Lymphatic and Blood Network Analysis During Obesity
Summary
Obesity is a growing global public health issue. It has been previously associated with lymphatic dysfunction, suggesting a vital crosstalk between the adipose tissue and the lymphatic system. Here, we propose an accessible methodology allowing the distinct labeling of blood and lymphatic vasculatures within the subcutaneous adipose tissue.
Abstract
Lymphatic collecting vessels and lymph nodes are inevitably embedded in adipose tissue. The physiological significance of this observation remains still not elucidated. However, obesity is characterized by impaired lymphatic function and increased vessel permeability. Inversely, lymphatic dysfunction induces obesity in mice, suggesting a significant interplay between lymphatic vessels and the adipose tissue. Therefore, understanding factors leading to lymphatic dysfunction might open new therapeutic windows to prevent obesity and associated comorbidities. The first step in this process requires a precise and detailed visualization of the lymphatic network in healthy and inflamed adipose tissue. Here, we describe a rapid, inexpensive, and efficient method that allows to label and analyze lymphatic and blood vessels. This approach takes advantage of the skin-draining brachial lymph node localization within the subcutaneous adipose tissue. The lymphatic arborization of this tissue can be revealed by injecting fluorochrome-conjugated lectins subcutaneously. Moreover, the in vivo labeling approach provides a way to evaluate lymphatic vessel density and functions. Coupled to blood vessel, adipocyte and immune cell staining, the protocol allows for high-resolution mapping of the subcutaneous adipose tissue by 3D imaging.
Introduction
The lymphatic circulatory system plays a crucial role in the maintenance of tissue homeostasis and the induction of efficient immune responses. Lymphatic vessels run parallel to blood vessels and carry interstitial fluid, metabolites, and immune cells to the local draining lymph node (LN) and finally towards the venous circulation1. Dysfunctional lymphatic drainage has been observed during infection, inflammation and metabolic diseases2,3,4,5. The lymphatic vasculature is composed of small size vessels named lymphatic capillaries. Lymphatic capillaries are formed by a single layer of thin lymphatic endothelial cells (LECs) characterized by open junctions (“button-like” junctions) facilitating interstitial fluid, metabolites, and immune cells, mainly dendritic cells (DCs) and T cells, entry into the lymphatic capillary lumen5. Lymphatic capillaries merge into larger vessels named lymphatic collecting vessels. Lymphatic collectors are characterized by a layer of LECs surrounded by a muscle layer providing autonomous contractile tonus and maintaining fluid flow5. Moreover, collecting vessels possess valves assuring a unidirectional lymph flow.
The LECs of collectors and capillaries express a specific set of markers that distinguish them from blood endothelial cells (BECs). Among those factors, Prox1 is a transcription factor guiding LECs generation and is highly expressed in LECs while absent in BECs. The critical involvement of Prox1 in LECs biology was illustrated by the generation and analysis of Prox1-deficient mice6. Prox1 heterozygous mice have a defective lymphatic vasculature development characterized by reduced lymphatic vessel density and increased vascular permeability6. LECs highly express VEGFR3, Podoplanin and CCL215. Those markers are not found on BECs and allow to separately analyze the network of lymphatic and blood vessels. Lyve1 is selectively expressed by lymphatic capillaries while absent on collecting vessels5.
Three types of adipose tissue have been described based on their mitochondrial content and subsequent color. Mitochondria-rich thermogenic brown adipose tissue plays a key role during cold exposure and is located in the interscapular region in mice7,8. White and beige adipocytes have lower mitochondrial density and are mainly involved in energy storing in the form of lipid droplets. White and beige adipocytes are located in visceral and subcutaneous depots9.
