Radiation Therapy in Alzheimer's Disease: A Systematic Review

PURPOSE

Pathophysiological hallmarks of Alzheimer's disease (AD) include extracellular amyloid plaques and intracellular neurofibrillary tangles. Recent studies also demonstrated a role of neuroinflammation in the progression of the disease. Clinical trials and animal studies using low-dose radiation therapy (LDRT) have shown therapeutic potential for AD. This systematic review summarizes the current evidence on the use of LDRT for the treatment of AD, outlines potential mechanisms of action, and discusses current challenges in the planning of future trials.

METHODS AND MATERIALS

A systematic review of human and animal studies as well as registered clinical trials describing outcomes for RT in the treatment of AD was conducted. We followed the 2020 Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Articles published until July 1, 2023, were included.

RESULTS

The initial search yielded 993 articles. After the removal of duplicates and ineligible publications, a total of 16 (12 animal, 4 human) studies were included. Various dose regimens were utilized in both animal and human trials. The results revealed that LDRT reduced the number of amyloid plaques and neurofibrillary tangles, and it has a role in the regulation of genes and protein expression involved in the pathological progression of AD. LDRT has demonstrated reduced astro- and microgliosis, anti-inflammatory and neuroprotective effects, and an alleviation of symptoms of cognitive deficits in animal models. Most studies in humans suggested improvements in cognition and behavior. None of the trials or studies described significant (>grade 2) toxicity.

CONCLUSIONS

Preclinical studies, animal studies, and early clinical trials in humans have shown a promising role for LDRT in the treatment of AD pathologies, although the underlying mechanisms are yet to be fully explored. Phase I/II/III trials are needed to assess the long-term safety, efficacy, and optimal treatment parameters of LDRT in AD treatment.

Spermidine reduces neuroinflammation and soluble amyloid beta in an Alzheimer's disease mouse model

Background

Deposition of amyloid beta (Aβ) and hyperphosphorylated tau along with glial cell-mediated neuroinflammation are prominent pathogenic hallmarks of Alzheimer’s disease (AD). In recent years, impairment of autophagy has been identified as another important feature contributing to AD progression. Therefore, the potential of the autophagy activator spermidine, a small body-endogenous polyamine often used as dietary supplement, was assessed on Aβ pathology and glial cell-mediated neuroinflammation.

Results

Oral treatment of the amyloid prone AD-like APPPS1 mice with spermidine reduced neurotoxic soluble Aβ and decreased AD-associated neuroinflammation. Mechanistically, single nuclei sequencing revealed AD-associated microglia to be the main target of spermidine. This microglia population was characterized by increased AXL levels and expression of genes implicated in cell migration and phagocytosis. A subsequent proteome analysis of isolated microglia confirmed the anti-inflammatory and cytoskeletal effects of spermidine in APPPS1 mice. In primary microglia and astrocytes, spermidine-induced autophagy subsequently affected TLR3- and TLR4-mediated inflammatory processes, phagocytosis of Aβ and motility. Interestingly, spermidine regulated the neuroinflammatory response of microglia beyond transcriptional control by interfering with the assembly of the inflammasome.

Conclusions

Our data highlight that the autophagy activator spermidine holds the potential to enhance Aβ degradation and to counteract glia-mediated neuroinflammation in AD pathology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02534-7.

Age-related dysfunction of the autophago-lysosomal pathway in human endothelial cells

Senescent cells, which are cells in a post-proliferative state, show an increased number of dysfunctional mitochondria and oxidatively damaged and aggregated proteins. The mitochondrial-lysosomal axis theory of aging proposes that the autophago-lysosomal system is unable to cope with the rising amount of damaged organelles and proteins. We used human umbilical vein endothelial cells (HUVEC) as in vitro model system to determine which part/s of the autophago-lysosomal pathway become deficient by aging. Senescent HUVEC contained a much larger population of autophagosomes and lysosomes compared to young cells. Transcriptome analysis comparing young and old cells demonstrated several age-related changes of autophagy gene expression. One reason for the observed increase of autophagosomes was an impairment of the autophagic flux in senescent cells due to reduced V-ATPase activity required for acidification of the lysosomes and thus functionality of lysosomal hydrolases. The hypothesis that reduced mitochondrial ATP production underlies low V-ATPase activity was supported by addition of exogenous ATP. This procedure rescued the lysosomal acidification and restored the autophagic flux. Thus, we propose impaired lysosomal acidification due to ATP shortage which may result from mitochondrial dysfunction as a mechanism underlying the accumulation of dysfunctional cellular constituents during aging.

Beclin1-driven autophagy modulates the inflammatory response of microglia via NLRP3

Alzheimer's disease is characterized not only by extracellular amyloid plaques and neurofibrillary tangles, but also by microglia-mediated neuroinflammation. Recently, autophagy has been linked to the regulation of the inflammatory response. Thus, we investigated how an impairment of autophagy mediated by BECN1/Beclin1 reduction, as described in Alzheimer's disease patients, would influence cytokine production of microglia. Acutely stimulated microglia from Becn1 ^+/- mice exhibited increased expression of IL-1beta and IL-18 compared to wild-type microglia. Becn1 ^+/- APPPS1 mice also contained enhanced IL-1beta levels. The investigation of the IL-1beta/IL-18 processing pathway showed an elevated number of cells with inflammasomes and increased levels of NLRP3 and cleaved CASP1/Caspase1 in Becn1 ^+/- microglia. Super-resolation microscopy revealed a very close association of NLRP3 aggregates and LC3-positive vesicles. Interestingly, CALCOCO2 colocalized with NLRP3 and its downregulation increased IL-1beta release. These data support the notion that selective autophagy can impact microglia activation by modulating IL-1beta and IL-18 production via NLRP3 degradation and thus present a mechanism how impaired autophagy could contribute to neuroinflammation in Alzheimer's disease.

