preparing mouse lung tissue sections for isolectin b4 staining to evaluate pulmonary microvascular density

Assessing Pulmonary Microvasculature with Isolectin B4 Staining in Mice

Endothelial cells (ECs) are a special type of cell located on the inner lining of blood vessels, covering the entire arterial and venous tree and playing a crucial role in maintaining the stability of blood vessels and organs1. The lungs are highly vascularized organs and play essential physiological and pathological roles in the lungs, such as forming the vascular wall, regulating blood flow, facilitating gas exchange, modulating inflammatory responses, controlling platelet activity, secreting regulatory substances involved in vascular growth, repair, and maintaining coagulation balance.
Lung microvascular endothelial cells (LMECs) are specific endothelial cells of pulmonary tissue, particularly in the microvasculature (capillaries) of the lungs, distinguishing them from the more generalized arterial and venous endothelial cells in the lungs. These cells have various functions, including regulating vascular tone, controlling vascular permeability, participating in the regulation of inflammatory responses, and regulating thrombus formation. They play a crucial role in pulmonary circulation, regulating gas exchange and the transport of nutrients, and are involved in various physiological and pathological processes related to the lungs, likely for aging2. Moreover, the abnormal alternation of pulmonary angiogenesis is related to lung microvascular dysfunction3. Employing the conventional endothelial cell marker CD31 and spatial localization (specifically, the peripheral regions of the lungs), Larissa L.&#160;et al. observed a significant decrease in microvascular endothelial cell density in aged mice (18 months old) compared to their younger counterparts (4 months old)4. In the context of the pulmonary pathology associated with asthma, Makoto H. et al. demonstrated a substantial increase in vessel induction in bronchial biopsy specimens stained with anti-collagen IV from asthmatic patients in comparison to control subjects5. Recently, by introducing the techniques of transmission and scanning electron microscopy, Maximilian A. et al. reported a notable increase in the numeric density of features related to intussusceptive and sprouting angiogenesis in patients who died from Covid-19 or Influenza A (H1N1)6. Evidently, the abnormal microvascular genesis is linked with pulmonary dysfunction. However, there is currently no simple, economical method available for quantifying changes in microvascular density.
In murine models, the lungs are conventionally segmented into five distinct lobes: right cranial, right middle, right caudal, left cranial, and left caudal. Each lobe exhibits unique characteristics in terms of size, shape, location, and likely vascularization, contributing to efficient gas exchange and potentially synergistic regulation of pulmonary circulation. However, to the best of our knowledge, no methodologies account for the differences among these lung lobes when investigating Lung microvascular changes.
This study presents a new method for sectioning lobules in mice, utilizing IB4, a well-defined marker of lung micro-endothelial cells7, for unbiased quantitative assessment of pulmonary microvascular density. This innovative approach addresses the need for a more comprehensive understanding of microvascular alterations in murine lungs by considering the distinct properties of individual lobes of mice. As a demonstration, in aging&#160;mice, a significant reduction in pulmonary 
microvascular density is observed specifically&#160;within both the caudal lobe and left lobe.&#160;The protocol underscores the importance of integrating lobe-specific analyses into investigations of changes in the microvascular landscape of murine lungs. Notably, this method provides valuable research references for investigators seeking a comprehensive understanding of both physiological and pathological progression of lung developments and lesions, extending beyond angiogenesis.

Author Spotlight: Advances in Quantifying Microvascular Density in Aging Murine Lungs

Quantifying Pulmonary Microvascular Density in Mice Across Lobules

Our research aims at quantifying pulmonary microvascular density in various mouse lung lobules considering anatomical differences and age-related changes using isolectin B4 staining and ImageJ analysis. Key questions that we are trying to address are how to quantify microvascular density in different lung lobules accurately, and what changes occur in microvascular density across lobules with aging. Recent advancements in pulmonary microvascular research, including improved tissue clearing and fixation techniques, advanced imaging, and software tools for accurate quantification.These developments enhance our understanding of microvascular changes in aging and related diseases, like asthma and COVID-19, offering insights for potential therapies. The main challenges are accurately locating lesions, properly sectioning lung tissues, mitigating fixative-related tissue damage, and lack of specific markers for lung endothelial cells. Traditional methods often overlook the variations inside shape and vascularization across different lung lobes.Our protocol addresses the gap by providing a comprehensive approach to sectioning lung lobes and quantifying microvascular density using isolectin before staining. The combination of IB4 staining with the free software ImageJ for advanced microvascular density analysis ensures accurate and useful results. The cellular and molecular mechanisms by which pulmonary microvascular lesions participate in the progression of lung-related diseases, and develop new strategies or targets for rescuing pulmonary microvascular function to promote lung repair and induce functional recovery.

Preparing Mouse Lung Tissue Sections for Isolectin B4 Staining to Evaluate Pulmonary Microvascular Density

