Amyloid and intracellular accumulation of BRI2

Familial British dementia (FBD) and familial Danish dementia (FDD) are caused by mutations in the BRI₂ gene. These diseases are characterized clinically by progressive dementia and ataxia and neuropathologically by amyloid deposits and neurofibrillary tangles. Herein, we investigate BRI₂ protein accumulation in FBD, FDD, Alzheimer disease and Gerstmann-Sträussler-Scheinker disease. In FBD and FDD, we observed reduced processing of the mutant BRI₂ pro-protein, which was found accumulating intracellularly in the Golgi of neurons and glial cells. In addition, we observed an accumulation of a mature form of BRI₂ protein in dystrophic neurites, surrounding amyloid cores. Accumulation of BRI₂ was also observed in dystrophic neurites of Alzheimer disease and Gerstmann-Sträussler-Scheinker disease cases. Although it remains to be determined whether intracellular accumulation of BRI₂ may lead to cell damage in these degenerative diseases, our study provides new insights into the role of mutant BRI₂ in the pathogenesis of FBD and FDD and implicates BRI₂ as a potential indicator of neuritic damage in diseases characterized by cerebral amyloid deposition.

Increased tau phosphorylation and tau truncation, and decreased synaptophysin levels in mutant BRI2/tau transgenic mice

Familial Danish dementia (FDD) is an autosomal dominant neurodegenerative disease caused by a 10-nucleotide duplication-insertion in the BRI₂ gene. FDD is clinically characterized by loss of vision, hearing impairment, cerebellar ataxia and dementia. The main neuropathologic findings in FDD are the deposition of Danish amyloid (ADan) and the presence of neurofibrillary tangles (NFTs). Here we investigated tau accumulation and truncation in double transgenic (Tg-FDD-Tau) mice generated by crossing transgenic mice expressing human Danish mutant BRI₂ (Tg-FDD) with mice expressing human 4-repeat mutant Tau-P301S (Tg-Tau). Compared to Tg-Tau mice, we observed a significant enhancement of tau deposition in Tg-FDD-Tau mice. In addition, a significant increase in tau cleaved at aspartic acid (Asp) 421 was observed in Tg-FDD-Tau mice. Tg-FDD-Tau mice also showed a significant decrease in synaptophysin levels, occurring before widespread deposition of fibrillar ADan and tau can be observed. Thus, the presence of soluble ADan/mutant BRI₂ can lead to significant changes in tau metabolism and synaptic dysfunction. Our data provide new in vivo insights into the pathogenesis of FDD and the pathogenic pathway(s) by which amyloidogenic peptides, regardless of their primary amino acid sequence, can cause neurodegeneration.

The Psen1-L166P-knock-in mutation leads to amyloid deposition in human wild-type amyloid precursor protein YAC transgenic mice

Genetically engineered mice have been generated to model cerebral β-amyloidosis, one of the hallmarks of Alzheimer disease (AD) pathology, based on the overexpression of a mutated cDNA of the amyloid-β precursor protein (AβPP) or by knock-in of the murine Aβpp gene alone or with presenilin1 mutations. Here we describe the generation and initial characterization of a new mouse line based on the presence of 2 copies of the human genomic region encoding the wild-type AβPP and the L166P presenilin 1 mutation. At ∼6 mo of age, double-mutant mice develop amyloid pathology, with signs of neuritic dystrophy, intracellular Aβ accumulation, and glial inflammation, an increase in AβPP C-terminal fragments, and an 8 times increase in Aβ42 levels with a 40% decrease in Aβ40 levels, leading to a significant increase (14 times) of Aβ42/Aβ40 ratios, with minimal effects on presenilin or the Notch1 pathway in the brain. We conclude that in mice, neither mutations in AβPP nor overexpression of an AβPP isoform are a prerequisite for Aβ pathology. This model will allow the study of AD pathogenesis and testing of therapeutic strategies in a more relevant environment without experimental artifacts due to the overexpression of a single-mutant AβPP isoform using exogenous promoters.

Chemical stress induces the unfolded protein response in olfactory sensory neurons

More than any other neuron, olfactory sensory neurons are exposed to environmental insults. Surprisingly, their only documented response to damaging stress is apoptosis and subsequent replacement by new neurons. However, they expressed unfolded protein response genes, a transcriptionally regulated defense mechanism activated by many types of insults. The unfolded protein response transcripts Xbp1, spliced Xbp1, Chop (Ddit3), and BiP (Hspa5) were decreased when external access of stressors was reduced by blocking a nostril (naris occlusion). These transcripts and Nrf2 (Nfe2l2) were increased by systemic application of tunicamycin or the selective olfactotoxic chemical methimazole. Methimazole's effects overcame naris occlusion, and the unfolded protein response was independent of odor-evoked neuronal activity. Chemical stress is therefore a major and chronic activator of the unfolded protein response in olfactory sensory neurons. Stress-dependent repression of the antiapoptotic gene Bcl2 was absent, however, suggesting a mechanism for disconnecting the UPR from apoptosis and tolerating a chronic unfolded protein response. Environmental stressors also affect both the sustentacular cells that support the neurons and the respiratory epithelia, because naris occlusion decreased expression of the xenobiotic chemical transformation enzyme Cyp2a5 in sustentacular cells, and both naris occlusion and methimazole altered the abundance of the antibacterial lectin Reg3g in respiratory epithelia.

Mouse olfactory sensory neurons express 10,000 genes

Olfactory epithelial cells from olfactory marker protein-green fluorescent protein (OMP-GFP) mice were separated by fluorescence-activated cell sorting into a GFP+ sample enriched in mature olfactory sensory neurons (OSNs) and a GFP- sample enriched in all other cells. GeneChip expression profiling of these samples provided a predictive measure of expression in OSNs. Validation tests comparing the ratio of GFP+/GFP- signal intensity against expression patterns from in situ hybridization for 189 mRNAs proved statistically significant and provided probabilities of expression in OSNs scaled according to the signal intensity ratios. These probabilities predict that, among 11,596 mRNAs detected in the GFP+ sample, more than 10,000 are expressed in OSNs. Transcripts and overrepresented categories of mRNAs detected in the GFP+ sample agreed with known properties of OSNs and predict additional properties. For example, ciliogenesis and spermatogenesis were overrepresented, consistent with similarities between OSN cilia and sperm flagella. Chromatin assembly mRNAs were expressed throughout the OSN cell lineage, consistent with the hypothesis that chromatin remodeling plays a role in OSN differentiation. We detected numerous signaling proteins and receptors, such as 30 nonchemosensory G-protein-coupled receptors, including the presynaptic glutamate receptor mGlur4 and the Wnt receptor Fzd3. The largest group of mRNAs, however, was the hundreds of transcriptional regulators that presumably determine the OSN phenotype. The absence of OMP protein in OMP-GFP mice had no detectable effect on mRNA abundance. Within limits prescribed by the nature of microarray data and the in situ hybridization validation, these data should be useful in directing further experiments on OSN function.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.