Genetic variation among 82 pharmacogenes: The PGRNseq data from the eMERGE network

Genetic variation can affect drug response in multiple ways, although it remains unclear how rare genetic variants affect drug response. The electronic Medical Records and Genomics (eMERGE) Network, collaborating with the Pharmacogenomics Research Network, began eMERGE‐PGx, a targeted sequencing study to assess genetic variation in 82 pharmacogenes critical for implementation of “precision medicine.” The February 2015 eMERGE‐PGx data release includes sequence‐derived data from ∼5,000 clinical subjects. We present the variant frequency spectrum categorized by variant type, ancestry, and predicted function. We found 95.12% of genes have variants with a scaled Combined Annotation‐Dependent Depletion score above 20, and 96.19% of all samples had one or more Clinical Pharmacogenetics Implementation Consortium Level A actionable variants. These data highlight the distribution and scope of genetic variation in relevant pharmacogenes, identifying challenges associated with implementing clinical sequencing for drug treatment at a broader level, underscoring the importance for multifaceted research in the execution of precision medicine.

Failure of hyperglycemia and hyperinsulinemia to compensate for impaired metabolic response to an oral glucose load

OBJECTIVE

To evaluate whether the augmented insulin and glucose response to a glucose challenge is sufficient to compensate for defects in glucose utilization in obesity and type 2 diabetes, using a breath test measurement of integrated glucose metabolism.

METHODS

Non-obese, obese normoglycemic and obese type 2 diabetic subjects were studied on 2 consecutive days. A 75g oral glucose load spiked with ¹³C-glucose was administered, measuring exhaled breath ¹³CO₂ as an integrated measure of glucose metabolism and oxidation. A hyperinsulinemic euglycemic clamp was performed, measuring whole body glucose disposal rate. Body composition was measured by DEXA. Multivariable analyses were performed to evaluate the determinants of the breath ¹³CO₂.

RESULTS

Breath ¹³CO₂ was reduced in obese and type 2 diabetic subjects despite hyperglycemia and hyperinsulinemia. The primary determinants of breath response were lean mass, fat mass, fasting FFA concentrations, and OGTT glucose excursion. Multiple approaches to analysis showed that hyperglycemia and hyperinsulinemia were not sufficient to compensate for the defect in glucose metabolism in obesity and diabetes.

CONCLUSIONS

Augmented insulin and glucose responses during an OGTT are not sufficient to overcome the underlying defects in glucose metabolism in obesity and diabetes.

Design and anticipated outcomes of the eMERGE-PGx project: a multicenter pilot for preemptive pharmacogenomics in electronic health record systems

We describe here the design and initial implementation of the eMERGE-PGx project. eMERGE-PGx, a partnership of the eMERGE and PGRN consortia, has three objectives : 1) Deploy PGRNseq, a next-generation sequencing platform assessing sequence variation in 84 proposed pharmacogenes, in nearly 9,000 patients likely to be prescribed drugs of interest in a 1–3 year timeframe across several clinical sites; 2) Integrate well-established clinically-validated pharmacogenetic genotypes into the electronic health record with associated clinical decision support and assess process and clinical outcomes of implementation; and 3) Develop a repository of pharmacogenetic variants of unknown significance linked to a repository of EHR-based clinical phenotype data for ongoing pharmacogenomics discovery. We describe site-specific project implementation and anticipated products, including genetic variant and phenotype data repositories, novel variant association studies, clinical decision support modules, clinical and process outcomes, approaches to manage incidental findings, and patient and clinician education methods.

Effects of losartan on whole body, skeletal muscle and vascular insulin responses in obesity/insulin resistance without hypertension

AIMS

Renin-angiotensin system antagonists have been found to improve glucose metabolism in obese hypertensive and type 2 diabetic subjects. The mechanism of these effects is not well understood. We hypothesized that the angiotensin receptor antagonist losartan would improve insulin-mediated vasodilation, and thereby improve insulin-stimulated glucose uptake in skeletal muscle of insulin-resistant subjects.

METHODS

We studied subjects with obesity and insulin resistance but without hypertension, hypercholesterolaemia or dysglycaemia [age 39.0 ± 9.6 yr (mean ± SD), body mass index (BMI) 33.2 ± 5.9 kg/m(2) , BP 115.8 ± 12.2/70.9 ± 7.2 mmHg, LDL 2.1 ± 0.5 mmol/l]. Subjects were randomized to 12 weeks' double-blind treatment with losartan 100 mg once daily (n = 9) or matching placebo (n = 8). Before and after treatment, under hyperinsulinaemic euglycaemic clamp conditions we measured whole-body insulin-stimulated glucose disposal, insulin-mediated vasodilation, and insulin-stimulated leg glucose uptake by the limb balance technique.

RESULTS

Whole-body insulin-stimulated glucose disposal was not significantly increased by losartan. Insulin-mediated vasodilation was augmented following both treatments [increase in leg vascular conductance: pretreatment 0.7 ± 0.3 l/min/mmHg (losartan, mean ± SEM) and 0.9 ± 0.3 (placebo), posttreatment 1.0 ± 0.4 (losartan) and 1.3 ± 0.6 (placebo)] but not different between treatment groups (p = 0.53). Insulin's action to augment nitric oxide (NO) production and to augment endothelium-dependent vasodilation was also not improved. Leg glucose uptake was not significantly changed by treatments, and not different between groups (p = 0.11).

CONCLUSIONS

These findings argue against the hypothesis that losartan might improve skeletal muscle glucose metabolism by improving insulin-mediated vasodilation in normotensive insulin-resistant obese subjects. The metabolic benefits of angiotensin receptor blockers may require the presence of hypertension in addition to obesity-associated insulin resistance.

Collaborative annotation of genes and proteins between UniProtKB/Swiss-Prot and dictyBase

UniProtKB/Swiss-Prot, a curated protein database, and dictyBase, the Model Organism Database for Dictyostelium discoideum, have established a collaboration to improve data sharing. One of the major steps in this effort was the ‘Dicty annotation marathon’, a week-long exercise with 30 annotators aimed at achieving a major increase in the number of D. discoideum proteins represented in UniProtKB/Swiss-Prot. The marathon led to the annotation of over 1000 D. discoideum proteins in UniProtKB/Swiss-Prot. Concomitantly, there were a large number of updates in dictyBase concerning gene symbols, protein names and gene models. This exercise demonstrates how UniProtKB/Swiss-Prot can work in very close cooperation with model organism databases and how the annotation of proteins can be accelerated through those collaborations.

