Progranulin regulates neuronal morphology through activation of ERK pathway

Abstract

Frontotemporal lobar degeneration (FTLD) is the second most common form of presenile dementia after Alzheimer's disease. FTLD is divided into two main subgroups: those with tau-positive pathology (FTDP-tau), and those with TDP-43- and ubiquitin-positive, but tau- and a-synulcein-negative, pathology (FTLD-U). Null mutations in progranulin (PGRN) are a major cause of FTLD-U, creating premature termination of the coding sequence and degradation of the mutant RNA by nonsense-mediated decay resulting in a haploinsuffiency. Recent studies report that PGRN acts as a protective neurotrophic factor regulating neuronal survival and growth. However, little is known about its function in the CNS or its relation to neurodegeneration. To study the impact of loss of PGRN on neurodegeneration, we utilized murine PGRN knockout and wildtype primary hippocampal neurons. Primary neurons plated on coverslips were stained with MAP2 and analyzed by MetaMorph software to detect changes in neuronal morphology. To further examine the effects of PGRN using this model, we treated these cultures with recombinant PGRN (rPGRN) and ERK inhibitors. We developed a primary cell culture model demonstrating that loss of PGRN in primary neurons produces morphological deficits when compared to wildtype neurons. These deficits were rescued by the overexpression of rPGRN. Additionally, we determined that rPGRN induced neurite outgrowth is dependent on the ERK pathway. We identified that loss of PGRN in primary neurons causes morphological changes which could explain how PGRN haploinsufficiency in patients leads to neurodegeneration. Treatment with rPGRN can rescue this phenotype and promote neurite outgrowth and branching. Interestingly, we determined that these neurotrophic effects of PGRN are dependent on activation of the ERK pathway. This data will shed further insight on the mechanisms underlying neurodegeneration due to loss of PGRN, and provide information into the development of FTLD-U and other neurodegenerative diseases. Establishment of this model, as well as mechanisms behind the neurotrophic functions of PGRN, will be a useful tool allowing us to assess how loss of PGRN may increase neuronal vulnerability and ultimately the functions of PGRN in the CNS.