Phosphofructokinase in the rat nervous system: regional differences in activity and characteristics of axonal transport

The fate and function of oligodendrocyte progenitor cells after traumatic spinal cord injury

Oligodendrocyte progenitor cells (OPCs) are the most proliferative and dispersed population of progenitor cells in the adult central nervous system, which allows these cells to rapidly respond to damage. Oligodendrocytes and myelin are lost after traumatic spinal cord injury (SCI), compromising efficient conduction and, potentially, the long-term health of axons. In response, OPCs proliferate and then differentiate into new oligodendrocytes and Schwann cells to remyelinate axons. This culminates in highly efficient remyelination following experimental SCI in which nearly all intact demyelinated axons are remyelinated in rodent models. However, myelin regeneration comprises only one role of OPCs following SCI. OPCs contribute to scar formation after SCI and restrict the regeneration of injured axons. Moreover, OPCs alter their gene expression following demyelination, express cytokines and perpetuate the immune response. Here, we review the functional contribution of myelin regeneration and other recently uncovered roles of OPCs and their progeny to repair following SCI.

Myelination and the trophic support of long axons

In addition to their role in providing myelin for rapid impulse propagation, the glia that ensheath long axons are required for the maintenance of normal axon transport and long-term survival. This presumably ancestral function seems to be independent of myelin membrane wrapping. Here, I propose that ensheathing glia provide trophic support to axons that are metabolically isolated, and that myelin itself might cause such isolation. This glial support of axonal integrity may be relevant for a number of neurological and psychiatric diseases.

Shifts in the myosin heavy chain isozymes in the mouse heart result in increased energy efficiency

Cardiac-specific transgenesis in the mouse is widely used to study the basic biology and chemistry of the heart and to model human cardiovascular disease. A fundamental difference between mouse and human hearts is the background motor protein: mouse hearts contain predominantly the alphaalpha-myosin heavy chain (MyHC) isozyme while human hearts contain predominantly the betabeta-MyHC isozyme. Although the intrinsic differences in mechanical and enzymatic properties of the alphaalpha- and betabeta-MyHC molecules are well known, the consequences of isozyme shifts on energetics of the intact beating heart remain unknown. Therefore, we compared the free energy of ATP hydrolysis (|DeltaG( approximately ATP)|) determined by (31)P-NMR spectroscopy in isolated perfused littermate mouse hearts containing the same amount of myosin comprised of either >95% alphaalpha-MyHC or approximately 83% betabeta-MyHC. |DeltaG( approximately ATP)| was approximately 2 kJ mol(-1) higher in the betabeta-MyHC hearts at all workloads. Furthermore, upon inotropic challenge, hearts containing predominantly betabeta-MyHC hearts increased developed pressure more than alphaalpha-MyHC hearts whereas heart rate increased more in alphaalpha-MyHC hearts. Thus, hearts containing predominantly the betabeta-MyHC isozyme are more energy efficient than alphaalpha-MyHC hearts. We suggest that these fundamental differences in the motor protein energy efficiency at the whole heart level should be considered when interpreting results using mouse-based cardiovascular modeling of normal and diseased human hearts.

Decreased contractile and metabolic reserve in peroxisome proliferator-activated receptor-alpha-null hearts can be rescued by increasing glucose transport and utilization

BACKGROUND

Downregulation of peroxisome proliferator-activated receptor-alpha (PPARalpha) in hypertrophied and failing hearts leads to the reappearance of the fetal metabolic pattern, ie, decreased fatty acid oxidation and increased reliance on carbohydrates. Here, we sought to elucidate the functional significance of this shift in substrate preference.

