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Wu Li

Publications and source records attributed to Wu Li.

17 recordsLinked to original sources
PubMed2006

Contour saliency in primary visual cortex.

Contour integration is an important intermediate stage of object recognition, in which line segments belonging to an object boundary are perceptually linked and segmented from complex backgrounds. Contextual influences observed in primary visual cortex (V1) suggest the involvement of V1 in contour integration. Here, we provide direct evidence that, in monkeys performing a contour detection task, there was a close correlation between the responses of V1 neurons and the perceptual saliency of contours. Receiver operating characteristic analysis showed that single neuronal responses encode the presence or absence of a contour as reliably as the animal's behavioral responses. We also show that the same visual contours elicited significantly weaker neuronal responses when they were not detected in the detection task, or when they were unattended. Our results demonstrate that contextual interactions in V1 play a pivotal role in contour integration and saliency.

Action Potentials↗
PubMed2006

Axons and synaptic boutons are highly dynamic in adult visual cortex.

While recent studies of synaptic stability in adult cerebral cortex have focused on dendrites, how much axons change is unknown. We have used advances in axon labeling by viruses and in vivo two-photon microscopy to investigate axon branching and bouton dynamics in primary visual cortex (V1) of adult Macaque monkeys. A nonreplicative adeno-associated virus bearing the gene for enhanced green fluorescent protein (AAV.EGFP) provided persistent labeling of axons, and a custom-designed two-photon microscope enabled repeated imaging of the intact brain over several weeks. We found that large-scale branching patterns were stable but that a subset of small branches associated with terminaux boutons, as well as a subset of en passant boutons, appeared and disappeared every week. Bouton losses and gains were both approximately 7% of the total population per week, with no net change in the overall density. These results suggest ongoing processes of synaptogenesis and elimination in adult V1.

Animals↗
PubMed2006

Effect of degradation rates of resorbable phosphate invert glasses on in vitro osteoblast proliferation.

Four resorbable phosphate invert glasses for use as bone replacement were synthesized in the system P2O5--CaO--MgO--Na2O. TiO2 and SiO2 were added at concentrations of 1 and 5.5 mol % to control solubility and crystallization. Both bulk glasses and samples with an open porosity of 65% and pore sizes of 150 to 400 microm were produced using a salt sintering process. Addition of TiO2 decreased the solubility in water and simulated body fluid, while the glass with addition of SiO2 showed a higher dissolution rate than did the original glass. The hypothesis that dissolution rates of the glasses will affect cell proliferation of osteoblastlike cells was tested using a MC3T3-E1.4 murine preosteoblast cell line. Cells were cultured on nonporous polished and porous glasses with tissue culture polystyrene (TCPS) as control. Cell proliferation was studied over 24 and 72 h in culture. Cells proliferated on all polished glasses, but proliferation on porous glasses showed variations with glass composition. Cell proliferation increased with decreased solubility of the glass. It is suggested that resorbable implant materials require the adjustment of dissolution rate so as to facilitate cell adhesion and proliferation and thus a gradual transition from artificial implant to new bone structure.

Animals↗
PubMed2006

Fluoride down-regulates the expression of matrix metalloproteinase-20 in human fetal tooth ameloblast-lineage cells in vitro.

Fluoride is associated with a decrease in the incidence of dental caries, but excessive fluoride intake during tooth enamel formation can result in enamel fluorosis. Fluorosed enamel has increased porosity, which has been related to a delay in the removal of amelogenin proteins as the enamel matures. This delay in protein removal suggests that fluoride may affect either the amount or the activity of enamel matrix proteinases. In this study, we investigated the role of fluoride in the synthesis and secretion of matrix metalloproteinase-20 (MMP-20), the proteinase primarily responsible for the initial hydrolysis of amelogenin during the secretory stage of enamel formation. Cultured human fetus tooth organ ameloblast-lineage cells were exposed to 10 microM fluoride and analyzed for synthesis of MMP-20. Immunoblotting showed that 10 microM NaF down-regulated the synthesis of MMP-20 by 21% compared with control cells, but did not alter the amount of amelogenin or kalikrein-4 (KLK-4) synthesized by the cells. Real-time polymerase chain reaction (PCR) showed that 10 microM NaF down-regulated MMP-20 mRNA expression to 28% of the levels found in the non-treated cells. These in vitro results suggest that fluoride can alter the expression of MMP-20 by ameloblasts, resulting in a disturbance of the balance between MMP-20 and its substrate that may contribute to the retention of amelogenins in the formation of fluorosed enamel.