CNS myeloid cells critically regulate heat hyperalgesia

Activation of non-neuronal microglia is thought to play a causal role in spinal processing of neuropathic pain. To specifically investigate microglia-mediated effects in a model of neuropathic pain and overcome the methodological limitations of previous approaches exploring microglia function upon nerve injury, we selectively ablated resident microglia by intracerebroventricular ganciclovir infusion into male CD11b-HSVTK-transgenic mice, which was followed by a rapid, complete, and persistent (23 weeks) repopulation of the CNS by peripheral myeloid cells. In repopulated mice that underwent sciatic nerve injury, we observed a normal response to mechanical stimuli, but an absence of thermal hypersensitivity ipsilateral to the injured nerve. Furthermore, we found that neuronal expression of calcitonin gene-related peptide (CGRP), which is a marker of neurons essential for heat responses, was diminished in the dorsal horn of the spinal cord in repopulated mice. These findings identify distinct mechanisms for heat and mechanical hypersensitivity and highlight a crucial contribution of CNS myeloid cells in the facilitation of noxious heat.

Ciglitazone Inhibits Plasmin-Induced Proinflammatory Monocyte Activation via Modulation of p38 MAP Kinase Activity

Plasmin triggers chemotaxis and NF-κBand AP-1-mediated proinflammatory gene expression in human peripheral monocytes (PM). Compared with macrophages and dendritic cells, PM express mainly the peroxisome proliferator-activated receptor (PPAR) γ and traces of PPARα as detected by semiquantitative RT-PCR and immunoblotting. The PPARγ agonist ciglitazone, but not the PPARα agonist clofibric acid, concentration-dependently inhibited the plasmin-, but not the FMLP-induced PM chemotaxis. Similarly, release of interleukin (IL)-1α, IL-1β and tumor necrosis factor (TNF)-α from plasmin-stimulated PM was concentration-dependently inhibited by ciglitazone, but not by clofibric acid, while the LPS-induced TNF-α release remained unaffected by any of both PPAR agonists. Ciglitazone activates PPARγ as shown by a novel surface plasmon resonance analysis and inhibits the plasmin-induced activation of NF-κB and AP-1. It also inhibits p38 MAPK phosphorylation essential for the plasmin-induced PM chemotaxis and gene activation. Thus, activation of PPARγ by ciglitazone may allow controling of the plasmin-mediated recruitment and activation of PM at sites of inflammation.

Blood RNA biomarkers in prodromal PARK4 and rapid eye movement sleep behavior disorder show role of complexin 1 loss for risk of Parkinson's disease

Parkinson's disease (PD) is a frequent neurodegenerative process in old age. Accumulation and aggregation of the lipid-binding SNARE complex component α-synuclein (SNCA) underlies this vulnerability and defines stages of disease progression. Determinants of SNCA levels and mechanisms of SNCA neurotoxicity have been intensely investigated. In view of the physiological roles of SNCA in blood to modulate vesicle release, we studied blood samples from a new large pedigree with SNCA gene duplication (PARK4 mutation) to identify effects of SNCA gain of function as potential disease biomarkers. Downregulation of complexin 1 (CPLX1) mRNA was correlated with genotype, but the expression of other Parkinson's disease genes was not. In global RNA-seq profiling of blood from presymptomatic PARK4 indviduals, bioinformatics detected significant upregulations for platelet activation, hemostasis, lipoproteins, endocytosis, lysosome, cytokine, Toll-like receptor signaling and extracellular pathways. In PARK4 platelets, stimulus-triggered degranulation was impaired. Strong SPP1, GZMH and PLTP mRNA upregulations were validated in PARK4. When analysing individuals with rapid eye movement sleep behavior disorder, the most specific known prodromal stage of general PD, only blood CPLX1 levels were altered. Validation experiments confirmed an inverse mutual regulation of SNCA and CPLX1 mRNA levels. In the 3'-UTR of the CPLX1 gene we identified a single nucleotide polymorphism that is significantly associated with PD risk. In summary, our data define CPLX1 as a PD risk factor and provide functional insights into the role and regulation of blood SNCA levels. The new blood biomarkers of PARK4 in this Turkish family might become useful for PD prediction.

