Constitutive and cytokine-regulated expression of presenilin-1 and presenilin-2 genes in human neural cell lines

Relevance of a Truncated PRESENILIN 2 Transcript to Alzheimer's Disease and Neurodegeneration

BACKGROUND

The PRESENILIN genes (PSEN1, PSEN2) encoding for their respective proteins have critical roles in many aspects of Alzheimer's disease (AD) pathogenesis. The PS2V transcript of PSEN2 encodes a truncated protein and is upregulated in AD brains; however, its relevance to AD and disease progression remains to be determined.

OBJECTIVE

Assess transcript levels in postmortem AD and non-AD brain tissue and in lymphocytes collected under the Australian Imaging Biomarker and Lifestyle (AIBL) study.

METHODS

Full length PSEN2 and PS2V transcript levels were assessed by quantitative digital PCR in postmortem brain tissue (frontal cortex and hippocampus) from control, AD, frontotemporal dementia (FTD), and Lewy body dementia (LBD). Transcript levels were also assessed in lymphocytes obtained from the Perth subset of the AIBL study (n = 160). Linear regression analysis was used to assess correlations between transcript copy number and brain volume and neocortical amyloid load.

RESULTS

PS2V levels increased in AD postmortem brain but PS2V was also present at significant levels in FTD and LBD brains. PS2V transcript was detected in lymphocytes and PS2V/PSEN2 ratios were increased in mild cognitive impairment (p = 0.024) and AD (p = 0.019) groups compared to control group. Increased ratios were significantly correlated with hippocampal volumes only (n = 62, β= -0.269, p = 0.03).

CONCLUSION

Taken together, these results suggest that PS2V may be a marker of overall neurodegeneration.

Cyclooxygenase-2 Induced the β-Amyloid Protein Deposition and Neuronal Apoptosis Via Upregulating the Synthesis of Prostaglandin E2 and 15-Deoxy-Δ12,14-prostaglandin J2

Elevated levels of cyclooxygenase-2 (COX-2) and prostaglandins (PGs) have been shown to be involved in the pathogenesis of Alzheimer's disease. Analysis of the underlying mechanisms elucidated a function of sequential PGE2 and PGD2 synthesis in regulating β-amyloid protein (Aβ) deposition by modulating tumor necrosis factor α (TNF-α)-dependent presenilin (PS)1/2 activity in COX-2 and APP/PS1 crossed mice. Specifically, COX-2 overexpression accelerates the expression of microsomal PGE synthase-1 (mPGES-1) and lipocalin-type prostaglandin D synthase (L-PGDS), leading to the synthesis of PGE2 and 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) in 6-month-old APP/PS1 mice. Consequently, PGE2 has the ability to increase Aβ production by enhancing the expression of PS1/2 in a TNF-α-dependent manner, which accelerates the cognitive decline of COX-2/APP/PS1 mice. More interestingly, low concentrations of 15d-PGJ2 treatment facilitate the effects of PGE2 on the deposition of Aβ via TNF-α-dependent PS1/2 mechanisms. In contrast, high concentrations of 15d-PGJ2 treatment inhibit the deposition of Aβ via suppressing the expression of TNF-α-dependent PS1/2. In this regard, a high concentration of 15d-PGJ2 appears to be a therapeutic agent against Alzheimer's disease. However, the high 15d-PGJ2 concentration treatment induces neuronal apoptosis via increasing the protein levels of Bax, cleaved caspase-3, and DFF45, which further impairs the learning ability of APP/PS1 mice.

Kainic Acid Impairs the Memory Behavior of APP23 Mice by Increasing Brain Amyloid Load through a Tumor Necrosis Factor-α-Dependent Mechanism

Kainic acid (KA) was recently identified as an epileptogenic and neuroexcitotoxic agent that is responsible for inducing learning and memory deficits in various neurodegenerative diseases, such as Alzheimer's disease (AD). However, the mechanism by which KA acts upon AD remains unclear. To this end, we presently investigated the roles of KA in processing amyloid-β protein precursor (AβPP) and amyloid-β protein (Aβ) loads during the course of AD development and progression. Specifically, KA treatment clearly caused the upregulation of tumor necrosis factor α (TNF-α) via activation of the PI3-K/AKT, ERK1/2, and p65 pathways in glial cells. TNF-α secreted from glial cells was then found to be responsible for stimulating the expression of BACE-1 and PS1/2, which resulted in the production and deposition of Aβ in neurons. Finally, the accumulation and aggregation of Aβ lead to the cognitive decline of APP23 mice. These results indicate that KA accelerates the progression of AD by inducing the crosstalk between glial cells and neurons.

