Localization and characterization of tubulin-like proteins associated with brain mitochondria: the presence of a membrane-specific isoform

The anti-apoptotic protein L(*) of Theiler's murine encephalomyelitis virus (TMEV) contains a mitochondrial targeting signal

L(*) protein of TMEV is out-of-frame with the viral polyprotein from an alternative initiation codon AUG, 13 nucleotides downstream from the authentic polyprotein AUG. Anti-apoptotic activity of L(*) was demonstrated by both 'loss of function' and 'gain of function' experiments. However, the precise mechanism(s) of anti-apoptotic activity of L(*) remains to be clarified. In this study, L(*) was demonstrated to be localized to mitochondria. It was also shown by the GFP fusion protein that N-terminal sequence of L(*) may contain a mitochondrial targeting signal (MTS). Surprisingly, L(*)((5-70))-GFP and L(*)((41-70))-GFP were localized to mitochondria although L(*)((1-70))-GFP was distributed in the cytosol, suggesting L(*) has an MTS between amino acid (AA) positions 41 and 70, and that L(*)((1-4)) inhibits its mitochondrial targeting. Furthermore, L(*)((1-70))-GFP was localized to the mitochondria by co-expression of L(*)((65-156)), indicating that L(*)((65-156)) suppresses the inhibition of mitochondrial targeting by L(*)((1-4)). These results suggest that the intra- or inter-molecular interaction of L(*) regulates its mitochondrial localization. It is also suggested that L(*) may inhibit the intrinsic apoptosis through the localization to mitochondria.

Identification and characterisation of novel tubulin-binding motifs located within the C-terminus of TRPV1

Previously, we reported that TRPV1, the vanilloid receptor, interacts with soluble alphabeta-tubulin dimers as well as microtubules via its C-terminal cytoplasmic domain. The interacting region of TRPV1, however, has not been defined. We found that the TRPV1 C-terminus preferably interacts with beta-tubulin and less with alpha-tubulin. Using a systematic deletion approach and biotinylated-peptides we identified two tubulin-binding sites present in TRPV1. These two sequence stretches are highly conserved in all known mammalian TRPV1 orthologues and partially conserved in some of the TRPV1 homologues. As these sequence stretches are not similar to any known tubulin-binding sequences, we conclude that TRPV1 interacts with tubulin and microtubule through two novel tubulin-binding motifs.

Direct interaction of Bcl-2 proteins with tubulin

A direct interaction between tubulin and several pro-apoptotic and anti-apoptotic members of the Bcl-2 family has been demonstrated by effects on the assembly of microtubules from pure rat brain tubulin. Bcl-2, Bid, and Bad inhibit assembly sub-stoichiometrically, whereas peptides from Bak and Bax promote tubulin polymerization at near stoichiometric concentrations. These opposite effects on microtubule assembly are mutually antagonistic. The BH3 homology domains, common to all members of the family, are involved in the interaction with tubulin but do not themselves affect polymerization. Pelleting experiments with paclitaxel-stabilized microtubules show that Bak is associated with the microtubule pellet, whereas Bid remains primarily with the unpolymerized fraction. These interactions require the presence of the anionic C-termini of alpha- and beta-tubulin as they do not occur with tubulin S in which the C-termini have been removed. While in no way ruling out other pathways, such direct associations are the simplest potential regulatory mechanism for apoptosis resulting from disturbances in microtubule or tubulin function.

Involvement of acetylated tubulin in the regulation of Na+,K+ -ATPase activity in cultured astrocytes

The results presented support the view that the modulation of Na(+),K(+)-ATPase activity in living cells involves the association/dissociation of acetylated tubulin with the enzyme. We found that the stimulation of Na(+),K(+)-ATPase activity by L-glutamate correlates with decreased acetylated tubulin quantity associated with the enzyme. The effect of L-glutamate was abolished by the glutamate transporter inhibitor DL-threo-beta-hydroxyaspartate but was not affected by either specific agonists or antagonists. The effect of L-glutamate seems to be mediated by Na(+) entry resulting from glutamate transport, since the Na(+) ionophore monensin produced stimulation of Na(+),K(+)-ATPase activity with concomitant decrease of acetylated tubulin quantity associated with the enzyme.

