Hsp90 as a capacitor for morphological evolution

Exposure of Caenorhabditis elegans to extremely low frequency high magnetic fields induces stress responses

Responses of the small heat shock protein gene, hsp-16, were examined in transgenic Caenorhabditis elegans exposed to electromagnetic fields. Expression of the hsp-16-lacZ gene was enhanced when transgenic animals were exposed to magnetic fields up to 0.5 T at 60 Hz. The hsp-16 promoter was more efficiently expressed at the embryonic than at the post-embryonic stage irrespective of exposure. Promoter activity was more sensitive to the stimulus in the intestine at the post-embryonic stage. Evidence is presented that the induction occurs at the transcriptional step of hsp-16.

The Hsp90 family of proteins in Arabidopsis thaliana

The 90-kDa heat shock protein (Hsp90) is an essential molecular chaperone in eukaryotic cells, with key roles in the folding and activation of proteins involved in signal transduction and control of the cell cycle. A search for Hsp90 sequences in the Arabidopsis thaliana genome revealed that this family includes 7 members. The AtHsp90-1 through AtHsp90-4 proteins constitute the cytoplasmic subfamily, whereas the AtHsp90-5, AtHsp90-6, and AtHsp90-7 proteins are predicted to be within the plastidial, mitochondrial, and endoplasmic reticulum compartments, respectively. The deduced amino acid sequences of each of the cytoplasmic proteins contains the highly conserved C-terminal pentapeptide MEEVD. All of the AtHsp90 sequences include a conserved adenosine triphosphate-binding domain, whereas only the cytoplasmic and endoplasmic reticulum-resident sequences include an adjacent charged linker domain that is common in mammalian and yeast sequences. The occurrence of multiple AtHsp90 proteins in the cytoplasm and of family members in other subcellular compartments suggests a range of specific functions and target polypeptides.

Regulation of metamorphosis in ascidians involves NO/cGMP signaling and HSP90

Treatment of larvae of the ascidians Boltenia villosa (Family: Pyuridae) and Cnemidocarpa finmarkiensis (Family: Styelidae) with drugs that inhibit the function of the molecular chaperone HSP90 increased the frequency of tail resorption, the primary morphogenetic event of metamorphosis. If treatment was initiated at hatching, metamorphic events subsequent to tail resorption failed to occur, indicating an ongoing role for HSP90 during morphogenesis. Removal of tails from heads of mature, but not newly hatched larvae, induced metamorphosis of the head. Decapitation experiments indicate that the capacity of tails to shorten in response to inhibition of HSP90 function requires communication with heads. To identify candidate proteins with which HSP90 may interact to regulate metamorphosis, we noted that in mammalian cells, nitric oxide synthase (NOS) interacts with HSP90 and its activity is sensitive to drugs that inhibit HSP90 function. In addition, nitric oxide (NO) signaling in the marine snail Ilyanassa obsoleta is an important regulator of metamorphosis. Inhibition of NOS activity in these ascidian larvae with L-NAME increased the frequency of metamorphosis, consistent with a putative interaction of NOS and HSP90. NOS is present in tail muscle cells, implicating them as targets for the drug treatments, consistent with the decapitation experiments. Inhibition of soluble guanylyl cyclase, the most common effector of NO signaling, also increased the frequency of metamorphosis. In contrast to treatment with anti-HSP90 drugs, metamorphosis induced with L-NAME or ODQ was complete. The results presented suggest that an HSP90-dependent, NO-based regulatory mechanism localized in tails represses ascidian metamorphosis. We discuss these results in relation to the induction of ascidian metamorphosis by several unrelated agents.

Proliferation, cell cycle and apoptosis in cancer

Beneath the complexity and idiopathy of every cancer lies a limited number of 'mission critical' events that have propelled the tumour cell and its progeny into uncontrolled expansion and invasion. One of these is deregulated cell proliferation, which, together with the obligate compensatory suppression of apoptosis needed to support it, provides a minimal 'platform' necessary to support further neoplastic progression. Adroit targeting of these critical events should have potent and specific therapeutic consequences.

