Disease-Modifying Targets in Neurodegenerative Disorders. Paving the Way for Disease-Modifying Therapies

307 Pages·2017·12.573 MB·English

by  Veerle Baekelandt and Evy Lobbestael (Eds.)

Disease-Modifying Targets in
Neurodegenerative Disorders
Paving the Way for
Disease-Modifying Therapies
Edited by
Veerle Baekelandt
Evy Lobbestael
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List of Contributors
Matthew Bird
Institute of Neuroscience, Catholic University of Louvain, Brussels, Belgium
Christel Claes
KU Leuven Stem Cell Institute; VIB Center for the Biology of Disease, Leuven, Belgium
Mark R. Cookson
National Institute on Aging, National Institutes of Health, Bethesda, MD, United States
Marc Cruts
VIB Center of Molecular Neurology; Institute Born–Bunge, University of Antwerp, Antwerp,
Belgium
Nicole Déglon
Neuroscience Research Center (CRN); Lausanne University Hospital (CHUV), Lausanne,
Switzerland
Marion Delenclos
Jacksonville, FL, United States
Ilse Dewachter
Institute of Neuroscience, Catholic University of Louvain, Brussels; University of Hasselt, Belgium
Richard Gordon
The University of Queensland, St. Lucia, QLD, Australia
Sarah Libbrecht
KU Leuven, Leuven, Belgium
Pamela J. McLean
Jacksonville, FL, United States
Ronald Melki
Paris–Saclay Institute of Neuroscience, French National Center for Scientific Research,
University of Paris–Saclay, Gif-sur-Yvette, France
Simon Moussaud
Jacksonville, FL, United States
Tiago F. Outeiro
Institute of Molecular Medicine, University of Lisbon, Lisboa, Portugal; Center for Nanoscale
Microscopy and Molecular Physiology of the Brain, University Medical Center Goettingen,
Goettingen, Germany
Wouter Peelaerts
KU Leuven, Leuven, Belgium
Kristel Sleegers
VIB Center of Molecular Neurology; Institute Born–Bunge, University of Antwerp, Antwerp,
Belgium
xi
xii List of Contributors
Ilie-Cosmin Stancu
Institute of Neuroscience, Catholic University of Louvain, Brussels, Belgium
Joke Terryn
KU Leuven Stem Cell Institute; VIB Vesalius Research Center; University Hospitals Leuven,
Leuven, Belgium
Dick Terwel
reMYND NV, Leuven, Belgium
Christine Van Broeckhoven
VIB Center of Molecular Neurology; Institute Born–Bunge, University of Antwerp, Antwerp,
Belgium
Ludo Van Den Bosch
Leuven Research Institute for Neuroscience and Disease (LIND), KU Leuven; VIB, Center for
Brain & Disease Research, Leuven, Belgium
Bruno Vasconcelos
Institute of Neuroscience, Catholic University of Louvain, Brussels, Belgium
Catherine M. Verfaillie
KU Leuven Stem Cell Institute, Leuven, Belgium
Eline Wauters
VIB Center of Molecular Neurology; Institute Born–Bunge, University of Antwerp, Antwerp,
Belgium
Trent M. Woodruff
The University of Queensland, St. Lucia, QLD, Australia
CHAPTER
1
THE MULTITUDE OF
THERAPEUTIC TARGETS
IN NEURODEGENERATIVE
PROTEINOPATHIES
Ronald Melki
Paris–Saclay Institute of Neuroscience, French National Center for Scientific Research (CNRS),
University of Paris–Saclay, Gif-sur-Yvette, France
CHAPTER OUTLINE
Protein Misfolding and Aggregation ...........................................................................................................1
Mechanism of Assembly ............................................................................................................................2
Prion-Like Propagation of Protein Assemblies ............................................................................................4
Propagation Routes of Pathogenic Protein Aggregates ................................................................................5
Limiting Steps in the Propagation of Pathogenic Protein Assemblies ............................................................6
Therapeutic Strategies Targeting the Physiological Levels of Aggregation-Prone
Proteins Involved in Neurodegeneration .....................................................................................................7
Therapeutic Strategies Targeting the Misfolding and Aggregation of Proteins
Involved in Neurodegeneration ..................................................................................................................7
Therapeutic Strategies Targeting the Accumulation of Misfolded Pathologic Protein Aggregates ...................8
Therapeutic Strategies Targeting the Cell-to-Cell Propagation of Pathogenic Protein Aggregates ..................9
Therapeutic Strategies Aimed at Restoring the Damage Pathogenic Protein Aggregates Induce .....................9
Therapeutic Strategies Targeting Misfolded Pathologic Protein
Aggregate–Mediated Neuroinflammation ..................................................................................................10
Limitations of the Different Therapeutic Strategies ....................................................................................11
Acknowledgments ...................................................................................................................................12
References .............................................................................................................................................12
PROTEIN MISFOLDING AND AGGREGATION
Within our cells, proteins are folded to be functional. Following or concomitant to synthesis, with or
without the assistance of molecular chaperones, newly synthesized proteins acquire native folds [1].
