Bruce T. Lamb

Increased App Expression in a Mouse Model of Down's Syndrome Disrupts NGF Transport and Causes Cholinergic Neuron Degeneration

Degeneration of basal forebrain cholinergic neurons (BFCNs) contributes to cognitive dysfunction in Alzheimers disease (AD) and Downs syndrome (DS). We used Ts65Dn and Ts1Cje mouse models of DS to show that the increased dose of the amyloid precursor protein gene, App, acts to markedly decrease NGF retrograde transport and cause degeneration of BFCNs. NGF transport was also decreased in mice expressing wild-type human APP or a familial AD-linked mutant APP; while significant, the decreases were less marked and there was no evident degeneration of BFCNs. Because of evidence suggesting that the NGF transport defect was intra-axonal, we explored within cholinergic axons the status of early endosomes (EEs). NGF-containing EEs were enlarged in Ts65Dn mice and their App content was increased. Our study thus provides evidence for a pathogenic mechanism for DS in which increased expression of App, in the context of trisomy, causes abnormal transport of NGF and cholinergic neurodegeneration.

The Ter mutation in the Dead-end gene causes germ cell loss and testicular germ cell tumours

In mice, the Ter mutation causes primordial germ cell (PGC) loss in all genetic backgrounds. Ter is also a potent modifier of spontaneous testicular germ cell tumour (TGCT) susceptibility in the 129 family of inbred strains, and markedly increases TGCT incidence in 129-Ter/Ter males. In 129-Ter/Ter mice, some of the remaining PGCs transform into undifferentiated pluripotent embryonal carcinoma cells, and after birth differentiate into various cells and tissues that compose TGCTs. Here, we report the positional cloning of Ter, revealing a point mutation that introduces a termination codon in the mouse orthologue (Dnd1) of the zebrafish dead end (dnd) gene. PGC deficiency is corrected both with bacterial artificial chromosomes that contain Dnd1 and with a Dnd1-encoding transgene. Dnd1 is expressed in fetal gonads during the critical period when TGCTs originate. DND1 has an RNA recognition motif and is most similar to the apobec complementation factor, a component of the cytidine to uridine RNA-editing complex. These results suggest that Ter may adversely affect essential aspects of RNA biology during PGC development. DND1 is the first protein known to have an RNA recognition motif directly implicated as a heritable cause of spontaneous tumorigenesis. TGCT development in the 129-Ter mouse strain models paediatric TGCT in humans. This work will have important implications for our understanding of the genetic control of TGCT pathogenesis and PGC biology.

TREM2 deficiency eliminates TREM2+ inflammatory macrophages and ameliorates pathology in Alzheimer’s disease mouse models

Jay and colleagues show that TREM2 deficiency reduces the number of macrophages infiltrating the brain and is protective against disease pathogenesis in mouse models of Alzheimer’s disease.

The role of oxidative stress in the pathophysiology of cerebrovascular lesions in Alzheimer's disease.

Alzheimers disease (AD) and stroke are two leading causes of age‐associated dementia. A rapidly growing body of evidence indicates that increased oxidative stress from reactive oxygen radicals is associated with the aging process and age‐related degenerative disorders such as atherosclerosis, ischemia/reperfusion, arthritis, stroke, and neurodegenerative diseases. New evidence has also indicated that vascular lesions are a key factor in the development of AD. This idea is based on a positive correlation between AD and cardiovascular and cerebrovascular diseases such as arterio‐ and atherosclerosis and ischemia/reperfusion injury. In this review we consider recent evidence supporting the existence of an intimate relationship between oxidative stress and vascular lesions in the pathobiology of AD. We also consider the opportunities for therapeutic interventions based on the molecular pathways involved with these causal relationships.

NSAIDs prevent, but do not reverse, neuronal cell cycle reentry in a mouse model of Alzheimer disease

Ectopic cell cycle events (CCEs) mark vulnerable neuronal populations in human Alzheimer disease (AD) and are observed early in disease progression. In transgenic mouse models of AD, CCEs are found before the onset of beta-amyloid peptide (Abeta) deposition to form senile plaques, a hallmark of AD. Here, we have demonstrated that alterations in brain microglia occur coincidently with the appearance of CCEs in the R1.40 transgenic mouse model of AD. Furthermore, promotion of inflammation with LPS at young ages in R1.40 mice induced the early appearance of neuronal CCEs, whereas treatment with 2 different nonsteroidal antiinflammatory drugs (NSAIDs) blocked neuronal CCEs and alterations in brain microglia without altering amyloid precursor protein (APP) processing and steady-state Abeta levels. In addition, NSAID treatment of older R1.40 animals prevented new neuronal CCEs, although it failed to reverse existing ones. Retrospective human epidemiological studies have identified long-term use of NSAIDs as protective against AD. Prospective clinical trials, however, have failed to demonstrate a similar benefit. Our use of CCEs as an outcome measure offers fresh insight into this discrepancy and provides important information for future clinical trials, as it suggests that NSAID use in human AD may need to be initiated as early as possible to prevent disease progression.

