Mesenchymal stem/progenitor cells are essential for tissue development and repair throughout life, but how they are maintained under chronic differentiation pressure is not known. Using single-cell transcriptomics of human progenitor cells we find that adipose differentiation stimuli elicit two cellular trajectories: one toward mature adipocytes and another toward a pool of non-differentiated cells that maintain progenitor characteristics. These cells are induced by transient Wnt pathway activation and express numerous extracellular matrix genes and are therefore named structural Wnt-regulated adipose tissue cells. We find that the genetic signature of structural Wnt-regulated adipose tissue cells is present in adult human adipose tissue and adipose tissue developed from human progenitor cells in mice. Our results suggest a mechanism whereby adipose differentiation occurs concurrently with the maintenance of a mesenchymal progenitor cell pool, ensuring tissue development, repair and appropriate metabolic control over the lifetime.

Mechanisms that control 'beige/brite' thermogenic adipose tissue development may be harnessed to improve human metabolic health. To define these mechanisms, we developed a species-hybrid model in which human mesenchymal progenitor cells were used to develop white or thermogenic/beige adipose tissue in mice. The hybrid adipose tissue developed distinctive features of human adipose tissue, such as larger adipocyte size, despite its neurovascular architecture being entirely of murine origin. Thermogenic adipose tissue recruited a denser, qualitatively distinct vascular network, differing in genes mapping to circadian rhythm pathways, and denser sympathetic innervation. The enhanced thermogenic neurovascular network was associated with human adipocyte expression of THBS4, TNC, NTRK3 and SPARCL1, which enhance neurogenesis, and decreased expression of MAOA and ACHE, which control neurotransmitter tone. Systemic inhibition of MAOA, which is present in human but absent in mouse adipocytes, induced browning of human but not mouse adipose tissue, revealing the physiological relevance of this pathway. Our results reveal species-specific cell type dependencies controlling the development of thermogenic adipose tissue and point to human adipocyte MAOA as a potential target for metabolic disease therapy.

Tissue development and repair throughout life depends on the availability of multipotent mesenchymal stem/progenitor cells capable of differentiating into specialized cell types. How an appropriately sized pool of such multipotent progenitors is maintained under varied signals for tissue growth and repair is unknown. We addressed this question by monitoring fate trajectories of human adipose tissue-derived multipotent progenitor cells using single-cell transcriptomics. Homogenous multipotent progenitors underwent two distinct fate trajectories rapidly upon induction of adipose differentiation– one toward the adipocyte fate, and the other towards a distinct, non-differentiated state characterized by up-regulation of canonical Wnt target genes. Upon isolation, this latter cell population was able to resume proliferation and display multipotency. Using canonical Wnt agonists and antagonists we find Wnt signaling is required for the maintenance of this multipotent pool under differentiation stimulus. In vivo, these cells are retained in adipose tissue developed from human multipotent progenitor cells in immunocompromised mice, and their transcriptomic signature is detected in human adult adipose tissue. Our study reveals a previously unrecognized mechanism for maintaining a functional pool of human mesenchymal progenitor cells under conditions of differentiation pressure, driven by Wnt signaling.

Human beige/brite thermogenic adipose tissue exerts beneficial metabolic effects and may be harnessed to improve metabolic health. To uncover mechanisms by which thermogenic adipose tissue is generated and maintained we developed a species-hybrid model in which human mesenchymal progenitor cells are induced in vitro to differentiate into white or thermogenic adipocytes and are then implanted into immuno-compromised mice. Upon implantation, thermogenic adipocytes form a more densely vascularized and innervated adipose tissue compared to non-thermogenic adipocytes. Mouse endothelial and stem/progenitor cells recruited by implanted human thermogenic adipocytes are also qualitatively different, with differentially expressed genes mapping predominantly to circadian rhythm pathways. We trace the formation of this enhanced neurovascular architecture to higher expression of a distinct set of genes directly associated with neurogenesis (THBS4, TNC, NTRK3 and SPARCL1), and to lower expression of genes associated with neurotransmitter degradation (MAOA, ACHE) by adipocytes in the developed tissue. Further analysis reveals that MAOA is abundant in human adipocytes but absent in mouse adipocytes, revealing species-specific mechanisms of neurotransmitter tone regulation. In summary, our work discovers specific neurogenic genes associated with development and maintenance of human thermogenic adipose tissue, reveals species-specific mechanisms of control of neurotransmitter tone, and suggests that targeting adipocyte MAOA may be a strategy for enhancing thermogenic adipose tissue activity in humans.

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