Assessing the understanding of biobank participants

Biobanks have been developed as a tool to better understand the genetic basis of disease by linking DNA samples to corresponding medical information. The broad scope of such projects presents a challenge to informed consent and participant understanding. To address this, 200 telephone interviews were conducted with participants in the NUgene Project, Northwestern University's biobank. Interviews included a modified version of the "quality of informed consent measure" (QuIC) and semi-structured questions which were analyzed thematically for 109 of the interviews. The QuIC, originally applied to cancer clinical trials, objectively assessed some of the components of informed consent for a biobank, and interview questions provided rich data to assist in interpreting participant understanding. The best understood domains included: the nature of the study, benefit to future patients, and the voluntary nature of participation. Lower knowledge scores included: potential risks and discomforts, experimental nature of the research, procedures in the event of study-related injury, and confidentiality issues. Qualitatively, confidentiality protections of the study were described as good by most (>50%). Although some cited concerns with employer (12%) or insurance discrimination (25%), most considered the risks to privacy low (25%) or none (approximately 60%). Only 10% of participants explicitly stated they had no expectation for personal benefit, and when asked whether they expected to be contacted with study results, respondents were split between having no expectation (39%), being hopeful for results (37%) and expecting to be contacted with results (12%). These findings are informative to those establishing and implementing biobanks, and to the IRBs reviewing such studies.

The genome of the social amoeba Dictyostelium discoideum

The social amoebae are exceptional in their ability to alternate between unicellular and multicellular forms. Here we describe the genome of the best-studied member of this group, Dictyostelium discoideum. The gene-dense chromosomes of this organism encode approximately 12,500 predicted proteins, a high proportion of which have long, repetitive amino acid tracts. There are many genes for polyketide synthases and ABC transporters, suggesting an extensive secondary metabolism for producing and exporting small molecules. The genome is rich in complex repeats, one class of which is clustered and may serve as centromeres. Partial copies of the extrachromosomal ribosomal DNA (rDNA) element are found at the ends of each chromosome, suggesting a novel telomere structure and the use of a common mechanism to maintain both the rDNA and chromosomal termini. A proteome-based phylogeny shows that the amoebozoa diverged from the animal-fungal lineage after the plant-animal split, but Dictyostelium seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi.

During multicellular migration, myosin ii serves a structural role independent of its motor function

We have shown previously that cells lacking myosin II are impaired in multicellular motility. We now extend these results by determining whether myosin contractile function is necessary for normal multicellular motility and shape control. Myosin from mutants lacking the essential (mlcE(-)) myosin light chain retains the ability to form bipolar filaments that bind actin, but shows no measurable in vitro or in vivo contractile function. The contractile function is necessary for cell shape control since mlcE(-) cells, like myosin heavy-chain null mutants (mhcA(-)), were defective in their ability to control their three-dimensional shape. When mixed with wild-type cells in chimeric aggregation streams, the mlcE(-) cells were able to move normally, unlike mhcA(-) cells which accumulated at the edges of the stream and became distorted by their interactions with wild-type cells. When mhcA(-) cells were mixed with mlcE(-) streams, the mhcA(-) cells were excluded. The normal behavior of the mlcE(-) cells in this assay suggests that myosin II, in the absence of motor function, is sufficient to allow movement in this constrained, multicellular environment. We hypothesize that myosin II is a major contributor to cortical integrity even in the absence of contractile function.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

Three-dimensional in vivo analysis of Dictyostelium mounds reveals directional sorting of prestalk cells and defines a role for the myosin II regulatory light chain in prestalk cell sorting and tip protrusion

During cell sorting in Dictyostelium, we observed that GFP-tagged prestalk cells (ecmAO-expressing cells) moved independently and directionally to form a cluster. This is consistent with a chemotaxis model for cell sorting (and not differential adhesion) in which a long-range signal attracts many of the prestalk cells to the site of cluster formation. Surprisingly, the ecmAO prestalk cluster that we observed was initially found at a random location within the mound of this Ax3 strain, defining an intermediate sorting stage not widely reported in Dictyostelium. The cluster then moved en masse to the top of the mound to produce the classic, apical pattern of ecmAO prestalk cells. Migration of the cluster was also directional, suggesting the presence of another long-range guidance cue. Once at the mound apex, the cluster continued moving upward leading to protrusion of the mound's tip. To investigate the role of the cluster in tip protrusion, we examined ecmAO prestalk-cell sorting in a myosin II regulatory light chain (RLC) null in which tips fail to form. In RLC-null mounds, ecmAO prestalk cells formed an initial cluster that began to move to the mound apex, but then arrested as a vertical column that extended from the mound's apex to its base. Mixing experiments with wild-type cells demonstrated that the RLC-null ecmAO prestalk-cell defect is cell autonomous. These observations define a specific mechanism for myosin's function in tip formation, namely a mechanical role in the upward movement of the ecmAO prestalk cluster. The wild-type data demonstrate that cell sorting can occur in two steps, suggesting that, in this Ax3 strain, spatially and temporally distinct cues may guide prestalk cells first to an initial cluster and then later to the tip.

Transrectal ultrasound assessment of murine aorta and iliac arteries

Recent research in arterial aneurysm formation has focused on animal model development. Mice are an ideal experimental organism due to their short life cycle, prolific progeny, and extensively studied genome. Most experiments require the sacrifice of the mice to observe and assess any morphological changes. Noninvasive or minimally invasive imaging is limited due to the relatively small size of the structures. The development of such a technique, therefore, is especially useful for allowing repeated measurement without sacrificing the mice. We introduce a novel technique of imaging and measuring the aorta, the aorta/inferior vena cava complex, and the right and the left common iliac artery/vein complex by the use of an intravascular ultrasound catheter. The catheter is inserted through the anus and rectum and into the sigmoid and left colon, where the aorta can be observed to fluctuate at approximately 500 beats/min. The aortic bifurcation can also be observed. The diameters of the aorta and the inferior vena cava were measured first with the transrectal ultrasound technique and then with direct visualization upon laparotomy for 10 mice. This revealed a percentage error between 13.7 and 14.2% for this novel technique. Fifteen more sets of vessel measurements were also made with 8 male and 7 female mice. The results demonstrated a correlation between vessel size and body weight in male but not female mice and suggested an intersex difference in vessel growth rate. We conclude that transrectal ultrasound is a useful tool in imaging and measuring the murine aorta and its bifurcation.

