Robin Elaine Duncan

Barth Syndrome (BTHS) is a debilitating X‐linked genetic disorder caused by mutations in the gene encoding TAFAZZIN, an enzyme responsible for the remodeling of cardiolipin. While cyclic neutropenia is a well‐recognized immunological feature of this disease, emerging evidence suggests that lymphopenia may also occur. The objective of this study was to examine the effects of cannabidiol (CBD) on growth, cardiolipin content, and mitochondrial abnormalities in BTHS patient‐derived B‐lymphoblastoid cells. CBD (1 μM) restored the growth of BTHS B‐lymphoblastoids to healthy control levels, but did not alter cell cycle distribution or sub‐G1 cell populations, which surprisingly also did not differ from healthy control B‐lymphoblastoids. CBD treatment also fully restored the total cellular cardiolipin concentration and reversed the elevation in monolysocardiolipin/cardiolipin ratio in BTHS B‐lymphoblastoids to healthy cell levels, but did not restore the cardiolipin fatty acyl composition. Assessment of mitochondrial markers suggested that increased cardiolipin did not result from increased mitochondrial content. This improvement in cardiolipin concentration was associated with a significant increase in the maximal coupled state III respiration of BTHS B‐lymphoblastoids, with all five tested BTHS donors exhibiting increased mitochondrial membrane potential following CBD treatment. CBD fully reversed the deficit in succinate dehydrogenase subunit A in BTHS cells, and partially reversed deficits in cytochrome c oxidase subunits I and IV, and partially restored supercomplex I/III ₂ levels, but did not rescue I/III ₂ /IV levels. This work suggested a potential role for CBD as a therapeutic in BTHS B‐lymphopenia that merits further investigation.

Adipose tissue stores triacylglycerol (TAG) in lipid droplets (LD) and release fatty acids upon lipolysis during energy shortage. We identify ApoL6 as a LD-associated protein mainly found in adipose tissue, specifically in adipocytes. ApoL6 expression is low during fasting but induced upon feeding. ApoL6 knockdown results in smaller LD with lower TAG content in adipocytes, while ApoL6 overexpression causes larger LD with higher TAG content. We show that the ApoL6 affects adipocytes through inhibition of lipolysis. While ApoL6, Perilipin 1 (Plin1), and HSL can form a complex on LD, C-terminal ApoL6 directly interacts with N-terminal Plin1 to prevent Plin1 binding to HSL, to inhibit lipolysis. Thus, ApoL6 ablation decreases white adipose tissue mass, protecting mice from diet-induced obesity, while ApoL6 overexpression in adipose brings obesity and insulin resistance, making ApoL6 a potential future target against obesity and diabetes.

Barth syndrome (BTHS) is caused by mutations in tafazzin resulting in deficits in cardiolipin remodeling that alter major metabolic processes. The tafazzin gene is encoded on the X chromosome, and therefore BTHS primarily affects males. Female carriers are typically considered asymptomatic, but age-related changes have been reported in female carriers of other X-linked disorders. Therefore, we examined the phenotype of female mice heterozygous for deletion of the tafazzin gene (Taz-HET) at 3 and 12 months of age. Food intakes, body masses, lean tissue and adipose depot weights, daily activity levels, metabolic measures, and exercise capacity were assessed. Age-related changes in mice resulted in small but significant genotype-specific differences in Taz-HET mice compared with their female Wt littermates. By 12 months, Taz-HET mice weighed less than Wt controls and had smaller gonadal, retroperitoneal, and brown adipose depots and liver and brain masses, despite similar food consumption. Daily movement, respiratory exchange ratio, and total energy expenditure did not vary significantly between the age-matched genotypes. Taz-HET mice displayed improved glucose tolerance and insulin sensitivity at 12 months compared with their Wt littermates but had evidence of slightly reduced exercise capacity. Tafazzin mRNA levels were significantly reduced in the cardiac muscle of 12-month-old Taz-HET mice, which was associated with minor but significant alterations in the heart cardiolipin profile. This work is the first to report the characterization of a model of female carriers of heterozygous tafazzin deficiency and suggests that additional study, particularly with advancing age, is warranted.

