The Maternal-to-Zygotic Transition Targets Actin to Promote Robustness during Morphogenesis

Robustness is a property built into biological systems to ensure stereotypical outcomes despite fluctuating inputs from gene dosage, biochemical noise, and the environment. During development, robustness safeguards embryos against structural and functional defects. Yet, our understanding of how robustness is achieved in embryos is limited. While much attention has been paid to the role of gene and signaling networks in promoting robust cell fate determination, little has been done to rigorously assay how mechanical processes like morphogenesis are designed to buffer against variable conditions. Here we show that the cell shape changes that drive morphogenesis can be made robust by mechanisms targeting the actin cytoskeleton. We identified two novel members of the Vinculin/α-Catenin Superfamily that work together to promote robustness during Drosophila cellularization, the dramatic tissue-building event that generates the primary epithelium of the embryo. We find that zygotically-expressed Serendipity-α (Sry-α) and maternally-loaded Spitting Image (Spt) share a redundant, actin-regulating activity during cellularization. Spt alone is sufficient for cellularization at an optimal temperature, but both Spt plus Sry-α are required at high temperature and when actin assembly is compromised by genetic perturbation. Our results offer a clear example of how the maternal and zygotic genomes interact to promote the robustness of early developmental events. Specifically, the Spt and Sry-α collaboration is informative when it comes to genes that show both a maternal and zygotic requirement during a given morphogenetic process. For the cellularization of Drosophilids, Sry-α and its expression profile may represent a genetic adaptive trait with the sole purpose of making this extreme event more reliable. Since all morphogenesis depends on cytoskeletal remodeling, both in embryos and adults, we suggest that robustness-promoting mechanisms aimed at actin could be effective at all life stages.     

Antifreeze protein from freeze-tolerant grass has a beta-roll fold with an irregularly structured ice-binding site

The grass Lolium perenne produces an ice-binding protein (LpIBP) that helps this perennial tolerate freezing by inhibiting the recrystallization of ice. Ice-binding proteins (IBPs) are also produced by freeze-avoiding organisms to halt the growth of ice and are better known as antifreeze proteins (AFPs). To examine the structural basis for the different roles of these two IBP types, we have solved the first crystal structure of a plant IBP. The 118-residue LpIBP folds as a novel left-handed beta-roll with eight 14- or 15-residue coils and is stabilized by a small hydrophobic core and two internal Asn ladders. The ice-binding site (IBS) is formed by a flat beta-sheet on one surface of the beta-roll. We show that LpIBP binds to both the basal and primary-prism planes of ice, which is the hallmark of hyperactive AFPs. However, the antifreeze activity of LpIBP is less than 10% of that measured for those hyperactive AFPs with convergently evolved beta-solenoid structures. Whereas these hyperactive AFPs have two rows of aligned Thr residues on their IBS, the equivalent arrays in LpIBP are populated by a mixture of Thr, Ser and Val with several side-chain conformations. Substitution of Ser or Val for Thr on the IBS of a hyperactive AFP reduced its antifreeze activity. LpIBP may have evolved an IBS that has low antifreeze activity to avoid damage from rapid ice growth that occurs when temperatures exceed the capacity of AFPs to block ice growth while retaining the ability to inhibit ice recrystallization.     

Stable and dynamic microtubules coordinately determine and maintain Drosophila bristle shape

Within interphase cells, microtubules (MTs) are organized in a cell-specific manner to support cell shape and function. Here, we report that coordination between stable and dynamic MTs determines and maintains the highly elongated bristle cell shape. By following MT-decorating hooks and by tracking EB1 we identified two MT populations within bristles: a stable MT population polarized with their minus ends distal to the cell body, and a dynamic MT population that exhibits mixed polarity. Manipulating MT dynamics by Klp10A downregulation demonstrates that MTs can initiate new shaft extensions and thus possess the ability to determine growth direction. Actin filament bundling subsequently supports the newly formed shaft extensions. Analysis of ik2 mutant bristles, established by elongation defects in the Drosophila ikkε homolog, led to the observation that stable and dynamic MT orientation and polarized organization are important for proper bristle elongation. Thus, we demonstrate for the first time that coordination between stable and dynamic MT sets that are axially organized yet differently polarized drives cell elongation.     

nMAT1, a nuclear-encoded maturase involved in the trans-splicing of nad1 intron 1, is essential for mitochondrial complex I assembly and function

Mitochondrial genomes (mtDNAs) in angiosperms contain numerous group II-type introns that reside mainly within protein-coding genes that are required for organellar genome expression and respiration. While splicing of group II introns in non-plant systems is facilitated by proteins encoded within the introns themselves (maturases), the mitochondrial introns in plants have diverged and have lost the vast majority of their intron-encoded ORFs. Only a single maturase gene (matR) is retained in plant mtDNAs, but its role(s) in the splicing of mitochondrial introns is currently unknown. In addition to matR, plants also harbor four nuclear maturase genes (nMat 1 to 4) encoding mitochondrial proteins that are expected to act in the splicing of group II introns. Recently, we established the role of one of these proteins, nMAT2, in the splicing of several mitochondrial introns in Arabidopsis. Here, we show that nMAT1 is required for trans-splicing of nad1 intron 1 and also functions in cis-splicing of nad2 intron 1 and nad4 intron 2. Homozygous nMat1 plants show retarded growth and developmental phenotypes, modified respiration activities and altered stress responses that are tightly correlated with mitochondrial complex I defects.     

