To GVB or not to GVB: a study on neuronal resilience

The balance of protein synthesis, folding and degradation - proteostasis - is essential for cellular survival. This is especially the case for post-mitotic neurons that cannot self-renew and thus rely on specialised mechanism to maintain this balance and deal with stresses like the aggregation of tau in many neurodegenerative diseases. The mechanisms that confer neuronal resilience to tau pathology remain poorly understood, yet their therapeutic potential is considerable. Therefore, the studies in this thesis focus on granulovacuolar degeneration bodies (GVBs) as a candidate resilience mechanism. In Chapter 2, we investigated the formation of GVBs in relation to tau and α-synuclein pathology. We developed a novel seed-independent primary neuron model for studying tau pathology and GVB formation. Single-cell analyses in both human brain tissue of patients with Alzheimer’s disease (AD) and primary mouse neurons confirmed a causal relation between tau pathology and GVBs: intracellular, neuronal aggregation of tau occurs prior to and is essential for GVB formation. GVBs were also observed in the substantia nigra of patients with Parkinson’s disease (PD) in neurons with α-synuclein aggregates in the absence of tau pathology. In our seeded α-synuclein primary mouse model, GVBs also formed upon the aggregation of α-synuclein. These results showed that GVB formation reflects a general neuronal response to cytosolic protein aggregation. In Chapter 3, we studied neuronal resilience in the context of tau pathology. Using the primary neuron model of tau aggregation, we showed that while tau aggregation induces neuronal toxicity, GVB-positive (GVB+) neurons survive. We showed that GVB formation requires basal autophagy and CK1δ activity. Moreover, we showed that tau aggregation reduces general protein synthesis and specific long-term potentiation (LTP)-induced protein translation, but that GVB+ neurons show similar levels of protein synthesis as tau-negative (tau-) neurons. We propose that this is due to observed increase in ribosomal levels in GVB+ neurons compared to GVB-negative (GVB-) neurons, rather than the activation of transient stress pathways like the ISR or mTOR. We proposed that GVB formation reflects a neuron-specific protective adaptation to proteostatic stress induced by tau aggregation. In Chapter 4, we explored how CK1δ regulates the formation of GVBs in more detail. Using the tau aggregation model in primary neurons and astrocytes, we demonstrated that only neurons form GVBs in response to tau aggregation, even when CK1δ is overexpressed in tau-positive (tau+) astrocytes, highlighting a neuron-specific regulatory mechanism. Furthermore, CK1δ localisation to GVBs is governed by its C-terminus, and truncation of this region - resulting in aberrant kinase activity - reduces both GVB formation and size. We also demonstrate that CK1δ protein levels are tightly controlled by proteasomal, but not auto-lysosomal, degradation dependent on CK1δ activity. Together, these findings position CK1δ as a critical and highly regulated driver of GVB formation, while also indicating that GVBs emerge to sequester and restrain CK1δ activity.