This work was supported by Grants from the National Institute of Mental Health (R00MH103531 and R01MH116901 to J.A.), a predoctoral training Grant from the National Institute of General Medicine (T32GM007635 to S.R.), and a Lyda Hill Gilliam Fellowship for Advanced Study (GT11021 to S.R.). We thank Dr. Kevin Woolfrey for help with the microscope, Dr. K Ulrich Bayer for the use of his epifluorescent microscope, and Thomas S&#252;dhof (Stanford University) for the LRRTM2 plasmid.

1. Cell culture and transfection
<ol>
	<li>Make HEK cell media with DMEM, 1x (Dulbecco&#39;s Modification of Eagle&#39;s Medium) supplemented with 4.5 g/L glucose, L-glutamine &#38; sodium pyruvate and 10% FBS. Sterile filter.</li>
	<li>Predetermine suitable ligands and receptors for aggregation assay. 
	NOTE: Neurexin3&#945; SS4+/- and one of its known ligands, LRRTM2, were used in this study. Ligands and receptors of interest were expressed from cDNAs in pcDNA3.1. A Gibson assembly was used to insert Neurexin3&#...

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C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Presynaptic+neurexin-3+alternative+splicing+trans-synaptically+controls+postsynaptic+AMPA+receptor+trafficking.">Presynaptic neurexin-3 alternative splicing trans-synaptically controls postsynaptic AMPA receptor trafficking.</a> Cell. 154 (1), 75-88 (2013).</li><li>Aoto, J., F&#246;ldy, C., Ilcus, S. M. C., Tabuchi, K., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distinct+circuit-dependent+functions+of+presynaptic+neurexin-3+at+GABAergic+and+glutamatergic+synapses.">Distinct circuit-dependent functions of presynaptic neurexin-3 at GABAergic and glutamatergic synapses.</a> Nature Neuroscience. 18 (7), 997-1007 (2015).</li><li>S&#252;dhof, T. 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The authors have nothing to disclose.

Dissecting the protein-protein interactions that occur in trans during cell adhesion can lead to a better understanding of the molecular mechanisms underlying basic cellular processes including the formation, function and maintenance of synapses during maturation and remodeling. The implications of cell-to-cell interactions expand beyond neurobiology and have broader roles in signal transduction, cell migration and tissue development14. Aberrations in cell adhesion can disrupt cellular processes i...

Synaptic interactions facilitated by synaptic adhesion molecules are foundational for the development, organization, specification, maintenance and function of synapses and the generation of neural networks. The identification of these transsynaptic cell adhesion molecules is rapidly increasing; thus, it is fundamentally important to identify binding partners and understand how these new adhesion molecules interact with each other. Additionally, genome sequencing has identified mutations in many of these adhesion molecules that are commonly linked to a multitude of neurodevelopmental, neuropsychiatric, and addiction disorders1. Mutations in genes that code for synaptic cell-adhesion molecules may detrimentally alter trans interactions and may contribute to pathophysiological alterations in synapse formation and or maintenance.
Multiple assays exist to quantitatively assess protein-protein interactions such as isothermal calorimetry, circular dichroism, surface plasmon resonance2 and although quantitative in nature, they have several limitations. First, they require recombinant protein, sometimes demanding lengthy and tedious purification steps. Second, they require sophisticated specialized equipment and technical expertise. Third, they can overestimate the extent of binding as they allow for both cis and trans interactions between proteins that are naturally tethered to a membrane in vivo. Here we propose a simple and relatively rapid assay that exclusively tests trans interactions.
To circumvent many of the complications associated with purified protein assays, we have optimized a cell-based protein interaction assay that recapitulates trans interactions in a reduced heterologous cell system. This assay has been previously used in various forms to study transcellular interactions. In this approach, candidate cell adhesion molecules are transfected into HEK293T cells. At physiological conditions, HEK293T cells do not exhibit self-aggregation, making them exemplary models for this assay. However, when individual populations of HEK cells expressing receptor and ligand are combined, the binding of the receptor and the ligand forces aggregation of HEK cells to occur. This aggregation is mediated exclusively by trans interactions and is usually observable in tens of minutes. No protein purification steps are required in this method, and the efficiency of the method relies on the paradigm that populations of HEK cells expressing cognate adhesion molecules are being combined and then imaged only tens of minutes later. Additionally, this method is relatively inexpensive, as neither antibodies nor costly equipment are required. The only equipment required for the acquisition of data is a standard fluorescent microscope. An additional advantage to this cell-based assay is the ability to quickly screen the effect of disease relevant point mutations on trans interactions. This can be performed by transfecting HEK cells with cDNAs of the mutant variants of the protein of interest.
In this protocol, we present an example in which we investigate whether a missense mutation in Neurexin3&#945; (Neurexin3&#945;A687T), identified in a patient diagnosed with profound intellectual disability and epilepsy, alters interactions in trans with leucine-rich repeat transmembrane protein 2 (LRRTM2). Neurexin3&#945; is a member of the evolutionarily conserved family of presynaptic cell-adhesion molecules and while recent work has identified multiple roles at the synapse3,4,5,6,7, our synaptic understanding of this molecule and all members of the neurexin family remains incomplete. LRRTM2 is an excitatory postsynaptic cell adhesion protein that participates in synapse formation and maintenance8,9,10. Importantly, LRRTM2 exclusively interacts with neurexin isoforms that lack the splice site 4 alternative exon (SS4-) but not with neurexin isoforms containing the splice site 4 alternative exon (SS4+). The human missense mutation (A687T) identified in Neurexin3&#945; is located in an unstudied extracellular region that is evolutionarily conserved and is conserved between all alpha neurexins7. As the interaction between these two molecules has been established8,9,11, we posed the question: is the binding capability of Neurexin3&#945; SS4- to LRRTM2 altered by an A687T point mutation? This assay revealed that the A687T point mutation unexpectedly enhanced the aggregation of Neurexin3&#945; to LRRTM2 suggesting that the extracellular region in which the point mutation is located, plays a role in mediating transsynaptic interactions.

