New hematologic, cardiovascular system link may have therapeutic implications

ATLANTA – An intriguing link between the sex steroid hormonal milieu and platelet mitochondria has potential implications for reducing thrombosis risk.

The link, which involves a mitochondrial protein known as optic atrophy 1 (OPA1), appears to play a role in the regulation of thrombosis, and provides a possible explanation for the marked differences in cardiovascular risks between men and women, according to E. Dale Abel, MD, chair of the department of internal medicine, and director of the Fraternal Order of Eagles Diabetes Research Center at the University of Iowa, Iowa City.

The findings could lead to risk-stratification strategies and the identification of therapeutic targets, Dr. Abel said in an interview.

^{Courtesy Dr. E. Dale Abel}

Dr. E. Dale Abel

OPA1 is an inner mitochondrial membrane protein involved in mitochondrial fusion, he explained.

“My laboratory, for a very long time, has been interested in the cardiovascular complications of diabetes, and a lot of our work has focused on the heart and on the relationship between changes in metabolism and mitochondrial biology in those complications. We got interested in platelets because of a collaboration that actually started with Dr. [Andrew] Weyrich when my lab was at the University of Utah. There was a request for proposals from the National Heart, Lung, and Blood Institute for projects that would seek to understand the increased risk of thrombosis that occurs in people with diabetes,” he said.

Specifically, Dr. Weyrich had some preliminary data showing a backup of intermediates of glucose metabolism occurring in the platelets of diabetics.

“This suggested either that there was increased import of glucose into those cells or a decreased ability of those cells to metabolize glucose,” Dr. Abel said, adding that a closer look at the expression of certain genes in platelets as they related to the risk of thrombosis showed that a number of mitochondrial genes were involved, including OPA1.

Since Dr. Abel’s lab was already involved with studying glucose metabolism and mitochondrial metabolism, and had created a number of tools for modifying alleles, which would enable the targeting of expression of some of these genes, he and his colleagues began to look closer at the role of OPA1.

“The relationship between OPA1 and platelet biology, at least based on epidemiological studies from Dr. [Jane] Freedman’s analysis of platelet RNA expression in the Framingham cohort, really seemed to suggest that this had more to do with events in females rather than males,” Dr. Abel said.

He and his colleagues then generated a mouse model in which OPA1 levels in platelets could be manipulated. The goal was to determine if such manipulation would affect platelet function or platelet biology, and also to see if the effects differed between males and females.

“Initially, we didn’t have an expectation that we would see a difference between males and females, but in retrospect, it actually fits very nicely with what the epidemiological data in humans would suggest,” he said, referring to the differences in thrombosis risk between men and women.

Mitochondria go through a process of fusion and fission; OPA1 is involved in the fusion of the inner mitochondrial membrane, which has many folds known as cristae.

“These cristae are very important in the ability of mitochondria to generate energy, and OPA1 plays a very important role in maintaining the structure of these cristae,” he explained.

He and his colleagues generated mice that lacked OPA1 specifically in platelets. They then characterized the mitochondria and platelet function in these knockout mice.

“We saw that there was a difference between males and females in terms of how they responded to OPA1 deletion. Specifically, the males appeared to get more overt mitochondrial damage in terms of their structure and function, whereas the mitochondria appeared remarkably normal in females,” he said.

A look at platelet function showed that platelets in males were somewhat hyperactive, while in females they were somewhat underactive.

When the researchers used a model of deep venous thrombosis (DVT), more than 90% of male knockout mice developed a DVT versus 50% of wild-type controls. In contrast, there was no increase in DVT in female knockout mice relative to wild-type controls.

“So they were really opposite phenotypes in terms of platelet activity, and whenever one sees a difference between sexes in any biological variable or phenotype, you wonder if this is because of sex hormones,” he said.

This question led to a number of additional experiments.

In one of those experiments, Dr. Abel and his colleagues used a mouse model in which platelets were depleted and replaced via transfusion with platelets from another animal.

“We took male mice that were wild type and we depleted their platelets, and then we took platelets from an OPA1 deficient female and transfused these back into male mice, and took OPA1 deficient platelets from males and transfused them back into platelet-depleted female hosts. The really interesting thing in those experiments was that the phenotype switched,” he said.

That is, platelets in male mice with OPA1 deficiency, which had increased platelet activation in the male mice, became hypoactive when they were transfused into female mice. Similarly, hypoactive platelets from female mice became hyperactive when transfused into platelet-depleted male mice.

“What this told us then, is that the hormonal milieu interacts with OPA1 deficiency to modulate the function of the platelets,” he said.

Additional hormonal manipulations, involving orchiectomy in male mice to lower testosterone levels and increase estrogen levels, and ovariectomy in female mice to lower estrogen levels, showed that this could also modify platelet response.

“So we have discovered that somehow the amount of OPA1 in platelets interacts with circulating estrogens to modify the activity of platelets. This is not a trivial issue, because, as in the epidemiological study, the relationship is something that seems to be particularly true in females, and it also turned out that the OPA1 tended to track with increased cardiovascular events,” he said.

Preliminary studies involving pregnant women, looking at both the first and third trimester (when estrogen levels spike), also showed a correlation between increased platelet activity in the third trimester and higher levels of OPA1 in their platelets.

“It seems there is a relationship between OPA1 and platelets in women and estrogen levels that may then increase the risk of thrombosis. Maybe our mouse model phenotype is explained by the fact that we did the opposite: We reduced OPA1 in the platelets of females, and we actually saw that this was protective,” he said.

The findings are generating excitement, according to Dr. Weyrich, professor of internal medicine and vice president for research at the University of Utah, Salt Lake City, who was involved in the earlier studies that led Dr. Abel and his team to delve into the OPA1 research.

During a presentation of Dr. Abel’s findings at the annual meeting of the American Society of Hematology, Dr. Weyrich called the work “really, really striking,” and said the gender-specific findings are of particular importance.

“It’s something we often overlook and don’t think about, but I think it’s something that’s probably going to be more and more important as we begin to understand all types of diseases both in benign and malignant hematology,” he said.

Dr. Abel and his team plan to do their part to further that understanding. They are awaiting word on a new National Institutes of Health grant that will allow for expansion of their mouse studies into humans. Specifically, those studies will look at correlations between levels of OPA1 expression in platelets in women and history of/risk for developing a thrombotic event.

“Thrombosis is a significant problem in women who are exposed to estrogens ... and with the exception of a small number of specific genetic disorders of platelets, very little is known about what the risk factors are for this estrogen dependent increase in thrombotic risk,” he said.

What needs to be uncovered, Dr. Abel said, is whether women with the highest levels of OPA1 carry the highest risk of thrombosis.