Presenilins are required for maintenance of neural stem cells in the developing brain

Lipophorin receptors genetically modulate neurodegeneration caused by reduction of Psn expression in the aging Drosophila brain

Mutations in the Presenilin (PSEN) genes are the most common cause of early-onset familial Alzheimer's disease (FAD). Studies in cell culture, in vitro biochemical systems, and knockin mice showed that PSEN mutations are loss-of-function mutations, impairing γ-secretase activity. Mouse genetic analysis highlighted the importance of Presenilin (PS) in learning and memory, synaptic plasticity and neurotransmitter release, and neuronal survival, and Drosophila studies further demonstrated an evolutionarily conserved role of PS in neuronal survival during aging. However, molecular pathways that interact with PS in neuronal survival remain unclear. To identify genetic modifiers that modulate PS-dependent neuronal survival, we developed a new DrosophilaPsn model that exhibits age-dependent neurodegeneration and increases of apoptosis. Following a bioinformatic analysis, we tested top ranked candidate genes by selective knockdown (KD) of each gene in neurons using two independent RNAi lines in Psn KD models. Interestingly, 4 of the 9 genes enhancing neurodegeneration in Psn KD flies are involved in lipid transport and metabolism. Specifically, neuron-specific KD of lipophorin receptors, lpr1 and lpr2, dramatically worsens neurodegeneration in Psn KD flies, and overexpression of lpr1 or lpr2 does not alleviate Psn KD-induced neurodegeneration. Furthermore, lpr1 or lpr2 KD alone also leads to neurodegeneration, increased apoptosis, climbing defects, and shortened lifespan. Lastly, heterozygotic deletions of lpr1 and lpr2 or homozygotic deletions of lpr1 or lpr2 similarly lead to age-dependent neurodegeneration and further exacerbate neurodegeneration in Psn KD flies. These findings show that LpRs modulate Psn-dependent neuronal survival and are critically important for neuronal integrity in the aging brain.

Human Presenilin-1 delivered by AAV9 rescues impaired γ-secretase activity, memory deficits, and neurodegeneration in Psen mutant mice

Mutations in the Presenilin (PSEN1 and PSEN2) genes are the major cause of early-onset familial Alzheimer's disease (FAD). Presenilin (PS) is the catalytic subunit of the γ-secretase complex, which cleaves type I transmembrane proteins, such as Notch and the amyloid precursor protein (APP), and plays an evolutionarily conserved role in the protection of neuronal survival during aging. FAD PSEN1 mutations exhibit impaired γ-secretase activity in cell culture, in vitro, and knockin (KI) mouse brains, and the L435F mutation is the most severe in reducing γ-secretase activity and is located closest to the active site of γ-secretase. Here, we report that introduction of the codon-optimized wild-type human PSEN1 cDNA by adeno-associated virus 9 (AAV9) results in broadly distributed, sustained, low to moderate levels of human PS1 (hPS1) expression and rescues impaired γ-secretase activity in the cerebral cortex of Psen mutant mice either lacking PS or expressing the Psen1 L435F KI allele, as evaluated by endogenous γ-secretase substrates of APP and recombinant γ-secretase products of Notch intracellular domain and Aβ peptides. Furthermore, introduction of hPS1 by AAV9 alleviates impairments of synaptic plasticity and learning and memory in Psen mutant mice. Importantly, AAV9 delivery of hPS1 ameliorates neurodegeneration in the cerebral cortex of aged Psen mutant mice, as shown by the reversal of age-dependent loss of cortical neurons and elevated microgliosis and astrogliosis. These results together show that moderate hPS1 expression by AAV9 is sufficient to rescue impaired γ-secretase activity, synaptic and memory deficits, and neurodegeneration caused by Psen mutations in mouse models.

Presenilins regulate synaptic plasticity in the perforant pathways of the hippocampus

Mutations in the Presenilin genes (PSEN1 and PSEN2) are the major cause of familial Alzheimer's disease (AD), highlighting the importance of Presenilin (PS) in AD pathogenesis. Previous studies of PS function in the hippocampus demonstrated that loss of PS results in the impairment of short- and long-term synaptic plasticity and neurotransmitter release at hippocampal Schaffer collateral (SC) and mossy fiber (MF) synapses. Cortical input to the hippocampus through the lateral perforant pathway (LPP) and the medial perforant pathway (MPP) is critical for normal cognitive functions and is particularly vulnerable during aging and early stages of AD. Whether PS regulates synaptic function in the perforant pathways, however, remained unknown. In the current study, we investigate PS function in the LPP and MPP by performing whole-cell and field-potential electrophysiological recordings using acute hippocampal slices from postnatal forebrain-restricted excitatory neuron-specific PS conditional double knockout (cDKO) mice. We found that paired-pulse ratio (PPR) is reduced in the LPP and MPP of PS cDKO mice. Moreover, synaptic frequency facilitation or depression in the LPP or MPP, respectively, is impaired in PS cDKO mice. Notably, depletion of intracellular Ca2+ stores by inhibition of sarcoendoplasmic reticulum Ca2+ ATPase (SERCA) minics and occludes the effects of PS inactivation, as evidenced by decreases of the evoked excitatory postsynaptic currents (EPSCs) amplitude in the LPP and MPP of control neurons but no effect on the EPSC amplitude in PS cDKO neurons, suggesting that impaired intracellular calcium homeostasis in the absence of PS may contribute to the observed deficits in synaptic transmission. While spontaneous synaptic events, such as both the frequency and the amplitude of spontaneous or miniature EPSCs, are similar between PS cDKO and control neurons, long-term potentiation (LTP) is impaired in the LPP and MPP of PS cDKO mice, accompanied with reduction of evoked NMDA receptor-mediated responses. These findings show the importance of PS in the regulation of synaptic plasticity and intracellular calcium homeostasis in the hippocampal perforant pathways.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca2+-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

Pen-2 Negatively Regulates the Differentiation of Oligodendrocyte Precursor Cells into Astrocytes in the Central Nervous System

Mutations on γ-secretase subunits are associated with neurologic diseases. Whereas the role of γ-secretase in neurogenesis has been intensively studied, little is known about its role in astrogliogenesis. Recent evidence has demonstrated that astrocytes can be generated from oligodendrocyte precursor cells (OPCs). However, it is not well understood what mechanism may control OPCs to differentiate into astrocytes. To address the above questions, we generated two independent lines of oligodendrocyte lineage-specific presenilin enhancer 2 (Pen-2) conditional KO mice. Both male and female mice were used. Here we demonstrate that conditional inactivation of Pen-2 mediated by Olig1-Cre or NG2-CreERT2 causes enhanced generation of astrocytes. Lineage-tracing experiments indicate that abnormally generated astrocytes are derived from Cre-expressing OPCs in the CNS in Pen-2 conditional KO mice. Mechanistic analysis reveals that deletion of Pen-2 inhibits the Notch signaling to upregulate signal transducer and activator of transcription 3, which triggers activation of GFAP to promote astrocyte differentiation. Together, these novel findings indicate that Pen-2 regulates the specification of astrocytes from OPCs through the signal transducer and activator of transcription 3 signaling.SIGNIFICANCE STATEMENT Astrocytes and oligodendrocyte (OLs) play critical roles in the brain. Recent evidence has demonstrated that astrocytes can be generated from OL precursor cells (OPCs). However, it remains poorly understood what mechanism governs the differentiation of OPCs into astrocytes. In this study, we took advantage of OL lineage cells specific presenilin enhancer 2 (Pen-2) conditional KO mice. We show that deletion of Pen-2 leads to dramatically enhanced astrocyte differentiation from OPCs in the CNS. Mechanistic analysis reveals that deletion of Pen-2 inhibits Hes1 and activates signal transducer and activator of transcription 3 to trigger GFAP activation which promotes astrocyte differentiation. Overall, this study identifies a novel function of Pen-2 in astrogliogenesis from OPCs.

