Differential effects of a Rab6 mutant on secretory versus amyloidogenic processing of Alzheimer's beta-amyloid precursor protein

Advances in the cell biology of the trafficking and processing of amyloid precursor protein: impact of familial Alzheimer's disease mutations

The production of neurotoxic amyloid-β peptides (Aβ) is central to the initiation and progression of Alzheimer's disease (AD) and involves sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. APP and the secretases are transmembrane proteins and their co-localisation in the same membrane-bound sub-compartment is necessary for APP cleavage. The intracellular trafficking of APP and the β-secretase, BACE1, is critical in regulating APP processing and Aβ production and has been studied in several cellular systems. Here, we summarise the intracellular distribution and transport of APP and its secretases, and the intracellular location for APP cleavage in non-polarised cells and neuronal models. In addition, we review recent advances on the potential impact of familial AD mutations on APP trafficking and processing. This is critical information in understanding the molecular mechanisms of AD progression and in supporting the development of novel strategies for clinical treatment.

Emerging Nanotechnology for the Treatment of Alzheimer's Disease

Nanotechnology is a great choice for medical research, and the green synthesis approach is a novel and better way to synthesize nanoparticles. Biological sources are cost-effective, environmentally friendly, and allow large-scale production of nanoparticles. Naturally obtained 3 β-hydroxy-urs- 12-en-28-oic acids reported for neuroprotective and dendritic structure are reported as solubility enhancers. Plants are free from toxic substances and act as natural capping agents. In this review, the pharmacological properties of ursolic acid (UA) and the structural properties of the dendritic structure are discussed. UA acid appears to have negligible toxicity and immunogenicity, as well as favorable biodistribution, according to the current study, and the dendritic structure improves drug solubility, prevents drug degradation, increases circulation time, and potentially targets by using different pathways with different routes of administration. Nanotechnology is a field in which materials are synthesized at the nanoscale. Nanotechnology could be the next frontier of humankind's technological advancement. Richard Feynman first used the term 'Nanotechnology' in his lecture, "There is Plenty of Room at the Bottom", on 29th December, 1959, and since then, interest has increased in the research on nanoparticles. Nanotechnology is capable of helping humanity by solving major challenges, particularly in neurological disorders like Alzheimer's disease (AD), the most prevalent type, which may account for 60-70% of cases. Other significant forms of dementia include vascular dementia, dementia with Lewy bodies (abnormal protein aggregates that form inside nerve cells), and a number of illnesses that exacerbate frontotemporal dementia. Dementia is an acquired loss of cognition in several cognitive domains that are severe enough to interfere with social or professional functioning. However, dementia frequently co-occurs with other neuropathologies, typically AD with cerebrovascular dysfunction. Clinical presentations show that neurodegenerative diseases are often incurable because patients permanently lose some neurons. A growing body of research suggests that they also advance our knowledge of the processes that are probably crucial for maintaining the health and functionality of the brain. Serious neurological impairment and neuronal death are the main features of neurodegenerative illnesses, which are also extremely crippling ailments. The most prevalent neurodegenerative disorders cause cognitive impairment and dementia, and as average life expectancy rises globally, their effects become more noticeable.

Crosstalk between the Rho and Rab family of small GTPases in neurodegenerative disorders

Neurodegeneration is associated with defects in cytoskeletal dynamics and dysfunctions of the vesicular trafficking and sorting systems. In the last few decades, studies have demonstrated that the key regulators of cytoskeletal dynamics are proteins from the Rho family GTPases, meanwhile, the central hub for vesicle sorting and transport between target membranes is the Rab family of GTPases. In this regard, the role of Rho and Rab GTPases in the induction and maintenance of distinct functional and morphological neuronal domains (such as dendrites and axons) has been extensively studied. Several members belonging to these two families of proteins have been associated with many neurodegenerative disorders ranging from dementia to motor neuron degeneration. In this analysis, we attempt to present a brief review of the potential crosstalk between the Rab and Rho family members in neurodegenerative pathologies such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease, and amyotrophic lateral sclerosis (ALS).

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer's Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer's disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Serum Glycoproteomics and Identification of Potential Mechanisms Underlying Alzheimer’s Disease

Objectives. This study compares glycoproteomes in Thai Alzheimer’s disease (AD) patients with those of cognitively normal individuals. Methods. Study participants included outpatients with clinically diagnosed AD ( $N=136$ ) and healthy controls without cognitive impairment ( $N=183$ ). Blood samples were collected from all participants for biochemical analysis and for $\text{Apolipoprotein}\text{E}$ (APOE) genotyping by real-time TaqMan PCR assays. Comparative serum glycoproteomic profiling by liquid chromatography-tandem mass spectrometry was then performed to identify differentially abundant proteins with functional relevance. Results. Statistical differences in age, educational level, and APOE ɛ3/ɛ4 and ɛ4/ɛ4 haplotype frequencies were found between the AD and control groups. The frequency of the APOE ɛ4 allele was significantly higher in the AD group than in the control group. In total, 871 glycoproteins were identified, including 266 and 259 unique proteins in control and AD groups, respectively. There were 49 and 297 upregulated and downregulated glycoproteins, respectively, in AD samples compared with the controls. Unique AD glycoproteins were associated with numerous pathways, including Alzheimer’s disease-presenilin pathway (16.6%), inflammation pathway mediated by chemokine and cytokine signaling (9.2%), Wnt signaling pathway (8.2%), and apoptosis signaling pathway (6.7%). Conclusion. Functions and pathways associated with protein-protein interactions were identified in AD. Significant changes in these proteins can indicate the molecular mechanisms involved in the pathogenesis of AD, and they have the potential to serve as AD biomarkers. Such findings could allow us to better understand AD pathology.

Role of Rab GTPases in Alzheimer's Disease

Alzheimer's disease (AD) comprises two major pathological hallmarks: extraneuronal deposition of β-amyloid (Aβ) peptides ("senile plaques") and intraneuronal aggregation of the microtubule-associated protein tau ("neurofibrillary tangles"). Aβ is derived from sequential cleavage of the β-amyloid precursor protein by β- and γ-secretases, while aggregated tau is hyperphosphorylated in AD. Mounting evidence suggests that dysregulated trafficking of these AD-related proteins contributes to AD pathogenesis. Rab proteins are small GTPases that function as master regulators of vesicular transport and membrane trafficking. Multiple Rab GTPases have been implicated in AD-related protein trafficking, and their expression has been observed to be altered in postmortem AD brain. Here we review current implicated roles of Rab GTPase dysregulation in AD pathogenesis. Further elucidation of the pathophysiological role of Rab GTPases will likely reveal novel targets for AD therapeutics.

