Promotion of microtubule assembly by oligocations: cooperativity between charged groups

Polyamines putrescine and spermidine as modulators of protein aggregation rate: The effect on DTT-induced aggregation of α-lactalbumin

Protein aggregation is undesirable for cells due to its possible toxicity, and is also undesirable in biotechnology and pharmaceuticals. Polyamines are known to be capable of both suppressing and stimulating protein aggregation. In the present work polyamines (spermidine, putrescine) have been shown to alter the pathway of α-lactalbumin aggregation induced by dithiothreitol, leading to the formation of larger protein particles during the initial stages of aggregation and promoting the later stage of sticking of aggregates. According to the aggregation kinetics data, polyamines accelerate protein aggregation in a concentration-dependent manner, with a maximum at 50 mM spermidine and 100 mM putrescine. With a further increase in polyamines concentration the effect of aggregation acceleration decreased, thus, the modulation of the aggregation rate by polyamines was shown. A comparison of the aggregation kinetics and hydrodynamic radii growth data registered by dynamic light scattering with the data obtained by asymmetric flow field-flow fractionation and analytical ultracentrifugation allowed us to describe the early stages of aggregation and formation of initial α-lactalbumin clusters. Our results provide a deeper insight into the mechanism of amorphous aggregation of α-lactalbumin and polyamines action on protein aggregation and protein-protein interaction in general.

Regulation of microtubule dynamic instability by the carboxy-terminal tail of β-tubulin

This work examines how the carboxy-terminal tail domain of β-tubulin governs microtubule dynamic instability and the structure of plus ends using complementary in vivo and in vitro experiments.

Dynamic instability is an intrinsic property of microtubules; however, we do not understand what domains of αβ-tubulins regulate this activity or how these regulate microtubule networks in cells. Here, we define a role for the negatively charged carboxy-terminal tail (CTT) domain of β-tubulin in regulating dynamic instability. By combining in vitro studies with purified mammalian tubulin and in vivo studies with tubulin mutants in budding yeast, we demonstrate that β-tubulin CTT inhibits microtubule stability and regulates the structure and stability of microtubule plus ends. Tubulin that lacks β-tubulin CTT polymerizes faster and depolymerizes slower in vitro and forms microtubules that are more prone to catastrophe. The ends of these microtubules exhibit a more blunted morphology and rapidly switch to disassembly after tubulin depletion. In addition, we show that β-tubulin CTT is required for magnesium cations to promote depolymerization. We propose that β-tubulin CTT regulates the assembly of stable microtubule ends and provides a tunable mechanism to coordinate dynamic instability with ionic strength in the cell.

Cyclic AMP signaling: a molecular determinant of peripheral nerve regeneration

Disruption of axonal integrity during injury to the peripheral nerve system (PNS) sets into motion a cascade of responses that includes inflammation, Schwann cell mobilization, and the degeneration of the nerve fibers distal to the injury site. Yet, the injured PNS differentiates itself from the injured central nervous system (CNS) in its remarkable capacity for self-recovery, which, depending upon the length and type of nerve injury, involves a series of molecular events in both the injured neuron and associated Schwann cells that leads to axon regeneration, remyelination repair, and functional restitution. Herein we discuss the essential function of the second messenger, cyclic adenosine monophosphate (cyclic AMP), in the PNS repair process, highlighting the important role the conditioning lesion paradigm has played in understanding the mechanism(s) by which cyclic AMP exerts its proregenerative action. Furthermore, we review the studies that have therapeutically targeted cyclic AMP to enhance endogenous nerve repair.

