Low levels of an enzyme associated with the rare childhood disorder sialidosis may lead to the development of idiopathic pulmonary fibrosis (IPF), according to new research. The findings also suggest that NEU1 enzyme levels may help in determining fibrosis’ risk and likely prognosis.
The study “Excessive exosome release is the pathogenic pathway linking a lysosomal deficiency to generalized fibrosis” was published in the journal Science Advances.
When activated, cells called fibroblasts and myofibroblasts are key players in the deposition of extracellular matrix (ECM) — which provides structural and biochemical support to cells — that leads to fibrosis, or scarring.
Molecules such as TGF-beta and WNT can induce the production of ECM components by myofibroblasts, and the expression of genes involved in cell proliferation, while also making cells resistant to apoptosis, or “programmed” cell death as opposed to death caused by injury.
Research has shown an association between a deficiency in the enzyme neuraminidase 1 (NEU1) and the buildup of connective tissue (fibrosis) in organs such as the kidney, liver, heart, and lungs.
Tiny vesicles known as exosomes are involved in ECM remodeling via their ability to serve as delivery agents between cells and to induce cell signaling. ECM remodeling also requires delivery of lysosomal enzymes — those produced by lysosomes, structures that break down and recycle membranes and other molecules — a process suppressed by the NEU1 enzyme.
Mutations in the NEU1 gene cause sialidosis, a pediatric lysosomal storage disease. Prior work showed that mice lacking this gene developed muscle atrophy (shrinkage) upon proliferation of connective tissue and ECM deposition. The loss of NEU1 is also associated with sarcoma, a tumor of the connective tissue. Still, the link between NEU1 deficiency and connective tissue disease remains poorly understood.
A team at St. Jude Children’s Research Hospital assessed the role of exosomes, released by muscle fibroblasts from mice lacking the NEU1 gene, on fibrosis.
The researchers initially found that fibroblasts lacking the NEU1 gene were activated into myofibroblasts in a self-perpetuating mechanism driven by exosomes, with marked deposition of collagen (a primary ECM component) as well as a greater ability to proliferate than controls.
Levels of TGF-beta and of other markers of fibrosis — such as E-cadherin, vimentin, and collagen — were significantly higher in connective tissue lacking NEU1 than in normal tissue. Periostin and osteopontin, two proteins involved in tissue repair, inflammation, and fibrosis, were detected only in connective tissue without NEU1.
Activated myofibroblasts were also seen to release excessive amounts of exosomes loaded with molecules capable of activating mouse and human fibroblasts, such as TGF-beta, WNT, and members of their signaling pathways. Similar results were observed with exosomes collected from mouse tissue lacking NEU1, and from fibroblasts derived from sialidosis patients.
Importantly, intravenous injection of viral vectors expressing the human NEU1 gene reversed these alterations in muscle connective tissue, “confirming NEU1 deficiency as the primary insult,” the researchers wrote.
Tests on lung fibroblasts from IPF patients found significantly lower NEU1 expression compared to healthy people; this was associated with higher amounts of molecules promoting cell differentiation into myofibroblasts.
“On the basis of our findings, deregulated levels of this enzyme [NEU1] should be sought as a contributing factor in some of the numerous idiopathic forms of fibrosis in humans,” the researchers wrote.
This work marks “the first time NEU1 has been associated with fibrotic conditions,” Alessandra d’Azzo, PhD, the study’s senior author, said in a press release.
“Based on these findings, it is tantalizing to hypothesize that NEU1 expression levels may help identify individuals at risk for fibrosis or inform their prognosis, particularly when information about the cause or possible treatment is lacking,” d’Azzo added.