Abstract GS5-05: Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state

Approximately 50% of patients with localized triple negative breast cancer (TNBC) have substantial residual cancer burden following treatment with neoadjuvant chemotherapy (NACT), resulting in distant metastasis and death for most of these patients. While genomic and phenotypic intra-tumor heterogeneity are pervasive features of TNBCs at the time of diagnosis, the functional contributions of heterogeneous tumor cell populations to chemoresistance have not been elucidated.

To investigate tumor evolution accompanying NACT, we employed orthotopic patient-derived xenograft (PDX) models of treatment-naïve TNBC, which retain intra-tumor heterogeneity characteristic of human TNBC. We discovered that some PDX models initially exhibited partial sensitivity to standard front-line NACT (Adriamycin plus Cytoxan, AC). Following AC, residual tumors were resistant to chemotherapy but repopulated tumors with chemo-sensitive cells if left untreated, indicating that tumor cells possessed inherent plasticity. To identify the tumor cell subpopulation(s) conferring chemoresistance, we conducted barcode-mediated clonal tracking in three independent PDX models by introducing a high-complexity pooled lentiviral barcode library into PDX tumor cells which were then orthotopically engrafted into recipient mice. Strikingly, residual tumors maintained the same heterogeneous clonal architecture as naïve tumors. Concordantly, whole-exome sequencing revealed conservation of genomic subclonal architecture throughout treatment. These results were corroborated by genomic sequencing of serial biopsies pre- and post-AC obtained directly from TNBC patients enrolled on an ongoing clinical trial at MD Anderson (ARTEMIS; NCT02276443). Together, these studies revealed that genomically distinct pre-treatment subclones were equally capable of surviving AC to reconstitute tumors after treatment.

To identify functional addictions of residual tumor cells, we conducted histologic and transcriptomic profiling. Residual tumors following AC-treatment exhibited extensive fibrotic desmoplasia and tumor cell pleomorphism in both PDX models and in serial biopsies obtained from TNBC patients enrolled on the ARTEMIS trial. Strikingly, these AC-induced features were reverted upon regrowth of residual tumors in PDXs and in patients' tumors. Similarly, residual tumors exhibited unique transcriptomic features, many of which are also de-regulated in cohorts of human TNBCs undergoing chemotherapy treatment. These features were nearly completely reverted after tumors regrew, suggesting that the residual tumor state may be a unique and transient therapeutic window. Gene set enrichment analyses revealed that residual tumors had increased activation of oxidative phosphorylation and decreased glycolytic signaling. Pharmacologic targeting of oxidative phosphorylation with a small-molecule inhibitor of mitochondrial electron transport chain complex I (IACS-010759) significantly delayed the regrowth of AC-treated residual tumors in three independent PDX models. Collectively, these studies reveal that a reversible phenotypic state can confer chemoresistance in the absence of genomic selection and that the residual tumor state is a novel therapeutic window for chemo-refractory TNBC.

Citation Format: Echeverria GV, Ge Z, Seth S, Jeter-Jones SL, Zhang X, Zhou X, Cai S, Tu Y, McCoy A, Peoples M, Lau R, Shao J, Sun Y, Bristow C, Carugo A, Ma X, Harris A, Wu Y, Moulder S, Symmans WF, Marszalek JR, Heffernan TP, Chang JT, Piwnica-Worms H. Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS5-05.

Abstract P4-03-02: Characterizing and targeting chemoresistant subclones in patient-derived xenograft models of triple negative breast cancer

Fifty percent of all triple negative breast cancer (TNBC) patients harbor significant residual tumor burden following treatment with standard neoadjuvant chemotherapy (NACT), resulting in poor prognosis. Recent studies in TNBC have revealed extensive intra-tumoral heterogeneity at the time of diagnosis and throughout disease progression, but the relative contributions of these heterogeneous populations of tumor cells to chemoresistance are not well understood.

