Inhibition of LRRK2 or Casein Kinase 1 Results in LRRK2 Protein Destabilization

Mutations and variations in the leucine-rich repeat kinase 2 (LRRK2) gene are strongly associated with an increased risk to develop Parkinson's disease (PD). Most pathogenic LRRK2 mutations display increased kinase activity, which is believed to underlie LRRK2-mediated toxicity. Therefore, major efforts have been invested in the development of potent and selective LRRK2 kinase inhibitors. Several of these compounds have proven beneficial in cells and in vivo, even in a LRRK2 wild-type background. Therefore, LRRK2 kinase inhibition holds great promise as disease-modifying PD therapy, and is currently tested in preclinical and early clinical studies. One of the safety concerns is the development of lung pathology in mice and non-human primates, which is most likely related to the strongly reduced LRRK2 protein levels after LRRK2 kinase inhibition. In this study, we aimed to better understand the molecular consequences of chronic LRRK2 kinase inhibition, which may be pivotal in the further development of a LRRK2 kinase inhibitor-based PD therapy. We found that LRRK2 protein levels are not restored during long-term LRRK2 kinase inhibition, but are recovered upon inhibitor withdrawal. Interestingly, LRRK2 kinase inhibitor-induced destabilization does not occur in all pathogenic LRRK2 variants and the N-terminal part of LRRK2 appears to play a crucial role in this process. In addition, we identified CK1, an upstream kinase of LRRK2, as a regulator of LRRK2 protein stability in cell culture and in vivo. We propose that pharmacological LRRK2 kinase inhibition triggers a cascade that results in reduced CK1-mediated phosphorylation of yet unidentified LRRK2 phosphorylation sites. This process involves the N-terminus of LRRK2 and ultimately leads to LRRK2 protein degradation.

ɑ-Synuclein strains and seeding in Parkinson's disease, incidental Lewy body disease, dementia with Lewy bodies and multiple system atrophy: similarities and differences

Several age-related neurodegenerative disorders are characterized by the deposition of aberrantly folded endogenous proteins. These proteins have prion-like propagation and amplification properties but so far appear nontransmissible between individuals. Because of the features they share with the prion protein, PrP, the characteristics of pathogenic protein aggregates in several progressive brain disorders, including different types of Lewy body diseases (LBDs), such as Parkinson's disease (PD), multiple system atrophy (MSA) and dementia with Lewy bodies (DLB), have been actively investigated. Even though the pleomorphic nature of these syndromes might suggest different underlying causes, ɑ-synuclein (ɑSyn) appears to play an important role in this heterogeneous group of diseases (the synucleinopathies). An attractive hypothesis is that different types of ɑSyn protein assemblies have a unique and causative role in distinct synucleinopathies. We will discuss the recent research progress on ɑSyn assemblies involved in PD, MSA and DLB; their behavior as strains; current spreading hypotheses; their ability to seed centrally and peripherally; and their implication for disease pathogenesis.

ATP13A2/PARK9 regulates endo-/lysosomal cargo sorting and proteostasis through a novel PI(3, 5)P2-mediated scaffolding function

ATP13A2 (also called PARK9), is a transmembrane endo-/lysosomal-associated P5 type transport ATPase. Loss-of-function mutations in ATP13A2 result in the Kufor-Rakeb Syndrome (KRS), a form of autosomal Parkinson's disease (PD). In spite of a growing interest in ATP13A2, very little is known about its physiological role in stressed cells. Recent studies suggest that the N-terminal domain of ATP13A2 may hold key regulatory functions, but their nature remains incompletely understood. To this end, we generated a set of melanoma and neuroblastoma cell lines stably overexpressing wild-type (WT), catalytically inactive (D508N) and N-terminal mutants, or shRNA against ATP13A2. We found that under proteotoxic stress conditions, evoked by the proteasome inhibitor Bortezomib, endo-/lysosomal associated full-length ATP13A2 WT, catalytically-inactive or N-terminal fragment mutants, reduced the intracellular accumulation of ubiquitin-conjugated (Ub) proteins, independent of autophagic degradation. In contrast, ATP13A2 silencing increased the intracellular accumulation of Ub-proteins, a pattern also observed in patient-derived fibroblasts harbouring ATP13A2 loss-of function mutations. In treated cells, ATP13A2 evoked endocytic vesicle relocation and increased cargo export through nanovesicles. Expression of an ATP13A2 mutant abrogating PI(3,5)P2 binding or chemical inhibition of the PI(3,5)P2-generating enzyme PIKfyve, compromised vesicular trafficking/nanovesicles export and rescued intracellular accumulation of Ub-proteins in response to proteasomal inhibition. Hence, our study unravels a novel activity-independent scaffolding role of ATP13A2 in trafficking/export of intracellular cargo in response to proteotoxic stress.

