Tracking differentiating neural progenitors in pluripotent cultures using microRNA-regulated lentiviral vectors. Proc Natl Acad Sci U S A. 2010;107:11602-11607. [PMID: 20534548 DOI: 10.1073/pnas.1006568107]

Forced LMX1A expression induces dorsal neural fates and disrupts patterning of human embryonic stem cells into ventral midbrain dopaminergic neurons

The differentiation of human pluripotent stem cells into ventral mesencephalic dopaminergic (DA) fate is relevant for the treatment of Parkinson's disease. Shortcuts to obtaining DA cells through direct reprogramming often include forced expression of the transcription factor LMX1A. Although reprogramming with LMX1A can generate tyrosine hydroxylase (TH)-positive cells, their regional identity remains elusive. Using an in vitro model of early human neural tube patterning, we report that forced LMX1A expression induced a ventral-to-dorsal fate shift along the entire neuroaxis with the emergence of roof plate fates despite the presence of ventralizing molecules. The LMX1A-expressing progenitors gave rise to grafts containing roof plate-derived choroid plexus cysts as well as ectopically induced TH-positive neurons of a forebrain identity. Early activation of LMX1A prior to floor plate specification was necessary for the dorsalizing effect. Our work suggests using caution in employing LMX1A for the induction of DA fate, as this factor may generate roof plate rather than midbrain fates.

miRNA-mediated control of exogenous OCT4 during mesenchymal-epithelial transition increases measles vector reprogramming efficiency

OCT4 is a key mediator of induced pluripotent stem cell (iPSC) reprogramming, but the mechanistic insights into the role of exogenous OCT4 and timelines that initiate pluripotency remain to be resolved. Here, using measles reprogramming vectors, we present microRNA (miRNA) targeting of exogenous OCT4 to shut down its expression during the mesenchymal to the epithelial transition phase of reprogramming. We showed that exogenous OCT4 is required only for the initiation of reprogramming and is dispensable for the maturation stage. However, the continuous expression of SOX2, KLF4, and c-MYC is necessary for the maturation stage of the iPSC. Additionally, we demonstrate a novel application of miRNA targeting in a viral vector to contextually control the vector/transgene, ultimately leading to an improved reprogramming efficiency. This novel approach could be applied to other systems for improving the efficiency of vector-induced processes.

Live-cell imaging of microRNA expression with post-transcriptional feedback control

MicroRNAs (miRNAs) are small noncoding RNAs that regulate complex gene expression networks in eukaryotic cells. Because of their unique expression patterns, miRNAs are potential molecular markers for specific cell states. Although a system capable of imaging miRNA in living cells is needed to visually detect miRNA expression, very few fluorescence signal-on sensors that respond to expression of target miRNA (miR-ON sensors) are available. Here we report an miR-ON sensor containing a bidirectional promoter-driven Csy4 endoribonuclease and green fluorescent protein, ZsGreen1, for live-cell imaging of miRNAs with post-transcriptional feedback control. Csy4-assisted miR-ON (Csy4-miR-ON) sensors generate negligible background but respond sensitively to target miRNAs, allowing high-contrast fluorescence detection of miRNAs in various human cells. We show that Csy4-miR-ON sensors enabled imaging of various miRNAs, including miR-21, miR-302a, and miR-133, in vitro as well as in vivo. This robust tool can be used to evaluate miRNA expression in diverse biological and medical applications.

Activation of endogenous retroviruses during brain development causes an inflammatory response

Endogenous retroviruses (ERVs) make up a large fraction of mammalian genomes and are thought to contribute to human disease, including brain disorders. In the brain, aberrant activation of ERVs is a potential trigger for an inflammatory response, but mechanistic insight into this phenomenon remains lacking. Using CRISPR/Cas9‐based gene disruption of the epigenetic co‐repressor protein Trim28, we found a dynamic H3K9me3‐dependent regulation of ERVs in proliferating neural progenitor cells (NPCs), but not in adult neurons. In vivo deletion of Trim28 in cortical NPCs during mouse brain development resulted in viable offspring expressing high levels of ERVs in excitatory neurons in the adult brain. Neuronal ERV expression was linked to activated microglia and the presence of ERV‐derived proteins in aggregate‐like structures. This study demonstrates that brain development is a critical period for the silencing of ERVs and provides causal in vivo evidence demonstrating that transcriptional activation of ERV in neurons results in an inflammatory response.

