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

The 2022 Lady Estelle Wolfson lectureship on neurofilaments

Neurofilament proteins (Nf) have been validated and established as a reliable body fluid biomarker for neurodegenerative pathology. This review covers seven Nf isoforms, Nf light (NfL), two splicing variants of Nf medium (NfM), two splicing variants of Nf heavy (NfH), α -internexin (INA) and peripherin (PRPH). The genetic and epigenetic aspects of Nf are discussed as relevant for neurodegenerative diseases and oncology. The comprehensive list of mutations for all Nf isoforms covers Amyotrophic Lateral Sclerosis, Charcot-Marie Tooth disease, Spinal muscular atrophy, Parkinson Disease and Lewy Body Dementia. Next, emphasis is given to the expanding field of post-translational modifications (PTM) of the Nf amino acid residues. Protein structural aspects are reviewed alongside PTMs causing neurodegenerative pathology and human autoimmunity. Molecular visualisations of NF PTMs, assembly and stoichiometry make use of Alphafold2 modelling. The implications for Nf function on the cellular level and axonal transport are discussed. Neurofilament aggregate formation and proteolytic breakdown are reviewed as relevant for biomarker tests and disease. Likewise, Nf stoichiometry is reviewed with regard to in vitro experiments and as a compensatory mechanism in neurodegeneration. The review of Nf across a spectrum of 87 diseases from all parts of medicine is followed by a critical appraisal of 33 meta-analyses on Nf body fluid levels. The review concludes with considerations for clinical trial design and an outlook for future research.

A Novel Based-Network Strategy to Identify Phytochemicals from Radix Salviae Miltiorrhizae (Danshen) for Treating Alzheimer's Disease

Alzheimer’s disease (AD) is a common age-related neurodegenerative disease that strikes millions worldwide. Herein, we demonstrate a new approach based on network target to identify anti-AD compounds from Danshen. Network pharmacology and molecular docking were employed to establish the DS-AD network, which mainly involved apoptosis of neuron cells. Then network scoring was confirmed via Connectivity Map analysis. M308 (Danshenxinkun D) was an anti-AD candidate with a high score (p < 0.01). Furthermore, we conducted ex vivo experiments with H2O2-treated PC12 cells to verify the neuroprotective effect of Salvia miltiorrhiza-containing plasma (SMP), and UPLC-Q-TOF/MS and RT-qPCR were performed to demonstrate the anti-AD activity of M308 from SMP. Results revealed that SMP could enhance cell viability and level of acetylcholine. AO/EB staining and Mitochondrial membrane potential (MMP) analysis showed that SMP significantly suppressed apoptosis, which may be due to anti-oxidative stress activity. Moreover, the effects of M308 and SMP on expressions of PSEN1, DRD2, and APP mRNA were consistent, and M308 can significantly reverse the expression of PSEN1 and DRD2 mRNA in H2O2-treated PC12 cells. The strategy based on the network could be employed to identify anti-AD compounds from Chinese herbs. Notably, M308 stands out as a promising anti-AD candidate for development.

Neurofilament Proteins as Biomarkers to Monitor Neurological Diseases and the Efficacy of Therapies

Biomarkers of neurodegeneration and neuronal injury have the potential to improve diagnostic accuracy, disease monitoring, prognosis, and measure treatment efficacy. Neurofilament proteins (NfPs) are well suited as biomarkers in these contexts because they are major neuron-specific components that maintain structural integrity and are sensitive to neurodegeneration and neuronal injury across a wide range of neurologic diseases. Low levels of NfPs are constantly released from neurons into the extracellular space and ultimately reach the cerebrospinal fluid (CSF) and blood under physiological conditions throughout normal brain development, maturation, and aging. NfP levels in CSF and blood rise above normal in response to neuronal injury and neurodegeneration independently of cause. NfPs in CSF measured by lumbar puncture are about 40-fold more concentrated than in blood in healthy individuals. New ultra-sensitive methods now allow minimally invasive measurement of these low levels of NfPs in serum or plasma to track disease onset and progression in neurological disorders or nervous system injury and assess responses to therapeutic interventions. Any of the five Nf subunits - neurofilament light chain (NfL), neurofilament medium chain (NfM), neurofilament heavy chain (NfH), alpha-internexin (INA) and peripherin (PRPH) may be altered in a given neuropathological condition. In familial and sporadic Alzheimer's disease (AD), plasma NfL levels may rise as early as 22 years before clinical onset in familial AD and 10 years before sporadic AD. The major determinants of elevated levels of NfPs and degradation fragments in CSF and blood are the magnitude of damaged or degenerating axons of fiber tracks, the affected axon caliber sizes and the rate of release of NfP and fragments at different stages of a given neurological disease or condition directly or indirectly affecting central nervous system (CNS) and/or peripheral nervous system (PNS). NfPs are rapidly emerging as transformative blood biomarkers in neurology providing novel insights into a wide range of neurological diseases and advancing clinical trials. Here we summarize the current understanding of intracellular NfP physiology, pathophysiology and extracellular kinetics of NfPs in biofluids and review the value and limitations of NfPs and degradation fragments as biomarkers of neurodegeneration and neuronal injury.

