Stress-induced switch in Numb isoforms enhances Notch-dependent expression of subtype-specific transient receptor potential channel

The PTB and PRR domains of numb regulate neurite outgrowth by influencing voltage-gated calcium channel expression and kinetics

Numb is an evolutionarily conserved protein that regulates the differentiation of neuronal progenitor cells through unknown mechanisms. Numb has four alternative splice variants with different lengths of phosphotyrosine-binding (PTB) and proline-rich regions (PRR) domains. In this study, we demonstrated that Numb expression was increased in the primary cultures of rat cortical and hippocampal neurons over time in vitro, and Numb antisense inhibited neurite outgrowth. We verified that cells overexpressing short PTB (SPTB) or long PTB (LPTB) domains exhibited differentiation or proliferation, respectively. SPTB-mediated differentiation was related to the PRR domains, as cells expressing SPTB/LPRR had longer dendrites and more branched dendrites than cells expressing SPTB/SPRR. The differentiation of both cell types was completely blocked by the Ca2+ chelator. Western blot analysis revealed the increased total protein expression of voltage-gated calcium channel (VGCC) subunit α1C and α1D in cells expressing SPTB and LPTB Numb. The increased expression of the VGCC β3 subunit was only observed in cells expressing SPTB Numb. Immunocytochemistry further showed that SPTB-mediated cell differentiation was associated with increased membrane expression of VGCC subunits α1C, α1D and β3, which corresponded to the higher Ca2+ current (ICa) densities. Furthermore, we found that VGCC of cells transfected with SPTB/SPRR or SPTB/LPRR Numb isoforms exhibit steady-state inactivation (SSI) in both differentiated and undifferentiated phenotypes. A similar SSI of VGCC was observed in the differentiated cells transfected with SPTB/SPRR or SPTB/LPRR Numb isoforms, whereas a left shift SSI of VGCC in cells expressing SPTB/LPRR was detected in the undifferentiated cells. Collectively, these data indicate that SPTB domain is essential for neurite outgrowth involving in membrane expression of VGCC subunits, and LPRR plays a role in neuronal branching and the regulation of VGCC inactivation kinetics.

Post-Developmental Roles of Notch Signaling in the Nervous System

Since its discovery in Drosophila, the Notch signaling pathway has been studied in numerous developmental contexts in diverse multicellular organisms. The role of Notch signaling in nervous system development has been extensively investigated by numerous scientists, partially because many of the core Notch signaling components were initially identified through their dramatic ‘neurogenic’ phenotype of developing fruit fly embryos. Components of the Notch signaling pathway continue to be expressed in mature neurons and glia cells, which is suggestive of a role in the post-developmental nervous system. The Notch pathway has been, so far, implicated in learning and memory, social behavior, addiction, and other complex behaviors using genetic model organisms including Drosophila and mice. Additionally, Notch signaling has been shown to play a modulatory role in several neurodegenerative disease model animals and in mediating neural toxicity of several environmental factors. In this paper, we summarize the knowledge pertaining to the post-developmental roles of Notch signaling in the nervous system with a focus on discoveries made using the fruit fly as a model system as well as relevant studies in C elegans, mouse, rat, and cellular models. Since components of this pathway have been implicated in the pathogenesis of numerous psychiatric and neurodegenerative disorders in human, understanding the role of Notch signaling in the mature brain using model organisms will likely provide novel insights into the mechanisms underlying these diseases.