METHODS AND RESULTS

We assessed contractile function and substrate utilization using 13C nuclear magnetic resonance spectroscopy and high-energy phosphate metabolism using 31P nuclear magnetic resonance spectroscopy in perfused hearts isolated from genetically modified mice (PPARalpha(-/-)) that mimic the metabolic profile in myocardial hypertrophy. We found that the substrate switch from fatty acid to glucose (3-fold down) and lactate (3-fold up) in PPARalpha(-/-) hearts was sufficient for sustaining normal energy metabolism and contractile function at baseline but depleted the metabolic reserve for supporting high workload. Decreased ATP synthesis (measured by 31P magnetization transfer) during high workload challenge resulted in progressive depletion of high-energy phosphate content and failure to sustain high contractile performance. Interestingly, the metabolic and functional defects in PPARalpha(-/-) hearts could be corrected by overexpressing the insulin-independent glucose transporter GLUT1, which increased the capacity for glucose utilization beyond the intrinsic response to PPARalpha deficiency.

CONCLUSIONS

These findings demonstrate that metabolic remodeling in hearts deficient in PPARalpha increases the susceptibility to functional deterioration during hemodynamic overload. Moreover, our results suggest that normalization of myocardial energetics by further enhancing myocardial glucose utilization is an effective strategy for preventing the progression of cardiac dysfunction in hearts with impaired PPARalpha activity such as hearts with pathological hypertrophy.

Mechanisms for increased glycolysis in the hypertrophied rat heart

Glycolysis increases in hypertrophied hearts but the mechanisms are unknown. We studied the regulation of glycolysis in hearts with pressure-overload LV hypertrophy (LVH), a model that showed marked increases in the rates of glycolysis (by 2-fold) and insulin-independent glucose uptake (by 3-fold). Although the V(max) of the key glycolytic enzymes was unchanged in this model, concentrations of free ADP, free AMP, inorganic phosphate (P(i)), and fructose-2,6-bisphosphate (F-2,6-P2), all activators of the rate-limiting enzyme phosphofructokinase (PFK), were increased (up to 10-fold). Concentrations of the inhibitors of PFK, ATP, citrate, and H+ were unaltered in LVH. Thus, our findings show that increased glucose entry and activation of the rate-limiting enzyme PFK both contribute to increased flux through the glycolytic pathway in hypertrophied hearts. Moreover, our results also suggest that these changes can be explained by increased intracellular free [ADP] and [AMP], due to decreased energy reserve in LVH, activating the AMP-activated protein kinase cascade. This, in turn, results in enhanced synthesis of F-2,6-P2 and increased sarcolemma localization of glucose transporters, leading to coordinated increases in glucose transport and activation of PFK.

Decreased energetics in murine hearts bearing the R92Q mutation in cardiac troponin T

The thin filament protein cardiac troponin T (cTnT) is an important regulator of myofilament activation. Here we report a significant change in cardiac energetics in transgenic mice bearing the missense mutation R92Q within the tropomyosin-binding domain of cTnT, a mutation associated with a clinically severe form of familial hypertrophic cardiomyopathy. This functional domain of cTnT has recently been shown to be a crucial modulator of contractile function despite the fact that it does not directly interact with the ATP hydrolysis site in the myosin head. Simultaneous measurements of cardiac energetics using 31P NMR spectroscopy and contractile performance of the intact beating heart revealed both a decrease in the free energy of ATP hydrolysis available to support contractile work and a marked inability to increase contractile performance upon acute inotropic challenge in hearts from R92Q mice. These results show that alterations in thin filament protein structure and function can lead to significant defects in myocardial energetics and contractile reserve.

Tubulin and microtubule are potential targets for brain hexokinase binding

The metabolite-modulated association of a fraction of hexokinase to mitochondria in brain is well documented, however, the involvement of other non-mitochondrial components in the binding of the hexokinase is controversial. Now we present evidence that the hexokinase binds both tubulin and microtubules in brain in vitro systems. The interaction of tubulin with purified bovine brain hexokinase was characterized by displacement enzyme-linked immunosorbent assay using specific anti-brain hexokinase serum (IC(50)=4.0+/-1.4 microM). This value virtually was not affected by specific ligands such as ATP or glucose 6-phosphate. Microtubule-bound hexokinase obtained in reconstituted systems using microtubule and purified hexokinase or brain extract was visualized by transmission and immunoelectron microscopy on the surface of tubules. The association of purified bovine brain hexokinase with either tubulin or microtubules caused about 30% increase in the activity of the enzyme. This activation was also observed in brain, but not in muscle cell-free extract. The possible physiological relevance of the multiple heteroassociation of brain hexokinase is discussed.

