We would like to thank Dina Aranovich for protein purification and labelling. G.L. is grateful to the Israel Ministry of Science, Technology and Space for the Jabotinsky PhD Scholarship. A.B.G. is grateful to the Israel Science Foundation (grant 2101/20) and to the Ministry of Science and Technology - State of Israel (grant 3-17491) for financial support.

1. Glass surface treatment and passivation:
NOTE: This section includes three major steps (see Figure 1): (i) cleaning and hydrophilization, (ii) silanization, and (iii) surface passivation.
<ol>
	<li>Cleaning and hydrophilization
	<ol>
		<li>Use Piranha solution for glass surface cleaning. 
		NOTE: Piranha solution is a mixture of 30% H2O2 and 70% H2SO4 (Table of Materials). Its prime goal is to ...

<ol>
	<li>Alberts, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Genesis, Modulation, and Regeneration of Skeletal Muscle In Molecular Biology of the Cell. 4th edition. , (2002).</li><li>Howard, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Mechanics of Motor Proteins and the Cytoskeleton. , (2001).</li><li>Mogilner, A., Manhart, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intracellular+fluid+mechanics:+Coupling+cytoplasmic+flow+with+active+cytoskeletal+gel.">Intracellular fluid mechanics: Coupling cytoplasmic flow with active cytoskeletal gel.</a> Annual Review of Fluid Mechanics. 50 (1), 347-370 (2018).</li><li>Bausch, A. R., Kroy, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+bottom-up+approach+to+cell+mechanics.">A bottom-up approach to cell mechanics.</a> Nature Physics. 2 (4), 231-238 (2006).</li><li>Moeendarbary, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+cytoplasm+of+living+cells+behaves+as+a+poroelastic+material.">The cytoplasm of living cells behaves as a poroelastic material.</a> Nature Materials. 12 (3), 253-261 (2013).</li><li>Charras, G. T., Mitchison, T. J., Mahadevan, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Animal+cell+hydraulics.">Animal cell hydraulics.</a> Journal of Cell Science. 122 (18), 3233-3241 (2009).</li><li>Charras, G. T., Yarrow, J. C., Horton, M. A., Mahadevan, L., Mitchison, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-equilibration+of+hydrostatic+pressure+in+blebbing+cells.">Non-equilibration of hydrostatic pressure in blebbing cells.</a> Nature. 435 (7040), 365-369 (2005).</li><li>Keren, K., Yam, P. T., Kinkhabwala, A., Mogilner, A., Theriot, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intracellular+fluid+flow+in+rapidly+moving+cells.">Intracellular fluid flow in rapidly moving cells.</a> Nature Cell Biology. 11 (10), 1219-1224 (2009).</li><li>Paluch, E., Piel, M., Prost, J., Bornens, M., Sykes, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cortical+actomyosin+breakage+triggers+shape+oscillations+in+cells+and+cell+fragments.">Cortical actomyosin breakage triggers shape oscillations in cells and cell fragments.</a> Biophysical Journal. 89 (1), 724-733 (2005).</li><li>Liu, A. P., Fletcher, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biology+under+construction:+In+vitro+reconstitution+of+cellular+function.">Biology under construction: In vitro reconstitution of cellular function.</a> Nature Reviews Molecular Cell Biology. 10 (9), 644-650 (2009).</li><li>Siton-Mendelson, O., Bernheim-Groswasser, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+the+reconstitution+of+synthetic+cell+motility.">Toward the reconstitution of synthetic cell motility.</a> Cell Adhesion &amp; Migration. 10 (5), 461-474 (2016).</li><li>Bernheim-Groswasser, A., Wiesner, S., Golsteyn, R. M., Carlier, M. -. F., Sykes, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+dynamics+of+actin-based+motility+depend+on+surface+parameters.">The dynamics of actin-based motility depend on surface parameters.</a> Nature. 417 (6886), 308-311 (2002).</li><li>Bieling, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Force+feedback+controls+motor+activity+and+mechanical+properties+of+self-assembling+branched+actin+networks.">Force feedback controls motor activity and mechanical properties of self-assembling branched actin networks.</a> Cell. 164 (1-2), 115-127 (2016).</li><li>Carvalho, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Actin+polymerization+or+myosin+contraction:+two+ways+to+build+up+cortical+tension+for+symmetry+breaking.">Actin polymerization or myosin contraction: two ways to build up cortical tension for symmetry breaking.</a> Philosophical Transactions of the Royal Society B: Biological Sciences. 368 (1629), 20130005 (2013).</li><li>Dayel, M. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+silico+reconstitution+of+actin-based+symmetry+breaking+and+motility.">In silico reconstitution of actin-based symmetry breaking and motility.</a> PLoS Biology. 7 (9), e1000201 (2009).</li><li>Manhart, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+regulation+of+the+dynamic+steady+state+of+actin+networks.">Quantitative regulation of the dynamic steady state of actin networks.</a> eLife. 8, 42413 (2019).</li><li>Siton, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cortactin+releases+the+brakes+in+actin-based+motility+by+enhancing+WASP-VCA+detachment+from+Arp2/3+branches.">Cortactin releases the brakes in actin-based motility by enhancing WASP-VCA detachment from Arp2/3 branches.</a> Current Biology. 21 (24), 2092-2097 (2011).</li><li>Pontani, L. -. