Principles of planar polarity in animal development

Planar polarity describes the coordinated polarisation of cells or structures in the plane of a tissue. The patterning mechanisms that underlie planar polarity are well characterised in Drosophila, where many events are regulated by two pathways: the 'core' planar polarity complex and the Fat/Dachsous system. Components of both pathways also function in vertebrates and are implicated in diverse morphogenetic processes, some of which self-evidently involve planar polarisation and some of which do not. Here, we review the molecular mechanisms and cellular consequences of planar polarisation in diverse contexts, seeking to identify the common principles across the animal kingdom.     

Distinct roles of two conserved Staufen domains in oskar mRNA localization and translation

Drosophila Staufen protein is required for the localization of oskar mRNA to the posterior of the oocyte, the anterior anchoring of bicoid mRNA and the basal localization of prospero mRNA in dividing neuroblasts. The only regions of Staufen that have been conserved throughout animal evolution are five double-stranded (ds)RNA-binding domains (dsRBDs) and a short region within an insertion that splits dsRBD2 into two halves. dsRBDs 1, 3 and 4 bind dsRNA in vitro, but dsRBDs 2 and 5 do not, although dsRBD2 does bind dsRNA when the insertion is removed. Full-length Staufen protein lacking this insertion is able to associate with oskar mRNA and activate its translation, but fails to localize the RNA to the posterior. In contrast, Staufen lacking dsRBD5 localizes oskar mRNA normally, but does not activate its translation. Thus, dsRBD2 is required for the microtubule-dependent localization of osk mRNA, and dsRBD5 for the derepression of oskar mRNA translation, once localized. Since dsRBD5 has been shown to direct the actin-dependent localization of prospero mRNA, distinct domains of Staufen mediate microtubule- and actin-based mRNA transport.     

In-situ localization of chalcone synthase mRNA in pea root nodule development

In this paper studies on the role of flavonoids in pea root nodule development are reported. Flavonoid synthesis was followed by localizing chalcone synthase (CHS) mRNA in infected pea roots and in root nodules. In a nodule primordium, CHS mRNA is present in all cells of the primordium. Therefore it is hypothesized that the Rhizobium Nod factor induces cell division in the root cortex by stimulating the production of flavonoids that function as auxin transport inhibitors. In nodules CHS mRNA is predominantly present in a region at the apex of the nodule consisting of meristematic and cortical cells. These cells are not infected by Rhizobium. Therefore it is postulated that CHS plays a role in nodule development rather than in a defence response. In roots CHS mRNA is located at a similar position as in nodules, suggesting that CHS has the same function in both root and nodule development. When nodules are formed by mutants of Rhizobium leguminosarum bv. viciae that are unable to secrete β(1-2) glucan and to synthesize the O-antigen containing LPS I, CHS genes are also expressed in regions of the nodule that are infected by Rhizobium. It is postulated that the impaired development of nodules formed by these mutants is due to an induction of a plant defence response.     

Cuts can kill: the roles of apoptotic nucleases in cell death and animal development

Chromosome fragmentation is one of the major biochemical hallmarks of apoptosis. However, until recently, its roles in apoptosis and mechanisms of action remained elusive. Recent biochemical and genetic studies have shown that chromosome fragmentation is a complex biochemical process that involves a plethora of conserved nucleases with distinct nuclease activities and substrate specificities. These apoptotic nucleases act cooperatively among themselves and with other nonnuclease cofactors to promote stepwise chromosome fragmentation and DNA degradation. Importantly, in addition to its direct contribution to the dismantling of the dying cell, apoptotic DNA degradation can facilitate cell killing and other apoptotic events such as clearance of apoptotic cells. Furthermore, some apoptotic nucleases apparently affect other aspects of animal development, including immune responses. The identification of new apoptotic nucleases and analysis of their functions in apoptosis and animal development should pave the way for future studies to uncover new functions for apoptotic nucleases and shed light on the hidden links between apoptotic DNA degradation and human diseases.     

