyloid-beta(1-42) rapidly forms protofibrils and oligomers by distinct pathways in low concentrations of sodium dodecylsulfate.

Alzheimer's disease (AD) is characterized by large numbers of senile plaques in the brain that consist of fibrillar aggregates of 40- and 42-residue amyloid-beta (Abeta) peptides. However, the degree of dementia in AD correlates better with the concentration of soluble Abeta species assayed biochemically than with histologically determined plaque counts, and several investigators now propose that soluble aggregates of Abeta are the neurotoxic agents that cause memory deficits and neuronal loss. These endogenous aggregates are minor components in brain extracts from AD patients and transgenic mice that express human Abeta, but several species have been detected by gel electrophoresis in sodium dodecylsulfate (SDS) and isolated by size exclusion chromatography (SEC). Endogenous Abeta aggregation is stimulated at cellular interfaces rich in lipid rafts, and anionic micelles that promote Abeta aggregation in vitro may be good models of these interfaces. We previously found that micelles formed in dilute SDS (2 mM) promote Abeta(1-40) fiber formation by supporting peptide interaction on the surface of a single micelle complex. In contrast, here we report that monomeric Abeta(1-42) undergoes an immediate conversion to a predominant beta-structured conformation in 2 mM SDS which does not proceed to amyloid fibrils. The conformational change is instead rapidly followed by the near quantitative conversion of the 4 kDa monomer SDS gel band to 8-14 kDa bands consistent with dimers through tetramers. Removal of SDS by dialysis gave a shift in the predominant SDS gel bands to 30-60 kDa. While these oligomers resemble the endogenous aggregates, they are less stable. In particular, they do not elute as discrete species on SEC, and they are completed disaggregated by boiling in 1% SDS. It appears that endogenous oligomeric Abeta aggregates are stabilized by undefined processes that have not yet been incorporated into in vitro Abeta aggregation procedures.

Secondary structure and interfacial aggregation of amyloid-beta(1-40) on sodium dodecyl sulfate micelles.

Alzheimer's disease (AD) is characterized by the presence of large numbers of fibrillar amyloid deposits in the form of senile plaques in the brain. The fibrils in senile plaques are composed of 40- and 42-residue amyloid-beta (Abeta) peptides. Several lines of evidence indicate that fibrillar Abeta and especially soluble Abeta aggregates are important in the pathogenesis of AD, and many laboratories have investigated soluble Abeta aggregates generated from monomeric Abeta in vitro. Of these in vitro aggregates, the best characterized are called protofibrils. They are composed of globules and short rods, show primarily beta-structure by circular dichroism (CD), enhance the fluorescence of bound thioflavin T, and readily seed the growth of long fibrils. However, one difficulty in correlating soluble Abeta aggregates formed in vitro with those in vivo is the high probability that cellular interfaces affect the aggregation rates and even the aggregate structures. Reports that focus on the features of interfaces that are important in Abeta aggregation have found that amphiphilic interactions and micellar-like Abeta structures may play a role. We previously described the formation of Abeta(1-40) aggregates at polar-nonpolar interfaces, including those generated at microdroplets formed in dilute hexafluoro-2-propanol (HFIP). Here we compared the Abeta(1-40) aggregates produced on sodium dodecyl sulfate (SDS) micelles, which may be a better model of biological membranes with phospholipids that have anionic headgroups. At both HFIP and SDS interfaces, changes in peptide secondary structure were observed by CD immediately when Abeta(1-40) was introduced. With HFIP, the change involved an increase in predominant beta-structure content and in fluorescence with thioflavin T, while with SDS, a partial alpha-helical conformation was adopted that gave no fluorescence. However, in both systems, initial amorphous clustered aggregates progressed to soluble fibers rich in beta-structure over a roughly 2 day period. Fiber formation was much faster than in the absence of an interface, presumably because of the close intermolecular proximity of peptides at the interfaces. While these fibers resembled protofibrils, they failed to seed the aggregation of Abeta(1-40) monomers effectively.

Rapid assembly of amyloid-beta peptide at a liquid/liquid interface produces unstable beta-sheet fibers.