Quantitative characterization of homo- and heteroassociations of muscle phosphofructokinase with aldolase

Dissociation of purified phosphofructokinase accompanied with inactivation was analyzed in the absence and presence of aldolase and the data were compared with those obtained with muscle extract. The kinetics of the decrease in enzymatic activity was highly dependent on the dilution factor in both cases, but the inactivation appeared to be biphasic only with extract. The inactivation of the phosphofructokinase was impeded by addition of excess of aldolase. Time courses of kinase inactivation were fitted by alternative kinetic models to characterize the multiple equilibria of several homo- and hetero-oligomers of phosphofructokinase. The combination of modeling data obtained with purified and extract systems suggests that aldolase binds to an intermediate dimer of phosphofructokinase and within this heterocomplex the kinase is completely active. The intermediate dimer is stabilized by association with microtubules and the kinase activity decreased due to dilution can be recovered by addition of excess aldolase. In extract, the phosphofructokinase is of sigmoidal character (Hill coefficient of 2.3); the addition of excess exogenous aldolase to phosphofructokinase resulted in heterocomplex formation displaying Michaelian kinetics. The possible physiological relevance of heterocomplex formation of phosphofructokinase in muscle extract is discussed.

Macromolecular compartmentation and channeling

One of the accepted characterizations of the living state is that it is complex to an extraordinary degree. Since our current understanding of the living condition is minimal and fragmentary, it is not surprising that our first descriptions are simplistic. However, in certain areas of metabolism, especially those that have been amenable to experimentation for the longest period of time, the simplistic explanations have been the most difficult to revise. For example, current texts of general biochemistry still view metabolism as occurring by a series of independent enzymes dispersed in a uniform aqueous environment. This notion has been shown to be deeply flawed by both experimental and theoretical considerations. Thus, there is ample evidence that, in many metabolic pathways, specific interactions between sequential enzymes occur as static and/or dynamic complexes. In addition, reversible interactions of enzymes with structural proteins and membranes is a common occurrence. The interactions of enzymes give rise to a higher level of complexity that must be accounted for when one wishes to understand the regulation of metabolism. One of the phenomena that occurs because of sequential enzyme interactions is the process of channeling. This article discusses enzyme interactions and channeling and summarizes experimental and theoretical results from a few well-studied examples.

Combined enhancement of microtubule assembly and glucose metabolism in neuronal systems in vitro: decreased sensitivity to copper toxicity

Brain cell-free extract greatly stimulates the polymerization rate of purified tubulin with a reduction of the nucleation period and without a significant alteration of the final assembly state. This effect is mimicked by neuroblastoma extract at 10-fold lower extract concentration, but not by excess muscle extract. Copper inhibits microtubule assembly in vitro but in the presence of brain extract the copper effect is suspended. Electron microscopic images showed that intact microtubules are formed and decorated by cytosolic proteins in the absence and presence of copper, while the copper alone induces the formation of S-shaped sheets and oligomeric threads. The flux of triosephosphate formation from glucose is enhanced by microtubules in brain extract, but not in muscle extract. Copper inhibits the glycolytic flux; however, the presence of microtubules not only suspends the inhibition by copper but the activation of glycolysis by microtubules is also preserved. We conclude that the organization of neuronal proteins modifies both the rates of microtubule assembly and glycolysis, and reduces their sensitivities against the inhibition caused by copper.

