The temporal expression of amyloid precursor protein mRNA in vitro in dissociated hippocampal neuron cultures

Aluminium and Alzheimer's disease

The hypothesis that aluminium (Al) is a cause of (or a risk factor in) the development of beta-amyloid plaques and neurofibrillary tangles (NFT) and dementia in Alzheimer's disease (AD) is based on studies by Wisniewski et al, Klatzo et al and Terry & Peña in 1965 that showed that injection of experimental animals with Al compounds induces the formation of NFT. Other publications revealed that Al affects cognitive functions in experimental animals and humans undergoing dialysis for renal failure. Electron probe and laser microprobe mass analysis (LAMMA) studies have demonstrated the presence of Al in NFT and cores of amyloid stars and nuclei of neurons in AD patients. Other studies have indicated the association between amyotrophic lateral sclerosis/Guam parkinsonism-dementia complex and Al in the environment. A recent report suggests that the chelating agent desferrioxamine slows the rate of cognitive decline in AD patients. Extensive studies of the pathology of AD and Al-induced encephalopathy by our group and others indicate that Al does not cause Alzheimer's disease neuropathology. However, under certain conditions, cognition can be affected when Al enters the brain. Therefore, for individuals with renal failure or undergoing dialysis or individuals with a damaged blood-brain barrier, the intake of Al should be controlled.

Characterization of mRNA species that are associated with postsynaptic density fraction by gene chip microarray analysis

We previously reported the partial identification by random sequencing of mRNA species that are associated with the postsynaptic density (PSD) fraction prepared from the rat forebrain [Tian et al., 1999. Mol. Brain Res. 72, 147-157]. We report here further characterization by gene chip analysis of the PSD fraction-associated mRNAs, which were prepared in the presence of RNase inhibitor. We found that mRNAs encoding various postsynaptic proteins, such as channels, receptors for neurotransmitters and neuromodulators, proteins involved in signaling, scaffold and adaptor proteins and cytoskeletal proteins, were highly concentrated in the PSD fraction, whereas those encoding housekeeping proteins, such as enzymes in the glycolytic pathway, were not. We extracted approximately 1900 mRNA species that were highly concentrated in the PSD fraction. mRNAs related to certain neuronal diseases were also enriched in the PSD fraction. We also constructed a cDNA library using the PSD fraction-associated mRNAs as templates, and identified 1152 randomly selected clones by sequencing. Our data suggested that the PSD fraction-associated mRNAs are a very useful resource, in which a number of as yet uncharacterized mRNAs are concentrated. Identification and functional characterization of them are essential for complete understanding of synaptic function.

Analysis of subcellularly localized mRNAs using in situ hybridization, mRNA amplification, and expression profiling

Targeting of mRNAs to distinct subcellular regions occurs in all polarized cells. The mechanisms by which RNA transport occurs are poorly understood. With the advent of RNA amplification methodologies and expression profiling it is now possible to catalogue the RNAs that are targeted to particular subcellular regions. In particular, neurons are polarized cells in which dendrites receive signals from presynaptic neurons. Upon stimulation (information receipt) the dendrite processes the information such that an immediate dendritic response is generated as well as a longer-term somatic response. The integrated cellular response results in a signal that can be propagated through the axon to the next post-synaptic neuron. Much previous work has shown that mRNAs can be localized in dendrites and that local translation in dendrites can occur. In this chapter the methods for analysis of RNAs that are localized to dendrites are reviewed and a partial list of dendritically localized RNAs is presented. This information may be useful in identifying RNA regulatory regions that are responsible for specifying rate of RNA transport and the dendritic sites at which targeted RNAs dock so that they can be translated.

BetaAPP and furin mRNA concentrates in immature senile plaques in the brain of Alzheimer patients

This study examined the possibility that in Alzheimer disease (AD) beta-amyloid precursor protein (betaAPP) mRNA is delivered to senile plaques (SPs) via dendritic processes. BetaAPP mRNA was detected in SPs by in situ hybridization, using a 1.4-kb cRNA in which both [35S]-UTP and [35S]-CTP were incorporated together. The betaAPP mRNA was compared with that of furin, a proteolytic enzyme putatively involved in betaAPP processing, and its orthologue proprotein convertase PCI served as a control. Human presenile AD cases with mostly immature SPs and AD cases generally with mature SPs were analyzed. To decrypt SPs after hybridization, brain sections were stained with thioflavin S. To establish relationships between the density of dystrophic fibers, the degree of plaque maturation, and the concentration of mRNA in SPs, the plaque maturity markers Abeta(1-42) and Abeta(1-40) peptides were co-localized with neurofilament protein 200 and compared with microtubule-associated protein 2 (MAP 2). The results suggest that immature, Abeta(1-42)- and dystrophic dendrite-containing SPs (but not mature SPs containing Abeta(1-40) and missing dystrophic dendrites) are capable of concentrating specific mRNAs. Dystrophic dendrites may thus serve as a route for the transport of specific mRNAs from the cell bodies to SPs.