Phosphatidylethanolamine (PE) is a major glycerophospholipid (PL) in cellular and organellar membranes, and dysregulation of PE synthesis has been associated with energy over-storage and insulin resistance.¹ Due to the small size of its ethanolamine headgroup relative to the volume occupied by its long and, in particular, unsaturated fatty acyl side chains (which impart a “kink” to the molecule), PE forms a conical shape.² This molecular structure is key to sculpting the curvature of the inner layer of cellular and organellar membranes, where PE is quantitatively the most abundant type of PL.³

Barth syndrome (BTHS) is an X-linked mitochondrial disease caused by mutations in the gene encoding for tafazzin (TAZ), a key enzyme in the remodeling of cardiolipin. Mice with a germline deficiency in Taz have been generated (Taz-KO) but not yet fully characterized. We performed physiological assessments of 3-, 6-, and 12-month-old male Taz-KO mice, including measures of perinatal survival, growth, lifespan, gross anatomy, whole-body energy and substrate metabolism, glucose homeostasis, and exercise capacity. Taz-KO mice displayed reduced viability, with lower-than-expected numbers of mice recorded at 4 weeks of age, and a shortened lifespan due to disease progression. At all ages, Taz-KO mice had lower body weights compared with wild-type (Wt) littermates despite similar absolute food intakes. This finding was attributed to reduced adiposity and diminutive organs and tissues, including heart and skeletal muscles. Although there were no differences in basal levels of locomotion between age-matched genotypes, indirect calorimetry studies showed higher energy expenditure measures and respiratory exchange ratios in Taz-KO mice. At the youngest age, Taz-KO mice had comparable glucose tolerance and insulin action to Wt mice, but while these measures indicated metabolic impairments in Wt mice with advancing age that were likely associated with increasing adiposity, Taz-KO mice were protected. Comparisons across the three age-cohorts revealed a significant and more severe deterioration of exercise capacity in Taz-KO mice than in their Wt littermate controls. The Taz-KO mouse model faithfully recapitulates important aspects of BTHS, and thus provides an important new tool to investigate pathophysiological mechanisms and potential therapies.

Lysophosphatidic acid acyltransferases/acylglycerophosphate acyltransferases (LPAATs/AGPATs) are a group of homologous enzymes that catalyze the formation of phosphatidic acid (PA) from lysophosphatidic acid. We have previously reported that LPAATδ/AGPAT4 localizes to mitochondria, suggesting a potential role in energy metabolism. However, in prior studies of young Lpaatδ-deficient mice (age 9–12 weeks old), we found no differences in body weights, food intakes, activity levels, respiratory gas exchange, or energy expenditure compared to their wildtype (Wt) littermates. To test whether Lpaatδ^−/− mice may develop differences in metabolic measures with advancing age, we recorded body weights and food intakes, and used metabolic chambers to assess ambulatory and locomotor activity levels, oxygen consumption (VO₂), carbon dioxide production (VCO₂), respiratory exchange ratio (RER), and total energy expenditure (heat). Fourteen-month-old Lpaatδ^−/− mice had significantly lower mean body weights compared to Wt littermate controls (44.6 ± 1.08 g vs. 53.5 ± 0.42 g, respectively), but no significant differences in food intake or activity levels. This phenotypic difference was accompanied by significantly elevated 24 h daily, and 12 h light and dark photoperiod average VO₂ (~20% higher) and VCO₂ (~30% higher) measures, as well as higher RER and total energy expenditure (heat) values compared to Wt control littermates. Thus, an age-related metabolic phenotype is evident in Lpaatδ^−/− mice. Future studies should examine the role of the lipid-modifying enzyme LPAATδ across the lifespan for greater insight into its role in normal and pathophysiology.