Targeted disruption of mouse conventional kinesin heavy chain, kif5B, results in abnormal perinuclear clustering of mitochondria

Dynamics of mitochondrial morphology in healthy cells and during apoptosis

Mitochondria exist as dynamic networks that often change shape and subcellular distribution. The number and morphology of mitochondria within a cell are controlled by precisely regulated rates of organelle fusion and fission. Recent reports have described dramatic alterations in mitochondrial morphology during the early stages of apoptotic cell death, a fragmentation of the network and the remodeling of the cristae. Surprisingly, proteins discovered to control mitochondrial morphology appear to also participate in apoptosis and proteins associated with the regulation of apoptosis have been shown to affect mitochondrial ultrastructure. In this review the recent progress in understanding the mechanisms governing mitochondrial morphology and the latest advances connecting the regulation of mitochondrial morphology with programmed cell death are discussed.

Alternatively spliced products of the human kinesin light chain 1 (KNS2) gene

Conventional kinesin is a microtubule-based molecular motor involved in the transport of membranous and non-membranous cargoes. The kinesin holoenzyme exists as a heterotetramer, consisting of two heavy chain and two light chain subunits. It is thought that one function of the light chains is to interact with the cargo. Alternative splicing of kinesin light chain pre-mRNA has been observed in lower organisms, although evidence for alternative splicing of the human gene has not been reported. We have identified 19 variants of the human KNS2 gene (KLC1) that are generated by alternative splicing of downstream exons, but calculate that KNS2 has the potential to produce 285 919 spliceforms. Corresponding spliceforms of the mouse KLC1 gene were also identified. The alternative exons are all located 3' of exon 12 and the novel spliceforms produce both alternative carboxy termini and alternative 3' untranslated regions. The observation of multiple light chain isoforms is consistent with their proposed role in specific cargo attachment.

Alzheimer's presenilin 1 mutations impair kinesin-based axonal transport

Several lines of evidence indicate that alterations in axonal transport play a critical role in Alzheimer's disease (AD) neuropathology, but the molecular mechanisms that control this process are not understood fully. Recent work indicates that presenilin 1 (PS1) interacts with glycogen synthase kinase 3beta (GSK3beta). In vivo, GSK3beta phosphorylates kinesin light chains (KLC) and causes the release of kinesin-I from membrane-bound organelles (MBOs), leading to a reduction in kinesin-I driven motility (Morfini et al., 2002b). To characterize a potential role for PS1 in the regulation of kinesin-based axonal transport, we used PS1-/- and PS1 knock-inM146V (KIM146V) mice and cultured cells. We show that relative levels of GSK3beta activity were increased in cells either in the presence of mutant PS1 or in the absence of PS1 (PS1-/-). Concomitant with increased GSK3beta activity, relative levels of KLC phosphorylation were increased, and the amount of kinesin-I bound to MBOs was reduced. Consistent with a deficit in kinesin-I-mediated fast axonal transport, densities of synaptophysin- and syntaxin-I-containing vesicles and mitochondria were reduced in neuritic processes of KIM146V hippocampal neurons. Similarly, we found reduced levels of PS1, amyloid precursor protein, and synaptophysin in sciatic nerves of KIM146V mice. Thus PS1 appears to modulate GSK3beta activity and the release of kinesin-I from MBOs at sites of vesicle delivery and membrane insertion. These findings suggest that mutations in PS1 may compromise neuronal function by affecting GSK-3 activity and kinesin-I-based motility.

Inhibition of proliferation of PC3 cells by the branched-chain fatty acid, 12-methyltetradecanoic acid, is associated with inhibition of 5-lipoxygenase

BACKGROUND

Branched-chain fatty acids or fatty alcohols have been reported to possess anti-tumor activity in various tumor models. Here we study 12-methyltetradecanoic acid (12-MTA), a branched-chain fatty acid, isolated from a sea cucumber extract, on the growth of prostate cancer cells and investigate the underlying mechanisms of its effect.

METHODS

12-MTA was evaluated by MTT assay for its ability to inhibit cell proliferation in various cancer types. The ability of 12-MTA to induce apoptosis of PC3 cells was examined by morphologic changes, propidium iodide (PI) staining, and caspase-3 activation. Furthermore, alteration of eicosanoid metabolism by 12-MTA was examined in PC3 and RBL-1 cells and in purified lipoxygenase (LOX) and cyclooxygenase (COX) enzymes.

RESULTS

12-MTA inhibited proliferation of various cell lines, with IC50s ranging from 17.99 to 35.44 microg/ml. PI staining clearly showed that 12-MTA caused PC3 cell death through induction of apoptosis. At 50 microg/ml, 12-MTA increased caspase-3 activity four to seven-fold compared with that in control cells. Examination of cellular arachidonate metabolism showed that at 25 microg/ml, 12-MTA reduced the level of 5-hydroxyeicosatetraenoic acid (5-HETE) by 45%. Furthermore, exogenous 5-HETE protects PC3 cells from 12-MTA induced cell death.

CONCLUSIONS

12-MTA inhibited proliferation of cancer cells via apoptosis, in which caspase-3 may play a role. At relevant concentrations, 12-MTA can selectively inhibit the formation of 5-HETE, a metabolite of 5-lipoxygenase. This agent may be a novel adjunctive therapy for selected malignancies including prostate cancer.

Conventional kinesin KIF5B mediates insulin-stimulated GLUT4 movements on microtubules

Insulin stimulates glucose uptake in muscle and adipose cells by mobilizing intracellular membrane vesicles containing GLUT4 glucose transporter proteins to the plasma membrane. Here we show in live cultured adipocytes that intracellular membranes containing GLUT4-yellow fluorescent protein (YFP) move along tubulin-cyan fluorescent protein-labeled microtubules in response to insulin by a mechanism that is insensitive to the phosphatidylinositol 3 (PI3)-kinase inhibitor wortmannin. Insulin increased by several fold the observed frequencies, but not velocities, of long-range movements of GLUT4-YFP on microtubules, both away from and towards the perinuclear region. Genomics screens show conventional kinesin KIF5B is highly expressed in adipocytes and this kinesin is partially co-localized with perinuclear GLUT4. Dominant-negative mutants of conventional kinesin light chain blocked outward GLUT4 vesicle movements and translocation of exofacial Myc-tagged GLUT4-green fluorescent protein to the plasma membrane in response to insulin. These data reveal that insulin signaling targets the engagement or initiates the movement of GLUT4-containing membranes on microtubules via conventional kinesin through a PI3-kinase-independent mechanism. This insulin signaling pathway regulating KIF5B function appears to be required for GLUT4 translocation to the plasma membrane.

