TNF-alpha is required for the attraction of mesenchymal precursors to white adipose tissue in Ob/ob mice

Abstract

Most adult tissues harbour a stem cell subpopulation (Mesenchymal Precursors or MPs) that represent a small proportion of the total cell number and have the potential to differentiate into several cell types within the mesenchymal lineage. In adipose tissue, adipocytes account for two-thirds of the total cell number. The remaining cells include blood and endothelial cells, along with adipocyte precursors (adipose MPs). Obesity is defined as an excess of body fat that frequently results in a significant impairment of health. The ob/ob mice bear a mutation in the ob gene that causes a deficiency in the hormone leptin and hence obesity. Here, we present evidence that ob/ob mice have a dramatic decrease in the resident MP pool of several tissues, including squeletal muscle, heart, lung and adipose tissue. Moreover, we show that that there is a migration of MP cells from distant organs, as well as homing of these cells to the adipose tissue mass of the ob/ob mice. We call this process adipotaxis. Once in the adipose tissue, migrant MPs undergoe adipose differentiation, giving rise to new differentiated adipocytes within the adipose mass. Finally, we provide evidence that adipotaxis is largely explained by the production of high levels of Tumor Necrosis Factor-alpha (TNF-alpha) within the ob/ob adipose tissue. The therapeutic implications for human obesity as well as for regenerative medicine are further discussed in this paper.

Figure 1. Isolation of Mesenchymal Precursors.

A. MPs from adipose tissue explants in C57 wt or ob/ob mice 7 days after the surgical extraction. B. Isolated MP clones growing at subconfluence in DMEM+10%.

Figure 2. Migration and engraftment of MPs into adipose tissue.

A. Results for epsilon chromosome RT-PCR, 6 h or 2 months after i.v. or i..m. male MP injection into female C57 or female ob/ob mice (*p<0.04). Note the recruitment of MPs to adipose tissue in ob/ob mice (see red arrows) (+p<0.05). B & C. Detection of intravenously injected male MPs by immunohistology in the adipose and muscle tissue of ob/ob mice 6 h or 2 months after the injection. Red fluorescence stains the injected cells, while the green color represents laminin staining (adipocytes). Hoescht dye (blue) stains all nuclei.

Figure 3. Analysis of the migration properties of MPs.

A. In vitro transmigration of MPs after 6 h in the presence or absence of different conditions: mock, SN (600 µl of adypocytes supernatants), recombinant murine TNFα (Rec TNF, 50 ng/ml) (*p<0.03), anti-TNF mAbs (600 µl of adypocytes supernatants in the presence of 10 µg/ml antibodies) (+p<0.05), and MCP1 (20 ng/ml). B. Images taken from the Transwells filters after 6 h under the different conditions. C. In vivo migration into the different tissues of MP population after 6 h of C57 or ob/ob iv injection, in the presence of anti-TNF-alpha mAbs treatment (*p<0.05). Note the reduction of MPs in the adipose tissue of ob/ob mice (red arrow). D. Images taken from the adipose tissue after 6 h. Note the disappearance of injected MPs around the tissue.