GAS2-like proteins mediate communication between microtubules and actin through interactions with end-binding proteins

Crosstalk between the microtubule (MT) and actin cytoskeletons is fundamental to many cellular processes including cell polarisation and cell motility. Previous work has shown that members of the growth-arrest-specific 2 (GAS2) family mediate the crosstalk between filamentous actin (F-actin) and MTs, but the molecular basis of this process remained unclear. By using fluorescence microscopy, we demonstrate that three members of this family, GAS2-like 1, GAS2-like 2 and GAS2-like 3 (G2L1, G2L2 and G2L3, also known as GAS2L1, GAS2L2 and GAS2L3, respectively) are differentially involved in mediating the crosstalk between F-actin and MTs. Although all localise to actin and MTs, only the exogenous expression of G2L1 and G2L2 influenced MT stability, dynamics and guidance along actin stress fibres. Biochemical analysis and live-cell imaging revealed that their functions are largely due to the association of these proteins with MT plus-end-binding proteins that bind to SxIP or SxLP motifs located at G2L C-termini. Our findings lead to a model in which end-binding (EB) proteins play a key role in mediating actin–MT crosstalk.

Enhancement of microtubule-associated protein-1 Alpha gene expression in osteoblasts by low level laser irradiation

BACKGROUND

Low level laser irradiation (LLLI) stimulates bone regeneration. However, the molecular mechanisms leading to this is not yet understood. The stepwise subtractive cDNA cloning technology has been developed, coupled with DNA homology searched in DNA database is useful to identify the certain gene.

AIM

In order to understand the mechanism, we attempted to identify genes whose expressions are enhanced by LLLI. MC3T3-E1 osteoblastic cells were irradiated with an 830 nm Ga-Al-As diode laser, and a cDNA library was constructed using subtractive gene cloning.

MATERIAL AND METHODS

The cDNA library of osteoblasts which was treated by LLLI was constructed. Nucleotide sequences were analyzed and homology searched in a DNA database using BLASTN program to identify the gene with altered expression. Altered mRNA levels by LLLI were confirmed by reverse transcription polymerase chain reaction (RT-PCR) and real-time PCR.

RESULTS

The DNA sequence of a subtracted gene clone MCL129 indicated high homology (99%) with the microtubule-associated protein 1A (MAP1A) gene. Increase in MAP-1A mRNA level by LLLI was successfully confirmed by RT-PCR and real-time PCR.

DISCUSSION

MAP1A has been shown to promote microtubule assembly and its functional expression. Microtubules play an important role in cell division, cell shape and polarity, cell movement, intracellular transport, signal transduction, and synthesis and secretion of collagen. Thus, enhancement of MAP1A gene expression by LLLI may be one of the molecular mechanisms involved in accelerating bone formation by LLLI.

CONCLUSION

LLLI irradiation enhances MAP1A gene expression and modulates microtubule assembly and the functional structure of microtubules, in turn, stimulates osteoblastic proliferation and differentiation.

Reversed phase ion-pairing chromatography of an oligolysine mixture in different mobile phases: effort of searching critical chromatography conditions

Our earlier study [J. Chromatogr. A 1218 (2011) 7765] on separation of an oligolysine mixture consisting of chains with 2-8 lysine residues (number of lysine residues, dp=2-8) by ion-pairing reversed-phase chromatography using heptafluorobutyric acid (HFBA) as an ion pairing reagent at fixed mobile phase acetonitrile (ACN) content was extended to isocratic elution conditions with different ACN percentages. The present work explored how manipulating the mobile phase HFBA concentration ([HFBA]) and %-ACN content influences separations of the oligolysine mixture. The closed pairing model was used to analyze variation of the retention factor as a function of [HFBA]. The partition coefficient of the paired peptide decreased with increasing %-ACN. Pairing of HFBA to oligolysine was cooperative, and the effect increased when %-ACN in the mobile phase was lowered. A plot of the partition coefficient as a function of %-ACN for oligolysines varying in dp converged at one ACN content, indicating a critical condition in which components of different dp co-elute.

