Unravelling a novel role for cannabidivarin in the modulation of subventricular zone postnatal neurogenesis

Postnatal neurogenesis has been shown to rely on the endocannabinoid system. Here we aimed at unravelling the role of Cannabidivarin (CBDV), a non-psychoactive cannabinoid, with high affinity for the non-classical cannabinoid receptor TRPV1, on subventricular zone (SVZ) postnatal neurogenesis. Using the neurosphere assay, SVZ-derived neural stem/progenitor cells (NSPCs) were incubated with CBDV and/or 5'-Iodoresinferotoxin (TRPV1 antagonist), and their role on cell viability, proliferation, and differentiation were dissected. CBDV was able to promote, through a TRPV1-dependent mechanism, cell survival, cell proliferation and neuronal differentiation. Furthermore, pulse-chase experiments revealed that CBDV-induced neuronal differentiation was a result of cell cycle exit of NSPCs. Regarding oligodendrocyte differentiation, CBDV inhibited oligodendrocyte differentiation and maturation. Since our data suggested that the CBDV-induced modulation of NSPCs acted via TRPV1, a sodium-calcium channel, and that intracellular calcium levels are known regulators of NSPCs fate and neuronal maturation, single cell calcium imaging was performed to evaluate the functional response of SVZ-derived cells. We observed that CBDV-responsive cells displayed a two-phase calcium influx profile, being the initial phase dependent on TRPV1 activation. Taken together, this work unveiled a novel and untapped neurogenic potential of CBDV via TRPV1 modulation. These findings pave the way to future neural stem cell biological studies and repair strategies by repurposing this non-psychoactive cannabinoid as a valuable therapeutic target.

Regulation of hippocampal postnatal and adult neurogenesis by adenosine A2A receptor: Interaction with brain-derived neurotrophic factor

Adenosine A_2A receptor (A_2A R) activation modulates several brain processes, ranging from neuronal maturation to synaptic plasticity. Most of these actions occur through the modulation of the actions of the neurotrophin brain-derived neurotrophic factor (BDNF). In this work, we studied the role of A_2A Rs in regulating postnatal and adult neurogenesis in the rat hippocampal dentate gyrus (DG). Here, we show that A_2A R activation with CGS 21680 promoted neural stem cell self-renewal, protected committed neuronal cells from cell death and contributed to a higher density of immature and mature neuronal cells, particularly glutamatergic neurons. Moreover, A_2A R endogenous activation was found to be essential for BDNF-mediated increase in cell proliferation and neuronal differentiation. Our findings contribute to further understand the role of adenosinergic signaling in the brain and may have an impact in the development of strategies for brain repair under pathological conditions.

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

Sustained Hippocampal Neural Plasticity Questions the Reproducibility of an Amyloid-β-Induced Alzheimer's Disease Model

BACKGROUND

The use of Alzheimer's disease (AD) models obtained by intracerebral infusion of amyloid-β (Aβ) has been increasingly reported in recent years. Nonetheless, these models may present important challenges.

OBJECTIVE

We have focused on canonical mechanisms of hippocampal-related neural plasticity to characterize a rat model obtained by an intracerebroventricular (icv) injection of soluble amyloid-β42 (Aβ42).

METHODS

Animal behavior was evaluated in the elevated plus maze, Y-Maze spontaneous or forced alternation, Morris water maze, and open field, starting 2 weeks post-Aβ42 infusion. Hippocampal neurogenesis was assessed 3 weeks after Aβ42 injection. Aβ deposition, tropomyosin receptor kinase B levels, and neuroinflammation were appraised at 3 and 14 days post-Aβ42 administration.

RESULTS

We found that immature neuronal dendritic morphology was abnormally enhanced, but proliferation and neuronal differentiation in the dentate gyrus was conserved one month after Aβ42 injection. Surprisingly, animal behavior did not reveal changes in cognitive performance nor in locomotor and anxious-related activity. Brain-derived neurotrophic factor related-signaling was also unchanged at 3 and 14 days post-Aβ icv injection. Likewise, astrocytic and microglial markers of neuroinflammation in the hippocampus were unaltered in these time points.

CONCLUSION

Taken together, our data emphasize a high variability and lack of behavioral reproducibility associated with these Aβ injection-based models, as well as the need for its further optimization, aiming at addressing the gap between preclinical AD models and the human disorder.

Diet-dependent gut microbiota impacts on adult neurogenesis through mitochondrial stress modulation

The influence of dietary factors on brain health and mental function is becoming increasingly recognized. Similarly, mounting evidence supports a role for gut microbiota in modulating central nervous system function and behaviour. Still, the molecular mechanisms responsible for the impact of diet and associated microbiome in adult neurodegeneration are still largely unclear. In this study, we aimed to investigate whether and how changes in diet-associated microbiome and its metabolites impact on adult neurogenesis. Mice were fed a high-fat, choline-deficient diet, developing obesity and several features of the metabolic syndrome, including non-alcoholic steatohepatitis. Strikingly, our results showed, for the first time, that animals fed with this specific diet display premature increased neurogenesis, possibly exhausting the available neural stem cell pool for long-term neurogenesis processes. The high-fat, choline-deficient diet further induced neuroinflammation, oxidative stress, synaptic loss and cell death in different regions of the brain. Notably, this diet-favoured gut dysbiosis in the small intestine and cecum, up-regulating metabolic pathways of short-chain fatty acids, such as propionate and butyrate and significantly increasing propionate levels in the liver. By dissecting the effect of these two specific short-chain fatty acids in vitro, we were able to show that propionate and butyrate enhance mitochondrial biogenesis and promote early neurogenic differentiation of neural stem cells through reactive oxygen species- and extracellular signal-regulated kinases 1/2-dependent mechanism. More importantly, neurogenic niches of high-fat, choline-deficient-fed mice showed increased expression of mitochondrial biogenesis markers, and decreased mitochondrial reactive oxygen species scavengers, corroborating the involvement of this mitochondrial stress-dependent pathway in mediating changes of adult neurogenesis by diet. Altogether, our results highlight a mitochondria-dependent pathway as a novel mediator of the gut microbiota–brain axis upon dietary influences.

Reprogramming of Lipid Metabolism as a New Driving Force Behind Tauroursodeoxycholic Acid-Induced Neural Stem Cell Proliferation

Recent evidence suggests that neural stem cell (NSC) fate is highly dependent on mitochondrial bioenergetics. Tauroursodeoxycholic acid (TUDCA), an endogenous neuroprotective bile acid and a metabolic regulator, stimulates NSC proliferation and enhances adult NSC pool in vitro and in vivo. In this study, we dissected the mechanism triggered by this proliferation-inducing molecule, namely in mediating metabolic reprogramming. Liquid chromatography coupled with mass spectrometry (LC-MS) based detection of differential proteomics revealed that TUDCA reduces the mitochondrial levels of the long-chain acyl-CoA dehydrogenase (LCAD), an enzyme crucial for β-oxidation of long-chain fatty acids (FA). TUDCA impact on NSC mitochondrial proteome was further confirmed, including in neurogenic regions of adult rats. We show that LCAD raises throughout NSC differentiation, while its silencing promotes NSC proliferation. In contrast, nuclear levels of sterol regulatory element-binding protein (SREBP-1), a major transcription factor of lipid biosynthesis, changes in the opposite manner of LCAD, being upregulated by TUDCA. In addition, alterations in some metabolic intermediates, such as palmitic acid, also supported the TUDCA-induced de novo lipogenesis. More interestingly, a metabolic shift from FA to glucose catabolism appears to occur in TUDCA-treated NSCs, since mitochondrial levels of pyruvate dehydrogenase E1-α (PDHE1-α) were significant enhanced by TUDCA. At last, the mitochondria-nucleus translocation of PDHE1-α was potentiated by TUDCA, associated with an increase of H3-histones and acetylated forms. In conclusion, TUDCA-induced proliferation of NSCs involves metabolic plasticity and mitochondria-nucleus crosstalk, in which nuclear PDHE1-α might be required to assure pyruvate-derived acetyl-CoA for histone acetylation and NSC cycle progression.

