Estimation of Nephron Number in Whole Kidney using the Acid Maceration Method

Summary

Estimates of whole kidney nephron number are important clinically and experimentally, as there is an inverse association between nephron number and an enhanced risk of renal and cardiovascular disease.Herein, the use of the acid maceration method, which provides fast and reliable estimates of whole kidney nephron number, is demonstrated.

Abstract

Nephron endowment refers to the total number of nephrons an individual is born with, as nephrogenesis in humans is completed by 36 weeks of gestation and no new nephrons are formed post-birth. Nephron number refers to the total number of nephrons measured at any point in time post-birth. Both genetic and environmental factors influence both nephron endowment and number. Understanding how specific genes or factors influence the process of nephrogenesis and nephron loss or demise is important as individuals with lower nephron endowment or number are thought to be at a higher risk of developing renal or cardiovascular disease. Understanding how environmental exposures over the course of a person's lifetime affects nephron number will also be vital in determining future disease risk. Thus, the ability to assess whole kidney nephron number quickly and reliably is a basic experimental requirement to better understand mechanisms that contribute to or promote nephrogenesis or nephron loss. Here, we describe the acid maceration method for the estimation of whole kidney nephron number based on the procedure described by Damadian, Shawayri, and Bricker, with slight modifications. The acid maceration method provides fast and reliable estimates of nephron number (as assessed by counting glomeruli) that are within 5% of those determined using more advanced, albeit expensive, methods such as magnetic resonance imaging. Moreover, the acid maceration method is an excellent high-throughput method to assess nephron number in large numbers of samples or experimental conditions.

Introduction

The nephron is both the basic structural and the functional unit of the kidney¹. Structurally, the nephron consists of the glomerulus (capillaries and podocytes) located within the Bowman's capsule and the renal tubule, consisting of the proximal tubule, the Loop of Henle, and the distal tubule which terminates into the collecting duct. Functionally, the role of the nephron is the filtration and reabsorption of water and electrolytes and the secretion of wastes. In general, nephrogenesis is completed at 36 weeks of gestation in humans and shortly after birth in several species such as the mouse and the rat². Nephron endowment refers to the total number of nephrons which an individual is born with, whereas nephron number is the total number of nephrons measured at any time post-birth³. The term nephron number and glomerular number are often used interchangeably. Because there is only one glomerulus per nephron, the assessment of glomeruli number is an important surrogate for estimating nephron number.

The assessment of nephron endowment and nephron number is of clinical interest as studies have demonstrated an association between nephron endowment and reduced nephron numbers with an increased incidence of cardiovascular disease^{4,5,6,7,8,9,10,11,12,13,14,15}. Based on findings in kidneys at autopsy, Brenner observed that hypertensive individuals presented with a lower total number of nephrons than normotensive individuals¹⁶. Thus, Brenner hypothesized that there is an inverse relationship between nephron number and the risk of developing hypertension later in life. Brenner also hypothesized that a reduction in nephron number was compensated for by the nephrons that remained. In order to maintain the normal filtration rate in the kidney, residual nephrons compensate by increasing their glomerular surface area (glomerular hypertrophy), thereby working to mitigate any adverse effect of nephron loss on renal function^4,16.

While protective in the short-term, glomerular hypertrophy, in the long-term, leads to increased sodium and fluid retention, increased extracellular fluid volume, and increases in arterial blood pressure, leading to a vicious cycle of further increases in glomerular capillary pressure, glomerular hyperfiltration, and nephron scarring (sclerosis) and injury^4,16.

Obtaining estimates or counts of nephron number offer a couple of experimental advantages: 1) it provides information regarding the process of nephrogenesis, which can then be linked to specific genes or factors in the embryo or maternal-fetal environment, and 2) there is an association of nephron number with cardiovascular disease and, thus, there is the potential that estimates of nephron number could be used to predict future cardiovascular risk^{2,17,18,19,20,21,22}. In addition to the maternal-fetal environment, several diseases directly impact nephron number and renal function, including atherosclerosis, diabetes, hypertension, and even normal aging^{2,9,10,11,12,22,23}. Thus, assessment of whole kidney nephron number is important to understand both the genetic and environmental factors that affect nephrogenesis (i.e., nephron endowment) and nephron number over the course of a person's life and the resulting effects on renal function and cardiovascular health.

Currently, there are several methods available for the determination and quantification of nephron number, each with its own advantages and limitations^{24,25,26,27,28,29,30}. Sophisticated methods for determining whole kidney nephron number include stereological methods, such as the dissector/fractionator method, and magnetic resonance imaging^25,26. Often considered the gold-standard for determining whole kidney nephron number, the dissector/fractionator method is both expensive and time-consuming. Recent advances and improvement in magnetic resonance imaging and processing have provided the tools to count each and every nephron individually. However, magnetic resonance imaging is not only time-consuming but also extremely expensive. In addition, both the dissector/fractionator method and magnetic resonance imaging requires advanced technical expertise, thus limiting the use of such methods in the majority of research laboratories.