Identification of mRNAs localizing in the postsynaptic region

Local protein synthesis using mRNAs readily distributed in the dendrites is believed to play an important role in maintaining the already expressed synaptic plasticity. To find proteins translated in the postsynaptic region, such as neuronal dendrites, we tried to identify the mRNAs associated with the postsynaptic density (PSD) fraction prepared from a rat's forebrain. The PSD-associated mRNAs were amplified by reverse transcriptase-based polymerase chain reaction (RT-PCR), separated by polyacrylamide gel electrophoresis, and sequenced. The database search revealed, among 130 mRNAs sequenced, 17 known and 108 unknown sequences, while five mRNAs were too short for the search. Of the mRNAs with unknown sequences, we selected 33 genes with a length longer than 150 bases, performed in situ hybridization, and found that at least 12 mRNA types were localized in the dendrites. These results suggest that a large number of mRNAs localize around the postsynaptic area of the neuronal cells in the central nervous system. In addition, our method proved efficient in identifying collectively the mRNAs localizing in the dendrites.

Dendritic localization of mRNAs

The dendritic localization of mRNAs has been proposed to underlie the structural and functional polarity of neurons, as well as certain aspects of synaptic plasticity. Even though there is no conclusive evidence that such a localization is a physiological requirement, studies of mRNA localization in relation to function in other cell types and recent experiments on synaptic plasticity suggest that this proposal may be correct.

Increased expression of dendritic mRNA following the induction of long-term potentiation

A small number of mRNAs, including Ca2+/calmodulin-dependent protein kinase II alpha-subunit (CamKIIalpha) mRNA and microtubule-associated protein 2 (MAP2) mRNA, are present in the dendrites of neurones as well as in the cell bodies. We show here that the induction of long-term potentiation (LTP) in the hippocampal perforant path/granule cell synapses in anaesthetised rats is associated with increased levels of CamKIIalpha mRNA and MAP2 mRNA in the granule cell dendrites after 2 h. Similarly, induction of LTP in the Schaffer collateral/CA1 pyramidal cell synapses in hippocampal slices maintained in vitro also results in elevated dendritic levels of CamKIIalpha mRNA and MAP2 mRNA 2 h later. In both models, the levels of various other mRNA species restricted to the cell body region were unaffected by the induction of LTP. Increased expression of dendritic CamKIIalpha mRNA and MAP2 mRNA appears to be a general feature of hippocampal plasticity, since it occurs following LTP induction in both the dentate gyrus and the CA1 region. The elevation of mRNA levels in a restricted region close to the afferent synapses would allow a highly-localised enhancement of the synthesis of the corresponding proteins, providing an elegant mechanism for protein-synthesis-dependent synaptic plasticity to maintain a high degree of anatomical specificity.

Cell-specific expression of beta-amyloid precursor protein isoform mRNAs and proteins in neurons and astrocytes

The abnormal accumulation of beta-amyloid (A beta) in senile plaques appears to be a central pathological process in Alzheimer's disease. A beta is formed by proteolysis of beta-amyloid precursor protein (APP) with several isoforms generated by alternative splicing of exons 7, 8 and 15. A semi-quantitative reverse transcription (RT)-polymerase chain reaction (PCR) analysis showed that APP695 mRNA lacking exon 7 and 8 was most abundant in primary cultures of rat neurons, while APP770 and APP751 representing, respectively, the full length and exon 8 lacking isoforms predominated in cultured astroglial cells. Antisera AP-2 and AP-4 were produced by immunizing rabbits with keyhole limpet haemocyanin coupled with synthetic peptides representing KPI region APP301-316 and A beta region APP670-686 of APP770, respectively. These polyclonal antisera were purified against the corresponding peptide using affinity chromatography. Western blot analysis of homogenates of relatively enriched neuronal and astroglial cultures showed that these antibodies discretely stained bands of proteins in a cell-specific manner. Dot-blot analysis using AP-2, AP-4 and 22C11 antibodies indicated that, in comparison with neurons, cultured astrocytes contained 3-fold greater KPI-containing APP isoform proteins. The amount of total APP proteins, which include both KPI-containing and KPI-lacking APP isoforms, was approximately 90% higher in astrocytes than in neurons. Consistent with these in vitro findings in cultured astrocytes, in fimbria-fornix lesioned rat hippocampus, labelling with AP-2 antibody, which specifically reacts with KPI-containing APP proteins, was mainly observed in glial fibrillary acidic protein-positive reactive astrocytes in vivo. The results showed that APP isoforms are expressed in a cell type-specific manner in the brain and, since deposition of A beta is closely associated with the expression of KPI-containing APP isoforms, provide further evidence for the involvement of astrocytes in plaque biogenesis.