Learning in the fast lane: new insights into neuroplasticity

The timescale of structural remodeling that accompanies functional neuroplasticity is largely unknown. Although structural remodeling of human brain tissue is known to occur following long-term (weeks) acquisition of a new skill, little is known as to what happens structurally when the brain needs to adopt new sequences of procedural rules or memorize?a cascade of events within minutes or hours. Using diffusion tensor imaging (DTI), an MRI-based framework, we examined subjects before and after a spatial learning and memory task. Microstructural changes (as reflected by DTI measures) of limbic system structures (hippocampus and parahippocampus) were significant after only 2?hr of training. This observation was also found in a supporting rat study. We conclude that cellular rearrangement of neural tissue can be detected by DTI, and that this modality may allow neuroplasticity to be localized over short timescales.     

Over-expression of <Emphasis Type="Italic">CONSTANS-LIKE 5</Emphasis> can induce flowering in short-day grown <Emphasis Type="Italic">Arabidopsis</Emphasis>

To ensure that the initiation of flowering occurs at the correct time of year, plants need to integrate a diverse range of external and internal signals. In Arabidopsis, the photoperiodic flowering pathway is controlled by a set of regulators that include CONSTANS (CO). In addition, Arabidopsis plants also have a family of genes with homologies to CO known as CO-LIKE (COL) about which relatively little is known. In this paper, we describe the regulation and interactions of a novel member of the family, COL5. The expression of COL5 is under circadian and diurnal regulation, but COL5 itself does not appear to affect circadian rhythms. COL5, like CO, is regulated by GIGANTEA. Furthermore, COL5 is expressed in the vascular tissue. Using COL5 over-expressing lines we show that, under short days, constitutive expression of COL5 affects flowering time and the expression of the floral integrator genes, FLOWERING LOCUS T and SUPPRESSOR OF OVEREXPRESSION OF CO 1. Constitutive expression of COL5 partially suppresses the late flowering phenotype of the co-mutant plants. However, plants with loss of COL5 function do not show altered flowering. Taken together, our results suggest that COL5 has COL activity, but may either not have a role in regulating flowering in wild-type plants or may act redundantly with other flowering regulators. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Insulin-like growth factor binding protein-1 levels are increased in patients with IgA nephropathy

The mechanisms underlying the pathogenesis of immunoglobulin A (IgA) nephropathy (IgAN) are not well understood. In this study, we examined gene expression profiles in kidneys obtained from mice with high serum IgA levels (HIGA mice), which exhibit features of human IgAN. Female inbred HIGA, established from the ddY line, were used in these experiments. Serum IgA levels, renal IgA deposition, mesangial proliferation, and glomerulosclerosis were increased in 32-week-old HIGA mice in comparison to ddY animals. By microarray analysis, five genes were observed to be increased by more than 2.5-fold in 32-week-old HIGA in comparison to 16-week-old HIGA; these same five genes were decreased more than 2.5-fold in 32-week-old ddY in comparison to 16-week-old ddY mice. Of these five genes, insulin-like growth factor (IGF) binding protein (IGFBP)-1 exhibited differential expression between these mouse lines, as confirmed by quantitative RT-PCR. In addition, serum IGFBP-1 levels were significantly higher in patients with IgAN than in healthy controls. In patients with IgAN, these levels correlated with measures of renal function, such as estimated glomerular filtration rate (eGFR), but not with sex, age, serum IgA, C3 levels, or IGF-1 levels. Pathologically, serum IGFBP-1 levels were significantly associated with the severity of renal injury, as assessed by mesangial cell proliferation and interstitial fibrosis. These results suggest that increased IGFBP-1 levels are associated with the severity of renal pathology in patients with IgAN.     

Candidate cis-elements for human renin gene expression in the promoter region

The regulation of renin gene expression, the rate‐limiting enzyme of the system, is thought to be fundamental to the total system. Previously, we mapped six putative cis‐elements in the promoter region of the human renin gene with nuclear proteins from human chorionic cells and human renal cortex by DNase I protection assay (footprint A–F). Each footprint contains Ets motif like site (A), HOXñPBX recognition sequence (B), unknown sequence as DNA binding consensus (C), CRE (D), COUP‐TFII (ARP‐1) motif like site (E), and AGE3 like site (F). Footprint D has been characterized by means of functional studies as the genuine human renin gene CRE interacting with CREB in cooperation with the site of footprint B. To obtain further clues to the specific expression in the promoter region, these putative cis‐elements were conducted to a consensus‐specific binding assay to compare renin‐producing and non‐renin‐producing cells by EMSA and electromobility super‐shift assay. Different sequence‐specific DNA/protein binding was obtained among the different cell lines with footprint B site, with COUP‐TFII (ARP‐1) motif like site and possibly with footprint F site. The results implicate these putative cis‐elements and each corresponding trans‐factor in the specific expression of the human renin gene in the promoter region. Further functional characterization of these elements would provide important data for a better understanding of human renin gene expression. © 2004 Wiley‐Liss, Inc.