Integrated communications between cyclooxygenase-2 and Alzheimer's disease

Elevated levels of cyclooxygenase-2 (COX-2) and prostaglandins (PGs) are involved in the pathogenesis of Alzheimer's disease (AD), which is characterized by the accumulation of β-amyloid protein (Aβ) and tau hyperphosphorylation. However, the gaps in our knowledge of the roles of COX-2 and PGs in AD have not been filled. Here, we summarized the literature showing that COX-2 dysregulation obviously influences abnormal cleavage of β-amyloid precursor protein, aggregation and deposition of Aβ in β-amyloid plaques and the inclusion of phosphorylated tau in neurofibrillary tangles. Neuroinflammation, oxidative stress, synaptic plasticity, neurotoxicity, autophagy, and apoptosis have been assessed to elucidate the mechanisms of COX-2 regulation of AD. Notably, an imbalance of these factors ultimately produces cognitive decline. The current review substantiates our understanding of the mechanisms of COX-2-induced AD and establishes foundations for the design of feasible therapeutic strategies to treat AD.-Guan, P.-P., Wang, P. Integrated communications between cyclooxygenase-2 and Alzheimer's disease.

Magnesium Ions Inhibit the Expression of Tumor Necrosis Factor α and the Activity of γ-Secretase in a β-Amyloid Protein-Dependent Mechanism in APP/PS1 Transgenic Mice

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by cognitive impairment. The neuropathological features of AD are the aggregation of extracellular amyloid β-protein (Aβ) and tau phosphorylation. Recently, AD was found to be associated with magnesium ion (Mg²⁺) deficit and tumor necrosis factor-alpha (TNF-α) elevation in the serum or brains of AD patients. To study the relationship between Mg²⁺ and TNF-α, we used human- or mouse-derived glial and neuronal cell lines or APP/PS1 transgenic (Tg) mice as in vitro and in vivo experimental models, respectively. Our data demonstrates that magnesium-L-threonate (MgT) can decrease the expression of TNF-α by restoring the levels of Mg²⁺ in glial cells. In addition, PI3-K/AKT and NF-κB signals play critical roles in mediating the effects of Mg²⁺ on suppressing the expression of TNF-α. In neurons, Mg²⁺ elevation showed similar suppressive effects on the expression of presenilin enhancer 2 (PEN2) and nicastrin (NCT) through a PI3-K/AKT and NF-κB-dependent mechanism. As the major components of γ-secretase, overexpression of presenilin 1 (PS1), PEN2 and NCT potentially promote the synthesis of Aβ, which in turn activates TNF-α in glial cells. Reciprocally, TNF-α stimulates the expression of PEN2 and NCT in neurons. The crosstalk between TNF-α and Aβ in glial cells and neurons could ultimately aggravate the development and progression of AD.

Prostaglandin I2 is responsible for ameliorating prostaglandin E2 stress in stimulating the expression of tumor necrosis factor α in a β-amyloid protein -dependent mechanism

Cyclooxygenase-2 (COX-2) has been found to be induced during the early stage of Alzheimer's disease (AD). Using mouse-derived astrocyte and APP/PS1 transgenic (Tg) mice as model systems, we firstly elucidated the mechanisms underlying COX-2 metabolic production including prostaglandin (PG)E₂- and PGI₂-mediated tumor necrosis factor α (TNF-α) regulation. Specifically, PGE₂ accumulation in astrocyte activated the p38 and JNK/c-Jun signaling pathways via phosphorylation, resulting in TNF-α expression. In contrast, the administration of PGI₂ attenuated the effects of PGE₂ in stimulating the production of TNF-α by inhibiting the activity of TNF-α promoter and the binding activity of AP1 on the promoter of TNF-α. Moreover, our data also showed that not only Aβ_1-42 oligomers but also Aβ_1-42 fibrils have the ability to involve in mediating the antagonistic effects of PGE₂ and PGI₂ on regulating the expression of TNF-α via a p38- and JNK/c-Jun-dependent, AP1-transactivating mechanism. Reciprocally, the production of TNF-α finally accelerated the deposition of β-amyloid protein (Aβ)_1-42 in β-amyloid plaques (APs), which contribute to the cognitive decline of AD.

