Loss of C9orf72 disrupts nucleoporins and contributes to TDP-43 mislocalization
Subcellular compartmentalization between the nucleus and cytoplasm is primarily governed by two systems; 1) active nucleocytoplasmic transport (NCT) regulated by Ran-GTPase and involving nuclear transport receptors such as Importin β-1, and 2) macromolecular nuclear pore complexes (NPCs), comprised of >30 nucleoporin (Nups) embedded in the nuclear envelope. FG-Nups, the largest family of Nups, line the NPC central channel and create a permeability barrier between the nucleus and cytoplasm. It has been proposed that abnormalities of either system could underlie nuclear depletion and cytoplasmic aggregation of TDP-43; the characteristic pathology of 97% of ALS cases, including sporadic ALS (sALS).
The most commonly known genetic cause of ALS are hexanucleotide repeat expansions in the C9orf72 gene, which induce gain-of-function (GOF) and loss-of-function (LOF) mechanisms. It has been speculated that both
GOF and LOF effects may be required for neurodegeneration. GOF mechanisms have been shown to affect NCT and NPCs, and I have also demonstrated that C9orf72 LOF disrupts NCT and causes mislocalization of Ran-GTPase, Importin β-1 and FG-Nups.
I hypothesize that C9orf72, via its interaction with Ran-GTPase and Importin β-1, has a crucial role in FG-Nup and NPC homeostasis. In this project, I will demonstrate that loss of C9orf72 affects FG-Nup localization, which has two downstream effects: 1) altered abundance and permeability of NPCs, leading to loss of subcellular compartmentalization, and 2) abnormal localization of FG-Nups, which will contribute to TDP-43 mislocalization.
I will confirm these phenotypes in cell lines and primary mouse neurons, before establishing them in a human neuronal model, which retains its age-related signatures. These studies will characterize a novel pathological cascade that contributes to TDP-43 mislocalization in ALS, expanding our knowledge of these pathomechanisms, further establishing the roles of dysfunctional NCT and NPCs in ALS pathogenesis and opening new avenues for therapeutic development.
Philip McGoldrick , University of Toronto