energy storage, membrane components, and signaling. Extracellular hydrolysis of dietary TG in circulating lipoproteins yields FFAs and sn -2 MG, which are then taken up by cells ( 1,2 ). MGs are also produced intracellularly from membrane phospholipids and the consecutive action of phospholipase C and diacylglycerol lipase, or from the hydrolysis of stored TG by adipose TG lipase (ATGL) and hormone sensitive lipase (HSL) ( 2-5 ). The ultimate fate of intracellular MGs is hydrolysis to FFAs and glycerol or reesterifi cation by acyltransferases into diacylglycerol and TG ( 6, 7 ).MG lipase (MGL) is considered the rate-determining enzyme in MG catabolism. MGL accounts for roughly 85% of MG hydrolysis in the brain, with the remainder being catalyzed by the enzymes ABHD6 and ABHD12 ( 8,9 ). MGL is expressed in many other tissues as well, including brain, liver, skeletal muscle, adipose, and intestine ( 10-13 ). Within cells, MGL localizes to both the cytosolic and membrane fractions and hydrolyzes sn -1 and sn -2 MGs of varying acyl chain lengths and degrees of unsaturation, with almost no activity toward other lipids, such as TG and lyso-phospholipids ( 10,(14)(15)(16)(17)(18).MGL is involved in energy balance through two important functions. Abbreviations: AA, arachidonic acid; AEA, arachidonoyl ethanolamide; 2-AG, 2-arachidonoyl glycerol; AUC, area under the curve; CB, cannabinoid; EC, endocannabinoid; HFD, high-fat diet; HOMA-IR, homeostatic model assessment of insulin resistance; iMGL, mice that overexpress monoacylglycerol lipase specifi cally in the intestinal mucosa; LFD, low-fat diet; MG, monoacylglycerol; MGL, monoacylglycerol lipase; OFTT, oral fat tolerance test; OGTT, oral glucose tolerance test; RER, respiratory exchange ratio .

The tissue microenvironment has profound effects on tissue-specific regeneration. The 3-dimensional extracellular matrix (ECM) niche influences the linage-specific differentiation of stem cells in tissue. To understand how ECM guides tissue-specific regeneration, we established a series of 3D composite scaffolds containing ECMs derived from different primary cells isolated from a single animal species and assessed their impact on the differentiation of human mesenchymal stem cells (hMSCs). Synthetic microfiber scaffolds (fiber mats) were fabricated by electrospinning tyrosine-derived polycarbonates (pDTEC). The bovine primary fibroblasts, chondrocytes and osteoblasts cultured on the fiber mats produced and assembled their ECMs, infiltrating the pores of the fibrous scaffold. The composite scaffolds were decellularized to remove cellular components, preserve ECM and minimally affect polymer integrity. Characterization of the ECMs derived from different primary cells in the composite scaffolds showed overlapping but distinct compositions. The chondrogenic and osteogenic differentiation of hMSCs on the different composite scaffolds were compared. Our results showed that ECM derived from chondrocytes cultured in synthetic fiber mats promoted the chondrogenic differentiation of hMSC in the presence or absence of soluble inducing factors. ECM derived from co-culture of osteoblasts and chondrocytes promoted osteogenic differentiation in hMSCs better than ECM derived from chondrocytes. This study demonstrated that decellularized ECMs derived from different cell types formed within synthetic fiber scaffolds guide the tissue-specific differentiation of hMSCs. These composite scaffolds may be developed into models to study the mechanisms of ECM-induced tissue regeneration.Graphical Abstract

Objective In vitro expansion of chondrocytes is required for cartilage tissue engineering and clinical cell-based cartilage repair practices. However, the dedifferentiation of chondrocytes during in vitro expansion continues to be a challenge. This study focuses on identifying a cell culture surface to support chondrocyte expansion with reduced dedifferentiation. Design A less adhesive culture surface, non-tissue culture treated surface (NTC), was tested for its suitability for culturing chondrocytes. The cell expansion and the expression of chondrocyte markers were monitored for at least 2 passages on NTC in comparison with conventional tissue culture treated polystyrene surface (TCP). The ability of expanded chondrocytes to form cartilage tissues was evaluated using pellet culturing and subcutaneous implantation in nude mice. Results NTC supported bovine chondrocyte proliferation to a clinically relevant expansion requirement within 2 passages. Chondrocyte phenotypes were better maintained when cultured on NTC than on TCP. In vitro pellet culture studies showed that chondrocytes expanded on NTC expressed a higher level of chondrocyte extracellular matrix. Furthermore, the cells expanded on NTC or TCP were implanted subcutaneously as pellets in nude mice for 6 weeks. The recovered pellets showed cartilage-like tissue formation from cells expanded on NTC but not from the cells expanded on TCP. Conclusions This study presents an innovative and easy culturing procedure to expand chondrocytes with reduced dedifferentiation. This procedure has potential to be developed to expand chondrocytes in vitro for basic research, tissue engineering, and possibly for clinical applications.

Mononuclear phagocytes, including monocytes, macrophages, and dendritic cells, function in tissue homeostasis, and also regulate immune responses. Tissue resident macrophages in a variety of tissues arise from the yolk sac, and are generated independently of hematopoietic stem cells. However, the origin of thymic macrophages remains poorly understood. Nr4a1 is an orphan nuclear receptor that limits pro-inflammatory cytokine production in macrophages, and functions as the master regulator of Ly6C- patrolling monocyte development. Here, we report a significant defect in the development of thymic resident macrophages in Nr4a1-deficient animals. Thymic macrophage levels were reduced over two-fold compared with wild-type controls. However, Nr4a1 does not globally regulate the development of tissue macrophages, as macrophages resident in the spleen, lung, brain, pancreas, and peritoneum from Nr4a1-deficient mice develop normally. Moreover, Nr4a1-deficient thymic macrophages fail to clear apoptotic thymocytes in vivo, and express reduced levels of genes important for apoptotic cell engulfment, including engulfment receptors Mertk, and Axl. Defective thymic macrophage development likely impairs proper T cell selection, thus altering the immune system as a whole. Together, these data show that Nr4a1 regulates the development and functional capacity of tissue resident macrophages in the thymus, and provide a novel mechanism for maintenance of thymic homeostasis.

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