<table border="0" cellpadding="0" cellspacing="0" style="width:788px;" width="787">
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	<tbody>
		<tr height="60">
			<td height="60" style="height:60px;width:339px;">1.5 mL disposable microtubes with snap caps</td>
			<td style="width:121px;">VWR</td>
			<td style="width:161px;">89000-028</td>
			<td style="width:167px;">Incubation of mixed population of HEK cells</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">1000 mL Rapid&#8212;Flow Filter Unit, 0.2 um aPES membrane</td>
			<td style="width:121px;">Thermo Fisher</td>
			<td style="width:161px;">567-0020</td>
			<td style="width:167px;">Sterilization of HEK media</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">15 mL SpectraTube centrifuge tubes</td>
			<td style="width:121px;">Ward&#8217;s Science</td>
			<td style="width:161px;">470224-998</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">6 well sterile tissue culture plates</td>
			<td style="width:121px;">VWR</td>
			<td style="width:161px;">100062-892</td>
			<td style="width:167px;">culturing HEK cells</td>
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:339px;">Calcium Chloride</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">223506-500G</td>
			<td style="width:167px;">Calcium phosphate transfection, HEK cell resuspension</td>
		</tr>
		<tr height="44">
			<td height="44" style="height:45px;width:339px;">Centrifuge- Sorvall Legend RT</td>
			<td style="width:121px;">Kendro Laboratory Products</td>
			<td align="right" style="width:161px;">75004377</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:22px;width:339px;">CO2 cell incubator</td>
			<td style="width:121px;">Thermo Scientific</td>
			<td style="width:161px;">HERACELL 150i</td>
			<td style="width:167px;">Incubation of HEK cells during growth</td>
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:339px;">DMEM, 1x (Dulbecco&#39;s Modification of Eagle&#39;s Medium) with 4.5 g/L glucose, L-glutamine &#38; sodium pyruvate</td>
			<td style="width:121px;">Corning</td>
			<td style="width:161px;">10-013-CV</td>
			<td style="width:167px;">HEK cell maintenance</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Dulbecco&#8217;s Phosphate Buffered Saline PBS (1X)</td>
			<td style="width:121px;">Gibco</td>
			<td style="width:161px;">14190-144</td>
			<td style="width:167px;">Passaging/harvesting HEK cell</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">Ethylenediaminetetraacetic acid</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">ED-500G</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:339px;">Falcon Vented culture flasks, 75cm2 growth area</td>
			<td style="width:121px;">Corning</td>
			<td style="width:161px;">9381M26</td>
			<td style="width:167px;">Culturing HEK cells</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Fetal Bovine Serum</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">17L184</td>
			<td style="width:167px;">HEK cell maintenance</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">HEK293T cells</td>
			<td style="width:121px;">ATCC</td>
			<td style="width:161px;"></td>
			<td style="width:167px;">Model system</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">ImageJ</td>
			<td style="width:121px;">NIH</td>
			<td style="width:161px;">V: 2.0.0-rc-69/1.52p</td>
			<td style="width:167px;">Image analysis</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Magnesium Chloride hexahydrate</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">M9272-500G</td>
			<td style="width:167px;">HEK cell resuspension</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Sodium phosphate dibasic anhydrous</td>
			<td style="width:121px;">Fisher BioReagents</td>
			<td style="width:161px;">BP332-500</td>
			<td style="width:167px;">Calcium phosphate transfection</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Trypsin 0.25% (1X) Solution</td>
			<td style="width:121px;">GE Healthcare Life Sciences</td>
			<td style="width:161px;">SH30042.01</td>
			<td style="width:167px;">Passaging HEK cells</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Tube rotator</td>
			<td style="width:121px;"></td>
			<td style="width:161px;"></td>
			<td style="width:167px;">Incubation of mixed population of HEK cells</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:339px;">UltraClear Microscope slides. White Frosted, Positive Charged</td>
			<td style="width:121px;">Denville Scientific Inc.</td>
			<td style="width:161px;">M1021</td>
			<td style="width:167px;">Image acquisition</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">Wide-field microscope</td>
			<td style="width:121px;">Zeiss</td>
			<td style="width:161px;">Axio Vert 200M</td>
			<td style="width:167px;">Image acquisition</td>
		</tr>
	</tbody>
</table>

measuring transcellular interactions through protein aggregation in a heterologous cell system