Spectraplakins promote microtubule-mediated axonal growth by functioning as structural microtubule-associated proteins and EB1-dependent +TIPs (tip interacting proteins)

The correct outgrowth of axons is essential for the development and regeneration of nervous systems. Axon growth is primarily driven by microtubules. Key regulators of microtubules in this context are the spectraplakins, a family of evolutionarily conserved actin-microtubule linkers. Loss of function of the mouse spectraplakin ACF7 or of its close Drosophila homolog Short stop/Shot similarly cause severe axon shortening and microtubule disorganization. How spectraplakins perform these functions is not known. Here we show that axonal growth-promoting roles of Shot require interaction with EB1 (End binding protein) at polymerizing plus ends of microtubules. We show that binding of Shot to EB1 requires SxIP motifs in Shot's C-terminal tail (Ctail), mutations of these motifs abolish Shot functions in axonal growth, loss of EB1 function phenocopies Shot loss, and genetic interaction studies reveal strong functional links between Shot and EB1 in axonal growth and microtubule organization. In addition, we report that Shot localizes along microtubule shafts and stabilizes them against pharmacologically induced depolymerization. This function is EB1-independent but requires net positive charges within Ctail which essentially contribute to the microtubule shaft association of Shot. Therefore, spectraplakins are true members of two important classes of neuronal microtubule regulating proteins: +TIPs (tip interacting proteins; plus end regulators) and structural MAPs (microtubule-associated proteins). From our data we deduce a model that relates the different features of the spectraplakin C terminus to the two functions of Shot during axonal growth.

Characterization of G2L3 (GAS2-like 3), a new microtubule- and actin-binding protein related to spectraplakins

The microtubule (MT) and actin cytoskeletons are fundamental to cell integrity, because they control a host of cellular activities, including cell division, growth, polarization, and migration. Proteins involved in mediating the cross-talk between MT and actin cytoskeletons are key to many cellular processes and play important physiological roles. We identified a new member of the GAS2 family of MT-actin cross-linking proteins, named G2L3 (GAS2-like 3). We show that GAS2-like 3 is widely conserved throughout evolution and is ubiquitously expressed in human tissues. GAS2-like 3 interacts with filamentous actin and MTs via its single calponin homology type 3 domain and C terminus, respectively. Interestingly, the role of the putative MT-binding GAS2-related domain is to modulate the binding of GAS2-like 3 to both filamentous actin and MTs. This is in contrast to GAS2-related domains found in related proteins, where it functions as a MT-binding domain. Furthermore, we show that tubulin acetylation drives GAS2-like 3 localization to MTs and may provide functional insights into the role of GAS2-like 3.

The C terminus of tubulin, a versatile partner for cationic molecules: binding of Tau, polyamines, and calcium

The C-terminal region of tubulin is involved in multiple aspects of the regulation of microtubule assembly. To elucidate the molecular mechanisms of this regulation, we study here, using different approaches, the interaction of Tau, spermine, and calcium, three representative partners of the tubulin C-terminal region, with a peptide composed of the last 42 residues of α1a-tubulin. The results show that their binding involves overlapping amino acid stretches in the C-terminal tubulin region: amino acid residues 421-441 for Tau, 430-432 and 444-451 for spermine, and 421-443 for calcium. Isothermal titration calorimetry, NMR, and cosedimentation experiments show that Tau and spermine have similar micromolar binding affinities, whereas their binding stoichiometry differs (C-terminal tubulin peptide/spermine stoichiometry 1:2, and C-terminal tubulin peptide/Tau stoichiometry 8:1). Interestingly, calcium, known as a negative regulator of microtubule assembly, can compete with the binding of Tau and spermine with the C-terminal domain of tubulin and with the positive effect of these two partners on microtubule assembly in vitro. This observation opens up the possibility that calcium may participate in the regulation of microtubule assembly in vivo through direct (still unknown) or indirect mechanism (displacement of microtubule partners). The functional importance of this part of tubulin was also underlined by the observation that an α-tubulin mutant deleted from the last 23 amino acid residues does not incorporate properly into the microtubule network of HeLa cells. Together, these results provide a structural basis for a better understanding of the complex interactions and putative competition of tubulin cationic partners with the C-terminal region of tubulin.