Loss of lysosome-associated membrane protein 3 (LAMP3) enhances cellular vulnerability against proteasomal inhibition

The family of lysosome-associated membrane proteins (LAMP) includes the ubiquitously expressed LAMP1 and LAMP2, which account for half of the proteins in the lysosomal membrane. Another member of the LAMP family is LAMP3, which is expressed only in certain cell types and differentiation stages. LAMP3 expression is linked with poor prognosis of certain cancers, and the locus where it is encoded was identified as a risk factor for Parkinson's disease (PD). Here, we investigated the role of LAMP3 in the two main cellular degradation pathways, the proteasome and autophagy. LAMP3 mRNA was not detected in mouse models of PD or in the brain of human patients. However, it was strongly induced upon proteasomal inhibition in the neuroblastoma cell line SH-SY5Y. Induction of LAMP3 mRNA following proteasomal inhibition was dependent on UPR transcription factor ATF4 signaling and induced autophagic flux. Prevention of LAMP3 induction enhanced apoptotic cell death. In summary, these data demonstrate that LAMP3 regulation as part of the UPR contributes to protein degradation and cell survival during proteasomal dysfunction. This link between autophagy and the proteasome may be of special importance for the treatment of tumor cells with proteasomal inhibitors.

Loss of PINK1 impairs stress-induced autophagy and cell survival

The mitochondrial kinase PINK1 and the ubiquitin ligase Parkin are participating in quality control after CCCP- or ROS-induced mitochondrial damage, and their dysfunction is associated with the development and progression of Parkinson's disease. Furthermore, PINK1 expression is also induced by starvation indicating an additional role for PINK1 in stress response. Therefore, the effects of PINK1 deficiency on the autophago-lysosomal pathway during stress were investigated. Under trophic deprivation SH-SY5Y cells with stable PINK1 knockdown showed downregulation of key autophagic genes, including Beclin, LC3 and LAMP-2. In good agreement, protein levels of LC3-II and LAMP-2 but not of LAMP-1 were reduced in different cell model systems with PINK1 knockdown or knockout after addition of different stressors. This downregulation of autophagic factors caused increased apoptosis, which could be rescued by overexpression of LC3 or PINK1. Taken together, the PINK1-mediated reduction of autophagic key factors during stress resulted in increased cell death, thus defining an additional pathway that could contribute to the progression of Parkinson's disease in patients with PINK1 mutations.

Mitochondrial acetylation and genetic models of Parkinson's disease

Parkinson's disease (PD) is frequent at old age, leading to atrophy of specific neurons and to early death. Lifespan and healthy aging of organisms depend on growth factor/nutrient signaling and on bioenergetics via mitochondria, all of which regulate downstream nuclear functions through FOXO and SIR proteins. Mammalian SIRtuins include the mitochondrial deacetylase SIRT3, and recently mitochondrial lysine acetylation (AcLys) was found to initiate mitochondrial degradation by autophagy. This mitophagy process is closely regulated by PINK1 and Parkin, two interacting proteins which relocalize to mitochondria with deficient proton gradients, and whose mutations cause autosomal recessive variants of PD. Strong generalized deacetylation of mitochondrial proteins and altered SIRT3 levels occur in rodent models of PD before the onset of toxic aggregate formation. We propose that the development of site-specific AcLys-antibodies and their characterization in patients will have medical value.

Loss of mitochondrial peptidase Clpp leads to infertility, hearing loss plus growth retardation via accumulation of CLPX, mtDNA and inflammatory factors

The caseinolytic peptidase P (CLPP) is conserved from bacteria to humans. In the mitochondrial matrix, it multimerizes and forms a macromolecular proteasome-like cylinder together with the chaperone CLPX. In spite of a known relevance for the mitochondrial unfolded protein response, its substrates and tissue-specific roles are unclear in mammals. Recessive CLPP mutations were recently observed in the human Perrault variant of ovarian failure and sensorineural hearing loss. Here, a first characterization of CLPP null mice demonstrated complete female and male infertility and auditory deficits. Disrupted spermatogenesis already at the spermatid stage and ovarian follicular differentiation failure were evident. Reduced pre-/post-natal survival and marked ubiquitous growth retardation contrasted with only light impairment of movement and respiratory activities. Interestingly, the mice showed resistance to ulcerative dermatitis. Systematic expression studies detected up-regulation of other mitochondrial chaperones, accumulation of CLPX and mtDNA as well as inflammatory factors throughout tissues. T-lymphocytes in the spleen were activated. Thus, murine Clpp deletion represents a faithful Perrault model. The disease mechanism probably involves deficient clearance of mitochondrial components and inflammatory tissue destruction.

Mitochondrion-derived reactive oxygen species lead to enhanced amyloid beta formation

AIMS

Intracellular amyloid beta (Aβ) oligomers and extracellular Aβ plaques are key players in the progression of sporadic Alzheimer's disease (AD). Still, the molecular signals triggering Aβ production are largely unclear. We asked whether mitochondrion-derived reactive oxygen species (ROS) are sufficient to increase Aβ generation and thereby initiate a vicious cycle further impairing mitochondrial function.

RESULTS

Complex I and III dysfunction was induced in a cell model using the respiratory inhibitors rotenone and antimycin, resulting in mitochondrial dysfunction and enhanced ROS levels. Both treatments lead to elevated levels of Aβ. Presence of an antioxidant rescued mitochondrial function and reduced formation of Aβ, demonstrating that the observed effects depended on ROS. Conversely, cells overproducing Aβ showed impairment of mitochondrial function such as comprised mitochondrial respiration, strongly altered morphology, and reduced intracellular mobility of mitochondria. Again, the capability of these cells to generate Aβ was partly reduced by an antioxidant, indicating that Aβ formation was also ROS dependent. Moreover, mice with a genetic defect in complex I, or AD mice treated with a complex I inhibitor, showed enhanced Aβ levels in vivo.

