We're Seeing the Light
News and Perspective from the Packard's Symposium 2011
Here's the News
In what’s to follow, you won’t see a summary of this year’s work from all 25 of Packard’s investigators. That’s because some of the research is work-in-progress and results either aren’t available or haven’t been analyzed. Also, because many of our investigators are reporting prior to publishing, we’ve steered clear of specifics, where necessary. Still, it’s clear: The progress is real. We’re beginning to see the light.
A Search Heats Up
Two burning questions in ALS research center on the two hottest genes, TDP-43 and FUS, whose mutated versions appear in both sporadic and familial patients. Both genes produce proteins that get mislocated in neurons, forming abnormal clumps in the cytoplasm.
The questions: What makes the proteins aggregate? Is that a problem?
Single-celled yeasts are a model of choice to find answers, according to investigators Aaron Gitler and Jim Shorter, both at the University of Pennsylvania. Either human FUS or TDP-43 gene can be engineered into the organisms and its repercussions followed.
And that’s what they’re doing, the scientists reported.
Recently, they made (and published) what Shorter calls the “striking” discovery that both of those genes, as they’re found in patients, contain areas most often found in the unusual agents of disease called prions.
True prions, such as those that cause “mad cow disease,” are incredibly infectious proteins and work by interacting with and causing host proteins to change shape and become toxic. While the parts of TDP-43 and FUS in question are only prion-like in that they don’t appear infectious, they do appear predisposed to cause trouble.
Earlier, Gitler and Shorter showed that most of the TDP-43 mutations that cause ALS come from — a surprise — the prion-like part of the TDP-43 gene. He found that, as in prions, the resulting TDP-43 protein is quick to clump.
And in that state, the now-toxic protein somehow kills neurons.
At the symposium, the researchers also discussed their findings for the FUS gene but, as the data are yet unpublished, that news must wait.
Flies, Eyes and a Surprise
Udai Pandey (Louisiana State University) made a revealing Drosophila (fruit fly) model of ALS using the recently-discovered FUS gene. Fortunately, the fly is engineered so that neurodegeneration is easy to see, either by looking at its eyes or its ability to move.
Some of the similarities between the model’s and real-life behavior of this gene — it causes a familial ALS and is also in sporadic patients — are especially striking. They make the model all the more realistic.
Flies bearing the toxic human gene showed characteristic clumped proteins in cells, for example. The model’s neurons degenerate and it becomes paralyzed, as occurs in human disease. Also as in patients, the mutant FUS protein that’s formed from the gene moves unnaturally to the cytoplasm rather than remain in the nucleus.
Pandey found that if he could block this migration, the flies stayed healthy.
Most important, he’s discovered an interaction between FUS and another human ALS gene, mutant TDP-43. Blocking the link between the two also appeared to keep the flies safe — an avenue to consider for therapy.
Judging the Company It Keeps
J. Paul Taylor became a Packard grantee because of his interests in the mechanism that the gene TDP-43 — either in its mutated or natural form — unleashes to bring about ALS. Whether the gene does damage by producing a toxic TDP-43 protein or by depriving cells of a TDP-43 that does useful work is a key question.
To find an answer, Taylor has recently been mapping TDP-43’s interactome. That’s the molecular equivalent of a dance card, a list of other cell proteins that come in contact with TDP-43. By knowing precisely how the interactions come about, he’s in a better place to understand what they accomplish — or disrupt.
This year, to get a still better grasp of the TDP’s usual role in cells, Taylor’s using the fruit fly he developed that carries the human TDP-43 gene, slightly altering that gene in specific ways. Seeing what then goes amiss in the fly sheds light on its normal function.
He’s also modeled another human ALS gene in flies, the newly-discovered VCP gene, and is carrying out similar studies.
TDP-43 — Too Much or Too Little?
Like Paul Taylor, Phil Wong is also trying to determine whether TDP-43’s ALS damage is due to the gain or loss of some important function, or if it’s both. This year, Wong has shown that in his mouse models where human TDP-43 is over-produced, there’s highly telling damage that, in part, involves the disappearance of energy-supplying mitochondria at the junction where neurons approach muscle. The junction itself is improperly formed, leading to early death.
Likewise, Wong has begun studies in mouse models where TDP-43 is modestly increased. Wong has a quiet belief that having the gene’s activity weakened may lead to a model that is a better mimic of ALS, than having it overactive. Details will come when this study is published.
Proteome Gone Awry?