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reconstitution+of+an+actin+cortex+inside+a+liposome.">Reconstitution of an actin cortex inside a liposome.</a> Biophysical Journal. 96 (1), 192-198 (2009).</li><li>Ideses, Y., Sonn-Segev, A., Roichman, Y., Bernheim-Groswasser, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Myosin+II+does+it+all:+Assembly,+remodeling,+and+disassembly+of+actin+networks+are+governed+by+myosin+II+activity.">Myosin II does it all: Assembly, remodeling, and disassembly of actin networks are governed by myosin II activity.</a> Soft Matter. 9 (29), 7127-7137 (2013).</li><li>Backouche, F., Haviv, L., Groswasser, D., Bernheim-Groswasser, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Active+gels:+Dynamics+of+patterning+and+self-organization.">Active gels: Dynamics of patterning and self-organization.</a> Physical Biology. 3 (4), 264 (2006).</li><li>Alvarado, J., Sheinman, M., Sharma, A., MacKintosh, F. C., Koenderink, G. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+motors+robustly+drive+active+gels+to+a+critically+connected+state.">Molecular motors robustly drive active gels to a critically connected state.</a> Nature Physics. 9 (9), 591-597 (2013).</li><li>Linsmeier, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disordered+actomyosin+networks+are+sufficient+to+produce+cooperative+and+telescopic+contractility.">Disordered actomyosin networks are sufficient to produce cooperative and telescopic contractility.</a> Nature Communications. 7 (1), 12615 (2016).</li><li>Ideses, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spontaneous+buckling+of+contractile+poroelastic+actomyosin+sheets.">Spontaneous buckling of contractile poroelastic actomyosin sheets.</a> Nature Communications. 9 (1), 2461 (2018).</li><li>Spudich, J. A., Watt, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+regulation+of+rabbit+skeletal+muscle+contraction:+I.+Biochemical+studies+of+the+interaction+of+the+tropomyosin-troponin+complex+with+actin+and+the+proteolytic+fragments+of+myosin.">The regulation of rabbit skeletal muscle contraction: I. Biochemical studies of the interaction of the tropomyosin-troponin complex with actin and the proteolytic fragments of myosin.</a> Journal of Biological Chemistry. 246 (15), 4866-4871 (1971).</li><li>Ono, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+an+actin+binding+region+and+a+protein+kinase+C+phosphorylation+site+on+human+fascin.">Identification of an actin binding region and a protein kinase C phosphorylation site on human fascin.</a> Journal of Biological Chemistry. 272 (4), 2527-2533 (1997).</li><li>Margossian, S. S., Lowey, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+myosin+and+its+subfragments+from+rabbit+skeletal+muscle.">Preparation of myosin and its subfragments from rabbit skeletal muscle.</a> In Methods in Enzymology. 85, 55-71 (1982).</li><li>Quinlan, M. E., Forkey, J. N., Goldman, Y. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Orientation+of+the+myosin+light+chain+region+by+single+molecule+total+internal+reflection+fluorescence+polarization+microscopy.">Orientation of the myosin light chain region by single molecule total internal reflection fluorescence polarization microscopy.</a> Biophysical Journal. 89 (2), 1132-1142 (2005).</li><li>Cheng, N. -. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Formula+for+the+viscosity+of+a+glycerol&#8722;water+mixture.">Formula for the viscosity of a glycerol&#8722;water mixture.</a> Industrial &amp; Engineering Chemistry Research. 47 (9), 3285-3288 (2008).</li><li>Marsh, D., Bartucci, R., Sportelli, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lipid+membranes+with+grafted+polymers:+Physicochemical+aspects.">Lipid membranes with grafted polymers: Physicochemical aspects.</a> Biochimica et Biophysica Acta (BBA) - Biomembranes. 1615 (1-2), 33-59 (2003).</li><li>Strychalski, W., Guy, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intracellular+pressure+dynamics+in+blebbing+cells.">Intracellular pressure dynamics in blebbing cells.</a> Biophysical Journal. 110 (5), 1168-1179 (2016).</li><li>Head, D. A., Levine, A. J., MacKintosh, F. 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+regimes+of+elastic+response+and+deformation+modes+of+crosslinked+cytoskeletal+and+semiflexible+polymer+networks.">Distinct regimes of elastic response and deformation modes of crosslinked cytoskeletal and semiflexible polymer networks.</a> Physical Review E. 68 (6), 061907 (2003).</li><li>MacKintosh, F. C., Levine, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonequilibrium+mechanics+and+dynamics+of+motor-activated+gels.">Nonequilibrium mechanics and dynamics of motor-activated gels.</a> Physical Review Letters. 100 (1), 018104 (2008).</li><li>Melero, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-induced+molecular+adsorption+of+proteins+using+the+PRIMO+system+for+micro-patterning+to+study+cell+responses+to+extracellular+matrix+proteins.">Light-induced molecular adsorption of proteins using the PRIMO system for micro-patterning to study cell responses to extracellular matrix proteins.</a> Journal of Visualized Experiments. (152), e60092 (2019).</li><li>Bendix, P. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+quantitative+analysis+of+contractility+in+active+cytoskeletal+protein+networks.">A quantitative analysis of contractility in active cytoskeletal protein networks.</a> Biophysical Journal. 94 (8), 3126-3136 (2008).</li></ol>

