首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1154篇
  免费   69篇
  1223篇
  2024年   1篇
  2023年   10篇
  2022年   25篇
  2021年   50篇
  2020年   14篇
  2019年   32篇
  2018年   42篇
  2017年   22篇
  2016年   44篇
  2015年   73篇
  2014年   85篇
  2013年   115篇
  2012年   121篇
  2011年   111篇
  2010年   51篇
  2009年   58篇
  2008年   69篇
  2007年   76篇
  2006年   58篇
  2005年   39篇
  2004年   31篇
  2003年   29篇
  2002年   22篇
  2001年   7篇
  2000年   2篇
  1999年   2篇
  1998年   4篇
  1997年   3篇
  1996年   6篇
  1995年   2篇
  1994年   3篇
  1993年   3篇
  1992年   1篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1985年   1篇
  1983年   1篇
  1980年   2篇
  1976年   1篇
  1973年   2篇
排序方式: 共有1223条查询结果,搜索用时 15 毫秒
1.
DNA from 40 unrelated familial hypercholesterolemia (FH) heterozygotes were subjected to analyses of single-strand conformation polymorphisms (SSCPs) of exon 10 of the low density lipoprotein receptor (LDLR) gene. Four different SSCP patterns were observed. The underlying mutations were characterized by DNA sequencing. Three of the patterns represented the three genotypes of a recently described sense mutation in codon 450. A method based upon the polymerase chain reaction (PCR) was developed to analyze this mutation. The frequencies of the wild-type (G at nucleotide 1413) and mutant (A at nucleotide 1413) alleles were 0.56 and 0.44, respectively. The fourth pattern was found in only one FH heterozygote and was caused by heterozygosity at nucleotide 1469 (G/A). Nucleotide 1469 is the second base of codon 469Trp(TGG). The GA mutation changes this codon into the amber stop codon, and is referred to as FH469Stop. The mutant receptor consists of the amino terminal 468 amino acids. Because the truncated receptor has lost the membrane-spanning domain, it will not be anchored in the cell membrane. FH469Stop destroys an AvaII restriction site, and this characteristic was used to develop a PCR method to establish its frequency in Norwegian FH subjects. Two out of 204 (1%) unrelated FH heterozygotes possessed the mutation.  相似文献   
2.
Phytochemistry Reviews - Oxyprenylated secondary metabolites of plant, fungal, and microbial origin have emerged as biologically active natural compounds with a great potential for the next future....  相似文献   
3.
Classically, peroxisome proliferator-activated receptor β/δ (PPARβ/δ) function was thought to be restricted to enhancing adipocyte differentiation and development of adipose-like cells from other lineages. However, recent studies have revealed a critical role for PPARβ/δ during skeletal muscle growth and regeneration. Although PPARβ/δ has been implicated in regulating myogenesis, little is presently known about the role and, for that matter, the mechanism(s) of action of PPARβ/δ in regulating postnatal myogenesis. Here we report for the first time, using a PPARβ/δ-specific ligand (L165041) and the PPARβ/δ-null mouse model, that PPARβ/δ enhances postnatal myogenesis through increasing both myoblast proliferation and differentiation. In addition, we have identified Gasp-1 (growth and differentiation factor-associated serum protein-1) as a novel downstream target of PPARβ/δ in skeletal muscle. In agreement, reduced Gasp-1 expression was detected in PPARβ/δ-null mice muscle tissue. We further report that a functional PPAR-responsive element within the 1.5-kb proximal Gasp-1 promoter region is critical for PPARβ/δ regulation of Gasp-1. Gasp-1 has been reported to bind to and inhibit the activity of myostatin; consistent with this, we found that enhanced secretion of Gasp-1, increased Gasp-1 myostatin interaction and significantly reduced myostatin activity upon L165041-mediated activation of PPARβ/δ. Moreover, we analyzed the ability of hGASP-1 to regulate myogenesis independently of PPARβ/δ activation. The results revealed that hGASP-1 protein treatment enhances myoblast proliferation and differentiation, whereas silencing of hGASP-1 results in defective myogenesis. Taken together these data revealed that PPARβ/δ is a positive regulator of skeletal muscle myogenesis, which functions through negatively modulating myostatin activity via a mechanism involving Gasp-1.  相似文献   
4.
Multiple system atrophy (MSA) is a fatal rapidly progressive α-synucleinopathy, characterized by α-synuclein accumulation in oligodendrocytes. It is accepted that the pathological α-synuclein accumulation in the brain of MSA patients plays a leading role in the disease process, but little is known about the events in the early stages of the disease. In this study we aimed to define potential roles of the miRNA-mRNA regulatory network in the early pre-motor stages of the disease, i.e., downstream of α-synuclein accumulation in oligodendroglia, as assessed in a transgenic mouse model of MSA. We investigated the expression patterns of miRNAs and their mRNA targets in substantia nigra (SN) and striatum, two brain regions that undergo neurodegeneration at a later stage in the MSA model, by microarray and RNA-seq analysis, respectively. Analysis was performed at a time point when α-synuclein accumulation was already present in oligodendrocytes at neuropathological examination, but no neuronal loss nor deficits of motor function had yet occurred. Our data provide a first evidence for the leading role of gene dysregulation associated with deficits in immune and inflammatory