Ameloblasts↗
PubMed2006

Differentiation of human ameloblast-lineage cells in vitro.

Previous studies have shown that ameloblast-like cells can be selectively cultured from the enamel organ in a serum-free medium with low calcium concentrations. The purpose of this study was to further characterize this culture system to identify differentiated ameloblast-lineage cells. Tooth organs from 19-24-wk-old fetal cadavers were either frozen and cryosectioned for immunostaining, or digested in collagenase/dispase for cell culture. The cells were grown in keratinocyte media supplemented with 0.05 mM calcium, and characterized by morphology and immunofluorescence. Epithelial clones with two distinct morphologies, including smaller cobblestone-shaped cells and larger (5-15 times in size) rounded cells, began to form between day 8 and day 12 after culture. The cobblestone-shaped cells continued to proliferate in culture, while the larger cells proliferated slowly or not at all. These larger cells formed filopodia, usually had two or more nuclei and a radiating cytoplasm at the cell margin, and were more abundant with increasing time in culture. Both cell types stained for cytokeratin 14, and the larger cells appeared more differentiated, showing stronger staining for amelogenin and ameloblastin. Immunofluorescence of the tooth bud sections showed staining for these matrix proteins as ameloblasts differentiated from the inner enamel epithelium. These results show the successful culture of differentiating ameloblast-lineage cells, and lay a foundation for use of these cells to further understand ameloblast biology with application to tooth enamel tissue engineering.

Ameloblasts↗
PubMed2006

Growth of ameloblast-lineage cells in a three-dimensional Matrigel environment.

Enamel organ epithelial cells grow in culture as two distinct cell populations--either stellate-shaped or polygonal-shaped cells. The polygonal cells have an ameloblast cell phenotype and are difficult to grow in culture beyond two passages. This study was designed to determine the effects of a Matrigel three-dimensional (3D) environment on polygonal cells, as compared with stellate cells, derived from porcine tooth enamel organ. Enamel organs were dissected free from the unerupted molars of 30-kg pigs and then grown in LCH-8e media, either with or without serum. Cells grown in serum-free media were primarily polygonal shaped, whereas cells grown in media containing serum were stellate shaped. Both types of cells were grown in a 3D Matrigel matrix. In addition, polygonal-shaped cells were mixed with hydroxyapatite powder and transplanted subcutaneously into nude mice. Polygonal-shaped epithelial cells formed cell groups, similar to epithelial pearls, both in vitro and in vivo. The stellate-shaped cells, in contrast, did not form similar structures, but remained suspended in the Matrigel and gradually disappeared from the culture. These results suggest that a Matrigel environment, rich in basement membrane and matrix proteins, selects for polygonal-shaped ameloblast-lineage cells and induces the formation of epithelial pearls.

Ameloblasts↗
PubMed2006

Self-assembly and effect on crystal growth of the leucine-rich amelogenin peptide.

Amelogenins are a unique group of alternatively spliced proteins. While the full-length amelogenin is known to assemble into nanospheres and alter apatite crystal growth and alignment, the function of the leucine-rich amelogenin peptide (LRAP) in biomineralization is not understood. This study tested the hypothesis that LRAP self-assembles into a supramolecular structure and guides crystal growth similarly to the full-length protein. Synthetic LRAP and recombinant full-length amelogenin (rH175) were used at different concentrations and either immobilized onto fluoroapatite substrates (FAP) or immersed into saturated calcium-phosphate solutions. The structure of the assembled protein and the height of apatite crystals formed on the FAP template were determined using atomic force microscopy. Both LRAP and rH175 assembled into nanospheres. LRAP self-assembly, however, was only observed at concentrations of >0.5 mg ml-1 and limited to sizes between 5 and 30 nm. Apatite crystal growth was not significantly affected by LRAP, while rH175 accelerated crystal growth by up to 50-fold. The increased growth rate was only observed when rH175 precipitated at concentrations of >0.8 mg ml-1. It was concluded that the ability of amelogenins to self-assemble into nanospheres and to bind to apatite in vitro is not inevitably an indication for the ability to control apatite crystal growth.

