Illustration by William Hogan
ention any disease and a few questions immediately
come to mind. Chief among them: Who is vulnerable and how does it occur? If
it’s an infectious disease, it may spread through the air or by touch. But the presiding
dogma for most of modern biomedical history tells us that the transmissible
agents contain nucleic acid and that replication is inextricably bound to DNA
or RNA. As information vehicles, these molecules power the dual aspects of
evolution: constancy and change. The constancy of faithful replication allows
the inheritance of traits that allow life forms (including classical disease
culprits) to survive. Through change, genes assure adaptability to a complex
environment. These properties confer pathogenicity by enabling them to prey
upon infectious agents and adapt to their hosts.
parallel universe of disease transmission ignores these rules. This is the
world of prions. In this world, the agent of transmission is a protein and the
information lies in the vast shape of the space within which proteins fold. How
big is that space? To get a handle on the size of chemical space, the chemistry
blogger Derek Lowe quoted Douglas Adams in The
Hitchhiker’s Guide to the Galaxy: “Space is big. Really big. You just won’t
believe how vastly, hugely, mind-boggling big it is.” The immense variety
within stretches of ATGCs (the chemicals adenine, cytosine, guanine, and
thymine) that build the language of the genetic code pales beside the alphabet
of the periodic table with its spelling and grammatical rules of chemical bond
formation and compound stability.
Tau, a Protean Protein
a normal protein used in neurons to shape a dynamic system of tracks that
traffic goods to various destinations inside the cell. Tau is one small
fraction of the bewilderingly large space within which all proteins fold. But
like a fractal image, zooming in on tau opens up a chemical space full of
molecular crevices and passageways that mirror the far greater universe from
which tau is but one tiny part. Like the early sea god, Proteus, tau can take
many forms. It was said that Proteus had the ability to foretell the future,
but would change his shape to avoid doing so. Indeed, some forms of tau are harbingers
of a future with neurodegeneration (more on this point later). The chemical
space that tau occupies in the nervous system must first be catalogued
according to the six different molecular isoforms of the tau protein, which
many different chemical processes can modify. These six isoforms each have
slightly different sequences with small stretches of amino acids either left in
cursory quantitative knowledge of the staggering number of possible modified
tau states does not exist. Beyond these molecular states lies a far more
extensive terrain of folding patterns, termed conformations (think of multiple
ways to a crumple a piece of paper). And the only constraint on these
conformations is the time they dwell in any one of them. Some shapes are stable
only for a few milliseconds, and how readily the protein can assume a
particular shape among all possible shapes is called the kinetic landscape.
the enormous realm of protein shapes, those that pertain to tau are little
studied and poorly understood. Tau is in a unique class of proteins called
intrinsically disordered proteins (IDPs). In contrast to enzymes, for example,
which adopt a precise three-dimensional structure to facilitate catalysis, IDPs
lack a unique three-dimensional structure and do not exhibit any stable
secondary structure in the free form. They can adopt a wide variety of extended
and compact conformations that facilitate many vital physiological functions by
folding after they bind to targets.
As with other IDPs, enzymes act on tau
proteins twice as often as on other proteins and can alter their binding
properties. Among these enzymes are a category called kinases that add a
phosphate to a protein.
IDPs are, on average, substrates of
twice as many kinases as structured proteins.
Tau is particularly singled out as a substrate of multiple kinases, and
some investigators believe that the multiple phosphates which decorate tau
contribute to its tendency toward misfolding.
use their lack of structure to their advantage. The protean shapes that these
proteins can assume provide a larger interaction surface area than globular
proteins of a similar length. The variety of shapes exposes short linear
peptide motifs that serve as molecular recognition features, and thereby allow
IDPs to scaffold and interact with numerous other proteins. It enables diverse
post-translational modifications that facilitate regulation of their function
and stability in a cell; and by folding upon binding, IDPs can interact with their
targets with relatively high specificity and low affinity. These features are
ideal for recognizing partners to interact with and for coordinating regulatory
events in space and time.
However, these properties require that cells assiduously monitor these
proteins because they are potentially dangerous and capable of inflicting
damage to cells by binding to each other.
cells tightly regulate IDPs throughout, from transcript synthesis to protein
degradation. Among the means that cells can use to adjust the levels of
proteins is by the process of post-transcriptional regulation through
microRNAs, which have been credited with helping to maintain tau homeostasis.
MicroRNAs target specific messenger RNAs, which encode proteins and
fine-tune the amount of protein that gets translated from the messenger RNA.
simple over-expression of tau in a variety of cell types, including neurons in
laboratory tissue culture, does not result in the replication and spread of tau
(aggregation), even with high expression levels. More often tau overexpression
induces the rampant assembly of a structure called microtubules, which are the
cell’s railroad tracks that ship cargo to different locations in the cell.