The varied proteins in a cell — a cell’s proteome — don’t each exist in isolated splendor, carrying out their dedicated function(s), but they can interact, they ebb and flow, often in subtle ways. Based on years of study, David Borchelt believes that delicate interaction may be upset in neurodegenerative diseases where a specific protein is known to go awry — Parkinson’s, Alzheimer’s and ALS all fit that mold.
In his ALS studies, Borchelt has examined behavior of mutant SOD1 proteins, to try to solve the too-long unanswered question of how they trigger disease. He advances the idea that having abnormally shaped SOD1 protein, as occurs in the ALS it causes, may influence other shape-vulnerable proteins, leading to a proteome gone downhill. One outward sign of this could be the clumps of aggregated proteins that typically increase as the disease worsens. Borchelt’s recent work is showing which proteins that interact with SOD1 are least stable and, thus most vulnerable.
Now in Animal Models? Mitochondria Still Not Happy.
Giovanni Manfredi and Jordi Magrané have been noting abnormalities in how mitochondria — the cells’ powerhouses — look and behave in familial ALS. What they’ve found suggests a problem with the ability of the cell structures to be situated at a right time and place to generate energy for motor neurons. In the past, they’ve studied mitochondrial behavior in lab cultures of neurons. Now they’ve moved to animal models of ALS with SOD1 and TDP-43 mutations. Findings reported at this year’s meeting showed abnormalities of mitochondrial movements and shape within neurons of liveALS mice. These abnormalities worsen as the disease advances.
Stem Cells Still Going Forward
Hongjun Song, collaborating with Nicholas Maragakis and Jeff Rothstein, has created multiple lines of induced pluripotent stem cells (iPSCs) for study, working from tissues gathered from familial ALS patients with an SOD1 mutation. The lines exist mostly for the next step: to create motor neurons and astrocytes as human cell models of the disease.
So far, things are on track. Song is pleased, for example, that iPSC-derived motor neurons carry impulses as do “natural” motor neurons. Their synapses appear normal as well. And the early-stage astrocytes carry markers of those cells.
By providing the human cells either in pure cultures or grown together as a closer mimic of nature, Song hopes to give researchers a more accurate way to explain ALS’s pathology and to screen possible therapies. He also believes iPSCs provide the most useful system so far to see how the human disease differs from what animal models experience — important to realize before moving drugs to human trials.
The cells also provide a potential system to study environmental agents that might affect onset of ALS.
An Explosion of Cells
The gray and white matter of the spinal cord has a good supply of NG2 cells, Dwight Bergles reports. They’re a distinct type of cell that can grow, as needed, into the oligodendrocytes that are companion cells crucial to neurons’ health. Bergles found that, in ALS, there’s an unexpected outpouring of NG2s, and their subsequent morphing into young oligodendrocytes takes a huge jump. Why? Is the change harmful? Is it an attempt to rescue the nervous system? That’s what he’s trying to find out.
A Stem Cell Rescue?
As part of a P2ALS project (a Packard collaboration), Nicholas Maragakis is investigating the promise of glial-restricted precursors (GRPs) to protect the nervous system from ALS. As their name says, these cells give rise to glia — cells both plentiful and crucial in maintaining the nervous system. Maragakis earlier showed that added rodent GRPs can help ALS rats, extending life and protecting their neurons.
Now, with a good supply of human GRPs, he’s working to see if those derived from ALS patients might have the same positive effect on animal models of ALS or if there are side-effects that question their potential for therapy. Early results await publication.
The Quicker Picker-Upper
The misfolding of certain proteins and their habit of clumping together as microscopically-visible, insoluble cell deposits in ALS and other neurodegenerative diseases has been a focus of Alfred Goldberg’s work. Two major “housekeeping” systems in cells exist to clear these potentially toxic masses. Goldberg’s Packard work aims to find ways to boost these systems, while it increases our knowledge of how they work. This year, he’s identified three new places within the systems as possible therapeutic targets.
Too Early to Describe Fully
We don’t mean to tease. We just want to give you a very general idea what’s coming down the pike.
- Thomas Lloyd and Alex Kolodkin have developed a fruit fly model of ALS based on the dynactin protein and continue to show how it develops pathology. They have currently found that this model mimics some key, early changes observed in motor neuron disease.
- One team of Packard investigators has uncovered a protein system not studied before in ALS that, when properly affected, appears to protect a wide variety of animal models of the disease against neurodegeneration.
- Another Packard scientist has found a possible pathway that may account for the major weight loss that occurs in ALS.
- One Packard investigator has uncovered a strong, unexpected immune link in ALS that suggests a useful therapeutic target.
- Another researcher has discovered a key molecule in animal models and, likely, in humans, that stimulates immature cells to form motor neurons and that also has a healthy protective effect on existing neurons.