Apatites↗
PubMed2006

Comparative calcium binding of leucine-rich amelogenin peptide and full-length amelogenin.

Leucine-rich amelogenin peptide (LRAP) is an alternately spliced amelogenin. LRAP is known to bind to hydroxyapatite, and has been shown to signal mesenchymal cells to proliferate, but its function in enamel formation is unclear. The purpose of this study was to determine the calcium-binding properties and structure of recombinant human LRAP (rLRAP) compared with full-length amelogenin (rH174). rLRAP and rH174 were synthesized in Escherichia coli and purified by affinity chromatography and reverse-phase high-performance liquid chromatography. Calcium binding was measured by isothermal titration calorimetry (ITC) at pH 7.5 and 25 degrees C, and raw data were analyzed by origin 7.0 software. The structure of rLRAP was analyzed by nuclear magnetic resonance (NMR) and circular dichroism (CD) in the absence or presence of Ca2+, pH 7.5 and 4.0, at 25 degrees C. Thermodynamic values showed that rLRAP had a Ca2+-binding affinity approximately 6.4-times greater than rH174. NMR and CD data revealed that rLRAP was randomly coiled, and that this structure was not altered by Ca2+, which bound to rLRAP and rH174 via ionic interactions. Unlike r174 (beta-spiral), rLRAP had a random-coiled structure. The calcium binding and structural differences between rLRAP and rH174 suggest that these proteins have different functions in enamel biomineralization.

Amelogenesis↗
PubMed2005

MEPE is downregulated as dental pulp stem cells differentiate.

UNLABELLED: Previous studies on dental pulp cell culture have described heterogenous mixtures of cells that differentiate into odontoblasts and form mineralized dentin. OBJECTIVE: The aim of this study was to characterize the matrix extracellular phosphoglycoprotein (MEPE) expression by dental pulp stem cells (DPSC), related to cell differentiation. DESIGN: DPSC differentiation to form mineralized nodules was characterized by Alizarin red staining and micro-Raman spectroscopy. Osteogenesis SuperArray analysis was used to broadly screen for osteogenesis-related genes altered by DPSC differentiation. Relative levels of expression of MEPE and DSP were determined by semiquantitative RT-PCR and Western blot. RESULTS: Mineral analysis showed that as DPSC differentiated, they formed a carbonated hydroxyapatite mineral. Differentiation was initially marked by upregulation by Runx2, TGFbeta-related genes, EGFR and genes involved in collagen metabolism. ALP activity first increased, as DPSCs reached confluence but later decreased when cells further differentiated three weeks after confluence. MEPE was the only marker that was downregulated as DPSCs differentiated. CONCLUSION: DPSC differentiation can be characterized by downregulation of MEPE as other markers of DPSC differentiation, such as DSP, are upregulated. Expression of MEPE related to DSP and can be used to monitor DPSC as they are used for studies of odontoblast differentiation, tissue engineering or vital pulp therapy. The downregulation of MEPE as DPSC differentiate, suggests that MEPE is an inhibitor of mineralization.

Biomarkers↗
PubMed2005

Analysis of the phosphoryl transfer mechanism of c-AMP dependent protein kinase (PKA) by penta-coodinate phosphoric transition state theory.