Brain plasma membrane Na+,K+-ATPase is inhibited by acetylated tubulin

Membranes from brain tissue contain tubulin that can be isolated as a hydrophobic compound by partitioning into Triton X-114. The hydrophobic behavior of this tubulin is due to the formation of a complex with the alpha-subunit of Na+,K+-ATPase. In the present work we show that the interaction of tubulin with Na+K+-ATPase inhibits the enzyme activity. We found that the magnitude of the inhibition is correlated with: (1) concentration of the acetylated tubulin isoform present in the tubulin preparation used, and (2) amount of acetylated tubulin isoform isolated as a hydrophobic compound. In addition, some compounds involved in the catalytic action of Na+K+-ATPase were assayed to determine their effects on the inhibitory capability of tubulin on this enzyme. The inhibitory effect of tubulin was only slightly decreased by ATP at relatively low nucleotide concentration (0.06 mM). NaCl (1-160 mM) and KCl (0.2-10 mM) showed no effect whereas inorganic phosphate abolished the inhibitory effect of tubulin in a concentration-dependent manner.

Na+,K+-ATPase was found to be the membrane component responsible for the hydrophobic behavior of the brain membrane tubulin

We have previously described that the tubulin isolated from brain membranes as a hydrophobic compound by partitioning into Triton X-114 is a peripheral membrane protein [corrected]. The hydrophobic behavior of this tubulin is due to its interaction with membrane protein(s) and the interaction occurs principally with the acetylated tubulin isotype. In the present work we identified the membrane protein that interacts with tubulin as the Na+,K+-ATPase alpha subunit by amino acid sequencing. Using purified brain Na+,K+-ATPase we were able to isolate part of the total hydrophilic tubulin as a hydrophobic compound which contains a high proportion of the acetylated tubulin isotype.

The relationship of hydrophobic tubulin with membranes in neural tissue

Brain membrane preparations contain tubulin that can be extracted with Triton X-114. After the extract is allowed to partition, 8% of the total brain tubulin is isolated as a hydrophobic compound in the detergent-rich phase. Cytosolic tubulin does not show this hydrophobic behaviour since it is recovered in the aqueous phase. Membrane tubulin can be released by 0.1 M Na2 CO3 treatment at pH > or = 11.5 in such a way that the hydrophobic tubulin is converted into the hydrophilic form. These results suggest that tubulin exists associated with some membrane component that confers the hydrophobic behaviour to tubulin. If the tissue is homogenized in microtubule-stabilizing buffer containing Triton X-100, the hydrophobic tubulin is isolated from the microtubule fraction. This result indicates that the hydrophobic tubulin isolated from membrane preparations belongs to microtubules that in vivo are associated to membranes. Therefore, hydrophobic tubulin (tubulin-membrane component complex) can be obtained from membranes or from microtubules depending on the conditions of brain homogenization.

Subcellular distribution and immunological detection of retrograde axonally transported proteins in acrylamide and diabetic neuropathies

Neuropathies produced by both acrylamide- and streptozotocin-induced diabetes in the rat are accompanied by a deficit in the retrograde axonal transport of a defined group of proteins that can be visualized on two-dimensional polyacrylamide gels. In this work, these proteins are identified as being primarily soluble and being absent from rat brain. They are not immunologically related to the major retrogradely transported protein synaptophysin. Polyclonal antiserum to the proteins was produced in mice and used to confirm the reduction in their retrograde transport in sciatic nerve of diabetic and acrylamide-treated rats.

