The nucellus: between cell elimination and sugar transport

The architecture of the seed is shaped by the processes of tissue partitioning, which determines the volume ratio of maternal and zygotic tissues, and nutrient partitioning, which regulates nutrient distribution among tissues. In angiosperms, early seed development is characterized by antagonistic development of the nucellus maternal tissue and the endosperm fertilization product to become the main sugar sink. This process marked the evolution of angiosperms and outlines the most ancient seed architectures. In Arabidopsis, the endosperm partially eliminates the nucellus and imports sugars from the seed coat. Here, we show that the nucellus is symplasmically connected to the chalaza, the seed nutrient unloading zone, and works as both a sugar sink and source alongside the seed coat. After fertilization, the transient nucellus accumulates starch early on and releases it in the apoplasmic space during its elimination. By contrast, the persistent nucellus exports sugars toward the endosperm through the SWEET4 hexose facilitator. Finally, we analyzed sugar metabolism and transport in the transparent testa 16 mutant, which fails to undergo nucellus cell elimination, which shed light on the coordination between tissue and nutrient partitioning. Overall, this study identifies a path of sugar transport in the Arabidopsis seed and describes a link between sugar redistribution and the nucellus cell-elimination program.

A path of sugar transport through the nucellus maternal tissue of the Arabidopsis seed is coordinated with the process of nucellus cell elimination.     

Towards precision medicine in diabetes? A critical review of glucotypes

In response to a study previously published in PLOS Biology, this Formal Comment thoroughly examines the concept of ’glucotypes’ with regard to its generalisability, interpretability and relationship to more traditional measures used to describe data from continuous glucose monitoring.

