Hsp90 as a capacitor for morphological evolution

Heat shock protein 90 function is essential for Plasmodium falciparum growth in human erythrocytes

Hsp90 is important for normal growth and development in eukaryotes. Together with Hsp70 and other accessory proteins, Hsp90 not only helps newly synthesized proteins to fold but also regulates activities of transcription factors and protein kinases. Although the gene coding for heat shock protein 90 from Plasmodium falciparum (PfHsp90) has been characterized previously, there is very little known regarding its function in the parasite. We have analyzed PfHsp90 complexes and addressed its role in parasite life cycle using Geldanamycin (GA), a drug known to interfere with Hsp90 function. Sedimentation analysis and size exclusion chromatography showed PfHsp90 to be in 11 s(20,(w)) complexes of approximately 300 kDa in size. Similar to the hetero-oligomeric complexes of Hsp90 in mammals, PfHsp70 was found to be present in PfHsp90 complexes. Homology modeling revealed a putative GA-binding pocket at the amino terminus of PfHsp90. The addition of GA inhibited parasite growth with LD(50) of 0.2 microm. GA inhibited parasite growth by arresting transition from Ring to trophozoite. Transition from trophozoite to schizonts and reinvasion of new erythrocytes were less significantly affected. While inducing the synthesis of PfHsp70 and PfHsp90, GA did not significantly alter the pattern of newly synthesized proteins. Pre-exposure to heat shock attenuated GA-mediated growth inhibition, suggesting the involvement of heat shock proteins. Specificity of GA action on PfHsp90 was evident from selective inhibition of PfHsp90 phosphorylation in GA-treated cultures. In addition to suggesting an essential role for PfHsp90 during parasite growth, our results highlight PfHsp90 as a potential drug target to control malaria.

Prokaryote and eukaryote evolvability

The concept of evolvability covers a broad spectrum of, often contradictory, ideas. At one end of the spectrum it is equivalent to the statement that evolution is possible, at the other end are untestable post hoc explanations, such as the suggestion that current evolutionary theory cannot explain the evolution of evolvability. We examine similarities and differences in eukaryote and prokaryote evolvability, and look for explanations that are compatible with a wide range of observations. Differences in genome organisation between eukaryotes and prokaryotes meets this criterion. The single origin of replication in prokaryote chromosomes (versus multiple origins in eukaryotes) accounts for many differences because the time to replicate a prokaryote genome limits its size (and the accumulation of junk DNA). Both prokaryotes and eukaryotes appear to switch from genetic stability to genetic change in response to stress. We examine a range of stress responses, and discuss how these impact on evolvability, particularly in unicellular organisms versus complex multicellular ones. Evolvability is also limited by environmental interactions (including competition) and we describe a model that places limits on potential evolvability. Examples are given of its application to predator competition and limits to lateral gene transfer. We suggest that unicellular organisms evolve largely through a process of metabolic change, resulting in biochemical diversity. Multicellular organisms evolve largely through morphological changes, not through extensive changes to cellular biochemistry.

Effect of heat shock, pretreatment and hsp70 copy number on wing development in Drosophila melanogaster

Naturally occurring heat shock (HS) during pupation induces abnormal wing development in Drosophila; we examined factors affecting the severity of this induction. The proportion of HS-surviving adults with abnormal wings varied with HS duration and intensity, and with the pupal age or stage at HS administration. Pretreatment (PT), mild hyperthermia delivered before HS, usually protected development against HS. Gradual heating resembling natural thermal regimes also protected wing development against thermal disruption. Because of the roles of the wings in flight and courtship and in view of natural thermal regimes that Drosophila experience, both HS-induction of wing abnormalities and its abatement by PT may have marked effects on Drosophila fitness in nature. Because PT is associated with expression of heat-inducible molecular chaperones such as Hsp70 in Drosophila, we compared thermal disruption of wing development among hsp70 mutants as well as among strains naturally varying in Hsp70 levels. Contrary to expectations, lines or strains with increased Hsp70 levels were no more resistant to HS-disruption of wing development than counterparts with lower Hsp70 levels. In fact, wing development was more resistant to HS in hsp70 deletion strains than control strains. We suggest that, while high Hsp70 levels may aid cells in surviving hyperthermia, high levels may also overly stimulate or inhibit numerous signalling pathways involved in cell proliferation, maturation and programmed death, resulting in developmental failure.