Prion protein region 23-32 interacts with tubulin and inhibits microtubule assembly

In previous studies we have demonstrated that prion protein (PrP) binds directly to tubulin and this interaction leads to the inhibition of microtubule formation by inducement of tubulin oligomerization. This report is aimed at mapping the regions of PrP and tubulin involved in the interaction and identification of PrP domains responsible for tubulin oligomerization. Preliminary studies focused our attention to the N-terminal flexible part of PrP encompassing residues 23-110. Using a panel of deletion mutants of PrP, we identified two microtubule-binding motifs at both ends of this part of the molecule. We found that residues 23-32 constitute a major site of interaction, whereas residues 101-110 represent a weak binding site. The crucial role of the 23-32 sequence in the interaction with tubulin was confirmed employing chymotryptic fragments of PrP. Surprisingly, the octarepeat region linking the above motifs plays only a supporting role in the interaction. The binding of Cu(2+) to PrP did not affect the interaction. We also demonstrate that PrP deletion mutants lacking residues 23-32 exhibit very low efficiency in the inducement of tubulin oligomerization. Moreover, a synthetic peptide corresponding to this sequence, but not that identical with fragment 101-110, mimics the effects of the full-length protein on tubulin oligomerization and microtubule assembly. At the cellular level, peptide composed of the PrP motive 23-30 and signal sequence (1-22) disrupted the microtubular cytoskeleton. Using tryptic and chymotryptic fragments of alpha- and beta-tubulin, we mapped the docking sites for PrP within the C-terminal domains constituting the outer surface of microtubule.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Polyamine sharing between tubulin dimers favours microtubule nucleation and elongation via facilitated diffusion

We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.

YB-1 promotes microtubule assembly in vitro through interaction with tubulin and microtubules

Background

YB-1 is a major regulator of gene expression in eukaryotic cells. In addition to its role in transcription, YB-1 plays a key role in translation and stabilization of mRNAs.

Results

We show here that YB-1 interacts with tubulin and microtubules and stimulates microtubule assembly in vitro. High resolution imaging via electron and atomic force microscopy revealed that microtubules assembled in the presence of YB-1 exhibited a normal single wall ultrastructure and indicated that YB-1 most probably coats the outer microtubule wall. Furthermore, we found that YB-1 also promotes the assembly of MAPs-tubulin and subtilisin-treated tubulin. Finally, we demonstrated that tubulin interferes with RNA:YB-1 complexes.

Conclusion

These results suggest that YB-1 may regulate microtubule assembly in vivo and that its interaction with tubulin may contribute to the control of mRNA translation.

Synergistic apoptosis of MCF-7 breast cancer cells by 2-methoxyestradiol and bis(ethyl)norspermine

2-Methoxyestradiol (2ME) is an estradiol metabolite with anti-tumor and anti-angiogenic properties. We studied the effect of 2ME on apoptosis of MCF-7 breast cancer cells and explored a combination therapy using 2ME and a polyamine analogue, bis(ethyl)norspermine (BE-3-3-3). Determination of viable cells on day 4 of treatment with 2ME/BE-3-3-3 combinations showed synergistic effects by Chou-Talalay analysis. APO-BRDU analysis showed that there was only 1.5+/-0.5% apoptosis at 200 nM 2ME and 3.7+/-1.7% in the presence of 2.5 microM BE-3-3-3. Combination of 200 nM 2ME and 2.5 microM BE-3-3-3 resulted in 52.2+/-2.6% apoptosis. Up to 90% of the cells underwent apoptosis in the presence of 1000 nM 2ME and 2.5 microM BE-3-3-3. Combination treatments resulted in total disruption of microtubules and depletion of putrescine, spermidine and spermine. In addition, phosphorylation of Akt and nuclear localization of cyclin D1 were altered by 2ME/BE-3-3-3 combination. Our results suggest an important strategy to induce apoptosis of breast cancer cells, with potential applications in therapy.

