Neuron-microglia crosstalk in FTLD-GRN models

Reich, Marvin (2024): Neuron-microglia crosstalk in FTLD-GRN models. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

Frontotemporal dementia (FTD) is a devastating neurodegenerative disease affecting more than a million people worldwide. Progranulin (PGRN) haploinsufficiency is the primary genetic cause of a subtype of frontotemporal lobar degeneration (FTLD-GRN) presenting with behavioral changes and TDP-43 pathology. FTLD-GRN, due to its monogenic nature, facilitates studies to better understand the molecular determinants of FTD and to develop novel treatment strategies. Nevertheless, to date, the pathobiology of FTLD-GRN is still only partially understood and there is no disease-modifying treatment available. Several treatment strategies for FTLD-GRN are already in clinical development, all aiming to raise PGRN levels in the brain. However, none of them proved efficacy on TDP-43 pathology, behavioral phenotypes, or neurodegeneration. In this study, a novel treatment approach was established by applying a protein replacement approach using recombinant PGRN engineered for transferrin receptor-mediated blood-brain-barrier transcytosis, combined with a liver-targeting adenovirus-associated virus (AAV) gene therapy approach (AAV(L):bPGRN). The AAV(L):bPGRN treatment was tested in two preclinical FTLD-GRN models, namely Grn knockout (KO), and double KO (DKO) mice, in which both Grn and the FTLD-GRN risk factor Tmem106b are knocked out. The recently described DKO model not only exacerbates the phenotypes of Grn KO mice, such as neuroinflammation, lipidome alterations, and autophagic and lysosomal dysfunction. Additionally, DKO mice have a reduced life span, strong neurodegeneration, motor impairment, as well as TDP-43 pathology, a hallmark of FTLD-GRN. Using this AAV-based approach, all of the phenotypes in Grn KO and DKO mice could be ameliorated or fully reversed. The PGRN replacement strategy was further translated to a human induced pluripotent stem cell (hiPSC)-based model, in which microglia, derived from newly generated DKO hiPSCs, were co-cultured with wildtype neurons. The phenotypes observed in this in vitro hiPSC model, such as neuronal degeneration, TDP-43 pathology, and lysosomal dysfunction were all successfully rescued as well. These findings strongly support both PGRN replacement therapy as a promising treatment approach, as well as the DKO mouse model and hiPSC models as useful tools to study FTLD-GRN. The second project addresses a related major open question in the FTD field, which is how loss of PGRN, a lysosomal protein primarily expressed in microglia, eventually leads to TDP-43 pathology in neurons, subsequently resulting in neurodegeneration. The lysosomal protease legumain (LGMN) appears to be a perfect link, as it is critical for the correct maturation of several other lysosomal proteases and was previously shown to be able to process PGRN, as well as TDP-43, into disease-associated fragments. Here, LGMN levels and activity were shown to be increased in both FTLD-GRN patient brains and in multiple FTLD-GRN models, including hiPSC-derived microglia and, age-independently, in Grn KO mice. In vitro, LGMN maturation was slowed down in the presence of full-length PGRN, but matured LGMN rapidly processed PGRN, showing a reciprocal regulation that could be disrupted in FTLD-GRN. Given the predominantly microglial expression of LGMN, a pathomechanism was proposed, in which excess LGMN in GRN KO microglia could be transferred to neurons resulting in TDP-43 processing and aggregation, as observed in FTLD-GRN. To test whether LGMN transfer from microglia to neurons could be possible, neuronal uptake and activation of LGMN from conditioned media by neurons was shown, causing increased TDP-43 processing, which could be ameliorated by LGMN inhibition. AAV-mediated overexpression of LGMN in mouse brain also resulted in increased TDP-43 processing and the accumulation of insoluble phosphorylated TDP-43, as well as motor impairment and neurodegeneration, similarly to the DKO mice. Therefore, LGMN is proved to be an important link between PGRN haploinsufficiency and TDP-43 pathology in FTLD-GRN. In summary, the two studies presented in this thesis address major gaps in the FTD field, establishing LGMN as a link between the loss of PGRN and TDP-43 pathology, and also provide support for PGRN replacement as a disease-modifying therapeutic approach for FTLD-GRN.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Neurodegeneration, FTD, Microglia, Therapeutic, iPSC
• Themengebiete: 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 28. August 2024
• 1. Bericht­erstatter:in: Haass, Christian