Understanding significance of tubulin detyrosination/re-tyrosination cycle in synaptic plasticity – SPEED-Y

A better insight into the molecular and cellular mechanisms underlying synaptic plasticity is important to our understanding of brain development and functions. Many brain disorders, including degenerative and psychiatric diseases, have been associated with alterations of synaptic connections. On the post-synaptic side, dendritic spines exhibit active structural changes which depend on both actin and microtubule cytoskeletons. While actin filaments are predominantly concentrated in spines, microtubules are mostly confine to the dendritic shaft. In the last decade several groups demonstrated however that dynamic microtubules enter transiently into the dendritic spines, and modulate their morphology and the plasticity mechanisms.
Microtubules are polarized fibers from the cytoskeleton formed from alpha/beta tubulin heterodimers which permanently assemble and disassemble. They have versatile architectures and functions in cells. The alpha-tubulin undergoes a specific post-translational modification, the cleavage of its C-terminal tyrosine and its re-addition, respectively carried out by a carboxypeptidase (TCP) and a ligase (TTL). This cycle (the deTyr/Tyr cycle), which is unique to tubulin, has been associated with cancer, cardiomyopathies and brain disorganization. Partner 1 demonstrated the vital role of TTL in brain, and identified long-sought TCP enzymes as enzymatic complexes composed of vasohibins (VASH1/2) associated to Small Vasohibin Binding protein (SVBP) (Aillaud et al., Science 2017). In recent collaborative studies, Partner 1 also analyzed structure-function of the TCP complex and showed that SVBP loss leads to structural brain abnormalities and cognitive deficiency in mice and humans (Wang et al., NSMB 2019 & Pagnamenta et al., HMG 2019). The deTyr/Tyr cycle has been linked since long to microtubule dynamics: tyrosinated and detyrosinated forms of alpha-tubulin are respectively present in dynamic and non-dynamic microtubules. Causal links were established by Partner 1, such as the higher sensitivity of tyrosinated microtubules to motors of the kinesin 13 family that catalyze microtubule depolymerization.
The SPEED-Y project aims at a better understanding of the biological mechanisms connecting microtubule dynamicity to dendritic spine function. We hypothesize that spine function depends on the tyrosination status of microtubules established by enzymes TCPs (detyrosination) and TTL (re-retyrosination). Our study of brains from Alzheimer patients corroborates this hypothesis: TTL decreases and modified tubulin (deTyr) accumulates during the neurodegeneration, inducing a loss of dynamic microtubules which is very likely deleterious. Even if TTL was identified in the 90s, its function in mature neurons has not been studied. With our recent discovery of long-sought TCPs, we are now able to unravel how both sides of this tubulin modification cycle could control synaptic plasticity. We propose to decipher the role of the deTyr/Tyr cycle in synaptic plasticity both at the cellular level (cultured hippocampal neurons) and in the brain tissue (hippocampus CA1 region) using transgenic TTL and TCPs mice that we recently developed. The objectives will be organized in 3 aims and will be performed in tight collaboration between the three partners, coupling the expertise of partner 1 on cytoskeleton and molecular and cellular neurobiology, of partner 2 on electrophysiology, and on Partner 3 on super-resolution microscopy. We intend to (a) establish causal relationships between tubulin tyrosination status and neuronal synaptic and plasticity parameters, (b) unravel the functional role of the deTyr/Tyr cycle in neuronal transmission, and (c) show how enzymes regulation is critical for synaptic plasticity. This fundamental project represents a prerequisite that could guide a novel therapeutic concept targeting the detyrosinating enzymes to ameliorate microtubules dynamicity in synapses of degenerative neurons.