Interaction of brain mitochondria with microtubules reconstituted from brain tubulin and MAP2 or TAU

To explore the behaviour of microtubule-associated proteins, MAP2 and TAU in the interactions of mitochondria with microtubules, an homologous acellular system has been reconstituted with organelles isolated from rat brain. We have established a quantitative in vitro binding assay based on the cosedimentation of 125I-labeled microtubules with mitochondria. We found that binding of microtubules to mitochondria was concentration dependent and saturable. Binding was insensitive to ATP. A comparison of taxol-stabilized microtubules prepared from MAP-free tubulin or tubulin coated with TAU or MAP2 showed that the microtubule-associated proteins diminished, or reduced to background levels, the formation of complexes with mitochondria. In contrast, the amount of MAP-free taxol microtubules that cosedimented with mitochondria increased two- and six-fold when mitochondria were coated with MAP2 or TAU. These studies suggest that the two major brain MAPs could have a crosslinking or a spacing role, depending on their organelle localization.

Tyrosinated, detyrosinated and acetylated tubulin isotypes in rat brain membranes. Their proportions in comparison with those in cytosol

The heterogeneity of alpha-tubulin and the relative proportions of the tubulin isotypes were investigated in brain membranes of rats of 1, 25 and 180 days of age by using four anti-alpha-tubulin antibodies: a) the monoclonal DM1A antibody, specific for alpha-tubulin; b) the monoclonal 6-11B-1 antibody, specific for acetylated tubulin; c) a polyclonal antibody (Glu antibody), specific for detyrosinated tubulin; and d) a polyclonal antibody (Tyr antibody), specific for tyrosinated tubulin. We found that rat brain membranes contain the three tubulin isotypes mentioned above. The proportions of tyrosinated and detyrosinated tubulin relative to total alpha-tubulin were somewhat lower in membrane than in cytosol in animals of 25 and 180 days of age. At day one of development, the proportions in membrane were similar to those found in cytosol. With respect to the acetylated form, it was about 20 times higher in membrane than in cytosol at the three ages studied. The proportion of acetylated tubulin was determined in different subcellular fractions: myelin, synaptic vesicles, mitochondria, microsomes, and plasma membrane. While the amount of total tubulin differed between the different subcellular fractions, the proportion of acetylated tubulin relative to total alpha-tubulin was constant and similar to that found in total membranes. The proportion of acetylated tubulin was also investigated in non-neural tissues (kidney, liver and lung). Although values for cytosol were about 10-fold higher than that found in brain cytosol, no detectable values for membranes could be obtained in these organs.

Associations between beta-tubulin and mitochondria in adult isolated heart myocytes as shown by immunofluorescence and immunoelectron microscopy

We have investigated the associations between beta-tubulin and mitochondria in freshly isolated cardiac myocytes from the rat. Beta-tubulin was identified by using monoclonal antibodies for immunofluorescence and high resolution immunogold electron microscopy. In addition, conventional transmission and scanning electron microscopic studies were performed. After chemical stabilization in a formaldehyde solution, the myocytes were shock-frozen at -150 degrees C, cryosectioned at -70 degrees C and subsequently processed for immunohistochemical and immunocytochemical microscopy. A characteristic of the rod shaped myocytes is the presence of a dense network of microtubules in the cytoplasm displaying a pattern of strong anti-beta-tubulin reaction. The complexity of this network however varies considerably among the myocytes reflecting microtubule dynamic instability. Further, our findings demonstrate that the beta-tubulin label in rod cells is confined to the perinuclear and interfibrillar spaces and, therefore, is largely colocalized with the cytoplasmic organelles. In myocytes undergoing severe contracture the distribution of beta-tubulin is entirely restricted to the outer mitochondrial-containing domain. This implies that, in a cell model with marked segregation of the contractile filaments and organelles, mitochondria are codistributed with microtubules in the total absence of desmin intermediate filaments. Moreover, our immunogold preparations demonstrate anti-beta-tubulin labelling in the outer mitochondrial membrane as well as of fibres in close apposition to this membrane. These results indicate the presence of a specific beta-tubulin binding to the outer mitochondrial membrane that probably also involves microtubule based translocators and/or MAPs.