Prostaglandin I₂ Attenuates Prostaglandin E₂-Stimulated Expression of Interferon γ in a β-Amyloid Protein- and NF-κB-Dependent Mechanism

Cyclooxygenase-2 (COX-2) has been recently identified as being involved in the pathogenesis of Alzheimer's disease (AD). However, the role of an important COX-2 metabolic product, prostaglandin (PG) I2, in AD development remains unknown. Using mouse-derived astrocytes as well as APP/PS1 transgenic mice as model systems, we firstly elucidated the mechanisms of interferon γ (IFNγ) regulation by PGE2 and PGI2. Specifically, PGE2 accumulation in astrocytes activated the ERK1/2 and NF-κB signaling pathways by phosphorylation, which resulted in IFNγ expression. In contrast, the administration of PGI2 attenuated the effects of PGE2 on stimulating the production of IFNγ via inhibiting the translocation of NF-κB from the cytosol to the nucleus. Due to these observations, we further studied these prostaglandins and found that both PGE2 and PGI2 increased Aβ1-42 levels. In detail, PGE2 induced IFNγ expression in an Aβ1-42-dependent manner, whereas PGI2-induced Aβ1-42 production did not alleviate cells from IFNγ inhibition by PGI2 treatment. More importantly, our data also revealed that not only Aβ1-42 oligomer but also fibrillar have the ability to induce the expression of IFNγ via stimulation of NF-κB nuclear translocation in astrocytes of APP/PS1 mice. The production of IFNγ finally accelerated the deposition of Aβ1-42 in β-amyloid plaques.

Hydrogen sulfide inhibits A2A adenosine receptor agonist induced β-amyloid production in SH-SY5Y neuroblastoma cells via a cAMP dependent pathway

Alzheimer's disease (AD) is the leading cause of senile dementia in today's society. Its debilitating symptoms are manifested by disturbances in many important brain functions, which are influenced by adenosine. Hence, adenosinergic system is considered as a potential therapeutic target in AD treatment. In the present study, we found that sodium hydrosulfide (NaHS, an H2S donor, 100 µM) attenuated HENECA (a selective A2A receptor agonist, 10-200 nM) induced β-amyloid (1-42) (Aβ42) production in SH-SY5Y cells. NaHS also interfered with HENECA-stimulated production and post-translational modification of amyloid precursor protein (APP) by inhibiting its maturation. Measurement of the C-terminal APP fragments generated from its enzymatic cleavage by β-site amyloid precursor protein cleaving enzyme 1 (BACE1) showed that NaHS did not have any significant effect on β-secretase activity. However, the direct measurements of HENECA-elevated γ-secretase activity and mRNA expressions of presenilins suggested that the suppression of Aβ42 production in NaHS pretreated cells was mediated by inhibiting γ-secretase. NaHS induced reductions were accompanied by similar decreases in intracellular cAMP levels and phosphorylation of cAMP responsive element binding protein (CREB). NaHS significantly reduced the elevated cAMP and Aβ42 production caused by forskolin (an adenylyl cyclase, AC agonist) alone or forskolin in combination with IBMX (a phosphodiesterase inhibitor), but had no effect on those caused by IBMX alone. Moreover, pretreatment with NaHS significantly attenuated HENECA-elevated AC activity and mRNA expressions of various AC isoforms. These data suggest that NaHS may preferentially suppress AC activity when it was stimulated. In conclusion, H2S attenuated HENECA induced Aβ42 production in SH-SY5Y neuroblastoma cells through inhibiting γ-secretase via a cAMP dependent pathway.

Presenilin 2 is the predominant γ-secretase in microglia and modulates cytokine release