During this process, polypeptides populate billions of folding intermediates, a fraction of which cannot
convert into functional proteins [2,3]. In addition, the folded state of functional proteins is not static.
Indeed, proteins oscillate between different shapes depending on, for example, their enzymatic activity
Disease-Modifying Targets in Neurodegenerative Disorders. 1
Copyright © 2017 Elsevier Inc. All rights reserved.
2 CHAPTER 1 THE MULTITUDE OF THERAPEUTIC TARGETS
or interaction with partner proteins and populate nonnative conformations that are trapped thermody­
namically in a misfolded state [4]. Some misfolded proteins or their degradation products have the
ability to interact and coalesce into intra­ or extracellular protein clumps, a fraction of which resembles
orderly elongated aggregates that are named amyloid fibrils [5]. These fibrillar protein clumps are the
common hallmark of Parkinson’s disease (PD) and other synucleinopathies (which are delineated in
Chapters 2, 6, and 7), Alzheimer’s disease (which is introduced in Chapter 5), amyotrophic lateral
sclerosis (Chapter 10), Creutzfeldt–Jakob, Huntington’s disease (Chapter 9), and other dementias [6,7]
(Chapter 8).
The diseases caused by misfolded proteins aggregates are either the consequence of the loss of
function of proteins following trapping into irreversible aggregates or of a gain of pathologic function
of the macromolecular assembly made of misfolded proteins [8–10]. Misfolded proteins aggregates
can indeed: (1) permeabilize the plasma membrane and/or intracellular membranous compartments,
(2) perturb membrane protein dynamics and distribution, (3) constitute novel pathogenic signaling plat­
forms, and (4) trap significant amounts of molecular chaperones and other partner proteins. In addition,
as misfolded protein aggregates recruit the soluble form of their constituting proteins, they grow in size
and amplify when they break into pieces.
MECHANISM OF ASSEMBLY
Under physiological pH, salt, and temperature conditions, a variety of proteins, or domains within
proteins, whose intrinsic structures are ill defined (i.e, they adopt many conformations in equilibrium)
self­associate into assemblies that can elongate by incorporation of additional monomers. Among these
proteins are the prion protein (PrP); the amyloid β (Aβ) peptide; the proteins Tau and α­synuclein
(α­syn), whose aggregation process is detailed in Chapter 2; huntingtin exon 1 (HTTExon1) with a
pathologic polyQ stretch; mutant superoxide dismutase (SOD1); and TAR DNA­binding protein 43
(TDP43). These assemblies grow in an unlimited manner, as the incorporation of a monomer creates
a new binding site for another monomer [11]. The early assemblies, considered as the precursors of
fibrils, are unstable, as they result from the transient longitudinal or lateral interactions between protein
molecules in the same conformation. When longitudinal and lateral interactions between polypeptides
in the same conformation are established, the intermolecular interactions outweigh the entropic energy
in solution under physiological conditions, yielding nearly irreversible seeds unless a destabilizing
irreversible reaction, such as nucleotide hydrolysis, occurs within the assemblies [11]. The latter can
grow indefinitely by the incorporation of protein molecules at their ends, as the binding of an additional
nonnative monomer to the seed ends generates an incorporation site for another subunit. The rate limit­
ing step in a nonnative polypeptide aggregation is, therefore, the formation of stable seeds. Brownian
movement [12] and severing and/or disaggregating factors [13–15] generate increased numbers of ends
and increase the likelihood of new subunits being incorporated. As the numerous folding intermedi­
ates are in equilibrium, a given conformer recruitment by seed or fibril ends displaces the equilibrium
toward repopulating this precise conformer. This leads ultimately to a near complete aggregation of the
protein. This cooperative aggregation process is schematized in Fig. 1.1.