The primary tumor, dermal metastasis, and germline reference were obtained from a patient with untreated metastatic TNBC. Tumor cells were engrafted into the humanized mammary fat pads of NOD/SCID mice to establish PDX models of the primary (PIM001-P) and metastatic (PIM001-M) tumors. RNA sequencing and whole-exome sequencing (WES), performed on the patient's primary and metastatic tumors and the first- and third- passage PDX models revealed transcriptomic profiles and subclonal heterogeneity of the patient's tumors were recapitulated in the PDX models.

Treatment of mice engrafted with PIM001-P tumors with NACT (Adriamycin plus cyclophosphamide, AC) resulted in partial response, the magnitude of which was diminished in mice bearing PIM001-M tumors. Tumor subclones were tracked during chemotherapy treatment in mice engrafted with PIM001-P tumors using lentiviral non-targeting DNA barcodes. Residual tumors maintained the clonal architecture of untreated tumors, and deep WES revealed stable maintenance of somatic mutant allele frequencies throughout treatment. Therefore, selection of pre-existing resistant clones did not lead to AC resistance in this model. Interestingly, only 25% of residual tumor clones contributed to primary relapse once treatment was halted, suggesting only a subpopulation of tumor cells was able to reconstitute the tumor.

RNA sequencing and reverse phase protein array revealed that while vehicle-treated and regrown tumors were highly similar, residual tumors harbored a unique profile characterized by numerous significant alterations in RNA and protein levels. Together, these results suggest that residual tumors enter into a transient drug-resistant state that is reversible. Residual tumors were enriched for alterations in pathways such as metabolism, extracellular matrix remodeling, and cell-cell communication. Pharmacologic targeting of the residual tumor state with an inhibitor of mitochondrial oxidative phosphorylation led to significant inhibition of tumor regrowth following AC treatment. Additional vulnerabilities identified in residual tumors are being targeted therapeutically with the goal of eradicating residual tumor cells.

Citation Format: Echeverria GV, Seth S, Ge Z, Sun Y, DiFrancesco E, Lau R, Marszalek J, Moulder S, Symmans F, Heffernan TP, Chang JT, Piwnica-Worms H. Characterizing and targeting chemoresistant subclones in patient-derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-03-02.

A specialized mitochondrial molecular chaperone system: a role in formation of Fe/S centers

Mitochondria contain a specialized system of molecular chaperones that plays a critical role in the biogenesis of Fe/S centers. This Hsp70:J-protein system shows many similarities to the system found in bacteria, but the precise role of neither chaperone system has been defined. However, evidence to date suggests an interaction with the scaffold protein on which a transient Fe/S center is assembled, and thus implies a role in either assembly of the center or its transfer to recipient proteins.

Jac1, a mitochondrial J-type chaperone, is involved in the biogenesis of Fe/S clusters in Saccharomyces cerevisiae

A minor Hsp70 chaperone of the mitochondrial matrix of Saccharomyces cerevisiae, Ssq1, is involved in the formation or repair of Fe/S clusters and/or mitochondrial iron metabolism. Here, we report evidence that Jac1, a J-type chaperone of the mitochondrial matrix, is the partner of Ssq1 in this process. Reduced activity of Jac1 results in a decrease in activity of Fe/S containing mitochondrial proteins and an accumulation of iron in mitochondria. Fe/S enzyme activities remain low in both jac1 and ssq1 mutant mitochondria even if normal mitochondrial iron levels are maintained. Therefore, the low activities observed are not solely due to oxidative damage caused by excess iron. Rather, these molecular chaperones likely play a direct role in the normal assembly process of Fe/S clusters.

Genetic evidence for selective transport of opsin and arrestin by kinesin-II in mammalian photoreceptors

To test whether kinesin-II is important for transport in the mammalian photoreceptor cilium, and to identify its potential cargoes, we used Cre-loxP mutagenesis to remove the kinesin-II subunit, KIF3A, specifically from photoreceptors. Complete loss of KIF3A caused large accumulations of opsin, arrestin, and membranes within the photoreceptor inner segment, while the localization of alpha-transducin was unaffected. Other membrane, organelle, and transport markers, as well as opsin processing appeared normal. Loss of KIF3A ultimately caused apoptotic photoreceptor cell death similar to a known opsin transport mutant. The data suggest that kinesin-II is required to transport opsin and arrestin from the inner to the outer segment and that blocks in this transport pathway lead to photoreceptor cell death as found in retinitis pigmentosa.

