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Biggest organelle gets image update: surprising structure of cell's endoplasmic reticulum seen.

Textbook drawings of the largest organelle in most cells might need to be revised based on new images. Super-resolution shots of the endoplasmic reticulum reveal tightly packed tubes where previous pictures showed plain flat sheets, scientists report in the Oct. 28 Science.

The finding helps explain how the endoplasmic reticulum, or ER, reshapes itself in response to changing conditions, says study coauthor Jennifer Lippincott-Schwartz, a cell biologist at the Howard Hughes Medical Institute's Janelia Research Campus in Ashburn, Va.

A snaking network of membranes that stretches from the nucleus of the cell to its edge, the ER is a cellular jack-of-all-trades. It provides scaffolding for protein-producing ribosomes and makes sure those proteins are folded properly. It churns out lipids. And it stores and releases calcium, which sends messages within and between cells.

New super-resolution microscopy techniques reveal details just tens of nanometers wide, far smaller than what conventional microscopes can see. That resolution upgrade showed that apparently flat sheets of membranes actually consist of dense clusters of tubules.

Some past ER imaging required killing the cells, capturing their inner structure at just one moment in time, notes cell biologist Mark Terasaki of the University of Connecticut Health Center in Farmington. These new imaging techniques capture the motion of the ER in living cells, showing how the tubes rapidly vibrate and shift shapes.

Those tiny tubules come together in three-way junctions, linking into a mesh network that resembles a stretchy spider web. To move into a new part of the cell, the tubes can expand or contract. And the junctions can also slide up and down the tubes like curtains on a rod, the team found. The tubes are packed to different densities throughout the ER, perhaps reflecting the various jobs that different parts of the sprawling organelle take on.

Bona fide sheets--stacked like pancakes--were still found in the part of the ER closest to the cell's nucleus, a feature other scientists have also reported.

Endoplasmic reticulum stress or malfunction can contribute to neurodegenerative diseases such as Alzheimer's and Parkinson's. "For us to really understand disease, we need to understand what normal is," says study coauthor Craig Blackstone, a cell biologist at the National Institute of Neurological Disorders and Stroke in Bethesda, Md.

Caption: Super-resolution imaging of the endoplasmic reticulum shows a tangled web of interconnected tubes, not a network of flat sheets.


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Title Annotation:LIFE & EVOLUTION
Author:Hamers, Laurel
Publication:Science News
Geographic Code:1USA
Date:Nov 26, 2016
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