Small-Molecule-Directed Endogenous Regeneration of Visual Function in a Mammalian Retinal Degeneration Model

Degenerative retinal diseases associated with photoreceptor loss are a leading cause of visual impairment worldwide, with limited treatment options. Phenotypic profiling coupled with medicinal chemistry were used to develop a small molecule with proliferative effects on retinal stem/progenitor cells, as assessed in vitro in a neurosphere assay and in vivo by measuring Msx1-positive ciliary body cell proliferation. The compound was identified as having kinase inhibitory activity and was subjected to cellular pathway analysis in non-retinal human primary cell systems. When tested in a disease-relevant murine model of adult retinal degeneration (MNU-induced retinal degeneration), we observed that four repeat intravitreal injections of the compound improved the thickness of the outer nuclear layer along with the regeneration of the visual function, as measured with ERG, visual acuity, and contrast sensitivity tests. This serves as a proof of concept for the use of a small molecule to promote endogenous regeneration in the eye.

Rewarding Effects of the Hallucinogen 4-AcO-DMT Administration and Withdrawal in Rats: A Challenge to the Opponent-Process Theory

According to the opponent-process theory of drug addiction, the intake of an addictive substance initiates two processes: a rapid primary process that results in the drug's rewarding effects, and a slower opponent process that leads to the aversive motivational state of drug aftereffects. This aversive state is integral in the desire, pursuit, and maintenance of drug use, potentially leading to dependence and addiction. However, current observational and experimental evidence suggests that the administration of a 5-hydroxytryptamine receptors-type 2A (5-HT2A) agonist, while capable of inducing a positive mental state in humans, may not generate the behavioral patterns typically associated with drugs of abuse. In this study, we found that administering the 5-HT2A agonist 4-Acetoxy-N,N-dimethyltryptamine fumarate (4-AcO-DMT) did not result in place preference in male rats compared to control saline administration 24 h later, after the drug has been cleared from the organism. However, in a modified place preference test where only the acute motivational effects of the drug were evaluated (excluding withdrawal), 4-AcO-DMT was found to be rewarding. Furthermore, in another modified place preference test where only the motivational effects of drug withdrawal were evaluated (excluding the acute effects of drug administration), the 24-hour aftereffect of 5-HT2A agonist administration also resulted in a robust place preference. Therefore, while 4-AcO-DMT administration was able to induce place preference, its 24-hour aftereffect also produced a strong reward. In the counterbalanced test, this reward from the aftereffect effectively overshadowed its acute rewarding properties, which could potentially create a false impression that 4-AcO-DMT lacks motivational properties. This suggests that 5-HT2A agonist administration follows a different dynamic than that proposed by the opponent-process theory of motivation and implies that the administration of 5-HT2A agonists may lead to behavioral patterns less typical of drugs associated with addiction.

Fate Specification of GFAP-Negative Primitive Neural Stem Cells and Their Progeny at Clonal Resolution

The mature brain contains an incredible number and diversity of cells that are produced and maintained by heterogeneous pools of neural stem cells (NSCs). Two distinct types of NSCs exist in the developing and adult mouse brain: Glial Fibrillary Acidic Protein (GFAP)-negative primitive (p)NSCs and downstream GFAP-positive definitive (d)NSCs. To better understand the embryonic functions of NSCs, we performed clonal lineage tracing within neurospheres grown from either pNSCs or dNSCs to enrich for their most immediate downstream neural progenitor cells (NPCs). These clonal progenitor lineage tracing data allowed us to construct a hierarchy of progenitor subtypes downstream of pNSCs and dNSCs that were then validated using single-cell transcriptomics. Further, we identify Nexn as required for neuronal specification from neuron/astrocyte progenitor cells downstream of rare pNSCs. Combined, these data provide single-cell resolution of NPC lineages downstream of rare pNSCs that likely would be missed from population-level analyses in vivo.