Interaction mapping of a dynein heavy chain. Identification of dimerization and intermediate-chain binding domains

Cytoplasmic dynein is a multisubunit microtubule-based motor protein that is involved in several eukaryotic cell motilities. Two dynein heavy chains each form a motor domain that connects to a common cargo-binding tail. Although this tail domain is composed of multiple polypeptides, subunit organization within this region is poorly understood. Here we present an in vitro dissection of the tail-forming region of the dynein heavy chain from Dictyostelium. Our work identifies a sequence important for dimerization and for binding the dynein intermediate chain. The core of this motif localizes within an approximately 150-amino acid region that is strongly conserved among other cytoplasmic dyneins. This level of conservation does not extend to the axonemal dynein heavy chains, suggesting functional differences between the two. Dimerization appears to occur through a different mechanism than the heavy chain-intermediate chain interaction. We corroborate the in vitro interactions with in vivo expression of heavy chain fragments in Dictyostelium. Fragments lacking the interaction domain express well, without an obvious phenotype. On the other hand, the region crucial for both interactions appears to be lethal when overexpressed.

Regulatory light chain mutations affect myosin motor function and kinetics

The actin-based motor protein myosin II plays a critical role in many cellular processes in both muscle and non-muscle cells. Targeted disruption of the Dictyostelium regulatory light chain (RLC) caused defects in cytokinesis and multicellular morphogenesis. In contrast, a myosin heavy chain mutant lacking the RLC binding site, and therefore bound RLC, showed normal cytokinesis and development. One interpretation of these apparently contradictory results is that the phenotypic defects in the RLC null mutant results from mislocalization of myosin caused by aggregation of RLC null myosin. To distinguish this from the alternative explanation that the RLC can directly influence myosin activity, we expressed three RLC point mutations (E12T, G18K and N94A) in a Dictyostelium RLC null mutant. The position of these mutations corresponds to the position of mutations that have been shown to result in familial hypertrophic cardiomyopathy in humans. Analysis of purified Dictyostelium myosin showed that while these mutations did not affect binding of the RLC to the MHC, its phosphorylation by myosin light chain kinase or regulation of its activity by phosphorylation, they resulted in decreased myosin function. All three mutants showed impaired cytokinesis in suspension, and one produced defective fruiting bodies with short stalks and decreased spore formation. The abnormal myosin localization seen in the RLC null mutant was restored to wild-type localization by expression of all three RLC mutants. Although two of the mutant myosins had wild-type actin-activated ATPase, they produced in vitro motility rates half that of wild type. N94A myosin showed a fivefold decrease in actin-ATPase and a similar decrease in the rate at which it moved actin in vitro. These results indicate that the RLC can play a direct role in determining the force transmission and kinetic properties of myosin.

Expression of chicken gizzard RLC complements the cytokinesis and developmental defects of Dictyostelium RLC null cells

Dictyostelium RLC null cells have defects in cytokinesis and development that can be rescued by expression of either the wild type Dictyostelium RLC or an RLC mutant that cannot be phosphorylated by MLCK (S13A) (Ostrow et al., 1994). The wild type and S13A mutant LCs rescued the cells equally well, despite the fact that RLC phosphorylation increases purified Dictyostelium myosin's activity 5-fold. In this report, we assess the ability of foreign RLCs to rescue the RLC null phenotype. The RLC from smooth muscle myosin, whose activity is tightly controlled by phosphorylation, rescued the null cell phenotype. The purified hybrid myosin had an activity and motility comparable to phosphorylated Dictyostelium myosin. In contrast, cells expressing skeletal muscle RLC were deficient in cytokinesis and development, despite having an activity and motility similar to that of myosin with the unposphorylatable S13A mutant RLC. Neither foreign LC was phosphorylated when expressed in Dictyostelium. These results suggest that the level of actin-activated ATPase activity and motility is not the sole determinant of proper myosin function in vivo. Other heavy chain/light chain interactions, which occur only with the native RLC and smooth muscle RLC, appear to be necessary for optimal function.

Regulation of cytokinesis

At the end of mitosis, daughter cells are separated from each other by cytokinesis. This process involves equal partitioning and segregation of cytoplasm between the two cells. Despite years of study, the mechanism driving cytokinesis in animal cells is not fully understood. Actin and myosin are major components of the contractile ring, the structure at the equator between the dividing cells that provides the force necessary to constrict the cytoplasm. Despite this, there are also tantalizing results suggesting that cytokinesis can occur in the absence of myosin. It is unclear what the roles are of the few other contractile ring components identified to date. While it has been difficult to identify important proteins involved in cytokinesis, it has been even more challenging to pinpoint the regulatory mechanisms that govern this vital process. Cytokinesis must be precisely controlled both spatially and temporally; potential regulators of these parameters are just beginning to be identified. This review discusses the recent progress in our understanding of cytokinesis in animal cells and the mechanisms that may regulate it.

Cell-type-specific rescue of myosin function during Dictyostelium development defines two distinct cell movements required for culmination

Mutant Dictyostelium cells lacking any of the component polypeptides of myosin II exhibit developmental defects. To define myosin's role in establishing Dictyostelium's developmental pattern, we have rescued myosin function in a myosin regulatory light chain null mutant (mlcR-) using cell-type-specific promoters. While mlcR- cells fail to progress beyond the mound stage, expression of RLC from the prestalk promoter, ecmA, produces culminants with normal stalks but with defects in spore cell localization. When GFP-marked prestalk and prespore cells expressing ecmA-RLC are mixed with wild-type cells, the mislocalization of prestalk cells, but not prespore cells, is rescued. Time-lapse video recording of ecmA-RLC cells showed that the posterior prespore zone failed to undergo a contraction important for the upward movement of prespore cells. Prespore cells marked with green fluorescent protein (GFP) failed to move toward the tip with the spiral motion typical of wild type. In contrast, expression of RLC in prespore cells using the psA promoter produced balloon-like structures reminiscent of sorocarps but lacking stalks. GFP-labeled prespore cells showed a spiral movement toward the top of the structures. Expression of RLC from the psA promoter restores the normal localization of psA-GFP cells, but not ecmA-GFP cells. These results define two distinct, myosin-dependent movements that are required for establishing a Dictyostelium fruiting body: stalk extension and active movement of the prespore zone that ensures proper placement of the spores atop the stalk. The approach used in these studies provides a direct means of testing the role of cell motility in distinct cell types during a morphogenetic program.

