Preparation of a recombinant cDNA library from poly(A+) RNA of the Alzheimer brain. Identification and characterization of a cDNA copy encoding a glial-specific protein

Structure and transcriptional regulation of the GFAP gene

Transcriptional regulation of the GFAP gene is intimately connected with astrocyte function: its initial activation marks the differentiation of astrocytes, and its up-regulation accompanies the reactive response to CNS injury. Studies of GFAP transcription should thus provide insights into multiple regulatory pathways operating in these cells. In addition, they should identify DNA elements that could be used to direct synthesis of other proteins to astrocytes in transgenic animals, permitting creation of disease models, and the testing of cause and effect relationships. This review describes several GFAP cDNA and genomic clones that have been isolated, including homology comparisons of the encoded RNAs and proteins. Cell transfection studies by several laboratories are summarized that have identified a DNA segment immediately upstream of the RNA start site that is essential for transcriptional activity, but which have yielded conflicting results concerning the importance of other segments located both further upstream and downstream of the RNA start site. Two procedures are recounted that have led to the successful expression of GFAP-transgenes in astrocytes in mice. One of these incorporates the transgene into the first exon of a fragment spanning the entire GFAP gene, while the other links it to a 2 kb 5'-flanking segment. Results already produced by GFAP-transgenic studies include demonstration of a neurotoxic effect of the HIV-1 gp120 coat protein, and creation of a hydrocephalic mouse model.

Profiling of endogenous brain peptides and small proteins: methodology, computer-assisted analysis, and application to aging and lesion models

Significant advances in the technology for the isolation of peptides and small proteins have permitted their identification as biologic markers and enhanced the study of the posttranslational life of proteins. The protocol described here examined large numbers of tissue-derived peptides and small proteins, extracted in low pH and boiled so that proteolysis was interrupted. These were then fractionated batchwise using size exclusion and ion-exchange chromatography. Profiles of species in the peptide pools were then generated on reverse-phase high-performance liquid chromatography (HPLC). The HPLC profiles were evaluated with chromatographic analysis software to identify and quantify peptide peaks and with data compilation programs to sort this information into spreadsheets for comparison of profiles among groups. Using rodent brain, the effects of postmortem delay or age were examined. Postmortem delay produced limited alterations to the profiles, but the effect of age was more pronounced. Many changes were apparent until 12 months, after which the profiles became more constant. Additional peptide profiling of the hippocampus demonstrated changes in peptide content as a function of perforant pathway ablation. The major strengths of HPLC-mediated peptide profiling are that it lends itself to automation and can be used to detect changes in peptides and small proteins among experimental groups or subjects without any prior assumptions concerning which ones might be altered.

On the stability of messenger RNA and ribosomal RNA in the brains of control human subjects and patients with Alzheimer's disease

The levels of the mRNAs encoding the G protein subunits GS alpha, G beta 1, and G beta 2 were measured by northern blotting in the frontal cortex and hippocampus of control subjects and of patients with a clinical and histopathological diagnosis of dementia of the Alzheimer type (DAT). There was no significant difference, in either brain region, between the control and DAT groups for any of the G protein mRNAs measured. The degree of intersubject variability was very high, e.g., GS alpha mRNA in the frontal cortex (mean optical density +/- SD) was 405 +/- 342 in the control group versus 305 +/- 207 in the DAT group. The extent of generalised RNA degradation was assessed by detecting the breakdown products of 28S rRNA. RNA degradation was present in tissue samples from every human subject studied. The extent of 28S rRNA degradation in each subject was found to be related to the levels of G protein mRNA detected. The degree of RNA degradation in human subjects was found to be very variable and unaffected by the presence of DAT. RNA degradation correlated poorly with postmortem interval and this was confirmed by a controlled study of postmortem degradation in rat tissue. The possibility that the relative hypoxia and ischaemia in patients immediately before death could influence RNA degradation is discussed. The variable extent of RNA degradation means that great care must be taken to ensure the validity of RNA analyses undertaken in human postmortem brain, particularly when techniques are employed (such as in situ hybridisation) that themselves give no indication of RNA integrity.

Characterization of human cDNA and genomic clones for glial fibrillary acidic protein

Both a partial cDNA clone and a complete genomic clone have been isolated for human gfa, the gene encoding the major component of astrocyte intermediate filaments, glial fibrillary acidic protein (GFAP). The nucleotide sequence of the entire coding region and 102 bp of the 5' flanking DNA was determined. The mRNA start site was identified by primer extension and probe protection experiments, and a novel in vitro transcription and translation procedure was then used to establish that the first ATG in the mRNA initiates GFAP synthesis. The predicted amino-terminal sequence for human GFAP differs greatly from that previously deduced for mouse GFAP from its gene sequence, despite otherwise high homology. This discrepancy was resolved by determining that the published mouse genomic sequence has an incorrect additional base. The corrected sequence produces strong homology between human and mouse GFAP in their predicted amino acid sequences, and suggests that human and mouse GFAP initiate at homologous positions. The beginning sequence deduced here for both proteins is matched closely by that previously obtained for porcine GFAP by direct sequencing of its amino-terminal end. This supports the protein initiation sites proposed, and also indicates that GFAP is not processed at its amino-terminal end.