Optimization of production and transgene expression of a retrogradely transported pseudotyped lentiviral vector

BACKGROUND

To target specific neuronal populations by gene transfer is challenging. A complicating fact is that populations of neurons may have opposing roles despite being found adjacent to each other. One example is the medium spiny neurons of the striatum. These cells have different projection patterns, a trait used in this study to specifically target one population.

NEW METHOD

Here we present a way of labeling and further studying neurons based on their projections. This was achieved by pseudotyping lentiviral vectors with a chimeric glycoprotein allowing for retrograde transport in combination with optimizing the promoter element used.

RESULTS

We transduced on average 4000 neurons of the direct pathway in the striatum, with the viral vector allowing for microscopy and miRNA immunoprecipitation. In addition, we were able to optimize vector production, reducing the time and material used.

COMPARISON WITH EXISTING METHOD

The optimized protocol is more reproducible compared to previously published protocols. Alternative methods to study specific populations of neurons are transgenic animals or, if available, specific promoter elements. However, very specific promoter elements are rarely available and often large, limiting the usefulness in viral vectors. Our optimized retrograde vectors allow for selection based on neuronal projections and are therefore independent of such elements.

CONCLUSION

We have developed a method that allows for specific analysis of neuronal subpopulations in the brain either by microscopy or by biochemical methods e.g. immunoprecipitation. This method is simple to use and can be combined with transgenic animals for studying disease models.

Labeling Microglia with Genetically Encoded Calcium Indicators

Genetically encoded calcium indicators (GECIs) have become widely used for Ca²⁺ imaging in cultured cells as well as in living organisms. Transduction of microglia with viral vectors encoding GECIs provides a convenient means to label microglia for in vivo Ca²⁺ imaging. We describe a method using microglia-specific microRNA-9-regulated viral vector, to label microglial cells with a ratiometric GECI (Twitch-2B). This method enables longitudinal recording of both transient and sustained elevations of Ca²⁺ in microglia in live animals.

MicroRNA-Regulated Gene Delivery Systems for Research and Therapeutic Purposes

Targeted gene delivery relies on the ability to limit the expression of a transgene within a defined cell/tissue population. MicroRNAs represent a class of highly powerful and effective regulators of gene expression that act by binding to a specific sequence present in the corresponding messenger RNA. Involved in almost every aspect of cellular function, many miRNAs have been discovered with expression patterns specific to developmental stage, lineage, cell-type, or disease stage. Exploiting the binding sites of these miRNAs allows for construction of targeted gene delivery platforms with a diverse range of applications. Here, we summarize studies that have utilized miRNA-regulated systems to achieve targeted gene delivery for both research and therapeutic purposes. Additionally, we identify criteria that are important for the effectiveness of a particular miRNA for such applications and we also discuss factors that have to be taken into consideration when designing miRNA-regulated expression cassettes.

Sensitive and long-term monitoring of intracellular microRNAs using a non-integrating cytoplasmic RNA vector

MicroRNAs (miRNAs) are small noncoding RNAs that modulate gene expression at the post-transcriptional level. Different types of cells express unique sets of miRNAs that can be exploited as potential molecular markers to identify specific cell types. Among the variety of miRNA detection methods, a fluorescence-based imaging system that utilises a fluorescent-reporter gene regulated by a target miRNA offers a major advantage for long-term tracking of the miRNA in living cells. In this study, we developed a novel fluorescence-based miRNA-monitoring system using a non-integrating cytoplasmic RNA vector based on a replication-defective and persistent Sendai virus (SeVdp). Because SeVdp vectors robustly and stably express transgenes, this system enabled sensitive monitoring of miRNAs by fluorescence microscopy. By applying this system for cellular reprogramming, we found that miR-124, but not miR-9, was significantly upregulated during direct neuronal conversion. Additionally, we were able to isolate integration-free human induced pluripotent stem cells by long-term tracking of let-7 expression. Notably, this system was easily expandable to allow detection of multiple miRNAs separately and simultaneously. Our findings provide insight into a powerful tool for evaluating miRNA expression during the cellular reprogramming process and for isolating reprogrammed cells potentially useful for medical applications.

