Three types of amyloid protein precursor mRNA in human brain: their differential expression in Alzheimer's disease

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Structural Determinant of β-Amyloid Formation: From Transmembrane Protein Dimerization to β-Amyloid Aggregates

Most neurodegenerative diseases have the characteristics of protein folding disorders, i.e., they cause lesions to appear in vulnerable regions of the nervous system, corresponding to protein aggregates that progressively spread through the neuronal network as the symptoms progress. Alzheimer's disease is one of these diseases. It is characterized by two types of lesions: neurofibrillary tangles (NFTs) composed of tau proteins and senile plaques, formed essentially of amyloid peptides (Aβ). A combination of factors ranging from genetic mutations to age-related changes in the cellular context converge in this disease to accelerate Aβ deposition. Over the last two decades, numerous studies have attempted to elucidate how structural determinants of its precursor (APP) modify Aβ production, and to understand the processes leading to the formation of different Aβ aggregates, e.g., fibrils and oligomers. The synthesis proposed in this review indicates that the same motifs can control APP function and Aβ production essentially by regulating membrane protein dimerization, and subsequently Aβ aggregation processes. The distinct properties of these motifs and the cellular context regulate the APP conformation to trigger the transition to the amyloid pathology. This concept is critical to better decipher the patterns switching APP protein conformation from physiological to pathological and improve our understanding of the mechanisms underpinning the formation of amyloid fibrils that devastate neuronal functions.

Amyloid precursor protein glycosylation is altered in the brain of patients with Alzheimer's disease

Background

The amyloid precursor protein (APP) is a transmembrane glycoprotein that undergoes alternative proteolytic processing. Its processing through the amyloidogenic pathway originates a large sAPPβ ectodomain fragment and the β-amyloid peptide, while non-amyloidogenic processing generates sAPPα and shorter non-fibrillar fragments. Hence, measuring sAPPα and sAPPβ has been proposed as a means to identify imbalances between the amyloidogenic/non-amyloidogenic pathways in the brain of Alzheimer’s disease (AD) patients. However, to date, no consistent changes in these proteolytic fragments have been identified in either the brain or cerebrospinal fluid of AD individuals.

Methods

In frontal cortex homogenates from AD patients (n = 7) and non-demented controls (NDC; n = 7), the expression of total APP mRNA and that of the APP isoforms generated by alternative splicing, APP695 and APP containing the Kunitz protease inhibitor (KPI), was analyzed by qRT-PCR using TaqMan and SYBR Green probes. The balance between the amyloidogenic/non-amyloidogenic pathways was examined in western blots estimating the sAPPα and sAPPβ fragments and their membrane-tethered C-terminal fragments CTFα and CTFβ. CHO-PS70 cells, stably over-expressing wild-type human APP, served to evaluate whether Aβ42 peptide treatment results in altered APP glycosylation. We determined the glycosylation pattern of sAPPα and sAPPβ in brain extracts and CHO-PS70 culture media by lectin-binding assays.

Results

In the cortex of AD patients, we detected an increase in total APP mRNA relative to the controls, due to an increase in both the APP695 and APP-KPI variants. However, the sAPPα or sAPPβ protein levels remained unchanged, as did those of CTFα and CTFβ. We studied the glycosylation of the brain sAPPα and sAPPβ using lectins and pan-specific antibodies to discriminate between the fragments originated from neuronal APP695 and glial/KPI variants. Lectin binding identified differences in the glycosylation of sAPPβ species derived from the APP695 and APP-KPI variants, probably reflecting their distinct cellular origin. Moreover, the lectin-binding pattern differed in the sAPPα and sAPPβ originated from all the variants. Finally, when the lectin-binding pattern was compared between AD and NDC groups, significant differences were evident in sAPPα glycosylation. Lectin binding of the soluble sAPPα and sAPPβ from CHO-PS70 cells were also altered in cells treated with the Aβ peptide.

Conclusion

Our analysis of the lectin binding to sAPPα and sAPPβ suggests that glycosylation dictates the proteolytic pathway for APP processing. Differences between the demented and controls indicate that changes in glycosylation may influence the generation of the different APP fragments and, consequently, the pathological progression of AD.

Deciphering the neuroprotective and neurogenic potential of soluble amyloid precursor protein alpha (sAPPα)

Amyloid precursor protein (APP) is a transmembrane protein expressed largely within the central nervous system. Upon cleavage, it does not produce the toxic amyloid peptide (Aβ) only, which is involved in neurodegenerative progressions but via a non-amyloidogenic pathway it is metabolized to produce a soluble fragment (sAPPα) through α-secretase. While a lot of studies are focusing on the role played by APP in the pathogenesis of Alzheimer's disease, sAPPα is reported to have numerous neuroprotective effects and it is being suggested as a candidate with possible therapeutic potential against Alzheimer's disease. However, the mechanisms through which sAPPα precisely works remain elusive. We have presented a comprehensive review of how sAPPα is regulating the neuroprotective effects in different biological models. Moreover, we have focused on the role of sAPPα during different developmental stages of the brain, neurogenic microenvironment in the brain and how this metabolite of APP is regulating the neurogenesis which is regarded as a compelling approach to ameliorate the impaired learning and memory deficits in dementia and diseases like Alzheimer's disease. sAPPα exerts beneficial physiological, biochemical and behavioral effects mitigating the detrimental effects of neurotoxic compounds. It has shown to increase the proliferation rate of numerous cell types and promised the synaptogenesis, neurite outgrowth, cell survival and cell adhesion. Taken together, we believe that further studies are warranted to investigate the exact mechanism of action so that sAPPα could be developed as a novel therapeutic target against neuronal deficits.

Alterations in the Balance of Amyloid-β Protein Precursor Species in the Cerebrospinal Fluid of Alzheimer's Disease Patients

We recently demonstrated that soluble forms of the amyloid-β protein precursor (sAβPP) assemble into multimeric complexes in cerebrospinal fluid (CSF), which contributes to the underestimation of specific sAβPP species when assessed by ELISA. To circumvent this issue, we analyzed by SDS-PAGE large fragments of sAβPP and their variants in the CSF from Alzheimer's disease (AD; n = 20) and control (n = 20) subjects, probing with specific antibodies against particular domains. Similar levels of sAβPPα and sAβPPβ protein were found in CSF samples from AD and controls, yet there appeared to be a shift in the balance of the soluble full-length AβPP (sAβPPf) species in AD samples, with a decrease in the proportion of the lower (∼100 kDa) band relative to the upper (∼120 kDa) band. Similar differences were observed in the contribution of the major KPI-immunoreactive AβPP species. CSF samples also displayed differences in the correlations of AβPP species with classical AD biomarkers, particularly with respect to the Aβ42 peptide. The differences reveal alterations that probably reflect pathophysiological changes in the brain.