Development and Mechanistic Insight into the Enhanced Cytotoxic Potential of Parvifloron D Albumin Nanoparticles in EGFR-Overexpressing Pancreatic Cancer Cells

Pancreatic cancer is one of the most lethal cancers, with an extremely poor prognosis. The development of more effective therapies is thus imperative. Natural origin compounds isolated from Plectranthus genus, such as parvifloron D (PvD), have cytotoxic and antiproliferative activity against human tumour cells. However, PvD is a very low water-soluble compound, being nanotechnology a promising alternative strategy to solve this problem. Therefore, the aim of this study was to optimize a nanosystem for preferential delivery of PvD to pancreatic tumour cells. Albumin nanoparticles (BSA NPs) were produced through a desolvation method. Glucose cross-linking and bioactive functionalization profiles of BSA platform were elucidated and analysed using static lattice atomistic simulations in vacuum. Using the optimized methodology, PvD was encapsulated (yield higher than 80%) while NPs were characterized in terms of size (100–400 nm) and morphology. Importantly, to achieve a preferential targeting to pancreatic cancer cells, erlotinib and cetuximab were attached to the PvD-loaded nanoparticle surface, and their antiproliferative effects were evaluated in BxPC3 and Panc-1 cell lines. Erlotinib conjugated NPs presented the highest antiproliferative effect toward pancreatic tumour cells. Accordingly, cell cycle analysis of the BxPC3 cell line showed marked accumulation of tumour cells in G1-phase and cell cycle arrest promoted by NPs. As a result, erlotinib conjugated PvD-loaded BSA NPs must be considered a suitable and promising carrier to deliver PvD at the tumour site, improving the treatment of pancreatic cancer.

A Novel Small Molecule p53 Stabilizer for Brain Cell Differentiation

Brain tumor, as any type of cancer, is assumed to be sustained by a small subpopulation of stem-like cells with distinctive properties that allow them to survive conventional therapies and drive tumor recurrence. Thus, the identification of new molecules capable of controlling stemness properties may be key in developing effective therapeutic strategies for cancer by inducing stem-like cells differentiation. Spiropyrazoline oxindoles have previously been shown to induce apoptosis and cell cycle arrest, as well as upregulate p53 steady-state levels, while decreasing its main inhibitor MDM2 in the HCT116 human colorectal carcinoma cell line. In this study, we made modifications in this scaffold by including combinations of different substituents in the pyrazoline ring in order to obtain novel small molecules that could modulate p53 activity and act as differentiation inducer agents. The antiproliferative activity of the synthesized compounds was assessed using the isogenic pair of HCT116 cell lines differing in the presence or absence of the p53 gene. Among the tested spirooxindoles, spiropyrazoline oxindole 1a was selective against the cancer cell line expressing wild-type p53 and presented low cytotoxicity. This small molecule induced neural stem cell (NSC) differentiation through reduced SOX2 (marker of multipotency) and increased βIII-tubulin (marker of neural differentiation) which suggests a great potential as a non-toxic inducer of cell differentiation. More importantly, in glioma cancer cells (GL-261), compound 1a reduced stemness, by decreasing SOX2 protein levels, while also promoting chemotherapy sensitization. These results highlight the potential of p53 modulators for brain cell differentiation, with spirooxindole 1a representing a promising lead molecule for the development of new brain antitumor drugs.

Amyloid β Peptide Compromises Neural Stem Cell Fate by Irreversibly Disturbing Mitochondrial Oxidative State and Blocking Mitochondrial Biogenesis and Dynamics

Alzheimer's disease (AD) is the most common neurodegenerative disease and is characterized by the accumulation of amyloid β peptide (Aβ). Although most AD mouse models present a decline in neurogenesis, they express mutated genes which regulate neurogenesis per se and are not present in most AD patients, thus masking the real impact of Aβ on adult neurogenesis. Mitochondrion, a well-known target of Aβ in neurons, is a main regulator of neural stem cell (NSC) fate. Here, we aimed to investigate the impact of Aβ on NSC mitochondria and cell fate decisions, namely whether and how Aβ affects neurogenesis. NSC fate and mitochondrial parameters, including biogenesis, dynamics, and oxidative stress, were evaluated. Our results showed that Aβ impaired NSC viability and proliferation and indirectly blocked neurogenic differentiation, by disrupting mitochondrial signaling of self-renewing NSCs. Importantly, Aβ decreased ATP levels, generated oxidative stress, and affected the radical scavenger system through SOD2 and SIRT3. Aβ also reduced mtDNA and mitochondrial biogenesis proteins, such as Tfam, PGC-1α, and NRF1, and inhibited activation of PGC-1α-positive regulator CREB. Moreover, Aβ triggered mitochondrial fragmentation in self-renewing NSCs and reduced mitochondrial fusion proteins, such as Mfn2 and ERRα. Notably, Aβ compromised NSC commitment and survival by irreversibly impairing mitochondria and thwarting any neurogenic rescue through mitochondrial biogenesis, dynamics, or radical scavenger system. Altogether, this study brings new perspective to rethink the molecular targets relevant for endogenous NSC-based strategies in AD.

Impaired cross-talk between the thioredoxin and glutathione systems is related to ASK-1 mediated apoptosis in neuronal cells exposed to mercury

Mercury (Hg) compounds target both cysteine (Cys) and selenocysteine (Sec) residues in peptides and proteins. Thus, the components of the two major cellular antioxidant systems - glutathione (GSH) and thioredoxin (Trx) systems - are likely targets for mercurials. Hg exposure results in GSH depletion and Trx and thioredoxin reductase (TrxR) are prime targets for mercury. These systems have a wide-range of common functions and interaction between their components has been reported. However, toxic effects over both systems are normally treated as isolated events. To study how the interaction between the glutathione and thioredoxin systems is affected by Hg, human neuroblastoma (SH-SY5Y) cells were exposed to 1 and 5μM of inorganic mercury (Hg2+), methylmercury (MeHg) or ethylmercury (EtHg) and examined for TrxR, GSH and Grx levels and activities, as well as for Trx redox state. Phosphorylation of apoptosis signalling kinase 1 (ASK1), caspase-3 activity and the number of apoptotic cells were evaluated to investigate the induction of Trx-mediated apoptotic cell death. Additionally, primary cerebellar neurons from mice depleted of mitochondrial Grx2 (mGrx2D) were used to examine the link between Grx activity and Trx function. Results showed that Trx was affected at higher exposure levels than TrxR, especially for EtHg. GSH levels were only significantly affected by exposure to a high concentration of EtHg. Depletion of GSH with buthionine sulfoximine (BSO) severely increased Trx oxidation by Hg. Notably, EtHg-induced oxidation of Trx was significantly enhanced in primary neurons of mGrx2D mice. Our results suggest that GSH/Grx acts as backups for TrxR in neuronal cells to maintain Trx turnover during Hg exposure, thus linking different mechanisms of molecular and cellular toxicity. Finally, Trx oxidation by Hg compounds was associated to apoptotic hallmarks, including increased ASK-1 phosphorylation, caspase-3 activation and increased number of apoptotic cells.

Tauroursodeoxycholic Acid Enhances Mitochondrial Biogenesis, Neural Stem Cell Pool, and Early Neurogenesis in Adult Rats

Although neurogenesis occurs in restricted regions of the adult mammalian brain, neural stem cells (NSCs) produce very few neurons during ageing or after injury. We have recently discovered that the endogenous bile acid tauroursodeoxycholic acid (TUDCA), a strong inhibitor of mitochondrial apoptosis and a neuroprotective in animal models of neurodegenerative disorders, also enhances NSC proliferation, self-renewal, and neuronal conversion by improving mitochondrial integrity and function of NSCs. In the present study, we explore the effect of TUDCA on regulation of NSC fate in neurogenic niches, the subventricular zone (SVZ) of the lateral ventricles and the hippocampal dentate gyrus (DG), using rat postnatal neurospheres and adult rats exposed to the bile acid. TUDCA significantly induced NSC proliferation, self-renewal, and neural differentiation in the SVZ, without affecting DG-derived NSCs. More importantly, expression levels of mitochondrial biogenesis-related proteins and mitochondrial antioxidant responses were significantly increased by TUDCA in SVZ-derived NSCs. Finally, intracerebroventricular administration of TUDCA in adult rats markedly enhanced both NSC proliferation and early differentiation in SVZ regions, corroborating in vitro data. Collectively, our results highlight a potential novel role for TUDCA in neurologic disorders associated with SVZ niche deterioration and impaired neurogenesis.

