Septin 7: Actin cross-organization is required for axonal association of Schwann cells

Myelin sheaths present two distinct domains: compacted myelin spirals and flanking non-compacted cytoplasmic channels, where lipid and protein segregation is established by unknown mechanisms. Septins, a conserved family of membrane and cytoskeletal interacting GTPases, form intracellular diffusion barriers during cell division and neurite extension and are expressed in myelinating cells. Septins, particularly septin 7 (Sept7), the central constituent of septin polymers, are associated with the cytoplasmic channels of myelinating cells. Here we show that Schwann cells deprived of Sept7 fail to wrap around axons from dorsal root ganglion neurons and exhibit disorganization of the actin cytoskeleton. Likewise, Sept7 distribution is dependent on microfilament but not microtubule organization.

Local regulation of the axonal phenotype, a case of merotrophism

In this essay, we show that several anatomical features of the axon, namely, microtubular content, caliber and extension of sprouts, correlate on a local basis with the particular condition of the glial cell, i.e., the anatomy of axons is dynamic, although it is seen usually in its `normal' state. The occurrence of ribosomes and messenger RNAs in the axon suggests that axoplasmic proteins are most likely synthesized locally, at variance with the accepted notion that they are supplied by the cell body. We propose that the supporting cell (oligodendrocyte or Schwann cell) regulates the axonal phenotype by fine-tuning the ongoing axonal protein synthesis.