Phosphofructokinase from Dictyostelium discoideum is a potent inhibitor of tubulin polymerization

We identified the nonallosteric phosphofructokinase from the slime mold Dictyostelium discoideum as a potent protein factor that inhibits the rate of polymerization of tubulin at a molar ratio of 1 molecule to about 300 tubulin dimers for half-maximal action (IC50 = 32 nM). This effect was (i) assessed by turbidity measurements, pelleting of microtubules, and electron microscopy, (ii) observed when tubulin assembly was induced by taxol as well as by GTP in the presence of microtubule-associated proteins or glutamate, and (iii) specific as it was not produced by the phosphofructokinase from rabbit muscle. Also in contrast to the latter, neither tubulin nor microtubules modified the catalytic activity of the slime mold isozyme. Immunoelectron microscopy provided further evidence that D. discoideumphosphofructokinase physically interacts with tubulin, leading to the formation of aggregates. The process seems to be reversible since microtubules eventually formed in the presence of the inhibitor with concomitant reduction of tubulin aggregates. Limited proteolysis by subtilisin showed that the hypervariable C-termini of tubulin is not involved in the interaction with the enzyme. The possible physiological relevance of this novel function of D. discoideum phosphofructokinase different from its glycolytic action is discussed.

Pyruvate kinase as a microtubule destabilizing factor in vitro

Endogenous control of microtubule dynamism is essential in many cell types. Numerous microtubule-adhering proteins stabilize the polymer status, while very few protein factors are described with opposite effects. The brain- and muscle-specific M1 isoform of the enzyme pyruvate kinase is investigated here in this respect. Three pieces of evidence indicate antimicrotubular effects of this protein. (1) Pyruvate kinase inhibits taxol-induced tubulin polymerization into microtubules as revealed by turbidimetry. (2) Pelleting experiments show that pyruvate kinase partially disassembles taxol-stabilized microtubules into less sedimentable oligomers leading to the appearance of tubulin in the supernatant fractions. (3) Electron microscopy reveals the kinase-induced formation of great amounts of thread-like tubulin oligomers which tend to accumulate in a light/less sedimentable fraction. Immunoelectron micrographs using labeled antibody against pyruvate kinase provide evidence for the binding of pyruvate kinase to the thread-like oligomeric forms. The present data allow the assumption that pyruvate kinase may display multiple regulatory functions as a glycolytic control enzyme and as a modulator of microtubule dynamism.

Impaired cardiac energetics in mice lacking muscle-specific isoenzymes of creatine kinase

Our purpose was to determine whether hearts from mice bioengineered to lack either the M isoform of creatine kinase (MCK-/- mice) or both the M and mitochondrial isoforms (M/MtCK-/- mice) have deficits in cardiac contractile function and energetics, which have previously been reported in skeletal muscle from these mice. The phenotype of hearts with deleted creatine kinase (CK) genes is of clinical interest, since heart failure is associated with decreased total CK activity and changes in the relative amounts of the CK isoforms in the heart. We measured isovolumic contractile performance in isolated perfused hearts from wild-type, MCK-/-, and M/MtCK-/- mice simultaneously with cardiac energetics (31P-nuclear magnetic resonance spectroscopy) at baseline, during increased cardiac work, and during recovery. Hearts from wild-type, MCK-/-, and M/MtCK-/- mice had comparable baseline function and responded to 10 minutes of increased heart rate and perfusate Ca2+ with similar increases in rate-pressure product (48+/-5%, 42+/-6%, and 51+/-6%, respectively). Despite a similar contractile response, M/MtCK-/- hearts increased [ADP] by 95%, whereas wild-type and MCK-/- hearts maintained [ADP] at baseline levels. The free energy released from ATP hydrolysis decreased by 3.6 kJ/mol in M/MtCK-/- hearts during increased cardiac work but only slightly in wild-type (1.7 kJ/mol) and MCK-/- (1.5 kJ/mol) hearts. In contrast to what has been reported in skeletal muscle, M/MtCK-/- hearts were able to hydrolyze and resynthesize phosphocreatine. Taken together, our results demonstrate that when CK activity is lowered below a certain level, increases in cardiac work become more "energetically costly" in terms of high-energy phosphate use, accumulation of ADP, and decreases in free energy released from ATP hydrolysis, but not in terms of myocardial oxygen consumption.

