Incapacitating the evolutionary capacitor: Hsp90 modulation of disease

Self-Improvising Memory: A Perspective on Memories as Agential, Dynamically Reinterpreting Cognitive Glue

Many studies on memory emphasize the material substrate and mechanisms by which data can be stored and reliably read out. Here, I focus on complementary aspects: the need for agents to dynamically reinterpret and modify memories to suit their ever-changing selves and environment. Using examples from developmental biology, evolution, and synthetic bioengineering, in addition to neuroscience, I propose that a perspective on memory as preserving salience, not fidelity, is applicable to many phenomena on scales from cells to societies. Continuous commitment to creative, adaptive confabulation, from the molecular to the behavioral levels, is the answer to the persistence paradox as it applies to individuals and whole lineages. I also speculate that a substrate-independent, processual view of life and mind suggests that memories, as patterns in the excitable medium of cognitive systems, could be seen as active agents in the sense-making process. I explore a view of life as a diverse set of embodied perspectives-nested agents who interpret each other's and their own past messages and actions as best as they can (polycomputation). This synthesis suggests unifying symmetries across scales and disciplines, which is of relevance to research programs in Diverse Intelligence and the engineering of novel embodied minds.

Hsp90 and complex birth defects: A plausible mechanism for the interaction of genes and environment

How genes and environment interact to cause birth defects is not well understood, but key to developing new strategies to modify risk. The threshold model has been proposed to represent this complex interaction. This model stipulates that while environmental exposure or genetic mutation alone may not result in a defect, factors in combination increase phenotypic variability resulting in more individuals crossing the disease threshold where birth defects manifest. Many environmental factors that contribute to birth defects induce widespread cellular stress and misfolding of proteins. Yet, the impact of the stress response on the threshold model is not typically considered in discephering the etiology of birth defects. This mini-review will explore a potential mechanism for gene-environment interactions co-opted from studies of evolution. This model stipulates that heat shock proteins that mediate the stress response induced by environmental factors can influence the number of individuals that cross disease thresholds resulting in increased incidence of birth defects. Studies in the field of evolutionary biology have demonstrated that heat shock proteins and Hsp90 in particular provide a link between environmental stress, genotype and phenotype. Hsp90 is a highly expressed molecular chaperone that assists a wide variety of protein clients with folding and conformational changes needed for proper function. Hsp90 also chaperones client proteins with potentially deleterious amino acid changes to suppress variation caused by genetic mutations. However, upon exposure to stress, Hsp90 abandons its normal physiological clients and is diverted to assist with the misfolded protein response. This can impact the activity of signaling pathways that involve Hsp90 clients as well as unmask suppressed protein variation, essentially creating complex traits in a single step. In this capacity Hsp90 acts as an evolutionary capacitor allowing stored variation to accumulate and then become expressed in times of stress. This mechanism provides a substrate which natural selection can act upon at the population level allowing survival of the species with selective pressure. However, at the level of the individual, this mechanism can result in simultaneous expression of deleterious variants as well as reduced activity of a variety of Hsp90 chaperoned pathways, potentially shifting phenotypic variability over the disease threshold resulting in birth defects.

Evolutionary justifications for human reproductive limitations

Common human reproductive inefficiencies have multiple etiologies. Going against chance, many effects, such as polycystic ovaries, endometriosis, and folate metabolic issues, have genetic components, while aneuploid losses arise from diverse mitotic and meiotic errors at different stages, some transitory. This can be advantageous, since greater overall survival with fewer offspring can increase reproductive success. Benefits primarily accrue to mothers, who bear most child related costs, and for whom early losses are less costly than late. Different adaptations to different situations reflect human evolutionary history. For early speciation, periodic climate extremes repeatedly reduced resources, favoring limitations while contracted populations helped fix relevant genes. Later, under better conditions, evolving social cooperation could increase fecundity faster than it added resources, further supporting reproductive suppression through mitotic aneuploidy, with very early losses minimizing maternal costs. The grandmother hypothesis suggests benefits in limiting reproduction as maternal age increased pregnancy risks in order to support grandchildren as they arrived, selecting for maternal age-related meiotic aneuploidy. Finally, with variable short-term agricultural shortages, acute reproductive responses arose through chromatin "nutrient sensor"-regulated epigenetic effects that also shifted some lethal effects earlier, reducing both maternal and mutation load costs. Overall, despite suggestions to the contrary, it is likely that human selective pressures have not decreased with civilization, but that many of the costs have been shifted to early reproduction.