Cell cycle transition under stress conditions controlled by vertebrate heat shock factors

The roles of heat shock transcription factors (HSFs) under physiological conditions have recently become the focus of intense study. We generated avian cells lacking two heat-inducible HSFs, HSF1 and HSF3. In addition to complete loss of activation of heat shock genes under stress conditions, these cells exhibited a marked reduction in Hsp90alpha expression under normal growth conditions. Reduction in Hsp90alpha expression caused instability of a cyclin-dependent kinase, Cdc2, and cell cycle progression was blocked mainly at the G2 phase, but also at G1 phase even at mild heat shock temperatures. Restoration of Hsp90alpha expression rescued the temperature sensitivity without induction of HSPS: We demonstrated for the first time a molecular target affected by heat shock in vivo that causes cell cycle arrest in vertebrates and a novel mechanism of stress resistance controlled by vertebrate HSFS:

Ecological developmental biology: developmental biology meets the real world

The production of phenotype is regulated by differential gene expression. However, the regulators of gene expression need not all reside within the embryo. Environmental factors, such as temperature, photoperiod, diet, population density, or the presence of predators, can produce specific phenotypes, presumably by altering gene-expression patterns. The field of ecological developmental biology seeks to look at development in the real world of predators, competitors, and changing seasons. Ecological concerns had played a major role in the formation of experimental embryology, and they are returning as the need for knowledge about the effects of environmental change on embryos and larvae becomes crucial. This essay reviews some of the areas of ecological developmental biology, concentrating on new studies of amphibia and Homo.

Glycoprotein folding in the endoplasmic reticulum

Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.

Odorants as cell-type specific activators of a heat shock response in the rat olfactory mucosa

Heat shock, or stress, proteins (HSPs) are induced in response to conditions that cause protein denaturation. Activation of cellular stress responses as a protective and survival mechanism is often associated with chemical exposure. One interface between the body and the external environment and chemical or biological agents therein is the olfactory epithelium (OE). To determine whether environmental odorants affect OE HSP expression, rats were exposed to a variety of odorants added to the cage bedding. Odorant exposure led to transient, selective induction of HSP70, HSC70, HSP25, and ubiquitin immunoreactivities (IRs) in supporting cells and subepithelial Bowman's gland acinar cells, two OE non-neuronal cell populations involved with inhalant biotransformation, detoxification, and maintenance of overall OE integrity. Responses exhibited odor specificity and dose dependency. HSP70 and HSC70 IRs occurred throughout the apical region of supporting cells; ubiquitin IR was confined to a supranuclear cone-shaped region. Electron microscopic examination confirmed these observations and, additionally, revealed odor-induced formation of dense vesicular arrays in the cone-like regions. HSP25 IR occurred throughout the entire supporting cell cytoplasm. In contrast to classical stress responses, in which the entire array of stress proteins is induced, no increases in HSP40 and HSP90 IRs were observed. Extended exposure to higher odorant doses caused prolonged activation of the same HSP subset in the non-neuronal cells and severe morphological damage in both supporting cells and olfactory receptor neurons (ORNs), suggesting that non-neuronal cytoprotective stress response mechanisms had been overwhelmed and could no longer adequately maintain OE integrity. Significantly, ORNs showed no stress responses in any of our studies. These findings suggest a novel role for these HSPs in olfaction and, in turn, possible involvement in other normal neurophysiological processes.

Antibodies specific for heat shock proteins in human and murine malaria

Heat shock proteins (HSPs) are immunodominant antigens recognized by the host immune system in various infectious diseases. We analyzed HSP-specific antibodies, including immunoglobulin G (IgG), IgM and IgA, in sera from malaria patients in Thailand by using an enzyme-linked immunosorbent assay. All of the antibodies to HSP90 were remarkably increased in the patients compared with those in controls, while only IgM to HSP70 or IgA to HSP65 was significantly elevated. Further experiments showed that anti-HSP IgG was significantly increased in C57BL/6 mice infected with a non-lethal strain of Plasmodium yoelii, with anti-HSP90 IgG being the most elevated. These results suggest that the antigenic potential of HSP90 is higher than those of HSP70 and HSP65 in malaria infection.

Both the N- and C-terminal chaperone sites of Hsp90 participate in protein refolding

Hsp90 is able to bind partially unfolded firefly luciferase and maintain it in a refoldable state; the subsequent successive action of the 20S proteasome activator PA28, Hsc70 and Hsp40 enables its refolding. Hsp90 possesses two chaperone sites in the N- and C-terminal domains that prevent the aggregation of denatured proteins. Here we show that both chaperone sites of Hsp90 are effective not only in capturing thermally denatured luciferase, but also in holding it in a state prerequisite for the successful refolding process mediated by PA28, Hsc70 and Hsp40. In contrast, the heat-induced activity of Hsp90 to bind chemically denature dihydrofolate reductase efficiently and prevent its rapid spontaneous refolding was detected in the N-terminal site of Hsp90 only, while the C-terminal site was without effect. Thus it is most likely that both the N- and C-terminal chaperone sites may contribute to Hsp90 function as holder chaperones, however, in a significantly distinct manner.

