Toward a Consensus View of Mammalian Adipocyte Stem and Progenitor Cell Heterogeneity

White adipose tissue (WAT) is a cellularly heterogeneous endocrine organ that not only serves as an energy reservoir, but also actively participates in metabolic homeostasis. Among the main constituents of adipose tissue are adipocytes, which arise from adipose stem and progenitor cells (ASPCs). While it is well known that these ASPCs reside in the stromal vascular fraction (SVF) of adipose tissue, their molecular heterogeneity and functional diversity is still poorly understood. Driven by the resolving power of single-cell transcriptomics, several recent studies provided new insights into the cellular complexity of ASPCs among different mammalian fat depots. In this review, we present current knowledge on ASPCs, their population structure, hierarchy, fat depot-specific nature, function, and regulatory mechanisms, and discuss not only the similarities, but also the differences between mouse and human ASPC biology.

Quantitative approaches to unravel bone marrow adipocyte site-specificity and its implication in hematopoiesis

Josefine Catharina Pedersen Tratwal

Over the last decade, bone marrow (BM) adipose tissue (BMAT) has emerged as a distinct adipose depot with unique metabolic properties and effects both in homeostasis and in the progression of disease. We provide a first comprehensive review of existing and emerging technologies including accompanying challenges for the study of bone marrow adipocytes (BMAds). The recommendations specified through this consensus opinion for standardization of methodological approaches are imperative on our quest to uncover the meaning, the sense, and the significance of BMAds. Due to its challenging localization within the bone and its fragile nature, imaging methods provide powerful tools for ex vivo or in vivo BMAT quantification as well as characterization through lipid profiling. While isolation methods are still evolving in the pursuit of the BMAd and its progenitor, lineage tracing and BMAT (environmental, pharmacological, or pathological) modulation studies have identified two sub-populations of BMAds dependent on skeletal location in the hematopoietic red or fatty yellow marrow. We uncover changes to these compartments in the mouse skeleton with age or irradiation and bone marrow transplantation induced aplastic conversion of the marrow, through our novel imaging quantification tool for histological sections. Following this progression by magnetic resonance imaging in vivo emphasized a differential hematopoietic recovery within the long bones of mice. In order to delineate the subtle differences between BMAds of red and fatty marrow, we established first an adipocytic differentiation, and then a co-culture system to recapitulate the BM niche in vitro with the OP9 stromal cell line. Notably, Raman microspectroscopic analysis revealed nuances of lipid saturation levels previously reported in primary BMAds at the single droplet level that we propose is indicative of their maturation state. This may have implication for the documented differential hematopoietic support capacity of BMAds that we elucidate through co-culture with hematopoietic stem and progenitor cells. We were able to translate the established co-culture system onto a three-dimensional scaffold for construction of a tissue-engineered BM model. In combination with a novel approach for preparation of its in vivo injection, we have established the direct in vitro to in vivo transfer of a biomimetic BM niche. The novel methodological approaches presented here to study BMAds and the hematopoietic microenvironment, have far-reaching applications for improving the outcome of BM disease modeling, biomolecular and drug screening, as well as in regenerative medicine at large.

EPFL2020

Redefining the metabolic landscape through TGF-beta/Activin signaling in Drosophila melanogaster

Wen Bin Chng

Multicellular organisms assess their nutritional state and adapt their metabolic landscape to match their energy demands. In this thesis, we address a longstanding conundrum of how an organism coordinates digestive enzyme expression in response to its nutritional status. Hydrolysis of food materials by endogenous or microbial-derived digestive enzymes precedes absorption through the gut lumen. Thus, modulation of endogenous digestive mechanism represents a pragmatic step to limit the uptake of nutrients when nutrients are in abundance. In Drosophila melanogaster, it is known that dietary glucose represses amylase transcript and enzymatic activity, in a process known as glucose repression. Here, we show that glucose reduces the expression of many carbohydrases, lipases, and glucose transporters, presumably to prevent nutritional overload after feeding. We found that the repression of carbohydrases is effected by nutritious sugars through the induction of the TGF-beta ligand, Dawdle. Interestingly, glucose repression of digestive enzymes is dependent upon the induction of Dawdle in the fat body, a tissue functionally analogous to the mammalian liver and adipose tissue. Dawdle is then transported via the circulation to activate TGFÎ²/Activin signaling in the midgut, leading to the repression of digestive enzymes expression. We later discovered that Dawdle expression in the fat body is regulated by the evolutionarily conserved Mio-bigmax transcription factor complex. In addition, expression of Dawdle must be tightly coordinated to reflect the nutritional state of adult flies, as flies overexpressing Dawdle are sensitive to starvation and have their TAG and glycogen stores rapidly depleted upon nutrient limitation. Thus, our results are consistent with a model whereby fat body derived Daw functions as a nutritional cue for carbohydrates to modulate metabolic responses in distal tissues. Its induction provides the nutritional signal to repress carbohydrate digestive capacity and stimulate peripheral insulin signaling. In conclusion, our study identifies the TGFÎ²/Activin signaling as a critical mechanism for carbohydrate homeostasis. Our results indicate that the fat body functions as a carbohydrate sensing tissue, which through the secretion of the TGFÎ² ligand, Daw, regulates organism-wide carbohydrate metabolism. Our work in Drosophila is likely to have a broader scientific impact by placing a pathway known for its role in development, immunity, and cancer, as an integral component for sugar sensing and metabolism.