Conserved Oligomeric Golgi and Neuronal Vesicular Trafficking

The conserved oligomeric Golgi (COG) complex is an evolutionary conserved multi-subunit vesicle tethering complex essential for the majority of Golgi apparatus functions: protein and lipid glycosylation and protein sorting. COG is present in neuronal cells, but the repertoire of COG function in different Golgi-like compartments is an enigma. Defects in COG subunits cause alteration of Golgi morphology, protein trafficking, and glycosylation resulting in human congenital disorders of glycosylation (CDG) type II. In this review we summarize and critically analyze recent advances in the function of Golgi and Golgi-like compartments in neuronal cells and functions and dysfunctions of the COG complex and its partner proteins.

Disruption of endolysosomal trafficking pathways in glioma cells by methuosis-inducing indole-based chalcones

Methuosis is a form of non-apoptotic cell death involving massive vacuolization of macropinosome-derived endocytic compartments, followed by a decline in metabolic activity and loss of membrane integrity. To explore the induction of methuosis as a potential therapeutic strategy for killing cancer cells, we have developed small molecules (indole-based chalcones) that trigger this form of cell death in glioblastoma and other cancer cell lines. Here, we report that in addition to causing fusion and expansion of macropinosome compartments, the lead compound, 3-(5-methoxy-2-methyl-1H-indol-3-yl)-1-(4-pyridinyl)-2-propen-1-one (MOMIPP), disrupts vesicular trafficking at the lysosomal nexus, manifested by impaired degradation of EGF and LDL receptors, defective processing of procathepsins, and accumulation of autophagosomes. In contrast, secretion of the ectodomain derived from a prototypical type-I membrane glycoprotein, β-amyloid precursor protein, is increased rather than diminished. A closely related MOMIPP analog, which causes substantial vacuolization without reducing cell viability, also impedes cathepsin processing and autophagic flux, but has more modest effects on receptor degradation. A third analog, which causes neither vacuolization nor loss of viability, has no effect on endolysosomal trafficking. The results suggest that differential cytotoxicity of structurally similar indole-based chalcones is related, at least in part, to the severity of their effects on endolysosomal trafficking pathways.

Simvastatin inhibits protein isoprenylation in the brain

Evidence suggests that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, or statins, may reduce the risk of Alzheimer's disease (AD). Statin action in patients with AD, as in those with heart disease, is likely to be at least partly independent of the effects of statins on cholesterol. Statins can alter cellular signaling and protein trafficking through inhibition of isoprenylation of Rho, Cdc42, and Rab family GTPases. The effects of statins on protein isoprenylation in vivo, particularly in the central nervous system, are poorly studied. We utilized two-dimensional gel electrophoresis approaches to directly monitor the levels of isoprenylated and non-isoprenylated forms of Rho and Rab family GTPases. We report that simvastatin significantly inhibits RhoA and Rab4, and Rab6 isoprenylation at doses as low as 50nM in vitro. We also provide the first in vivo evidence that statins inhibit the isoprenylation of RhoA in the brains of rats and RhoA, Cdc42, and H-Ras in the brains of mice treated with clinically relevant doses of simvastatin.

Defects in the COG complex and COG-related trafficking regulators affect neuronal Golgi function

The Conserved Oligomeric Golgi (COG) complex is an evolutionarily conserved hetero-octameric protein complex that has been proposed to organize vesicle tethering at the Golgi apparatus. Defects in seven of the eight COG subunits are linked to Congenital Disorders of Glycosylation (CDG)-type II, a family of rare diseases involving misregulation of protein glycosylation, alterations in Golgi structure, variations in retrograde trafficking through the Golgi and system-wide clinical pathologies. A troublesome aspect of these diseases are the neurological pathologies such as low IQ, microcephaly, and cerebellar atrophy. The essential function of the COG complex is dependent upon interactions with other components of trafficking machinery, such as Rab-GTPases and SNAREs. COG-interacting Rabs and SNAREs have been implicated in neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Defects in Golgi maintenance disrupts trafficking and processing of essential proteins, frequently associated with and contributing to compromised neuron function and human disease. Despite the recent advances in molecular neuroscience, the subcellular bases for most neurodegenerative diseases are poorly understood. This article gives an overview of the potential contributions of the COG complex and its Rab and SNARE partners in the pathogenesis of different neurodegenerative disorders.

β-Amyloid: the key peptide in the pathogenesis of Alzheimer's disease

The amyloid β peptide (Aβ) is a critical initiator that triggers the progression of Alzheimer's Disease (AD) via accumulation and aggregation, of which the process may be caused by Aβ overproduction or perturbation clearance. Aβ is generated from amyloid precursor protein through sequential cleavage of β- and γ-secretases while Aβ removal is dependent on the proteolysis and lysosome degradation system. Here, we overviewed the biogenesis and toxicity of Aβ as well as the regulation of Aβ production and clearance. Moreover, we also summarized the animal models correlated with Aβ that are essential in AD research. In addition, we discussed current immunotherapeutic approaches targeting Aβ to give some clues for exploring the more potentially efficient drugs for treatment of AD.

First effects of rising amyloid-β in transgenic mouse brain: synaptic transmission and gene expression

Understanding the earliest changes in Alzheimer’s disease may help in the prevention of cognitive impairment. In a transgenic mouse model, Cummings et al. show that synaptic changes occur shortly after soluble amyloid-β levels become measurable, and before the rapid increases in total Aβ and Aβ₄₂:Aβ₄₀ that lead to detectable plaque deposition.

Detecting and treating Alzheimer’s disease, before cognitive deficits occur, has become the health challenge of our time. The earliest known event in Alzheimer’s disease is rising amyloid-β. Previous studies have suggested that effects on synaptic transmission may precede plaque deposition. Here we report how relative levels of different soluble amyloid-β peptides in hippocampus, preceding plaque deposition, relate to synaptic and genomic changes. Immunoprecipitation-mass spectrometry was used to measure the early rise of different amyloid-β peptides in a mouse model of increasing amyloid-β (‘TASTPM’, transgenic for familial Alzheimer’s disease genes APP/PSEN1). In the third postnatal week, several amyloid-β peptides were above the limit of detection, including amyloid-β₄₀, amyloid-β₃₈ and amyloid-β₄₂ with an intensity ratio of 6:3:2, respectively. By 2 months amyloid-β levels had only increased by 50% and although the ratio of the different peptides remained constant, the first changes in synaptic currents, compared to wild-type mice could be detected with patch-clamp recordings. Between 2 and 4 months old, levels of amyloid-β₄₀ rose by ∼7-fold, but amyloid-β₄₂ rose by 25-fold, increasing the amyloid-β₄₂:amyloid-β₄₀ ratio to 1:1. Only at 4 months did plaque deposition become detectable and only in some mice; however, synaptic changes were evident in all hippocampal fields. These changes included increased glutamate release probability (P < 0.001, n = 7–9; consistent with the proposed physiological effect of amyloid-β) and loss of spontaneous action potential-mediated activity in the cornu ammonis 1 (CA1) and dentate gyrus regions of the hippocampus (P < 0.001, n = 7). Hence synaptic changes occur when the amyloid-β levels and amyloid-β₄₂:amyloid-β₄₀ ratio are still low compared to those necessary for plaque deposition. Genome-wide microarray analysis revealed changes in gene expression at 2–4 months including synaptic genes being strongly affected but often showing significant changes only by 4 months. We thus demonstrate that, in a mouse model of rising amyloid-β, the initial deposition of plaques does not occur until several months after the first amyloid-β becomes detectable but coincides with a rapid acceleration in the rise of amyloid-β levels and the amyloid-β₄₂:amyloid-β₄₀ ratio. Prior to acceleration, however, there is already a pronounced synaptic dysfunction, reflected as changes in synaptic transmission and altered gene expression, indicating that restoring synaptic function early in the disease progression may represent the earliest possible target for intervention in the onset of Alzheimer’s disease.