Blood neurofilament light: a critical review of its application to neurologic disease

Neuronal injury is a universal event that occurs in disease processes that affect both the central and peripheral nervous systems. A blood biomarker linked to neuronal injury would provide a critical measure to understand and treat neurologic diseases. Neurofilament light chain (NfL), a cytoskeletal protein expressed only in neurons, has emerged as such a biomarker. With the ability to quantify neuronal damage in blood, NfL is being applied to a wide range of neurologic conditions to investigate and monitor disease including assessment of treatment efficacy. Blood NfL is not specific for one disease and its release can also be induced by physiological processes. Longitudinal studies in multiple sclerosis, traumatic brain injury, and stroke show accumulation of NfL over days followed by elevated levels over months. Therefore, it may be hard to determine with a single measurement when the peak of NfL is reached and when the levels are normalized. Nonetheless, measurement of blood NfL provides a new blood biomarker for neurologic diseases overcoming the invasiveness of CSF sampling that restricted NfL clinical application. In this review, we examine the use of blood NfL as a biologic test for neurologic disease.

Gene Expression of Quaking in Sporadic Alzheimer's Disease Patients is Both Upregulated and Related to Expression Levels of Genes Involved in Amyloid Plaque and Neurofibrillary Tangle Formation

Quaking (QKI) is a gene exclusively expressed within glial cells. QKI has previously been implicated in various neurological disorders and diseases, including Alzheimer’s disease (AD), a condition for which increasing evidence suggests a central role of glia cells. The objective of the present study was to investigate the expression levels of QKI and three QKI isoforms (QKI5, QKI6, and QKI7) in AD. Genes that have previously been related to the ontogeny and progression of AD, specifically APP, PSEN1, PSEN2, and MAPT, were also investigated. A real-time PCR assay of 123 samples from human postmortem sporadic AD patients and control brains was performed. The expression values were analyzed with an analysis of covariance model and subsequent multiple regressions to explore the possibility of related expression values between QKI, QKI isoforms, and AD-related genes. Further, the sequences of AD-related genes were analyzed for the presence of QKI binding domains. QKI and all measured QKI isoforms were found to be significantly upregulated in AD samples, relative to control samples. However, APP, PSEN1, PSEN2, and MAPT were not found to be significantly different. QKI and QKI isoforms were found to be predictive for the variance of APP, PSEN1, PSEN2, and MAPT, and putative QKI binding sites suggests an interaction with QKI. Overall, these results implicate a possible role of QKI in AD, although the exact mechanism by which this occurs remains to be uncovered.

Novel antioxidants protect mitochondria from the effects of oligomeric amyloid beta and contribute to the maintenance of epigenome function

Alzheimer's disease is associated with metabolic deficits and reduced mitochondrial function, with the latter due to the effects of oligomeric amyloid beta peptide (AβO) on the respiratory chain. Recent evidence has demonstrated reduction of epigenetic markers, such as DNA methylation, in Alzheimer's disease. Here we demonstrate a link between metabolic and epigenetic deficits via reduction of mitochondrial function which alters the expression of mediators of epigenetic modifications. AβO-induced loss of mitochondrial function in differentiated neuronal cells was reversed using two novel antioxidants (1 and 2); both have been shown to mitigate the effects of reactive oxygen species (ROS), and compound 1 also restores adenosine triphosphate (ATP) levels. While both compounds were effective in reducing ROS, restoration of ATP levels was associated with a more robust response to AβO treatment. Our in vitro system recapitulates key aspects of data from Alzheimer's brain samples, the expression of epigenetic genes in which are also shown to be normalized by the novel analogues.

