Hematopoietic stem cells (HSCs), which sustain production of all blood cell lineages,1 rely on glycolysis for ATP production,2,3 yet little attention has been paid to the role of mitochondria. We show here that the short isoform of a critical regulator of HSCs, Prdm16,4,5 induces mitofusin 2 (Mfn2), a protein involved in mitochondrial fusion and in tethering of mitochondria to the endoplasmic reticulum (ER). Overexpression and deletion studies, including single cell transplantation assays, revealed that Mfn2 is specifically required for the maintenance of HSCs with extensive lymphoid potential, but not, or less so for the maintenance of myeloid-dominant HSCs. Mfn2 increased buffering of Cai2+, an effect mediated through its ER-mitochondria tethering activity,6,7 thereby negatively regulating nuclear translocation and transcriptional activity of Nuclear Factor of Activated T cells (NFAT). NFAT inhibition rescued the effects of Mfn2 deletion in HSCs, demonstrating that negative regulation of NFAT is the prime downstream mechanism of Mfn2 in the maintenance of HSCs with extensive lymphoid potential. Mitochondria therefore play an important role in HSCs. These findings provide a mechanism underlying clonal heterogeneity among HSCs8–11 and may lead to the design of approaches to bias HSC differentiation into desired lineages after transplantation.

Hematopoietic stem cells (HSCs) produce most cellular energy through glycolysis rather than through mitochondrial respiration. Consistent with this notion, mitochondrial mass has been reported to be low in HSCs. However, we found that staining with mitotracker green, a commonly used dye to measure mitochondrial content, leads to artifactually low fluorescence specifically in HSCs because of dye efflux. Using mtDNA quantification, enumeration of mitochondrial nucleoids and fluorescence intensity of a genetically encoded mitochondrial reporter we unequivocally show here that HSCs and multipotential progenitors (MPPs) have higher mitochondrial mass than lineage-committed progenitors and mature cells. Despite similar mitochondrial mass, respiratory capacity of MPPs exceeds that of HSCs. Furthermore, although elevated mitophagy has been invoked to explain low mitochondrial mass in HSCs, we observed that mitochondrial turnover capacity is comparatively low in HSCs. We propose that the role of mitochondria in HSC biology may have to be revisited in light of these findings.

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Diploid (2N) and triploid (3N) sibling post-smolts were divided between six sea pens and fed: a standard nutrient package diet (2 x 2N SP, 2 x 3N SP), or an iso-energetic nutrient boosted package (2 x 3N BP) until market size. 3N groups initially grew significantly faster than 2N, and by harvest, 3N BP weighed significantly more (3210 ± 87g) than 2N SP or 3N SP (3007± 64g; 2965 ± 88g), while there was no significant difference in weight between ploidy in SP diet. Higher visible vertebral (9.6 ± 0.4%) and jaw deformities (10.6 ± 1.2%) were observed in 3N compared to 2N (0.9 ± 0.1%; 1.3 ± 0.5%).However, x-ray radiography revealed that 3N BP and 2N SP had comparable levels of severely affected individuals to that at time of sea transfer, while 3N SP showed a 3 fold increase in the severity of malformed individuals. The tail region (R3) in 3N SP fish had both the lowest vertebral strength and stiffness and the highest number of deformed vertebrae. Fillet quality attributes were comparable between diet and ploidy. These findings showed that triploid growth rate can be sustained until harvest throughout the seawater phase by using a nutrient boosted diet, and furthermore, the progression of spinal deformity beyond that at sea transfer can be stabilised by increasing dietary P during the marine phase. The study generates new data on dietary requirements of triploid Atlantic salmon that provides a significant improvement in farmed triploid welfare, allowing exploitation of faster growth rates and highlights the need to develop "triploid specific" aquafeeds rather than the use of conventional diploid diets.We look forward to receiving your feedback. Cover Letter  Triploid Atlantic salmon growth rate can be sustained during marine rearing using nutrient boosted diets  Progression of skeletal malformation development can be prevented during marine rearing of triploid Atlantic salmon by increasing dietary phosphorous  The occurrence of skeletal malformation in triploid Atlantic salmon must be addressed during freshwater rearing in the first instance Highlights (for review)This study demonstrates that triploid Atlantic salmon have higher dietary requirements than their diploid siblings and that supplementing dietary phosphorous can prevent further progression of deformity during marine rearing. Tailored triploid specific aquafeeds must be formulated to support growth and prevent deformity in order to minimise welfare implications and allow exploitation of faster growth potential of triploid salmon within industry. were observed in 3N compared to 2N (0.9 ± 0.1%; 1.3 ± 0.5%). However, x-ray radiography 7 revealed that 3N BP and 2N SP had comparable levels of severely affected individuals to that comparable between diet and ploidy. These findings showed that triploid growth rate can be 12 sustained until harvest throughout the seawater phase by using a nutrient boosted diet, and 13 furthermore, the progression of spinal deformity beyond that at sea transfer can be stabilised 14 by increasing dietary P during the m...

PRDM16 is a transcriptional coregulator involved in translocations in acute myeloblastic leukemia (AML), myelodysplastic syndromes, and T acute lymphoblastic leukemia that is highly expressed in and required for the maintenance of hematopoietic stem cells (HSCs), and can be aberrantly expressed in AML. Prdm16 is expressed as full-length (fPrdm16) and short (sPrdm16) isoforms, the latter lacking the N-terminal PR domain. The role of both isoforms in normal and malignant hematopoiesis is unclear. We show here that fPrdm16 was critical for HSC maintenance, induced multiple genes involved in GTPase signaling, and repressed inflammation, while sPrdm16 supported B cell development biased toward marginal zone B cells and induced an inflammatory signature. In a mouse model of human MLL-AF9 leukemia, fPrdm16 extended latency, while sPrdm16 shortened latency and induced a strong inflammatory signature, including several cytokines and chemokines that are associated with myelodysplasia and with a worse prognosis in human AML. Finally, in human NPM1-mutant and in MLL-translocated AML, high expression of PRDM16, which negatively impacts outcome, was associated with inflammatory gene expression, thus corroborating the mouse data. Our observations demonstrate distinct roles for Prdm16 isoforms in normal HSCs and AML, and identify sPrdm16 as one of the drivers of prognostically adverse inflammation in leukemia.