Ph.D. Projects
The role of synapsins in the physiopathology of epilepsy: regulation of synaptic vesicle pools
Background and Rationale.
Epilepsy affects more than 1% of the world population. It is believed that seizures in epilepsy are the manifestation of abnormalities in the formation and/or activity of cortical networks. The dissection of the genetic basis of the variable phenotypes of epilepsy has been so far very difficult. In this scenario, animal models are essential to elucidate the pathogenetic role of mutations affecting neuron-specific genes implicated in the control of synaptic transmission. This proposal originates from the observation that null mutant mice for Syns (a family of synaptic vesicle proteins involved in the regulation of neurotransmitter (NT) release and synaptogenesis) are epileptic. The recent identification of mutations of human synapsin genes associated with familial epilepsy supports a role of Syns in this disease.
Synapsins (syn) are a family of brain phosphoproteins involved in neurotransmitter release through regulation of synaptic vesicle (SV) availability during stimulus-driven exocytosis. The function of these proteins is exerted by reversibly anchoring SVs to each other and to the actin cytoskeleton in a phosphorylation-dependent manner. To date, the Syns are the first presynaptic molecules involved in the regulation of NT release whose mutation leads to epilepsy in humans.
Aims
Our central interest is to investigate the molecular role(s) of syn in the etiology of epilepsy. Recent data from our laboratory show that synapsins have a fundamental function in the organization of synaptic contacts and in the dynamic regulation of SV pools before the onset of epilepsy. These findings suggest that the alteration of the presynaptic architecture is an early process in the establishment of the pathology.
The specific objective of this project will be to investigate the role of syn in the maturation of SVs and in the regulation/maintenance of actively cycling SV pools during sustained stimulation. Suitable molecular tools including fluorescently tagged SV proteins, lentiviruses and animal models are available in the laboratory.
Research Plan
One interesting feature of Syn KO mice is that these animals show decreased SV proteins and SVs at a young age, preceding the appearance of epilepsy. In addition, under high frequency stimulation Syn KO mice exhibit increased synaptic fatigue, raising the possibility that the decreased number of SVs has a functional consequence. Recent results from our laboratories suggest an explanation for these observations, indicating that SV proteins are severely mistargeted to non-synaptic areas of Syn KO mice. SV dispersion can be a cause of synaptic disfunction, either because: (i) the altered organization of SVs impairs the efficiency of synaptic transmission; (ii) it affects spreading of the signal; and/or (iii) dispersion leads to increased SV degradation. It has been shown that cycling SVs can be shared by adjacent synapses. Even though the relevance of SV sharing is unclear, it is possible that repetitive stimuli modulate local synaptic plasticity through increased sharing of SV from active synapses to neighbouring terminals. Consistent with the reduced targeting of SVs, this mechanism might be altered in the absence of Syns. It is also possible that SV sharing follows different dynamics at excitatory and inhibitory synapses. Because of differences in the physiology of GLUergic and GABAergic terminals, SV unclustering could lead to differential or even opposite effects at the level of small, slowly firing excitatory terminals and large, high-rate firing inhibitory boutons, causing an imbalance in neurotransmission and epilepsy. Thus, we intend to clarify whether the SV dispersion observed in Syn KOs is accompanied by enhanced SV sharing and whether this has a role in the modulation of neuronal activity. The dispersion of SV components observed in Syn KO neurons affects several SV markers, raising the possibility that the overall turnover of SVs is altered. This could cause the SV number decrease observed in mice and be a crucial factor for epileptic alterations. We will investigate whether SV protein diffusion is linked to SV depletion, possibly through increased degradation. We plan to determine whether SV protein degradation is altered in Syn KO mice, whether alterations in transcription or translation of SV protein genes occur and whether these phenomena are related to SV loss.
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