Development and maintenance of the neuronal cytoskeleton in aggregated cell cultures of fetal rat telencephalon and influence of elevated K+ concentrations

Circadian and sleep/wake-dependent variations in tau phosphorylation are driven by temperature

Study Objectives

Aggregates of hyperphosphorylated tau protein are a hallmark of Alzheimer’s disease (AD) and other tauopathies. Sleep disturbances are common in AD patients, and insufficient sleep may be a risk factor for AD. Recent evidence suggests that tau phosphorylation is dysregulated by sleep disturbances in mice. However, the physiological regulation of tau phosphorylation during the sleep–wake cycle is currently unknown. We thus aimed to determine whether tau phosphorylation is regulated by circadian rhythms, inherently linked to the sleep–wake cycle.

Methods

To answer these questions, we analyzed by Western blotting tau protein and associated kinases and phosphatases in the brains of awake, sleeping, and sleep-deprived B6 mice. We also recorded their temperature.

Results

We found that tau phosphorylation undergoes sleep-driven circadian variations as it is hyperphosphorylated during sleep but not during acute sleep deprivation. Moreover, we demonstrate that the mechanism behind these changes involves temperature, as tau phosphorylation was inversely correlated with circadian- and sleep deprivation-induced variations in body temperature, and prevented by housing the animals at a warmer temperature. Notably, similar changes in tau phosphorylation were reproduced in neuronal cells exposed to temperatures recorded during the sleep–wake cycle. Our results also suggest that inhibition of protein phosphatase 2A (PP2A) may explain the hyperphosphorylation of tau during sleep-induced hypothermia.

Conclusion

Taken together, our results demonstrate that tau phosphorylation follows a circadian rhythm driven mostly by body temperature and sleep, and provide the physiological basis for further understanding how sleep deregulation can affect tau and ultimately AD pathology.

Lead acetate toxicity in vitro: Dependence on the cell composition of the cultures

It is well known that exposure to low doses of lead causes long-lasting neurobehavioural deficits, but the cellular changes underlying these behavioural changes remain to be elucidated. A protective role of glial cells on neurons through lead sequestration by astrocytes has been proposed. The possible modulation of lead neurotoxicity by neuron-glia interactions was examined in three-dimensional cultures of foetal rat telencephalon. Mixed-brain cell cultures or cultures enriched in either neurons or glial cells were treated for 10 days with lead acetate (10(-6) m), a concentration below the limit of cytotoxicity. Intracellular lead content and cell type-specific enzyme activities were determined. It was found that in enriched cultures neurons stored more lead than glial cells, and each cell type alone stored more lead than in co-culture. Moreover, glial cells but not neurons were more affected by lead in enriched culture than in co-culture. These results show that neuron-glia interactions attenuate the cellular lead uptake and the glial susceptibility to lead, but they do not support the idea of a protective role of astrocytes.

Memory impairments and hippocampal modifications in adult rats with neonatal isolation stress experience

Early life events have profound consequences. Our research demonstrates that the early life stress of neonatal isolation (1-h individual isolation on postnatal days 2-9) in rats has immediate and enduring neural and behavioral effects. Recently, we showed neonatal isolation impaired hippocampal-dependent context conditioned fear in adult rats. We now expand upon this finding to test whether neonatal isolation impairs performance in inhibitory avoidance and in the non-aversive, hippocampal-dependent object recognition task. In addition to assessments of hippocampal-dependent memory, we examined if neonatal isolation results in cellular alterations in the adult hippocampus. This was measured with antibodies that selectively label calpain-mediated spectrin breakdown product (BDP), a marker of cytoskeletal modification that can have neuronal consequences. Neonatally isolated male and female rats showed impaired performance in both memory tasks as well as elevated BDP levels in hippocampal immunoblot samples. In tissue sections stained for BDP, the cytoskeletal fragmentation was localized to pyramidal neurons and their proximal dendrites. Interestingly, the hippocampal samples also exhibited reduced staining for the postsynaptic marker, GluR1. Neonatal isolation may render those neurons involved in memory encoding to be vulnerable to calpain deregulation and synaptic compromise as shown previously with brain injury. Together with our prior research showing enhanced striatal-dependent learning and neurochemical responsivity, these results indicate that the early experience of neonatal isolation causes enduring yet opposing region-specific neural and behavioral alterations.

