Crosstalk between biochemical signaling and trafficking determines AMPAR dynamics in synaptic plasticity

Synaptic plasticity involves the modification of both biochemical and structural components of neurons. Many studies have revealed that the change in the number density of the glutamatergic receptor AMPAR at the synapse is proportional to synaptic weight update; increase in AMPAR corresponds to strengthening of synapses while decrease in AMPAR density weakens synaptic connections. The dynamics of AMPAR are thought to be regulated by upstream signaling, primarily the calcium-CaMKII pathway, trafficking to and from the synapse, and influx from extrasynaptic sources. Here, we have developed a set of models using compartmental ordinary differential equations to systematically investigate contributions of signaling and trafficking variations on AMPAR dynamics at the synaptic site. We find that the model properties including network architecture and parameters significantly affect the integration of fast upstream species by slower downstream species. Furthermore, we predict that the model outcome, as determined by bound AMPAR at the synaptic site, depends on (a) the choice of signaling model (bistable CaMKII or monostable CaMKII dynamics), (b) trafficking versus influx contributions, and (c) frequency of stimulus. Therefore, AMPAR dynamics can have unexpected dependencies when upstream signaling dynamics (such as CaMKII and PP1) are coupled with trafficking modalities.

Automated in vivo drug screen in zebrafish identifies synapse-stabilising drugs with relevance to spinal muscular atrophy

ABSTRACT Synapses are particularly vulnerable in many neurodegenerative diseases and often the first to degenerate, for example in the motor neuron disease spinal muscular atrophy (SMA). Compounds that can counteract synaptic destabilisation are rare. Here, we describe an automated screening paradigm in zebrafish for small-molecule compounds that stabilize the neuromuscular synapse in vivo. We make use of a mutant for the axonal C-type lectin chondrolectin (chodl), one of the main genes dysregulated in SMA. In chodl−/− mutants, neuromuscular synapses that are formed at the first synaptic site by growing axons are not fully mature, causing axons to stall, thereby impeding further axon growth beyond that synaptic site. This makes axon length a convenient read-out for synapse stability. We screened 982 small-molecule compounds in chodl chodl−/− mutants and found four that strongly rescued motor axon length. Aberrant presynaptic neuromuscular synapse morphology was also corrected. The most-effective compound, the adenosine uptake inhibitor drug dipyridamole, also rescued axon growth defects in the UBA1-dependent zebrafish model of SMA. Hence, we describe an automated screening pipeline that can detect compounds with relevance to SMA. This versatile platform can be used for drug and genetic screens, with wider relevance to synapse formation and stabilisation.

Spinal P2X receptor modulates reflex pressor response to activation of muscle afferents

Static contraction of skeletal muscle evokes increases in blood pressure and heart rate. Previous studies suggested that the dorsal horn of the spinal cord is the first synaptic site responsible for those cardiovascular responses. In this study, we examined the role of ATP-sensitive P2X receptors in the cardiovascular responses to contraction by microdialyzing the P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) into the L7 level of the dorsal horn of nine anesthetized cats. Contraction was elicited by electrical stimulation of the L7 and S1 ventral roots. Blockade of P2X receptor attenuated the contraction induced-pressor response [change in mean arterial pressure (ΔMAP): 16 ± 4 mmHg after 10 mM PPADS vs. 42 ± 8 mmHg in control; P < 0.05]. In addition, the pressor response to muscle stretch was also blunted by PPADS (ΔMAP: 27 ± 5 mmHg after PPADS vs. 49 ± 8 mmHg in control; P < 0.05). Finally, activation of P2X receptor by microdialyzing 0.5 mM α,β-methylene into the dorsal horn significantly augmented the pressor response to contraction. This effect was antagonized by prior PPADS dialysis. These data demonstrate that blockade of P2X receptors in the dorsal horn attenuates the pressor response to activation of muscle afferents and that stimulation of P2X receptors enhances the reflex response, indicating that P2X receptors play a role in mediating the muscle pressor reflex at the first synaptic site of this reflex.