Pharmacological LRRK2 kinase inhibition induces LRRK2 protein destabilization and proteasomal degradation

Leucine-rich repeat kinase 2 (LRRK2) kinase activity is increased in several pathogenic mutations, including the most common mutation, G2019S, and is known to play a role in Parkinson’s disease (PD) pathobiology. This has stimulated the development of potent, selective LRRK2 kinase inhibitors as one of the most prevailing disease-modifying therapeutic PD strategies. Although several lines of evidence support beneficial effects of LRRK2 kinase inhibitors, many questions need to be answered before clinical applications can be envisaged. Using six different LRRK2 kinase inhibitors, we show that LRRK2 kinase inhibition induces LRRK2 dephosphorylation and can reduce LRRK2 protein levels of overexpressed wild type and G2019S, but not A2016T or K1906M, LRRK2 as well as endogenous LRRK2 in mouse brain, lung and kidney. The inhibitor-induced reduction in LRRK2 levels could be reversed by proteasomal inhibition, but not by lysosomal inhibition, while mRNA levels remained unaffected. In addition, using LRRK2 S910A and S935A phosphorylation mutants, we show that dephosphorylation of these sites is not required for LRRK2 degradation. Increasing our insight in the molecular and cellular consequences of LRRK2 kinase inhibition will be crucial in the further development of LRRK2-based PD therapies.

Depletion of RIPK3 or MLKL blocks TNF-driven necroptosis and switches towards a delayed RIPK1 kinase-dependent apoptosis

In human cells, the RIPK1–RIPK3–MLKL–PGAM5–Drp1 axis drives tumor necrosis factor (TNF)-induced necroptosis through mitochondrial fission, but whether this pathway is conserved among mammals is not known. To answer this question, we analyzed the presence and functionality of the reported necroptotic axis in mice. As in humans, knockdown of receptor-interacting kinase-3 (RIPK3) or mixed lineage kinase domain like (MLKL) blocks TNF-induced necroptosis in L929 fibrosarcoma cells. However, repression of either of these proteins did not protect the cells from death, but instead induced a switch from TNF-induced necroptosis to receptor-interacting kinase-1 (RIPK1) kinase-dependent apoptosis. In addition, although mitochondrial fission also occurs during TNF-induced necroptosis in L929 cells, we found that knockdown of phosphoglycerate mutase 5 (PGAM5) and dynamin 1 like protein (Drp1) did not markedly protect the cells from TNF-induced necroptosis. Depletion of Pink1, a reported interactor of both PGAM5 and Drp1, did not affect TNF-induced necroptosis. These results indicate that in these murine cells mitochondrial fission and Pink1 dependent processes, including Pink-Parkin dependent mitophagy, apparently do not promote necroptosis. Our data demonstrate that the core components of the necrosome (RIPK1, RIPK3 and MLKL) are crucial to induce TNF-dependent necroptosis both in human and in mouse cells, but the associated mechanisms may differ between the two species or cell types.

Melatonin prevents dopaminergic cell loss induced by lentiviral vectors expressing A30P mutant alpha-synuclein

Two hallmarks of Parkinson's disease (PD) are dopaminergic cell loss and the presence of cytoplasmic inclusions (Lewy bodies). Different point mutations in alpha-synuclein, the main constituent of Lewy bodies, have been identified in familial PD. Alpha-synuclein also constitutes one of the main components of Lewy bodies in sporadic cases of PD. Moreover, oxidant stress and generation of free radicals from both mitochondrial impairment and dopamine metabolism are considered to play critical roles in PD etiopathogenesis. Melatonin, a known potent antioxidant secreted by the pineal gland, may protect against the effect of several Parkinsonogenic compounds that are associated with progressive impairment of mitochondrial function and increased oxidative damage. However, the neuroprotective effect of melatonin has never been tested in the newly available genetic models of PD based on the viral expression of mutated alpha-synuclein. Lentiviral vectors encoding A30P mutant human alpha-synuclein (lenti-A30P) were stereotactically injected into the right substantia nigra of adult male Sprague-Dawley rats and neuroprotection was examined by administration of melatonin or vehicle from two days before nigral administration of lenti-A30P until eight weeks after injection. It was found that lenti-A30P induced a significant TH⁺ cell-loss both in the medial and lateral substantia nigra versus the contrallateral side injected with lenti-eGFP. However, melatonin administration showed a total neuroprotective effect in both regions of the substantia nigra. In conclusion, the data here show that melatonin is neuroprotective against mutant alpha-synuclein-induced injury in the substantia nigra.