Substitution mutations in the myosin essential light chain lead to reduced actin-activated ATPase activity despite stoichiometric binding to the heavy chain

Myosin essential light chain (ELC) wraps around an alpha-helix that extends from the myosin head, where it is believed to play a structural support role. To identify other role(s) of the ELC in myosin function, we have used an alanine scanning mutagenesis approach to convert charged residues in loops I, II, III, and helix G of the Dictyostelium ELC into uncharged alanines. Dictyostelium was used as a host system to study the phenotypic and biochemical consequences associated with the mutations. The ELC carrying loop mutations bound with normal stoichiometry to the myosin heavy chain when expressed in ELC-minus cells. When expressed in wild type cells these mutants competed efficiently with the endogenous ELC for binding, suggesting that the affinity of their interaction with the heavy chain is comparable to that of wild type. However, despite apparently normal association of ELC the cells still exhibited a reduced efficiency to undergo cytokinesis in suspension. Myosin purified from these cells exhibited 4-5-fold reduction in actin-activated ATPase activity and a decrease in motor function as assessed by an in vitro motility assay. These results suggest that the ELC contributes to myosin's enzymatic activity in addition to providing structural support for the alpha-helical neck region of myosin heavy chain.

Both the amino and carboxyl termini of Dictyostelium myosin essential light chain are required for binding to myosin heavy chain

Dictyostelium myosin deficient in the essential light chain (ELC) does not function normally either in vivo or in vitro (Pollenz, R. S., Chen, T. L., Trivinos-Lagos, L., and Chisholm, R. L. (1992) Cell 69, 951-962). Since normal myosin function requires association of ELC, we investigated the domains of ELC that are necessary for binding to the myosin heavy chain (MHC). Deleting the NH2-terminal 11 or 28 amino acid residues (delta N11 or delta N28) or the COOH-terminal 15 amino acid residues (delta C15) abolished binding of the ELC to the MHC when the mutants were expressed in wild-type (WT) cells. In contrast, the ELC carrying deletion or insertion of four amino acid residues (D4 or I4) in the central linker segment bound the MHC in WT cells, although less efficient competition with WT ELC suggested that the affinity for the MHC is reduced. When these mutants were expressed in ELC-minus (mlcE-) cells, where the binding to the heavy chain is not dependent on efficient competition with the endogenous ELC, delta N28 and delta N11 bound to the MHC at 15% of WT levels and delta C15 did not bind to a significant degree. I4 and D4, however, bound with normal stoichiometry. These data indicate that residues at both termini of the ELC are required for association with the MHC, while the central linker domain appears to be less critical for binding. When the mutants were analyzed for their ability to complement the cytokinesis defect displayed by mlcE- cells, a correlation to the level of ELC carried by the MHC was observed, indicating that a stoichiometric ELC-MHC association is necessary for normal myosin function in vivo.

Targeted disruption of the Dictyostelium myosin essential light chain gene produces cells defective in cytokinesis and morphogenesis

We have previously demonstrated that the myosin essential light chain (ELC) is required for myosin function in a Dictyostelium cell line, 7–11, in which the expression of ELC was inhibited by antisense RNA overexpression. We have now disrupted the gene encoding the ELC (mlcE) in Dictyostelium by gene targeting. The mlcE- mutants provide a clean genetic background for phenotypic analysis and biochemical characterization by removing complications arising from the residual ELC present in 7–11 cells, as well as the possibility of mutations due to insertion of the antisense construct at multiple sites in the genome. The mlcE- mutants, when grown in suspension, exhibited the typical multinucleate phenotype observed in both myosin heavy chain mutants and 7–11 cells. This phenotype was rescued by introducing a construct that expressed the wild-type Dictyostelium ELC cDNA. Myosin purified from the mlcE- cells exhibited significant calcium ATPase activity, but the actin-activated ATPase activity was greatly reduced. The results obtained from the mlcE- mutants strengthen our previous conclusion based on the antisense cell line 7–11 that ELC is critical for myosin function. The proper localization of myosin in mlcE- cells suggests that its phenotypic defects primarily arise from defective contractile function of myosin rather than its mislocalization. The enzymatic defect of myosin in mlcE- cells also suggests a possible mechanism for the observed chemotactic defect of mlcE- cells. We have shown that while mlcE- cells were able to respond to chemoattractant with proper directionality, their rate of movement was reduced. During chemotaxis, proper directionality toward chemoattractant may depend primarily on proper localization of myosin, while efficient motility requires contractile function. In addition, we have analyzed the morphogenetic events during the development of mlcE- cells using lacZ reporter constructs expressed from cell type specific promoters. By analyzing the morphogenetic patterns of the two major cell types arising during Dictyostelium development, prespore and prestalk cells, we have shown that the localization of prespore cells is more susceptible to the loss of ELC than prestalk cells, although localization of both cell types is abnormal when developed in chimeras formed by mixing equal numbers of wild-type and mutant cells. These results suggest that the morphogenetic events during Dictyostelium development have different requirements for myosin.