Kif1Bbeta isoform is enriched in motor neurons but does not change in a mouse model of amyotrophic lateral sclerosis

The kinesin superfamily motor protein Kif1B is expressed in two isoforms, Kif1Balpha and Kif1Bbeta, with distinct cargo-binding domains. We examined the mRNA distribution of the two isoforms in adjacent sections of brain and spinal cord of adult mice using in situ hybridization analysis. Kif1Bbeta mRNA is enriched in several regions of brain and spinal cord. Its levels are four to five times higher than that of the alpha isoform, which was barely detectable. The highest mRNA levels of Kif1Bbeta were found in the cortex, hippocampus, cerebellum and the grey matter of the spinal cord. At the cellular level the highest signal was found in motor neurons in the motor nuclei of medulla oblongata and the ventral horn of spinal cord. Because expression of other Kif genes is altered in amyotrophic lateral sclerosis (ALS) models, we examined the expression level of Kif1Bbeta mRNA in the spinal cord of transgenic mice carrying the SOD1G93A mutation, a model of familial ALS, at presymptomatic and early stages of the disease. No changes were observed in Kif1Bbeta mRNA in motor neurons or in other regions of the spinal cord. These findings indicate that Kif1Balpha, which modulates the transport of mitochondria, may play a major role in tissues other than the central nervous system. Instead Kif1Bbeta, responsible for the transport of synaptic vesicle precursors, seems to play an important role in the nervous system, particularly in the lower motor neurons. The absence of changes of Kif1Bbeta mRNA in transgenic SOD1G93A mice suggests that other molecular mechanisms may play a role in the disruption of axonal transport occurring in the motor neurons of these mice.

Science in motion: common molecular pathological themes emerge in the hereditary spastic paraplegias

The hereditary spastic paraplegias are a group of neurodegenerative conditions that all share the principal clinical feature of progressive lower limb spastic paralysis, caused by either failure of development or progressive degeneration of the corticospinal tract. The conditions are characterised by extreme genetic heterogeneity, with at least 20 genes involved. Until recently, no functional overlap was apparent in the associated molecular pathological mechanisms. However, with recent progress in hereditary spastic paraplegia gene identification, common pathological themes are now emerging.

Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development

Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population.

Disruption of mitochondrial networks by the human cytomegalovirus UL37 gene product viral mitochondrion-localized inhibitor of apoptosis

By 24 h after infection with human cytomegalovirus, the reticular mitochondrial network characteristic of uninfected fibroblasts was disrupted as mitochondria became punctate and dispersed. These alterations were associated with expression of the immediate-early (alpha) antiapoptotic UL37x1 gene product viral mitochondrion-localized inhibitor of apoptosis (vMIA). Similar alterations in mitochondrial morphology were induced directly by vMIA in transfected cells. A 68-amino-acid antiapoptotic derivative of vMIA containing the mitochondrial localization and antiapoptotic domains also induced disruption, whereas a mutant lacking the antiapoptotic domain failed to cause disruption. These data suggest that the fission and/or fusion process that normally controls mitochondrial networks is altered by vMIA. Mitochondrial fission has been implicated in the induction of apoptosis and vMIA-mediated inhibition of apoptosis may occur subsequent to this event.

Follicle-stimulating hormone-responsive cytoskeletal genes in rat granulosa cells: class I beta-tubulin, tropomyosin-4, and kinesin heavy chain

FSH regulates gene expression for granulosa cell differentiation and follicular development. Therefore, FSH-responsive genes are crucial, but only a few genes have been identified for the early stage of follicular development. In particular, little is known about cytoskeletal genes, which likely play essential roles in the morphological changes such as the antrum formation, a major landmark. FSH is also known to induce the differentiation of an immature, undifferentiated rat ovary granulosa (ROG) cell line. Our data show that FSH induced massive yet distinct reorganization of microtubules and the actin cytoskeletons as well as morphological changes. To identify those genes responding to FSH during the differentiation, differential display was performed on ROG cells. Of the 80 FSH-responsive genes identified, there were three cytoskeleton-related genes (class I beta-tubulin, tropomyosin 4, and kinesin heavy chain), which are crucial for intracellular morphogenesis, transport, and differentiation. Northern blots show that the level of these gene transcripts reached a peak at 6 h after FSH treatment and subsided at 24 h. FSH induced the similar temporal expression not only in granulosa cells isolated from immature rats, but also in vivo. For instance, in situ hybridization showed that beta-tubulin mRNA was transiently expressed in the granulosa cells of large preantral and early antral follicles. Despite the same temporal expression, the regulatory mechanisms of the three genes were strikingly different. As an example, cycloheximide blocked the beta-tubulin mRNA expression, whereas it increased tropomyosin-4 (TM4) mRNA. Yet, it did not impact kinesin heavy chain (Khc) mRNA. In conclusion, FSH induces the massive reorganization of the cytoskeletons and morphological changes by the selective regulation of the gene expression, protein synthesis, and rearrangement of the cytoskeletal proteins in the ROG cells and probably, specific follicles and granulosa cells.

Axonal transport versus dendritic transport

Neurons have polarized processes for information output and input, axons, and dendrites. This polarized architecture is essential for the neuronal function. An increasing number of molecular components that mediate neuronal polarity establishment have been characterized over the past few years. The vast majority of these molecules include proteins that act in scaffolding protein complexes to sustain the polarized anchoring of molecules. In addition, more signaling and cytoskeleton-associated proteins have been proposed for establishment of polarity. It has become evident that dendritic and axonal transport of molecules depends on scaffolding/adaptor proteins that are recognized by molecular motors. Current and future research in the neuronal cell polarity will be focused on how different cargo molecules transmit their signals to the cytoskeleton and change its dynamic properties to affect the rate and direction of vesicular movement. In this review, we discuss recent evidence that scaffolding proteins can regulate motor motility and guidance by a mechanism of substrate-cytoskeletal coupling and amino acid modifications during polarized transport.

