The authors are thankful to Dr. Jeffrey Golden (Cedars-Sinai Medical Center) for the critical review of the manuscript. This work was funded in part by the National Institute of Neurological Disease and Stroke (NINDS, R01NS113516).

1. Cell maintenance and seeding (Day 1)
<ol>
	<li>Maintain HEK293T cells in cell culture media containing Dulbecco&#39;s Modified Eagle Medium (DMEM) with 10% fetal bovine serum (FBS) (in 100 mm culture dishes) in a humidified incubator at 37 &#176;C with 5% CO2.</li>
	<li>Before starting, check the confluency of the plate by viewing under the microscope. When the cells reach approximately 80-90% confluency, prepare to seed the cells in a 6-well culture plate by removing the media and washin...

A Simple Assay to Measure ER-Mitochondria Contacts for High-Throughput Screens

<ol>
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A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+the+connection:+How+membrane+contact+sites+have+changed+our+view+of+organelle+biology.">Making the connection: How membrane contact sites have changed our view of organelle biology.</a> Cell. 187 (2), 257-270 (2024).</li><li>Paillusson, 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=There's+something+wrong+with+my+MAM;+the+ER-mitochondria+axis+and+neurodegenerative+diseases.">There's something wrong with my MAM; the ER-mitochondria axis and neurodegenerative diseases.</a> Trends Neurosci. 39 (3), 146-157 (2016).</li><li>Sasi, U. S. S., Sindhu, G., Raj, P. S., Raghu, K. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+associated+membranes+(MAMs):+emerging+drug+targets+for+diabetes.">Mitochondria associated membranes (MAMs): emerging drug targets for diabetes.</a> Curr Med Chem. 27 (20), 3362-3385 (2019).</li><li>Joshi, A. U., Kornfeld, O. S., Mochly-Rosen, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+entangled+ER-mitochondrial+axis+as+a+potential+therapeutic+strategy+in+neurodegeneration:+A+tangled+duo+unchained.">The entangled ER-mitochondrial axis as a potential therapeutic strategy in neurodegeneration: A tangled duo unchained.</a> Cell Calcium. 60 (3), 218-234 (2016).</li><li>Moltedo, O., Remondelli, P., Amodio, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+mitochondria-endoplasmic+reticulum+contacts+and+their+critical+role+in+aging+and+age-associated+diseases.">The mitochondria-endoplasmic reticulum contacts and their critical role in aging and age-associated diseases.</a> Front Cell Dev Biol. 7, 172 (2019).</li><li>Rieusset, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+role+of+endoplasmic+reticulum-mitochondria+contact+sites+in+the+control+of+glucose+homeostasis:+an+update.">The role of endoplasmic reticulum-mitochondria contact sites in the control of glucose homeostasis: an update.</a> Cell Death Dis. 9 (3), 388 (2018).</li><li>Bouguerra, M. D., Lalli, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ER-mitochondria+interactions:+Both+strength+and+weakness+within+cancer+cells.">ER-mitochondria interactions: Both strength and weakness within cancer cells.</a> Biochim Biophys Acta Mol Cell Res. 1866 (4), 650-662 (2019).</li><li>Tepikin, A. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+junctions+with+cellular+organelles:+Ca2++signalling+perspective.">Mitochondrial junctions with cellular organelles: Ca2+ signalling perspective.</a> Pfl&#252;gers Arch. 470 (8), 1181-1192 (2018).