Alberto Guiggiani for developing the real time control system, Evelina Chieregatti and Hanako Tsushima for insightful discussions, Giacomo Pruzzo and Alessandro Parodi for the development of custom electronics and software, and Claudia Chiabrera and Marina Nanni for their expert advice and assistance in cell culture preparation.

1. Optical Setup
The entire optical system was described earlier15. Briefly, the optical tweezers system is based on an ytterbium continuous wave (CW) fiber laser operating at 1064 nm (IPG Laser GmbH). A spatial light modulator (SLM) (LCOS-SLM, model X10468-07 - Hamamatsu) varies the phase of the incoming IR laser beam to control the position of the trapping focus spot on the culture dish by computer generated holograms. The freely available Blue-tweezers software (web link on equipme...

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We report in this work a quantitative method to compare the neurite adhesion to the culture substrate, by performing simultaneous force spectroscopy measurement during laser induced cell lesion. The measured release of tension is related to the degree of adhesion of the cell to the substrate: cells with a higher number of focal adhesions should release less tension. Measuring the release of tension in terms of piconewtons provides a physical quantity by which to evaluate the axonal adhesion to the culture support in diff...

Optical microscopy is one of the less invasive imaging system available to observe living cells. With the exploitation of effects such as radiation pressure (as in optical tweezers1), or high-energy photon flux (as in laser dissector2), this technology was extended to nano-manipulation. The optical imaging system furnishes a precise control to visualize and manipulate sub cellular targets3. At the same time, thanks to the accurate calibration of the delivered laser power, optical tools accomplish either soft or invasive sample manipulation with unprecedented reproducibility.
Several laboratories integrated, in the same experimental setup, optical tweezers and laser dissector in order to ablate organelles4, to fuse together different cells5, or to stimulate cells by optically driven cargos6,7. While optical tweezers, after calibration of the optical stiffness, allow for the control of applied force to the cell on a piconewton scale, laser dissection systems can modulate optical manipulation, which ranges from membrane photo-poration to ablation of single organelles or dissection of sub-cellular structures. However, laser dissection calibration relies on qualitative assessment of the entity of optical manipulation with respect to the energy delivered to the sample, mainly based on image analysis illustrating morphological changes caused to the specimen8. In the presented method, we demonstrate how to perform force spectroscopy measurement during the laser axonal dissection of a developing neuron, to quantify, on piconewton scale, the force produced by an altered equilibrium in the cytoskeleton structure of a sub-cellular compartment9. Cultured neurons adhere to the substrate, and polarize during development. The polarization phase occurs during the first five days in vitro. At stage two of polarization, one of the extruding neurites becomes longer, and it will differentiate to become the axon10. Axonal elongation in response to towing force at the growth cone has been previously modeled by Dennerl's model11. Recently, this model has been extended12 to include the role of neurite adhesion to the extracellular matrix substrates. This biophysical model, proposed after experimental observations13, showed that pulling forces on growth cone, propagating along the neurite, are modulated by focal adhesions to the substrate. Likewise, axonal lesion produces a local release of tension propagating toward the cell body. Thus, we proposed that measuring such released tension in a location along the axon between the lesion and the cell soma offers the possibility to assess the dampening outcome of unaffected focal adhesions.
We calibrate the necessary energy photon-flux of the laser dissector to control the extent of the inflicted axonal damage, from complete transection to partial lesion. Following the calibration, we repeated partial lesion to the axons of several differentiating neurons and developed the protocol to quantify the tension release, and thus obtained a quantitative parameter to estimate the adhesion of the axon to the substrate14.
In the present work, we describe in detail the developed protocol, which represents a precise experimental procedure to evaluate and compare with piconewton sensitivity the axonal adhesion to the substrate in different experimental conditions such as chemical treatment14, or different types of cell culture support.