Genomic analysis of immediate/early response to shear stress in human coronary artery endothelial cells

The involvement of shear stress in the pathogenesis of vascular disease has motivated efforts to define the endothelial cell response to applied shear stress in vitro. A central question has been the mechanisms by which endothelial cells perceive and respond to changes in fluid flow. We have utilized cDNA microarrays to characterize the immediate/early genomic response to applied laminar shear stress (LSS) in primary cultures of human coronary artery endothelial cells (HCAECs). Cells were exposed, in a parallel plate flow chamber, to 0, 15, or 45 dyn/cm2 LSS for 1 h, and gene expression profiles were determined using human GEM1 cDNA microarrays. We find that a high proportion of LSS-responsive genes are transcription factors, and these are related by their involvement in growth arrest. These likely play a central role in the reprogramming of endothelial homeostasis following the switch from a static to a shear-stressed environment. LSS-responsive genes were also found to encode factors involved in vasoreactivity, signal transduction, antioxidants, cell cycle-associated genes, and markers of cytoskeletal function and dynamics.

Axonal transport of mitochondria to synapses depends on milton, a novel Drosophila protein

A protein required to localize mitochondria to Drosophila nerve terminals has been identified genetically. Photoreceptors mutant for milton show aberrant synaptic transmission despite normal phototransduction. Without Milton, synaptic terminals and axons lack mitochondria, although mitochondria are numerous in neuronal cell bodies. In contrast, synaptic vesicles continue to be transported to and concentrated at synapses. Milton protein is associated with mitochondria and is present primarily in axons and synapses. A likely explanation of the apparent trafficking defect is offered by the coimmunoprecipitation of Milton and kinesin heavy chain. Transfected into HEK293T cells, Milton induces a redistribution of mitochondria within the cell. We propose that Milton is a mitochondria-associated protein required for kinesin-mediated transport of mitochondria to nerve terminals.

Genetic and comparative mapping of genes dysregulated in mouse hearts lacking the Hand2 transcription factor gene

The helix-loop-helix transcription factor HAND2 plays a vital role in the development of the heart, limb, facies, and other neural crest-derived structures. We used differential display analysis to identify 33 putative HAND2-regulated ESTs that are differentially expressed in Hand2(-/-) vs wild-type mice. We determined the positions on mouse and human genetic maps of 29 of these by using the T31 mouse Radiation Hybrid panel, comparison to human genomic sequence, and comparative mapping. We examined the conserved chromosomal locations for phenotypes that involve development of heart, face, and limb structures that are affected by HAND2. One EST mapped to a region of conserved synteny between mouse chromosome 2 and human chromosome 10p. RACE analysis extended the sequence and identified this cDNA as the mouse ortholog of human nebulette, an actin-binding protein expressed in fetal heart. Nebulette was shown to be deleted in DiGeorge Syndrome 2 patients with the proximal deletion of human 10p13-p14 that is associated with cardiac and craniofacial abnormalities.

Involvement of conventional kinesin in glucose-stimulated secretory granule movements and exocytosis in clonal pancreatic beta-cells

Recruitment of secretory vesicles to the cell surface is essential for the sustained secretion of insulin in response to glucose. At present, the molecular motors involved in this movement, and the mechanisms whereby they may be regulated, are undefined. To investigate the role of kinesin family members, we labelled densecore vesicles in clonal beta-cells using an adenovirally expressed, vesicle-targeted green fluorescent protein (phogrin.EGFP), and employed immunoadsorption to obtain highly purified insulin-containing vesicles. Whereas several kinesin family members were expressed in this cell type, only conventional kinesin heavy chain (KHC) was detected in vesicle preparations. Expression of a dominant-negative KHC motor domain (KHC(mut)) blocked all vesicular movements with velocity >0.4 micro m second(-1), which demonstrates that kinesin activity was essential for vesicle motility in live beta-cells. Moreover, expression of KHC(mut) strongly inhibited the sustained, but not acute, stimulation of secretion by glucose. Finally, vesicle movement was stimulated by ATP dose-dependently in permeabilized cells, which suggests that glucose-induced increases in cytosolic [ATP] mediate the effects of the sugar in vivo, by enhancing kinesin activity. These data therefore provide evidence for a novel mechanism whereby glucose may enhance insulin release.

A kinesin heavy chain (KIF5A) mutation in hereditary spastic paraplegia (SPG10)

We have identified a missense mutation in the motor domain of the neuronal kinesin heavy chain gene KIF5A, in a family with hereditary spastic paraplegia. The mutation occurs in the family in which the SPG10 locus was originally identified, at an invariant asparagine residue that, when mutated in orthologous kinesin heavy chain motor proteins, prevents stimulation of the motor ATPase by microtubule-binding. Mutation of kinesin orthologues in various species leads to phenotypes resembling hereditary spastic paraplegia. The conventional kinesin motor powers intracellular movement of membranous organelles and other macromolecular cargo from the neuronal cell body to the distal tip of the axon. This finding suggests that the underlying pathology of SPG10 and possibly of other forms of hereditary spastic paraplegia may involve perturbation of neuronal anterograde (or retrograde) axoplasmic flow, leading to axonal degeneration, especially in the longest axons of the central nervous system.

Mitochondrial dynamics in filamentous fungi

Mitochondria are essential organelles of eukaryotic cells. They grow continuously throughout the cell cycle and are inherited by daughter cells upon cell division. Inheritance of mitochondria and maintenance of mitochondrial distribution and morphology require active transport of the organelles along the cytoskeleton and depend on membrane fission and fusion events. Many of the molecular components and cellular mechanisms mediating these complex processes have been conserved during evolution across the borders of the fungal and animal kingdoms. During the past few decades, several constituents of the cellular machinery mediating mitochondrial behavior have been identified and functionally characterized. Here, we review the contributions of fungi, with special emphasis on the filamentous fungus Neurospora crassa, to our current understanding of mitochondrial morphogenesis and inheritance.