</li><li>Masson, G. L., Przedborski, S., Abbott, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+computational+model+of+motor+neuron+degeneration.">A computational model of motor neuron degeneration.</a> Neuron. 83 (4), 975-988 (2014).</li><li>Giamogante, F., Barazzuol, L., Brini, M., Cal&#236;, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ER-mitochondria+contact+sites+reporters:+strengths+and+weaknesses+of+the+available+approaches.">ER-mitochondria contact sites reporters: strengths and weaknesses of the available approaches.</a> Int J Mol Sci. 21 (21), 8157 (2020).</li><li>Rizzuto, R., 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=Close+contacts+with+the+endoplasmic+reticulum+as+determinants+of+mitochondrial+Ca2++responses.">Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2+ responses.</a> Science. 280 (5370), 1763-1766 (1998).</li><li>Valm, A. 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=Applying+systems-level+spectral+imaging+and+analysis+to+reveal+the+organelle+interactome.">Applying systems-level spectral imaging and analysis to reveal the organelle interactome.</a> Nature. 546 (7656), 162-167 (2017).</li><li>Csord&#225;s, G., 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=Imaging+interorganelle+contacts+and+local+calcium+dynamics+at+the+ER-mitochondrial+interface.">Imaging interorganelle contacts and local calcium dynamics at the ER-mitochondrial interface.</a> Mol Cell. 39 (1), 121-132 (2010).</li><li>Cieri, D., 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=SPLICS:+a+split+green+fluorescent+protein-based+contact+site+sensor+for+narrow+and+wide+heterotypic+organelle+juxtaposition.">SPLICS: a split green fluorescent protein-based contact site sensor for narrow and wide heterotypic organelle juxtaposition.</a> Cell Death Differ. 25 (6), 1131-1145 (2018).</li><li>Kakimoto, 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=Visualizing+multiple+inter-organelle+contact+sites+using+the+organelle-+targeted+split-GFP+system.">Visualizing multiple inter-organelle contact sites using the organelle- targeted split-GFP system.</a> Sci Rep. 8 (1), 6175 (2018).</li><li>Wu, M. 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M., Scorrano, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitofusin+2+tethers+endoplasmic+reticulum+to+mitochondria.">Mitofusin 2 tethers endoplasmic reticulum to mitochondria.</a> Nature. 456 (7222), 605-610 (2008).</li><li>Kremer, 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=Developing+3D+SEM+in+a+broad+biological+context.">Developing 3D SEM in a broad biological context.</a> J Microsc. 259 (2), 80-96 (2015).</li><li>S&#246;derberg, 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=Direct+observation+of+individual+endogenous+protein+complexes+in+situ+by+proximity+ligation.">Direct observation of individual endogenous protein complexes in situ by proximity ligation.</a> Nat Methods. 3 (12), 995-1000 (2006).</li><li>Lim, Y., Cho, I. -. 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We have used a split-Renilla luciferase 8 reassembly assay (split-Rluc assay) to quantify the level of ER-mitochondria couplings. In this study, we have modified the original split-Rluc construct24 by generating a single vector, pCAG-MitoRlucN-T2A-RlucCER-IRES-mCherry, encoding each split-Rluc component (MitoRlucN and RlucCER) and a self-cleaving peptide 2A sequence from Thosea a...