Normally, tau promotes the assembly of microtubules with the goal of building a
protrusion from the cell that will become an axon. These long cylindrical
structures use their microtubule railroads to carry cargo over the vast
distances that axons traverse, such as the axons that travel from the lower
spinal cord all the way to the muscles of the big toe. In the brain, axons
connect the two hemispheres and are key elements of brain circuitry. When tau
is overexpressed in nonneural cells, the out-of-control microtubule assembly
results in numerous microtubule bundles spiraling around the perimeter of the
cell, which is unable to form a protrusion in this foreign environment.
Curiously, when tau is expressed in a type of cell called Sf9, taken from the
ovary of an insect, it acquires many of the modifications seen in
neurofibrillary tangles. With tau, these cells extrude a single, very long
process that resembles an axon in its shape, but has none of the electrical
conduction properties of an axon.
Thus, tau makes an axon ghost in these cells. One conclusion from these
studies is that simply lowering tau levels across the board is not the most
strategic way to approach therapeutics for diseases termed tauopathies. Nor is
simply overexpressing tau the way to make aggregates.
proteins can misfold into shapes called prions (as noted above) that have the
unusual property of inducing other copies of the same protein to misfold
similarly. The prion guides similar conformations in additional copies of the
same protein. The prototypical prion, known as PrP, is responsible for several
human diseases: Creutzfeldt-Jakob disease, Gerstmann–Sträussler–Scheinker
syndrome, fatal familial insomnia, kuru, and bovine spongiform encephalopathy
(also known as mad cow disease). Each of these diseases has very different
clinical presentations, and they are distinguished from other infectious
diseases mainly by their mode of transmission. Given that PrP causes all of
these conditions and others found in nonhuman species, the idea that prion
strains with distinct phenotypes exist has gained traction and experimental
validation. This view suggests that a subset of PrP shapes is transmissible. Depending
on the particular folding of a strain, a specific phenotype or species predisposition
arises. Faithfully propagating strains, therefore, is a prerequisite for
clinically defined presentations, and spread through the brain via specific
has been peculiar since the discovery of PrP is that only one human prion is
known. Certainly other proteins must have kinetic troughs into which a protein
can fall, get stuck, and spread their conformation to others. But only in yeast
was a similar phenomenon clearly observed, and remarkably the prion state of
the yeast protein confers a selective advantage.
For many years, investigators nibbled at the concept of prions to
explain numerous neurodegenerative diseases in which a misfolded protein
aggregates and remains trapped inside the cell. Among these conditions are the
tau aggregates in the tauopathies, synuclein aggregates in Parkinson’s disease,
huntingtin aggregates in Huntington’s disease, TDP-43 in frontotemporal lobar
degeneration and amyotrophic lateral sclerosis, and transthyretin in familial
amyloidotic neuropathy. Prions conceptually unify the neurodegenerative
diseases, which otherwise lack a fundamental disease mechanism akin to a virus
or a malignant cell or an autoimmune process in other disease categories. At
the core of neurodegeneration lies an unknown process as mysterious as aging.
aggregates are puzzling entities that lie on the wrong side of what in other
contexts we call protein-protein interactions. Normal cellular function
requires that proteins bind to each other as pairs or somewhat larger
complexes. On the other hand, when tau aggregates, it grows massively, with
molecular weights in the millions, and may capture other proteins within the
In many cases, including tau inclusions, some feature of tau that is
predisposed to self-assemble triggers recognition by a class of enzymes called
ubiquitin ligases, which mark the protein for degradation. These enzymes enable
a set of reactions that attach a chain of ubiquitin peptides to a protein. The
ubiquitins tell the cell that the protein to which they are attached is trash
and should be thrown away in the cell’s trash can (called the proteasome).
However, for unknown reasons, when ubiquitins attach to tau, it does not get
degraded in the proteasome. Instead, it gets stuck in the cell as an aggregate.
Maybe the size of the aggregate does not fit into the opening of proteasome,
which is shaped like a barrel into which proteins enter for destruction inside.
The Spreading of Tau
addition to the ability to evade the cellular surveillance systems that rid the
cell of damaged proteins, prions have the even more insidious property of
spreading to contiguous cells. Postmortem brain extracts from humans who had
died with various tauopathies were injected into the hippocampus and cerebral
cortex of mice and could be propagated between mouse brains.
The passage of a particular tau conformer appeared to faithfully replicate
the pathology specific to each one of three clinically distinct types of
replication of tau aggregates was also demonstrated at the cellular level.
