PLD3 and PLD4 synthesize S,S-BMP, a key phospholipid enabling lipid degradation in lysosomes.

Bis(monoacylglycero)phosphate (BMP) is an abundant lysosomal phospholipid required for degradation of lipids, particularly gangliosides. Alterations in BMP levels are associated with neurodegenerative diseases. Unlike typical glycerophospholipids, lysosomal BMP has two chiral glycerol carbons in the S (rather than the R) stereo-conformation, protecting it from lysosomal degradation. How this unusual and yet crucial S,S-stereochemistry is achieved is unknown. Here, we report that phospholipases D3 and D4 (PLD3 and PLD4) synthesize lysosomal S,S-BMP, with either enzyme catalyzing the critical glycerol stereo-inversion reaction in vitro. Deletion of PLD3 or PLD4 markedly reduced BMP levels in cells or in murine tissues where either enzyme is highly expressed (brain for PLD3; spleen for PLD4), leading to gangliosidosis and lysosomal abnormalities. PLD3 mutants associated with neurodegenerative diseases, including risk of Alzheimer's disease, diminished PLD3 catalytic activity. We conclude that PLD3/4 enzymes synthesize lysosomal S,S-BMP, a crucial lipid for maintaining brain health.

Carboxy-terminal blockade of sortilin binding enhances progranulin gene therapy, a potential treatment for frontotemporal dementia.

Frontotemporal dementia is commonly caused by loss-of-function mutations in the progranulin gene. Potential therapies for this disorder have entered clinical trials, including progranulin gene therapy and drugs that reduce progranulin interactions with sortilin. Both approaches ameliorate functional and pathological abnormalities in mouse models of progranulin insufficiency. Here we investigated whether modifying the progranulin carboxy terminus to block sortilin interactions would improve the efficacy of progranulin gene therapy. We compared the effects of treating progranulin-deficient mice with gene therapy vectors expressing progranulin with intact sortilin interactions, progranulin with the carboxy terminus blocked to reduce sortilin interactions, or GFP control. We found that expressing carboxy-terminally blocked progranulin generated higher levels of progranulin both at the injection site and in more distant regions. Carboxy-terminally blocked progranulin was also more effective at ameliorating microgliosis, microglial lipofuscinosis, and lipid abnormalities including ganglioside accumulation and loss of bis(monoacylglycero)phosphate lipids. Finally, only carboxy-terminally blocked progranulin reduced plasma neurofilament light chain, a biomarker of neurodegeneration, in progranulin-deficient mice. These results demonstrate that modifying the progranulin cargo to block sortilin interactions may be important for increasing the effectiveness of progranulin gene therapy. One-sentence Summary: The effectiveness of progranulin gene therapy in models of FTD is improved by blocking the protein's carboxy terminus, which prevents sortilin binding.

Transcriptional determinants of lipid mobilization in human adipocytes.

Defects in adipocyte lipolysis drive multiple aspects of cardiometabolic disease, but the transcriptional framework controlling this process has not been established. To address this, we performed a targeted perturbation screen in primary human adipocytes. Our analyses identified 37 transcriptional regulators of lipid mobilization, which we classified as (i) transcription factors, (ii) histone chaperones, and (iii) mRNA processing proteins. On the basis of its strong relationship with multiple readouts of lipolysis in patient samples, we performed mechanistic studies on one hit, ZNF189, which encodes the zinc finger protein 189. Using mass spectrometry and chromatin profiling techniques, we show that ZNF189 interacts with the tripartite motif family member TRIM28 and represses the transcription of an adipocyte-specific isoform of phosphodiesterase 1B (PDE1B2). The regulation of lipid mobilization by ZNF189 requires PDE1B2, and the overexpression of PDE1B2 is sufficient to attenuate hormone-stimulated lipolysis. Thus, our work identifies the ZNF189-PDE1B2 axis as a determinant of human adipocyte lipolysis and highlights a link between chromatin architecture and lipid mobilization.

Mice lacking triglyceride synthesis enzymes in adipose tissue are resistant to diet-induced obesity.

Triglycerides (TGs) in adipocytes provide the major stores of metabolic energy in the body. Optimal amounts of TG stores are desirable as insufficient capacity to store TG, as in lipodystrophy, or exceeding the capacity for storage, as in obesity, results in metabolic disease. We hypothesized that mice lacking TG storage in adipocytes would result in excess TG storage in cell types other than adipocytes and severe lipotoxicity accompanied by metabolic disease. To test this hypothesis, we selectively deleted both TG synthesis enzymes, DGAT1 and DGAT2, in adipocytes (ADGAT DKO mice). As expected with depleted energy stores, ADGAT DKO mice did not tolerate fasting well and, with prolonged fasting, entered torpor. However, ADGAT DKO mice were unexpectedly otherwise metabolically healthy and did not accumulate TGs ectopically or develop associated metabolic perturbations, even when fed a high-fat diet. The favorable metabolic phenotype resulted from activation of energy expenditure, in part via BAT (brown adipose tissue) activation and beiging of white adipose tissue. Thus, the ADGAT DKO mice provide a fascinating new model to study the coupling of metabolic energy storage to energy expenditure.

Deficiency of the frontotemporal dementia gene GRN results in gangliosidosis.

Haploinsufficiency of GRN causes frontotemporal dementia (FTD). The GRN locus produces progranulin (PGRN), which is cleaved to lysosomal granulin polypeptides. The function of lysosomal granulins and why their absence causes neurodegeneration are unclear. Here we discover that PGRN-deficient human cells and murine brains, as well as human frontal lobes from GRN-mutation FTD patients have increased levels of gangliosides, glycosphingolipids that contain sialic acid. In these cells and tissues, levels of lysosomal enzymes that catabolize gangliosides were normal, but levels of bis(monoacylglycero)phosphates (BMP), lipids required for ganglioside catabolism, were reduced with PGRN deficiency. Our findings indicate that granulins are required to maintain BMP levels to support ganglioside catabolism, and that PGRN deficiency in lysosomes leads to gangliosidosis. Lysosomal ganglioside accumulation may contribute to neuroinflammation and neurodegeneration susceptibility observed in FTD due to PGRN deficiency and other neurodegenerative diseases.