Glutamate-receptor-interacting protein GRIP1 directly steers kinesin to dendrites

In cells, molecular motors operate in polarized sorting of molecules, although the steering mechanisms of motors remain elusive. In neurons, the kinesin motor conducts vesicular transport such as the transport of synaptic vesicle components to axons and of neurotransmitter receptors to dendrites, indicating that vesicles may have to drive the motor for the direction to be correct. Here we show that an AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor subunit--GluR2-interacting protein (GRIP1)--can directly interact and steer kinesin heavy chains to dendrites as a motor for AMPA receptors. As would be expected if this complex is functional, both gene targeting and dominant negative experiments of heavy chains of mouse kinesin showed abnormal localization of GRIP1. Moreover, expression of the kinesin-binding domain of GRIP1 resulted in accumulation of the endogenous kinesin predominantly in the somatodendritic area. This pattern was different from that generated by the overexpression of the kinesin-binding scaffold protein JSAP1 (JNK/SAPK-associated protein-1, also known as Mapk8ip3), which occurred predominantly in the somatoaxon area. These results indicate that directly binding proteins can determine the traffic direction of a motor protein.

Interaction of mitochondria with microtubules in the filamentous fungus Neurospora crassa

The establishment and maintenance of the 3D structure of eukaryotic cells depends on active transport and positioning of organelles along cytoskeletal elements. The biochemical basis of these processes is only poorly understood. We analysed the interaction of mitochondria with microtubules in the filamentous fungus Neurospora crassa. Mitochondria were fluorescently labelled by expression of matrix-targeted green fluorescent protein. Upon isolation, mitochondria collapsed to round spherical structures that were still able to interact with microtubules in vitro. Binding of mitochondria to microtubules was dependent on peripherally associated proteins on the organellar surface, and was sensitive to adenine nucleotides. MMM1, a mitochondrial outer membrane protein important for maintenance of normal mitochondrial morphology, was not required. This suggests that the interaction of mitochondria with the cytoskeleton is independent of MMM1. We conclude that mitochondrial morphology is maintained by a complex interplay of extrinsic and intrinsic factors, including ATP-dependent proteins on the organellar surface.

Late endosome motility depends on lipids via the small GTPase Rab7

We report that lipids contribute to regulate the bidirectional motility of late endocytic compartments. Late endocytic vesicles loaded with cholesterol lose their dynamic properties, and become essentially immobile, including in cells from Niemann-Pick C patients. These vesicles then retain cytoplasmic dynein activity, but seem to be unable to acquire kinesin activity, eventually leading to paralysis. Our data suggest that this defect depends on the small GTPase Rab7, since the motility of vesicles loaded with cholesterol can be restored by the Rab7 inhibitory mutant N125I. Conversely, wild-type Rab7 overexpression mimics the effects of cholesterol on motility in control cells. Consistently, cholesterol accumulation increases the amounts of membrane-associated Rab7, and inhibits Rab7 membrane extraction by the guanine nucleotide dissociation inhibitor. Our observations thus indicate that cholesterol contributes to regulate the Rab7 cycle, and that Rab7 in turn controls the net movement of late endocytic elements. We conclude that motor functions can be regulated by the membrane lipid composition via the Rab7 cycle.

Mitochondrial disappearance from cells: a clue to the role of autophagy in programmed cell death and disease?

When cells are induced to undergo apoptosis in the presence of general caspase inhibitors and then returned to their normal growth environment, there follows an extended period of life during which the entire cohort of mitochondria (including mitochondrial DNA) disappears from the cells. This phenomenon is widespread; it occurs in NGF-deprived sympathetic neurons, in NGF-maintained neurons treated with cytosine arabinoside, and in diverse cell lines treated with staurosporine, including HeLa, CHO, 3T3 and Rat 1 cells. Mitochondrial removal is highly selective since the structure of all other organelles remains unperturbed. Since Bcl2 overexpression blocks the removal of mitochondria without preventing death-inducing signals, it appears that the mitochondria are responsible for initiating their own demise. Degradation of mitochondria is not in itself a rare event. It occurs in large part by autophagy during normal cell house-keeping, during ecdysis in insects, as well as after induction of apoptosis. However, the complete and selective removal of an entire cohort of mitochondria in otherwise living mammalian cells has not been described previously. These findings raise several questions. What are the mechanisms which remove mitochondria in such a 'clean' fashion? What are the signals that target mitochondria for such selective degradation? How are cells that have lost their mitochondria different from rho0 cells (which retain mitochondria but lack mitochondrial DNA, and cannot carry out oxidative phosphorylation)? Are the cells which have lost mitochondria absolutely committed to die or might they be repaired by mitochondrial therapy? The answers will be especially relevant when considering treatment of diseases affecting long-lived and non-renewable organs such as the nervous system.

Molecular motor KIF1C is not essential for mouse survival and motor-dependent retrograde Golgi apparatus-to-endoplasmic reticulum transport

KIF1C is a new member of the kinesin superfamily of proteins (KIFs), which act as microtubule-based molecular motors involved in intracellular transport. We cloned full-length mouse kif1C cDNA, which turned out to have a high homology to a mitochondrial motor KIF1Balpha and to be expressed ubiquitously. To investigate the in vivo significance of KIF1C, we generated kif1C(-/-) mice by knocking in the beta-galactosidase gene into the motor domain of kif1C gene. On staining of LacZ, we detected its expression in the heart, liver, hippocampus, and cerebellum. Unexpectedly, kif1C(-/-) mice were viable and showed no obvious abnormalities. Because immunocytochemistry showed partial colocalization of KIF1C with the Golgi marker protein, we compared the organelle distribution in primary lung fibroblasts from kif1C(+/+) and kif1C(-/-) mice. We found that there was no significant difference in the distribution of the Golgi apparatus or in the transport from the Golgi apparatus to the endoplasmic reticulum (ER) facilitated by brefeldin A between the two cells. This retrograde membrane transport was further confirmed to be normal by time-lapse analysis. Consequently, KIF1C is dispensable for the motor-dependent retrograde transport from the Golgi apparatus to the ER.

