The Fe65 adaptor protein interacts through its PID1 domain with the transcription factor CP2/LSF/LBP1

Uroporphyrinogen III synthase erythroid promoter mutations in adjacent GATA1 and CP2 elements cause congenital erythropoietic porphyria

Congenital erythropoietic porphyria, an autosomal recessive inborn error of heme biosynthesis, results from the markedly deficient activity of uroporphyrinogen III synthase. Extensive mutation analyses of 40 unrelated patients only identified approximately 90% of mutant alleles. Sequencing the recently discovered erythroid-specific promoter in six patients with a single undefined allele identified four novel mutations clustered in a 20-bp region: (a) a -70T to C transition in a putative GATA-1 consensus binding element, (b) a -76G to A transition, (c) a -86C to A transversion in three unrelated patients, and (d) a -90C to A transversion in a putative CP2 binding motif. Also, a -224T to C polymorphism was present in approximately 4% of 200 unrelated Caucasian alleles. We inserted these mutant sequences into luciferase reporter constructs. When transfected into K562 erythroid cells, these constructs yielded 3 +/- 1, 54 +/- 3, 43 +/- 6, and 8 +/- 1%, respectively, of the reporter activity conferred by the wild-type promoter. Electrophoretic mobility shift assays indicated that the -70C mutation altered GATA1 binding, whereas the adjacent -76A mutation did not. Similarly, the -90C mutation altered CP2 binding, whereas the -86A mutation did not. Thus, these four pathogenic erythroid promoter mutations impaired erythroid-specific transcription, caused CEP, and identified functionally important GATA1 and CP2 transcriptional binding elements for erythroid-specific heme biosynthesis.

The beta-amyloid precursor protein functions as a cytosolic anchoring site that prevents Fe65 nuclear translocation

In this study we addressed the question of the intracellular localization of Fe65, an adaptor protein interacting with the beta-amyloid precursor protein (APP) and with the transcription factor CP2/LSF/LBP1. By using tagged Fe65 expression vectors, we observed that a significant fraction of Fe65 is localized in the nucleus of transfected COS7 cells. Furthermore, the isolation of nuclei from untransfected PC12 cells allowed us to observe that a part of the endogenous Fe65 is present in the nuclear extract. The analysis of Fe65 mutant constructs demonstrated that the region of the protein required for its nuclear translocation includes the WW domain, and that, on the other hand, a small fragment of 100 residues, including this WW domain, contains enough structural information to target a reporter protein (green fluorescent protein (GFP)-GFP) to the nucleus. To evaluate whether the Fe65-APP interaction could affect Fe65 intracellular trafficking, COS7 cells were cotransfected with APP(695) or APP(751) and with GFP-Fe65 expression vectors. These experiments demonstrated that Fe65 is no longer translocated to the nucleus when the cells overexpress APP, whereas the nuclear targeting of GFP-Fe65 mutants, unable to interact with APP, is unaffected by the coexpression of APP, thus suggesting that the interaction with APP anchors Fe65 in the cytosol.

Functions of WW domains in the nucleus

The WW domain is a protein module found in a wide range of signaling proteins. It is one of the smallest protein modules that folds as a monomer without disulfide bridges or cofactors. WW domains bind proteins containing short linear peptide motifs that are proline-rich or contain at least one proline. Although the WW domain was initially considered a 'cytoplasmic module', the proteins containing WW domains have also been localized in the cell nucleus. Moreover, these proteins have been documented to participate in co-activation of transcription and modulation of RNA polymerase II activity. The carboxy-terminal domain (CTD) of RNA polymerase II acts as an assembly platform for distinct WW domain-containing proteins that affect the function of the RNA polymerase II. The formation of complexes between CTD and WW domain-containing proteins is regulated by phosphorylation of the CTD. Since the CTD sequence is highly repetitive and a target of several post-translational modifications and conformational changes, it presents a unique structure capable of enormous molecular diversity. The WW domain has been implicated in several human diseases including Alzheimer's disease. The WW domain-containing iso-prolyl isomerase named Pin1, a protein known to be essential for cell cycle progression, was shown to be active in restoration of the microtubule-binding activity of Tau, a protein of neurofibrillar tangles found in the brains of Alzheimer's patients. It is the WW domain of Pin1 that interacts directly with Tau protein. In addition, the WW domain-containing adapter protein FE65 was shown to regulate processing of Alzheimer's amyloid precursor protein. It is expected that by understanding the details of the WW domain-mediated protein-protein interactions, we will be able to illuminate numerous signaling pathways which control certain aspects of transcription and cell cycle.