The effects of dietary restriction and aging on amyloid precursor protein and presenilin-1 mRNA and protein expression in rat brain

The objective of this study was to examine the effects of aging and long-term dietary restriction (DR) on the level of amyloid precursor protein (APP) and presenilin-1 (PS-1), proteins that are critically involved in Alzheimer's disease. Changes in mRNA and protein expression were assessed by real-time PCR and western blot analysis during aging and long-term DR in the cortex and hippocampus of 6-, 12-, 18-, and 24-month-old rats. Prominent regional changes in expression were observed in response to aging and DR. Although the hippocampus displayed significant alterations in APP mRNA and protein expression and no significant changes in PS-1 expression, an opposite pattern was observed in the cortex. DR counteracted the age-related changes in APP mRNA expression in both structures of old animals. The observed DR-induced increase in mRNA levels in the hippocampus was accompanied by an increase in the level of full-length protein APP. These results indicate that although both structures are very sensitive to aging, a specific spatial pattern of changes in APP and PS-1 occurs during aging. Furthermore, these findings provide evidence that DR can affect APP and PS-1 expression in a manner consistent with its proposed 'antiaging' effect.

Housekeepers for accurate transcript expression analysis in Alzheimer's disease autopsy brain tissue

BACKGROUND

Quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) is a popular technique for mRNA expression studies. Normalization to an endogenous reference transcript (housekeeper) is widely used to correct for differences in loading and RNA quality. Alzheimer's disease (AD) alters brain metabolism. The stability of housekeeper transcript expression must be carefully validated.

METHODS

qRT-PCR was used to assess eight putative housekeeper transcripts in four brain regions from 15 control, 12 AD, and 10 AD/Lewy body disease (LBD) cases.

RESULTS

RNA quality is lower in AD and AD/LBD than in controls. Frequently used housekeepers such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and β-actin had lower overall expression in AD and AD/LBD cases than in controls. RPL13 and 18S were the most stably expressed housekeepers tested. Synaptophysin and glial fibrillary acidic protein were used to evaluate normalized quantification. By using different housekeepers we confirmed that synaptophysin expression was down-regulated in AD cases, whereas glial fibrillary acidic protein expression was increased.

CONCLUSIONS

Among all candidates tested, RPL13 was the best housekeeper for qRT-PCR studies in autopsy brain tissue samples from controls and AD cases. RNA quality should be assessed and data normalized on this index as well.

Lifespan profiles of Alzheimer's disease-associated genes and products in monkeys and mice

Alzheimer's disease (AD) is characterized by plaques of amyloid-beta (Abeta) peptide, cleaved from amyloid-beta protein precursor (AbetaPP). Our hypothesis is that lifespan profiles of AD-associated mRNA and protein levels in monkeys would differ from mice and that differential lifespan expression profiles would be useful to understand human AD pathogenesis. We compared profiles of AbetaPP mRNA, AbetaPP protein, and Abeta levels in rodents and primates. We also tracked a transcriptional regulator of the AbetaPP gene, specificity protein 1 (SP1), and the beta amyloid precursor cleaving enzyme (BACE1). In mice, AbetaPP and SP1 mRNA and their protein products were elevated late in life; Abeta levels declined in old age. In monkeys, SP1, AbetaPP, and BACE1 mRNA declined in old age, while protein products and Abeta levels rose. Proteolytic processing in both species did not match production of Abeta. In primates, AbetaPP and SP1 mRNA levels coordinate, but an inverse relationship exists with corresponding protein products as well as Abeta levels. Comparison of human DNA and mRNA sequences to monkey and mouse counterparts revealed structural features that may explain differences in transcriptional and translational processing. These findings are important for selecting appropriate models for AD and other age-related diseases.

Epigenetic changes in Alzheimer's disease: decrements in DNA methylation

DNA methylation is a vital component of the epigenetic machinery that orchestrates changes in multiple genes and helps regulate gene expression in all known vertebrates. We evaluated immunoreactivity for two markers of DNA methylation and eight methylation maintenance factors in entorhinal cortex layer II, a region exhibiting substantial Alzheimer's disease (AD) pathology in which expression changes have been reported for a wide variety of genes. We show, for the first time, neuronal immunoreactivity for all 10 of the epigenetic markers and factors, with highly significant decrements in AD cases. These decrements were particularly marked in PHF1/PS396 immunoreactive, neurofibrillary tangle-bearing neurons. In addition, two of the DNA methylation maintenance factors, DNMT1 and MBD2, have been reported also to interact with ribosomal RNAs and ribosome synthesis. Consistent with these findings, DNMT1 and MBD2, as well as p66α, exhibited punctate cytoplasmic immunoreactivity that co-localized with the ribosome markers RPL26 and 5.8s rRNA in ND neurons. By contrast, AD neurons generally lacked such staining, and there was a qualitative decrease in RPL26 and 5.8s rRNA immunoreactivity. Collectively, these findings suggest epigenetic dysfunction in AD-vulnerable neurons.