Transmembrane Amyloid-Related Proteins in CSF as Potential Biomarkers for Alzheimer's Disease

In the continuing search for new cerebrospinal fluid (CSF) biomarkers for Alzheimer’s disease (AD), reasonable candidates are the secretase enzymes involved in the processing of the amyloid precursor protein (APP), as well as the large proteolytic cleavage fragments sAPPα and sAPPβ. The enzymatic activities of some of these secretases, such as BACE1 and TACE, have been investigated as potential AD biomarkers, and it has been assumed that these activities present in human CSF result from the soluble truncated forms of the membrane-bound enzymes. However, we and others recently identified soluble forms of BACE1 and APP in CSF containing the intracellular domains, as well as the multi-pass transmembrane presenilin-1 (PS1) and other subunits of γ-secretase. We also review recent findings that suggest that most of these soluble transmembrane proteins could display self-association properties based on hydrophobic and/or ionic interactions leading to the formation of heteromeric complexes. The oligomerization state of these potential new biomarkers needs to be taken into consideration for assessing their real potential as CSF biomarkers for AD by adequate molecular tools.

Chondroitin Sulfate Accelerates Trans-Golgi-to-Surface Transport of Proteoglycan Amyloid Precursor Protein

The amyloid precursor protein (APP) is a membrane protein implicated in the pathogenesis of Alzheimer's disease. APP is a part-time proteoglycan, as splice variants lacking exon 15 are modified by a chondroitin sulfate glycosaminoglycan (GAG) chain. Investigating the effect of the GAG chain on the trafficking of APP in non-polarized cells, we found it to increase the steady-state surface-to-intracellular distribution, to reduce the rate of endocytosis and to accelerate transport kinetics from the trans-Golgi network (TGN) to the plasma membrane. Deletion of the cytosolic domain resulted in delayed surface arrival of GAG-free APP, but did not affect the rapid export kinetics of the proteoglycan form. Protein-free GAG chains showed the same TGN-to-cell surface transport kinetics as proteoglycan APP. Endosome ablation experiments were performed to distinguish between indirect endosomal and direct pathways to the cell surface. Surprisingly, TGN-to-cell surface transport of both GAG-free and proteoglycan APP was found to be indirect via transferrin-positive endosomes. Our results show that GAGs act as alternative sorting determinants in cellular APP transport that are dominant over cytoplasmic signals and involve distinct sorting mechanisms.

Heteromers of amyloid precursor protein in cerebrospinal fluid

BACKGROUND

Soluble fragments of the amyloid precursor protein (APP) generated by α- and β-secretases, sAPPα and sAPPβ, have been postulated as promising new cerebrospinal fluid (CSF) biomarkers for the clinical diagnosis of Alzheimer's disease (AD). However, the capacity of these soluble proteins to assemble has not been explored and could be relevant. Our aim is to characterize possible sAPP oligomers that could contribute to the quantification of sAPPα and sAPPβ in CSF by ELISA, as well as to characterize the possible presence of soluble full-length APP (sAPPf).

RESULTS

We employed co-immunoprecipitation, native polyacrylamide gel electrophoresis and ultracentrifugation in sucrose density gradients to characterize sAPP oligomers in CSF. We have characterized the presence of sAPPf in CSF from NDC and AD subjects and demonstrated that all forms, including sAPPα and sAPPβ, are capable of assembling into heteromers, which differ from brain APP membrane-dimers. We measured sAPPf, sAPPα and sAPPβ by ELISA in CSF samples from AD (n = 13) and non-disease subjects (NDC, n = 13) before and after immunoprecipitation with antibodies against the C-terminal APP or against sAPPβ. We demonstrated that these sAPP heteromers participate in the quantification of sAPPα and sAPPβ by ELISA. Immunoprecipitation with a C-terminal antibody to remove sAPPf reduced by ~30% the determinations of sAPPα and sAPPβ by ELISA, whereas immunoprecipitation with an APPβ antibody reduced by ~80% the determination of sAPPf and sAPPα.

CONCLUSIONS

The presence of sAPPf and sAPP heteromers should be taken into consideration when exploring the levels of sAPPα and sAPPβ as potential CSF biomarkers.

Alternative splicing regulation of APP exon 7 by RBFox proteins

RBFox proteins are well-known alternative splicing regulators. We have shown previously that during neuronal differentiation of P19 cells induced by all-trans retinoic acid and cell aggregation, RBFox1 shows markedly increased temporal expression. To find its key splicing regulation, we examined the effect of RBFox1 on 33 previously reported and validated neuronal splicing events of P19 cells. We observed that alternative splicing of three genes, specifically, amyloid precursor protein (APP), disks large homolog 3 (DLG3), and G protein, alpha activating activity polypeptide O (GNAO1), was altered by transient RBFox1 expression in HEK293 and HeLa cells. Moreover, an RBFox1 mutant (RBFox1FA) that was unable to bind the target RNA sequence ((U)GCAUG) did not induce these splicing events. APP generates amyloid beta peptides that are involved in the pathology of Alzheimer's disease, and therefore we examined APP alternative splicing regulation by RBFox1 and other splicing regulators. Our results indicated that RBFox proteins promote the skipping of APP exon 7, but not the inclusion of exon 8. We made APP6789 minigenes and observed that two (U)GCAUG sequences, located upstream of exon 7 and in exon 7, functioned to induce skipping of exon 7 by RBFox proteins. Overall, RBFox proteins may shift APP from exon 7 containing isoforms, APP770 and APP751, toward the exon 7 lacking isoform, APP695, which is predominant in neural tissues.

Expression and function of APP and its metabolites outside the central nervous system

Amyloid precursor protein (APP) derived amyloid beta (Aβ) peptides have been extensively investigated in Alzheimer's disease pathology of the brain. However, the function of full length APP in the central nervous system remains unclear. Even less is known about the function of this ubiquitously expressed protein and its metabolites outside of the central nervous system. This review summarizes key aspects of the current understanding of the expression and function of APP and its proteolytic fragments in specific non-neuronal tissues.