The authors have nothing to disclose.

Here, an in vitro approach is employed to characterize the mechanics of poroelastic actomyosin gels as a model system of the cell cytoskeleton and, more generally, of the cell cytoplasm, which has been shown to behave as a poroelastic material3,5. The rheology of the cell cytoskeleton (cytoplasm) has been characterized by a poroelastic diffusion constant, which dictates how long it takes for the intracellular cytosolic fluid to redistribute within the ce...

Living cells have unique mechanical properties. Besides the ability to passively react to applied forces, they are also capable of actively generating forces in response to external stimuli1. These characteristics, which are essential for a variety of cellular processes, notably during cell motility, are primarily attributed to the mechanical and dynamic properties of the cell cytoskeleton, especially the actomyosin cytoskeleton, which is an active gel of polar actin filaments, myosin molecular motors, and accessory proteins. These actomyosin networks exhibit intrinsic self-organization and contraction properties driven by the myosin motor proteins, which crosslink the actin filaments and actively generate mechanical stresses in the network fueled by ATP hydrolysis2.
Numerous experimental and theoretical studies have been conducted to study the material properties of the cytoskeleton3. The commonly accepted view is that the cytoskeleton behaves as a viscoelastic material4. This means that on short timescales, the cytoskeleton behaves as an elastic material, and on long timescales, it behaves as a viscous fluid due to the crosslinking proteins and myosin motor detachment (and reattachment), which allows the network to dynamically turnover. In many situations, however, the viscoelastic model cannot describe the experimental results, which are more consistent with a picture describing the cytoskeleton and, more generally, the cell cytoplasm being described as a poroelastic active material5,6. Two main features characterize these types of materials. (i) The first main feature is the generation of a flow of the penetrating cytosol (the &#34;solvent&#34;) across the gel pores by contractility gradients driven by the myosin motors, which underlies processes such as cell blebbing7, motility8, and cell shape oscillations9. The emergence of such cytosolic flows can be local, for blebbing, or global, like in cell motility. In the latter case, the contractile-applied stresses at the cell rear drive the flow of the cytosolic fluid toward the cell front, which replenishes the protein pool needed for lamellipodia assembly8. (ii) The second main feature is that the relaxation of stresses is diffusive and is characterized by an effective diffusion constant, , which depends on the gel elastic modulus, gel porosity, and solvent viscosity5. The poroelastic diffusion constant determines how fast the system responds to an applied stress. Higher diffusion constants correspond to faster stress redistribution. This, in turn, determines how long it takes for the intracellular cytosolic fluid to be redistributed within the cell following applied mechanical stress, be it external or internal, such as the active contractile stresses generated by myosin motors. These examples, thus, demonstrate that the mechanics of the cytoskeleton and the cytosol are tightly coupled and cannot be treated separately3.