Presenilin 1 (PS1) and Presenilin 2 (PS2) are the enzymatic component of the γ-secretase complex that cleaves amyloid precursor protein (APP) to release amyloid beta (Aβ) peptide. PS deficiency in mice results in neuroinflammation and neurodegeneration in the absence of accumulated Aβ. We hypothesize that PS influences neuroinflammation through its γ-secretase action in CNS innate immune cells. We exposed primary murine microglia to a pharmacological γ-secretase inhibitor which resulted in exaggerated release of TNFα and IL-6 in response to lipopolysaccharide. To determine if this response was mediated by PS1, PS2 or both we used shRNA to knockdown each PS in a murine microglia cell line. Knockdown of PS1 did not lead to decreased γ-secretase activity while PS2 knockdown caused markedly decreased γ-secretase activity. Augmented proinflammatory cytokine release was observed after knockdown of PS2 but not PS1. Proinflammatory stimuli increased microglial PS2 gene transcription and protein in vitro. This is the first demonstration that PS2 regulates CNS innate immunity. Taken together, our findings suggest that PS2 is the predominant γ-secretase in microglia and modulates release of proinflammatory cytokines. We propose PS2 may participate in a negative feedback loop regulating inflammatory behavior in microglia.

PS2 protein expression is upregulated by sex steroids in the cerebral cortex of aging mice

Mutations in presenilin (PS) genes cause majority of early onset Alzheimer's disease (AD), an age related neurodegenerative disorder. PS proteins undergo proteolytic cleavage to produce biologically active fragments, which constitute the catalytic core of the gamma-secretase enzyme. This enzyme cleaves beta-amyloid precursor protein (betaAPP) to generate Abeta peptides, which are influenced by sex steroids. Recently we have reported the downregulation of PS1 expression by sex steroids in the brain of adult mice. Here we have examined the effect of gonadectomy and subsequent administration of gonadal hormones 17beta-estradiol and testosterone on the level of PS2 C-terminal fragment (CTF) in the cerebral cortex of adult and old AKR strain mice of both sexes. PS2 expression was downregulated following gonadectomy, but upregulated by supplementation of gonadal steroids in both age groups and sexes. Thus these results demonstrate up-regulation of PS2 protein expression by sex steroids, which in turn may influence PS2 associated brain functions.

Age and Sex Dependent Alteration in Presenilin Expression in Mouse Cerebral Cortex

(1) Presenilin (PS) expression is regulated by several cellular and extracellular factors which change with age and sex. Both age and sex are key risk factors for Alzheimer's disease (AD), which is linked to mutations in PS genes. (2) We have analyzed the effect of age and sex on PS expression by northern hybridization and western blot analysis using the cerebral cortex of adult (24 +/- 2 weeks) and old (65 +/- 5 weeks) mice. (3) Our results demonstrate that PS1 was downregulated and PS 2 was upregulated in old mice of both sexes. The level of PS 1 was relatively higher and that of PS 2 was lower in female than male mice of same age group. Taken together, these findings show age and sex dependent alteration in PS expression, which in turn may influence the signal transduction pathways and consequently brain functions.

Studies on brain volume, Alzheimer-related proteins and cytokines in mice with chronic overexpression of IL-1 receptor antagonist

Inflammation is associated with both acute and chronic neurological disorders, including stroke and Alzheimer's disease (AD). Cytokines such as interleukin (IL)-1 have several activities in the brain both under physiological and pathophysiological conditions. The objective of this study was to evaluate consequences of the central blockade of IL-1 transmission in a previously developed transgenic mouse strain with brain-directed overexpression of human soluble IL-1 receptor antagonist (Tg hsIL-1ra). Effects on brain morphology and brain levels of the AD-related proteins beta-amyloid precursor protein (APP) and presenilin 1(PS1), as well as the levels of IL-1beta, IL-6 and tumour necrosis factor-alpha (TNF-alpha) were analysed in homozygotic and heterozygotic mice and wild type (WT) controls, of both genders and of young (30-40 days) and adult (13-14 months) age. A marked reduction in brain volume was observed in transgenic mice as determined by volumetry. Western blot analysis showed higher levels of APP, but lower levels of PS1, in adult animals than in young ones. In the cerebellum, heterozygotic (Tg hsIL-1ra(+/-)) mice had lower levels of APP and PS1 than WT mice. With one exception, there were no genotypic differences in the levels of IL-1beta, IL-6 and TNF-alpha. The cytokine levels were generally higher in adult than in young mice. In conclusion, the chronic blockade of IL-1 signalling in the brain was associated with an atrophic phenotype of the brain, and with modified levels of APP and PS1. Brain-directed overexpression of hsIL-1ra was not followed by major compensatory changes in the levels of pro-inflammatory cytokines.