Clinical observations established a link between obesity and lymphatic dysfunction10. Obesity induces morphological changes of adipose tissue lymphatic vasculature and leads to an impaired lymph transport11. Data obtained in pre-clinical models revealed that High Fat Diet (HFD) induces a lymphatic remodeling and obese mice have smaller lymph nodes and fewer number of lymphatic vessels12. Nevertheless, the precise molecular mechanisms governing this phenotype remain to be elucidated. LECs involvement during obesity is further supported by observations in genetic models with impaired lymphatic vessel development. As discussed earlier, Prox1 heterozygous mice (Prox1+/-) present an ill-functioning lymphatic system, and coincidently develop excessive visceral adipose tissue accumulation in comparison to Prox1 sufficient animals6. Interestingly, this adipose tissue phenotype is rescued by restoration of lymphatic function13. Together, these results have brought to light strong inter-connections between lymphatic vessels and the adipose tissue, which need further investigation.
In the context of inflammation, a hallmark of obesity, the altered expression of LEC and BEC markers compromises the analysis of these cells via classical antibody staining14,15. Genetic models to label specifically LECs and BECs have been developed and allow to palliate this problem16,17,18,19. However, the usage of genetic reporter lines requires multiple steps of breeding and considerably increases the length and cost of a project. Thus, we propose to use fluorochrome-conjugated lectin injections to investigate blood and lymphatic circulatory systems in the subcutaneous adipose tissue, a simple and relatively non-expensive approach. Lectin conjugated to various fluorochromes are commercially available and can be injected intravenously to label blood vessels, or subcutaneously to label the skin-draining lymphatic vessels embedded in the subcutaneous adipose tissue. This approach relies on the use of separate fluorochrome-lectin conjugates for each injection and allows the distinct labeling of each vasculature. This method is also compatible with the use of genetic models to label the lymphatic or blood vasculature network. Importantly, it provides multiple readouts to analyze the overall health status of the subcutaneous adipose tissue and the blood and lymphatics vasculatures perfusing it. This procedure could be easily applied to analyze the lymphatic and blood vasculature networks during acute and chronic skin diseases including psoriasis and infections.
Protocol
Representative Results
Discussion
This approach provides efficient and robust labeling of the blood and lymphatic vasculatures of the subcutaneous adipose tissue. The separate analysis of blood and lymphatic endothelial networks might unravel pathological mechanisms affecting one or both of the circulatory systems during obesity or other pathological conditions. This protocol aims to analyze the architecture of the vascular systems, their interaction with stromal and immune cells, and their functionality during health and disease.
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The authors have nothing to disclose.
Acknowledgements
SI is supported by Institut National de la Sante et de la Recherche Medicale (INSERM) and Agence Nationale de la Recherche (ANR-17-CE14-0017-01 and ANR-19-ECVD-0005-01). AG is supported by the French government, through the UCAJedi Investments in the Future projects managed by the National Research Agency (ANR) with the reference number ANR-15-IDEX-01. RSC is supported by FA-2020-01-IBD-1 from the Lawrence C. Pakula, MD IBD Education & Innovation Fund”.
Materials
| Lectin DyLight 649 | Vector Labs | DL-1178-1 | Described in protocol |
| Lectin DyLight 488 | Vector Labs | DL-1174 | Described in protocol |
| Paraformaldehyde | VWR Chemicals | 9713.1000 | |
| Sucrose | Euromedex | CAS Number 57-50-1 | |
| Anti-Podoplanin | AngioBio | 11-033 | Dilution : 1/50 |
| Lectin DyLight 594 | Vector Labs | DL-1177 | Described in protocol |
| Anti-MHCII (Clone M5/114.15.2) | Biolegend | 107618 | Dilution : 1/100 |
| Anti-CD11b (Clone M1/70) | Biolegend | 101218 | Dilution : 1/100 |
| Anti-CD68 (Clone FA.11) | Biolegend | 137004 | Dilution : 1/100 |
| Anti-B220 (Clone RA3-6B2) | Biolegend | 103225 | Dilution : 1/100 |
| Anti-Perilipin (Clone PERI 112.17) | Progen | 651156 | Dilution : 1/50 |
| Anti-CD3 (Clone 17A2) | Biolegend | 100210 | Dilution : 1/100 |
References
- Baluk, P., et al. Functionally specialized junctions between endothelial cells of lymphatic vessels. Journal of Experimental Medicine. 204 (10), 2349-2362 (2007).
