How type 2 diabetes develops remains poorly understood. One reason is that many of the tissues involved are deep inside the abdominal cavity, making studying living people very risky. In a new study, researchers used paired tissue samples from organ donors to create an atlas of how type 2 diabetes changes protein expression across multiple organ systems. A researcher hopes the method will generate hypotheses for future studies.
Although the effects of type 2 diabetes are well documented, the road to type 2 – the cellular and organ-level changes between normal function and full-blown disease – remain poorly understood. Because of standard practices at university tissue banks, researchers must look at a single organ in isolation – for instance, just the pancreas or liver – without understanding how it fits into the broader context of the other organs and tissues.
New research, published in Cell Reports Medicine, used paired tissue samples from organ donors to create an atlas of how diabetes changes protein expression across multiple organ systems.
“This is a unique resource,” says lead author Klev Diamanti, a Data Scientist at the Department of Immunology, Genetics and Pathology at Uppsala University in Sweden. The opportunity to “combine the changes that happen in different organs in different stages of diabetes” could go a long way in pinning down the mysterious mechanisms of the disease.
An extraordinary gift
Researchers hoping to study the development of type 2 diabetes have uneven access to the tissues involved. “Fat, for example, and also of course blood are easy to get” from living people, Diamanti says. But organs key to understanding diabetes – such as the pancreatic islets – are buried deep in the abdominal cavity and nearly impossible to access without risking damage to other internal organs.
But about a decade ago, collaboration with the Swedish organ transplantation network opened new doors for diabetes researchers, explains co-author Claes Wadelius, Professor of Medical Genetics, who studies the development of diabetes at Uppsala University.
The new programme enabled people who had elected to donate their organs for transplantation to choose to donate their tissues to scientific research. This created a unique opportunity to acquire paired samples – essentially sets of organ and tissue samples known to be from the same donor – that enable researchers to better understand how organ systems interact and create a more full-body picture of the disease.
For this study, the researchers investigated five key tissues from 43 individual donors – blood serum; the liver, which is responsible for glucose production; visceral adipose tissue, fat that surrounds the internal organs and can affect insulin sensitivity; the pancreatic islets, clusters of insulin-secreting cells considered the epicentre of diabetes; and skeletal muscle, which in addition to all our voluntary movements also stores significant energy.
Many people do not show any symptoms
The donors were grouped according to their ability to regulate blood sugar – people diagnosed with type 2 diabetes, people with normal blood sugar regulation (normoglycaemics) and a third category of people with prediabetes.
People with prediabetes have disrupted blood sugar regulation but not as severe as type 2 diabetes. “Many people do not show any symptoms at that stage and do not feel anything wrong with their body,” Diamanti explains. “So they continue their lives as usual.” Prediabetes is diagnosed by tracking levels of glycated haemoglobin for 3–4 months, and the researchers had access to those data and limited other information through the transplantation network.
The researchers emphasise that the samples from people with normoglycaemia, prediabetes and diabetes are not a time sequence, and taking samples as the disease progresses in an individual would be the gold standard. But failing that, looking at these groups can give scientists the lay of the land.
A biological logbook
The researchers turned to protein analysis to determine the condition of these five organs and tissues. Proteins act as a logbook of what genes are being expressed and the rate of activity of various metabolic pathways – such as the breakdown and storage of glucose.
But proteins are not easy to study. “When we started this project, we were considering technologies that could measure perhaps 100 proteins” from a single sample, Wadelius says – a drop in the bucket of the (tens of thousands of?) proteins at work in any given tissue. But during the study, the lab of co-author Matthias Mann at the University of Copenhagen developed a new protein analysis technique that Wadelius says elevates proteomics to new heights. With Mann’s new technology, “we could measure 3000, 4000 or 5000 proteins from each tissue,” Wadelius says.
Change in fits and starts
What did the researchers glean from their unexpected wealth of protein data?
Unsurprisingly, people with normal blood sugar regulation and people with type 2 diabetes had the biggest differences in protein expression. But if we consider the three groups as a makeshift time series, we can reverse-engineer a timeline of change.
Most of the differences found between healthy donors and those with prediabetes are in the pancreatic islets and, to a lesser extent, the liver. That did not surprise the researchers – both organs have well-documented involvement in the early development of diabetes. The “vast majority” of changes in protein expression in the islets and liver involved immune responses, the authors write.
The scientists were intrigued to find that protein expression in the pancreatic islets and liver did not differ much between those with prediabetes and type 2 diabetes – suggesting that these metabolic changes are already established well before symptoms emerge.
By the time full-blown type 2 diabetes develops, there are also changes to skeletal muscle and visceral adipose tissue compared with prediabetes. These changes centre around cellular respiration – the process if converting glucose into ATP, the body’s energy currency.
Notably, the researchers observed that changes to protein expression were not necessarily linear – the expression of a certain group of proteins could be dramatically higher in prediabetes than in healthy tissue but could fall back to an intermediate level in type 2 diabetes.
A jumping-off point
Although collecting five distinct samples from 43 organ donors was a feat, Wadelius says that this number is a fraction of the sample size necessary to draw any statistically meaningful conclusions – especially since there are multiple ways to arrive at type 2 diabetes. He points to a study by Lund University that analysed tens of thousands of individuals and ultimately identified distinct subcategories of the disease that behave in different ways. These subcategories almost certainly have different protein and gene activity patterns too.
Although the specific results of the study are intriguing, the researchers say its true value is as a proof of concept for the protein analysis techniques and the opportunities presented by paired samples. “We believe that this is more of a map, more of a hypothesis generator” for future studies, Diamanti explains.