Treatment with a copper-zinc chelator markedly and rapidly inhibits beta-amyloid accumulation in Alzheimer's disease transgenic mice

Inhibition of neocortical beta-amyloid (Abeta) accumulation may be essential in an effective therapeutic intervention for Alzheimer's disease (AD). Cu and Zn are enriched in Abeta deposits in AD, which are solubilized by Cu/Zn-selective chelators in vitro. Here we report a 49% decrease in brain Abeta deposition (-375 microg/g wet weight, p = 0.0001) in a blinded study of APP2576 transgenic mice treated orally for 9 weeks with clioquinol, an antibiotic and bioavailable Cu/Zn chelator. This was accompanied by a modest increase in soluble Abeta (1.45% of total cerebral Abeta); APP, synaptophysin, and GFAP levels were unaffected. General health and body weight parameters were significantly more stable in the treated animals. These results support targeting the interactions of Cu and Zn with Abeta as a novel therapy for the prevention and treatment of AD.

The A beta peptide of Alzheimer's disease directly produces hydrogen peroxide through metal ion reduction

Oxidative stress markers characterize the neuropathology both of Alzheimer's disease and of amyloid-bearing transgenic mice. The neurotoxicity of amyloid A beta peptides has been linked to peroxide generation in cell cultures by an unknown mechanism. We now show that human A beta directly produces hydrogen peroxide (H2O2) by a mechanism that involves the reduction of metal ions, Fe(III) or Cu(II), setting up conditions for Fenton-type chemistry. Spectrophotometric experiments establish that the A beta peptide reduces Fe(III) and Cu(II) to Fe(II) and Cu(I), respectively. Spectrochemical techniques are used to show that molecular oxygen is then trapped by A beta and reduced to H2O2 in a reaction that is driven by substoichiometric amounts of Fe(II) or Cu(I). In the presence of Cu(II) or Fe(III), A beta produces a positive thiobarbituric-reactive substance (TBARS) assay, compatible with the generation of the hydroxyl radical (OH.). The amounts of both reduced metal and TBARS reactivity are greatest when generated by A beta 1-42 >> A beta 1-40 > rat A beta 1-40, a chemical relationship that correlates with the participation of the native peptides in amyloid pathology. These findings indicate that the accumulation of A beta could be a direct source of oxidative stress in Alzheimer's disease.

Dramatic aggregation of Alzheimer abeta by Cu(II) is induced by conditions representing physiological acidosis

The cortical deposition of Abeta is an event that occurs in Alzheimer's disease, Down's syndrome, head injury, and normal aging. Previously, in appraising the effects of different neurochemical factors that impact upon the solubility of Abeta, we observed that Zn2+ was the predominant bioessential metal to induce the aggregation of soluble Abeta at pH 7.4 in vitro and that this reaction is totally reversible with chelation. We now report that unlike other biometals tested at maximal biological concentrations, marked Cu2+-induced aggregation of Abeta1-40 emerged as the solution pH was lowered from 7.4 to 6.8 and that the reaction was completely reversible with either chelation or alkalinization. This interaction was comparable to the pH-dependent effect of Cu2+ on insulin aggregation but was not seen for aprotinin or albumin. Abeta1-40 bound three to four Cu2+ ions when precipitated at pH 7.0. Rapid, pH-sensitive aggregation occurred at low nanomolar concentrations of both Abeta1-40 and Abeta1-42 with submicromolar concentrations of Cu2+. Unlike Abeta1-40, Abeta1-42 was precipitated by submicromolar Cu2+ concentrations at pH 7.4. Rat Abeta1-40 and histidine-modified human Abeta1-40 were not aggregated by Zn2+, Cu2+, or Fe3+, indicating that histidine residues are essential for metal-mediated Abeta assembly. These results indicate that H+-induced conformational changes unmask a metal-binding site on Abeta that mediates reversible assembly of the peptide. Since a mildly acidic environment together with increased Zn2+ and Cu2+ are common features of inflammation, we propose that Abeta aggregation by these factors may be a response to local injury. Cu2+, Zn2+, and Fe3+ association with Abeta explains the recently reported enrichment of these metal ions in amyloid plaques in Alzheimer's disease.

