Journey to the Center of the Cell

The animal cell nucleus houses the genetic material of the organism and therefore protects and maintains the blueprint for the cell and all its progeny. However, the nucleus is more than a simple repository for chromosomes. A dynamic organelle, the nucleus goes through astonishing transformations during each cell cycle, breaking down completely during mitosis and reforming afresh in each daughter cell after cell division. Within the nucleus, chromosomes are replicated and their DNA is transcribed to provide information that programs the physiology of the cell. Also, ribosomes assemble in the nucleus, then leave and carry out protein translation in the cytoplasm. All of this activity requires complex machineries that can respond to the changing needs of the cell throughout the cell cycle and may vary during development and by cell type.
One of the defining features of the nucleus is its unique architecture. The nucleus is bounded by a nuclear envelope, a double layer of membranes punctuated by nuclear pores, which allow the passage of a huge variety of molecules into and out of the nucleus. The nuclear envelope is contiguous with the endoplasmic reticulum, a membranous labyrinth that provides the entry portal to the secretory pathway. The nuclear envelope is lined with a protein meshwork known as the nuclear lamina, which is composed of lamin proteins related to cytoplasmic intermediate filament proteins. Defects in the nuclear lamina have been linked to progeric diseases.

In this issue, three Reviews examine various aspects of nuclear organization and dynamics. Trinkle-Mulcahy and Lamond (p. 1402) describe how a combination of state-of-the-art proteomics and imaging technologies is contributing to a greater understanding of the dynamics of nuclear function in living cells in real time. Stewart et al. (p. 1408) describe how the nuclear envelope influences events within the nucleus and throughout the cell. Terry et al. (p. 1412) explain how the bidirectional transport of proteins and nucleic acids through nuclear pores can be regulated at many levels, from individual cargoes to global changes in nuclear pore transport characteristics.

In a News story, Travis (p. 1400) describes how a few researchers are making the case that proteins from the nucleus do double duty as components of a spindle matrix, a controversial structure alleged to help move chromosomes during cell division.

Two Perspectives at Science's Signal Transduction Knowledge Environment (http://www.sciencemag.org/sciext/subnucleus/) highlight mechanisms controlling nuclear transport. Alvarez-Gonzalez describes how poly(ADP-ribosyl)ation of p53 in response to DNA damage interferes with its nuclear export, and Swanson and Kopchick describe how the cytokine receptor GHR may traffic to the nucleus to regulate gene expression.

In the future, it will be important to continue to advance our understanding of how the nucleus interacts with the rest of the cytoplasm throughout the cell cycle and how these interactions program organismal physiology and development.




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CREDIT: PAM ENGEBRETSON AND JEFF MOORE (MONTAGE)




Grand Tour
Traveling out to the farthest reaches of the solar system, to Pluto and the Kuiper belt where it will arrive in 2015, the New Horizons probe has to endure a long and mostly uneventful journey. But luckily there are some spectacular sights along the way. On 28 February 2007, New Horizons flew past Jupiter, where it used the gas giant's gravity to slingshot it to even greater speeds and also test its instruments in flight. New Horizons' transit took it to unvisited areas of the planet's spacescape. The papers in this special issue record how the probe witnessed lightning and aurorae in Jupiter's atmosphere, volcanic eruptions on the moon Io, and the pulsing of Jupiter's magnetosphere, a cocoon of charged particles that swathes the entire system.
On Earth, although seen planetwide, the most powerful thunderstorms concentrate near the equator and in the tropics. Not so on Jupiter. Lightning flashed near both poles as well as elsewhere, suggesting that convective electrical storms bubble up everywhere in Jupiter's atmosphere because of global heat imbalances. Nighttime auroral glows, on the other hand, were not as widespread as expected.

Skirting the giant planet, New Horizons also flew by Jupiter's rings and attendant moons, big and small. Surprisingly, no moonlets smaller than a kilometer in size were seen in Jupiter's faint rings, a puzzle if they are built from the debris of shattered moons. Rubble also clumps together in locations favored by gravity resonances with larger moons.

An eruption of the Tvashtar volcano on the satellite Io was caught in the act, allowing the mechanics of the sulfurous plume and the lava temperature to be measured. Pollution from Io's volcanoes has even reached the shores of Europa, an icy moon that may harbor oceans beneath its frozen surface. Io's volcanic emissions feed extra sulfur and oxygen ions into a vast particle cloud that circles the entire Jupiter system, held in place by the planet's strong magnetic field. Behind the planet, it is pulled into a magnetic shadow billions of kilometers long, streaming away from the Sun as the solar wind deflects around Jupiter. Acting like a giant pipe, this magnetic tail drains half a metric ton of charged particles out of the jovian system each second. New Horizons' route took it down the magnetotail, to regions unexplored by earlier Galileo or Voyager missions (see the Perspective on p. 216). Pulses of energetic particles flow along the tail in synchrony with Jupiter's 10-hour rotation rate and also every few days as plasma blobs are fed down the tube.

With Pluto still in its sights, New Horizons' snapshots show that Jupiter inhabits an active landscape, experiencing storms, the pumping of the magnetosphere, and volcanic ash falls. A pity then that it is the last time we will visit Jupiter until the Juno mission in 2016. So sit back, enjoy these views, and think of New Horizons as it races along the solar system's back roads to an even stranger destination.




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