Accumulation of aggregated amyloid-beta peptide (Abeta) in the brain is a pathological hallmark of Alzheimer's disease (AD). In vitro studies indicate that the 40- to 42-residue Abeta peptide in solution will undergo self-assembly leading to the transient appearance of soluble protofibrils and ultimately to insoluble fibrils. The Abeta peptide is amphiphilic and accumulates preferentially at a hydrophilic/hydrophobic interface. Solid surfaces and air-water interfaces have been shown previously to promote Abeta aggregation, but detailed characterization of these aggregates has not been presented. In this study Abeta(1-40) introduced to aqueous buffer in a two-phase system with chloroform aggregated 1-2 orders of magnitude more rapidly than Abeta in the buffer alone. The interface-induced aggregates were released into the aqueous phase and persisted for 24-72 h before settling as a visible precipitate at the interface. Thioflavin T fluorescence and circular dichroism analyses confirmed that the Abeta aggregates had a beta-sheet secondary structure. However, these aggregates were far less stable than Abeta(1-40) protofibrils prepared in buffer alone and disaggregated completely within 3 min on dilution. Atomic force microscopy revealed that the aggregates consisted of small globules 4-5 nm in height and long flexible fibers composed of these globules aligned roughly along a longitudinal axis, a morphology distinct from that of Abeta protofibrils prepared in buffer alone. The relative instability of the fibers was supported by fiber interruptions apparently introduced by brief washing of the AFM grids. To our knowledge, unstable aggregates of Abeta with beta-sheet structure and fibrous morphology have not been reported previously. Our results provide the clearest evidence yet that the intrinsic beta-sheet structure of an in vitro Abeta aggregate depends on the aggregation conditions and is reflected in the stability of the aggregate and the morphology observed by atomic force microscopy. Resolution of these structural differences at the molecular level may provide important clues to the further understanding of amyloid formation in vivo.

Amyloid-beta protofibrils differ from amyloid-beta aggregates induced in dilute hexafluoroisopropanol in stability and morphology.

The brains of Alzheimer's disease (AD) patients contain large numbers of amyloid plaques that are rich in fibrils composed of 40- and 42-residue amyloid-beta (Abeta) peptides. Several lines of evidence indicate that fibrillar Abeta and especially soluble Abeta aggregates are important in the etiology of AD. Recent reports also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we demonstrate that Abeta-(1-40) can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta-(1-40) at low ionic strength. Dilution of these aggregation reactions induced disaggregation to monomers as measured by size exclusion chromatography. Protofibril concentrations monitored by thioflavin T fluorescence decreased in at least two kinetic phases, with initial disaggregation (rate constant approximately 1 h(-1)) followed by a much slower secondary phase. Incubation of the reactions without agitation resulted in less disaggregation at slower rates, indicating that the protofibrils became progressively more stable over time. In fact, protofibrils isolated by size exclusion chromatography were completely stable and gave no disaggregation. A second class of soluble Abeta aggregates was generated rapidly (<10 min) in buffered 2% hexafluoroisopropanol (HFIP). These aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. Electron microscopy and atomic force microscopy revealed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP, these aggregates initially were very unstable and disaggregated completely within 2 min. However, their stability increased as they progressed to fibers. Relative to Abeta protofibrils, the HFIP-induced aggregates seeded elongation by Abeta monomer deposition very poorly. The techniques used to distinguish these two classes of soluble Abeta aggregates may be useful in characterizing Abeta aggregates formed in vivo.

Nordihydroguaiaretic acid does not disaggregate beta-amyloid(1-40) protofibrils but does inhibit growth arising from direct protofibril association.

Nordihydroguaiaretic acid (NDGA) was observed by Ono et al. (J Neurochem 87:172-181, 2002) to decrease the fluorescence of thioflavin T associated with freshly extended amyloid beta-protein (Abeta) fibrils. They concluded that NDGA could disaggregate Abeta fibrils into aggregates that were larger than monomers or oligomers and did not bind thioflavin T. Such an effect could be of therapeutic importance in the treatment of Alzheimer's disease. In the current study, we confirmed that NDGA induces a decrease in the fluorescence of thioflavin T associated with Abeta(1-40) fibrils and extended this observation to Abeta(1-40) protofibrils. However, attempts to identify protofibril disaggregation products using dynamic light scattering, electron microscopy, and size exclusion chromatography failed to demonstrate any decrease in aggregate size or concentration or a parallel increase in Abeta monomers or small oligomers when protofibrils were incubated with excess NDGA. We propose instead that the decreases in thioflavin T fluorescence resulted from either displacement or conformational alteration of thioflavin T upon the binding of NDGA to these aggregates. In fact, the same equilibrium fluorescence values were observed regardless of the order in which NDGA, thioflavin T, and Abeta protofibrils were added to the incubation. Although NDGA failed to disaggregate Abeta protofibrils, it did inhibit direct protofibril-protofibril association but did not alter protofibril elongation by monomer addition. These results suggest that NDGA might bind along the lateral surface of Abeta protofibrils. In addition, the binding of NDGA to Abeta protofibrils increased their nonspecific adherence to Superdex 75 resin and diminished their effects on cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide.