Effect of tributyltin chloride (TBT-Cl) exposure on expression of HSP90β1 in the river pufferfish (Takifugu obscurus): Evidences for its immunologic function involving in exploring process

HSP90β1 (known as glyco-protein 96, GP96) is a vital endoplasmic reticulum (ER) depended chaperonin among the HSPs (heat shock proteins) family. Furthermore, it always processes and presents antigen of the tumor and keeps balance for the intracellular environment. In the present study, we explored the effect of tributyltin chloride (TBT-Cl) exposure on HSP90β1 expression in river pufferfish, Takifugu obscurus. The full length of To-HSP90β1 was gained with 2775 bp in length, with an ORF (open reading frame) encoding an 803 aa polypeptide. A phylogenetic tree was constructed and showed the close relationship to other fish species. The HSP90β1 mRNA transcript was expressed in all tissues investigated with higher level in the gill and liver. After the acute and chronic exposure of TBT-Cl, the To-HSP90β1 mRNA transcript significantly was up-regulated in gills. Moreover, the histology study indicated the different injury degree of TBT-Cl in liver and gill. Immunohistochemistry (IHC) staining results implied the cytoplasm reorganization after TBT-Cl stress and the function of immunoregulation for To-HSP90β1 to TBT-Cl exposure. All the results indicated that HSP90β1 may be involved in the resistance to the invasion of TBT-Cl for keeping autoimmune homeostasis.

Morphological homeostasis in the fossil record

Morphological homeostasis limits the extent to which genetic and/or environmental variation is translated into phenotypic variation, providing generation-to-generation fitness advantage under a stabilizing selection regime. Depending on its lability, morphological homeostasis might also have a longer-term impact on evolution by restricting the variation-and thus the response to directional selection-of a trait. The fossil record offers an inviting opportunity to investigate whether and how morphological homeostasis constrained trait evolution in lineages or clades on long timescales (thousands to millions of years) that are not accessible to neontological studies. Fossils can also reveal insight into the nature of primitive developmental systems that might not be predictable from the study of modern organisms. The ability to study morphological homeostasis in fossils is strongly limited by taphonomic processes that can destroy, blur, or distort the original biological signal: genetic data are unavailable; phenotypic data can be modified by tectonic or compaction-related deformation; time-averaging limits temporal resolution; and environmental variation is hard to study and impossible to control. As a result of these processes, neither allelic sensitivity (and thus genetic canalization) nor macroenvironmental sensitivity (and thus environmental canalization) can be unambiguously assessed in the fossil record. However, homeorhesis-robustness against microenvironmental variation (developmental noise)-can be assessed in ancient developmental systems by measuring the level of fluctuating asymmetry (FA) in a nominally symmetric trait. This requires the analysis of multiple, minimally time-averaged samples of exquisite preservational quality. Studies of FA in fossils stand to make valuable contributions to our understanding of the deep-time significance of homeorhesis. Few empirical studies have been conducted to date, and future paleontological research focusing on how homeorhesis relates to evolutionary rate (including stasis), species survivorship, and purported macroevolutionary trends in evolvability would reap high reward.

Multiple modes of canalization: Links between genetic, environmental canalizations and developmental stability, and their trait-specificity

The robustness of biological systems against mutational and environmental perturbations is termed canalization. Because reducing phenotypic variability under environmental and genetic perturbations can be adaptive and facilitated by natural selection, it has been suggested that once canalization mechanisms have evolved to buffer the effects of environmental perturbations, they may act to buffer any and all sources of variation. Although whether canalization mechanisms are general or specific to the types of perturbation or phenotypic traits that they buffer is often addressed, the links between different canalization mechanisms remain unclear. In this review, three major sources of phenotypic variation, associated canalization concepts and indicators of the degree of canalization are first outlined. Then, the molecular bases of canalization mechanisms based on recent empirical studies are overviewed. Finally, the links between the underlying processes of different canalization mechanisms are explored.

