SUMMARY In adult white adipose tissue, cold or β3-adrenoceptor activation promotes the appearance of thermogenic beige adipocytes. Our comprehensive single-cell analysis revealed that these cells arise through the reprogramming of existing adipogenic trajectories, rather than from a single precursor. These trajectories predominantly arise from SM22-expressing vascular mural progenitor cells. Central in this transition is the activation of Adrb3 in mature adipocytes, leading to subsequent upregulation of Adrb1 in primed progenitors. Under thermoneutral conditions, synergistic activation of both Adrb3 and Adrb1 recapitulates the pattern of cold-induced SM22+ cell recruitment. Lipolysis-derived eicosanoids, specifically docosahexaenoic acid (DHA) and arachidonic acid (AA) prime these processes and in vitro, were sufficient to recapitulate progenitor cells priming. Collectively, our findings provide a robust model for cold-induced beige adipogenesis, emphasizing a profound relationship between mature adipocytes and mural cells during cold acclimation, and revealing the metabolic potential of this unique cellular reservoir.

Embryonic development of the alveolar sac of the lung is dependent upon multiple signaling pathways to coordinate cell growth, migration, and the formation of the extracellular matrix. Here, we identify GORAB as a regulator of embryonic alveolar sac formation as genetically disrupting the Gorab gene in mice resulted in fatal saccular maturation defects characterized by a thickened lung mesenchyme. This abnormality is not associated with impairments in cellular proliferation and death, but aberrantly increased protein kinase B (AKT) phosphorylation, elevated Vcan transcription, and enhanced migration of mesenchymal fibroblasts. Genetically augmenting PDGFRα, a potent activator of AKT in lung mesenchymal cells, recapitulated the alveolar phenotypes, whereas disrupting PDGFRα partially rescued alveolar phenotypes in Gorab-deficient mice. Overexpressing or suppressing Vcan in primary embryonic lung fibroblasts could, respectively, mimic or attenuate alveolar sac-like phenotypes in a co-culture model. These findings suggest a role of GORAB in negatively regulating AKT phosphorylation, the expression of Vcan, and the migration of lung mesenchyme fibroblasts, and suggest that alveolar sac formation resembles a patterning event that is orchestrated by molecular signaling and the extracellular matrix in the mesenchyme.
© 2020 Federation of American Societies for Experimental Biology.

Conditions of extended bed rest and limb immobilization can initiate rapid and significant loss of skeletal muscle mass and function. Physical rehabilitation is standard practice following a period of disuse, yet mobility may be severely compromised, and recovery is commonly delayed or incomplete in special populations. Thus, a novel approach toward recovery of muscle mass is highly desired. Pericytes [neuron-glial antigen 2 (NG2)+CD31-CD45- (Lineage- [Lin-]) and CD146+Lin-] demonstrate capacity to facilitate muscle repair, yet the ability to enhance myofiber growth following disuse is unknown. In the current study, 3-4-mo-old mice were unilaterally immobilized for 14 d (IM) or immobilized for 14 d followed by 14 d of remobilization (RE). Flow cytometry and targeted gene expression analyses were completed to assess pericyte quantity and function following IM and RE. In addition, a transplantation study was conducted to assess the impact of pericytes on recovery. Results from targeted analyses suggest minimal impact of disuse on pericyte gene expression, yet NG2+Lin- pericyte quantity is reduced following IM (P < 0.05). Remarkably, pericyte transplantation recovered losses in myofiber cross-sectional area and the capillary-to-fiber ratio following RE, whereas deficits remained with vehicle alone (P = 0.01). These findings provide the first evidence that pericytes effectively rehabilitate skeletal muscle mass following disuse atrophy.-Munroe, M., Dvoretskiy, S., Lopez, A., Leong, J., Dyle, M. C., Kong, H., Adams, C. M., Boppart, M. D. Pericyte transplantation improves skeletal muscle recovery following hindlimb immobilization.

Large-scale expansion of myogenic progenitors is necessary to support the development of high-throughput cellular assays in vitro and to advance genetic engineering approaches necessary to develop cellular therapies for rare muscle diseases. However, optimization has not been performed in order to maintain the differentiation capacity of myogenic cells undergoing long-term cell culture. Multiple extracellular matrices have been utilized for myogenic cell studies, but it remains unclear how different matrices influence long-term myogenic activity in culture. To address this challenge, we have evaluated multiple extracellular matrices in myogenic studies over long-term expansion.
We evaluated the consequence of propagating mouse and human myogenic stem cell progenitors on various extracellular matrices to determine if they could enhance long-term myogenic potential. For the first time reported, we comprehensively examine the effect of physiologically relevant laminins, laminin 211 and laminin 521, compared to traditionally utilized ECMs (e.g., laminin 111, gelatin, and Matrigel) to assess their capacity to preserve myogenic differentiation potential.
Laminin 521 supported increased proliferation in early phases of expansion and was the only substrate facilitating high-level fusion following eight passages in mouse myoblast cell cultures. In human myoblast cell cultures, laminin 521 supported increased proliferation during expansion and superior differentiation with myotube hypertrophy. Counterintuitively however, laminin 211, the native laminin isoform in resting skeletal muscle, resulted in low proliferation and poor differentiation in mouse and human cultures. Matrigel performed excellent in short-term mouse studies but showed high amounts of variability following long-term expansion.
These results demonstrate laminin 521 is a superior substrate for both short-term and long-term myogenic cell culture applications compared to other commonly utilized substrates. Since Matrigel cannot be used for clinical applications, we propose that laminin 521 could possibly be employed in the future to provide myoblasts for cellular therapy directed clinical studies.