Specific cellular localization of tyrosinase mRNA during Ciona intestinalis larval development

A Ciona intestinalis cDNA clone that encodes a protein highly homologous to other tyrosinases was isolated. Northern blot analysis showed that expression of Ciona tyrosinase starts at the early neurula stage and continues throughout the tail-bud and tadpole larval stages. The earliest tyrosinase expression was detected, by in situ hybridization, at the neural plate stage, in pigment precursor cells located along the two neural folds, in the animal region of the embryo. In the course of embryonic development the strong hybridization signal was always localized, within the rostral part of the developing brain, in the pigment precursor cells and was later detected in the otolith and ocellus. These results are discussed in relation to tyrosinase as an early marker of neural induction.     

Determinants of mRNA localization

RNA localization provides a mechanism for protein targeting within developing or differentiating cells. Specific cis-acting sequences on mRNA mediate this process. Such 'localizer' or 'zipcode' nucleic acid sequences have been restricted to the 3' untranslated region of several mRNAs. The presence of genetic information denoting a spatial component of translation adds a new dimension to gene expression.     

mRNA localization and content in bovine kininogen

The bradykinin (BK) gene chemically synthesized and cloned into pBR 322 was used for the study of tissue localization and quantitation of mRNA coding for the BK precursor, kininogen. Poly(A+)-mRNAs from bovine liver, spleen, kidney, mammary gland and pancreas were used for dot-hybridization to [32P]DNA of the BK gene or to the plasmid-containing BK gene. The experimental results demonstrate that [32P]DNA of BK is hybridized only to the liver poly(A+)-RNA, which proves the liver to be the main kininogen mRNA-producing tissue. In other tissues, the kininogen mRNA synthesis is either altogether absent or its level is two orders of magnitude less as compared to the liver. Several approaches for the quantitation of the kininogen mRNA were developed. The amount of this mRNA was shown to be about 0.6% of the total cellular poly(A+)-RNA. Poly(A+)-RNA which is bound to BK DNA-cellulose and is enriched with BK-coding sequences was used for the study of the hybridization kinetics and translation in a cell-free system from rabbit reticulocytes. The polypeptides synthesized contain BK as was shown by the use of a bio-test in rat uterus.     

Differential roles of distracters in reflexive and memory-based localization

We investigated the effects of spatial and temporal factors on manual localization of a visual target by measuring accuracy, precision, and bias. Spatial factors included manipulation of display as with or without distracters, with invariant or variant distracters, and with near or far distracters, respectively, in Experiments 1, 2, and 3. The target and distracters were of 1degrees dots differing only by luminance parameter; they were presented concurrently for 150 or 1000 ms while observers had to memorize the target location maintaining a fixed gaze. The observers' task was to reproduce the location of the target with a mouse cursor available 150 ms following stimuli offset. Results from all experiments showed that localization performance for a briefly exposed target was as accurate and precise as that for a long exposed target. Moreover, manipulation of spatial factors had no systematic effects on accuracy and precision except that near distracters yielded higher precision. Interestingly, localization performance was unbiased in 150 ms condition when there were distracters in the display, while being biased towards the fovea in 1000 ms condition regardless of their presence or absence. These results suggest a temporal dynamics in dominance-suppression between egocentric and exocentric cues in the construction of memory for location.     

Evolutionary development of redundant nuclear localization signals in the mRNA export factor NXF1

In human cells, the mRNA export factor NXF1 resides in the nucleoplasm and at nuclear pore complexes. Karyopherin β2 or transportin recognizes a proline-tyrosine nuclear localization signal (PY-NLS) in the N-terminal tail of NXF1 and imports it into the nucleus. Here biochemical and cellular studies to understand the energetic organization of the NXF1 PY-NLS reveal unexpected redundancy in the nuclear import pathways used by NXF1. Human NXF1 can be imported via importin β, karyopherin β2, importin 4, importin 11, and importin α. Two NLS epitopes within the N-terminal tail, an N-terminal basic segment and a C-terminal R-X(2-5)-P-Y motif, provide the majority of binding energy for all five karyopherins. Mutation of both NLS epitopes abolishes binding to the karyopherins, mislocalized NXF1 to the cytoplasm, and significantly compromised its mRNA export function. The understanding of how different karyopherins recognize human NXF1, the examination of NXF1 sequences from divergent eukaryotes, and the interactions of NXF1 homologues with various karyopherins reveals the evolutionary development of redundant NLSs in NXF1 of higher eukaryotes. Redundancy of nuclear import pathways for NXF1 increases progressively from fungi to nematodes and insects to chordates, potentially paralleling the increasing complexity in mRNA export regulation and the evolution of new nuclear functions for NXF1.     