Cloning of HSP90, expression and localization of HSP70/90 in different tissues including lactating/non-lactating yak (Bos grunniens) breast tissue

The aim of this study is to investigate the expression and localization of HSP70/90 in different tissues and explore the regulation effects of HSP70/90 at lactation period of female yaks. HSP90 mRNA was cloned from the heart samples of female yaks, Quantitative real-time (qRT-PCR), Western blotting (WB), immunohistochemistry and immunofluorescence assays were utilized to analyze the expressions of HSP70/90 mRNA and protein in different tissues. Sequence analysis showed that HSP90 is a conserved molecular chaperone of female yaks. The qRT-PCR, WB results showed that the expressions of HSP70/90 mRNA and protein were significantly different in different tissues, and 3-fold higher expression during the lactation period than the non-lactation period of breast tissue (P < 0.01). Immunohistochemistry and immunofluorescence assays results showed that HSP70/90 were located in the cardiac muscle cells, cerebellar medulla, theca cells lining at the reproductive system, and the mammary epithelia of the breasts. In addition, the expression level of HSP70 was higher than those of HSP90 in all examined tissues. Therefore, our results strongly suggest that the expression and localization of HSP70/90 could provide significant evidence to further research in tissue specific expression, and lactation function of female yaks.

Little effect of HSP90 inhibition on the quantitative wing traits variation in Drosophila melanogaster

Drosophila wings have been a model system to study the effect of HSP90 on quantitative trait variation. The effect of HSP90 inhibition on environmental buffering of wing morphology varies among studies while the genetic buffering effect of it was examined in only one study and was not detected. Variable results so far might show that the genetic background influences the environmental and genetic buffering effect of HSP90. In the previous studies, the number of the genetic backgrounds used is limited. To examine the effect of HSP90 inhibition with a larger number of genetic backgrounds than the previous studies, 20 wild-type strains of Drosophila melanogaster were used in this study. Here I investigated the effect of HSP90 inhibition on the environmental buffering of wing shape and size by assessing within-individual and among-individual variations, and as a result, I found little or very weak effects on environmental and genetic buffering. The current results suggest that the role of HSP90 as a global regulator of environmental and genetic buffering is limited at least in quantitative traits.

Novel genetic capacitors and potentiators for the natural genetic variation of sensory bristles and their trait specificity in Drosophila melanogaster

Cryptic genetic variation (CGV) is defined as the genetic variation that has little effect on phenotypic variation under a normal condition, but contributes to heritable variation under environmental or genetic perturbations. Genetic buffering systems that suppress the expression of CGV and store it in a population are called genetic capacitors, and the opposite systems are called genetic potentiators. One of the best-known candidates for a genetic capacitor and potentiator is the molecular chaperone protein, HSP90, and one of its characteristics is that it affects the genetic variation in various morphological traits. However, it remains unclear whether the wide-ranging effects of HSP90 on a broad range of traits are a general feature of genetic capacitors and potentiators. In the current study, I searched for novel genetic capacitors and potentiators for quantitative bristle traits of Drosophila melanogaster and then investigated the trait specificity of their genetic buffering effect. Three bristle traits of D. melanogaster were used as the target traits, and the genomic regions with genetic buffering effects were screened using the 61 genomic deficiencies examined previously for genetic buffering effects in wing shape. As a result, four and six deficiencies with significant effects on increasing and decreasing the broad-sense heritability of the bristle traits were identified, respectively. Of the 18 deficiencies with significant effects detected in the current study and/or by the previous study, 14 showed trait-specific effects, and four affected the genetic buffering of both bristle traits and wing shape. This suggests that most genetic capacitors and potentiators exert trait-specific effects, but that general capacitors and potentiators with effects on multiple traits also exist.