Secreted amyloid β-proteins in a cell culture model include N-terminally extended peptides that impair synaptic plasticity

Evidence for a central role of amyloid β-protein (Aβ) in the genesis of Alzheimer’s disease (AD) has led to advanced human trials of Aβ-lowering agents. The “amyloid hypothesis” of AD postulates deleterious effects of small, soluble forms of Aβ on synaptic form and function. Because selectively targeting synaptotoxic forms of soluble Aβ could be therapeutically advantageous, it is important to understand the full range of soluble Aβ derivatives. We previously described a Chinese hamster ovary (CHO) cell line (7PA2 cells) that stably expresses mutant human amyloid precursor protein (APP). Here, we extend this work by purifying an sodium dodecyl sulfate (SDS)-stable, ∼8 kDa Aβ species from the 7PA2 medium. Mass spectrometry confirmed its identity as a noncovalently bonded Aβ40 homodimer that impaired hippocampal long-term potentiation (LTP) in vivo. We further report the detection of Aβ-containing fragments of APP in the 7PA2 medium that extend N-terminal from Asp1 of Aβ. These N-terminally extended Aβ-containing monomeric fragments are distinct from soluble Aβ oligomers formed from Aβ1-40/42 monomers and are bioactive synaptotoxins secreted by 7PA2 cells. Importantly, decreasing β-secretase processing of APP elevated these alternative synaptotoxic APP fragments. We conclude that certain synaptotoxic Aβ-containing species can arise from APP processing events N-terminal to the classical β-secretase cleavage site.

Dysregulation of protein trafficking in neurodegeneration

Intracellular protein trafficking plays an important role in neuronal function and survival. Protein misfolding is a common theme found in many neurodegenerative diseases, and intracellular trafficking machinery contributes to the pathological accumulation and clearance of misfolded proteins. Although neurodegenerative diseases exhibit distinct pathological features, abnormal endocytic trafficking is apparent in several neurodegenerative diseases, such as Alzheimer’s disease (AD), Down syndrome (DS) and Parkinson’s disease (PD). In this review, we will focus on protein sorting defects in three major neurodegenerative diseases, including AD, DS and PD. An important pathological feature of AD is the presence of extracellular senile plaques in the brain. Senile plaques are composed of β-amyloid (Aβ) peptide aggregates. Multiple lines of evidence demonstrate that over-production/aggregation of Aβ in the brain is a primary cause of AD and attenuation of Aβ generation has become a topic of extreme interest in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavage by β-secretase and the γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα which precludes Aβ generation. DS patients display a strikingly similar pathology to AD patients, including the generation of neuronal amyloid plaques. Moreover, all DS patients develop an AD-like neuropathology by their 40 s. Therefore, understanding the metabolism/processing of APP and how these underlying mechanisms may be pathologically compromised is crucial for future AD and DS therapeutic strategies. Evidence accumulated thus far reveals that synaptic vesicle regulation, endocytic trafficking, and lysosome-mediated autophagy are involved in increased susceptibility to PD. Here we review current knowledge of endosomal trafficking regulation in AD, DS and PD.

Trafficking regulation of proteins in Alzheimer's disease

The β-amyloid (Aβ) peptide has been postulated to be a key determinant in the pathogenesis of Alzheimer’s disease (AD). Aβ is produced through sequential cleavage of the β-amyloid precursor protein (APP) by β- and γ-secretases. APP and relevant secretases are transmembrane proteins and traffic through the secretory pathway in a highly regulated fashion. Perturbation of their intracellular trafficking may affect dynamic interactions among these proteins, thus altering Aβ generation and accelerating disease pathogenesis. Herein, we review recent progress elucidating the regulation of intracellular trafficking of these essential protein components in AD.

Molecular Dissection of Cyclosporin A's Neuroprotective Effect Reveals Potential Therapeutics for Ischemic Brain Injury

After the onset of brain ischemia, a series of events leads ultimately to the death of neurons. Many molecules can be pharmacologically targeted to protect neurons during these events, which include glutamate release, glutamate receptor activation, excitotoxicity, Ca²⁺ influx into cells, mitochondrial dysfunction, activation of intracellular enzymes, free radical production, nitric oxide production, and inflammation. There have been a number of attempts to develop neuroprotectants for brain ischemia, but many of these attempts have failed. It was reported that cyclosporin A (CsA) dramatically ameliorates neuronal cell damage during ischemia. Some researchers consider ischemic cell death as a unique process that is distinct from both apoptosis and necrosis, and suggested that mitochondrial dysfunction and Δψ collapse are key steps for ischemic cell death. It was also suggested that CsA has a unique neuroprotective effect that is related to mitochondrial dysfunction. Here, I will exhibit examples of neuroprotectants that are now being developed or in clinical trials, and will discuss previous researches about the mechanism underlying the unique CsA action. I will then introduce the results of our cDNA subtraction experiment with or without CsA administration in the rat brain, along with our hypothesis about the mechanism underlying CsA’s effect on transcriptional regulation.

A new Mint1 isoform, but not the conventional Mint1, interacts with the small GTPase Rab6

Small GTPases of the Rab family are important regulators of a large variety of different cellular functions such as membrane organization and vesicle trafficking. They have been shown to play a role in several human diseases. One prominent member, Rab6, is thought to be involved in the development of Alzheimer's Disease, the most prevalent mental disorder worldwide. Previous studies have shown that Rab6 impairs the processing of the amyloid precursor protein (APP), which is cleaved to β-amyloid in brains of patients suffering from Alzheimer's Disease. Additionally, all three members of the Mint adaptor family are implied to participate in the amyloidogenic pathway. Here, we report the identification of a new Mint1 isoform in a yeast two-hybrid screening, Mint1 826, which lacks an eleven amino acid (aa) sequence in the conserved C-terminal region. Mint1 826, but not the conventional Mint1, interacts with Rab6 via the PTB domain. This interaction is nucleotide-dependent, Rab6-specific and influences the subcellular localization of Mint1 826. We were able to detect and sequence a corresponding proteolytic peptide derived from cellular Mint1 826 by mass spectrometry proving the absence of aa 495-505 and could show that the deletion does not influence the ability of this adaptor protein to interact with APP. Taking into account that APP interacts and co-localizes with Mint1 826 and is transported in Rab6 positive vesicles, our data suggest that Mint1 826 bridges APP to the small GTPase at distinct cellular sorting points, establishing Mint1 826 as an important player in regulation of APP trafficking and processing.