Cu(II) potentiation of alzheimer abeta neurotoxicity. Correlation with cell-free hydrogen peroxide production and metal reduction

Oxidative stress markers as well as high concentrations of copper are found in the vicinity of Abeta amyloid deposits in Alzheimer's disease. The neurotoxicity of Abeta in cell culture has been linked to H(2)O(2) generation by an unknown mechanism. We now report that Cu(II) markedly potentiates the neurotoxicity exhibited by Abeta in cell culture. The potentiation of toxicity is greatest for Abeta1-42 > Abeta1-40 >> mouse/rat Abeta1-40, corresponding to their relative capacities to reduce Cu(II) to Cu(I), form H(2)O(2) in cell-free assays and to exhibit amyloid pathology. The copper complex of Abeta1-42 has a highly positive formal reduction potential ( approximately +500-550 mV versus Ag/AgCl) characteristic of strongly reducing cuproproteins. These findings suggest that certain redox active metal ions may be important in exacerbating and perhaps facilitating Abeta-mediated oxidative damage in Alzheimer's disease.

Characterization of copper interactions with alzheimer amyloid beta peptides: identification of an attomolar-affinity copper binding site on amyloid beta1-42

Cu and Zn have been shown to accumulate in the brains of Alzheimer's disease patients. We have previously reported that Cu(2+) and Zn(2+) bind amyloid beta (Abeta), explaining their enrichment in plaque pathology. Here we detail the stoichiometries and binding affinities of multiple cooperative Cu(2+)-binding sites on synthetic Abeta1-40 and Abeta1-42. We have developed a ligand displacement technique (competitive metal capture analysis) that uses metal-chelator complexes to evaluate metal ion binding to Abeta, a notoriously self-aggregating peptide. This analysis indicated that there is a very-high-affinity Cu(2+)-binding site on Abeta1-42 (log K(app) = 17.2) that mediates peptide precipitation and that the tendency of this peptide to self-aggregate in aqueous solutions is due to the presence of trace Cu(2+) contamination (customarily approximately 0.1 microM). In contrast, Abeta1-40 has much lower affinity for Cu(2+) at this site (estimated log K(app) = 10.3), explaining why this peptide is less self-aggregating. The greater Cu(2+)-binding affinity of Abeta1-42 compared with Abeta1-40 is associated with significantly diminished negative cooperativity. The role of trace metal contamination in inducing Abeta precipitation was confirmed by the demonstration that Abeta peptide (10 microM) remained soluble for 5 days only in the presence of high-affinity Cu(2+)-selective chelators.

Alzheimer-associated presenilins 1 and 2: neuronal expression in brain and localization to intracellular membranes in mammalian cells

Mutations in two recently identified genes appear to cause the majority of early-onset familial Alzheimer's disease (FAD). These two novel genes, presenilin 1 (PS1) and presenilin 2 (PS2) are members of an evolutionarily conserved gene family. The normal biological role(s) of the presenilins and the mechanism(s) by which the FAD-associated mutations exert their effect remain unknown. Employing in situ hybridization, we demonstrate that the expression patterns of PS1 and PS2 in the brain are extremely similar to each other and that messages for both are primarily detectable in neuronal populations. Immunochemical analyses indicate that PS1 and PS2 are similar in size and localized to similar intracellular compartments (endoplasmic reticulum and Golgi complex). FAD-associated mutations in PS1 and PS2 do not significantly modify either their migration patterns on SDS-polyacrylamide gel electrophoresis or their overall subcellular localization, although subtle differences in perinuclear staining were noted for mutant PS1.

Cleavage of lamin A by Mch2 alpha but not CPP32: multiple interleukin 1 beta-converting enzyme-related proteases with distinct substrate recognition properties are active in apoptosis

