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Huntingtin functions in the regulation of axonal transport: consequences on neuronal network homeostasis and behavior, in health and disease

on the December 11, 2020
2PM visio

Thesis defense of Hélène Vitet

Friday, December the 11th 2020, Hélène Vitet will defend her thesis "Huntingtin functions in the regulation of axonal transport: consequences on neuronal network homeostasis and behavior, in health and disease".

This thesis was prepared at GIN in Frédéric SAUDOU team.

Supervisors:

  • Frédéric Saudou
  • Chiara Scaramuzzino

Jury:

  • Rapportrices: Karine Merienne (LNCA, Strasbourg) and Coralie Fassier (Institut de la vision, Paris)
  • Examinateurs: Alain Buisson (GIN) and Alain Marty (SPPIN)
  • Guest: Chiara Scaramuzzino (GIN)

Abstract:

Neuronal circuits are at the basis of behaviors such as motor coordination or learning and memory. As being part of a network, neurons communicate at synapses through finely tuned molecular and cellular processes. One key mechanism regulating synapse homeostasis involves the transport of vesicles within axons and dendrites, which is dysregulated in many neurological disorders such as Rett syndrome, Alzheimer’s (AD) and Huntington’s diseases (HD). Thus, deciphering the regulation of vesicular transport within neurites in physiological context is crucial to understand, and potentially restore, the consequences of these dysregulations in pathological contexts.

Huntingtin (HTT) protein, known for its devastating role in HD when mutated, is a key actor of axonal transport. It promotes and regulates vesicular transport in neurites by scaffolding adaptors and molecular motors. Particularly, HTT phosphorylation status at S421 regulates the directionality of BDNF, APP and VAMP-7 vesicles within neurites in cultured and transfected neurons. However, several questions remain to be elucidated regarding the mechanisms and the consequences of this HTT-dependent regulation of vesicular transport such as the neuritic specificity (axons or dendrites) and the behavioral consequences of such modification. Finally, we do not know whether transport regulation can be influenced in pathological conditions to restore disease-associated phenotypes in vivo.


This thesis aims at characterizing in vivo the mechanisms and the consequences of axonal transport regulation of three different types of vesicles through the phosphorylation of Huntingtin at S421 and to investigate its propensity to restore disease-associated phenotypes in mouse models of human neurological disorders.


In order to reproduce in vitro the in vivo networks associated with neurological disorders we used microfluidic devices that allow the reconstitution of neuronal networks in vitro. We investigated the transport of Amyloid Precursor Protein (APP) vesicles, precursors of synaptic vesicles (SVPs) or dense-core vesicles (DCVs) in neurons in which the HTT phosphorylation status was modified. These neurons came from mice in which Serine 421 has been replaced by an aspartic acid to mimic the phosphorylated form of HTT (HTTS421D) or by an alanine to mimic the unphosphorylatable form of HTT (HTTS421A).

APP homeostasis is impaired in AD. We investigated APP transport and accumulation at corticocortical synapses. We found that Akt-mediated HTT phosphorylation at S421 regulates the directionality of APP containing vesicles in axons but not in dendrites: the unphosphorylatable form of HTT decreases axonal anterograde flux of APP and reduces its levels at presynaptic zones both in vitro and in vivo. Reducing anterograde flux of APP in familial AD mouse model restored synapse homeostasis in vivo and memory deficits (Publication 1; Bruyere*, Abada*, Vitet* et al., eLife, 2020).

BDNF transport within DCVs is dysregulated in the corticostriatal network of Rett syndrome’s patients. We found that endogenous HTT phosphorylation at S421 or a chemical inhibitor of calcineurin (FK506) rescue BDNF transport in the corticostriatal network, neuronal communication, and behaviors of Rett syndrome model mice (Publication 2; Ehinger et al., Embo Mol Med, 2020).

Finally, it has been shown that SVP axonal transport regulates the number of SVs at the synapse, which, within a corticostriatal synapse, is essential for motor skill learning. We found that HTT phosphorylation increases the recruitment of the molecular motor KIF1A on SVPs, thus promoting anterograde transport and the probability of release. Silencing KIF1A in the corticostriatal network of HTTS421D mice, we found that pHTTS421 increases the number of SVs at the synapse and impairs procedural memory through a specific HTT-KIF1A dependent mechanism. This study defines a pathway by which axonal transport of SVP impact the behavioral phenotype (Publication 3; Vitet et al., in prep).

Updated on November 30, 2020

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