Protein interactions at cellular interfaces dictate a multitude of biological outcomes ranging from tissue development and cancer progression to synapse formation and maintenance. Many of these fundamental interactions occur in trans and are typically induced by heterophilic or homophilic interactions between cells expressing membrane anchored binding pairs. Elucidating how disease relevant mutations disrupt these fundamental protein interactions can provide insight into a myriad of cell biology fields. Many protein-protein interaction assays do not typically disambiguate between cis and trans interactions, which potentially leads to an overestimation of the extent of binding that is occurring in vivo and involve labor intensive purification of protein and/or specialized monitoring equipment. Here, we present an optimized simple protocol that allows for the observation and quantification of only trans interactions without the need for lengthy protein purifications or specialized equipment. The HEK cell aggregation assay involves the mixing of two independent populations of HEK cells, each expressing membrane-bound cognate ligands. After a short incubation period, samples are imaged and the resulting aggregates are quantified.

The A687T mutation increases Neurexin3&#945; SS4- binding to LRRTM27 
To investigate how intercellular interactions of two known synaptic proteins are affected by the introduction of a point mutation found in a patient with intellectual disability and epilepsy, we used the above HEK cell aggregation assay (Figure 1). Cells were transfected according to section 1 and prepared for imaging according to sections 1 and 2 of the protocol. Cells were imaged at basel...

Watch this Scientific Journal Video about Measuring Transcellular Interactions through Protein Aggregation in a Heterologous Cell System at JoVE.com

Measuring Transcellular Interactions through Protein Aggregation in a Heterologous Cell System

Here, we present an optimized protocol to rapidly and semiquantitatively measure ligand-receptor interactions in trans in a heterologous cell system using fluorescence microscopy.

Protein interactions at cellular interfaces dictate a multitude of biological outcomes ranging from tissue development and cancer progression to synapse ...

The cell-based assay allows for the observation and quantification of trans adhesion interactions without the need for lengthy protein purifications or specialized equipment. Although the protein interactions tested here are relevant to neurobiology, this method may be applied to any area of research where protein adhesion is occurring intercellularly. 48 hours after transfecting the HEK-293T cells, harvest the cells from the six-well plate for aggregation.First, wash each well twice with PBS. Next, to gently disassociate the cells, add one milliliter of 10 millimolar EDTA to each well, then incubate the plate at 37 degrees Celsius. After five minutes of incubation, gently tap the plate to detach the cells and harvest each well into a separate 15 milliliter conical tube.Centrifuge the tubes at 500 x g at room temperature for five minutes. While the cells are pelleting, prepare six incubation tubes by labeling the tops of microcentrifuge tubes with the experimental conditions. Every permutation of GFP and mCherry should be used.Remove the supernatant from the 15 milliliter conical tubes and resuspend the cells in 500 microliters of medium. Using a hemocytometer, count the cells in each conical tube. Then aliquot 200, 000 cells from each condition into the appropriate incubation tube for a one-to-one mix in a total volume of 500 microliters.After assessing baseline aggregation described in the next section, place the incubation tubes in a slow tube rotator at room temperature. To assess aggregation, at baseline and again after 60 minutes, pipette 40 microliters from the incubation tube onto a charged microscope slide. Baseline acquisition should be done as quickly as possible after the addition of cells to the microcentrifuge tube.Image the slide under fluorescence in both the green and red channels. For each slide, capture images of three different fields of view in one focal plane. HEK-293T cells were transfected with a protein of interest, a mutated protein of interest, or a ligand of interest and co-transfected with a fluorescent protein.Cell populations expressing the protein of interest were mixed with cell populations expressing the ligand of interest and assessed for aggregation after 60 minutes. In overlays of images captured in the green and red channels, aggregation appears as yellow puncta. Conditions in which cells were not expressing any synaptic ligands showed minimal aggregation.Also, minimal aggregation was exhibited when only one of the two populations were expressing synaptic ligands. No aggregation was exhibited when the two populations of cells expressed incompatible adhesion molecules. Conditions with compatible adhesion molecules exhibited significant aggregation after a 60 minute incubation.Surprisingly, the mutated protein of interest exhibited significantly more aggregation than its wild-type counterpart, suggesting that the point mutation enhances the proteins binding capabilities. Mutations in many adhesion molecules are commonly linked to neurodevelopmental, neuropsychiatric, and addiction disorders. With this technique, one can screen the effects of disease relevant point mutations on trans interactions.

measuring-transcellular-interactions-through-protein-aggregation

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Neuroscience

3.4K Views.  University of Colorado Denver School of Medicine. The cell-based assay allows for the observation and quantification of trans adhesion interactions without the need for lengthy protein purifications or specialized equipment. Although the protein interactions tested here are relevant to neurobiology, this method may be applied to any area of research where protein adhesion is occurring intercellularly. 48 hours after transfecting the HEK-293T cells, harvest the cells from the six-well plate for aggregation.First, wash each well twice w...