Regulated binding of adenomatous polyposis coli protein to actin

Adenomatous polyposis coli (APC) protein is a large tumor suppressor that is truncated in most colorectal cancers. The carboxyl-terminal third of APC protein mediates direct interactions with microtubules and the microtubule plus-end tracking protein EB1. In addition, APC has been localized to actin-rich regions of cells, but the mechanism and functional significance of this localization have remained unclear. Here we show that purified carboxyl-terminal basic domain of human APC protein (APC-basic) bound directly to and bundled actin filaments and associated with actin stress fibers in microinjected cells. Actin filaments and microtubules competed for binding to APC-basic, but APC-basic also could cross-link actin filaments and microtubules at specific concentrations, suggesting a possible role in cytoskeletal cross-talk. APC interactions with actin in vitro were inhibited by its ligand EB1, and co-microinjection of EB1 prevented APC association with stress fibers. Point mutations in EB1 that disrupted APC binding relieved the inhibition in vitro and restored APC localization to stress fibers in vivo, demonstrating that EB1-APC regulation is direct. Because tumor formation and metastasis involve coordinated changes in the actin and microtubule cytoskeletons, this novel function for APC and its regulation by EB1 may have direct implications for understanding the molecular basis of tumor suppression.

Intracellular distribution of the lysyl oxidase propeptide in osteoblastic cells

Lysyl oxidase plays a critical role in the formation of the extracellular matrix, and its activity is required for the normal maturation and cross-linking of collagen and elastin. An 18-kDa lysyl oxidase propeptide (LOPP) is generated from 50-kDa prolysyl oxidase by extracellular proteolytic cleavage during the biosynthesis of active 30-kDa lysyl oxidase enzyme. The fate and the functions of the LOPP are largely unknown, although intact LOPP was previously observed in osteoblast cultures. We investigated the spatial localization of molecular forms of lysyl oxidase, including LOPP in proliferating and differentiating osteoblasts, by using confocal immunofluorescence microscopy and Western blots of cytoplasmic and nuclear extracts. In the present study, a stage-dependent intracellular distribution of LOPP in the osteoblastic cell was observed. In proliferating osteoblasts, LOPP epitopes were principally associated with the Golgi and endoplasmic reticulum, and mature lysyl oxidase epitopes were found principally in the nucleus and perinuclear region. In differentiating cells, LOPP and mature lysyl oxidase immunostaining showed clear colocalization with the microtubule network. The subcellular distribution of LOPP and its temporal and physical association with microtubules were confirmed by Western blot and far Western blot studies. We also report that N-glycosylated and nonglycosylated LOPP are present in MC3T3-E1 cell cultures. We conclude that LOPP has a stage-dependent intracellular distribution in osteoblastic cells. Future studies are needed to investigate whether the LOPP associations with microtubules or the osteoblast nucleus have functional effects for osteoblast differentiation and bone formation.

Polyamine and aminoguanidine treatments to promote structural and functional recovery in the adult mammalian brain after injury: a brief literature review and preliminary data about their combined administration

The regeneration potential of the adult mammalian central nervous system (CNS) is very modest, due to, among other factors, the presence of either a glial scar, or myelin-associated regeneration inhibitors such as Nogo-A, MAG and OMgp, which all interact with the same receptor (NgR). After a brief review of the key proteins (Rho and PKC) implicated in NgR-mediated signalling cascades, we will tackle the implications of cAMP and Arginase I in overcoming myelin growth-inhibitory influence, and then will focus on the effects of polyamines and aminoguanidine to propose (and to briefly support this proposal by our own preliminary data) that their association might be a potent way to enable functionally-relevant regeneration in the adult mammalian CNS.

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.

p25alpha is flexible but natively folded and binds tubulin with oligomeric stoichiometry

