抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

In both metazoan development and metastatic cancer, migrating cells must carry out a detailed, complex program of sensing cues, binding substrates, and moving their cytoskeletons. The linker cell in Caenorhabditis elegans males undergoes a stereotyped migration that guides gonad organogenesis, occurs with precise timing, and requires the nuclear hormone receptor NHR-67. To better understand how this occurs, we performed RNA-seq of individually staged and dissected linker cells, comparing transcriptomes from linker cells of third-stage (L3) larvae, fourth-stage (L4) larvae, and nhr-67-RNAi–treated L4 larvae. We observed expression of 8,000–10,000 genes in the linker cell, 22–25% of which were up- or down-regulated 20-fold during development by NHR-67. Of genes that we tested by RNAi, 22% (45 of 204) were required for normal shape and migration, suggesting that many NHR-67–dependent, linker cell-enriched genes play roles in this migration. One unexpected class of genes up-regulated by NHR-67 was tandem pore potassium channels, which are required for normal linker-cell migration. We also found phenotypes for genes with human orthologs but no previously described migratory function. Our results provide an extensive catalog of genes that act in a migrating cell, identify unique molecular functions involved in nematode cell migration, and suggest similar functions in humans.

https://science.sciencemag.org/content/sci/256/5054/184.full.pdf

Alzheimer's disease: the amyloid cascade hypothesis

Rapid progress in deciphering the biological mechanism of Alzheimer's disease (AD) has arisen from the application of molecular and cell biology to this complex disorder of the limbic and association cortices. In turn, new insights into fundamental aspects of protein biology have resulted from research on the disease. This beneficial interplay between basic and applied cell biology is well illustrated by advances in understanding the genotype-to-phenotype relationships of familial Alzheimer's disease. All four genes definitively linked to inherited forms of the disease to date have been shown to increase the production and/or deposition of amyloid β-protein in the brain. In particular, evidence that the presenilin proteins, mutations in which cause the most aggressive form of inherited AD, lead to altered intramembranous cleavage of the β-amyloid precursor protein by the protease called γ-secretase has spurred progress toward novel therapeutics. The finding that presenilin itself may be the long-sought γ-...

Alzheimer's Disease: Genes, Proteins, and Therapy

Parkinson's disease is a common neurodegenerative disease with complex clinical features1. Autosomal recessive juvenile parkinsonism (AR-JP)2,3 maps to the long arm of chromosome 6 (6q25.2-q27) and is linked strongly to the markers D6S305 and D6S253 (ref. 4); the former is deleted in one Japanese AR-JP patient5. By positional cloning within this microdeletion, we have now isolated a complementary DNA clone of 2,960 base pairs with a 1,395-base-pair open reading frame, encoding a protein of 465 amino acids with moderate similarity to ubiquitin at the amino terminus and a RING-finger motif at the carboxy terminus. The gene spans more than 500 kilobases and has 12 exons, five of which (exons 3–7) are deleted in the patient. Four other AR-JP patients from three unrelated families have a deletion affecting exon 4 alone. A 4.5-kilobase transcript that is expressed in many human tissues but is abundant in the brain, including the substantia nigra, is shorter in brain tissue from one of the groups of exon-4-deleted patients. Mutations in the newly identified gene appear to be responsible for the pathogenesis of AR-JP, and we have therefore named the protein product ‘Parkin’.

Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism

Parkinson's disease: Mechanisms and models

The human mas oncogene, which renders transfected NIH/3T3 cells tumorigenic, was identified as a subtype of angiotensin receptor by transient expression in Xenopus oocytes and stable expression in the mammalian neuronal cell line, NG115-401L. The mas receptor preferentially recognizes angiotensin III, and is expressed at high levels in brain. The mas/angiotensin receptor functions through the breakdown of inositol lipids and can drive DNA synthesis, unlike another inositol-linked peptide receptor, that for bradykinin. Comparative analysis of several early biochemical events elicited by either angiotensin or bradykinin stimulation of mas-transfected cells has not indicated a specific difference correlated with mitogenic activity. In particular, the inositol lipid kinase, phosphatidylinositol-3-kinase, thought to be involved in the mitogenic mechanism of platelet-derived growth factor receptors, is unaffected by activation of mas. These results have shown that a proto-oncogene encodes a neural peptide receptor, indicating that peptide receptors may be involved in differentiation and proliferation processes, as are other identified proto-oncogenes.

The mas oncogene as a neural peptide receptor: expression, regulation and mechanism of action.

The present inventions relates to substances which are capable of controlling the inclusion of exon (10) of the tau gene. The substances can reverse 'wrong' tau splicing pattern and. Since 'wrong' tau splicing patterns are associated with tauopathies that cause dementia, the substances can be used as therapeutic agents. The invention also provides a method for testing substances that control tau exon (10) inclusion. The method is suitable for a high throughput screening. Additional products which are useful for changing tau exon (10) expression or for performing the test method are described as well.

