Binhui Ni

Functional gamma‐secretase inhibitors reduce beta‐amyloid peptide levels in brain

Converging lines of evidence implicate the beta‐amyloid peptide (Aβ) as causative in Alzheimers disease. We describe a novel class of compounds that reduce Aβ production by functionally inhibiting γ‐secretase, the activity responsible for the carboxy‐terminal cleavage required for Aβ production. These molecules are active in both 293 HEK cells and neuronal cultures, and exert their effect upon Aβ production without affecting protein secretion, most notably in the secreted forms of the amyloid precursor protein (APP). Oral administration of one of these compounds, N‐[N‐(3,5‐difluorophenacetyl)‐l‐alanyl]‐S‐phenylglycine t‐butyl ester, to mice transgenic for human APPV717F reduces brain levels of Aβ in a dose‐dependent manner within 3 h. These studies represent the first demonstration of a reduction of brain Aβin vivo. Development of such novel functional γ‐secretase inhibitors will enable a clinical examination of the Aβ hypothesis that Aβ peptide drives the neuropathology observed in Alzheimers disease.

Depolarization or glutamate receptor activation blocks apoptotic cell death of cultured cerebellar granule neurons

Cerebellar granule neurons can be readily maintained in culture if depolarized with high concentrations of K+ or subtoxic concentrations of various excitatory amino acids. We now report that these depolarizing stimuli promote cerebellar granule neuron survival by blocking their programmed death via apoptosis. Cerebellar granule neurons maintained in depolarizing conditions and then changed to non-depolarizing conditions, exhibit the morphological and biochemical features of apoptosis, including cytoplasmic blebbing, condensation and aggregation of nuclear chromatin and internucleosomal DNA fragmentation. Inhibitors of RNA or protein synthesis greatly attenuate cell death induced by non-depolarizing culture conditions. In contrast, cerebellar granule neurons, when exposed to fresh serum-containing medium or to high concentrations of glutamate, exhibit a delayed-type of neurotoxicity which is non-apoptotic in nature. Given the actions of excitatory amino acid receptor agonists in preventing apoptosis of cultured cerebellar granule neurons, we hypothesize that the functional innervation of postmigratory granule neurons during cerebellar development may prevent further elimination of these neurons by blocking their programmed death.

α2-Macroglobulin as a β-amyloid peptide-binding plasma protein

Abstract: The β‐amyloid peptide (Aβ) is a normal proteolytic processing product of the amyloid precursor protein, which is constitutively expressed by many, if not most, cells. For reasons that are still unclear, Aβ is deposited in an aggregated fibrillar form in both diffuse and senile plaques in the brains of patients with Alzheimers disease (AD). The factor(s) responsible for the clearance of soluble Aβ from biological fluids or tissues are poorly understood. We now report that human α2‐macroglobulin (α2M), a major circulating endoproteinase inhibitor, which has recently been shown to be present in senile plaques in AD, binds 125I‐Aβ(1–42) with high affinity (apparent dissociation constant of 3.8 × 10−10M). Approximately 1 mol of Aβ is bound per mole of α2M. Both native and methylamine‐activated α2M bind 125I‐Aβ(1–42). The binding of 125I‐Aβ(1–42) to α2M is enhanced by micromolar concentrations of Zn2+ (but not Ca2+) and is inhibited by noniodinated Aβ(1–42) and Aβ(1–40) but not by the reverse peptide Aβ(40‐1) or the cytokines interleukin 1β or interleukin 2. α1‐Antichymotrypsin, another plaque‐associated protein, inhibits both the binding of 125I‐Aβ(1–42) to α2M as well as the degradation of 125I‐Aβ(1–42) by proteinase‐activated α2M. Moreover, the binding of 125I‐Aβ(1–42) to α2M protects the peptide from proteolysis by exogenous trypsin. These data suggest that α2M may function as a carrier protein for Aβ and could serve to either facilitate or impede clearance of Aβ from tissues such as the brain.

Raloxifene and estradiol benzoate both fully restore hippocampal choline acetyltransferase activity in ovariectomized rats.

Selective estrogen receptor modulators (SERMs) demonstrate tissue-specific estrogen receptor (ER) agonist or antagonist properties. Raloxifene, a prototypical SERM, has ER agonist properties in bone and on cholesterol metabolism but full antagonist properties in the uterus and breast. To characterize the ER agonist/antagonist profile of raloxifene in the brain, we have examined its effect on the activity of a known estrogen-responsive gene product, choline acetyltransferase (ChAT), in the hippocampus and other brain regions of 6-month-old ovariectomized (OVX) Sprague-Dawley rats. Three weeks post-ovariectomy, animals received estradiol benzoate (EB, 0.03 mg or 0.3 mg kg(-1) day(-1) for 3 or 10 days); raloxifene HCl (3.0 mg kg(-1) day(-1) for 3 or 10 days), tamoxifen (3.0 mg kg(-1) day(-1) for 10 days) or vehicle (20% CDX). As previously reported, ChAT activity decreased by approximately 20%-50% in the hippocampus of OVX compared with SHAM-operated control rats with no change in ChAT activity observed in the hypothalamus. Raloxifene or EB reversed the OVX-induced decrease in ChAT activity in the hippocampus but did not change ChAT activity in the hypothalamus. Animals that received combined EB (0.03 mg/kg) plus raloxifene (1 mg/kg) or tamoxifen alone (3.0 or 10 mg/kg) also showed increased hippocampal ChAT activity. Raloxifene failed to increase uterine weight and blocked the estrogen-induced increase in uterine weight, while another SERM, tamoxifen, increased uterine weight. These data demonstrate that raloxifene has estrogen-like properties on hippocampal ChAT activity in vivo, and suggest that benzothiophene SERMs may exert estrogen-like beneficial effects on cholinergic neurotransmission in brain without producing peripheral stimulation of breast or uterine tissue.