Cellular prion protein expression is not regulated by the Alzheimer's amyloid precursor protein intracellular domain

There is increasing evidence of molecular and cellular links between Alzheimer's disease (AD) and prion diseases. The cellular prion protein, PrP^C, modulates the post-translational processing of the AD amyloid precursor protein (APP), through its inhibition of the β-secretase BACE1, and oligomers of amyloid-β bind to PrP^C which may mediate amyloid-β neurotoxicity. In addition, the APP intracellular domain (AICD), which acts as a transcriptional regulator, has been reported to control the expression of PrP^C. Through the use of transgenic mice, cell culture models and manipulation of APP expression and processing, this study aimed to clarify the role of AICD in regulating PrP^C. Over-expression of the three major isoforms of human APP (APP₆₉₅, APP₇₅₁ and APP₇₇₀) in cultured neuronal and non-neuronal cells had no effect on the level of endogenous PrP^C. Furthermore, analysis of brain tissue from transgenic mice over-expressing either wild type or familial AD associated mutant human APP revealed unaltered PrP^C levels. Knockdown of endogenous APP expression in cells by siRNA or inhibition of γ-secretase activity also had no effect on PrP^C levels. Overall, we did not detect any significant difference in the expression of PrP^C in any of the cell or animal-based paradigms considered, indicating that the control of cellular PrP^C levels by AICD is not as straightforward as previously suggested.

Genetics of Alzheimer disease

Alzheimer disease (AD) is the most common causes of neurodegenerative disorder in the elderly individuals. Clinically, patients initially present with short-term memory loss, subsequently followed by executive dysfunction, confusion, agitation, and behavioral disturbances. Three causative genes have been associated with autosomal dominant familial AD (APP, PSEN1, and PSEN2) and 1 genetic risk factor (APOEε4 allele). Identification of these genes has led to a number of animal models that have been useful to study the pathogenesis underlying AD. In this article, we provide an overview of the clinical and genetic features of AD.

Copper binding to the Alzheimer's disease amyloid precursor protein

Alzheimer’s disease is the fourth biggest killer in developed countries. Amyloid precursor protein (APP) plays a central role in the development of the disease, through the generation of a peptide called Aβ by proteolysis of the precursor protein. APP can function as a metalloprotein and modulate copper transport via its extracellular copper binding domain (CuBD). Copper binding to this domain has been shown to reduce Aβ levels and hence a molecular understanding of the interaction between metal and protein could lead to the development of novel therapeutics to treat the disease. We have recently determined the three-dimensional structures of apo and copper bound forms of CuBD. The structures provide a mechanism by which CuBD could readily transfer copper ions to other proteins. Importantly, the lack of significant conformational changes to CuBD on copper binding suggests a model in which copper binding affects the dimerisation state of APP leading to reduction in Aβ production. We thus predict that disruption of APP dimers may be a novel therapeutic approach to treat Alzheimer’s disease.

Increased KPI containing amyloid precursor protein in experimental autoimmune encephalomyelitis brains

Amyloid precursor protein can be translated from three alternatively spliced mRNAs. We measured levels of amyloid precursor protein isoforms containing the Kunitz protease inhibitor domain (KPIAPP), and amyloid precursor protein without the Kunitz protease inhibitor domain (KPIAPP) in brain homogenates of acute experimental autoimmune encephalomyelitis mice. At the preclinical phase of the disease, both KPIAPP and KPIAPP levels were significantly higher in homogenates from brains of autoimmune encephalomyelitis mice, whereas at the acute phase of the disease only KPIAPP remained significantly elevated compared with controls. At the recovery phase, no differences were observed between the groups. The early and isoform-specific elevation of KPIAPP in autoimmune encephalomyelitis mice suggests a possible role for amyloid precursor protein in the immune response mediating the disease.

Structural Studies of the Alzheimer’s Amyloid Precursor Protein Copper-binding Domain Reveal How it Binds Copper Ions

Alzheimer's disease (AD) is the major cause of dementia. Amyloid beta peptide (Abeta), generated by proteolytic cleavage of the amyloid precursor protein (APP), is central to AD pathogenesis. APP can function as a metalloprotein and modulate copper (Cu) transport, presumably via its extracellular Cu-binding domain (CuBD). Cu binding to the CuBD reduces Abeta levels, suggesting that a Cu mimetic may have therapeutic potential. We describe here the atomic structures of apo CuBD from three crystal forms and found they have identical Cu-binding sites despite the different crystal lattices. The structure of Cu(2+)-bound CuBD reveals that the metal ligands are His147, His151, Tyr168 and two water molecules, which are arranged in a square pyramidal geometry. The site resembles a Type 2 non-blue Cu center and is supported by electron paramagnetic resonance and extended X-ray absorption fine structure studies. A previous study suggested that Met170 might be a ligand but we suggest that this residue plays a critical role as an electron donor in CuBDs ability to reduce Cu ions. The structure of Cu(+)-bound CuBD is almost identical to the Cu(2+)-bound structure except for the loss of one of the water ligands. The geometry of the site is unfavorable for Cu(+), thus providing a mechanism by which CuBD could readily transfer Cu ions to other proteins.

Porcine APP cDNAs: Molecular cloning and characterization, expression analysis, chromosomal localization and SNP analysis

The human amyloid precursor protein (APP) is the precursor of Abeta, a peptide with the potential to create amyloid plaques in neurons. Mutations in the human APP gene are associated with the familial form of Alzheimer's disease. In addition, differential expression of three alternative pre-mRNA APP splicing variants of 695, 751, and 770 amino acids is linked to the pathogenesis. In this study, two novel transcript variants of porcine APP have been identified, producing isoforms of 695 and 751 amino acids, respectively. These are highly homologous to APP orthologues from other vertebrate species. Expression analyses revealed that the gene is expressed in all 30 examined porcine tissues and in a selected subset of these, differential representation of the three major APP transcript variants was observed. The APP isoform of 770 amino acids clearly predominates in non-neuronal tissues while in porcine cerebellum, the APP isoforms of 695 and 770 amino acids are expressed at equivalent levels. Employing a somatic cell hybrid panel, the APP gene was mapped to porcine chromosome 13 in either the 13q41 or 13q46-q49 region. A large pig population was screened for single nucleotide polymorphisms (SNPs) in APP exon 17 and flanking intron sequences. No missense mutations were detected; however, the allele frequencies of two silent mutations and two intron polymorphisms varied significantly among breeds.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Amyloid precursor protein mRNA levels in Alzheimer's disease brain

