Name: studying the activity of neuropeptides and other regulators of the excretory system in the adult mosquito
Uploaded: 2021-08-24T13:00:00
Description: Watch this Scientific Journal Video about Studying the Activity of Neuropeptides and Other Regulators of the Excretory System in the Adult Mosquito at JoVE.com

This research was funded by Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants to AD and J-PP. AL and FS received NSERC CGS-M awards in support of their graduate research.

1. Making silicone-lined dishes
NOTE: This step should be done prior to the experiments. These dishes will be made to prepare the assay dish for dissections, and for contraction assay experiments.
<ol>
	<li>Prepare the silicone using a silicone elastomer kit following the manufacturer&#8217;s recommendations. Mix the two-part liquid components at a ratio of 10 to 1. Mix gently and thoroughly by inverting but be sure to minimize bubble formation.</li>
	<li>Pour the silicone elastomer into sta...

When ingesting a blood meal, haematophagus insects face the challenge of excess solutes and water in their haemolymph2. To cope with this, they have a specialized excretory system, which is tightly controlled by hormonal factors, allowing the insects to rapidly initiate post-prandial diuresis. The Ramsay assay and use of ion-selective microelectrodes allows for measurement of fluid secretion rates along with ion concentrations and transport rates in isolated insect MTs. Critical steps within these...