MECHANISM OF ASSEMbLy 3
FIGURE 1.1 Model for Protein Misfolding and Aggregation into Fibrillar Assemblies that can Grow Indefinitely
Native or natively unfolded polypeptides (sphere) undergo conformational changes that lead to an abnormal
assembly­competent conformation (cube). The latter form is in equilibrium with the native or natively unfolded
polypeptide. The different surfaces of the abnormal, assembly­competent form are represented (in six different
colors). The abnormal form has the ability to interact transiently either longitudinally (Lon) or laterally (Lat) with
another molecule in the same conformation. Here the longitudinal interaction occurs between the surfaces colored
yellow and blue and the lateral interactions occur between two surfaces colored green. As long as the interactions
are only longitudinal or lateral, the oligomers remain unstable and dissociate because the intermolecular interactions
do not outweigh the entropic cost of binding. Once longitudinal and lateral interactions have been established
between abnormal assembly­competent protein molecules, stable seeds are formed. Seed growth is unlimited by the
incorporation of molecules in the same conformation, yielding fibrils. The fibrils can break into smaller fragments
because, among other things, of Brownian movement. The rate of breakage depends on the number of bonds
established between the molecules. Each resulting fragment acts as a seed. The fibrils represented here have distinct
surfaces: red, white, and black. They interact through these defined surfaces with protein partners or lipids.
4 CHAPTER 1 THE MULTITUDE OF THERAPEUTIC TARGETS
PRION-LIKE PROPAGATION OF PROTEIN ASSEMBLIES
Until recently, the spread and transmission of disease via misfolded protein aggregates was thought
to be restricted to PrP [16]. A wide range of recent experimental evidence suggest that other protein
aggregates that are the hallmarks of major neurodegenerative diseases spread in a prion­like manner
[7,17–19]. The first evidence for the transmission of misfolded protein aggregates not involving PrP in
man came from the observation that the injection of brain homogenates containing amyloid plaques,
whose main constituent is the aggregated form of the Aβ peptide, into the brains of model mice induced
lesions characteristic of Alzheimer’s disease [20,21], while no such lesions where observed when con­
trol brain extracts lacking protein aggregates were injected. The induced lesions, initially confined to
the injected brain region, propagated to neighboring and/or axonally connected areas over time. The
intraperitoneal injection of brain homogenates containing amyloid plaques also induced cerebral le­
sions characteristic of Aβ deposits [22]. Finally, the insertion of stainless steel needles preincubated in
brain homogenates containing aggregated Aβ also triggered plaque formation in recipient animals [23]
in a manner similar to what was observed decades before when stainless steel needles contaminated by
the infectious form of the PrP were inserted in the brain of recipient animals [24].