Comparison of motor performance, brain biochemistry and histology of two A30P α-synuclein transgenic mouse strains

Three point mutations in the SNCA gene encoding α-synuclein (aSyn) have been associated with autosomal dominant forms of Parkinson's disease. To better understand the role of the A30P mutant aSyn, we compared two transgenic mouse strains: a knock-in mouse with an introduced A30P point mutation in the wild-type (WT) gene (Snca(tm(A30P))) and a transgenic (Tg) mouse overexpressing the human A30P aSyn gene under the prion promoter [tg(Prnp-SNCA A30P)]. The brain aSyn load, motor performance, brain dopamine (DA) and sensitivity to 6-hydroxydopamine (6-OHDA) were studied in these mice. aSyn was evidently accumulating with age in all mice, particularly in tg(Prnp-SNCA A30P) Tg mice. There were no robust changes in basal locomotor activities of the mice of either line at 6 months, but after 1 year, tg(Prnp-SNCA A30P) Tg mice developed severe problems with vertical movements. However, the younger Tg mice had a reduced locomotor response to 1mg/kg of d-amphetamine. Snca(tm(A30P)) mice with the targeted mutation (Tm) were slightly hyperactive at all ages. Less 6-OHDA was required in tg(Prnp-SNCA A30P) Tg (1 μg) than in WT (3μg) mice for an ipsilateral rotational bias by d-amphetamine. That was not seen with the Snca(tm(A30P)) strain. A small dose of 6-OHDA (0.33 μg) led to contralateral rotations and elevated striatal DA in Tg/Tm mice of both lines but otherwise 6-OHDA-induced striatal DA depletion was similar in all mice, indicating no A30P-aSyn-related toxin sensitivity. 3,4-Dihydroxyphenylacetic acid/DA-ratio was elevated in tg(Prnp-SNCA A30P) mice, suggesting an enhanced DA turnover. This ratio and homovanillic acid/DA-ratio were declined in Snca(tm(A30P)) mice. Our results demonstrate that the two differently constructed A30P-aSyn mouse strains have distinct behavioral and biochemical characteristics, some of which are opposite. Since the two lines with the same background were not identically produced, the deviations found may be partially caused by factors other than aSyn-related genetic differences.

A prolyl oligopeptidase inhibitor, KYP-2047, reduces α-synuclein protein levels and aggregates in cellular and animal models of Parkinson's disease

BACKGROUND AND PURPOSE

The aggregation of α-synuclein is connected to the pathology of Parkinson's disease and prolyl oligopeptidase (PREP) accelerates the aggregation of α-synuclein in vitro. The aim of this study was to investigate the effects of a PREP inhibitor, KYP-2047, on α-synuclein aggregation in cell lines overexpressing wild-type or A30P/A53T mutant human α-syn and in the brains of two A30P α-synuclein transgenic mouse strains.

EXPERIMENTAL APPROACH

Cells were exposed to oxidative stress and then incubated with the PREP inhibitor during or after the stress. Wild-type or transgenic mice were treated for 5 days with KYP-2047 (2 × 3 mg·kg(-1) a day). Besides immunohistochemistry and thioflavin S staining, soluble and insoluble α-synuclein protein levels were measured by Western blot. α-synuclein mRNA levels were quantified by PCR. The colocalization of PREP and α-synuclein,and the effect of KYP-2047 on cell viability were also investigated.

KEY RESULTS

In cell lines, oxidative stress induced a robust aggregation of α-synuclein,and low concentrations of KYP-2047 significantly reduced the number of cells with α-synuclein inclusions while abolishing the colocalization of α-synuclein and PREP. KYP-2047 significantly reduced the amount of aggregated α-synuclein,and it had beneficial effects on cell viability. In the transgenic mice, a 5-day treatment with the PREP inhibitor reduced the amount of α-synuclein immunoreactivity and soluble α-synuclein protein in the brain.