Dopaminergic regulation of progesterone receptors: brain D5 dopamine receptors mediate induction of lordosis by D1-like agonists in rats

To characterize the signaling pathway by which the neurotransmitter dopamine modulates progesterone receptor (PR) activation, the steroid-dependent behavior lordosis was used in estrogen-primed ovariectomized Sprague-Dawley rats with stereotaxic implanted third ventricle cannulas. Lordosis was observed in response to solicitous males in females after central administration of the D1-like agonist SKF38393 and three of its analogs (SKF77434, SKF75640, and SKF85174). In contrast, D1-like antagonist SCH23390 and D1-like/D2 repopulation inhibitor EEDQ blocked behavior inducible by the D1-like agonists. Further, antisense oligonucleotides to D5, but not D1, dopamine receptor mRNA suppressed reproductive behavior associated with D1-like stimulation. This finding provides strong evidence that dopaminergic modulation of lordosis is mediated by the novel D5 dopamine receptor. Although D1, but not D5, dopamine receptor mRNAs were detected in the ventromedial nucleus (VMN) by in situ hybridization, agonists microinjected into the VMN, but not into the arcuate nucleus or preoptic area, induced lordosis, suggesting the functional presence of D5 dopamine receptors in the VMN. Also in support, D5 receptor mRNA antisense microinjected into the VMN blocked the subsequent induction of lordosis by D1-like agonists. Finally, facilitation of sex behavior by D1-like agonists was blocked by the antiprogestin RU38486 and PR antisense oligonucleotide. Collectively, the data provide strong evidence for dopaminergic modulation of reproductive behavior through D5 dopamine receptor-mediated modulation of PR-dependent behavior in rat CNS.

Ultrastructural basis for gene expression at the synapse: synapse-associated polyribosome complexes

This review summarizes what is known about the protein synthetic machinery that is selectively localized beneath postsynaptic sites on the dendrites of CNS neurons. This machinery, made up of polyribosomes and associated membranous cisterns, allows a local synthesis of key proteins at individual postsynaptic sites.

mRNA distribution within dendrites: relationship to afferent innervation

The majority of neuronal mRNAs are confined to cell bodies, but a few mRNAs are present at high levels in dendrites. Here we report an initial analysis of the relationship between afferent innervation and the distribution of mRNA within dendritic fields. In situ hybridization techniques were used to compare the subcellular distribution of dendritic mRNAs in principal neurons of the hippocampal formation in vivo. The mRNA encoding the alpha subunit of calcium/calmodulin dependent protein kinase II (CAMII kinase) was present at high levels throughout the layers that contain the dendrites of hippocampal pyramidal cells and dentate granule cells. In contrast, the mRNA encoding the high molecular weight microtubule-associated protein MAP2 had a more limited distribution. In the dentate gyrus, labeling for MAP2 was present in a discrete band in the lamina containing proximal dendrites and decreased to low levels in laminae containing distal dendrites. This laminar pattern resembles the distinct terminations of the commissural/associational projection (high MAP2 labeling) and the entorhinal projection (lower MAP2 labeling) upon dendrites of granule cells. To determine if the differential distribution of dendritic mRNAs was regulated by either the presence or activity of afferents, we evaluated mRNA distribution in the dentate molecular layer following (1) removal of the entorhinal input by lesions of the entorhinal cortex or (2) prolonged delivery of potentiating stimulation to entorhinal afferents. Denervation led to modest decreases in the levels of mRNAs for both CAMII and MAP2 but did not lead to detectable alterations in mRNA distribution. Also, prolonged stimulation did not lead to detectable alterations in MAP2 or CAMII mRNA distribution although such stimulation clearly elevated the expression of mRNA for glial fibrillary acidic protein (GFAP).

Targeting of mRNAs to subsynaptic microdomains in dendrites

Recent studies have revealed that a heterogeneous population of mRNAs is present in neuronal dendrites (including mRNAs that encode proteins involved in intracellular signaling), that different types of neurons have different assortments of dendritic mRNAs, and that the levels of some dendritic mRNAs are up-regulated by activity. These findings reinforce and extend the hypothesis that the localization of mRNA in dendrites provides a means of synthesizing proteins locally that are important for synaptic function.