Efficient neural differentiation of mouse pluripotent stem cells in a serum-free medium and development of a novel strategy for enrichment of neural cells

Pluripotent stem cells (PSCs) offer an excellent model to study neural development and function. Although various protocols have been developed to direct the differentiation of PSCs into desired neural cell types, many of them suffer from limitations including low efficiency, long duration of culture, and the use of expensive, labile, and undefined growth supplements. In this study, we achieved efficient differentiation of mouse PSCs to neural lineage, in the absence of exogenous molecules, by employing a serum-free culture medium containing knockout serum replacement (KSR). Embryoid bodies (EBs) cultured in this medium predominantly produced neural cells which included neural progenitors (15-18%), immature neurons (8-24%), mature neurons (10-26%), astrocytes (27-61%), and oligodendrocytes (∼1%). Different neuronal subtypes including glutamatergic, GABAergic, cholinergic, serotonergic, and dopaminergic neurons were generated. Importantly, neurons generated in the KSR medium were electrically active. Further, the EB scooping strategy, involving the removal of the EB core region from the peripheral EB outgrowth, resulted in the enrichment of PSC-derived neural cells. Taken together, this study provides the evidence that the KSR medium is ideal for the rapid and efficient generation of neural cells, including functional neurons, from PSCs without the requirement of any other additional molecule.

let-7 regulates radial migration of new-born neurons through positive regulation of autophagy

During adult neurogenesis, newly formed olfactory bulb (OB) interneurons migrate radially to integrate into specific layers of the OB Despite the importance of this process, the intracellular mechanisms that regulate radial migration remain poorly understood. Here, we find that microRNA (miRNA) let-7 regulates radial migration by modulating autophagy in new-born neurons. Using Argonaute2 immunoprecipitation, we performed global profiling of miRNAs in adult-born OB neurons and identified let-7 as a highly abundant miRNA family. Knockdown of let-7 in migrating neuroblasts prevented radial migration and led to an immature morphology of newly formed interneurons. This phenotype was accompanied by a decrease in autophagic activity. Overexpression of Beclin-1 or TFEB in new-born neurons lacking let-7 resulted in re-activation of autophagy and restored radial migration. Thus, these results reveal a miRNA-dependent link between autophagy and adult neurogenesis with implications for neurodegenerative diseases where these processes are impaired.

MicroRNA-regulated viral vectors for gene therapy

Safe and effective gene therapy approaches require targeted tissue-specific transfer of a therapeutic transgene. Besides traditional approaches, such as transcriptional and transductional targeting, microRNA-dependent post-transcriptional suppression of transgene expression has been emerging as powerful new technology to increase the specificity of vector-mediated transgene expression. MicroRNAs are small non-coding RNAs and often expressed in a tissue-, lineage-, activation- or differentiation-specific pattern. They typically regulate gene expression by binding to imperfectly complementary sequences in the 3' untranslated region (UTR) of the mRNA. To control exogenous transgene expression, tandem repeats of artificial microRNA target sites are usually incorporated into the 3' UTR of the transgene expression cassette, leading to subsequent degradation of transgene mRNA in cells expressing the corresponding microRNA. This targeting strategy, first shown for lentiviral vectors in antigen presenting cells, has now been used for tissue-specific expression of vector-encoded therapeutic transgenes, to reduce immune response against the transgene, to control virus tropism for oncolytic virotherapy, to increase safety of live attenuated virus vaccines and to identify and select cell subsets for pluripotent stem cell therapies, respectively. This review provides an introduction into the technical mechanism underlying microRNA-regulation, highlights new developments in this field and gives an overview of applications of microRNA-regulated viral vectors for cardiac, suicide gene cancer and hematopoietic stem cell therapy, as well as for treatment of neurological and eye diseases.

Identification of the miRNA targetome in hippocampal neurons using RIP-seq

MicroRNAs (miRNAs) are key players in the regulation of neuronal processes by targeting a large network of target messenger RNAs (mRNAs). However, the identity and function of mRNAs targeted by miRNAs in specific cells of the brain are largely unknown. Here, we established an adeno-associated viral vector (AAV)-based neuron-specific Argonaute2:GFP-RNA immunoprecipitation followed by high-throughput sequencing to analyse the regulatory role of miRNAs in mouse hippocampal neurons. Using this approach, we identified more than two thousand miRNA targets in hippocampal neurons, regulating essential neuronal features such as cell signalling, transcription and axon guidance. Furthermore, we found that stable inhibition of the highly expressed miR-124 and miR-125 in hippocampal neurons led to significant but distinct changes in the AGO2 binding of target mRNAs, resulting in subsequent upregulation of numerous miRNA target genes. These findings greatly enhance our understanding of the miRNA targetome in hippocampal neurons.

