We have analyzed the polarity orientation of microtubules in the axons and dendrites of cultured rat hippocampal neurons. As previously reported of axons from other neurons, microtubules in these axons are uniform with respect to polarity; (+)-ends are directed away from the cell body toward the growth cone. In sharp contrast, microtubules in the mid-region of the dendrite, -75 Jtm from the cell body, are not of uniform polarity orientation. Roughly equal proportions of these microtubules are oriented with (+)-ends directed toward the growth cone and ( + )-ends directed .toward the cell body. At distances within 15 ,um of the growth cone, however, microtubule polarity orientation in dendrites is similar to that in axons; (+)-ends are uniformly directed toward the growth cone. These findings indicate a clear difference between axons and dendrites with respect to microtubule organization, a difference that may underlie the differential distribution of organelles within the neuron.Vertebrate neurons generate and maintain two morphologically and functionally distinct types of neurites, axons and dendrites (1-6). It has long been recognized that axons and dendrites differ in their complements of cytoplasmic organelles (1, 6). Most notable in this regard, ribosomes and Golgi elements are present in dendrites but are absent from axons. What is the basis for the nonuniform distribution of organelles in neurons? Several lines of evidence indicate that the distribution of organelles in a cell reflects active transport processes that selectively convey organelles from their sites of synthesis and assembly to other locations in the cell (7, 8).These observations raise the possibility that many of the differences between the organelle composition of axons and dendrites are produced by differences in the organization of the transport systems that convey materials from the cell body into the axon or dendrite.The transport of organelles is a microtubule-based process; microtubules provide the substrate for organelle translocation and, by virtue of their intrinsic polarity, influence the directionality of transport (7-9). The intrinsic polarity of microtubules is based on the asymmetry of the tubulin subunit and its self-assembly characteristics; the (+)-end is preferred for subunit addition over the (-)-end (10, 11). Microtubule-based translocators convey organelles specifically toward either the ( + )-or the ( -)-end ofthe microtubule (7-9). In the axon, microtubules are uniform with respect to polarity, with the (+ )-ends directed away from the cell body (12-15). Thus, only those organelles that translocate toward (+ )-ends of microtubules will be conveyed from the cell body into the axon.Do microtubules in dendrites have the same polarity orientation as those in axons? To date, information concerning the polarity orientation of dendritic microtubules derives from a few atypical cell types. In the dendrite-like processes of teleost retinal cone cells (16) and frog primary olfactory neurons (17), microtubules are unifo...

Little is known about which components of the axonal cytoskeleton might break during rapid mechanical deformation, such as occurs in traumatic brain injury. Here, we micropatterned neuronal cell cultures on silicone membranes to induce dynamic stretch exclusively of axon fascicles. After stretch, undulating distortions formed along the axons that gradually relaxed back to a straight orientation, demonstrating a delayed elastic response. Subsequently, swellings developed, leading to degeneration of almost all axons by 24 h. Stabilizing the microtubules with taxol maintained the undulating geometry after injury but greatly reduced axon degeneration. Conversely, destabilizing microtubules with nocodazole prevented undulations but greatly increased the rate of axon loss. Ultrastructural analyses of axons postinjury revealed immediate breakage and buckling of microtubules in axon undulations and progressive loss of microtubules. Collectively, these data suggest that dynamic stretch of axons induces direct mechanical failure at specific points along microtubules. This microtubule disorganization impedes normal relaxation of the axons, resulting in undulations. However, this physical damage also triggers progressive disassembly of the microtubules around the breakage points. While the disintegration of microtubules allows delayed recovery of the "normal" straight axon morphology, it comes at a great cost by interrupting axonal transport, leading to axonal swelling and degeneration.

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.

Abstract. We have explored the composition and stability properties of individual microtubules (MTs) in the axons of cultured sympathetic neurons. Using morphometric means to quantify the MT mass remaining in axons after various times in 2 #g/ml nocodazole, we observed that ~48% of the MT mass in the axon is labile, depolymerizing with a t1/2 of "-,5 min, whereas the remaining 52% of the MT mass is stable, depolymerizing with a tt/2 of r~ 240 min. Immunofluorescence analyses show that the labile MTs in the axon are rich in tyrosinated ot-tubulin, whereas the stable MTs contain little or no tyrosinated ot-tubulin and are instead rich in posttranslationally detyrosinated and acetylated o~-tubulin. These results were confirmed quantitatively by immunoelectron microscopic analyses of the distribution of tyrosinated ot-tubulin among axonal MTs. Individual MT profiles were typically either uniformly labeled for tyrosinated ot-tubulin all along their length, or were completely unlabeled. Roughly 48% of the MT mass was tyrosinated, ~52% was detyrosinated, and ~85% of the tyrosinated MTs were depleted within 15 min of nocodazole treatment. Thus, the proportion of MT profiles that were either tyrosinated or detyrosinated corresponded precisely with the proportion of MTs that were either labile or stable respectively. We also observed. MT profiles that were densely labeled for tyrosinated a-tubulin at one end but completely unlabeled at the other end. In all of these latter cases, the tyrosinated, and therefore labile domain, was situated at the plus end of the MT, whereas the detyrosinated, and therefore stable domain was situated at the minus end of the MT, and in each case there was an abrupt transition between the two domains. Based on the frequency with which these latter MT profiles were observed, we estimate that minimally 40% of the MTs in the axon are composite, consisting of a stable detyrosinated domain in direct continuity with a labile tyrosinated domain. The extreme drug sensitivity of the labile domains suggests that they are very dynamic, turning over rapidly within the axon. The direct continuity between the labile and stable domains indicates that labile MTs assemble directly from stable MTs. We propose that stable MTs act as MT nucleating structures that spatially regulate MT dynamics in the axon.