Variation of tooth number in mammalian dentition: connecting genetics, development, and evolution

A major question in modern biology is how gene mutations affect development and are translated into macroevolutionary changes in morphology. Variations in tooth number, a strategy used by many mammals to develop specialized dentitions, has been an important factor for species diversification. Changes in the number of teeth tend to occur in the reverse of the order teeth are formed during development, which also characterizes the general pattern of tooth loss observed during the evolution of placental mammals. To understand how changes at the molecular level affect the distinct stages of tooth development, we analyzed the ontogenesis of tooth growth arrest in sciurids and mice and in single and double knockout mutant mice. We show that the complexity of the genetic network that governs tooth development can change during ontogenetic trajectory, and these changes may be related to macroevolutionary changes. Furthermore, we show that the variation in tooth number in the affected members of human families bearing mutations in the MSX1 and PAX9 genes can help to understand how the genetic variations within a population can modulate evolutionary changes in dental patterning.

Genetics and the making of Homo sapiens

Understanding the genetic basis of the physical and behavioural traits that distinguish humans from other primates presents one of the great new challenges in biology. Of the millions of base-pair differences between humans and chimpanzees, which particular changes contributed to the evolution of human features after the separation of the Pan and Homo lineages 5-7 million years ago? How can we identify the 'smoking guns' of human genetic evolution from neutral ticks of the molecular evolutionary clock? The magnitude and rate of morphological evolution in hominids suggests that many independent and incremental developmental changes have occurred that, on the basis of recent findings in model animals, are expected to be polygenic and regulatory in nature. Comparative genomics, population genetics, gene-expression analyses and medical genetics have begun to make complementary inroads into the complex genetic architecture of human evolution.

Between genotype and phenotype: protein chaperones and evolvability

Protein chaperones direct the folding of polypeptides into functional proteins, facilitate developmental signalling and, as heat-shock proteins (HSPs), can be indispensable for survival in unpredictable environments. Recent work shows that the main HSP chaperone families also buffer phenotypic variation. Chaperones can do this either directly through masking the phenotypic effects of mutant polypeptides by allowing their correct folding, or indirectly through buffering the expression of morphogenic variation in threshold traits by regulating signal transduction. Environmentally sensitive chaperone functions in protein folding and signal transduction have different potential consequences for the evolution of populations and lineages under selection in changing environments.

The unfolding story of three-dimensional domain swapping

Three-dimensional domain swapping is the event by which a monomer exchanges part of its structure with identical monomers to form an oligomer where each subunit has a similar structure to the monomer. The accumulating number of observations of this phenomenon in crystal structures has prompted speculation as to its biological relevance. Domain swapping was originally proposed to be a mechanism for the emergence of oligomeric proteins and as a means for functional regulation, but also to be a potentially harmful process leading to misfolding and aggregation. We highlight experimental studies carried out within the last few years that have led to a much greater understanding of the mechanism of domain swapping and of the residue- and structure-specific features that facilitate the process. We discuss the potential biological implications of domain swapping in light of these findings.

Hsp90 reaches new heights. Conference on the Hsp90 chaperone machine

Conference on the Hsp90 Chaperone Machine

Apoptosis, necrosis and cellular senescence: chaperone occupancy as a potential switch

Chaperone function plays a key role in repairing proteotoxic damage and in the maintenance of cell survival. Here we compare the regulatory role of molecular chaperones (heat shock proteins, stress proteins) in cellular senescence, apoptosis and necrosis. We also review the current data on chaperone level and function in aging cells, and list some possible therapeutic interventions. Finally, we postulate a hypothesis, that increasing chaperone occupancy might be an important event which forces cells out of the normal cell cycle towards senescence. In the case of severe stress, this may lead to apoptosis or, following lethal stress, to cell necrosis.

Characterization of a plant homolog of hop, a cochaperone of hsp90

The 90-kD molecular chaperone hsp90 is the key component of a multiprotein chaperone complex that facilitates folding, stabilization, and functional modulation of a number of signaling proteins. The components of the animal chaperone complex include hsp90, hsp70, hsp40, Hop, and p23. The animal Hop functions to link hsp90 and hsp70, and it can also inhibit the ATPase activity of hsp90. We have demonstrated the presence of an hsp90 chaperone complex in plant cells, but not all components of the complex have been identified. Here, we report the isolation and characterization of soybean (Glycine max) GmHop-1, a soybean homolog of mammalian Hop. An analysis of soybean expressed sequence tags, combined with preexisting data in literature, suggested the presence of at least three related genes encoding Hop-like proteins in soybean. Transcripts corresponding to Hop-like proteins in soybean were detected under normal growth conditions, and their levels increased further in response to stress. A recombinant GmHop-1 bound hsp90 and its binding to hsp90 could be blocked by the tetratricopeptide repeat (TPR) domain of rat (Rattus norvegicus) protein phosphatase 5. Deletion of amino acids 325 to 395, adjacent to the TPR2A domain in GmHop-1, resulted in loss of hsp90 binding. In a minimal assembly system, GmHop-1 was able to stimulate mammalian steroid receptor folding. These data show that plant and animal Hop homologs are conserved in their general characteristics, and suggest that a Hop-like protein in plants is an important cochaperone of plant hsp90.