Excitotoxic protection by polyanionic polysaccharide: evidence of a cell survival pathway involving AMPA receptor-MAPK Interactions

Growing numbers of studies indicate that polysaccharides influence signaling events important for brain function. It has been speculated that such polysaccharide modulation of neuronal signals can promote synaptogenesis and cell maintenance. Here, we tested whether dextran sulfate, a polyanion that mimics natural mucopolysaccharides, protects hippocampal neurons against excitotoxic insults. An excitotoxin was applied to primary hippocampal cultures in the absence or presence of a large 500-kDa dextran sulfate (DS-L), a smaller 5-8-kDa species (DS-S), or sulfate-free dextran of 500 kDa. Only DS-L prevented neuronal damage as determined by a membrane permeability assay and phase contrast morphology. The sulfate and size dependence is also characteristic of DS-L's modulatory action on the channel activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors. The extent of neuroprotection correlates with the level of modulation of AMPA responses, and DS-L exhibits comparable EC(50) values for the two effects (3-7 nM). DS-L also modulates the link between AMPA receptors and mitogen-activated protein kinase (MAPK) involving extracellular signal-regulated protein kinase (ERK), well known for its involvement in cell survival and repair. Correspondingly, protection against N-methyl-D-aspartate (NMDA) excitotoxicity was evident in hippocampal slice cultures when DS-L was applied 30 min postinsult. These findings suggest that polysaccharides elicit neuroprotection in the brain, including enhanced repair responses through the AMPA receptor-MAPK axis.

Dual modulation of endocannabinoid transport and fatty acid amide hydrolase protects against excitotoxicity

The endocannabinoid system has been suggested to elicit signals that defend against several disease states including excitotoxic brain damage. Besides direct activation with CB1 receptor agonists, cannabinergic signaling can be modulated through inhibition of endocannabinoid transport and fatty acid amide hydrolase (FAAH), two mechanisms of endocannabinoid inactivation. To test whether the transporter and FAAH can be targeted pharmacologically to modulate survival/repair responses, the transport inhibitor N-(4-hydroxyphenyl)-arachidonamide (AM404) and the FAAH inhibitor palmitylsulfonyl fluoride (AM374) were assessed for protection against excitotoxicity in vitro and in vivo. AM374 and AM404 both enhanced mitogen-activated protein kinase (MAPK) activation in cultured hippocampal slices. Interestingly, combining the distinct inhibitors produced additive effects on CB1 signaling and associated neuroprotection. After an excitotoxic insult in the slices, infusing the AM374/AM404 combination protected against cytoskeletal damage and synaptic decline, and the protection was similar to that produced by the stable CB1 agonist AM356 (R-methanandamide). AM374/AM404 and the agonist also elicited cytoskeletal and synaptic protection in vivo when coinjected with excitotoxin into the dorsal hippocampus. Correspondingly, potentiating endocannabinoid responses with the AM374/AM404 combination prevented behavioral alterations and memory impairment that are characteristic of excitotoxic damage. The protective effects mediated by AM374/AM404 were (1) evident 7 d after insult, (2) correlated with the preservation of CB1-linked MAPK signaling, and (3) were blocked by a selective CB1 antagonist. These results indicate that dual modulation of the endocannabinoid system with AM374/AM404 elicits neuroprotection through the CB1 receptor. The transporter and FAAH are modulatory sites that may be exploited to enhance cannabinergic signaling for therapeutic purposes.

Neuronal activity and neurotrophic factors regulate GAD-65/67 mRNA and protein expression in organotypic cultures of rat visual cortex