Although proteases related to the interleukin 1 beta-converting enzyme (ICE) are known to be essential for apoptotic execution, the number of enzymes involved, their substrate specificities, and their specific roles in the characteristic biochemical and morphological changes of apoptosis are currently unknown. These questions were addressed using cloned recombinant ICE-related proteases (IRPs) and a cell-free model system for apoptosis (S/M extracts). First, we compared the substrate specificities of two recombinant human IRPs, CPP32 and Mch2 alpha. Both enzymes cleaved poly-(ADP-ribose) polymerase, albeit with different efficiencies. Mch2 alpha also cleaved recombinant and nuclear lamin A at a conserved VEID decreases NG sequence located in the middle of the coiled-coil rod domain, producing a fragment that was indistinguishable from the lamin A fragment observed in S/M extracts and in apoptotic cells. In contrast, CPP32 did not cleave lamin A. The cleavage of lamin A by Mch2 alpha and by S/M extracts was inhibited by millimolar concentrations of Zn2+, which had a minimal effect on cleavage of poly (ADP-ribose) polymerase by CPP32 and by S/M extracts. We also found that N-(acetyltyrosinylvalinyl-N epsilon-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy]methyl ketone, which derivatizes the larger subunit of active ICE, can affinity label up to five active IRPs in S/M extracts. Together, these observations indicate that the processing of nuclear proteins in apoptosis involves multiple IRPs having distinct preferences for their apoptosis-associated substrates.

Studies of the lamin proteinase reveal multiple parallel biochemical pathways during apoptotic execution

Although specific proteinases play a critical role in the active phase of apoptosis, their substrates are largely unknown. We previously identified poly(ADP-ribose) polymerase (PARP) as an apoptosis-associated substrate for proteinase(s) related to interleukin 1 beta-converting enzyme (ICE). Now we have used a cell-free system to characterize proteinase(s) that cleave the nuclear lamins during apoptosis. Lamin cleavage during apoptosis requires the action of a second ICE-like enyzme, which exhibits kinetics of cleavage and a profile of sensitivity to specific inhibitors that is distinct from the PARP proteinase. Thus, multiple ICE-like enzymes are required for apoptotic events in these cell-free extracts. Inhibition of the lamin proteinase with tosyllysine "chloromethyl ketone" blocks nuclear apoptosis prior to the packaging of condensed chromatin into apoptotic bodies. Under these conditions, the nuclear DNA is fully cleaved to a nucleosomal ladder. Our studies reveal that the lamin proteinase and the fragmentation nuclease function in independent parallel pathways during the final stages of apoptotic execution. Neither pathway alone is sufficient for completion of nuclear apoptosis. Instead, the various activities cooperate to drive the disassembly of the nucleus.

LDL receptor-related protein, a multifunctional ApoE receptor, binds secreted beta-amyloid precursor protein and mediates its degradation

The secreted form of beta-amyloid precursor protein (APP) containing the Kunitz proteinase inhibitor (KPI) domain, also called protease nexin II, is internalized and degraded by cells. We show that the low density lipoprotein (LDL) receptor-related protein (LRP) is responsible for the endocytosis of secreted APP. APPs770 degradation is inhibited by an LRP antagonist called the receptor-associated protein (RAP) and by LRP antibodies and is greatly diminished in fibroblasts genetically deficient in LRP. APPs695, which lacks the KPI domain, is a poor LRP ligand. Since LRP also binds apolipoprotein E (apoE)-enriched lipoproteins and inheritance of the epsilon 4 allele of the apoE gene is a risk factor for Alzheimer's disease (AD), these data link in a single metabolic pathway two molecules strongly implicated in the pathophysiology of AD.

The amyloid protein precursor of Alzheimer's disease is a mediator of the effects of nerve growth factor on neurite outgrowth

The beta A4 protein, the major component of the amyloid deposition characterizing Alzheimer's disease, derives from the amyloid protein precursor (APP), an integral membrane protein with soluble derivatives. The function of APP is unknown. Both soluble and membrane-associated human brain APP (10(-10) M) significantly increased (P less than 0.025) neurite length and branching in pheochromocytoma PC12 cells, but did not affect the number of neurites per cell. At higher concentrations, APP was cytotoxic, with a half-maximal concentration of 5 x 10(-9) M. Nerve growth factor (NGF) is known to affect APP expression in vivo and in vitro. Antibodies to APP specifically diminished the effects of NGF on neurite length and branching. Thus APP may act to mediate neurite outgrowth promotion by NGF.