Alternative binding of two sequential glycolytic enzymes to microtubules. Molecular studies in the phosphofructokinase/aldolase/microtubule system

Simultaneous binding of two sequential glycolytic enzymes, phosphofructokinase and aldolase, to a microtubular network was investigated. The binding of the phosphofructokinase to microtubules and its bundling activity has been previously characterized (Lehotzky, A., Telegdi, M., Liliom, K., and Ovádi, J. (1993) J. Biol. Chem. 268, 10888-10894). Aldolase binding to microtubules at near physiological ionic strength is weak (Kd = 20 microM) as compared with that of the kinase (Kd = 1 microM). The interactions of both enzymes with microtubules are modulated by their common intermediate, fructose-1,6-bisphosphate. Pelleting and electron microscopic measurements have revealed that the aldolase binding interferes with that of phosphofructokinase, although they have distinct binding domains on microtubules. The underlying molecular mechanism responsible for this finding is that in the solution phase aldolase and phosphofructokinase form a bienzyme complex that does not bind to the microtubule. The bienzyme complex formation does not influence the catalytic activity of aldolase, however, it inhibits the dissociation-induced inactivation of the kinase by stabilizing a catalytically active molecular form. The present data suggest the first experimental evidence that two sequential glycolytic enzymes do not associate simultaneously to microtubules, but their complexation in solution provides kinetic advantage for glycolysis.

Characterization of microtubule-phosphofructokinase complex: specific effects of MgATP and vinblastine

Phosphofructokinase interacts with both microtubules and microtubules containing microtubule-associated proteins to produce bundling and periodical cross-bridging of tubules. Immunoelectron microscopy using anti-phosphofructokinase antibodies provided direct evidence that the kinase molecules are responsible for the cross-bridging of microtubules. Limited proteolysis by subtilisin, a procedure that cleaves the N-terminal segment of the free enzyme as well as the C-terminal "tails" of tubulin subunits exposed on microtubules, showed that while phosphofructokinase becomes resistant, tubulin retains sensitivity against proteolysis within the heterologous complex. These data suggest that the N-terminal segment of the enzyme, but not the C-terminal "tail" of tubulin subunits, is involved in the interaction between the microtubule and the kinase. The phosphorylation of phosphofructokinase or microtubules containing microtubule-associated proteins by the cAMP-dependent protein kinase did not interfere with the heterologous complex formation. MgATP prevents phosphofructokinase binding to the microtubules, and it can displace the enzyme from the single microtubules. However, the bundled microtubules are apparently resistant to the MgATP dissociation effect. Modelling of the assembly process suggests that the tubulin-kinase complex is able to polymerize as the free tubulin. Vinblastine, an anti-mitotic agent, inhibits tubulin assembly; however, its inhibitory effect is partially suppressed in the presence of phosphofructokinase. Fluorescence anisotropy measurements indicated that kinase and vinblastine compete for tubulin binding with no evidence for ternary complex formation. This competitive mechanism and the ability of the tubulin-enzyme complex to polymerize into microtubules may result in the resistance of the tubulin-enzyme complex against the inhibition of assembly induced by vinblastine. Microtubules formed in the presence of vinblastine plus phosphofructokinase can be visualized by electron microscopy. A molecular model is suggested that summarizes the effects of MgATP and vinblastine on the multiple equilibria in the tubulin/microtubules/phosphofructokinase system.

Role of MgADP in the development of diastolic dysfunction in the intact beating rat heart

Sarcomere relaxation depends on dissociation of actin and myosin, which is regulated by a number of factors, including intracellular [MgATP] as well as MgATP hydrolysis products [MgADP] and inorganic phosphate [Pi], pHi, and cytosolic calcium concentration ([Ca2+]c). To distinguish the contribution of MgADP from the other regulators in the development of diastolic dysfunction, we used a strategy to increase free [MgADP] without changing [MgATP], [Pi], or pHi. This was achieved by applying a low dose of iodoacetamide to selectively inhibit the creatine kinase activity in isolated perfused rat hearts. [MgATP], [MgADP], [Pi], and [H+] were determined using 31P NMR spectroscopy. The [Ca2+]c and the glycolytic rate were also measured. We observed an approximately threefold increase in left ventricular end diastolic pressure (LVEDP) and 38% increase in the time constant of pressure decay (P < 0.05) in these hearts, indicating a significant impairment of diastolic function. The increase in LVEDP was closely related to the increase in free [MgADP]. Rate of glycolysis was not changed, and [Ca2+]c increased by 16%, which cannot explain the severity of diastolic dysfunction. Thus, our data indicate that MgADP contributes significantly to diastolic dysfunction, possibly by slowing the rate of cross-bridge cycling.