Role of the mitochondrial Hsp70s, Ssc1 and Ssq1, in the maturation of Yfh1

The mitochondrial matrix of the yeast Saccharomyces cerevisiae contains two molecular chaperones of the Hsp70 class, Ssc1 and Ssq1. We report that Ssc1 and Ssq1 play sequential roles in the import and maturation of the yeast frataxin homologue (Yfh1). In vitro, radiolabeled Yfh1 was not imported into ssc1-3 mutant mitochondria, remaining in a protease-sensitive precursor form. As reported earlier, the Yfh1 intermediate form was only slowly processed to the mature form in Deltassq1 mitochondria (S. A. B. Knight, N. B. V. Sepuri, D. Pain, and A. Dancis, J. Biol. Chem. 273:18389-18393, 1998). However, the intermediate form in both wild-type and Deltassq1 mitochondria was entirely within the inner membrane, as it was resistant to digestion with protease after disruption of the outer membrane. Therefore, we conclude that Ssc1, which is present in mitochondria in approximately a 1,000-fold excess over Ssq1, is required for Yfh1 import into the matrix, while Ssq1 is necessary for the efficient processing of the intermediate to the mature form in isolated mitochondria. However, the steady-state level of mature Yfh1 in Deltassq1 mitochondria is approximately 75% of that found in wild-type mitochondria, indicating that this retardation in processing does not dramatically affect cellular concentrations. Therefore, Ssq1 likely has roles in addition to facilitating the processing of Yfh1. Twofold overexpression of Ssc1 partially suppresses the cold-sensitive growth phenotype of Deltassq1 cells, as well as the accumulation of mitochondrial iron and the defects in Fe/S enzyme activities normally found in Deltassq1 mitochondria. Deltassq1 mitochondria containing twofold-more Ssc1 efficiently converted the intermediate form of Yfh1 to the mature form. This correlation between the observed processing defect and suppression of in vivo phenotypes suggests that Ssc1 is able to carry out the functions of Ssq1, but only when present in approximately a 2,000-fold excess over normal levels of Ssq1.

Understanding the functions of kinesin-II

Species ranging from Chlamydomonas to humans possess the heterotrimeric kinesin-II holoenzyme composed of two different motor subunits and one non-motor accessory subunit. An important function of kinesin-II is that it transports the components needed for the construction and maintenance of cilia and flagella from the site of synthesis in the cell body to the site of growth at the distal tip. Recent work suggests that kinesin-II does not directly interact with these components, but rather via a large protein complex, which has been termed a raft (intraflagellar transport (IFT)). While ciliary transport is the best-established function for kinesin-II, evidence has been reported for possible roles in neuronal transport, melanosome transport, the secretory pathway and during mitosis.

Dual role of the mitochondrial chaperone Mdj1p in inheritance of mitochondrial DNA in yeast

Mdj1p, a homolog of the bacterial DnaJ chaperone protein, plays an essential role in the biogenesis of functional mitochondria in the yeast Saccharomyces cerevisiae. We analyzed the role of Mdj1p in the inheritance of mitochondrial DNA (mtDNA). Mitochondrial genomes were rapidly lost in a temperature-sensitive mdj1 mutant under nonpermissive conditions. The activity of mtDNA polymerase was severely reduced in the absence of functional Mdj1p at a nonpermissive temperature, demonstrating the dependence of the enzyme on Mdj1p. At a permissive temperature, the activity of mtDNA polymerase was not affected by the absence of Mdj1p. However, under these conditions, intact [rho(+)] genomes were rapidly converted to nonfunctional [rho(-)] genomes which were stably propagated in an mdj1 deletion strain. We propose that mtDNA polymerase depends on Mdj1p as a chaperone in order to acquire and/or maintain an active conformation at an elevated temperature. In addition, Mdj1p is required for the inheritance of intact mitochondrial genomes at a temperature supporting optimal growth; this second function appears to be unrelated to the function of Mdj1p in maintaining mtDNA polymerase activity.