TRIM28 represses transcription of endogenous retroviruses in neural progenitor cells

TRIM28 is a corepressor that mediates transcriptional silencing by establishing local heterochromatin. Here, we show that deletion of TRIM28 in neural progenitor cells (NPCs) results in high-level expression of two groups of endogenous retroviruses (ERVs): IAP1 and MMERVK10C. We find that NPCs use TRIM28-mediated histone modifications to dynamically regulate transcription and silencing of ERVs, which is in contrast to other somatic cell types using DNA methylation. We also show that derepression of ERVs influences transcriptional dynamics in NPCs through the activation of nearby genes and the expression of long noncoding RNAs. These findings demonstrate a unique dynamic transcriptional regulation of ERVs in NPCs. Our results warrant future studies on the role of ERVs in the healthy and diseased brain.

Direct neural conversion from human fibroblasts using self-regulating and nonintegrating viral vectors

Recent findings show that human fibroblasts can be directly programmed into functional neurons without passing via a proliferative stem cell intermediate. These findings open up the possibility of generating subtype-specific neurons of human origin for therapeutic use from fetal cell, from patients themselves, or from matched donors. In this study, we present an improved system for direct neural conversion of human fibroblasts. The neural reprogramming genes are regulated by the neuron-specific microRNA, miR-124, such that each cell turns off expression of the reprogramming genes once the cell has reached a stable neuronal fate. The regulated system can be combined with integrase-deficient vectors, providing a nonintegrative and self-regulated conversion system that rids problems associated with the integration of viral transgenes into the host genome. These modifications make the system suitable for clinical use and therefore represent a major step forward in the development of induced neurons for cell therapy.

Human ESC-derived dopamine neurons show similar preclinical efficacy and potency to fetal neurons when grafted in a rat model of Parkinson's disease

Considerable progress has been made in generating fully functional and transplantable dopamine neurons from human embryonic stem cells (hESCs). Before these cells can be used for cell replacement therapy in Parkinson’s disease (PD), it is important to verify their functional properties and efficacy in animal models. Here we provide a comprehensive preclinical assessment of hESC-derived midbrain dopamine neurons in a rat model of PD. We show long-term survival and functionality using clinically relevant MRI and PET imaging techniques and demonstrate efficacy in restoration of motor function with a potency comparable to that seen with human fetal dopamine neurons. Furthermore, we show that hESC-derived dopamine neurons can project sufficiently long distances for use in humans, fully regenerate midbrain-to-forebrain projections, and innervate correct target structures. This provides strong preclinical support for clinical translation of hESC-derived dopamine neurons using approaches similar to those established with fetal cells for the treatment of Parkinson’s disease.

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Transplants of hESC-DA survive long term and restore DA neurotransmission in vivo

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The functional potency of hESC-DA is similar to human fetal midbrain DA neurons

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hESC-DA are capable of long-distance, target-specific innervation of the host brain

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The axonal outgrowth capacity of hESC-DA meets the requirements for use in humans

microRNA-125 distinguishes developmentally generated and adult-born olfactory bulb interneurons

New neurons, originating from the subventricular zone, are continuously integrating into neuronal circuitry in the olfactory bulb (OB). Using a transgenic sensor mouse, we found that adult-born OB interneurons express microRNA-125 (miR-125), whereas the pre-existing developmentally generated OB interneurons represent a unique population of cells in the adult brain, without miR-125 activity. Stable inhibition of miR-125 in newborn OB neurons resulted in enhanced dendritic morphogenesis, as well as in increased synaptic activation in response to odour sensory stimuli. These data demonstrate that miR-125 controls functional synaptic integration of adult-born OB interneurons. Our results also suggest that absence of an otherwise broadly expressed miRNA is a novel mechanism with which to achieve neuronal subtype specification.