Neuronal gene expression in non-demented individuals with intermediate Alzheimer's Disease neuropathology

While the clinical and neuropathological characterization of Alzheimer's Disease (AD) is well defined, our understanding of the progression of pathologic mechanisms in AD remains unclear. Post-mortem brains from individuals who did not fulfill clinical criteria for AD may still demonstrate measurable levels of AD pathologies to suggest that they may have presented with clinical symptoms had they lived longer or are able to stave off disease progression. Comparison between such individuals and those clinically diagnosed and pathologically confirmed to have AD will be key in delineating AD pathogenesis and neuroprotection. In this study, we expression profiled laser capture microdissected non-tangle bearing neurons in 6 post-mortem brain regions that are differentially affected in the AD brain from 10 non-demented individuals demonstrating intermediate AD neuropathologies (NDAD; Braak stage of II through IV and CERAD rating of moderate to frequent) and evaluated this data against that from individuals who have been diagnosed with late onset AD as well as healthy elderly controls. We identified common statistically significant expression changes in both NDAD and AD brains that may establish a degenerative link between the two cohorts, in addition to NDAD specific transcriptomic changes. These findings pinpoint novel targets for developing earlier diagnostics and preventative therapies for AD prior to diagnosis of probable AD. We also provide this high-quality, low post-mortem interval (PMI), cell-specific, and region-specific NDAD/AD reference data set to the community as a public resource.

Cytoskeletal Transport in the Aging Brain: Focus on the Cholinergic System

There is now compelling evidence for the aging-related breakdown of cytoskeletal support in neurons. Similarly affected are the principal components of the intracellular microtubule system, the transport units involved in active shuttle of organelles and molecules in an antero- and retrograde manner, and the proteins stabilizing the cytoskeleton and providing trophic support. Here, we review the basic organization of the cytoskeleton, and describe its elements and their interactions. We then critically assess the role of these cytoskeletal proteins in physiological aging and aging-related malfunction. Our focus is on the microtubule-associated protein tau, for which comprehensive investigations suggest a critical role in neurodegenerative diseases, for instance tauopathies. These diseases frequently lead to cognitive decline and are often paralleled by reductions in cholinergic neurotransmission. We propose this reduction to be due to destabilization of the cytoskeleton and protein transport mechanisms in these neurons. Therefore, maintenance of the neuronal cytoskeleton during aging may prevent or delay neurodegeneration as well as cognitive decline during physiological aging.

Hyperphosphorylation of RNA polymerase II and reduced neuronal RNA levels precede neurofibrillary tangles in Alzheimer disease

Affected neurons of Alzheimer disease (AD) brain are distinguished by the presence of the cell cycle cdc2 kinase and mitotic phosphoepitopes. A significant body of previous data has documented a decrease in neuronal RNA levels and nucleolar volume in AD brain. Here we present evidence that integrates these seemingly distinct findings and offers an explanation for the degenerative outcome of the disease. During mitosis cdc2 phosphorylates and inhibits the major transcriptional regulator RNA polymerase II (RNAP II). We therefore investigated cdc2 phosphorylation of RNAP II in AD brain. Using the H5 and H14 monoclonal antibodies specific for the cdc2-phosphorylated sites in RNAP II, we found that the polymerase is highly phosphorylated in AD. Moreover, RNAP II in AD translocates from its normally nuclear compartment to the cytoplasm of affected neurons, where it colocalizes with cdc2. These M phase-like changes in RNAP II correlate with decreased levels of poly-A RNA in affected neurons. Significantly, they precede tau phosphorylation and neurofibrillary tangle formation. Our data support the hypothesis that inappropriate activation of the cell cycle cdc2 kinase in differentiated neurons contributes to neuronal dysfunction and degeneration in part by inhibiting RNAP II and cellular processes dependent on transcription.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Quantitative analysis of neurofilament proteins in Alzheimer brain by enzyme linked immunosorbent assay system

The abnormality of cytoskeletal proteins is related to Alzheimer's disease. Because neurofilament proteins (NF) are major cytoskeletal components of neurones, abnormality of NF may be involved in the pathology of disease. In this study, insoluble NF in the grey matter of temporal lobes of Alzheimer and control brains were dissolved in a urea buffer and quantitatively measured by an enzyme linked immunosorbent assay system. No apparent quantitative changes of NF-L and NF-H were found between the Alzheimer and control brains, and there were also no significant differences in the mean molar ratio of NF-L to NF-H between them. However, the relative amount of phosphorylated NF-H in Alzheimer brains was increased in comparison with that in control brains. These results suggest that the increase of phosphorylated NF-H might be accompanied with Alzheimer's disease.

