Reproductive response of the copepods Calanoides carinatus and Calanus agulhensis to varying periods of starvation in the southern Benguela upwelling region

Egg production by the calanoid copepods Calanoides carinatusand Calanus agulhensis fed excess Thalassiosira weissflogiiwas monitored in the laboratory following starvation periodsof 1, 3, 5, 7 and 9 days. Following short (1–3 day) periodsof starvation, egg production by C.agulhensis returned to thesatiated rate (51.1 eggs {female} day^–1) more rapidly(after 0.9–2.4 days of excess food) than that of Ca. carinatus(after 2.8–3.1 days). However, following longer (5–9day) periods of starvation, Ca. carinatus regained satiatedlevels of egg production (55.8 eggs {female}^–1 day^–1)more rapidly (after 3.1–4.0 days of excess food) thanC. agulhensis (after 3.8–5.2 days of feeding following5–7 days of starvation). Moreover, many C. agulhensisfemales did not regain normal rates of egg production after9 days of starvation. For both species, the time required foregg production to recover was proportional to the starvationperiod, although only up to 7 days for C. agulhensis, and wasthe same following 4.25 days of starvation. Previously fed Ca.carinatus terminated egg production more rapidly than C. agulhensiswhen starved. The ability of Ca. carinatus to tolerate, andrecover rapidly from, prolonged periods of starvation, combinedwith a comparatively fast development time and high rate ofegg production, provides this species with a strong competitiveadvantage over C. agulhensis in the highly pulsed food environmentof the southern Benguela upwelling region.     

Thymocytes between the beta-selection and positive selection checkpoints are nonresponsive to IL-7 as assessed by STAT-5 phosphorylation

Interleukin-7 is widely accepted as a major homeostatic factor involved in T cell development. To assess the IL-7 responsiveness of thymocytes involved in selection processes, we used a new sensitive flow cytometry-based assay to detect intracellular phosphorylation of STAT-5 induced by IL-7 in defined mouse thymocyte subsets. Using this method, we found the earliest thymocyte subset (CD4(-)CD8(-)CD25(-)CD44(+)) to contain both IL-7-responsive and nonresponsive cells. Transition through the next stages of development (CD4(-)CD8(-)CD25(+)CD44(+ and -)) was associated with responsiveness of all thymocytes within these populations. Passage of thymocytes through beta-selection resulted in a significant reduction in IL-7 sensitivity. In the next phases of development (TCR(-) and TCR(low)CD69(-)), thymocytes were completely insensitive to the effects of IL-7. STAT-5 phosphorylation in response to IL-7 was again observed, however, in thymocytes involved in the positive selection process (TCR(low)CD69(+) and TCR(intermediate)). As expected, CD4 and CD8 single-positive thymocytes were responsive to IL-7. These findings delineate an IL-7-insensitive population between the beta-selection and positive selection checkpoints encompassing thymocytes predicted to die by neglect due to failure of positive selection. This pattern of sensitivity suggests a two-signal mechanism by which survival of thymocytes at these checkpoints is governed.     

Diego interacts with Prickle and Strabismus/Van Gogh to localize planar cell polarity complexes

Planar cell polarity (PCP) in the Drosophila eye is established by the distinct fate specifications of photoreceptors R3 and R4, and is regulated by the Frizzled (Fz)/PCP signaling pathway. Before the PCP proteins become asymmetrically localized to opposite poles of the cell in response to Fz/PCP signaling, they are uniformly apically colocalized. Little is known about how the apical localization is maintained. We provide evidence that the PCP protein Diego (Dgo) promotes the maintenance of apical localization of Flamingo (Fmi), an atypical Cadherin-family member, which itself is required for the apical localization of the other PCP factors. This function of Dgo is redundant with Prickle (Pk) and Strabismus (Stbm), and only appreciable in double mutant tissue. We show that the initial membrane association of Dgo depends on Fz, and that Dgo physically interacts with Stbm and Pk through its Ankyrin repeats, providing evidence for a PCP multiprotein complex. These interactions suggest a positive feedback loop initiated by Fz that results in the apical maintenance of other PCP factors through Fmi.     

A search for amino acid substitutions that universally activate response regulators

Two-component regulatory systems, typically composed of a sensor kinase to detect a stimulus and a response regulator to execute a response, are widely used by microorganisms for signal transduction. Response regulators exhibit a high degree of structural similarity and undergo analogous activating conformational changes upon phosphorylation. The activity of particular response regulators can be increased by specific amino acid substitutions, which either prolong the lifetime or mimic key features of the phosphorylated state. We probed the universality of response regulator activation by amino acid substitution. Thirty-six mutations that activate 11 different response regulators were identified from the literature. To determine whether the activated phenotypes would be retained in the context of a different response regulator, we recreated 51 analogous amino acid substitutions at corresponding positions of CheY. About 55% of the tested substitutions completely or partially inactivated CheY, approximately 30% were phenotypically silent, and approximately 15% activated CheY. Three previously uncharacterized activated CheY mutants were found. The 94NS (and presumably 94NT) substitutions resulted in resistance to CheZ-mediated dephosphorylation. The 113AP substitution led to enhanced autophosphorylation and may increase the fraction of non-phosphorylated CheY molecules that populate the activated conformation. The locations of activating substitutions on the response regulator three-dimensional structure are generally consistent with current understanding of the activation mechanism. The best candidates for potentially universal activating substitutions of response regulators identified in this study were 13DK and 113AP.     

Quantitative trait loci for steady-state platelet count in mice

Platelet count in humans is a strongly genetically regulated trait, with approximately 85% of the interindividual variance in platelet numbers attributable to genetic factors. Inbred mouse strains also have strain-specific platelet count ranges. As part of a project to identify novel factors that regulate platelet count, we identified two inbred mouse strains, CBA/CaH and QSi5, with substantial differences in platelet count (mean values of 581 vs. 1062 × 10⁹/L). An F₂ intercross resource of 1126 animals was bred from these two parental strains for a genomewide scan for quantitative trait loci (QTL) for platelet count. QTL were identified on MMU1 (LOD 6.8, p < 0.0005) and MMU11 (LOD 11.2, p < 0.0005) by selectively genotyping animals from the extremes of the F₂ platelet count distribution. Three other QTL of suggestive statistical significance were also detected on MMU7, 13, and 17. It is noteworthy that no QTL were detected in the vicinity of the genes encoding thrombopoietin (Thpo), and its receptor (c-Mpl), both known to influence platelet production. Comparison of gene expression levels between the parental mouse strains by microarrays also showed little difference in the mRNA levels of these known candidate genes. These results represent the first published use of a genetic linkage-based approach in a mouse model toward the identification of genetic factors that regulate platelet count.