Flexible and Accurate Substrate Processing with Distinct Presenilin/γ-Secretases in Human Cortical Neurons

Mutations in the presenilin genes (PS1, PS2) have been linked to the majority of familial Alzheimer’s disease (AD). Although great efforts have been made to investigate pathogenic PS mutations, which ultimately cause an increase in the toxic form of β-amyloid (Aβ), the intrinsic physiological functions of PS in human neurons remain to be determined. In this study, to investigate the physiological roles of PS in human neurons, we generated PS1 conditional knock-out (KO) induced pluripotent stem cells (iPSCs), in which PS1 can be selectively abrogated under Cre transduction with or without additional PS2 KO. We showed that iPSC-derived neural progenitor cells (NPCs) do not confer a maintenance ability in the absence of both PS1 and PS2, showing the essential role of PS in Notch signaling. We then generated PS-null human cortical neurons, where PS1 was intact until full neuronal differentiation occurred. Aβ40 production was reduced exclusively in human PS1/PS2-null neurons along with a concomitant accumulation of amyloid β precursor protein (APP)-C-terminal fragments CTFs, whereas Aβ42 was decreased in neurons devoid of PS2. Unlike previous studies in mice, in which APP cleavage is largely attributable to PS1, γ-secretase activity seemed to be comparable between PS1 and PS2. In contrast, cleavage of another substrate, N-cadherin, was impaired only in neurons devoid of PS1. Moreover, PS2/γ-secretase exists largely in late endosomes/lysosomes, as measured by specific antibody against the γ-secretase complex, in which Aβ42 species are supposedly produced. Using this novel stem cell-based platform, we assessed important physiological PS1/PS2 functions in mature human neurons, the dysfunction of which could underlie AD pathogenesis.

A multifactorial model of pathology for age of onset heterogeneity in familial Alzheimer's disease

Presenilin-1 (PSEN1) mutations cause familial Alzheimer's disease (FAD) characterized by early age of onset (AoO). Examination of a large kindred harboring the PSEN1-E280A mutation reveals a range of AoO spanning 30 years. The pathophysiological drivers and clinical impact of AoO variation in this population are unknown. We examined brains of 23 patients focusing on generation and deposition of beta-amyloid (Aβ) and Tau pathology profile. In 14 patients distributed at the extremes of AoO, we performed whole-exome capture to identify genotype-phenotype correlations. We also studied kinome activity, proteasome activity, and protein polyubiquitination in brain tissue, associating it with Tau phosphorylation profiles. PSEN1-E280A patients showed a bimodal distribution for AoO. Besides AoO, there were no clinical differences between analyzed groups. Despite the effect of mutant PSEN1 on production of Aβ, there were no relevant differences between groups in generation and deposition of Aβ. However, differences were found in hyperphosphorylated Tau (pTau) pathology, where early onset patients showed severe pathology with diffuse aggregation pattern associated with increased activation of stress kinases. In contrast, late-onset patients showed lesser pTau pathology and a distinctive kinase activity. Furthermore, we identified new protective genetic variants affecting ubiquitin-proteasome function in early onset patients, resulting in higher ubiquitin-dependent degradation of differentially phosphorylated Tau. In PSEN1-E280A carriers, altered γ-secretase activity and resulting Aβ accumulation are prerequisites for early AoO. However, Tau hyperphosphorylation pattern, and its degradation by the proteasome, drastically influences disease onset in individuals with otherwise similar Aβ pathology, hinting toward a multifactorial model of disease for FAD. In sporadic AD (SAD), a wide range of heterogeneity, also influenced by Tau pathology, has been identified. Thus, Tau-induced heterogeneity is a common feature in both AD variants, suggesting that a multi-target therapeutic approach should be used to treat AD.

Targeting a Lipid Desaturation Enzyme, SCD1, Selectively Eliminates Colon Cancer Stem Cells through the Suppression of Wnt and NOTCH Signaling

The elimination of the cancer stem cell (CSC) population may be required to achieve better outcomes of cancer therapy. We evaluated stearoyl-CoA desaturase 1 (SCD1) as a novel target for CSC-selective elimination in colon cancer. CSCs expressed more SCD1 than bulk cultured cells (BCCs), and blocking SCD1 expression or function revealed an essential role for SCD1 in the survival of CSCs, but not BCCs. The CSC potential selectively decreased after treatment with the SCD1 inhibitor in vitro and in vivo. The CSC-selective suppression was mediated through the induction of apoptosis. The mechanism leading to selective CSC death was investigated by performing a quantitative RT-PCR analysis of 14 CSC-specific signaling and marker genes after 24 and 48 h of treatment with two concentrations of an inhibitor. The decrease in the expression of Notch1 and AXIN2 preceded changes in the expression of all other genes, at 24 h of treatment in a dose-dependent manner, followed by the downregulation of most Wnt- and NOTCH-signaling genes. Collectively, we showed that not only Wnt but also NOTCH signaling is a primary target of suppression by SCD1 inhibition in CSCs, suggesting the possibility of targeting SCD1 against colon cancer in clinical settings.

Cell-autonomous role of Presenilin in age-dependent survival of cortical interneurons

BACKGROUND

Mutations in the PSEN1 and PSEN2 genes are the major cause of familial Alzheimer's disease. Previous studies demonstrated that Presenilin (PS), the catalytic subunit of γ-secretase, is required for survival of excitatory neurons in the cerebral cortex during aging. However, the role of PS in inhibitory interneurons had not been explored.

METHODS

To determine PS function in GABAergic neurons, we generated inhibitory neuron-specific PS conditional double knockout (IN-PS cDKO) mice, in which PS is selectively inactivated by Cre recombinase expressed under the control of the endogenous GAD2 promoter. We then performed behavioral, biochemical, and histological analyses to evaluate the consequences of selective PS inactivation in inhibitory neurons.

RESULTS

IN-PS cDKO mice exhibit earlier mortality and lower body weight despite normal food intake and basal activity. Western analysis of protein lysates from various brain sub-regions of IN-PS cDKO mice showed significant reduction of PS1 levels and dramatic accumulation of γ-secretase substrates. Interestingly, IN-PS cDKO mice develop age-dependent loss of GABAergic neurons, as shown by normal number of GAD67-immunoreactive interneurons in the cerebral cortex at 2-3 months of age but reduced number of cortical interneurons at 9 months. Moreover, age-dependent reduction of Parvalbumin- and Somatostatin-immunoreactive interneurons is more pronounced in the neocortex and hippocampus of IN-PS cDKO mice. Consistent with these findings, the number of apoptotic cells is elevated in the cerebral cortex of IN-PS cDKO mice, and the enhanced apoptosis is due to dramatic increases of apoptotic interneurons, whereas the number of apoptotic excitatory neurons is unaffected. Furthermore, progressive loss of interneurons in the cerebral cortex of IN-PS cDKO mice is accompanied with astrogliosis and microgliosis.

CONCLUSION

Our results together support a cell-autonomous role of PS in the survival of cortical interneurons during aging. Together with earlier studies, these findings demonstrate a universal, essential requirement of PS in the survival of both excitatory and inhibitory neurons during aging.

A Review of the Familial Alzheimer's Disease Locus PRESENILIN 2 and Its Relationship to PRESENILIN 1

PRESENILIN 1 (PSEN1) and PRESENILIN 2 (PSEN2) genes are loci for mutations causing familial Alzheimer's disease (fAD). However, the function of these genes and how they contribute to fAD pathogenesis has not been fully determined. This review provides a summary of the overlapping and independent functions of the PRESENILINS with a focus on the lesser studied PSEN2. As a core component of the γ-secretase complex, the PSEN2 protein is involved in many γ-secretase-related physiological activities, including innate immunity, Notch signaling, autophagy, and mitochondrial function. These physiological activities have all been associated with AD progression, indicating that PSEN2 plays a particular role in AD pathogenesis.

Conditional Inactivation of Pen-2 in the Developing Neocortex Leads to Rapid Switch of Apical Progenitors to Basal Progenitors

The transition of apical progenitors (APs) to basal progenitors (BPs) is an important neurogenic process during cortical expansion. Presenilin enhancer 2 (Pen-2, also named as Psenen) is a key subunit of γ-secretase and has been implicated in neurodevelopmental disease. However, it remains unknown how Pen-2 may regulate the maintenance of APs. To address this question, we generated a conditional KO (cKO) mouse in which Pen-2 is specifically inactivated in neural progenitor cells in the telencephalon. Both male and female embryos were used. We show that Pen-2 cKO cortices display remarkable depletion of Aps, but transient increase on BPs, compared with controls. We demonstrate that the proliferation rate of APs or BPs is not changed, but the switch of APs to BPs is dramatically accelerated in Pen-2 cKO cortices. Molecular analyses reveal decreased levels of Hes1 and Hes5 but increased levels of Ngn2 and NeuroD1 in Pen-2 KO cells. We report that expression of Notch1 intracellular domain in Pen-2 cKO cortices restores the population of APs and BPs. In summary, these findings highlight a central role of the Notch signaling in Pen-2-dependent maintenance of neural stem cells in the developing neocortex.SIGNIFICANCE STATEMENT Presenilin enhancer 2 (Pen-2) has been implicated in neurodevelopmental disease. However, mechanisms by which Pen-2 regulates cortical development are not understood. In this study, we generated neural progenitor cell-specific Pen-2 conditional KO mice. We observe depletion of apical progenitors and transiently increased the number of basal progenitors in the developing neocortex of Pen-2 mutant mice. Mechanistic analyses reveal decreased levels of Hes1 and Hes5, but increased levels of neurogenic transcription factors in Pen-2 mutant cortices, compared with controls. We demonstrate that reintroduction of Notch intracellular domain into mutant mice restores the population of apical progenitors to basal progenitors. The above findings strongly suggest that the Pen-2-Notch pathway plays an essential role in the maintenance of neural stem cells during cortical development.