Insoluble beta-amyloid deposits in Alzheimer's disease (AD) brain are proteolytically derived from the membrane bound amyloid precursor protein (APP). The APP gene is differentially spliced to produce isoforms that can be classified into those containing a Kunitz-type serine protease inhibitor domain (K(+), APP(751), APP(770), APRP(365) and APRP(563)), and those without (K(-), APP(695) and APP(714)). Given the hypothesis that Abeta is a result of aberrant catabolism of APP, differential expression of mRNA isoforms containing protease inhibitors might play an active role in the pathology of AD. We took 513 cerebral cortex samples from 90 AD and 81 control brains and quantified the mRNA isoforms of APP with TaqMan real-time RT-PCR. After adjustment for age at death, brain pH and gender we found a change in the ratio of KPI(+) to KPI(-) mRNA isoforms of APP. Three separate probes, designed to recognise only KPI(+) mRNA species, gave increases of between 28% and 50% in AD brains relative to controls (p=0.002). There was no change in the mRNA levels of KPI-(APP 695) (p=0.898). Therefore, whilst KPI-mRNA levels remained stable the KPI(+) species increased specifically in the AD brains.

Protein phosphorylation and APP metabolism

Numerous lines of evidence place signal transduction cascades at the core of many processes having a direct role in neurodegeneration and associated disorders. Key players include neurotransmitters, growth factors, cytokines, hormones, and even binding and targeting proteins. Indeed, abnormal phosphorylation of key control proteins has been detected in many cases and is thought to underlie the associated cellular dysfunctions. Several signaling cascades have been implicated, affecting processes as varied as protein processing, protein expression, and subcellular protein localization, among others. The Alzheimer's amyloid precursor protein (APP) is a phosphoprotein, with well-defined phosphorylation sites but whose function is not clearly understood. The factors and pathways regulating the processing of APP have been particularly elusive, both in normal ageing and the Alzheimer's disease (AD) condition. Not surprisingly, the physiological function(s) of the protein remain(s) to be elucidated, although many hypotheses have been advanced. Nonetheless, considerable data has accumulated over the last decade, placing APP in key positions to be modulated both directly and indirectly by phosphorylation and phosphorylation-dependent events. The pathological end product of APP processing is the main proteinaceous component of the hallmark senile plaques found in the brains of AD patients, that is, a toxic peptide termed Abeta. In this minireview we address the importance of phosphorylation and signal transduction cascades in relation to APP processing and Abeta production. The possible use of the identified molecular alterations as therapeutic targets is also addressed.

Identification of a novel alternative splicing isoform of human amyloid precursor protein gene, APP639

Senile plaques, neurofibrillary tangles and amyloid-laden cerebral vessels are characteristic features in the brains of individuals with Alzheimer's disease (AD). The principal component of amyloid in senile plaque and amyloid-laden cerebral vessels is beta-amyloid (Abeta), a peptide proteolytically derived from a large amyloid precursor protein (APP). To date, several alternatively spliced human APP transcripts have been described. Here, we report the identification of a novel alternative splicing isoform of the human APP gene, APP639, which excludes exon 2 as well as exons 7 and 8. RT-PCR and Southern blot analysis show that APP639 mRNA is expressed in many human fetal tissues. In contrast, the APP639 transcript is hardly detected in the aged human cerebral cortex from both pathologically confirmed sporadic AD cases and nondemented controls. However, APP639 mRNA exists in the adult human liver. Western blot analysis shows that the protein product produced from the APP639 cDNA could be recognized by the APP antibody, and it does lack the exon 2 coding region. These results suggest that APP639, a novel alternative splicing isoform of human APP gene, does exist in human tissues in vivo.

Regulation of beta-amyloid precursor protein and inositol 1,4,5-trisphosphate receptor gene expression during differentiation of a human neuronal cell line

Retinoic acid-induced differentiation of SH-SY5Y human neuroblastoma cells results in the development of extensive neurite processes as well as changes in cell body morphology toward a neuronal phenotype. The authors have examined concurrent regulation of beta-amyloid precursor protein (APP) and inositol 1,4,5-trisphosphate receptor (insP(3)R) gene expression in SY5Y cells during neuronal differentiation. Of the multiple APP mRNA transcripts expressed in this cell line, retinoic acid treatment significantly increased the expression of APP(695) transcript while the level of total APP remained unchanged. In the same time course, neuronal differentiation decreased the expression of insP(3)R at both the mRNA and protein levels. These findings demonstrate an inverse relationship between APP and insP(3)R gene expression during neuronal differentiation of SH-SY5Y cells and suggest a possible change in intracellular calcium homeostasis.

Long-term accumulation of amyloid-beta in axons following brain trauma without persistent upregulation of amyloid precursor protein genes

Brain trauma has been shown to be a risk factor for developing Alzheimer disease (AD), and AD-like plaques containing amyloid-beta (Abeta) peptides have been found in the brain shortly following trauma. Here, we evaluated the effects of brain trauma on the accumulation of Abeta and expression of amyloid precursor protein (APP) genes (APP695 and APP751/ 770) over 1 yr in a non-transgenic rodent model. Anesthetized male Sprague-Dawley rats were subjected to parasagittal fluid percussion brain injury of moderate severity (2.5-2.9 atm) or sham treatment and their brains were evaluated at 2, 4, 7, 14 days, and 1, 2, 6, 12 months following injury. Immunohistochemical analysis detected only weak Abeta staining by 2 wk following injury. However, by 1 month to 1 yr following injury, strong immunoreactivity for Abeta was found in damaged axons throughout the thalamus and white matter. Western blot analysis confirmed the accumulation of Abeta peptides in tissue from injured brains. Although in situ hybridization demonstrated an increased gene expression of APP751/770 surrounding the cortical lesion at 2 to 7 days following injury, this expression returned to baseline levels at all subsequent time points and no increase in the expression of APP695 was detected at any time point. These results demonstrate that long-termAbeta accumulation in damaged axons can be induced in a non-transgenic rodent model of brain trauma. Surprisingly, the extent of this Abeta production appeared to be dependent on the maturity of the injury, but uncoupled from the gene expression of APP. Together, these data suggest a mechanism that may contribute to long-term neurodegeneration following brain trauma.