Uptake of HIV-1 tat protein mediated by low-density lipoprotein receptor-related protein disrupts the neuronal metabolic balance of the receptor ligands

Neurological disorders develop in most people infected with human immunodeficiency virus type 1 (HIV-1). However, the underlying mechanisms remain largely unknown. Here we report that binding of HIV-1 transactivator (Tat) protein to low-density lipoprotein receptor-related protein (LRP) promoted efficient uptake of Tat into neurons. LRP-mediated uptake of Tat was followed by translocation to the neuronal nucleus. Furthermore, the binding of Tat to LRP resulted in substantial inhibition of neuronal binding, uptake and degradation of physiological ligands for LRP, including alpha2-macroglobulin, apolipoprotein E4, amyloid precursor protein and amyloid beta-protein. In a model of macaques infected with a chimeric strain of simian-human immunodeficiency virus, increased staining of amyloid precursor protein was associated with Tat expression in the brains of simian-human immunodeficiency virus-infected macaques with encephalitis. These results indicate that HIV-1 Tat may mediate HIV-1-induced neuropathology through a pathway involving disruption of the metabolic balance of LRP ligands and direct activation of neuronal genes.

Rapid movements of vimentin on microtubule tracks: kinesin-dependent assembly of intermediate filament networks

The assembly and maintenance of an extended intermediate filament (IF) network in fibroblasts requires microtubule (MT) integrity. Using a green fluorescent protein-vimentin construct, and spreading BHK-21 cells as a model system to study IF-MT interactions, we have discovered a novel mechanism involved in the assembly of the vimentin IF cytoskeleton. This entails the rapid, discontinuous, and MT-dependent movement of IF precursors towards the peripheral regions of the cytoplasm where they appear to assemble into short fibrils. These precursors, or vimentin dots, move at speeds averaging 0.55 +/- 0.24 micrometer/s. The vimentin dots colocalize with MT and their motility is inhibited after treatment with nocodazole. Our studies further implicate a conventional kinesin in the movement of the vimentin dots. The dots colocalize with conventional kinesin as shown by indirect immunofluorescence, and IF preparations from spreading cells are enriched in kinesin. Furthermore, microinjection of kinesin antibodies into spreading cells prevents the assembly of an extended IF network. These studies provide insights into the interactions between the IF and MT systems. They also suggest a role for conventional kinesin in the distribution of non-membranous protein cargo, and the local regulation of IF assembly.

Zinc-induced Alzheimer's Abeta1-40 aggregation is mediated by conformational factors

The heterogeneous precipitates of Abeta that accumulate in the brain cortex in Alzheimer's disease possess varying degrees of resistance to resolubilization. We previously found that Abeta1-40 is rapidly precipitated in vitro by physiological concentrations of zinc, a neurochemical that is highly abundant in brain compartments where Abeta is most likely to precipitate. We now present evidence that the zinc-induced precipitation of Abeta is mediated by a peptide dimer and favored by conditions that promote alpha-helical and diminish beta-sheet conformations. The manner in which the synthetic peptide is solubilized was critical to its behavior in vitro. Zinc-induced Abeta aggregation was dependent upon the presence of NaCl, was enhanced by alpha-helical-promoting solvents, but was abolished when the peptide stock solution was stored frozen. The Abeta aggregates induced by zinc were reversible by chelation, but could then be reprecipitated by zinc for several cycles, indicating that the peptide's conformation is probably preserved in the zinc-mediated assembly. In contrast, Abeta aggregates induced by low pH (5.5) were not resolubilized by returning the pH milieu to 7.4. The zinc-Abeta interaction exhibits features resembling the gelation process of zinc-mediated fibrin assembly, suggesting that, in events such as clot formation or injury, reversible Abeta assembly could be physiologically purposive. Such a mechanism is contemplated in the early evolution of diffuse plaques in Alzheimer's disease and suggests a possible therapeutic strategy for the resolubilization of some forms of Abeta deposit in the disease.