The long-term effects of a single injection of taxol upon peripheral nerve axons

To study the long-term effects of a single injection of the microtubule stabilizing drug taxol in vivo, the compound was injected into rat sciatic nerve and the ensuing morphological changes followed for 8-25 weeks after injection. In accord with previously published works, taxol-induced giant axonal bulbs were common and were most marked at 8-10 weeks. From 12 weeks onwards these giant axons decreased in diameter with concomitant remyelination. By 20 weeks axonal bulbs could not be seen. The recovery of axons from taxol intoxication began 8-12 weeks after injection with the growth of axonal sprouts, longitudinally and laterally, from the distal aspect of the proximal stump. During recovery, from 12 weeks onwards, axons showed apparent reorganization of the axoplasmic cytoskeleton where microtubules diminished and neurofilaments became more numerous. By 16 weeks only small groups of microtubules remained, often encircling a mitochondrion. By 25 weeks taxol-treated nerves showed no apparent taxol-induced changes. A common ultrastructural finding up to 16 weeks was the appearance within axons of tubular profiles covered by a double membrane. These structures were sometimes arranged as crystalloid aggregates. The diameter of these profiles was 85 nm, they were most common at 12 weeks and it is proposed that they may be derived from mitochondria. The present results show taxol to have a long-lasting and local effect upon axoplasmic organization in vivo. The cytoskeletal reorganization described supports the concept of the differential movement of axoplasmic neurofilaments and that neurofilaments stabilize axonal structures.

Mechanism of action of antimitotic drugs: a new hypothesis based on the role of cellular calcium

The antimitotic drugs such as colchicine, podophyllotoxin, etc. are currently believed to exert their cytotoxic and antimitotic effects due to binding of the drug-tubulin complex to the growing ends of microtubules (MTs), leading to an "end-capping or poisoning" effect. However, to account for a number of apparently puzzling observations regarding antimitotic drugs (which cannot be readily explained by the current model) and the mitotic process, a new hypothesis regarding the mechanism of action of antimitotic drugs is proposed. The key observations in this context are as follows: (i) The antimitotic drugs bind specifically to free tubulin. (ii) Cell growth by these drugs is specifically blocked in metaphase, and interphase microtubules do not seem to play any role in the drugs' cytotoxic or antimitotic effects. (iii) Tubulin is specifically associated with a number of membranous organelles (viz. mitochondria, plasma membranes, endoplasmic reticulum) which are responsible for intracellular Ca+2 homeostasis. (iv) Fluorescent derivatives of antimitotic drugs also bind to the above membranous organelles and not to MTs. (v) Ca+2 plays a central role in the control of MT assembly/disassembly in vivo and a Ca+2 pulse is necessary for the metaphase to anaphase transition. (vi) Cellular mutants which exhibit specific resistance to various antimitotic drugs are altered in either tubulin(s) or mitochondrial matrix proteins. To account for these observations, it is suggested that free tubulin present in the above membranous organelles serves as the cellular receptor for these drugs and this binding interferes with the Ca+2 regulatory/signalling mechanism essential for anaphase chromosome movement. The effect of these drugs on interphase MTs appears to be a secondary consequence of this alteration in Ca+2 regulation. The observed changes in mitochondrial matrix proteins in many of the mutants resistant to antimitotic drugs further indicate that mitochondria should play an important role in Ca+2 homeostasis, as it relates to mitosis. The possible mechanisms by which these drugs may interfere with the Ca+2 regulation and some implications of this hypothesis are discussed.

Binding of microtubule-associated proteins (MAPs) to rat brain mitochondria: a comparative study of the binding of MAP2, its microtubule-binding and projection domains, and tau proteins

Two major brain microtubule-associated proteins (MAPs), MAP2 and tau, were found to be able to bind to purified rat brain mitochondria. The apparent dissociation constants of the binding of thermostable 32P-labeled MAP2 and tau are 0.9 +/- 0.04 x 10(-7) and 3.8 +/- 0.7 x 10(-7) M, respectively. 32P-labeled MAP2 and tau bound to the mitochondria can be displaced by phosphorylated, nonradioactive MAP2. The binding parameters of MAP2 prepared without heat treatment and those of the thermostable MAP2 were of the same order of magnitude. Microtubule-binding and projection domains of MAP2 were obtained by chymotryptic digestion of rat brain microtubules (Vallee, Proc. Natl. Acad. Sci. USA, 77:3206-3210, 1980). Displacement studies with these two domains show that MAP2 bound to mitochondria can be displaced by the microtubule-binding domain, whereas the projection domain does not displace MAP2. The two domains of MAP2 bind to the mitochondria with similar affinity constants; however, the Bmax for the projection domain was 10 times and 35 times lower than the Bmax of the binding of the intact MAP2 and the microtubule-binding domain, respectively. Chymotryptic digestion of MAP2 bound to the mitochondria yielded peptide fragments with molecular masses similar to those obtained by the digestion of MAP2 bound to the microtubules. The fragments corresponding to the projection domain were released into the extramitochondrial supernatant, whereas the fragments originating from the microtubule-binding domain remained bound to the mitochondria. These results suggest that MAP2 binds to mitochondria preferentially via its microtubule-binding domain.