CONCLUSIONS AND IMPLICATIONS

The results suggest that the PREP may play a role in brain accumulation and aggregation of α-synuclein, while KYP-2047 seems to effectively prevent these processes.

A mathematical model of adult subventricular neurogenesis

Neurogenesis has been the subject of active research in recent years and many authors have explored the phenomenology of the process, its regulation and its purported purpose. Recent developments in bioluminescent imaging (BLI) allow direct in vivo imaging of neurogenesis, and in order to interpret the experimental results, mathematical models are necessary. This study proposes such a mathematical model that describes adult mammalian neurogenesis occurring in the subventricular zone and the subsequent migration of cells through the rostral migratory stream to the olfactory bulb (OB). This model assumes that a single chemoattractant is responsible for cell migration, secreted both by the OB and in an endocrine fashion by the cells involved in neurogenesis. The solutions to the system of partial differential equations are compared with the physiological rodent process, as previously documented in the literature and quantified through the use of BLI, and a parameter space is described, the corresponding solution to which matches that of the rodent model. A sensitivity analysis shows that this parameter space is stable to perturbation and furthermore that the system as a whole is sloppy. A large number of parameter sets are stochastically generated, and it is found that parameter spaces corresponding to physiologically plausible solutions generally obey constraints similar to the conditions reported in vivo. This further corroborates the model and its underlying assumptions based on the current understanding of the investigated phenomenon. Concomitantly, this leaves room for further quantitative predictions pertinent to the design of future proposed experiments.

Evaluation of the specificity and sensitivity of ferritin as an MRI reporter gene in the mouse brain using lentiviral and adeno-associated viral vectors

The development of in vivo imaging protocols to reliably track transplanted cells or to report on gene expression is critical for treatment monitoring in (pre)clinical cell and gene therapy protocols. Therefore, we evaluated the potential of lentiviral vectors (LVs) and adeno-associated viral vectors (AAVs) to express the magnetic resonance imaging (MRI) reporter gene ferritin in the rodent brain. First, we compared the induction of background MRI contrast for both vector systems in immune-deficient and immune-competent mice. LV injection resulted in hypointense (that is, dark) changes of T(2)/T(2)(*) (spin-spin relaxation time)-weighted MRI contrast at the injection site, which can be partially explained by an inflammatory response against the vector injection. In contrast to LVs, AAV injection resulted in reduced background contrast. Moreover, AAV-mediated ferritin overexpression resulted in significantly enhanced contrast to background on T(2)(*)-weighted MRI. Although sensitivity associated with the ferritin reporter remains modest, AAVs seem to be the most promising vector system for in vivo MRI reporter gene imaging.

Magnetic resonance imaging and spectroscopy methods for molecular imaging

Magnetic resonance imaging (MRI), one of the most powerful imaging modalities available for clinical diagnosis, has contributed significantly to phenotyping of transgenic organisms and to cellular imaging and is now gaining importance in the field of molecular imaging. Its advantage is the ability to provide in vivo information with high resolution and good soft tissue contrast as compared to established other molecular imaging methods. MRI can non-invasively report on cell localisation and migration with detailed anatomical background information, which is of great interest in cellular therapies. Recent technological advances and contrast generation strategies aim to bring MRI beyond cellular imaging to the detection of functional changes in vivo. MR based monitoring of molecular processes, requires the development of contrast agents and targeting methods as well as improvements in the methods sensitivity. Here, an overview is provided on advanced MR technologies and contrast generation strategies for this purpose. This includes MRI and MR spectroscopic methods for molecular imaging and various approaches for targeted and responsive contrast generation to visualize functional changes of particular cells. A description of different methods is provided, as well as the potentials and challenges of MR techniques for the visualization of molecular processes in vivo.