Visualization and genetic modification of resident brain microglia using lentiviral vectors regulated by microRNA-9

Functional studies of resident microglia require molecular tools for their genetic manipulation. Here we show that microRNA-9-regulated lentiviral vectors can be used for the targeted genetic modification of resident microglia in the rodent brain. Using transgenic reporter mice, we demonstrate that murine microglia lack microRNA-9 activity, whereas most other cells in the brain express microRNA-9. Injection of microRNA-9-regulated vectors into the adult rat brain induces transgene expression specifically in cells with morphological features typical of ramified microglia. The majority of transgene-expressing cells colabels with the microglia marker Iba1. We use this approach to visualize and isolate activated resident microglia without affecting circulating and infiltrating monocytes or macrophages in an excitotoxic lesion model in rat striatum. The microRNA-9-regulated vectors described here are a straightforward and powerful tool that facilitates functional studies of resident microglia.

MicroRNA target sites as genetic tools to enhance promoter-reporter specificity for the purification of pancreatic progenitor cells from differentiated embryonic stem cells

Pluripotent cells hold great promise for cell replacement therapies in regenerative medicine. All known protocols for directed in vitro differentiation of pluripotent cells did not yield pure populations complicating the characterization of the derived cells. In addition, the risk of tumor formation due to residual undifferentiated cells is a serious unresolved problem. In the present study the tissue-specific mouse Pdx1 promoter was used to control the expression of the reporter gene GFP2 in mouse ES cells in order to purify them via FACS during in vitro differentiation. The background fluorescence of transduced ES cells hampered the purification of Pdx1-positive cells due to a contaminating population of partially undifferentiated cells. MicroRNAs (mir) are important regulators of gene expression and were used to enhance promoter specificity during differentiation towards pancreatic progenitor cells. The mouse mmu-mir-294 was found to be mainly expressed during pluripotency, whereas the expression of the mir-302 cluster was increased during early differentiation. Integration of a microRNA target site for the mmu-mir-294 into the lentiviral vector reduced the background fluorescence specifically during pluripotency and permitted re-occurrence of GFP2 expression upon differentiation. A combination of the microRNA target site with the Pdx1 promoter fragment allowed the purification of pancreatic progenitors from differentiated ES cells. This population reflected an early pancreatic progenitor population without other contaminating cell lineages. In conclusion, microRNA target sites are efficient regulatory elements to control transgene expression and to enhance tissue specificity as presented in this study facilitating the sorting and purification of Pdx1-positive pancreatic progenitor cells.

RILES, a novel method for temporal analysis of the in vivo regulation of miRNA expression

Novel methods are required to investigate the complexity of microRNA (miRNA) biology and particularly their dynamic regulation under physiopathological conditions. Herein, a novel plasmid-based RNAi-Inducible Luciferase Expression System (RILES) was engineered to monitor the activity of endogenous RNAi machinery. When RILES is transfected in a target cell, the miRNA of interest suppresses the expression of a transcriptional repressor and consequently switch-ON the expression of the luciferase reporter gene. Hence, miRNA expression in cells is signed by the emission of bioluminescence signals that can be monitored using standard bioluminescence equipment. We validated this approach by monitoring in mice the expression of myomiRs-133, −206 and −1 in skeletal muscles and miRNA-122 in liver. Bioluminescence experiments demonstrated robust qualitative and quantitative data that correlate with the miRNA expression pattern detected by quantitative RT-PCR (qPCR). We further demonstrated that the regulation of miRNA-206 expression during the development of muscular atrophy is individual-dependent, time-regulated and more complex than the information generated by qPCR. As RILES is simple and versatile, we believe that this methodology will contribute to a better understanding of miRNA biology and could serve as a rationale for the development of a novel generation of regulatable gene expression systems with potential therapeutic applications.

