Mutant huntingtin (mHTT), the causative protein in Huntington’s disease (HD), associates with the translocase of mitochondrial inner membrane 23 (TIM23) complex, resulting in inhibition of synaptic mitochondrial protein import first detected in presymptomatic HD mice. The early timing of this event suggests that it is a relevant and direct pathophysiologic consequence of mHTT expression. We show that, of the 4 TIM23 complex proteins, mHTT specifically binds to the TIM23 subunit and that full-length wild-type huntingtin (wtHTT) and mHTT reside in the mitochondrial intermembrane space. We investigated differences in mitochondrial proteome between wtHTT and mHTT cells and found numerous proteomic disparities between mHTT and wtHTT mitochondria. We validated these data by quantitative immunoblotting in striatal cell lines and human HD brain tissue. The level of soluble matrix mitochondrial proteins imported through the TIM23 complex is lower in mHTT-expressing cell lines and brain tissues of HD patients compared with controls. In mHTT-expressing cell lines, membrane-bound TIM23-imported proteins have lower intramitochondrial levels, whereas inner membrane multispan proteins that are imported via the TIM22 pathway and proteins integrated into the outer membrane generally remain unchanged. In summary, we show that, in mitochondria, huntingtin is located in the intermembrane space, that mHTT binds with high-affinity to TIM23, and that mitochondria from mHTT-expressing cells and brain tissues of HD patients have reduced levels of nuclearly encoded proteins imported through TIM23. These data demonstrate the mechanism and biological significance of mHTT-mediated inhibition of mitochondrial protein import, a mechanism likely broadly relevant to other neurodegenerative diseases.

Objectives EZH2 regulates B cell development and differentiation. We previously demonstrated increased EZH2 expression in peripheral blood mononuclear cells from lupus patients. The goal of this study was to evaluate the role of EZH2 expression in B cells in the pathogenesis of lupus. Methods We generated an MRL/lpr mouse with floxed Ezh2, which was crossed with CD19‐Cre mice to examine the effect of B cell EZH2 deficiency in MRL/lpr lupus‐prone mice. Differentiation of B cells was assessed using flow cytometry. Single‐cell RNA sequencing and single‐cell B cell receptor sequencing were performed. In vitro B cell culture with an X‐box binding protein 1 (XBP1) inhibitor was performed. EZH2 and XBP1 messenger RNA levels in CD19+ B cells isolated from lupus patients and healthy controls were analyzed. Results We show that Ezh2 deletion in B cells significantly decreased autoantibody production and improved glomerulonephritis. B cell development was altered in the bone marrow and spleen of EZH2‐deficient mice. Differentiation of germinal center B cells and plasmablasts was impaired. Single‐cell RNA sequencing showed that XBP1, a key transcription factor in B cell development, is down‐regulated in the absence of EZH2. Inhibiting XBP1 in vitro impairs plasmablast development similar to EZH2 deficiency in mice. Single‐cell B cell receptor RNA sequencing revealed defective immunoglobulin class‐switch recombination in EZH2‐deficient mice. In human lupus B cells, we observed a strong correlation between EZH2 and XBP1 messenger RNA expression levels. Conclusion EZH2 overexpression in B cells contributes to disease pathogenesis in lupus.

Enhancer of zeste homolog 2 (EZH2) has been shown to regulate early B cell development and the differentiation of antibody secreting cells (ASCs). We have previously demonstrated increased EZH2 expression in peripheral blood mononuclear cells isolated from lupus patients, and that pharmacological inhibition of EZH2 alleviates lupus-like disease in mouse models. In this study, we generated a conditional knockout mouse to examine the effect of EZH2 deficiency in B cells in the MRL/lpr lupus-prone mouse. We show that Ezh2 deletion in B cells significantly decreased autoantibody production and improved glomerulonephritis. B cell development was altered in the bone marrow and spleen in EZH2-deficient mice. Differentiation of ASCs was impaired. Single cell RNA sequencing showed that XBP1, a key transcription factor in B cell development, is downregulated in the absence of EZH2. Inhibiting XBP1 in vitro impairs ASC development similar to EZH2-deficient mice. Single cell B cell receptor RNA sequencing revealed defective immunoglobulin class switch recombination in EZH2- deficient mice. In human lupus B cells, we observed a strong correlation between EZH2 and XBP1 mRNA expression levels. Taken together, our results suggest that EZH2 overexpression in B cells contributes to disease pathogenesis in lupus.