In vivo bioluminescence imaging in an experimental mouse model for dendritic cell based immunotherapy against malignant glioma

The value of bioluminescence imaging (BLI) for experimental cancer models has become firmly established. We applied BLI to the GL261 glioma model in the context of dendritic cell (DC) immunotherapy. Initial validation revealed robust linear correlations between in vivo, ex vivo and in vitro luciferase activity measurements. Ex vivo BLI demonstrated midline crossing and leakage of tumor cells. Orthotopically challenged mice followed with BLI showed an initial adaptation phase, after which imaging data correlated linearly with stereologically determined tumor dimensions. Transition from healthy to moribund state corresponded with an increasing in vivo flux but the onset of neurological deficit was clearly delayed compared to the onset of in vivo flux increase. BLI was implemented in prophylactic immunotherapy and imaging data were prognostic for therapy outcome. Three distinct response patterns were detected. Our data underscore the feasibility of in vivo BLI in an experimental immunotherapeutic setting in the GL261 glioma model.

Alpha-synuclein A30P point-mutation generates age-dependent nigrostriatal deficiency in mice

Lewy bodies are mainly composed of alpha-synuclein (SNCA) and specific mutations in SNCA gene are related to familial forms of Parkinson's disease (PD). The purpose of our study was to generate a mouse line with A30P knock-in point mutation in SNCA gene and to test if a single point-mutation is able to turn otherwise normal SNCA into a toxic form. The behavioral profile of SNCA A30P mice was followed for 16 months. Generally, these mice are healthy and viable without any obvious abnormalities. Starting from the age of 13 months mice developed a significant deficit in motor performance tests related to nigrostriatal function (ink-test and beam walk). In other tests (motility boxes, rotarod) mice continuously performed normally. Moreover, SNCA A30P mice expressed the altered sensitivity to VMAT2 inhibitor reserpine, possibly reflecting a functional deficiency of dopamine. Indeed, mice at 15 months of age had significantly reduced levels of dopamine and its major metabolite DOPAC in the striatum, and reduced levels of dopamine in the mesolimbic system. The present study confirms that SNCA plays an important role in the development of PD and an insertion of a single point mutation is sufficient to generate age-related decline in specific motor performance. The generated mouse line has a potential to become a model for PD with comparable time course and phenotype.

Optimized lentiviral vector production and purification procedure prevents immune response after transduction of mouse brain

HIV-derived lentiviral vectors are efficient vehicula to deliver genes into the brain and hold great promise for future gene therapy of neurodegenerative disorders. However, administration of the current vector preparations in mouse brain was found to induce a systemic immune response to vector proteins and a modest inflammation in the brain. Moreover, serum antibodies from vector-treated animals were capable of partially neutralizing lentiviral vector-mediated transduction in cell culture. To avoid this unexpected immune reaction, we have optimized new vector production and purification protocols. Purification by sucrose gradient ultracentrifugation abolished the immune response, but vector titers also decreased substantially. Lentiviral vector production in the absence of serum in the cell culture medium equally reduced immunogenicity without affecting transduction efficiency. These results have important implications for future clinical use of lentiviral vectors, and for the use of lentiviral vectors to create animal models for neurodegenerative diseases that have an important neuroinflammatory component.

Neuroprotective gene therapy for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterised by a progressive loss of the dopaminergic neurones in the substantia nigra pars compacta. Accumulating evidence indicates that apoptosis contributes to neuronal cell death in PD patients' brain. Excitotoxicity, oxidative stress, and mitochondrial respiratory failure are thought to be the key inducers of the apoptotic cascade. Even though the initial cause and the mechanism of degeneration are poorly understood, neuroprotection can be achieved by interfering with neuronal cell death either directly or by preventing neuronal dysfunction. Potential agents for neuroprotection are neurotrophic factors, inhibitors of apoptosis or anti-oxidative agents. However, the existence of the blood-brain barrier precludes systemic delivery of these factors. In situ gene delivery provides strategies for local and sustained administration of protective factors at physiologically relevant doses. Viral vectors mediating stable gene expression in the central nervous system exist and are still under development. Efficacy of these vectors has repeatedly been demonstrated in the animal models both ex vivo and in vivo. Ex vivo gene delivery could furthermore be combined with cell replacement therapies by transplanting genetically modified cells compensating for the lost neuronal cell population in order to provide neuroprotection to both the grafted cells and degenerating host neurones. However, several aspects of gene transfer, such as uncontrolled diffusion, axonal transport, unpredictable site of integration and immunological responses, still raise safety concerns and justify further development of viral and non-viral vectors as well as genetic elements with tightly controlled gene expression. Various relevant animal models for Parkinson's disease are available for the evaluation of gene therapy strategies. These include induction of cell death in specific neurone population through administration of toxins either directly in the brain or systemically, as well as transgenic mice expressing human disease-associated mutations.