Age-related atrophy of motor axons in mice deficient in the mid-sized neurofilament subunit

Neurofilaments are central determinants of the diameter of myelinated axons. It is less clear whether neurofilaments serve other functional roles such as maintaining the structural integrity of axons over time. Here we show that an age-dependent axonal atrophy develops in the lumbar ventral roots of mice with a null mutation in the mid-sized neurofilament subunit (NF-M) but not in animals with a null mutation in the heavy neurofilament subunit (NF-H). Mice with null mutations in both genes develop atrophy in ventral and dorsal roots as well as a hind limb paralysis with aging. The atrophic process is not accompanied by significant axonal loss or anterior horn cell pathology. In the NF-M-null mutant atrophic ventral root, axons show an age-related depletion of neurofilaments and an increased ratio of microtubules/neurofilaments. By contrast, the preserved dorsal root axons of NF-M-null mutant animals do not show a similar depletion of neurofilaments. Thus, the lack of an NF-M subunit renders some axons selectively vulnerable to an age-dependent atrophic process. These studies argue that neurofilaments are necessary for the structural maintenance of some populations of axons during aging and that the NF-M subunit is especially critical.

Expression profiles of multiple genes in single neurons of Alzheimer's disease

Many changes have been described in the brains of Alzheimer's disease (AD) patients, including loss of neurons and formation of senile plaques and neurofibrillary tangles. The molecular mechanisms underlying these pathologies are unclear. Northern blot, dot-blot, and reverse transcription-coupled PCR analyses have demonstrated altered expression levels of multiple messages in AD brain. Because not all cells are equally affected by the disease, these methods obviously cannot study the changes in relation to disease states of individual cells. We address this problem by using antisense RNA profiling of single cells. We present expression profiles of single neurons at early and late stages of AD and describe statistical tools for data analysis. With multivariate canonical analysis, we were able to distinguish the disease state on the basis of altered expression of multiple messages. To validate this approach, we compared results obtained by this approach with results obtained by in situ hybridization analysis. When the neurofilament medium subunit was used as a marker, our results from an antisense RNA profiling revealed no change in neurofilament medium subunit expression between early- and late-stage AD, consistent with findings obtained with in situ hybridization. However, our results obtained by either analysis at the single-cell level differed from the reported decrease in AD neocortex obtained by Northern blot analysis [Kittur, S., Hoh, J., Endo, H., Tourtellotte, W., Weeks, B. S., Markesbery, W. & Adler, W. (1994) J. Geriatr. Psychiatry Neurol. 7, 153-158]. Thus, the strategy of using the single-cell antisense RNA approach to identify altered gene expression in postmortem AD brain, followed by detailed in situ hybridization studies for genes of interest, is valuable in the study of the molecular mechanisms underlying AD neuropathology.

Presenilin-1, amyloid precursor protein and amyloid precursor-like protein 2 mRNA levels in human superior frontal cortex during aging

The presenilin-1 (PS-1) and amyloid precursor protein (APP) genes carry mutations which co-segregate with early-onset familial Alzheimer's disease. The APP and PS-1 gene products may be involved in the aetiology of the more common late onset form of Alzheimer's disease, where increasing age is a major risk factor. To investigate whether age affected mRNA expression of these genes, we quantified PS-1, total APP, APP containing the kunitz-type protease inhibitor (KPI) domain and amyloid precursor-like protein 2 (APLP2) mRNAs in post-mortem superior frontal cortices from 23 control subjects aged 38 to 89 years using solution hybridisation-RNase protection assays. PS-1, total APP, APP KPI and APLP2 mRNA levels were unchanged over this age range. PS-1 was the least abundant mRNA, at approximately 7% of total APP, the most highly expressed mRNA studied (10.8 copies/pg total RNA). The proportion of total APP encoding the KPI domain (approximately 52%) was unaffected by age. APLP2 mRNA was present at approximately 29% of the total APP mRNA level. Significant positive correlations were present between total APP, APP KPI and APLP2 mRNA levels. These results indicate that the increased prevalence of Alzheimer's disease cannot be attributed to alterations in cortical PS-1, APP and APLP2 mRNA levels or APP KPI splicing during aging.

