|M.Sc Student||Kahana Roni|
|Subject||Dependence of Presynaptic Stability on Active Zone|
|Department||Department of Medicine||Supervisor||Professor Noam Ziv|
|Full Thesis text|
The human brain consists of a vast number of neurons interconnected by specialized communication devices known as synapses. It is widely believed that the ability of our brain to learn and memorize is based, to a large extent, on changes made to these synaptic connections. This belief also implies, however, that synapses, when not driven to change their characteristics should retain these over time. Otherwise, physiologically relevant changes would be gradually lost due to spurious changes or spontaneous drift.
Synaptic tenacity - the capacity of synapses to retain their characteristics over time - is not at all obvious. Imaging studies reveal that synapses, and in particularly presynaptic compartments, are sites of intense molecular dynamics and membrane trafficking processes. On top of this, most presynaptic compartments lack obvious barriers that confine their constituents or isolate them from neighboring synapses. How, then, do presynaptic compartments maintain their structural and functional identities in face of such challenges? How stable are synapses to start with?
Here we examined the hypothesis that Bassoon, a huge protein found exclusively within the presynaptic active zone cytoskeletal matrix, is an important component of a relatively stable “core scaffold” that conserves presynaptic organization. To that end we used time-lapse confocal microscopy, immunocytochemistry, cultured cortical neurons and viral vectors carrying the gene for EGFP-tagged ERC2 - another active zone molecule that served as a reporter of presynaptic active zone size - to measure the baseline stability of individual presynaptic compartments over many hours and days. We then examined the stability of presynaptic sites in which Bassoon expression was suppressed by means of RNA interference. We found that most presynaptic sites survive for more than 60 hours. During this period, some AZs exhibited dramatic changes in their sizes, whereas other remained rather stable. On average size fluctuations were of 17% from the mean size of the AZ. Suppression of Bassoon expression led to a dramatic increase in presynaptic turnover rates, as well as a moderate effect on presynaptic size fluctuation in terms of their time scales (shorter) and magnitude (larger fluctuations).
These findings suggest that Bassoon has an important role in stabilizing the presynaptic compartment that is mainly manifested in protecting the presynaptic specialization, and perhaps the entire synapse, from processes that promote its elimination.