Gamma-secretase: substrates and inhibitors

The amyloid beta-protein (Abeta) deposited in Alzheimer's disease (AD), the most common form of dementia in the elderly, is a secreted proteolytic product of the amyloid beta-protein precursor (APP). Generation of Abeta from the APP requires two sequential proteolytic events, beta-secretase cleavage to generate the amino terminus, followed by gamma-secretase cleavage to generate the carboxyl terminus. Because this process is a central event in the pathogenesis of AD, gamma-secretase is believed to be an excellent therapeutic target. Gamma-secretase activity has been demonstrated to be membrane-associated, with the cleavage site primarily determined by the location of the substrate with respect to the membrane. It has also been shown that this unusual proteolytic activity not only occurs for APP, but also for proteins involved in morphogenic processes or cell proliferation and differentiation such as Notch and ErbB4. Thus far, all gamma-secretase substrates are involved in some form of nuclear signaling. These recent findings have important implications for the development of pharmacological interventions that target gamma-secretase.

The molecular bases of Alzheimer's disease and other neurodegenerative disorders

Alzheimer's disease, the cause of one of the most common types of dementia, is a brain disorder affecting the elderly and is characterized by the formation of two main protein aggregates: senile plaques and neurofibrillary tangles, which are involved in the process leading to progressive neuronal degeneration and death. Neurodegeneration in Alzheimer's disease is a pathologic condition of cells rather than an accelerated way of aging. The senile plaques are generated by a deposition in the human brain of fibrils of the beta-amyloid peptide (Abeta), a fragment derived from the proteolytic processing of the amyloid precursor protein (APP). Tau protein is the major component of paired helical filaments (PHFs), which form a compact filamentous network described as neurofibrillary tangles (NFTs). Experiments with hippocampal cells in culture have indicated a relationship between fibrillary amyloid and the cascade of molecular signals that trigger tau hyperphosphorylations. Two main protein kinases have been shown to be involved in anomalous tau phosphorylations: the cyclin-dependent kinase Cdk5 and glycogen synthase kinase GSK3beta. Cdk5 plays a critical role in brain development and is associated with neurogenesis as revealed by studies in brain cells in culture and neuroblastoma cells. Deregulation of this protein kinase as induced by extracellular amyloid loading results in tau hyperphosphorylations, thus triggering a sequence of molecular events that lead to neuronal degeneration. Inhibitors of Cdk5 and GSK3beta and antisense oligonucleotides exert protection against neuronal death. On the other hand, there is cumulative evidence from studies in cultured brain cells and on brains that oxidative stress constitutes a main factor in the modification of normal signaling pathways in neuronal cells, leading to biochemical and structural abnormalities and neurodegeneration as related to the pathogenesis of Alzheimer's disease. This review is focused on the main protein aggregates responsible for neuronal death in both sporadic and familial forms of Alzheimer's disease, as well as on the alterations in the normal signaling pathways of functional neurons directly involved in neurodegeneration. The analysis is extended to the action of neuroprotective factors including selective inhibitors of tau phosphorylating protein kinases, estrogens, and antioxidants among other molecules that apparently prevent neuronal degeneration.

Secretases as therapeutic targets for the treatment of Alzheimer's disease

The extracellular deposition of short amyloid peptides in the brain of patients is thought to be a central event in the pathogenesis of Alzheimer's Disease. The generation of the amyloid peptide occurs via a regulated cascade of cleavage events in its precursor protein, A beta PP. At least three enzymes are responsible for A beta PP proteolysis and have been tentatively named alpha-, beta- and gamma-secretases. The recent identification of several of these secretases is a major leap in the understanding how these secretases regulate amyloid peptide formation. Members of the ADAM family of metalloproteases are involved in the non-amyloidogenic alpha-secretase pathway. The amyloidogenic counterpart pathway is initiated by the recently cloned novel aspartate protease named BACE. The available data are conclusive and crown BACE as the long-sought beta-secretase. This enzyme is a prime candidate drug target for the development of therapy aiming to lower the amyloid burden in the disease. Finally, the gamma-secretases are intimately linked to the function of the presenilins. These multi-transmembrane domain proteins remain intriguing study objects. The hypothesis that the presenilins constitute a complete novel type of protease family, and are cleaving A beta PP within the transmembrane region, remains an issue of debate. Several questions remain unanswered and direct proof that they exert catalytic activity is still lacking. The subcellular localization of presenilins in neurons, their integration in functional multiprotein complexes and the recent identification of additional modulators of gamma-secretase, like nicastrin, indicate already that several players are involved. Nevertheless, the rapidly increasing knowledge in this area is already paving the road towards selective inhibitors of this secretase as well. It is hoped that such drugs, possibly in concert with the experimental vaccination therapies that are currently tested, will lead to a cure of this inexorable disease.

Analysis of mouse intron 7 DNA sequence of the APP gene: comparison with the human homologue

Mutations in the beta-amyloid precursor protein gene (APP) cause Alzheimer disease (AD) in certain families. The mature protein (APP) exists in several different isoforms resulting from alternative splicing of the primary transcript. Several lines of evidence indicate that particular isoform(s) of APP may contribute to the etiology of AD. One of the isoforms, APP695, lacks the Kunitz protease inhibitor (KPI) domain encoded by exon 7. APP695 is expressed predominantly in neurons, whereas the KPI domain containing isoforms, APP751 and APP770, are expressed ubiquitously. The ratio of APP751/APP695 mRNA tends to increase in the brain of AD patients. Furthermore, this ratio in mouse brain is much lower than that in human brain, and mice are resistant to the spontaneous development of beta-amyloidosis. In addition, transgenic mice that develop pathological changes similar to those of AD expressed more KPI-domain containing APP mRNA than transgenic mice without the changes. Previous studies imply that the controlling elements exist in the flanking sequences of the alternatively-spliced exons. Therefore, we have determined the DNA sequences of intron 7 and made a comparison between mouse and human DNA sequences of intron 7. Mouse intron 7 shares about 50% sequence identity with the human homologue, with higher sequence identity (approximately 85%) mainly in the 5' end (approximately 250 bp) of the intron. A palindromic sequence was found in both human and mouse intron 7 and showed subtle differences in their structure between the two species. Whether this sequence plays any roles in regulating alternative splicing of exon 7 remains to be determined. Human intron 7 contains a Alu element, which possesses potential retinoic acid and thyroid hormone responsive elements that might be involved in the regulation of alternative splicing. Mouse intron 7 sequence also contains a few repeat sequences which are specific to the genome of mice and rats. Homologies shared between human and mouse intron 7 sequences may contribute to the common characteristics of neuron-specific splicing of APP in both species. The unique features of the intron may account for differences between human and mouse brain in fine tuning of alternative splicing of the APP transcript, which may lead to their different susceptibilities to beta-amyloidosis.