As cells can regulate their mechanical properties in a variety of ways, the interplay between network mechanics and fluid flow dynamics remains poorly understood. A powerful alternative approach is to use in vitro reconstituted systems that allow for full control of the various microscopic constituents and the system parameters, which renders these model systems optimal for physical analysis10,11. This approach has been successfully employed to study the impact of protein composition and system geometry on actin-based motility12,13,14,15,16,17,18, the 2D patterning of actomyosin networks19,20,21,22, and the interplay between network contractility and fluid flow dynamics of poroelastic actomyosin gels, which is the focus of this paper23.
In this manuscript, the preparation of contractile elastic actomyosin networks of controllable dimensions and material properties is discussed based on the work of Ideses et al.23. The dynamics of the contracting gel and the drained solvent are analyzed and quantified, through which it is demonstrated that these actomyosin gels can be described as a poroelastic active material. Studying the effect of solvent viscosity on stress diffusivity further confirms the poroelastic nature of these networks. The various scaling relations used for data quantification are provided. Finally, the experimental challenges, the common pitfalls, and the relevance of the experimental results to the cell cytoskeleton are also discussed.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>(3-Mercaptopropyl)trimethoxysilane</td>
			<td>Sigma-Aldrich Company</td>
			<td>175617</td>
			<td>Stored under Argon atmosphere at 4 &#176;C&#160;</td>
		</tr>
		<tr>
			<td>Acetic acid</td>
			<td>Bio-Lab ltd</td>
			<td>1070521</td>
			<td></td>
		</tr>
		<tr>
			<td>Alexa-Fluor 488</td>
			<td>Invitrogene</td>
			<td>A10254</td>
			<td>Diluted with DMSO, stored under Argon atmosphere at -20 &#176;C&#160;</td>
		</tr>
		<tr>
			<td>Alexa-Fluor 647</td>
			<td>Invitrogene</td>
			<td>A20347</td>
			<td>Diluted with DMSO, stored under Argon atmosphere at -20 &#176;C&#160;</td>
		</tr>
		<tr>
			<td>BSA</td>
			<td>Sigma -Aldrich Company</td>
			<td>A3059</td>
			<td>Stored at 4 &#176;C&#160;</td>
		</tr>
		<tr>
			<td>Catalase</td>
			<td>Sigma -Aldrich Company</td>
			<td>C9322</td>
			<td>The stock bottle is kept under dry atmosphere (silica gel) at -20 &#176;C</td>
		</tr>
		<tr>
			<td>Coverslips</td>
			<td>Mezel-glaser</td>
			<td>CG2222-1.5</td>
			<td>Kept in milliQ-water after the Piranha treatment and used within 3 weeks</td>
		</tr>
		<tr>
			<td>Creatine kinase</td>
			<td>Roche Life Science Products</td>
			<td>10736988001</td>
			<td>Prepared fresh in glycine buffer, kep on ice, and used within 3 days.&#160; The stock bottle is kept under dry atmosphere (silica gel) at 4 &#176;C</td>
		</tr>
		<tr>
			<td>Creatine phosphate</td>
			<td>Roche Life Science Products</td>
			<td>10621714001</td>
			<td>When dissolved should be kept at -20 &#176;C and used within 3 months. The stock bottle is kept under Argon atmosphere and stored at 4 &#176;C</td>
		</tr>
		<tr>
			<td>DTT</td>
			<td>Roche Life Science Products</td>
			<td>10708984001</td>
			<td>When dissolved should be kept at -20 &#176;C and used within 3 months</td>
		</tr>
		<tr>
			<td>Dual view Simultaneous Imaging System&#160;</td>
			<td>Photometrics</td>
			<td>DV2-CUBE</td>
			<td></td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>MP Biomedicals</td>
			<td>195174</td>
			<td></td>
		</tr>
		<tr>
			<td>EM-CCD Camera</td>
			<td>Andor Technology Ltd</td>
			<td>DV 887</td>
			<td></td>
		</tr>
		<tr>
			<td>EM-CCD Camera</td>
			<td>Photometrics</td>