This review briefly covers recent literature of research on the phosphoryl transfer mechanism of PKA. Combining experimental and theoretical calculation results on enzymes with experimentally observed biomimic activities of phosphoryl amino acids and a small molecular model of catalytic core in PKA, a novel mechanism was proposed. The cooperative participation roles of both Asp166 and Lys168 via a penta-coodinate phosphoric intermediate was elucidated to conciliate the current different views of the phosphoryl transfer mechanism of PKA. Since many ATP-binding enzymes may share a similar phosphoryl transfer mechanism, this proposed mechanism might also apply to the mechanism of these enzymes, e.g., molecular motor and phosphatase among others.

Catalysis↗
PubMed2004

Perceptual learning and top-down influences in primary visual cortex.

Neuronal responses at early stages in visual cortical processing, including those in primary visual cortex (V1), are subject to the influences of visual context, experience and attention. Here we show that for monkeys trained in a shape discrimination task, V1 neurons took on novel functional properties related to the attributes of the trained shapes. Furthermore, these properties depended on the perceptual task being performed; neurons responded very differently to an identical visual stimulus under different visual discrimination tasks. These top-down influences were seen from the very beginning and throughout the entire time course of the neural responses. Information theoretic analysis showed that neurons carried more information about a stimulus attribute when the animals were performing a task related to that attribute. Our findings suggest that the output from V1 reflects both sensory and behavioral context.

Action Potentials↗
PubMed2004

Robust unsupervised segmentation of infarct lesion from diffusion tensor MR images using multiscale statistical classification and partial volume voxel reclassification.

Manual region tracing method for segmentation of infarction lesions in images from diffusion tensor magnetic resonance imaging (DT-MRI) is usually used in clinical works, but it is time consuming. A new unsupervised method has been developed, which is a multistage procedure, involving image preprocessing, calculation of tensor field and measurement of diffusion anisotropy, segmentation of infarction volume based on adaptive multiscale statistical classification (MSSC), and partial volume voxel reclassification (PVVR). The method accounts for random noise, intensity overlapping, partial volume effect (PVE), and intensity shading artifacts, which always appear in DT-MR images. The proposed method was applied to 20 patients with clinically diagnosed brain infarction by DT-MRI scans. The accuracy and reproducibility in terms of identifying the infarction lesion have been confirmed by clinical experts. This automatic segmentation method is promising not only in detecting the location and the size of infarction lesion in stroke patient but also in quantitatively analyzing diffusion anisotropy of lesion to guide clinical diagnoses and therapy.

Algorithms↗
PubMed2003

X-linked amelogenesis imperfecta may result from decreased formation of tyrosine rich amelogenin peptide (TRAP).

Amelogenesis imperfecta (AI) is a group of inherited disorders with defective tooth enamel formation caused by various gene mutations. One of the mutations substitutes a cytidine for an adenine in exon 6 of the X-chromosomal amelogenin gene, which results in a proline to threonine change in the expressed amelogenin. This transformation is four amino acids N-terminal to the cleavage site for enamel matrix metalloproteinase-20 (MMP-20) in amelogenin. MMP-20 releases the tyrosine rich amelogenin peptide (TRAP) from amelogenin. This study evaluated the rate at which MMP-20 hydrolyses mutated amelogenin relative to unmutated amelogenin. A full-length recombinant human amelogenin and a mutated amelogenin with a substitution of proline by threonine were expressed and purified by ammonium sulphate precipitation and reverse phase HPLC. Recombinant metalloproteinase-20 (rMMP-20) was used to digest the recombinant proteins, which resulted in fragments with a mass predicted for TRAP. The proteolytic site was also modelled as substrates by two synthetic peptides, SYGYEPMGGWLHHQ and SYGYETMGGWLHHQ, selected from residues 36 to 49 of the amino acid sequence for amelogenin and the respective X-linked amelogenin mutant. These two peptides were labelled at their N- and C-termini respectively by using rhodamine and biotin. After digestion with MMP-20, the truncated peptides were separated by avidin-labelled magnetic Dynal beads and were identified by mass spectrometry. These results demonstrated that both oligopeptides were cleaved between tryptophan and leucine, matching the TRAP cutting site found in tooth enamel. Enzyme kinetics showed that the k(cat)/K(m) of rMMP-20 against the unmutated amelogenin peptide was 21 times greater than that against the mutated peptide. This study suggests that the reduced rate of TRAP formation by a single amino acid substitution alters enamel matrix hydrolysis by MMP-20, which may result in amelogenesis imperfecta.