A novel C-terminal kinesin is essential for maintaining functional acidocalcisomes in Trypanosoma brucei

Kinesins are cytoskeletal motor proteins that play roles in a variety of fundamental cellular processes including cell division and the anterograde transport of vesicles and organelles. We purified, cloned, and functionally characterized in Trypanosoma brucei a new member of the C-terminal kinesin family, TbKIFC1. Kinetic constants of the recombinant motor domain of TbKIFC1 were estimated at 0.56 microm for the microtubule dissociation constant (K(d)) with a k(cat) of 0.2 s(-1). Immunolocalization analysis showed an association of TbKIFC1 with punctate structures. Because they were rapidly transported to the negative pole of the microtubule after NH(4)Cl treatment, these structures were considered to be associated with acidic vesicles. To determine the role of the kinesin in vivo, we produced an inducible kinesin-deficient strain by double-stranded RNA interference methodology. Mutant cells were loaded with the fluorescent reagent fura2/acetoxymethylester to measure intracellular free calcium ([Ca(2+)](i)). The resting [Ca(2+)](i) was unchanged in mutant cells; however, alkalinization of acidic vesicles induced by NH(4)Cl or nigericin was not followed by release of Ca(2+). These data and the relative importance of the ionomycin-releasable and the ionomycin-plus-NH(4)Cl-releasable Ca(2+) pools suggest a lower Ca(2+) content in acidocalcisomes and dysfunctional Ca(2+) release.

Disruption of axonal transport and neuronal viability by amyloid precursor protein mutations in Drosophila

We tested the hypothesis that amyloid precursor protein (APP) and its relatives function as vesicular receptor proteins for kinesin-I. Deletion of the Drosophila APP-like gene (Appl) or overexpression of human APP695 or APPL constructs caused axonal transport phenotypes similar to kinesin and dynein mutants. Genetic reduction of kinesin-I expression enhanced while genetic reduction of dynein expression suppressed these phenotypes. Deletion of the C terminus of APP695 or APPL, including the kinesin binding region, disrupted axonal transport of APP695 and APPL and abolished the organelle accumulation phenotype. Neuronal apoptosis was induced only by overexpression of constructs containing both the C-terminal and Abeta regions of APP695. We discuss the possibility that axonal transport disruption may play a role in the neurodegenerative pathology of Alzheimer's disease.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

cRel induces mitochondrial alterations in correlation with proliferation arrest

We have previously shown that overexpressing cRel, a transcription factor of the Rel/NF-kappa B family, concomitantly inhibits proliferation of HeLa cells and makes them resistant against TNF alpha-induced apoptosis. Both effects rely on the upregulation of the manganese superoxide dismutase (MnSOD), a mitochondrial enzyme that converts O(2)(*-) in H(2)O(2). Here we describe additional alterations induced by cRel, namely mitochondrial clustering and accumulation of dense dark granules near the nucleus. These changes preferentially occur in cells that display a sustained cRel expression in the nucleus and that are cell-cycle arrested. As the cell-cycle arrest, these changes are reproduced by directly overexpressing MnSOD or by treating cells with H(2)O(2), suggesting they are due to MnSOD induction and ensuing H(2)O(2) accumulation. We propose that mitochondria cluster because they are damaged by the H(2)O(2) they overproduce. They would then be autophagocytosed and degraded in secondary lysosomes. In support of this scenario, we documented the occurrence of oxidative damage and the presence of lysosomes in the area of mitochondrial clustering. In addition, we identified the dense dark granules as lipofuscin, based on their autofluorescence. Lipofuscin could directly originate from the mitochondrial degradation products that would aggregate and become indigestible because of the presence of H(2)O(2) in the secondary lysosomes. Altogether, our findings show that cRel overexpression in HeLa cells creates, via the induction of MnSOD, an oxidative injury that culminates in mitochondrial degeneration, proliferation blockage, and resistance against TNF alpha-induced apoptosis.

Genetic evidence for a microtubule-destabilizing effect of conventional kinesin and analysis of its consequences for the control of nuclear distribution in Aspergillus nidulans

Conventional kinesin is a microtubule-dependent motor protein believed to be involved in a variety of intracellular transport processes. In filamentous fungi, conventional kinesin has been implicated in different processes, such as vesicle migration, polarized growth, nuclear distribution, mitochondrial movement and vacuole formation. To gain further insights into the functions of this kinesin motor, we identified and characterized the conventional kinesin gene, kinA, of the established model organism Aspergillus nidulans. Disruption of the gene leads to a reduced growth rate and a nuclear positioning defect, resulting in nuclear cluster formation. These clusters are mobile and display a dynamic behaviour. The mutant phenotypes are pronounced at 37 degrees C, but rescued at 25 degrees C. The hyphal growth rate at 25 degrees C was even higher than that of the wild type at the same temperature. In addition, kinesin-deficient strains were less sensitive to the microtubule destabilizing drug benomyl, and disruption of conventional kinesin suppressed the cold sensitivity of an alpha-tubulin mutation (tubA4). These results suggest that conventional kinesin of A. nidulans plays a role in cytoskeletal dynamics, by destabilizing microtubules. This new role of conventional kinesin in microtubule stability could explain the various phenotypes observed in different fungi.

Intact lysosome transport and phagosome function despite kinectin deficiency

The mechanism of cargo coupling to kinesin motor proteins is a fundamental issue in organelle transport along microtubules. Kinectin has been postulated to function as a membrane anchor protein that attaches various organelles to the prototype motor protein kinesin. To verify the biological relevance of kinectin in vivo, the murine kinectin gene was disrupted by homologous recombination. Unexpectedly, kinectin-deficient mice were viable and fertile, and no gross abnormalities were observed up to 1 year of age. The assembly of the endoplasmic reticulum was essentially unaffected in kinectin-deficient cells. Mitochondria appeared to be correctly distributed throughout the cytoplasm along the microtubules. Furthermore, the stationary distribution and the bidirectional movement of lysosomes did not depend on kinectin. Kinectin-deficient phagocytes internalized and cleared bacteria, indicating that phagosome trafficking and maturation are functional without kinectin. Thus, these data unequivocally indicate that kinectin is not essential for trafficking of lysosomes, phagosomes, and mitochondria in vivo.

