Summary: New research shows that progranulin deficiency may contribute to neurodegeneration in frontotemporal dementia by reducing lysosomal support for key proteases.
Source: VIB Flanders
Progranulin stabilizes lysosomal enzyme cathepsin D, study finds
In a study published in Human Molecular Genetics, researchers at VIB and KU Leuven, led by Professor Philip Van Damme in collaboration with Professor Paul Saftig (University of Kiel), describe a previously unrecognized lysosomal function of progranulin. The team demonstrates that progranulin acts as a chaperone for the lysosomal aspartic protease cathepsin D (CTSD), stabilizing the enzyme and enhancing its proteolytic activity. These findings link progranulin deficiency to impaired lysosomal enzyme function and provide a potential mechanism for neuronal dysfunction in frontotemporal dementia (FTD).
Frontotemporal dementia: clinical background
Frontotemporal dementia (FTD) is the second most common cause of early-onset dementia after Alzheimer’s disease. It primarily affects people in mid-40s to mid-60s and is marked by progressive loss of neurons in the frontal and anterior temporal lobes. Clinically, FTD often presents with profound changes in behavior—such as apathy or disinhibition—that disrupt normal social functioning, and with language disturbances that can range from semantic impairment to primary progressive aphasia. There is currently no disease-modifying treatment for FTD, and average survival after symptom onset is limited.
Approximately half of FTD cases show a familial pattern. Mutations in the progranulin gene (GRN) are among the most frequent genetic causes of FTD and typically result in haploinsufficiency, producing roughly 50% of normal progranulin protein levels. Attempts to model full progranulin loss in mice have produced only mild phenotypes without overt neurodegeneration, complicating efforts to clarify how GRN deficiency leads to neuronal loss in humans.
Progranulin as a neurotrophic factor in vivo
Previous in vitro work established that progranulin promotes neuronal survival and neurite outgrowth, suggesting a neurotrophic role. To test this in a living system, the authors used a facial nerve crush model to study axonal regeneration in vivo. Mice lacking progranulin (GRN-/-) showed delayed recovery after facial nerve injury, indicating impaired axonal outgrowth. Reintroducing human progranulin into these mice fully rescued the regenerative deficit, demonstrating that progranulin is specifically required for axonal regrowth after nerve injury.
Using cell-type specific knockout models, the researchers found that basal progranulin expression in the brain is predominantly neuronal, and that deletion of neuronal progranulin—rather than microglial progranulin—was sufficient to reproduce the axonal outgrowth impairment. This confirms that neuronal progranulin plays a critical role in promoting axonal regeneration in vivo.
Mechanism: progranulin acts as a chaperone for cathepsin D
To identify molecular mediators underlying the regenerative defect, the team performed transcriptome analysis of the facial motor nucleus after injury. The lysosomal protease cathepsin D (CTSD) emerged as the most upregulated gene in progranulin-deficient mice. Despite increased CTSD expression, aged GRN-/- cortical tissue displayed reduced relative CTSD enzymatic activity, suggesting a post-translational impairment.
Biochemical experiments demonstrated a direct interaction between progranulin and CTSD. Addition of progranulin enhanced CTSD proteolytic activity in a dose-dependent manner in vitro. The C-terminal granulin domain (GrnE) also stimulated CTSD activity. Progranulin binding prevented heat-induced CTSD degradation and stabilized the enzyme across temperatures. These findings support a model in which progranulin functions as a lysosomal chaperone that preserves CTSD stability and proteolytic capacity.
Functionally, the progranulin–CTSD interaction proved necessary for recovery after facial nerve injury: combined partial reductions of progranulin and CTSD led to synergistic impairment of axonal outgrowth. This links the neurotrophic effects of progranulin to its role in maintaining lysosomal protease function.
Significance and implications
These results provide a mechanistic explanation for how progranulin haploinsufficiency may contribute to neuronal dysfunction in FTD: reduced progranulin levels compromise lysosomal protease stability and activity, potentially impairing protein turnover and cellular homeostasis in neurons. By identifying progranulin as a chaperone for CTSD, this work highlights lysosomal enzyme support as a candidate pathway for therapeutic development in progranulin-related FTD.
About this research
This research was conducted by Sander Beel, Matthieu Moisse, Markus Damme, Louis De Muynck, Wim Robberecht, Ludo Van Den Bosch, Paul Saftig, and Philip Van Damme and was published in Human Molecular Genetics (online April 26, 2017). The study links progranulin’s neurotrophic properties to a lysosomal chaperone function that stabilizes cathepsin D and supports axonal outgrowth in vivo.
Abstract (condensed)
Loss-of-function mutations in progranulin (GRN) cause frontotemporal dementia, but the mechanism of GRN haploinsufficiency remains unclear. Using an in vivo axonal regeneration model, GRN-/- mice showed delayed recovery after facial nerve injury that was rescued by human GRN and depended on neuronal GRN production and its C-terminal domain. Transcriptomic analysis identified cathepsin D (CTSD) as strongly upregulated, while aged GRN-/- cortices exhibited reduced CTSD activity that improved with exogenous GRN. Both progranulin and its granulinE domain stimulated CTSD activity in vitro, and pull-down assays confirmed a direct interaction that stabilized CTSD. Combined reductions in GRN and CTSD synergistically impaired axonal outgrowth, linking GRN’s neurotrophic effect to a lysosomal chaperone role maintaining CTSD proteolytic capacity.