<table border="1">
	<tbody>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>REAGENTS</td>
		</tr>
		<tr>
			<td>Polymer microspheres, &Oslash; 4 &mu;m, COOH coated</td>
			<td>Bangs laboratories</td>
			<td>PC05N/6700</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>PolyLink Protein Coupling Kit</td>
			<td>Polyscience</td>
			<td>19539</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>EQUIPMENT</td>
		</tr>
		<tr>
			<td>IR laser</td>
			<td>IPG Laser GmbH</td>
			<td>YLM-5-SC-LP</td>
			<td>ytterbium continuous wave (CW) fiber laser operating at 1064 nm, with linear polarization</td>
		</tr>
		<tr>
			<td>Spatial light modulator</td>
			<td>Hamamatsu</td>
			<td>LCOS-SLM 10468-07</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Blue-tweezers software</td>
			<td>Optics group, University of Glasgow</td>
			<td>Free downloadable software</td>
			<td><a href="http://www.physics.gla.ac.uk/Optics/projects/tweezers/slmcontrol/" target="_blank">http://www.physics.gla.ac.uk/Optics/projects/tweezers/slmcontrol/</a></td>
		</tr>
		<tr>
			<td>ImageJ</td>
			<td>NIH</td>
			<td>Free downloadable software</td>
			<td><a href="http://rsbweb.nih.gov/ij/" target="_blank">http://rsbweb.nih.gov/ij/ </a></td>
		</tr>
		<tr>
			<td>QPD</td>
			<td>Thorlabs</td>
			<td>S5980 with C5460SPL 6041 board</td>
			<td>Four quadrant photo-diode to measure x, y trapped probe displacement</td>
		</tr>
		<tr>
			<td>PD</td>
			<td>Teem Photonics</td>
			<td>PDA100A-EC</td>
			<td>Photodiode to measure z trapped probe displacement</td>
		</tr>
		<tr>
			<td>nano-Pulse UV laser</td>
			<td>AA-optoelctronics</td>
			<td>PNV-001525-040</td>
			<td>Pulsed UVA laser, pulse length 400 ps</td>
		</tr>
		<tr>
			<td>Acoustic Optic Modulator</td>
			<td>Olympus</td>
			<td>MQ110-A3-UV, 355 nm fused silica</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Upright microscope</td>
			<td>Andor</td>
			<td>BX51</td>
			<td>Equipped with a 60X, 0.9 NA, water dipping objective</td>
		</tr>
		<tr>
			<td>CCD</td>
			<td>Warner Instruments</td>
			<td>V887ECSUVB EMCCD</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Peltier device</td>
			<td>Physic Instruments</td>
			<td>QE1 resistive heating with TC-344B dual channel heater controller</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Microscope stage: micro+piezo stage</td>
			<td>National Instruments</td>
			<td>Three linear stages M-126.CG1 carrying a separate 3-axis piezoelectric nano-positioning stage P-733.3DD</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Daq</td>
			<td>&nbsp;</td>
			<td>NI PCI-6229</td>
			<td>Acquiring the x, y, z position of the trapped probe, and sending feedback loop signals to microscope stage</td>
		</tr>
		<tr>
			<td>Linux Real Time Application Interface (RTAI) machine</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>Real time feedback loop system, to control stage position, developed on a dedicated PC desktop</td>
		</tr>
	</tbody>
</table>

measurement of tension release during laser induced axon lesion to evaluate axonal adhesion to the substrate at piconewton and millisecond resolution

The formation of functional connections in a developing neuronal network is influenced by extrinsic cues. The neurite growth of developing neurons is subject to chemical and mechanical signals, and the mechanisms by which it senses and responds to mechanical signals are poorly understood. Elucidating the role of forces in cell maturation will enable the design of scaffolds that can promote cell adhesion and cytoskeletal coupling to the substrate, and therefore improve the capacity of different neuronal types to regenerate after injury.
Here, we describe a method to apply simultaneous force spectroscopy measurements during laser induced cell lesion. We measure tension release in the partially lesioned axon by simultaneous interferometric tracking of an optically trapped probe adhered to the membrane of the axon. Our experimental protocol detects the tension release with piconewton sensitivity, and the dynamic of the tension release at millisecond time resolution. Therefore, it offers a high-resolution method to study how the mechanical coupling between cells and substrates can be modulated by pharmacological treatment and/or by distinct mechanical properties of the substrate.

The cell generates traction forces on the substrate by its focal adhesions. Force generated by cytoskeletal elements are in equilibrium with the counteracting force of the culture substrate. After laser induced lesion of the neurite, some of the tensed cytoskeleton cables are disrupted and their equilibrated tension is released because the opposing force of the substrate adhesion is eliminated. The released tension is partially distributed on the unaffected focal adhesions, and the bead attached to the cell membrane, hel...

Watch this Scientific Journal Video about Measurement of Tension Release During Laser Induced Axon Lesion to Evaluate Axonal Adhesion to the Substrate at Piconewton and Millisecond Resolution at JoVE.

Measurement of Tension Release During Laser Induced Axon Lesion to Evaluate Axonal Adhesion to the Substrate at Piconewton and Millisecond Resolution

We measured the tension release in an axon that was partially lesioned with a laser dissector by simultaneous force spectroscopy measurement performed on an optically-trapped probe adhered to the membrane of the axon. The developed experimental protocol evaluates the axon adhesion to the culture substrate.

The formation of functional connections in a developing neuronal network is influenced by extrinsic cues. The neurite growth of developing neurons is ...

measurement-tension-release-during-laser-induced-axon-lesion-to

Research

JoVE Journal

Bioengineering

10.2K Views.  National Research Council of Italy. Abbiamo misurato il rilascio della tensione in un assone, parzialmente lesionati con un dissettore laser per la misurazione simultanea della spettroscopia di forza eseguita su una sonda otticamente intrappolati aderito alla membrana dell'assone. Il protocollo sperimentale sviluppato valuta l'assone adesione al substrato cultura.

Video: Measurement of Tension Release During Laser Induced Axon Lesion to Evaluate Axonal Adhesion to the Substrate at Piconewton and Millisecond Resolution