INNOVATION

We show for the first time that mitochondrion-derived ROS are sufficient to trigger Aβ production in vitro and in vivo.

CONCLUSION

Several lines of evidence show that mitochondrion-derived ROS result in enhanced amyloidogenic amyloid precursor protein processing, and that Aβ itself leads to mitochondrial dysfunction and increased ROS levels. We propose that starting from mitochondrial dysfunction a vicious cycle is triggered that contributes to the pathogenesis of sporadic AD.

Guidelines for the use and interpretation of assays for monitoring autophagy

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.

Primary skin fibroblasts as a model of Parkinson's disease

Parkinson's disease is the second most frequent neurodegenerative disorder. While most cases occur sporadic mutations in a growing number of genes including Parkin (PARK2) and PINK1 (PARK6) have been associated with the disease. Different animal models and cell models like patient skin fibroblasts and recombinant cell lines can be used as model systems for Parkinson's disease. Skin fibroblasts present a system with defined mutations and the cumulative cellular damage of the patients. PINK1 and Parkin genes show relevant expression levels in human fibroblasts and since both genes participate in stress response pathways, we believe fibroblasts advantageous in order to assess, e.g. the effect of stressors. Furthermore, since a bioenergetic deficit underlies early stage Parkinson's disease, while atrophy underlies later stages, the use of primary cells seems preferable over the use of tumor cell lines. The new option to use fibroblast-derived induced pluripotent stem cells redifferentiated into dopaminergic neurons is an additional benefit. However, the use of fibroblast has also some drawbacks. We have investigated PARK6 fibroblasts and they mirror closely the respiratory alterations, the expression profiles, the mitochondrial dynamics pathology and the vulnerability to proteasomal stress that has been documented in other model systems. Fibroblasts from patients with PARK2, PARK6, idiopathic Parkinson's disease, Alzheimer's disease, and spinocerebellar ataxia type 2 demonstrated a distinct and unique mRNA expression pattern of key genes in neurodegeneration. Thus, primary skin fibroblasts are a useful Parkinson's disease model, able to serve as a complement to animal mutants, transformed cell lines and patient tissues.

Autophagy proteins LC3B, ATG5 and ATG12 participate in quality control after mitochondrial damage and influence lifespan

Mitochondrial health is maintained by the quality control mechanisms of mitochondrial dynamics (fission and fusion) and mitophagy. Decline of these processes is thought to contribute to aging and neurodegenerative diseases. To investigate the role of mitochondrial quality control in aging on the cellular level, human umbilical vein endothelial cells (HUVEC) were subjected to mitochondria-targeted damage by combining staining of mitochondria and irradiation. This treatment induced a short boost of reactive oxygen species, which resulted in transient fragmentation of mitochondria followed by mitophagy, while mitochondrial dynamics were impaired. Furthermore, targeted mitochondrial damage upregulated autophagy factors LC3B, ATG5 and ATG12. Consequently these proteins were overexpressed in HUVEC as an in vitro aging model, which significantly enhanced the replicative life span up to 150% and the number of population doublings up to 200%, whereas overexpression of LAMP-1 did not alter the life span. Overexpression of LC3B, ATG5 and ATG12 resulted in an improved mitochondrial membrane potential, enhanced ATP production and generated anti-apoptotic effects, while ROS levels remained unchanged and the amount of oxidized proteins increased. Taken together, these data relate LC3B, ATG5 and ATG12 to mitochondrial quality control after oxidative damage, and to cellular longevity.

Restriction of trophic factors and nutrients induces PARKIN expression

Parkinson’s disease (PD) is the most frequent neurodegenerative movement disorder and manifests at old age. While many details of its pathogenesis remain to be elucidated, in particular the protein and mitochondrial quality control during stress responses have been implicated in monogenic PD variants. Especially the mitochondrial kinase PINK1 and the ubiquitin ligase PARKIN are known to cooperate in autophagy after mitochondrial damage. As autophagy is also induced by loss of trophic signaling and PINK1 gene expression is modulated after deprivation of cytokines, we analyzed to what extent trophic signals and starvation stress regulate PINK1 and PARKIN expression. Time course experiments with serum deprivation and nutrient starvation of human SH-SY5Y neuroblastoma cells and primary mouse neurons demonstrated phasic induction of PINK1 transcript up to twofold and PARKIN transcript levels up to sixfold. The corresponding threefold starvation induction of PARKIN protein was limited by its translocation to lysosomes. Analysis of primary mouse cells from PINK1-knockout mice indicated that PARKIN induction and lysosomal translocation occurred independent of PINK1. Suppression of the PI3K-Akt-mTOR signaling by pharmacological agents modulated PARKIN expression accordingly. In conclusion, this expression survey demonstrates that PARKIN and PINK1 are coregulated during starvation and suggest a role of both PD genes in response to trophic signals and starvation stress.

Do UCP2 and mild uncoupling improve longevity?