Molecular Cloning, Expression, and Chromosomal Localization of a Human Brain-Specific Na+-Dependent Inorganic Phosphate Cotransporter

Abstract: We describe the molecular cloning of a cDNA encoding a human brain Na+‐dependent inorganic phosphate (Pi) cotransporter (hBNPI). The nucleotide and deduced amino acid sequences of hBNPI reveal a protein of 560 amino acids with six to eight putative transmembrane segments. hBNPI shares a high degree of homology with other Na+‐dependent inorganic Pi cotransporters, including those found in rat brain and human and rabbit kidney. Expression of hBNPI in COS‐1 cells results in Na+‐dependent Pi uptake. Northern blot analysis demonstrates that hBNPI mRNA is expressed predominantly in brain and most abundantly in neuron‐enriched regions such as the amygdala and hippocampus. Moderate levels of expression are also observed in glia‐enriched areas such as the corpus callosum, and low levels are observed in the substantia nigra, subthalamic nuclei, and thalamus. In situ hybridization histochemistry reveals relatively high levels of hBNPI mRNA in pyramidal neurons of the cerebral cortex and hippocampus and in granule neurons of dentate gyrus. The level of hBNPI mRNA is quite low in fetal compared with adult human brain, suggesting developmental regulation of hBNPI gene expression. Southern analyses of nine eukaryotic genomic DNAs probed under stringent conditions with hBNPI cDNA revealed that the hBNPI gene is highly conserved during vertebrate evolution and that each gene is most likely present as a single copy. Using fluorescent in situ hybridization, we localized hBNPI to the long arm of chromosome 19 (19q13) in close proximity to the late‐onset familial Alzheimers disease locus.

Inhibition of Aβ production and APP maturation by a specific PKA inhibitor

Alzheimers disease is characterized pathologically by extracellular amyloid β protein (Aβ) deposition in the brain. The Aβ peptide, a 39–42 amino acid fragment, is derived from defined proteolysis of the amyloid precursor protein (APP) [Glenner et al., Appl. Pathol. 2 (1984) 357–369; Selkoe, Neuron 6 (1991) 487–498] and is the primary component of senile plaques. Although it is known that intracellular APP is subjected to posttranslational modification, the molecular mechanism that regulates the APP processing is not completely clear. In the present study, we demonstrates that H89, a specific inhibitor for cAMP dependent protein kinase A (PKA), inhibits Aβ production and APP secretion in a dose dependent manner in cells stably transfected with human APP bearing a ‘Swedish mutation’. Concurrent with the effect, H89 inhibits C‐terminal fragment of the APP. We also found that the PKA inhibitor abolishes the mature form of intracellular APP and accumulates the immature form. Finally, direct administration of H89 into brains of transgenic mice overexpressing human APP shows that the compound inhibits Aβ production in the hippocampal region. Our data suggests that PKA plays an important role in the maturation of APP associated with APP processing.

Overexpression of GSK3βS9A Resulted in Tau Hyperphosphorylation and Morphology Reminiscent of Pretangle-Like Neurons in the Brain of PDGSK3β Transgenic Mice

It has been demonstrated that GSK3β is involved in Alzheimer Disease (AD) pathogenesis. In order to understand the underlying mechanism, we have generated and characterized transgenic mice in which the constitutively active human GSK3β(with S9A mutation) was overexpressed in the brain under the control of the platelet-derived growth factor (PDGF) B-chain promoter. Varying levels of human GSK3βS9A transgene protein expression was observed in six of the seven founders generated. Line 3083, 3107, 3112 and 3125 displayed higher GSK3βS9A protein expression levels. Immunostaining analysis demonstrated that transgene expression was observed mainly in cortex and hippocampus of transgenic brain. Expression of human GSK3β transgene did not significantly change the brain total GSK3β protein levels in any of the generated mouse lines, as comparing to age matched wild type mice. Although significant kinase activity was detected in human GSK3βS9A transgene protein extracted from brains of all six expressing lines, significant increase in total GSK3βS9A kinase activity was observed only in the offspring of line 3083 and 3107. By analyzing the offspring from several transgenic mouse lines, including lines other than 3083 and 3107, it was found that overexpressed constitutively active human GSK3βS9A resulted in hyperphosphorylation of tau and morphology reminiscent of pretangle-like neurons in cortex and hippocampus.