Disruption of nuclear lamin organization alters the distribution of replication factors and inhibits DNA synthesis

The nuclear lamina is a fibrous structure that lies at the interface between the nuclear envelope and the nucleoplasm. The major proteins comprising the lamina, the nuclear lamins, are also found in foci in the nucleoplasm, distinct from the peripheral lamina. The nuclear lamins have been associated with a number of processes in the nucleus, including DNA replication. To further characterize the specific role of lamins in DNA replication, we have used a truncated human lamin as a dominant negative mutant to perturb lamin organization. This protein disrupts the lamin organization of nuclei when microinjected into mammalian cells and also disrupts the lamin organization of in vitro assembled nuclei when added to Xenopus laevis interphase egg extracts. In both cases, the lamina appears to be completely absent, and instead the endogenous lamins and the mutant lamin protein are found in nucleoplasmic aggregates. Coincident with the disruption of lamin organization, there is a dramatic reduction in DNA replication. As a consequence of this disruption, the distributions of PCNA and the large subunit of the RFC complex, proteins required for the elongation phase of DNA replication, are altered such that they are found within the intranucleoplasmic lamin aggregates. In contrast, the distribution of XMCM3, XORC2, and DNA polymerase alpha, proteins required for the initiation stage of DNA replication, remains unaltered. The data presented demonstrate that the nuclear lamins may be required for the elongation phase of DNA replication.

Dynamic disruptions in nuclear envelope architecture and integrity induced by HIV-1 Vpr

Human immunodeficiency virus-1 (HIV-1) Vpr expression halts the proliferation of human cells at or near the G2 cell-cycle checkpoint. The transition from G2 to mitosis is normally controlled by changes in the state of phosphorylation and subcellular compartmentalization of key cell-cycle regulatory proteins. In studies of the intracellular trafficking of these regulators, we unexpectedly found that wild-type Vpr, but not Vpr mutants impaired for G2 arrest, induced transient, localized herniations in the nuclear envelope (NE). These herniations were associated with defects in the nuclear lamina. Intermittently, these herniations ruptured, resulting in the mixing of nuclear and cytoplasmic components. These Vpr-induced NE changes probably contribute to the observed cell-cycle arrest.

Dynamic properties of nuclear lamins: lamin B is associated with sites of DNA replication

The nuclear lamins form a fibrous structure, the nuclear lamina, at the periphery of the nucleus. Recent results suggest that lamins are also present as foci or spots in the nucleoplasm at various times during interphase of the cell cycle (Goldman, A. E., R. D. Moir, M. Montag-Lowy, M. Stewart, and R. D. Goldman. 1992. J. Cell Biol. 104:725-732; Bridger, J. M., I. R. Kill, M. O'Farrell, and C. J. Hutchison. 1993. J. Cell Sci. 104:297-306). In this report we demonstrate that during mid-late S-phase, nuclear foci detected with lamin B antibodies are coincident with sites of DNA replication as detected by the colocalization of sites of incorporation of bromodeoxyuridine (BrDU) or proliferating cell nuclear antigen (PCNA). The relationship between lamin B and BrDU is not maintained in the following G1 stage of the cell cycle. Furthermore, the nuclear staining patterns seen with antibodies directed against lamins A and C in mid-late S-phase do not coalign with the lamin B/BrDU-containing structures. These results imply that there is a role for lamin B in the organization of replicating chromatin during S phase.

Motile properties of vimentin intermediate filament networks in living cells

The motile properties of intermediate filament (IF) networks have been studied in living cells expressing vimentin tagged with green fluorescent protein (GFP-vimentin). In interphase and mitotic cells, GFP-vimentin is incorporated into the endogenous IF network, and accurately reports the behavior of IF. Time-lapse observations of interphase arrays of vimentin fibrils demonstrate that they are constantly changing their configurations in the absence of alterations in cell shape. Intersecting points of vimentin fibrils, or foci, frequently move towards or away from each other, indicating that the fibrils can lengthen or shorten. Fluorescence recovery after photobleaching shows that bleach zones across fibrils rapidly recover their fluorescence. During this recovery, bleached zones frequently move, indicating translocation of fibrils. Intriguingly, neighboring fibrils within a cell can exhibit different rates and directions of movement, and they often appear to extend or elongate into the peripheral regions of the cytoplasm. In these same regions, short filamentous structures are also seen actively translocating. All of these motile properties require energy, and the majority appear to be mediated by interactions of IF with microtubules and microfilaments.