Mild uncoupling of mitochondrial respiration is considered to prolong life span of organisms by reducing the production of reactive oxygen species (ROS). Experimental evidence against this hypothesis has been brought forward by premature senescence in cell cultures treated with uncouplers. Exposing HUVEC to a mixture of nutritionally important fatty acids (oil extract of chicken yolk) mild uncoupling with "naturally acting substances" was performed. This treatment also resulted in premature senescence although ROS production did not increase. Fatty acids activate uncoupling proteins (UCP) in the inner mitochondrial membrane. UCP2 expression proved to be sensitive to the presence of fatty acids but remains unchanged during the ageing process. UCP3 expression in senescent HUVEC and avUCP expression in senescent CEF were considerably less than in young cultures. No indication for protonophoric reduction of mitochondrial membrane potential was found in UCP2 overexpressing HeLa cells and only little in HUVEC. ROS levels increased instead of being reduced in these cells. Stable transfection with UCP2-GFP was possible only in chick embryo fibroblasts and HeLa cells and resulted in decreased proliferation. Stable transfection of HUVEC with UCP2-GFP resulted in death of cultures within one or two weeks. The reason for this behaviour most probably is apoptosis preceded by mitochondrial fragmentation and loss of membrane potential.

Decreased expression of Drp1 and Fis1 mediates mitochondrial elongation in senescent cells and enhances resistance to oxidative stress through PINK1

Mitochondria display different morphologies, depending on cell type and physiological situation. In many senescent cell types, an extensive elongation of mitochondria occurs, implying that the increase of mitochondrial length in senescence could have a functional role. To test this hypothesis, human endothelial cells (HUVECs) were aged in vitro. Young HUVECs had tubular mitochondria, whereas senescent cells were characterized by long interconnected mitochondria. The change in mitochondrial morphology was caused by downregulation of the expression of Fis1 and Drp1, two proteins regulating mitochondrial fission. Targeted photodamage of mitochondria induced the formation of reactive oxygen species (ROS), which triggered mitochondrial fragmentation and loss of membrane potential in young cells, whereas senescent cells proved to be resistant. Alterations of the Fis1 and Drp1 expression levels also influenced the expression of the putative serine-threonine kinase PINK1, which is associated with the PARK6 variant of Parkinson's disease. Downregulation of PINK1 or overexpression of a PINK1 mutant (G309D) increased the sensitivity against ROS in young cells. These results indicate that there is a Drp1- and Fis1-induced, and PINK1-mediated protection mechanism in senescent cells, which, when compromised, could contribute to the age-related progression of Parkinson's disease and arteriosclerosis.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

H2O2-dependent translocation of TCTP into the nucleus enables its interaction with VDR in human keratinocytes: TCTP as a further module in calcitriol signalling

Translationally controlled tumour protein (TCTP) is an evolutionarily highly conserved molecule implicated in many processes related to cell cycle progression, proliferation and growth, to the protection against harmful conditions including apoptosis and to the human allergic response. We are showing here that after application of mild oxidative stress, human TCTP relocates from the cytoplasm to the nuclei of HaCaT keratinocytes where it directly associates with the ligand-binding domain of endogenous vitamin D(3) receptor (VDR) through its helical domain 2 (AA 71-132). Interestingly, the latter harbours a putative nuclear hormone receptor coregulatory LxxLL-like motif which seems to be involved in the interaction. Moreover, we demonstrate that VDR transcriptionally induces the expression of TCTP by binding to a previously unknown VDR response element within the TCTP promotor. Conversely, ectopically overexpressed TCTP downregulates the amount of VDR on both mRNA as well as protein level. These data, to conclude, suggest a kind of feedback regulation between TCTP and VDR to regulate a variety of (Ca(2+) dependent) cellular effects and in this way further underscore the physiological relevance of this novel protein-protein interaction.

Parkinson phenotype in aged PINK1-deficient mice is accompanied by progressive mitochondrial dysfunction in absence of neurodegeneration

BACKGROUND

Parkinson's disease (PD) is an adult-onset movement disorder of largely unknown etiology. We have previously shown that loss-of-function mutations of the mitochondrial protein kinase PINK1 (PTEN induced putative kinase 1) cause the recessive PARK6 variant of PD.

METHODOLOGY/PRINCIPAL FINDINGS

Now we generated a PINK1 deficient mouse and observed several novel phenotypes: A progressive reduction of weight and of locomotor activity selectively for spontaneous movements occurred at old age. As in PD, abnormal dopamine levels in the aged nigrostriatal projection accompanied the reduced movements. Possibly in line with the PARK6 syndrome but in contrast to sporadic PD, a reduced lifespan, dysfunction of brainstem and sympathetic nerves, visible aggregates of alpha-synuclein within Lewy bodies or nigrostriatal neurodegeneration were not present in aged PINK1-deficient mice. However, we demonstrate PINK1 mutant mice to exhibit a progressive reduction in mitochondrial preprotein import correlating with defects of core mitochondrial functions like ATP-generation and respiration. In contrast to the strong effect of PINK1 on mitochondrial dynamics in Drosophila melanogaster and in spite of reduced expression of fission factor Mtp18, we show reduced fission and increased aggregation of mitochondria only under stress in PINK1-deficient mouse neurons.

CONCLUSION

Thus, aging Pink1(-/-) mice show increasing mitochondrial dysfunction resulting in impaired neural activity similar to PD, in absence of overt neuronal death.