			<td>Evolve Delta</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Bio-Lab ltd</td>
			<td>525050300</td>
			<td></td>
		</tr>
		<tr>
			<td>Flourescence Lamp</td>
			<td>Rapp Optoelectronic</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fluoresbrite YG Microspheres</td>
			<td>Polysciences</td>
			<td>17151-10</td>
			<td>200 nm diameter</td>
		</tr>
		<tr>
			<td>Glucose</td>
			<td>ICN Biomedicals Inc</td>
			<td>194024</td>
			<td>When dissolved should be kept at -20 &#176;C and used within 3 months.</td>
		</tr>
		<tr>
			<td>Glucose oxidase</td>
			<td>Sigma-Aldrich Company</td>
			<td>G7141</td>
			<td>Kept in -20 &#176;C and used within 3 months. The stock powder is kept under Argon atmosphere and kept at -20 &#176;C</td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>ICN Biomedicals Inc</td>
			<td>800687</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycine</td>
			<td>MP Biomedicals</td>
			<td>808822</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen Peroxide</td>
			<td>Sigma-Aldrich Company</td>
			<td>216763</td>
			<td>Stored at 4 &#176;C&#160;</td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>EMD Millipore Corp.</td>
			<td>529552</td>
			<td></td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Bio-Lab ltd</td>
			<td>1368052100</td>
			<td></td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>EMD Millipore Corp.</td>
			<td>442615</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope</td>
			<td>Leica Microsystems</td>
			<td>DMI3000</td>
			<td></td>
		</tr>
		<tr>
			<td>mPEG-mal</td>
			<td>Nanocs</td>
			<td>PG1-ML-5k&#160;</td>
			<td>Mw = 5000 Da. Divided to small batches by weight. Stored under Argon atmosphere at -20 &#176;C</td>
		</tr>
		<tr>
			<td>Nile red microspheres</td>
			<td>Spherotech</td>
			<td>FP-2056-2&#160;</td>
			<td>2300 nm diameter</td>
		</tr>
		<tr>
			<td>Objective (10x)</td>
			<td>Leica Germany</td>
			<td>HC PL AP0</td>
			<td>UPlanFL Numerical Aperture = 0.3</td>
		</tr>
		<tr>
			<td>Objective (2.5x)</td>
			<td>Leica Germany</td>
			<td>506304</td>
			<td>&#160;Plan-NEOFLUAR Numerical Aperture = 0.075</td>
		</tr>
		<tr>
			<td>Oven</td>
			<td>WTC Binder</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>Amcor</td>
			<td>PM-996</td>
			<td></td>
		</tr>
		<tr>
			<td>PBS Buffer</td>
			<td>Sigma-Aldrich Company</td>
			<td>P4417</td>
			<td></td>
		</tr>
		<tr>
			<td>Shutter Driver</td>
			<td>Vincet Associates</td>
			<td>VMM D1</td>
			<td></td>
		</tr>
		<tr>
			<td>Silica gel</td>
			<td>Merck</td>
			<td>1.01907.5000</td>
			<td></td>
		</tr>
		<tr>
			<td>Sonicator</td>
			<td>Elma</td>
			<td>Elmasonic P</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfuric acid</td>
			<td>Carlo Erba reagents</td>
			<td>410301</td>
			<td></td>
		</tr>
		<tr>
			<td>DV2 Dual-Channel Simultaneous-Imaging System</td>
			<td>Photometrics</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>TRIS</td>
			<td>MP Biomedicals</td>
			<td>819620</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-VIS Spectrophotometer</td>
			<td>Pharmacia</td>
			<td>Ultraspec 2100 pro</td>
			<td></td>
		</tr>
		<tr>
			<td>MICROMAN E</td>
			<td>Gilson</td>
			<td>FD10001</td>
			<td>1&#8211;10 uL</td>
		</tr>
		<tr>
			<td>MATLAB R2017b</td>
			<td>MathWorks</td>
			<td></td>
			<td>Data quantification&#160;</td>
		</tr>
		<tr>
			<td>MetaMorph&#160;</td>
			<td>Molecular devices</td>
			<td></td>
			<td>Control software of the optical imaging system; data quantification (particle tracking analysis, network mesh size)</td>
		</tr>
	</tbody>
</table>