An evaluation of Hsp90 as a mediator of cortical patterning in Tetrahymena

This study asks two questions: 1) whether Hsp90 is involved in the regulation of cortical patterning in Tetrahymena, and 2) if it is, whether specific defects in this regulation can be attributed to functional insufficiency of the Hsp90 molecule. To address question 1, we compared the effects of a specific inhibitor of Hsp90, geldanamycin, on population growth and on development of the oral apparatus in two Tetrahymena species, T. pyriformis and T. thermophila. We observed that geldanamycin inhibits population growth in both species at very low concentrations, and that it has far more severe effects on oral patterning in T. pyriformis than in T. thermophila. These effects are parallel to those of high temperature in the same two species, and provide a tentative affirmative answer to the first question. To address question 2, we ascertained the base sequence of the genes that encode the Hsp90 molecules which are induced at high temperatures in both Tetrahymena species, as well as corresponding sequences in Paramecium tetraurelia. Extensive comparative analyses of the deduced amino acid sequences of the Hsp90 molecules of the two Tetrahymena species indicate that on the basis of what we currently know about Hsp90 both proteins are equally likely to be functional. Phylogenetic analyses of Hsp90 amino acid sequences indicate that the two Tetrahymena Hsp90 molecules have undergone a similar number of amino acid substitutions from their most recent common ancestor, with none of these corresponding to any known functionally critical region of the molecule. Thus there is no evidence that the Hsp90 molecule of T. pyriformis is functionally impaired; the flaw in the control of cortical patterning is more likely to be caused by defects in mechanism(s) that mediate the response to Hsp90, as would be expected from the "Hsp90 capacitor" model of Rutherford and Lindquist.

Bag1-Hsp70 mediates a physiological stress signalling pathway that regulates Raf-1/ERK and cell growth

Survival after stress requires the precise orchestration of cell-signalling events to ensure that biosynthetic processes are alerted and cell survival pathways are initiated. Here we show that Bag1, a co-chaperone for heat-shock protein 70 (Hsp70), coordinates signals for cell growth in response to cell stress, by downregulating the activity of Raf-1 kinase. Raf-1 and Hsp70 compete for binding to Bag1, such that Bag1 binds to and activates Raf-1, subsequently activating the downstream extracellular signal-related kinases (ERKs). When levels of Hsp70 are elevated after heat shock, or in cells conditionally overexpressing Hsp70, Bag1-Raf-1 is displaced by Bag1-Hsp70, and DNA synthesis is arrested. Mutants Bag1C204A and Bag1E208A, which cannot bind Hsp70, constitutively activate Raf-1/ERK kinases but are unaffected by Hsp70; consequently neither Bag1-Raf-1 nor DNA synthesis is negatively affected during heat shock. Likewise, mutants Hsp70F245S, Hsp70R262W and Hsp70L282R, which retain chaperone activity but do not bind to Bag1, fail to repress Bag1 activation of Raf-1/ERK kinase. We propose that Bag1 functions in the heat-shock response to coordinate cell growth signals and mitogenesis, and that Hsp70 functions as a sensor in stress signalling.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Opposite effects of the Hsp90 inhibitor Geldanamycin: induction of apoptosis in PC12, and differentiation in N2A cells

The inhibitor of the Hsp90 chaperone Geldanamycin has been reported to have several cellular effects, such as inhibition of v-src activity or destabilization of Raf-1 among others. We show now that Geldanamycin treatment induces different phenotypes in different cell lines. In PC12 cells, it triggers apoptosis, whereas in the murine neuroblastoma N2A, it induces differentiation with neurite outgrowth. Geldanamycin effects cannot be mimicked by inhibition of the c-src protein tyrosine kinases, and nerve growth factor does not protect PC12 cells from apoptosis. Mitogen-activated protein kinase activities ERK and JNK are activated differently according to cell type: in PC12 cells JNK is activated, and its inhibition abolishes apoptosis, but not ERK; in N2A cells, both ERK and JNK are activated, but with peak activities at different times.