Environmental factors are known to regulate the molecular differentiation of neocortical interneurons. Their class-defining transmitter synthetic enzymes are the glutamic acid decarboxylases (GAD); yet, fairly little is known about the developmental regulation of transcription and translation of the GAD-65/67 isoforms. We have characterized the role of neuronal activity, neurotrophins and afferent systems for GAD-65/67 expression in visual cortex in organotypic cultures (OTC) compared with in vivo in order to identify cortex-intrinsic regulatory mechanisms. Spontaneously active OTC prepared at postnatal day 0 displayed from 10 days in vitro (DIV) onwards 12-14% GAD-65/GAD-67 neurons similar to in vivo. However, GAD-65 mRNA was higher, whereas GAD-67 protein was lower, than in vivo. During the first week neurotrophins increased whereas the Trk receptor inhibitor K252a and MEK inhibitors decreased both GAD mRNAs and proteins. After 10 DIV GAD expression no longer depended on neurotrophin signalling. Activity-deprived OTC revealed only 6% GAD-67 neurons and mRNA and protein were reduced by 50%. GAD-65 mRNA was less reduced, but protein was reduced by half, suggesting translational regulation. Upon recovery of activity GAD mRNAs, cell numbers, and both proteins quickly returned to normal and these 'adult' levels were resistant to late-onset deprivation. In 20 DIV activity-deprived OTC, only neurotrophin 4 increased GAD-65/67 mRNAs, rescued the percentage of GAD-67 neurons and increased both proteins in a TrkB-dependent manner. Activity deprivation had thus shifted the period of neurotrophin sensitivity to older ages. The results suggested neuronal activity as a major regulator differentially affecting transcription and translation of the GAD isoforms. The early presence of neuronal activity promoted the GAD expression in OTC to a neurotrophin-independent state suggesting that neurotrophins play a context-dependent role.

Changes of KCl sensitivity of proliferating neural progenitors during in vitro neurogenesis

The effects of KCl-treatment on the survival and proliferation of NE-4C self-renewing neural progenitor cells were investigated during early phases of in vitro induced neurogenesis. NE-4C cells, derived from the anterior brain vesicles of embryonic mouse (E9), divided continuously under non-inducing conditions, but acquired neuronal features within 6 days, if induced by all-trans retinoic acid (RA). During the first 2 days of induction, the cells went on proliferating and did not show signs of morphological differentiation. In this stage, the resting membrane potential of RA-induced cells adopted more negative values in comparison to non-induced ones. Despite the increased membrane polarity and K+ conductance, addition of 20-50 mM KCl failed to elicit inward Na+ currents and did not induce an increase in the intracellular Ca+ level. Long-term treatment with 25 mM KCl, on the other hand, resulted in a selective loss of cells committed to neuronal fate by both decreasing the rate of cell proliferation and increasing the rate of cell death. The data indicate that the viability and proliferation of neural progenitors are influenced by extracellular K+-level in a differentiation stage-dependent manner.

Long-term depolarization changes morphological parameters of PC12 cells

It is well known that neuronal differentiation is strongly dependent on the intracellular level of free calcium ions ([Ca2+]i). In the present study the morphological and intracellular free calcium concentration changes were compared on PC12 pheochromocytoma cells cultured in control conditions and in a medium with high KCl level. Culturing PC12 cells in a medium with 20-30 mM KCl deprived of nerve growth factor supported cell proliferation and rapid growth of small neurite-like processes. However, their lengths did not increase with prolongation of the time of culturing. During culturing with 40 mM KCl the growth of these processes became blocked; the cells stopped proliferating and showed signs of degeneration. Measurements of [Ca2+]i level during the first days of PC12 cells culturing in a hyperpotassium medium indicate that such changes in this level could be an important factor in the induction of the observed morphological alterations; however, other effects induced by membrane depolarization may also be responsible for them.

Differentiation of rat striatal embryonic stem cells in vitro: monolayer culture vs. three-dimensional coculture with differentiated brain cells

Several groups have demonstrated the existence of self-renewing stem cells in embryonic and adult mouse brain. In vitro, these cells proliferate in response to epidermal growth factor, forming clusters of nestin-positive cells that may be dissociated and subcultured repetitively. Here we show that, in stem cell clusters derived from rat embryonic striatum, cell proliferation decreased with increasing number of passages and in response to elevated concentrations of potassium (30 mM KCl). In monolayer culture, the appearance of microtubule-associated protein type-5-immunoreactive (MAP-5(+)) cells (presumptive neurons) in response to basic fibroblast growth factor (bFGF) was reduced at low cell density and with increasing number of passages. In the presence of bFGF, elevated potassium caused a more differentiated neuronal phenotype, characterized by an increased proportion of MAP-5(+) cells, extensive neuritic branching, and higher specific activity of glutamic acid decarboxylase. Dissociated stem cells were able to invade cultured brain cell aggregates containing different proportions of neurons and glial cells, whereas they required the presence of a considerable proportion of glial cells in the host cultures to become neurofilament H-positive. The latter observation supports the view that astrocyte-derived factors influence early differentiation of the neuronal cell lineage.