Novel dendritic kinesin sorting identified by different process targeting of two related kinesins: KIF21A and KIF21B

Neurons use kinesin and dynein microtubule-dependent motor proteins to transport essential cellular components along axonal and dendritic microtubules. In a search for new kinesin-like proteins, we identified two neuronally enriched mouse kinesins that provide insight into a unique intracellular kinesin targeting mechanism in neurons. KIF21A and KIF21B share colinear amino acid similarity to each other, but not to any previously identified kinesins outside of the motor domain. Each protein also contains a domain of seven WD-40 repeats, which may be involved in binding to cargoes. Despite the amino acid sequence similarity between KIF21A and KIF21B, these proteins localize differently to dendrites and axons. KIF21A protein is localized throughout neurons, while KIF21B protein is highly enriched in dendrites. The plus end-directed motor activity of KIF21B and its enrichment in dendrites indicate that models suggesting that minus end-directed motor activity is sufficient for dendrite specific motor localization are inadequate. We suggest that a novel kinesin sorting mechanism is used by neurons to localize KIF21B protein to dendrites since its mRNA is restricted to the cell body.

Situs inversus and embryonic ciliary morphogenesis defects in mouse mutants lacking the KIF3A subunit of kinesin-II

The embryonic cellular events that set the asymmetry of the genetic control circuit controlling left-right (L-R) axis determination in mammals are poorly understood. New insight into this problem was obtained by analyzing mouse mutants lacking the KIF3A motor subunit of the kinesin-II motor complex. Embryos lacking KIF3A die at 10 days postcoitum, exhibit randomized establishment of L-R asymmetry, and display numerous structural abnormalities. The earliest detectable abnormality in KIF3A mutant embryos is found at day 7.5, where scanning electron microscopy reveals loss of cilia ordinarily present on cells of the wild-type embryonic node, which is thought to play an important role in setting the initial L-R asymmetry. This cellular phenotype is observed before the earliest reported time of asymmetric expression of markers of the L-R signaling pathway. These observations demonstrate that the kinesin-based transport pathway needed for flagellar and ciliary morphogenesis is conserved from Chlamydomonas to mammals and support the view that embryonic cilia play a role in the earliest cellular determinative events establishing L-R asymmetry.

Role of adenine nucleotides, molecular chaperones and chaperonins in stabilization of DnaA initiator protein of Escherichia coli

DnaA protein of Escherichia coli is a sequence-specific DNA binding protein required for the initiation of DNA replication from the chromosomal origin, oriC, and of several E. coli plasmids. At a moderate ionic strength, purified DnaA protein has a strong tendency to aggregate; the self-aggregate form is inactive in DNA replication. Binding of ATP or ADP to DnaA protein protected it from aggregation to maintain its replication activity. AMP or cyclic AMP had no protective effect. The molecular chaperone DnaK protected DnaA protein from aggregation with or without ATP. DnaJ and GrpE were not stimulatory. Chaperonins GroEL and GroES were also able to prevent aggregation but only in the presence of ATP. The studies presented here show that for DnaA protein to be active in the initiation of DNA replication, it must be prevented from forming a self-aggregate by the binding of adenine nucleotides, and/or by the action of molecular chaperones.

Interaction of the Escherichia coli DnaA protein with bacteriophage lambda DNA

Interaction of the Escherichia coli DnaA (replication initiator) protein with restriction fragments of phage lambda DNA demonstrated differential binding of DnaA along the whole lambda DNA. Interaction of DnaA with the lambda replication region (from the promoter pR to the origin of replication, orilambda) demonstrated a strong binding of DnaA to the region around the p(o) promoter where synthesis of a short antisense oop RNA is initiated. The four sequences protected by DnaA (two 9mers and two 5mers) are not related even to a relaxed DnaA box. The pattern of protection of these four sequences and the location of three DNase I hypersensitive sites in the lambda DNA r strand, together with results of mobility shift assays and electron microscopy studies, may indicate an interaction involving DnaA monomers bound to different DNA positions on one side of the helix and the formation of higher-order nucleoprotein structures. Therefore, it is tempting to suggest that DnaA, in addition to its activity in regulation of replication and transcription, could be considered as a factor which structures certain chromosomal regions.