Hsp90: A New Player in DNA Repair?

Heat shock protein 90 (Hsp90) is an evolutionary conserved molecular chaperone that, together with Hsp70 and co-chaperones makes up the Hsp90 chaperone machinery, stabilizing and activating more than 200 proteins, involved in protein homeostasis (i.e., proteostasis), transcriptional regulation, chromatin remodeling, and DNA repair. Cells respond to DNA damage by activating complex DNA damage response (DDR) pathways that include: (i) cell cycle arrest; (ii) transcriptional and post-translational activation of a subset of genes, including those associated with DNA repair; and (iii) triggering of programmed cell death. The efficacy of the DDR pathways is influenced by the nuclear levels of DNA repair proteins, which are regulated by balancing between protein synthesis and degradation as well as by nuclear import and export. The inability to respond properly to either DNA damage or to DNA repair leads to genetic instability, which in turn may enhance the rate of cancer development. Multiple components of the DNA double strand breaks repair machinery, including BRCA1, BRCA2, CHK1, DNA-PKcs, FANCA, and the MRE11/RAD50/NBN complex, have been described to be client proteins of Hsp90, which acts as a regulator of the diverse DDR pathways. Inhibition of Hsp90 actions leads to the altered localization and stabilization of DDR proteins after DNA damage and may represent a cell-specific and tumor-selective radiosensibilizer. Here, the role of Hsp90-dependent molecular mechanisms involved in cancer onset and in the maintenance of the genome integrity is discussed and highlighted.

Hectd1 is required for development of the junctional zone of the placenta

The placenta plays a critical role in the growth and survival of the fetus. Here we demonstrate that the Homologous to the E6-AP Carboxyl Terminus (HECT) domain E3 ubiquitin ligase, Hectd1, is essential for development of the mouse placenta. Hectd1 is widely expressed during placentation with enrichment in trophoblast giant cells (TGCs) and other trophoblast-derived cell subtypes in the junctional and labyrinth zones of the placenta. Disruption of Hectd1 results in mid-gestation lethality and intrauterine growth restriction (IUGR). Variable defects in the gross structure of the mutant placenta are found including alterations in diameter, thickness and lamination. The number and nuclear size of TGCs is reduced. Examination of subtype specific markers reveals altered TGC development with decreased expression of Placental lactogen-1 and -2 (Pl1 and Pl2) and increased expression of Proliferin (Plf). Reduced numbers of spongiotrophoblasts and glycogen trophoblasts were also found at the junctional zone of the Hectd1 mutant placenta. Finally, there was an increase in immature uterine natural killer (uNK) cells in the maternal decidua of the Hectd1 mutant placenta. Proliferation and apoptosis are differentially altered in the layers of the placenta with an increase in both apoptosis and proliferation in the maternal decidua, a decrease in proliferation and increase in apoptosis in the labyrinth layer and both unchanged in the junctional zone. Together these data demonstrate that Hectd1 is required for development of multiple cell types within the junctional zone of the placenta.

Multiple capacitors for natural genetic variation in Drosophila melanogaster

Cryptic genetic variation (CGV) or a standing genetic variation that is not ordinarily expressed as a phenotype is released when the robustness of organisms is impaired under environmental or genetic perturbations. Evolutionary capacitors modulate the amount of genetic variation exposed to natural selection and hidden cryptically; they have a fundamental effect on the evolvability of traits on evolutionary timescales. In this study, I have demonstrated the effects of multiple genomic regions of Drosophila melanogaster on CGV in wing shape. I examined the effects of 61 genomic deficiencies on quantitative and qualitative natural genetic variation in the wing shape of D. melanogaster. I have identified 10 genomic deficiencies that do not encompass a known candidate evolutionary capacitor, Hsp90, exposing natural CGV differently depending on the location of the deficiencies in the genome. Furthermore, five genomic deficiencies uncovered qualitative CGV in wing morphology. These findings suggest that CGV in wing shape of wild-type D. melanogaster is regulated by multiple capacitors with divergent functions. Future analysis of genes encompassed by these genomic regions would help elucidate novel capacitor genes and better understand the general features of capacitors regarding natural genetic variation.

