Cell Size In Response Research Articles

Glucose‐dependent cell size is regulated by a G protein‐coupled receptor system in yeast Saccharomyces cerevisiae

In the yeast, Saccharomyces cerevisiae, cell size is affected by the kind of carbon source in the medium. Here, we present evidence that the Gpr1 receptor and Gpa2 Galpha subunit are required for both maintenance and modulation of cell size in response to glucose. In the presence of glucose, mutants lacking GPR1 or GPA2 gene showed smaller cells than the wild-type strain. Physiological studies revealed that protein synthesis rate was reduced in the mutant strains indicating that reduced growth rate, while the level of mRNAs for CLN1, 2 and 3 was not affected in all strains. Gene chip analysis also revealed a down-regulation in the expression of genes related to biosynthesis of not only protein but also other cellular component in the mutant strains. We also show that GPR1 and GPA2 are required for a rapid increase in cell size in response to glucose. Wild-type cells grown in ethanol quickly increased in size by addition of glucose, while little change was observed in the mutant strains, in which glucose-dependent cell cycle arrest caused by CLN1 repression was somewhat alleviated. Our study indicates that the yeast G-protein coupled receptor system consisting of Gpr1 and Gpa2 regulates cell size by affecting both growth rate and cell division.

HhLIM protein is involved in cardiac hypertrophy

Proteins of the LIM family are critical regulators of development and differentiation in various cell types. Here we examined the roles of one new member of LIM family, hhLIM, in cardiac hypertrophic growth and cardiac muscle-specific gene expression. To model the increase in endogenous hhLIM transcriptional activity that occurs in response to hypertrophic stimulation, hhLIM was overexpressed using a recombinant plasmid for hhLIM. The results showed that overexpression of hhLIM resulted in increased cell volume in both C2C12 muscle cells (>1.5-fold) and cardiac myocytes (>2.49-fold), a phenotype commonly associated with cardiac hypertrophy. RT-PCR and Western blot showed that transfection of hhLIM into C2C12 muscle cells and cardiomyocytes increased skeletal alpha-actin levels and triggered the expression of the embryonic-related gene BNP, which is associated with cardiac hypertrophy. Inhibition of hhLIM expression by antisense transcripts blocked the induction of skeletal alpha-actin and BNP expression by endothelin-1. These data indicated that hhLIM played a role in regulation of cardiomyocyte growth and cell size in response to hypertrophic stimuli through its modulation of skeletal alpha-actin and BNP expression. We also determined by confocal laser scanning microscopy and immunoprecipitation that hhLIM was associated with alpha-actin and localized in the cytoplasm in unstimulated cells, and was relocalized from the cytoplasm to the nucleus upon hypertrophic stimulation. These studies suggest that hhLIM protein is involved in cardiac hypertrophy.

The mTOR Signaling Complex

The protein kinase mTOR phosphorylates S6 kinase (S6K) and eIF-4E binding protein, two regulators of protein synthesis, and an mTOR signaling pathway has been implicated in controlling mammalian cell growth and cell size in response to nutrients and growth factors. The activity of mTOR is controlled in part by its association with raptor (regulatory associated protein of TOR), an interaction that seems to have both positive and negative effects on mTOR activity. For example, overexpression of raptor has been observed to both stimulate and inhibit mTOR activity. A nutrient-sensitive interaction between mTOR and raptor has also been unclear. Kim et al. now report that nutrient-sensitivity of the mTOR-raptor complex requires the interaction of mTOR with another protein called GβL (G protein β subunit-like protein), a widely expressed protein of previously unknown function. An endogenous mTOR-GβL complex was detected in cultured mammalian cells and GβL bound to the kinase domain of mTOR. The mTOR-GβL interaction was more stable, and independent from, mTOR-raptor interaction. GβL appears to be a positive regulator of the mTOR signaling pathway, because overexpression of GβL stimulated mTOR kinase activity and decreased either GβL expression by small interfering RNA or expression of mutants that do not bind mTOR reduced phosphorylation of S6K. In addition, decreased GβL expression reduced raptor association with mTOR, implying that GβL stabilizes the mTOR-raptor complex. Furthermore, the mTOR-raptor complex was only nutrient-sensitive if GβL was expressed. The authors propose opposing actions of GβL and raptor on mTOR in which binding of raptor to a GβL-mTOR complex inhibits GβL-mediated activation of mTOR. Low nutrient conditions promote raptor association with mTOR only when GβL is also docked onto mTOR. Increased nutrients would weaken the raptor-mTOR interaction, allowing GβL to stimulate mTOR. D.-H. Kim, D. D. Sarbassov, S. M. Ali, R. R. Latek, K. V. P. Guntur, H. Erdjument-Bromage, P. Tempst, D. M. Sabatini, GβL, a positive regulator of the rapamycin-sensitive pathway required for the nutrient-sensitive interaction between raptor and mTOR. Mol. Cell 11 , 895-904 (2003). [Online Journal]

