where the precursor liquid phase can be either
the dilute solution of macromolecules or the
dense phase formed via LLPS.
Many proteins associated with neurode-
generation, including tau,a-synuclein, TDP-43,
hnRNPA1, TIA1, and FUS, are capable of fibril
formation in vitro ( 1 – 7 ). Although this process
is often referred to as aggregation, phase tran-
sitions and aggregation are distinct phenomena:
Whereas fibril formation is mediated by homo-
typic interactions and governed by the princi-
ples of phase transitions, aggregation refers to
the sticking of molecules to one another, un-
constrained by concentration thresholds or
accompanied by symmetry breaking. This dis-
tinction between pathological phase transi-
tion and aggregation is more than academic
because it is informative with respect to how
such fibrils may arise in a pathological context,
the cellular processes that may be disturbed by
pathological phase transitions, and how such
pathology may be reversed.
Beginning in 2015, it was appreciated that
many neurodegenerative disease-related pro-
teins not only assemble into fibrillar solids
but also undergo LLPS to form liquid droplets.
This phenomenon was first illustrated for
hnRNPA1 ( 8 , 9 ), TDP-43 ( 8 ), and FUS ( 10 , 11 ).
These results indicated that many disease
proteins exhibit distinct concentration thresh-
olds that correspond to the onset of two types
of phase transitions: one threshold for LLPS,
and a higher threshold for liquid-to-solid phase
transition ( 8 , 10 ). It was also shown that the
liquid-to-solid phase transition can be enhanced
within the liquid phase ( 8 , 10 ). Similar observa-
tions have been made for tau ( 12 ),a-synuclein
( 13 ), huntingtin ( 14 ), and TIA1 ( 7 ). These ob-
servations have highlighted two distinct routes
to fibril formation (Fig. 1). Fibril formation may
be initiated by a combination of primary and
secondary nucleation ( 15 ) in dilute solution.
Alternatively, fibril formation may occur via
liquid-to-solid phase transition within the dense
liquid phase. In the latter case, the condensed
liquid state that arises from LLPS facilitates
fibril formation by concentrating proteins and
enabling the crossing of the threshold con-
centration for liquid-to-solid phase transition.
These routes are not mutually exclusive and
might differ for different proteins and different
contexts (e.g., in vitro versus in living cells).
Neurodegenerative disease–causing muta-
tions in the low-complexity domains (LCDs)
of hnRNPA1 ( 6 ), hnRNPA2B1 ( 6 ), TDP-43 ( 16 ),
FUS ( 10 , 11 ), and TIA1 ( 7 ) are known to reduce
the concentration threshold for LLPS. These
mutations can also reduce the threshold for
liquid-to-solid phase transitions within dense
liquid phases, giving rise to pathological phase
transitions. Likewise, it was recently shown
that disease-causing mutations ina-synuclein
also alter the threshold for liquid-to-solid phase
transition ( 13 ).
The biophysical properties of proteins can
be regulated by local changes in the cellular
milieu (e.g., pH) or chemical modifications.
Indeed, a number of posttranslational mod-
ifications associated with disease in these
proteins also reduce the threshold for phase
transitions driven by homotypic interactions
that promote fibril formation ( 12 , 13 , 17 ).Neurodegenerative disease proteins are
constituents of complex condensates
A common feature of purified disease proteins
is their ability to undergo phase transitions
driven by homotypic interactions, and such
phase transitions are promoted by disease-
causing mutations. However, the situation is
far more complex in living cells. Moreover, the
degeneration of neurons cannot be explained
solely by fibril formation by a disease protein.
Rather, understanding the pathogenesis of
neurodegeneration requires consideration of
underlying cellular processes that are corrupted
over time. Notably, many proteins associated
with neurodegeneration reside primarily within
cellular bodies known as biomolecular con-
densates that assemble via phase separation
and encompass hundreds of distinct protein and
nucleic acid components ( 13 , 18 – 22 ) (Fig. 2).
Biomolecular condensates are distinct from
simple liquid droplets or solids formed via
phase transitions mediated by homotypic in-
teractions of specific proteins. Instead, they
form and dissolve via reversible phase transi-
tions of multicomponent systems that are con-
trolled by dynamic networks of homotypic
and heterotypic interactions ( 23 ).
In cells, biomolecular condensation provides
spatial and temporal control over cellular com-
ponents and biochemical reactions ( 24 ). A
plethora of condensates are found in cells
spanning a vast range of sizes and composi-
tions. For example, the central channels of nu-
clear pores are small condensates composed of
a few different biomolecules, whereas ribonu-
cleoprotein (RNP) granules are large, complexSCIENCEsciencemag.org 2 OCTOBER 2020•VOL 370 ISSUE 6512 57
Box 1. Glossary of terms.Condensate:A generic term used to refer to membraneless cellular structures
that concentrate biomolecules. These structures can form through reversible
phase transitions.Phase transition:An abrupt, highly cooperative change to order parameters
caused by the breaking of symmetries that in turn leads to a change in the
state of matter.Symmetry:The invariance of a physical system to operations such as trans-
lations of molecules along, or rotations about, defined axes. Disorder is char-
acterized by a state of high symmetry; disorder-to-order transitions occur by
the breaking of specific symmetries.Phase separation:A type of phase transition in which a system separates
into one or more coexisting phases. In a binary mixture, the coexisting phases are
dense and dilute phases. If the dense and dilute phases that result from phase
separation are liquids, then the transition is referred to as liquid-liquid phase
separation (LLPS).Percolation:Multivalent macromolecules behave like associative polymers
that form reversible, noncovalent cross-links. When the number of cross-links
crosses a threshold known as the percolation threshold, a majority of the
molecules are incorporated into a system-spanning (percolated) network.Material properties:With respect to biomolecular condensates, this refers to
features such as viscosity, elasticity, and surface tension of the dense phase.
These features are manifestations of elastic and dissipative moduli that are
governed by the extent of physical cross-linking and the time scales for making
and breaking cross-links within condensates. These material properties influence
the spatial organization and diffusion of macromolecules within the dense phase,
as well as selective permeability to molecules entering the condensate. These
material properties are tightly regulated and directly linked to condensate
function.Dynamical arrest:The situation in which making and breaking of cross-links
within a condensate becomes so inefficient that dynamism is lost. A network of
physical cross-links can trap macromolecules in arrested phases characterized
by irregular (aspherical) morphologies and immobile molecules. When cross-
links are made and broken efficiently, the resulting condensate can have liquid
properties. Excessive cross-linking or reduced efficiency in the breaking of
cross-links—such as accompanies many disease-causing mutations—results
in arrested dynamism, altered material properties, and impaired function.Heterotypic buffering:The ability of heterotypic interactions to buffer against
the deleterious effects of homotypic interactions that can drive pathological
liquid-to-solid phase transitions. This term also refers to the positive effects of
heterotypic interactions that suppress dynamical arrest.