MicroRNA-145 Regulates Neural Stem Cell Differentiation Through the Sox2-Lin28/let-7 Signaling Pathway

MicroRNAs (miRNAs or miRs) regulate several biological functions, including cell fate determination and differentiation. Although miR-145 has already been described to regulate glioma development, its precise role in neurogenesis has never been addressed. miR-145 represses sex-determining region Y-box 2 (Sox2), a core transcription factor of embryonic stem cells (ESCs), to inhibit pluripotency and self-renewal in human ESCs. In addition, the Sox2-Lin28/let-7 signaling pathway regulates proliferation and neurogenesis of neural precursors. In this study, we aimed to investigate the precise role of miR-145 in neural stem cell (NSC) fate decision, and the possible involvement of the Sox2-Lin28/let-7 signaling pathway in miR-145 regulatory network. Our results show for the first time that miR-145 expression significantly increased after induction of mouse NSC differentiation, remaining elevated throughout this process. Forced miR-145 downregulation decreased neuronal markers, namely βIII-tubulin, NeuN, and MAP2. Interestingly, throughout NSC differentiation, protein levels of Sox2 and Lin28, a well-known suppressor of let-7 biogenesis, decreased. Of note, neuronal differentiation also resulted in let-7a and let-7b upregulation. Transfection of NSCs with anti-miR-145, in turn, increased both Sox2 and Lin28 protein levels, while decreasing both let-7a and let-7b. More importantly, Sox2 and Lin28 silencing partially rescued the impairment of neuronal differentiation induced by miR-145 downregulation. In conclusion, our results demonstrate a novel role for miR-145 during NSC differentiation, where miR-145 modulation of Sox2-Lin28/let-7 network is crucial for neurogenesis progression. Stem Cells 2016;34:1386-1395.

Tauroursodeoxycholic acid increases neural stem cell pool and neuronal conversion by regulating mitochondria-cell cycle retrograde signaling

The low survival and differentiation rates of stem cells after either transplantation or neural injury have been a major concern of stem cell-based therapy. Thus, further understanding long-term survival and differentiation of stem cells may uncover new targets for discovery and development of novel therapeutic approaches. We have previously described the impact of mitochondrial apoptosis-related events in modulating neural stem cell (NSC) fate. In addition, the endogenous bile acid, tauroursodeoxycholic acid (TUDCA) was shown to be neuroprotective in several animal models of neurodegenerative disorders by acting as an anti-apoptotic and anti-oxidant molecule at the mitochondrial level. Here, we hypothesize that TUDCA might also play a role on NSC fate decision. We found that TUDCA prevents mitochondrial apoptotic events typical of early-stage mouse NSC differentiation, preserves mitochondrial integrity and function, while enhancing self-renewal potential and accelerating cell cycle exit of NSCs. Interestingly, TUDCA prevention of mitochondrial alterations interfered with NSC differentiation potential by favoring neuronal rather than astroglial conversion. Finally, inhibition of mitochondrial reactive oxygen species (mtROS) scavenger and adenosine triphosphate (ATP) synthase revealed that the effect of TUDCA is dependent on mtROS and ATP regulation levels. Collectively, these data underline the importance of mitochondrial stress control of NSC fate decision and support a new role for TUDCA in this process.

Distinct kinetics and pathways of apoptosis in influenza A and B virus infection

Annual influenza epidemics are associated with high incidence and mortality rates, and are an important cause of work absenteeism and productivity losses. For successful replication, influenza viruses have evolved as to counteract and/or take a part on host defense mechanisms. Manipulation of apoptosis is one of such mechanisms that have been subject of attention, particularly in relation to influenza type A viruses over the past years. However, this knowledge has not been extended to include influenza type B viruses. In this study, MDCK-SIAT1 cells were infected with influenza A and B strains and the kinetics and pathways of apoptosis post infection were studied, through LDH measurements, Hoechst dye staining, caspase activity assays and protein expression analysis. The resulting data points to a difference in induction of apoptosis profiles between influenza A and B strains. While influenza A strain induced apoptosis later in the course of infection and mainly by the intrinsic pathway, influenza B strain induced apoptosis early in infection by both intrinsic and extrinsic pathways. Also, data revealed the IκB/NF-κB pathway as the major contributor for the observed differences. The study of the virus-host interactions, particularly those that could have an impact on viral replication, are essential in both influenza A and B viruses, as they will allow the identification of viral/host targets common to both influenza types, which could affect viral replication. This information may prove useful for vaccine and antiviral research.

Mitochondria: Major Regulators of Neural Development

Mitochondria are organelles derived from primitive symbiosis between archeon ancestors and prokaryotic α-proteobacteria species, which lost the capacity of synthetizing most proteins encoded the bacterial DNA, along the evolutionary process of eukaryotes. Nowadays, mitochondria are constituted by small circular mitochondrial DNA of 16 kb, responsible for the control of several proteins, including polypeptides of the electron transport chain. Throughout evolution, these organelles acquired the capacity of regulating energy production and metabolism, thus becoming central modulators of cell fate. In fact, mitochondria are crucial for a variety of cellular processes, including adenosine triphosphate production by oxidative phosphorylation, intracellular Ca(2+) homeostasis, generation of reactive oxygen species, and also cellular specialization in a variety of tissues that ultimately relies on specific mitochondrial specialization and maturation. In this review, we discuss recent evidence extending the importance of mitochondrial function and energy metabolism to the context of neuronal development and adult neurogenesis.

Mitochondrial translocation of p53 modulates neuronal fate by preventing differentiation-induced mitochondrial stress

AIMS

Apoptosis regulatory proteins, such as p53, play a pivotal role in neural differentiation, through mechanisms independent of cell death. In addition, p53 has been identified as an important regulator of mitochondrial survival response, maintaining mitochondrial DNA (mtDNA) integrity and oxidative protection. The aim of this study was to determine the role of mitochondrial p53 in organelle damage and neural differentiation.

RESULTS

Our results show that mitochondrial apoptotic events such as reactive oxygen species production, mitochondrial membrane permeabilization, and cytochrome c release are typical of early-stage mouse neural stem cell differentiation, which occurs 3-18 h after induction of differentiation, with no evidence of cell death. In addition, decreased mtDNA content, lipidated LC3 (LC3-II), colocalization of mitochondria and LC3-II puncta, and mitochondria-associated Parkin are consistent with activation of mitophagy. Importantly, at early stages of neural differentiation, p53 was actively translocated to mitochondria and attenuated mitochondrial oxidative stress, cytochrome c release, and mitophagy. Forced mitochondrial translocation of p53 increased neurogenic potential and neurite outgrowth.

INNOVATION AND CONCLUSION

In conclusion, our results reveal a novel role for mitochondrial p53, which modulates mitochondrial damage and apoptosis-related events in the context of neural differentiation, thus enhancing neuronal fate.

TAp63γ demethylation regulates protein stability and cellular distribution during neural stem cell differentiation

p63 is a close relative of the p53 tumor suppressor and transcription factor that modulates cell fate. The full-length isoform of p63, containing a transactivation (TA) domain (TAp63) is an essential proapoptotic protein in neural development. The role of p63 in epithelial development is also well established; however, its precise function during neural differentiation remains largely controversial. Recently, it has been demonstrated that several conserved elements of apoptosis are also integral components of cellular differentiation; p53 directly interacts with key regulators of neurogenesis. The aim of this study was to evaluate the role of p63 during mouse neural stem cell (NSC) differentiation and test whether the histone H3 lysine 27-specific demethylase JMJD3 interacts with p63 to redirect NSCs to neurogenesis. Our results showed that JMJD3 and TAp63γ are coordinately regulated to establish neural-specific gene expression programs in NSCs undergoing differentiation. JMJD3 overexpression increased TAp63γ levels in a demethylase activity-dependent manner. Importantly, overexpression of TAp63γ increased β-III tubulin whereas downregulation of TAp63γ by specific p63 siRNA decreased β-III tubulin. Immunoprecipitation assays demonstrated direct interaction between TAp63γ and JMJD3, and modulation of TAp63γ methylation status by JMJD3-demethylase activity. Importantly, the demethylase activity of JMJD3 influenced TAp63γ protein stabilization and cellular distribution, as well as TAp63γ-regulated neurogenesis. These findings clarify the role of p63 in adult neural progenitor cells and reveal TAp63γ as a direct target for JMJD3-mediated neuronal commitment.

Death receptors and mitochondria: two prime triggers of neural apoptosis and differentiation

BACKGROUND

Stem cell therapy is a strategy far from being satisfactory and applied in the clinic. Poor survival and differentiation levels of stem cells after transplantation or neural injury have been major problems. Recently, it has been recognized that cell death-relevant proteins, notably those that operate in the core of the executioner apoptosis machinery are functionally involved in differentiation of a wide range of cell types, including neural cells.

SCOPE OF REVIEW

This article will review recent studies on the mechanisms underlying the non-apoptotic function of mitochondrial and death receptor signaling pathways during neural differentiation. In addition, we will discuss how these major apoptosis-regulatory pathways control the decision between differentiation, self-renewal and cell death in neural stem cells and how levels of activity are restrained to prevent cell loss as final outcome.

MAJOR CONCLUSIONS

Emerging evidence suggests that, much like p53, caspases and Bcl-2 family members, the two prime triggers of cell death pathways, death receptors and mitochondria, may influence proliferation and differentiation potential of stem cells, neuronal plasticity, and astrocytic versus neuronal stem cell fate decision.