Lysosomal fusion dysfunction as a unifying hypothesis for Alzheimer's disease pathology

Alzheimer's disease is characterized pathologically by extracellular senile plaques, intracellular neurofibrillary tangles, and granulovacuolar degeneration. It has been debated whether these hallmark lesions are markers or mediators of disease progression, and numerous paradigms have been proposed to explain the appearance of each lesion individually. However, the unfaltering predictability of these lesions suggests a single pathological nidus central to disease onset and progression. One of the earliest pathologies observed in Alzheimer's disease is endocytic dysfunction. Here we review the recent literature of endocytic dysfunction with particular focus on disrupted lysosomal fusion and propose it as a unifying hypothesis for the three most-studied lesions of Alzheimer's disease.

Effects of cyclosporin A administration on gene expression in rat brain

PRIMARY OBJECTIVE

The immunosuppressant cyclosporin A (CsA) is reported to have a strong anti-ischemic effect. Although this neuroprotective effect is speculated to be related to the blockade of a mitochondrial permeability transition pore (mPTP), the underlying molecular mechanism remains to be elucidated. This study focused on the effect of CsA on transcriptional regulation in brain cells.

METHODS

CsA and a control substance were injected into rat brains and purified extracted mRNA. Both mRNAs were compared using a cDNA subtraction technique.

RESULTS

Nine significantly up-regulated genes and seven significantly down-regulated genes were detected following CsA administration. All of the up-regulated genes are neurotrophic or reported to have roles in regeneration of brain tissue. Among the down-regulated genes, three are known to be detrimental to neuronal cells and are also reported to facilitate the pathology of Alzheimer's disease (AD) and four genes are related to oxidative metabolism.

CONCLUSIONS

Strong immunosuppression would present as a side-effect during CsA use as a neuroprotectant. The results of this study will help to discriminate between the CsA immunosuppressive effect and the neuroprotective effect at the molecular level and may lead to the development of new conceptual and pharmacological tools.

Insulin-degrading enzyme sorting in exosomes: a secretory pathway for a key brain amyloid-beta degrading protease

The accumulation of amyloid-beta (Abeta) peptides in senile plaques is one of the hallmarks of Alzheimer's disease (AD) progression. The endocytic pathway has been proposed as a major subcellular site for Abeta generation while the compartments in which Abeta-degrading proteases interact with Abeta are still elusive. It was suggested that extracellular Abeta degradation may take place by plasma-membrane associated proteases or by extracellular proteases, among which insulin-degrading enzyme (IDE) is the most relevant. However, the mechanisms of IDE secretion are poorly understood. In the present study we used N2a cells to explore if IDE is indeed released through exosomes and the effect of exosomes release on extracellular levels of Abeta. We demonstrated that proteolytically-active plasma membrane associated-IDE is routed in living N2a cells to multivesicular bodies and subsequently, a major fraction is sorted to exosomes. We described that extracellular IDE levels decrease if the generation of multivesicular bodies is interfered and may be positively modulated by exosomes release under stress-induced conditions. Our results reinforce the relevance of functional IDE in the catabolism of extracellular Abeta.

Neurite-like structures induced by mevalonate pathway blockade are due to the stability of cell adhesion foci and are enhanced by the presence of APP

Epidemiological studies have shown an association between statin use and a decreased risk of dementia. However, the mechanism by which this beneficial effect is brought about is unclear. In the context of Alzheimer's disease, at least three possibilities have been studied; reduction in amyloid beta peptide (Abeta) production, the promotion of alpha-secretase cleavage and positive effects on neurite outgrowth. By investigating the effects of mevalonate pathway blockade on neurite outgrowth using real-time imaging, we found that rather than promote the production of neurite extensions, inhibition rapidly induced cell rounding. Crucially, neurite-like structures were generated through the persistence of cell-cell and cell-substrate adhesions and not through a mechanism of positive outgrowth. This effect can be strikingly enhanced by the over-expression of human amyloid precursor protein and is isoprenoid rather than cholesterol dependent.

A Computational Approach for the Identification of Small GTPases Based on Preprocessed Amino Acid Sequences

The prediction of essential biological features based on a given protein sequence is a challenging task in computational biology. To limit the amount of in vitro verification, the prediction of essential biological activities gives the opportunity to detect so far unknown sequences with similar properties. Besides the application within the identification of proteins being involved in tumorigenesis, other functional classes of proteins can be predicted. The prediction accuracy depends on the selected machine learning approach and even more on the composition of the descriptor set used. A computational approach based on feedforward neural networks was applied for the prediction of small GTPases. Consequently, this was realized by taking secondary structure and hydrophobicity information as a preprocessing architecture and thus, as descriptors for the neural networks. We developed a neural network cluster, which consists of a filter network and four subfamily networks. The filter network was trained to identify small GTPases and the subfamily networks were trained to assign a small GTPase to one of the subfamilies. The accuracy of the prediction, whether a given sequence represents a small GTPase is very high (98.25%). The classifications of the subfamily networks yield comparable accuracy. The high prediction accuracy of the neural network cluster developed, gives the opportunity to suggest the use of hydrophobicity and secondary structure prediction in combination with a neural network cluster, as a promising method for the prediction of essential biological activities.

Chapter 5: rab proteins and their interaction partners

The Ras superfamily consists of over 150 low molecular weight proteins that cycle between an inactive guanosine diphosphate (GDP)-bound state and an active guanosine triphosphate (GTP)-bound state. They are involved in a variety of signal transduction pathways that regulate cell growth, intracellular trafficking, cell migration, and apoptosis. Several methods have been devised to detect and characterize the interacting partners of small GTPases with the aim of better understanding their physiological function in normal cells and tumor cells. The Rab (Ras analog in brain) proteins form the largest family within the Ras superfamily. Rab proteins regulate vesicular trafficking pathways, behaving as membrane-associated molecular switches. The guanine nucleotide-binding status of Rab proteins is modulated by three different classes of regulatory proteins, which have been extensively studied for the Rab molecules but also for other subfamilies of the Ras superfamily. Furthermore, numerous effector molecules have been isolated especially for the Rab subfamily of proteins, which interact via a Rab-binding domain (RBD) and are recruited afterwards to specific sub-cellular compartments by the Rab proteins.