The distribution of heat shock proteins in the nervous system of the unstressed mouse embryo suggests a role in neuronal and non-neuronal differentiation

Heat shock proteins (Hsps) act as molecular chaperones and are generally constitutively expressed in the absence of stress. Hsps are also inducible by a variety of stressors whose effects could be disastrous on the brain. It has been shown previously that Hsps are differentially expressed in glial and neuronal cells, as well as in the different structures of the brain. This differential expression has been related to specific functions distinct from their general chaperone function, such as intracellular transport. We investigated here the constitutive expression of 5 Hsps (the small Hsp, Hsp25, the constitutive Hsc70 and Hsp90beta, the mainly inducible Hsp70 and Hsp90alpha), and of a molecular chaperone, TCP-1alpha during mouse nervous system development. We analyzed, by immunohistochemistry, their distribution in the central nervous system and in the ganglia of the peripheral nervous system from day 9.5 (E9.5) to day 17.5 (E17.5) of gestation. Hsps are expressed in different cell classes (neuronal, glial, and vascular). The different proteins display different but often overlapping patterns of expression in different regions of the developing nervous system, suggesting unique roles at different stages of neural maturation. Their putative function in cell remodeling during migration or differentiation and in protein transport is discussed. Moreover we consider Hsp90 function in cell signaling and the role of Hsp25 in apoptosis protection.

Sensitivity of mature Erbb2 to geldanamycin is conferred by its kinase domain and is mediated by the chaperone protein Hsp90

ErbB receptors are a family of ligand-activated tyrosine kinases that play a central role in proliferation, differentiation, and oncogenesis. ErbB2 is overexpressed in >25% of breast and ovarian cancers and is correlated with poor prognosis. Although ErbB2 and ErbB1 are highly homologous, they respond quite differently to geldanamycin (GA), an antibiotic that is a specific inhibitor of the chaperone protein Hsp90. Thus, although both mature and nascent ErbB2 proteins are down-regulated by GA, only nascent ErbB1 is sensitive to the drug. To reveal the underlying mechanism behind these divergent responses, we made a chimeric receptor (ErbB1/2) composed of the extracellular and transmembrane domains of ErbB1 and the intracellular domain of ErbB2. The ErbB1/2 protein is functional since its kinase activity was stimulated by epidermal growth factor. The sensitivity of ErbB1/2 to GA was similar to that of ErbB2 and unlike that of ErbB1, indicating that the intracellular domain of the chimera confers GA sensitivity. This finding also suggests that the GA sensitivity of mature ErbB2 depends on cytosolic Hsp90, rather than Grp94, a homolog of Hsp90 that is restricted to the lumen of the endoplasmic reticulum, although both chaperones bind to and are inhibited by GA. Lack of Grp94 involvement in mediating ErbB2 sensitivity to GA is further suggested by the fact that a GA derivative with low affinity for Grp94 efficiently depleted ErbB2 protein in treated cells. To localize the specific region of ErbB2 that confers GA sensitivity, we made truncated receptors with progressive deletions of the cytoplasmic domain and tested the GA sensitivity of these molecules. We found that ErbB2 constructs containing an intact kinase domain retained GA sensitivity, whereas those lacking the kinase domain (ErbB2/DK) lost responsiveness to GA completely. Hsp90 co-immunoprecipitated with all ErbB2 constructs that were sensitive to GA, but not with ErbB2/DK or ErbB1. Both tyrosine-phosphorylated and non-phosphorylated ErbB2 proteins were similarly sensitive to GA, as was a kinase-dead ErbB2 mutant. These data suggest that Hsp90 uniquely stabilizes ErbB2 via interaction with its kinase domain and that GA stimulates ErbB2 degradation secondary to disruption of ErbB2/Hsp90 association.

Perinatal expression of heat-shock protein 90 in different regions of the brain and in non-neural tissues of the piglet

Important stressful events occur at birth or within the few hours that follow. To establish a possible involvement of stress proteins, expression of heat-shock protein 90 was determined by Western blotting in several regions of the brain and in non-neural tissues of the developing piglet (fetal to 10 days and adult). Expression was found in all the tissues studied. While comparable values were found in the whole brain during development, decreased expressions were observed from 4 to 8 h to 2 days after birth in cerebellum, cortex, hypothalamus and striatum. In hippocampus, low expression was observed from 4 h postnatally onward. In non-neural tissues, low expression was observed after birth and in the adult for heart, liver and lungs. In kidney, low values were found from birth to 1 day of age. Changes in environmental parameters like temperature and/or hypoxia can be related to differential expressions of heat-shock proteins and they possibly result in severe developmental outcomes. The results are discussed in terms of using the newborn piglet as a model for the study of different forms of stress on the heat-shock protein expression during postnatal development.

Modeling genetic architecture: a multilinear theory of gene interaction

The map from genotype to phenotype is an exceedingly complex function of central importance in biology. In this work we derive and analyze a mathematically tractable model of the genotype-phenotype map that allows for any order of gene interaction. By assuming that the alterations of the effect of a gene substitution due to changes in the genetic background can be described as a linear transformation, we show that the genotype-phenotype map is a sum of linear and multilinear terms of operationally defined "reference" effects at each locus. The "multilinear" model is used to study the effect of epistasis on quantitative genetic variation, on the response to selection, and on genetic canalization. It is shown how the model can be used to estimate the strength of "functional" epistasis from a variety of genetic experiments.