The interactions between the ER and the mitochondria are vital for cellular homeostasis and survival1,2,3,4. Previous evidence indicates that any type of disruption or dysregulation in ER-mitochondria contact sites can contribute to several neurodegenerative, metabolic, and cardiovascular diseases, as well as cancer5,6,7,8,9,10. For example, an abnormal increase of Ca2+ uptake into the mitochondria can lead to cell death by opening mitochondria permeability transition pores, which are commonly seen in some models of Alzheimer&#39;s disease5,11. Similarly, reduced ER-mitochondria contacts can result in a decrease in ATP production&#160;and impairment of Ca2+ intake, as seen in models of amyotrophic lateral sclerosis5,11,12. As more studies are being conducted in the realm of ER-mitochondria contacts, additional disease-related proteins and genes that could affect these contacts are being discovered. Despite current knowledge and evidence showcasing the role of ER-mitochondria contact sites, much work is still needed to elucidate how these contacts could lead to loss of cellular function and ultimately cell death.
There have been various methods developed to evaluate the proximity of the two membranes, structural morphology, and the distance between the two organelle contact sites3,4,13. The approaches to monitor ER-mitochondria coupling include fluorescence marker-based imaging14,15, FRET-reporter-based imaging16, and split-fluorescence-probe-based imaging17,18, which use epifluorescence and confocal microscopy. Super-resolution and atomic resolution microscopy are also powerful tools for accurately visualizing inter-organelle contacts, although their utilization in contact site analysis is still limited since they require highly dedicated microscopes and technical expertise19. In addition, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and other EM techniques such as electron tomography (ET) and cryo-electron microscopy, are commonly used as they provide high-resolution ultrastructural imaging of the contact sites, which are often impossible to explore using other experimental approaches20,21,22. However, these EM-based methods are a very low-throughput technique that can also be affected by chemical &#64257;xation procedures. More recently, proximity labeling-based methods have been used to detect contact sites as well as to identify new contact-site proteins. For example, proximity ligation assay (PLA) has been used to quantify organelle proximities23,24, while a revised version of the ascorbate peroxidase (APEX) assay has been utilized in identifying new contact-site proteins25,26. It is important to recognize that all these methods described above have strengths and intrinsic limitations in detecting the contacts between the organelles. Thus, the pairing of different techniques is required to obtain a thorough interpretation of the organelle contact sites.
Previously we have established the split-Renilla luciferase 8 reassembly assay (split-Rluc assay) to monitor the level of ER-mitochondria membrane contacts (Figure 1A)24,26,27. Briefly, each split half of Renilla luciferase is conjugated with an ER- or mitochondria- targeting sequence. When transfected together, each split half of the enzyme is expressed either in the ER or mitochondrial membrane. When the ER and mitochondria are positioned in close proximity to each other, the split halves come together and reconstitute the whole enzyme with luciferase activity. For the split-Rluc construct, we used Renilla luciferase 8 (Rluc8) in pBAD/Myc-His27 for the initial template. The split site (between amino acids 91 and 92) was determined based on previous reports27. For the N-terminal half of the Rluc8, DNA sequences for amino acids 1-91 of the Rluc8 were fused to the 3&#39; end of the FLAG tag and the mouse AKAP1 mitochondrial-targeting sequence in pcDNA3.1 TOPO vector by PCR27. For the C-terminal half targeted to the ER, DNA sequences that encode amino acids 92-311 were fused to the 5&#39; end of the myc tag and the yeast UBC6 ER localization sequence. Here, we have upgraded the split-Rluc plasmid construct such that split halves of the Renilla luciferase are expressed in a single vector (pCAG) under the same promoter and subsequently cleaved into two fragments as T2A, a self-cleaving peptide 2A sequence from Thosea asigna virus, is inserted in between the two split halves (Figure 1B). The plasmid DNA map and sequences are provided in Supplemental File 1 and Supplemental Figure S1. Using this system, we have measured the effects of three chemicals (inhibiting GTPases involved in actin polymerization) on ER-mitochondria contacts. This split-Rluc assay is a simple but robust assay system for high-throughput screening for inter-organelle contact modulators24.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>1.7 mL SafeSeal Microcentrifuge Tube</td>
			<td>Sorenson</td>