- Fonseca, D. M., et al. Microbiota-Dependent Sequelae of Acute Infection Compromise Tissue-Specific Immunity. Cell. 163 (2), 354-366 (2015).
- Thomas, S. N., et al. Impaired humoral immunity and tolerance in K14-VEGFR-3-Ig mice that lack dermal lymphatic drainage. The Journal of Immunology. 189 (5), 2181-2190 (2012).
- Kuan, E. L., et al. Collecting lymphatic vessel permeability facilitates adipose tissue inflammation and distribution of antigen to lymph node-homing adipose tissue dendritic cells. The Journal of Immunology. 194 (11), 5200-5210 (2015).
- Randolph, G. J., Ivanov, S., Zinselmeyer, B. H., Scallan, J. P. The Lymphatic System: Integral Roles in Immunity. Annual Review of Immunology. 35, 31-52 (2017).
- Harvey, N. L., et al. Lymphatic vascular defects promoted by Prox1 haploinsufficiency cause adult-onset obesity. Nature Genetics. 37 (10), 1072-1081 (2005).
- Kajimura, S., Spiegelman, B. M., Seale, P. Brown and Beige Fat: Physiological Roles beyond Heat Generation. Cell Metabolism. 22 (4), 546-559 (2015).
- Wu, J., Cohen, P., Spiegelman, B. M. Adaptive thermogenesis in adipocytes: is beige the new brown. Genes and Development. 27 (3), 234-250 (2013).
- Cinti, S. The adipose organ. Prostaglandins, Leukotrienes & Essential Fatty Acids. 73 (1), 9-15 (2005).
- Kataru, R. P., et al. Regulation of Lymphatic Function in Obesity. Frontiers in Physiology. 11, 459 (2020).
- Escobedo, N., Oliver, G. The Lymphatic Vasculature: Its Role in Adipose Metabolism and Obesity. Cell Metabolism. 26 (4), 598-609 (2017).
- Weitman, E. S., et al. Obesity impairs lymphatic fluid transport and dendritic cell migration to lymph nodes. PLoS One. 8 (8), 70703 (2013).
- Escobedo, N., et al. Restoration of lymphatic function rescues obesity in Prox1-haploinsufficient mice. JCI Insight. 1 (2), (2016).
- Commerford, C. D., et al. Mechanisms of Tumor-Induced Lymphovascular Niche Formation in Draining Lymph Nodes. Cell Reports. 25 (13), 3554-3563 (2018).
- Gregory, J. L., et al. Infection Programs Sustained Lymphoid Stromal Cell Responses and Shapes Lymph Node Remodeling upon Secondary Challenge. Cell Reports. 18 (2), 406-418 (2017).
- Ivanov, S., et al. CCR7 and IRF4-dependent dendritic cells regulate lymphatic collecting vessel permeability. Journal of Clinical Investigation. 126 (4), 1581-1591 (2016).
- Zhong, W., et al. Prox1-GFP/Flt1-DsRed transgenic mice: an animal model for simultaneous live imaging of angiogenesis and lymphangiogenesis. Angiogenesis. 20 (4), 581-598 (2017).
- Choi, I., et al. Visualization of lymphatic vessels by Prox1-promoter directed GFP reporter in a bacterial artificial chromosome-based transgenic mouse. Blood. 117 (1), 362-365 (2011).
- Pham, T. H., et al. Lymphatic endothelial cell sphingosine kinase activity is required for lymphocyte egress and lymphatic patterning. Journal of Experimental Medicine. 207 (1), 17-27 (2010).
- Gilleron, J., et al. Exploring Adipose Tissue Structure by Methylsalicylate Clearing and 3D Imaging. Journal of Visualized Experiments. (162), e61640 (2020).
- Ivanov, S., Merlin, J., Lee, M. K. S., Murphy, A. J., Guinamard, R. R. Biology and function of adipose tissue macrophages, dendritic cells and B cells. Atherosclerosis. 271, 102-110 (2018).
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