Quantification of APP and APLP2 mRNA in APOE genotyped Alzheimer's disease brains

Amyloid precursor protein (APP) is metabolised to produce A beta, a peptide found aggregated in Alzheimer's disease neuritic plaques. APP is a member of a multigene protein family which includes amyloid precursor-like protein 2 (APLP2). Since A beta accumulation can be triggered by factors acting up- or downstream of APP processing, we investigated whether APP mRNA expression was altered in Alzheimer's disease post-mortem cerebral cortex. In addition, we characterised cortical APLP2 mRNA levels. Quantitative RNA-RNA solution hybridisation-RNase protection was used to assay total APP. APP containing the Kunitz-type protease inhibitor (KPI) insert and APLP2 mRNA in mid-temporal and superior frontal cortices from apolipoprotein E-genotyped subjects with Alzheimer's disease, other neurological diseases and non-demented controls. Approximately 3 times more APP than APLP2 mRNA was detected and about 70% of total APP mRNA contained the KPI insert in the control subjects. Total APP and APLP2 mRNA levels were significantly reduced in Alzheimer's disease mid-temporal, but not superior frontal cortex, suggesting that regional reductions in these mRNA correlate with severity of disease pathology. A small significant increase in the proportion of APP KPI mRNA was seen in both cortical regions in Alzheimer's disease. Apolipoprotein E genotype did not influence cortical levels of total APP, APP KPI or APLP2 mRNA. Alzheimer's disease-related increases in tissue DNA content were seen in both regions studied, while tissue RNA levels were reduced in the positive disease controls. In summary, these results indicate that Alzheimer's disease is not associated with over-expression of either APP or APLP2 mRNA. Our findings reveal a disease-associated increase in the proportion of APP KPI-containing isoforms, and further investigation should clarify whether this predisposes affected individuals to A beta production and aggregation, or reflects later events such as gliosis and neuronal cell death.

Aging is associated with divergent effects on Nf-L and GFAP transcription in rat brain

We studied the effects of advancing age on the expression of several proteins important in the structure and function of the nervous system. Brains of young (3 month), middle-aged (13 month), and old (29 month) male Fischer 344 rats were examined. Run-on transcription and Northern blot hybridizations were used to determine gene-specific transcription rates and mRNA levels, respectively. With advancing age, there was a decrement in the transcription rate and mRNA levels for neurofilament-light subunit (Nf-L), but an increment in the transcription rate and mRNA levels for glial fibrillary acidic protein (GFAP). Proteolipid protein (PLP) mRNA levels were attenuated between 3 and 13 months of age, whereas amyloid precursor protein (APP) mRNA levels were attenuated in the middle-aged but not the old animals. Transcription rates for alpha-actin and fos, and mRNA levels for alpha-actin, were unaffected. These observations indicate divergent transcriptional regulation of several genes, notably Nf-L and GFAP, in the aging mammalian forebrain.

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.

Axonal atrophy in aging is associated with a decline in neurofilament gene expression

Neurofilaments (Nfs) are major determinants of axonal caliber. Nf transcript levels increase during development and maturation, and are associated with an increase in Nf protein, Nf numbers, and caliber of axons. With aging there is axonal atrophy. In this study we asked whether the axonal atrophy of aging was associated with a decline in Nf transcript expression, Nf protein levels, and Nf numbers. Expression of transcripts for the three Nf subunits was evaluated in dorsal root ganglia (DRG) of Fischer-344 rats aged 3-32 months by Northern and in situ hybridization. There was an approximately 50% decrease in Nf subunit mRNA levels in DRG of aged (> 23 months) as compared to young and mature (3 and 12 months) rats, whereas expression of another neuronal mRNA, GAP-43, showed no decline. Western analysis showed a corresponding decrease in Nf subunit proteins and no decline in GAP-43. Morphometric analysis showed a 50% decrease in Nf numbers within axons. The decrease in Nf gene expression and Nf numbers was accompanied by a decrease in cross-sectional area and circularity of all myelinated fibers, with the largest fibers showing the most marked changes, and a shrinkage in the perikaryal area of large neurons. Furthermore, we found a concomitant decrease in the expression of transcripts for the nerve growth factor receptors trkA and p75 with aging. Although the mechanisms leading to the decrease in Nf gene expression with aging are not known, a decrease in the availability of growth factors, or the neuron's ability to respond to them, may play a role in this process.