Peroxisome proliferator-activated receptor beta regulates acyl-CoA synthetase 2 in reaggregated rat brain cell cultures

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate the expression of many genes involved in lipid metabolism. The biological roles of PPARalpha and PPARgamma are relatively well understood, but little is known about the function of PPARbeta. To address this question, and because PPARbeta is expressed to a high level in the developing brain, we used reaggregated brain cell cultures prepared from dissociated fetal rat telencephalon as experimental model. In these primary cultures, the fetal cells initially form random aggregates, which progressively acquire a tissue-specific pattern resembling that of the brain. PPARs are differentially expressed in these aggregates, with PPARbeta being the prevalent isotype. PPARalpha is present at a very low level, and PPARgamma is absent. Cell type-specific expression analyses revealed that PPARbeta is ubiquitous and most abundant in some neurons, whereas PPARalpha is predominantly astrocytic. We chose acyl-CoA synthetases (ACSs) 1, 2, and 3 as potential target genes of PPARbeta and first analyzed their temporal and cell type-specific pattern. This analysis indicated that ACS2 and PPARbeta mRNAs have overlapping expression patterns, thus designating the ACS2 gene as a putative target of PPARbeta. Using a selective PPARbeta activator, we found that the ACS2 gene is transcriptionally regulated by PPARbeta, demonstrating a role for PPARbeta in brain lipid metabolism.

Separate neuronal and glial Na+,K+-ATPase isoforms regulate glucose utilization in response to membrane depolarization and elevated extracellular potassium

The role of cell type-specific Na+,K+-ATPase isozymes in function-related glucose metabolism was studied using differentiated rat brain cell aggregate cultures. In mixed neuron-glia cultures, glucose utilization, determined by measuring the rate of radiolabeled 2-deoxyglucose accumulation, was markedly stimulated by the voltage-dependent sodium channel agonist veratridine (0.75 micromol/L), as well as by glutamate (100 micromol/L) and the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) (10 micromol/L). Significant stimulation also was elicited by elevated extracellular potassium (12 mmol/L KCl), which was even more pronounced at 30 mmol/L KCl. In neuron-enriched cultures, a similar stimulation of glucose utilization was obtained with veratridine, specific ionotropic glutamate receptor agonists, and 30 mmol/L but not 12 mmol/L KCl. The effects of veratridine, glutamate, and NMDA were blocked by specific antagonists (tetrodotoxin, CNQX, or MK801, respectively). Low concentrations of ouabain (10(-6) mol/L) prevented stimulation by the depolarizing agents but reduced only partially the response to 12 mmol/L KCl. Together with previous data showing cell type-specific expression of Na+,K+-ATPase subunit isoforms in these cultures, the current results support the view that distinct isoforms of Na+,K+-ATPase regulate glucose utilization in neurons in response to membrane depolarization, and in glial cells in response to elevated extracellular potassium.

Aggregating Brain Cell Cultures as a Model to Study Ischaemia-induced Neurodegeneration

Rotation-mediated aggregating brain cell cultures at two different maturational stages (DIV 11 and DIV 20) were subjected for 1 or 2 hours to ischaemic conditions by transient immobilization (arrest of media circulation). During recovery, cell damage was evaluated by measuring changes in cell type-specific enzyme activities and total protein content. It was found that in immature cultures (DIV 11), immobilization for 1 or 2 hours did not affect the parameters measured. By contrast, at DIV 20, ischaemic conditions for 1 hour caused a pronounced decrease in the activities of glutamic acid decarboxylase and choline acetyltransferase. A significant decrease in these neuron-specific enzyme activities was found at post-ischaemic days 1-14, indicating immediate and irreversible neuronal damage. The activity of the astrocyte-specific enzyme, glutamine synthetase, was significantly increased at 4 days post-treatment; equal to control values at 6 days; and significantly decreased at 14 days after the ischaemic insult. Immobilization of DIV 20 cultures for 2 hours caused a drastic reduction in all the parameters measured at post-ischaemic day 6. Generally, the ischaemic conditions appeared to be more detrimental to neurons than to astrocytes, and GABAergic neurons were more affected than cholinergic neurons.