p25alpha is a 219-residue protein which stimulates aberrant tubulin polymerization and is implicated in a variety of other functions. The protein has unusual secondary structure involving significant amounts of random coil, and binding to microtubules is accompanied by a large structural change, suggesting a high degree of plasticity. p25alpha has been proposed to be natively unfolded, so that folding is coupled to interaction with its physiological partners. Here we show that recombinant human p25alpha is folded under physiological conditions, since it has a well structured and solvent-sequestered aromatic environment and considerable chemical shift dispersion of amide and aliphatic protons. With increasing urea concentrations, p25alpha undergoes clear spectral changes suggesting significant loss of structure. p25alpha unfolds cooperatively in urea according to a simple two-state transition with a stability in water of approximately 5 kcal/mol. The protein behaves as a monomer and refolds with a transient on-pathway folding intermediate. However, high sensitivity to proteolytic attack and abnormal gel filtration migration behavior suggests a relatively extended structure, possibly organized in distinct domains. A deletion mutant of p25alpha lacking residues 3-43 also unfolds cooperatively and with similar stability, suggesting that the N-terminal region is largely unstructured. Both proteins undergo significant loss of structure when bound to monomeric tubulin. The stoichiometry of binding is estimated to be 3-4 molecules of tubulin per p25alpha and is not significantly affected by the deletion of residues 3-43. In conclusion, we dismiss the proposal that p25alpha is natively unfolded, although the protein is relatively flexible. This flexibility may be linked to its tubulin-binding properties.

A role for cAMP in regeneration of the adult mammalian CNS

Injury to the adult mammalian central nervous system (CNS) often results in permanent loss of sensory and motor function. This is due to the failure of injured axons to regenerate. The inhibitory nature of the CNS can be attributed to several factors, including formation of the glial scar, the presence of several molecules, associated with myelin, which inhibit axonal regrowth, and the intrinsic growth state of these neurons. Encouraging regeneration in the adult mammalian CNS therefore will require targeting one or all of these factors following injury. Here we illustrate recent work from our laboratory that identifies some of the signalling components involved in modulation of the intrinsic growth state of adult neurons. When activated, these signalling pathways can induce axonal regeneration in the presence of the myelin-associated inhibitors both in vitro and in vivo.

Charge variants of tubulin, tubulin S, membrane-bound and palmitoylated tubulin from brain and pheochromocytoma cells

Isoelectric focusing (IEF) of only approximately 1 microg of rat brain tubulin yields 27-30 distinct charge variants in the pH range of 4.5-5.4 with band separations of 0.01-0.02 pH units as detected by silver staining. Variants can be efficiently transferred from the immobilized gradient strip to polyvinylidene difluoride (PVDF) membranes for reaction with monoclonal antibodies. C-terminal-directed antibodies to alpha- and beta-tubulin yield patterns similar to N-terminal-directed antibodies. Removal of the acidic C-termini with subtilisin to form tubulin S increases the pI values by approximately 1 pH unit, leads to a loss in the isoelectric distinction between the alpha- and beta-tubulin variants seen by N-terminal-directed antibodies, and abolishes reactions with all beta-variants and all but three alpha variants by C-terminal-directed antibodies (TU-04 and TU-14). Many, but not all, of the variants are substrates for autopalmitoylation of rat brain tubulin. The distribution of isoelectric variants differs between cytoplasm and membrane fractions from PC12 pheochromocytoma cells. A potential role for different variants is suggested.

Arginase I and polyamines act downstream from cyclic AMP in overcoming inhibition of axonal growth MAG and myelin in vitro

Elevation of cAMP can overcome myelin inhibitors to encourage regeneration of the CNS. We show that a consequence of elevated cAMP is the synthesis of polyamines, resulting from an up-regulation of Arginase I, a key enzyme in their synthesis. Inhibiting polyamine synthesis blocks the cAMP effect on regeneration. Either over-expression of Arginase I or exogenous polyamines can overcome inhibition by MAG and by myelin in general. While MAG/myelin support the growth of young DRG neurons, they become inhibitory as DRGs mature. Endogenous Arginase I levels are high in young DRGs but drop spontaneously at an age that coincides with the switch from promotion to inhibition by MAG/myelin. Over-expressing Arginase I in maturing DRGs blocks that switch. Arginase I and polyamines are more specific targets than cAMP for intervention to encourage regeneration after CNS injury.

