(mentions CMT) New clues about mitochondrial 'growth spurts'

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New clues about mitochondrial 'growth spurts'

http://www.brightsurf.com/news/headlines/43449/New_clues_about_mitocho

ndrial_growth_spurts.html

Mitochondria are restless, continually merging and splitting. But

contrary to conventional wisdom, the size of these organelles depends

on more than fusion and fission, as Berman et al. show. Mitochondrial

growth and degradation are also part of the equation.

The study will appear online March 2, 2009 (www.jcb.org) and in the

March 9, 2009 print issue of The Journal of Cell Biology (JCB).

Fission is necessary to produce new mitochondria, such as those that

power synaptic activity in healthy neurons. Fusion is also important.

It goes awry in one form of Charcot-Marie-Tooth disease, in which

peripheral nerves deteriorate, and in other neurodegenerative

diseases. How cells manage mitochondrial size and number remains

unclear.

Berman et al. found a clue when they started refining measurements of

mitochondrial dynamics. The team labeled the organelles with a red

fluorescent protein and a light-activated green fluorescent protein.

By switching on the green marker with a laser and then looking for

the mixing of colors, the researchers could distinguish mitochondrial

mergers from near misses. To their surprise, they found that in

healthy neurons, fission occurs up to six times more often than

fusion.

So why aren't the cells cluttered with tiny mitochondria? Because the

organelles grow longer, the researchers determined. This size

increase offsets the higher fission rate. The researchers also

surmised that to " balance the books, " another process has to be

operating-mitochondrial degradation. Together, fusion, fission,

growth, and breakdown determine mitochondrial size and shape, Berman

et al. propose.

Orchestrating many of these changes is the protein Bcl-xL. The team

found that it spurred mitochondrial elongation and sped up fission

and fusion. Without Bcl-xL, mitochondria became stumpy and seemingly

less energy efficient. Bcl-xL belongs to the Bcl-2 protein family,

whose members can protect mitochondria or shatter them to drive

apoptosis. Berman et al.'s results suggest that Bcl-xL manages the

number, size, and energy-producing capacity of mitochondria long

before the cell is faced with a life-or-death decision. Still a

mystery, the scientists say, is how Bcl-xL sparks mitochondrial

growth.