Neuronal protein trafficking associated with Alzheimer disease: from APP and BACE1 to glutamate receptors

Aberrant and/or cumulative amyloid-beta (Abeta) production, resulting from proteolytic processing of the amyloid precursor protein (APP) by beta and gamma-secretases, have been postulated to be a main etiological basis of Alzheimer disease (AD). A number of proteins influence the subcellular trafficking itinerary of APP and the beta-site APP-cleaving enzyme (BACE1) between the cell surface, endosomes and the trans-Golgi network (TGN). Available evidence suggests that co-residence of APP and BACE1 in the endosomal compartments promotes amyloidogenesis. Retrograde transport of APP out of the endosome to the TGN reduces Abeta production, while APP routed to and kept at the cell surface enhances its non-amyloidogenic, alpha-secretase-mediated processing. Changes in post-Golgi membrane trafficking in aging neurons that may influence APP processing is particularly relevant to late-onset, idiopathic AD. Dystrophic axons are key features of AD pathology, and impaired axonal transport could play crucial roles in the pathogenesis of idiopathic AD. Recent evidence has also indicated that Abeta-induced synaptic defects and memory impairment could be explained by a loss of both AMPA and NMDA receptors through endocytosis. Detail understanding of factors that influence these neuronal trafficking processes will open up novel therapeutic avenues for preventing or delaying the onset of symptomatic AD.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.

Statins Reduce Amyloid-β Production through Inhibition of Protein Isoprenylation

Epidemiological evidence suggests that long term treatment with hydroxymethylglutaryl-CoA reductase inhibitors, or statins, decreases the risk for developing Alzheimer disease (AD). However, statin-mediated AD protection cannot be fully explained by reduction of cholesterol levels. In addition to their cholesterol lowering effects, statins have pleiotropic actions and act to lower the concentrations of isoprenoid intermediates, such as geranylgeranyl pyrophosphate and farnesyl pyrophosphate. The Rho and Rab family small G-proteins require addition of these isoprenyl moieties at their C termini for normal GTPase function. In neuroblastoma cell lines, treatment with statins inhibits the membrane localization of Rho and Rab proteins at statin doses as low as 200 nm, without affecting cellular cholesterol levels. In addition, we show for the first time that at low, physiologically relevant, doses statins preferentially inhibit the isoprenylation of a subset of GTPases. The amyloid precursor protein (APP) is proteolytically cleaved to generate beta-amyloid (Abeta), which is the major component of senile plaques found in AD. We show that inhibition of protein isoprenylation by statins causes the accumulation of APP within the cell through inhibition of Rab family proteins involved in vesicular trafficking. Moreover, inhibition of Rho family protein function reduces levels of APP C-terminal fragments due to enhanced lysosomal dependent degradation. Statin inhibition of protein isoprenylation results in decreased Abeta secretion. In summary, we show that statins selectively inhibit GTPase isoprenylation at clinically relevant doses, leading to reduced Abeta production in an isoprenoid-dependent manner. These studies provide insight into the mechanisms by which statins may reduce AD pathogenesis.

Rab6 is increased in Alzheimer's disease brain and correlates with endoplasmic reticulum stress

Alzheimer's disease (AD) is characterized by deposits of aggregated proteins. Accumulation of aggregation-prone proteins activates protein quality control mechanisms, such as the unfolded protein response (UPR) in the endoplasmic reticulum (ER). We previously reported upregulation of the UPR marker BiP in AD brain. In this study, we investigated the small GTPase Rab6, which is involved in retrograde Golgi-ER trafficking and may function as a post-ER quality control system. Using immunohistochemistry and semiquantitative Western blotting, the expression of Rab6 was analysed in hippocampus, entorhinal and temporal cortex of 10 AD patients and six nondemented control subjects. Rab6 is upregulated in AD temporal cortex from Braak stage 3/4, the same stage that UPR activation is found. We observe increased neuronal Rab6 immunoreactivity in all brain areas examined. Although some neurones show colocalization of immunoreactivity for Rab6 and hyperphosphorylated tau, strong Rab6 staining does not colocalize with tangles. We find a highly significant correlation between the Rab6 and BiP levels. In vitro data show that Rab6 is not upregulated as a result of UPR activation or proteasome inhibition indicating an independent regulatory mechanism. Our data suggest that ER and post-ER protein quality control mechanisms are activated early in the pathology of AD.

COPI-mediated retrograde transport is required for efficient γ-secretase cleavage of the amyloid precursor protein

Sequential cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases results in the production of beta-amyloid peptide, which is a key determinant in Alzheimer's disease. Since several putative locations for gamma-secretase cleavage have been identified along the secretory pathway, trafficking of APP may be of importance for beta-amyloid peptide production. Here we have studied the role of retrograde transport in APP processing. We found that APP interacts with the beta subunit of the coatomer protein I (COPI) complex, which is involved in retrograde transport. In line with a role of retrograde trafficking in APP transport, inhibition of COPI-dependent transport altered APP trafficking, decreased APP cell surface expression, and coincided with a profound reduction in gamma-secretase cleavage. These results suggest that COPI-dependent retrograde transport is important for APP processing and influences production of beta-amyloid peptide.

Pathological and physiological functions of presenilins

Mutations in PSEN1 and PSEN2 genes account for the majority of cases of early-onset familial Alzheimer disease. Since the first prediction of a genetic link between PSEN1 and PSEN2 with Alzheimer's disease, many research groups from both academia and pharmaceutical industry have sought to unravel how pathogenic mutations in PSEN cause presenile dementia. PSEN genes encode polytopic membrane proteins termed presenilins (PS1 and PS2), which function as the catalytic subunit of γ-secretase, an intramembrane protease that has a wide spectrum of type I membrane protein substrates. Sequential cleavage of amyloid precursor protein by BACE and γ-secretase releases highly fibrillogenic β-amyloid peptides, which accumulate in the brains of aged individuals and patients with Alzheimer's disease. Familial Alzheimer's disease-associated presenilin variants are thought to exert their pathogenic function by selectively elevating the levels of highly amyloidogenic Aβ42 peptides. In addition to Alzheimer's disease, several recent studies have linked PSEN1 to familiar frontotemporal dementia. Here, we review the biology of PS1, its role in γ-secretase activity, and discuss recent developments in the cell biology of PS1 with respect to Alzheimer's disease pathogenesis.