Remodeling of yeast genome expression in response to environmental changes

We used genome-wide expression analysis to explore how gene expression in Saccharomyces cerevisiae is remodeled in response to various changes in extracellular environment, including changes in temperature, oxidation, nutrients, pH, and osmolarity. The results demonstrate that more than half of the genome is involved in various responses to environmental change and identify the global set of genes induced and repressed by each condition. These data implicate a substantial number of previously uncharacterized genes in these responses and reveal a signature common to environmental responses that involves approximately 10% of yeast genes. The results of expression analysis with MSN2/MSN4 mutants support the model that the Msn2/Msn4 activators induce the common response to environmental change. These results provide a global description of the transcriptional response to environmental change and extend our understanding of the role of activators in effecting this response.

The development and evolution of crossveins in insect wings

The formation of crossveins in Drosophila was an important early case study in understanding the role of the environment in the development and evolution of morphological structures. More recent work has shown that signalling processes play a crucial role in the formation of crossveins in Drosophila and that the interaction of a heat shock factor, Hsp90, with components of signal transduction pathways may account for the sensitivity of these structures to environmental perturbations. A new model for the development of crossveins is presented that divides the formation of crossveins into 3 separate stages. First, the number and placement of the crossveins is determined by signalling along the proximal-distal axis of the wing. This signal may involve the cdc42 gene product and the Jun-N-terminal Kinase signal transduction pathway. Then, during the second stage, an inductive signal from the dorsal wing epithelium is sent to the ventral wing epithelium at locations specified by the first signal. The second signal appears to involve the BMP-like signalling pathway. Finally, in the third stage, a domain of vein competent cells is defined by the signalling from the EGF-receptor-Map Kinase signal transduction pathway, and the exact location of the veins is eventually determined within that domain by Notch-Delta signalling interactions. By altering components of these 3 stages, insects can independently regulate the presence or absence, the number and placement, and the thickness and flexibility of the crossveins. This capacity for the modulation of crossvein structure in many different ways may have contributed to the evolution of different modes of insect flight.

Gene conversion may aid adaptive peak shifts

Gene conversion is often viewed as a homogenizing force that opposes adaptive evolution. The objective of this study is to suggest a potential role for gene conversion in adaptive evolution of proteins through aiding the transfer of a population from one adaptive peak to another. Our hypothesis starts with the observation that a tandem gene duplication may result in an extra gene copy that is released from selective constraints. In such cases, individually deleterious mutations may accumulate on the extra copy of the gene, and through gene conversion these mutations may subsequently be presented to the functioning gene for selection en masse. Thus, groups of mutations that jointly confer a selective advantage may regularly be made available for selection. We present a mathematical model of this process and identify the range of rates of gene conversion, gene duplication and mutation under which it may operate. The results indicate that the process may be biologically feasible if the rate of appearance of the potentially beneficial mutations is not too small in relation to the rates of null mutation and of gene conversion. This process appears to be a possible mechanism for effecting adaptive peak shifts in large populations. We show that all the evolutionary steps in the proposed model may have occurred in the evolution of primate gamma -globin genes. We suggest that hide-and-release mechanisms for genetic variation may constitute a more general principal of evolvability.

The Fas-induced apoptosis analyzed by high throughput proteome analysis

The fate of cytosolic proteins was studied during Fas-induced cell death of Jurkat T-lymphocytes by proteome analysis. Among 1000 spots resolved in two-dimensional gels, comparison of control versus apoptotic cells revealed that the signal intensity of 19 spots decreased or even disappeared, whereas 38 novel spots emerged. These proteins were further analyzed with respect to de novo protein synthesis, phosphorylation status, and intracellular localization by metabolic labeling and analysis of subcellular protein fractions in combination with two-dimensional Western blots and mass spectrometry analysis of tryptic digests. We found that e.g. hsp27, hsp70B, calmodulin, and H-ras synthesis was induced upon Fas signaling. 34 proteins were affected by dephosphorylation (e.g. endoplasmin) and phosphorylation (e.g. hsc70, hsp57, and hsp90). Nuclear annexin IV translocated to the cytosol, whereas decreasing cytosolic TCP-1alpha became detectable in the nucleus. In addition, degradation of 12 proteins was observed; among them myosin heavy chain was identified as a novel caspase target. Fas-induced proteome alterations were compared with those of other cell death inducers, indicating specific physiological characteristics of different cell death mechanisms, consequent to as well as independent of caspase activation. Characteristic proteome alterations of apoptotic cells at early time points were found reminiscent of those of malignant cells in vivo.