Amelogenesis Imperfecta↗
PubMed2003

Are metastable, precrystallisation, density-fluctuations a universal phenomena?

In-situ observations of crystallisation in minerals and organic polymers have been made by simultaneous, time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) techniques. In isotactic polypropylene slow quiescent crystallisation shows the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with long range order before the development of crystalline peaks in the WAXS region. Block copolymers self-assemble into mesophases in polymer melts above a critical chain length (or above a critical temperature) and this self-assembly process is shown to be susceptible to an incipient crystallisation. Mesophase formation is observed at anomalously high temperatures in ethylene-oxide containing block copolymers below the normal melting point of the polyoxy ethylene chains. Formation of calcium carbonate from aqueous solutions of sodium carbonate and calcium nitrate is observed to be a two-stage process and precipitation proceeds by the production of an amorphous metastable phase. This phase grows until it is volume filling and leads to the formation of the two polymorphs Calcite and Vaterite. These three sets of results suggest pre-nucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation. In due course, this phase undergoes transformation to "normal" crystals.

Journal Article↗
PubMed2002

Global contour saliency and local colinear interactions.

Our visual system can link components of contours and segregate contours from complex backgrounds based on geometric grouping rules. This is an important intermediate step in object recognition. The substrate for contour integration may be based on contextual interactions and intrinsic horizontal connections seen in primary visual cortex (V1). We examined the perceptual rules governing contour saliency to determine whether the spatial extents of contextual interactions and horizontal connections match those mediating saliency. To quantify these rules, we used stimuli composed of randomly oriented nonoverlapping line segments. Salient contours within this complex background were formed by colinear alignment of nearby segments. Contour detectability was measured using a 2-interval-forced-choice design. Contour detectability deteriorated with increasing spacing between contour elements and improved as the number of colinear line elements was increased. At short contour spacing, the detectability reached a plateau with alignment of a few line segments that together formed a contour subtending several visual degrees. At intermediate spacing, saliency built up progressively with a greater number of colinear lines, extending up to 30 degrees. When contour spacing was beyond a critical range (about 2 degrees ), however, the detectability dropped to chance levels, regardless of the number of colinear lines. Contour detectability was found to be a function not only of the relative spacing of contour elements with respect to the noise elements but also of the average density of the overall pattern. Furthermore, training significantly improved contour detection, increasing the critical spacing of line elements beyond which contours were no longer detectable. Our data suggest that global contour integration is based on mechanisms of limited spatial extent, comparable to the interactions observed in V1. These interactions can cascade over larger distances provided the spacing of stimulus elements is kept within a limited range.

Adult↗
PubMed2002

Expression of alternatively spliced RNA transcripts of amelogenin gene exons 8 and 9 and its end products in the rat incisor.

In addition to seven known exons of the amelogenin gene, recent studies have identified two exons downstream of amelogenin exon 7 in genomic DNA of mouse and rat. Here the spatial and temporal expression of mRNAs and of the translated proteins derived from alternative splicing of the amelogenin gene ending with exon 8 and exon 9 were examined by in situ hybridization (ISH) and immunohistochemistry (IHC). RNA signals for exons 8 and 9 were expressed in the ameloblast layer extending from early presecretory to postsecretory transitional stages of amelogenesis. IHC of amelogenin proteins that include sequences encoded by these exons demonstrated identical localization of these proteins in the ameloblast layer corresponding to RNA signals identified by ISH. There was intense immunostaining of the enamel matrix secreted by these cells. Western blotting analysis of rat enamel proteins revealed three distinct protein bands with sequences encoded by the new exons. These data confirmed the existence of the transcripts of alternatively spliced mRNAs coding for exons 8 and 9 of the amelogenin gene in rat tooth germs and suggest that the translated proteins contribute to the heterogeneity of amelogenins and have some significant roles in enamel formation and mineralization.

Alternative Splicing↗