Egr-1 upregulates the Alzheimer's disease presenilin-2 gene in neuronal cells

Regulation of the Alzheimer's disease (AD)-related gene, presenilin-2 (PSEN2), was analyzed in neuronal (SK-N-SH) and non-neuronal (human embryonic kidney 293, HEK293) cells. We show that the PSEN2 regulatory region includes two separate promoter elements, each located upstream of multiple transcription start sites in the first and second exons. The stronger upstream promoter, P1, has housekeeping characteristics: it resides in a CpG island, is TATA-less, and up to 83% of PSEN2-P1 activity depends on a stimulating protein 1 (Sp1) site at the most 5' initiation site. However, the downstream promoter P2 includes neuronal-specific elements and two sites for early growth response gene-1 (Egr-1), a transcription factor upregulated in learning paradigms and implicated in neuronal plasticity, in response to injury. We show that Egr-1 binds to PSEN-P2, and that PSEN-P2 activity is increased threefold by overexpression of Egr-1, and by 12-O-tetradecanoylphorbol-13-acetate (TPA), which induces physiological Egr-1 levels. Egr-1 represses PSEN2-P1 activity by 50% in neuronal cells, suggesting it partially shifts promoter usage from PSEN2-P1 to PSEN2-P2. This could lead to a relative increase in shorter exon 2 transcripts, which may be more efficiently translated than exon 1 transcripts. Identification of PSEN2 as an Egr-1 target suggests a link between PSEN2 expression and Egr-1-related processes, which may impact on understanding PSEN-2's physiological function and its role in Alzheimer's disease.

Chronic gliosis triggers Alzheimer's disease-like processing of amyloid precursor protein

Alzheimer's disease is a progressively dementing illness characterized by the extracellular accumulation and deposition of beta-amyloid. Early onset Alzheimer's disease is linked to mutations in three genes, all of which lead to increased beta-amyloid production. Inflammatory changes and gliosis may also play a role in the disease process, but the importance of these reactive events remains unclear. We recently reported that chronic cortical gliosis in heterotopic fetal rat cortical transplants is associated with significant changes in the levels of some of the proteins implicated in the pathogenesis of Alzheimer's disease. Because rodent beta-amyloid does not form extracellular amyloid deposits, we have now extended this model of chronic cortical gliosis to transgenic mice expressing the Swedish mutant form of human amyloid precursor protein. In addition, apolipoprotein E knockout mice were used to elucidate the role of this protein in reactive gliosis. The expression of mutant and murine proteins was assayed 6 or 10 months after transplantation using immunohistochemical and western blot methods. Heterotopic transplantation of fetal cortex onto the midbrain of neonatal mice consistently resulted in reactive gliosis, independent of apolipoprotein E status. In contrast, in homotopic cortex-to-cortex grafts there was little alteration in glial reactivity, a result similar to that obtained previously in rats. By 10 months post-transplantation the level of presenilin-1 expression was lower in heterotopic grafts than in host cortex and there was increased expression of transgenic amyloid precursor protein, but only in the gliotic cortex-to-midbrain grafts. Most importantly, increased levels of beta-amyloid, and particularly its precursor, C-99, were selectively found in these heterotopic transplants. Our results show that chronic gliosis is associated with altered processing of the amyloid precursor protein in vivo and thus may initiate or exacerbate pathological changes associated with Alzheimer's disease.

Cytokines and neurotrophic factors fail to affect Nogo-A mRNA expression in differentiated human neurones: implications for inflammation-related axonal regeneration in the central nervous system

Nogo is a novel myelin-associated inhibitor of neurite outgrowth which regulates stable neuronal connections during axonal regeneration following injury in the adult mammalian central nervous system (CNS). Because cytokines and neurotrophic factors play a key role in inflammation-related axonal regeneration, we investigated: (i) the constitutive expression of Nogo and the Nogo receptor (NgR) mRNA in human neural cell lines; (ii) Nogo and NgR mRNA levels in the NTera2 human teratocarcinoma cell line during retinoic acid (RA)-induced neuronal differentiation; and (iii) their regulation in NTera2-derived differentiated neurones (NTera2-N) after exposure to a battery of cytokines and growth factors potentially produced by activated glial cells at post-traumatic inflammatory lesions in the CNS. By reverse transcriptase-polymerase chain reaction analysis, the constitutive expression of Nogo-A, the longest isoform of three distinct Nogo transcripts and NgR mRNA was identified in a wide variety of human neural and non-neural cell lines. By Northern blot analysis, the levels of Nogo-A mRNA were elevated markedly in NTera2 cells following RA-induced neuronal differentiation, accompanied by an increased expression of the neurite growth-associated protein GAP-43 mRNA. In contrast, Nogo-A, Nogo-B, NgR and GAP-43 mRNA levels were unaltered in NTera2-N cells by exposure to basic fibroblast growth factor, brain-derived neurotrophic factor, glia-derived neurotrophic factor, tumour necrosis factor-alpha, interleukin-1beta, dibutyryl cyclic AMP or phorbol 12-myristate 13-acetate. These results indicate that both Nogo-A and NgR mRNA are coexpressed in various human cell types, including differentiated neurones, where their expression is unaffected by exposure to a panel of cytokines and neurotrophic factors which might be involved in inflammation-related axonal regeneration in the CNS.