Dynamic Activity of miR-125b and miR-93 during Murine Neural Stem Cell Differentiation In Vitro and in the Subventricular Zone Neurogenic Niche

Several microRNAs (miRNAs) that are either specifically enriched or highly expressed in neurons and glia have been described, but the identification of miRNAs modulating neural stem cell (NSC) biology remains elusive. In this study, we exploited high throughput miRNA expression profiling to identify candidate miRNAs enriched in NSC/early progenitors derived from the murine subventricular zone (SVZ). Then, we used lentiviral miRNA sensor vectors (LV.miRT) to monitor the activity of shortlisted miRNAs with cellular and temporal resolution during NSC differentiation, taking advantage of in vitro and in vivo models that recapitulate physiological neurogenesis and gliogenesis and using known neuronal- and glial-specific miRNAs as reference. The LV.miRT platform allowed us monitoring endogenous miRNA activity in low represented cell populations within a bulk culture or within the complexity of CNS tissue, with high sensitivity and specificity. In this way we validated and extended previous results on the neuronal-specific miR-124 and the astroglial-specific miR-23a. Importantly, we describe for the first time a cell type- and differentiation stage-specific modulation of miR-93 and miR-125b in SVZ-derived NSC cultures and in the SVZ neurogenic niche in vivo, suggesting key roles of these miRNAs in regulating NSC function.

Exploiting microRNA regulation for genetic engineering

RNA interference (RNAi) has been a landmark discovery in science. A typical application is to knock down the expression of endogenous genes by delivering small interfering RNA (siRNA) into cells triggering the degradation of complementary mRNA. However, RNAi can also be exploited the other way round: making use of the huge diversity of endogenous microRNAs (miRNA), the expression of exogenously introduced genes tagged with artificial miRNA target sequences can be negatively regulated according to the activity of a given miRNA which can be tissue-, lineage-, activation- or differentiation stage specific. This has significantly expanded the regulatory potential of gene transfer vectors and will benefit both basic science and therapeutic applications. This review briefly introduces the reader to the technical basis for exploiting miRNA regulation, followed by a discussion of specific applications for miRNA-regulated vectors/viruses in basic research, gene- and virotherapy.

A double-switch vector system positively regulates transgene expression by endogenous microRNA expression (miR-ON vector)

To better understand and exploit microRNA (miR) regulation, a more precise characterization of miR expression patterns within a tissue or a lineage during development, differentiation, and homeostasis is needed. We previously showed that lentiviral vectors (LV) can be made responsive to miR to stringently control transgene expression as well as to report miR activity "live" and at the single-cell level. Although very useful, this approach reports miR activity by transgene suppression, hampering the direct identification and selection of miR-expressing cells. Here, we describe a strategy to couple transgene expression to the activity of the miR of interest. To this aim, we generated LV encoding two in-series OFF switches: a transcriptional repressor tagged with miR target sequences and a reporter cassette under the control of the repressor. Reporter expression is ON only when the miR is active and represses translation of the transcriptional repressor. We successfully applied this design to different types of repressors, multiple gene encoding vectors and delivered the system either by two separate or a self-contained vector. We demonstrated its performance by live monitoring of two miRs in different stages of human primary hematopoietic stem/progenitor cell differentiation in vivo. Further applications of this approach include imaging of rare miR-expressing cells and positive regulation of a therapeutic or selector gene in target cells identified by the expression of selected miRs.

MicroRNA-124 is a subventricular zone neuronal fate determinant

New neurons are continuously generated from neural stem cells with astrocyte properties, which reside in close proximity to the ventricle in the postnatal and adult brain. In this study we found that microRNA-124 (miR-124) dictates postnatal neurogenesis in the mouse subventricular zone. Using a transgenic reporter mouse we show that miR-124 expression is initiated in the rapid amplifying progenitors and remains expressed in the resulting neurons. When we stably inhibited miR-124 in vivo, neurogenesis was blocked, leading to the appearance of ectopic cells with astrocyte characteristics in the olfactory bulb. Conversely, when we overexpressed miR-124, neural stem cells were not maintained in the subventricular zone and neurogenesis was lost. In summary, our results demonstrate that miR-124 is a neuronal fate determinant in the subventricular zone.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Functional Studies of microRNAs in Neural Stem Cells: Problems and Perspectives

In adult mammals, neural stem cells (NSCs) are found in two niches of the brain; the subventricular zone by the lateral ventricles and the subgranular zone of the dentate gyrus in the hippocampus. Neurogenesis is a complex process that is tightly controlled on a molecular level. Recently, microRNAs (miRNAs) have been implicated to play a central role in the regulation of NCSs. miRNAs are small, endogenously expressed RNAs that regulate gene expression at the post-transcriptional level. However, functional studies of miRNAs are complicated due to current technical limitations. In this review we describe recent findings about miRNAs in NSCs looking closely at miR-124, miR-9, and let-7. In addition, we highlight technical strategies used to investigate miRNA function, accentuating limitations, and potentials.