Adult hippocampal neurogenesis occurs in the absence of Presenilin 1 and Presenilin 2

Mutations in the presenilin genes (PS1 and PS2) are a major cause of familial-Alzheimer's disease (FAD). Presenilins regulate neurogenesis in the developing brain, with loss of PS1 inducing aberrant premature differentiation of neural progenitor cells, and additional loss of PS2 exacerbating this effect. It is unclear, however, whether presenilins are involved in adult neurogenesis, a process that may be impaired in Alzheimer's disease within the hippocampus. To investigate the requirement of presenilins in adult-generated dentate granule neurons, we examined adult neurogenesis in the PS2-/- adult brain and then employ a retroviral approach to ablate PS1 selectively in dividing progenitor cells of the PS2-/- adult brain. Surprisingly, the in vivo ablation of both presenilins resulted in no defects in the survival and differentiation of adult-generated neurons. There was also no change in the morphology or functional properties of the retroviral-labeled presenilin-null cells, as assessed by dendritic morphology and whole-cell electrophysiology analyses. Furthermore, while FACS analysis showed that stem and progenitor cells express presenilins, inactivation of presenilins from these cells, using a NestinCreER^T2 inducible genetic approach, demonstrated no changes in the proliferation, survival, or differentiation of adult-generated cells. Therefore, unlike their significant role in neurogenesis during embryonic development, presenilins are not required for cell-intrinsic regulation of adult hippocampal neurogenesis.

Therapeutic targeting of lipid synthesis metabolism for selective elimination of cancer stem cells

Cancer stem cells (CSCs) are believed to have an essential role in tumor resistance and metastasis; however, no therapeutic strategy for the selective elimination of CSCs has been established. Recently, several studies have shown that the metabolic regulation for ATP synthesis and biological building block generation in CSCs are different from that in bulk cancer cells and rather similar to that in normal tissue stem cells. To take advantage of this difference for CSC elimination therapy, many studies have tested the effect of blocking these metabolism. Two specific processes for lipid biosynthesis, i.e., fatty acid unsaturation and cholesterol biosynthesis, have been shown to be very effective and selective for CSC targets. In this review, lipid metabolism specific to CSCs are summarized. In addition, how monounsaturated fatty acid and cholesterol synthesis may contribute to CSC maintenance are discussed. Specifically, the molecular mechanism required for lipid synthesis and essential for stem cell biology is highlighted. The limit and preview of the lipid metabolism targeting for CSCs are also discussed.

microRNA-137 promotes endothelial progenitor cell proliferation and angiogenesis in cerebral ischemic stroke mice by targeting NR4A2 through the Notch pathway

Cerebral ischemic stroke (CIS) is one of the common causes of death and disability worldwide. This study aims to investigate effect of miR-137 on endothelial progenitor cells and angiogenesis in CIS by targeting NR4A2 via the Notch pathway. Brain tissues were extracted from CIS and normal mice. Immunohistochemistry was used to determine positive rate of NR4A2 expression. Serum VEGF, Ang, HGF, and IκBα levels were determined by ELISA. RT-qPCR and Western blotting were used to determine expression of related factors. Endothelial progenitor cells in CIS mice were treated and grouped into blank, NC, miR-137 mimic, miR-137 inhibitor, siRNA-NR4A2, and miR-137 inhibitor + siRNA-NR4A2 groups, and cells in normal mice into normal group. Proliferation and apoptosis were determined by MTT and flow cytometry, respectively. NR4A2 protein expression was strongly positive in CIS mice, which showed higher serum levels of VEGF, Ang, and HGF but lower IκBα than normal mice. Compared with normal group, the rest groups (endothelial progenitor cells from CIS mice) showed decreased expressions of miR-137, Hes1, Hes5, and IκBα but elevated NR4A2, Notch, Jagged1, Hey-2, VEGF, Ang, and HGF, inhibited proliferation and enhanced apoptosis. Compared with blank and NC groups, the miR-137 mimic and siRNA-NR4A2 groups exhibited increased expression of miR-137, Hes1, Hes5, and IκBα, but decreased NR4A2, Notch, Jagged1, and Hey-2, with enhanced proliferation and attenuated apoptosis. The miR-137 inhibitor group reversed the conditions. miR-137 enhances the endothelial progenitor cell proliferation and angiogenesis in CIS mice by targeting NR4A2 through the Notch signaling pathway.

Increased adult neurogenesis associated with reactive astrocytosis occurs prior to neuron loss in a mouse model of neurodegenerative disease

AIMS

This study was to investigate whether cell proliferation and adult neurogenesis are affected at early neurodegenerative stage when neuron loss has not begun to display.

METHODS AND RESULTS

Forebrain-specific nicastrin (NCT) conditional knockout (cKO) mice were generated by crossing NCT^f/f with CaMKIIα-Cre Tg mice. BrdU was used as a lineage tracer to label proliferating neural progenitor cells (NPCs). Immunohistochemistry (IHC) on BrdU indicated that the total number of BrdU positive (+) cells was increased in NCT cKO mice. IHC on doublecortin (DCX) showed that the total number of DCX+ cells was also increased in NCT cKO mice. NCT cKO mice displayed significant astrogliosis as well. However, NCT cKO mice at 3 months did not show significant neuronal death or synaptic loss.

CONCLUSIONS

NCT-dependent γ-secretase activity plays an important role in cell proliferation and immature neuron generation. Enhanced neurogenesis and astrogliosis may be early cellular events prior to the occurrence of neuronal death in neurodegenerative disease.

Presenilins regulate synaptic plasticity and mitochondrial calcium homeostasis in the hippocampal mossy fiber pathway

Background

Presenilins play a major role in the pathogenesis of Alzheimer’s disease, in which the hippocampus is particularly vulnerable. Previous studies of Presenilin function in the synapse, however, focused exclusively on the hippocampal Schaffer collateral (SC) pathway. Whether Presenilins play similar or distinct roles in other hippocampal synapses is unknown.

Methods

To investigate the role of Presenilins at mossy fiber (MF) synapses we performed field and whole-cell electrophysiological recordings and Ca²⁺ imaging using acute hippocampal slices of postnatal forebrain-restricted Presenilin conditional double knockout (PS cDKO) and control mice at 2 months of age. We also performed quantitative electron microscopy (EM) analysis to determine whether mitochondrial content is affected at presynaptic MF boutons of PS cDKO mice. We further conducted behavioral analysis to assess spatial learning and memory of PS cDKO and control mice at 2 months in the Morris water maze.

Results

We found that long-term potentiation and short-term plasticity, such as paired-pulse and frequency facilitation, are impaired at MF synapses of PS cDKO mice. Moreover, post-tetanic potentiation (PTP), another form of short-term plasticity, is also impaired at MF synapses of PS cDKO mice. Furthermore, blockade of mitochondrial Ca²⁺ efflux mimics and occludes the PTP deficits at MF synapses of PS cDKO mice, suggesting that mitochondrial Ca²⁺ homeostasis is impaired in the absence of PS. Quantitative EM analysis showed normal number and area of mitochondria at presynaptic MF boutons of PS cDKO mice, indicating unchanged mitochondrial content. Ca²⁺ imaging of dentate gyrus granule neurons further revealed that cytosolic Ca²⁺ increases induced by tetanic stimulation are reduced in PS cDKO granule neurons in acute hippocampal slices, and that inhibition of mitochondrial Ca²⁺ release during high frequency stimulation mimics and occludes the Ca²⁺ defects observed in PS cDKO neurons. Consistent with synaptic plasticity impairment observed at MF and SC synapses in acute PS cDKO hippocampal slices, PS cDKO mice exhibit profound spatial learning and memory deficits in the Morris water maze.

Conclusions

Our findings demonstrate the importance of PS in the regulation of synaptic plasticity and mitochondrial Ca²⁺ homeostasis in the hippocampal MF pathway.