Targeted disruption of the Dictyostelium RMLC gene produces cells defective in cytokinesis and development

Conventional myosin has two different light chains bound to the neck region of the molecule. It has been suggested that the light chains contribute to myosin function by providing structural support to the neck region, therefore amplifying the conformational changes in the head following ATP hydrolysis (Rayment et al., 1993). The regulatory light chain is also believed to be important in regulating the actin-activated ATPase and myosin motor function as assayed by an in vitro motility assay (Griffith et al., 1987). Despite extensive in vitro biochemical study, little is known regarding RMLC function and its regulatory role in vivo. To better understand the importance and contribution of RMLC in vivo, we engineered Dictyostelium cell lines with a disrupted RMLC gene. Homologous recombination between the introduced gene disruption vector and the chromosomal RMLC locus (mlcR) resulted in disruption of the RMLC-coding region, leading to cells devoid of both the RMLC transcript and the 18-kD RMLC polypeptide. RMLC-deficient cells failed to divide in suspension, becoming large and multinucleate, and could not complete development following starvation. These results, similar to those from myosin heavy chain mutants (DeLozanne et al., 1987; Manstein et al., 1989), suggest the RMLC subunit is required for normal cytokinesis and cell motility. In contrast to the myosin heavy chain mutants, however, the mlcR cells are able to cap cell surface receptors following concanavilin A treatment. By immunofluorescence microscopy, RMLC null cells exhibited myosin localization patterns different from that of wild-type cells. The myosin localization in RMLC null cells also varied depending upon whether the cells were cultured in suspension or on a solid substrate. In vitro, purified RMLC- myosin assembled to form thick filaments comparable to wild-type myosin, but the filaments then exhibit abnormal disassembly properties. These results indicate that in vivo RMLC is necessary for myosin function.

Expression of a myosin regulatory light chain phosphorylation site mutant complements the cytokinesis and developmental defects of Dictyostelium RMLC null cells

In a number of systems phosphorylation of the regulatory light chain (RMLC) of myosin regulates the activity of myosin. In smooth muscle and vertebrate nonmuscle systems RMLC phosphorylation is required for contractile activity. In Dictyostelium discoideum phosphorylation of the RMLC regulates both ATPase activity and motor function. We have determined the site of phosphorylation on the Dictyostelium RMLC and used site-directed mutagenesis to replace the phosphorylated serine with an alanine. The mutant light chain was then expressed in RMLC null Dictyostelium cells (mLCR-) from an actin promoter on an integrating vector. The mutant RMLC was expressed at high levels and associated with the myosin heavy chain. RMLC bearing a ser13ala substitution was not phosphorylated in vitro by purified myosin light chain kinase, nor could phosphate be detected on the mutant RMLC in vivo. The mutant myosin had reduced actin-activated ATPase activity, comparable to fully dephosphorylated myosin. Unexpectedly, expression of the mutant RMLC rescued the primary phenotypic defects of the mlcR- cells to the same extent as did expression of wild-type RMLC. These results suggest that while phosphorylation of the Dictyostelium RMLC appears to be tightly regulated in vivo, it is not essential for myosin-dependent cellular functions.

The highly divergent alpha- and beta-tubulins from Dictyostelium discoideum are encoded by single genes

As a step in the characterization of the microtubule system of Dictyostelium discoideum, we have isolated and sequenced full-length cDNA clones that encode the Dictyostelium alpha- and beta-tubulins, as well as the Dictyostelium alpha-tubulin gene. Southern blot analysis suggests that Dictyostelium is unusual in that its genome contains single alpha- and beta-tubulin genes, rather than the multi-gene family common in most eukaryotic organisms. The complete alpha-tubulin cDNA contains 1558 nucleotides, with an open reading frame, that encode a protein of 457 amino acids. The complete beta-tubulin cDNA contains 1572 nucleotides and encodes a protein of 456 amino acids. Analysis of the deduced protein sequences indicates that while there is a significant degree of sequence similarity between the Dictyostelium tubulins and other known tubulins, the Dictyostelium alpha-tubulin displays the greatest sequence divergence yet described. Single alpha- and beta-tubulin transcripts are detected by northern blot analysis during all stages of Dictyostelium development. The highest levels of message accumulate late in germinating spores and vegetative amoebae. Despite changes in alpha- and beta-tubulin mRNA levels, protein levels remain constant throughout development. We have expressed the carboxy-terminal two-thirds of the alpha- and beta-tubulins as trpE fusions in Escherichia coli and used this protein to produce polyclonal antisera specific for the Dictyostelium alpha- and beta-tubulins. These antisera recognize one alpha- and two beta-tubulin spots on western blots of 2-D gels and, by indirect immunofluorescence, both recognize the interphase and mitotic microtubule arrays in vegetative amoebae.

Collagen and elastin gene expression in aortic aneurysms

BACKGROUND

The decreased elastin concentration found in abdominal aortic aneurysms (AAAs) may result from a differential synthetic response wherein elastin gene expression fails to increase in parallel with type I procollagen (COL I) gene expression. The purpose of this study is to determine tissue mRNA levels for elastin and COL I in AAAs compared with levels in normal, age-matched aorta and to determine the relationship between aging and COL I gene expression.

METHODS

Total RNA exacted from normal infrarenal aortic tissue (n = 7) and AAA (n = 10) tissue was subjected to Northern analysis. Mean values for COL I, elastin, and alpha-tubulin mRNA levels were compared by use of the Student t test. Age and COL I mRNA levels were analyzed by regression analysis.

RESULTS

COL I mRNA was increased significantly in AAAs (1.18 +/- 0.13) compared with normal aortas (0.14 +/- 0.05). A commensurate increase in elastin mRNA (AAAs, 0.11 +/- 0.02, vs normal aortas, 0.39 +/- 0.2) was absent. There was no correlation between age and COL gene expression.

CONCLUSIONS

The decreased elastin concentration relative to collagen in AAAs may be explained, in part, by the changes in message level of elastin and collagen. The enhanced COL I gene expression in AAAs is unrelated to age.

The Dictyostelium essential light chain is required for myosin function

A Dictyostelium mutant (7-11) that expresses less than 0.5% of wild-type levels of the myosin essential light chain (EMLC) has been created by overexpression of antisense RNA. Cells from 7-11 contain wild-type levels of the myosin heavy chain (MHC) and regulatory light chain (RMLC). Myosin isolated from 7-11 cells consists of the MHC with the RMLC associated in reduced stoichiometry, and binds to purified actin in an ATP-sensitive fashion. Purified 7-11 myosin displays calcium-activated ATPase activity with a Vmax about 15%-25% of that of wild type, and a Km for ATP of 27 +/- 5 microM versus 83 +/- 30 microM for wild type. At actin concentrations as high as 17 microM, 7-11 myosin displays greatly reduced actin-activated ATPase activity. Phenotypically, 7-11 cells resemble MHC mutants, growing poorly in suspension and becoming large and multinucleate. When starved for multicellular development, 7-11 cells take several hours longer than wild-type cells to aggregate. Although multicellular aggregates eventually form, they fail to develop further. The cells are also unable to cap receptors in response to Con A treatment. Since cells expressing the EMLC are phenotypically similar to MHC null mutants, the EMLC appears necessary for myosin function, at least in part because it is required for normal actin-activated ATPase activity.