CRTR-1, a developmentally regulated transcriptional repressor related to the CP2 family of transcription factors

CP2-related proteins comprise a family of DNA-binding transcription factors that are generally activators of transcription and expressed ubiquitously. We reported a differential display polymerase chain reaction fragment, Psc2, which was expressed in a regulated fashion in mouse pluripotent cells in vitro and in vivo. Here, we report further characterization of the Psc2 cDNA and function. The Psc2 cDNA contained an open reading frame homologous to CP2 family proteins. Regions implicated in DNA binding and oligomeric complex formation, but not transcription activation, were conserved. Psc2 expression in vivo during embryogenesis and in the adult mouse demonstrated tight spatial and temporal regulation, with the highest levels of expression in the epithelial lining of distal convoluted tubules in embryonic and adult kidneys. Functional analysis demonstrated that PSC2 repressed transcription 2.5-15-fold when bound to a heterologous promoter in ES, 293T, and COS-1 cells. The N-terminal 52 amino acids of PSC2 were shown to be necessary and sufficient for this activity and did not share obvious homology with reported repressor motifs. These results represent the first report of a CP2 family member that is expressed in a developmentally regulated fashion in vivo and that acts as a direct repressor of transcription. Accordingly, the protein has been named CP2-Related Transcriptional Repressor-1 (CRTR-1).

A FE65 polymorphism associated with risk of developing sporadic late-onset alzheimer's disease but not with Abeta loading in brains

The FE65 protein was previously described interacting with amyloid protein precursor (APP) and mediating its internalization. Hu et al. (Hum. Genet., 103 (1998) 295) recently reported that a deletion polymorphism in intron 13 of the FE65 gene may be protective for sporadic Alzheimer's disease (AD) forms and suggested that this deletion may modify splicing between exon 13 and 14 (the two exons encoding the interaction domain of FE65 with APP). We tested the impact of this polymorphism in 646 controls and 639 sporadic AD cases. We were only able to detect a protective effect of the deletion in the population over 75 years (odds ratio = 0.53, 95% confidence interval (0.35-0.82), P= 0.002). Furthermore, no association of this polymorphism with Abeta40, Abeta42(43) and total Abeta loads were detected in 74 AD brains, although, we could expect that this deletion was associated with modifications of the APP metabolism. In conclusion, the FE65 gene may be a minor genetic determinant only for sporadic late-onset AD forms, although, we cannot conclude that this impact is mediated by a modulation of the APP process and/or Abeta peptide deposition.

NFkappa B interacts with serum amyloid A3 enhancer factor to synergistically activate mouse serum amyloid A3 gene transcription

We had previously identified a distal regulatory element (DRE) in the mouse serum amyloid A3 (SAA3) promoter that functions as a cytokine-inducible transcription enhancer. Within this DRE, three functional elements interact with CCAAT/enhancer-binding protein (C/EBP) and SAA3 enhancer factor (SEF) transcription factors. In this study, we show that cotransfection of the SEF expression plasmid with an SAA3/luciferase reporter resulted in 3-5-fold activation of the SAA3 promoter. When SEF-transfected cells were further stimulated with conditioned medium or interleukin-1, SAA3 promoter activity was dramatically increased, suggesting that SEF may cooperate functionally with other interleukin-1-inducible transcription factors to synergistically up-regulate SAA3 gene transcription. Indeed, cotransfection of SEF and NFkappaBp65 expression DNAs resulted in synergistic activation of the SAA3 promoter. Intriguingly, no consensus NFkappaB-binding site was found in the SAA3 promoter region; rather a putative NFkappaB-binding sequence with 3-base pair mismatches was identified in the DRE. When this sequence was used in an electrophoretic mobility shift assay, it interacted with NFkappaBp50, albeit with binding affinities that were several hundredfold lower than that with the consensus NFkappaB probe. Functional cooperation between SEF and NFkappaB was further strengthened by the finding that SEF and NFkappaB formed stable cytokine-inducible protein-protein complexes. Finally, despite its weak binding, mutation of this NFkappaB-binding site nevertheless dramatically reduced both NFkappaBp65- and cytokine-mediated induction of SAA3 promoter. Therefore, the molecular basis for the functional synergy between SEF and NFkappaB may, in part, be the ability of SEF to recruit NFkappaB through physical interactions that lead to enhancement or stabilization of NFkappaB binding to the SAA3 promoter element.