EPFL2015

Metabolic and microenvironmental strategies to accelerate hematopoietic recovery post-aplasia

Vasco Ledebur de Antas de Campos

The procedure-associated mortality rate of bone marrow transplantations (BMT) remains close to 25%. The BMT is therefore only indicated for life-threatening diseases, with no replacement of this technology to be expected in the foreseeable future. Modest improvements such as the use of G-CSF to accelerate hematopoietic recovery significantly reduce mortality. Hematopoietic stem cells (HSC) reside in the bone marrow (BM) and are mainly quiescent, dividing only to self-renew. Via extracellular signals and interactions with other cells of the BM (microenvironment), a stable HSC pool is maintained, while hematopoietic progenitor cells (HPC) proliferate and differentiate to mature blood cell types. Marrow adipocytes (BMA) have recently been recognized as a hematopoietic regulatory entity, although the mechanisms by which it interacts with HSCs and HPCs (HSPC) in vivo, remain unknown. During the hematopoietic recovery period post-BMT, HSPCs increase their metabolism as a response to hematopoietic stress, partly modulated by the microenvironment. In this work we identified novel pharmacological approaches to accelerate hematopoietic recovery in the post-BMT period, addressed via two strategies: (i) by directly modulating HSC metabolism, and (ii) by regulating the differentiation of BMA. Increasing oxidative phosphorylation, combined with an impaired mitochondrial stress response, can severely compromise the regenerative capacity of HSCs. Here we demonstrate that the NAD+-boosting agent Nicotinamide Riboside (NR) reduces mitochondrial activity within HSCs through increased mitophagy, the recycling mechanism of damaged mitochondria. We observed in vitro NR treatment of HSCs to lead to increased asymmetric HSC divisions, which resulted in a significantly enlarged pool of progenitor cells, without HSC exhaustion. Dietary supplementation of NR to mice undergoing BMT accelerated blood recovery and improved survival. We therefore established a novel link between HSC mitochondrial stress, mitophagy and stem cell fate decision. Recent studies have suggested BMA maintain HSCs quiescent, which, may be detrimental to the increased expansion of HPCs in the recovery phase post-BMT. BM stromal cells (BMSC) are the cellular precursors of both adipocytes and osteoblasts, and they have been strongly implicated in supporting hematopoiesis by secreting cytokines such as SCF and Cxcl12. We observed that adipocytes quickly accumulate the marrow space of mice undergoing BMT, followed by an expansion hematopoiesis-supportive multipotent Sca1+/CD24+ stromal cells, coinciding with hematopoietic recovery. We developed a novel screening platform using Digital Holographic Microscopy (DHM), quantifying in vitro adipocytic differentiation non-invasively, allowing for follow-up co-cultures with HSPCs. Using the OP9 BMSC-line, we modulated adipocyte differentiation prior to co-culture using a pharmacological approach and observed that high adipocytic content was associated to a decrease in HPC expansion. We perfomed a screening of over 4000 FDA-approved drugs and natural compounds and identified one to efficiently prevent adipocytic differentiation and maintain the hematopoietic supportive capacity of OP9 stroma. Altogether, this work opens the door to alternative strategies accelerating hematopoietic reconstitution and unveils the potential to improve recovery of patients undergoing BMT.

EPFL2019