GENERAL SIGNIFICANCE

A better understanding of the molecular mechanisms underlying key checkpoints responsible for neural differentiation as an alternative to cell death will surely contribute to improve neuro-replacement strategies.

Distinct regulatory functions of calpain 1 and 2 during neural stem cell self-renewal and differentiation

Calpains are calcium regulated cysteine proteases that have been described in a wide range of cellular processes, including apoptosis, migration and cell cycle regulation. In addition, calpains have been implicated in differentiation, but their impact on neural differentiation requires further investigation. Here, we addressed the role of calpain 1 and calpain 2 in neural stem cell (NSC) self-renewal and differentiation. We found that calpain inhibition using either the chemical inhibitor calpeptin or the endogenous calpain inhibitor calpastatin favored differentiation of NSCs. This effect was associated with significant changes in cell cycle-related proteins and may be regulated by calcium. Interestingly, calpain 1 and calpain 2 were found to play distinct roles in NSC fate decision. Calpain 1 expression levels were higher in self-renewing NSC and decreased with differentiation, while calpain 2 increased throughout differentiation. In addition, calpain 1 silencing resulted in increased levels of both neuronal and glial markers, β-III Tubulin and glial fibrillary acidic protein (GFAP). Calpain 2 silencing elicited decreased levels of GFAP. These results support a role for calpain 1 in repressing differentiation, thus maintaining a proliferative NSC pool, and suggest that calpain 2 is involved in glial differentiation.

miR-34a regulates mouse neural stem cell differentiation

Background

MicroRNAs (miRNAs or miRs) participate in the regulation of several biological processes, including cell differentiation. Recently, miR-34a has been implicated in the differentiation of monocyte-derived dendritic cells, human erythroleukemia cells, and mouse embryonic stem cells. In addition, members of the miR-34 family have been identified as direct p53 targets. However, the function of miR-34a in the control of the differentiation program of specific neural cell types remains largely unknown. Here, we investigated the role of miR-34a in regulating mouse neural stem (NS) cell differentiation.

Methodology/Principal Findings

miR-34a overexpression increased postmitotic neurons and neurite elongation of mouse NS cells, whereas anti-miR-34a had the opposite effect. SIRT1 was identified as a target of miR-34a, which may mediate the effect of miR-34a on neurite elongation. In addition, acetylation of p53 (Lys 379) and p53-DNA binding activity were increased and cell death unchanged after miR-34a overexpression, thus reinforcing the role of p53 during neural differentiation. Interestingly, in conditions where SIRT1 was activated by pharmacologic treatment with resveratrol, miR-34a promoted astrocytic differentiation, through a SIRT1-independent mechanism.

Conclusions

Our results provide new insight into the molecular mechanisms by which miR-34a modulates neural differentiation, suggesting that miR-34a is required for proper neuronal differentiation, in part, by targeting SIRT1 and modulating p53 activity.

p53 interaction with JMJD3 results in its nuclear distribution during mouse neural stem cell differentiation

Conserved elements of apoptosis are also integral components of cellular differentiation. In this regard, p53 is involved in neurogenesis, being required for neurite outgrowth in primary neurons and for axonal regeneration in mice. Interestingly, demethylases regulate p53 activity and its interaction with co-activators by acting on non-histone proteins. In addition, the histone H3 lysine 27-specific demethylase JMJD3 induces ARF expression, thereby stabilizing p53 in mouse embryonic fibroblasts. We hypothesized that p53 interacts with key regulators of neurogenesis to redirect stem cells to differentiation, as an alternative to cell death. Specifically, we investigated the potential cross-talk between p53 and JMJD3 during mouse neural stem cell (NSC) differentiation. Our results demonstrated that JMJD3 mRNA and protein levels were increased early in mouse NSC differentiation, when JMJD3 activity was readily detected. Importantly, modulation of JMJD3 in NSCs resulted in changes of total p53 protein, coincident with increased ARF mRNA and protein expression. ChIP analysis revealed that JMJD3 was present at the promoter and exon 1 regions of ARF during neural differentiation, although without changes in H3K27me3. Immunoprecipitation assays demonstrated a direct interaction between p53 and JMJD3, independent of the C-terminal region of JMJD3, and modulation of p53 methylation by JMJD3-demethylase activity. Finally, transfection of mutant JMJD3 showed that the demethylase activity of JMJD3 was crucial in regulating p53 cellular distribution and function. In conclusion, JMJD3 induces p53 stabilization in mouse NSCs through ARF-dependent mechanisms, directly interacts with p53 and, importantly, causes nuclear accumulation of p53. This suggests that JMJD3 and p53 act in a common pathway during neurogenesis.

Apoptosis-associated microRNAs are modulated in mouse, rat and human neural differentiation

Background

MicroRNAs (miRs or miRNAs) regulate several biological processes in the cell. However, evidence for miRNAs that control the differentiation program of specific neural cell types has been elusive. Recently, we have shown that apoptosis-associated factors, such as p53 and caspases participate in the differentiation process of mouse neural stem (NS) cells. To identify apoptosis-associated miRNAs that might play a role in neuronal development, we performed global miRNA expression profiling experiments in NS cells. Next, we characterized the expression of proapoptotic miRNAs, including miR-16, let-7a and miR-34a in distinct models of neural differentiation, including mouse embryonic stem cells, PC12 and NT2N cells. In addition, the expression of antiapoptotic miR-19a and 20a was also evaluated.

Results

The expression of miR-16, let-7a and miR-34a was consistently upregulated in neural differentiation models. In contrast, expression of miR-19a and miR-20a was downregulated in mouse NS cell differentiation. Importantly, differential expression of specific apoptosis-related miRNAs was not associated with increased cell death. Overexpression of miR-34a increased the proportion of postmitotic neurons of mouse NS cells.

Conclusions

In conclusion, the identification of miR-16, let-7a and miR-34a, whose expression patterns are conserved in mouse, rat and human neural differentiation, implicates these specific miRNAs in mammalian neuronal development. The results provide new insights into the regulation of neuronal differentiation by apoptosis-associated miRNAs.

Role of nuclear steroid receptors in apoptosis

Nuclear steroid receptors (NSR) are ligand-activated transcription factors that play a key role in a variety of vital physiological phenomena including developmental or endocrine signaling, reproduction, and homeostasis. In addition, they are implicated in other important biological processes, such as apoptosis. Modulation of apoptosis by NSR is mostly associated with control of pro-apoptotic versus anti-apoptotic gene expression, and includes both induction and prevention of apoptosis depending on cell type. However, it is unclear how NSR can affect opposing expression of the same gene in different cells. Of note, recently described nongenomic mechanisms of NSR, in particular glucocorticoid receptor translocation to mitochondria, were suggested to be crucial steps for triggering apoptosis. NSR often act solely as nuclear transporters of other regulatory molecules, thus indirectly regulating several apoptosis-related genes. Curiously, NSR are thought to cooperate with the anti-apoptotic endogenous bile acid, ursodeoxycholic acid (UDCA), to prevent programmed cell death. In fact, as cholesterol-derived molecules and due to their chemical and structural similarities to steroid hormones, bile acids also modulate NSR activation. Although the precise link between NSR and UDCA remains unclear, we have demonstrated that the bile acid requires NSR for translocation to the cell nucleus as part of a ligand-receptor complex, using a mechanism similar to that of steroid hormones. Interestingly, other studies revealed that UDCA interacts with the glucocorticoid receptor as a novel and selective NSR modifier. The huge diversity of natural ligands and xenobiotics that bind to NSR and regulate their function represents one of the most exciting drug targets for potential therapeutic intervention. The next decade will almost certainly unveil the remarkable role of NSR in modulating cell fate in human health and disease.