Identification, partial characterization, and genetic mapping of kinesin-like protein genes in mouse

Microtubule-dependent motors of the kinesin superfamily have undergone structural and functional diversification during evolution and play crucial roles in cell division and intracellular transport. Degenerate oligonucleotides homologous to highly conserved regions of sequence within the motor domain were used in a polymerase chain reaction to isolate five new members (KIF3C, KIFC2, KIFC3, KIFC4, and KIF22) of the kinesin superfamily from a mouse brain cDNA library. Northern analysis showed that KIF3C and KIFC2 are expressed mainly in neural tissues, that KIFC4 and KIF22 are expressed primarily in proliferative tissues and cell lines, and that KIFC3 is apparently ubiquitous. To elucidate the organization of genes encoding kinesin-like motors in the mouse genome and to explore the potential associations of these genes with classical mouse mutations or human genetic diseases, these new genes as well as genes encoding the previously reported KIF3A and KIF3B motors were mapped to mouse chromosomes by using an interspecific backcross panel of DNAs from The Jackson Laboratory. The data indicate that the gene KIFC4 is present in three copies in the mouse genome on chromosomes 13 (KIFC4A), 10 (KIFC4B), and 17 (KIFC4C). The gene KIF22 is present in two copies on chromosomes 7 (KIF22A) and 1 (KIF22B). The genes KIF3A, KIF3B, KIF3C, KIFC2, and KIFC3 are each single loci and map to chromosomes 11, 2, 12, 15, and 8, respectively.

Neurofilament subunit NF-H modulates axonal diameter by selectively slowing neurofilament transport

To examine the mechanism through which neurofilaments regulate the caliber of myelinated axons and to test how aberrant accumulations of neurofilaments cause motor neuron disease, mice have been constructed that express wild-type mouse NF-H up to 4.5 times the normal level. Small increases in NF-H expression lead to increased total neurofilament content and larger myelinated axons, whereas larger increases in NF-H decrease total neurofilament content and strongly inhibit radial growth. Increasing NF-H expression selectively slow neurofilament transport into and along axons, resulting in severe perikaryal accumulation of neurofilaments and proximal axonal swellings in motor neurons. Unlike the situation in transgenic mice expressing modest levels of human NF-H (Cote, F., J.F. Collard, and J.P. Julien. 1993. Cell. 73:35-46), even 4.5 times the normal level of wild-type mouse NF-H does not result in any overt phenotype or enhanced motor neuron degeneration or loss. Rather, motor neurons are extraordinarily tolerant of wild-type murine NF-H, whereas wild-type human NF-H, which differs from the mouse homolog at > 160 residue positions, mediates motor neuron disease in mice by acting as an aberrant, mutant subunit.

Mechanisms of selective motor neuron death in transgenic mouse models of motor neuron disease

To examine the mechanism(s) of disease underlying ALS, transgenic mouse models have been constructed that express aberrant neurofilaments or mutations in the abundant, cytoplasmic enzyme superoxide dismutase 1 (SOD1). In addition to progressive weakness arising from selective motor neuron death, mice expressing a modest level of a point mutant in neurofilament subunit NF-L show most of the pathologic hallmarks observed in familial and sporadic ALS, including perikaryal proximal axonal swellings, axonal degeneration, and severe skeletal muscle atrophy. Additional mice expressing familial ALS-linked mutations in the cytoplasmic enzyme SOD1, the only proven cause of ALS and which accounts for approximately 20% of familial disease, have demonstrated that at least one mutation causes disease through acquisition of an adverse property by the mutant enzyme, rather than elevation or loss of SOD1 activity. These animals not only provide a detailed look at the pathogenic progression of disease, but also represent a tool for testing hypotheses concerning the specific mechanism(s) of neuronal death and for testing therapeutic strategies.