An Evolutionarily Conserved Role of Presenilin in Neuronal Protection in the Aging Drosophila Brain

Mutations in the Presenilin genes are the major genetic cause of Alzheimer's disease. Presenilin and Nicastrin are essential components of γ-secretase, a multi-subunit protease that cleaves Type I transmembrane proteins. Genetic studies in mice previously demonstrated that conditional inactivation of Presenilin or Nicastrin in excitatory neurons of the postnatal forebrain results in memory deficits, synaptic impairment, and age-dependent neurodegeneration. The roles of Drosophila Presenilin (Psn) and Nicastrin (Nct) in the adult fly brain, however, are unknown. To knockdown (KD) Psn or Nct selectively in neurons of the adult brain, we generated multiple shRNA lines. Using a ubiquitous driver, these shRNA lines resulted in 80-90% reduction of mRNA and pupal lethality-a phenotype that is shared with Psn and Nct mutants carrying nonsense mutations. Furthermore, expression of these shRNAs in the wing disc caused notching wing phenotypes, which are also shared with Psn and Nct mutants. Similar to Nct, neuron-specific Psn KD using two independent shRNA lines led to early mortality and rough eye phenotypes, which were rescued by a fly Psn transgene. Interestingly, conditional KD (cKD) of Psn or Nct in adult neurons using the elav-Gal4 and tubulin-Gal80^ts system caused shortened lifespan, climbing defects, increases in apoptosis, and age-dependent neurodegeneration. Together, these findings demonstrate that, similar to their mammalian counterparts, Drosophila Psn and Nct are required for neuronal survival during aging and normal lifespan, highlighting an evolutionarily conserved role of Presenilin in neuronal protection in the aging brain.

Repurposing L-Menthol for Systems Medicine and Cancer Therapeutics? L-Menthol Induces Apoptosis through Caspase 10 and by Suppressing HSP90

The objective of the present study was to repurpose L-menthol, which is frequently used in oral health and topical formulations, for cancer therapeutics. In this article, we argue that monoterpenes such as L-menthol might offer veritable potentials in systems medicine, for example, as cheaper anti-cancer compounds. Other monoterpenes such as limonene, perillyl alcohol, and geraniol have been shown to induce apoptosis in various cancer cell lines, but their mechanisms of action are yet to be completely elucidated. Earlier, we showed that L-menthol modulates tubulin polymerization and apoptosis to inhibit cancer cell proliferation. In the present report, we used an apoptosis-related gene microarray in conjunction with proteomics analyses, as well as in silico interpretations, to study gene expression modulation in human adenocarcinoma Caco-2 cell line in response to L-menthol treatment. The microarray analysis identified caspase 10 as the important initiator caspase, instead of caspase 8. The proteomics analyses showed downregulation of HSP90 protein (also corroborated by its low transcript abundance), which in turn indicated inhibition of AKT-mediated survival pathway, release of pro-apoptotic factor BAD from BAD and BCLxL complex, besides regulation of other factors related to apoptosis. Based on the combined microarray, proteomics, and in silico data, a signaling pathway for L-menthol-induced apoptosis is being presented for the first time here. These data and literature analysis have significant implications for "repurposing" L-menthol beyond oral medicine, and in understanding the mode of action of plant-derived monoterpenes towards development of cheaper anticancer drugs in future.

Presenilin-1 knockin mice reveal loss-of-function mechanism for familial Alzheimer's disease

Presenilins play essential roles in memory formation, synaptic function, and neuronal survival. Mutations in the Presenilin-1 (PSEN1) gene are the major cause of familial Alzheimer's disease (FAD). How PSEN1 mutations cause FAD is unclear, and pathogenic mechanisms based on gain or loss of function have been proposed. Here, we generated Psen1 knockin (KI) mice carrying the FAD mutation L435F or C410Y. Remarkably, KI mice homozygous for either mutation recapitulate the phenotypes of Psen1(-/-) mice. Neither mutation altered Psen1 mRNA expression, but both abolished γ-secretase activity. Heterozygosity for the KI mutation decreased production of Aβ40 and Aβ42, increased the Aβ42/Aβ40 ratio, and exacerbated Aβ deposition. Furthermore, the L435F mutation impairs hippocampal synaptic plasticity and memory and causes age-dependent neurodegeneration in the aging cerebral cortex. Collectively, our findings reveal that FAD mutations can cause complete loss of Presenilin-1 function in vivo, suggesting that clinical PSEN mutations produce FAD through a loss-of-function mechanism.

γ-Secretase modulators reduce endogenous amyloid β42 levels in human neural progenitor cells without altering neuronal differentiation

Soluble γ-secretase modulators (SGSMs) selectively decrease toxic amyloid β (Aβ) peptides (Aβ42). However, their effect on the physiologic functions of γ-secretase has not been tested in human model systems. γ-Secretase regulates fate determination of neural progenitor cells. Thus, we studied the impact of SGSMs on the neuronal differentiation of ReNcell VM (ReN) human neural progenitor cells (hNPCs). Quantitative PCR analysis showed that treatment of neurosphere-like ReN cell aggregate cultures with γ-secretase inhibitors (GSIs), but not SGSMs, induced a 2- to 4-fold increase in the expression of the neuronal markers Tuj1 and doublecortin. GSI treatment also induced neuronal marker protein expression, as shown by Western blot analysis. In the same conditions, SGSM treatment selectively reduced endogenous Aβ42 levels by ∼80%. Mechanistically, we found that Notch target gene expressions were selectively inhibited by a GSI, not by SGSM treatment. We can assert, for the first time, that SGSMs do not affect the neuronal differentiation of hNPCs while selectively decreasing endogenous Aβ42 levels in the same conditions. Our results suggest that our hNPC differentiation system can serve as a useful model to test the impact of GSIs and SGSMs on both endogenous Aβ levels and γ-secretase physiologic functions including endogenous Notch signaling.

Partial loss of presenilin impairs age-dependent neuronal survival in the cerebral cortex

Mutations in the presenilin (PSEN1 and PSEN2) genes are linked to familial Alzheimer's disease (AD) and cause loss of its essential function. Complete inactivation of presenilins in excitatory neurons of the adult mouse cerebral cortex results in progressive memory impairment and age-dependent neurodegeneration, recapitulating key features of AD. In this study, we examine the effects of varying presenilin dosage on cortical neuron survival by generating presenilin-1 conditional knock-out (PS1 cKO) mice carrying two, one, or zero copies of the PS2 gene. We found that PS1 cKO;PS2(+/-) mice at 16 months exhibit marked neurodegeneration in the cerebral cortex with ∼17% reduction of cortical volume and neuron number, as well as astrogliosis and microgliosis compared with ∼50% reduction of cortical volume and neuron number in PS1 cKO;PS2(-/-) mice. Moreover, there are more apoptotic neurons labeled by activated caspase-3 immunoreactivity and TUNEL assay in PS1 cKO;PS2(+/-) mice at 16 months, whereas apoptotic neurons are increased in the PS1 cKO;PS2(-/-) cerebral cortex at 4 months. The accumulation of the C-terminal fragments of the amyloid precursor protein is inversely correlated with PS dosage. Interestingly, levels of PS2 are higher in the cerebral cortex of PS1 cKO mice, suggesting a compensatory upregulation that may provide protection against neurodegeneration in these mice. Together, our findings show that partial to complete loss of presenilin activity causes progressively more severe neurodegeneration in the mouse cerebral cortex during aging, suggesting that impaired presenilin function by PSEN mutations may lead to neurodegeneration and dementia in AD.