Epigenetic mechanisms of character origination

The close mapping between genotype and morphological phenotype in many contemporary metazoans has led to the general notion that the evolution of organismal form is a direct consequence of evolving genetic programs. In contrast to this view, we propose that the present relationship between genes and form is a highly derived condition, a product of evolution rather than its precondition. Prior to the biochemical canalization of developmental pathways, and the stabilization of phenotypes, interaction of multicellular organisms with their physicochemical environments dictated a many-to-many mapping between genomes and forms. These forms would have been generated by epigenetic mechanisms: initially physical processes characteristic of condensed, chemically active materials, and later conditional, inductive interactions among the organism's constituent tissues. This concept, that epigenetic mechanisms are the generative agents of morphological character origination, helps to explain findings that are difficult to reconcile with the standard neo-Darwinian model, e.g., the burst of body plans in the early Cambrian, the origins of morphological innovation, homology, and rapid change of form. Our concept entails a new interpretation of the relationship between genes and biological form.

From genotype to phenotype: buffering mechanisms and the storage of genetic information

DNA sequence variation is abundant in wild populations. While molecular biologists use genetically homogeneous strains of model organisms to avoid this variation, evolutionary biologists embrace genetic variation as the material of evolution since heritable differences in fitness drive evolutionary change. Yet, the relationship between the phenotypic variation affecting fitness and the genotypic variation producing it is complex. Genetic buffering mechanisms modify this relationship by concealing the effects of genetic and environmental variation on phenotype. Genetic buffering allows the build-up and storage of genetic variation in phenotypically normal populations. When buffering breaks down, thresholds governing the expression of previously silent variation are crossed. At these thresholds, phenotypic differences suddenly appear and are available for selection. Thus, buffering mechanisms modulate evolution and regulate a balance between evolutionary stasis and change. Recent work provides a glimpse of the molecular details governing some types of genetic buffering.

Progress and potential of Drosophila protein interaction maps

Protein-protein interactions mediate many important cellular processes and are central to the mechanisms by which most proteins function. Charting the interactions among the proteins involved in a process has been an essential step in characterising the function of proteins and pathways. The yeast two-hybrid system is one approach to detecting protein interactions that can now be scaled-up to assay large sets of proteins systematically, such as those being identified from genome sequencing efforts. The system has already been extensively used to acquire data that have enabled construction of large protein interaction maps (PIMs). When combined with other data, including data being generated by other functional genomics approaches, PIMs help assign function to new proteins and delineate functional networks. Hypotheses generated in such a manner often must be tested by additional experimentation, preferably in vivo. The model organism Drosophila melanogaster has a wealth of genetic and bioinformatic tools available for such analyses. The proteome predicted from the recently sequenced Drosophila genome indicates that humans have more genes in common with Drosophila than with any other invertebrate model organism characterised to date. Thus, the construction and characterisation of Drosophila PIMs will help define the functions of many conserved genes and pathways, and will provide the pharmaceutical research industry with invaluable data to assist with drug target identification and validation.

The distribution of heat shock proteins in the nervous system of the unstressed mouse embryo suggests a role in neuronal and non-neuronal differentiation

Heat shock proteins (Hsps) act as molecular chaperones and are generally constitutively expressed in the absence of stress. Hsps are also inducible by a variety of stressors whose effects could be disastrous on the brain. It has been shown previously that Hsps are differentially expressed in glial and neuronal cells, as well as in the different structures of the brain. This differential expression has been related to specific functions distinct from their general chaperone function, such as intracellular transport. We investigated here the constitutive expression of 5 Hsps (the small Hsp, Hsp25, the constitutive Hsc70 and Hsp90beta, the mainly inducible Hsp70 and Hsp90alpha), and of a molecular chaperone, TCP-1alpha during mouse nervous system development. We analyzed, by immunohistochemistry, their distribution in the central nervous system and in the ganglia of the peripheral nervous system from day 9.5 (E9.5) to day 17.5 (E17.5) of gestation. Hsps are expressed in different cell classes (neuronal, glial, and vascular). The different proteins display different but often overlapping patterns of expression in different regions of the developing nervous system, suggesting unique roles at different stages of neural maturation. Their putative function in cell remodeling during migration or differentiation and in protein transport is discussed. Moreover we consider Hsp90 function in cell signaling and the role of Hsp25 in apoptosis protection.