Rab6 interacts with the mint3 adaptor protein

The Rab6 GTPase regulates a retrograde transport route connecting endosomes and the endoplasmic reticulum (ER) via the Golgi apparatus. Recently it was shown that active (GTP-loaded) Rab6A regulates intracellular processing of the amyloid precursor protein (APP). To characterize the role of Rab6A in APP trafficking and to identify effector proteins of the active Rab6A protein, we screened a human placenta cDNA library using the yeast two-hybrid system. We isolated an interacting cDNA clone encoding part of the adaptor protein mint3. The interaction between Rab6A and mint3 is GTP-dependent and requires the complete phosphotyrosine-binding (PTB) domain of the mint protein, which also mediates the association with APP. By confocal microscopy we show that Rab6A, mint3 and APP co-localize at Golgi membranes in HeLa cells. Density gradient centrifugation of cytosolic extracts confirms a common distribution of these three proteins. Our data suggest that mint3 links Rab6A to APP traffic.

Statins cause intracellular accumulation of amyloid precursor protein, beta-secretase-cleaved fragments, and amyloid beta-peptide via an isoprenoid-dependent mechanism

The use of statins, 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors that block the synthesis of mevalonate (and downstream products such as cholesterol and nonsterol isoprenoids), as a therapy for Alzheimer disease is currently the subject of intense debate. It has been reported that statins reduce the risk of developing the disorder, and a link between cholesterol and Alzheimer disease pathophysiology has been proposed. Moreover, experimental studies focusing on the cholesterol-dependent effects of statins have demonstrated a close association between cellular cholesterol levels and amyloid production. However, evidence suggests that statins are pleiotropic, and the potential cholesterol-independent effects of statins on amyloid precursor protein (APP) metabolism and amyloid beta-peptide (A beta) genesis are unknown. In this study, we developed a novel in vitro system that enabled the discrete analysis of cholesterol-dependent and -independent (i.e. isoprenoid-dependent) statin effects on APP cleavage and A beta formation. Given the recent interest in the role that intracellular A beta may play in Alzheimer disease, we analyzed statin effects on both secreted and cell-associated A beta. As reported previously, low cellular cholesterol levels favored the alpha-secretase pathway and decreased A beta secretion presumably within the endocytic pathway. In contrast, low isoprenoid levels resulted in the accumulation of APP, amyloidogenic fragments, and A beta likely within biosynthetic compartments. Importantly, low cholesterol and low isoprenoid levels appeared to have completely independent effects on APP metabolism and A beta formation. Although the implications of these effects for Alzheimer disease pathophysiology have yet to be investigated, to our knowledge, these results provide the first evidence that isoprenylation is involved in determining levels of intracellular A beta.

Rab6 membrane association is dependent of Presenilin 1 and cellular phosphorylation events

Processing of the amyloid precursor protein (APP) by alpha-secretase precludes the formation of beta-amyloid (Abeta). Therefore, the increase of cleavage by alpha-secretase upon stimulation by protein kinase C (PKC) is of potential therapeutic interest for Alzheimer's disease (AD). Unknown is whether phosphorylation by PKC increases alpha-secretase-mediated cleavage directly or indirectly, for example, by modulation of APP trafficking. Because modulation of Rab6-mediated transport has been shown to affect APP processing, we investigated the regulation of Rab6 membrane association by PKC and its relation to PS1. We show that in fibroblasts, Rab6 membrane association is PKC dependent, an effect strongly potentiated by inhibition of calcineurin. Moreover, we demonstrate that this regulation of Rab6 membrane association is dependent on PS1. The possible implications for APP processing and AD are discussed.

Tyrosine phosphorylation of the Rab24 GTPase in cultured mammalian cells

Several members of the large family of Rab GTPases have been shown to function in vesicular trafficking in mammalian cells. However, the exact role of Rab24 remains poorly defined. Rab24 differs from other Rab proteins in that it has a low intrinsic GTPase activity and is not efficiently prenylated. Here we report an additional unique property of Rab24; i.e., the protein can undergo tyrosine phosphorylation when overexpressed in cultured cells. Immunoblot analyses with specific anti-phosphotyrosine monoclonal antibodies revealed the presence of phosphotyrosine (pTyr) on myc-Rab24 in whole cell lysates and immunoprecipitated samples. No pTyr was detected on other overexpressed myc-tagged GTPases (H-Ras, Rab1b, Rab6, Rab11 or Rab13). Comparisons of myc-Rab24 in the soluble and particulate fractions from HEK293 and HEp-2 cells indicated that the cytosolic pool of Rab24 was more heavily phosphorylated than the membrane pool. Treatment of transfected cells with the broad-spectrum tyrosine kinase inhibitor, genistein, as well as the specific Src-family kinase inhibitor, PP2, eliminated the pTyr signal from Rab24. In contrast the receptor tyrosine kinase inhibitor, tyrphostin A25, had no effect. Tyrosine phosphorylation of Rab24 was reduced by alanine substitution of two unique tyrosines, one found in a strong consensus phosphorylation motif (Y [Formula: see text] ) in the hypervariable domain (Y172) and the other falling within the GXXXGK(S/T) motif known as the P-loop (Y17). The latter region is known to influence GTP hydrolysis in Rab proteins, so the phosphorylation of Y17 could contribute to the low intrinsic GTPase activity of Rab24. This is the first report of tyrosine phosphorylation in any member of the Ras superfamily and it raises the possibility that this type of modification could influence Rab24 targeting and interactions with effector protein complexes.

A new functional screening system for identification of regulators for the generation of amyloid beta-protein

Presenilin (PS) is essential for gamma-cleavage, which is required for the generation of amyloid beta-protein (Abeta) from the beta-amyloid precursor protein. However, it remains to be clarified how gamma-cleavage is regulated. To elucidate the regulation of PS-mediated gamma-cleavage, we developed a new functional screening method for identifying cDNA that enhances gamma-cleavage. This screening system utilizes our own developed cell line, where the expression of cDNA that enhances gamma-cleavage confers puromycin resistance. The cDNA library is retrovirally delivered to the above-mentioned cell line, allowing the identification of our target cDNAs by a combination of puromycin resistance selection and Abeta assay screening. With this screening method, we isolated several cDNAs enhancing gamma-cleavage, including the previously reported Herp. Here we also demonstrate that Rab1A, identified with this screening, can be a regulator of Abeta generation. Thus, our established screening method is a powerful tool for identifying multiple regulators involved in gamma-cleavage in the Abeta generation pathway, including modulators of gamma-secretase activity or the intracellular trafficking of factors necessary for gamma-cleavage.

Mutant Rab24 GTPase is targeted to nuclear inclusions

BACKGROUND

Members of the Rab GTPase family regulate intracellular protein trafficking, but the specific function of Rab24 remains unknown. Several attributes distinguish this protein from other members of the Rab family, including a low intrinsic GTPase activity.