			<td>16070</td>
			<td></td>
		</tr>
		<tr>
			<td>6-well plate TC Treated</td>
			<td>USA Scientific</td>
			<td>CC7682-7506</td>
			<td></td>
		</tr>
		<tr>
			<td>10 mL Pipette Tips OneTip</td>
			<td>USA Scientific</td>
			<td>1110-3700</td>
			<td></td>
		</tr>
		<tr>
			<td>10 &#956;L pipette tips OneTip</td>
			<td>USA Scientific</td>
			<td>1110-3700</td>
			<td></td>
		</tr>
		<tr>
			<td>20-200 &#956;L Beveled tips OneTip</td>
			<td>USA Scientific</td>
			<td>1111-1210</td>
			<td></td>
		</tr>
		<tr>
			<td>50 mL Polypropylene Conical Tube</td>
			<td>Falcon</td>
			<td>352070</td>
			<td></td>
		</tr>
		<tr>
			<td>96-Well Flipper Microtube Racks</td>
			<td>ThermoFisher Scientific</td>
			<td>8770-11</td>
			<td></td>
		</tr>
		<tr>
			<td>96-well plate TC Treated&#160;</td>
			<td>USA Scientific</td>
			<td>CC7682-7596</td>
			<td></td>
		</tr>
		<tr>
			<td>100 mm x 20 mm TC Treated Dish</td>
			<td>USA Scientific</td>
			<td>CC7682-3394</td>
			<td></td>
		</tr>
		<tr>
			<td>1250 &#956;L Tips OneTip</td>
			<td>USA Scientific</td>
			<td>1112-1720</td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge 5910&#160;Ri - Refrigerated Centrifuge</td>
			<td>Eppendorf</td>
			<td>5943000131</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide, anhydrous, &#8805;99.9%</td>
			<td>Sigma-Aldrich</td>
			<td>276855-100ML</td>
			<td></td>
		</tr>
		<tr>
			<td>DMEM, high glucose</td>
			<td>ThermoFisher Scientific</td>
			<td>11965092</td>
			<td></td>
		</tr>
		<tr>
			<td>DPBS, no calcium, no magnesium</td>
			<td>ThermoFisher Scientific</td>
			<td>14190144</td>
			<td></td>
		</tr>
		<tr>
			<td>EHop 016</td>
			<td>Bio-Techne Tocris</td>
			<td>6248</td>
			<td>Dissolve in DMSO; store at -70 &#176;C</td>
		</tr>
		<tr>
			<td>EnduRen Live Cell Substrate</td>
			<td>Promega</td>
			<td>E6481</td>
			<td>Store aliquots at -70 &#176;C</td>
		</tr>
		<tr>
			<td>Eppendorf 2-20 &#956;L pipette</td>
			<td>Eppendorf</td>
			<td>3123000039</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Research plus 100-1000 &#956;L pipette</td>
			<td>Eppendorf</td>
			<td>3123000063</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Research Plus 1-10 &#181;L pipette</td>
			<td>Eppendorf</td>
			<td>3123000020</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Research plus 12-channel</td>
			<td>Eppendorf</td>
			<td>3125000028</td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf Research plus 200 &#956;L pipette</td>
			<td>Eppendorf&#160;</td>
			<td>3123000055</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal Bovine Serum, qualified, USDA-approved regions</td>
			<td>ThermoFisher Scientific</td>
			<td>10437028</td>
			<td></td>
		</tr>
		<tr>
			<td>Forma Steri-Cycle CO2&#160;Incubator, 184 L, Polished Stainless Steel</td>
			<td>ThermoFisher Scientific</td>
			<td>381</td>
			<td></td>
		</tr>
		<tr>
			<td>Hand tally counter</td>
			<td>Sigma-Aldrich</td>
			<td>HS6594</td>
			<td></td>
		</tr>
		<tr>
			<td>HEK 293T Cells</td>
			<td>ATCC</td>
			<td>CRL-3216</td>
			<td></td>
		</tr>
		<tr>
			<td>Hemacytometer - Neubauer Bright Line, Double-Counting Chamber</td>
			<td>LW Scientific</td>
			<td>CTL-HEMM-GLDR</td>
			<td></td>
		</tr>
		<tr>
			<td>Invitrogen TE Buffer</td>
			<td>ThermoFisher Scientific</td>
			<td>8019005</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope</td>
			<td>Zeiss</td>
			<td>Axiovert 25 CFL</td>
			<td></td>
		</tr>
		<tr>
			<td>Mini centrifuge</td>
			<td>Benchmark Scientific</td>
			<td>C1012</td>
			<td></td>
		</tr>
		<tr>
			<td>Multi Tube Rack For 50ml Conical, 15ml Conical, And Microcentrifuge Tubes</td>
			<td>Boekel Scientific</td>
			<td>120008</td>
			<td></td>
		</tr>
		<tr>
			<td>PEI MAX - Transfection Grade Linear Polyethylenimine Hydrochloride (MW 40,000)</td>
			<td>Polysciences</td>
			<td>24765-100MG</td>
			<td></td>
		</tr>
		<tr>
			<td>Pipet-Aid XP</td>
			<td>USA Scientific</td>
			<td>4440-0101</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly-D-lysine hydrobromide</td>
			<td>Sigma-Aldrich</td>
			<td>P6407-5MG</td>
			<td></td>
		</tr>
		<tr>
			<td>Rhosin hydrochloride</td>
			<td>Bio-Techne Tocris</td>
			<td>5003</td>
			<td>Dissolve in DMSO; store at -70 &#176;C</td>
		</tr>
		<tr>
			<td>Trypsin-EDTA (0.05%), phenol red</td>
			<td>ThermoFisher Scientific</td>
			<td>25300054</td>
			<td></td>
		</tr>
		<tr>
			<td>Varioskan LUX multimode microplate reader</td>
			<td>ThermoFisher Scientific</td>
			<td>VL0000D0</td>
			<td></td>
		</tr>
		<tr>
			<td>Vortex</td>
			<td>ThermoFisher Scientific</td>
			<td>2215365</td>
			<td>level 8</td>
		</tr>
		<tr>
			<td>VWR Vacuum Aspiration System</td>
			<td>VWR</td>
			<td>75870-734</td>
			<td></td>
		</tr>
		<tr>
			<td>ZCL 278</td>
			<td>Bio-Techne Tocris</td>
			<td>4794</td>
			<td>Dissolve in DMSO; store at -70 &#176;C</td>
		</tr>
	</tbody>
</table>