The characterization of phospholipids associated with microtubules, purified tubulin and microtubule associated proteins in vitro

1. Significant levels of total phospholipid phosphate were detected in highly purified microtubule protein preparations. 2. While the phospholipid profiles of total microtubule proteins and microtubule-associated proteins showed both similarities and differences to that of a whole brain homogenate, purified tubulin was associated only with phospholipids that were not detectable in the latter. 3. Phosphatidyl ethanolamine, found exclusively in a fraction of microtubule associated proteins, stimulated microtubule assembly in vitro.

Microtubule-associated proteins bind to 30 kDa and 60 kDa proteins of rat brain mitochondria: visualization by ligand blotting

Axonal transport of mitochondria is a microtubule-associated movement. Microtubule-mitochondria interactions were studied in vitro using organelles isolated from rat brain. Thanks to the ligand blotting method we were able to show two mitochondrial membrane proteins with apparent molecular masses of 30 kDa and 60 kDa that bind microtubule-associated proteins. The binding of the 30 kDa protein has an apparent Kd of 8 x 10(-8) M. Digitonin fractionation of mitochondria reveals a bimodal localization of the 30 kDa and the 60 kDa proteins within the outer membrane. The data suggest that these polypeptides could participate to the interactions observed in situ between microtubules and mitochondria.

Microtubule-mitochondrial associations in regenerating axons after taxol intoxication

Ultrastructural examination of regenerating axons within rat sciatic nerve injected locally with the microtubule assembly-promoting compound, taxol, has revealed the frequent association of microtubules with axoplasmic organelles, in particular mitochondria. This was characterized by the prodigious accumulation of axonal microtubules, some of which became aligned to form multilayered channels within which axoplasmic organelles were sequestered. Between each layer of microtubules forming the walls of these channels, a 5 nm filament, believed to derive from microtubule side-arm material, was present. The findings suggest that these microtubule channels might play a role in mitochondrial traffic across the lesion area. Similar but much less elaborate associations within axons between microtubules and axoplasmic organelles, including mitochondria, have been described previously. Within regenerating axonal sprouts, intermediate filaments were found only at later timepoints when they commonly occurred within the microtubule channels. It is proposed that taxol impedes axonal regrowth at an early stage of cytoskeleton formation and that that the present observations represent drug-induced exaggerations of a normal phenomenon.

Changes in the beta-subunit of mitochondrial F1 ATPase during neurogenesis

A polypeptide migrating in the area of the isotubulin in 2 D-gel electrophoresis of extracts from neuronal cells was characterized as the beta-subunit of the F1 ATPase matrix component. The synthesis of this subunit is enhanced during neurogenesis and the presence of an isoform was detected in adult mouse brain.

Phosphorylation of tubulin enhances its interaction with membranes

Tubulin, the main component of intracellular microtubules, is also a major protein in subcellular membrane preparations and can interact with biological and artificial membranes in vitro. Of particular interest is the association of tubulin with postsynaptic junctional lattices enriched in a polypeptide of relative molecular mass (Mr) 50,000 (50K), recently identified as the major subunit of the calmodulin-dependent protein kinase. Phosphorylation of tubulin with a calmodulin-dependent protein kinase similar to that found in postsynaptic densities inhibits its ability to self-assemble into microtubules in a reversible fashion. This involves the phosphorylation of residues in its 4K carboxy-terminal region, a domain that seems to regulate its self-assembly. The results presented here suggest that the phosphorylation of tubulin with this kinase enhances its ability to interact with membranes. This effect is reversible.