Mice with a homozygous gene trap vector insertion in mgcRacGAP die during pre-implantation development

In a phenotypic screen in mice using a gene trap approach in embryonic stem cells, we have identified a recessive loss-of-function mutation in the mgcRacGAP gene. Maternal protein is present in the oocyte, and mgcRacGAP gene transcription starts at the four-cell stage and persists throughout mouse pre-implantation development. Total mgcRacGAP deficiency results in pre-implantation lethality. Such E3.5 embryos display a dramatic reduction in cell number, but undergo compaction and form a blastocoel. At E3.0-3.5, binucleated blastomeres in which the nuclei are partially interconnected are frequently observed, suggesting that mgcRacGAP is required for normal mitosis and cytokinesis in the pre-implantation embryo. All homozygous mutant blastocysts fail to grow out on fibronectin-coated substrates, but a fraction of them can still induce decidual swelling in vivo. The mgcRacGAP mRNA expression pattern in post-implantation embryos and adult mouse brain suggests a role in neuronal cells. Our results indicate that mgcRacGAP is essential for the earliest stages of mouse embryogenesis, and add evidence that CYK-4-like proteins also play a role in microtubule-dependent steps in the cytokinesis of vertebrate cells. In addition, the severe phenotype of null embryos indicates that mgcRacGAP is functionally non-redundant and cannot be substituted by other GAPs during early cleavage of the mammalian embryo.

DNA-Dependent protein kinase is not required for efficient lentivirus integration

How DNA is repaired after retrovirus integration is not well understood. DNA-dependent protein kinase (DNA-PK) is known to play a central role in the repair of double-stranded DNA breaks. Recently, a role for DNA-PK in retroviral DNA integration has been proposed (R. Daniel, R. A. Katz, and A. M. Skalka, Science 284:644-647, 1999). Reduced transduction efficiency and increased cell death by apoptosis were observed upon retrovirus infection of cultured scid cells. We have used a human immunodeficiency virus (HIV) type 1 (HIV-1)-derived lentivirus vector system to further investigate the role of DNA-PK during integration. We measured lentivirus transduction of scid mouse embryonic fibroblasts (MEF) and xrs-5 or xrs-6 cells. These cells are deficient in the catalytic subunit of DNA-PK and in Ku, the DNA-binding subunit of DNA-PK, respectively. At low vector titers, efficient and stable lentivirus transduction was obtained, excluding an essential role for DNA-PK in lentivirus integration. Likewise, the efficiency of transduction of HIV-derived vectors in scid mouse brain was as efficient as that in control mice, without evidence of apoptosis. We observed increased cell death in scid MEF and xrs-5 or xrs-6 cells, but only after transduction with high vector titers (multiplicity of infection [MOI], >1 transducing unit [TU]/cell) and subsequent passage of the transduced cells. At an MOI of <1 TU/cell, however, transduction efficiency was even higher in DNA-PK-deficient cells than in control cells. Taken together, the data suggest a protective role of DNA-PK against cellular toxicity induced by high levels of retrovirus integrase or integration. Another candidate cellular enzyme that has been claimed to play an important role during retrovirus integration is poly(ADP-ribose) polymerase (PARP). However, no inhibition of lentivirus vector-mediated transduction or HIV-1 replication by 3-methoxybenzamide, a known PARP inhibitor, was observed. In conclusion, DNA-PK and PARP are not essential for lentivirus integration.

Gene therapeutic strategies for neurodegenerative diseases

Gene transfer into the central nervous system by ex vivo or in vivo techniques is a rapidly emerging field in neuroscience. Potential applications of gene therapy for the nervous system include not only congenital single gene disorders, but also brain tumors and acquired chronic diseases. Considerable progress has been made in the understanding of neurodegenerative diseases such as Parkinson's and Alzheimer's disease. As a result, gene therapy for Parkinson's, and possibly Alzheimer's disease could be regarded as a realistic alternative to the limited treatment options currently available. In this review, we highlight the most important developments in gene transfer techniques as well as the newest insights in the mechanisms of some neurodegenerative disorders and put these into the perspective of gene therapeutic strategies for the central nervous system.

