Asymmetrically Segregated Mitochondria Provide Cellular Memory of Hematopoietic Stem Cell Replicative History and Drive HSC Attrition.

The metabolic requirements of hematopoietic stem cells (HSCs) change with their cell cycle activity. However, the underlying role of mitochondria remains ill-defined. Here we found that, after mitochondrial activation with replication, HSCs irreversibly remodel the mitochondrial network and that this network is not repaired after HSC re-entry into quiescence, contrary to hematopoietic progenitors. HSCs keep and accumulate dysfunctional mitochondria through asymmetric segregation during active division. Mechanistically, mitochondria aggregate and depolarize after stress because of loss of activity of the mitochondrial fission regulator Drp1 onto mitochondria. Genetic and pharmacological studies indicate that inactivation of Drp1 causes loss of HSC regenerative potential while maintaining HSC quiescence. Molecularly, HSCs carrying dysfunctional mitochondria can re-enter quiescence but fail to synchronize the transcriptional control of core cell cycle and metabolic components in subsequent division. Thus, loss of fidelity of mitochondrial morphology and segregation is one type of HSC divisional memory and drives HSC attrition.

Genetic, Inflammatory, and Epithelial Cell Differentiation Factors Control Expression of Human Calpain-14.

Eosinophilic esophagitis (EoE) is a chronic, food-driven allergic disease resulting in eosinophilic esophageal inflammation. We recently found that EoE susceptibility is associated with genetic variants in the promoter of CAPN14, a gene with reported esophagus-specific expression. CAPN14 is dynamically up-regulated as a function of EoE disease activity and after exposure of epithelial cells to interleukin-13 (IL-13). Herein, we aimed to explore molecular modulation of CAPN14 expression. We identified three putative binding sites for the IL-13-activated transcription factor STAT6 in the promoter and first intron of CAPN14 Luciferase reporter assays revealed that the two most distal STAT6 elements were required for the ∼10-fold increase in promoter activity subsequent to stimulation with IL-13 or IL-4, and also for the genotype-dependent reduction in IL-13-induced promoter activity. One of the STAT6 elements in the promoter was necessary for IL-13-mediated induction of CAPN14 promoter activity while the other STAT6 promoter element was necessary for full induction. Chromatin immunoprecipitation in IL-13 stimulated esophageal epithelial cells was used to further support STAT6 binding to the promoter of CAPN14 at these STAT6 binding sites. The highest CAPN14 and calpain-14 expression occurred with IL-13 or IL-4 stimulation of esophageal epithelial cells under culture conditions that allow the cells to differentiate into a stratified epithelium. This work corroborates a candidate molecular mechanism for EoE disease etiology in which the risk variant at 2p23 dampens CAPN14 expression in differentiated esophageal epithelial cells following IL-13/STAT6 induction of CAPN14 promoter activity.