Caspases and p53 modulate FOXO3A/Id1 signaling during mouse neural stem cell differentiation

Neural stem cells (NSCs) differentiate into neurons and glia, and a large percentage undergoes apoptosis. The engagement and activity of apoptotic pathways may favor either cell death or differentiation. In addition, Akt represses differentiation by up-regulating the inhibitor of differentiation 1 (Id1), through phosphorylation of its repressor FOXO3A. The aim of this study was to investigate the potential cross-talk between apoptosis and proliferation during mouse NSC differentiation. We determined the time of neurogenesis and gliogenesis using neuronal beta-III tubulin and astroglial GFAP to confirm that both processes occurred at approximately 3 and 8 days, respectively. p-Akt, p-FOXO3A, and Id1 were significantly reduced throughout differentiation. Caspase-3 processing, p53 phosphorylation, and p53 transcriptional activation increased at 3 days of differentiation, with no evidence of apoptosis. Importantly, in cells exposed to the pancaspase inhibitor z-VAD.fmk, p-FOXO3A and Id1 were no longer down-regulated, p53 phosphorylation and transcriptional activation were reduced, while neurogenesis and gliogenesis were significantly delayed. The effect of siRNA-mediated silencing of p53 on FOXO3A/Id1 was similar to that of z-VAD.fmk only at 3 days of differentiation. Interestingly, caspase inhibition further increased the effect of p53 knockdown during neurogenesis. In conclusion, apoptosis-associated factors such as caspases and p53 temporally modulate FOXO3A/Id1 signaling and differentiation of mouse NSCs.

p53 is a key molecular target of ursodeoxycholic acid in regulating apoptosis

p53 plays an important role in regulating expression of genes that mediate cell cycle progression and/or apoptosis. In addition, we have previously shown that the hydrophilic bile acid ursodeoxycholic acid (UDCA) prevents transforming growth factor beta1-induced p53 stabilization and apoptosis in primary rat hepatocytes. Therefore, we hypothesized that p53 may represent an important target in bile acid-induced modulation of apoptosis and cell survival. In this study we demonstrated that UDCA reduces p53 transcriptional activity, thereby preventing its ability to induce Bax expression, mitochondrial translocation, cytochrome c release, and apoptosis in primary rat hepatocytes. More importantly, bile acid inhibition of p53-induced apoptosis was associated with decreased p53 DNA binding activity. Subcellular localization of p53 was also altered by UDCA. Both events appear to be related with increased association between p53 and its direct repressor, Mdm-2. In conclusion, these results further clarify the antiapoptotic mechanism of UDCA and suggest that modulation of Mdm-2/p53 interaction is a prime target for this bile acid.

Progesterone and Caspase-3 Activation in Equine Cyclic Corpora Lutea

Soon after ovulation, the newly formed corpus luteum (CL) starts secreting progesterone (P(4)), necessary for implantation. The CL, an ovarian transient endocrine organ, undergoes growth and regression throughout its life span. The objective of this study was to evaluate if caspase-3 mediates cell death in the equine cyclic luteal structures and relate it to luteal endocrine function. Blood and luteal tissue were collected during the breeding season after slaughter from 38 randomly assigned cycling mares. Luteal tissues were classified as corpora haemorrhagica (CH; n = 7); mid luteal phase corpora lutea (Mid-CL; n = 17); late or regressing corpora lutea (Late-CL; n = 9) and corpora albicans (CA; n = 5). Plasma P(4) concentration, determined by radioimmunoassay, showed a significant increase from CH to Mid-CL (p < 0.001), followed by a decrease to Late-CL (p < 0.001) and CA (p < 0.001). Caspase-3 processing and poly (ADP) ribose polymerase (PARP) degradation were assessed by western blotting. Active caspase-3 was twofold increased in Mid-CL, Late-CL and CA as compared with CH (p < 0.05). Immunocytochemistry also showed a significant increase in caspase-3 expression in large luteal cells in all structures when compared with CH (p < 0.05). Consistently, the endogenous caspase-3 substrate, PARP, was markedly degraded from CH to CA (p < 0.05). In fact, the ratio of full-length to degraded PARP showed a significant decrease from CH to Mid-CL, Late-CL and CA (p < 0.05). Finally, the decrease in P(4) from Mid- to Late-CL coincided with no further increases in apoptosis. In conclusion, these results suggest that the effector caspase-3 of apoptosis, might play an important role during luteal tissue involution in the mare, even though its relationship with P(4) remains to be elucidated.

Function of nuclear steroid receptors in apoptosis: role of ursodeoxycholic acid

Nuclear steroid receptors such as the glucocorticoid and the mineralocorticoid receptors modulate apoptosis in different cell types through transactivation-dependent and -independent mechanisms. They are involved in both the induction and prevention of apoptosis depending on cell type. However, it is unclear how nuclear steroid receptors can affect expression of the same gene in opposing ways for different cells. In addition to their function as modulators of gene expression, nuclear steroid receptors often act as nuclear transporters of other regulatory molecules, thus indirectly regulating several apoptosis-related genes. Curiously, nuclear steroid receptors are thought to cooperate with the antiapoptotic endogenous bile acid, ursodeoxycholic acid, to prevent programmed cell death. The next decade will almost certainly unveil the remarkable role of nuclear steroid receptors in modulating the life and death struggle of cells and organ systems in human development and function.

Differential regulation of cyclin D1 and cell death by bile acids in primary rat hepatocytes

Ursodeoxycholic (UDCA) and tauroursodeoxycholic (TUDCA) acids modulate apoptosis and regulate cell-cycle effectors, including cyclin D1. In contrast, deoxycholic acid (DCA) induces cell death and cyclin D1. In this study, we explored the role of cyclin D1 in DCA-induced toxicity and further elucidated the antiapoptotic function of UDCA and TUDCA in primary rat hepatocytes. Cells were incubated with DCA and with or without UDCA or TUDCA for 8-30 h. In addition, hepatocytes were transfected with either an adenovirus expressing cyclin D1 or with a cyclin D1 reporter plasmid with or without bile acids. Finally, cells were cotransfected with short interfering RNA targeting p53. Unlike DCA, both UDCA and TUDCA reduced cyclin D1 expression and transcriptional activation, confirming our previous DNA microarray data. Furthermore, UDCA and TUDCA prevented DCA-induced cyclin D1 and cell death. Cyclin D1 overexpression increased DCA-induced Bax translocation, cytochrome c release, and apoptosis. However, UDCA and TUDCA were less efficient at decreasing cyclin D1 levels as well as DCA-induced changes with overexpression. Finally, after p53 silencing, the effects of cyclin D1 overexpression were almost completely abrogated, whereas UDCA and TUDCA cytoprotective potential was reestablished. In conclusion, cyclin D1 is a relevant player in modulating apoptosis by bile acids, in part through a p53-dependent mechanism.

Game and players: mitochondrial apoptosis and the therapeutic potential of ursodeoxycholic acid

Apoptosis represents a universal and exquisitely efficient cellular suicide pathway essential for a variety of normal biological processes ranging from embryonic development to ageing. In fact, tissue homeostasis is dependent on the perfect balance between positive and negative signals that determines the decision between life and death. Therefore, any imbalance can result in a wide range of pathologic disorders associated with unwanted apoptosis or cell growth. During the apoptotic process, the molecular players interact closely with each other in ways relevant to accelerate or interrupt the cellular death process. In addition, two major pathways of apoptosis activation have been recognized as the "intrinsic" mitochondrial pathway and the "extrinsic" death receptor pathway. Although these pathways act independently to initiate apoptosis, a delicate balance and cross-talk between the extrinsic and intrinsic pathways is thought to occur in many cell types. Interestingly, we have shown that ursodeoxycholic acid (UDCA), an endogenous hydrophilic bile acid, is a potent inhibitor of apoptosis by either stabilizing the mitochondrial membrane or modulating the expression of specific upstream targets. Herein, we review the main effectors involved in the death machinery, describe how they interact to regulate apoptosis, and discuss the main pathways that control cell death and survival. Further, we address multiple interesting targets as well as the potential application of UDCA as a therapeutic modality for apoptosis-related disorders.

Modulation of hepatocyte apoptosis: cross-talk between bile acids and nuclear steroid receptors

The efficient removal of unwanted cells, such as senescent, damaged, mutated or infected cells is crucial for the maintenance of normal liver function. In fact, apoptosis has emerged as a potential contributor to the pathogenesis of a number of hepatic disorders, such as viral hepatitis, autoimmune diseases, ethanol-induced injury, cholestasis, and hepatocellular carcinoma. In contrast to the effect of cytotoxic bile acids in the liver, ursodeoxycholic acid (UDCA) has increasingly been used for the treatment of various liver disorders. The clinical efficacy of this hydrophilic bile acid was first recognized by its use in traditional Asian medicine. However, many studies have subsequently confirmed that UDCA improves liver function by three major mechanisms of action, including protection of cholangiocytes against the cytotoxicity of hydrophobic bile acids, stimulation of hepatobiliary secretion, and inhibition of liver cell apoptosis. UDCA acts as a potent inhibitor of the classical mitochondrial pathway of apoptosis, but also interferes with alternate and upstream molecular targets such as the E2F-1/p53 pathway. Together, there is growing evidence that this hydrophilic bile acid may modulate gene expression to prevent cell death. Curiously, as a cholesterol-derived molecule, UDCA interacts with nuclear steroid receptors, such as the glucocorticoid receptor. Nuclear steroid receptors play crucial roles in mediating steroid hormone signaling involved in many biological processes, including apoptosis. Here, we review the anti-apoptotic mechanisms of UDCA in hepatic cells, and discuss a potential involvement of nuclear steroid receptors in mediating the survival effects of UDCA.