Maintenance of salt and water levels in insects allows them to succeed in many ecological and environmental niches, utilizing a variety of feeding strategies1. Most insects have evolved mechanisms to regulate the composition of their haemolymph within narrow limits in order to withstand the different challenges associated with their particular environment2. Terrestrial insects are often faced with the challenge of conserving water and the excretory system undergoes anti-diuresis to prevent loss of water and some essential salts, therefore, avoiding desiccation. In contrast, diuresis occurs when the insect feeds and is challenged with excess water and potentially salts3,4. Through their specialized and highly active excretory system, insects have evolved regulatory mechanisms acting to counter their osmoregulatory challenges. In adult Aedes aegypti mosquitoes, the excretory system is comprised of the Malpighian tubules (MTs) and hindgut, the latter of which is made up of the anterior ileum and posterior rectum5. MTs are responsible for generating primary urine, usually rich in NaCl and/or KCl. The primary urine is then modified through secretory and reabsorptive processes as it travels downstream of the tubule and enters the hindgut5. The final excreta can be hyper- or hypoosmotic to the haemolymph, depending on feeding/environmental conditions, and is enriched in toxic and nitrogenous wastes2.
MTs are ideal for studying many features of epithelial fluid and solute transport as they carry out a great variety of transport and excretory functions2,6. Through hormonal regulation2, MTs function by secreting ions and other solutes from the blood into the tubule lumen7, providing an osmotic gradient allowing water to be transported by aquaporins8,9, which collectively creates the primary urine, before traveling toward the reabsorptive hindgut2. Thus, by collecting the secreted fluid from isolated MTs, one can continuously monitor transepithelial transport of fluid and ions. Measuring secretion rate and urine composition provides insight on mechanisms responsible for transepithelial ion and fluid transport. A popular method for studying fluid secretion rates is the Ramsay assay, which was first introduced by Ramsay in 195310. In this method, the distal (closed) end of the tubule is treated with a hormone (or other test compound/drug), while the proximal (open) end is wrapped around a pin in water-saturated paraffin oil, which secretes the primary urine, accumulating as a droplet on the tip of the pin. Isolated MTs are able to survive and function for long periods (up to 24 h) under optimized in vitro conditions, which make them suitable and efficient models for fluid secretion measurement. Insects have open circulatory systems, thus the MTs are easily dissected and removed as they are usually freely floating in the haemolymph6. Additionally, with the exception of aphids&#8212;which lack MTs11&#8212;the number of MTs in a given insect species can vary considerably from four to hundreds (five in Aedes mosquitoes) allowing for multiple measurements from one insect.
The MTs in Aedes mosquitoes, in common with other endopterygote insects, are composed of two cell types forming a simple epithelium2,12; large principal cells, which facilitate active transport of cations (i.e., Na+ and K+) into the lumen, and thin stellate cells, which aid in transepithelial Cl- secretion13. The MTs are not innervated2, and instead are regulated by several hormones including both diuretic and anti-diuretic factors, allowing for the control of ion transport (mainly Na+, K+, and Cl-) and osmotically-obliged water2. Numerous studies have examined the hormonal regulation of Aedes MTs to understand the role of endocrine factors on transepithelial transport14,15,16,17,18. As shown in the representative results, the protocols herein demonstrate the effects of different hormonal factors on isolated MTs from adult female A. aegypti mosquitoes, including both diuretic and anti-diuretic control (Figure 1). The Ramsay assay is used to demonstrate how an anti-diuretic hormone, AedaeCAPA-1, inhibits fluid secretion of MTs stimulated by diuretic hormone 31 (DH31) (Figure 1).
The smaller size of insects has required the development of micro methods for measuring ionic activity and concentrations in fluid samples, or near the surface of isolated tissues such as the MTs and gut. Varying methods have been implemented, including the use of radioisotopes of ions19, which requires collection of the secreted fluid drops for measurement of ion concentrations20. Stimulated Aedes tubules in vitro typically secrete ~0.5 nL/min21, thus handling of such small volumes can pose a challenge and potentially introduce error upon transfer. As a result, ion-selective microelectrodes (ISMEs) have been extensively used to measure ion concentrations in secreted droplets of MTs in vitro. In this method, a reference electrode and ISME, filled with the appropriate backfill solution and ionophore, are positioned into the secreted urine droplet to determine ion concentrations22. Adapted from Donini and colleagues23, this current protocol uses a Na+-selective ionophore to measure ion activity in secreted droplets from stimulated MTs in adult Aedes mosquitoes. Since ion-selective microelectrodes measure ion activity, this data can be expressed as ion concentrations following the assumption that the calibration solutions and experimental samples share the same ion activity coefficient21 (Figure 1B,C).
The Scanning Ion-selective Electrode Technique (SIET) also makes use of ISMEs to measure ion concentration gradients in the unstirred layer adjacent to organs, tissues, or cells that are transporting ions. The ISMEs measure voltage gradients which can then be used to calculate the ion concentration gradients and direction and magnitude of ion flux across the organ, tissue, or cell20. In this technique, the ISME is mounted to a three axes manipulator controlled by computerized micro-stepper motors so that its 3D position is controlled to the micrometer level20. Voltages are measured at two points within the unstirred layer using a sampling protocol programmed into and controlled by computer software. The two points are typically separated by a distance of 20&#8211;100 &#181;m with one point within 5&#8211;10 &#181;m of the surface of the organ, tissue, or cell and the second point a further 20&#8211;100 &#181;m away. The difference in magnitude of voltages between the two points is calculated to obtain a voltage gradient24,25,26, which is then used to calculate the concentration gradient and subsequently the net flux using Fick&#8217;s Law24,27. This method is useful for assessing the transport of specific ions across different regions of the insect gut and MTs, or at specific timepoints following a bloodmeal or treatment exposure. For instance, the SIET can be used to understand how absorptive and secretory processes in the mosquito excretory system are regulated by hormones28 as well as different feeding behaviors and rearing conditions25. Previous work utilizing the SIET revealed sites involved in ion transport along the anal papillae and rectum of larval and adult mosquitoes24,28. The current protocol, described previously by Paluzzi and colleagues26, measures Na+ flux across the rectal pad epithelia of the adult female rectum (Figure 2).
The final segment of the mosquito excretory system requires coordinated muscular movement to help mix food and secrete waste26. Non-absorbable products of digestion from the midgut, along with primary urine secreted by the MTs, are passed through the pyloric valve and delivered to the hindgut2. Spontaneous hindgut contractions begin at the pyloric valve and occur in peristaltic waves, which are relayed over the ileum through the coordinated contraction of circular and longitudinal muscles surrounding the basal surface of epithelial cells26. Finally, the muscles within the rectum help to propel and eliminate waste through the anal canal. Although insect hindgut motility is myogenic, requiring extracellular Ca2+ to produce spontaneous contractions, these processes can also be regulated neuronally26,29,30. This exogenous regulation by the nervous system is important after feeding, as the animal must expel wastes from the gut and restore haemolymph balance31. As a result, performing in vitro bioassays to identify myostimulatory or myoinhibitory neuropeptides is useful in assessing how neurochemicals influence hindgut motility. The current protocol, performed by Lajevardi and Paluzzi28, uses video recordings to examine ileal motility in response to neuropeptides (Figure 3). Similarly, a force transducer or impedance converter may also be used to observe traces of contractions through a data acquisition software32,33. However, using video technology allows us to visually assess the organ and further analyze using a subset of parameters to identify the role of hormones on hindgut motility.
Using these techniques can help characterize factors that regulate and coordinate fluid and ion transport along the excretory system along with hindgut motility. Importantly, a functional link between the diuretic response by the MTs and hindgut motility is supported, as diuretic hormones, such as DH31 and 5HT, characterized by their ability to stimulate fluid secretion by the MTs, have also been found to exhibit myotropic actions along the mosquito hindgut21,34,35. These findings highlight the importance of stringent coordination between the MTs and hindgut during events such as post-prandial diuresis in insects requiring rapid waste elimination.
Herein, the detailed approach behind the Ramsay assay technique to measure fluid secretion rate in the mosquito, A. aegypti, and the use of ion-selective microelectrodes to determine Na+ concentrations within the secreted fluid of the MTs are described, which when combined allows for transepithelial ion transport rates to be determined. Additionally, the Scanning Ion-selective Electrode Technique and hindgut contraction assays are described to measure ion flux and motility, respectively, which helps to elucidate hormonal regulation of the hindgut (Figure 4).