The γ-secretase complex: from structure to function

One of the most critical pathological features of Alzheimer’s disease (AD) is the accumulation of β-amyloid (Aβ) peptides that form extracellular senile plaques in the brain. Aβ is derived from β-amyloid precursor protein (APP) through sequential cleavage by β- and γ-secretases. γ-secretase is a high molecular weight complex minimally composed of four components: presenilins (PS), nicastrin, anterior pharynx defective 1 (APH-1), and presenilin enhancer 2 (PEN-2). In addition to APP, γ-secretase also cleaves many other type I transmembrane (TM) protein substrates. As a crucial enzyme for Aβ production, γ-secretase is an appealing therapeutic target for AD. Here, we summarize current knowledge on the structure and function of γ-secretase, as well as recent progress in developing γ-secretase targeting drugs for AD treatment.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Presenilins regulate neurotrypsin gene expression and neurotrypsin-dependent agrin cleavage via cyclic AMP response element-binding protein (CREB) modulation

Presenilins, the catalytic components of the γ-secretase complex, are upstream regulators of multiple cellular pathways via regulation of gene transcription. However, the underlying mechanisms and the genes regulated by these pathways are poorly characterized. In this study, we identify Tequila and its mammalian ortholog Prss12 as genes negatively regulated by presenilins in Drosophila larval brains and mouse embryonic fibroblasts, respectively. Prss12 encodes the serine protease neurotrypsin, which cleaves the heparan sulfate proteoglycan agrin. Altered neurotrypsin activity causes serious synaptic and cognitive defects; despite this, the molecular processes regulating neurotrypsin expression and activity are poorly understood. Using γ-secretase drug inhibitors and presenilin mutants in mouse embryonic fibroblasts, we found that a mature γ-secretase complex was required to repress neurotrypsin expression and agrin cleavage. We also determined that PSEN1 endoproteolysis or processing of well known γ-secretase substrates was not essential for this process. At the transcriptional level, PSEN1/2 removal induced cyclic AMP response element-binding protein (CREB)/CREB-binding protein binding, accumulation of activating histone marks at the neurotrypsin promoter, and neurotrypsin transcriptional and functional up-regulation that was dependent on GSK3 activity. Upon PSEN1/2 reintroduction, this active epigenetic state was replaced by a methyl CpG-binding protein 2 (MeCP2)-containing repressive state and reduced neurotrypsin expression. Genome-wide analysis revealed hundreds of other mouse promoters in which CREB binding is similarly modulated by the presence/absence of presenilins. Our study thus identifies Tequila and neurotrypsin as new genes repressed by presenilins and reveals a novel mechanism used by presenilins to modulate CREB signaling based on controlling CREB recruitment.

Function and dysfunction of presenilin

The presenilin(PS) genes harbor approximately 90% of the identified mutations linked to familial forms of Alzheimer's disease, and the presenilin (PS) proteins are essential components of the γ-secretase complex involved in the proteolytic cleavage of type I receptors, such as Notch and the amyloid precursor protein. Genetic analysis employing cell type-specific conditional knockout technology highlighted the importance of PS in the adult brain, including learning and memory, synaptic function and age-dependent neuronal survival. In the central synapse, PS regulates neurotransmitter release, short- and long-term synaptic plasticity and calcium homeostasis. However, the molecular mechanisms by which PS maintains these essential functions are less clear. Although many γ-secretase substrates have been identified, their physiological relevance is often unclear. The findings that nicastrin and PS conditional knockout mice exhibit similar deficits in memory and age-dependent neurodegeneration are consistent with the notion that γ-secretase-dependent activities of PS are required for the maintenance of memory and neuronal survival, though the γ-secretase physiological substrates, Notch receptors, are not targets of PS in the adult brain. Thus, despite of the intense interest in PS since its identification in 1995, more work is needed to define the molecular and cellular mechanisms by which PS controls brain functions and the dysfunction conferred by disease-causing mutations.

Bmp7 maintains undifferentiated kidney progenitor population and determines nephron numbers at birth

The number of nephrons, the functional units of the kidney, varies among individuals. A low nephron number at birth is associated with a risk of hypertension and the progression of renal insufficiency. The molecular mechanisms determining nephron number during embryogenesis have not yet been clarified. Germline knockout of bone morphogenetic protein 7 (Bmp7) results in massive apoptosis of the kidney progenitor cells and defects in early stages of nephrogenesis. This phenotype has precluded analysis of Bmp7 function in the later stage of nephrogenesis. In this study, utilization of conditional null allele of Bmp7 in combination with systemic inducible Cre deleter mice enabled us to analyze Bmp7 function at desired time points during kidney development, and to discover the novel function of Bmp7 to inhibit the precocious differentiation of the progenitor cells to nephron. Systemic knockout of Bmp7 in vivo after the initiation of kidney development results in the precocious differentiation of the kidney progenitor cells to nephron, in addition to the prominent apoptosis of progenitor cells. We also confirmed that in vitro knockout of Bmp7 in kidney explant culture results in the accelerated differentiation of progenitor population. Finally we utilized colony-forming assays and demonstrated that Bmp7 inhibits epithelialization and differentiation of the kidney progenitor cells. These results indicate that the function of Bmp7 to inhibit the precocious differentiation of the progenitor cells together with its anti-apoptotic effect on progenitor cells coordinately maintains renal progenitor pool in undifferentiated status, and determines the nephron number at birth.

Presenilins regulate calcium homeostasis and presynaptic function via ryanodine receptors in hippocampal neurons

Presenilin (PS) plays a central role in the pathogenesis of Alzheimer's disease, and loss of PS causes progressive memory impairment and age-related neurodegeneration in the mouse cerebral cortex. In hippocampal neurons, PS is essential for neurotransmitter release, NMDA receptor-mediated responses, and long-term potentiation. PS is also involved in the regulation of calcium homeostasis, although the precise site of its action is less clear. Here we investigate the mechanism by which PS regulates synaptic function and calcium homeostasis using acute hippocampal slices from PS conditional knockout mice and primary cultured postnatal hippocampal neurons, in which PS is inducibly inactivated. Using two different calcium probes, Fura-2 and Mag-Fura-2, we found that inactivation of PS in primary hippocampal neurons does not affect calcium concentration in the endoplasmic reticulum. Rather, in the absence of PS, levels of ryanodine receptor (RyR) are reduced in the hippocampus, measured by Western analysis and radioligand binding assay, although the mRNA expression is unaffected. RyR-mediated function is also impaired, as indicated by reduced RyR agonist-induced calcium release from the ER and RyR-mediated synaptic responses in the absence of PS. Furthermore, knockdown of RyR expression in wild-type hippocampal neurons by two independent shRNAs to levels comparable with the RyR protein reduction in PS-deficient hippocampal neurons mimics the defects exhibited in calcium homeostasis and presynaptic function. Collectively, our findings show that PS regulates calcium homeostasis and synaptic function via RyR and suggest that disruption of intracellular calcium homeostasis may be an early pathogenic event leading to presynaptic dysfunction in Alzheimer's disease.

Neddylation dysfunction in Alzheimer's disease

Ubiquitin-dependent proteolysis is a major mechanism that downregulates misfolded proteins or those that have finished a programmed task. In the last two decades, neddylation has emerged as a major regulatory pathway for ubiquitination. Central to the neddylation pathway is the amyloid precursor protein (APP)-binding protein APP-BP1, which together with Uba3, plays an analogous role to the ubiquitin-activating enzyme E1 in nedd8 activation. Activated nedd8 covalently modifies and activates a major class of ubiquitin ligases called Cullin-RING ligases (CRLs). New evidence suggests that neddylation also modifies Type-1 transmembrane receptors such as APP. Here we review the functions of neddylation and summarize evidence suggesting that dysfunction of neddylation is involved in Alzheimer's disease.

The cellular composition of neurogenic periventricular zones in the adult zebrafish forebrain

A central goal of adult neurogenesis research is to characterize the cellular constituents of a neurogenic niche and to understand how these cells regulate the production of new neurons. Because the generation of adult-born neurons may be tightly coupled to their functional requirement, the organization and output of neurogenic niches may vary across different regions of the brain or between species. We have undertaken a comparative study of six (D, Vd, Vv, Dm, Dl, Ppa) periventricular zones (PVZs) harboring proliferative cells present in the adult forebrain of the zebrafish (Danio rerio), a species known to possess widespread neurogenesis throughout life. Using electron microscopy, we have documented for the first time the detailed cytoarchitecture of these zones, and propose a model of the cellular composition of pallial and subpallial PVZs, as well as a classification scheme for identifying morphologically distinct cell types. Immunolabeling of resin-embedded tissue confirmed the phenotype of three constitutively proliferating (bromodeoxyuridine [BrdU]+) cell populations, including a radial glial-like (type IIa) cell immunopositive for both S100β and glutamine synthetase (GS). Our data revealed rostrocaudal differences in the density of distinct proliferative populations, and cumulative labeling studies suggested that the cell cycle kinetics of these populations are not uniform between PVZs. Although the peak numbers of differentiated neurons were generated after ~2 weeks among most PVZs, niche-specific decline in the number of newborn neurons in some regions occurred after 4 weeks. Our data suggest that the cytoarchitecture of neurogenic niches and the tempo of neuronal production are regionally distinct in the adult zebrafish forebrain.

