Alessandra d'Azzo Lab

Cathepsin A, neuraminidase 1 (NEU1), and b-galactosidase (b-GAL) are three lysosomal enzymes that assemble into a high-molecular weight lysosomal complex. In this configuration, the enzymes modulate each other’s activity to optimally tackle the substrates they need to process. Because of their dependency on one another, single or combined deficiency in any of these three enzymes results in disease. Our goal is to dissect the features these enzymes share and uncover the unique aspects of pathogenesis in the associated diseases.

• Galactosialidosis – This lysosomal storage disease primarily affects glycoprotein catabolism, hence it belongs to the glycoprotein storage disease subgroup. A deficit in the cathepsin A enzyme causes the disease, which leads to a complete loss of associated NEU1 activity and partial deficiency of b-GAL. As a result of this triple enzyme deficiency, undigested materials abnormally accumulate in lysosomes and affect multiple organs of the body and nervous system. The earlier the age of symptom onset, the more widespread and fatal the disease proves to be. 
  
  
• Sialidosis – Similar to galactosialidosis, sialidosis is a glycoprotein storage disease that links exclusively to the defect of NEU1 and leads to an abnormal accumulation of undigested, sialylated metabolites in multiple organs of the body and nervous system. Using an experimental laboratory model of sialidosis, we discovered several pathologic features in the brain that closely mimic Alzheimer’s disease. These include progressive accumulation of toxic amyloid proteins in the hippocampus region of the brain—important for learning and memory—severely impaired microglia function, and extensive neuroinflammation.
  
  There is no effective treatment for these two catastrophic lysosomal storage diseases in the clinic, but several therapeutic approaches, including gene therapy, have proven successful in laboratory models of these diseases.
  
  
• GM1 gangliosidosis – This disease associates with isolated deficiency of b-GAL. The enzyme is pivotal for the proper processing and degradation of a specific lipid, which is abundant in the brain, called GM1-ganglioside. Thus, loss of b-GAL function leads to the accumulation of GM1 ganglioside in neurons of the brain and spinal cord, which influences the overall lipid and protein composition of neuronal membranes, including the plasma membrane (PM). This ultimately provokes neuronal cell death and is accompanied by a massive, widespread neuroinflammatory response. 
  
  An AAV-mediated gene therapy for infantile GM1-gangliosidosis is currently in the clinic.

While studying the cause of pathogenesis in a sialidosis model, we identified a previously unknown function of NEU1 that controls a ubiquitous process called lysosomal exocytosis. This process is primarily used by cells to repair their plasma membrane (PM) when small tears occur. It entails the movement of lysosomes to the site of damage at the PM, where they dock and eventually fuse with the PM. The docking step of the pathway is controlled by the lysosome-associated membrane protein (LAMP1), which is a natural substrate of NEU1. The fusion event results in the redistribution of lysosomal membrane proteins at the PM and releases part of the lysosome contents extracellularly. 

Impaired functionality of lysosomal exocytosis links to several diseases. In contrast, in sialidosis, NEU1 deficiency provokes lysosomal accumulation of LAMP1 in an unprocessed sialylated state, which eventually causes lysosomes to dock in exceeding number at the PM, ready to fuse with the PM and abnormally release their contents extracellularly. This exacerbated lysosomal exocytosis, which occurs in cells of virtually all organs of the body and the nervous system, initiates and perpetuates a pathogenic cascade that leads to organ dysfunction and neurodegeneration. In neurons, for instance, the abnormal extracellular deposition of amyloid occurs via excessive lysosomal exocytosis, which in turn elicits a massive neuroinflammatory response. 

Most importantly, this dysregulated pathway downstream of NEU1 deficiency can explain aspects of disease pathogenesis not only in this pediatric lysosomal disease but also in more common adult conditions, like Alzheimer’s disease, idiopathic fibrosis, and cancer. These studies are the main research focus in the d’Azzo laboratory.

In all our work, we strive to identify the correct starting point of pathogenesis in these genetic neurodegenerative diseases that raise awareness to the lysosomal system’s role in both pediatric and adult diseases. We apply a variety of approaches—ex vivo gene therapy, AAV-mediated therapy, enzyme replacement, drug therapy—that strive to restore enzyme function and aid the development of effective therapies for pediatric and adult diseases affecting the lysosomal system. A consistent, relentlessly curious approach guides all the work our laboratory leads.