Strains differed with respect to the morphology, size, and subcellular localization
of the aggregates as well as their sedimentation profile, seeding capacity,
protease digestion patterns, and toxicity.
these features provide reliable “bar codes” for diagnosis will be known in the
next few years.
engineered to express a pathogenic human tau transgene in the entorhinal
cortex, a highly vulnerable region involved in the sense of smell where tau
pathology often begins, can spread to neuroanatomically connected regions.
Furthermore, mouse tau was bound up in the aggregates, suggesting that the
pathological human tau induced normal mouse tau to misfold. Among the cells
into which tau spread were dentate granule cells that are separated from the
entorhinal cortical cells by a synapse. Whether spread occurs transsynaptically
remains a fascinating, open question.
findings and related tissue culture system studies point to four areas for
deeper investigation: (a) how tau exits through the cell’s membrane; (b) whether
the existence of tau in an extracellular compartment offers a rationale for
removing transmissible tau with an antibody; (c) how tau passes into the
membrane of into a neighboring cell; and (d) what potential way stations—such
as microglia—could interface with tau during its spread.
let’s return to the myriad shapes tau can assume. Among these shapes are a few
that expose some sticky surfaces normally kept folded and concealed within the
protein. When this happens, tau can bind to another tau protein in a process
called “seeding.” As more and more tau proteins join the pack, eventually the
aggregate becomes quite large and often appears like a fibril. Proteins, such
as tau, that can wiggle and squirm in numerous ways are intrinsically
disordered proteins (IDPs), and they have generated great interest in the
scientific community. By changing their shape rapidly, they present different
surfaces to other proteins and engage in a variety of binding interactions for
their normal function.
tau transitions on and off microtubules by folding in different ways and, in so
doing, can stabilize and elongate the microtubule. However, during these on-off
transitions, there are vulnerable moments with the potential for misfolding.
Or, while tau is being synthesized from its mRNA, inopportune moments might
allow tau to fold in a way that permits binding to another tau and seed the growth
of a tau aggregate.
aggregation- prone motif observed in tau is called a steric zipper, in which a
pair of sheetlike sequences is held together by the interlinking of small
Researchers have designed inhibitors that slow tau fibrillation by
targeting a set of six amino acids in tau involved in this interaction.
Many unlikely events have to occur together for tau to form an
aggregate: It must drop into a rare conformation or shape, it must remain in
that shape for sufficient time to seed assembly of other tau proteins, and
other tau proteins must be sufficiently close to serve as substrates upon which
misfolded tau can template its nefarious shape.
mistakes in folding are monitored by classes of proteins called chaperones and
co-chaperones. These proteins can refold a protein correctly or, if irreversibly
damaged, direct the protein to the proteasome for complete degradation. The
tethering of one such complex called BAG2/Hsp70 to the microtubule may provide
a protective capture function for misfolded tau.
conformations may occur rarely, but in the presence of a tau mutation or
traumatic brain injury or beta-amyloid deposition among other precipitants, tau
is more likely to assume an aggregation-prone conformation. In vitro, polyanions (molecules or
chemicals with negative charges) are capable of inducing tau self-assembly.
Similarly, in living cells, contact with charged membrane phosphates or RNA may
predispose tau toward an aggregation-prone conformation.
A compelling biophysical mechanism that may initiate the misfolding is
the elimination of water that normally surrounds each tau molecule, and thereby
makes tau sticky and prone to the aggregation into threads.
Once an oligomer or fibril is formed, it can seed subsequent reactions
within the cell and enter neighboring cells through massive vesicles, termed
tau forms an aggregate, it also appears capable of transmissibility from cell
of tau spread constitute a neuroanatomical network, and these networks are
associated with clinical features.
Given the described thinking concerning tau strains, the patterns of
spread may arise from the specificity of a particular tau strain for a specific
network. These selectively vulnerable patterns can be predicted by a diffusion
mechanism modeled by a graph theoretic analysis using tractography data.
Neural network compromise may begin long before there is
neuropathological evidence of disease in the form of misfolded tau aggregates.
study that recorded from neocortical pyramidal cells in a mouse model of
tauopathy found numerous significant physiologic alterations when only a
fraction of the neurons showed pathological tau.
Membrane potential oscillations were slower during slow-wave sleep and under
anesthesia. Firing rates were reduced with longer latencies and interspike
intervals. These changes reduce the activity of the neocortical network and
suggest that conduction and synaptic transmission deficits may be among the
earliest changes induced by tau spread at a resolution below light microscopy.
The interest level in tau
among scientists has had the kinds of peaks and valleys that one might compare
to the stock market. And like the last few years of the stock market,
investment and the trajectory of growth have risen considerably. As we gain a
deeper understanding of the molecule, as well as the ability to image tau
pathology in the living human brain, we stand on the threshold of treatments.