Three-Dimensional Culture of Vascularized Thermogenic Adipose Tissue from Microvascular Fragments

Summary

Here, we present a detailed protocol outlining the use of microvascular fragments isolated from rodent or human fat tissue as a straightforward approach to engineer functional, vascularized beige adipose tissue.

Abstract

Engineering thermogenic adipose tissue (e.g., beige or brown adipose tissues) has been investigated as a potential therapy for metabolic diseases or for the design of personalized microtissues for health screening and drug testing. Current strategies are often quite complex and fail to accurately fully depict the multicellular and functional properties of thermogenic adipose tissue. Microvascular fragments, small intact microvessels comprised of arteriole, venules, and capillaries isolated from adipose tissue, serve as a single autologous source of cells that enable vascularization and adipose tissue formation. This article describes methods for optimizing culture conditions to enable the generation of three-dimensional, vascularized, and functional thermogenic adipose tissues from microvascular fragments, including protocols for isolating microvascular fragments from adipose tissue and culture conditions. Additionally, best practices are discussed, as are techniques for characterizing the engineered tissues, and sample results from both rodent and human microvascular fragments are provided. This approach has the potential to be utilized for the understanding and development of treatments for obesity and metabolic disease.

Introduction

The goal of this protocol is to describe an approach for developing vascularized beige adipose tissue from a single, potentially autologous source, microvascular fragment (MVF). Brown and beige adipose tissues have been demonstrated to display beneficial properties related to metabolic regulation; however, the small volume of these adipose tissue depots in adults limits the potential impact on systemic metabolism, particularly in diseased conditions such as obesity or type 2 diabetes^{1,2,3,4,5,6,7}. There is significant interest in brown/beige fat as a therapeutic target for preventing the harmful metabolic effects linked with obesity and its comorbidities^8,9,10,11,12.

MVFs are vessel structures that can be directly isolated from adipose tissue, cultured, and maintained in a three-dimensional configuration for extended periods of time^13,14,15. Previous work from our group, and others, have begun to exploit the multicellular and multipotent capacity of MVFs, specifically as it relates to adipose tissue formation^16,17,18. As a buildup of this work, we recently demonstrated that MVFs derived from rodent models of healthy and type 2 diabetes¹⁹ and from human subjects (adults over 50 years of age)²⁰ contained cells capable of being induced to form thermogenic, or beige, adipose tissue.

Herein is an innovative approach from which a single source MVF is utilized, not only capable of creating beige adipose tissue but also its associated and critical vascular component²¹. The use of this technique could be of great value for studies looking for a straightforward tissue-engineered approach for thermogenic adipose tissue formation. Unlike other methods aspiring to engineer beige adipose tissue^{22,23,24,25,26,27,28}, the process described in this study does not require using multiple cell types or complex induction regimens. Vascularized beige and white fat models can be created with MVFs originating from rodent and human sources, demonstrating great translation potential. The end product of this protocol is an engineered beige thermogenic fat tissue with a structure and metabolic function comparable to brown adipose tissue. Overall, this protocol presents the idea that an easily accessible and possibly autologous source MVF may be a worthwhile therapeutic intervention and tool for studying metabolic disorders.

Protocol

This study was conducted in compliance with the Animal Welfare Act and the Implementing Animal Welfare Regulations in accordance with the principles of the Guide for the Care and Use of Laboratory Animals. All animal procedures were approved by the Institutional Animal Care and Use Committee at the University of Texas at San Antonio. NOTE: For the steps described below, male Lewis Rats are utilized. Slight protocol adjustments must be made for a female, as well as mouse microvascular fragment …

Representative Results

There are a few key phenotypic morphological characteristics of beige/brown adipose tissue: it is multilocular/contains small lipid droplets, possesses a large number of mitochondria (the reason for its characteristically "brownish" appearance in vivo), correspondingly has a high oxygen consumption rate/mitochondrial bioenergetics, is highly vascularized, has increased lipolysis/insulin-stimulated glucose uptake, and, most notoriously, expresses high levels of uncoupling protein 1 (UCP1), a mitochondrial…

Discussion

The field of brown/beige adipose tissue engineering is largely immature^{22,23,24,25,26,27,28}, with the bulk of adipose models being developed for white adipose tissue^8,22,31. Engineered brown/beige m…