A yeast prion provides a mechanism for genetic variation and phenotypic diversity

A major enigma in evolutionary biology is that new forms or functions often require the concerted effects of several independent genetic changes. It is unclear how such changes might accumulate when they are likely to be deleterious individually and be lost by selective pressure. The Saccharomyces cerevisiae prion [PSI+] is an epigenetic modifier of the fidelity of translation termination, but its impact on yeast biology has been unclear. Here we show that [PSI+] provides the means to uncover hidden genetic variation and produce new heritable phenotypes. Moreover, in each of the seven genetic backgrounds tested, the constellation of phenotypes produced was unique. We propose that the epigenetic and metastable nature of [PSI+] inheritance allows yeast cells to exploit pre-existing genetic variation to thrive in fluctuating environments. Further, the capacity of [PSI+] to convert previously neutral genetic variation to a non-neutral state may facilitate the evolution of new traits.

Evolutionary transition pathways for changing peptide ligand specificity and structure

We identified evolutionary pathways for the inter- conversion of three sequentially and structurally unrelated peptides, GATPEDLNQKL, GLYEWGGARI and FDKEWNLIEQN, binding to the same site of the hypervariable region of the anti-p24 (HIV-1) monoclonal antibody CB4-1. Conversion of these peptides into each other could be achieved in nine or 10 single amino acid substitution steps without loss of antibody binding. Such pathways were identified by analyzing all 7 620 480 pathways connecting 2560 different peptides, and testing them for CB4-1 binding. The binding modes of intermediate peptides of selected optimal pathways were characterized using complete sets of substitution analogs, revealing that a number of sequential substitutions accumulated without changing the pattern of key interacting residues. At a distinct step, however, one single amino acid exchange induces a sudden change in the binding mode, indicating a flip in specificity and conformation. Our data represent a model of how different specificities, structures and functions might evolve in protein-protein recognition.

Torsin A and its torsion dystonia-associated mutant forms are lumenal glycoproteins that exhibit distinct subcellular localizations

Early-onset torsion dystonia is an autosomal dominant hyperkinetic movement disorder that has recently been linked to a 3-base pair deletion in the DYT1 gene. The DYT1 gene encodes a 332-amino acid protein, torsin A, that bears low but significant homology to the Hsp100/Clp family of ATPase chaperones. The deletion in DYT1 associated with torsion dystonia results in the loss of one of a pair of glutamic acid residues residing near the C terminus of torsin A (DeltaE-torsin A). At present, little is known about the expression, subcellular distribution, and/or function of either the torsin A or DeltaE-torsin A protein. When transfected into mammalian cells, both torsin A and DeltaE-torsin A were found to behave as lumenally oriented glycoproteins. Immunofluorescence studies revealed that torsin A localized to a diffuse network of intracellular membranes displaying significant co-immunoreactivity for the endoplasmic reticulum resident protein BiP, whereas DeltaE-torsin A resided in large spheroid intracellular structures exclusive of BiP immunoreactivity. These results initially suggested that DeltaE-torsin A might exist as insoluble aggregates. However, both torsin A and DeltaE-torsin A were readily solubilized by nonionic detergents, were similarly accessible to proteases, and displayed equivalent migration patterns on sucrose gradients. Collectively, these data support that both the wild type and torsion dystonia-associated forms of torsin A are properly folded, lumenal proteins of similar oligomeric states. The potential relationship between the altered subcellular distribution of DeltaE-torsin A and the disease-inducing phenotype of the protein is discussed.

p50(Cdc37) can buffer the temperature-sensitive properties of a mutant of Hck

Genetic studies have previously revealed that Cdc37p is required for the catalytic competence of v-Src in yeast. We have reasoned that temperature-sensitive mutants of Src family kinases might be more sensitive to the cellular level of p50(Cdc37), the mammalian homolog of Cdc37p, than their wild-type counterpart, thus potentially providing a unique opportunity to elucidate the involvement of p50(Cdc37) in the folding and stabilization of Src family kinases. A temperature-sensitive mutant of a constitutively active form of Hck (i.e., tsHck499F) was created by mutating two amino acids within the kinase domain of Hck499F. Significantly, overexpression of p50(Cdc37) rescues the catalytic activity of tsHck499F at 33 degrees C, while partially buffering it against inactivation at higher temperatures (e.g., 37 and 39 degrees C). Hsp90 function is required for tsHck499F activity and its stabilization by p50(Cdc37), but overexpression of Hsp90 is not sufficient to stabilize tsHck499F. Overexpression of p50(Cdc37) promotes the association of tsHck499F with Hsp90, suggesting that the cellular level of p50(Cdc37) might be the rate-limiting step in the association of tsHck499F with Hsp90. A truncation mutant of p50(Cdc37) that cannot bind Hsp90 still has a limited capacity to rescue the catalytic activity of tsHck499F and promote its association with Hsp90. This is a particularly important observation, since it argues that rather than solely acting as a passive adapter protein to tether tsHck499F to Hsp90, p50(Cdc37) may also act allosterically to enhance the association of tsHck499F with Hsp90. The findings presented here might also have implications for our understanding of the evolution of protein kinases and tumor development.