RESULTS

The functions of other Rab proteins have been defined through the use of dominant-negative mutants with amino acid substitutions in the conserved N(T)KxD nucleotide binding motif. Surprisingly, when such Rab24 constructs were expressed in cultured cells, they accumulated in nuclear inclusions which disrupted the integrity of the nuclear envelope. The inclusions reacted positively with antibodies against ubiquitin and Hsp70, similar to protein aggregates observed in polyglutamine disorders. They also appeared to sequester importin-beta and GFP-coupled glucocorticoid receptor. Other Rab GTPases with similar mutations in the N(T)KxD motif were never found in inclusions, suggesting that the unusual localization of Rab24 is not related solely to misfolding of its nucleotide-free form. Studies with Rab24/Rab1B chimeras indicated that targeting of the mutant protein to inclusions requires the unique C-terminal domain of Rab24.

CONCLUSION

These studies demonstrate that mutations in Rab24 can trigger a cytopathic cellular response involving accumulation of nuclear inclusions. If the N(T)KxD mutants of Rab24 function as dominant suppressors, these studies may point to a unique role for Rab24 in degradation of misfolded cellular proteins or trafficking of proteins to the nuclear envelope. However, we cannot yet eliminate the possibility that these phenomena are related to unusual non-physiological protein interactions with the mutant form of Rab24.

Altered metabolism of the amyloid beta precursor protein is associated with mitochondrial dysfunction in Down's syndrome

Most Down's syndrome (DS) patients develop Alzheimer's disease (AD) neuropathology. Astrocyte and neuronal cultures derived from fetal DS brain show alterations in the processing of amyloid beta precursor protein (AbetaPP), including increased levels of AbetaPP and C99, reduced levels of secreted AbetaPP (AbetaPPs) and C83, and intracellular accumulation of insoluble Abeta42. This pattern of AbetaPP processing is recapitulated in normal astrocytes by inhibition of mitochondrial metabolism, consistent with impaired mitochondrial function in DS astrocytes. Intracellular Abeta42 and reduced AbetaPPs are also detected in DS and AD brains. The survival of DS neurons is markedly increased by recombinant or astrocyte-produced AbetaPPs, suggesting that AbetaPPs may be a neuronal survival factor. Thus, mitochondrial dysfunction in DS may lead to intracellular deposition of Abeta42, reduced levels of AbetaPPs, and a chronic state of increased neuronal vulnerability.

Retention of the Alzheimer's amyloid precursor fragment C99 in the endoplasmic reticulum prevents formation of amyloid beta-peptide

gamma-Secretase is a membrane-associated endoprotease that catalyzes the final step in the processing of Alzheimer's beta-amyloid precursor protein (APP), resulting in the release of amyloid beta-peptide (Abeta). The molecular identity of gamma-secretase remains in question, although recent studies have implicated the presenilins, which are membrane-spanning proteins localized predominantly in the endoplasmic reticulum (ER). Based on these observations, we have tested the hypothesis that gamma-secretase cleavage of the membrane-anchored C-terminal stump of APP (i.e. C99) occurs in the ER compartment. When recombinant C99 was expressed in 293 cells, it was localized mainly in the Golgi apparatus and gave rise to abundant amounts of Abeta. Co-expression of C99 with mutant forms of presenilin-1 (PS1) found in familial Alzheimer's disease resulted in a characteristic elevation of the Abeta(42)/Abeta(40) ratio, indicating that the N-terminal exodomain of APP is not required for mutant PS1 to influence the site of gamma-secretase cleavage. Biogenesis of both Abeta(40) and Abeta(42) was almost completely eliminated when C99 was prevented from leaving the ER by addition of a di-lysine retention motif (KKQN) or by co-expression with a dominant-negative mutant of the Rab1B GTPase. These findings indicate that the ER is not a major intracellular site for gamma-secretase cleavage of C99. Thus, by inference, PS1 localized in this compartment does not appear to be active as gamma-secretase. The results suggest that presenilins may acquire the characteristics of gamma-secretase after leaving the ER, possibly by assembling with other proteins in peripheral membranes.

Overexpression of Rab11 or constitutively active Rab11 does not affect sAPPalpha and Abeta secretions by wild-type and Swedish mutated betaAPP-expressing HEK293 cells

Presenilins 1 and 2 are two homologous proteins which, when mutated, appear responsible for most of the early-onset familial forms of Alzheimer's disease. Among various functional aspects, presenilins appear to behave as chaperoning partners of a series of proteins including the beta-amyloid precursor protein. Recently, presenilins were shown to interact with Rab11, a GTPase involved in intracellular transport. This suggested that Rab11-presenilin interaction could influence the routing of betaAPP and thereby modulate its maturation. In this context, we examined whether overexpression of Rab11 or its constitutively active mutant Rab11Q70L could affect betaAPP maturation in human HEK293 cells. We show here that the overexpression of both Rab11-related proteins does not modify the recovery of secreted sAPPalpha or Abeta in HEK293 cells expressing wild-type betaAPP or betaAPP harboring the Swedish double mutation. These data indicate that Rab11 does not influence betaAPP processing in HEK293 cells. However, it does not preclude the possibility for Rab11 to modulate other presenilin-mediated functions in human cells.

hFE65L influences amyloid precursor protein maturation and secretion

The amyloid precursor protein (APP) is processed in the secretory and endocytic pathways, where both the neuroprotective alpha-secretase-derived secreted APP (APPs alpha) and the Alzheimer's disease-associated beta-amyloid peptide are generated. All three members of the FE65 protein family bind the cytoplasmic domain of APP, which contains two sorting signals, YTS and YENPTY. We show here that binding of APP to the C-terminal phosphotyrosine interaction domain of hFE65L requires an intact YENPTY clathrin-coated pit internalization sequence. To study the effects of the hFE65L/APP interaction on APP trafficking and processing, we performed pulse/chase experiments and examined APP maturation and secretion in an H4 neuroglioma cell line inducible for expression of the hFE65L protein. Pulse/chase analysis of endogenous APP in these cells showed that the ratio of mature to total cellular APP increased after the induction of hFE65L. We also observed a three-fold increase in the amount of APPs alpha recovered from conditioned media of cells overexpressing hFE65L compared with uninduced controls. The effect of hFE65L on the levels of APPs alpha secreted is due neither to a simple increase in the steady-state levels of APP nor to activation of the protein kinase C-regulated APP secretion pathway. We conclude that the effect of hFE65L on APP processing is due to altered trafficking of APP as it transits through the secretory pathway.

The developmental role of warthog, the notch modifier encoding Drab6

The warthog (wrt) gene, recovered as a modifier for Notch signaling, was found to encode the Drosophila homologue of rab6, Drab6. Vertebrate and yeast homologues of this protein have been shown to regulate Golgi network to TGN trafficking. To study the function of this protein in the development of a multicellular organism, we analyzed three different warthog mutants. The first was an R62C point mutation, the second a genomic null, and the third was an engineered GTP-bound form. Our studies show, contrary to yeast, that the Drosophila homologue of rab6 is an essential gene. However, it has limited effects on development beyond the larval stage. Only the mechanosensory bristles on the head, notum, and scutellum are affected by warthog mutations. We present models for the modifying effect of Drab6 on Notch signaling.

