Sardi, J. | Search bioRxiv

bioRxiv2019

Mad dephosphorylation at the nuclear envelope is essential for asymmetric stem cell division

Stem cell niche signals act over a short range so that only stem cells but not the differentiating daughter cells receive the self-renewal signals. Drosophila female germline stem cells (GSCs) are maintained by short range BMP signaling; BMP ligands Dpp/Gbb activate receptor Tkv to phosphorylate Mad (phosphor-Mad or pMad) which accumulates in the GSC nucleus and activates the stem cell transcription program. pMad is highly concentrated in the nucleus of the GSC, but is immediately downregulated in the nucleus of the pre-cystoblast (preCB), a differentiating daughter cell, that is displaced away from the niche. Here we show that this asymmetry in the intensity of pMad is formed even before the completion of cytokinesis. A delay in establishing the pMad asymmetry leads to germline tumors through conversion of differentiating cells into a stem cell-like state. We show that a Mad phosphatase Dullard (Dd) interacts with Mad at the nuclear pore, where it may dephosphorylate Mad. A mathematical model explains how an asymmetry can be established in a common cytoplasm. It also demonstrates that the ratio of pMad concentrations in GSC/preCB is highly sensitive to Mad dephosphorylation rate. Our study reveals a previously unappreciated mechanism for breaking symmetry between daughter cells during asymmetric stem cell division.

developmental biology↗

bioRxiv2019

Analysis of the three dimensional structure of the kidney glomerulus capillary network

The capillary network of the kidney glomerulus filters small molecules from the blood. The glomerular 3D structure should help to understand its function, but it is poorly characterized. We therefore devised a new approach in which an automated tape collecting microtome(ATUM) was used to collect 0.5 micron thick serial sections from fixed mouse kidneys. The sections were imaged by scanning electron microscopy at [~]50 nm / pixel resolution. With this approach, 12 glomeruli were reconstructed at an x-y-z resolution [~]10x higher than that of paraffin sections. We found a no-cross zone between afferent and efferent branches on the vascular pole side; connections here could allow blood to exit without being adequately filtered. Network analysis indicates that the glomerular network does not form by repetitive longitudinal splitting of capillaries. It also suggests that capillaries vary their diameter to make flow more efficient. The shortest path (minimum number of branches to travel from afferent to efferent arterioles) is relatively independent of glomerular size and is present primarily on the vascular pole size. This suggests that the shortest path is established on the vascular pole side, after which new branches and longer paths form on the urinary pole side. Thus the 3D structure of the glomerular capillary network provides useful information with which to understand glomerular function. Other tissue structures in the body may benefit from this new three dimensional approach.

cell biology↗