Alzheimer cortical neurons containing abundant amyloid mRNA. Relationship to amyloid deposition and senile plaques

Since the detailed molecular events leading to the formation of amyloid-containing senile plaques of the Alzheimer's disease (AD) brain are incompletely understood, the present studies were undertaken to address this issue using a combination of molecular and cytochemical approaches. Amyloid precursor protein riboprobes containing the A4 (beta-amyloid) domain were applied to cortex using the in situ hybridization method to examine the distribution of neuronal amyloid mRNA in relation to the laminar pattern of amyloid deposition and the localization of plaques. The derived data indicated that high levels of amyloid mRNA can be synthesized by AD cortical neurons that appeared to be morphologically intact. The distribution of these cells was not coincident with the cortical laminar pattern that is typical of amyloid deposits observed after immunostaining with anti-A4 monoclonal antibodies. Further, there was no obvious relationship between neurons containing abundant amyloid mRNA and the distribution of plaques identified by thioflavin S staining. While the neuronal synthesis of amyloid may be a significant factor at some point during plaque formation, it may not be the exclusive determinant. The possibility is raised that processes affecting secretion, diffusion, and/or transport of amyloid away from neuronal or non-neuronal cells of origin to sites of deposition may be meaningful aspects of the molecular pathology of Alzheimer's disease.

Altered gene expression in Alzheimer's disease brain tissue

We review the evidence for altered gene expression in Alzheimer's disease brain and identify alternative molecular approaches for isolating additional novel markers. One marker, pADHC-9, was isolated from a human hippocampal cDNA library by differential screening with AD and control cDNA probes. This clone hybridizes to a 2 Kb RNA which is increased 2 fold in AD hippocampus. The deduced amino acid sequence of pADHC-9 codes for a 52 kDAL protein similar to a testicular sulfated glycoprotein secreted by rat Sertoli cells. The normal function of this protein in brain and whether that function is altered in Alzheimer's disease is unknown.

Molecular cloning and primary structure of human glial fibrillary acidic protein

Glial fibrillary acidic protein (GFAP) is an intermediate-filament (IF) protein that is highly specific for cells of astroglial lineage, although its tissue-specific role is speculative. Determination of the primary structure of this protein should be of importance for understanding the functional role it plays in astroglia. Therefore, we isolated a cDNA clone encoding this protein and determined its nucleotide sequence. The predicted amino acid sequence indicates that GFAP shares structural similarities--particularly in the central rod domain and to a lesser degree in the carboxyl-terminal domain--with other IF proteins found in nonepithelial cell types. Considerable sequence divergence in the amino-terminal region of GFAP suggests that the tissue-specific functions of this IF protein might be mediated through this region of the molecule. In contrast, conservation of structural characteristics and a moderate degree of sequence conservation in the carboxyl-terminal region suggest functional similarities. Blot hybridization analysis using the GFAP cDNA as a probe failed to detect GFAP mRNA in both normal and neoplastic human tissues in which IF proteins other than GFAP are known to be expressed.

Molecular biology in psychiatric research: Alzheimer's disease as a paradigm

As illustration of the integration of molecular biological studies into psychiatric research, use of molecular techniques in the study of Alzheimer's disease is described, and the resultant observation of otherwise undetectable pathological changes is noted. Using postmortem Alzheimer brain messenger RNA, recombinant DNA studies led to the ability to clone and characterize the expressed genetic transcript for amyloid. Application of this methodology is discussed in terms of establishing cellular and animal models for a neuropsychiatric disease.

Overexpression of amyloid precursor protein A4 (beta-amyloid) immunoreactivity in genetically transformed cells: implications for a cellular model of Alzheimer amyloidosis

Among the major obstacles to clarifying molecular mechanisms involved in amyloid metabolism in Alzheimer disease has been the unavailability of laboratory models for this uniquely human disorder. The present studies were aimed at establishing genetically engineered cell lines that overexpress amyloid immunoreactivity and that may be relevant to amyloid accumulation in the Alzheimer disease brain. We used cloned amyloid cDNA that contains a region encoding A4 (beta-polypeptide) amino acids along with recently developed tumor virus vectors derived from simian virus 40 to prepare transformed cells. After transient and permanent transfection, a variety of cell types overexpressed A4 immunoreactivity that was detected by highly specific monoclonal antibodies. We observed that the use of an amyloid subdomain containing the A4 region, but lacking the sequence of a Kunitz-type protease inhibitor found in amyloid precursor protein variants, was sufficient to obtain cells that overproduced an A4 epitope. The transformed cells were readily propagated in culture and may provide an experimental medium to elucidate aspects of the molecular pathogenesis of Alzheimer disease. The cellular models may also serve as tools for deriving potentially useful therapeutic agents.