The formation and plasticity of synaptic connections rely on regulatory interactions between pre- and postsynaptic cells. We show that the Drosophila heparan sulfate proteoglycans (HSPGs) Syndecan (Sdc) and Dallylike (Dlp) are synaptic proteins necessary to control distinct aspects of synaptic biology. Sdc promotes the growth of presynaptic terminals, whereas Dlp regulates active zone form and function. Both Sdc and Dlp bind at high affinity to the protein tyrosine phosphatase LAR, a conserved receptor that controls both NMJ growth and active zone morphogenesis. These data and double mutant assays showing a requirement of LAR for actions of both HSPGs lead to a model in which presynaptic LAR is under complex control, with Sdc promoting and Dlp inhibiting LAR in order to control synapse morphogenesis and function.

Synaptotagmin is one of the major integral membrane proteins of synaptic vesicles. It has been postulated to dock vesicles to their release sites, to act as the Ca2+ sensor for the release process, and to be a fusion protein during exocytosis. To clarify the function of this protein, we have undertaken a genetic analysis of the synaptotagmin gene in Drosophila. We have identified five lethal alleles of synaptotagmin, at least one of which lacks detectable protein. Surprisingly, however, many embryos homozygous for this null allele hatch and, as larvae, crawl, feed, and respond to stimuli. Electrophysiological recordings in embryonic cultures confirmed that synaptic transmission persists in the null allele. Therefore, synaptotagmin is not absolutely required for the regulated exocytosis of synaptic vesicles. The lethality of synaptotagmin in late first instar larvae is probably due to a perturbation of transmission that leaves the main apparatus for vesicle docking and fusion intact.

SUMMARY1. The effects of phorbol esters on evoked and spontaneous excitatory neurotransmission were studied in the CAI area in the in vitro hippocampal slice preparation of the rat. Experiments were conducted using field potential recording and whole-cell voltage clamp of CAI pyramidal neurons.2. Pyramidal cells dialysed during whole-cell recording with EGTA-containing electrode solutions, unable to support the induction of long-term potentiation (LTP), still showed robust phorbol ester-induced potentiation of excitatory synaptic transmission.3. Spontaneous miniature excitatory postsynaptic currents (EPSCs), recorded in whole-cell voltage clamp in the presence of tetrodotoxin and picrotoxin, had amplitudes ranging from 4 to 40 pA and occurred at an average frequency of 0-8-5 Hz. Neither the amplitude nor the frequency of spontaneous EPSCs was altered by cadmium, dihydropyridines, or wo-conotoxin GVIA.4. The phorbol ester 4-,/-phorbol 12,13-diacetate increased the frequency of spontaneous miniature EPSCs without changing the shape of the EPSC amplitude distribution, suggesting that phorbol esters exert their potentiating effects presynaptically.5. Blockade of voltage-dependent calcium channels with cadmium attenuated the phorbol-induced increase in spontaneous miniature EPSCs frequency. The phorbol ester-induced increase in miniature EPSC frequency was also attenuated by dihydropyridines, but not by w-conotoxin GVIA.6. Unlike spontaneous synaptic currents, stimulus-evoked synaptic currents were reduced by w-conotoxin but not by nifedipine.7. We conclude that the phorbol ester increases spontaneous release of glutamate by modulating an L-type channel that does not participate in stimulus-evoked neurotransmitter release.

Secreted amyloid precursor protein-alpha (sAPP␣) has growth factor-like properties and can modulate long-term potentiation (LTP) and memory. Here, we demonstrate that exposure to sAPP␣ converts short-lasting LTP into protein-synthesis-dependent late LTP in hippocampal slices from male rats. sAPP␤ had no discernable effect. We hypothesized that sAPP␣ facilitated LTP via regulated glutamate receptor trafficking and de novo protein synthesis. We found using a linear mixed model that sAPP␣ stimulated trafficking of GluA2lacking AMPARs, as well as NMDARs to the extrasynaptic cell surface, in a calcium/calmodulin-dependent kinase II and protein kinase G-dependent manner. Both cell surface receptor accumulation and LTP facilitation were present even after sAPP␣ washout and inhibition of receptor trafficking or protein synthesis prevented all these effects. Direct visualization of newly synthesized proteins (FUNCAT-PLA) confirmed the ability of sAPP␣ to stimulate de novo protein synthesis and revealed GluA1 as one of the upregulated proteins. Therefore, sAPP␣ generates a coordinated synthesis and trafficking of glutamate receptors to the cell surface that facilitate LTP. Significance StatementSecreted amyloid precursor protein-alpha (sAPP␣) is a neurotrophic and neuroprotective protein that can promote synaptic plasticity and memory, yet the molecular mechanisms underlying these effects are still not well understood. Here, we show that sAPP␣ facilitates long-term potentiation (LTP) in a concentration-dependent fashion through cellular processes involving de novo protein synthesis and trafficking of both GluA2-lacking AMPARs and NMDARs to the extrasynaptic cell surface. sAPP␣ also enhances GluA1, but not GluA2, synthesis. The trafficking effects, along with the LTP facilitation, persist after sAPP␣ washout, revealing a metaplastic capability of exogenous sAPP␣ administration. sAPP␣ thus facilitates LTP through coordinated activation of protein synthesis and trafficking of glutamate receptors to the cell surface, where they are positioned for priming LTP.