Presenilin 2 deficiency causes a mild pulmonary phenotype and no changes in amyloid precursor protein processing but enhances the embryonic lethal phenotype of presenilin 1 deficiency

Mutations in the homologous presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of familial Alzheimer's disease. Although PS1 function and dysfunction have been extensively studied, little is known about the function of PS2 in vivo. To delineate the relationships of PS2 and PS1 activities and whether PS2 mutations involve gain or loss of function, we generated PS2 homozygous deficient (-/-) and PS1/PS2 double homozygous deficient mice. In contrast to PS1(-/-) mice, PS2(-/-) mice are viable and fertile and develop only mild pulmonary fibrosis and hemorrhage with age. Absence of PS2 does not detectably alter processing of amyloid precursor protein and has little or no effect on physiologically important apoptotic processes, indicating that Alzheimer's disease-causing mutations in PS2, as in PS1, result in gain of function. Although PS1(+/-) PS2( -/-) mice survive in relatively good health, complete deletion of both PS2 and PS1 genes causes a phenotype closely resembling full Notch-1 deficiency. These results demonstrate in vivo that PS1 and PS2 have partially overlapping functions and that PS1 is essential and PS2 is redundant for normal Notch signaling during mammalian embryological development.

Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain

Transgenic mice overexpressing different forms of amyloid precursor protein (APP), i.e. wild type or clinical mutants, displayed an essentially comparable early phenotype in terms of behavior, differential glutamatergic responses, deficits in maintenance of long term potentiation, and premature death. The cognitive impairment, demonstrated in F1 hybrids of the different APP transgenic lines, was significantly different from nontransgenic littermates as early as 3 months of age. Biochemical analysis of secreted and membrane-bound APP, C-terminal "stubs," and Abeta(40) and Abeta(42) peptides in brain indicated that no single intermediate can be responsible for the complex of phenotypic dysfunctions. As expected, the Abeta(42) levels were most prominent in APP/London transgenic mice and correlated directly with the formation of amyloid plaques in older mice of this line. Plaques were associated with immunoreactivity for hyperphosphorylated tau, eventually signaling some form of tau pathology. In conclusion, the different APP transgenic mouse lines studied display cognitive deficits and phenotypic traits early in life that dissociated in time from the formation of amyloid plaques and will be good models for both early and late neuropathological and clinical aspects of Alzheimer's disease.

Transgenic mice expressing an alpha-secretion mutant of the amyloid precursor protein in the brain develop a progressive CNS disorder

Expression of alpha-secretion mutant APP/RK in mouse brain results in a progressive disorganization of the central nervous system, exemplified by behavioral deficits, premature death and neuropathology. Here we report on the progressive nature of this CNS disorder as indicated by the age dependency of the neophobic reaction in the open-field test. The earlier reported NMDA hypo-sensitivity in the transgenic APP/RK mice is likely to represent a subtle functional disturbance, since no changes in NMDA receptor density or distribution could be detected. None of the typical neuropathological hallmarks of Alzheimer's Disease, i.e. amyloid deposits and neurofibrillary tangles are detected in the brain of these transgenic mice. Nevertheless, the progressive CNS disorder elicited in the transgenic APP/RK mice recapitulates certain features and symptoms of patients with Alzheimer's disease as discussed.

Identification of two proteins that bind to a pyrimidine-rich sequence in the 3'-untranslated region of GAP-43 mRNA

GAP-43 is a membrane phosphoprotein that is important for the development and plasticity of neural connections. In undifferentiated PC12 pheochromocytoma cells, GAP-43 mRNA degrades rapidly ( t = 5 h), but becomes stable when cells are treated with nerve growth factor. To identify trans- acting factors that may influence mRNA stability, we combined column chromatography and gel mobility shift assays to isolate GAP-43 mRNA binding proteins from neonatal bovine brain tissue. This resulted in the isolation of two proteins that bind specifically and competitively to a pyrimidine-rich sequence in the 3'-untranslated region of GAP-43 mRNA. Partial amino acid sequencing revealed that one of the RNA binding proteins coincides with FBP (far upstream element binding protein), previously characterized as a protein that resembles hnRNP K and which binds to a single-stranded, pyrimidine-rich DNA sequence upstream of the c -myc gene to activate its expression. The other binding protein shares sequence homology with PTB, a polypyrimidine tract binding protein implicated in RNA splicing and regulation of translation initiation. The two proteins bind to a 26 nt pyrimidine-rich sequence lying 300 nt downstream of the end of the coding region, in an area shown by others to confer instability on a reporter mRNA in transient transfection assays. We therefore propose that FBP and the PTB-like protein may compete for binding at the same site to influence the stability of GAP-43 mRNA.