The first evidence supporting the propagation of aggregated α­syn from cell to cell came from the
finding that Lewy bodies from PD brains invade grafts of fetal dopaminergic neurons transplanted into
these brains, years after transplantation [25,26]. Intercellular transfer of α­syn has subsequently been
demonstrated in cultured cells [27–30]. Aggregated α­syn assemblies were shown to be taken up by neu­
rons and transported anterogradely and retrogradely in vitro and in vivo [31–33]. Injections of recombi­
nant α­syn and brain homogenates of transgenic mice or patients in transgenic and wild type mice, rats,
and nonhuman primates clearly showed spreading and acceleration of α­syn pathology, with significant
differences in the nature of the pathologic α­syn species, the time frame, and robustness of the seeding/
amplification reported between different studies [30,34–39]. α­Syn assemblies amplify when injected
within the brain [35,37,39] and reach the central nervous system (CNS) where they amplify by seeding
the aggregation of soluble α­syn when injected in the muscle [38] or the blood stream [40].
Recent in vitro and in vivo studies have shown that Tau aggregates seed–soluble Tau aggregation
within neurons [41–44]. Tau assemblies spread to neighboring cells and functionally connected brain
regions [41,45]. Tau transsynaptic transfer has been shown, for instance, to allow the spread of neurofi­
brillary degeneration in a hierarchical pattern along existing neural networks [46–48]. Tau fibrils made
in vitro were shown to trigger Tau aggregation in cultured cells [49] and in Tau transgenic mice [50],
and the intracerebral injections of brain extracts from various human tauopathies have shown that Tau
lesions in mice can be induced to resemble those in the corresponding human diseases [51].
Similarly, huntingtin with expanded polyQ stretches aggregates have been shown to transfer to
neighboring cells and seed misfolding of the wild type protein in vitro [52–54] and in vivo [55–57],
TDP43 pathologic aggregates from patient brains revealed capable seeding the aggregation of soluble
TDP43 in vitro [58]. Finally, mutant SOD1 aggregation was demonstrated to be self­perpetuating, and
the aggregate shown to propagate from cell to cell [59].
These and other data support the idea that neurodegenerative diseases associated with protein
misfolding and aggregation could propagate in a prion­like manner. This process is schematized in
Fig. 1.2. Furthermore, considering that these diseases follow anatomical pathways for their propagation
in the brain, it is tempting to speculate on the existence of common spreading mechanisms of different
proteinaceous aggregates that might contribute to the progression of neurodegeneration.
PROPAGATION ROUTES OF PATHOGENIC PROTEIN AGGREGATES 5
FIGURE 1.2 Schematic Representation of the Intercellular Propagation of Protein Assemblies
Misfolded pathologic protein assemblies (stars) can be released from neurons (colored red) after anterograde
or retrograde active transport or from astrocytes (colored blue) in the medium and taken up by naïve neurons or
astrocytes. They can also move from neuron to neuron or neuron to astrocytes, and vice versa, through cell­to­
cell contacts. While propagating, misfolded pathologic protein assemblies grow and multiply by recruitment of
misfolded protein monomers.
PROPAGATION ROUTES OF PATHOGENIC PROTEIN AGGREGATES
For extracellular assemblies of insoluble nonnative proteins to come in contact with the cytoplasm,
where they can amplify, they must cross a lipid bilayer. Biological membranes are selectively perme­
able to ions and small solutes, such as sugars and amino acids, but are generally impermeable to large
polar macromolecules. The only known nonvesicular mechanisms for protein translocation across elec­
trically tight bilayers are either ABC transporters [60] or translocons [61] that only can accommodate
completely unfolded monomeric polypeptides. Given the high­thermodynamic stability of the protein
assemblies involved in disease, the possibility that aggregates disassemble outside the cell, cross the
membrane as monomers or oligomers, and reassemble inside the cell is excluded. Thus, one needs to
consider that pathologic protein assemblies enter the cells either by nonconventional cellular mecha­
nisms, such as cell membrane puncturing or hijacking of existing cellular processes.