The role of myosin I and II in cell motility

It has been recognized since the turn of the century that cell motility by non-muscle cells requires virtually continuous restructuring of the cytoskeleton (see refs [1-4]). It is also clear that cell motility requires a mechanism for converting chemical energy into mechanical work. The proteins actin and myosin, two important constituents of the cytoskeleton, have been postulated to act as the chemicomechanical transducer in motile cells. Central to their role as a force generating mechanism in motile cells is the ability of myosin (a) to hydrolyze ATP when it interacts with actin and (b) to form filaments. Recent studies on mammalian cells and on the cellular slime mold Dictyostelium discoideum have shed light and at the same time raised questions regarding the involvement of myosin in cell motility. Moreover, they have demonstrated the presence of two types of myosins, called myosin II and myosin I, that have unique biochemical and regulatory properties and that may play different roles in mediating cell motility. In this chapter we will discuss the properties of these two myosins and then describe what is known about their involvement in Dictyostelium and mammalian cell motility.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

A pleiotropic defect in cAMP-regulated gene expression in the Dictyostelium agg- mutant synag 7

During differentiation of Dictyostelium discoideum, cAMP functions as a diffusible, extracellular signal to direct chemotaxis and regulate developmental gene expression. The availability of signal-transduction mutants of Dictyostelium now makes it feasible to pursue a genetic analysis of cAMP signaling. The synag 7 mutant is defective in receptor-mediated adenylate cyclase stimulation and cannot relay a cAMP signal. To further characterize this mutant, mRNA levels of several cAMP-regulated genes were measured during development. cAMP-regulated gene expression was found to be dramatically altered in synag 7:several different genes which require cAMP for expression in wild-type cells were induced in synag 7 in the absence of cAMP. In addition, the gene-encoding discoidin I, which is normally expressed in starved cells and repressed by cAMP, is expressed at very low levels in starved synag 7 cells, possibly due to precocious repression. These results suggest that a pleiotropic regulator of cAMP-regulated gene expression is uncoupled from its normal controls during development in synag 7.

Expression and organization of BP74, a cyclic AMP-regulated gene expressed during Dictyostelium discoideum development

We have characterized a cDNA and the corresponding gene for a cyclic AMP-inducible gene expressed during Dictyostelium development. This gene, BP74, was found to be first expressed about the time of aggregate formation, approximately 6 h after starvation. Accumulation of BP74 mRNA did not occur in Dictyostelium cells that had been starved in fast-shaken suspension cultures but was induced in similar cultures to which cyclic AMP pulses had been added. The BP74 cDNA and gene were characterized by DNA sequence analysis and transcriptional mapping. When the BP74 promoter region was fused with a chloramphenicol acetyltransferase reporter gene and reintroduced into Dictyostelium cells, the transfected chloramphenicol acetyltransferase gene displayed the same developmentally regulated pattern of expression as did the endogenous BP74 gene, suggesting that all of the cis-acting elements required for regulated expression were carried by a 2-kilobase cloned genomic fragment. On the basis of sequence analysis, the gene appeared to encode a protein containing a 20-residue hydrophobic sequence at the amino-terminal end and 26 copies of a 20-amino-acid repeat.

Dictyostelium discoideum myosin: isolation and characterization of cDNAs encoding the regulatory light chain

Phosphorylation of the regulatory light chains (RMLC) of nonmuscle myosin can increase the actin-activated ATPase activity and filament formation. Little is known about these regulatory mechanisms and how the RMLC are involved in ATP hydrolysis. To better characterize the nonmuscle RMLC, we isolated cDNAs encoding the Dictyostelium RMLC. Using an antibody specific for the RMLC, we screened a lambda gt11 expression library and obtained a 200-base-pair clone that encoded a portion of the RMLC. The remainder of the sequence was obtained from two clones identified by DNA hybridization, using the 200-base-pair cDNA. The composite RMLC cDNA was 645 nucleotides long. It contained 60 base pairs of 5' untranslated, 483 bases of coding, and 102 base pairs of 3' untranslated sequence. The amino acid sequence predicted an 18,300-dalton protein that shares 42% amino acid identity with Dictyostelium calmodulin and 30% identity with the chicken skeletal myosin RMLC. This sequence contained three regions that were similar to the E-F hand calcium-binding domains found in calmodulin, troponin C, and other myosin light chains. A sequence similar to the phosphorylation sequence found in chicken gizzard and skeletal myosin light chains was found at the amino terminus. Genomic Southern blot analysis suggested that the Dictyostelium genome contains a single gene encoding the RMLC. Analysis of RMLC expression patterns during Dictyostelium development indicated that accumulation of this mRNA increases just before aggregation and again during culmination. This pattern is similar to that obtained for the Dictyostelium essential myosin light chain and suggests that expression of the two light chains is coordinated during development.

A type III intermediate filament gene is expressed in mature neurons

A cDNA (199E) specific for the 57 kd neural IF protein has been isolated from a PC12 cell lambda gt11 library. Antibody eluted from the fusion protein produced by 199E recognizes the 57 kd protein on immunoblots and, in PC12 cells, labels a pattern of fibrillar structures identical to that seen with 57 kd antiserum. In situ hybridization using antisense RNA transcripts labels areas of the nervous system known to contain the 57 kd protein. 199E hybridizes with a single mRNA species of approximately 2.0 kb from PC12 cells. A 199E-reactive message can be detected as early as E10 in rat embryos. Southern analyses suggest that there is only one gene for this protein. Amino acid sequence predicted from 199E indicates that the 57 kd protein is a type III IF protein like vimentin and desmin. Thus, expression of IF structural genes in neurons is not limited to the type IV neuronal IF triplet proteins.

