The actin cytoskeleton during adipocyte formation

In addition to providing heat insulation and mechanical cushioning, adipose tissue regulates overall metabolic homeostasis and serves as an essential energy storage site. Excess adipose tissue, or obesity, is on the rise in the US among all demographics. The expansion of adipose tissue results from both adipocyte hypertrophy and hyperplasia but the mechanisms that regulate these processes are not fully understood. Destabilizing actin has been shown to promote adipogenesis while actin stabilization inhibits this process. In addition, decreased actin synthesis is known to occur. However, these studies examined total actin and did not consider that actin is in fact a family of functionally diverse isoforms and that individual isoforms may have different functions in adipogenesis. I hypothesized that actin isoforms contribute differently to adipogenic actin reorganization. To measure this, I developed a novel fractionation method that allowed for the reliable quantification of actin polymerization. I used this actin fractionation method to identify an early loss in polymerized α-smooth muscle actin (α-SMA) relative to polymerized β-actin and γ-actin and to also rule out a role for the actin severing protein gelsolin in the loss of polymerized actin. Furthermore, I showed that the loss of α-SMA expression precedes the loss of β-actin and γ-actin expression. A known regulator of actin cytoskeleton genes is the transcription factor serum response factor (SRF) and its co-activator, myocardin related transcription factor (MRTF). I identified a role for MRTF/SRF in the downregulation of actin expression during adipogenesis, particularly α-SMA. There was an additional cAMP-responsive decrease in α-SMA expression during the initiation of adipogenesis by exposure to established inducers. Overall, my findings are consistent with growing evidence suggesting that genetic markers of smooth muscle cells, including α-SMA, help control adipogenic commitment. Understanding these early stages of adipogenesis could open new therapeutic avenues for obesity and its co-morbidities

Cardiolipin synthesis in brown and beige fat mitochondria is essential for systemic energy homeostasis

Activation of energy expenditure in thermogenic fat is a promising strategy to improve metabolic health, yet the dynamic processes that evoke this response are poorly understood. Here we show that synthesis of the mitochondrial phospholipid cardiolipin is indispensable for stimulating and sustaining thermogenic fat function. Cardiolipin biosynthesis is robustly induced in brown and beige adipose upon cold exposure. Mimicking this response through overexpression of cardiolipin synthase (Crls1) enhances energy consumption in mouse and human adipocytes. Crls1 deficiency in thermogenic adipocytes diminishes inducible mitochondrial uncoupling and elicits a nuclear transcriptional response through endoplasmic reticulum stress-mediated retrograde communication. Cardiolipin depletion in brown and beige fat abolishes adipose thermogenesis and glucose uptake, which renders animals insulin resistant. We further identify a rare human CRLS1 variant associated with insulin resistance and show that adipose CRLS1 levels positively correlate with insulin sensitivity. Thus, adipose cardiolipin has a powerful impact on organismal energy homeostasis through thermogenic fat bioenergetics

Mosaic fungal individuals have the potential to evolve within a single generation

Although cells of mushroom-producing fungi typically contain paired haploid nuclei (n + n), most Armillaria gallica vegetative cells are uninucleate. As vegetative nuclei are produced by fusions of paired haploid nuclei, they are thought to be diploid (2n). Here we report finding haploid vegetative nuclei in A. gallica at multiple sites in southeastern Massachusetts, USA. Sequencing multiple clones of a single-copy gene isolated from single hyphal filaments revealed nuclear heterogeneity both among and within hyphae. Cytoplasmic bridges connected hyphae in field-collected and cultured samples, and we propose nuclear migration through bridges maintains this nuclear heterogeneity. Growth studies demonstrate among- and within-hypha phenotypic variation for growth in response to gallic acid, a plant-produced antifungal compound. The existence of both genetic and phenotypic variation within vegetative hyphae suggests that fungal individuals have the potential to evolve within a single generation in response to environmental variation over time and space

Cardiolipin synthesis in brown and beige fat mitochondria is essential for systemic energy homeostasis.

Activation of energy expenditure in thermogenic fat is a promising strategy to improve metabolic health, yet the dynamic processes that evoke this response are poorly understood. Here we show that synthesis of the mitochondrial phospholipid cardiolipin is indispensable for stimulating and sustaining thermogenic fat function. Cardiolipin biosynthesis is robustly induced in brown and beige adipose upon cold exposure. Mimicking this response through overexpression of cardiolipin synthase (Crls1) enhances energy consumption in mouse and human adipocytes. Crls1 deficiency in thermogenic adipocytes diminishes inducible mitochondrial uncoupling and elicits a nuclear transcriptional response through endoplasmic reticulum stress-mediated retrograde communication. Cardiolipin depletion in brown and beige fat abolishes adipose thermogenesis and glucose uptake, which renders animals insulin resistant. We further identify a rare human CRLS1 variant associated with insulin resistance and show that adipose CRLS1 levels positively correlate with insulin sensitivity. Thus, adipose cardiolipin has a powerful impact on organismal energy homeostasis through thermogenic fat bioenergetics