In the United States today, nearly six million people are living with Alzheimer’s disease. That number is expected to reach 14 million by 2050. Alzheimer’s disease is the sixth leading cause of death in the country and drains an estimated $290 billion a year from our economy.
Despite a relentless worldwide effort to find an effective treatment, clinical trials have been largely unsuccessful. There are at least two barriers to progress.
First, the highly protected human brain is inherently difficult to study. The cells and tissues impacted by Alzheimer’s disease are inaccessible by design. Few patients want doctors opening up their skulls. Animals are imperfect models for study: a mouse brain has 70 million neurons; human brains have 85 billion neurons.
Then, at the tissue level, there are many different cell types in the brain that must all work together for healthy cognitive functioning. This increases the degree of difficulty. Understanding how Alzheimer’s disease begins and how to stop it requires scientists to study all of these cell types at the same time while recognizing that the disease likely affects each type differently.
Despite these challenges, scientists have identified several possible culprits. Although no smoking gun has been found, one research team at the UW Institute for Stem Cell and Regenerative Medicine (ISCRM) has been particularly focused on a gene, known as SORL1, that has been linked to increased Alzheimer’s disease risk in family and population studies.
Now, a new paper published in the journal Cell Reports details how the ISCRM investigators used stem cell technology to produce new evidence that implicates SORL1 in some some types of Alzheimer’s. The research team showed that a loss of SORL1 in certain cells of the central nervous system leads to debilitating neural traffic jams. Furthermore, by demonstrating that the molecules tested in numerous clinical trials are not effective at clearing these traffic jams, the researchers shed new light on why many promising treatments have hit dead ends in clinical trials.
Jessica Young PhD, an Assistant Professor of Pathology and an ISCRM faculty member, led the recent study in partnership with her ISCRM colleague Sumie Jayadev MD. According to Young, one of the main goals of the investigation was to gather more evidence to that SORL1 is a risk factor for Alzheimer’s disease.
“We know SORL1 is a critical part of the machinery that controls the movement of proteins in cells,” says Young. “And while we have previously shown that a loss of SORL1 is associated with Alzheimer’s disease, how, where, and when this happens in cells and in which cells in the brain this happens has not been clearly demonstrated.”
To better understand how Alzheimer’s disease begins without actually extracting brain tissue, Young and her team use stem cells derived from patients to create mature cells like those found in the brain and the central nervous system. (This is know as induced pluripotent stem cell technology.)
Specifically, the researchers were interested in two cell types: neurons and microglia (immune cells native to the brain). To test their hunch that SORL1 plays a key role in early onset of Alzheimer’s disease, the researchers used CRISPR gene editing technology to reduce the level of the gene in both cell types and watch the chain of events unfold.
What they saw seems to confirm the hunch. In the neurons, the neural machinery broke down as SORL1 could no longer regulate the proteins responsible for trafficking proteins and other structures critical for cellular functioning. The researchers hypothesized that a loss of SORL1 led to problems with endosomes, cellular structures that serve as receiving-sorting centers for proteins, lipids, nutrients, and other molecules coming in and out of cells.
“When we reduced levels of SORL1, we saw the endosomes became enlarged,” says Young. “Enlarged endosomes fail to mature and function properly. With all kinds of cargo going in and out of the cells, we believe this defect may contribute to neurodegeneration and symptoms we associate with Alzheimer’s disease.”
However, the researchers did not see the same results in microglia. Unlike in neurons, reduced SORL1 in microglia did not result in traffic jams associated with enlarged endosomes. “Neurons and microglia are fundamentally different cell types,” explains Swati Mishra PhD, a postdoc in the Young Lab, and a co-author of the paper. “Neurons are known to have a single sorting station – what we call the early endosomes – while microglia have an additional sorting compartment – known as phagosomes, which aid in trafficking.”
Mishra notes that microglia are not entirely unaffected by changes in SORL1 levels. She points to current studies in the Young Lab indicating that in both neurons and microglia, loss of SORL1 results in altered trafficking to and from lysosomes, structures that break down molecules inside cells.
“Overall,” says Mishra, “Our research is exciting because it shows that the loss of SORL1, which is observed in Alzheimer’s disease, may affect different cell types in the brain differently. This suggests that any therapy has to consider how genetic risk factors for Alzheimer’s impact each cell type because alteration of the same gene could be beneficial in one cell type and detrimental in another.”
Young and her team also explored the connection between SORL1 and a protein known as APP – aka amyloid precursor protein. Amyloid, a molecule found in plaque form in Alzheimer’s patients, has been the focus of many unsuccessful clinical trials that have left researchers frustrated. Allison Knupp, a graduate student in the Young Lab, and a co-author of the study, says that the new study suggests amyloid plaque may not be the only culprit.
“It is known that problems with APP can cause enlarged endosomes,” explains Knupp. “In our experiments, however, we found that we continued to see enlarged endosomes even when we blocked the pathways that make amyloid from APP. We believe this shows that not all enlarged endosomes are caused by APP complications.” In real world terms this finding suggests someone who has a SORL1 variant of Alzheimer’s may not respond to a treatment that just targets APP.
One challenge for the team was pinpointing the size of the endosomes and the location of the APP fragments. Endosomes are difficult to study, particularly in neurons, where they are packed together more densely than they would be in other cell types. Expert help came from Dale Hailey, the Director of ISCRM’s Garvey Imaging Core, where high resolution imaging technology allowed the researchers to map and measure the endosomes and determine whether APP was aggregating in the tiny cellular structures.
Both insights have crucial therapeutic implications. Knowing that various cell types are affected differently means that a one-size-fits-all approach to treating Alzheimer’s disease will probably be ineffective. And, recognizing that the disease may be progressing before amyloid plaque buildup occurs helps explain why so many previous trials have failed.
“Our data really points to issues with endosomal trafficking as an independent event that can lead to dysfunctional Alzheimer’s disease,” says Young. “It doesn’t completely exonerate amyloid. What this tells us is that there are multiple pathways. We think we may be seeing an earlier stage of disease progression and that the two pathways could converge. This is going to affect how we design new treatments.”
Next, the researchers want to analyze the brains of people known to have genetic risk for the SORL1 mutation and compare signatures seen in the brain with the models of the central nervous system being derived in the lab.
This work was supported by NIH grants ( R01AG062148 ) and BrightFocus Foundation grant ( A2018656S ) to J.E.Y., a Biogen Sponsored Research Agreement to J.E.Y., an NIH training grant ( T32AG052354 ) to A.K., and a generous gift from the Ellison Foundation (to UW).