ROLE OF PHASE SEPARATION IN DISEASE
In addition to its role in normal cellular function, liquid-liquid phase
separation could be a key player in disease pathogenesis. For example,
aberrant phase separation may be a critical step in aggregation-related
neurodegenerative disease.
The protein tau—long identified as a major constituent of
neurofibrillary tangles characteristic of Alzheimer’s disease—may
transition from a soluble state into phase-separated liquid droplets
in neuronal cells. Separate experiments by Susanne Wegmann and
Markus Zweckstetter, now at the Berlin and Göttingen campuses of
the German Center for Neurodegenerative Diseases, respectively,
showed that tau droplets change over time in vitro, becoming
more solid, with tau aggregates forming after a few days (EMBO J,
e98049, 2018; Nat Commun, 8:275, 2017). Mutations known to be
associated with tangle formation led to an increased propensity to
phase separate, implying that formation of phase-separated liquid
droplets may be a critical step for developing tau aggregates.
Further evidence for this mechanism of pathogenesis
has come from studying amyotrophic lateral sclerosis (ALS)
and frontotemporal dementia (FTD). Work by several groups
indicates that disease-related proteins, including FUS, hnRNPA1,
and TDP43, form liquid droplets before aggregating into
pathological clumps. Genetic mutations found in patients with
more aggressive forms of ALS or FTD increase the speed with
which droplets of purified FUS, hnRNPA1, and TDP43 morph into
plaque-like tangles. There is now a concerted effort to identify
ways of preventing these proteins from undergoing pathological phase separation in cells, and even reversing the process after it has occurred.
A more complicated system can malfunction in more ways, and so it seems with liquid-liquid phase separation. Although it increases
cellular functionality through greater compartmentalization, it leaves the cell vulnerable to pathological phases. Understanding the cellular
mechanisms that keep pathological liquid-liquid phase separation in check, and why these protective mechanisms break down with age,
are major topics of future biomedical research. However, with a better conceptual framework, we are at least beginning to know what new
questions to ask.
in germline cells protect the genome from
transposon activity.
To better understand how the chem-
ical environment inside the membrane-
less organelle droplet can support these
functions, researchers have developed
model membraneless organelles com-
posed of one or two highly flexible pro-
tein types. For example, using regions of
a protein called Ddx4—a primary pro-
tein component of germ granules—I
(T.N.) and my University of Oxford col-
league Andrew Baldwin found that the
organelle interior is in some ways more
like an organic solvent than water.^2
Reactions that would not occur in the
rest of the cell could therefore proceed
inside a phase-separated liquid droplet,
where chemical conditions may be far
more favorable.
Princeton University chemical and
biological engineer Clifford Brangwynne
has advanced this concept even further
by conducting experiments demonstrat-
ing that the nucleolus consists of at least
three distinct phase-separated layers—
droplets within droplets within droplets.
The nucleolus is responsible for ribo-
some biogenesis, a complex process that
involves folding, modifying, and assem-
bling RNA and hundreds of different
proteins. Brangwynne and his colleagues
suggest that these tasks may be carried
out sequentially through the specializedzones, likening the nucleolar layout to an
assembly line.^3
To date, the nucleolus is the only
characterized example of this type of
multiphase organization, but in 2016,
researchers imaged membraneless
organelles called stress granules with
super-resolution microscopy and found
evidence that they may have similarly
concentric internal structures,^4 suggest-
ing that droplets within droplets could be
a common theme of cellular organization.Controlled access without
a membrane
In order to perform specific biological func-
tions, membraneless organelles must be© ISTOCK.COM, RALWEL