Depletion of acetylated alpha-tubulin during microtubule purification from bovine brain gray and white matter regions

The Altered Hepatic Tubulin Code in Alcoholic Liver Disease

The molecular mechanisms that lead to the progression of alcoholic liver disease have been actively examined for decades. Because the hepatic microtubule cytoskeleton supports innumerable cellular processes, it has been the focus of many such mechanistic studies. It has long been appreciated that α-tubulin is a major target for modification by highly reactive ethanol metabolites and reactive oxygen species. It is also now apparent that alcohol exposure induces post-translational modifications that are part of the natural repertoire, mainly acetylation. In this review, the modifications of the "tubulin code" are described as well as those adducts by ethanol metabolites. The potential cellular consequences of microtubule modification are described with a focus on alcohol-induced defects in protein trafficking and enhanced steatosis. Possible mechanisms that can explain hepatic dysfunction are described and how this relates to the onset of liver injury is discussed. Finally, we propose that agents that alter the cellular acetylation state may represent a novel therapeutic strategy for treating liver disease.

A novel method for purification of polymerizable tubulin with a high content of the acetylated isotype

Tubulin can be acetylated/deacetylated on Lys40 of the α-subunit. Studies of the post-translational acetylation/deacetylation of tubulin using biochemical techniques require tubulin preparations that are enriched in AcTubulin (acetylated tubulin) and (for comparison) preparations lacking AcTubulin. Assembly–disassembly cycling of microtubules gives tubulin preparations that contain little or no AcTubulin. In the present study we demonstrated that this result is owing to the presence of high deacetylating activity in the extracts. This deacetylating activity in rat brain homogenates was inhibited by TSA (Trichostatin A) and tubacin, but not by nicotinamide, indicating that HDAC6 (histone deacetylase 6) is involved. TSA showed no effect on microtubule polymerization or depolymerization. We utilized these properties of TSA to prevent deacetylation during the assembly–disassembly procedure. The effective inhibitory concentration of TSA was 3 μM in the homogenate and 1 μM in the subsequent cycling steps. By comparison with immunopurified AcTubulin, we estimated that ~64% of the tubulin molecules in the three cycled preparations were acetylated. The protein profiles of these tubulin preparations, as assessed by SDS/PAGE and Coomassie Blue staining, were identical to that of a preparation completely lacking AcTubulin obtained by assembly–disassembly cycles in the absence of TSA. The tyrosination state and in vitro assembly–disassembly kinetics were the same regardless of the degree of acetylation.

Post-translational regulation of the microtubule cytoskeleton: mechanisms and functions

Half a century of biochemical and biophysical experiments has provided attractive models that may explain the diverse functions of microtubules within cells and organisms. However, the notion of functionally distinct microtubule types has not been explored with similar intensity, mostly because mechanisms for generating divergent microtubule species were not yet known. Cells generate distinct microtubule subtypes through expression of different tubulin isotypes and through post-translational modifications, such as detyrosination and further cleavage to Δ2-tubulin, acetylation, polyglutamylation and polyglycylation. The recent discovery of enzymes responsible for many tubulin post-translational modifications has enabled functional studies demonstrating that these post-translational modifications may regulate microtubule functions through an amazing range of mechanisms.

Attenuation of neurodegeneration-relevant modifications of brain proteins by dietary soy

Epidemiological studies show that postmenopausal women who undertake estrogen-replacement therapy have significantly lower risk for the onset of Alzheimer's disease (AD) than women who do not. Animal behavior studies have shown that ovariectomy results in the development of cognitive dysfunction that is prevented by estrogen-replacement, suggesting that normal mammalian cognitive function is impaired by estrogen reduction. Soy isoflavones in particular genistein have been demonstrated to have weak and selective estrogenic actions in various models of human chronic diseases. A hallmark of several human dementias including AD and fronto temporal dementia with Parkinsonism on chromosome 17 (FTDP-17) is the hyperphosphorylation of the microtubule-associated protein tau. Preliminary experiments are discussed here which show that isoflavones delivered in a soy protein matrix attenuated selected AD-relevant tau phosphorylations in a primate model of menopause. The rationale is discussed for the use of soy-based foods for protection against postmenopausal neurodegeneration.

Biochemical characterization of related microtubule proteins in Drosophila melanogaster and adult rat brain

We describe the biochemical characteristics of three proteins isolated from Drosophila embryos and the rat brain. We refer to these proteins as DMAPs (Drosophila microtubule-associated proteins) since they were identified by monoclonal antibodies generated against microtubule protein (MTP) purified from Drosophila melanogaster embryos. DMAP-45 is a 45 kDa protein that binds microtubules in an ATP dependent manner. Preliminary biochemical evidence suggests that DMAP-45 may be an actin-related protein. DMAP-55 is a 55 kDa protein and based on its molecular weight and isoelectric point, may be a novel isoform of tubulin. DMAP-66 is a 66 kDa protein that binds strongly to microtubules in vitro and has multiple isoforms. Analyses of proteins in rat brain tissue extracts and purified rat brain MTP identified proteins of similar molecular weight and isoelectric points and are designated DMAP-45R, -55R and -66R. The presence of proteins with common biochemical properties in these widely divergent animal species suggests that they are related proteins.

In vivo and in vitro effects of diisopropyl phosphorofluoridate (DFP) on the rate of hen brain tubulin polymerization

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus ester-induced delayed neurotoxicity (OPIDN) in sensitive species. We have investigated the in vivo and in vitro effects of DFP on hen brain tubulin polymerization. Hens were treated with a single dose of DFP (1.7 mg/kg, sc.), and were sacrificed after 18-21 days. Tubulin from DFP-treated hen brains showed small but significant decrease (14.42%) in the rate of polymerization and 11.05% decrease in rise in O.D. at 340 nm in 30 min. DFP in vivo treatment also resulted in decreased concentration of tau and an enhanced concentration of two peptides (45 kDa, 35 kDa) in the brain supernatant. These peptides seemed to be the degradation products of MAP-2. The decrease in the rate of brain tubulin polymerization in treated hens is consistent with neurochemical alterations and the focal degeneration and aggregation of these filamentous structures in OPIDN.

Organization and slow axonal transport of cytoskeletal proteins under normal and regenerating conditions

The organization of the axonal cytoskeleton was investigated by analyzing the solubility and transport profile of the major cytoskeletal proteins in motor axons of the rat sciatic nerve under normal and regenerating conditions. When extracted with the Triton-containing buffer at low temperature, 50% of tubulin and 30% of actin were recovered in the insoluble form resistant to further depolymerizing treatments. Most of this cold-insoluble form was transported in slow component a (SCa), the slower of the two subcomponents of slow axonal transport, whereas the cold-soluble form showed a biphasic distribution between SCa and SCb (slow component b). Changes in slow transport during regeneration were studied by injuring the nerve either prior to (experiment I) or after (experiment II) radioactive labeling. In experiment I where the transport of proteins synthesized in response to injury was examined, selective acceleration of SCb was detected together with an increase in the relative proportion of this component. In experiment II where the response of the preexisting cytoskeleton was examined, a shift from SCa to SCb of the cold-soluble form was observed. The differential distribution and response of the two forms of tubulin and actin suggest that the cold-soluble form may be more directly involved in axonal transport.