Most methods of determining nephron number make counts or estimates based on the identification of glomeruli, as they are readily identifiable structurally. In this paper, the acid maceration method for estimating nephron number in whole kidney is described and demonstrated²⁷. The acid maceration method is fast, reliable, and significantly less expensive than other methods, such as the dissector/fractionator method and magnetic resonance imaging. Moreover, the acid maceration method provides highly repeatable estimates of nephron number that have been reported to be within the range of those determined using magnetic resonance imaging²⁶.

Protocol

Supplies and reagents listed below are for the determination of the whole kidney nephron number in one mouse, that is, two kidneys. Modifications for the use of the acid maceration method for rat are identified with asterisks. All experimental protocols conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee at The University of Mississippi Medical Center.

1. Kidney Isolation Procedure

1. Weigh the mouse (or other species) and euthanize it with an isoflurane (5% - 8%) overdose or pentobarbital (150 mg/kg intraperitoneal injection).
2. Once the mouse is euthanized, open its abdominal cavity using fine surgical scissors along the midline.
3. Carefully lift the intestines and reproductive adipose to the right side of the abdominal cavity. By gross dissection, isolate the left kidney. Using fine surgical scissors, cut the left renal artery and vein and carefully remove the left kidney, placing the kidney into an appropriately labeled (mouse number/identifier) weigh boat containing phosphor-buffered saline (PBS).
4. Repeat the procedure for the right kidney.
5. Remove each kidney from its respective weigh boat and place it onto a surgical gauze pre-moistened with PBS.
6. Leaving the kidney on the surgical gauze, quickly remove any adherent non-renal tissue (such as perirenal adipose or adrenal gland) followed by the removal of the renal capsule. Weigh each kidney individually, recording the weight of the left and right kidney separately in a laboratory notebook.

2. Homogenization, Incubation, and Straining Procedures

1. Once each kidney is weighed, drain each weigh boat of PBS and place the kidneys back into the appropriately labeled weigh boat. Using a clean razor blade, cut the kidney in half, lengthwise. Place each kidney half facing down and cut each half into 2-mm or smaller pieces.
2. Using the same razorblade, carefully collect and place the chopped kidney pieces into a labeled 15-mL conical tube (mouse number/identifier; left versus right kidney).
3. Repeat the procedure for the opposite kidney, using a new razor blade. Place the chopped kidney into a separately labeled 15-mL conical tube.
4. In a well-ventilated fume hood, add 5 mL of 6 M hydrochloric (HCl) acid to each 15-mL conical tube.
5. Replace the cap to the conical tube, gently agitate the kidney/HCl mixture, and place the 15-mL conical tube into a preheated water bath set at 37 °C for 90 min (*120 min for rat kidneys).
6. Briefly agitate each 15-mL tube every 15 min during the incubation in order to ensure that all tissue is exposed to HCl acid.
7. Insert an 18-G needle into a 5-mL syringe (*10-mL syringe for rat) and carefully remove the syringe plunger. Place the syringe in a 50-mL conical tube (tube #1) in a fume hood.
8. Remove the kidney/HCl solution from the water bath and pour the tissue solution into the open end of the syringe and set the 15-mL conical tube aside in a test tube rack. Carefully replace the plunger and slowly push the plunger so as to extrude the solution through the needle and into tube #1.
9. Wash the 15-mL conical tube with 5 mL of PBS solution. Swirl the PBS in the 15-mL conical tube so as to solubilize any remaining kidney tissue.
10. Again, carefully remove the plunger from the 5-mL syringe containing the 18-G needle and pour the contents from the 15-mL conical tube into the open end of the syringe. Carefully replace the plunger and flush the syringe by gently pushing down on the plunger, into tube #1. Repeat this process 2x (performed 3x in total).
11. Insert a 21-G needle into a new 5-mL syringe (*10-mL syringe for rat) and carefully remove the syringe plunger. Place the syringe with the 21-G needle attached into a new 50-mL conical tube (tube #2).
12. Pour the contents from tube #1 into the open end of the syringe containing the 21-G needle. Carefully insert the plunger and flush the syringe by gently pushing down on the syringe plunger and placing the extruded solution into tube #2.
13. Wash tube #1 with 5 mL of PBS solution. Swirl the PBS in tube #1 so as to solubilize any remaining kidney tissue.
14. Again, carefully remove the plunger from the 5-mL syringe containing the 18-G needle and pour the contents from tube #1 into the open end of the syringe. Carefully replace the plunger and flush the syringe by gently pushing down on the plunger, extruding solution into tube #2. Repeat this process 2x (performed 3x in total).
15. Bring the total volume of tube #2 up to 50 mL by adding additional PBS, up to the 50-mL line on tube #2.
16. Incubate tube #2 containing the kidney tissue solution in a tube rack on a rocker plate in a refrigerator set at 4 °C overnight (minimum 8 - 10 h).