Substance capable of controlling the inclusion of exon 10 of the tau gene, and its therapeutic use against tauopathies

The ontogenetic development of neurotensin-like immunoreactivity and neurotensin receptors in the cat striatum.

Abundant neurofibrillary tangles, neuropil threads and senile plaque neurites constitute the neurofibrillary pathology of Alzheimer's disease. They form in the nerve cells that undergo degeneration in the disease, in which their regional distribution correlates with the degree of dementia. Each lesion contains the paired helical filament (PHF) as its major fibrous component. Recent work has shown that PHFs are composed of the microtubule-associated protein tau in a hyperphosphorylated state. PHF-tau is hyperphosphorylated on six adult brain tau isoforms. As a consequence, tau is unable to bind to microtubules and is believed to self-assemble into the PHF. Current evidence suggests that protein kinases or protein phosphatases with a specificity for serine/threonine-proline residues are instrumental in the hyperphosphorylation of tau. Candidate protein kinases include mitogen-activated protein kinase, glycogen synthase kinase 3 and cyclin-dependent kinase 5, whereas the trimeric form of protein phosphatase 2A is a candidate phosphatase.

Molecular dissection of the neurofibrillary lesions of Alzheimer's disease.

The most common neurodegenerative diseases are characterized by the presence of abnormal filamentous protein inclusions in nerve cells of the brain. In Alzheimer's disease, these inclusions are made of hyperphosphorylated tau protein (1). Together with the extracellular β-amyloid deposits, they constitute the defining neuropathological characteristics of Alzheimer's disease. Tau inclusions, in the absence of extracellular deposits, are characteristic of progressive supranuclear palsy, corticobasal degeneration, Pick's disease, argyrophilic grain disease, and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) (1). The identification of mutations in Tau in FTDP-17 (2–4) has established that dysfunction of tau protein is central to the neurodegenerative process. At an experimental level, the expression of mutant human tau in nerve cells is leading to improved models of neurodegeneration. In this issue of PNAS, Kraemer et al. (5) describe lines of Caenorhabditis elegans expressing transgenic wild-type and mutant human tau protein. They represent an important addition to existing transgenic models for the human tauopathies.

Tau protein is widely expressed in the mammalian nervous system, where it plays a role in the assembly and stabilization of microtubules (1). In adult human brain, there are six isoforms of tau, produced from a single gene by alternative mRNA splicing (6, 7). They differ by the presence or absence of a 29- or 58-aa insert in the N-terminal half and by the inclusion, or not, of a 31-aa repeat, encoded by exon 10 of Tau, in the C-terminal half of the protein (Fig. 1a). The exclusion of exon 10 leads to the production of three isoforms, each containing three repeats, and its inclusion leads to a further three isoforms, each containing four repeats. The repeats constitute the microtubule-binding region of tau, and similar levels of Three- And four-repeat isoforms are expressed in adult human brain. Repeat sequences homologous to those in tau are also present in the high-molecular-weight microtubule-associated proteins MAP2 and MAP4. The genomes of C. elegans and Drosophila melanogaster encode only one protein with tau-like repeats.



Fig. 1.

Mutations in Tau in FTDP-17. (a) Schematic diagram of the six tau isoforms (352–441 aa) expressed in adult human brain, with mutations in the coding region indicated by using the numbering of the 441-aa isoform. The six isoforms are produced ...



Tau mutations in FTDP-17 are either missense, deletion, or silent mutations in the coding region, or intronic mutations located close to the splice-donor site of the intron following exon 10 (1). So far, 31 different mutations have been described in >80 families with FTDP-17 (Fig. 1). Functionally, Tau mutations fall into two largely nonoverlapping categories, those that influence the alternative splicing of tau pre-mRNA and those whose primary effect is at the protein level. The intronic mutations and most coding region mutations in exon 10 increase the splicing of this exon, changing the ratio between three- and four-repeat isoforms (3, 4). Approximately half of the known mutations have their primary effect at the RNA level. They affect exon splicing enhancer and silencer sequences in exon 10 (8) or destabilize a predicted stem-loop structure located at the boundary between exon 10 and the intron that follows it (3, 4, 9) (Fig. 1b). Thus, to a significant extent, FTDP-17 is a disease of alternative mRNA splicing. The other mutations affect tau isoforms directly. In accordance with their location in the microtubule-binding region, most missense mutations and the deletion mutations lead to a reduced ability of tau to promote microtubule assembly (10, 11). A number of mutations may cause FTDP-17, at least in part, by promoting the assembly of tau into filaments (12, 13).

