ER-diffusion barriers exist in hematopoietic stem cell mitoses, but are not required for lysosomal asymmetric cell division

Hematopoietic stem cells (HSCs) maintain the blood system by balancing self-renewal versus mature blood cell generation. One mechanism contributing to this balance is asymmetric cell division (ACD), which relies on tightly regulated intracellular compartmentalization. In other cell types, endoplasmic reticulum diffusion barriers (ER-DBs) contribute to the targeted distribution of cellular components and cell fate regulators during ACDs. Here, we identified ER-DBs as a feature of a subset of HSC divisions. Using fluorescence recovery after photobleaching (FRAP) and time-lapse confocal microscopy, we observe ER-DBs in around 30% of mitotic HSCs. These ER-DBs are significantly weakened by fingolimod, a potent inhibitor of sphingosine-1-phosphate receptor and ceramide synthesis, implicating sphingolipid metabolism in their regulation. We found that strong ER-DBs are not required for the asymmetric inheritance of lysosomes during HSC ACD. This demonstrates that ER-diffusion barriers are present and regulated during HSC division and are an additional mechanism orchestrating molecular polarization and asymmetric inheritance in HSC divisions, independently of the mechanism regulating lysosomal asymmetry.