3. Counting of Glomeruli and Extrapolation of the Nephron Number

1. Remove tube #2 from the refrigerator and resuspend the pelleted tissue by gently inverting the tube several times in order to create a homogenous solution. We recommend counting glomeruli within 5 d after processing.
2. Carefully aliquot 500 µL of the kidney solution into a single well of a 12-well plate. Repeat this 2x, placing each aliquot into a separate well so that there are three wells of kidney solution per kidney, for analysis in triplicate.
3. Add 500 µL of PBS to each of the three wells containing the kidney solution, for a 1:1 dilution.
4. Using an inverted microscope, count the number of glomeruli per well. Counting is aided by using a grid of 16 separate sections placed on the bottom of each well. Count the number of glomeruli per each gridded section and then sum the count per grid to get the total number of glomeruli per well. Glomeruli are readily identifiable by their spherical structure. Additional identifiers included a reddish hue due to blood-filled capillaries, as well as pre- or post-arterioles that remain attached to the body of individual glomeruli (Figure 1).
5. Add the total number of glomeruli counted per each of the three wells and then divide them by three for the average number of glomeruli per 500 µL of kidney solution. If the variance in the average number of glomeruli per well is greater than 10%, repeat the nephron-counting procedure, paying close attention to the homogeneous nature of the kidney solution. Multiply the number of glomeruli counted per well times 100 for the average number of glomeruli per kidney. Total nephron number can be expressed per kidney or, using kidney weight, per mg or g of tissue.

Results

Below are representative estimates of whole kidney nephron number from an established mouse model of hypertension and a genetic rat model of age-related chronic kidney disease. Key identifying characteristics of glomeruli, such as a spherical structure with or without attached pre- or post-arteriolar or tubular structures, are highlighted for those new to the acid maceration method (Figure 1).

In th...

Discussion

With good experimental technique, the acid maceration method is ideal for estimating nephron number in whole kidney. Although the kidney is dissolved in acid, glomeruli remain largely intact and are readily identifiable, making the counting of individual glomeruli relatively easy and straightforward. The acid maceration technique is particularly advantageous for several reasons. First, the acid maceration method is a rapid and convenient method that requires relatively little in terms of expense and physical effort. All ...

Materials

Name	Company	Catalog Number	Comments
Isoflurane anesthesia	Abbott Laboratories	05260-05
Isoflurane vaporizor system & flow gauge	Braintree Scientific	VP I	Include medical grade oxygen supply
Leica Inverted Microscope DMIL LED	Leica Microsystems	DMIL LED	Any make also suitable
Digital water bath	Fisher Scientific	2239	Any make also suitable
ToughCut Fine surgical scissors	Fine Science Tools	14058-11	25 mm cutting edge, 11.5 cm length; Tips: sharp-sharp; Tip shape: straight
Micro dissecting forceps 4 1/4 in.	Biomed Res Instruments, Inc	10-1760	Curved tip
Plexiglass board 5 in. x 7 in.	any source suitable	n/a	Any make also suitable
Hexagonal polystyrene weighing dish	Fisher Scientific	02-2002-100	Any make also suitable
Razor blades	Fisher Scientific	12-640	Single edge carbon steel 0.009
Gauze sponges 4 x 4 in. 8 ply	Fisher Scientific	MSD-1400250
10x concentrate phosphate buffered saline (PBS)	Sigma Aldrich	P5493-4L	Dilute to 1x
6 N Hydrocholric acid solution	Sigma Aldrich	3750-32
15 mL conical centrifuge tube	Fisher Scientific	14-959-70C	Any make also suitable
50 mL conical centrifuge tube	Fisher Scientific	14-959-49A	Any make also suitable
Disposable 5 mL syringe	Cole Palmer	EW-07944-06	Any make also suitable
18G1.5 disposable needle	Fisher Scientific	14-826-5D	Any make also suitable
21G1.5 disposable needle	Fisher Scientific	14-826-5B	Any make also suitable
12-well multiple-well cell culture plates with lid	Cole Palmer	#FW-01959-06	Any make also suitable
Polypropylene modular test tube rack	Cole Palmer	#EW-06733-00	Capable of accommodating 15 and 50 mL conical tubes; any make also suitable