Characterization of Na(+)-dependent phosphate uptake in cultured fetal rat cortical neurons.

Abstract: Our laboratory has recently cloned and expressed a brain‐ and neuron‐specific Na+‐dependent inorganic phosphate (Pi) cotransporter that is constitutively expressed in neurons of the rat cerebral cortex, hippocampus, and cerebellum. We have now characterized Na+‐dependent 32Pi cotransport in cultured fetal rat cortical neurons, where >90% of saturable Pi uptake is Na+‐dependent. Saturable, Na+‐dependent 32Pi uptake was first observed in primary cultures of cortical neurons at 7 days in vitro (DIV) and was maximal at 12 DIV. Na+‐dependent Pi transport was optimal at physiological temperature (37°C) and pH (7.0–7.5), with apparent Km values for Pi and Na+ of 54 ± 12.7 µM and 35 ± 4.2 mM, respectively. A reduction in extracellular Ca2+ markedly reduced (>60%) Na+‐dependent Pi uptake, with a threshold for maximal Pi import of 1–2.5 mM CaCl2. Primary cultures of fetal cortical neurons incubated in medium where equimolar concentrations of choline were substituted for Na+ had lower levels of ATP and ADP and higher levels of AMP than did those incubated in the presence of Na+. Furthermore, a substantial fraction of the 32Pi cotransported with Na+ was concentrated in the adenine nucleotides. Inhibitors of oxidative metabolism, such as rotenone, oligomycin, or dinitrophenol, dramatically decreased Na+‐dependent Pi import rates. These data establish the presence of a Na+‐dependent Pi cotransport system in neurons of the CNS, demonstrate the Ca2+‐dependent nature of 32Pi uptake, and suggest that the neuronal Na+‐dependent Pi cotransporter may import Pi required for the production of high‐energy compounds vital to neuronal metabolism.

Regulation of amyloid precursor protein expression and secretion via activation of ERK1/2 by hepatocyte growth factor in HEK293 cells transfected with APP751

The increased intracellular levels and aberrant processing of the amyloid precursor protein (APP) are associated with beta-amyloid peptide (A beta) production, cerebrovascular amyloid deposition, and amyloid plaque formation. Here we report that APP level, soluble APP (sAPP) secretion, and A beta production in HEK293 cells transfected with either wild-type APP(751) or APP(751) carrying the Swedish mutation are all elevated by hepatocyte growth factor (HGF). We investigated the potential molecular mechanisms underlying the HGF effect. Our data show that HGF stimulated extended activation of extracellular signal-regulated protein kinases (ERK1/2). Pretreatment of cells with inhibitors (UO126 or PD98059) for MEK, the upstream kinase of ERK1/2, abolished ERK1/2 activation evoked by HGF, and abrogated HGF-induced increases in APP levels and sAPP secretion. In addition, transient expression of active MEK1 activated ERK1/2 and increased intracellular APP levels and sAPP secretion. Inhibition of ERK1/2 activity, however, failed to block HGF-stimulated A beta production. Consistently, transient expression of active MEK1 did not increase A beta accumulation. Taken together, these results suggest that: (1) HGF regulates the intracellular levels of APP and the secretion of sAPP and A beta; (2) the modulation of APP levels and sAPP secretion induced by HGF is mediated via the MEK1/ERK1/2 signaling pathway; (3) HGF-stimulated A beta production is independent of ERK activity and, therefore, independent of HGF-evoked elevation of intracellular APP levels.

Inorganic Pi Increases Neuronal Survival in the Acute Early Phase Following Excitotoxic/Oxidative Insults

Abstract: Inorganic phosphate (Pi) plays a vital role in intracellular energy metabolism. Its many effects include stimulation of glucose use, enhancement of high‐energy phosphate concentrations, and modulation of cytosolic free [Ca2+]. Cultured fetal rat cortical neurons constitutively import Pi, and cytosolic levels positively correlate with [ATP], [NADPH], and energy charge. In the present study, we demonstrate that the concentration of intracellular Pi is an important determinant of acute neuronal survival after an excitotoxic or oxidative insult to cultured fetal rat cortical neurons. Extracellular Pi dose‐dependently enhanced survival of cortical neurons after exposure to NMDA at early (≤6 h) time points after termination of the insult. Pi similarly increased neuronal survival after exposure to kainic acid or H2O2. Pi‐exposed neurons had higher basal intracellular [Pi], [ATP], and [GSH], and slightly lower cytosolic free [Ca2+], compared with Pi‐deprived neurons. Pi‐exposed neurons maintained increased [ATP] after exposure to NMDA and displayed reduced formation of reactive oxygen species after exposure to kainic acid or H2O2, compared with Pi‐deprived neurons. These findings demonstrate that changes in extracellular and intracellular Pi can affect neuronal survival after excitotoxic or oxidative insults.