Atherosclerotic lesions and mitochondria DNA deletions in brain microvessels as a central target for the development of human AD and AD-like pathology in aged transgenic mice

Abstract: We have studied the ultrastructural features of vascular lesions and mitochondria in brain vascular wall cells from human AD brain biopsy, human short postmortem brain tissues, and yeast artificial chromosome (YAC) and C57B6/SJL transgenic positive (Tg+) mice overexpressing amyloid beta precursor protein (AβPP). In situ hybridization using mitochondrial DNA (mtDNA) probes for human wild type, 5 kb deleted, and mouse mtDNA was performed, along with immunocytochemistry using antibodies against amyloid precursor protein (APP), 8‐hydroxy‐2′‐guanosine (8‐OHG), and cytochrome c oxidase (COX). There was a higher degree of amyloid deposition in the vascular walls of the human AD, YAC, and C57B6/SJL Tg (+) mice compared to age‐matched controls. In addition, vessels with more severe lesions showed immunopositive staining for APP and possessed large, lipid‐laden vacuoles in the cytoplasm of endothelial cells (EC). Significantly more mitochondrial abnormalities were seen in human AD, YAC, and C57B6/SJL Tg (+) mouse microvessels where lesions occurred. In situ hybridization using wild and chimera (5 kb) mtDNA probes revealed positive signals in damaged mitochondria from the vascular endothelium and in perivascular cells of lesioned microvessels close to regions of large amyloid deposition. These features were absent in undamaged regions of human AD tissues, YAC and C57B6/SJL Tg (+) mouse tissues, and in age‐matched control subjects. In addition, vessels with atherosclerotic lesions revealed endothelium and perivascular cells possessing clusters of wild and deleted mtDNA positive probes. These mtDNA deletions were accompanied by increased amounts of immunoreactive APP, 8‐OHG, and COX in the same cellular compartment. Our observations demonstrate that vascular wall cells, especially their mitochondria, appear to be a central target for oxidative stress‐induced damage.

Transgenic mouse models of Alzheimer's disease.

Recent advances in the understanding of the genetic basis of Alzheimers disease have enabled the production of transgenic mouse models of the disease. Utilizing both cDNA- and genomic-based approaches, these mouse models for Alzheimers disease have already provided valuable insights into the pathogenesis of the disease and potential therapeutic interventions.

Nitric Oxide as an Initiator of Brain Lesions During the Development of Alzheimer Disease

Nitric oxide (NO) is an important regulatory molecule for the host defense that plays a fundamental role in the cardiovascular, immune, and nervous systems. NO is synthesized through the conversion of l-arginine to l-citrulline by the enzyme NO synthase (NOS), which is found in three isoforms classified as neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). Recent evidence supports the theory that this bioactive molecule has an influential role in the disruption of normal brain and vascular homeostasis, a condition known to elucidate chronic hypoperfusion which ultimately causes the development of brain lesions and the pathology that typify Alzheimer disease (AD). In addition, vascular NO activity appears to be a major contributor to this pathology before any overexpression of NOS isoforms is observed in the neuron, glia, and microglia of the brain tree, where the overexpression the NOS isoforms causes the formation of a large amount of NO. We hypothesize that since an imbalance between the NOS isoforms and endothelin-1 (ET-1), a human gene that encodes for blood vessel constriction, can cause antioxidant system insufficiency; by using pharmacological intervention with NO donors and/or NO suppressors, the brain lesions and the downstream progression of brain pathology and dementia in AD should be delayed or minimized.

The Cleavage Products of Amyloid-β Precursor Protein Are Sorted to Distinct Carrier Vesicles That Are Independently Transported within Neurites

The amyloid-β (Aβ) precursor protein (APP), a transmembrane protein that undergoes proteolytic cleavage into defined fragments, has been implicated in axonal transport. The proposed role of APP as a vesicle receptor for the microtubule motor kinesin-1 has relevance for the pathogenesis of Alzheimers disease. Nevertheless, this function, which relies on the transport to the cell periphery of full-length APP rather than its cleavage fragments, remains controversial. Other proposed functions of APP, such as regulating transcription, neurogenesis, cell movement, or neurite growth also rely on APPs presence as a full-length protein at the cell surface, implying that APP cleavage occurs after its transport to the cell periphery. To test this hypothesis, we mapped the localization of various APP epitopes in neurons in culture and in the mouse brain. Surprisingly, epitopes from the N-terminal, C-terminal, and central (Aβ) domains of APP each showed a distinct distribution throughout the cell and rarely colocalized. Within neurites, these epitopes were localized to distinct transport vesicles that associated with different sets of microtubules and, occasionally, actin filaments. C-terminal APP fragments were preferentially transported into neurites as phosphorylated forms, entered the lamellipodium and filopodia of growth cones, and concentrated in regions of growth cone turning and advancement (unlike the N-terminal and Aβ fragments). We conclude that, under normal conditions, the proteolytic cleavage of APP primarily occurs before its sorting into axonal transport vesicles and the cleaved fragments segregate into separate vesicle populations that reach different destinations, and thus have different functions.

YAC transgenics and the study of genetics and human disease

Advances in yeast artificial chromosome (YAC) technologies over the past decade have enabled the precise identification and manipulation of large genomic regions (>100 kb) of DNA. Introduction of YACs into the mouse germline has now been accomplished through transfection of mouse embryonic stem cells as well as through pronuclear microinjection, allowing the efficient transfer defined genomic loci into mice. YAC transgenics will have a profound impact on the development of transgenic mice as bioreactors and as models of human disease, and on the functional analysis of higher order genomic structure.