Characterization of an apoptosis inhibitory domain at the C-termini of FE65-like protein

TR2(L) is a 56-amino-acid polypeptide that has been shown to block TNF cytotoxicity. FE65-like (FE65L) proteins possess this conserved TR2(L) sequence at their C-termini, whereas variations in the sequences are found in the FE65 proteins. To further analyze the antiapoptotic function of TR2(L), here we utilized an isolated murine partial FE65L cDNA that encodes an N-terminal phosphotyrosine-binding domain (PTB) and the conserved C-terminal TR2(L) sequence. When L929 cells were stably transfected with the FE65L cDNA or its 3' end TR2(L) DNA sequence, these cells became resistant to TNF killing. Replacement of the N-terminal PTB domain with GFP failed to abolish the FE65L-mediated TNF resistance. Ablation of the C-terminal TR2(L) sequence through frame-shift mutation resulted in a complete loss of the FE65L function against TNF. Various protein kinase inhibitors, including lavendustin A, tyrphostin, H7, and staurosporine, which may affect the PTB domain function, could not abolish the FE65L-mediated TNF resistance. A prolonged exposure of L929 cells to these inhibitors for 24 h resulted in cell death, whereas FE65L significantly blocked the cell death. Polyclonal antibodies were generated against a synthetic peptide and shown to interact with a 38-kDa FE65L in L929 cells. Hyaluronidase downregulates the expression of FE65L gene and protein in L929 cells, and this correlates with its enhancement of TNF killing of these cells. Together, our data indicate that the TR2(L) amino acid sequence is an apoptosis-inhibitory domain commonly present in the FE65 and FE65-like family proteins.

Apolipoprotein E receptors: linking brain development and Alzheimer's disease

Alzheimer's disease is a debilitating neurodegenerative disorder that afflicts an increasing part of our ageing population. An isoform of apolipoprotein E, a protein that mediates the transport of lipids and cholesterol in the circulatory system, predisposes carriers of this allele to the common late-onset form of the disease. How this protein is related to a neurodegenerative disorder is an enigma. Mounting evidence indicates that apolipoprotein E receptors, which are abundantly expressed in most neurons in the central nervous system, also fulfill critical functions during brain development and may profoundly influence the pathogenesis of Alzheimer's disease.

The transcriptional factor LBP-1c/CP2/LSF gene on chromosome 12 is a genetic determinant of Alzheimer's disease

Although the varepsilon4 allele of the apolipoprotein E gene appears as an important biological marker for Alzheimer's disease (AD) susceptibility, other genetic determinants are clearly implicated in the AD process. Here, we propose that a genetic variation in the transcriptional factor LBP-1c/CP2/LSF gene, located close to the LRP locus, is a genetic susceptibility factor for AD. We report an association between a non-coding polymorphism (G-->A) in the 3'-untranslated region of this gene and sporadic AD in French and British populations and a similar trend in a North American population. The combined analysis of these three independent populations provides evidence of a protective effect of the A allele (OR = 0.58, 95% CI 0.44-0.75). We describe a potential biologically relevant role for the A allele whereby it reduces binding to nuclear protein(s). The absence of the A allele was associated with a lower LBP-1c/CP2/LSF gene expression in lymphocytes from AD cases compared with controls. Our data suggest that polymorphic variation in the implication of the LBP-1c/CP2/LSF gene may be important for the pathogenesis of AD, particularly since LBP-1c/CP2/LSF interacts with proteins such as GSKbeta, Fe65 and certain factors involved in the inflammatory response.

Proteolytic processing and cell biological functions of the amyloid precursor protein

Recent research has identified some key players involved in the proteolytic processing of amyloid precursor protein (APP) to amyloid beta-peptide, the principal component of the amyloid plaques in Alzheimer patients. Interesting parallels exists with the proteolysis of other proteins involved in cell differentiation, cholesterol homeostasis and stress responses. Since the cytoplasmic domain of APP is anchored to a complex protein network that might function in axonal elongation, dendritic arborisation and neuronal cell migration, the proteolysis of APP might be critically involved in intracellular signalling events.

Binding partners of Alzheimer's disease proteins: are they physiologically relevant?