Kraemer et al. (5) expressed the 412-aa isoform of human tau in nerve cells of C. elegans, either in the wild-type form or with a missense mutation of FTDP-17 (P301L or V337M, Fig. 1a). This resulted in a reduced lifespan, behavioral impairment, defective cholinergic neurotransmission, the accumulation of insoluble phosphorylated tau, and neurodegeneration, as indicated by axonal damage and nerve cell loss. Although similar changes were observed in all transgenic lines, expression of mutant tau resulted in a more severe neurodegenerative phenotype than expression of wild-type tau. Soluble, phosphorylated tau was expressed 1 day after hatching, when the nervous system looked normal. However, uncoordinated behavior was already apparent in some lines, indicating that the simple expression of mutant human tau was sufficient to cause nerve cell dysfunction. By immunohistochemistry, tau staining was observed in most nerve cells and their processes. Insoluble, phosphorylated tau was first detected 5–7 days after hatching. Its appearance was paralleled by progressive axonal degeneration and nerve cell loss, which were more severe in mutant than in wild-type tau lines. By electron microscopy, 9-day-old worms from the mutant lines showed degenerating axons throughout the ventral and dorsal nerve cords. By immunoelectron microscopy, tau staining in the axons was largely diffuse. Amorphous, tau-positive aggregates were only observed in worms transgenic for V337M tau.

How do these molecular and cellular changes compare with what has been observed in human diseases with tau pathology? Tauopathies in worms and humans share the progressive accumulation of insoluble tau and extensive neurodegeneration. However, there also some differences, the most conspicuous of which are a more modest phosphorylation and the absence of tau filaments in C. elegans. This is reminiscent of work in D. melanogaster, where the expression of wild-type and mutant human tau proteins in nerve cells led to a reduced lifespan and the loss of nerve cells, in the absence of tau filaments (14, 15). Phosphorylation of tau appeared to be more extensive in the fly than in the worm. Moreover, coexpression of human tau with the fly homologue of glycogen synthase kinase-3β, a candidate protein kinase for the hyperphosphorylation of tau, resulted in accelerated neurodegeneration and the formation of tau-immunoreactive inclusions (15). Filamentous structures were observed, but they were not shown to be decorated by anti-tau antibodies. In contrast to what has been described in FTDP-17 (16), tau-induced neurodegeneration involved programmed cell death (15). Taken together, it appears that conformationally altered, nonfilamentous human tau protein is neurotoxic in invertebrates.


Tauopathies in worms and humans share the accumulation of insoluble tau and extensive neurodegeneration.


What about vertebrate models for the human tauopathies? Expression of wild-type human tau in nerve cells of the sea lamprey has been shown to lead to the accumulation of filaments made of hyperphosphorylated tau and degenerative changes (17, 18), indicating a possible link between the formation of tau filaments and the degeneration of nerve cells. Most work has been done in the mouse (19–30), where the transgenic expression of single isoforms of wild-type human tau resulted in large numbers of nerve cells with abnormal tau-immunoreactive cell bodies and dendrites, as well as signs of axonal degeneration and muscle weakness (19–23). Abundant tau filaments were not observed, neither was substantial nerve cell loss. The level of tau phosphorylation was lower than in the human tauopathies. Overexpression of nonfilamentous tau can thus cause axonopathy and muscle weakness in the mouse. However, these transgenic lines are incomplete models for the human tauopathies.

Mouse lines expressing single tau isoforms with missense mutations of FTDP-17 in nerve cells have provided more complete models (25–30). Some of these lines exhibited the essential molecular and cellular features of human tauopathies, including the formation of abundant filaments made of hyperphosphorylated tau protein and nonapoptotic nerve cell loss (25, 29). Filamentous tau was hyperphosphorylated to the same extent as in human tauopathy brains, and most of the same sites were also phosphorylated in soluble tau, consistent with the view that hyperphosphorylation precedes filament assembly. In a recent study, an increase in the phosphorylation of soluble tau resulted in increased filament formation, suggesting that phosphorylation of tau can drive filament formation (31).

Work on mouse models has shown a correlation between the formation of tau filaments and neurodegeneration, but it is not known whether the two phenomena are causally related. The findings in C. elegans and D. melanogaster suggest that this may not be the case, because neurodegeneration was effected by nonfilamentous tau of reduced solubility. Alternatively, the mechanisms of neurodegeneration in worms and flies may differ from those in mice and humans.

Disease models in C. elegans offer some advantages over mouse models, in particular with regard to the speed and relative ease with which genetic modifiers of disease phenotype can be discovered and pharmacological modifiers can be screened. Once identified, their relevance for mammalian model systems can be tested. In the future, this combined approach is likely to lead to a better understanding of the detailed molecular mechanisms by which the dysfunction of tau protein can cause neurodegeneration.

https://www.pnas.org/content/pnas/100/17/9653.full.pdf

Michel Goedert

Papers

The mas oncogene as a neural peptide receptor: expression, regulation and mechanism of action.

Substance capable of controlling the inclusion of exon 10 of the tau gene, and its therapeutic use against tauopathies

The ontogenetic development of neurotensin-like immunoreactivity and neurotensin receptors in the cat striatum.

Molecular dissection of the neurofibrillary lesions of Alzheimer's disease.

Neurodegenerative tauopathy in the worm.