Evolution of mutational robustness

We review recent advances in the understanding of the mutation-selection balance of asexual replicators. For over 30 years, population geneticists thought that an expression derived by Kimura and Maruyama in 1966 fully solved this problem. However, Kimura and Maruyama's result is only correct in the absence of neutral mutations. The inclusion of neutral mutations leads to a wealth of interesting new effects, and, in particular, to a selective pressure to evolve robustness against mutations. We cover recent literature on the population dynamics of asexual replicators on networks of neutral genotypes, on the outcompetition of fast replicators by slower ones with better mutational support, and on the probability of fixation at high mutation rates. We discuss empirical evidence for the evolution of mutational robustness, and speculate on its relevance for higher organisms.

Fluctuating asymmetry--indicator of what?

In a population the optimal phenotype is promoted by buffering mechanisms that keep inter- and intra-individual variation low. A link exists between canalization, that controls phenotypic variation, and developmental stability, mostly measured as fluctuating asymmetry of bilateral traits (FA). Both types of variation are associated with the functional importance of a trait, and both are increased by stress of various kinds. But there are also several instances of non-congruence. The concept of developmental stability has been found elusive, and low FA is not the unambiguous measure of well being and good genes that has been claimed. It can be concluded that developmental stability is partly governed by specific, as yet unknown, molecular processes.

Stress proteins and glial functions: possible therapeutic targets for neurodegenerative disorders

Recent findings suggest that unfolded or misfolded proteins participate in the pathology of several neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. Usually, several stress proteins and glial cells act as intracellular molecular chaperones and show chaperoning neuronal function, respectively. In the brains of patients with neurodegenerative disorders, however, stress proteins are expressed and frequently associated with protein aggregates, and glial cells are activated around degenerative regions. In addition, several stress proteins and glial cells may also regulate neuronal cell death and loss. Therefore, some types of stress proteins and glial cells are considered to be neuroprotective targets. We summarize the current findings regarding the neuroprotective effects of stress proteins and glial cells, and discuss the possibility of using this knowledge to develop new therapeutic strategies to treat neurodegeneration.

Evidence for an epigenetic mechanism by which Hsp90 acts as a capacitor for morphological evolution

Morphological alterations have been shown to occur in Drosophila melanogaster when function of Hsp90 (heat shock 90-kDa protein 1alpha, encoded by Hsp83) is compromised during development. Genetic selection maintains the altered phenotypes in subsequent generations. Recent experiments have shown, however, that phenotypic variation still occurs in nearly isogenic recombinant inbred strains of Arabidopsis thaliana. Using a sensitized isogenic D. melanogaster strain, iso-Kr(If-1), we confirm this finding and present evidence supporting an epigenetic mechanism for Hsp90's capacitor function, whereby reduced activity of Hsp90 induces a heritably altered chromatin state. The altered chromatin state is evidenced by ectopic expression of the morphogen wingless in eye imaginal discs and a corresponding abnormal eye phenotype, both of which are epigenetically heritable in subsequent generations, even when function of Hsp90 is restored. Mutations in nine different genes of the trithorax group that encode chromatin-remodeling proteins also induce the abnormal phenotype. These findings suggest that Hsp90 acts as a capacitor for morphological evolution through epigenetic and genetic mechanisms.

Molecular chaperones, stress proteins and redox homeostasis

Protection against oxidative stress is highly interrelated with the function of the most ancient cellular defense system, the network of molecular chaperones, heat shock, or stress-proteins. These ubiquitous, conserved proteins help other proteins and macromolecules to fold or re-fold and reach their final, native conformation. Redox regulation of protein folding becomes especially important during the preparation of extracellular proteins to the outside oxidative milieu, which should take place in a gradual and step-by-step controlled manner in the endoplasmic reticulum or in the periplasm. Several chaperones, such as members of the Hsp33 family in yeast and the plethora of small heat shock proteins as well as one of the major chaperones, Hsp70 are able to act against cytoplasmic oxidative damage. Abrupt changes of cellular redox status lead to chaperone induction. The function of several chaperones is tightly regulated by the surrounding redox conditions. Moreover, our recent data suggest that chaperones may act as a central switchboard for the transmission of redox changes in the life of the cell.