Familial frontotemporal dementia-associated presenilin-1 c.548G>T mutation causes decreased mRNA expression and reduced presenilin function in knock-in mice

Mutations in the presenilin-1 (PSEN1) gene are associated with familial Alzheimer's disease and frontotemporal dementia (FTD). Interestingly, neuropathological analysis of a Belgian FTD family carrying a PSEN1 c.548G>T mutation confirmed neurodegeneration in the absence of amyloid plaques. To investigate the impact of the c.548G>T mutation on presenilin-1 (PS1) function in vivo, we introduced this mutation into the genomic Psen1 locus. The resulting c.548G>T knock-in (KI) mice are viable but express markedly lower levels of Psen1 mRNA and protein in the brain. This reduction is due to production of aberrantly spliced transcripts lacking either exon 6 or exons 6 and 7 and their subsequent degradation via non-sense-mediated decay (NMD); inhibition of NMD by cycloheximide treatment stabilized these transcripts and restored the level of Psen1 mRNA in KI/KI brains. Interestingly, the reduction of Psen1 mRNA expression and the degradation of aberrant Psen1 splice products occur exclusively in the brain but not in other tissues. Consistent with decreased Psen1 expression, γ-secretase activity was strongly reduced in the cerebral cortex of KI mice, as measured by de novo γ-secretase-mediated cleavage of APP and Notch. Moreover, PS1 expressed from Psen1 cDNA carrying the c.548G>T mutation displayed normal γ-secretase activity in cultured cells, indicating that the corresponding p.183G>V amino acid substitution does not affect γ-secretase activity. Finally, Psen1 c.548G>T(KI/KI);Psen2(-/-) mice exhibited mild spatial memory deficits in the Morris water maze task. Together, our findings demonstrate that the c.548G>T mutation results in a brain-specific loss of presenilin function due to decreased Psen1 mRNA expression.

Conditional deletion of Notch1 and Notch2 genes in excitatory neurons of postnatal forebrain does not cause neurodegeneration or reduction of Notch mRNAs and proteins

Activation of Notch signaling requires intramembranous cleavage by γ-secretase to release the intracellular domain. We previously demonstrated that presenilin and nicastrin, components of the γ-secretase complex, are required for neuronal survival in the adult cerebral cortex. Here we investigate whether Notch1 and/or Notch2 are functional targets of presenilin/γ-secretase in promoting survival of excitatory neurons in the adult cerebral cortex by generating Notch1, Notch2, and Notch1/Notch2 conditional knock-out (cKO) mice. Unexpectedly, we did not detect any neuronal degeneration in the adult cerebral cortex of these Notch cKO mice up to ∼2 years of age, whereas conditional inactivation of presenilin or nicastrin using the same αCaMKII-Cre transgenic mouse caused progressive, striking neuronal loss beginning at 4 months of age. More surprisingly, we failed to detect any reduction of Notch1 and Notch2 mRNAs and proteins in the cerebral cortex of Notch1 and Notch2 cKO mice, respectively, even though Cre-mediated genomic deletion of the floxed Notch1 and Notch2 exons clearly took place in the cerebral cortex of these cKO mice. Furthermore, introduction of Cre recombinase into primary cortical cultures prepared from postnatal floxed Notch1/Notch2 pups, where Notch1 and Notch2 are highly expressed, completely eliminated their expression, indicating that the floxed Notch1 and Notch2 alleles can be efficiently inactivated in the presence of Cre. Together, these results demonstrate that Notch1 and Notch2 are not involved in the age-related neurodegeneration caused by loss of presenilin or γ-secretase and suggest that there is no detectable expression of Notch1 and Notch2 in pyramidal neurons of the adult cerebral cortex.

Presenilins in synaptic function and disease

The presenilin genes harbor approximately 90% of mutations linked to early-onset familial Alzheimer's disease (FAD), but how these mutations cause the disease is still being debated. Genetic analysis in Drosophila and mice demonstrate that presenilin plays essential roles in synaptic function, learning and memory, as well as neuronal survival in the adult brain, and the FAD-linked mutations alter the normal function of presenilin in these processes. Presenilin has also been reported to regulate the calcium homeostasis of intracellular stores, and presynaptic presenilin controls neurotransmitter release and long-term potentiation through modulation of calcium release from intracellular stores. In this review, we highlight recent advances in deciphering the role of presenilin in synaptic function, calcium regulation and disease, and pose key questions for future studies.

Presenilin-1 regulates neural progenitor cell differentiation in the adult brain

Presenilin-1 (PS1) is the catalytic core of the aspartyl protease γ-secretase. Previous genetic studies using germ-line deletion of PS1 and conditional knock-out mice demonstrated that PS1 plays an essential role in neuronal differentiation during neural development, but it remained unclear whether PS1 plays a similar role in neurogenesis in the adult brain. Here we show that neural progenitor cells infected with lentiviral vectors-expressing short interfering RNA (siRNA) for the exclusive knockdown of PS1 in the neurogenic microenvironments, exhibit a dramatic enhancement of cell differentiation. Infected cells differentiated into neurons, astrocytes and oligodendrocytes, suggesting that multipotentiality of neural progenitor cells is not affected by reduced levels of PS1. Neurosphere cultures treated with γ-secretase inhibitors exhibit a similar phenotype of enhanced cell differentiation, suggesting that PS1 function in neural progenitor cells is γ-secretase-dependent. Neurospheres infected with lentiviral vectors expressing siRNA for the targeting of PS1 differentiated even in the presence of the proliferation factors epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), suggesting that PS1 dominates EFG and bFGF signaling pathways. Reduction of PS1 expression in neural progenitor cells was accompanied by a decrease in EGF receptor and β-catenin expression level, suggesting that they are downstream essential transducers of PS1 signaling in adult neural progenitor cells. These findings suggest a physiological role for PS1 in adult neurogenesis, and a potential target for the manipulation of neural progenitor cell differentiation.

Cell cycle deregulation in the neurons of Alzheimer's disease

The cell cycle consists of four main phases: G(1), S, G(2), and M. Most cells undergo these cycles up to 40-60 times in their life. However, neurons remain in a nondividing, nonreplicating phase, G(0). Neurons initiate but do not complete cell division, eventually entering apoptosis. Research has suggested that like cancer, Alzheimer's disease (AD) involves dysfunction in neuronal cell cycle reentry, leading to the development of the two-hit hypothesis of AD. The first hit is abnormal cell cycle reentry, which typically results in neuronal apoptosis and prevention of AD. However, with the second hit of chronic oxidative damage preventing apoptosis, neurons gain "immortality" analogous to tumor cells. Once both of these hits are activated, AD can develop and produce senile plaques and neurofibrillary tangles throughout brain tissue. In this review, we propose a mechanism for neuronal cell cycle reentry and the development of AD.

Presenilin mouse and zebrafish models for dementia: focus on neurogenesis

Autosomal dominant mutations in the presenilin gene PSEN cause familial Alzheimer's disease (AD), a neurological disorder pathologically characterized by intraneuronal accumulation and extracellular deposition of amyloid-β in plaques and intraneuronal, hyperphosphorylated tau aggregation in neurofibrillary tangles. Presenilins (PS/PSENs) are part of the proteolytic γ-secretase complex, which cleaves substrate proteins within the membrane. Cleavage of the amyloid precursor protein (APP) by γ-secretase releases amyloid-β peptides. Besides its role in the processing of APP and other transmembrane proteins, presenilin plays an important role in neural progenitor cell maintenance and neurogenesis. In this review, we discuss the role of presenilin in relation to neurogenesis and neurodegeneration and review the currently available presenilin animal models. In addition to established mouse models, zebrafish are emerging as an attractive vertebrate model organism to study the role of presenilin during the development of the nervous system and in neurodegenerative disorders involving presenilin. Zebrafish is a suitable model organism for large-scale drug screening, making this a valuable model to identify novel therapeutic targets for AD.

Inactivation of presenilins causes pre-synaptic impairment prior to post-synaptic dysfunction

Abstract

Synaptic dysfunction is widely thought to be a pathogenic precursor to neurodegeneration in Alzheimer’s disease (AD), and the extent of synaptic loss provides the best correlate for the severity of dementia in AD patients. Presenilins 1 and 2 are the major causative genes of early-onset familial AD. Conditional inactivation of presenilins in the adult cerebral cortex results in synaptic dysfunction and memory impairment, followed by age-dependent neurodegeneration. To characterize further the consequence of presenilin inactivation in the synapse, we evaluated the temporal development of pre-synaptic and post-synaptic deficits in the Schaeffer-collateral pathway of presenilin conditional double knockout (PS cDKO) mice prior to onset of neurodegeneration. Following presenilin inactivation at 4 weeks, synaptic facilitation and probability of neurotransmitter release are impaired in PS cDKO mice at 5 weeks of age, whereas post-synaptic NMDA receptor (NMDAR)-mediated responses are normal at 5 weeks but impaired at 6 weeks of age. Long-term potentiation induced by theta burst stimulation is also reduced in PS cDKO mice at 6 weeks of age. These results show that loss of presenilins results in pre-synaptic deficits in short-term plasticity and probability of neurotransmitter release prior to post-synaptic NMDAR dysfunction, raising the possibility that presenilins may regulate post-synaptic NMDAR function in part via a trans-synaptic mechanism.

