TSPAN5 Enriched Microdomains Provide a Platform for Dendritic Spine Maturation through Neuroligin-1 Clustering

Tetraspanins are a class of evolutionarily conserved transmembrane proteins with 33 members identified in mammals that have the ability to organize specific membrane domains, named tetraspanin-enriched microdomains (TEMs). Despite the relative abundance of different tetraspanins in the CNS, few studies have explored their role at synapses. Here, we investigate the function of TSPAN5, a member of the tetraspanin superfamily for which mRNA transcripts are found at high levels in the mouse brain. We demonstrate that TSPAN5 is localized in dendritic spines of pyramidal excitatory neurons and that TSPAN5 knockdown induces a dramatic decrease in spine number because of defects in the spine maturation process. Moreover, we show that TSPAN5 interacts with the postsynaptic adhesion molecule neuroligin-1, promoting its correct surface clustering. We propose that membrane compartmentalization by tetraspanins represents an additional mechanism for regulating excitatory synapses

Fluorescent amino acids as versatile building blocks for chemical biology

Fluorophores have transformed the way we study biological systems, enabling non-invasive studies in cells and intact organisms, which increase our understanding of complex processes at the molecular level. Fluorescent amino acids have become an essential chemical tool because they can be used to construct fluorescent macromolecules, such as peptides and proteins, without disrupting their native biomolecular properties. Fluorescent and fluorogenic amino acids with unique photophysical properties have been designed for tracking protein–protein interactions in situ or imaging nanoscopic events in real time with high spatial resolution. In this Review, we discuss advances in the design and synthesis of fluorescent amino acids and how they have contributed to the field of chemical biology in the past 10 years. Important areas of research that we review include novel methodologies to synthesize building blocks with tunable spectral properties, their integration into peptide and protein scaffolds using site-specific genetic encoding and bioorthogonal approaches, and their application to design novel artificial proteins, as well as to investigate biological processes in cells by means of optical imaging. [Figure not available: see fulltext.]

Dynamic and specific interaction between synaptic NR2-NMDA receptor and PDZ proteins

The relative content of NR2 subunits in the NMDA receptor confers specific signaling properties and plasticity to synapses. However, the mechanisms that dynamically govern the retention of synaptic NMDARs, in particular 2A-NMDARs, remain poorly understood. Here, we investigate the dynamic interaction between NR2 C termini and proteins containing PSD-95/Discs-large/ZO-1 homology (PDZ) scaffold proteins at the single molecule level by using high-resolution imaging. We report that a biomimetic divalent competing ligand, mimicking the last 15 amino acids of NR2A C terminus, specifically and efficiently disrupts the interaction between 2A-NMDARs, but not 2B-NMDARs, and PDZ proteins on the time scale of minutes. Furthermore, displacing 2A-NMDARs out of synapses lead to a compensatory increase in synaptic NR2B-NMDARs, providing functional evidence that the anchoring mechanism of 2A- or 2B-NMDARs is different. These data reveal an unexpected role of the NR2 subunit divalent arrangement in providing specific anchoring within synapses, highlighting the need to study such dynamic interactions in native conditions

Biomimetic divalent ligands for the acute disruption of synaptic AMPAR stabilization

The interactions of the AMPA receptor (AMPAR) auxiliary subunit Stargazin with PDZ domain–containing scaffold proteins such as PSD-95 are critical for the synaptic stabilization of AMPARs. To investigate these interactions, we have developed biomimetic competing ligands that are assembled from two Stargazin-derived PSD-95/DLG/ZO-1 (PDZ) domain–binding motifs using 'click' chemistry. Characterization of the ligands in vitro and in a cellular FRET-based model revealed an enhanced affinity for the multiple PDZ domains of PSD-95 compared to monovalent peptides. In cultured neurons, the divalent ligands competed with transmembrane AMPAR regulatory protein (TARP) for the intracellular membrane-associated guanylate kinase resulting in increased lateral diffusion and endocytosis of surface AMPARs, while showing strong inhibition of synaptic AMPAR currents. This provides evidence for a model in which the TARP-containing AMPARs are stabilized at the synapse by engaging in multivalent interactions. In light of the prevalence of PDZ domain clusters, these new biomimetic chemical tools could find broad application for acutely perturbing multivalent complexes

Self-assembled lamellar complexes of siRNA with lipidic aminoglycoside derivatives promote efficient siRNA delivery and interference

RNA interference requires efficient delivery of small double-stranded RNA molecules into the target cells and their subsequent incorporation into RNA-induced silencing complexes. Although current cationic lipids commonly used for DNA transfection have also been used for siRNA transfection, a clear need still exists for better siRNA delivery to improve the gene silencing efficiency. We synthesized a series of cationic lipids characterized by head groups bearing various aminoglycosides for specific interaction with RNA. siRNA complexation with such lipidic aminoglycoside derivatives exhibited three lipid/siRNA ratio-dependent domains of colloidal stability. Fluorescence and dynamic light-scattering experiments showed that cationic lipid/siRNA complexes were formed at lower charge ratios, exhibited a reduced zone of colloidal instability, and had smaller mean diameters compared with our previously described guanidinium-based cationic lipids. Cryo-transmission electron microscopy and x-ray-scattering experiments showed that, although the final in toto morphology of the lipid/siRNA complexes depended on the aminoglycoside type, there was a general supramolecular arrangement consisting of ordered lamellar domains with an even spacing of 67 Å. The most active cationic lipid/siRNA complexes for gene silencing were obtained with 4,5-disubstituted 2-deoxystreptamine aminoglycoside derivatives and were characterized by the siRNA being entrapped in small particles exhibiting lamellar microdomains corresponding to siRNA molecules sandwiched between the lipid bilayers. These results clearly show that lipidic aminoglycoside derivatives constitute a versatile class of siRNA nanocarriers allowing efficient gene silencing