2',3'-Cyclic nucleotide 3'-phosphodiesterase: a membrane-bound, microtubule-associated protein and membrane anchor for tubulin

2',3'-Cyclic nucleotide-3'-phosphodiesterase (CNP) is firmly associated with tubulin from brain tissue and FRTL-5 thyroid cells as demonstrated by copolymerization with microtubules through several warm/cold cycles, the presence of CNP activity in purified tubulin preparations, and identical behavior during various extraction procedures. CNP acts as a microtubule-associated protein in promoting microtubule assembly at low mole ratios. This activity resides in the C terminus of the enzyme, which, by itself, promotes microtubule assembly at higher mole ratios. Phosphorylation of CNP interferes with its assembly-promoting activity, as does deletion of the C terminus, which leads to abnormal microtubule distribution in the cell. Submembranous colocalization of the proteins and CNP-dependent microtubule organization suggest that CNP is a membrane-bound microtubule-associated protein that can link tubulin to membranes and may regulate cytoplasmic microtubule distribution.

Amino acid/spermine conjugates: polyamine amides as potent spermidine uptake inhibitors

In this paper we describe the synthesis and characterization of a series of simple spermine/amino acid conjugates, some of which potently inhibit the uptake of spermidine into MDA-MB-231 breast cancer cells. The presence of an amide in the functionalized polyamine appeared to add to the affinity for the polyamine transporter. The extensive biological characterization of an especially potent analogue from this series, the Lys-Spm conjugate (31), showed this molecule will be an extremely useful tool for use in polyamine research. It was shown that the use of 31 in combination with DFMO led to a cytostatic growth inhibition of a variety of cancer cells, even when used in the presence of an extracellular source of transportable spermidine. It was furthermore shown that this combination effectively reduced the cellular levels of putrescine and spermidine while not affecting the levels of spermine. These facts together with the nontoxic nature of 31 make it a novel lead for further anticancer development.

Structural insight into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of alpha/beta-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Structural basis for the interaction of tubulin with proteins and drugs that affect microtubule dynamics

The microtubule cytoskeleton is a highly regulated system. At different times in the cell cycle and positions within the organism, microtubules can be very stable or highly dynamic. Stability and dynamics are regulated by interaction with a large number of proteins that themselves may change at specific points in the cell cycle. Exogenous ligands can disrupt the normal processes by either increasing or decreasing microtubule stability and inhibiting their dynamic behavior. The recent determination of the structure of tubulin, the main component of microtubules, makes it possible now to begin to understand the details of these interactions. We review here the structure of the tubulin dimer, with particular regard to how proteins and drugs may bind and modulate microtubule dynamics.

Structural insights into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of /¿-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

The carboxy terminus of Tub4p is required for gamma-tubulin function in budding yeast

The role of gamma-tubulin in microtubule nucleation is well established, however, its function in other aspects of microtubule organization is unknown. The carboxy termini of alpha/beta-tubulins influence the assembly and stability of microtubules. We investigated the role of the carboxy terminus of yeast gamma-tubulin (Tub4p) in microtubule organization. This region consists of a conserved domain (DSYLD), and acidic tail. Cells expressing truncations lacking the DSYLD domain, tail or both regions are temperature sensitive for growth. Growth defects of tub4 mutants lacking either or both carboxy-terminal domains are suppressed by the microtubule destabilizing drug benomyl. tub4 carboxy-terminal mutants arrest as large budded cells with short bipolar spindles positioned at the bud neck. Electron microscopic analysis of wild-type and CTR mutant cells reveals that SPBs are tightly associated with the bud neck/cortex by cytoplasmic microtubules in mutants lacking the tail region (tub4-delta 444, tub4-delta 448). Mutants lacking the DSYLD residues (tub4-delta 444, tub4-delta DSYLD) form many cytoplasmic microtubules. We propose that the carboxy terminus of Tub4p is required for re-organization of the microtubules upon completion of nuclear migration, and facilitates spindle elongation into the bud.