Translocation and rearrangement of myeloperoxidase gene in acute promyelocytic leukemia

Acute promyelocytic leukemia (subtype M3) is characterized by malignant promyelocytes exhibiting an abundance of abnormally large or aberrant primary granules. Myeloperoxidase (MPO) activity of these azurophilic granules, as assessed by cytochemical staining, is unusually intense. In addition, M3 is universally associated with a chromosomal translocation, t(15;17)(q22;q11.2). In this report, the MPO gene was localized to human chromosome 17 (q12-q21), the region of the breakpoint on chromosome 17 in the t(15;17), by somatic cell hybrid analysis and in situ chromosomal hybridization. By means of MPO complementary DNA clones for in situ hybridization and Southern blot analysis, the effect of this specific translocation on the MPO gene was examined. In all cases of M3 examined, MPO is translocated to chromosome 15. Genomic blot analyses indicate rearrangement of MPO in leukemia cells of two of four cases examined. These findings suggest that MPO may be pivotal in the pathogenesis of acute promyelocytic leukemia.

Isolation of cDNAs encoding desmosomal plaque proteins: evidence that bovine desmoplakins I and II are derived from two mRNAs and a single gene

Desmoplakins (DPs) I and II (approximately equal to 240 and approximately equal to 210 kDa) are major components of the internal portion of the desmosomal cytoplasmic plaque. Desmosomes play a crucial role in cell-cell adhesion and serve as specific attachment sites for cytoplasmic intermediate filaments. Although DP-I and -II are closely related molecules, their structure (i.e., amino acid or DNA sequence) has not been determined. In addition, it is not known whether these proteins are derived from one or more genes or whether they result from posttranscriptional or posttranslational events. This paper describes the isolation and characterization of eight DP cDNA clones from a bovine lambda gt11 expression library. Fusion proteins from six of these clones selected antibodies that reacted with DP-I and -II and two selected antibodies that reacted with DP-I alone. Antibodies made against fusion protein produced by the DP1A clone reacted specifically with DP-I and -II on immunoblots. When used for indirect immunofluorescence on bovine tongue cryostat sections and cultured mouse keratinocytes, these antibodies produced a typical desmosomal staining pattern. RNA blot analysis demonstrated hybridization of three DP-I/II cDNA probes with two messages of approximately equal to 7.5 and approximately equal to 9.5 kilobases in bovine tongue RNA. In contrast, a cDNA clone that affinity-purified antibodies reacting with DP-I only hybridized exclusively with the 9.5-kilobase band. Southern blots of genomic DNA digested with a panel of restriction enzymes were hybridized with one probe derived from a DP-I/II clone and with one from a DP-I clone. Both probes hybridized with single bands of the same size in each digested sample of DNA. Together, these data suggest that DP-I and DP-II are translated from two separate messages in bovine tongue and that these messages may be derived from a single gene.

Dictyostelium discoideum myosin: isolation and characterization of cDNAs encoding the essential light chain

We used an antibody specific for Dictyostelium discoideum myosin to screen a lambda gt11 cDNA expression library to obtain cDNA clones which encode the Dictyostelium essential myosin light chain (EMLC). The amino acid sequence predicted from the sequence of the cDNA clone showed 31.5% identity with the amino acid sequence of the chicken EMLC. Comparisons of the Dictyostelium EMLC, a nonmuscle cell type, with EMLC sequences from similar MLCs of skeletal- and smooth-muscle origin, showed distinct regions of homology. Much of the observed homology was localized to regions corresponding to consensus Ca2+-binding of E-F hand domains. Southern blot analysis suggested that the Dictyostelium genome contains a single gene encoding the EMLC. Examination of the pattern of EMLC mRNA expression showed that a significant increase in EMLC message levels occurred during the first few hours of development, coinciding with increased actin expression and immediately preceding the period of maximal chemotactic activity.

The effect of caffeine, adenosine, and buffer ionic composition on the induction of cell-surface cAMP binding during starvation of Dictyostelium discoideum

We have analyzed the effects of the cAMP relay inhibitor, caffeine, and the receptor antagonist, adenosine, on the regulation of the cell-surface cAMP receptor in suspension-starved Dictyostelium discoideum cells by measuring ammonium sulfate-stabilized binding of [3-H]cAMP to intact cells. When cells were starved in fast (230 r.p.m.) shaken suspension in 10 mM Na+/5 mM K+ phosphate buffer, pH 6.5, plus 1 mM CaCl2 and 2.5 mM MgCl2, and assayed for specific cAMP binding, receptor accumulation peaked at approximately 6 hours, reaching a maximum of 1.5 pmol cAMP bound/10(7) cells (saturation binding). Neither caffeine nor adenosine inhibited the accumulation of cAMP receptors. Similar results were obtained in caffeine-treated, slow shaken (90 r.p.m.) suspension cultures. These results suggest that starvation alone is sufficient stimulus to induce the cAMP receptor. We have also tested the effects of different buffer ionic compositions on the accumulation of cAMP receptors. Elevation of the monovalent ion concentration to 30-40 mM was found to significantly inhibit the induction of cAMP receptors.

cDNA cloning of human myeloperoxidase: decrease in myeloperoxidase mRNA upon induction of HL-60 cells

Myeloperoxidase (MPO), the most abundant neutrophil protein, is a bacteriocidal component of the primary granules and a critical marker in distinguishing acute myelogenous leukemia from acute lymphoid leukemia. A cDNA clone for human MPO was isolated by immunologic screening of human hematopoietic lambda gt11 expression vector libraries with specific anti-MPO antibody. The identity of the cDNA clone was confirmed by finding that epitope-selected antibody against this clone recognizes purified MPO and MPO in human promyelocytic (HL-60) cell lysates by immunoblot analysis, and that hybrid selection of HL-60 mRNA with this cDNA clone and translation in vitro results in the synthesis of an 80-kDa protein recognized by the anti-MPO antiserum. RNA blot analysis with this MPO cDNA clone detects hybridization to two polyadenylylated transcripts of approximately 3.6 and approximately 2.9 kilobases in HL-60 cells. No hybridization is detected to human placenta mRNA. Upon induction of HL-60 cells to differentiate by incubation for 4 days with dimethyl sulfoxide, a drastic decrease in the hybridization intensity of these two bands is seen. This is consistent with previous data suggesting a decrease in MPO synthesis upon such induction of these cells. The MPO cDNA should be useful for further molecular and genetic characterization of MPO and its expression and biosynthesis in normal and leukemic granulocytic differentiation.