Notch activation of Ca2+-sensing receptor mediates hypoxia-induced pulmonary hypertension

A recent study from our group demonstrated that the Ca²⁺-sensing receptor (CaSR) was upregulated and that the extracellular Ca²⁺-induced increase in the cytosolic Ca²⁺ concentration [Ca²⁺]_cyt was enhanced in pulmonary arterial smooth muscle cells (PASMCs) from patients with idiopathic pulmonary arterial hypertension. Here, we examined whether hypoxia-induced activation of Notch signaling leads to the activation and upregulation of CaSR in hypoxia-induced pulmonary hypertension (HPH). The activation of Notch signaling with Jag-1, a Notch ligand, can activate the function and increase the expression of CaSR in acute and chronic hypoxic PASMCs. Downregulation of Notch3 with a siRNA attenuates the extracellular Ca²⁺-induced increase in [Ca²⁺]_cyt and the increase in hypoxia-induced PASMC proliferation in acute hypoxic rat PASMCs. Furthermore, we tested the prevention and rescue effects of a γ-secretase inhibitor (DAPT) in HPH rats. For the Jag-1-treated group, right ventricular systolic pressure (RVSP), right heart hypertrophy (RV/LV+S ratio), and the level of right ventricular myocardial fibrosis were higher than the hypoxia alone group. Meanwhile, DAPT treatment prevented and rescued pulmonary hypertension in HPH rats. The Notch activation of CaSR mediates hypoxia-induced pulmonary hypertension. Understanding the new molecular mechanisms that regulate [Ca²⁺]_cyt and PASMC proliferation is critical to elucidating the pathogenesis of HPH and the development of novel therapies for pulmonary hypertension.

Notch Activation of Ca(2+) Signaling in the Development of Hypoxic Pulmonary Vasoconstriction and Pulmonary Hypertension

Hypoxic pulmonary vasoconstriction (HPV) is an important physiological response that optimizes the ventilation/perfusion ratio. Chronic hypoxia causes vascular remodeling, which is central to the pathogenesis of hypoxia-induced pulmonary hypertension (HPH). We have previously shown that Notch3 is up-regulated in HPH and that activation of Notch signaling enhances store-operated Ca(2+) entry (SOCE), an important mechanism that contributes to pulmonary arterial smooth muscle cell (PASMC) proliferation and contraction. Here, we investigate the role of Notch signaling in HPV and hypoxia-induced enhancement of SOCE. We examined SOCE in human PASMCs exposed to hypoxia and pulmonary arterial pressure in mice using the isolated perfused/ventilated lung method. Wild-type and canonical transient receptor potential (TRPC) 6(-/-) mice were exposed to chronic hypoxia to induce HPH. Inhibition of Notch signaling with a γ-secretase inhibitor attenuates hypoxia-enhanced SOCE in PASMCs and hypoxia-induced increase in pulmonary arterial pressure. Our results demonstrate that hypoxia activates Notch signaling and up-regulates TRPC6 channels. Additionally, treatment with a Notch ligand can mimic hypoxic responses. Finally, inhibition of TRPC6, either pharmacologically or genetically, attenuates HPV, hypoxia-enhanced SOCE, and the development of HPH. These results demonstrate that hypoxia-induced activation of Notch signaling mediates HPV and the development of HPH via functional activation and up-regulation of TRPC6 channels. Understanding the molecular mechanisms that regulate cytosolic free Ca(2+) concentration and PASMC proliferation is critical to elucidation of the pathogenesis of HPH. Targeting Notch regulation of TRPC6 will be beneficial in the development of novel therapies for pulmonary hypertension associated with hypoxia.

Genome-wide copy-number variation study of psychosis in Alzheimer's disease

About 40-60% of patients with late-onset Alzheimer's disease (AD) develop psychosis, which represents a distinct phenotype of more severe cognitive and functional deficits. The estimated heritability of AD+P is ~61%, which makes it a good target for genetic mapping. We performed a genome-wide copy-number variation (CNV) study on 496 AD cases with psychosis (AD+P), 639 AD subjects with intermediate psychosis (AD intermediate P) and 156 AD subjects without psychosis (AD-P) who were recruited at the University of Pittsburgh Alzheimer's Disease Research Center using over 1 million single-nucleotide polymorphisms (SNPs) and CNV markers. CNV load analysis found no significant difference in total and average CNV length and CNV number in the AD+P or AD intermediate P groups compared with the AD-P group. Our analysis revealed a marginally significant lower number of duplication events in AD+P cases compared with AD-P controls (P=0.059) using multivariable regression model. The most interesting finding was the presence of a genome-wide significant duplication in the APC2 gene on chromosome 19, which was protective against developing AD+P (odds ratio=0.42; P=7.2E-10). We also observed suggestive associations of duplications with AD+P in the SET (P=1.95E-06), JAG2 (P=5.01E-07) and ZFPM1 (P=2.13E-07) genes and marginal association of a deletion in CNTLN (P=8.87E-04). We have identified potential novel loci for psychosis in Alzheimer's disease that warrant follow-up in large-scale independent studies.