Duplicate dmbx1 genes regulate progenitor cell cycle and differentiation during zebrafish midbrain and retinal development

Background

The Dmbx1 gene is important for the development of the midbrain and hindbrain, and mouse gene targeting experiments reveal that this gene is required for mediating postnatal and adult feeding behaviours. A single Dmbx1 gene exists in terrestrial vertebrate genomes, while teleost genomes have at least two paralogs. We compared the loss of function of the zebrafish dmbx1a and dmbx1b genes in order to gain insight into the molecular mechanism by which dmbx1 regulates neurogenesis, and to begin to understand why these duplicate genes have been retained in the zebrafish genome.

Results

Using gene knockdown experiments we examined the function of the dmbx1 gene paralogs in zebrafish, dmbx1a and dmbx1b in regulating neurogenesis in the developing retina and midbrain. Dose-dependent loss of dmbx1a and dmbx1b function causes a significant reduction in growth of the midbrain and retina that is evident between 48-72 hpf. We show that this phenotype is not due to patterning defects or persistent cell death, but rather a deficit in progenitor cell cycle exit and differentiation. Analyses of the morphant retina or anterior hindbrain indicate that paralogous function is partially diverged since loss of dmbx1a is more severe than loss of dmbx1b. Molecular evolutionary analyses of the Dmbx1 genes suggest that while this gene family is conservative in its evolution, there was a dramatic change in selective constraint after the duplication event that gave rise to the dmbx1a and dmbx1b gene families in teleost fish, suggestive of positive selection. Interestingly, in contrast to zebrafish dmbx1a, over expression of the mouse Dmbx1 gene does not functionally compensate for the zebrafish dmbx1a knockdown phenotype, while over expression of the dmbx1b gene only partially compensates for the dmbx1a knockdown phenotype.

Conclusion

Our data suggest that both zebrafish dmbx1a and dmbx1b genes are retained in the fish genome due to their requirement during midbrain and retinal neurogenesis, although their function is partially diverged. At the cellular level, Dmbx1 regulates cell cycle exit and differentiation of progenitor cells. The unexpected observation of putative post-duplication positive selection of teleost Dmbx1 genes, especially dmbx1a, and the differences in functionality between the mouse and zebrafish genes suggests that the teleost Dmbx1 genes may have evolved a diverged function in the regulation of neurogenesis.

Presenilin 1 mutants impair the self-renewal and differentiation of adult murine subventricular zone-neuronal progenitors via cell-autonomous mechanisms involving notch signaling

The vast majority of pedigrees with familial Alzheimer's disease (FAD) are caused by inheritance of mutations in the PSEN1 1 gene. While genetic ablation studies have revealed a role for presenilin 1 (PS1) in embryonic neurogenesis, little information has emerged regarding the potential effects of FAD-linked PS1 variants on proliferation, self-renewal and differentiation, key events that control cell fate commitment of adult brain neural progenitors (NPCs). We used adult brain subventricular zone (SVZ)-derived NPC cultures transduced with recombinant lentivirus as a means to investigate the effects of various PS1 mutants on self-renewal and differentiation properties. We now show that viral expression of several PS1 mutants in NPCs leads to impaired self-renewal and altered differentiation toward neuronal lineage, in vitro. In line with these observations, diminished constitutive proliferation and steady-state SVZ progenitor pool size was observed in vivo in transgenic mice expressing the PS1DeltaE9 variant. Moreover, NPC cultures established from the SVZ of adult mice expressing PS1DeltaE9 exhibit reduced self-renewal capacity and premature exit toward neuronal fates. To these findings, we show that both the levels of endogenous Notch/CBF-1-transcriptional activity and transcripts encoding Notch target genes are diminished in SVZ NPCs expressing PS1DeltaE9. The deficits in self-renewal and multipotency are restored by expression of Notch1-ICD or a downstream target of the Notch pathway, Hes1. Hence, we argue that a partial reduction in PS-dependent gamma-secretase processing of the Notch, at least in part, accounts for the impairments observed in SVZ NPCs expressing the FAD-linked PS1DeltaE9 variant.

The disintegrin/metalloproteinase ADAM10 is essential for the establishment of the brain cortex

The metalloproteinase and major amyloid precursor protein (APP) alpha-secretase candidate ADAM10 is responsible for the shedding of proteins important for brain development, such as cadherins, ephrins, and Notch receptors. Adam10(-/-) mice die at embryonic day 9.5, due to major defects in development of somites and vasculogenesis. To investigate the function of ADAM10 in brain, we generated Adam10 conditional knock-out (cKO) mice using a Nestin-Cre promotor, limiting ADAM10 inactivation to neural progenitor cells (NPCs) and NPC-derived neurons and glial cells. The cKO mice die perinatally with a disrupted neocortex and a severely reduced ganglionic eminence, due to precocious neuronal differentiation resulting in an early depletion of progenitor cells. Premature neuronal differentiation is associated with aberrant neuronal migration and a disorganized laminar architecture in the neocortex. Neurospheres derived from Adam10 cKO mice have a disrupted sphere organization and segregated more neurons at the expense of astrocytes. We found that Notch-1 processing was affected, leading to downregulation of several Notch-regulated genes in Adam10 cKO brains, in accordance with the central role of ADAM10 in this signaling pathway and explaining the neurogenic phenotype. Finally, we found that alpha-secretase-mediated processing of APP was largely reduced in these neurons, demonstrating that ADAM10 represents the most important APP alpha-secretase in brain. Our study reveals that ADAM10 plays a central role in the developing brain by controlling mainly Notch-dependent pathways but likely also by reducing surface shedding of other neuronal membrane proteins including APP.

Characterization of age-dependent and progressive cortical neuronal degeneration in presenilin conditional mutant mice

Presenilins are the major causative genes of familial Alzheimer's disease (AD). Our previous study has demonstrated essential roles of presenilins in memory and neuronal survival. Here, we explore further how loss of presenilins results in age-related, progressive neurodegeneration in the adult cerebral cortex, where the pathogenesis of AD occurs. To circumvent the requirement of presenilins for embryonic development, we used presenilin conditional double knockout (Psen cDKO) mice, in which presenilin inactivation is restricted temporally and spatially to excitatory neurons of the postnatal forebrain beginning at 4 weeks of age. Increases in the number of degenerating (Fluoro-Jade B+, 7.6-fold) and apoptotic (TUNEL+, 7.4-fold) neurons, which represent approximately 0.1% of all cortical neurons, were first detected at 2 months of age when there is still no significant loss of cortical neurons and volume in Psen cDKO mice. By 4 months of age, significant loss of cortical neurons (approximately 9%) and gliosis was found in Psen cDKO mice. The apoptotic cell death is associated with caspase activation, as shown by increased numbers of cells immunoreactive for active caspases 9 and 3 in the Psen cDKO cortex. The vulnerability of cortical neurons to loss of presenilins is region-specific with cortical neurons in the lateral cortex most susceptible. Compared to the neocortex, the increase in apoptotic cell death and the extent of neurodegeneration are less dramatic in the Psen cDKO hippocampus, possibly in part due to increased neurogenesis in the aging dentate gyrus. Neurodegeneration is also accompanied with mitochondrial defects, as indicated by reduced mitochondrial density and altered mitochondrial size distribution in aging Psen cortical neurons. Together, our findings show that loss of presenilins in cortical neurons causes apoptotic cell death occurring in a very small percentage of neurons, which accumulates over time and leads to substantial loss of cortical neurons in the aging brain. The low occurrence and significant delay of apoptosis among cortical neurons lacking presenilins suggest that loss of presenilins may induce apoptotic neuronal death through disruption of cellular homeostasis rather than direct activation of apoptosis pathways.

