Understanding how tumor cells transition to an invasive and drug-resistant phenotype is central to cancer biology, but the mechanisms underlying this transition remain unclear. We show that sarcomas gain these malignant traits by inducing lysosomal exocytosis, a ubiquitous physiological process. During lysosomal exocytosis, the movement of exocytic lysosomes along the cytoskeleton and their docking at the plasma membrane involve LAMP1, a sialylated membrane glycoprotein and target of the sialidase NEU1. Cleavage of LAMP1 sialic acids by NEU1 limits the extent of lysosomal exocytosis. We found that by down-regulation of NEU1 and accumulation of oversialylated LAMP1, tumor cells exacerbate lysosomal exocytosis of soluble hydrolases and exosomes. This facilitates matrix invasion and propagation of invasive signals, and purging of lysosomotropic chemotherapeutics. In Arf^−⁄− mice, Neu1 haploinsufficiency fostered the development of invasive, pleomorphic sarcomas, expressing epithelial and mesenchymal markers, and lysosomal exocytosis effectors, LAMP1 and Myosin-11. These features are analogous to those of metastatic, pleomorphic human sarcomas, where low NEU1 levels correlate with high expression of lysosomal exocytosis markers. In a therapeutic proof of principle, we demonstrate that inhibiting lysosomal exocytosis reversed invasiveness and chemoresistance in aggressive sarcoma cells. Thus, we reveal that this unconventional, lysosome-regulated pathway plays a primary role in tumor progression and chemoresistance.

Alzheimer’s disease (AD) belongs to a category of adult neurodegenerative conditions, which are associated with intracellular and extracellular accumulation of neurotoxic protein aggregates. Understanding how these aggregates are formed, secreted and propagated by neurons has been the subject of intensive research, but so far no preventive or curative therapy for AD is available, and clinical trials have been largely unsuccessful. Here we show that deficiency of the lysosomal sialidase NEU1 leads to the spontaneous occurrence of an AD-like amyloidogenic process in mice. This involves two consecutive events linked to NEU1 loss-of-function—accumulation and amyloidogenic processing of an oversialylated amyloid precursor protein in lysosomes, and extracellular release of Aβ peptides by excessive lysosomal exocytosis. Furthermore, cerebral injection of NEU1 in an established AD mouse model substantially reduces β-amyloid plaques. Our findings identify an additional pathway for the secretion of Aβ and define NEU1 as a potential therapeutic molecule for AD.

Expansion of fibrous connective tissue and abnormal deposition of extracellular matrix (ECM) are at the basis of many fibrotic diseases. Fibrosis can occur in response to both physiological and pathological cues, including wound healing, tissue remodeling/repair and inflammation. Chronic fibrosis can lead to severe tissue damage, organ failure and death. Assessing the extent of organ fibrosis is crucial for accurate diagnosis of this condition. The use of Masson’s trichrome staining of tissue sections from skeletal muscle is a fast method for detection of morphological alterations indicative of a fibrotic phenotype in this organ. This staining method detects the extent of collagen fibers deposition and, because it employs the combination of three dyes, can also distinguish muscle fibers (red), from collagen (blue) and nuclei (black), simultaneously. Bio-protocol is an online peer-reviewed protocol journal. Its mission is to make life science research more efficient and reproducible by curating and hosting high quality, free access protocols.

Neuraminidase initiates the hydrolysis of sialo-glycoconjugates by removing their terminal sialic acid residues. In humans, primary or secondary deficiency of this enzyme leads to two clinically similar neurodegenerative lysosomal storage disorders: sialidosis and galactosialidosis (GS). Mice nullizygous at the Neu1 locus develop clinical abnormalities reminiscent of early-onset sialidosis in children, including severe nephropathy, progressive edema, splenomegaly, kyphosis and urinary excretion of sialylated oligosaccharides. Although the sialidosis mouse model shares clinical and histopathological features with GS mice and GS patients, we have identified phenotypic abnormalities that seem specific for sialidosis mice. These include progressive deformity of the spine, high incidence of premature death, age-related extramedullary hematopoiesis, and lack of early degeneration of cerebellar Purkinje cells. The differences and similarities identified in these sialidosis and GS mice may help to better understand the pathophysiology of these diseases in children and to identify more targeted therapies for each of these diseases.