responses in the very early, non-symptomatic disease stages of MSA. While dysfunctional homeostasis and oxidative stress were prominent in SN in the early stages of MSA, in striatum differential gene expression in the non-symptomatic phase was linked to oligodendroglial dysfunction, disturbed protein handling, lipid metabolism, transmembrane transport and altered cell death control, respectively. A large number of putative miRNA-mRNAs interaction partners were identified in relation to the control of these processes in the MSA model. Our results support the role of early changes in the miRNA-mRNA regulatory network in the pathogenesis of MSA preceding the clinical onset of the disease. The findings thus contribute to understanding the disease process and are likely to pave the way towards identifying disease biomarkers for early diagnosis of MSA.  相似文献   
5.
Global climate change scenarios predict lake water temperatures to increase up to 4°C and extreme weather events, including heat waves and large temperature fluctuations, to occur more frequently. Such changes may result in a reorganization of the plankton community structure, causing shifts in diversity and structure toward a community dominated by fewer species that are more adapted to endure warmer and irregular temperature conditions. We designed a long‐term (8 months) mesocosm experiment to explore how ambient water temperature (C: control), induced increased temperature (T: +4°C), and temperature fluctuations (F: ±4°C relative to T) change phytoplankton phenology, taxonomical diversity, and community structure, and how such changes affected zooplankton abundance and composition. Synthesis. Our results show that T and F relative to C significantly decreased phytoplankton diversity. Moreover, there was a clear effect of the temperature treatments (T and F) on phytoplankton size structure that resulted in a significantly lower growth of large species (i.e., large Chlorophyta) compared to C. Decreased diversity and evenness in the T and F treatments pushed the community toward the dominance of only a few phytoplankton taxa (mainly Cyanobacteria and Chlorophyta) that are better adapted to endure warmer and more irregular temperature conditions. The observed shift toward Cyanobacteria dominance may affect trophic energy transfer along the aquatic food web.  相似文献   
6.
Diatoms represent one of the most abundant groups of microalgae in the ocean and are responsible for approximately 20% of photosynthetically fixed CO2 on Earth. Due to their complex evolutionary history and ability to adapt to different environments, diatoms are endowed with striking molecular biodiversity and unique metabolic activities. Their high growth rate and the possibility to optimize their biomass make them very promising ‘biofactories’ for biotechnological applications. Among bioactive compounds, diatoms can produce ovothiols, histidine-derivatives, endowed with unique antioxidant and anti-inflammatory properties, and occurring in many marine invertebrates, bacteria and pathogenic protozoa. However, the functional role of ovothiols biosynthesis in organisms remains almost unexplored. In this work, we have characterized the thiol fraction of Phaeodactylum tricornutum, providing the first evidence of the presence of ovothiol B in pennate diatoms. We have used P. tricornutum to overexpress the 5-histidylcysteine sulfoxide synthase ovoA, the gene encoding the key enzyme involved in ovothiol biosynthesis and we have discovered that OvoA localizes in the mitochondria, a finding that uncovers new concepts in cellular redox biochemistry. We have also obtained engineered biolistic clones that can produce higher amount of ovothiol B compared to wild-type cells, suggesting a new strategy for the eco-sustainable production of these molecules.  相似文献   
7.
Chronic inflammation is associated with the occurrence of several diseases. However, the side effects of anti‐inflammatory drugs prompt the identification of new therapeutic strategies. Plant‐derived extracellular vesicles (PDEVs) are gaining increasing interest in the scientific community for their biological properties. We isolated PDEVs from the juice of Citrus limon L. (LEVs) and characterized their flavonoid, limonoid and lipid contents through reversed‐phase high‐performance liquid chromatography coupled to electrospray ionization quadrupole time‐of‐flight mass spectrometry (RP‐HPLC–ESI‐Q‐TOF‐MS). To investigate whether LEVs have a protective role on the inflammatory process, murine and primary human macrophages were pre‐treated with LEVs for 24 h and then were stimulated with lipopolysaccharide (LPS). We found that pre‐treatment with LEVs decreased gene and protein expression of pro‐inflammatory cytokines, such as IL‐6, IL1‐β and TNF‐α, and reduced the nuclear translocation and phosphorylation of NF‐κB in LPS‐stimulated murine macrophages. The inhibition of NF‐κB activation was associated with the reduction in ERK1‐2 phosphorylation. Furthermore, the ability of LEVs to decrease pro‐inflammatory cytokines and increase anti‐inflammatory molecules was confirmed ex vivo in human primary T lymphocytes. In conclusion, we demonstrated that LEVs exert anti‐inflammatory effects both in vitro and ex vivo by inhibiting the ERK1‐2/NF‐κB signalling pathway.  相似文献   
8.