The C-terminal domain of TRPV4 is essential for plasma membrane localization

Many members of the TRP superfamily oligomerize in the ER before trafficking to the plasma membrane. For membrane localization of the non-selective cation channel TRPV4 specific domains in the N-terminus are required, but the role of the C-terminus in the oligomerization and trafficking process has been not determined until now. Therefore, the localization of recombinant TRPV4 in two cell models was analyzed: HaCaT keratinocytes that express TRPV4 endogenously were compared to CHO cells that are devoid of endogenous TRPV4. When deletions were introduced in the C-terminal domain three states of TRPV4 localization were defined: a truncated TRPV4 protein of 855 amino acids was exported to the plasma membrane like the full-length channel (871 aa) and was also functional. Mutants with a length of 828 to 844 amino acids remained in the ER of CHO cells, but in HaCaT cells plasma membrane localization was partially rescued by oligomerization with endogenous TRPV4. This was confirmed by coexpression of recombinant full-length TRPV4 together with these deletion mutants, which resulted in an almost complete plasma membrane localization of both proteins and significant FRET in the plasma membrane and the ER. All deletions upstream of amino acid 828 resulted in total ER retention that could not rescued by coexpression with the full-length protein. However, these deletion mutants did not impair export of full-length TRPV4, implying that no oligomerization took place. These data indicate that the C-terminus of TRPV4 is required for oligomerization, which takes place in the ER and precedes plasma membrane trafficking.

Short- and long-term alterations of mitochondrial morphology, dynamics and mtDNA after transient oxidative stress

Cells are exposed during their life span to fluctuating levels of reactive oxygen species (ROS). To investigate the effects of a single ROS boost in vitro, human endothelial cells (HUVEC) were treated with one short-term dose of hydrogen peroxide. This treatment resulted in a short, dose-dependent ROS peak that caused transient changes in the mitochondrial morphology and fine structure, in the frequency of mitochondrial fission and fusion and in the mRNA levels of distinct fission and fusion factors. Treatment with a higher dose induced prolonged mtDNA damage; these cells exhibited a significantly shortened replicative lifespan, indicating dose-dependent effects of oxidative stress on mitochondria.

Identification of cultivation-independent markers of human endothelial cell senescence in vitro

Human aging processes are regulated by many divergent pathways and on many levels. Thus, to understand such a complex system and define conserved mechanisms of aging, the use of cell culture-based models is a widespread practice. An often stated advantage of in vitro aging of primary cells is the high reproducibility compared to the much more intricate aging of organisms. However, the aging process of cultured cells is, like aging of organisms, not only defined by genetic but also by environmental factors, making it difficult to distinguish between cell culture condition-induced artefacts and true aspects of aging. Therefore we investigated aging of HUVEC (human umbilical vascular endothelial cells), a well-known and widely used model system for in vitro aging, with different, already well-established cell culture protocols. Culturing conditions had indeed a strong impact on cell proliferation, the replicative lifespan and apoptosis rates. However, despite these significant differences, we found also various robust markers that define senescent HUVEC: morphological changes, increased senescence-associated beta-galactosidase staining, cell cycle arrest in the G1 phase, lowered mitochondrial membrane potential and increased oxidatively modified proteins were displayed independent of cell culture protocols and could therefore be considered also as markers for in vivo aging.

Spectrally and spatially resolved fluorescence lifetime imaging in living cells: TRPV4-microfilament interactions

Time- and space-correlated single photon counting method has been used to demonstrate the interactions of cation channel "transient receptor potential vanilloid 4" (TRPV4) and microfilaments. Living cells co-expressing TRPV4-CFP and actin-YFP, when excited for the donor molecules (CFP) exhibited an emission peak at 527 nm and decrease of the lifetime in the wavelength band 460-490 nm; corresponding to resonance energy transfer to YFP. CFP fluorescence decay was fitted best by a dual mode decay model. Considering the average lifetime of the donor, both in the presence and absence of acceptor yielded an apparent FRET efficiency of approximately 20%. This is rather high placing the minimum distance of chromophores in the two fluorescent proteins in the range of 4 nm. Thus, this study shows for the first time that TRPV4 and actin intimately associate within living cells. The significance of this finding for cell volume regulation is highlighted.

Mitochondrial dynamics generate equal distribution but patchwork localization of respiratory Complex I

Highly dynamic mitochondrial morphology is a prerequisite for fusion and fission. Mitochondrial fusion may represent a rescue mechanism for impaired mitochondria by exchanging constituents (proteins, lipids and mitochondrial DNA) and thus maintaining functionality. Here we followed for the first time the dynamics of a protein complex of the respiratory chain during fusion and fission. HeLa cells with differently labelled respiratory Complex I were fused and the dynamics of Complex I were investigated. The mitochondrial proteins spread throughout the whole mitochondrial population within 3 to 6 h after induction of cell fusion. Mitochondria of fused cells displayed a patchy substructure where the differently labelled proteins occupied separate and distinct spaces. This patchy appearance was already--although less pronounced--observed within single mitochondria before fusion, indicating a specific localization of Complex I with restricted diffusion within the inner membrane. These findings substantiate the view of a homogenous mitochondrial population due to constantly rearranging mitochondria, but also indicate the existence of distinct inner mitochondrial sub-compartments for respiratory chain complexes.