What the evolution of the amyloid protein precursor supergene family tells us about its function

The Alzheimer's disease amyloid protein precursor (APP) gene is part of a multi-gene super-family from which sixteen homologous amyloid precursor-like proteins (APLP) and APP species homologues have been isolated and characterised. Comparison of exon structure (including the uncharacterised APL-1 gene), construction of phylogenetic trees, and analysis of the protein sequence alignment of known homologues of the APP super-family were performed to reconstruct the evolution of the family and to assess the functional significance of conserved protein sequences between homologues. This analysis supports an adhesion function for all members of the APP super family, with specificity determined by those sequences which are not conserved between APLP lineages, and provides evidence for an increasingly complex APP superfamily during evolution. The analysis also suggests that Drosophila APPL and Caenorhabditis elegans APL-1 may be a fourth APLP lineage indicating that these proteins, while not functional homologues of human APP, are similarly likely to regulate cell adhesion. Furthermore, the betaA4 sequence is highly conserved only in APP orthologues, strongly suggesting this sequence is of significant functional importance in this lineage.

Relative increase in Alzheimer's disease of soluble forms of cerebral Abeta amyloid protein precursor containing the Kunitz protease inhibitory domain

Although a number of studies have examined amyloid precursor protein (APP) mRNA levels in Alzheimer's disease (AD), no clear consensus has emerged as to whether the levels of transcripts for isoforms containing a Kunitz protease inhibitory (KPI)-encoded region are increased or decreased in AD. Here we compare AD and control brain for the relative amounts of APP protein containing KPI to APP protein lacking this domain. APP protein was purified from the soluble subcellular fraction and Triton X-100 membrane pellet extract of one hemisphere of AD (n = 10), normal (n = 7), and neurological control (n = 5) brains. The amount of KPI-containing APP in the purified protein samples was determined using two independent assay methods. The first assay exploited the inhibitory action of KPI-containing APP on trypsin. The second assay employed reflectance analysis of Western blots. The proportion of KPI-containing forms of APP in the soluble subcellular fraction of AD brains is significantly elevated (p < 0.01) compared with controls. Species containing a KPI domain comprise 32-41 and 76-77% of purified soluble APP from control and AD brains, respectively. For purified membrane-associated APP, 72-77 and 65-82% of control and AD samples, respectively, contain a KPI domain. Since KPI-containing species of APP may be more amyloidogenic (Ho, L., Fukuchi, K., and Yonkin, S. G. (1996) J. Biol. Chem. 271, 30929-30934), our findings support an imbalance of isoforms as one possible mechanism for amyloid deposition in sporadic AD.

Identification and cloning of human placental bikunin, a novel serine protease inhibitor containing two Kunitz domains

Interrogation of the public expressed sequence tag (EST) data base with the sequence of preproaprotinin identified ESTs encoding two potential new members of the Kunitz family of serine protease inhibitors. Through reiterative interrogation, an EST contig was obtained, the consensus sequence from which encoded both of the novel Kunitz domains in a single open reading frame. This consensus sequence was used to direct the isolation of a full-length cDNA clone from a placental library. The resulting cDNA sequence predicted a 252-residue protein containing a putative NH2-terminal signal peptide followed sequentially by each of the two Kunitz domains within a 170-residue ectodomain, a putative transmembrane domain, and a 31-residue hydrophilic COOH terminus. The gene for this putative novel protein was mapped by use of a radiation hybrid panel to chromosome 19q13, and Northern analysis showed that the corresponding mRNA was expressed at high levels in human placenta and pancreas and at lower levels in brain, lung, and kidney. An endogenous soluble form of this protein, which was designated as placental bikunin, was highly purified from human placenta by sequential kallikrein-Sepharose affinity, gel filtration, and C18 reverse-phase chromatography. The natural protein exhibited the same NH2 terminus as predicted from the cloned cDNA and inhibited trypsin, plasma kallikrein, and plasmin with IC50 values in the nanomolar range.

Serum deprivation alters the expression and the splicing at exons 7, 8 and 15 of the beta-amyloid precursor protein in the C6 glioma cell line

Amyloid deposition characterizes the pathological lesions of Alzheimer's disease. We investigated the effect of serum deprivation on the regulation of beta-amyloid precursor protein (APP) mRNA expression in C6 glioma cells. Serum deprivation increased APP mRNA levels approximately 4-fold over controls. This increase was accompanied by changes in the pattern of alternative splicing, including the novel alternatively spliced site at exon 15. The proportion of isoforms containing exons 7 and 8 significantly increased from 61% to 68%, while isoforms lacking these exons decreased from 14% to 8%. The proportion of leukocyte-derived APP, which is a novel alternatively spliced isoform lacking exon 15, significantly increased from 19% to 40%. Among the six major isoforms produced by the two independent splicing sites, L-APP752 which contains exons 7 and 8, but lacks exon 15, increased the most (approximately 10-fold). Our findings provide evidence linking APP expression to alterations in alternative splicing at exon 15. These results demonstrate that in glial cells, APP mRNA regulation involves the alteration in alternative splicing at exons 7, 8 and 15, suggesting that not only increased expression but also an imbalance in the relative abundance of the six APP isoforms in stressed condition might affect the amyloidogenesis in Alzheimer's disease.

