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Mice expressing human but not murine beta3-adrenergic receptors under the control of human gene regulatory elements
1Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA.
Abstract:
Beta-adrenergic receptors (ARs) are expressed predominantly in adipose tissue, and beta3-selective agonists are effective anti-obesity drugs in rodents. Rodent and human beta3-ARs differ with respect to expression in white versus brown adipocytes as well as their ability to be stimulated by beta3-AR-selective agonists. Humans express beta3-AR mRNA abundantly in brown but not white adipocytes, while rodents express beta3-AR mRNA abundantly in both sites. To determine the basis for this difference, we have transgenically introduced 74 kilobases (kb) of human beta3-AR genomic sequence into gene knockout mice lacking beta3-ARs. Importantly, human beta3-AR mRNA was expressed only in brown adipose tissue (BAT) of transgenic mice, with little or no expression being detected in white adipose tissue (WAT), liver, stomach, small intestine, skeletal muscle, and heart. This pattern of expression differed from that observed in mice bearing a murine beta3-AR genomic transgene in which beta3-AR mRNA was expressed in both WAT and BAT, but not in other sites. Furthermore, we have transgenically introduced smaller human constructs containing -14.5 and -0.6 kb of upstream sequence into beta3-AR gene knockout mice. Both -14.5 and -0.6 kb constructs were expressed in BAT but not WAT. Thus, human but not murine cis-regulatory elements direct beta3-AR gene expression preferentially to brown adipocytes. Identification of responsible cis-regulatory element(s) and relevant trans-acting factor(s) should provide insight into mechanisms controlling human beta3-AR gene expression. In addition, the beta3-AR agonist, CGP-12177, stimulated oxygen consumption in mice expressing human but not murine beta3-ARs by 91% compared with only 49% in control beta3-AR gene knockout mice, demonstrating that the human beta3-AR can functionally couple with energy expenditure. These "humanized" mice should assist us in the development of drugs that may become effective anti-obesity agents in humans.
Insights
Human beta3-AR gene expression is directed to brown fat by human regulatory elements, unlike in mice. These findings aid in developing anti-obesity drugs targeting human beta3-adrenergic receptors (ARs).
Area of Science:
- Molecular biology
- Genetics
- Pharmacology
Background:
- Beta-adrenergic receptors (ARs) are key in adipose tissue metabolism.
- Beta3-AR agonists show anti-obesity effects in rodents, but human and rodent receptors differ.
- Human beta3-ARs are primarily in brown adipocytes, unlike rodent beta3-ARs found in both white and brown fat.
Purpose of the Study:
- To investigate the genetic basis for preferential human beta3-AR expression in brown adipose tissue (BAT).
- To create "humanized" mouse models for studying human beta3-AR function and developing anti-obesity drugs.
Main Methods:
- Transgenic mice were created by introducing human beta3-AR genomic sequences into beta3-AR knockout mice.
- Expression patterns of human and murine beta3-AR mRNA were analyzed in various tissues.
- The functional response of human beta3-AR to a selective agonist was assessed by measuring oxygen consumption.
Main Results:
- Human beta3-AR mRNA was exclusively detected in brown adipose tissue (BAT) of transgenic mice, irrespective of the construct size.
- Murine beta3-AR mRNA was expressed in both white adipose tissue (WAT) and BAT.
- The human beta3-AR agonist CGP-12177 significantly stimulated oxygen consumption in humanized mice, confirming functional coupling.
Conclusions:
- Human cis-regulatory elements preferentially direct beta3-AR gene expression to brown adipocytes.
- The developed "humanized" mice accurately model human beta3-AR expression and function.
- These models are valuable for developing targeted anti-obesity therapies for humans.