The impact of progranulin species on TDP-43 proteolysis and pathobiology

Neurodegenerative disorders such as Alzheimer’s Disease (AD), frontotemporal dementia (FTD) and limbic-predominant, age-associated TDP-43 encephalopathy (LATE), are age-associated diseases characterized by aberrant protein homeostasis. Lysosomes and the lysosome-dependent autophagic processes degrade and recycle macromolecules, thereby promoting cellular protein homeostasis and preventing inclusion formation. Proteolysis in the lysosome is carried out by a suite of proteases, known as cathepsins, that each recognize specific linear amino acid sequences. Declines in lysosomal function with age have been shown to contribute to neurodegenerative disease pathogenesis and genetic evidence also strongly supports the importance of lysosomes in AD, FTD and LATE. For example, loss-of-function mutations in the Pgrn gene lead to protein haploinsufficiency and FTD with TDP-43 aggregates. PGRN is a lysosome-resident protein that can be cleaved first into bioactive multi- granulin fragments (MGFs) and then individual granulins (henceforth collectively referred to as PGRN species). We have demonstrated that 1) relative levels of PGRN species change with age, 2) PGRN and MGFs promote the activity of a key lysosomal protease, CTSD, while 3) granulins inhibit cathepsin activity and TDP-43 clearance. Still unclear are precisely how PGRN species alter TDP-43 cleavage and why these processes change with age and Pgrn haploinsufficiency. The major focus of my research group is to utilize multi-disciplinary approaches to understand how lysosomes contribute to aberrant proteostasis in aging and neurodegeneration. Our objective here is to determine how PGRN species contribute to TDP-43 accumulation with age and Pgrn haploinsufficiency. Our central hypothesis is that age and Pgrn haploinsufficiency alter the relative levels of intact vs cleaved PGRN species, thereby impacting the ability of lysosomal proteases to degrade TDP-43. The rationale for this work is that if TDP-43 is not efficiently degraded, steady-state levels will increase, promoting TDP-43 aggregation and potentially contributing to selective vulnerability to TDP-based diseases. To address our hypothesis, we propose the following specific aims. Aim 1: Test the ability of PGRN species to modulate TDP-43 proteolysis in vitro. Aim 2: Define the capacity of induced neurons, astrocytes and microglia with and without Pgrn deficiency to degrade and clear TDP-43. Aim 3: Determine how PGRN species and proteolysis of TDP-43 changes with age and PGRN deficiency in murine models. Successful completion of the proposed aims will enhance understanding of the factors governing TDP-43 proteolysis with age and disease. This information could reveal new therapeutic avenues for treating TDP-43 diseases and has implications for precision medicine in other forms of neurodegeneration.