Background

Homeodomain interacting protein kinase 2 (HIPK2) is an evolutionary conserved serine/threonine kinase whose activity is fundamental in maintaining wild-type p53 function, thereby controlling the destiny of cells when exposed to DNA damaging agents. We recently reported an altered conformational state of p53 in tissues from patients with Alzheimer''s Disease (AD) that led to an impaired and dysfunctional response to stressors.

Methodology/Principal Findings

Here we examined the molecular mechanisms underlying the impairment of p53 activity in two cellular models, HEK-293 cells overexpressing the amyloid precursor protein and fibroblasts from AD patients, starting from recent findings showing that p53 conformation may be regulated by HIPK2. We demonstrated that beta-amyloid 1–40 induces HIPK2 degradation and alters HIPK2 binding activity to DNA, in turn regulating the p53 conformational state and vulnerability to a noxious stimulus. Expression of HIPK2 was analysed by western blot experiments, whereas HIPK2 DNA binding was examined by chromatin immunoprecipitation analysis. In particular, we evaluated the recruitment of HIPK2 onto some target promoters, including hypoxia inducible factor-1α and metallothionein 2A.

Conclusions/Significance

These results support the existence of a novel amyloid-based pathogenetic mechanism in AD potentially leading to the survival of injured dysfunctional cells.  相似文献   
9.
Glioblastoma is a highly heterogeneous aggressive primary brain tumor, with the glioma stem‐like cells (GSC) being more sensitive to cytotoxic lymphocyte‐mediated killing than glioma differentiated cells (GDC). However, the mechanism behind this higher sensitivity is unclear. Here, we found that the mitochondrial morphology of GSCs modulates the ER–mitochondria contacts that regulate the surface expression of sialylated glycans and their recognition by cytotoxic T lymphocytes and natural killer cells. GSCs displayed diminished ER–mitochondria contacts compared to GDCs. Forced ER–mitochondria contacts in GSCs increased their cell surface expression of sialylated glycans and reduced their susceptibility to cytotoxic lymphocytes. Therefore, mitochondrial morphology and dynamism dictate the ER–mitochondria contacts in order to regulate the surface expression of certain glycans and thus play a role in GSC recognition and elimination by immune effector cells. Targeting the mitochondrial morphology, dynamism, and contacts with the ER could be an innovative strategy to deplete the cancer stem cell compartment to successfully treat glioblastoma.  相似文献   
10.
Directed evolution by error-prone PCR was applied to stabilize the cold-active lipase from Pseudomonas fragi (PFL). PFL displays high activity at 10 degrees C, but it is highly unstable even at moderate temperatures. After two rounds of evolution, a variant was generated with a 5-fold increase in half-life at 42 degrees C and a shift of 10 degrees C in the temperature optimum, nevertheless retaining cold-activity. The evolved lipase displayed specific activity higher than the wild type enzyme in the temperature range 29-42 degrees C. Biophysical measurements did not indicate any obvious difference between the improved variant and the wild type enzyme in terms of loss of secondary structure upon heat treatment, nor a shift in the apparent melting temperature.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号