Placing ion channels into a signaling network of T cells: from maturing thymocytes to healthy T lymphocytes or leukemic T lymphoblasts

T leukemogenesis is a multistep process, where the genetic errors during T cell maturation cause the healthy progenitor to convert into the leukemic precursor that lost its ability to differentiate but possesses high potential for proliferation, self-renewal, and migration. A new misdirecting "leukemogenic" signaling network appears, composed by three types of participants which are encoded by (1) genes implicated in determined stages of T cell development but deregulated by translocations or mutations, (2) genes which normally do not participate in T cell development but are upregulated, and (3) nondifferentially expressed genes which become highly interconnected with genes expressed differentially. It appears that each of three groups may contain genes coding ion channels. In T cells, ion channels are implicated in regulation of cell cycle progression, differentiation, activation, migration, and cell death. In the present review we are going to reveal a relationship between different genetic defects, which drive the T cell neoplasias, with calcium signaling and ion channels. We suggest that changes in regulation of various ion channels in different types of the T leukemias may provide the intracellular ion microenvironment favorable to maintain self-renewal capacity, arrest differentiation, induce proliferation, and enhance motility.

Activation of Notch signaling by short-term treatment with Jagged-1 enhances store-operated Ca(2+) entry in human pulmonary arterial smooth muscle cells

Notch signaling plays a critical role in controlling proliferation and differentiation of pulmonary arterial smooth muscle cells (PASMC). Upregulated Notch ligands and Notch3 receptors in PASMC have been reported to promote the development of pulmonary vascular remodeling in patients with pulmonary arterial hypertension (PAH) and in animals with experimental pulmonary hypertension. Activation of Notch receptors by their ligands leads to the cleavage of the Notch intracellular domain (NICD) to the cytosol by γ-secretase; NICD then translocates into the nucleus to regulate gene transcription. In this study, we examined whether short-term activation of Notch functionally regulates store-operated Ca(2+) entry (SOCE) in human PASMC. Treatment of PASMC with the active fragment of human Jagged-1 protein (Jag-1) for 15-60 min significantly increased the amplitude of SOCE induced by passive deletion of Ca(2+) from the intracellular stores, the sarcoplasmic reticulum (SR). The Jag-1-induced enhancement of SOCE was time dependent: the amplitude was maximized at 30 min of treatment with Jag-1, which was closely correlated with the time course of Jag-1-mediated increase in NICD protein level. The scrambled peptide of Jag-1 active fragment had no effect on SOCE. Inhibition of γ-secretase by N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT) significantly attenuated the Jag-1-induced augmentation of SOCE. In addition to the short-term effect, prolonged treatment of PASMC with Jag-1 for 48 h also markedly enhanced the amplitude of SOCE. These data demonstrate that short-term activation of Notch signaling enhances SOCE in PASMC; the NICD-mediated functional interaction with store-operated Ca(2+) channels (SOC) may be involved in the Jag-1-mediated enhancement of SOCE in human PASMC.