[Molecular chaperone HSP90 as a novel target for cancer chemotherapy]

HSP90 is one of the major molecular chaperones whose expression level increases by environmental stresses. Even under normal conditions, HSP90 is a highly abundant cytosolic protein and is essential for cell viability. HSP90 is involved in the maintenance of appropriate folding and conformation of many cellular functional proteins. These "HSP90 client proteins" are associated with HSP90 and they include a wide variety of signal-transducing proteins that regulate cell growth and differentiation, such as protein kinases and steroid hormone receptors. HSP90 functions in an ATP-dependent manner with other molecular chaperones such as Cdc37 and FKBP52. An HSP90 inhibitor, geldanamycin, binds the ATP-binding pocket of HSP90 and specifically inhibits the essential ATPase activity of HSP90. Thus, treatment of cells with geldanamycin results in inactivation, destabilization, and degradation of HSP90 client proteins. Because HSP90 client proteins play important roles in the regulation of the cell cycle, cell growth, cell survival, apoptosis, and oncogenesis, geldanamycin obstructs the proliferation of cultured cancer cells and shows anti-cancer activity in experimental animals. Although the precise mechanism of the effect of HSP90 inhibitors on cancer cells remains to be clarified, HSP90 inhibitors will be potential and effective cancer chemotherapeutic drugs with a unique profile. In fact, a modified geldanamycin with lower toxicity, 17-allylaminogeldanamycin (17-AAG), has been examined in phase I clinic trials with encouraging results.

Expression of a unique drug-resistant Hsp90 ortholog by the nematode Caenorhabditis elegans

In all species studied to date, the function of heat shock protein 90 (Hsp90), a ubiquitous and evolutionarily conserved molecular chaperone, is inhibited selectively by the natural product drugs geldanamycin (GA) and radicicol. Crystal structures of the N-terminal region of yeast and human Hsp90 have revealed that these compounds interact with the chaperone in a Bergerat-type adenine nucleotide-binding fold shared throughout the gyrase, Hsp90, histidine kinase mutL (GHKL) superfamily of adenosine triphosphatases. To better understand the consequences of disrupting Hsp90 function in a genetically tractable multicellular organism, we exposed the soil-dwelling nematode Caenorhabditis elegans to GA under a variety of conditions designed to optimize drug uptake. Mutations in the gene encoding C elegans Hsp90 affect larval viability, dauer development, fertility, and life span. However, exposure of worms to GA produced no discernable phenotypes, although the amino acid sequence of worm Hsp90 is 85% homologous to that of human Hsp90. Consistent with this observation, we found that solid phase-immobilized GA failed to bind worm Hsp90 from worm protein extracts or when translated in a rabbit reticulocyte lysate system. Further, affinity precipitation studies using chimeric worm-vertebrate fusion proteins or worm C-terminal truncations expressed in reticulocyte lysate revealed that the conserved nucleotide-binding fold of worm Hsp90 exhibits the novel ability to bind adenosine triphosphate but not GA. Despite its unusual GA resistance, worm Hsp90 appeared fully functional when expressed in a vertebrate background. It heterodimerized with its vertebrate counterpart and showed no evidence of compromising its essential cellular functions. Heterologous expression of worm Hsp90 in tumor cells, however, did not render them GA resistant. These findings provide new insights into the nature of unusual N-terminal nucleotide-binding fold of Hsp90 and suggest that target-related drug resistance is unlikely to emerge in patients receiving GA-like chemotherapeutic agents.

The chlorate-resistant and photomorphogenesis-defective mutant cr88 encodes a chloroplast-targeted HSP90

The cr88 mutant of Arabidopsis is a novel chlorate-resistant mutant that displays long hypocotyls in red light, but not in far red or blue light, and is delayed in the greening process. In cotyledons and young leaves, plastids are less developed compared with those of the wild type. In addition, a subset of light-regulated genes are under-expressed in this mutant. To understand the pleiotropic phenotypes of cr88, we isolated the CR88 gene through map-based cloning. We found that CR88 encodes a chloroplast-targeted 90-kDa heat shock protein (HSP90). The CR88 gene is expressed at highest levels during early post-germination stages and in leaves and reproductive organs. It is constitutively expressed but is also light and heat shock inducible. Chloroplast import experiments showed that the protein is localized to the stroma compartment of the chloroplast. The possible function of an HSP90 in the chloroplast and a plausible explanation of the pleiotropic phenotypes observed in cr88 are discussed.

A subclass of plant heat shock cognate 70 chaperones carries a motif that facilitates trafficking through plasmodesmata

Plasmodesmata establish a pathway for the trafficking of non-cell-autonomously acting proteins and ribonucleoprotein complexes. Plasmodesmal enriched cell fractions and the contents of enucleate sieve elements, in the form of phloem sap, were used to isolate and characterize heat shock cognate 70 (Hsc70) chaperones associated with this cell-to-cell transport pathway. Three Cucurbita maxima Hsc70 chaperones were cloned and functional and sequence analysis led to the identification of a previously uncharacterized subclass of non-cell-autonomous chaperones. The highly conserved nature of the heat shock protein 70 (Hsp70) family, in conjunction with mutant analysis, permitted the characterization of a motif that allows these Hsc70 chaperones to engage the plasmodesmal non-cell-autonomous translocation machinery. Proof of concept that this motif is necessary for Hsp70 gain-of-movement function was obtained through the engineering of a human Hsp70 that acquired the capacity to traffic through plasmodesmata. These results are discussed in terms of the roles likely played by this subclass of Hsc70 chaperones in the trafficking of non-cell-autonomous proteins.