Domains of DnaA protein involved in interaction with DnaB protein, and in unwinding the Escherichia coli chromosomal origin

DnaA protein of Escherichia coli is a sequence-specific DNA-binding protein required for the initiation of DNA replication from the chromosomal origin, oriC. It is also required for replication of several plasmids including pSC101, F, P-1, and R6K. A collection of monoclonal antibodies to DnaA protein has been produced and the primary epitopes recognized by them have been determined. These antibodies have also been examined for the ability to inhibit activities of DNA binding, ATP binding, unwinding of oriC, and replication of both an oriC plasmid, and an M13 single-stranded DNA with a proposed hairpin structure containing a DnaA protein-binding site. Replication of the latter DNA is dependent on DnaA protein by a mechanism termed ABC priming. These studies suggest regions of DnaA protein involved in interaction with DnaB protein, and in unwinding of oriC, or low-affinity binding of ATP.

Subunit composition of neurofilaments specifies axonal diameter

Neurofilaments (NFs), which are composed of NF-L, NF-M, and NF-H, are required for the development of normal axonal caliber, a property that in turn is a critical determinant of axonal conduction velocity. To investigate how each subunit contributes to the radial growth of axons, we used transgenic mice to alter the subunit composition of NFs. Increasing each NF subunit individually inhibits radial axonal growth, while increasing both NF-M and NF-H reduces growth even more severely. An increase in NF-L results in an increased filament number but reduced interfilament distance. Conversely, increasing NF-M, NF-H, or both reduces filament number, but does not alter nearest neighbor interfilament distance. Only a combined increase of NF-L with either NF-M or NF-H promotes radial axonal growth. These results demonstrate that both NF-M and NF-H play complementary roles with NF-L in determining normal axonal calibers.

Increasing neurofilament subunit NF-M expression reduces axonal NF-H, inhibits radial growth, and results in neurofilamentous accumulation in motor neurons

The carboxy-terminal tail domains of neurofilament subunits neurofilament NF-M and NF-H have been postulated to be responsible for the modulation of axonal caliber. To test how subunit composition affects caliber, transgenic mice were generated to increase axonal NF-M. Total neurofilament subunit content in motor and sensory axons remained essentially unchanged, but increases in NF-M were offset by proportionate decreases in both NF-H and axonal cross-sectional area. Increase in NF-M did not affect the level of phosphorylation of NF-H. This indicates that (a) in vivo NF-H and NF-M compete either for coassembly with a limiting amount of NF-L or as substrates for axonal transport, and (b) NF-H abundance is a primary determinant of axonal caliber. Despite inhibition of radial growth, increase in NF-M and reduction in axonal NF-H did not affect nearest neighbor spacing between neurofilaments, indicating that cross-bridging between nearest neighbors does not play a crucial role in radial growth. Increase in NF-M did not result in an overt phenotype or neuronal loss, although filamentous swellings in perikarya and proximal axons of motor neurons were frequently found.

The ClpX heat-shock protein of Escherichia coli, the ATP-dependent substrate specificity component of the ClpP-ClpX protease, is a novel molecular chaperone

All major classes of protein chaperones, including DnaK (the Hsp70 eukaryotic equivalent) and GroEL (the Hsp60 eukaryotic equivalent) have been found in Escherichia coli. Molecular chaperones enhance the yields of correctly folded polypeptides by preventing aggregation and even by disaggregating certain protein aggregates. Previously, we identified the ClpX heat-shock protein of E. coli because it enables the ClpP catalytic protease to degrade the bacteriophage lambda O replication protein. Here we report that ClpX alone possesses all the properties expected of a molecular chaperone protein. Specifically, it can protect the lambda O protein from heat-induced aggregation, disaggregate preformed lambda O aggregates, and even promote efficient binding of lambda O to its DNA recognition sequence. A lambda O-ClpX specific protein-protein interaction can be detected either by a modified ELISA assay or through the stimulation of ClpX's weak ATPase activity by lambda O. Unlike the behaviour of the major DnaK and GroEL chaperones, ClpX requires the presence of ATP or its non-hydrolysable analogue ATP-gamma-S for efficient interaction with other proteins including the protection of lambda O from aggregation. However, ClpX's ability to disaggregate lambda O aggregates requires hydrolysable ATP. We propose that the ClpX protein is a bona fide chaperone, whose biological role includes the maintenance of certain polypeptides in a form competent for proteolysis by the ClpP protease. Furthermore, our results suggest that the ClpX protein also performs typical chaperone protein functions independent of ClpP.

