Name: super-resolution imaging of neuronal dense-core vesicles
Uploaded: 2014-07-02T14:00:00
Description: Watch this Scientific Journal Video about Super-resolution Imaging of Neuronal Dense-core Vesicles at JoVE.com

This work was supported by National Institutes of Health grants 2 R15 GM061539-02 (to B.A.S.), 2 R15 NS40425-03 (to J.E.L.), MH 66179 (to Dr. Gary Banker of Oregon Health &#160;&#38;&#160;Science&#160;University/OHSU), and P30 NS061800 (to Dr. Sue Aicher of OHSU). We thank Barbara Smoody for extensive support with the culture of hippocampal neurons, and Drs. Brian Long and James Abney for a critical reading of this manuscript.

1. Sample Preparation
<ol>
	<li>Prepare DNA encoding a photoconvertible or photoactivatable chimera targeted to DCVs, using standard molecular biology techniques. 
	Note: One possibility is DNA encoding a tissue plasminogen activator-Dendra2 chimera (tPA-Dendra2). Dendra2 is a photoconvertible protein that switches from green to red emission upon exposure to ultraviolet light12.</li>
	<li>Culture hippocampal neurons on high performance #1.5 cover slips for ~5-10 days, following standard protocols13...

<ol>
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PALM and related super-resolution fluorescence microscopy techniques have recently emerged as a valuable complement to better-established forms of optical microscopy and electron microscopy (EM)22-24. Positive attributes of PALM include relatively simple sample preparation and minimal sample perturbation. In principle, PALM also can be used to study living cells. Probably the main negative attribute of PALM is time-consuming data acquisition, which significantly hampers studies of faster dynamic processes in l...

A number of cellular processes depend on accurate and efficient vesicle-mediated trafficking of biomolecules to specific subcellular destinations. One prominent example is synaptic assembly, which is preceded by long-ranged, vesicle-mediated delivery of synaptic constituents from sites of biogenesis in the neuronal soma to potentially distal pre- and postsynaptic sites1.
Fluorescence microscopy is a powerful and popular method of studying vesicle trafficking. Strengths of the technique include its sensitivity, specificity, and compatibility with live imaging2. Unfortunately, until relatively recently, the technique has suffered from one major weakness, diffraction-limited resolution2, which hampers studies of structures with dimensions smaller than ~250 nm. Recently, lateral resolution in fluorescence microscopy surpassed the diffraction barrier with the introduction of super-resolution fluorescence microscopy techniques, such as PALM3. The lateral resolution of PALM, tens of nanometers, is ideally suited to the study of vesicles, which have dimensions that typically range from ~50-250 nm4. It is thus now possible to use fluorescence, with its myriad strengths, to elucidate a spectrum of previously inaccessible attributes of vesicles, including some aspects of their trafficking to specific subcellular sites.
PALM is not trivial to implement, and successful strategies often must be tailored to the type of system under study. Here we describe how to implement PALM studies of vesicular structures, and we demonstrate the efficacy of our approach for the case of DCVs in hippocampal neurons. In particular, we use PALM to address the hypothesis that trafficking of DCVs to synapses in hippocampal neurons is mediated by DCV clusters5-8.
Cluster-mediated trafficking of vesicles to synapses in developing neurons is an intriguing possibility because it may facilitate rapid synaptic stabilization and assembly9,10. Proponents of clustered trafficking of DCVs cite the large apparent size of extrasynaptic fluorescent puncta harboring exogenous DCV cargo as evidence supporting clustering6. However, these puncta appear in images generated using diffraction-limited fluorescence microscopy techniques, which are not suited to distinguishing size effects arising from diffraction from those arising from clustering.
To resolve this issue, we collected conventional widefield fluorescence and PALM images of hippocampal neurons expressing chimeras targeted to DCVs. Analysis of these images revealed that &#62;92% of putative extrasynaptic DCV clusters in conventional images are resolved as 80 nm (individual DCV-sized)11 puncta in PALM images. Thus, these data largely invalidate the clustering hypothesis as applied to DCVs in developing hippocampal neurons.