Near-neutrality in evolution of genes and gene regulation

The nearly neutral theory contends that the interaction of drift and selection is important and occurs at various levels, including synonymous and nonsynonymous substitutions in protein coding regions and sequence turnover of regulatory elements. Recent progress of the theory is reviewed, and the interaction between drift and selection is suggested to differ at these different levels. Weak selective force on synonymous changes is stable, whereas its consequence on nonsynonymous changes depends on environmental factors. Selection on differentiation of regulatory elements is even more dependent on environmental factors than on amino acid changes. Of particular significance is the role of drift in the evolution of gene regulation that directly participates in morphological evolution. The range of near neutrality depends on the effective size of the population that is influenced by selected linked loci. In addition to the effective population size, molecular chaperones such as heat shock protein 90 have significant effects on the range of near neutrality.

The relations between genetics and epigenetics: a historical point of view

I have tried to unpack the polysemy of the word epigenetics by adopting a historical point of view and by focusing on the models that were proposed at the beginning of the 1960s to explain variations in gene activity during cell differentiation and development. Most of the questions that were or are at the core of epigenetics were posed in this period. This was due to the fact that the regulatory models and their extension to the notion of the genetic program were proposed as genetic answers to the questions raised by Waddington when he defined epigenetics in the 1940s. Studies of DNA methylation and chromatin structure, which became increasingly important in the 1960s and 1970s, were seen as alternative explanations to the regulatory mechanisms that had been previously proposed. This historical detour shows that epigenetics cannot be defined per se, but only as an evolving opposition to the piecemeal, reductionist approach of genetics.

Chaperones and aging: role in neurodegeneration and in other civilizational diseases

Chaperones are highly conserved proteins responsible for the preservation and repair of the correct conformation of cellular macromolecules, such as proteins, RNAs, etc. Environmental stress leads to chaperone (heat-shock protein, stress protein) induction reflecting the protective role of chaperones as a key factor for cell survival and in repairing cellular damage after stress. The present review summarizes our current knowledge about the chaperone-deficiency in the aging process, as well as the possible involvement of chaperones in neurodegenerative diseases, such as in Alzheimer's, Parkinson's, Huntington- and prion-related diseases. We also summarize a recent theory implying chaperones as "buffers" of variations in the human genome, which role probably increased during the last 200 years of successful medical practice minimizing natural selection. Chaperone-buffered, silent mutations may be activated during the aging process, which leads to the phenotypic exposure of previously hidden features and might contribute to the onset of polygenic diseases, such as atherosclerosis, cancer, diabetes and several neurodegenerative diseases.

Evolution of development in closely related species of flies and worms

One of the main challenges in evolutionary biology is to identify the molecular changes that underlie phenotypic differences that are of evolutionary significance. Comparative studies of early development have shown that changes in the spatio-temporal use of regulatory genes, as well as changes in the specificity of regulatory proteins, are correlated with important differences in morphology between phylogenetically distant species. However, it is not known how such changes take place in natural populations, and whether they result from a single, or many small, additive events. Extending this approach to the study of development of closely related species promises to enrich this debate.

Identification of Hsp90 as a stimulatory host factor involved in influenza virus RNA synthesis

Efficient transcription and replication of the influenza virus genome are dependent upon host-derived factors. Using an in vitro RNA synthesis system, we have purified and identified Hsp90 as one of the host factors that stimulate viral RNA polymerase activity. Hsp90 interacted with the PB2 subunit of the viral RNA polymerase through the amino-terminal chaperone domain and the middle region containing a highly acidic domain. The acidic middle region was also responsible for its stimulatory activity. We found that a portion of Hsp90 is re-localized to the cell nucleus after viral infection. A PB2 fragment containing a Hsp90 binding domain inhibited viral gene expression in a dominant-negative manner. These results suggest that Hsp90 is a host factor for the influenza virus RNA polymerase.

Development of a purine-scaffold novel class of Hsp90 binders that inhibit the proliferation of cancer cells and induce the degradation of Her2 tyrosine kinase

The first published synthesis and characterization of a purine-scaffold library of hsp90 inhibitors is presented. The purine-scaffold represents a platform for the creation of easily synthesizable and derivatizable soluble molecules that are amenable for oral administration. The most active compound of the series (71) exhibits binding to hsp90 comparable to the natural product derivative 17AAG that is now in Phase I clinical trial as a cancer therapeutic. Induces the degradation of Her2 tyrosine kinase and arrests the MCF-7 breast cancer cell line at low micromolar concentrations (IC50=2 microM).

