Venue: Auditorium 12, Campus Carreire
Language of defense: english
IINS
Thesis supervisor: David Perrais
Title
Spatial organization and dynamics of neuronal membrane trafficking in dendrites and role in synaptic plasticity
Abstract
Endocytosis is a fundamental cellular mechanism by which neurons regulate intercellular signaling, nutrient uptake, and synaptic transmission. The endocytosis and recycling of postsynaptic receptors are essential for the expression of synaptic plasticity, a key cellular mechanism underlying learning and memory. Receptor internalization occurs at endocytic zones (EZs), which are localized in dendritic spines near postsynaptic densities (PSDs) or along dendritic shafts. The spatial organization of EZs near PSDs is mediated by Shank proteins, whose dysfunctions are associated with neurological disorders such as autism. While EZs, marked by clathrin, appear highly static in mature neurons in vitro, imaging of EZs in neurons within organotypic slices reveals significantly greater dynamism. This discrepancy may reflect differences in mechanical strain experienced by neurons, given that the brain is among the softest biological tissues, with a rigidity of approximately 0.17 kPa, compared to 2-4 GPa for glass.
To investigate these effects, we developed a method to culture neurons on hydrogels with controlled rigidity, mimicking the mechanical properties of the brain and other organs. We characterized neurons at 17 days in vitro (DIV) using transfection and immunofluorescence. Neurons cultured on softer hydrogels exhibited comparable dendritic arborization but displayed a higher density of larger dendritic spines, as well as elongated axon initial segments (AIS), the site of action potential that modulate neurons excitability. Confocal microscopy analysis revealed that PSD95, a marker of PSDs, showed a non-significant increase in density on soft hydrogels, whereas high-resolution microscopy detected a significant increase in the density of Shank2, another PSD marker. Additionally, neurons grown on softer substrates had a higher proportion of PSDs associated with an EZ in spines. Moreover, the association between pre- and postsynaptic markers remained consistent across rigidity conditions.
To assess neuronal function, we performed patch-clamp electrophysiology. The amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs) remained unchanged across all rigidity conditions, consistent with the data on pre- and postsynaptic association. However, neurons on softer hydrogels exhibited minor modifications in excitatory activity, with neuronal responses initiating at a higher injected current (one step of 50 pA higher than other conditions). Interestingly, these differences were rapidly compensated at higher injected currents, where no distinctions were observable between conditions.
These findings indicate that neurons cultured on soft hydrogels exhibit structural modifications in spines and AIS while preserving neuronal function, potentially due to homeostasis process. For future perspectives, we will explore the mechanical forces and interactions between extracellular matrix (ECM) proteins, such as integrins, proteins involved in focal and reticular adhesions, and spines to elucidate the mechanisms underlying the maintenance of neuronal function.
Keywords: Synapse, endocytic zone, dynamics, high resolution microscopy, extracellular matrix, plasticity
Jury
- Harold MacGillavry
- Amélie Fréal
- Stéphane Vassilopoulos
- Fekrije Selimi