Protein-protein interactions are a molecular basis for the structural and functional organization within cells. They are mediated by a growing number of protein modules that bind peptide targets. Alterations in binding affinities can have serious consequences for some essential cellular processes. The three proteins identified to have mutations in their corresponding genes leading to presenile Alzheimer dementia (AD)-the amyloid precursor protein (APP) and presenilin 1 and 2-all interact with other proteins. The nature and function of these interacting proteins may contribute to elucidating the proper physiological functions of the AD proteins. APP-interacting proteins are pointing toward a function of APP in cell adhesion and neurite outgrowth and signaling. Proteins interacting with the presenilins however are more diverse in nature linking presenilin function to regulation in different signaling pathways including Wnt and Notch but also in apoptosis and Ca(2+) homeostasis. Further research however is still needed to delineate the exact functional relevance of these interactions with respect to the physiological functions of the AD proteins in particular and the contribution of these proteins to AD pathogenesis in general.

Broadly altered expression of the mRNA isoforms of FE65, a facilitator of beta amyloidogenesis, in Alzheimer cerebellum and other brain regions

FE65 is a key "adapter" protein that links a multiprotein complex to an intracellular domain of beta-amyloid precursor protein (betaPP). Its overexpression modulates the trafficking of betaPP and facilitates the generation of beta-amyloid (Abeta). FE65 is predominantly expressed in brain tissues. An exon 9-inclusive isoform is exclusively expressed in neurons, and an exon 9-exclusive isoform is only expressed in non-neuronal cells. We quantitated the two isoforms in middle temporal cortex, middle frontal cortex, cerebellar cortex and caudate nucleus of 17 Alzheimer disease (AD) patients, 12 normal controls and 9 non-AD neurodegenerative disease controls by reverse transcription-competitive polymerase chain reaction (RT-cPCR). Expression of the two isoforms was significantly and differentially altered, with a 30-57% decrease in levels of the neuronal form (P < 0.05-0.002) and a 73-135% increase in levels of non-neuronal form (P < 0.02-0.001), in the temporal and frontal cortex of AD brains. These alterations presumably reflect advanced neurodegenerative processes of these regions. Surprisingly, expression of both isoforms was significantly up-regulated by 42-66% in the cerebellar cortex and caudate nucleus of AD brains when compared to normal brains (P < 0.05-0.005). Diffuse Abeta-positive plaques were observed in the cerebellum of these AD subjects but not in the normal controls. Selective up-regulation of only the FE65 neuronal isoform was seen in the cerebellar cortex in association with other neurodegenerative diseases (largely Parkinson's disease). Because FE65 modulates trafficking of betaPP toward the production of Abeta, the up-regulation of FE65 in AD cerebellum may be relevant to the genesis of diffuse plaques. Thus, early biochemical alterations in AD, not complicated by advanced pathology, may be beneficially investigated in the less-affected regions of the brain, such as the cerebellum.

Alternatively spliced isoforms of FE65 serve as neuron-specific and non-neuronal markers

FE65 is predominantly expressed in brain and is especially rich in the regions with the highest densities of neurons. The FE65 protein binds to an intracellular domain of the beta-amyloid precursor protein (betaPP) and may modulate the production of beta-amyloid peptide (AP). One of FE65 exons, a mini-exon (exon 9, 6 bp), is alternatively spliced, giving rise to two isoforms varying only in 6 base pairs. We quantitated the two isoforms by a sensitive reverse transcription-competitive polymerase chain reaction technique, and characterized their expressions in various tissues and cell cultures, and the kinetics of expression of the two isoforms in P19 embryonal carcinoma cell lines during neuronal differentiation. Our results show that the exon 9-inclusive (E9) form, the more abundant form in brain, was exclusively expressed in neurons, while the exon 9-exclusive (DeltaE9) form was widely expressed in all non-neuronal cells, but was not expressed in differentiated neurons. When P19 cells were differentiated to neurons, expression of FE65 was significantly up regulated ( approximately 30-fold) and the splicing pattern of the FE65 pre-mRNA was switched from the DeltaE9 pattern to the E9 form. Based upon their distinctive expression patterns, these two isoforms may serve as neuronal and non-neuronal markers, and determination of their ratios may have applications in neuropathological diagnosis.