Materials

Aminocaproic Acid	Sigma Aldrich	A2504-100G	Added in DMEM at the concentration of 1 mg/mL
Blunt-Tipped Scissors	Fisher scientific	12-000-172	Sterilize in autoclave
Bovin Serum Albumin (BSA)	Millipore	126575-10GM	Diluted in PBS to 4 mg/mL and 1 mg/mL
Collagenase Type 1	Fisher scientific	NC9633623	Diluted to 6 mg/mL in BSA 4 mg/mL, Digestion of minced fat
Dexamethasone	Thermo Scientific	AC230302500	Diluted in ethanol at a 2 mg/ml stock concentration
Disposable underpads	Fisher scientific	23-666-062	For fluid absorption during surgery
Dissecting Scissors	Fisher scientific	08-951-5	Sterilize in autoclave
Dulbecco′s Modified Eagle′s Medium (DMEM)	Fisher scientific	11885092
Dulbecco′s Modified Eagle′s Medium/Nutrient Mixture F-12 Ham (DMEM/F12)	Sigma Aldrich	D8062
Fetal Bovine Serum	Fisher scientific	16140089	Added in DMEM to 20% v/v.
Fibrinogen	Sigma Aldrich	F8630-25G	Solubilized in DMEM at the concentration of 20 mg/mL, Protein found in blood plasma and main component of hydrogel
Flask, 250 mL	Fisher scientific	FB500250	Allows for digestion of fat using a large surface area
Forceps	Fisher scientific	50-264-21	Sterilize in autoclave, For handling of tissue and filters
Forskolin	Sigma Aldrich	F6886	Diluted in ethanol at a 10 mM stock concentration
Human MVF	Advanced Solutions Life Scienes, LLC	https://www.advancedsolutions.com/microvessels	Human MVFs (hMVFs) isolated from three different patients (52-, 54-, and 56-year old females) were used in the current study.
Indomethacine	Sigma Aldrich	I7378	Diluted in ethanol at a 12.5 mM stock concentration
Insulin from porcine pancreas	Sigma Aldrich	I5523	Diluted in 0.01 N HCl at a 5 mg/ml stock concentration
MycoZap	Fisher scientific	NC9023832	Added in DMEM to 0.2% w/v, Mycoplasma Prophylactic
Pennycilin/Streptomycin (10,000 U/mL)	Fisher scientific	15140122	Added in DMEM to 1% v/v.
Petri dishes, polystyrene (100 mm x 15 mm).	Fisher scientific	351029	3 for removal of blood vessels and mincing, 8 (lid) for presoaking of screens & 8 (dish) for use when filtering with 500 or 37 µM screens
Petri dishes, polystyrene (35 mm x 10 mm).	Fisher scientific	50-202-036	For counting fragments
Phosphate Buffer Saline (PBS)	Fisher scientific	14-190-250	Diluted to 1x with sterile deionized water.
Rat Clippers (Andwin Mini Arco Pet Trimmer)	Fisher scientific	NC0854141
Rosiglitazone	Fisher scientific	R0106200MG	Diluted in DMSO at a 10 mM stock concentration
Scissors	Fine Science Tools	14059-11	1 for initial incision, 1 for epididymal incision, 1 for tip clipping
Screen  37 µM	Carolina Biological Supply Company	652222R	Cut into 3" rounded squares and sterilized in ethylene oxide, Fragment entrapment and removal of very small fragments/single cells and debris
Screen 500 µM	Carolina Biological Supply Company	652222F	Cut into 3" rounded squares and sterilized in ethylene oxide, Removes larger fragments/debris
Serrated Hemostat	Fisher scientific	12-000-171	Sterilize in autoclave, For clamping of skin before incision
Steriflip Filter 0.22 μm	Millipore	SE1M179M6
Thrombin	Fisher scientific	6051601KU	Diluted in deionzed water to 10 U/mL, Used as a clotting agent turning fibrinogen to fibrin
Thyroid hormone (T3)	Sigma Aldrich	T2877	Diluted in 1N NaOH at a 0.02 mM stock concentration
Zucker diabetic fatty (ZDF) rats – obese (FA/FA) or lean (FA/+) male	Charles River	https://www.criver.com/products-services/find-model/zdf-rat-lean-fa?region=3611 https://www.criver.com/products-services/find-model/zdf-rat-obese?region=3611	Obtained from Charles River (Wilmington, MA). Rats were acquired at 4 weeks of age and fed Purina 5008 until euthanasia (15-19 weeks of age). Glucose levels (blood from the lateral saphenous vein) were greater than 300 mg/dL in all FA/FA rats used in the study. All animals were housed in a temperature-controlled environment with a 12-h light-dark cycle and fed ad libitum.