Dichotomous effects on lymphatic transport with loss of caveolae in mice.

AIM: Fluid and macromolecule transport from the interstitium into and through lymphatic vessels is necessary for tissue homeostasis. While lymphatic capillary structure suggests that passive, paracellular transport would be the predominant route of macromolecule entry, active caveolae-mediated transcellular transport has been identified in lymphatic endothelial cells (LECs) in vitro. Caveolae also mediate a wide array of endothelial cell processes, including nitric oxide regulation. Thus, how does the lack of caveolae impact "lymphatic function"? METHODS: Various aspects of lymphatic transport were measured in mice constitutively lacking caveolin-1 ("CavKO"), the protein required for caveolae formation in endothelial cells, and in mice with a LEC-specific Cav1 gene deletion (Lyve1-Cre x Cav1flox/flox ; "LyCav") and ex vivo in their vessels and cells. RESULTS: In each model, lymphatic architecture was largely unchanged. The lymphatic conductance, or initial tissue uptake, was significantly higher in both CavKO mice and LyCav mice by quantitative microlymphangiography and the permeability to 70 kDa dextran was significantly increased in monolayers of LECs isolated from CavKO mice. Conversely, transport within the lymphatic system to the sentinel node was significantly reduced in anaesthetized CavKO and LyCav mice. Isolated, cannulated collecting vessel studies identified significantly reduced phasic contractility when lymphatic endothelium lacks caveolae. Inhibition of nitric oxide synthase was able to partially restore ex vivo vessel contractility. CONCLUSION: Macromolecule transport across lymphatics is increased with loss of caveolae, yet phasic contractility reduced, resulting in reduced overall lymphatic transport function. These studies identify lymphatic caveolar biology as a key regulator of active lymphatic transport functions.

Cellular Origins of Beige Fat Cells Revisited.

Activated beige adipocytes have therapeutic potential due to their ability to improve glucose and lipid homeostasis. To date, the origin of beige adipocytes remains enigmatic. Whether beige cells arise through de novo differentiation from resident precursors or through reprogramming of mature white adipocytes has been a topic of intense discussion. Here, we offer our perspective on the natural origin of beige adipocytes in mice. In particular, we revisit recent lineage-tracing studies that shed light on this issue and offer new insight into how environmental housing temperatures early in life influence the mode of beige adipocyte biogenesis upon cold exposure later in life. We suggest a unified model in which beige adipocytes (UCP1+ multilocular cells) in rodents initially arise predominantly from progenitors (i.e., de novo beige adipogenesis) upon the first exposure to cold temperatures and then interconvert between "dormant beige" and "active beige" phenotypes (i.e., beige cell activation) upon subsequent changes in environmental temperature. Importantly, we highlight experimental considerations needed to visualize de novo adipogenesis versus beige cell activation in mice. A precise understanding of the cellular origins of beige adipocytes emanating in response to physiological and pharmacological stimuli may better inform therapeutic strategies to recruit beige adipocytes in vivo.

De novo adipocyte differentiation from Pdgfrß+ preadipocytes protects against pathologic visceral adipose expansion in obesity.

Pathologic expansion of white adipose tissue (WAT) in obesity is characterized by adipocyte hypertrophy, inflammation, and fibrosis; however, factors triggering this maladaptive remodeling are largely unknown. Here, we test the hypothesis that the potential to recruit new adipocytes from Pdgfrß+ preadipocytes determines visceral WAT health in obesity. We manipulate levels of Pparg, the master regulator of adipogenesis, in Pdgfrß+ precursors of adult mice. Increasing the adipogenic capacity of Pdgfrß+ precursors through Pparg overexpression results in healthy visceral WAT expansion in obesity and adiponectin-dependent improvements in glucose homeostasis. Loss of mural cell Pparg triggers pathologic visceral WAT expansion upon high-fat diet feeding. Moreover, the ability of the TZD class of anti-diabetic drugs to promote healthy visceral WAT remodeling is dependent on mural cell Pparg. These data highlight the protective effects of de novo visceral adipocyte differentiation in these settings, and suggest Pdgfrß+ adipocyte precursors as targets for therapeutic intervention in diabetes.

Fetal development of subcutaneous white adipose tissue is dependent on Zfp423.

OBJECTIVE: Zfp423 is a multi zinc-finger transcription factor expressed in preadipocytes and mature adipocytes in vivo. Our recent work has revealed a critical role for Zfp423 in maintaining the fate of white adipocytes in adult mice through suppression of the beige cell thermogenic gene program; loss of Zfp423 in mature adipocytes of adult mice results in a white-to-beige phenotypic switch. However, the exact requirements of Zfp423 in the fetal stages of early adipose development in vivo have not been clarified. METHOD: Here, we utilize two models that confer adipose-specific Zfp423 inactivation during fetal adipose development (Adiponectin-Cre; Zfp423loxP/loxP and Adiponectin-rtTA; TRE-Cre; Zfp423loxP/loxP). We assess the impact of fetal adipose Zfp423 deletion on the initial formation of adipose tissue and evaluate the metabolic consequences of challenging these animals with high-fat diet feeding. RESULTS: Deletion of Zfp423 during fetal adipose development results in a different phenotype than is observed when deleting Zfp423 in adipocytes of adult mice. Inactivation of Zfp423 during fetal adipose development results in arrested differentiation, specifically of inguinal white adipocytes, rather than a white-to-beige phenotypic switch that occurs when Zfp423 is inactivated in adult mice. This is likely explained by the observation that adiponectin driven Cre expression is active at an earlier stage of the adipocyte life cycle during fetal subcutaneous adipose development than in adult mice. Upon high-fat diet feeding, obese adipose Zfp423-deficient animals undergo a pathological adipose tissue expansion, associated with ectopic lipid deposition and systemic insulin resistance. CONCLUSIONS: Our results reveal that Zfp423 is essential for the terminal differentiation of subcutaneous white adipocytes during fetal adipose tissue development. Moreover, our data highlight the striking adverse effects of pathological subcutaneous adipose tissue remodeling on visceral adipose function and systemic nutrient homeostasis in obesity. Importantly, these data reveal the distinct phenotypes that can occur when adiponectin driven transgenes are activated in fetal vs. adult adipose tissue.