Science - USA (2020-10-02)

condensates containing hundreds of distinct
biomolecules that function as discrete mem-
braneless organelles. Indeed, a cell may be
viewed as a complex, dynamic network of con-
densates that are in constant communication
through exchange of materials. Biomolecular
condensates provide advantages over membrane-
mediated compartments in that they concen-
trate macromolecules in space while enabling
rapid exchange of constituents with the sur-
rounding intracellular milieu ( 25 ). Moreover,
many condensates can assemble or disassemble
rapidly in response to changes in cellular state.
The functional consequences of this dynamic
organization include positive and negative reg-
ulation of biochemical processes. For example,
condensation of multiple enzymes in a com-
mon pathway can promote“substrate chan-
neling”wherein the intermediary metabolic
product of one enzyme is passed directly to
another enzyme without its release into solu-
tion, thereby increasing overall efficiency of the
pathway ( 26 ). Such a mechanism may underlie
the regulation of multiple glycolytic enzymes
at neuronal synapses in an activity-dependent
manner ( 27 ). Condensate formation via phase
separation may also have the opposite effect,
wherein sequestering one or more constituents
in the dense phase may negatively regulate biol-
ogical activities in the dilute phase ( 28 ). Con-
densates can also orchestrate the assembly of
complex higher-order structures, such as ribo-
some subunit assembly in the nucleolus ( 29 ).

Dynamism in complex biomolecular condensates

The dynamic behavior of condensates reflects
the nature of the interactions that underlie
their assembly: weak, transient interactions
among multivalent biomolecules that form
noncovalent cross-links of varying strengths
and durations. Above a system-specific thresh-

old, the system becomes populated with suf-

ficient cross-links to form a system-spanning

network that holds the condensate together,

a phenomenon known as percolation (Box 1).

Thus, unlike phase transitions that are driven

purely by homotypic interactions, condensate

formation requires the crossing of a collective

threshold defined by condensate-specific net-

works of homotypic and heterotypic inter-

actions ( 23 ). The macromolecular partners

engaging in physical cross-links will evolve dy-

namically, and if such cross-links are made and

broken efficiently, the condensate can be highly

dynamic and exhibit liquid properties ( 30 ). The

extent of networking and the time scales for

making and breaking cross-links contribute

directly to the material properties (Box 1) of a

condensate. These include properties such as

viscosity, elasticity, and surface tension. These

material properties are tightly regulated because

they influence the spatial organization and diffu-

sion of macromolecules within the dense phase,

selective permeability to molecules entering the

dense phase, exchange of constituents with the

light phase, and ultimately condensate function.

Many proteins associated with neurode-

generative diseases reside within distinct

biomolecular condensates. For example, disease-

related RNA-binding proteins such as TDP-43,

FUS, hnRNPA1, hnRNPA2B1, and TIA1 are

constituents of multiple types of RNP conden-

sates that control the fate of RNA molecules as

they transit through processes such as splicing,

nuclear export, trafficking in the cytoplasm,

translation, and degradation ( 31 ). Disease-

causing mutations alter the balance of homo-

typic and heterotypic interactions in these RNP

assemblies, thereby changing their material

properties, even in the absence of pathological

liquid-to-solid phase transitions ( 32 ). Indeed, a

recurrent observation is that disease-causing

mutations lead to dynamical arrest (Box 1) of

RNP granules that can impair functions with

adverse consequences for RNA metabolism

( 6 , 7 , 10 – 13 , 33 ). At the same time, concen-

tration of proteins in the dense liquid phase

increases the probability of a liquid-to-solid

phase transition—particularly in proteins har-

boring prion-like LCDs.

However, the rarity of this pathological event

points to the presence of mechanisms that hold

such pathological transitions in check. The pri-

mary factor suppressing potentially deleterious

excess homotypic interactions is the collective

effect of functional networks of heterotypic

interactions within condensates, which we term

heterotypic buffering (Box 1). Additional checks,

most notably the activities of chaperones, are

in place to reverse any excess homotypic inter-

actions that escape heterotypic buffering. The

concept of heterotypic buffering is particularly

useful as a framework to understand sporadic

neurodegenerative disease, which culminates in

the same pathology as disease arising from rare

genetic mutations. According to this view, a va-

riety of insults may intersect at a common point,

collectively altering the dynamic network of con-

densate interactions in such a way as to impair

heterotypic buffering, leading to pathological

liquid-to-solid transitions and/or dynamical

arrest (Fig. 3).

Defining the relationship between pathological

phase transitions and disease

Understanding the process of neurodegeneration

requires consideration not only of how end-stage

proteinaceous deposits arise, but how specific cel-

lular processes are corrupted over time to give rise

to neuronal dysfunction and death. Such disturb-

ances can be considered from two perspectives:

first, a mode of toxicity in which pathological

consequences arise directly from unchecked

58 2 OCTOBER 2020•VOL 370 ISSUE 6512 sciencemag.org SCIENCE

Nucleation in the dilute phase of cytoplasm or nucleus

Disease protein

Fibril deposition

Loss of

heterotypic bufering

Heterotypic bufering

prevents nucleation of

concentrated protein

Nucleation

LLPS

Templating Fibril growth

Fibril deposition

Fibril growth

Disease protein

Liquid-to-solid phase transition in the dense liquid phase of a condensate

Fig. 1. Two nonexclusive routes lead to fibril formation in neurodegenerative diseases.Fibril formation may be initiated by a primary and secondary nucleation
in dilute solution, with subsequent growth through templating of additional units. Alternatively, fibril formation may occur via a liquid-to-solid phase transition
within the dense liquid phase. In the condensed liquid state, fibril formation is facilitated by concentrating proteins and bringing them closer to the threshold for liquid-
to-solid phase transition. These routes are not mutually exclusive and may be influenced by context.

NEURODEGENERATION