Neural Plasticity in the Ventral Tegmental Area, Aversive Motivation during Drug Withdrawal and Hallucinogenic Therapy

Aberrant glutamatergic signaling has been closely related to several pathologies of the central nervous system. Glutamatergic activity can induce an increase in neural plasticity mediated by brain-derived neurotrophic factor (BDNF) in the ventral tegmental area (VTA), a nodal point in the mesolimbic dopamine system. Recent studies have related BDNF dependent plasticity in the VTA with the modulation of aversive motivation to deal with noxious environmental stimuli. The disarray of these learning mechanisms would produce an abnormal augmentation in the representation of the emotional information related to aversion, sometimes even in the absence of external environmental trigger, inducing pathologies linked to mood disorders such as depression and drug addiction. Recent studies point out that serotonin (5-hydroxytryptamine, 5-HT) receptors, especially the 2a (5-HT2a) subtype, play an important role in BDNF-related neural plasticity in the VTA. It has been observed that a single administration of a 5HT2a agonist can both revert an animal to a nondependent state from a drug-dependent state (produced by the chronic administration of a substance of abuse). The 5HT2a agonist also reverted the BDNF-induced neural plasticity in the VTA, suggesting that the administration of 5-HT2a agonists could be used as effective therapeutic agents to treat drug addiction. These findings could explain the neurobiological correlate of the therapeutic use of 5HT2a agonists, which can be found in animals, plants and fungi during traditional medicine ceremonies and rituals to treat mood related disorders.

Administration of brain-derived neurotrophic factor in the ventral tegmental area produces a switch from a nicotine nondependent D1R-mediated motivational state to a nicotine dependent-like D2R-mediated motivational state

Brain-derived neurotrophic factor (BDNF) has been implicated in the transition from a nondependent motivational state to a drug-dependent and -withdrawn motivational state. Chronic nicotine can increase BDNF in the rodent brain and is associated with smoking severity in humans; however, it is unknown whether this increased BDNF is linked functionally to the switch from a nicotine-nondependent to a nicotine-dependent state. We used a place conditioning paradigm to measure the conditioned responses to nicotine, showing that a dose of acute nicotine that nondependent male mice find aversive is found rewarding in chronic nicotine-treated mice experiencing withdrawal. A single BDNF injection in the ventral tegmental area (in the absence of chronic nicotine treatment) caused mice to behave as if they were nicotine-dependent and in withdrawal, switching the neurobiological substrate mediating the conditioned motivational effects from dopamine D1 receptors to D2 receptors. Quantification of gene expression of BDNF and its receptor, tropomyosin-receptor-kinase B (TrkB), revealed an increase in TrkB mRNA but not BDNF mRNA in the VTA in nicotine-dependent and -withdrawn mice. These results suggest that BDNF signaling in the VTA is a critical neurobiological substrate for the transition to nicotine dependence. The modulation of BDNF signaling may be a promising new pharmacological avenue for the treatment of addictive behavior.

A microfluidic platform enables comprehensive gene expression profiling of mouse retinal stem cells

Loss of photoreceptors due to retinal degeneration is a major cause of untreatable visual impairment and blindness. Cell replacement therapy, using retinal stem cell (RSC)-derived photoreceptors, holds promise for reconstituting damaged cell populations in the retina. One major obstacle preventing translation to the clinic is the lack of validated markers or strategies to prospectively identify these rare cells in the retina and subsequently enrich them. Here, we introduce a microfluidic platform that combines nickel micromagnets, herringbone structures, and a design enabling varying flow velocities among three compartments to facilitate a highly efficient enrichment of RSCs. In addition, we developed an affinity enrichment strategy based on cell-surface markers that was utilized to isolate RSCs from the adult ciliary epithelium. We showed that targeting a panel of three cell surface markers simultaneously facilitates the enrichment of RSCs to 1 : 3 relative to unsorted cells. Combining the microfluidic platform with single-cell whole-transcriptome profiling, we successfully identified four differentially expressed cell surface markers that can be targeted simultaneously to yield an unprecedented 1 : 2 enrichment of RSCs relative to unsorted cells. We also identified transcription factors (TFs) that play functional roles in maintenance, quiescence, and proliferation of RSCs. This level of analysis for the first time identified a spectrum of molecular and functional properties of RSCs.

Activation of adult mammalian retinal stem cells in vivo via antagonism of BMP and sFRP2

Background

The adult mammalian retina does not have the capacity to regenerate cells lost due to damage or disease. Therefore, retinal injuries and blinding diseases result in irreversible vision loss. However, retinal stem cells (RSCs), which participate in retinogenesis during development, persist in a quiescent state in the ciliary epithelium (CE) of the adult mammalian eye. Moreover, RSCs retain the ability to generate all retinal cell types when cultured in vitro, including photoreceptors. Therefore, it may be possible to activate endogenous RSCs to induce retinal neurogenesis in vivo and restore vision in the adult mammalian eye.