The peptide KLVFF-K(6) promotes beta-amyloid(1-40) protofibril growth by association but does not alter protofibril effects on cellular reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT).

The peptide KLVFF-K6 was observed by Lowe et al. to simultaneously enhance amyloid beta-protein (Abeta) fibrillogenesis and decrease cellular toxicity, as measured in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay. It was postulated that accelerated Abeta aggregation and precipitation induced by KLVFF-K6 may lead to an increase in less toxic insoluble fibrils at the expense of more toxic soluble protofibrils. In a previous study, we distinguished between two modes of protofibril growth: elongation by monomer deposition and direct protofibril-protofibril association. These growth mechanisms could be resolved by varying Abeta monomer and NaCl concentrations. Using assays designed to isolate these distinct modes of protofibril growth, we report here that larger Abeta aggregates formed in the presence of KLVFF-K6 resulted from enhanced protofibril association. 3H-Radiomethylated KLVFF-K6 bound to associated protofibrils with an apparent Kd of 180 nM, and concentrations of free [3H]KLVFF-K6 in this range were sufficient to convert soluble protofibrils to sedimentable fibrils. However, promotion of Abeta protofibril association by KLVFF-K6 had no effect on Abeta-induced decreases in cellular MTT reduction. Therefore, our data do not support the proposal that insoluble fibrils formed with KLVFF-K6 are less toxic than soluble protofibrils. KLVFF-K6 did not alter rates of protofibril elongation by monomer deposition. In contrast, when added to Abeta monomers isolated with the use of size-exclusion chromatography, KLVFF-K6 inhibited fibrillogenesis, as measured by thioflavin T fluorescence, and this inhibition was paralleled by a failure to alter cellular MTT reduction.

Growth of beta-amyloid(1-40) protofibrils by monomer elongation and lateral association. Characterization of distinct products by light scattering and atomic force microscopy.

Amyloid plaques in brain tissue are a hallmark of Alzheimer's disease. Primary components of these plaques are 40- and 42-residue peptides, denoted A beta(1-40) and A beta(1-42), that are derived by proteolysis of cellular amyloid precursor protein. Synthetic A beta(1-40) and A beta(1-42) form amyloid fibrils in vitro that share many features with the amyloid in plaques. Soluble intermediates in A beta fibrillogenesis, termed protofibrils, have been identified previously, and here we describe the in vitro formation and isolation of A beta(1-40) protofibrils by size exclusion chromatography. In some experiments, the A beta(1-40) was radiomethylated to better quantify various A beta species. Mechanistic studies clarified two separate modes of protofibril growth, elongation by monomer deposition and protofibril-protofibril association, that could be resolved by varying the NaCl concentration. Small isolated protofibrils in dilute Tris-HCl buffers were directed along the elongation pathway by addition of A beta(1-40) monomer or along the association pathway by addition of NaCl. Multi-angle light scattering analysis revealed that protofibrils with initial molecular masses M(w) of (7-30) x 10(3) kDa grew to M(w) values of up to 250 x 10(3) kDa by these two growth processes. However, the mass per unit length of the associated protofibrils was about 2-3 times that of the elongated protofibrils. Rate constants for further elongation by monomer deposition with the elongated, associated, and initial protofibril pools were identical when equal number concentrations of original protofibrils were compared, indicating that the original number of protofibril ends had not been altered by the elongation or association processes. Atomic force microscopy revealed heterogeneous initial protofibrils that became more rodlike following the elongation reaction. Our data indicate that protofibril elongation in the absence of NaCl results from monomer deposition only at the ends of protofibrils and proceeds without an increase in protofibril diameter. In contrast, protofibril association occurs in the absence of monomer when NaCl is introduced, but this association involves lateral interactions that result in a relatively disordered fibril structure.