Tauroursodeoxycholic acid modulates p53-mediated apoptosis in Alzheimer's disease mutant neuroblastoma cells

Early onset familial Alzheimer's disease (FAD) is linked to autosomal dominant mutations in the amyloid precursor protein (APP) and presenilin 1 and 2 (PS1 and PS2) genes. These are critical mediators of total amyloid beta-peptide (Abeta) production, inducing cell death through uncertain mechanisms. Tauroursodeoxycholic acid (TUDCA) modulates exogenous Abeta-induced apoptosis by interfering with E2F-1/p53/Bax. Here, we used mouse neuroblastoma cells that express either wild-type APP, APP with the Swedish mutation (APPswe), or double-mutated human APP and PS1 (APPswe/DeltaE9), all exhibiting increased Abeta production and aggregation. Cell viability was decreased in APPswe and APPswe/DeltaE9 but was partially reversed by z-VAD.fmk. Nuclear fragmentation and caspase 2, 6 and 8 activation were also readily detected. TUDCA reduced nuclear fragmentation as well as caspase 2 and 6, but not caspase 8 activities. p53 activity, and Bcl-2 and Bax changes, were also modulated by TUDCA. Overexpression of p53, but not mutant p53, in wild-type and mutant neuroblastoma cells was sufficient to induce apoptosis, which, in turn, was reduced by TUDCA. In addition, inhibition of the phosphatidylinositide 3'-OH kinase pathway reduced TUDCA protection against p53-induced apoptosis. In conclusion, FAD mutations are associated with the activation of classical apoptotic pathways. TUDCA reduces p53-induced apoptosis and modulates expression of Bcl-2 family.

Functional Modulation of Nuclear Steroid Receptors by Tauroursodeoxycholic Acid Reduces Amyloid β-Peptide-Induced Apoptosis

Tauroursodeoxycholic acid (TUDCA) prevents amyloid beta-peptide (Abeta)-induced neuronal apoptosis, by modulating both classical mitochondrial pathways and specific upstream targets. In addition, activation of nuclear steroid receptors (NSRs), such as the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) differentially regulates apoptosis in the brain. In this study we investigated whether TUDCA, a cholesterol-derived endogenous molecule, requires NSRs for inhibiting Abeta-induced apoptosis in primary neurons. Our results confirmed that TUDCA significantly reduced Abeta-induced apoptosis; in addition, the fluorescently labeled bile acid molecule was detected diffusely in both cytoplasm and nucleus of rat cortical neurons. Interestingly, experiments using small interfering RNAs (siRNAs) revealed that, in contrast to GR siRNA, MR siRNA abolished the antiapoptotic effect of TUDCA. Abeta incubation reduced MR nuclear translocation while increasing nuclear GR levels. Notably, pretreatment with TUDCA markedly altered Abeta-induced changes in NSRs, including MR dissociation from its cytosolic chaperone, heat shock protein 90, and subsequent translocation to the nucleus. Furthermore, when a carboxy terminus-deleted form of MR was used, nuclear trafficking of both MR and the bile acid was abrogated, suggesting that they translocate to the nucleus as a steroid-receptor complex. Transfection experiments with wild-type or mutant MR confirmed that this interaction was required for TUDCA protection against Abeta-induced apoptosis. Finally, in cotransfection experiments with NSR response element reporter and overexpression constructs, pretreatment with TUDCA significantly modulated Abeta-induced changes in MR and GR transactivation. In conclusion, these results provide novel insights into the specific cellular mechanism of TUDCA antiapoptotic function against Abeta-induced apoptosis and suggest targets for potential therapeutic intervention.

Plasma levels of ursodeoxycholic acid in black bears, Ursus americanus: seasonal changes

To date, no other studies have examined the seasonal changes in circulating levels of various bile acids in the plasma of wild North American black bears, Ursus americanus. Using gas chromatography, bile acid concentrations were measured in plasma samples obtained during either early or late hibernation, and during summer active periods. Thus, specific compositional changes from individual animals were examined through a given year. Total bile acid concentrations in the plasma of these normal animals were found to range between 0.2 and 3.1 micromol/L (0.9 +/- 0.2 micromol/L, mean +/- SEM). Cholic, ursodeoxycholic and chenodeoxycholic acids were the major bile acid species identified. Ursodeoxycholic acid represented 28.0 +/- 2.6% of the total bile acid pool. Deoxycholic and lithocholic acids were found only in small amounts. In addition, total bile acid concentrations were lower in plasma samples obtained during hibernation compared with those obtained during summer active periods (0.6 +/- 0.1 and 1.2 +/- 0.4 micromol/L, respectively; p < 0.05). However, the relative proportion of ursodeoxycholic acid, was significantly greater in winter than in summer (31.5 +/- 3.2% and 22.2 +/- 4.5%, p < 0.05). Finally, taurine-conjugated bile acids were the predominant species in bear plasma, accounting for >67% of the total bile acids. These data demonstrate that ursodeoxycholic acid is a major bile acid in black bear plasma, mostly conjugated with taurine. Further, the finding of seasonal variation in plasma bile acid composition provides evidence to support the possible role that ursodeoxycholic acid may play in cellular protection in hibernating black bears.

Apoptosis and Bcl-2 expression in the livers of patients with steatohepatitis

OBJECTIVES

Apoptosis may play a role in the pathogenesis of alcoholic (ASH) and non-alcoholic steatohepatitis (NASH). In this study, we investigated the modulation of apoptosis-related liver proteins in steatohepatitis.

METHODS

Hepatocyte apoptosis was evaluated by the TUNEL assay in liver tissue of 12 patients with NASH, 12 with ASH and in histologically normal controls. In addition, caspase-3 processing was evaluated by immunoblot analysis. Expression of death receptors, Bcl-2 family members, and NF-kappaB inhibitor (IkappaB) were determined by western blot. Liver biopsies were also graded for inflammation and fibrosis.

RESULTS

Apoptotic hepatocytes were markedly increased in NASH (P<0.05) and ASH (P<0.001) as compared to controls. Active caspase-3 was also elevated in steatohepatitis (P<0.01), coinciding with upregulation of pro-apoptotic Bax (P<0.001). Further, production of tumour necrosis factor-receptor 1 was increased up to 4-fold (P<0.05). Degradation of IkappaB increased >70% in steatohepatitis (P<0.001). Notably, Bcl-2 was also strongly expressed (>100-fold; P<0.001). These data were significantly correlated with relative degrees of portal and lobular inflammation.

CONCLUSION

The results show that liver injury in NASH and ASH is associated with apoptosis and NF-kappaB activation. Anti-apoptotic Bcl-2 is strongly expressed, probably reflecting an adaptive response to obesity or alcohol-related stress.

Nuclear translocation of UDCA by the glucocorticoid receptor is required to reduce TGF-beta1-induced apoptosis in rat hepatocytes

Ursodeoxycholic acid (UDCA) inhibits classical mitochondrial pathways of apoptosis by either directly stabilizing mitochondrial membranes or modulating specific upstream targets. Furthermore, UDCA regulates apoptosis-related genes from transforming growth factor beta1 (TGF-beta1)-induced hepatocyte apoptosis by a nuclear steroid receptor (NSR)-dependent mechanism. In this study, we further investigated the potential role of the glucocorticoid receptor (GR) in the anti-apoptotic function of UDCA. Our results with short interference RNA (siRNA) technology confirmed that UDCA significantly reduces TGF-beta1-induced apoptosis of primary rat hepatocytes through a GR-dependent effect. Immunoprecipitation assays and confocal microscopy showed that UDCA enhanced free GR levels with subsequent GR nuclear translocation. Interestingly, when a carboxy-terminus deleted form of GR was used, UDCA no longer increased free GR and/or GR translocation, nor did it protect against TGF-beta1-induced apoptosis. In co-transfection experiments with GR response element reporter and overexpression constructs, UDCA did not enhance the transactivation of GR with TGF-beta1. Finally, using a fluorescently labeled UDCA molecule, the bile acid appeared diffuse in the cytosol but was aggregated in the nucleus of hepatocytes. Both siRNA assays and transfection experiments with either wild-type or mutant forms of GR showed that nuclear trafficking occurs through a GR-dependent mechanism. In conclusion, these results further clarify the anti-apoptotic mechanism(s) of UDCA and suggest that GR is crucial for the nuclear translocation of this bile acid for reducing apoptosis.