On the relevance of mitochondrial fusions for the accumulation of mitochondrial deletion mutants: a modelling study

The molecular mechanisms underlying the aging process are still unclear, but the clonal accumulation of mitochondrial deletion mutants is one of the prime candidates. An important question for the mitochondrial theory of aging is to discover how defective organelles might be selected at the expense of wild-type mitochondria. We propose that mitochondrial fission and fusion events are of critical importance for resolving this apparent contradiction. We show that the occurrence of fusions removes the problems associated with the idea that smaller DNA molecules accumulate because they replicate in a shorter time--the survival of the tiny (SOT) hypothesis. Furthermore, stochastic simulations of mitochondrial replication, mutation and degradation show that two important experimental findings, namely the overall low mosaic pattern of oxidative phosphorylation (OXPHOS) impaired cells in old organisms and the distribution of deletion sizes, can be reproduced and explained by this hypothesis. Finally, we make predictions that can be tested experimentally to further verify our explanation for the age-related accumulation of mitochondrial deletion mutants.

TRPV4 exhibits a functional role in cell-volume regulation

Tight regulation of the cell volume is important for the maintenance of cellular homeostasis. In a hypotonic environment, cells swell owing to osmosis. With many vertebrate cells, swelling is followed by an active reduction of volume, a process called regulatory volume decrease (RVD). A possible participant in RVD is the non-selective cation channel TRPV4, a member of the TRP superfamily that has been shown to react to hypotonic stimuli with a conductance for Ca2+. As a model for cell-volume regulation, we used a human keratinocyte cell line (HaCaT) that produces TRPV4 endogenously. When HaCaT cells were exposed to a hypotonic solution (200 mOsm) maximal swelling was followed by RVD. During swelling and volume regulation, a strong Ca2+ influx was measured. Gd3+, an inhibitor of TRPV4, blocked RVD of HaCaT cells and the accompanying rise of cytosolic Ca2+. To define the role of TRPV4 in volume regulation, a TRPV4-EGFP fusion protein was produced in CHO cells. CHO cells are unable to undergo RVD under hypotonic conditions and do not produce TRPV4 endogenously. Fluorescence imaging revealed that recombinant TRPV4 was localized to the cell membrane. Production of TRPV4 enabled CHO cells to undergo typical RVD after hypo-osmolarity-induced cell swelling. RVD of TRPV4-transfected CHO cells was significantly reduced by Gd3+ treatment or in Ca2+-free solution. Taken together, these results show a direct participation of TRPV4 in RVD.

Morpho-dynamic changes of mitochondria during ageing of human endothelial cells

Mitochondrial morphology is regulated in many cultured eukaryotic cells by fusion and fission of mitochondria. A tightly controlled balance between fission and fusion events is required to ensure normal mitochondrial and cellular functions. During ageing, mitochondria are undergoing significant changes on the functional and morphological level. The effect of ageing on fusion and fission of mitochondria and consequences of altered fission and fusion activity are still unknown although theoretical models on ageing consider the significance of these processes. Human umbilical vein endothelial cells (HUVECs) have been established as a cell culture model to follow mitochondrial activity and dysfunction during the ageing process. Mitochondria of old and postmitotic HUVECs showed distinct alterations in overall morphology and fine structure, and furthermore, loss of mitochondrial membrane potential. In parallel, a decrease of intact mitochondrial DNA (mtDNA) was observed. Fission and fusion activity of mitochondria were quantified in living cells. Mitochondria of old HUVECs showed a significant and equal decrease of both fusion and fission activity indicating that these processes are sensitive to ageing and could contribute to the accumulation of damaged mitochondria during ageing.

Differential expression and regulation of protease-activated receptors in human peripheral monocytes and monocyte-derived antigen-presenting cells

Protease-activated receptors (PARs) are stimulated by proteolytic cleavage of their extracellular domain, unmasking a new N-terminus acting as tethered ligand. Whereas the role of PARs in platelets is well known, their presence and function in human monocytes and other antigen-presenting cells has not been characterized. Here it is demonstrated that human peripheral monocytes and monocyte-derived macrophages and dendritic cells differentially express PARs. Human monocytes express mainly PAR1 and less PAR3. Differentiation of monocytes into macrophages by either macrophage colony-stimulating factor (M-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF) elicits enhanced expression of PAR1, PAR2, and PAR3. In contrast, dendritic cells differentiated from monocytes by GM-CSF and interleukin-4 (IL-4) strongly down-regulated PAR1, PAR2, and PAR3, both at the mRNA and the protein level. Down-regulation of the PAR expression was apparently due to IL-4, because treatment of macrophages with IL-4 caused down-regulation of PAR1, PAR2, and PAR3. PAR4 mRNA expression remained undetectable in any of the cell types investigated. Stimulation of PAR1, PAR2, and PAR3 with thrombin, trypsin, or established receptor-activating peptides (PAR-APs) triggered cytosolic Ca2+ responses, indicating functionally active PARs. Further, stimulation of monocytes or macrophages with thrombin or PAR1-AP, but not with PAR2-or PAR4-AP, triggers expression of monocyte chemoattractant protein-1 (MCP-1) both at the mRNA and the protein level. These data demonstrate that differentiation of human monocytes is associated with differential expression of functionally active PARs that mediate distinct regulatory functions in inflammation and atherogenesis.