Notch signaling in the brain: in good and bad times

Notch signaling is an evolutionarily conserved pathway, which is fundamental for neuronal development and specification. In the last decade, increasing evidence has pointed out an important role of this pathway beyond embryonic development, indicating that Notch also displays a critical function in the mature brain of vertebrates and invertebrates. This pathway appears to be involved in neural progenitor regulation, neuronal connectivity, synaptic plasticity and learning/memory. In addition, Notch appears to be aberrantly regulated in neurodegenerative diseases, including Alzheimer's disease and ischemic injury. The molecular mechanisms by which Notch displays these functions in the mature brain are not fully understood, but are currently the subject of intense research. In this review, we will discuss old and novel Notch targets and molecular mediators that contribute to Notch function in the mature brain and will summarize recent findings that explore the two facets of Notch signaling in brain physiology and pathology.

Notch signaling is involved in neurogenic commitment of human periodontal ligament-derived mesenchymal stem cells

Notch signaling plays critical roles in stem cells by regulating cell fate determination and differentiation. The aim of this study was to evaluate the participation of Notch signaling in neurogenic commitment of human periodontal ligament-derived mesenchymal stem cells (hPDLSCs) and to examine the ability to control differentiation of these cells using modified surfaces containing affinity immobilized Notch ligands. Neurogenic induction of hPDLSCs was performed via neurosphere formation. Cells were aggregated and form spheres as early 1 day in culture. In addition, the induced cells exhibited increased mRNA and protein expression of neuronal markers that is, β3-tubulin and neurofilament. During neuronal differentiation, a significant increase of Hes1 and Hey1 mRNA expression was noted. Using pharmacological inhibition (γ-secretase inhibitor) or genetic manipulation (overexpression of dominant negative mastermind-like transcription co-activators), neurosphere formation was attenuated and a marked decrease in neurogenic mRNA expression was observed. To confirm the role of Notch signaling in neuronal differentiation of hPDLSCs, the Notch ligand, Jagged-1, is bound to the surface using an affinity immobilization technique. The hPDLSC cultured on a Jagged-1-modified surface had increased expression of Notch signaling target genes, Hes-1 and Hey-1, confirming the activity and potency of surface-bound Jagged-1. Further, hPDLSC on surface-bound Jagged-1 under serum-free conditions showed multiple long and thin neurite-like extensions, and an increase in the expression of neurogenic mRNA markers was observed. Pretreatment of the cells with γ-secretase inhibitor, DAPT, before seeding on the Jagged-1-modified surface blocked development of the neurite-like morphology. Together, the results in this study suggest the involvement of Notch signaling in neurogenic commitment of hPDLSCs.

Regulation of calcium influx and signaling pathway in cancer cells via TRPV6-Numb1 interaction

Ca(2+) is a critical factor in the regulation of signal transduction and Ca(2+) homeostasis is altered in different human diseases. The level of Ca(2+) in cells is highly regulated through a diverse class of regulators. Among them is the transient receptor potential vanilloid 6 (TRPV6), which is a Ca(2+) selective channel that absorbs Ca(2+) in the small intestine. TRPV6 is overexpressed in some cancers and exhibits oncogenic potential, but its exact mechanism is still poorly understood. The Numb protein is a cell fate determinant that functions in endocytosis and as a tumor suppressor via the stabilization of p53. Numb protein consisted of four isoforms. Here, we showed a novel function of Numb1, which negatively regulates TRPV6 activity. The expression of Numb1 decreased cytosolic Ca(2+) concentrations in TRPV6-transfected HEK293 cells. When all the isoforms of Numb were depleted using siRNA in a TRPV6 stable cell line, the levels of cytosolic Ca(2+) increased. We observed an interaction between Numb1 and TRPV6 using co-immunoprecipitation. We confirmed this interaction using Fluorescence Resolution Energy Transfer (FRET). We identified the TRPV6 and Numb1 binding site using TRPV6 C-terminal truncation mutants and Numb1 deletion mutants. The binding site in TRPV6 was an aspartic acid at amino acid residue 716, and that binding site in Numb1 was arginine at amino acid residue 434. A Numb1 mutant, lacking TRPV6 binding activity, failed to inhibit TRPV6 activity. Every isoform of Numb knockdown, using an siRNA-based approach in MCF-7 breast cancer cells, not only showed enhanced TRPV6 expression but also both the cytosolic Ca(2+) concentration and cell proliferation were increased. The down-regulated expression of TRPV6 using siRNA increased Numb protein expression; however, the cytosolic influx of Ca(2+) and proliferation of the cell were decreased. To examine downstream signaling during Ca(2+) influx, we performed Western blotting analysis on TRPV6 upregulated cancer cells (MCF-7, PC-3). Taken together, these results demonstrated that Numb1 interacts with TRPV6 through charged residues and inhibits its activity via the regulation of protein expression. Moreover, we provided evidence for a Ca(2+)-regulated cancer cell signaling pathway and that the Ca(2+) channel is a target of cancer cells.