Does the central dogma still stand?

Prions are agents of analog, protein conformation-based inheritance that can confer beneficial phenotypes to cells, especially under stress. Combined with genetic variation, prion-mediated inheritance can be channeled into prion-independent genomic inheritance. Latest screening shows that prions are common, at least in fungi. Thus, there is non-negligible flow of information from proteins to the genome in modern cells, in a direct violation of the Central Dogma of molecular biology. The prion-mediated heredity that violates the Central Dogma appears to be a specific, most radical manifestation of the widespread assimilation of protein (epigenetic) variation into genetic variation. The epigenetic variation precedes and facilitates genetic adaptation through a general 'look-ahead effect' of phenotypic mutations. This direction of the information flow is likely to be one of the important routes of environment-genome interaction and could substantially contribute to the evolution of complex adaptive traits.

Soluble guanylyl cyclase requires heat shock protein 90 for heme insertion during maturation of the NO-active enzyme

Heme insertion is key during maturation of soluble guanylyl cyclase (sGC) because it enables sGC to recognize NO and transduce its multiple biological effects. Although sGC is often associated with the 90-kDa heat shock protein (hsp90) in cells, the implications are unclear. The present study reveals that hsp90 is required to drive heme insertion into sGC and complete its maturation. We used a mammalian cell culture approach and followed heme insertion into transiently and endogenously expressed heme-free sGC. We used pharmacological hsp90 inhibitors, an ATP-ase inactive hsp90 mutant, and heme-dependent or heme-independent sGC activators as tools to decipher the role of hsp90. Our findings suggest that hsp90 complexes with apo-sGC, drives heme insertion through its inherent ATPase activity, and then dissociates from the mature, heme-replete sGC. Together, this improves our understanding of sGC maturation and reveals a unique means to control sGC activity in cells, and it has important implications for hsp90 inhibitor-based cancer therapy.

Interplay between gene expression noise and regulatory network architecture

Complex regulatory networks orchestrate most cellular processes in biological systems. Genes in such networks are subject to expression noise, resulting in isogenic cell populations exhibiting cell-to-cell variation in protein levels. Increasing evidence suggests that cells have evolved regulatory strategies to limit, tolerate or amplify expression noise. In this context, fundamental questions arise: how can the architecture of gene regulatory networks generate, make use of or be constrained by expression noise? Here, we discuss the interplay between expression noise and gene regulatory network at different levels of organization, ranging from a single regulatory interaction to entire regulatory networks. We then consider how this interplay impacts a variety of phenomena, such as pathogenicity, disease, adaptation to changing environments, differential cell-fate outcome and incomplete or partial penetrance effects. Finally, we highlight recent technological developments that permit measurements at the single-cell level, and discuss directions for future research.

Selfish cellular networks and the evolution of complex organisms

Human gametogenesis takes years and involves many cellular divisions, particularly in males. Consequently, gametogenesis provides the opportunity to acquire multiple de novo mutations. A significant portion of these is likely to impact the cellular networks linking genes, proteins, RNA and metabolites, which constitute the functional units of cells. A wealth of literature shows that these individual cellular networks are complex, robust and evolvable. To some extent, they are able to monitor their own performance, and display sufficient autonomy to be termed "selfish". Their robustness is linked to quality control mechanisms which are embedded in and act upon the individual networks, thereby providing a basis for selection during gametogenesis. These selective processes are equally likely to affect cellular functions that are not gamete-specific, and the evolution of the most complex organisms, including man, is therefore likely to occur via two pathways: essential housekeeping functions would be regulated and evolve during gametogenesis within the parents before being transmitted to their progeny, while classical selection would operate on other traits of the organisms that shape their fitness with respect to the environment.