Rab6 regulation of rhodopsin transport in Drosophila

Rab6 is a GTP binding protein that regulates vesicular trafficking within the Golgi and post-Golgi compartments. We overexpressed wild-type, a GTPase defective (Q71L), and a guanine nucleotide binding defective (N125I) Rab6 protein in Drosophila photoreceptors to assess the in vivo role of Rab6 in the trafficking of rhodopsin and other proteins. Expression of Drab6(Q71L) greatly reduced the steady state levels of two rhodopsins, Rh1 and Rh3, whereas Drab6(wt) and Drab6(N125I) showed weaker effects. Analysis of a strain carrying Rh1 rhodopsin under a heat shock promoter showed that Drab6(Q71L), but not Drab6(wt) or Drab6(N125I), prevents the maturation of rhodopsin beyond an immature 40 kDa form. Drab6(Q71L) is a GTPase defective mutant, indicating that anterograde transport of rhodopsin requires Rab6 GTPase function. The three Drab6 strains had no effect on the expression of several other photoreceptor proteins. The Drab6(Q71L) photoreceptors show marked histological defects at young ages and degenerate over a two week time span. These results establish that rhodopsin is transported via a Rab6 regulated pathway and that defects in trafficking pathways lead to retinal degeneration.

Presenilin-1 mutations associated with familial Alzheimer's disease do not disrupt protein transport from the endoplasmic reticulum to the Golgi apparatus

Mutations in genes encoding presenilin-1 (PS1) and presenilin-2 (PS2) have been linked to familial forms of Alzheimer's disease (AD). Cells expressing mutant presenilins produce elevated levels of Abeta42, the major amyloid peptide found in AD plaques. The mechanism whereby this occurs remains unknown, but the localization of presenilins to endoplasmic reticulum (ER) and Golgi compartments has suggested that they may function in intracellular trafficking pathways involved in processing beta-amyloid precursor proteins (APP). To test this possibility, we coexpressed PS1(wt), PS1(M146L), or PS1(L286V) in HEK293 cells together with the LDL receptor, a classic glycoprotein marker that undergoes post-translational O-glycosylation in the Golgi compartment. Pulse-chase analysis of the receptor indicated that mutant presenilins had no effect on ER-->Golgi transport. Similar results were obtained when the studies were carried out with cells expressing the Swedish variant of APP (SWAPP751) instead of the LDL receptor. Moreover, secretion of the soluble exodomain polypeptide fragments of SWAPP751 that arise from alpha-secretase and beta-secretase cleavage was not markedly affected by the PS1 mutants. Despite the lack of discernible effect of the PS1 mutants on trafficking of proteins through the Golgi apparatus, they caused a substantial increase in the proportion of Abeta42 relative to total Abeta in the culture medium. The results suggest that mutant forms of PS1 cause elevated production of Abeta42 by a mechanism that is independent of a major disruption of exocytic trafficking of APP.

Expression of Rab3A GTPase and other synaptic proteins is induced in differentiated NT2N neurons

Postmitotic NT2N cells, which are derived from human NT2 teratocarcinoma cells by treatment with retinoic acid (RA) and mitotic inhibitors, are viewed as a good in vitro model for mature neurons of the human central nervous system. Although NT2N cells exhibit many morphological and biochemical characteristics of neurons, the expression of key protein components involved in regulated exocytosis have not been firmly established. Here we show by immunoblot analysis that mature morphologically differentiated NT2N cells contain readily detectable quantities of the synaptic vesicle-associated proteins, synaptobrevin, synapsin, and synaptophysin. They also express the presynaptic plasma membrane protein, SNAP-25, and a Rab GTPase implicated in the control of Ca(2+)-dependent exocytosis, Rab3A. These proteins were not detected in untreated NT2 cells or cells exposed to RA for only 6 d. The induction of an array of proteins known to be involved in the docking and fusion of synaptic vesicles with the plasma membrane provides further support for the validity of NT2N cells as a model for human cortical neurons and suggests that these cells may be useful for in vitro molecular studies of the Ca(2+)-regulated exocytic pathway in nerve terminals.

Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells

Polarized sorting of rhodopsin in retinal rod photoreceptors is mediated by post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialized domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. To dissect the sorting machinery that regulates budding, targeting, and fusion of rhodopsin carrier vesicles, their GTP-binding protein composition has been studied using multiple means including high-resolution two-dimensional gel electrophoresis and [32P]GTP overlays of renatured proteins. These studies indicate a succession on rhodopsin-bearing vesicles of rab6, rab11, rab3 and rab8, all members of the small GTP-binding protein family of the known regulators of membrane trafficking. In this review the role of rab proteins in post-Golgi trafficking of rhodopsin is discussed.

Small GTP-binding protein, Rab6, is associated with secretory granules in atrial myocytes

Rab proteins, a subfamily of small GTP-binding proteins, have been shown to play key roles in regulation of vesicular traffic in eukaryotic cells. In this study, we have intended to identify, the atrial granule-associated Rab proteins that seem to be required for formation or intracellular transport of the granules. Atrial granules contained at least four small GTP-binding proteins, and we have demonstrated by biochemical analysis that one of the small GTP-binding proteins associated with the atrial granules is a Rab6 protein (Rab6p). Rab6p was also detected in highly purified zymogen granules of pancreatic exocrine gland. Immunogold electron microscopy performed on ultrathin cryosections of rat auricle revealed that Rab6p was associated with the atrial granule membranes. Association of Rab6p with the atrial granule membranes was also confirmed by immunodiffusion electron microscopy in agarose-embedded atrial granules. These data indicate that Rab6p is associated with the atrial granules and that it might function in the intracellular traffic of the secretory granules in the atrial myocytes.

The biology of Alzheimer's disease

The pathological hallmarks of Alzheimer's disease (AD) are amyloid angiopathy (AA), neutritic plaques (NP), and neurofibrillary tangles (NFT). This article will provide an update on Alzheimer's disease as well as discuss the key elements of a proposed multi-step pathogenic pathway. In an attempt to simplify this complex process, the focus will be on the production of NP/AA and NFT and the mechanisms of disease underlying their formation. In particular, this review will explore the possibility that AD is in part an inflammatory or immunological process, the potential role of oxidative DNA damage from oxygen free radical metabolites, and/or the putative role of excitotoxicity or ischemic neurological injury. Several genes have been identified as causative of AD and the evidence supports multiple mechanisms of disease. Alzheimer's disease may represent a final common pathway of different disease processes.