The gene defects responsible for familial Alzheimer's disease

Four different genes have now been found to contain AD-associated mutations or polymorphisms. While the pathogenic mutations in the early-onset FAD genes, APP, PS1, and PS2 directly cause AD with nearly 100% penetrance, in a larger subset of AD cases with onset over 60 years (maximally for onset at 61-65 years), inheritance of the APOE4 allele confers increased risk for AD but is not sufficient to cause the disease. Together, these four genes appear to account for approximately 50% of FAD cases. We are actively screening the genome for additional FAD loci by genotyping markers in over 400 FAD nuclear pedigrees and affected sib-pairs (83% late-onset and 17% early-onset). We have recently discovered genetic linkage to a novel FAD locus on chromosome 12 as well as another putative locus on chromosome 3 (unpublished findings). Positional cloning strategies are currently under way to identify these potentially novel FAD genes. A common event which is associated with all of the known FAD genes is the excessive accumulation of the A beta peptide and deposition of beta-amyloid in the brain. Thus, a common pathogenic pathway for AD neuropathogenesis appears to center around the cellular trafficking, maturation, and processing of APP, and the subsequent generation, aggregation, and deposition of A beta (or more specifically, A beta 1-42). APP and presenilin gene mutations most likely act as either gain-of-function or dominant negative gene defects which may ultimately lead to the transport of APP into intracellular compartments that promote the enhanced production of A beta or A beta 1-42. AD patients who carry an APOE4 allele experience increased amyloid burden in their brains compared to APOE4-negative AD cases. Thus, the presence of APOE4 would also appear to lead to abnormal generation, aggregation, or clearance of A beta in the brain A beta, perhaps by working in concert with its neuronal receptor, LRP. While the exact mechanisms by which the known FAD gene changes lead to the onset of AD remain unclear, the available data indicate that novel therapies aimed at curbing the generation, aggregation, and deposition of A beta would appear to carry the greatest potential for the effective treatment of this formidable disease.

Clearance of Alzheimer's Abeta peptide: the many roads to perdition

The amyloid hypothesis of Alzheimer's disease (AD) maintains that the accumulation of the amyloid beta protein (Abeta) is a critical event in disease pathogenesis. A great deal of both academic and commercial research has focused on the mechanisms by which Abeta is generated. However, investigations into the mechanisms underlying Abeta clearance have blossomed over the last several years. This minireview will summarize pathways involved in the removal of cerebral Abeta, including enzymatic degradation and receptor-mediated efflux out of the brain.

Endoproteolytic cleavage and proteasomal degradation of presenilin 2 in transfected cells

Mutations in the presenilin genes, PS1 and PS2, cause a major portion of early onset familial Alzheimer's disease (FAD). The biological roles of the presenilins and how their pathological mutations confer FAD are unknown. In this study, we set out to examine the processing and degradation pathways of PS2. For regulated expression of PS2, we have established inducible cell lines expressing PS2 under the tight control of the tetracycline-responsive transactivator. Western blot analysis revealed that PS2 was detected as an approximately 53-55-kDa polypeptide (54-kDa PS2) as well as a high molecular mass form (HMW-PS2). Using a stably transfected, inducible cell system, we have found that PS2 is proteolytically cleaved into two stable cellular polypeptides including an approximately 20-kDa C-terminal fragment and an approximately 34-kDa N-terminal fragment. PS2 is polyubiquitinated in vivo, and the degradation of PS2 is inhibited by proteasome inhibitors, N-acetyl-L-leucinal-L-norleucinal and lactacystin. Our studies suggest that PS2 normally undergoes endoproteolytic cleavage and is degraded via the proteasome pathway.

The role of parvovirus B19 in aplastic crisis and erythema infectiosum (fifth disease)

In 1984, simultaneous outbreaks of aplastic crisis and erythema infectiosum occurred in northeastern Ohio. Sera were analyzed from 26 patients with aplastic crisis: 24 had IgM specific for parvovirus B19, five had B19-like particles by electron microscopy, and 13 had DNA from B19; no sera from 33 controls had evidence of recent infection with B19 (P less than .0001). DNA from B19 was also detected in specimens of throat gargle and urine from two patients with aplastic crisis. Sera from 36 of 51 children with erythema infectiosum had B19-specific IgM, compared with serum from one of 42 susceptible controls (P less than .0001). DNA from B19 was detected in sera from only two of 51 patients who had erythema infectiosum. The secondary attack rates among susceptible contacts decreased with age (overall total, 49.6%). Differential rates of asymptomatic infection were observed among black (68.8%) and white (20.0%) household members (P = .003). These were the first identified simultaneous outbreaks of aplastic crisis and erythema infectiosum. Their occurrence provided an opportunity to study the epidemiology and spectrum of B19 infection with geographically and temporally matched comparison groups; our results support the hypothesis that infection with parvovirus causes these two clinical entities.