A distinct microarray gene expression profile in primary rat hepatocytes incubated with ursodeoxycholic acid

BACKGROUND/AIMS

Ursodeoxycholic acid (UDCA) and its taurine-conjugated derivative, TUDCA, modulate cell death and cell cycle regulators, such as E2F-1 and p53. However, precise pathways underlying UDCA's effects are not fully understood. The aim of this study was to identify specific cellular targets of UDCA.

METHODS

The expression profile of primary rat hepatocytes incubated with UDCA was determined using Affymetrix GeneChip Rat 230A arrays. Hybridization data were processed to identify genes with significant expression changes. RT-PCR and immunoblot analyses of a selected target confirmed microarray data.

RESULTS

The results showed that >440 genes were modulated with UDCA by >1.5-fold; approximately 25% were significantly different from controls. Genes affected by UDCA included new regulatory molecules, such as Apaf-1. RT-PCR and immunoblotting confirmed a decrease in Apaf-1. Other altered genes were directly involved in cell cycle (cyclin D1, cadherin 1, HMG-box containing protein 1) and apoptosis (prothymosin-alpha) events. The E2F-1/p53/Apaf-1 pathway appears to be targeted by UDCA. Finally, transcripts for proteins with kinase activity and transcription factors were specifically modulated by TUDCA.

CONCLUSIONS

This study expands our knowledge of the biological effects of UDCA in hepatocytes. Most of the identified genes represent novel potential targets of UDCA, which may ultimately explain its therapeutic properties.

Tauroursodeoxycholic acid prevents amyloid-beta peptide-induced neuronal death via a phosphatidylinositol 3-kinase-dependent signaling pathway

Tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, modulates cell death by interrupting classic pathways of apoptosis. Amyloid-beta (Abeta) peptide has been implicated in the pathogenesis of Alzheimer's disease, where a significant loss of neuronal cells is thought to occur by apoptosis. In this study, we explored the cell death pathway and signaling mechanisms involved in Abeta-induced toxicity and further investigated the anti-apoptotic effect(s) of TUDCA. Our data show significant induction of apoptosis in isolated cortical neurons incubated with Abeta peptide. Apoptosis was associated with translocation of pro-apoptotic Bax to the mitochondria, followed by cytochrome c release, caspase activation, and DNA and nuclear fragmentation. In addition, there was almost immediate but weak activation of the serine/threonine protein kinase Akt. Inhibition of the phosphatidylinositide 3 prime-OH kinase (PI3K) pathway with wortmannin did not markedly affect Abeta-induced cell death, suggesting that this signaling pathway is not crucial for Abeta-mediated toxicity. Notably, co-incubation with TUDCA significantly modulated each of the Abeta-induced apoptotic events. Moreover, wortmannin decreased TUDCA protection against Abeta-induced apoptosis, reduced Akt phosphorylation, and increased Bax translocation to mitochondria. Together, these findings indicate that Abeta-induced apoptosis of cortical neurons proceeds through a Bax mitochondrial pathway. Further, the PI3K signaling cascade plays a role in regulating the anti-apoptotic effects of TUDCA.

Inhibition of the E2F-1/p53/Bax pathway by tauroursodeoxycholic acid in amyloid beta-peptide-induced apoptosis of PC12 cells

Amyloid beta-peptide (Abeta)-induced cell death may involve activation of the E2F-1 transcription factor and other cell cycle-related proteins. In previous studies, we have shown that tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, modulates Abeta-induced apoptosis by interfering with crucial events of the mitochondrial pathway. In this study, we examined the role of E2F and p53 activation in the induction of apoptosis by Abeta, and investigated novel molecular targets for TUDCA. The results showed that despite Bcl-2 up-regulation, PC12 neuronal cells underwent significant apoptosis after incubation with the active fragment Abeta (25-35), as assessed by DNA fragmentation, nuclear morphology and caspase-3-like activation. In addition, transcription through the E2F-1 promoter was significantly induced and associated with loss of the retinoblastoma protein. In contrast, levels of E2F-1, p53 and Bax proteins were markedly increased. Overexpression of E2F-1 in PC12 cells was sufficient to induce p53 and Bax proteins, as well as nuclear fragmentation. Notably, TUDCA modulated Abeta-induced apoptosis, E2F-1 induction, p53 stabilization and Bax expression. Further, TUDCA protected PC12 cells against p53- and Bax-dependent apoptosis induced by E2F-1 and p53 overexpression, respectively. In conclusion, the results demonstrate that Abeta-induced apoptosis of PC12 cells proceeds through an E2F-1/p53/Bax pathway, which, in turn, can be specifically inhibited by TUDCA, thus underscoring its potential therapeutic use.

Modulation of nuclear steroid receptors by ursodeoxycholic acid inhibits TGF-beta1-induced E2F-1/p53-mediated apoptosis of rat hepatocytes

We have recently shown that both ursodeoxycholic acid (UDCA) and tauroursodeoxycholic acid (TUDCA) prevent transforming growth factor beta1 (TGF-beta1)-induced hepatocyte apoptosis by modulating the E2F-1/p53/Bax pathway. In addition, activation of glucocorticoid (GR) and mineralocorticoid receptors (MR) inhibits apoptosis in various systems. UDCA induces a ligand-independent activation of the GR, thus potentially regulating a number of targets. In this study, we investigated the role of GR and MR during TGF-beta1-induced hepatocyte apoptosis, and identified additional antiapoptotic targets for UDCA. Our results showed that in primary hepatocytes, TGF-beta1 induced 40-50% decreases in gr and mr mRNA expression (p < 0.01), together with up to 10-fold reductions in their protein levels (p < 0.01). Notably, pretreatment with UDCA resulted in a significant upregulation of nuclear steroid receptors (p < 0.05), which coincided with 2- and 3-fold increases in the level of GR and MR nuclear translocation, respectively, when compared with that of TGF-beta1 alone (p < 0.05). Similarly, TUDCA induced GR and MR nuclear translocations (p < 0.05) and markedly prevented MR protein changes associated with TGF-beta1 (p < 0.05) without affecting GR protein levels. Moreover, when interference RNA was used to inhibit GR and MR, UDCA no longer protected hepatocytes against TGF-beta1-induced apoptosis. In fact, the protective effect of UDCA in TGF-beta1-associated caspase activation decreased from 65 to <10% when GR or MR function was blocked. Finally, the TGF-beta1-induced E2F-1/Mdm-2/p53 apoptotic pathway, normally inhibited by UDCA, was not regulated by the bile acid after GR or MR silencing. These results demonstrate that UDCA protects against apoptosis through an additional pathway that involves nuclear receptors GR and MR as key factors. Further, the E2F-1/Mdm-2/p53 apoptotic pathway appears to be a prime target for UDCA-induced steroid receptor activation.

Hepatocyte apoptosis, expression of death receptors, and activation of NF-kappaB in the liver of nonalcoholic and alcoholic steatohepatitis patients

OBJECTIVES

The increasing incidence of nonalcoholic (NASH) and alcoholic steatohepatitis (ASH), associated with lack of effective treatment, has prompted intensive studies on disease pathogenesis. Apoptosis is recognized as common in liver injury and may also contribute to tissue inflammation, fibrogenesis, and development of cirrhosis. In this study, we identified mechanisms of apoptosis induction in human steatohepatitis, and evaluated potential associations between apoptosis, liver pathology, and clinical presentation in NASH and ASH.

METHODS

Hepatocyte apoptosis was evaluated by the TUNEL assay in 20 patients with NASH (all ambulatory), 40 with ASH (20 ambulatory, 20 hospitalized), and 20 controls. Liver biopsies were also graded for inflammation and fibrosis. Immunohistochemistry was performed for death receptors (Fas and TNF-R1), activated caspase-3, NF-kappaB p65, antiapoptotic Bcl-2 protein, and uncoupling protein 2 (UCP-2).

RESULTS

TUNEL-positive hepatocytes were markedly increased in NASH (p < 0.05) and ASH (p < 0.01). Similar results were obtained for activated caspase-3, confirming the occurrence of apoptosis. The Fas receptor was upregulated in ASH, especially in hospitalized patients (p < 0.01), whereas TNF-R1 was expressed both in NASH and ASH (p < 0.01). In addition, patients with ASH showed a remarkable expression of active NF-kappaB, as compared to NASH and controls (p < 0.01). Degrees of inflammation and fibrosis correlated with NF-kappaB p65 expression, which in turn coincided with apoptosis albeit Bcl-2 and UCP-2 expression.

CONCLUSIONS

Liver injury in NASH and ASH is associated with increased hepatocyte apoptosis mediated by death receptors. Further, apoptosis correlates with active NF-kappaB expression, and disease severity. This potential mechanistic link might provide multiple interesting targets for therapeutic intervention.