Long-term effects of retinal lesions on growth-associated protein 43 (GAP-43) expression in the visual system of adult cats

We have investigated the role of growth-associated protein 43 (GAP-43) in synaptic reorganization in the visual system of adult cats that received binocular central retinal lesions. Different survival times between 3 and 8 months after induction of the lesion were chosen. In the deafferented part of the dorsal lateral geniculate nucleus (dLGN) we found a long-lasting increase in GAP-43 protein, while glial fibrillary acidic protein (GFAP) immunoreactivity, which initially increased due to the degeneration of retinal ganglion cells, slowly subsided over this period. In area 17, the pattern of GAP-43 expression did not provide indications for morphological changes in the cortical architecture following retinal lesions.

Alterations in GAP-43 and synapsin immunoreactivity provide evidence for synaptic reorganization in adult cat dorsal lateral geniculate nucleus following retinal lesions

Growth-associated protein-43 (GAP-43) and synapsin were used as molecular markers for synaptic reorganization in the adult cat visual system following sensory deprivation. Small binocular retinal lesions (central 10 degrees) were made with a xenon light photocoagulator in adult cats. One, 3, 5 and 7 weeks after induction of the lesion, the neuropil levels of synapsin and GAP-43 in the dorsal lateral geniculate nucleus (dLGN) and area 17 were determined by immunocytochemistry. GAP-43 displayed a moderately low basal level in the dLGN of normal adult cats. The parvocellular C layers and the interlaminar plexi were characterized by higher immunoreactivity for GAP-43. Lesion-induced alterations were observed in all layers: GAP-43 immunoreactivity increased in the part of the dLGN representing central vision. This increase was maximal 3 weeks after the lesion. Under our experimental conditions, sensory deprivation did not significantly alter GAP-43 levels in the visual cortex. The changes in synapsin immunoreactivity were also restricted to the dLGN. In this nucleus, synapsin immunoreactivity decreased in all layers in the part subserving central vision 1 week after lesion. By 3 weeks after lesion, the level of synapsin had already returned to normal. This study provides evidence for a capacity for structural remodelling in primary sensory brain areas such as the dLGN throughout adult life. The observed changes in GAP-43 and synapsin in the dLGN suggest that synaptic reorganization is induced by retinal lesions. Normalization of synaptic density and activity could be important for the survival of the partially deafferented geniculate neurons.

Kinetics of different human immunodeficiency virus type 1 reverse transcriptases resistant to human immunodeficiency virus type 1-specific reverse transcriptase inhibitors

The human immunodeficiency virus type 1 (HIV-1)-specific reverse transcriptase (RT) inhibitors [tetrahydroimidazo[4,5,1-jk] [1,4]benzodiazepin-2(1H)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine, nevirapine, pyridinone, bis(heteroaryl)piperazine, etc.] are potent inhibitors of HIV-1 replication in cell culture. The rapid emergence of drug-resistant escape mutants in vitro (cell culture) and in vivo (patients) is predominantly linked to the Y181C mutation. Because amino acids Y181 and Y188 appear to be located within the drug binding site of the enzyme, we studied the impact of mutations of both amino acids on the enzyme kinetics and on the susceptibility of the enzyme to different HIV-1-specific RT inhibitors. Mutations Y181C, Y181I, and Y188L led to reduced sensitivity, albeit of varying extents, to all HIV-1-specific RT inhibitors. No resistance was observed to 2',3'-dideoxyguanosine 5'-triphosphate or phosphonoformic acid. The kcat of the Y181C mutant was similar to that of the wild-type RT (18 sec-1 x 10(-3)). The catalytic constant of the Y181I mutant was 6-fold higher and that of the Y188L mutant was 6-fold lower. Whereas TIBO displayed a linear mixed-type (noncompetitive) inhibition with respect to the deoxynucleotide substrate when wild-type p66/p51 was used, the pattern of inhibition became competitive or uncompetitive with Y181C or Y181I, respectively. Thus, the TIBO binding site of HIV-1 RT seems to be functionally and/or spatially related to the natural deoxynucleoside triphosphate binding site.