Characterization of the 105-kDa molecular chaperone. Identification, biochemical properties, and localization

We have characterized the biochemical properties of the testis and brain-specific 105-kDa protein which is cross-reacted with an anti-bovine HSP90 antibody. The protein was induced in germ cells by heat stress, resulting in a protein which is one of the heat shock proteins [Kumagai, J., Fukuda, J., Kodama, H., Murata, M., Kawamura, K., Itoh, H. & Tanaka, T. (2000) Eur. J. Biochem.267, 3073-3078]. In the present study, we characterized the biochemical properties of the protein. The 105-kDa protein inhibited the aggregation of citrate synthase as a molecular chaperone in vitro. ATP/MgCl2 has a slight influence of the suppression of the citrate synthase aggregation by the 105-kDa protein. The protein possessed chaperone activity. The protein was able to bind to ATP-Sepharose like the other molecular chaperone HSP70. A partial amino-acid sequence (24 amino-acid residues) of the protein was determined and coincided with those of the mouse testis- and brain-specific APG-1 and osmotic stress protein 94 (OSP94). The 105-kDa protein was detected only in the medulla of the rat kidney sections similar to OSP94 upon immunoblotting. The purified 105-kDa protein was cross-reacted with an antibody against APG-1. These results suggested that APG-1 and OSP94 are both identical to the 105-kDa protein. There were highly homologous regions between the 105-kDa protein/APG-1/OSP94 and HSP90. The region of HSP90 was also an immunoreactive site. An anti-bovine HSP90 antibody may cross-react with the 105-kDa protein similar to HSP90 in the rat testis and brain. We have investigated the localization and developmental induction of the protein in the rat brain. In the immunohistochemical analysis, the protein was mainly detected in the cytoplasm of the nerve and glial cells of the rat brain. Although the 105-kDa protein was localized in all rat brain segments, the expression pattern was fast in the cerebral cortex and hippocampus and slow in the cerebellum.

Unique behavior of a dictyostelium homologue of TRAP-1, coupling with differentiation of D. discoideum cells

Dd-TRAP1 is a Dictyostelium homologue of TRAP-1, a human protein that binds to the type 1 tumor necrosis factor (TNF) receptor. TRAP-1 has a putative mitochondrial localization sequence and shows significant homology to members of the HSP90 family. Although TRAP-1 is mainly localized to mitochondria in several mammalian cells, in certain tissues it is also localized at specific extramitochondrial sites. In Dictyostelium cells, Dd-TRAP1 is predominantly located in the cell membrane/cortex during growth and just after starvation. Double staining of vegetatively growing cells with the anti-Dd-TRAP1 antibody and TRITC-phalloidin has demonstrated colocalization of Dd-TRAP1 and F-actin at the leading edge of cortical protrusions such as pseudopodes. Coupled with differentiation, however, Dd-TRAP1 located at the cortical region is translocated to mitochondria in spite of the absence of the mitochondrial localization sequence at its N-terminus. The translocation of this protein raises interesting and fundamental questions regarding possible mechanisms by which Dd-TRAP1 is involved in cellular differentiation.

Review. The genotype-phenotype link

The basic knowledge which has opened for the new discipline of functional genomics stems from one hundred years of debates and experimentation among devoted geneticists, who tried to understand the processing of gene expression into phenotypic design without the molecular tools that are now available. Here I recapitulate some old and some newer results from this research, which illustrate the ways in which changes in genetic variation can be induced, how it can be kept latent or, when needed, be expressed in the phenotype.

Heat shock protein and proteasome targeting agents

Proteasome inhibitors such as PS-341 are novel agents with great potential as anticancer drugs. In early clinical studies, PS-341 was tolerated well with promising evidence of antitumor activity in diseases such as multiple myeloma. Studies also are ongoing in solid tumors, as single agent therapy and in combination with standard agents such as carboplatin. Although more research is needed to clarify the precise spectrum of antitumor activity of proteasome inhibitors, this novel approach to targeting human malignancies is highly promising.

Evolution of yellow gene regulation and pigmentation in Drosophila

BACKGROUND

Changes in developmental gene expression are central to phenotypic evolution, but the genetic mechanisms underlying these changes are not well understood. Interspecific differences in gene expression can arise from evolutionary changes in cis-regulatory DNA and/or in the expression of trans-acting regulatory proteins, but few case studies have distinguished between these mechanisms. Here, we compare the regulation of the yellow gene, which is required for melanization, among distantly related Drosophila species with different pigment patterns and determine the phenotypic effects of divergent Yellow expression.