the mechanics of (poro-)elastic contractile actomyosin networks as a model system of the cell cytoskeleton

Cells can actively change their shapes and become motile, a property that depends on their ability to actively reorganize their internal structure. This feature is attributed to the mechanical and dynamic properties of the cell cytoskeleton, notably, the actomyosin cytoskeleton, which is an active gel of polar actin filaments, myosin motors, and accessory proteins that exhibit intrinsic contraction properties. The usually accepted view is that the cytoskeleton behaves as a viscoelastic material. However, this model cannot always explain the experimental results, which are more consistent with a picture describing the cytoskeleton as a poroelastic active material-an elastic network embedded with cytosol. Contractility gradients generated by the myosin motors drive the flow of the cytosol across the gel pores, which infers that the mechanics of the cytoskeleton and the cytosol are tightly coupled. One main feature of poroelasticity is the diffusive relaxation of stresses in the network, characterized by an effective diffusion constant that depends on the gel elastic modulus, porosity, and cytosol (solvent) viscosity. As cells have many ways to regulate their structure and material properties, our current understanding of how cytoskeleton mechanics and cytosol flow dynamics are coupled remains poorly understood. Here, an in vitro reconstitution approach is employed to characterize the material properties of poroelastic actomyosin gels as a model system for the cell cytoskeleton. Gel contraction is driven by myosin motor contractility, which leads to the emergence of a flow of the penetrating solvent. The paper describes how to prepare these gels and run experiments. We also discuss how to measure and analyze the solvent flow and gel contraction both at the local and global scales. The various scaling relations used for data quantification are given. Finally, the experimental challenges and common pitfalls are discussed, including their relevance to cell cytoskeleton mechanics.

Two glass coverslips are used per experiment. The glass coverslips are cleaned and passivated with PEG polymers. Passivation is essential for preventing the solubilized proteins from adhering to the glass surfaces at the early experimental stages and for minimizing the interaction of the contracting network with the glass walls. Failure to achieve good passivation can lead to inefficient contraction and, in extreme cases, can even inhibit actin network formation.
Figure 1<...

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The Mechanics of Poro-Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton

In this work, an in vitro reconstitution approach is employed to study the poroelasticity of actomyosin gels under controlled conditions. The dynamics of the actomyosin gel and the embedded solvent are quantified, through which the network poroelasticity is demonstrated. We also discuss the experimental challenges, common pitfalls, and relevance to cell cytoskeleton mechanics.

The Mechanics of (Poro-)Elastic Contractile Actomyosin Networks As a Model System of the Cell Cytoskeleton

Cells can actively change their shapes and become motile, a property that depends on their ability to actively reorganize their internal structure. This ...