Musashi-1 post-transcriptionally enhances phosphotyrosine-binding domain-containing m-Numb protein expression in regenerating gastric mucosa

OBJECTIVE

Upregulation of the RNA-binding protein Musashi-1 (Msi1) has been shown to occur in rat gastric corpus mucosa after ethanol-induced mucosal injury. However, there is no direct evidence linking Msi1 with gastric regeneration. We examined the process of tissue repair after acute gastric mucosal injury with Msi1-knock-out (KO) mice to clarify the role of Msi1 and Msi1-dependent regulation of m-Numb expression in regenerating gastric mucosa.

METHODS

Acute gastric injury was induced in Msi1-KO and wild-type ICR mice by administering absolute ethanol. Expression of the splicing variants of m-Numb mRNA and protein in the gastric mucosa were analyzed by quantitative RT-PCR and western blotting, respectively.

RESULTS

We demonstrated that phosphotyrosine-binding domain-containing m-Numb expression was significantly upregulated at both the mRNA and protein levels in wild-type mice at 3 h after ethanol-induced acute gastric injury. In contrast, in Msi1-KO mice, the m-Numb protein was expressed weakly, and was associated with delayed regeneration of the injured gastric mucosal epithelium. In the Msi1-KO mouse, the ratio of m-Numb mRNA to total m-Numb mRNA in the heavy polysome fractions was lower than that in the wild-type mouse. Further, we showed that m-Numb-enhancement in gastric mucous cells induced the expression of prostate stem cell antigen and metallothionein-2. Under the m-Numb enhancing condition, the gastric cells exhibited enhanced cell proliferation and were significantly more resistant to H(2)O(2)-induced cell death than control cells.

CONCLUSIONS

Msi1-dependent post-transcriptional enhancement of m-Numb is crucial in gastric epithelial regeneration.

Numb and Alzheimer's disease: the current picture

Twenty-three years ago, numb was identified as a critical regulator in Drosophila sensory organ precursor cell asymmetric divisions. Beyond the recently recognized role in carcinogenesis, Numb seems to be important in Alzheimer’s disease. This assertion comes from the involvement in various processes such as synapse morphogenesis, amyloid precursor protein trafficking, notch signaling, and neurogenesis. The purpose of the present mini-review is to provide the current picture of Numb’s participation in mechanisms underlying Alzheimer’s disease pathogenesis and emphasize potential aspects for future research.

The homocysteine-inducible endoplasmic reticulum (ER) stress protein Herp counteracts mutant α-synuclein-induced ER stress via the homeostatic regulation of ER-resident calcium release channel proteins