Transcription factor CP2 is crucial in hemoglobin synthesis during erythroid terminal differentiation in vitro

The transcription factor CP2 was initially identified to bind to the promoter region of the murine alpha-globin gene and known to stimulate the expression of alpha-globin by increasing CP2 transcripts 3- to 5-fold during induced differentiation of mouse erythroleukemic (MEL) cells in vitro. Here, we report that this increment of CP2 expression is crucial in erythroid-specific globin gene expression and hemoglobin synthesis. When antisense CP2 was overexpressed in MEL cells, production of endogenous CP2 protein was reduced 70-80%, and significant loss of its promoter binding activity was observed. During HMBA-induced terminal differentiation of antisense CP2 expressing MEL cells, the transcription of endogenous alpha-globin gene was suppressed as expected. Moreover, both beta-globin gene expression and hemoglobin synthesis were also severely impaired, without affecting the expression of key heme enzyme genes or HMBA-induced proliferation and viability.

Molecular cloning and characterization of the human glycogen synthase kinase-3beta promoter

Glycogen synthase kinase 3beta (GSK-3beta) is a proline-directed kinase that forms part of the wingless signaling pathway. Recent studies have shown that GSK-3beta phosphorylates the microtubule-associated protein tau in vitro and in cell culture. Tau is the principal component of the paired helical filaments (PHFs) found in the brains of patients with Alzheimer disease, and PHF-tau is hyperphosphorylated. GSK-3beta is therefore one of the candidate kinases for phosphorylating tau in Alzheimer disease. GSK-3beta activity is negatively regulated by phosphorylation on serine 9 and positively regulated by phosphorylation on tyrosine 216. However, since overexpression of GSK-3beta by transfection leads to increased activity in the absence of any stimuli, GSK-3beta activity may also be regulated at the transcriptional level. Indeed, increased GSK-3beta protein levels are found in Alzheimer disease brains, and GSK-3beta is found associated with PHFs in Alzheimer disease. To understand how GSK-3beta activity may be regulated at the transcriptional level, we have isolated the human GSK-3beta promoter. The GSK-3beta promoter does not contain a conventional TATA box although several other transcription factor binding sites were identified. A putative transcription start site was mapped by 5' RACE. Transfection of various GSK-3beta promoter CAT reporter genes into both COS-7 cells and SHSY5Y neuronal cells revealed that the GSK-3beta promoter is more active in neuronal cells. Such transfection studies involving promoter deletion mutants revealed that a negative transcriptional response element may be present at position -1421 to -1363 and an activator sequence at position -427 to -384. CP2 binding sites were also present within the promoter. CP2 has recently been shown to interact with the Alzheimer disease amyloid precursor protein binding protein Fe65. The significance of these results with respect to Alzheimer disease pathogenesis are discussed.

Npw38, a novel nuclear protein possessing a WW domain capable of activating basal transcription

We have found a novel cDNA encoding a 265 amino acid protein possessing a WW domain in our full-length cDNA bank. The WW domain was sandwiched between an acidic region and an acidic-basic amino acid repetitive region. In vitro transcription/translation of the cDNA produced a 38 kDa product that was also found in the cell lysate by western blot analysis. Thus this protein is named the nuclear protein containing a WW domain with a molecular mass of 38 kDa, Npw38. Immunofluorescence studies and expression of a fusion protein to a green fluorescent protein revealed that this protein is localized in the nucleus. Npw38 was shown to be capable of binding to a poly(rG) resin. Interestingly, the WW domain of Npw38 was found to function as a transcriptional activator in CHO cells using the GAL4 DNA-binding fusion system. Furthermore, the WW domains of human YAP and Pin1 were demonstrated to have a similar transcription-promoting activity. Combined mutation of the conserved first and second Trp residues and a hydrophobic triplet of TyrTyrTrp in the WW domain of Npw38 abolished the transcription-promoting activity, but single mutations of these sites did not. These results suggest that some WW domains potentially possess transcription-promoting activity in mammalian cells.

Molecular cloning of human Fe65L2 and its interaction with the Alzheimer's beta-amyloid precursor protein

We report the cDNA sequence of human Fe65L2. The human Fe65L2 encoded 486 amino acids; the deduced amino acid sequence was shorter by 18 amino acids than the rat protein and had 86% identity to the rat protein Three protein-protein interaction domains, a WW and two PID/PTB elements, were conserved among the Fe65 protein family. Human Fe65L2 mRNA was expressed in various tissues; a transcript of about 2.2 kb was mainly expressed in the brain. A splicing variant lacking two amino acids in the first PID/PTB element was detected. We also confirmed that the carboxyl-terminal region of PID/PTB of the Fe65L2 interacted with the intracellular domain of the Alzheimer's beta-amyloid precursor protein (APP) and APP-like proteins.