Fates of murine pluripotent stem cell-derived neural progenitors following transplantation into mouse cochleae

This study evaluated the tumorigenesis risk of induced pluripotent stem (iPS) cells after transplantation into the cochlea. One mouse embryonic stem (ES) cell line and three mouse iPS cell lines, one derived from adult mouse tail-tip fibroblasts (TTFs) and two from mouse embryonic fibroblasts (MEFs), were neurally induced by stromal cell-inducing activity. Before transplantation, the efficiency of neural induction and the proportion of residual undifferentiated cells were evaluated using immunocytochemistry, and no significant differences were observed in the ratios of colonies expressing βIII tubulin, nestin, or octamer (Oct)3/4. Four weeks after transplantation into the cochleae of neonatal mice, the number of surviving transplants of TTF-derived iPS cells generated by retroviral infection was significantly higher than those of MEF-derived iPS cells generated by plasmid transfection. Teratoma formation was identified in one of five cochleae transplanted with TTF-derived iPS cells. However, no significant differences were found in the cell proliferation activity or the extent of differentiation into mature neurons among the cell lines. These findings emphasize the necessity of selecting appropriate iPS cell lines and developing methods to eliminate undifferentiated cells after neural induction, in order to establish safe iPS cell-based therapy for the inner ear.

Cell and Tissue Gene Targeting with Lentiviral Vectors

One of the main advantages of using lentivectors is their capacity to transduce a wide range of cell types, independently from the cell cycle stage. However, transgene expression in certain cell types is sometimes not desirable, either because of toxicity, cell transformation, or induction of transgene-specific immune responses. In other cases, specific targeting of only cancerous cells within a tumor is sought after for the delivery of suicide genes. Consequently, great effort has been invested in developing strategies to control transgene delivery/expression in a cell/tissue-specific manner. These strategies can broadly be divided in three; particle pseudotyping (surface targeting), which entails modification of the envelope glycoprotein (ENV); transcriptional targeting, which utilizes cell-specific promoters and/or inducible promoters; and posttranscriptional targeting, recently applied in lentivectors by introducing sequence targets for cell-specific microRNAs. In this chapter we describe each of these strategies providing some illustrative examples.

Gene and cell therapy for cystic fibrosis: from bench to bedside

Clinical trials in cystic fibrosis (CF) patients established proof-of-principle for transfer of the wild-type cystic fibrosis transmembrane conductance regulator (CFTR) gene to airway epithelial cells. However, the limited efficacy of gene transfer vectors as well as extra- and intracellular barriers have prevented the development of a gene therapy-based treatment for CF. Here, we review the use of new viral and nonviral gene therapy vectors, as well as human artificial chromosomes, to overcome barriers to successful CFTR expression. Pre-clinical studies will surely benefit from novel animal models, such as CF pigs and ferrets. Prenatal gene therapy is a potential alternative to gene transfer to fully developed lungs. However, unresolved issues, including the possibility of adverse effects on pre- and postnatal development, the risk of initiating oncogenic or degenerative processes and germ line transmission require further investigation. Finally, we discuss the therapeutic potential of stem cells for CF lung disease.

Knockdown of GAD67 protein levels normalizes neuronal activity in a rat model of Parkinson's disease

BACKGROUND

Dopamine depletion of the striatum is one of the hallmarks of Parkinson's disease. The loss of dopamine upregulates GAD67 expression in the striatal projection neurons and causes other changes in the activity of the basal ganglia circuit.

METHODS

To normalize the GAD67 expression in the striatum after dopamine depletion, we developed several lentiviral vectors that express RNA interference (RNAi) directed against GAD67 mitochondrial RNA. The vectors were injected into the striatum of hemiparkinsonian rats and the level of GAD67 protein as well as a marker of neuronal activity, mtCO1, was analyzed using western blots.

RESULTS

Unilateral lesions of the dopamine neurons in substantia nigra resulted in an increased level of GAD67 protein in the ipsilateral striatum. Furthermore, we detected significantly higher levels of mtCO1, after dopamine depletion in the striatum. Using a lentiviral vectors with a synthetic miRNA scaffold to deliver RNAi, we were able to normalize the GAD67 protein levels in the parkinsonian rat striatum. In addition, we were able to normalize the increased neural activity, which resulted from the loss of dopamine as measured by the marker mtCO1.