Expression of the intermediate filament-associated protein related to beta-amyloid precursor protein is developmentally regulated in cultured cells

It was previously reported that a monoclonal antibody to beta-amyloid precursor protein (mab22C11; Boehringer Mannheim, Indianapolis, IN) labels an intermediate filament-associated protein (beta APP-IFAP) in cultured human skin fibroblasts (Dooley et al.: J Neurosci Res 33:60-67, 1992). The time course of its expression and association with different classes of intermediate filaments has been assessed in neurons, Schwann cells, and astrocytes in dissociated cultures of murine brain and spinal cord-dorsal root ganglia; in primary cultures of human muscle; and in the epithelial cell line PtK1. beta APP-IFAP was expressed in all non-neuronal cell types examined. Mab22C11 immunoreactivity was minimal or absent following dissociation or subculture, but gradually increased with time. In fibroblasts, myoblasts, and epithelial cells, the distribution eventually resembled that of vimentin. With the exception of glial fibrillary acidic protein (GFAP), beta APP-IFAP was not associated with the intermediate filament proteins characteristically found in differentiated cells, i.e., desmin, the cytokeratins, and neurofilament proteins. No labeling of neurons by mab22C11 was observed at any stage of in vitro maturation. In sections of Alzheimer's brain, the antibody labeled a subpopulation of reactive astrocytes. It is suggested that beta APP-IFAP may be the product of a member of the beta APP multigene family expressed developmentally in non-neuronal cells.

Evaluation of RNAs present in synaptodendrosomes: dendritic, glial, and neuronal cell body contribution

"Synaptodendrosomes" are subcellular fractions that contain pinched-off axon terminals and dendrites. These fractions are a potential source of RNAs that are localized in dendrites. However, these fractions may also contain RNAs that are seen in situ in neuronal cell bodies and glia. To evaluate whether synaptodendrosomes could be used as a source of dendritic RNA, we studied the RNA content of this fraction as compared with RNA isolated from total forebrain and a cell body-enriched fraction. RNA was analyzed by gel electrophoresis, oligo-dT chromatography, and northern blot hybridization. RNA from synaptodendrosomes contained a greater proportion of low-molecular-weight nonpolyadenylated RNAs. RNAs known to be present in dendrites (mRNA for the alpha subunit of the calcium/calmodulin-dependent protein kinase II and the polymerase III transcript BC1) were detected in synaptodendrosomes; RNAs that are restricted to neuronal and glial cell bodies (mRNAs for the 68-kDa neurofilament protein, 43-kDa growth-associated protein, beta-tubulin, and beta-actin) were present only at low levels. However, the mRNA for glial fibrillary acidic protein (seen in situ in glial cell bodies and processes) was present at high levels in the synaptodendrosomes. These results support and extend previous studies indicating that a limited subset of mRNAs is present in neuronal and astrocyte processes and reveal that both of these types of mRNAs are present in synaptodendrosomes. Thus, synaptodendrosomes may be useful as a source of dendritic RNAs, but it will be necessary to develop strategies to subtract mRNAs present in astroglial processes.

Altered levels and splicing of the amyloid precursor protein in the adult rat hippocampus after treatment with DMSO or retinoic acid

Alzheimer's disease and cognitive impairment in rats has been associated with an increase in the percentage of amyloid precursor protein (APP) containing the KPI domain. It has recently been reported that retinoic acid (RA) is capable of increasing the levels and altering the splicing ratio of APP in cultured SH-SY5Y cells. The effects of peripherally administered RA (64 or 640 micrograms/kg; i.p.; q.d.) on the abundance of APP, the ratio of the three major isoforms, and the relative abundance of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined by rtPCR in the hippocampus of aged rats. Corresponding changes in choline acetyltransferase (ChAT) activity were also measured. Vehicle (DMSO) treated rats exhibited a 2 x (P < 0.01) increase in total APP and an 8 x (P < 0.001) decrease in the cyclophilin transcript. In addition, DMSO increased the percentage of APP 695 from 89% in saline treated rats to 94%. Treatment of RA in DMSO decreased the accumulation of total APP relative to cyclophilin at both the low (6.4 x; P < 0.01) and high (8 x; P < 0.05) dosages when compared to DMSO treated rats. Furthermore, the level of APP-695 decreased to 82% with low dosage of RA and 75% at high dosage of the total APP transcripts. No significant change in either NGF, NT-3, or BDNF transcripts were observed following low or high dosage RA administration relative to cyclophilin RNA nor was a change in ChAT activity detected at either of the dosages tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of growth factors and cytokines on expression of amyloid beta protein precursor mRNAs in cultured neural cells

