You just honed your expensive German steel chef’s knife and are steadily julienning away your carrots, when you feel the cold metal scrape along your skin. The scene on your cutting board is marred red and the pile of perfectly cut carrots is now smattered with blood droplets. Thankfully, the human body has evolved to not allow a small finger gash to be a major traumatic event due to unnecessary blood loss. Platelets morph in shape from round to spiny, sticking to each other and to the injured blood vessel walls, to begin patching the wound. The platelets join with other proteins to form a mesh-like plug, or clot, to stop the bleeding.
Yet, for the 1 in 25,000 people estimated to have MYH9-related disorders, caused by mutations in the MYH9 (myosin) gene, their blood doesn’t clot so well, resulting in a range of health issues—kidney failure, heavy menstrual periods, cataracts, and hearing loss, to list a few. However now, investigators at the University of Delaware (UD) along with collaborators at the National Institutes of Health reveal a number of wrong moves by blood cells on their creeping, crawling journey toward platelet formation.
Findings from the new study new published recently in Blood through an article titled “Megakaryocyte migration defects due to nonmuscle myosin IIA mutations underly thrombocytopenia in MYH9-Related Disease.”
While platelets normally are tiny, less than one-tenth the size of red blood cells, and the average person generates some 40 billion of these clotting cells a day, people with MYH9-related disorders have scant numbers of them. What’s more, their platelets are jumbo-sized and can even be as big as red blood cells.
“In people with MYH9-related disorders, the platelets are few and they are just way too big,” explained senior study investigator Velia Fowler, PhD, professor and chair of UD’s department of biological sciences. “They look like bluish giants in our stains under the microscope.”
Studying cells from mice that mimic the human disease, Fowler and her team began looking for defects in the platelet-making process. Platelets originate from humongous cells called megakaryocytes, which, in turn, are derived from stem cells, in the soft, gelatinous marrow at the center of your bones. These megakaryocytes creep and crawl from the bone marrow to neighboring blood vessels—the sinusoids—which have leaky walls that allow other cells to squeeze through. This is where the megakaryocytes extend branch-like arms called proplatelets into the blood vessel, and the circulating blood shears them off into many small platelets.
“Megakaryocytes are really, really large cells that glide forward pulling their large cell body along behind them like a snail dragging its shell,” Fowler said. “They use proteins like myosin and actin, which regulate muscle contractions, to move from here to there in a process called cell motility.”
How the cells know to go in a certain direction is still somewhat a mystery but involves the ability of the megakaryocyte to sense chemoattractant molecules released from the blood vessels, similar to a dog following a scent.
The team tracked and filmed these megakaryocytes during their migration from bone to blood vessel. Their focus was three mouse-cell lines, each representing a different known mutation in the myosin-9 protein molecule (for which the MYH9 blood disorder is named).
Using an inverted microscope, which allows researchers to view and film samples of live cells from below rather than from the top down, the researchers recorded the direction and distance these megakaryocytes traveled. When they plotted the data, they could see that the mutant cells were all going in the wrong direction like a pack of bloodhounds that had lost their sense of smell.
“They have lost their way somehow, but we don’t really know why,” Fowler remarked.
The mutant cells also move in erratic ways: too slowly or too randomly or much faster than they should, sometimes almost in a hyper state.
“The megakaryocyte cells can’t get there—to the blood vessel—so you can’t get platelets,” Fowler noted, “but the reason they can’t get there is different for each mutation.”
Specifically, cells with the R702C mutation experience a loss of myosin contractility—the ability of their microscopic muscle-like cellular structures to contract—making them too slow; cells with the D1424N mutation gain greater contractility resulting in rapid and at times hyperactive movement; and cells with the E1841K mutation produce contractility at random.
Based on these findings, Fowler said, personalized drug therapies and treatments would be needed to enhance or reduce the cells’ directionality and movement issues, depending on the patient-specific mutation.
“Just as megakaryocyte migration properties are affected by improper MYH9 myosin function, it is also possible that clots formed by the platelets carrying these mutations are unstable,” Fowler concluded. “Further hematological analysis of platelet properties from MYH9-RD patients will be required to determine if these mutations affect clot formation. Since many patients with MYH9-RD also develop cataracts, hearing loss, and kidney problems, our study can also shed light on the causes of other defects associated with this disease in patients.”