Although the promise of precision medicine has led to advances in the recognition and treatment of rare monogenic forms of diabetes, its impact on prevention and treatment of more common forms of diabetes has been underwhelming [1]. Several approaches to the subclassification of individuals with, or at high risk of, type 2 diabetes have been published recently [2–4]. Hall and colleagues introduced the concept of “glucotypes” in a research article [3] that has received enormous attention in the highest impact scientific journals [5–8], mostly in relation to precision medicine. The authors developed an algorithm to identify patterns of glucose fluctuations based on continuous glucose monitoring (CGM). They named the 3 identified patterns: “low variability,” “moderate variability,” and “severe variability” glucotypes. Each individual was characterised by the proportion of time spent in the 3 glucotypes and was assigned to an overall glucotype based on the highest proportion. They argued that glucotypes provide the advantage of taking into account a more detailed picture of glucose dynamics, in contrast to commonly used single time point or average-based measures, revealing subphenotypes within traditional diagnostic categories of glucose regulation. Even though the study was based on data from only 57 individuals without a prior diabetes diagnosis, others have interpreted the results as indicating that glucotypes might identify individuals at an early stage of glucose dysregulation, suggesting a potential role in diabetes risk stratification and prevention [5]. However, before glucotypes can become “an important tool in early identification of those at risk for type 2 diabetes” [3], the concept requires thorough validation. Therefore, we explore the generalisability and interpretability of glucotypes and their relationship to traditional CGM-based measures.We used data from The Maastricht Study [9] and the PRE-D Trial [10] comprising a total number of 770 diabetes-free individuals with a 7-day CGM registration. We observed that the average proportion of time spent in the low variability glucotype was low both in The Maastricht Study (6%) and the PRE-D Trial (4%), compared to 20% in the original study. A reason for the difference may be that our study populations were on average 11 to 12 years older and that the PRE-D Trial (n = 116) included only overweight and obese individuals with prediabetes. In The Maastricht Study, the median (Q1 to Q3) body mass index was 25.9 kg/m² (23.4 to 28.7), and 72% had normal glucose tolerance. As a logical consequence, the severe glucotype was most common in the PRE-D Trial (55%). Regardless, our data show that the initial estimates of the different glucotype prevalences do not necessarily generalise to other populations, especially in age groups at increased risk of type 2 diabetes.Hall and colleagues described glucotypes as a new measure of glucose variability, a clinically relevant metric of glycaemic patterns [3]. In the figures accompanying the original publication, the low variability pattern was characterised by both the lowest mean glucose level and variation, while the severe pattern had both the highest mean glucose level and variation. As such, these examples did not give an intuition whether glucotypes were predominantly driven by glucose variability or by mean glucose levels. We therefore present 3 examples from the PRE-D Trial (Fig 1). The first 2 profiles are very similar with regard to glucose variability. Thus, the driver of the most severe glucotype of the second participant is clearly the slightly higher mean glycaemic level. Also, even though the third participant has a much larger variation than the first two, the proportion of time in the severe glucotype is not higher than for the second participant as one would expect from a classical measure of glucose variability. To investigate this further, we assessed the association between glucotypes and classical CGM measures, i.e., the mean CGM glucose level (Fig 2A) and the coefficient of variation (Fig 2B) in The Maastricht Study. The scatterplots show a clear association between the mean CGM glucose and glucotypes. They also suggest that participants with a high proportion of time in the moderate glucotype do not have high variation in glucose. Rather than a biological feature, this may well be a methodological consequence of being assigned to the middle cluster. If large fluctuations were present, glucose levels would reach either low or high values, resulting in a higher proportion of time spent in the low or severe glucotypes, respectively (assuming a strong association between glucotypes and mean CGM glucose). Therefore, we decided to quantify this association using regression analysis where glucotype proportions were the outcomes, and the mean CGM glucose concentration was the independent variable modelled with natural cubic splines (more details on the specification of the models are given in Supporting information S1–S3 Codes). Then, we used the equation estimated in The Maastricht Study to predict glucotypes in the external validation sample (PRE-D Trial, Fig 2C). First, similarly to Hall and colleagues, we assigned individuals to the pattern with the highest proportion of time and then compared the predicted and the observed glucotypes. We found that in 107 out of 116 individuals, the glucotype was predicted correctly when using only the mean CGM glucose value. When considering the glucotypes as continuous proportions of time, the root mean squared errors (RMSEs) were 0.05, 0.09, and 0.07 for the low, moderate, and severe variability glucotypes, respectively, indicating good predictive ability. These results demonstrate that glucotypes either mainly reflect the mean CGM glucose level or do not translate to external datasets (e.g., due to overfitting). To investigate this further, we conducted the same analyses as described for the PRE-D Trial in the original data from Hall and colleagues and found a slightly weaker, but still strong association between mean CGM glucose levels and glucotypes. Using the regression model from The Maastricht Study, we could correctly predict 79% of the glucotypes, while the RMSEs were 0.11, 0.15, and 0.13.Open in a separate windowFig 1Example CGM profiles of participants in the PRE-D Trial with corresponding proportion of time spent in different glucotypes and conventional measures (mean and CV).CGM, continuous glucose monitoring; CV, coefficient of variation.Open in a separate windowFig 2Observed proportion of time spent in the 3 glucotypes by mean CGM glucose (A) and coefficient of variation (B) in The Maastricht Study, and by mean CGM glucose in the PRE-D Trial (C) alongside predicted proportions based on the regression analysis in The Maastricht Study. CGM, continuous glucose monitoring.Although the transformation of continuous measures into categorical ones is a common procedure in clinical research, assigning individuals to the glucotype with the highest proportion of time runs very much against the “precision” tenet of precision medicine. In line with this, a recent study has demonstrated how simple clinical features outperformed clusters in predicting relevant clinical outcomes [11]. This is especially problematic when a method does not provide clear separation between clusters, which can be quantified by calculating relative entropy [12]. A relative entropy of zero would mean that all individuals spend one-third of the time in each of the 3 glucotypes, while a value of one would indicate that each individual spends the entire time period in only one of the 3 glucotypes. In the original cohort of Hall and colleagues [3], we calculated a relative entropy of 0.24 indicating that cluster separation is far from optimal and together with the previous results question the claim that the glucotype is really a “more comprehensive measure of the pattern of glucose excursions than the standard laboratory tests in current use” [3].In conclusion, we demonstrate in 2 large, external datasets, that the assessment of glucotypes does not offer more novel insights than the mean CGM glucose, highlighting the importance of large development datasets and external validation for data-driven algorithms. As CGM is becoming more widely used in large clinical studies also among individuals without diabetes, glucose patterns derived from CGMs will be an important focus area in future diabetes research. However, it is important that scientific scrutiny precedes the introduction of emerging tools with a promise of identifying individuals at high risk of type 2 diabetes and its late complications at an earlier stage of disease progression, especially in an observational setting. Furthermore, future efforts towards precision medicine for diabetes prevention and treatment should go beyond the glucocentric approach we have seen so far. We know that hyperglycaemia is a late feature of diabetes development and that patients benefit most from a multifactorial treatment approach [13]. A multifactorial approach, with relevance to the aetiology of micro- and macrovascular complications, may also yield a more clinically useful risk stratification of nondiabetic individuals [14]. Even so, if we aim for precision medicine, we should aim to retain as much precision as possible at every step of the process, by treating determinants and outcomes as continuous measures if possible and by retaining information on the uncertainty of any hard classification such as cluster membership.     