Levels and alternative splicing of amyloid beta protein precursor (APP) transcripts in brains of APP transgenic mice and humans with Alzheimer's disease

Abnormal expression of human amyloid precursor protein (hAPP) gene products may play a critical role in Alzheimer's disease (AD). Recently, a transgenic model was established in which platelet-derived growth factor (PDGF) promoter-driven neuronal expression of an alternatively spliced hAPP minigene resulted in prominent AD-type neuropathology (Games, D., Adams, D., Alessandrini, R., Barbour, R., Berthelette, P., Blackwell, C., Carr, T., Clemens, J., Donaldson, T., Gillespie, F., Guido, T., Hagopian, S., Johnson-Wood, K., Khan, K., Lee, M., Leibowitz, P., Lieberburg, I., Little, S., Masliah, E., McConlogue, L., Montoya-Zavala, M., Mucke, L., Paganini, L., and Penniman, E. (1995) Nature 373, 523-527). Here we compared the levels and alternative splicing of APP transcripts in brain tissue of hAPP transgenic and nontransgenic mice and of humans with and without AD. PDGF-hAPP mice showed severalfold higher levels of total APP mRNA than did nontransgenic mice or humans, whereas their endogenous mouse APP mRNA levels were decreased. This resulted in a high ratio of mRNAs encoding mutated hAPP versus wild-type mouse APP. Modifications of hAPP introns 6, 7, and 8 in the PDGF-hAPP construct resulted in a prominent change in alternative splice site selection with transcripts encoding hAPP770 or hAPP751 being expressed at substantially higher levels than hAPP695 mRNA. Frontal cortex of humans with AD showed a subtle increase in the relative abundance of hAPP751 mRNA compared with normal controls. These data identify specific intron sequences that may contribute to the normal neuronspecific alternative splicing of APP pre-mRNA in vivo and support a causal role of hAPP gene products in the development of AD-type brain alterations.

Conventional protein kinase C (PKC)-alpha and novel PKC epsilon, but not -delta, increase the secretion of an N-terminal fragment of Alzheimer's disease amyloid precursor protein from PKC cDNA transfected 3Y1 fibroblasts

A large soluble N-terminal fragment of Alzheimer's disease amyloid precursor protein (secreted form of APP: APPs) is produced by constitutive processing in the middle of the amyloid beta-protein portion of APP. Recent studies indicate that the activation of endogenous protein kinase C (PKC) with phorbol ester raises the rate of secretion of APPs. We constructed rat fibroblast 3Y1 cells that stably overexpress PKC isoenzymes alpha, delta, or epsilon, and analyzed the amount of APPs released from these PKC transfectants. The levels of APPs released from 3Y1 cells overexpressing PKC alpha and -epsilon were higher than those from PKC delta-transfected and control cells expressing vector only. These results suggest that specific isoforms of PKC regulate the secretion of APPs through a signaling pathway.

beta-Amyloid peptide produced in vitro is degraded by proteinases released by cultured cells

The primary histopathological feature of Alzheimer's disease is the accumulation of beta-amyloid in the brains of afflicted individuals. This peptide has been shown to be produced and liberated both in vitro and in vivo by normal physiological processes. The mechanism by which beta-amyloid is formed, as well as that by which it may be cleared, are events likely to impact on the development and progression of this disease. Thus, the fate of beta-amyloid peptides secreted by cultured mammalian cells was investigated. It was found that levels of the soluble peptide are reduced over time due to the activity of multiple types of proteinases including those from the metallo, aspartyl, and thiol classes. Inhibitors to each class of proteinase can only partially block beta-amyloid degradation, but, if used in combination, they can fully prevent its catabolism. The Kunitz serine proteinase inhibitor domain, present on two beta-amyloid precursor protein isoforms, was found to be an effective inhibitor of beta-amyloid peptide degradation. These data indicate that modulations in expression of secreted proteinases and/or beta-amyloid precursor isoforms may influence levels of beta-amyloid.

Mitosis of Schwann cells and demyelination are induced by the amyloid precursor protein and other protease inhibitors in the rat sciatic nerve

We studied the cytological alterations produced in the rat sciatic nerve by the amyloid precursor protein (APP) containing the Kunitz insert (APP K+) and other protease inhibitors. Conditioning of nerve segments with APP K+, aprotinin or leupeptin for 5 days or more resulted in mitosis of Schwann cells, demyelination of fibres, and a < 10-fold increase in Schwann cells, associated with demyelinated fibres. Altered fibres nevertheless involved a small part of the population. Nerve segments proximal and distal to the conditioned region showed almost no alteration. Conditioning with saline, heated APP K+, or APP without the Kunitz insert was not effective. We conclude that APP K+ and other protease inhibitors induce Schwann cells to enter the cell cycle, and once committed to proliferate they resorb their myelin. These functional properties of APP may be relevant to the pathogenesis of Alzheimer's disease.

Gene expression in Alzheimer neocortex as a function of age and pathologic severity

Previous studies have shown a marked decline in neuronal and an increase in glial gene expression in Alzheimer's disease (AD) neocortex. Severity of pathologic changes may be greater in presenile AD (PAD) than in senile AD (SAD). We evaluated whether changes in transcript expression were altered as a function of age or pathologic severity. Northern analysis revealed a marked (> 50%) decline in expression of transcripts for the neurofilament light subunit and the major amyloid precursor protein (APP) isoforms in both PAD and SAD. Expression of these neuronal transcripts declined as a function of age in AD and control cases. Expression of the glial fibrillary acidic protein (GFAP) transcript was increased in AD, particularly in the presenile group. AD cases with larger numbers of neurofibrillary tangles had higher levels of GFAP transcript; AD cases with larger numbers of senile plaques had higher levels of APP695 transcript. We conclude that the neuronal mRNA decrements of AD are superimposed on an age-related decline. Age-related shift in expression of certain genes may account for the differences in pathologic severity of PAD and SAD.

Testing for mutations in exon 17 of the beta-amyloid precursor protein gene in Finnish Alzheimer patients and normal subjects

Mutations in the beta-amyloid precursor protein gene on chromosome 21 were shown to cause a small proportion of Alzheimer's disease. We studied the occurrence of the point mutations in exon 17 of the beta-amyloid precursor protein gene in a sample of Finnish familial Alzheimer patients and nondemented controls using polymerase chain reaction and a single strand conformation polymorphism technique. In addition, mutations in familial Alzheimer's disease patients were studied by sequencing the amplified products. Interestingly, two probable polymerase chain reaction errors were detected in codons 717 and 693 of the exon 17. However, no mutations in the exon 17 were confirmed adding the study to the body of literature that mutations in the exon 17 are a rare cause of familial Alzheimer's disease.