To begin, place the mouse lung tissue in 4%paraformaldehyde, which is 10 times the volume of the tissue. After three hours of fixation, separate the five pulmonary lobes and embed the cranial, middle, and accessory lobes of the right lung separately, orienting in the direction of the vertical lobe-to-bronchus connection. Then cut the caudal lobe of the right lung into two pieces at four millimeters from left to right in the direction perpendicular to the lung-lobe-bronchus connection, and embed the slices in a wax block, with the cut surface facing upward.To make three cuts in the left lung lobe, cut transversely above the main bronchial junction, two millimeters below the top. Make the second cut transversely above the main bronchial junction, five millimeters below the top. Make the third cut at the end of the lobe, eight millimeters below the top.After discarding the uppermost piece of tissue, encase the remaining three pieces in a wax block, with the cut surface facing upward. Following refixation, rinse the tissue block under flowing water for 15 minutes. Sequentially immerse the tissue in the escalating ethanol concentrations, followed by xylene and soft wax.Fill a mold with an appropriate amount of liquid paraffin and, using forceps, position the tissue in the embedding frame at the center. Place the mold and tissue in the freezing area of the embedding machine. When the paraffin starts to whiten slightly, quickly position the uncovered embedding frame on top of the mold and perform the second wax injection, sealing the holes at the bottom of the embedding frame.Then transfer the entire mold to the freezing table. After securing the wax block onto the slicer, trim the block to reveal a smooth and flat cut surface. Rotate the microtome's wheel crank to cut slices with a thickness of four micrometers.Smoothly place the removed sections onto the water surface sequentially. Once the sections are fully extended, separate the broken pieces at the junctions between each section. Tilt a glass slide underneath the section, and, upon contact, slowly and evenly lift the slide out of the water surface in a vertical motion.

preparing-mouse-lung-tissue-sections-for-isolectin-b4-staining-to

Research

JoVE Journal

Biology

The abnormal alternation of pulmonary angiogenesis is related to lung microvascular dysfunction and is deeply linked to vascular wall integrity, blood flow regulation, and gas exchange. In murine models, lung lobes exhibit significant differences in size, shape, location, and vascularization, yet existing methods lack consideration for these variations when quantifying microvascular density. This limitation hinders the comprehensive study of lung microvascular dysfunction and the potential remodeling of microvasculature circulation across different lobules. Our protocol addresses this gap by employing two sectioning methods to quantify pulmonary microvascular density changes, leveraging the size, shape, and distribution of airway branches across distinct lobes in mice. We then utilize Isolectin B4 (IB4) staining to label lung microvascular endothelial cells on different slices, followed by unbiased microvascular density analysis using the freely available software ImageJ. The results presented here highlight varying degrees of microvascular density changes across lung lobules with aging, comparing young and old mice. This protocol offers a straightforward and cost-effective approach for unbiased quantification of pulmonary microvascular density, facilitating research on both physiological and pathological aspects of lung microvasculature.

Here, we describe a simple and economical protocol to perform unbiased quantification of pulmonary microvascular density for whole mice lung tissue using single staining of Isolectin B4.

The abnormal alternation of pulmonary angiogenesis is related to lung microvascular dysfunction and is deeply linked to vascular wall integrity, blood ...

Staining the Mouse Lung Tissue Sections with Isolectin B4 and DAPI to Visualize Pulmonary Microvascular Density

To begin, obtain paraffin-embedded slice of mouse lung tissue and place it in a 65 degree Celsius oven for 60 minutes. For staining, immerse the slide with the tissue section sequentially in the staining jars containing appropriate solutions. Dilute the modified sodium citrate antigen retrieval solution 50-fold with deionized water.After preheating the antigen extract in the microwave on high power, place the slide in it for 15 minutes to boil. After the slide cools to room temperature, wash it two times with PBS for five minutes each. Then permeabilize the tissue with 0.25%triton two times for 10 minutes each.Encircle the tissue with an immunohistochemistry pen and incubate the slide with 5%BSA at room temperature for one hour. Next, dilute the isolectin B4, or IB4, with 1%BSA to a ratio of 1 to 50. After removing excess water, incubate the tissue section with 50 to 100 microliters of the diluted IB4 overnight at four degrees Celsius.Wash the slides three times with PBS for five minutes each and permeabilize with 0.25%triton for 10 minutes. Then incubate the tissue section with 50 to 100 microliters DAPI at room temperature for five minutes. After three PBS washes, apply a drop of anti-fade mounting medium to the slide, avoiding light exposure.Mount the air-dried slide under a fluorescence microscope to capture images of the nucleus and target signals. This protocol enables precise localization and observation of pulmonary microvasculature using IB4 for staining micro endothelial cells in different mouse lung lobes under a fluorescence microscope.

Unbiased Analysis of Microvascular Changes in Isolectin B4 Stained Murine Lung Lobules Using ImageJ

To begin, obtain Isolectin B4 or IB4 and DAPI-stained mouse lung tissue sections. Acquire images under a fluorescence microscope with a 10x objective. Import the images to ImageJ software and select them.Split the channels accordingly and load the file in ND2 format for DAPI and IB4 channels. Navigate to Analyze, Set Scale. Configure the image parameters, and then select Global, followed by OK.To set the same display range, go to Image, adjust brightness contrast, and click Set.Set display range for IB4 as zero to 10, 000, and that for DAPI as zero to 20, 000. To eliminate the background, click Process, followed by math. Subtract the background signal based on the values detected by the magic wand tool on the background.For selecting the best threshold value, go to Image and click Duplicate. Select both Original and Duplicate Image type to 8-bit. Next, under the Image tab, select Adjust followed by Threshold and Enter Modes.After choosing the most realistic threshold, select Dark Background and apply. Using the magic wand tool, select larger blood vessels compared to the duplicated image, then click Delete to eliminate IB4-positive signals from large arteries and veins. To count the number of target signals, click Analyze, followed by Set Measurements, and tick Area, Limit to threshold and Display label.Then select Analyze Particles and modify Size as zero to infinity, Circularity as zero to one, and Show as Overlay Masks. Tick Summarize, and click OK.Finally, choose Summary. Click File and save the results.The percentage of the IB4-positive foci in the cranial, middle, caudal, accessory, and left lung lobes could be distinguished between the young and middle-aged mice using ImageJ analysis.