Specific characteristics of phosphofructokinase-microtubule interaction

Muscle phosphofructokinase interacts with microtubule-associated protein-free microtubules resulting in a reduction of the overall activity of the enzyme [Lehotzky et al. (1993) J. Biol. Chem. 268, 10888-10894] and periodical cross-linking of the tubules [Lehotzky et al. (1994) Biochem. Biophys. Res. Commun. 204, 585-591]. Microtubule polymers of 'tail-free' tubulin obtained by removal of the carboxy-termini with limited subtilisin digestion retain the binding domains for phosphofructokinase that cross-bridges microtubule 'bodies'. Microtubule-associated proteins bound on tubulin 'tails' do not perturb the kinase binding. These data suggest that the tubulin carboxy-terminal domain is not involved in microtubule-phosphofructokinase interactions and phosphofructokinase and microtubule-associated proteins have distinct binding domains on microtubules. Of different isoforms of phosphofructokinase, occurring mainly in brain and tumor cells, the muscle isoform exhibits selective adsorption behaviour on microtubules. Phosphofructokinase M and C isoforms with different associative and allosteric properties may represent an auxiliary pathway to modulate energy production via glycolysis.

Impairment of energy metabolism in intact residual myocardium of rat hearts with chronic myocardial infarction

The purpose of this study was to test the hypothesis that energy metabolism is impaired in residual intact myocardium of chronically infarcted rat heart, contributing to contractile dysfunction. Myocardial infarction (MI) was induced in rats by coronary artery ligation. Hearts were isolated 8 wk later and buffer-perfused isovolumically. MI hearts showed reduced left ventricular developed pressure, but oxygen consumption was unchanged. High-energy phosphate contents were measured chemically and by 31P-NMR spectroscopy. In residual intact left ventricular tissue, ATP was unchanged after MI, while creatine phosphate was reduced by 31%. Total creatine kinase (CK) activity was reduced by 17%, the fetal CK isoenzymes BB and MB increased, while the "adult" mitochondrial CK isoenzyme activity decreased by 44%. Total creatine content decreased by 35%. Phosphoryl exchange between ATP and creatine phosphate, measured by 31P-NMR magnetization transfer, fell by 50% in MI hearts. Thus, energy reserve is substantially impaired in residual intact myocardium of chronically infarcted rats. Because phosphoryl exchange was still five times higher than ATP synthesis rates calculated from oxygen consumption, phosphoryl transfer via CK may not limit baseline contractile performance 2 mo after MI. In contrast, when MI hearts were subjected to acute stress (hypoxia), mechanical recovery during reoxygenation was impaired, suggesting that reduced energy reserve contributes to increased susceptibility of MI hearts to acute metabolic stress.

Fast axonal transport of kinesin in the rat visual system: functionality of kinesin heavy chain isoforms

The mechanochemical ATPase kinesin is thought to move membrane-bounded organelles along microtubules in fast axonal transport. However, fast transport includes several classes of organelles moving at rates that differ by an order of magnitude. Further, the fact that cytoplasmic forms of kinesin exist suggests that kinesins might move cytoplasmic structures such as the cytoskeleton. To define cellular roles for kinesin, the axonal transport of kinesin was characterized. Retinal proteins were pulse-labeled, and movement of radiolabeled kinesin through optic nerve and tract into the terminals was monitored by immunoprecipitation. Heavy and light chains of kinesin appeared in nerve and tract at times consistent with fast transport. Little or no kinesin moved with slow axonal transport indicating that effectively all axonal kinesin is associated with membranous organelles. Both kinesin heavy chain molecular weight variants of 130,000 and 124,000 M(r) (KHC-A and KHC-B) moved in fast anterograde transport, but KHC-A moved at 5-6 times the rate of KHC-B. KHC-A cotransported with the synaptic vesicle marker synaptophysin, while a portion of KHC-B cotransported with the mitochondrial marker hexokinase. These results suggest that KHC-A is enriched on small tubulovesicular structures like synaptic vesicles and that at least one form of KHC-B is predominantly on mitochondria. Biochemical specialization may target kinesins to appropriate organelles and facilitate differential regulation of transport.