Genetic screens for factors involved in the notum bristle loss of interspecific hybrids between Drosophila melanogaster and D. simulans

Interspecific cross is a powerful means to uncover hidden within- and between-species variation in populations. One example is a bristle loss phenotype of hybrids between Drosophila melanogaster and D. simulans, although both the pure species have exactly the same pattern of bristle formation on the notum. There exists a large amount of genetic variability in the simulans populations with respect to the number of missing bristles in hybrids, and the variation is largely attributable to simulans X chromosomes. Using nine molecular markers, I screened the simulans X chromosome for genetic factors that were responsible for the differences between a pair of simulans lines with high (H) and low (L) missing bristle numbers. Together with duplication-rescue experiments, a single major quantitative locus was mapped to a 13F-14F region. Importantly, this region accounted for most of the differences between H and L lines in three other independent pairs, suggesting segregation of H and L alleles at the single locus in different populations. Moreover, a deficiency screening uncovered several regions with factors that potentially cause the hybrid bristle loss due to epistatic interactions with the other factors.

Evolutionary novelties in islands: Drosophila santomea, a new melanogaster sister species from São Tomé

The finding of new melanogaster sister species may help us in understanding more about how the emergence of genetic novelties, particularly in insular habitats, can result in speciation. Here we report on the discovery of Drosophila santomea, which is the first melanogaster sibling found off West-equatorial Africa, on São Tomé, one of the Gulf of Guinea islands. Although the eight other melanogaster sister species are remarkably conservative in their morphology except for their terminalia, the new find has a morphological trait distinguishing it from all of these: a pure yellow body coloration of both sexes without the normal black abdominal banding. Evidence from the terminalia, polytene and mitotic chromosomes, period gene and allozymes are provided indicating that it is nonetheless the nearest relative of Drosophila yakuba with which it coexists on the island. The new find is a clear-cut taxon as shown by the production of sterile male hybrids, eventually with developmental defects, in both directions of cross with yakuba and by the existence of an altitudinal divide accompanied by a hybrid zone at mid-elevation on the island. Molecular and karyotypic data further support this conclusion. In contrast to the significant divergence of their nuclear DNAs, an intriguing similarity in their cytochrome b sequences was observed indicating a recent coalescence common to santomea, yakuba and also teissieri cytoplasms. These were shown to harbour the same Wolbachia endosymbiotic bacteria which could possibly be responsible for mitochondrial DNA hitchhiking across the species barrier.

Stress and transposable elements: co-evolution or useful parasites?

The activity of transposable elements can be induced by environmental and population factors and in particular by stresses in various organisms. A consequence of the increase in transposable element mobility is the creation of new genetic variability that can be useful in the face of stressful conditions. In this review, results supporting this hypothesis are presented and discussed. The main question is how stress induces the activity of transposable elements. We discuss hypotheses based upon the existence of promoters or fixation sites of transcription activators in the untranslated regions of transposable elements, similar to those found in regulatory regions of host defence genes.

A glucosinolate mutant of Arabidopsis is thermosensitive and defective in cytosolic Hsp90 expression after heat stress

The TU8 mutant of Arabidopsis previously described to be deficient in glucosinolate metabolism and pathogen-induced auxin accumulation was found to be remarkably less tolerant upon exposure to elevated temperatures than wild-type plants. Although moderately increased temperature only affected shoot growth, exposure to severe heat stress led to a dramatic decay of mutant plants. By contrast, wild-type seedlings showed little or no damage under the same conditions. Analysis of different heat stress proteins (Hsps) in TU8 seedlings revealed that only expression of cytoplasmic Hsp90 was affected in these plants. Although Hsp90 was present under control conditions, its level declined in mutant plants at elevated temperatures. Northern-blot analysis indicated that the decrease in Hsp90 protein was accompanied with a reduction of hsp90 transcript levels. Transient expression of Hsp90 in mutant protoplasts increased their survival rate at higher temperatures to near equivalent that of wild-type protoplasts. These data suggest that the reduced level of Hsp90 in TU8 mutants may be the primary cause for the observed reduction in thermostability.