Pathway of incorporation of microinjected lamin A into the nuclear envelope

When microinjected into the cytoplasm of 3T3 cells, biotinylated human lamin A rapidly enters the nucleus and gradually becomes incorporated into the nuclear lamina region as determined by immunofluorescence. The incorporation of the microinjected material takes several hours and progresses through a series of morphologically identifiable stages. Within minutes after microinjection, lamin A is found in spots distributed throughout the nucleus, except in nucleolar regions. Over a time course of up to 6 h, these spots appear to decrease in size and number as the biotinylated lamin A becomes associated with the endogenous nuclear lamina. Eventually, the typical nuclear rim staining pattern normally revealed by immunofluorescence with nuclear lamin antibodies is seen with antibiotin. This latter rim staining property is passed on to daughter cells following mitosis. These results indicate that the microinjected biotinylated nuclear lamin A retains those properties required for its integration into the lamina, as well as those necessary for the disassembly and subsequent reassembly of the nuclear lamina during cell division. The initial rapid accumulation into foci and the subsequent slower incorporation into the nuclear lamina appear to be analogous to the stages of incorporation following the microinjection of cytoskeletal intermediate filament proteins such as vimentin and keratin (Vikstrom, K., G. G. Borisy, and R. D. Goldman. 1989. Proc. Natl. Acad. Sci. USA. 86:549-553; Miller, R. K., K. Vikstrom, and R. D. Goldman. 1991. J. Cell Biol. 113:843-855). Foci are also observed in some uninjected cells using nuclear lamin antibodies, indicating that these features are a genuine component of nuclear substructure. Evidence is presented that shows the appearance of these nuclear structures is cell cycle dependent.

Repression of ribosome and tRNA synthesis in secretion-defective cells is signaled by a novel branch of the cell integrity pathway

The transcription of ribosomal DNA, ribosomal protein (RP) genes, and 5S and tRNA genes by RNA polymerases (Pols) I, II, and III, respectively, is rapidly and coordinately repressed upon interruption of the secretory pathway in Saccharomyces cerevisiae. We find that repression of ribosome and tRNA synthesis in secretion-defective cells involves activation of the cell integrity pathway. Transcriptional repression requires the upstream components of this pathway, including the Wsc family of putative plasma membrane sensors and protein kinase C (PKC), but not the downstream Bck1-Mkk1/2-Slt2 mitogen-activated protein kinase cascade. These findings reveal a novel PKC effector pathway that controls more than 85% of nuclear transcription. It is proposed that the coordination of ribosome and tRNA synthesis with cell growth may be achieved, in part, by monitoring the turgor pressure of the cell.

Review: the dynamics of the nuclear lamins during the cell cycle-- relationship between structure and function

The nuclear lamins are members of the intermediate filament (IF) family of proteins. The lamins have an essential role in maintaining nuclear integrity, as do the other IF family members in the cytoplasm. Also like cytoplasmic IFs, the organization of lamins is dynamic. The lamins are found not only at the nuclear periphery but also in the interior of the nucleus, as distinct nucleoplasmic foci and possibly as a network throughout the nucleus. Nuclear processes such as DNA replication may be organized around these structures. In this review, we discuss changes in the structure and organization of the nuclear lamins during the cell cycle and during cell differentiation. These changes are correlated with changes in nuclear structure and function. For example, the interactions of lamins with chromatin and nuclear envelope components occur very early during nuclear assembly following mitosis. During S-phase, the lamins colocalize with markers of DNA replication, and proper lamin organization must be maintained for replication to proceed. When cells differentiate, the expression pattern of lamin isotypes changes. In addition, changes in lamin organization and expression patterns accompany the nuclear alterations observed in transformed cells. These lamin structures may modulate nuclear function in each of these processes.