Impaired neurotransmitter release in Alzheimer's and Parkinson's diseases

Mutations in several causative genes have been linked to monogenic forms of Alzheimer's disease (AD) or Parkinson's disease (PD). To look for possible common pathogenic mechanisms underlying age-related neurodegeneration in AD and PD, we employed genetic approaches to investigate systematically the roles of these gene products (e.g. presenilins (PS) for AD; Parkin, DJ-1, PINK1 and LRRK2 for PD) in the mouse brain, especially in neural circuits that are particularly vulnerable in AD or PD. Our series of genetic studies revealed that PS play cell type-specific roles in the developing brain with the most prominent function in the maintenance of neural progenitor cells. In the adult cerebral cortex, where the pathogenesis of AD occurs, loss of PS results in progressive memory impairment and age-related neurodegeneration. Specifically, PS are involved in the regulation of long-term potentiation and NMDA receptor functions. Interestingly, our further genetic dissection in the hippocampal Schaeffer collateral pathway highlighted the importance of presynaptic PS in the activity-dependent regulation of glutamate release and long-term potentiation induction via modulation of calcium release from intracellular stores. Intriguingly, our independent genetic analysis of Parkin, DJ-1, PINK1 and LRRK2 showed a common defect in activity-dependent dopamine release caused by PD-linked mutations in these genes. Together, our genetic studies suggest that presynaptic dysfunction might be a converging early pathogenic event before neurodegeneration in AD and PD.

From birth till death: neurogenesis, cell cycle, and neurodegeneration

Neurogenesis in the embryo involves many signaling pathways and transcriptional programs and an elaborate orchestration of cell cycle exit in differentiating precursors. However, while the neurons differentiate into a plethora of different subtypes and different identities, they also presume a highly polar structure with a particular morphology of the cytoskeleton, thereby making it almost impossible for any differentiated cell to re-enter the cell cycle. It has been observed that dysregulated or forced cell cycle reentry is closely linked to neurodegeneration and apoptosis in neurons, most likely through changes in the neurocytoskeleton. However, proliferative cells still exist within the nervous system, and adult neural stem cells (NSCs) have been identified in the Central Nervous System (CNS) in the past decade, raising a great stir in the neuroscience community. NSCs present a new therapeutic potential, and much effort has since gone into understanding the molecular mechanisms driving differentiation of specific neuronal lineages, such as dopaminergic neurons, for use in regenerative medicine, either through transplanted NSCs or manipulation of existing ones. Nevertheless, differentiation and proliferation are two sides of the same coin, just like tumorigenesis and degeneration. Tumor formation may be regarded as a de-differentiation of tissues, where cell cycle mechanisms are reactivated in differentiated cell types. It is thus important to understand the molecular mechanisms underlying various brain tumors in this perspective. The recent Cancer Stem Cell (CSC) hypothesis also suggests the presence of Brain Tumor Initiating Cells (BTICs) within a tumor population, although the exact origin of these rare and mostly elusive BTICs are yet to be identified. This review attempts to investigate the correlation of neural stem cells/precursors, mature neurons, BTICs and brain tumors with respect to cell cycle regulation and the impact of cell cycle in neurodegeneration.

Transcriptional regulation of the murine Presenilin-2 gene reveals similarities and differences to its human orthologue

Inherited Presenilin-2 mutations cause familial Alzheimer's disease, and its regulation may play a role in sporadic cases. The human Presenilin-2 (PSEN2) regulatory region includes two separate promoters modulated by Egr-1, a transcription factor involved in learning and memory. To enable in-vivo analysis of Presenilin-2 regulation, we characterized the murine Presenilin-2 (Psen2) promoter. We identified novel Psen2 Transcription start sites (TSSs) 10 kb upstream of previously reported sites, along with two new alternatively transcribed exons (1A, and 1BC) in the 5' untranslated region. Transcripts initiating in Exon 1A are ubiquitous, whereas exon 1BC-initiated transcripts are non-neuronal. Only the sequence surrounding exon 1A, which includes homologous sequences to the human PSEN2 promoter, harbored significant promoter activity. Sequences upstream of exon 1A and a downstream enhancer were specifically important in neuronal cells, but similar to the human promoter, the murine promoter was characteristic of a housekeeping gene, and its activity depended on Sp1 binding. Egr-1 did not bind the murine promoter. Egr-1 over-expression and down-regulation, as well as in-vivo examination of Egr-1 and Psen2 expression during fear conditioning in mice, showed that Egr-1 does not regulate the murine Psen2 promoter. Differential Psen2 regulation in human and mouse has implications for Alzheimer disease mouse models.

Gender- and age-dependent gamma-secretase activity in mouse brain and its implication in sporadic Alzheimer disease

Alzheimer disease (AD) is an age-related disorder. Aging and female gender are two important risk factors associated with sporadic AD. However, the mechanism by which aging and gender contribute to the pathogenesis of sporadic AD is unclear. It is well known that genetic mutations in γ-secretase result in rare forms of early onset AD due to the aberrant production of Aβ42 peptides, which are the major constituents of senile plaques. However, the effect of age and gender on γ-secretase has not been fully investigated. Here, using normal wild-type mice, we show mouse brain γ-secretase exhibits gender- and age-dependent activity. Both male and female mice exhibit increased Aβ42∶Aβ40 ratios in aged brain, which mimics the effect of familial mutations of Presenilin-1, Presenlin-2, and the amyloid precursor protein on Aβ production. Additionally, female mice exhibit much higher γ-secretase activity in aged brain compared to male mice. Furthermore, both male and female mice exhibit a steady decline in Notch1 γ-secretase activity with aging. Using a small molecule affinity probe we demonstrate that male mice have less active γ-secretase complexes than female mice, which may account for the gender-associated differences in activity in aged brain. These findings demonstrate that aging can affect γ-secretase activity and specificity, suggesting a role for γ-secretase in sporadic AD. Furthermore, the increased APP γ-secretase activity seen in aged females may contribute to the increased incidence of sporadic AD in women and the aggressive Aβ plaque pathology seen in female mouse models of AD. In addition, deceased Notch γ-secretase activity may also contribute to neurodegeneration. Therefore, this study implicates altered γ-secretase activity and specificity as a possible mechanism of sporadic AD during aging.

Indirect regulation of presenilins in CREB-mediated transcription

Presenilins are essential for synaptic function, memory formation, and neuronal survival. Previously, we reported that expression of cAMP response element-binding protein (CREB) target genes is reduced in the cerebral cortex of presenilin (PS) conditional double knock-out (cDKO) mice. To determine whether the reduced expression of the CREB target genes in these mutant mice is due to loss of presenilin directly or secondary to the impaired neuronal activity, we established a sensitive luciferase reporter system to assess direct transcriptional regulation in cultured cells. We first used immortalized PS-deficient mouse embryonic fibroblasts (MEFs), and found that both CREB-mediated transcription and Notch-mediated HES1 transcription are decreased. However, the ubiquitin-C promoter-mediated transcription is also reduced, and among these three reporters, transfection of exogenous PS1 can rescue only the Notch-mediated HES1 transcription. Further Northern analysis revealed transcriptional alterations of Creb, ubiquitin-C, and other housekeeping genes in PS-deficient MEFs, indicating transcriptional dysregulation in these cells. We then used the Cre/loxP system to develop a postnatal PS-deficient cortical neuronal culture. Surprisingly, in these PS-null neurons, CREB-mediated transcription is not significantly decreased, and levels of total and phosphorylated CREB proteins are unchanged as well. Notch-mediated HES1 transcription is markedly reduced, and this reduction can be rescued by exogenous PS1. Together, our findings suggest that CREB-mediated transcription is regulated indirectly by PS in the adult cerebral cortex, and that attenuation of CREB target gene expression in PS cDKO mice is likely due to reduced neuronal activity in these mutant brains.

Cortical development in the presenilin-1 null mutant mouse fails after splitting of the preplate and is not due to a failure of reelin-dependent signaling

Cortical development is disrupted in presenilin-1 null mutant (Psen1-/-) mice. Prior studies have commented on similarities between Psen1-/- and reeler mice. Reelin induces phosphorylation of Dab1 and activates the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Psen1 is known to modulate PI3K/Akt signaling and both known reelin receptors (apoER2 and VLDLR) are substrates for Psen1 associated gamma-secretase activity. The purpose of this study was to determine whether reelin signaling is disrupted in Psen1-/- mice. We show that, while Dab1 is hypophosphorylated late in cortical development in Psen1-/- mice, it is normally phosphorylated at earlier ages and reelin signaling is intact in Psen1-/- primary neuronal cultures. gamma-secretase activity was also not required for reelin-induced phosphorylation of Dab1. Unlike reeler mice the preplate splits in Psen1-/- brain. Thus cortical development in Psen1-/- mice fails only after splitting of the preplate and is not due to an intrinsic failure of reelin signaling.