Dynamin-related protein Drp1 is required for mitochondrial division in mammalian cells

Mutations in the human dynamin-related protein Drp1 cause mitochondria to form perinuclear clusters. We show here that these mitochondrial clusters consist of highly interconnected mitochondrial tubules. The increased connectivity between mitochondria indicates that the balance between mitochondrial division and fusion is shifted toward fusion. Such a shift is consistent with a block in mitochondrial division. Immunofluorescence and subcellular fractionation show that endogenous Drp1 is localized to mitochondria, which is also consistent with a role in mitochondrial division. A direct role in mitochondrial division is suggested by time-lapse photography of transfected cells, in which green fluorescent protein fused to Drp1 is concentrated in spots that mark actual mitochondrial division events. We find that purified human Drp1 can self-assemble into multimeric ring-like structures with dimensions similar to those of dynamin multimers. The structural and functional similarities between dynamin and Drp1 suggest that Drp1 wraps around the constriction points of dividing mitochondria, analogous to dynamin collars at the necks of budding vesicles. We conclude that Drp1 contributes to mitochondrial division in mammalian cells.

Mitochondria make a come back

This review attempts to summarize our present state of knowledge of mitochondria in relation to a number of areas of biology, and to indicate where future research might be directed. In the evolution of eukaryotic cells mitochondria have for a long time played a prominent role. Nowadays their integration into many activities of a cell, and their dynamic behavior as subcellular organelles within a cell and during cell division are a major focus of attention. The crystal structures of the major complexes of the electron transport chain (except complex I) have been established, permitting increasingly detailed analyses of the important mechanism of proton pumping coupled to electron transport. The mitochondrial genome and its replication and expression are beginning to be understood in considerable detail, but more questions remain with regard to mutations and their repair, and the segregation of the mtDNA in oogenesis and development. Much emphasis and a large effort have recently been devoted to understand the role of mitochondria in programmed cell death (apoptosis). The understanding of their central role in mitochondrial diseases is a major achievement of the past decade. Finally, various drugs have traditionally played a part in understanding biochemical mechanisms within mitochondria; the repertoire of drugs with novel and interesting targets is expanding.

All kinesin superfamily protein, KIF, genes in mouse and human

Intracellular transport is essential for morphogenesis and functioning of the cell. The kinesin superfamily proteins (KIFs) have been shown to transport membranous organelles and protein complexes in a microtubule- and ATP-dependent manner. More than 30 KIFs have been reported in mice. However, the nomenclature of KIFs has not been clearly established, resulting in various designations and redundant names for a single KIF. Here, we report the identification and classification of all KIFs in mouse and human genome transcripts. Previously unidentified murine KIFs were found by a PCR-based search. The identification of all KIFs was confirmed by a database search of the total human genome. As a result, there are a total of 45 KIFs. The nomenclature of all KIFs is presented. To understand the function of KIFs in intracellular transport in a single tissue, we focused on the brain. The expression of 38 KIFs was detected in brain tissue by Northern blotting or PCR using cDNA. The brain, mainly composed of highly differentiated and polarized cells such as neurons and glia, requires a highly complex intracellular transport system as indicated by the increased number of KIFs for their sophisticated functions. It is becoming increasingly clear that the cell uses a number of KIFs and tightly controls the direction, destination, and velocity of transportation of various important functional molecules, including mRNA. This report will set the foundation of KIF and intracellular transport research.

Peroxisomes exist in growth cones and move anterogradely and retrogradely in neurites of PC12D cells

Localization and movement of peroxisomes have been investigated in neurites of a subline of PC12 pheochromocytoma cells (PC12D cells). The cells were transfected with a construct encoding the green fluorescent protein and bearing the C-terminal peroxisomal targeting signal 1 SKL motif (-Ser-Lys-Leu-COOH). Peroxisomes were detected as green punctate fluorescent signals. Many peroxisomes were observed in neurites of PC12D cells, especially in neural terminal-like structures, growth cones, varicosities, and branch points. Growth cones containing many peroxisomes were active, since they extended several long filopodias. Existence of peroxisomes in growth cones and neuronal terminal-like structures suggests that peroxisomes might have some role in neuronal extension and nerve terminal functioning. Peroxisomal motility was analyzed by time-lapse imaging using a fluorescence microscope at 25 degrees C. Peroxisomes were transported bidirectionally in neurites, i.e., through anterograde and retrograde transport. This result suggests that peroxisomes move to growth cones and neural terminals from the PC12D cell body, play some role in these parts, and go back to cell body.

Mitochondrial inheritance in budding yeast

During the past decade significant advances were made toward understanding the mechanism of mitochondrial inheritance in the yeast Saccharomyces cerevisiae. A combination of genetics, cell-free assays and microscopy has led to the discovery of a great number of components. These fall into three major categories: cytoskeletal elements, mitochondrial membrane components and regulatory proteins. These proteins mediate activities, including movement of mitochondria from mother cells to buds, segregation of mitochondria and mitochondrial DNA, and equal distribution of the organelle between mother cells and buds during yeast cell division.

Charcot-Marie-Tooth disease type 2A caused by mutation in a microtubule motor KIF1Bbeta

The kinesin superfamily motor protein KIF1B has been shown to transport mitochondria. Here, we describe an isoform of KIF1B, KIF1Bbeta, that is distinct from KIF1B in its cargo binding domain. KIF1B knockout mice die at birth from apnea due to nervous system defects. Death of knockout neurons in culture can be rescued by expression of the beta isoform. The KIF1B heterozygotes have a defect in transporting synaptic vesicle precursors and suffer from progressive muscle weakness similar to human neuropathies. Charcot-Marie-Tooth disease type 2A was previously mapped to an interval containing KIF1B. We show that CMT2A patients contain a loss-of-function mutation in the motor domain of the KIF1B gene. This is clear indication that defects in axonal transport due to a mutated motor protein can underlie human peripheral neuropathy.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

A role for kinesin in insulin-stimulated GLUT4 glucose transporter translocation in 3T3-L1 adipocytes

Insulin regulates glucose uptake in adipocytes and muscle by stimulating the movement of sequestered glucose transporter 4 (GLUT4) proteins from intracellular membranes to the cell surface. Here we report that optimal insulin-mediated GLUT4 translocation is dependent upon both microtubule and actin-based cytoskeletal structures in cultured adipocytes. Depolymerization of microtubules and F-actin in 3T3-L1 adipocytes causes the dispersion of perinuclear GLUT4-containing membranes and abolishes insulin action on GLUT4 movements to the plasma membrane. Furthermore, heterologous expression in 3T3-L1 adipocytes of the microtubule-binding protein hTau40, which impairs kinesin motors that move toward the plus ends of microtubules, markedly delayed the appearance of GLUT4 at the plasma membrane in response to insulin. The hTau40 protein had no detectable effect on microtubule structure or perinuclear GLUT4 localization under these conditions. These results are consistent with the hypothesis that both the actin and microtubule-based cytoskeleton, as well as a kinesin motor, direct the translocation of GLUT4 to the plasma membrane in response to insulin.