MTOR Interacts with Raptor to Form a Nutrient-Sensitive Complex that Signals to the Cell Growth Machinery

mTOR/RAFT1/FRAP is the target of the immunosuppressive drug rapamycin and the central component of a nutrient- and hormone-sensitive signaling pathway that regulates cell growth. We report that mTOR forms a stoichiometric complex with raptor, an evolutionarily conserved protein with at least two roles in the mTOR pathway. Raptor has a positive role in nutrient-stimulated signaling to the downstream effector S6K1, maintenance of cell size, and mTOR protein expression. The association of raptor with mTOR also negatively regulates the mTOR kinase activity. Conditions that repress the pathway, such as nutrient deprivation and mitochondrial uncoupling, stabilize the mTOR-raptor association and inhibit mTOR kinase activity. We propose that raptor is a missing component of the mTOR pathway that through its association with mTOR regulates cell size in response to nutrient levels.

C-Jun N-terminal kinase-interacting protein 1 inhibits gene expression in response to hypertrophic agonists in neonatal rat ventricular myocytes

Gq-coupled receptor agonists, such as endothelin-1 (ET-1) and phenylephrine (PE), initiate a hypertrophic response in cardiac myocytes that is characterized by increased expression of atrial natriuretic factor (ANF), β-myosin heavy chain (β-MHC), skeletal muscle α-actin (SkαA) and ventricular myosin light chain-2 (vMLC2). ET-1 and PE activate both the extracellular signal-regulated kinases and c-Jun N-terminal kinases (JNKs) in cardiac myocytes, but the extent to which each contributes to the hypertrophic response is uncertain. Here we have used the JNK-binding domain of JNK-interacting protein 1 (JIP-1), a cytosolic scaffold protein that binds to JNK and inhibits its signalling when overexpressed, to assess the contribution of JNK activation to the hypertrophic response. Expression of JIP-1 inhibited the increase in ANF, β-MHC, SkαA and vMLC2 reporter gene expression in response to ET-1 (by 45–86%) and PE (by 56–60%). However, activation of these reporter genes by PMA, which does not activate JNK significantly in myocytes, was much less affected by overexpression of JIP-1. JIP-1 also failed to inhibit reporter gene activation in response to constitutively active Ras or Raf, but attenuated reporter gene activation induced by a constitutively active mutant of mitogen-activated protein kinase kinase kinase 1 (MEKK1), an upstream kinase that preferentially activates JNKs, by 50%. Overexpression of JIP-1 also significantly reduced the increase in cell area in response to PE from 63% to 56%, but had no effect on the increase in cell size in response to ET-1 (38%). These results suggest that activation of the JNK pathway contributes to the transcriptional and morphological responses to Gq receptor-coupled hypertrophic agonists.