Brief potassium depolarization decreases levels of neurofilament proteins in CNS culture

Little is known about the effects of brief potassium depolarization that occurs concurrently with transient ischemia, epilepsy and head trauma. To investigate the effect of short-term depolarization on light (NF-L), middle (NF-M), and heavy (NF-H) neurofilament proteins and determine the role played by calcium in that effect, mixed septo-hippocampal cultures were exposed to 60 mM K+ for 6 min, in the presence of 0 to 11.8 mM Ca2+. Twenty-four hours later, neurofilament immunoreactivity in Western blots of depolarized cultures was decreased to 60% or less of control levels. Decreases were Ca2+-dependent, not due to cell loss, and affected both phosphorylated and nonphosphorylated proteins. The phosphorylation state of NF-M and NF-H influenced the degree of loss observed. Changes in the pattern of immunolabelling of neuritic processes were also associated with depolarization. Thus, brief potassium depolarization may contribute to cytoskeletal disruption following brain injury.

Time course of neurofilament protein loss following depolarization-induced injury in CNS culture

In septo-hippocampal cell cultures, brief potassium depolarization produces calcium-dependent decreases in neurofilament proteins and loss of fine neuritic processes within 24 h. It is not known whether neurons later exhibit delayed degeneration and die, live with enduring neurofilament loss, or restore neurofilament protein levels. Therefore, we exposed septohippocampal cultures to 6 min potassium depolarization (60 mM) with 2.8-11.8 mM extracellular Ca2+ and evaluated immunoreactivity for low, medium and heavy neurofilament proteins, neuronal number, and neuronal morphology for 10 days. Neuronal number remained unchanged; neurofilament protein levels recovered to between 31% and 99% of control levels, and fine neuritic processes reappeared.

Microglial responsiveness as a sensitive marker for trimethyltin (TMT) neurotoxicity

Activation of microglia is a well-documented phenomenon associated with diverse pathological conditions of the central nervous system. In order to investigate the involvement of microglial cells in the neurotoxic action of the heavy metal compound trimethyltin, three-dimensional brain cell cultures were treated during an early developmental period, using concentrations at or below the limit of cytotoxicity. Microglial cells were studied by cytochemical staining, using horseradish peroxidase-conjugated B4 isolectin (GSI-B4). In parallel, neurotoxic effects were assessed by determining the content of synaptophysin and synapsin I, both in the total homogenates and in the synaptosomal fraction of the cultures. Changes in the content of the specific growth cone protein, GAP-43, were also analyzed. It was found that low, non-cytotoxic concentrations of TMT (10(-9) to 10(-8) M) caused a significant increase in the number and/or the clustering of microglial cells. A decrease in the synaptic protein (synapsin I, synaptophysin) content was detected at 10(-8) M of TMT in synaptosomal fractions, whereas in the total homogenates, changes in synaptic proteins and GAP-43 were observed only at the cytotoxic TMT concentration (10(-6) M). Although it remains to be shown whether the microglial response is caused by direct or indirect action of TMT, the present findings show that microglial responsiveness can be detected prior to any sign of neuronal degeneration, and may serve as a sensitive indicator for heavy metal neurotoxicity in the brain.

Differentiation is induced in three-dimensional cultures of brain cells immortalized by the LAP mammalian regulatory system

Immortalized neuroectodermal precursor cell lines were generated from mouse brain by the SV40 large T antigen expressed under the control of the LAP (lac activating protein) mammalian regulatory system. The LAP system permits the reversible expression of T antigen as a function of the exogenous inducer, isopropyl-beta-D-thiogalactopyranoside. Immortalized cells can be stably maintained in an undifferentiated state in monolayer cultures. Cell lines expressed the early neurofilament-like protein nestin, but not markers characteristic for mature cells such as the neurofilament light protein and glial fibrillary acidic protein. Downregulating the LAP-controlled T antigen with isopropyl-beta-D-thiogalactopyranoside was not sufficient to induce differentiation. However, when cells formed three-dimensional aggregates, differentiation to a neuronal phenotype occurred, indicating that cell-cell interaction plays an important role in their differentiation. Cells in aggregates did not proliferate, even in the presence of T antigen, suggesting that an aggregation-induced signal to cease growth was dominant over the growth signal of T antigen. Further morphological differentiation was induced by basic fibroblast growth factor. These immortalized cells should facilitate molecular and cellular studies concerned with the mechanism of commitment, fate determination, and mitotic arrest of neuronal precursor cells in the developing mammalian CNS.