Heterogeneity reduces sensitivity of cell death for TNF-stimuli

Background

Apoptosis is a form of programmed cell death essential for the maintenance of homeostasis and the removal of potentially damaged cells in multicellular organisms. By binding its cognate membrane receptor, TNF receptor type 1 (TNF-R1), the proinflammatory cytokine Tumor Necrosis Factor (TNF) activates pro-apoptotic signaling via caspase activation, but at the same time also stimulates nuclear factor κB (NF-κB)-mediated survival pathways. Differential dose-response relationships of these two major TNF signaling pathways have been described experimentally and using mathematical modeling. However, the quantitative analysis of the complex interplay between pro- and anti-apoptotic signaling pathways is an open question as it is challenging for several reasons: the overall signaling network is complex, various time scales are present, and cells respond quantitatively and qualitatively in a heterogeneous manner.

Results

This study analyzes the complex interplay of the crosstalk of TNF-R1 induced pro- and anti-apoptotic signaling pathways based on an experimentally validated mathematical model. The mathematical model describes the temporal responses on both the single cell level as well as the level of a heterogeneous cell population, as observed in the respective quantitative experiments using TNF-R1 stimuli of different strengths and durations. Global sensitivity of the heterogeneous population was quantified by measuring the average gradient of time of death versus each population parameter. This global sensitivity analysis uncovers the concentrations of Caspase-8 and Caspase-3, and their respective inhibitors BAR and XIAP, as key elements for deciding the cell's fate. A simulated knockout of the NF-κB-mediated anti-apoptotic signaling reveals the importance of this pathway for delaying the time of death, reducing the death rate in the case of pulse stimulation and significantly increasing cell-to-cell variability.

Conclusions

Cell ensemble modeling of a heterogeneous cell population including a global sensitivity analysis presented here allowed us to illuminate the role of the different elements and parameters on apoptotic signaling. The receptors serve to transmit the external stimulus; procaspases and their inhibitors control the switching from life to death, while NF-κB enhances the heterogeneity of the cell population. The global sensitivity analysis of the cell population model further revealed an unexpected impact of heterogeneity, i.e. the reduction of parametric sensitivity.

Somatic mosaicism in healthy human tissues

From the fertilization of an egg until the death of an individual, somatic cells can accumulate genetic changes, such that cells from different tissues or even within the same tissue differ genetically. The presence of multiple cell clones with distinct genotypes in the same individual is referred to as 'somatic mosaicism'. Many endogenous factors such as mobile elements, DNA polymerase slippage, DNA double-strand break, inefficient DNA repair, unbalanced chromosomal segregation and some exogenous factors such as nicotine and UV exposure can contribute to the generation of somatic mutations, thereby leading to somatic mosaicism. Such changes can potentially affect the epigenetic patterns and levels of gene expression, and ultimately the phenotypes of cells. Although recent studies suggest that somatic mosaicism is widespread during normal development and aging, its implications for heightened disease risks are incompletely understood. Here, I discuss the origins, prevalence and implications of somatic mosaicism in healthy human tissues.

Heat shock proteins and Drosophila aging

Since their discovery in Drosophila, the heat shock proteins (Hsps) have been shown to regulate both stress resistance and life-span. Aging is characterized by increased oxidative stress and the accumulation of abnormal (malfolded) proteins, and these stresses induce Hsp gene expression through the transcription factor HSF. In addition, a subset of Hsps is induced by oxidative stress through the JNK signaling pathway and the transcription factor Foxo. The Hsps counteract the toxicity of abnormal proteins by facilitating protein refolding and turnover, and through other mechanisms including inhibition of apoptosis. The Hsps are up-regulated in tissue-specific patterns during aging, and their expression correlates with, and sometimes predicts, life span, making them ideal biomarkers of aging. The tools available for experimentally manipulating gene function and assaying healthspan in Drosophila provides an unparalleled opportunity to further study the role of Hsps in aging.