Alpha-synuclein expression is up-regulated in NTera2 cells during neuronal differentiation but unaffected by exposure to cytokines and neurotrophic factors

Increasing evidence has indicated that proinflammatory cytokines such as TNF-alpha and IL-1beta, produced by activated microglia and astrocytes, play a key role in progressive degeneration of the nigrostriatal dopaminergic neurons in Parkinson's disease (PD). Since alpha-synuclein is a major component of Lewy bodies in PD brains, we studied the constitutive and cytokine/neurotrophic factor-regulated expression of alpha-synuclein in cultured human neurons by Northern blot and Western blot analyses. The constitutive expression of alpha-synuclein mRNA was identified in a variety of human neural and non-neural cell lines. The levels of alpha-synuclein expression were elevated markedly in NTera2 teratocarcinoma cells following retinoic acid-induced neuronal differentiation, accompanied with an increased expression of synphilin-1, while they were unaltered in NTera2-derived differentiated neurons by exposure to TNF-alpha, IL-1beta, BDNF or GDNF. These results indicate that alpha-synuclein expression in human neurons is up-regulated during differentiation, but is unaffected by a panel of cytokines and neurotrophic factors which are supposed to be involved in the nigral neuronal death and survival.

Nicastrin, a key regulator of presenilin function, is expressed constitutively in human neural cell lines

Nicastrin acts as a key regulator for presenilin (PS)-mediated gamma-secretase cleavage of beta-amyloid precursor protein by forming a functional complex with PS1 and PS2. Both TNF-alpha and IL-1, aberrantly produced by activated microglia and astrocytes, play a role in amyloidogenesis and neurodegeneration in the brains of Alzheimer's disease (AD) patients, while BDNF synthesized chiefly by neurons has been found to be substantially reduced in AD brains. To investigate the constitutive and cytokine/neurotrophic factor-regulated expression of nicastrin in human neural cells, its mRNA levels were studied by RT-PCR and northern blot analysis in SK-N-SH neuroblastoma cells, IMR-32 neuroblastoma cells, U-373MG astrocytoma cells, and NTera2 teratocarcinoma-derived differentiated neurons (NTera2-N) following exposure to TNF-alpha, IL-1beta, BDNF, dibutyryl cyclic AMP, or phorbol 12-myristate 13-acetate. Nicastrin mRNA expression was identified in all human neural and nonneural cell lines and tissues examined. The levels of nicastrin mRNA, however, were unaltered in SK-N-SH, IMR-32, U-373MG, and NTera2-N cells by exposure to the factors tested, and unchanged in NTera2 cells during retinoic acid-induced neuronal differentiation. These results indicate that nicastrin mRNA is expressed constitutively in human neural cell lines, where its expression is not regulated at the transcriptional level by a battery of cytokines and growth/differentiation factors which are supposed to be involved in amyloidogenesis, neurodegeneration or neuroprotection in AD brains.