The requirement for molecular chaperones in lambda DNA replication is reduced by the mutation pi in lambda P gene, which weakens the interaction between lambda P protein and DnaB helicase

During the initiation of lambda DNA replication, the host DnaB helicase is complexed with phage lambda P protein in order to be properly positioned near the ori lambda-lambda O initiation complex. However, the lambda P-DnaB interaction inhibits the activities of DnaB. Thus, the concerted action of bacterial heat shock proteins, DnaK, DnaJ, and GrpE, is required to activate the helicase. Wild-type phage lambda cannot grow on the E. coli dnaB, dnaK, dnaJ, and grpE mutants. However, lambda phage with a mutation pi in the lambda P gene, is able to produce progeny in these mutants as well as in the wild-type bacteria. Purified mutant lambda pi protein reveals a much lower affinity to DnaB than wild-type lambda P, and the lambda pi-DnaB complex is unstable. Also, a very low concentration of DnaK protein is sufficient to activate the helicase in a replication system based on lambda dv dsDNA. In that system, the mutant DnaK756 protein, inactive in the lambda P-dependent replication, revealed its activity in the lambda pi-dependent reaction. The lambda O-lambda P-dependent replication system based on M13 ssDNA efficiently replicates DNA in the absence of any chaperone protein, unless lambda P is substituted by the lambda pi mutant protein. Data presented in this paper explain why lambda pi phage is able to grow on wild-type and dnaK756 bacteria.

A mutant neurofilament subunit causes massive, selective motor neuron death: implications for the pathogenesis of human motor neuron disease

A direct role of aberrant neurofilament accumulation in the etiology of human motor neuron diseases, including amyotrophic lateral sclerosis, is suggested by the presence of abnormal accumulations of neurofilaments as an early hallmark of the pathogenic process. Furthermore, forcing increased expression of neurofilament subunits in transgenic mouse models leads to motor neuron dysfunction, albeit without the widespread motor neuron death typical of human disease. We now show that accumulation of a modest level of a point mutant in the smallest neurofilament subunit (NF-L) causes massive, selective degeneration of spinal motor neurons accompanied by abnormal accumulations of neurofilaments and severe neurogenic atrophy of skeletal muscles. As in human disease, sensory neurons show only a modest level of degenerative changes. Thus, neurofilament mutations can cause selective motor neuron death, and neurofilamentous abnormalities may be a common toxic intermediate that significantly contributes to the motor neuron death in human disease.

DnaA protein directs the binding of DnaB protein in initiation of DNA replication in Escherichia coli

DnaA protein of Escherichia coli acts in the initiation of chromosomal replication to bind to sequences in the chromosomal origin. On binding, it promotes the assembly of other replication proteins that serve to prime DNA replication and assemble the replication apparatus for bidirectional replication fork movement. A collection of monoclonal antibodies to DnaA protein have been produced, one of which is described here, that interferes with the action of DnaA protein in promoting formation of a prepriming complex. On the analysis of this process, the antibody appears to interfere with the physical interaction between DnaA and DnaB protein in the DnaB.DnaC complex. Cross-linking studies confirm that DnaA and DnaB proteins interact directly. These results provide the first direct evidence that one of the roles of DnaA protein is to act as a site for binding of DnaB protein to the DNA and perhaps orients DnaB helicase to account for the directionality of replication fork movement.

