Downregulation of protein phosphatase 2A by apolipoprotein E: Implications for Alzheimer's disease.

The apolipoprotein E ε4 allele is the single most important genetic risk factor associated with Alzheimer's disease (AD). Tau phosphorylation and hyperphosphorylation is an underlying feature of AD and is regulated by specific kinases and phosphatases. Among phosphatases, protein phosphatase 2A (PP2A) is the principal tau dephosphorylating enzyme in the brain. Several abnormalities of PP2A have been reported in AD, including among others decreased protein levels of PP2A, decreased mRNA and protein levels of the catalytic subunit PP2AC and variable regulatory B subunits and reduced methylation of the catalytic subunit, all of which results in disruption of the PP2A phosphatase activity. In earlier studies we described a novel mechanism for ApoE as a transcription factor that binds regions of double-stranded DNA with high affinity, including the promoter regions of ~3000 different genes. The list of genes also included PPP2R5E (B56ε), a regulatory B' subunit of protein phosphatase 2A. Using a combination of A172 human glioblastoma cells, ApoE3/4 and ApoE-/- NSC and human postmortem tissue, we now demonstrate that ApoE not only binds to the PPP2R5E promoter but also triggers a significant reduction in PP2A activity by two mechanisms: 1) ApoE transcriptionally represses PPP2R5E and reduces protein expression, and 2) ApoE triggers demethylation of the catalytic subunit (PP2AC) of PP2A, resulting in the disruption of the PPP2R5E-PP2AC complex. Our results indicated a significant down-regulation of PPP2R5E gene expression and reduction in PP2A activity by ApoE4 compared with ApoE3. This may also explain an elevated Tau phosphorylation in AD human brains that featured at least one ApoE4 allele. Thus, our present work links ApoE and PPP2R5E expression to a reduction in the PP2A catalytic activity that has implications for Alzheimer's disease.

Direct Transcriptional Effects of Apolipoprotein E.

A major unanswered question in biology and medicine is the mechanism by which the product of the apolipoprotein E ε4 allele, the lipid-binding protein apolipoprotein E4 (ApoE4), plays a pivotal role in processes as disparate as Alzheimer's disease (AD; in which it is the single most important genetic risk factor), atherosclerotic cardiovascular disease, Lewy body dementia, hominid evolution, and inflammation. Using a combination of neural cell lines, skin fibroblasts from AD patients, and ApoE targeted replacement mouse brains, we show in the present report that ApoE4 undergoes nuclear translocation, binds double-stranded DNA with high affinity (low nanomolar), and functions as a transcription factor. Using chromatin immunoprecipitation and high-throughput DNA sequencing, our results indicate that the ApoE4 DNA binding sites include ∼1700 gene promoter regions. The genes associated with these promoters provide new insight into the mechanism by which AD risk is conferred by ApoE4, because they include genes associated with trophic support, programmed cell death, microtubule disassembly, synaptic function, aging, and insulin resistance, all processes that have been implicated in AD pathogenesis. Significance statement: This study shows for the first time that apolipoprotein E4 binds DNA with high affinity and that its binding sites include 1700 promoter regions that include genes associated with neurotrophins, programmed cell death, synaptic function, sirtuins and aging, and insulin resistance, all processes that have been implicated in Alzheimer's disease pathogenesis.

Neuroprotective Sirtuin ratio reversed by ApoE4.

The canonical pathogenesis of Alzheimer's disease links the expression of apolipoprotein E ε4 allele (ApoE) to amyloid precursor protein (APP) processing and Aß peptide accumulation by a set of mechanisms that is incompletely defined. The development of a simple system that focuses not on a single variable but on multiple factors and pathways would be valuable both for dissecting the underlying mechanisms and for identifying candidate therapeutics. Here we show that, although both ApoE3 and ApoE4 associate with APP with nanomolar affinities, only ApoE4 significantly (i) reduces the ratio of soluble amyloid precursor protein alpha (sAPPα) to Aß; (ii) reduces Sirtuin T1 (SirT1) expression, resulting in markedly differing ratios of neuroprotective SirT1 to neurotoxic SirT2; (iii) triggers Tau phosphorylation and APP phosphorylation; and (iv) induces programmed cell death. We describe a subset of drug candidates that interferes with the APP-ApoE interaction and returns the parameters noted above to normal. Our data support the hypothesis that neuronal connectivity, as reflected in the ratios of critical mediators such as sAPPα:Aß, SirT1:SirT2, APP:phosphorylated (p)-APP, and Tau:p-Tau, is programmatically altered by ApoE4 and offer a simple system for the identification of program mediators and therapeutic candidates.