We analyzed the effect of several growth factors and cytokines on the expression of amyloid beta protein precursor (APP) mRNAs in cultured mouse neuronal and glial cells. In neuronal cultures from embryonic day-15 brain. Northern blotting revealed that APP mRNAs increased by 1.3- to 2.6-fold when treated with nerve growth factor, basic fibroblast growth factor, interleukin 1, interleukin 2, interleukin 3, interleukin 6 or granulocyte-macrophage colony-stimulating factor but not with tumor necrosis factor alpha. An S1 nuclease protection assay revealed that the enhanced APP mRNA in neuronal cultures was exclusively APP695 mRNA. On the other hand, astrocyte-enriched cultures prepared from postnatal day-2 brain did not show any significant alteration among these factors. We conclude that certain growth factors and cytokines could enhance APP 695 mRNA expression in neurons in vitro.

Differential distribution of cellular forms of beta-amyloid precursor protein in murine glial cell cultures

The production and localization of cell-associated forms of beta-amyloid precursor protein (APP) of Alzheimer's disease was investigated in primary cultures of mouse glial cells. In both oligodendrocytes and astrocytes, immunofluorescence staining with an antibody against the carboxy terminus of APP revealed an intense cytoplasmic immunoreactivity. Immunoblotting of the cell extracts detected differences in the composition of APP between oligodendrocytes and astrocytes, notably the abundance of 107 kDa subtype in oligodendrocytes. Differences in immunoblot patterns were also noted between two buffer-insoluble, membrane-rich subcellular fractions of the glial cells, nuclear-mitochondrial and microsomal; the 119 kDa APP was enriched in the former, whereas the 73 and 115 kDa APPs in the latter. The results suggest that each APP subspecies may play a distinct functional role in different cell types and subcellular fractions.

Getting the message from the gene to the synapse: sorting and intracellular transport of RNA in neurons

A key question in cellular neurobiology is how neurons target molecules to cellular microdomains at a distance from the nucleus. Of special importance are the thousands of postsynaptic sites that form the basis for synaptic communication. Recent evidence suggests that an important aspect of molecular trafficking involves differential sorting, selective intracellular transport, and docking of particular mRNA molecules and associated protein synthetic machinery at postsynaptic sites. This offers the potential for local regulation of the production of key proteins in response to conditions at individual synapses. This article reviews what is known about the mechanisms of mRNA trafficking in neurons and in other cells ranging from oocytes to oligodendrocytes, and considers the possible role that mRNA trafficking and the resulting local synthesis of particular proteins may play in cellular function.

APP-collagen interaction is mediated by a heparin bridge mechanism

The amyloid precursor protein (APP) is a glycoprotein consisting of at least four isoforms derived from a single gene by a process of alternative splicing. The membrane-bound forms of APP have been suggested to have adhesive properties and to mediate neural cell adhesion. Previous studies have demonstrated the ability of Fab' fragments of antibodies to extracellular domains of APP to inhibit neural cell binding to a collagen substrate, suggesting a physiological role for the collagen-binding properties of APP. The binding of APP has been demonstrated to be specific for type IV collagen, and no binding to other extracellular matrix components, including fibronectin and laminin, was detected. The APP-collagen binding appeared to be mediated by a heparin-bridge mechanism, since the binding was abolished by the addition of excess heparan or heparinase. These results were observed by both a homogenate-collagen binding assay and a cell-surface adhesion assay, thus providing further evidence for the adhesion role of APP. They also pose the question of the possible role of the heparin-binding properties of APP in the genesis of the neuritic plaques characteristic of Alzheimer's disease.

Differential APP gene expression in rat cerebral cortex, meninges, and primary astroglial, microglial and neuronal cultures

Differential amyloid precursor protein (APP) gene expression was investigated in primary cultures of astrocytes, neurons and microglia from neonatal rat cerebral cortex as well as in meninges, and young and adult cerebral cortex tissues in order to define the possible contribution of individual CNS cell types in beta AP deposition. Meninges and neurons contained higher levels of total APP mRNA than glial cells and APP695 mRNA was abundant in neurons while glial cells and meninges contained higher levels of KPI-containing mRNAs. These results demonstrate cell-specific transcriptional and post-transcriptional regulation of APP gene expression in CNS cell types. In addition, the steady-state level of APPs in each cell type did not reflect mRNA levels indicating translational or post-translational regulation.