Defective replication activity of a dominant-lethal dnaB gene product from Escherichia coli

dnaB protein of Escherichia coli is an essential replication protein. A missense mutant has been obtained which results in replacement of an arginine residue with cysteine at position 231 of the protein (P. Shrimankar, L. Shortle, and R. Maurer, unpublished data). This mutant displays a dominant-lethal phenotype in strains that are heterodiploid for dnaB. Biochemical analysis of the altered form of dnaB protein revealed that it was inactive in replication in several purified enzyme systems which involve specific and nonspecific primer formation on single-stranded DNAs, and in replication of plasmids containing the E. coli chromosomal origin. Inactivity in replication appeared to be due to its inability to bind to single-stranded DNA. The altered dnaB protein was inhibitory to the activity of wild type dnaB protein in replication by sequestering dnaC protein which is also required for replication. By contrast, it was not inhibitory to dnaB protein in priming of single-stranded DNA by primase in the absence of single-stranded DNA binding protein. Sequestering of dnaC protein into inactive complexes may relate to the dominant-lethal phenotype of this dnaB mutant.

Escherichia coli DnaJ and GrpE heat shock proteins jointly stimulate ATPase activity of DnaK

The products of the Escherichia coli dnaK, dnaJ, and grpE heat shock genes have been previously shown to be essential for bacteriophage lambda DNA replication at all temperatures and for bacterial survival under certain conditions. DnaK, the bacterial heat shock protein hsp70 analogue and putative chaperonin, possesses a weak ATPase activity. Previous work has shown that ATP hydrolysis allows the release of various polypeptides complexed with DnaK. Here we demonstrate that the ATPase activity of DnaK can be greatly stimulated, up to 50-fold, in the simultaneous presence of the DnaJ and GrpE heat shock proteins. The presence of either DnaJ or GrpE alone results in a slight stimulation of the ATPase activity of DnaK. The action of the DnaJ and GrpE proteins may be sequential, since the presence of DnaJ alone leads to an acceleration in the rate of hydrolysis of the DnaK-bound ATP. The presence of GrpE alone increases the rate of release of bound ATP or ADP without affecting the rate of hydrolysis. The stimulation of the ATPase activity of DnaK may contribute to its more efficient recycling, and it helps explain why mutations in dnaK, dnaJ, or grpE genes often exhibit similar pleiotropic phenotypes.

Regulatory properties of AMP deaminases from rat tissues

1. Phosphocellulose column chromatography under double gradient conditions (phosphate and KCl) revealed two forms of AMP deaminase in rat heart and brain and a single form in the liver and skeletal muscle. 2. Kinetically all purified AMP deaminases were classified into two categories: those, which elute from the column at lower KCl and Pi concentrations, display low S0.5 value are only moderately affected by MgATP, MgGTP and Pi; and those which elute at higher KCl and Pi concentrations, display high S0.5 values and are strongly regulated by allosteric effectors. 3. Physiological significance of the occurrence of two kinetic forms of AMP deaminase in some tissues is discussed.

Regulation of platelet AMP deaminase activity in situ

The regulation of platelet AMP deaminase activity by ATP, GTP and phosphate was studied in human platelets in situ, and in vitro after partial purification. In intact platelets, a similar 50% decrease in cytosolic ATP was induced by either glucose starvation or treatment with H2O2. During starvation, AMP deaminase was in the inhibited state, as ATP consumption was mostly balanced by the accumulation of AMP. During H2O2 treatment, however, the enzyme was in the stimulated state, as the AMP formed was almost completely deaminated to IMP. Cytosolic GTP fell by 40-50% in both starvation and H2O2 treatment. In contrast, intracellular phosphate was 4-5-fold higher in starved than in H2O2-treated cells. These data point to phosphate as the main regulator of AMP deaminase activity in situ. This conclusion was verified by kinetic analysis of partially purified AMP deaminase. At near-physiological concentrations of MgATP, MgGTP and phosphate, the S0.5 (substrate half-saturation constant) for AMP was 0.35 mM. Half-maximal stimulation by MgATP occurred at a concn. between 2 and 3 mM. This stimulation was antagonized by the inhibitory effects of phosphate (IC50 = 2.0 mM) and MgGTP (IC50 = 0.2-0.3 mM), which acted in synergism (IC50 is the concentration causing 50% inhibition). We conclude that the difference in adenylate catabolism between starved and H2O2-treated platelets is due to the distinct phosphate concentrations. During starvation, refeeding and H2O2 treatment, the values of the adenylate charge and the phosphorylation potential were kept closely co-ordinated, which may be effected by AMP deaminase.