Ubiquitin C-terminal hydrolase-L1 (PGP9.5) expression in human neural cell lines following induction of neuronal differentiation and exposure to cytokines, neurotrophic factors or heat stress

Dysfunction of the ubiquitin-dependent proteolytic pathway contributes to progressive accumulation of ubiquitinated protein inclusions in neurodegenerative disorders, such as Parkinson's disease (PD). Ubiquitin C-terminal hydrolase-L1 (UCH-L1), alternatively designated protein gene product 9.5 (PGP9.5), is a neural deubiquitinating enzyme which is identified as a principal constituent of Lewy bodies. To clarify the regulatory mechanism of UCH-L1 expression in human neural cells, we studied the constitutive, cytokine/neurotrophic factor-regulated, and heat stress-induced expression of UCH-L1 in cultured human neural cell lines by Western blot analysis. The constitutive expression of UCH-L1 was identified in SK-N-SH neuroblastoma cells, IMR-32 neuroblastoma cells, U-373MG astrocytoma cells, and NTera2 teratocarcinoma-derived differentiated neurones (NTera2-N). The levels of UCH-L1 expression were unaltered in these cell lines following treatment with TNF-alpha, IL-1beta, BDNF, GDNF, dibutyryl cyclic AMP, or phorbol 12-myristate 13-acetate, and remained unchanged by exposure to heat stress. In contrast, its levels were elevated substantially in NTera2 teratocarcinoma cells following retinoic acid-induced neuronal differentiation, accompanied with an increased expression of alpha-synuclein and synaptophysin. These results indicate that UCH-L1 is expressed constitutively in human neual cell lines, where it is upregulated following induction of neuronal differentiation, but unaffected by exposure to heat stress, cytokines, or growth/differentiation factors which are supposed to be invloved in the nigral neuronal death and survival in PD.

Amyloid precursor protein beta-secretase (BACE) mRNA expression in human neural cell lines following induction of neuronal differentiation and exposure to cytokines and growth factors

Recently, a novel amyloid precursor protein beta-secretase (designated BACE) was identified. Because activated microglia and astrocytes play a role in amyloidogenesis in Alzheimer's disease, the constitutive and glial cytokine/growth factor-regulated expression of BACE was studied in human neural cell lines. By reverse transcription-polymerase chain reaction (RT-PCR) analysis, BACE mRNA expression was identified in various human neural and non-neural cell lines. By northern blot analysis, the expression of BACE mRNA composed of five distinct transcripts (>8.0, 7.0, 6.0, 4.4 and 2.6 kb) was elevated markedly in NTera2 teratocarcinoma cells following retinoic acid-induced neuronal differentiation. But the levels of three major BACE mRNA species (7.0, 6.0 and 4.4 kb) were not significantly altered in NTera2-derived neurons, SK-N-SH neuroblastoma or U-373MG astrocytoma following exposure to tumor necrosis factor-alpha, interleukin (IL)-1beta, IL-6, interferon-gamma, transforming growth factor-beta1, epidermal growth factor, basic fibroblast growth factor, brain-derived neurotrophic factor, dibutyryl cyclic adenosine monophosphate or phorbol 12-myristate 13-acetate. These results indicate that BACE mRNA is expressed constitutively in human neural cells and its expression is upregulated during neuronal differentiation, but it is unlikely to be regulated by activated glia-derived cytokines and growth factors.

Beta-catenin expression in human neural cell lines following exposure to cytokines and growth factors

Beta-catenin acts as a key mediator of the Wnt/Wingless signaling pathway involved in cell proliferation, differentiation and survival. Recent studies have shown that an unstable interaction between beta-catenin and the mutant presenilin-1 induces neuronal apoptosis, and that beta-catenin levels are decreased in the brains of patients with Alzheimer's disease (AD). Since activated microglia and astrocytes play a role in the process of neuronal degeneration in AD, the cytokine/growth factor-regulated expression of beta-catenin in human neural cell lines, including NTera2 teratocarcinoma-derived differentiated neurons (NTera2-N), IMR-32 neuroblastoma, SKN-SH neuroblastoma and U-373MG astrocytoma, was studied quantitatively following exposure to epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, interferon (IFN)-gamma, transforming growth factor (TGF)-beta1, dibutyryl cyclic adenosine 3',5'-cyclic monophosphate (cAMP) (dbcAMP) or phorbol 12-myristate 13-acetate (PMA). Beta-catenin mRNA expressed constitutively in all of these cell lines was unaffected by treatment with any factors examined. In contrast, beta-catenin protein levels were reduced markedly in NTera2-N cells by exposure to dbcAMP, EGF or bFGF, and in U-373MG cells by treatment with dbcAMP or PMA, but were unaffected in any cell lines by BDNF, TNF-alpha, IL-1beta, IL-6, IFN-gamma or TGF-beta1. These results indicate that beta-catenin is expressed constitutively in human neural cells and downregulated at a protein level by a set of growth factors in a cell type-specific manner.