Expression of interleukin-3 and tumor necrosis factor-beta mRNAs in cultured microglia

The function of interleukin-3 (or multi-CSF) in the hemopoietic system has been studied in great detail. Although its growth promoting activity on brain microglial cells has been confirmed both in vitro and in vivo, its presence in the brain and even in cultured brain cells has repeatedly been questioned. We have shown recently that isolated rat microglia express mRNA(IL-3) and synthesize IL-3 polypeptide. It is shown here by use of the PCR method, that mRNA(IL-3) is found also in C6 glioblastoma, in rat aggregate cultures, and in newborn and adult rat brain. Quantitation of amplified cDNA(IL-3) was achieved by non-competitive RT-PCR using an elongated internal standard. IL-3 messenger RNA was almost undetectable in vivo and low in (serum-free) aggregate cultures. In isolated microglia, mRNA(IL-3) was increased upon treatment with LPS, PHA, with the cytokines IL-1 or TNF-alpha, with retinoic acid, dbcAMP or the phorbol ester TPA. Effects of LPS were inhibited by dexamethasone, while the glucocorticoid by itself had no effect on basal IL-3 expression. LPS increased mRNA(IL-3) in a concentration-dependent manner beginning with 10 pg/ml and reaching plateau levels at 10 ng/ml. LPS also increased mRNAs of TNF-alpha and TNF-beta. TNF-alpha mRNA was already detectable in untreated microglia and LPS-increased levels were sustained for a few days. In contrast, TNF-beta mRNA was observed only between 4 and 16 h of LPS incubation. It was absent in LPS-free microglia, and after 24 h of LPS-treatment or later.(ABSTRACT TRUNCATED AT 250 WORDS)

Polyreactive autoimmune response induced by PC 12 cell grafts into rat striatum

In order to better characterize the autoantibodies induced by PC12 cells grafted into rat brain, we have tested sera from these animals by immunoblotting with several preparations, including phosphorylated and dephosphorylated neurofilaments, keratins, PC12 cells and proteins from various rat tissues, and by immunofluorescence of rat spinal cord neurons in culture. Sera from grafted rats reacted with several antigens present in all tissues tested and stained in cultured neurons not only NF but also cell bodies and membranous granular structures. These observations suggest either the polyreactivity of autoantibodies or the induction of a polyclonal B cell activation consecutive to the release of central nervous system antigens into the blood stream. These results are discussed with regard to the role of NF autoantibodies in neurodegenerative diseases.

The survival of cultured mouse cerebellar granule cells is not dependent on elevated potassium-ion concentration

The effects of K(+)-induced membrane depolarization were studied on the survival and biochemical parameters in mouse and rat cerebellar granule cells grown in micro-well cultures. Cell numbers were determined by estimating DNA content using the Hoechst 33258 fluorochrome binding assay. DNA from degenerated cells was removed by prior DNAase treatment. These DNA estimates of cell numbers were comparable with values obtained by direct counting of fluorescein diacetate-stained viable cells. In agreement with previous studies, the survival of rat granule cells was promoted by increasing the concentration of K+ in the medium from 5 to 25 mM throughout a 7-day culture period. In contrast, mouse granule cells survived in culture containing 'low' K+ (5 or 10 mM), as well as in the presence of 'high' K+ (25 mM). On the other hand, several biochemical parameters in mouse granule cells were markedly increased by cultivation in 'high' as compared with 'low' K(+)-containing media, demonstrated by increased fluorescein diacetate esterase activity, enhanced rate of NADPH-dependent tetrazolium reduction, augmented 2-deoxy-D-glucose accumulation and increased N-methyl-D-aspartate-evoked 45Ca2+ influx. It was concluded that although cultivation in 'high' K+ promotes biochemical differentiation in mouse cerebellar granule cells, these cells differ from their rat counterparts in that they do not develop a survival requirement for K(+)-induced membrane depolarization.

Changes of beta-amyloid precursor protein splice patterns in brain cell aggregate cultures

The splice pattern of beta-amyloid precursor protein (beta-APP) has been studied in a variety of neuronal and glial cells and in brain cell aggregate cultures by the polymerase chain reaction (PCR). The brain-typical pattern, in which beta-APP695 is the dominant form, has been found only in aggregate cultures but not in any of the other cell types including neuronal cell lines. Selective elimination of glial cells from aggregates resulted in increased quantities of beta-APP695, whereas removal of neurons led to a reduction of beta-APP695 and to an elevation of beta-APP751 and beta-APP770. This shift of splice pattern was not observed in cocultures of the neuronal cell line PC 12 with primary astrocytes combined in a variety of cellular ratios. Blood serum, which is an essential component of these cultures, tested on aggregates, did not reduce the amount of beta-APP695 or have any marked effects on splice patterns generally. From these results it is concluded that investigations on brain-typical splicing of beta-APP require primary neurons. Neuronal cell lines may be no suitable model systems. Splicing events favoring production of beta-APP695 may mark an important, very early step of amyloid formation in the brain.