Ciglitazone inhibits plasmin-induced proinflammatory monocyte activation via modulation of p38 MAP kinase activity

Plasmin triggers chemotaxis and NF-kappa B- and AP-1-mediated proinflammatory gene expression in human peripheral monocytes (PM). Compared with macrophages and dendritic cells, PM express mainly the peroxisome proliferator-activated receptor (PPAR) gamma and traces of PPAR alpha as detected by semiquantitative RT-PCR and immunoblotting. The PPAR gamma agonist ciglitazone, but not the PPAR alpha agonist clofibric acid, concentration-dependently inhibited the plasmin-, but not the FMLP-induced PM chemotaxis. Similarly, release of interleukin (IL)-1 alpha, IL-1 beta and tumor necrosis factor (TNF)-alpha from plasmin-stimulated PM was concentration-dependently inhibited by ciglitazone, but not by clofibric acid, while the LPS-induced TNF-alpha release remained unaffected by any of both PPAR agonists. Ciglitazone activates PPAR gamma as shown by a novel surface plasmon resonance analysis and inhibits the plasmin-induced activation of NF-kappa B and AP-1. It also inhibits p38 MAPK phosphorylation essential for the plasmin-induced PM chemotaxis and gene activation. Thus, activation of PPAR gamma by ciglitazone may allow controLling of the plasmin-mediated recruitment and activation of PM at sites of inflammation.

Plasmin-induced expression of cytokines and tissue factor in human monocytes involves AP-1 and IKKbeta-mediated NF-kappaB activation

It was previously shown that plasmin activates human peripheral monocytes in terms of lipid mediator release and chemotactic migration. Here it is demonstrated that plasmin induces proinflammatory cytokine release and tissue factor (TF) expression by monocytes. Plasmin 0.043 to 1.43 CTA U/mL, but not active site-blocked plasmin, triggered concentration-dependent expression of mRNA for interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), and TF with maximum responses after 4 hours. Plasmin-mediated mRNA expression was inhibited in a concentration-dependent manner by the lysine analogue trans-4-(aminomethyl)cyclohexane-1-carboxylic acid (t-AMCA). Increases in mRNA levels were followed by concentration- and time-dependent release of IL-1alpha, IL-1beta and TNF-alpha and by TF expression on monocyte surfaces. Neither cytokines nor TF could be detected when monocytes were preincubated with actinomycin D or cycloheximide. Electrophoretic mobility shift assays indicated plasmin-induced activation of NF-kappaB; DNA-binding complexes were composed of p50, p65, and c-Rel, as shown by supershift experiments. Nuclear translocation of NF-kappaB/Rel proteins coincided with IkappaBalpha degradation. At variance with endotoxic lipopolysaccharide, plasmin elicited the rapid degradation of another cytoplasmic NF-kappaB inhibitor, p105. Proteolysis of NF-kappaB inhibitors was apparently due to transient activation of IkappaB kinase (IKK) beta that reached maximum activity at 1 hour after plasmin stimulation. In addition, AP-1 binding was increased in plasmin-treated monocytes, with most complexes composed of JunD, c-Fos, and FosB. These findings further substantiate the role of plasmin as a proinflammatory activator of human monocytes and reveal an important new link between the plasminogen-plasmin system and inflammation. (Blood. 2001;97:3941-3950)

Characterization of an internal ribosome entry site within mRNA 5 of murine hepatitis virus

The unique region of mRNA 5 of murine hepatitis virus contains two open reading frames, ORF 5a and ORF 5b. The downstream ORF 5b encodes the envelope (E) protein, an integral membrane protein of the virus. We have shown previously that the expression of ORF 5b is mediated by the internal entry of ribosomes. In the experiments reported here, we have used the in vitro translation of synthetic mRNAs to identify the region of mRNA 5 that mediates internal ribosome entry. Our results show that the 5' border of the MHV mRNA 5 IRES element is located between nucleotides 227 and 244 in ORF 5a, while the 3' border is located between nucleotides 140 and 172 in ORF 5b. The MHV mRNA 5 IRES element, therefore, contains not more than 280 nucleotides and encompasses the ORF 5b initiation codon. As evidenced by electrophoretic mobility shift assays, the IRES element of mRNA 5 interacts specifically with protein factors present in an L-cell lysate.

Characterization and transformation of plasmid pAA-1 found in an antarctic cryptoendolithic bacterium

Sixty-three bacterial isolates from antarctic sandstone samples (Linnaeus Terrace, Asgard Range, McMurdo Dry Valleys) were screened for the presence of plasmid DNA. Twenty-seven percent of all the isolates (mainly Gram-positives) harbored one or more plasmids of low molecular mass (1.1–16.3 kb). Strain AA-341 contained plasmid pAA-1 (2.9 kb), as demonstrated by agarose gel electrophoresis and restriction endonuclease digests. This plasmid conferred resistance to chromium and ampicillin. It was not conjugative, but it could be transferred by electroporation to chromium- and ampicillin-sensitive strains AA-330, AA-338, and E. coli HB101. A restriction map of pAA-1 was constructed with HindIII, SalI, ScaI, AvaII, EcoRI, PvuII, BamHI, and DraI. Electrotransfer of this plasmid from E. coli HB101(pAA-1) to strain AA-330 was demonstrated. By natural genetic transformation, plasmid pAA-1 could be transferred into the sensitive strain AA-334, which thereby became resistant to chromium and ampicillin. The importance of such processes for the colonization of stressed environments is discussed.Key words: Antarctica, cryptoendolithic bacteria, plasmids, resistance to chromium, natural transformation.