Ayala and her lab study TDP-43, a protein that binds to RNA and plays a role in gene
expression. In 2006, researchers discovered that TDP-43 was the main feature of neurological
disorders like ALS and frontotemporal dementia.
TDP-43 is an important protein tied to cell survival, metabolism and function that
has been conserved by evolution in animals from flies to humans. Without TDP-43, scientists
see animals develop locomotive defects and shortened lifespans. Mice that lack this
protein do not survive beyond the embryonic stage. It regulates many genes and controls
the processing of messenger RNA.
When TDP-43 builds up in cells, it causes damaging plaque to accumulate; this is a
hallmark of ALS and frontotemporal lobar degeneration. It also is present in other
types of neurodegenerative diseases, including Alzheimer's disease.
ALS is a late onset disease; the average age of diagnosis is 55. A devastating illness
with no cure, most people die within two to five years after diagnosis. ALS kills
motorneurons in brain and spinal cord cells, causing a lack of connectivity to muscles
and muscle atrophy. It also affects neurons in the tongue and limbs.
In addition to ALS, TDP-43 is found in cases of frontotemporal dementia, a type of
dementia that is distinct from Alzheimer’s disease. Where Alzheimer’s patients develop
memory problems, those with frontotemporal dementia lose inhibition and undergo a
gradual decline in behavioral and speaking ability.
TDP-43 also is tied to minor neurodegenerative disorders and, in a recent discovery,
is present in many Alzheimer’s disease patients, as well. While the connection to
Alzheimer’s is not yet understood, scientists speculate that TDP-43 may be a secondary
pathology or a marker of Alzheimer’s disease.
Among all of these neurodegenerative disorders is a common factor: they are characterized
by the irreversible accumulation of plaques. A protein or set of proteins aggregates,
causing plaque to build up. And, scientists know that in ALS and frontotemporal dementia,
the aggregating protein is TDP-43.
Inside our cells: What Is TDP-43 Doing?
Ayala and her team aimed to learn more about how TDP-43 works within cells. The researchers’
first question is a matter of balance. TDP-43 is an indispensable protein that needs
to accumulate in order for vital cell processes to occur. Too much, though, and the
protein aggregation causes plaques to build up.
“We want to learn about the process of protein balance, how healthy cells maintain
balance, how the protein aggregates and what conditions promote or limit aggregations,”
Ayala said. “Most of us go around with our TDP-43 working well. When does it not work
well? What are those conditions? And, what are the really important functions of TDP-43
that happen in cells, functions that would cause the degeneration of neurons without
“And, finally we want to know more about the location of where TDP-43 accumulates
within the cell. In the nuclei, the cell recruits the protein into organelles or foci
that are necessary for normal gene expression and cell survival.
“We wanted to understand how the assembly into these organelles is regulated. For
TDP-43 and other RNA binding proteins, very little is known about when they will form,
how many will form and how big they are. How do you control the size of these things?”
The SLU team decided to see if phosphorylation – one of the most common ways that
proteins are regulated – may be responsible for managing the activity, location and
how tightly the protein binds. Scientists know that when they stress cells in a certain
way, they activate a heat shock response, triggering phosphorylation. Ayala used this
procedure to see if phosphorylation was involved with TDP-43’s actions.
“Heat shock response is the first stress conditions that will alter cell metabolism
that was ever discovered, and it is found in organisms ranging from yeast to humans.
The idea is that when you increase the temperature of the cells, the cells have a
response that is similar across many creatures. That is the signal that triggers many
cellular changes including phosphorylation of some proteins.
“What we found is that there is a specific kinase, the enzyme that is going to add
the phosphorylation, at work. It translates the signal, internal or external, to the
cell. This enzyme is called MEK and it is a really important kinase that has been
studied for many years. It is involved in cancer. But one of the new findings is that
it also is involved in protein balance in cells.”
Ayala and her team found that this kinase, MEK, phosphorylates at two residues.
“We were happy we identified this signal,” Ayala said. “It was nice to find this.
It can take a long time to find.”
Next, the researchers wanted to know if phosphorylation was affecting protein functions,
and they found that it regulates the ability to control processing.
“It made TDP-43 go to a compartment where it hadn’t previously been,” Ayala said.
“It may be involved in regulating a type of RNA we didn’t know it regulated and it
may be very important for cellular metabolism.
“Now we want to understand what it is doing in this compartment, its specific function
and how that translates to ALS and dementia.”
On the Horizon: Better Diagnosis and New Cures
Ayala is encouraged by the headway her team has made in understanding the underlying
processes that instigate these neurodegenerative diseases. And, she hopes that this
work will set the stage for studies that may be able to leverage these findings in
order to develop new cures.
“We had clues pointing in this direction. You had mutations and you had this compartment.
Now, I think we need to figure out how these two things converge to understand how
ALS happens. This is a critical place to look, to examine the function of that specific
organelle that is affected in this disease.”
“We may be able to use that kinase pathway as a target to develop drugs that will
make TDP-43 happier, preventing disease and preventing the aggregation of other proteins.
So I think this work will really help us understand what leads to the development
of ALS and dementia, to help us know the principle causes of neurotoxicity in these
cells. It may help us test therapies against the disease and adapt some of the drugs
that are already available.”
“MEK is studied a lot for cancer, so we may have a head start. Researchers already
have developed molecules to inhibit MEK, so potentially we could use that already
In addition to new cures, the diagnosis itself of neurodegenerative disorders can
be difficult, and it can be challenging for clinicians to tell one dementia from another.
It is only in postmortem examinations that doctors can see the distinguishing proteins
and shapes that mark the specific illnesses. Ayala and her team believe their research
holds promise not only for cures but also for good diagnostic methods, to diagnosis
earlier and more accurately.
Other researchers on the study include Wen Li, Ashley N. Reeb, Binyan Lin, Praveen
Subramanian, Erin E. Fey, Catherine R. Knoverek, Rachel L. French and Eileen H. Bigio.
The work is funded by the National Institutes of Health (NINDS Grant K01 NS082391)
and the Saint Louis University President’s Research Fund (Grant 230175).
Established in 1836, Saint Louis University School of Medicine has the distinction
of awarding the first medical degree west of the Mississippi River. The school educates
physicians and biomedical scientists, conducts medical research, and provides health
care on a local, national and international level. Research at the school seeks new
cures and treatments in five key areas: cancer, liver disease, heart/lung disease,
aging and brain disease, and infectious diseases.