The TMEM106B-GRN-TDP43 triad: leveraging human samples and humanized models to unravel FTLD-TDP disease mechanisms - ABSTRACT Frontotemporal lobar degeneration (FTLD) is a group of progressive neurodegenerative disorders that represents 10-20% of all dementias. FTLD profoundly impacts core human qualities including personality, insight and verbal communication and often affects individuals before the age of 65. Aggregates of the TAR DNA-binding protein 43 (TDP-43) in the frontal and temporal lobes are the most common FTLD pathology (FTLD- TDP); however, in-vivo models to study TDP-43 aggregation and/or dysfunction are not currently available, hampering our ability to perform mechanistic studies with translational potential. For example, Grn-deficient mice, which model the loss of progranulin (PGRN), a major cause of FTLD-TDP in humans, develop neuroinflammatory changes but not TDP-43 pathology or overt neuronal loss. This may be partly explained by the fact that mouse microglia cannot fully recapitulate all aspects of human microglial physiology. It is also possible that additional genetic factors, such as the transmembrane protein 106B (TMEM106B) risk haplotype, contribute to disease susceptibility in patients. In support of this theory, it was shown that in GRN carriers the presence of at least one copy of the TMEM106B risk haplotype is a near prerequisite for developing FTLD-TDP. While the specific functional variant(s) that characterize the TMEM106B haplotypes has remained elusive, accumulating evidence suggests the risk haplotype exerts its disease-modifying effect by increasing TMEM106B levels. Considering the findings above, we hypothesize that the genetic risk profile of an individual shapes its cellular responses and phenotypes, thereby promoting particular disease states and modifying the vulnerability of neurons to degeneration. We propose to study this concept in the context of FTLD-TDP using the established disease gene GRN and its genetic modifier TMEM106B in the following specific aims: 1) Generate a humanized FTLD-TDP model. In this aim, the novel MIGRATE protocol developed in the Mancuso lab will be used to transplant patient-derived GRN-deficient microglia in the brain of human TMEM106B overexpressing mice, followed by in-depth neuropathological and functional characterization; 2) Identify molecular players that microglia secrete to induce FTLD-related changes in neurons. Here, we will use our humanized FTLD-TDP mouse model and apply a proximity labeling strategy to define the human-specific secretome associated with GRN deficiency. Candidate disease proteins will be further evaluated as therapeutic targets using antibody treatment in mice and/or biomarkers in human CSF; and 3) Determine the functional variant(s) and regulatory mechanism of the TMEM106B risk haplotype in individual brain cell-types using human samples. Here, we will use our unique cohorts of FTLD brain tissues to perform unbiased single nuclei analyses of transcriptomic and epigenomic changes resulting from haplotype-associated TMEM106B variants. Functional validation of candidate variants will be performed in human iPSCs. Together, the proposed studies will lead to a better understanding and improved ability to diagnose and treat patients with FTLD and other TDP-43 proteinopathies.