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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:245px;">Zeiss PALM</td>
			<td style="width:169px;">Carl Zeiss, Inc.</td>
			<td style="width:119px;">Elyra PS.1</td>
			<td style="width:572px;">With Zeiss Efficient Navigation (ZEN) software and fluorescence filter Set 77 HE GFP/mRFP/Alexa633&#160;</td>
		</tr>
		<tr height="28">
			<td height="28" style="height:28px;width:245px;">Lipofectamine 2000 Transfection Reagent</td>
			<td style="width:169px;">Life Technoligies</td>
			<td style="width:119px;">11668-019</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:245px;">Minimum Essential Medium</td>
			<td style="width:169px;">Life Technologies</td>
			<td style="width:119px;">11095-080</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:245px;">Phosphate-Buffered Saline</td>
			<td style="width:169px;">Life Technologies</td>
			<td style="width:119px;">10010049</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:245px;">Paraformaldehyde</td>
			<td style="width:169px;">Electron Microscopy Sciences</td>
			<td style="width:119px;">19208</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:245px;">Sucrose</td>
			<td style="width:169px;">Sigma-Aldrich</td>
			<td style="width:119px;">S-8501</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:245px;">growth glass coverslips 18mm 1.5D</td>
			<td style="width:169px;">Fisher Scientific</td>
			<td style="width:119px;">NC0059095</td>
			<td></td>
		</tr>
	</tbody>
</table>

super-resolution imaging of neuronal dense-core vesicles

Detection of fluorescence provides the foundation for many widely utilized and rapidly advancing microscopy techniques employed in modern biological and medical applications. Strengths of fluorescence include its sensitivity, specificity, and compatibility with live imaging. Unfortunately, conventional forms of fluorescence microscopy suffer from one major weakness, diffraction-limited resolution in the imaging plane, which hampers studies of structures with dimensions smaller than ~250 nm. Recently, this limitation has been overcome with the introduction of super-resolution fluorescence microscopy techniques, such as photoactivated localization microscopy (PALM). Unlike its conventional counterparts, PALM can produce images with a lateral resolution of tens of nanometers. It is thus now possible to use fluorescence, with its myriad strengths, to elucidate a spectrum of previously inaccessible attributes of cellular structure and organization.
Unfortunately, PALM is not trivial to implement, and successful strategies often must be tailored to the type of system under study. In this article, we show how to implement single-color PALM studies of vesicular structures in fixed, cultured neurons. PALM is ideally suited to the study of vesicles, which have dimensions that typically range from ~50-250 nm. Key steps in our approach include labeling neurons with photoconvertible (green to red) chimeras of vesicle cargo, collecting sparsely sampled raw images with a super-resolution microscopy system, and processing the raw images to produce a high-resolution PALM image. We also demonstrate the efficacy of our approach by presenting exceptionally well-resolved images of dense-core vesicles (DCVs) in cultured hippocampal neurons, which refute the hypothesis that extrasynaptic trafficking of DCVs is mediated largely by DCV clusters.

Figure 1 shows one end product of imaging and processing. In this PALM image, lateral coordinates of localized fluorophores are shown using the centroid display mode, and the super-resolution image of the associated DCV is shown using the Gaussian display mode.
Figure 2A shows analogous widefield and PALM images of the soma and proximal processes of an eight days in vitro hippocampal neuron expressing tPA-Dendra2. Important features of the images incl...

Watch this Scientific Journal Video about Super-resolution Imaging of Neuronal Dense-core Vesicles at JoVE.com

Super-resolution Imaging of Neuronal Dense-core Vesicles

We describe how to implement photoactivated localization microscopy (PALM)-based studies of vesicles in fixed, cultured neurons. Key components of our protocol include labeling vesicles with photoconvertible chimeras, collecting sparsely sampled raw images with a super-resolution microscopy system, and processing the raw images to produce a super-resolution image.

Detection of fluorescence provides the foundation for many widely utilized and rapidly advancing microscopy techniques employed in modern biological and ...

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