split-luciferase reassembly assay to measure endoplasmic reticulum-mitochondria contacts in live cells

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid homeostasis, mitochondrial dynamics, autophagosome and mitophagosome biogenesis, and apoptosis. Failure to maintain normal ER-mitochondrial coupling is implicated in many neurodegenerative diseases such as Alzheimer&#39;s disease, Parkinson&#39;s disease, amyotrophic lateral sclerosis, and hereditary spastic paraplegia. It is of considerable significance to explore how the dysregulation of ER-mitochondrial contacts could lead to cell death and whether repairing these contacts to the normal level could ameliorate neurodegenerative conditions. Thus, improved assays that measure the level of these contacts could help to illuminate the pathogenic mechanisms of these diseases. Ultimately, establishing simple and reliable assays will facilitate the development of new therapeutic strategies. Here we describe&#160;a split-luciferase assay to quantitatively measure the level of ER-mitochondria contacts in live cells. This assay can be used to study the pathophysiological role of these contacts as well as to identify their modulators in high-throughput screening.

Author Spotlight: Regulation and Dysregulation of ER-Mitochondria Contacts &#8212; Implications for Neurodegenerative Disease Pathogenesis

Split-Luciferase Reassembly Assay to Measure Endoplasmic Reticulum-Mitochondria Contacts in Live Cells

In my lab, we study interorganal communication that takes a form of membrane contact and their roles in maintaining cellular health, as well as in disease pathogenesis. We're particularly interested in membrane contacts between mitochondria and endoplasmic reticulum and trying to understand how they're regulated and how their dysregulation can lead to neurodegeneration. Although we know that ER mitochondria contacts are important for cell health and their dysregulation is associated with many diseases, we do not know much about how their level is regulated.The first step to address this is to establish a simple, reliable protocol to measure the level of ER mitochondria contacts in cells. Our split luciferase assay offers an easier and faster way to quantify ER mitochondria contacts in live cells compared to most assays. We take advantage of two split halves of renilla luciferase, one half targeted to ER and the other to mitochondria.They can be reassembled together only where the ER and mitochondria membranes have close contact, which results in reconstituted luciferase with a full enzyme activity. This method is simple, suitable for high throughput screenings, and easily accessible for many labs. We hope that it will advance the neurodegenerative disease field by identifying molecular and genetic modulators of ER mitochondria contacts that can be further developed into therapeutic agents.Our results have paved the way for important questions such as how do different drugs or drug combinations affect ER mitochondria contacts, and what are the important molecular players behind these contacts. From these questions, we can gain a better understanding of the mechanism of ER mitochondria contacts, and find ways to combat related diseases.

We have used the protocol described above to measure the level of ER-mitochondria contacts upon the addition of three compounds known to inhibit specific GTPases. CDC42, RHO, and RAC are GTPases that promote actin polymerization28&#160;when activated&#160;and are inhibited by ZCL278, Rhosin, and Ehop-016, respectively24. HEK293T cells transfected with split-Rluc were treated with DMSO (control), ZCL278 (50 &#181;M), Rhosin (50 &#181;M), or Ehop-016 (25 &#181;M) and...

We have established a split-luciferase reassembly assay to monitor the endoplasmic reticulum-mitochondria contacts in live cells. Using this assay, we describe a protocol to quantitatively measure the level of these inter-organelle couplings in HEK293T cells, under the condition of chemical treatment.

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid ...

split-luciferase-reassembly-assay-to-measure-endoplasmic-reticulum