<table>
	<tbody>
		<tr>
			<td>1 mL syringes</td>
			<td>Fisher Scientific</td>
			<td>14955456</td>
			<td></td>
		</tr>
		<tr>
			<td>35 mm Petri dishes</td>
			<td>Corning Falcon (Fisher Scientific)</td>
			<td>C351008</td>
			<td></td>
		</tr>
		<tr>
			<td>Borosillicate glass capillary filamented tubes 
			(OD 1 mm, ID 0.58 mm, length 100 mm)</td>
			<td>World Precision Instruments</td>
			<td>1B100F-4</td>
			<td>used for ISME reference electrodes</td>
		</tr>
		<tr>
			<td>Borosillicate glass capillary filamented tubes 
			(OD 2 mm, ID 1.12 mm, length 102 mm)</td>
			<td>World Precision Instruments</td>
			<td>1B200F-4</td>
			<td>used for SIET reference electrodes</td>
		</tr>
		<tr>
			<td>Borosillicate glass capillary unfilamented tubes 
			(OD 1.5 mm, ID 1.12 mm, length 100 mm)</td>
			<td>World Precision Instruments</td>
			<td>TW150-4</td>
			<td>used for ISME and SIET electrodes</td>
		</tr>
		<tr>
			<td>CO2 pad</td>
			<td>Diamed</td>
			<td>GEN59-114</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyltrimethylsilylamine solution</td>
			<td>Sigma-Aldrich</td>
			<td>41716</td>
			<td></td>
		</tr>
		<tr>
			<td>Faraday cage</td>
			<td>Custom</td>
			<td></td>
			<td>Can be fabricated by local machine shop</td>
		</tr>
		<tr>
			<td>Ferric chloride</td>
			<td>Sigma-Aldrich</td>
			<td>157740</td>
			<td></td>
		</tr>
		<tr>
			<td>Forceps (Dumont #5)</td>
			<td>Fine Science Tools</td>
			<td>91150-20</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Petri dish</td>
			<td>Fisher Scientific</td>
			<td>08-748A</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrated mineral oil</td>
			<td>Fisher Scientific</td>
			<td>8042-47-5</td>
			<td>Specific brand is not important</td>
		</tr>
		<tr>
			<td>INFINITY1-2CB video camera</td>
			<td>Luminera</td>
			<td>INFINITY1-2CB</td>
			<td></td>
		</tr>
		<tr>
			<td>Micromanipulators (left and right handed)</td>
			<td>World Precision Instruments</td>
			<td>MMJL and MMJR</td>
			<td>Specific brand is not important so long as high quality manipulator</td>
		</tr>
		<tr>
			<td>Mineral Oil, Light</td>
			<td>Fisher Scientific</td>
			<td>0121-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Minutien pins (0.1 mm stainless steel)</td>
			<td>Fine Science Tools</td>
			<td>26002-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Non-hardening modeling clay</td>
			<td>Sargent Art</td>
			<td></td>
			<td>Specific brand is not important</td>
		</tr>
		<tr>
			<td>Olympus light microscope (FOR SIET)</td>
			<td>Olympus</td>
			<td>customized system</td>
			<td></td>
		</tr>
		<tr>
			<td>Plastic Pasteur (transfer) pipette</td>
			<td>Fisher Scientific</td>
			<td>13-711-7M</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly-L-lysine solution (0.1 mg/mL)</td>
			<td>Sigma-Aldrich</td>
			<td>A-005-M</td>
			<td>84 kDa</td>
		</tr>
		<tr>
			<td>Polyvinyl chloride (PVC)</td>
			<td>Sigma-Aldrich</td>
			<td>81395</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel Blade</td>
			<td>Fine Science Tools</td>
			<td>10050-00</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel Handle</td>
			<td>Fine Science Tools</td>
			<td>10053-09</td>
			<td></td>
		</tr>
		<tr>
			<td>Schneider&#39;s Drosophila medium</td>
			<td>Sigma-Aldrich</td>
			<td>S0146</td>
			<td></td>
		</tr>
		<tr>
			<td>SIET system</td>
			<td>Applicable Electronics</td>
			<td>customized system</td>
			<td>Details available at: http://www.applicableelectronics.com/overview</td>
		</tr>
		<tr>
			<td>Silver wire</td>
			<td>World Precision Instruments</td>
			<td>AGW1010</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium ionophore II cocktail A</td>
			<td>Fluka</td>
			<td>99357</td>
			<td></td>
		</tr>
		<tr>
			<td>Standard polystyrene Petri (culture) dishes</td>
			<td>Fisherbrand</td>
			<td>FB012921</td>
			<td>Any size would work, but 60 mm dishes are good for both dissections and assay</td>
		</tr>
		<tr>
			<td>Stereomicroscope with ocular micrometer</td>
			<td>Nikon</td>
			<td>SMZ800</td>
			<td></td>
		</tr>
		<tr>
			<td>Sutter P-97 Flaming Brown Pipette puller</td>
			<td>Sutter Instruments</td>
			<td>FGPN7</td>
			<td></td>
		</tr>
		<tr>
			<td>Sylgard 184 Silicone Elastomer Kit</td>
			<td>Dow Chemical Company</td>
			<td>NC9285739</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetrahydrofuran</td>
			<td>Sigma-Aldrich</td>
			<td>401757</td>
			<td></td>
		</tr>
		<tr>
			<td>VWR advanced hotplate stirrer - aluminum</td>
			<td>VWR</td>
			<td>9578</td>
			<td>Specific brand is not important</td>
		</tr>
	</tbody>
</table>

