homotypic phase transition leading to fibril for-
mation; and second, a mode of toxicity in which
pathological phase transitions have broader func-
tional consequences by altering key properties
of biomolecular condensates, thereby impair-
ing the ability of condensates to regulate bio-
logical activities.
With regard to the first mode of toxicity, the
accumulation of disease proteins in fibrillar
deposits may directly impair cellular function.
One possible mechanism is that pathological
protein deposits create a sink that depletes
cells of that particular protein. For example,
accumulation of fibrillar TDP-43 or FUS pa-
thology in the cytoplasm is accompanied by
gradual depletion from nuclei ( 34 ), and this
nuclear depletion may lead to a partial loss of
function ( 35 , 36 ). Pathological proteinaceous
deposits may also sequester additional factors
leading to their functional depletion. For ex-
ample, it has been suggested that recruitment
of chaperones to such pathology may deplete
the capacity of protein quality control mech-
anisms, with widespread implications ( 37 ).
Alternatively, pathological phase transitions
may exert broad cellular toxicity by influenc-
ing the network of interactions that define the
nature and function of biomolecular conden-
sates. A prominent example is amyotrophic
lateral sclerosis with frontotemporal dementia
(ALS-FTD), which arises from pathological var-
iants in at least eight different RNA-binding
proteins, including TDP-43 ( 38 ), FUS ( 39 , 40 ),
hnRNPA1 ( 6 ), hnRNPA2 ( 6 ), TIA1 ( 7 , 41 ),
matrin 3 ( 42 ), ataxin 2 ( 43 ), and annexin A11
( 44 ). All of these proteins reside within bio-
molecular condensates that are distributed
throughout the nucleus and cytoplasm of cells
and govern many aspects of RNA metabolism.
Disease-associated mutations in these RNA-
binding proteins alter the material properties
of their native condensates, and it is therefore
unsurprising that ALS-FTD is associated with
widespread disturbance of RNA metabolism
( 45 ). It remains to be determined whether
disease-causing mutations in other condensate-
resident proteins (e.g., tau anda-synuclein)
also impair functions of specific condensates.
Beyond impairing function, perturbation of
condensate material properties can simultane-
ously enhance the driving forces for additional
symmetry-breaking operations, notably liquid-
to-solid phase transition. As a result, dynam-
ically arrested condensates can also become
crucibles driving the formation of fibrillar de-
posits that arise from homotypic interactions,
as described above (Fig. 1). Thus, disturbance of
the material properties of condensates can drive
pathology through two consequences that are
inextricably linked: impairment of native con-
densate function and production of proteina-
ceous pathology.
Remarkably, disease-causing mutations
in a variety of proteins that are not typically
thought of as constituents of condensates can
also cause dynamical arrest. These include
proteins involved in the maintenance and
clearance of condensates, such as valosin-
containing protein (VCP) ( 46 – 48 ), UBQLN2
( 49 , 50 ), and OPTN ( 51 , 52 ). Furthermore,
pathological polydipeptides arising from ex-
pandedC9ORF72produce widespread distur-
bances in biomolecular condensate function.
Specifically, arginine-containing polydipeptides
(polyGR and polyPR) become concentrated
within biomolecular condensates and alter their
material properties through extensive interactions
with LCDs ( 53 ). For example,C9ORF72-related
polydipeptides impair the central channel of the
nuclear pore ( 54 ), disturb the dynamics of stress
granules and RNA transport granules ( 53 , 55 ),
and impair the dynamics and material proper-
ties of nucleoli, resulting in reduced ribosome
biogenesis ( 53 , 56 , 57 )andadecreaseinthe
ability of the nucleolus to buffer against nuclear
protein misfolding ( 58 ).
The mechanisms described above focus
on pathological phase transitions of proteins.
However, we note that RNA molecules are
also well suited to driving phase transitions.
Indeed, pathological expansion of RNA repeats,
such as those observed in myotonic dystrophy
types 1 and 2 andC9ORF72-related ALS-FTD, is
marked by pathological RNA phase transition
resulting in RNA foci ( 59 ). These RNA-driven
pathological phase transitions can also initiatetoxicity via mechanisms that are remarkably
similar to those observed with protein depo-
sits, including sequestration of RNAs and RNA-
binding proteins ( 60 ).Perspectives
In light of the past decade’s worth of research,
the commonly held perspective that neuro-
degeneration is caused by aggregation of mis-
folded, toxic proteins is evolving toward the view
that pathological phase transitions represent a
common principle underlying neurodegenera-
tion. This insight is important because it focuses
our attention squarely on the dynamic cellular
condensates that are assembled from these pro-
teins. Pathological phase transitions of disease
proteins, irrespective of which route they take
to fibril formation, are inextricably linked to the
functions of the condensates in which they reside.
According to this view, the primary manifes-
tations of cellular dysfunction in the context of
disease are twofold: (i) altered material prop-
erties due to dynamical arrest of condensates,
and (ii) pathological liquid-to-solid transitions.
Accordingly, reversing these defects should be
the objective of therapeutic intervention.
As described above, the percolation thresh-
old and material properties of condensates are
defined by the dynamic network of homotypic
and heterotypic interactions that define them.
Indeed, it is now evident that manipulation of
individual constituents is sufficient to alter theSCIENCEsciencemag.org 2 OCTOBER 2020•VOL 370 ISSUE 6512 59
RNA:RNA
interactionsProtein:Protein
interactionsProtein:RNA interactions
Fig. 2. Condensates arise through a network of heterotypic and homotypic interactions.Condensates
form through phase separation of multiple types of macromolecules. In RNP granules, for example, multivalent
proteins and RNA molecules participate in a network of homotypic and heterotypic interactions that collectively
determine the concentration threshold for LLPS and the material properties of the resulting condensate. The
material properties of biomolecular condensates, such as viscosity, elasticity, and surfacetension of the dense
phase, are governed by the extent of physical cross-linking and the time scales for making and breaking
cross-links within condensates. These material properties influencethe 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.