September 11, 2012

If I had to describe myself as a cell, I would want to say something about being a fired-up neuron, or a nurturing nurse cell, or a chameleon stem cell. Realistically, though, I’d be a muscle fiber—painfully organized and precise. When things aren’t so organized I’d just be a twitching little muscle cell until my closet was organized by sleeve length, my pantry in alphabetical order, my various to-do lists organized by priority. A recent paper describes how the nuclei and organelles within a muscle fiber are so regularly spaced (but sadly does not describe any suggestions for my own personal organization).

Striated muscle fibers have densely packed myofibrils, which are the structures that make muscle contractions happen. The muscle fiber is a large multinucleated cell, meaning that there are more than one nucleus. These nuclei, as well as other organelles, must be evenly distributed along the length of the fiber. KASH domain proteins play a role in positioning nuclei and other organelles in various cell types, and a recent paper found that two KASH proteins play this role in muscle cells in the fruit fly. Elhanany-Tamir and colleagues found that these two proteins, called Klar and MSP-300, cooperate to ensure the even spacing of nuclei within a muscle fiber. These two proteins link a nuclear ring of MSP-300 to astral microtubules. Without either of these KASH proteins, the nuclear spacing is irregular. MSP-300 also is important in the spacing of other organelles—mitochondria and ER—within the muscle fiber. In the images above, muscle fibers from wild-type (left), msp-300 mutant, and klar mutant fruit fly larvae are stained for MSP-300 (red) and a nuclear marker (green). In the mutants, the MSP-300 nuclear ring (arrowheads) is dissociated from the nucleus.

Elhanany-Tamir H, Yu YV, Shnayder M, Jain A, Welte M, & Volk T (2012). Organelle positioning in muscles requires cooperation between two KASH proteins and microtubules. originally published in the Journal of Cell Biology, 198 (5), 833-46 PMID: 22927463