C-Jun N-terminal kinase-interacting protein 1 inhibits gene expression in response to hypertrophic agonists in neonatal rat ventricular myocytes

G(q)-coupled receptor agonists, such as endothelin-1 (ET-1) and phenylephrine (PE), initiate a hypertrophic response in cardiac myocytes that is characterized by increased expression of atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), skeletal muscle alpha-actin (SkalphaA) and ventricular myosin light chain-2 (vMLC2). ET-1 and PE activate both the extracellular signal-regulated kinases and c-Jun N-terminal kinases (JNKs) in cardiac myocytes, but the extent to which each contributes to the hypertrophic response is uncertain. Here we have used the JNK-binding domain of JNK-interacting protein 1 (JIP-1), a cytosolic scaffold protein that binds to JNK and inhibits its signalling when overexpressed, to assess the contribution of JNK activation to the hypertrophic response. Expression of JIP-1 inhibited the increase in ANF, beta-MHC, SkalphaA and vMLC2 reporter gene expression in response to ET-1 (by 45-86%) and PE (by 56-60%). However, activation of these reporter genes by PMA, which does not activate JNK significantly in myocytes, was much less affected by overexpression of JIP-1. JIP-1 also failed to inhibit reporter gene activation in response to constitutively active Ras or Raf, but attenuated reporter gene activation induced by a constitutively active mutant of mitogen-activated protein kinase kinase kinase 1 (MEKK1), an upstream kinase that preferentially activates JNKs, by 50%. Overexpression of JIP-1 also significantly reduced the increase in cell area in response to PE from 63% to 56%, but had no effect on the increase in cell size in response to ET-1 (38%). These results suggest that activation of the JNK pathway contributes to the transcriptional and morphological responses to G(q) receptor-coupled hypertrophic agonists.

Chronic Hypoosmolality Induces a Selective Decrease in Magnocellular Neurone Soma and Nuclear Size in the Rat Hypothalamic Supraoptic Nucleus

AbstractThe magnocellular neurones of the hypothalamo‐neurohypophysial system (HNS) play a vital role in the maintenance of body homeostasis by regulating oxytocin (OT) and vasopressin (VP) secretion from the posterior pituitary. During hyperosmolality, OT and VP mRNA levels are known to increase by approximately two‐fold, whereas during chronic hypoosmolality, OT and VP mRNA levels decrease to approximately 10–20% of basal levels. In these studies, we evaluated changes in cell size associated with these physiological conditions. Cell and nuclear sizes of neurones in the supraoptic nucleus (SON), the nucleus of the lateral olfactory tract (LOT) and the medial habenular nucleus (MHB) were measured from neurones identified by in situ hybridization histochemistry for βIII‐tubulin mRNA, and measurements were made from OT and AVP magnocellular neurones in the SON after phenotypic identification by immunohistochemistry. Under hypoosmolar conditions, the cell and nuclear sizes of OT and VP magnocellular neurones decreased to approximately 60% of basal values, whereas cell and nuclear sizes of OT and VP neurones in hyperosmolar rats increased to approximately 170% of basal values. In contrast, neither hyperosmolality, nor hypoosmolality significantly affected cell and nuclear sizes in the LOT and MHB. These results confirm previous studies that showed that magnocellular neurones increase cell size in response to hyperosmolar conditions and, for the first time, demonstrate a marked decrease in cell size in the SON in response to chronic hypoosmolar conditions. These dramatic changes in cell and nuclear size directly parallel changes in OT and VP gene expression in the magnocellular neurones of the SON and, consequently, are consistent with the pronounced bidirectional changes in gene expression and cellular activity found during these osmotic perturbations. Our results therefore support the concept of global alterations in the synthetic activity of magnocellular OT and AVP neurones in response to extracellular osmolality.

Chronic hypoosmolality induces a selective decrease in magnocellular neurone soma and nuclear size in the rat hypothalamic supraoptic nucleus.