Preparation and properties of pure tubulin S

Limited proteolysis of the tubulin dimer (alphabeta) by subtilisin occurs more rapidly with beta than with alpha tubulin. This leads to the formation of an intermediate hybrid dimer, alphabeta(s), before both C termini are cleaved to form tubulin S(alpha(s)beta(s)). The three forms of tubulin usually coexist in subtilisin-treated preparations and such cross-contamination can be reliably detected only by running SDS-polyacrylamide gels well beyond expulsion of the dye front. Previously published preparations have not ruled out such contamination or have formed poorly reversible polymers. Because ion exchange separation incurred substantial protein losses, we have developed a new protocol for rapid preparation of tubulin S (alpha(s)beta(s), free of alphabeta or alphabeta(s)) that is based on proteolysis at low ionic strength. This increases the relative rate of C terminal cleavage of beta tubulin. The product forms sheets, bundles, or rings that are depolymerized by cold, salt, and podophyllotoxin, partially depolymerized by Ca2+, and has a decreased critical concentration for polymerization that can be further decreased by taxol. We have also found a method for forming nearly pure alphabeta(s) dimers by using methods that retard proteolysis of the C terminus of alpha tubulin.

1-(N-alkylamino)-11-(N-ethylamino)-4,8-diazaundecanes: simple synthetic polyamine analogues that differentially alter tubulin polymerization

Polyamine analogues such as bis(ethyl)norspermine and N1-(cyclopropylmethyl)-N11-ethyl-4,8-diazaundecane (CPENSpm) act as potent modulators of cellular polyamine metabolism in vitro and possess impressive antitumor activity against a number of cell lines. Some of these polyamine analogues appear to produce their cell-type-specific cytotoxic activity through the superinduction of spermidine/spermine N1-acetyltransferase (SSAT). However, there are several analogues (e.g., N1-(cycloheptylmethyl)-N11-ethyl-4, 8-diazaundecane (CHENSpm)) which are effective cytotoxic agents but do not superinduce SSAT. We have previously demonstrated that CPENSpm and CHENSpm both initiate the cell death program, although by different mechanisms, and that CHENSpm (but not CPENSpm) induces a G2/M cell cycle arrest. We now report that one potential mechanism by which some polyamine analogues can retard growth and ultimately produce cytotoxicity is through interference with normal tubulin polymerization. In these studies, we compare the effects of the polyamine analogues CHENSpm, CPENSpm, and (S)-N1-(2-methyl-1-butyl)-N11-ethyl-4,8-diazaundecane (IPENSpm) on in vitro tubulin polymerization. These spermine analogues behave very differently from spermine and from each other in terms of tubulin polymerization rate, equilibrium levels, and time of polymerization initiation. These results demonstrate that structurally similar polyamine analogues with potent antitumor effects can produce significantly different cellular effects. The discovery of polyamine analogues that can alter tubulin polymerization provides a series of promising lead compounds that may have a similar spectrum of activity to more difficult to synthesize compounds typified by paclitaxel.

Localization of critical histidyl residues required for vinblastine-induced tubulin polymerization and for microtubule assembly

Vinblastine-induced tubulin polymerization is electrostatically regulated and shows pH dependence with a pI approximately 7.0 suggesting the involvement of histidyl residues. Modification of histidyl residues of tubulin with diethylpyrocarbonate (DEPC) at a mole ratio of 0.74 (DEPC/total His residues) for 3 min at 25 degreesC completely inhibited vinblastine-induced polymerization with little effect on microtubule assembly. Under these conditions DEPC reacts only with histidyl residues. For complete inhibition two histidyl residues have to be modified. Demodification of the carboxyethyl histidyl derivatives by hydroxylamine led to nearly complete recovery of polymerization competence. Labeling with [14C]DEPC localized both of these histidyl residues on beta-tubulin at beta227 and beta264. Similarly, tubulin modification with DEPC for longer times (8 min) resulted in complete inhibition of microtubule assembly, at which time approximately 4 histidyl residues had been modified. This inhibition by DEPC was also reversed by hydroxylamine. The third histidyl residue was found on alpha-tubulin at alpha88. Thus, two charged histidyl residues are obligatorily involved in vinblastine-induced polymerization, whereas a different histidyl residue on a different tubulin monomer is involved in microtubule assembly.