RESULTS

Yellow expression has diverged among D. melanogaster, D. subobscura, and D. virilis and, in all cases, correlates with the distribution of black melanin. Species-specific Yellow expression patterns were retained in D. melanogaster transformants carrying the D. subobscura and D. virilis yellow genes, indicating that sequence evolution within the yellow gene underlies the divergence of Yellow expression. Evolutionary changes in the activity of orthologous cis-regulatory elements are responsible for differences in abdominal Yellow expression; however, cis-regulatory element evolution is not the sole cause of divergent Yellow expression patterns. Transformation of the D. melanogaster yellow gene into D. virilis altered its expression pattern, indicating that trans-acting factors that regulate the D. melanogaster yellow gene have also diverged between these two species. Finally, we found that the phenotypic effects of evolutionary changes in Yellow expression depend on epistatic interactions with other genes.

CONCLUSIONS

Evolutionary changes in Yellow expression correlate with divergent melanin patterns and are a result of evolution in both cis- and trans-regulation. These changes were likely necessary for the divergence of pigmentation, but evolutionary changes in other genes were also required.

Molecular chaperones and their roles in neural cell differentiation

During the development of the nervous system, a large number of neurons are eliminated through naturally occurring neuronal death. Many morphological and biochemical properties of such dying neurons are reminiscent not only of apoptosis, a type of death involving the action of genetically programmed events, but also of epigenetic phenomena such as oxidative stress. Increasing evidence demonstrates that oxidative stress alters the expression of antioxidant enzymes and enhances expression and/or DNA binding of numerous transcription factors, including heat shock factor. The latter is a transcription factor for specific promoter elements located upstream of the heat shock genes. Heat shock proteins (Hsps) are essential, highly conserved proteins that are needed for normal cell growth and maintenance, and expression of Hsps has been detected during embryogenesis in various organisms. Developmental profiles of Hsps indicate that they are differentially regulated during neural maturation, suggesting a role for Hsps in neural cell differentiation. Their putative function in cell remodeling during migration and differentiation, as well as during postnatal development, a time of extensive cell differentiation, is considered in the present review. Moreover, the function of Hsps in cell signaling, protein transport and the effect of heat shock on neural plate induction and brain development are discussed.

Biochemical networking contributes more to genetic buffering in human and mouse metabolic pathways than does gene duplication

During evolution different genes evolve at unequal rates, reflecting the varying functional constraints on phenotype. An important contributor to this variation is genetic buffering, which reduces the potential detrimental effects of mutations. We studied whether gene duplication and redundant metabolic networks affect genetic buffering by comparing the evolutionary rate of 242 human and mouse orthologous genes involved in metabolic pathways. A gene with a redundant network is defined as one for which the structural layout of metabolic pathways provides an alternative metabolic route that can, in principle, compensate for the loss of a protein function encoded by the gene. We found that genes with redundant networks evolve at similar rates as did genes without redundant networks, [corrected] but no significant difference was detected between single-copy genes and gene families. This implies that redundancy in metabolic networks provides significantly more genetic buffering than do gene families. We also found that genes encoding proteins involved in glycolysis and gluconeogenesis showed as a group a distinct pattern of variation, in contrast to genes involved in other pathways. These results suggest that redundant networks provide genetic buffering and contribute to the functional diversification of metabolic pathways.

Hsp90 inhibitors as novel cancer chemotherapeutic agents

Heat shock protein 90 (Hsp90) is a molecular chaperone whose association is required for the stability and function of multiple mutated, chimeric and over-expressed signaling proteins that promote the growth and/or survival of cancer cells. Hsp90 client proteins include mutated p53, Bcr-Abl, Raf-1, Akt, ErbB2 and hypoxia-inducible factor 1 alpha (HIF-1 alpha). Hsp90 inhibitors, by interacting specifically with a single molecular target, cause the destabilization and eventual degradation of Hsp90 client proteins, and they have shown promising antitumor activity in preclinical model systems. One Hsp90 inhibitor, 17-allylaminogeldanamycin (17AAG), is currently in phase I clinical trial. Because of the chemoprotective activity of several proteins that are Hsp90 clients, the combination of an Hsp90 inhibitor with a standard chemotherapeutic agent could dramatically increase the in vivo efficacy of the therapeutic agent.

Waddington's canalization revisited: developmental stability and evolution

Most species maintain abundant genetic variation and experience a range of environmental conditions, yet phenotypic variation is low. That is, development is robust to changes in genotype and environment. It has been claimed that this robustness, termed canalization, evolves because of long-term natural selection for optimal phenotypes. We show that the developmental process, here modeled as a network of interacting transcriptional regulators, constrains the genetic system to produce canalization, even without selection toward an optimum. The extent of canalization, measured as the insensitivity to mutation of a network's equilibrium state, depends on the complexity of the network, such that more highly connected networks evolve to be more canalized. We argue that canalization may be an inevitable consequence of complex developmental-genetic processes and thus requires no explanation in terms of evolution to suppress phenotypic variation.