Characterization of a membrane-specific tubulin isoform by peptide mapping

A membrane-specific tubulin-like protein, found in preparations of synaptic plasma membranes and brain mitochondria, was analyzed by chemical and proteolytic peptide mapping to determine which part of the molecule was different from cytoplasmic tubulin. The membrane polypeptide was identical to alpha tubulin in the first two-thirds of the molecule containing the amino terminal, as found by peptide mapping. However, some differences were observed in the peptide maps of the carboxy terminal one third of the molecule which includes a domain that is important in the regulation of tubulin self-assembly.

Interaction of tubulin with octyl glucoside and deoxycholate. 1. Binding and hydrodynamic studies

Tubulin purified from calf brain cytoplasm, normally a compact water-soluble dimer, is able to interact with the mild detergents octyl glucoside (a minimum of 60 detergent molecules) and deoxycholate (95 +/- 8 molecules). Binding is cooperative and approaches saturation below the critical micelle concentration of the amphiphiles. Binding is accompanied by a quenching of the intrinsic protein fluorescence, but no spectral shape changes indicating denaturation such as in the case of sodium dodecyl sulfate are observed. Glycerol, which is known to be preferentially excluded from the tubulin domain and to favor the folded and associated forms of this protein, inhibits the binding of the mild detergents. Octyl glucoside induces a rapidly equilibrating tubulin self-association reaction characterized by a bimodal sedimentation velocity profile with boundaries at approximately 5 and 12 S. Full dissociation of this detergent restores the normal sedimentation behavior to 90% of the protein. Binding of deoxycholate slows the sedimentation velocity of tubulin from s(0)20,w = 5.6 +/- 0.2 S to s(0)20,w = 4.8 +/- 0.3 S. Measurements of the molecular weight of the tubulin-deoxycholate complex indicate an increase from 100,000 to 143,000 +/- 5,000. The diffusion rate consistently decreases from (5.3 +/- 0.5) X 10(-7) to (3.8 +/- 0.2) X 10(-7) cm2 S-1. This is most simply interpreted as an expansion of the undissociated tubulin dimer upon detergent binding (a change in the frictional ratio, f/f min, from 1.35 to 1.86). It is concluded that tubulin shows a reversible transition between the water-soluble state and amphipathic detergent-bound forms which constitute a model system of tubulin-membrane interactions.

Taxol-induced neuropathy: chronic effects of local injection

The long-term neurotoxic effects of taxol, a compound known to promote microtubule protein polymerization, injected subepineurially into rat sciatic nerve were studied up to 10 weeks post-injection. At the site of injection, taxol caused local axonal reactions and degeneration which were causally related to the slow progressive accumulation of microtubules and other axoplasmic constituents. This culminated in the appearance of giant axonal spheroids and profiles similar to the retraction bulbs of Wallerian degeneration. From these axonal bulbs, many of which arose at nodes of Ranvier, groups of regenerating sprouts emanated. During the acute phase of taxol neurotoxicity, some swollen axons were divested of their myelin sheaths and remained demyelinated for many weeks. After 4 weeks, remyelination was apparent along some fibres. In addition to the accumulation of profiles usually associated with retraction bulbs, there was a vast increase in microtubules, some of which were aligned in concentric rings and formed channels for mitochondria. Microtubule anomalies were also visualized in distal portions of affected fibres and in regenerating sprouts. In contrast, Schwann cells displayed microtubule abnormalities only at the site of the lesion where excessive microtubule polymerization caused the displacement of ribosomes from rough endoplasmic reticulum. Distally, Schwann cells were essentially normal. Axonal depletion and regenerating sprouts were noted further downstream in the tibial nerve, and the gastrocnemius muscle showed changes similar to denervation atrophy. These results extend previous observations by demonstrating chronic, reparatory and reversible phenomena, the implications of which are discussed vis à vis axoplasmic transport and nerve regeneration.