Conformational properties of the pentapeptide fragment [32-36] of the thymic hormone thymopoietin

The conformational energy for the pentapeptide Arg-Lys-Asp-Val-Tyr (TP5) is calculated using empirical potential functions. Calculation of the local interactions for each independent residue gives a local energy term for which the probabilities as a function of phi, psi are plotted on Ramachandran-type maps. The interaction energy between residues is calculated only for these points in the maps with maximum probability. The most probable conformation for TP5 is found to have an extended backbone arrangement having the Arg and Tyr sidechains folded over the backbone. 13C n.m.r. spin lattice relaxation time measurements show no increase in T1 of the alpha-carbons at the first and terminal amino acids. The increase in T1 along the sidechain as found for Lys does not occur for Arg and Tyr. These signs of reduced mobility are consistent with a set of folded conformations in which the Arg and Tyr sidechains have hindered internal rotations. The vicinal NH-C alpha H couplings agree well with those calculated for the most probable conformer. This is not so for the C alpha H-C beta H couplings. These data are consistent with previous n.m.r. and structure activity studies.     

Flower development

Several homeotic genes controlling flower development have been characterized in Antirrhinum and Arabidopsis. Comparisons of their mutant phenotypes, expression patterns and genetic interactions have revealed that many of the basic mechanisms controlling flower development have been conserved in evolution, although important differences in the balance and interactions of genes also exist.     

Design,synthesis and structure–activity relationships of some novel,highly potent anti-invasive (E)- and (Z)-stilbenes

In our ongoing exploration of the structure–activity landscape of anti-invasive chalcones, we have prepared and evaluated a number of structurally related (E)- and (Z)-stilbenes. These molecules exhibited an extraordinary high in vitro potency in the chick heart invasion assay, being active up to 10 nmol L^?1, a concentration level a 100-fold lower than the lowest effective doses that have been reported for natural analogues. Furthermore, they possess an interesting pharmacological profile in silico.     

Maternal inheritance of P cytotype in Drosophila melanogaster: a “pre-P cytotype” is strictly extra-chromosomally transmitted

In Drosophila melanogaster, transposition of the P element is under the control of a cellular state known as cytotype. The P cytotype represses P transposition whereas the M cytotype is permissive for transposition. In the long-term, the P cytotype is determined by chromosomal P elements but over a small number of generations it is maternally inherited. In order to analyse the nature of this maternal inheritance, we tested whether a maternal component can be transmitted without chromosomal P elements. We used a stable determinant of P cytotype, linked to the presence of two P elements at the tip of the X chromosome (IA site) in a genome devoid of other P elements. We measured P repression capacity using two different assays: gonadal dysgenic sterility (GD) and P-lacZ transgene repression. We show that zygotes derived from a P cytotype female (heterozygous for P (1A)/balancer devoid of P copies) and which inherit no chromosomal P elements from the mother, have, however, maternally received a P-type extra-chromosomal component: this component is insufficient to specify the P cytotype if the zygote formed does not carry chromosomal P elements but can promote P cytotype determination if regulatory P elements have been introduced paternally. We refer to this strictly extra-chromosomally inherited state as the “pre-P cytotype”. In addition, we show that a zygote that has the pre-P cytotype but which has not inherited any chromosomal P elements, does not transmit the pre-P cytotype to the following generation. The nature of the molecular determinants of the pre-P cytotype is discussed.     

Comparative microanatomy of the branchial sieve in three sympatric cyprinid species,related to filter-feeding mechanisms

The chimaeroid holocephalian fishes are distinguished among extant chondrichthyans by the possession of three pairs of tooth plates, evergrowing and partially hypermineralized, that are not shed and replaced like the teeth of living elasmobranchs. Although derivation of the chimaeroid tooth plate from the fusion of members of a plesiomorphic chondrichthyan tooth family has been proposed, evidence for this hypothesis has been lacking. A new analysis of the development and structure of the tooth plates in Callorhinchus milii (Holocephali, Chimaeriformes) reveals the compound nature of the tooth plates in a chimaeroid fish. Each tooth plate consists of an oral and aboral territory that form independently in the embryo and maintain separate growth surfaces through life. The descending lamina on the aboral surface of the tooth plate demarcates the growth surface of the aboral territory. Comparison with the tooth plates of Chimaera monstrosa indicates that compound tooth plates may be a feature of all chimaeroids in which a descending lamina is present. The tooth plates in these fishes represent the fusion of two members of a reduced tooth family. The condition of the tooth plates in C. milii is plesiomorphic for chimaeroids and is of evolutionary significance in that it provides further evidence to support a lyodont dentition in chimaeroid fishes similar to that found in other chondrichthyans. © 1994 Wiley-Liss, Inc.