Cargo of kinesin identified as JIP scaffolding proteins and associated signaling molecules

The cargo that the molecular motor kinesin moves along microtubules has been elusive. We searched for binding partners of the COOH terminus of kinesin light chain, which contains tetratricopeptide repeat (TPR) motifs. Three proteins were found, the c-jun NH(2)-terminal kinase (JNK)-interacting proteins (JIPs) JIP-1, JIP-2, and JIP-3, which are scaffolding proteins for the JNK signaling pathway. Concentration of JIPs in nerve terminals requires kinesin, as evident from the analysis of JIP COOH-terminal mutants and dominant negative kinesin constructs. Coprecipitation experiments suggest that kinesin carries the JIP scaffolds preloaded with cytoplasmic (dual leucine zipper-bearing kinase) and transmembrane signaling molecules (the Reelin receptor, ApoER2). These results demonstrate a direct interaction between conventional kinesin and a cargo, indicate that motor proteins are linked to their membranous cargo via scaffolding proteins, and support a role for motor proteins in spatial regulation of signal transduction pathways.

Control of mitochondrial morphology by a human mitofusin

Although changes in mitochondrial size and arrangement accompany both cellular differentiation and human disease, the mechanisms that mediate mitochondrial fusion, fission and morphogenesis in mammalian cells are not understood. We have identified two human genes encoding potential mediators of mitochondrial fusion. The mitofusins (Mfn1 and Mfn2) are homologs of the Drosophila protein fuzzy onion (Fzo) that associate with mitochondria and alter mitochondrial morphology when expressed by transient transfection in tissue culture cells. An internal region including a predicted bipartite transmembrane domain (TM) is sufficient to target Mfn2 to mitochondria and requires hydrophobic residues within the TM. Co-expression of Mfn2 with a dominant interfering mutant dynamin-related protein (Drp1(K38A)) proposed to block mitochondrial fission resulted in long mitochondrial filaments and networks. Formation of mitochondrial filaments and networks required a wild-type Mfn2 GTPase domain, suggesting that the Mfn2 GTPase regulates or mediates mitochondrial fusion and that mitofusins and dynamin related GTPases play opposing roles in mitochondrial fusion and fission in mammals, as in yeast.

Animal mitochondrial biogenesis and function: a regulatory cross-talk between two genomes

Mitochondria play a pivotal role in cell physiology, producing the cellular energy and other essential metabolites as well as controlling apoptosis by integrating numerous death signals. The biogenesis of the oxidative phosphorylation system (OXPHOS) depends on the coordinated expression of two genomes, nuclear and mitochondrial. As a consequence, the control of mitochondrial biogenesis and function depends on extremely complex processes that require a variety of well orchestrated regulatory mechanisms. It is now clear that in order to provide cells with the correct number of structural and functional differentiated mitochondria, a variety of intracellular and extracellular signals including hormones and environmental stimuli need to be integrated. During the last few years a considerable effort has been devoted to study the factors that regulate mtDNA replication and transcription as well as the expression of nuclear-encoded mitochondrial genes in physiological and pathological conditions. Although still in their infancy, these studies are starting to provide the molecular basis that will allow to understand the mechanisms involved in the nucleo-mitochondrial communication, a cross-talk essential for cell life and death.

Opposite effects of microtubule-stabilizing and microtubule-destabilizing drugs on biogenesis of mitochondria in mammalian cells

Distribution of mitochondria as well as other intracellular organelles in mammalian cells is regulated by interphase microtubules. Here, we demonstrate a role of microtubules in the mitochondrial biogenesis using various microtubule-active drugs and human osteosarcoma cell line 143B cells and rat liver-derived RL-34 cells. Depolymerization of microtubules by nocodazole or colchicine, as well as 2-methoxyestradiol, a natural estrogen metabolite, arrested asynchronously cultured cells in G(2)/M phase of cell cycle and at the same time inhibited the mitochondrial mass increase and mtDNA replication. These drugs also inhibited the mitochondrial mass increase in the cells that were synchronized in cell cycle, which should occur during G(1) to G(2) phase progression in normal conditions. However, stabilization of microtubules by taxol did not affect the proliferation of mitochondria during the cell cycle, yet a prolonged incubation of cells with taxol induced an abnormal accumulation of mitochondria in cells arrested in G(2)/M phase of cell cycle. Taxol-induced accumulation of mitochondria was not only demonstrated by mitochondria-specific fluorescent dyes but also evidenced by the examination of cells transfected with yellow fluorescent protein fused with mitochondrial targeting sequence from subunit VIII of human cytochrome c oxidase (pEYFP) and by enhanced mtDNA replication. Two subpopulations of mitochondria were detected in taxol-treated cells: mitochondria with high Delta(psi)(m), detectable either by Mito Tracker Red CMXRos or by Green FM, and those with low Delta(psi)(m), detectable only by Green FM. However, taxol-induced increases in the mitochondrial mass and in the level of acetylated (alpha)-tubulin were abrogated by a co-treatment with taxol and nocodazole or taxol and colchicine. These data strongly suggest that interphase microtubules may be essential for the regulation of mitochondrial biogenesis in mammalian cells.