Due to their viscoelastic nature, white matter axons are susceptible to damage by high strain rates produced during traumatic brain injury (TBI). Indeed, diffuse axonal injury (DAI) is one of the most common features of TBI, characterized by the hallmark pathological profiles of axonal bulbs at disconnected terminal ends of axons and periodic swellings along axons, known as “varicosities.” Although transport interruption underlies axonal bulb formation, it is unclear how varicosities arise, with multiple sites accumulating transported materials along one axon. Recently, axonal microtubules have been found to physically break during dynamic stretch-injury of cortical axons in vitro. Here, the same in vitro model was used in parallel with histopathological analyses of human brains acquired acutely following TBI to examine the potential role of mechanical microtubule damage in varicosity formation post-trauma. Transmission electron microscopy (TEM) following in vitro stretch-injury revealed periodic breaks of individual microtubules along axons that regionally corresponded with undulations in axon morphology. However, typically less than a third of microtubules were broken in any region of an axon. Within hours, these sites of microtubule breaks evolved into periodic swellings. This suggests axonal transport may be halted along one broken microtubule, yet can proceed through the same region via other intact microtubules. Similar axonal undulations and varicosities were observed following TBI in humans, suggesting primary microtubule failure may also be a feature of DAI. These data indicate a novel mechanism of mechanical microtubule damage leading to partial transport interruption and varicosity formation in traumatic axonal injury.

Recent studies have shown that the transport of microtubules (MTs) and neurofilaments (NFs) within the axon is rapid, infrequent, asynchronous, and bidirectional. Here, we used RNA interference to investigate the role of cytoplasmic dynein in powering these transport events. To reveal transport of MTs and NFs, we expressed EGFP-tagged tubulin or NF proteins in cultured rat sympathetic neurons and performed live-cell imaging of the fluorescent cytoskeletal elements in photobleached regions of the axon. The occurrence of anterograde MT and retrograde NF movements was significantly diminished in neurons that had been depleted of dynein heavy chain, whereas the occurrence of retrograde MT and anterograde NF movements was unaffected. These results support a cargo model for NF transport and a sliding filament model for MT transport.

Microtubules in the axon are more resistant to severing by katanin than microtubules elsewhere in the neuron. We have hypothesized that this is because of the presence of tau on axonal microtubules. When katanin is overexpressed in fibroblasts, the microtubules are severed into short pieces, but this phenomenon is suppressed by the coexpression of tau. Protection against severing is also afforded by microtubule-associated protein 2 (MAP2), which has a tau-like microtubule-binding domain, but not by MAP1b, which has a different microtubule-binding domain. The microtubule-binding domain of tau is required for the protection, but within itself, provides less protection than the entire molecule. When tau (but not MAP2 or MAP1b) is experimentally depleted from neurons, the microtubules in the axon lose their characteristic resistance to katanin. These results, which validate our hypothesis, also suggest a potential explanation for why axonal microtubules deteriorate in neuropathies involving the dissociation of tau from the microtubules.

Neurons are terminally differentiated cells that use their microtubule arrays not for cell division but rather as architectural elements required for the elaboration of elongated axons and dendrites. In addition to acting as compression-bearing struts that provide for the shape of the neuron, microtubules also act as directional railways for organelle transport. The stability properties of neuronal microtubules are commonly discussed in the biomedical literature as crucial to the development and maintenance of the nervous system, and have recently gained attention as central to the etiology of neurodegenerative diseases. Drugs that affect microtubule stability are currently under investigation as potential therapies for disease and injury of the nervous system. There is often a lack of consistency, however, in how the issue of microtubule stability is discussed in the literature, and this can affect the design and interpretation of experiments as well as potential therapeutic regimens. Neuronal microtubules are considered to be more stable than microtubules in dividing cells. On average, this is true, but in addition to an abundant stable microtubule fraction in neurons, there is also an abundant labile microtubule fraction. Both are functionally important. Individual microtubules consist of domains that differ in their stability properties, and these domains can also differ markedly in their composition as well as how they interact with various microtubule-related proteins in the neuron. Myriad proteins and pathways have been discussed as potential contributors to microtubule stability in neurons.