Principles of structures of animal and plant lectins

Lectins form a diverse group of protein families that have in common their ability to specifically recognize certain carbohydrates. Crystal structures of members of the different animal and plant lectin families have revealed a wide variety of lectin folds and carbohydrate binding site architectures. Despite this large variability, a number of interesting cases of convergent as well as divergent evolution among animal and plant lectin families can be noted. These similarities exist at the levels of the protein fold, the architecture of the binding site as well as quaternary structure and may be derived from similar functional needs.     

Occurrence and biological roles of 'proximal glycanases' in animal cells

Glycosylation of particular proteins and lipids has become generallyacknowledged as being important for these molecules to expresstheir functions in various biological events. However, muchless attention has been paid to the biological significanceof deglycosylation of such onceglycosylated molecules in thecontext other than catabolism and recycling in the lysosome.Recently, in various kinds of animal cells and tissues we foundnon-lysosomal peptide: N-glycanase (PNGase) activities. Beforethese findings, PNGase was only known in plants and bacteria,and our findings indicated that de-N-glycosylation reactioncatalysed by PNGase occurred universally in bioorganisms, andmight function as a certain biologically important modification,not as a degradative pathway. Now, we put forward and extendthe concept to all the glycoconjugates that deglycosylationas well as glycosylation occur as a universal cellular systemto modulate the function of the present molecules, and postulate‘proximal glycanases’ (PROXIases) as enzymes thatare responsible for the detachment of intact glycan from glycoconjugatesand form free glycan and apo-glycoconjugates. In this article,we review the occurrence and possible function of proximal glycanasesin animal cells. glycosylation deglycosylatio PNGase     

Cell polarity and actin mRNA localization

In many species, intracellular mRNA localization is linked to cell polarity. In many cases however, mRNAs become localized as a result of a pre-existing cell-polarity, and they do not modify it. Remarkably, in the case beta actin mRNA in vertebrate, it has been shown that the transport and localization of this RNA is required for the establishment and maintenance of cell polarity. This occurs in fibroblasts, but, very interestingly, in immature neurons as well. This review will describe the functions and mechanisms of actin mRNA localization.     

The cytoskeleton and mRNA localization.

The localization of mRNA appears to facilitate protein sorting, so that proteins are synthesized in specific cellular regions. The spatial information on the mRNA may be transduced by proteins that recognize specific localizing sequences on the 3' end and then chaperone the mRNA, presumably along filaments, to its destination. Additional sequences such as poly(A), or the nascent chains of cytoskeleton-associated proteins, may then anchor mRNAs on the cytoskeleton.     

mRNA localization patterns in zebrafish oocytes

In both invertebrate and vertebrate systems, the localization of maternal mRNAs is a common mechanism used to influence developmental processes, including the establishment of the dorsal/ventral axis, anterior/posterior axis, and the germ line (for review, see Bashirullah et al., 1998. Annu. Rev. Biochem. 67, 335-394). While the existence of localized maternal mRNAs has been reported in the zebrafish, Danio rerio, the precise localization patterns of these molecules during oogenesis has not been determined. In this study, in situ hybridization experiments were performed on zebrafish ovaries and activated eggs to examine different mRNA localization patterns. The results establish that while some maternal mRNAs remain ubiquitously distributed throughout the oocyte, other mRNAs follow specific localization patterns, including localization to the animal pole, localization to the vegetal pole, and cortical localization. The animal/vegetal axis is first apparent in stage II oocytes when the earliest mRNA localization is seen. Unique patterns of localization are seen in mature eggs as well. Some mRNAs maintain their oocyte localization patterns, while others localize upon egg activation (fertilization).