Perinuclear localization and insulin responsiveness of GLUT4 requires cytoskeletal integrity in 3T3-L1 adipocytes

The GLUT4 glucose transporter resides mostly in perinuclear membranes in unstimulated 3T3-L1 adipocytes and is acutely translocated to the cell surface in response to insulin. Using a novel method to purify intracellular GLUT4-enriched membranes, we identified by mass spectrometry the intermediate filament protein vimentin and the microtubule protein alpha-tubulin as components of these membranes. Immunoelectron microscopy of the GLUT4-containing membranes also revealed their association with these cytoskeletal proteins. Disruption of intermediate filaments and microtubules in 3T3-L1 adipocytes by microinjection of a vimentin-derived peptide of the helix initiation 1A domain caused marked dispersion of perinuclear GLUT4 to peripheral regions of the cells. Inhibition of the microtubule-based motor dynein by brief cytoplasmic acidification of cultured adipocytes also dispersed perinuclear GLUT4 and inhibited insulin-stimulated GLUT4 translocation to the cell surface. Insulin sensitivity was restored as GLUT4 was again concentrated near the nucleus upon recovery of cells in physiological buffer. These data suggest that GLUT4 trafficking to perinuclear membranes of cultured adipocytes is directed by dynein and is required for optimal GLUT4 regulation by insulin.

A novel motor, KIF13A, transports mannose-6-phosphate receptor to plasma membrane through direct interaction with AP-1 complex

Intracellular transport mediated by kinesin superfamily proteins (KIFs) is a highly regulated process. The molecular mechanism of KIFs binding to their respective cargoes remains unclear. We report that KIF13A is a novel plus end-directed microtubule-dependent motor protein and associates with beta 1-adaptin, a subunit of the AP-1 adaptor complex. The cargo vesicles of KIF13A contained AP-1 and mannnose-6-phosphate receptor (M6PR). Overexpression of KIF13A resulted in mislocalization of the AP-1 and the M6PR. Functional blockade of KIF13A reduced cell surface expression of the M6PR. Thus, KIF13A transports M6PR-containing vesicles and targets the M6PR from TGN to the plasma membrane via direct interaction with the AP-1 adaptor complex.

Dynamics of the mitochondrial reticulum in live cells using Fourier imaging correlation spectroscopy and digital video microscopy

We report detailed studies of the dynamics of the mitochondrial reticulum in live cells using two independent experimental techniques: Fourier imaging correlation spectroscopy and digital video fluorescence microscopy. When both methods are used to study the same system, it is possible to directly compare measurements of preaveraged statistical dynamical quantities with their microscopic counterparts. This approach allows the underlying mechanism of the observed rates to be determined. Our results indicate that the dynamics of the reticulum structure is composed of two independent contributions, each important on very different time and length scales. During short time intervals (1-15 sec), local regions of the reticulum primarily undergo constrained thermally activated motion. During long time intervals (>15 sec), local regions of the reticulum undergo long-range "jump" motions that are associated with the action of cytoskeletal filaments. Although the frequency of the jumps depend on the physiological state of the cells, the average jump distance ( approximately 0.8 microm) is unaffected by metabolic activity. During short time intervals, the dynamics appear to be spatially heterogeneous, whereas the cumulative effect of the infrequent jumps leads to the appearance of diffusive motion in the limit of long time intervals.

KIF5C, a novel neuronal kinesin enriched in motor neurons

Kinesin superfamily proteins (KIFs) are the molecular motors conveying cargos along microtubules. KIF5s, the heavy chains of conventional kinesin (KHC), are originally identified members of KIFs, and neuronal KIF5A and ubiquitous KIF5B have been identified so far. In the present work, we cloned a novel member of KIF5, KIF5C, and generated specific antibodies against three KIF5s to investigate their distribution and functions. KIF5A showed pan-neuronal distribution in the nervous system. KIF5B showed a glial cell distribution pattern in general; however, interestingly, its expression was strongly upregulated in axon-elongating neurons, such as olfactory primary neurons and mossy fibers. KIF5C was also a neuronal KIF5 like KIF5A but was highly expressed in lower motor neurons in 2-week-old or older mice, suggesting its important roles in the maintenance of motor neurons rather than in their formation, such as axonal elongation. Because a large part of KIF5s in adult motor neurons were expected to be KIF5C, we generated mice lacking the kif5C gene to investigate the functions of KIF5C in neurons in living animals. The mutant mice showed smaller brain size but were viable and did not show gross changes in the nervous system. Closer examinations revealed the relative loss of motor neurons to sensory neurons. Because three KIF5s showed high similarity in the amino acid sequence, could rescue the KIF5B mutant cells, and could form heterodimers, we think that there are functional redundancy among the three KIF5s and that KIF5A and KIF5B prevented the KIF5C null mice from the severe phenotype.

Extramitochondrial localization of mortalin/mthsp70/PBP74/GRP75

Subcellular fractionation and immunofluorescence microscopy were used to identify the specific sites of intracellular residence of mortalin, also called a mitochondrial homologue of the hsp70 family, in immortal human cell lines previously assigned to four distinct complementation groups (A-D) for indefinite cell division. In addition to the mitochondria it was seen in the endoplasmic reticulum (ER) fractions of all the cell lines analyzed. Interestingly, three of the group A cells lines (EJ, GM639, and HT1080), in addition to the mitochondria and ER, exhibited cytosolically (extra-organelle) localized pool of mortalin. These findings demonstrate that mortalin is not present exclusively in mitochondria. Its residence in different organelles may be the basis of differential distribution observed previously in different human cell lines.

Sorting and transport in C. elegans: aA model system with a sequenced genome

In the past few years, yeast and cultured cells have been the model systems of choice for the study of protein sorting and transport. Recently, there has been a surge in research in these areas in Caenorhabditis elegans, with advances in experimental techniques and genomics. New in vivo assays that monitor endocytosis and neuronal transport have been used to delineate roles for several genes in these processes.

Tumor necrosis factor induces hyperphosphorylation of kinesin light chain and inhibits kinesin-mediated transport of mitochondria

The molecular motor kinesin is an ATPase that mediates plus end-directed transport of organelles along microtubules. Although the biochemical properties of kinesin are extensively studied, conclusive data on regulation of kinesin-mediated transport are largely lacking. Previously, we showed that the proinflammatory cytokine tumor necrosis factor induces perinuclear clustering of mitochondria. Here, we show that tumor necrosis factor impairs kinesin motor activity and hyperphosphorylates kinesin light chain through activation of two putative kinesin light chain kinases. Inactivation of kinesin, hyperphosphorylation of kinesin light chain, and perinuclear clustering of mitochondria exhibit the same p38 mitogen-activated kinase dependence, indicating their functional relationship. These data provide evidence for direct regulation of kinesin-mediated organelle transport by extracellular stimuli via cytokine receptor signaling pathways.