The magnocellular neurones of the hypothalamo-neurohypophysial system (HNS) play a vital role in the maintenance of body homeostasis by regulating oxytocin (OT) and vasopressin (VP) secretion from the posterior pituitary. During hyperosmolality, OT and VP mRNA levels are known to increase by approximately two-fold, whereas during chronic hypoosmolality, OT and VP mRNA levels decrease to approximately 10-20% of basal levels. In these studies, we evaluated changes in cell size associated with these physiological conditions. Cell and nuclear sizes of neurones in the supraoptic nucleus (SON), the nucleus of the lateral olfactory tract (LOT) and the medial habenular nucleus (MHB) were measured from neurones identified by in situ hybridization histochemistry for beta(III)-tubulin mRNA, and measurements were made from OT and AVP magnocellular neurones in the SON after phenotypic identification by immunohistochemistry. Under hypoosmolar conditions, the cell and nuclear sizes of OT and VP magnocellular neurones decreased to approximately 60% of basal values, whereas cell and nuclear sizes of OT and VP neurones in hyperosmolar rats increased to approximately 170% of basal values. In contrast, neither hyperosmolality, nor hypoosmolality significantly affected cell and nuclear sizes in the LOT and MHB. These results confirm previous studies that showed that magnocellular neurones increase cell size in response to hyperosmolar conditions and, for the first time, demonstrate a marked decrease in cell size in the SON in response to chronic hypoosmolar conditions. These dramatic changes in cell and nuclear size directly parallel changes in OT and VP gene expression in the magnocellular neurones of the SON and, consequently, are consistent with the pronounced bidirectional changes in gene expression and cellular activity found during these osmotic perturbations. Our results therefore support the concept of global alterations in the synthetic activity of magnocellular OT and AVP neurones in response to extracellular osmolality.

Estrogen increases the permeability of the cultured human cervical epithelium by modulating cell deformability.

Estrogens increase secretion of cervical mucus in females. The objective of this research was to study the mechanisms of estrogen action. The experimental models were human CaSki (endocervical) and hECE (ectocervical) epithelial cells cultured on filters. Incubation in steroid-free medium increased transepithelial electrical resistance (RTE) and decreased epithelial permeability to the cell-impermeant acid pyranine. Estrogen treatment reversed the effects, indicating estrogen decreases epithelial paracellular resistance. The estrogen effect was time and dose related (EC50 approximately 1 nM) and specific (estradiol = diethylstilbestrol > estrone, estriol; no effect by progesterone, testosterone, or cortisol) and was blocked by progesterone, tamoxifen, and ICI-182780 (an estrogen receptor antagonist). Estrogen treatment did not modulate dilution potential or changes in RTE in response to diC8 or to low extracellular Ca2+ (modulators of tight junctional resistance). In contrast, estrogen augmented decreases in RTE in response to hydrostatic and hypertonic gradients [modulators of resistance of lateral intercellular space (RLIS)], suggesting estrogen decreases RLIS. Estrogen decreased cervical cell size, shortened response time relative to changes in cell size after hypertonic challenge, and augmented the decrease in cell size in response to hypertonic and hydrostatic gradients. Lowering luminal NaCl had no significant effect on RTE, and the Cl- channel blocker diphenylamine-2-carboxylate attenuated the hypertonicity-induced decrease in cell size to the same degree in control and estrogen-treated cells, suggesting estrogen effects on permeability and cell size are not mediated by modulating Na+ or Cl- transport. In contrast, estrogen increased cellular G-actin levels, suggesting estrogens shift actin steady-state toward G-actin and the cervical cell cytoskeleton toward a more flexible structure. We suggest that the mechanism by which estrogens decrease RLIS and increase permeability is by fragmenting the cytoskeleton and facilitating deformability and decreases in cervical cell size.

