Molecular Switches at the Synapse Emerge from Receptor and Kinase Traffic

Changes in the synaptic connection strengths between neurons are believed to play a role in memory formation. An important mechanism for changing synaptic strength is through movement of neurotransmitter receptors and regulatory proteins to and from the synapse. Several activity-triggered biochemical events control these movements. Here we use computer models to explore how these putative memory-related changes can be stabilised long after the initial trigger, and beyond the lifetime of synaptic molecules. We base our models on published biochemical data and experiments on the activity-dependent movement of a glutamate receptor, AMPAR, and a calcium-dependent kinase, CaMKII. We find that both of these molecules participate in distinct bistable switches. These simulated switches are effective for long periods despite molecular turnover and biochemical fluctuations arising from the small numbers of molecules in the synapse. The AMPAR switch arises from a novel self-recruitment process where the presence of sufficient receptors biases the receptor movement cycle to insert still more receptors into the synapse. The CaMKII switch arises from autophosphorylation of the kinase. The switches may function in a tightly coupled manner, or relatively independently. The latter case leads to multiple stable states of the synapse. We propose that similar self-recruitment cycles may be important for maintaining levels of many molecules that undergo regulated movement, and that these may lead to combinatorial possible stable states of systems like the synapse

Caveolin-1 is ubiquitinated and targeted to intralumenal vesicles in endolysosomes for degradation

Identification of the pathway by which caveolin-1 is degraded when caveolae assembly is compromised suggests that “caveosomes” may be endosomal accumulations of the protein awaiting degradation

Engulfed cadherin fingers are polarized junctional structures between collectively migrating endothelial cells

The development and maintenance of tissues requires collective cell movement, during which neighbouring cells coordinate the polarity of their migration machineries. Here, we ask how polarity signals are transmitted from one cell to another across symmetrical cadherin junctions, during collective migration. We demonstrate that collectively migrating endothelial cells have polarized VE-cadherin-rich membrane protrusions, 'cadherin fingers', which leading cells extend from their rear and follower cells engulf at their front, thereby generating opposite membrane curvatures and asymmetric recruitment of curvature-sensing proteins. In follower cells, engulfment of cadherin fingers occurs along with the formation of a lamellipodia-like zone with low actomyosin contractility, and requires VE-cadherin/catenin complexes and Arp2/3-driven actin polymerization. Lateral accumulation of cadherin fingers in follower cells precedes turning, and increased actomyosin contractility can initiate cadherin finger extension as well as engulfment by a neighbouring cell, to promote follower behaviour. We propose that cadherin fingers serve as guidance cues that direct collective cell migration

Bistability for Tightly Coupled Switches

<div><p>(A) Schematic of PSD-localised PP1 acting on both CaMKII and AMPAR substrates in the PSD. The asterisks on CaMKII and AMPAR represent phosphate groups.</p><p>(B) Time course of response to Ca²⁺ (2.7 μM, 500-s duration), then cAMP (0.108 μM, 2,000-s duration) stimuli. The initial Ca²⁺ stimulus turns on CaMKII transiently, but it eventually returns to baseline. The subsequent cAMP stimulus turns on both switches.</p><p>(C) Time course of response to cAMP (0.108 μM, 2,000-s duration), then Ca²⁺ (2.7 μM, 500-s duration) stimuli. The initial AMPAR stimulus (cAMP elevation) is sufficient to turn both the AMPAR and the CaMKII switches on.</p><p>(D) Stochastic run in the low state. The figure illustrates a transient event that did not result in complete turn on.</p><p>(E) Stochastic run in the high state. There is a spontaneous turn off, but the average on time is over 100 h.</p></div

Nested Bistability for Weakly Coupled Switches

<div><p>(A) Schematic of independent PSD-localised PP1 enzyme activities for CaMKII and AMPAR. The two PP1 activities are labelled PP1-PSD-CaMKII and PP1-PSD-AMPAR, respectively. The asterisks represent phosphorylation.</p><p>(B) Time course of system response to Ca²⁺ (2.7 μM, 500-s duration), then cAMP (0.108 μM, 2,000-s duration) stimulus. The initial activation of CaMKII leads to a slow turn on of the AMPAR system.</p><p>(C) Time course of system response to cAMP (0.108 μM, 2,000-s duration), then Ca²⁺ (2.7 μM, 500-s duration) stimulus. First the AMPAR system turns on, then, following the Ca²⁺ stimulus, the CaMKII turns on. The conductance of the synapse has different levels in each of these states.</p><p>(D) Stochastic run for 60 h, showing resting, AMPAR only, and AMPAR + CaMKII activity states.</p></div

AMPAR and CaMKII Trafficking and Dependence on Steady Ca²⁺ Concentrations

<div><p>(A) Number of AMPARs in internal and synaptic membrane pools; AMPARs complexed to enzymes are not counted.</p><p>(B) Number of CaMKII molecules in the cytosol and PSD. The activity in the cytosol and PSD starts to rise at about 0.5 μM Ca²⁺, but translocation occurs around 1 μM.</p><p>(C) Conductance of membrane-inserted AMPARs. Receptor conductance is calculated by assuming that CaMKII phosphorylation of a single GluR1-Ser831 of the tetramer gives 1.5-fold basal conductance, and of two Ser831 gives 2-fold basal conductance. The conductance dips at around 300 nM Ca²⁺, when PP2B is active but CaMKII has yet to become fully active.</p></div