The Bile Acid Tauroursodeoxycholic Acid Modulates Phosphorylation and Translocation of Bad via Phosphatidylinositol 3-Kinase in Glutamate-Induced Apoptosis of Rat Cortical Neurons

Neurotoxicity associated with increased glutamate release results in cell death through both necrotic and apoptotic processes. In addition, tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, is a strong modulator of apoptosis in several cell types. The aims of this study were to test the hypothesis that TUDCA reduces the apoptotic threshold induced by glutamate in rat cortical neurons and examine potential transduction pathways involved in both apoptotic signaling and neuroprotection by TUDCA. The results demonstrated that exposure of cortical neurons to glutamate induced cytochrome c release and caspase activation, as well as morphologic changes of apoptosis. These events were associated with down-regulation of antiapoptotic members of the Bcl-2 family, Bcl-2 and Bcl-x(L), and dephosphorylation of the serine/threonine protein kinase Akt. Pretreatment with TUDCA significantly reduced glutamate-induced apoptosis of rat cortical neurons. In addition, TUDCA induced marked phosphorylation and translocation of Bad from mitochondria to the cytosol. Moreover, inhibition of the phosphatidylinositol 3-kinase (PI3K) survival pathway abrogated the protective effects of TUDCA, including phosphorylation and translocation of Bad. In conclusion, TUDCA appears to modulate glutamate-induced neuronal apoptosis, in part, by activating a PI3K-dependent Bad signaling pathway. These data suggest that TUDCA may be beneficial in treating neurodegenerative disorders in which increased glutamate levels contribute to the pathogenesis of the disease.

Tauroursodeoxycholic acid prevents Bax-induced membrane perturbation and cytochrome C release in isolated mitochondria

Bax is a potent pro-apoptotic member of the Bcl-2 protein family that localizes to the mitochondrial membrane during apoptosis. Tauroursodeoxycholic acid (TUDCA) modulates the apoptotic threshold, in part, by preventing Bax translocation both in vitro and in vivo. The mechanisms by which Bax induces and TUDCA inhibits release of cytochrome c are unclear. We show here that recombinant Bax protein induced cytochrome c release in isolated mitochondria without detectable swelling. Co-incubation with TUDCA prevented efflux of mitochondrial factors and proteolytic processing of caspases in cytosolic extracts. Spectroscopic analyses of mitochondria exposed to Bax revealed increased polarity and fluidity of the membrane lipid core as well as altered protein order, indicative of Bax binding, together with loss of spin-label paramagnetism, characteristic of oxidative damage. TUDCA markedly abrogated the Bax-induced membrane perturbation. In conclusion, our results indicate that Bax protein directly induces cytochrome c release from mitochondria through a mechanism that does not require the permeability transition. Rather, it is accompanied by changes in the organization of membrane lipids and proteins. TUDCA is a potent inhibitor of Bax association with mitochondria. Thus, TUDCA modulates apoptosis by suppressing mitochondrial membrane perturbation through pathways that are also independent of the mitochondrial permeability transition.

Membrane structural changes support the involvement of mitochondria in the bile salt-induced apoptosis of rat hepatocytes

The accumulation of toxic bile salts within the hepatocyte plays a key role in organ injury during liver disease. Deoxycholate (DC) and glycochenodeoxycholate (GCDC) induce apoptosis in vitro and in vivo, perhaps through direct perturbation of mitochondrial membrane structure and function. In contrast, ursodeoxycholate (UDC) and its taurine-conjugated form (TUDC) appear to be protective. We show here that hydrophobic bile salts induced apoptosis in cultured rat hepatocytes, without modulating the expression of pro-apoptotic Bax protein, and caused cytochrome c release in isolated mitochondria. Co-incubation with UDC and TUDC prevented cell death and efflux of mitochondrial factors. Using spin-labelling techniques and EPR spectroscopy analysis of isolated rat liver mitochondria, we found significant structural changes at the membrane-water surface in mitochondria exposed to hydrophobic bile salts, including modified lipid polarity and fluidity, altered protein order and increased oxidative injury. UDC, TUDC and cyclosporin A almost completely abrogated DC- and GCDC-induced membrane perturbations. We conclude that the toxicity of hydrophobic bile salts to hepatocytes is mediated by cytochrome c release, through a mechanism associated with marked direct effects on mitochondrial membrane lipid polarity and fluidity, protein order and redox status, without modulation of pro-apoptotic Bax expression. UDC and TUDC can directly suppress disruption of mitochondrial membrane structure, which may represent an important mechanism of hepatoprotection by these bile salts.

[Release of cytochrome C with the interaction of bilirubin, amyloid beta-peptide and glycochenodeoxycholate from isolated mitochondria]

The pathogenesis of cholestasis, bilirubin encephalopathy, and Alzheimer's disease appears to result from accumulation of diverse cytotoxic agents, which in turn may cause apoptotic cell death. In addition, mitochondria has lately been considered as a central executioner of programmed cell death, through the release of caspase activating factors. The aims of this study were to: (a) investigate mitochondrial perturbation during incubation of isolated mitochondria with unconjugated bilirubin (Bb), amyloid beta-peptide (A beta), and glycochenodeoxycholate (GCDC); (b) characterize membrane perturbation in isolated mitochondria induced by each toxic agent, and determine whether the mitochondrial permeabilization is required for cytochrome c redistribution. Mitochondria were isolated from rat liver and brain. Swelling and cytochrome c release were evaluated by spectrophotometry and western blot, respectively. The results showed that Bb as well as A beta and GCDC act directly at the mitochondrial level causing increased organelle volume, permeabilization, as well as cytochrome c release from the intermembrane space in a dose-dependent manner (P < 0.01). Moreover, cyclosporine A inhibited mitochondrial permeability, particularly after Bb- and GCDC-induced swelling (P < 0.01). Cytochrome c efflux was invariably prevented by cyclosporine A (P < 0.05). In conclusion, the results indicate that Bb-, A beta-, and GCDC-induced toxicity, culminating in apoptosis, may result from enhanced mitochondrial permeability, followed by cytochrome c efflux, which can be explained at least in part by the megapore opening.

Perturbation of membrane dynamics in nerve cells as an early event during bilirubin-induced apoptosis

Increased levels of unconjugated bilirubin, the end product of heme catabolism, impair crucial aspects of nerve cell function. In previous studies, we demonstrated that bilirubin toxicity may be due to cell death by apoptosis. To characterize the sequence of events leading to neurotoxicity, we exposed developing rat brain astrocytes and neurons to unconjugated bilirubin and investigated whether changes in membrane dynamic properties can mediate apoptosis. Bilirubin induced a rapid, dose-dependent increase in apoptosis, which was nevertheless preceded by impaired mitochondrial metabolism. Using spin labels and electron paramagnetic resonance spectroscopy analysis of whole cell and isolated mitochondrial membranes exposed to bilirubin, we detected major membrane perturbation. By physically interacting with cell membranes, bilirubin induced an almost immediate increase in lipid polarity sensed at a superficial level. The enhanced membrane permeability coincided with an increase in lipid fluidity and protein mobility and was associated with significant oxidative injury to membrane lipids. In conclusion, apoptosis of nerve cells induced by bilirubin is mediated by its primary effect at physically perturbing the cell membrane. Bilirubin directly interacts with membranes influencing lipid polarity and fluidity, protein order, and redox status. These data suggest that nerve cell membranes are primary targets of bilirubin toxicity.

Bilirubin induces apoptosis via the mitochondrial pathway in developing rat brain neurons

Increased levels of unconjugated bilirubin, the end-product of heme catabolism, are detrimental to the central nervous system. To examine the role of apoptosis in bilirubin-induced toxicity and to characterize the biochemical pathway of cell death, we exposed developing rat brain neurons to purified unconjugated bilirubin at concentrations below and above saturation of human serum albumin. Isolated neurons treated with bilirubin showed increased levels of apoptosis. Mitochondrial cytochrome c was extensively released and accumulated in cytosol. Consistent with this observation, caspase-3 was activated and the full-length substrate poly(ADP)ribose polymerase (PARP) degraded, even in the presence of very modestly elevated concentrations of bilirubin. In parallel, all events were prevented in cells preincubated with ursodeoxycholate. Further experiments showed that bilirubin diminished mitochondrial transmembrane potential (DeltaPsi(m)) and increased mitochondrial-associated Bax protein levels, while directly disrupting membrane lipid and protein structure. In conclusion, bilirubin induces mitochondrial depolarization and Bax translocation via physical interaction with membranes, mediating the mitochondrial pathway of apoptosis in neurons exposed to bilirubin. These results provide a novel insight into the mechanism of bilirubin-induced toxicity.