The mineralocorticoid receptor is a constitutive nuclear factor in cardiomyocytes due to hyperactive nuclear localization signals

The mineralocorticoid receptor (MR), a member of the nuclear receptor family, mediates the action of aldosterone in target epithelia, enhancing sodium reabsorption. In addition, MR may have other physiological functions in nonepithelial tissues. Altered expression or inappropriate activation of cardiac MR is directly linked to the development of cardiac fibrosis, and MR blockade is beneficial for the treatment of heart failure. However, the physiological role, activation status, and target genes of MR in the heart are poorly known. Because ligand-free steroid receptors are typically cytoplasmic and translocate to the nucleus upon ligand binding, we examined the subcellular localization of MR under different corticosteroid levels using subcellular fractionation and immunostaining. Our results demonstrate that MR is a chromatin-bound factor in mouse left ventricle and in a cultured model of cardiomyocytes, HL-1 cells, regardless of circulating corticosteroid levels. Immunohistochemical localization of MR in human heart confirms the subcellular localization pattern. Mutation of nuclear localization signals (NLSs) demonstrates that MR constitutive nuclear localization mainly depends on the synergistic contribution of NLS0 and NLS1. Constitutive nuclear localization in HL-1 cells can be reverted by cotransfection of heat shock protein 90. Heat shock protein 90 expression levels in the mouse heart and HL-1 cells are lower than those found in other tissues, suggesting that low levels of cochaperones render MR NLSs hyperactive in cardiomyocytes. Even though MR is constitutively nuclear, corticosteroids still control the transactivation properties of the receptor in a model promoter, although other MR ligand-independent activities cannot be excluded.

Hsp90 phosphorylation, Wee1 and the cell cycle

Heat Shock Protein 90 (Hsp90) is an essential molecular chaperone in eukaryotic cells, and it maintains the functional conformation of a subset of proteins that are typically key components of multiple regulatory and signaling networks mediating cancer cell proliferation, survival, and metastasis. It is possible to selectively inhibit Hsp90 using natural products such as geldanamycin (GA) or radicicol (RD), which have served as prototypes for development of synthetic Hsp90 inhibitors. These compounds bind within the ADP/ATP-binding site of the Hsp90 N-terminal domain to inhibit its ATPase activity. As numerous N-terminal domain inhibitors are currently undergoing extensive clinical evaluation, it is important to understand the factors that may modulate in vivo susceptibility to these drugs. We recently reported that Wee1Swe1-mediated, cell cycle-dependent, tyrosine phosphorylation of Hsp90 affects GA binding and impacts cancer cell sensitivity to Hsp90 inhibition. This phosphorylation also affects Hsp90 ATPase activity and its ability to chaperone a selected group of clients, comprised primarily of protein kinases. Wee1 regulates the G2/M transition. Here we present additional data demonstrating that tyrosine phosphorylation of Hsp90 by Wee1Swe1 is important for Wee1Swe1 association with Hsp90 and for Wee1Swe1 stability. Yeast expressing non-phosphorylatable yHsp90-Y24F, like swe1∆ yeast, undergo premature nuclear division that is insensitive to G2/M checkpoint arrest. These findings demonstrate the importance of Hsp90 phosphorylation for proper cell cycle regulation.

The impact of genomic neighborhood on the evolution of human and chimpanzee transcriptome

Divergence of gene expression can result in phenotypic variation, which contributes to the evolution of new species. Although the influence of trans- and cis-regulatory mutations is well known, the genome-wide impact of changes in genomic neighborhood of genes on expression divergence between species remains largely unexplored. Here, we compare the neighborhood of orthologous genes (within a window of 2 MB) in human and chimpanzee with the expression levels of their transcripts from several equivalent tissues and demonstrate that genes with altered neighborhood are more likely to undergo expression divergence than genes with conserved neighborhood. We observe the same trend when expression divergence data were analyzed from six different brain parts that are equivalent between human and chimpanzee. Additionally, we find enrichment for genes with altered neighborhood to be expressed in a tissue-specific manner in the human brain. These results suggest that expression divergence induced by this mechanism could have contributed to the phenotypic differences between human and chimpanzee. We propose that, in addition to other molecular mechanisms, change in genomic neighborhood is an important factor that drives transcriptome evolution.