The lack of a single protein usually thought of as a run-of-the-mill enzyme that helps to recycle molecules in cells causes an incurable and often fatal disease of children, according to St. Jude Children's Research Hospital investigators.
Children with this disease, called sialidosis, suffer from enlarged spleens and often develop vision problems, loss of coordination and seizures, among other symptoms. The patients generally die within the first few years of life.
St. Jude investigators showed in test tube experiments and mouse models of sialidosis that the loss of the protein NEU1 triggers a catastrophic falling of biochemical dominos that ultimately leads to disruption of normal formation of mature blood cells. A report on this work appears in the July 8, 2008, issue of the journal Developmental Cell.
"The discovery is important because it explains why patients with sialidosis have enlarged spleens and suggests that new drugs or gene therapies that target that problem might be an effective therapy," said Alessandra d'Azzo, Ph.D., a member in the St. Jude Department of Genetics and Tumor Cell Biology and the paper's senior author. "The results also explain how the loss of NEU1 can cause bone marrow transplantations to fail, and therefore suggests that such failures might also be corrected by target therapeutics."
The researchers showed that NEU1 controls how bags of digestive enzymes inside white blood cells, neutrophils and macrophages, discharge their contents into the bone marrow environment in a highly regulated process known as lysosomal exocytosis. These bags of enzymes, called lysosomes, rarely discharge their content outside of the cell. Instead, they use their enzymes inside the cell to digest no longer needed products into small building blocks that the cell can reuse or dispose.
The St. Jude team found that in the absence of NEU1, white cell lysosomes are more prone than normal lysosomes to dock and eventually fuse with the cell membrane and subsequently spill their active enzymes into the bone marrow environment. This aberrant behavior hampers the ability of hematopoietic stem cells (HSCs) to be correctly retained within the bone niche. HSCs are immature cells that give rise to all the types of blood cells in the body. The researchers showed that the released enzymes prematurely digest a protein on bone marrow stromal cells called VCAM-1, a molecule that these cells use to hold onto HSCs in the bone marrow.
Deprived of their normal nurturing environment, the HSCs migrate out of the bone marrow and into the spleen, crowding into the organ until it becomes severely enlarged. "Our work represents an unexpected and important clue to one of the prominent clinical manifestations of sialidosis patients," d'Azzo said. "We were surprised to discover that an old, ubiquitous enzyme in lysosomes better known for digesting cellular waste products plays such an important role in a basic biological process that when exacerbated contributes to the outcome of such a terrible disease in children."
In a series of experiments, d'Azzo's team discovered that the way NEU1 regulates the pool of lysosomes destined for lysosomal exocytosis is by cutting off a sugar called sialic acid from a structural protein of the lysosomal membrane, known as LAMP-1. They found that LAMP-1 is involved in the docking of lysosomes at the cell membrane, a prerequisite for these organelles to fuse with the cell membrane and release their content outside the cell. When NEU1 strips the sialic acids off LAMP-1, this protein is rapidly turned over so that its total amount is reduced. Less LAMP-1 at the lysosomal membrane influences the capacity of lysosomes to dock at the cell membrane and to engage in lysosomal exocytosis into the bone marrow environment.
"The evidence strongly suggests that in children lacking a normal gene for NEU1, disruption of the bone marrow environment causes the exodus of hematopoietic cells from the marrow to the spleen," d'Azzo said. "That leads to development of the symptoms of sialidosis. Although we don't have a cure for this terrible disease, we are now beginning to identify alternative ways to improve the disease outcome in affected children."
These findings offer new insight into why bone marrow transplants do not work in humans who lack the neu1 gene, d'Azzo said. Bone marrow cells transplanted into patients should normally home in the bone marrow niche and stay there until they mature. But if the bone marrow environment is hostile because of the loss of NEU1, the donated stem cells migrate out of the marrow and the transplant fails.
"The exciting thing about this work is that it sheds light on two major issues: the cause of sialidosis and the reason for bone marrow transplantation failure in the absence of NEU1," d'Azzo said. "This wealth of new information gives us a better understanding of the physiological function of NEU1, which appears to be much broader than originally thought. This illustrates the important role of basic research in making discoveries that have major implications for medical problems."
Other authors of this paper include Erik Bonten, Diantha van de Viekkert, Huimin Hu, Simon Moshiach and Samuel Connell (St. Jude) and Gouri Yogalingam (formerly of St. Jude).
This work was supported by the National Institutes of Health, the Assisi Foundation of Memphis and ALSAC.