studying the activity of neuropeptides and other regulators of the excretory system in the adult mosquito

Studies of insect physiology, particularly in those species that are vectors of pathogens causing disease in humans and other vertebrates, provide the foundation to develop novel strategies for pest control. Here, a series of methods are described that are routinely utilized to determine the functional roles of neuropeptides and other neuronal factors (i.e., biogenic amines) on the excretory system of the mosquito, Aedes aegypti. The Malpighian tubules (MTs), responsible for primary urine formation, can continue functioning for hours when removed from the mosquito, allowing for fluid secretion measurements following hormone treatments. As such, the Ramsay assay is a useful technique to measure secretion rates from isolated MTs. Ion-selective microelectrodes (ISME) can sequentially be used to measure ion concentrations (i.e., Na+ and K+) in the secreted fluid. This assay allows for the measurement of several MTs at a given time, determining the effects of various hormones and drugs. The Scanning Ion-selective Electrode Technique uses ISME to measure voltage representative of ionic activity in the unstirred layer adjacent to the surface of ion transporting organs to determine transepithelial transport of ions in near real time. This method can be used to understand the role of hormones and other regulators on ion absorption or secretion across epithelia. Hindgut contraction assays are also a useful tool to characterize myoactive neuropeptides, that may enhance or reduce the ability of this organ to remove excess fluid and waste. Collectively, these methods provide insight into how the excretory system is regulated in adult mosquitoes. This is important because functional coordination of the excretory organs is crucial in overcoming challenges such as desiccation stress after eclosion and before finding a suitable vertebrate host to obtain a bloodmeal.

Application of DH31 against unstimulated MTs results in a significant increase in fluid secretion rate, confirming its role as a diuretic hormone in Aedes mosquitoes (Figure 1A). When tubules are treated with AedaeCAPA-1, a reduction in secretion rate is observed in DH31-stimulated MTs. Figure 1B demonstrates the use of ion-selective electrodes to measure Na+ concentrations in the secreted droplets. Treatment of...

Watch this Scientific Journal Video about Studying the Activity of Neuropeptides and Other Regulators of the Excretory System in the Adult Mosquito at JoVE.com

Studying the Activity of Neuropeptides and Other Regulators of the Excretory System in the Adult Mosquito

This protocol outlines methodologies behind the Ramsay assay, ion-selective microelectrodes, Scanning Ion-selective Electrode Technique (SIET) and in vitro contraction assays, applied to study the adult mosquito excretory system, comprised of the Malpighian tubules and hindgut, to collectively measure ion and fluid secretion rates, contractile activity, and transepithelial ion transport.

Studies of insect physiology, particularly in those species that are vectors of pathogens causing disease in humans and other vertebrates, provide the ...

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