Selective apoptosis of tandemly duplicated FLT3-transformed leukemia cells by Hsp90 inhibitors

An internal tandem duplication of the juxtamembrane (JM) domain of FLT3, a family of ligand-activated receptor tyrosine kinases, has been found in 20% of cases of acute myeloid leukemia (AML), and this mutation is correlated with leukocytosis and a poor prognosis. As a therapeutic approach, we previously reported that herbimycin A (HA) inhibited the growth of tandemly duplicated FLT3 (TDFLT3)-transformed cells (Leukemia 2000; 14: 374). Here, we have investigated the mechanism behind the cytotoxicity of HA, an ansamycin derivative which is now known to target Hsp90. The treatment with HA or another Hsp90 inhibitor, radicicol, induced selective apoptosis in TDFLT3-transformed 32D cells (TDFLT3/32D). The tyrosine-phosphorylation of TDFLT3 was inhibited by HA, whereas FLT3 ligand-induced phosphorylation of wild-type FLT3 (WtFLT3) was not. The downstream signal molecules MAPK, Akt and STAT5a were also dephosphorylated by HA in TDFLT3/32D. Immunoprecipitation analysis showed that TDFLT3 but not WtFLT3 formed a complex with Hsp90, and that the HA treatment dissociated TDFLT3 from the Hsp90 chaperone complex. These findings imply that targeting of Hsp90 will facilitate the development of anti-TDFLT3 therapy, and that Hsp90 is closely involved in the oncogenic activation of FLT3.

Potential genetic variance and the domestication of maize

Since Darwin, there has been a long and arduous struggle to understand the source and maintenance of natural genetic variation and its relationship to phenotype. The reason that this task is so difficult is that it requires integration of detailed, and as yet incomplete, knowledge from several biological disciplines, including evolutionary, population, and developmental genetics. In this 'post-genomic' era, it is relatively easy to identify differences in the DNA sequence between individuals. However, the task remains to delineate how this abundant genetic diversity actually contributes to phenotypic diversity. This necessitates tackling the problem of hidden genetic variation. Genetic polymorphisms can be conditionally cryptic, but have the potential to contribute to phenotypic variation in particular genetic backgrounds or under specific environmental conditions. A recent paper by Lauter and Doebley highlights the contribution of hidden genetic variation to traits characterizing the morphological evolution of modern maize from its wild grass-like progenitor teosinte.1 This work is the first to demonstrate hidden variance for selected (agronomically 'adaptive') traits in a well-characterized model for morphological evolution.

Chaperoning signaling pathways: molecular chaperones as stress-sensing`heat shock' proteins

Heat shock proteins interact with multiple key components of signaling pathways that regulate growth and development. The molecular relationships between heat shock proteins, various signaling proteins and partner proteins appear to be critical for the normal function of signal transduction pathways. The relative levels of these proteins may be important, as too little or too much Hsp70 or Hsp90 can result in aberrant growth control, developmental malformations and cell death. Although the functions of heat shock proteins as molecular chaperones have been well characterized, their complementary role as a `stress-induced' proteins to monitor changes and alter the biochemical environment of the cell remains elusive. Genetic and molecular interactions between heat shock proteins, their co-chaperones and components of signaling pathways suggest that crosstalk between these proteins can regulate proliferation and development by preventing or enhancing cell growth and cell death as the levels of heat shock proteins vary in response to environmental stress or disease.

Hsp70 and thermal pretreatment mitigate developmental damage caused by mitotic poisons in Drosophila

To assess the ability of the heat-inducible molecular chaperone heat-shock protein 70 (Hsp70) to mitigate a specific developmental lesion, we administered the antimicrotubule drugs vinblastine (VB) and colchicine (COL) to larvae of Drosophila engineered to express differing levels of Hsp70 after heat pretreatment (HP). VB and COL decreased survival during metamorphosis, disrupted development of the adult eye and other structures as well as their precursor imaginal disks, and induced chromosome nondisjunction in the wing imaginal disk as indicated by the somatic mutation and recombination test (SMART) assay. Hsp70-inducing HP reduced many of these effects. For the traits viability, adult eye morphology, eye imaginal disk morphology, cell death in the eye imaginal disks, and single and total mutant clone formation in the SMART assay, HP reduced the impact of VB to a greater extent in Drosophila with 6 hsp70 transgenes than in a sister strain from which the transgenes had been excised. Because the extra-copy strain has higher levels of Hsp70 than does the excision strain but is otherwise almost identical in genetic background to the excision strain, these outcomes are attributable to Hsp70. The hsp70 copy number had a variable interaction with HP and COL administration.