Virus infection triggers insulin-dependent diabetes mellitus in a transgenic model: role of anti-self (virus) immune response

The characteristic glycerol dibiphytanyl glycerol tetraether membrane lipids (GDGTs) of marine ammonia-oxidizing archaea (AOA) are widely used as biomarkers for studying their occurrence and distribution in marine environments and for reconstructing past sea surface temperatures using the TEX86 index. Despite an increasing use of GDGT biomarkers in microbial ecology and paleoceanography, the physiological and environmental factors influencing lipid composition in AOA, in particular the cyclization of GDGTs, remain unconstrained. We investigated the effect of metabolic state on the composition of intact polar and core lipids and the resulting TEX86 paleothermometer in pure cultures of the marine AOA Nitrosopumilus maritimus as a function of growth phase. The cellular lipid content ranged from 0.9 to 1.9 fg cell−1 and increased during growth but was lower in the stationary phases, indicating changes in average cell size in response to metabolic status. The relative abundances of monoglycosidic GDGTs increased from 27% in early growth phase to 60% in late stationary phase, while monohydroxylated GDGTs increased only slightly. The proportions of characteristic hexose-phosphohexose GDGTs were up to 7-fold higher during growth than in stationary phase, suggesting that they are valuable biomarkers for the metabolically active fraction of AOA assemblages in the environment. Methoxy archaeol was identified as novel, genuine archaeal lipid of yet unknown function; it is one of the most abundant single compounds in the lipidome of N. maritimus. TEX86 values of individual intact GDGTs and total GDGTs differed substantially, were generally lower during early and late growth phases than in stationary phase, and did not reflect growth temperature. Consequently, our results strongly suggest that biosynthesis is at least partially responsible for the systematic offsets in TEX86 values between different intact polar GDGT classes observed previously in environmental samples. Nevertheless, differences in degradation rates of intact polar GDGTs may influence the TEX86 index because the intact polar lipid precursors differ for individual core GDGTs and moreover their relative abundances change with growth stage, which may result in distinct release rates of core GDGTs from their polar precursors. Overall, our findings stress the need to accurately describe the factors influencing GDGT cyclization in thaumarchaea and thus paleotemperature reconstructions.

Long-term protein and calorie restriction: Alterations in nucleic acid levels of organs of male rats

The effects of dietary restriction (calorie and/or protein) imposed at weaning on tissue concentrations of nucleic acids and protein were determined in young (one year) and old (two year) rats. In liver, calorie restriction increased protein concentration and decreased RNA/DNA and RNA/protein, while protein restriction elevated liver DNA and reduced protein and protein/DNA. These changes are consistent with decreased liver cell size in response to protein, but not calorie, restriction. Age also influenced these parameters, since DNA (elevated), RNA/protein (elevated), protein (reduced), and protein/DNA (reduced) differed in livers of old as compared to young rats. In kidney, DNA concentrations were unaffected by age or dietary restriction. Although protein and nucleic acid concentrations are commonly used as baselines for expressing other biochemical parameters, our results indicate that in liver, these factors vary both with age and nutritional status.

Nutrient Scarcity as an Evolutionary Cause of Haploidy

Diploidy is characteristic of complex organisms, although it also appears in numerous groups of simple organisms. Many groups of simple organisms are haploid or haplo-diploid. Diploidy is widely considered to have certain categorical advantages that account for its appearance in the more complex members of all evolutionary lines. Any categorical advantage of diploidy begs an explanation of the widespread retention of haploidy among simple organisms, however. The retention of haploidy can be explained on a nutritional basis. There is an essential connection of unknown cause between the amount of DNA and cell size; more DNA maintains more cytoplasm. Factors favoring small size will thus favor haploidy (minimal DNA) in a single-celled organism. Multicellular organisms are not similarly affected because they can reduce the number of cells, rather than reducing cell size in response to size-reducing selection pressure. The most general of size-reducing selection pressures is nutrient scarcity. For unicellular organisms, this selection pressure is probably most pronounced in taxa that obtain their energy and their nutrients by processes subject to temporal separation (autotrophs) and that live in environments where nutrients are frequently exhausted (plankton environments). In contrast, unicells that obtain energy and nutrients simultaneously (e.g., Protozoa) are not subject to the same selection pressures and are expected, therefore, to be diploid, as they are. The nutrient-sparing hypothesis is generally consistent with the sporadic distribution of haploidy among unicellular organisms, and thus appears to explain why haploidy is retained in some groups and not others.