Extensive Remodeling of the Presynaptic Cytomatrix upon Homeostatic Adaptation to Network Activity Silencing

Abstract

Global changes of activity in neuronal networks induce homeostatic adaptations of synaptic strengths, which involve functional remodeling of both presynaptic and postsynaptic apparatuses. Despite considerable advances in understanding cellular properties of homeostatic synaptic plasticity, the underlying molecular mechanisms are not fully understood. Here, we explored the hypothesis that adaptive homeostatic adjustment of presynaptic efficacy involves molecular remodeling of the release apparatus including the presynaptic cytomatrix, which spatially and functionally coordinates neurotransmitter release. We found significant downregulation of cellular expression levels of presynaptic scaffolding proteins Bassoon, Piccolo, ELKS/CAST, Munc13, RIM, liprin-␣, and synapsin upon prolonged (48 h) activity depletion in rat neuronal cultures. This was accompanied by a general reduction of Bassoon, Piccolo, ELKS/CAST, Munc13, and synapsin levels at synaptic sites. Interestingly, RIM was upregulated in a subpopulation of synapses. At the level of individual synapses, RIM quantities correlated well with synaptic activity, and a constant relationship between RIM levels and synaptic activity was preserved upon silencing. Silencing also induced synaptic enrichment of other previously identified regulators of presynaptic release probability, i.e., synaptotagmin1, SV2B, and P/Q-type calcium channels. Seeking responsible cellular mechanisms, we revealed a complex role of the ubiquitin-proteasome system in the functional presynaptic remodeling and enhanced degradation rates of Bassoon and liprin-␣ upon silencing. Together, our data indicate a significant molecular reorganization of the presynaptic release apparatus during homeostatic adaptation to network inactivity and identify RIM, synaptotagmin1, Ca v 2.1, and SV2B as molecular candidates underlying the main silencing-induced functional hallmark at presynapse, i.e., increase of neurotransmitter release probability.

GABA B receptor deficiency causes failure of neuronal homeostasis in hippocampal networks

“…Strong evidence exists on homeostatic augmentation of Pr and readily releasable pool size in response to prolong inactivity in hippocampal neurons (19,22,(34)(35)(36)(37)(38)(39)(40)(41)(42). These homeostatic adaptations are associated with modulation of presynaptic Ca 2+ flux (29) and remodeling of a large number of proteins in presynaptic cytomatrix (7). Recent studies have identified the mechanisms underlying presynaptic homeostatic signaling in Drosophila neuromuscular junction, implicating epithelial sodium leak channels (43) and endostatin (44) as homeostatic regulators of the presynaptic Ca V 2 channels (for review, see ref.…”

“…In the last 2 decades, major progress has been made toward understanding the homeostatic negative feedback systems underlying restoration of a baseline neuronal function after prolonged activity perturbations (2)(3)(4). Homeostatic processes may counteract the instability by adjusting intrinsic neuronal excitability, inhibition-to-excitation balance, and synaptic strength via postsynaptic or presynaptic modifications (5, 6) through a profound molecular reorganization of synaptic proteins (7,8). These stabilizing mechanisms have been collectively termed homeostatic plasticity.…”

Deletion of the Presynaptic Scaffold CAST Reduces Active Zone Size in Rod Photoreceptors and Impairs Visual Processing

“…Recent evidence suggests that the molecules involved in early formation of active zones, such as liprins and CAST/ELKS/ERC proteins, also contribute to or correlate with changes in active zone size in mature synapses (Zhen and Jin, 1999;Kaufmann et al, 2002, p 2;Olsen et al, 2005;Dai et al, 2006;Astigarraga et al, 2010;Lazarevic et al, 2011). Active zone size can change over different timescales from minutes (Matz et al, 2010) to hours (SpiwoksBecker et al, 2004;Hull et al, 2006) to days (Lazarevic et al, 2011), accompanying changes of synaptic activity and the circadian cycle. The hierarchy of regulation is likely to differ between synapse formation and structural plasticity and among different synapses.…”

“…The structure of presynaptic active zones is adapted to accommodate the specific functional requirements of a given synapse (Zhai and Bellen, 2004;Moser et al, 2006;Matthews and Fuchs, 2010;Gundelfinger and Fejtova, 2011). The mechanisms regulating active zone size are not well understood but likely to involve Liprins/Syd proteins (Stigloher et al, 2011), cytomatrix of the active zone (CAZ) proteins, such as CAST/ELKS (Kittel et al, 2006), Bassoon and Piccolo (Zhai et al, 2000(Zhai et al, , 2001, and, at ribbon synapses, also RIBEYE (Sheets et al, 2011).…”

The Presynaptic Active Zone Protein RIM1α Controls Epileptogenesis following Status Epilepticus

“…Whereas RIM1␣ and RIM1␤ function redundantly in the basic processes of synaptic vesicle exocytosis, RIM1␣ is furthermore a key mediator of presynaptically expressed forms of synaptic plasticity at different CNS synapses Schoch et al, 2002;Lonart et al, 2003;Calakos et al, 2004;Fourcaudot et al, 2008;Kaeser et al, 2008a,b;Pelkey et al, 2008) (endocannabinoid-mediated long-term depression) (Chevaleyre et al, 2007). Interestingly, the amount of RIM1 present at a single synapse has been found to correlate with its activity (Jiang et al, 2010;Lazarevic et al, 2011), suggesting a potential role in the regulation of plasticity itself.…”