Endoplasmic reticulum (ER) stress has been implicated as an initiator or contributing factor in neurodegenerative diseases. The mechanisms that lead to ER stress and whereby ER stress contributes to the degenerative cascades remain unclear but their understanding is critical to devising effective therapies. Here we show that knockdown of Herp (Homocysteine-inducible ER stress protein), an ER stress-inducible protein with an ubiquitin-like (UBL) domain, aggravates ER stress-mediated cell death induced by mutant α-synuclein (αSyn) that causes an inherited form of Parkinson's disease (PD). Functionally, Herp plays a role in maintaining ER homeostasis by facilitating proteasome-mediated degradation of ER-resident Ca(2+) release channels. Deletion of the UBL domain or pharmacological inhibition of proteasomes abolishes the Herp-mediated stabilization of ER Ca(2+) homeostasis. Furthermore, knockdown or pharmacological inhibition of ER Ca(2+) release channels ameliorates ER stress, suggesting that impaired homeostatic regulation of Ca(2+) channels promotes a protracted ER stress with the consequent activation of ER stress-associated apoptotic pathways. Interestingly, sustained upregulation of ER stress markers and aberrant accumulation of ER Ca(2+) release channels were detected in transgenic mutant A53T-αSyn mice. Collectively, these data establish a causative link between impaired ER Ca(2+) homeostasis and chronic ER stress in the degenerative cascades induced by mutant αSyn and suggest that Herp is essential for the resolution of ER stress through maintenance of ER Ca(2+) homeostasis. Our findings suggest a therapeutic potential in PD for agents that increase Herp levels or its ER Ca(2+)-stabilizing action.

Expression of aquaporin 1 and 4 in a congenital hydrocephalus rat model

BACKGROUND

Hydrocephalus occurs because of an imbalance of bulk fluid flow in the brain, and aquaporins (AQPs) play pivotal roles in cerebral water movement as essential mediators during edema and fluid accumulation. AQP1 is a water channel found in the choroid plexus (CP), and AQP4 is expressed at the brain-CSF interfaces and astrocytic end feet; excessive fluid accumulation may involve expression of changes in these AQPs during various stages of hydrocephalus.

OBJECTIVE

To determine the alterations of CP AQP1 expression in congenital hydrocephalus; detect hydrocephalus-induced AQP1 expression in the cortical parenchyma, ependyma, and pia mater of hydrocephalic animals; and evaluate AQP4 expression in congenital hydrocephalus through progressive stages of the condition.

METHODS

We evaluated differential expression of AQPs 1 and 4 in the congenital hydrocephalus Texas rat at postnatal days 5, 10, and 26 in isolated CP and cortex by enzyme-linked immunosorbent assay, Western blot, quantitative reverse transcriptase polymerase chain reaction, and immunohistochemistry.

RESULTS

The CP exhibited a 34% decrease in AQP1 expression in young hydrocephalic pups (postnatal days 5 and 10), which became normal (postnatal day 26) just before death. With advancing hydrocephalus, expression of AQPs 1 and 4 increased at the brain-CSF interfaces; AQP1 was localized to the endothelium of cortical capillaries with increased AQP4 expression in surrounding astrocytes end feet. AQP1 expression level was increased in the pia mater, with prominent AQP4 expression in the subpial layers. Subependymal capillaries expressed AQP1 in the endothelium, with increasing AQP4 expression in surrounding astrocytes. Hydrocephalic animals (postnatal day 26) had significant nonendothelial (CD34) AQP1 expression in the septal nucleus of the basal forebrain, an area affected by increased intracranial pressure.

CONCLUSION

Biphasic AQP1 expression in the CP with increased AQPs 1 and 4 at the brain-fluid interfaces may indicate compensatory mechanisms to regulate choroidal cerebrospinal fluid secretion and increase parenchymal fluid absorption in the high-pressure hydrocephalic condition.

Not(ch) just development: Notch signalling in the adult brain

The Notch pathway is often regarded as a developmental pathway, but components of Notch signalling are expressed and active in the adult brain. With the advent of more sophisticated genetic manipulations, evidence has emerged that suggests both conserved and novel roles for Notch signalling in the adult brain. Not surprisingly, Notch is a key regulator of adult neural stem cells, but it is increasingly clear that Notch signalling also has roles in the regulation of migration, morphology, synaptic plasticity and survival of immature and mature neurons. Understanding the many functions of Notch signalling in the adult brain, and its dysfunction in neurodegenerative disease and malignancy, is crucial to the development of new therapeutics that are centred around this pathway.

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.