Differential distribution of tau proteins in developing cat cerebellum

The differential distribution and phosphorylation of tau proteins in cat cerebellum was studied with two well characterized antibodies, TAU-1 and TAU-2. TAU-1 detects tau proteins in axons, and the epitope in perikarya and dendrites is masked by phosphorylation. TAU-2 detects a phosphorylation-independent epitope on tau proteins. The molecular composition of tau proteins in the range of 45 kD to 64 kD at birth changed after the first postnatal month to a set of several adult variants of higher molecular weights in the range of 59 kD to 95 kD. The appearance of tau proteins in subsets of axons corresponds to the axonal maturation of cerebellar local-circuit neurons in granular and molecular layers and confirms previous studies. Tau proteins were also identified in synapses by immunofluorescent double-staining with synapsin I, located in the pinceau around the Purkinje cells, and in glomeruli. Dephosphorylation of juvenile cerebellar tissue by alkaline phosphatase indicated indirectly the presence of differentially phosphorylated tau forms mainly in juvenile ages. Additional TAU-1 immunoreactivity was unmasked in numerous perikarya and dendrites of stellate cells, and in cell bodies of granule cells. Purkinje cell bodies were stained transiently at juvenile ages. During postnatal development, the intensity of the phosphate-dependent staining decreased, suggesting that phosphorylation of tau proteins in perikarya and dendrites may be essential for early steps in neuronal morphogenesis during cat cerebellum development.

An isoform of microtubule-associated protein 2 (MAP2) containing four repeats of the tubulin-binding motif

Microtubule-associated protein 2 (MAP2) exists in both high- and low-molecular mass isoforms, each of which has a tubulin-binding domain consisting of 3 imperfect tandem repeats of 31 amino acids containing a more highly conserved 18 amino acid ‘core’ sequence. We describe here a novel form of low molecular mass MAP2 (MAP2c) that contains an additional 4th repeat of this tubulin-binding motif. Like the 3 previously known repeat sequences, this 4th copy is highly conserved between MAP2 and the two other known members of the same gene family, tau and MAP4. In each of these three genes the additional 4th repeat is inserted between the 1st and 2nd repeats of the 3-repeat form of the molecule. Experiments with brain cell cultures, in which the relative proportions of neurons and glia had been manipulated by drug treatment, showed that 4-repeat MAP2c is associated with glial cells whereas 3-repeat MAP2c is expressed in neurons. Whereas 3-repeat MAP2c is expressed early in development and then declines, the level of 4-repeat MAP2c increases later in development, corresponding to the relatively late differentiation of glial cells compared to neurons. When transfected into non-neuronal cells, the 4-repeat version of MAP2c behaved indistinguishably from the 3-repeat form in stabilising and rearranging cellular microtubules. The presence of an additional 4th repeat of the tubulin-binding motif in all three members of the MAP2 gene family suggests that this variant arose prior to their differentiation from an ancestral gene.

Differential phosphorylation of some proteins of the neuronal cytoskeleton during brain development

The cytoskeleton is important for neuronal morphogenesis. During the postnatal development of cat brain, the molecular composition of the neuronal cytoskeleton changes with maturation. Several of its proteins change in their rate of expression, in their degree of phosphorylation, in their subcellular distribution, or in their biochemical properties. It is proposed that phosphorylation is an essential mechanism to regulate the plasticity of the early, juvenile-type cytoskeleton. Among such proteins are several microtubule-associated proteins (MAPs), such as MAP5a, MAP2c or the juvenile tau proteins. Phosphorylation may also act on neurofilaments, postulated to be involved in the adult-type stabilization of axons. These observations imply that phosphorylation may affect cytoskeleton function in axons and dendrites at various developmental stages. Yet, the mechanisms of phosphorylation and its regulation cascades are largely unknown. In view of the topic of this issue on CD15, the potential role of matrix molecules being involved in the modulation of phosphorylation activity and of cytoskeletal properties is addressed.