A Notch toward Progenitor D(G)FRentiation: Defining a NOTCH-PDGFR axis in brown adipogenesis

Abstract

Brown adipocytes are found in several fat depots, however, the origins and contributions of different lineages of adipogenic progenitor cells (APCs) to these depots are unclear. In this issue of Developmental Cell, Shi et al. show that platelet-derived growth factor receptor β (PDGFRβ)-lineage and T-box transcription factor 18 (TBX18)-lineage APCs differentially contribute to brown adipogenesis across these depots.

Adipocytes, or fat cells, store lipids such as triglycerides in lipid droplets in response to an energy surplus. There are three main types of adipocytes that possess different metabolic signatures and exert divergent functions. These are white, beige, and brown adipocytes, which exhibit sequentially higher metabolic activities and less fat content. An increase of white adipocytes and a reduction of brown adipocytes often leads to obesity and metabolic dysfunction. Conversely, studies in preclinical animal models have shown that increasing brown adipocytes confers systemic metabolic benefits, ranging from improving insulin sensitivity to protecting against cardiovascular diseases.¹ A better understanding of the origins and signaling pathways that govern the development of brown versus white adipocytes would therefore empower the development of therapies to treat a spectrum of diseases.

The classical location of brown adipocytes is the intrascapular brown adipose tissue (iBAT). In addition, there are four other brown adipocyte-containing depots located at periaortic (paBAT), perirenal (prBAT), supraclavicular (scBAT), and thoracic perivascular (tpVAT) regions. It is well known that brown adipocytes mainly arise from Myf5⁺ progenitor cells during embryogenesis.² Additionally, Pax7⁺ and Pax3⁺ progenitor cells are also shown to give rise to brown adipocytes during development.² More recently, single-cell RNA sequencing has led to the discovery that PDGFRα⁺ fibro-adipogenic progenitors and CD68⁺ myeloid progenitors both give rise to brown adipocytes in a post-natal BAT regeneration model.³ Apart from these, PDGFRβ⁺ mural cells have also shown potential to generate adipocytes.⁴ However, the contribution of various APCs to BAT depots during post-natal growth is yet to be understood.

To understand the relative contributions of two APC populations to postnatal brown adipocytes, Shi et al.⁵ performed a series of comprehensive pulse-chase experiments using PDGFRβ and TBX18 lineage reporter mice at different developmental time points (Figure 1A). Of note, PDGFRβ⁺ cells contribute the least to iBAT adipogenesis, compared to a slightly higher contribution to paBAT and scBAT at postnatal day 5 (P5). Interestingly, PDGFRβ⁺ cells contribute more substantially to tpVAT and prBAT at P1. Similarly, TBX18⁺ APCs do not give rise to adipocytes in the iBAT regardless of the time point but contributes to paBAT, scBAT, tpVAT, and prBAT at P1 (Figure 1A). These lineage tracing results uncover dynamic contributions of PDGFRβ⁺ and TBX18⁺ APCs to postnatal BAT development.

Figure 1. Lineage origins of various brown fat depots and the role of NOTCH-PDGFRb signaling in adipogenesis.

(A) Experimental scheme and results of the PDGFRβ/TBX18-lineage tracing in perinatal mice. APCs were pulsed with Tamoxifen at E14.5, P1, P5, and P10, and chased until P30. Relative contributions of the PDGFRβ/TBX18-lineage APCs to the different BAT depots are shown in the line graphs. The numbers in the graph were based on results in Shi et al.⁵

(B) Diagram illustrating the role of the NOTCH–PDGFRβ signaling axis in adipogenesis and mature adipocytes based on previous and the current studies cited in the graph. The level of NOTCH signaling in the APCs is higher than the mature adipocytes. Down-regulation of the NOTCH signaling in the APCs leads to its differentiation into mature adipocytes. Up-regulation of the NOTCH signaling in the mature adipocytes leads to the de-differentiation. NICD: NOTCH Intercellular Domain; Rbpj: Recombination Signal Binding Protein for Immunoglobulin Kappa J Region.

Figures were created with BioRender

Despite the low PDGFRβ-lineage contribution to BAT depots, diphtheria toxin (DT)-induced eradication of PDGFRβ-expressing cells at P10 reduces iBAT mass in P30 mice. Surprisingly, the DT-treated prBAT still contains PDGFRβ-derived adipocytes at a level comparable to the control groups. This may be due to a lack of DT penetration into the prBAT, or because the PDGFRβ progenitors in prBAT are less susceptible to DT-induced apoptosis. Moreover, the authors show that the PDGFRβ⁺ APCs have higher NOTCH signaling compared to PDGFRβ^– cells. They further demonstrate that blocking NOTCH signaling in the PDGFRβ-expressing cells by Rbpj deletion leads to an expansion of the BAT depots. This BAT mass expansion further confers a metabolic improvement and protects against diet-induced obesity during both neonatal and juvenile stages.

To explore how Notch signaling regulates PDGFRβ, the authors performed chromatin immunoprecipitation followed by quantitative PCR and showed that RBPJ binds to the Pdgfrb gene locus to promote its transcription. The authors proposed that PDGFRβ acts downstream of NOTCH, because the PDGFRβ activator blunts the pro-brown adipogenic effects of NOTCH inhibition. These results thus establish Notch signaling as an upstream regulator of PDGFRβ expression during postnatal brown adipogenesis (Figure 1B).

The finding of this paper resonates with previous reports on the crucial role of NOTCH signaling in governing brown adipogenesis. Specifically, adipocyte-specific deletion of Notch1 or Rbpj genes promotes brown adipogenesis, while constitutive activation of NOTCH signaling impairs early brown adipogenesis.⁶ On the other hand, constitutive NOTCH activation in white adipocytes leads to their de-differentiation and development of lipodystrophy.^{7, 8} The de-differentiated white adipocytes further transform to liposarcoma as the animals age.^{7, 9} Together, these findings highlight the bidirectional regulation of NOTCH signaling in adipogenesis and mature adipocytes. In this scenario, APCs maintain high levels of NOTCH signaling to prevent differentiation, yet NOTCH signaling needs to be downregulated for their differentiation (Figure 1B).

Several questions emerge from this study. First, the relative low contribution of PDGFRβ⁺ and TBX18⁺ APCs to adipogenesis at E14.5 and P1 compared to later stages (P5 and P10) is surprising. This may be due to a lower efficiency of tamoxifen pulsing of prenatal and perinatal embryos, which is typically through maternal circulation or milk feeding. It is also unclear why the loss of PDGFRβ⁺ APCs impairs juvenile brown adipogenesis so profoundly, despite their relative low contribution to the various BAT depots at P10. Furthermore, it remains to be determined what are the general functions of TBX family transcription factors in APCs, given that TBX1-lineage progenitors have also been reported to contribute to BAT development.¹⁰ Lastly, what are the cellular sources of PDGF and the molecular targets of PDGFRβ signaling in the APC that controls brown adipogenesis? Answers to these questions will pave the way to target the NOTCH-PDGFRβ axis to expand brown fat to counteract obesity and associated metabolic syndrome.