We have developed an in vitro approach to characterize the mechanics of poroelastic actomyosin gels as a model system of the cell cytoskeleton, and more generally, the cytoplasm, which have been shown to behave as a poroelastic active material. This method provides insights into how myosin contractility contributes to the emergence of a fluid flow and how network and cytosol velocities are correlated in time and space. This simple approach allows us to extract the mechanical properties of dynamically evolving systems without using any sophisticated equipment or techniques where conventional tools like AFIM cannot be employed.This method can provide insights into the rheology of poroelastic active materials regardless of the specificity of the active gel and the embedded fluid. To begin, place 10 to 12 number 1.5 glass coverslips in a homemade polytetrafluoroethylene holder and prepare Piranha solution in a 400 milliliter beaker. Prepare the solution on ice in a chemical hood.Transfer the holder into the Piranha filled beaker. After three hours, transfer the holder into a clean beaker filled with fresh double distilled water to wash the excess Piranha from the coverslips and transfer the beaker into a sonication bath at 80 hertz and full power for 10 minutes at 25 degrees Celsius. Repeat this step two more times with fresh double distilled water.For salinization, transfer the holder into a clean beaker filled with 300 milliliters of pure methanol, then into a sonication bath for 30 minutes. Then transfer the holder into a silane solution. Seal the beaker with parafilm and keep it in the refrigerator at four degrees Celsius overnight.On the next day, transfer the holder into a clean beaker filled with 300 milliliters of pure methanol for 15 seconds. Dry the bottom of the holder to remove the excess methanol and transfer it into a clean beaker. Then transfer the beaker to the oven to dry the glass coverslips for five minutes at 120 degrees Celsius.To achieve surface passivation, take two coverslips for each experiment and place them in a Petri dish coated with a parafilm layer initially cleaned with ethanol. Incubate each coverslip with one milliliter of methoxypolyethylene glycol maleimide in 1X PBS for one hour at 22 degrees Celsius. At the end of the incubation process, tilt the Petri dish to remove and peg.Rinse each coverslip with five milliliters of double distilled water and dry with a flow of nitrogen gas. Since the coverslips must be kept wet after passivation, immediately put one milliliter of 10 millimolar Tris on the pegylated surfaces and use the coverslips with two hours. To form large myosin aggregates, dilute the myosin stock solution with 10 millimolar Tris to reach a final concentration of 25 millimolar potassium chloride.Keep the diluted myosin solution for 10 minutes at room temperature and then transfer it to ice until use. The motors are fully active for up to two hours. to track the solvent flow across the gel pores, prepare passivated fluorescent beads and reduce the interaction between the beads and the actomyosin network by incubating one microliter of beads with five micromolar G-actin for 20 minutes.Remove the excess G-actin by centrifugation. Repeat this step with 10 milligrams per milliliter of bovine serum albumin to block the remaining uncoated surface and use the beads at a final dilution of one to 10, 000 in the experiments. To prepare the actomyosin solution, thaw the protein aliquots stored at minus 80 degrees Celsius and place them on ice.Then prepare the solution to reach a final concentration of 10 millimolar Tris, five micromolar G-actin, 280 nanomolar GST Fascin, and 1.67 nanomolar myosin II.Take one of the polyethylene glycol passivated coverslips. Place a greased paraform spacer on top of it and place it in the sample holder. Add 1.1 microliters of the actomyosin solution to the holder mounted coverslip and place the second coverslip on top.Screw the holder to seal the sample. Place the sample holder on the microscope and start the acquisition. Prepare the microscope in advance to reduce the initial acquisition time.To image the samples with an inverted fluorescent microscope, visualize the whole gel area by using low magnifications of 2.5X objective and follow its macroscopic contraction with time. Use a 10X objective to characterize the network porosity and structure and to follow the network self-organization and contraction with time. For the simultaneous two-color imaging, excite the actin at 488 nanometers and myosin II motors at 561 nanometers and record the images simultaneously on an EMCCD camera using a dual emission apparatus.Use the same imaging system to simultaneously image the actin gel and the fluorescent myosin II motors. For simultaneous two-color imaging, excite the actin at 488 nanometers and 2, 300 nanometer beads at 561 nanometers. And record the images simultaneously on an EMCCD camera using a dual emission apparatus.Use the same imaging system to simultaneously image the actin gel and the fluorescent beads. The first day involves demonstrating that the actomyosin network behaves as an elastic material. A schematic representation of an actomyosin network is depicted here.Fluorescence microscopy images demonstrate that the actin filaments spontaneously nucleate and polymerize into an isotropic interconnected network that coarsens with time and eventually contracts macroscopically. Network attraction initiates at the gel periphery and propagates inward into the gel bulk. The gel center is marked with a C.The actin filament bundles remain straight during network contraction.The distribution of the ratio between the contour length and the end-to-end distance at T316 seconds and T327 seconds is shown here. The second aim involves demonstrating that an outward solvent flow is generated by myosin contractility. The image of the gel at low magnification at intermediate stages of contraction is shown here.These circles mark the positions of four beads. The arrows mark the global direction of the bead motion. The trajectories of the nine selected beads are depicted here.Resolving the network porosity and the movement of the solvent across the gel pores at advanced stages of contraction are depicted here. The third aim involves demonstrating that stress relaxation is characterized by an effective poroelastic diffusion constant. Top view fluorescence images of a contracting actomyosin gel at low magnification from the mixing time up to the steady state are shown here.The actomyosin networks behave as a poroelastic active material. While attempting this procedure, work fast and accurately and keep in mind the importance of surface preservation.

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689 Views.  Ben Gurion University of the Negev. We have developed an in vitro approach to characterize the mechanics of poroelastic actomyosin gels as a model system of the cell cytoskeleton, and more generally, the cytoplasm, which have been shown to behave as a poroelastic active material. This method provides insights into how myosin contractility contributes to the emergence of a fluid flow and how network and cytosol velocities are correlated in time and space. This simple approach allows us to extract the mechanical properties of dyn...