CONCLUSIONS

We conclude that RNAi directed against GAD67 may be a valid approach to correct the dysregulation of the basal ganglia circuit in a rat model of Parkinson's disease. The possibility to correct for a loss of dopamine using nondopamimetic tools is interesting because it may be more directed towards the casual mechanisms of the motor symptoms.

Imaging of Induced Pluripotent Stem Cells: From Cellular Reprogramming to Transplantation

Successful reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) ushered in a new era of regenerative medicine. Human iPSCs provide powerful new approaches for disease modeling, drug testing, developmental studies, and therapeutic applications. Investigating iPSC behavior in vivo and the ultimate feasibility of cell transplantation therapy necessitates the development of novel imaging techniques to longitudinally monitor iPSC localization, proliferation, integration, and differentiation in living subjects. At this five year mark of initial iPSC discovery, we review the current status of imaging iPSCs which ranges from in vitro studies, where imaging was used to study the processes/mechanisms of cellular reprogramming, to in vivo imaging of the survival of transplanted cells. To date, most imaging studies of iPSCs have been based on optical techniques, which include fluorescence and bioluminescence imaging. Since each imaging technique has its advantages and limitations, a combination of multiple imaging modalities may provide complementary information. The ideal imaging approach for tracking iPSCs or their derivatives in patients requires the imaging tag to be non-toxic, biocompatible, and highly specific to reduce perturbation of these cells. In few other scenarios can "personalized medicine" be better illustrated than the use of individual patient-specific iPSCs. Much future effort will be required before this can become a reality and clinical routine, where imaging will play an indispensible role in many facets of iPSC-based research and therapies.

MicroRNA-restricted transgene expression in the retina

Background

Gene transfer using adeno-associated viral (AAV) vectors has been successfully applied in the retina for the treatment of inherited retinal dystrophies. Recently, microRNAs have been exploited to fine-tune transgene expression improving therapeutic outcomes. Here we evaluated the ability of retinal-expressed microRNAs to restrict AAV-mediated transgene expression to specific retinal cell types that represent the main targets of common inherited blinding conditions.

Methodology/Principal Findings

To this end, we generated AAV2/5 vectors expressing EGFP and containing four tandem copies of miR-124 or miR-204 complementary sequences in the 3′UTR of the transgene expression cassette. These vectors were administered subretinally to adult C57BL/6 mice and Large White pigs. Our results demonstrate that miR-124 and miR-204 target sequences can efficiently restrict AAV2/5-mediated transgene expression to retinal pigment epithelium and photoreceptors, respectively, in mice and pigs. Interestingly, transgene restriction was observed at low vector doses relevant to therapy.

Conclusions

We conclude that microRNA-mediated regulation of transgene expression can be applied in the retina to either restrict to a specific cell type the robust expression obtained using ubiquitous promoters or to provide an additional layer of gene expression regulation when using cell-specific promoters.

Patient-specific pluripotent stem cells in neurological diseases

Many human neurological diseases are not currently curable and result in devastating neurologic sequelae. The increasing availability of induced pluripotent stem cells (iPSCs) derived from adult human somatic cells provides new prospects for cellreplacement strategies and disease-related basic research in a broad spectrum of human neurologic diseases. Patient-specific iPSC-based modeling of neurogenetic and neurodegenerative diseases is an emerging efficient tool for in vitro modeling to understand disease and to screen for genes and drugs that modify the disease process. With the exponential increase in iPSC research in recent years, human iPSCs have been successfully derived with different technologies and from various cell types. Although there remain a great deal to learn about patient-specific iPSC safety, the reprogramming mechanisms, better ways to direct a specific reprogramming, ideal cell source for cellular grafts, and the mechanisms by which transplanted stem cells lead to an enhanced functional recovery and structural reorganization, the discovery of the therapeutic potential of iPSCs offers new opportunities for the treatment of incurable neurologic diseases. However, iPSC-based therapeutic strategies need to be thoroughly evaluated in preclinical animal models of neurological diseases before they can be applied in a clinical setting.