The observation of “unroofed” cells that had been exposed briefly to fibrillar polyQ assemblies by
deep­etch transmission electron microscopy revealed the existence of fibrils, neither surrounded by
endomembranous structures nor by clathrin, that were attached to the inner surface of the plasma mem­
brane [52]. This strongly suggests that amyloid fibrils can reach the cytosol after the physical breaching
of the plasma membrane. This, together with the passive release of cytosolic aggregates upon cell death
within the extracellular milieu, may contribute to the cell­to­cell propagation of protein aggregates as­
sociated with neurodegenerative diseases [17].
6 CHAPTER 1 THE MULTITUDE OF THERAPEUTIC TARGETS
Proteins and larger macromolecular assemblies, including viruses, enter and leave eukaryotic cells
through encapsulation within and fusion of vesicles, for example via endocytosis and phagocytosis.
Cells secrete membrane vesicles in the extracellular space [62–64]. These extracellular vesicles com­
prise exosomes (∼30–100 nm), which originate from the fusion of multivesicular bodies with the
plasma membrane. They also secrete microvesicles, or ectosomes (∼100–1000 nm), which form di­
rectly from the plasma membrane [62–64]. These extracellular vesicles act as a interneuronal commu­
nication route [65] and as vehicles for the intercellular transfer of nucleic acids, signaling molecules,
and pathogenic factors [66,67] and clearance/dilution of these vehicles. Evidence for the presence of
proteins involved in neurodegenerative diseases within extracellular vesicles has been documented
[68–70]. Thus, they may play a role in cell­to­cell transfer of protein aggregates associated with neu­
rodegenerative diseases [71]. Interestingly and in agreement with this, in many cases, the propagation
of pathologic protein aggregates from one cell to another does not require contact between cells but
includes extracellular release by affected cells and take­up by naïve cells [30,31,59].
Evidence for the presence of pathologic protein aggregates within dynamic filamentous actin­
containing bridges, named tunneling nanotubes (TNTs) [72] that directly connect the cytoplasm of
distant cells, from immune to neuronal cells exists. These long­range intercellular communication
routes selectively transfer a wide variety of cellular materials, such as cytoplasmic macromolecules,
plasma membrane components, vesicles, and organelles [73]. TNTs were shown to allow the passage
of infectious prions particles and that of HTTExon1, α­syn and Tau aggregates from affected donor
to naïve recipient cells [74–77], and may contribute to the spread of pathogenic protein assemblies in­
volved in neurodegenerative diseases. Other yet to be described cellular processes may also contribute
to the uptake and propagation of protein aggregates involved in neurodegenerative diseases.
LIMITING STEPS IN THE PROPAGATION OF PATHOGENIC
PROTEIN ASSEMBLIES
The binding and docking of naked or membrane­encapsulated misfolded pathogenic protein assem­
blies to the plasma membrane are not only rate limiting steps in their prion­like propagation process
but also key steps that define their tropism for specific cell types within the CNS. Their penetration into
neurons controls the amplification of aggregates by recruitment of the endogenous soluble form of their
constituting proteins and disease progression.
A number of proteins that are prone to aggregate into pathogenic assemblies have been shown to
sense membrane curvature [78] and alter the distribution of lipid molecules within the membrane to
the extent where it is remodeled and/or disrupted [79]. Membrane­bound naked misfolded pathogenic
protein assemblies impinge on the physiologic distribution of membrane proteins and lipids fluidity,
thus inducing the reorganization of membrane components and properties. Upon encountering/binding
the plasma membrane of neurons, astrocytes, oligodendrocytes, or glial cells, the diffusion of naked
misfolded pathogenic protein assemblies is “restricted” to a two­dimensional space. Bound pathogenic
protein assemblies diffuse in the plane of the plasma membrane. Their lateral diffusion favors the for­
mation of clusters that affects the overall distribution of extracellularly exposed membranous proteins
through a selective recruitment process. This may not only lead to membrane lipid bilayer curving and/
or rupture following molecular crowding, but also to the formation of platforms with undesirable prop­
erties, including calcium signaling [80,81]. Indeed, the increased confinement of pathogenic aggregates