Transcriptional Effects of ApoE4: Relevance to Alzheimer's Disease.

The major genetic risk factor for sporadic Alzheimer's disease (AD) is the lipid binding and transporting carrier protein apolipoprotein E, epsilon 4 allele (ApoE4). One of the unsolved mysteries of AD is how the presence of ApoE4 elicits this age-associated, currently incurable neurodegenerative disease. Recently, we showed that ApoE4 acts as a transcription factor and binds to the promoters of genes involved in a range of processes linked to aging and AD disease pathogenesis. These findings point to novel therapeutic strategies for AD and aging, resulting in an extension of human healthspan, the disease-free and functional period of life. Here, we review the effects and implications of the putative transcriptional role of ApoE4 and propose a model of Alzheimer's disease that focuses on the transcriptional nature of ApoE4 and its downstream effects, with the aim that this knowledge will help to define the role ApoE4 plays as a risk factor for AD, aging, and other processes such as inflammation and cardiovascular disease.

Sequence-specific binding of recombinant Zbed4 to DNA: insights into Zbed4 participation in gene transcription and its association with other proteins.

Zbed4, a member of the BED subclass of Zinc-finger proteins, is expressed in cone photoreceptors and glial Müller cells of human retina whereas it is only present in Müller cells of mouse retina. To characterize structural and functional properties of Zbed4, enough amounts of purified protein were needed. Thus, recombinant Zbed4 was expressed in E. coli and its refolding conditions optimized for the production of homogenous and functionally active protein. Zbed4's secondary structure, determined by circular dichroism spectroscopy, showed that this protein contains 32% α-helices, 18% ß-sheets, 20% turns and 30% unordered structures. CASTing was used to identify the target sites of Zbed4 in DNA. The majority of the DNA fragments obtained contained poly-Gs and some of them had, in addition, the core signature of GC boxes; a few clones had only GC-boxes. With electrophoretic mobility shift assays we demonstrated that Zbed4 binds both not only to DNA and but also to RNA oligonucleotides with very high affinity, interacting with poly-G tracts that have a minimum of 5 Gs; its binding to and GC-box consensus sequences. However, the latter binding depends on the GC-box flanking nucleotides. We also found that Zbed4 interacts in Y79 retinoblastoma cells with nuclear and cytoplasmic proteins Scaffold Attachment Factor B1 (SAFB1), estrogen receptor alpha (ERα), and cellular myosin 9 (MYH9), as shown with immunoprecipitation and mass spectrometry studies as well as gel overlay assays. In addition, immunostaining corroborated the co-localization of Zbed4 with these proteins. Most importantly, in vitro experiments using constructs containing promoters of genes directing expression of the luciferase gene, showed that Zbed4 transactivates the transcription of those promoters with poly-G tracts.

Overexpression of human γC-crystallin 5 bp duplication disrupts lens morphology in transgenic mice.

PURPOSE: To delineate the molecular mechanisms underlying autosomal dominant congenital cataracts caused by a 5 bp duplication in human CRYGC. METHODS: c.119_123dup (CRYGC5bpd) and wild-type human γC-crystallin (CRYGC) were expressed in transgenic mouse lenses by the chicken ßB1-crystallin promoter. Lenses were characterized histologically, by real-time PCR, SDS-PAGE, and Western blot. pET and Tet-on expression systems were used to express human CRYGC and CRYGC5bpd proteins in Escherichia coliand HeLa cells, respectively. RESULTS: Transgenic expression of CRYGC5bpd mutant γC-crystallin results in nuclear cataracts in which lens fiber cells begin to show variable degrees of degeneration and vacuolization by postnatal day 21. By 6 weeks of age all CRYGC5bpd lenses exhibit abnormalities of varying severity, comprising large vacuoles in cortical fiber cells, swelling and disorganization of fiber cells, and defective fiber cell migration and elongation. Levels of CRYGC5bpd mRNA are 3.7- and 14.1-fold higher than endogenous Crygc mRNA in postnatal day 1 and 6-week CRYGC5bpd mice lens, respectively. Crygc, Crygb, Crybb2, and Crybb3 mRNA levels are decreased in CRYGC5bpd mice compared with wild-type and CRYGC mice. Both wild-type and mutant human γC crystallin are uniformly distributed in the cytosol of HeLa cells, but CRYGC5bpd is degraded when expressed in E. coli BL21(DE3). CONCLUSIONS: Transgenic expression of mutant CRYGC5bpd γ-crystallin at near-physiological levels causes lens opacities and fiber cell defects, confirming the pathogenicity of this mutation. These results further suggest that HCG5pbd γ-crystallin causes cataracts through a direct toxic or developmental effect on lens cells causing damaged microstructure rather than through formation of HMW aggregates with resultant light scattering.