The effects of aging and hormonal manipulation on amyloid precursor protein APP695 mRNA expression in the rat hippocampus

In the rat hippocampus, neuronal morphology and survival are profoundly affected by adrenal steroids, and synaptic plasticity can be modulated by the ovarian sex steroids estrogen and progesterone. beta-amyloid peptides, which accumulate in neuritic plaques and are derived from the amyloid precursor protein (APP), have been shown to be both trophic and toxic for hippocampal neurons. Of the various APP isoforms, APP695 is the predominant form found in rat brain and the APP695 mRNA is abundantly expressed in the hippocampus. In order to investigate the hypothesis that APP may serve as a mediator of the steroid effects, we have monitored the hippocampal expression of APP695 mRNA by in situ hybridization, with aging and with steroid manipulation. In aged female rats we observed a decrease in the level of APP695 mRNA relative to young female rats, while no such age difference was evident in male rats. Physiological, surgical and pharmacological manipulation of glucocorticoids appeared to have no effect on APP695 mRNA levels in the hippocampus. Treatment of young, ovariectomized female rats with estrogen and progesterone, resulted in an increase in hippocampal APP695 expression compared to untreated, ovariectomized controls.

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.

Down's syndrome: up-regulation of beta-amyloid protein precursor and tau mRNAs and their defective coordination

Almost all patients > 40 years of age with Down's syndrome (DS) develop the pathology characteristic of Alzheimer's disease: abundant beta-amyloid plaques and neurofibrillary tangles. We have investigated the gene expression of beta-amyloid protein precursor (APP) and tau in DS and age-matched control brains and found that levels of both mRNAs were significantly elevated in DS. Such up-regulation was not observed in two other neuronal proteins. A correlation between total APP and tau mRNA levels was also found in DS brain but distinct from the pattern observed in normal brain. Although a proportionality existed between APP-695 mRNA and three-repeat tau mRNA in DS, the proportionality between APP-751 mRNA and four-repeat tau mRNA, which is normally present, was not observed. Thus, DS brains are primarily characterized by the up-regulation of tau mRNA as well as APP mRNA and disruption of the coordinate expression between APP-751 and four-repeat tau.

Age-related change in the proportion of amyloid precursor protein mRNAs in the gray matter of cerebral cortex

Alternative splicing of a transcript of amyloid precursor protein (APP) gene generates at least three types of mRNA coding for APP770, APP751, and APP695; the former two harbor, while the latter one lacks a Kunitz-type serine protease inhibitor (KPI). We compared, by using the RNase protection technique, APP mRNAs expression between gray and white matters in the frontal lobe, with special reference to Alzheimer's disease (AD) and aging. The proportions (y) of APP770 plus APP751 mRNAs in the white matter were nearly twice as much as those in the gray matter, both in control (non-AD) and AD brains; the difference between the two matters was statistically significant. Furthermore, in the gray matter of control, there was a positive correlation (y = 1.07 x-57.1, r = 0.899) between the age (x) and the proportion (y), but not in the white matter where the proportion varied markedly among individuals. In AD brains, no significant correlation was found in either of the two matters. These results indicated that the APP mRNAs proportion in the gray matter may serve as a molecular index of the brain aging in non-AD persons.

Regional distribution of amyloid beta-protein precursor, growth-associated phosphoprotein-43 and microtubule-associated protein 2 messenger RNAs in the nigrostriatal system of normal and Weaver mutant mice and effects of ventral mesencephalic grafts

Using in situ hybridization histochemistry with [32P]oligonucleotide probes, we studied the cellular localization of RNA transcripts for amyloid beta-protein precursor (beta APP), growth-associated phosphoprotein-43 (GAP-43) and microtubule-associated protein 2 (MAP2) in the mesostriatal system of normal (+/+) and weaver (wv/wv) mutant mice, which lose mesencephalic dopamine neurons. In addition, expression of the same messages was studied in ventral mesencephalic cell suspensions transplanted to the weaver striatum. Transcripts encoding GAP-43, MAP2 and isoforms beta APP695, beta APP714 and beta APP751 were present in normal substantia nigra and progressively reduced in weaver substantia nigra; such a reduction was correlated with dopamine neuron loss. The survival of dopamine neurons in unilateral intrastriatal grafts was documented by methamphetamine-induced rotational asymmetry tests and by tyrosine hydroxylase immunocytochemistry. High hybridization signals were obtained for GAP-43, MAP2, beta APP695, beta APP714 and beta APP751 RNA transcripts in the grafted tissue; the beta APP770 species--normally seen in striatum and not substantia nigra--was not expressed in the grafts, but it was present in the recipient striatum. Following immunocytochemical labelling with antibodies, GAP-43 and MAP2 immunoreactivities were seen in cell processes in the grafts and surrounding tissue, whereas beta APP immunoreactivity was mainly found in grafted cell bodies. These results suggest that the transplanted mesencephalic cells mature very similarly to those in the normal substantia nigra, expressing different mRNAs that are normally present in the ventral midbrain and which are reduced in the weaver mutant as a consequence of dopamine neuron loss.

Immunohistochemical distribution of amyloid precursor protein during normal rat development

This study focused on the immunohistochemical identification of the beta/A4 amyloid precursor protein (APP) in various developmental stages of both the rat central nervous system (CNS) and the peripheral nervous system (PNS). A comparative study with myelin basic protein (MBP) and synaptophysin (SYP) facilitated the understanding of neuronal maturation and synaptogenesis on both prenatal and postnatal development. Our immunohistochemical study revealed APP to be widely distributed through the nervous system while existing mainly in the cytoplasm, dendrites and axons of the neurons. However, immunoreactivity was also observed in either the ependymal cells or the choroid plexus epithelial cells. Our immunostaining was carried out by the hydrated autoclaving method and revealed the expression of APP at embryonic day 15 in the neuron of the mesencephalic nucleus of the trigeminal nerve and the anterior horn of the spinal cord, trigeminal and spinal ganglion, ependymal cells and the choroid plexus. We thus observed dramatic changes of APP expression in the cerebellum from the embryonic stage. The maturation of synaptogenesis in the cerebellar molecular layer was parallel to the extension of the dendrites of Purkinje cells, which revealed immunoreactivity for APP. These findings suggested that APP played an important role in neuronal maturation and synaptogenesis. Thus, APP is considered to be a useful marker for neuronal development.