The Microtubule-severing Proteins Spastin and Katanin Participate Differently in the Formation of Axonal Branches

Abstract

Neurons express two different microtubule-severing proteins, namely P60-katanin and spastin. Here, we performed studies on cultured neurons to ascertain whether these two proteins participate differently in axonal branch formation. P60-katanin is more highly expressed in the neuron, but spastin is more concentrated at sites of branch formation. Overexpression of spastin dramatically enhances the formation of branches, whereas overexpression of P60-katanin does not. The excess spastin results in large numbers of short microtubules, whereas the excess P60-katanin results in short microtubules intermingled with longer microtubules. We hypothesized that these different microtubule-severing patterns may be due to the presence of molecules such as tau on the microtubules that more strongly shield them from being severed by P60-katanin than by spastin. Consistent with this hypothesis, we found that axons depleted of tau show a greater propensity to branch, and that this is true whether or not the axons are also depleted of spastin. We propose that there are two modes by which microtubule severing is orchestrated during axonal branch formation, one based on the local concentration of spastin at branch sites and the other based on local detachment from microtubules of molecules such as tau that regulate the severing properties of P60-katanin.

Microtubule Severing by Katanin p60 AAA+ ATPase Requires the C-terminal Acidic Tails of Both α- and β-Tubulins and Basic Amino Acid Residues in the AAA+ Ring Pore

“…Thus, interactions of katanin p60 and spastin with MTs may occur in a similar manner. However, it has been shown that Tau, a MT-associating protein, protects MTs against severing by katanin p60 more efficiently than that by spastin in neurons (45). It was also shown that Tau could not interact with subtilisin-treated MTs (46), indicating that Tau binds to either C-terminal end of tubulins.…”

APC2 Plays an Essential Role in Axonal Projections through the Regulation of Microtubule Stability

“…Studies with cultured hippocampal neurons suggest that microtubules begin to fragment when and where axonal branches form (Yu et al, 1994). In the fragmentation of microtubules, microtubule-severing proteins such as katanin and spastin are involved, and both are known to be highly expressed in neurons (Ahmad et al, 1999;Errico et al, 2002;Karabay et al, 2004;Yu et al, 2008). Furthermore, MAPs are proposed to protect microtubules from these enzymes (McNally et al, 2002;Qiang et al, 2006).…”

Microtubule nucleation and organization in dendrites

“…37,38 Evidence that microtubule fragment-based nucleation occurs in dendrites comes from the genetic disruption of severing enzymes Spastin, Katanin 60 and Katanin p60-like1, all of which are required for correct dendrite patterning. [39][40][41][42] Moreover, loss of Katanin p60-like1 reduces the frequency of microtubule polymerization events in the terminal branches of class IV neurons. 39 The activity of microtubule-severing enzymes is not restricted to dendrites; they are also required for axonal growth.…”

“…39 The activity of microtubule-severing enzymes is not restricted to dendrites; they are also required for axonal growth. 40,42,43 Spatial regulation of microtubule-severing enzyme activity in neurons is controlled by mechanisms such as the binding of the microtubule-associated protein Tau to microtubules in hippocampal neuron axons 44 and the post-translational modification of dendrite microtubules (polyglutamylation). 45 g-tubulin and microtubule-severing pathways interact in nonneuronal cells, for example to organize meiotic spindle microtubules in C. elegans.…”