Superinduction of the Dictyostelium discoideum cell surface cAMP receptor by pulses of cAMP

Extracellular cAMP plays a crucial role in regulating the developmental program of Dictyostelium discoideum, functioning as a chemotactic agent, as well as a signal that regulates expression of developmentally expressed genes. These activities appear to be mediated by a cell-surface receptor for cAMP. We have studied the regulation of this receptor in cells developed in starved suspension cultures exposed to 50 nM pulses of cAMP every 6 min. cAMP-pulsed cells display roughly 10-fold higher cAMP receptor levels than cells that developed on filters or that were starved in suspension without cAMP pulses. Based on saturation binding analysis, the superinduced binding activity represents an increase in receptor number, while receptor affinity for cAMP is unaffected. Photoaffinity labeling of superinduced cells results in specific labeling of the same molecules that are labeled in starved cells. This increased cAMP binding activity was also detected in membrane preparations from cAMP-pulsed cells. These results provide evidence for an unusual mode of receptor regulation: autogenous induction of the receptor by its ligand.

The two alpha-tubulin genes of Chlamydomonas reinhardi code for slightly different proteins

Full-length cDNA clones corresponding to the transcripts of the two alpha-tubulin genes in Chlamydomonas reinhardi were isolated. DNA sequence analysis of the cDNA clones and cloned gene fragments showed that each gene contains 1,356 base pairs of coding sequence, predicting alpha-tubulin products of 451 amino acids. Of the 27 nucleotide differences between the two genes, only two result in predicted amino acid differences between the two gene products. In the more divergent alpha 2 gene, a leucine replaces an arginine at amino acid 308, and a valine replaces a glycine at amino acid 366. The results predicted that two alpha-tubulin proteins with different net charges are produced as primary gene products. The predicted amino acid sequences are 86 and 70% homologous with alpha-tubulins from rat brain and Schizosaccharomyces pombe, respectively. Each gene had two intervening sequences, located at identical positions. Portions of an intervening sequence highly conserved between the two beta-tubulin genes are also found in the second intervening sequence of each of the alpha genes. These results, together with our earlier report of the beta-tubulin sequences in C. reinhardi, present a picture of the total complement of genetic information for tubulin in this organism.

Mechanism of sequential induction of cell-type specific mRNAs in Dictyostelium differentiation

Upon starvation, the cellular slime mould Dictyostelium discoideum initiates a 24-h programme of differentiation. Within 6 h, cells move towards aggregation centres in response to pulsatile synthesis and secretion of cyclic AMP. At about 12 h, aggregates of 10(5) cells are formed, held together by newly made surface adhesion molecules. The cells then differentiate into the two principal types found in the terminal stage of development, spores and stalks. Here we show that the chemotaxis and aggregation stages of this developmental programme can be described as a series of sequential events in which these extracellular signals--starvation, cyclic AMP and cell-cell contact--induce specific, sequential changes in the pattern of gene expression.

A repetitive Dictyostelium gene family that is induced during differentiation and by heat shock

Clone pB41-6 (2.5 kb) contains sequences that are repeated 200-300 times in the Dictyostelium genome; about 40 of these sequences are part of a 4.5 kb repeated and apparently transposable genomic element. Clone pB41-6 hybridizes to a large number of cytoplasmic polyadenylated RNAs whose accumulation begins in the first hour of differentiation. In order to understand the regulation of these repeated sequences, we have sequenced pB41-6. It contains three long open reading frames in the "sense" strand. Remarkably, about 70 bases upstream of the transcription initiation site is a sequence identical to that responsible for induction of the Drosophila heat shock genes. A search of published sequences also generated a similar sequence upstream of one of the Dictyostelium actin genes. Indeed, we found that both pB41-6-related RNAs and actin mRNAs are increased as a result of heat shocking growing cells, and that transcription of pB41-6 sequences is induced by heat shock. Thus Dictyostelium contains a set of genes that are induced as a response to heat shock or to the stresses that trigger the initiation of development. We show here that the principal component of this "stress" is not amino acid starvation but the high density of the cells.

Different mRNAs have different nuclear transit times in Dictyostelium discoideum aggregates

Nuclear processing of mRNA precursors in differentiating multicellular Dictyostelium discoideum aggregates is markedly slower than in growing amoebae. Thus, we have been able to determine the time of nuclear processing of individual mRNA species in postaggregating cells by following the incorporation of 32PO4 into nuclear and cytoplasmic RNA complementary to cloned cDNAs. Precursors of mRNAs synthesized during both growth and differentiation remain in the nucleus for about 25 to 60 min. By contrast, typical mRNAs which are synthesized only by postaggregative cells have nuclear processing times between 50 and 100 min. Depending on the particular mRNA, between 20 and 60% of nuclear transcripts are converted into cytoplasmic mRNA. A third class of mRNAs are transcribed from a set of repetitive DNA segments and are expressed predominantly during differentiation. Nuclear precursors of these mRNAs are extensively degraded within the nucleus or very rapidly after transport to the cytoplasm. Those sequences that are stable in the cytoplasm exit from the nucleus only after a lag of over 2 h. Thus, mRNAs encoded by different genes that are subject to different types of developmental controls display different times of transit to the cytoplasm and different efficiencies of nuclear processing. Differential nuclear processing may contribute to the regulation of the level of individual cytoplasmic mRNAs.

The significance of varying SRBC/lymphocyte ratio in T cell rosette formation

The incubation ratio of sheep red blood cells (SRBC) to lymphocytes is a critical factor in rosette formation, whereas the length of time SRBC and lymphocytes are incubated together does not significantly affect the percentage of lymphocytes forming rosettes. The graph obtained by plotting percentage of rosette formation against the ratio of SRBC to lymphocytes is similar to that resulting from the formation of bimolecular complexes. If rosette formation is analogous to formation of bimolecular complexes, maximal rosette formation occurs when the system is saturated, i.e., with excess SRBC, and is a measure of the total capacity of a lymphocyte population to form rosettes. In addition, the percentage of rosette formation observed at a limiting SRBC/lymphocyte ratio gives an indication of the avidity of the lymphocytes for SRBC. This interpretation may provide an explanation for the difference between the "active" and "total" rosettes. When the log of the SRBC/lymphocyte ratio is plotted against percentage of rosette formation, a straight line is obtained, suggesting that within a given normal lymphocyte sample, T cell subsets with different avidities are not detected by rosette formation at different SRBC/lymphocyte ratios.