ZBED4, a BED-type zinc-finger protein in the cones of the human retina.

PURPOSE: To characterize the ZBED4 cDNA identified by subtractive hybridization and microarray of retinal cone degeneration (cd) adult dog mRNA from mRNA of normal dog retina. METHODS: The cDNA library obtained from subtractive hybridization was arrayed and screened with labeled amplicons from normal and cd dog retinas. Northern blot analysis was used to verify ZBED4 mRNA expression in human retina. Flow cytometry sorted peanut agglutinin (PNA)-labeled cones from dissociated mouse retinas, and quantitative RT-PCR (QPCR) was used to measure ZBED4 mRNA levels in these cone cells. Immunohistochemistry localized ZBED4 in human retinas. Expression of ZBED4 mRNA transiently transfected into HEK293 cells was analyzed by immunofluorescence. ZBED4 subcellular localization was determined with Western blot analysis. RESULTS: One of 80 cDNAs differentially expressed in normal and cd dog retinas corresponded to a novel gene, ZBED4, which is also expressed in human and mouse retinas. ZBED4 mRNA was found to be present in cone photoreceptors. When ZBED4 cDNA was transfected into HEK293 cells, the expressed protein showed nuclear localization. However, in human retinas, ZBED4 was localized to cone nuclei, inner segments, and pedicles, as well as to Müller cell endfeet. Confirming these immunohistochemical results, the 135-kDa ZBED4 was found in both the nuclear and cytosolic extracts of human retinas. ZBED4 has four predicted DNA-binding domains, a dimerization domain, and two LXXLL motifs characteristic of coactivators/corepressors of nuclear hormone receptors. CONCLUSIONS: ZBED4 cellular/subcellular localization and domains suggest a regulatory role for this protein, which may exert its effects in cones and Müller cells through multiple ways of action.

Allelic and locus heterogeneity in autosomal recessive gelatinous drop-like corneal dystrophy.

Gelatinous drop-like corneal dystrophy (GDLD) is a rare autosomal recessive disease characterized by the deposition of amyloid beneath the corneal epithelium and by severely impaired visual acuity leading to blindness. Although gelatinous corneal dystrophy has previously been mapped to chromosome 1p and seems to be associated with mutations in the M1S1 gene, molecular genetic studies have been limited to Japanese patients. To investigate the cause of GDLD in patients with diverse ethnic backgrounds, we performed linkage analyses in eight unrelated GDLD families from India, USA, Europe, and Tunisia. In seven of these families, the disease locus mapped to a 16-cM interval on the short arm of chromosome 1 between markers D1S519 and D1S2835, a region including the M1S1 gene. In addition, a 1.2-kb fragment containing the entire coding region of M1S1 gene was sequenced in affected individuals. Seven novel mutations (M1R, 8-bp ins., Q118 E, V194 E, C119 S, 870delC, and 1117delA) were identified in six families and two unrelated individuals. No sequence abnormalities were detected in a single family in which the GDLD locus was also excluded from the M1S1 region by linkage analysis. These findings demonstrate allelic and locus heterogeneity for GDLD.

Fine mapping of the Schnyder's crystalline corneal dystrophy locus.

Schnyder's crystalline corneal dystrophy (SCCD) is a rare autosomal dominant eye disease with a spectrum of clinical manifestations that may include bilateral corneal clouding, arcus lipoides, and anterior corneal crystalline cholesterol deposition. We have previously performed a genome-wide linkage analysis on two large Swede-Finn families and mapped the SCCD locus to a 16-cM interval between markers D1S2633 and D1S228 on chromosome 1p36. We have collected 11 additional families from Finland, Germany, Turkey, and USA to narrow the critical region for SCCD. Here, we have used haplotype analysis with densely spaced microsatellite markers in a total of 13 families to refine the candidate interval. A common disease haplotype was observed among the four Swede-Finn families indicating the presence of a founder effect. Recombination results from all 13 families refined the SCCD locus to 2.32 Mbp between markers D1S1160 and D1S1635. Within this interval, identity-by-state was present in all 13 families for two markers D1S244 and D1S3153, further refining the candidate region to 1.58 Mbp.