Biology subjects

Levy, D. L.

Publications and source records attributed to Levy, D. L..

4 recordsLinked to original sources

bioRxiv2019

Rtn4a promotes exocytosis in mammalian cells while ER morphology does not necessarily affect exocytosis and translation

ER tubules and sheets conventionally correspond to smooth and rough ER, respectively. The ratio of ER tubules-to-sheets varies in different cell types and changes in response to cellular conditions, potentially impacting the functional output of the ER. To directly test if ER morphology impacts ER function, we increased the tubule-to-sheet ratio by Rtn4a overexpression and monitored effects on protein translation and trafficking. While expression levels of several cell surface and secreted proteins were unchanged, their exocytosis was increased. Rtn4a depletion reduced cell surface trafficking without affecting ER morphology, and increasing the tubule-to-sheet ratio by other means did not affect trafficking. These data suggest that Rtn4a enhances exocytosis independently of changes in ER morphology. We demonstrate that Rtn4a enhances ER-to-Golgi trafficking and co-localizes with COPII vesicles. We propose that Rtn4a promotes COPII vesicle formation by inducing membrane curvature. Taken together, we show that altering ER morphology does not necessarily affect protein synthesis or trafficking, but that Rtn4a specifically enhances exocytosis.

cell biology↗

bioRxiv2019

Cytoplasmic volume and limiting nucleoplasmin scale nuclear size during Xenopus laevis development

How nuclear size is regulated relative to cell size is a fundamental cell biological question. Reductions in both cell and nuclear sizes during Xenopus laevis embryogenesis provide a robust scaling system to study mechanisms of nuclear size regulation. To test if the volume of embryonic cytoplasm is limiting for nuclear growth, we encapsulated gastrula stage embryonic cytoplasm and nuclei in droplets of defined volume using microfluidics. Nuclei grew and reached new steady-state sizes as a function of cytoplasmic volume, supporting a limiting component mechanism of nuclear size control. Through biochemical fractionation, we identified the histone chaperone nucleoplasmin (Npm2) as a putative nuclear size-scaling factor. Cellular amounts of Npm2 decrease over development, and nuclear size was sensitive to Npm2 levels both in vitro and in vivo, affecting nuclear histone levels and chromatin organization. Thus, reductions in cell volume with concomitant decreases in Npm2 amounts represent a developmental mechanism of nuclear size-scaling that may also be relevant to cancers with increased nuclear size.

cell biology↗

bioRxiv2019

The nucleoporin ELYS regulates nuclear size by controlling NPC number and nuclear import capacity

How intracellular organelles acquire their characteristic sizes is a fundamental cell biological question. Given the stereotypical changes in nuclear size in cancer, it is particularly important to understand the mechanisms that control nuclear size in human cells. Here we use a high-throughput imaging RNAi screen to identify and mechanistically characterize ELYS, a nucleoporin required for postmitotic nuclear pore complex (NPC) assembly, as a determinant of nuclear size in mammalian cells. We show that ELYS knockdown results in small nuclei, the accumulation of cytoplasmic lamin aggregates, reduced nuclear lamin B2 localization, lower NPC density, and decreased nuclear import. Increasing nuclear import by importin overexpression rescues nuclear size and lamin B2 import, while inhibiting importin /{beta} nuclear import decreases nuclear size. Conversely, ELYS overexpression leads to increased nuclear size, enrichment of nuclear lamin B2 staining at the nuclear periphery, and elevated NPC density and nuclear import. Consistent with these observations, knockdown or inhibition of exportin 1 increases nuclear size. In summary, we identify ELYS and NPC density as novel positive effectors of mammalian nuclear size and propose that nuclear size is controlled by nuclear import capacity.

cell biology↗

bioRxiv2019

Altering nuclear import in early Xenopus laevis embryos affects later development

More than just a container for DNA, the nucleus carries out a wide variety of critical and highly regulated cellular functions. One of these functions is nuclear import, and in this study we investigate how altering nuclear import impacts developmental progression and organismal size. During early Xenopus laevis embryogenesis, the timing of a key developmental event, the midblastula transition (MBT), is sensitive to nuclear import factor levels. How might altering nuclear import and MBT timing in the early embryo affect downstream development of the organism? We microinjected X.laevis two-cell embryos to increase levels of importin or NTF2, resulting in differential amounts of nuclear import factors in the two halves of the embryo. Compared to controls, these embryos exhibited delayed gastrulation, curved neural plates, and bent tadpoles with different sized eyes. Furthermore, embryos microinjected with NTF2 developed into smaller froglets compared to control microinjected embryos. We propose that altering nuclear import and size affects MBT timing, cell size, and cell number, subsequently disrupting later development. Thus, altering nuclear import early in development can affect function and size at the organismal level.

developmental biology↗