Microtubule transport and assembly cooperate to generate the microtubule array of growing axons

MTs are major architectural elements in growing axons. MTs overlap with each other along the axon, forming an array that is continuous from the cell body to the tip of the axon. The MT array constitutes a scaffolding that mechanically supports the elongate shape of the axon and also contributes directly to its shape. MTs also direct the transport of vesicular organelles between the cell body and the axon, and thereby determine, in part, the composition of the axon. In this article, I have discussed mechanisms involved in the elaboration of the MT array in growing axons, and I have emphasized the distinct but complementary roles of polymer transport mechanisms and local assembly dynamics. MTs for the axon originate in the cell body, and they are delivered to the axon by the polymer transport mechanisms. These mechanisms thus contribute directly to the shape of the axon by supplying it with essential architectural elements. The shape of the axon is further modulated by dynamic processes that alter cytoskeletal structure locally along its length. These dynamic processes include the assembly/disassembly mechanisms which influence polymer length and possibly number locally along the axon by subunit exchange between the monomer and polymer pools. In addition, the polymer transport mechanisms themselves are subject to modulation along the axon, as demonstrated by the observation that transport rate of MTs varies along the length of individual axons (Reinsch et al., 1991). Such local variations can, in and of themselves, change the number of MTs along the axon, and thereby focally affect axon shape. Thus, the dynamic processes of polymer transport and local assembly act cooperatively to shape the MT array of the axon, and thereby contribute directly to the elaboration of axonal morphology.

Brain isozyme of glycogen phosphorylase: immunohistological localization within the central nervous system

An antibody specific for the predicted carboxyterminal sequence of the human brain isozyme of glycogen phosphorylase (alpha-1,4-D-glucan:orthophosphate D-glucosyltransferase, EC 2.4.1.1) was generated to verify the carboxyterminal amino acid sequence of this protein. The isozyme-specific antibody was used to examine the localization of this protein in primate and non-primate brain. The highest levels of the brain isozyme in cerebrum and cerebellum were found in fibrous astrocytes, many with glial processes that appear to terminate upon blood vessels.

Axonal transport of a clathrin uncoating ATPase (HSC70): a role for HSC70 in the modulation of coated vesicle assembly in vivo

Clathrin plays an important role in many cellular processes, including endocytosis, secretion, and sorting of membranous organelles. Both the neuronal cell body and presynaptic terminals contain numerous coated vesicles, but few are detectable in the axonal regions that connect these two regions of the neuron. Clathrin heavy chains, light chains, and assembly proteins have all been shown to be axonally transported as part of slow component b (SCb). However, the paucity of coated vesicles present in the axon indicates the existence of a mechanism regulating clathrin coated assembly in vivo. A clathrin uncoating ATPase has been described that binds in stoichiometric amounts to clathrin and dissociates clathrin coats from vesicles in the presence of ATP in vitro. This clathrin uncoating ATPase is a major cytosolic protein of Mr 70 kD in bovine brain, forming 1% of soluble brain protein, and appears to be homologous with a constitutively expressed 70 kd heat shock protein (HSC70). We report here that a major 70 kD protein present in the SCb rate component of axonal transport is identical with HSC70 by both biochemical and immunochemical criteria. The cotransport of HSC70 with clathrin in SCb of axonal transport is consistent with a role for HSC70 in vivo in the regulation of clathrin function during axonal transport.