Methods

To investigate if endogenous RSCs can be activated, we performed combinatorial intravitreal injections of antagonists to BMP and sFRP2 proteins (two proposed mediators of RSC quiescence in vivo), with or without growth factors FGF and Insulin. We also investigated the effects of chemically-induced N-methyl-N-Nitrosourea (MNU) retinal degeneration on RSC activation, both alone and in combination withthe injected factors. Further, we employed inducible Msx1-Cre^ERT2 genetic lineage labeling of the CE followed by stimulation paradigms to determine if activated endogenous RSCs could migrate into the retina and differentiate into retinal neurons.

Results

We found that in vivo antagonism of BMP and sFRP2 proteins induced CE cells in the RSC niche to proliferate and expanded the RSC population. BMP and sFRP2 antagonism also enhanced CE cell proliferation in response to exogenous growth factor stimulation and MNU-induced retinal degeneration. Furthermore, Msx1-Cre^ERT2 genetic lineage tracing revealed that CE cells migrated into the retina following stimulation and/or injury, where they expressed markers of mature photoreceptors and retinal ganglion cells.

Conclusions

Together, these results indicate that endogenous adult mammalian RSCs may have latent regenerative potential that can be activated by modulating the RSC niche and hold promise as a means for endogenous retinal cell therapy to repair the retina and improve vision.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02630-0.

Stable oxime-crosslinked hyaluronan-based hydrogel as a biomimetic vitreous substitute

Vitreous substitutes are clinically used to maintain retinal apposition and preserve retinal function; yet the most used substitutes are gases and oils which have disadvantages including strict face-down positioning post-surgery and the need for subsequent surgical removal, respectively. We have engineered a vitreous substitute comprised of a novel hyaluronan-oxime crosslinked hydrogel. Hyaluronan, which is naturally abundant in the vitreous of the eye, is chemically modified to crosslink with poly(ethylene glycol)-tetraoxyamine via oxime chemistry to produce a vitreous substitute that has similar physical properties to the native vitreous including refractive index, density and transparency. The oxime hydrogel is cytocompatible in vitro with photoreceptors from mouse retinal explants and biocompatible in rabbit eyes as determined by histology of the inner nuclear layer and photoreceptors in the outer nuclear layer. The ocular pressure in the rabbit eyes was consistent over 56 d, demonstrating limited to no swelling. Our vitreous substitute was stable in vivo over 28 d after which it began to degrade, with approximately 50% loss by day 56. We confirmed that the implanted hydrogel did not impact retina function using electroretinography over 90 days versus eyes injected with balanced saline solution. This new oxime hydrogel provides a significant improvement over the status quo as a vitreous substitute.

Glucocorticoid agonists enhance retinal stem cell self-renewal and proliferation

BACKGROUND

Adult mammalian retinal stem cells (RSCs) readily proliferate, self-renew, and generate progeny that differentiate into all retinal cell types in vitro. RSC-derived progeny can be induced to differentiate into photoreceptors, making them a potential source for retinal cell transplant therapies. Despite their proliferative propensity in vitro, RSCs in the adult mammalian eye do not proliferate and do not have a regenerative response to injury. Thus, identifying and modulating the mechanisms that regulate RSC proliferation may enhance the capacity to produce RSC-derived progeny in vitro and enable RSC activation in vivo.

METHODS

Here, we used medium-throughput screening to identify small molecules that can expand the number of RSCs and their progeny in culture. In vitro differentiation assays were used to assess the effects of synthetic glucocorticoid agonist dexamethasone on RSC-derived progenitor cell fate. Intravitreal injections of dexamethasone into adult mouse eyes were used to investigate the effects on endogenous RSCs.

RESULTS

We discovered that high-affinity synthetic glucocorticoid agonists increase RSC self-renewal and increase retinal progenitor proliferation up to 6-fold without influencing their differentiation in vitro. Intravitreal injection of synthetic glucocorticoid agonist dexamethasone induced in vivo proliferation in the ciliary epithelium-the niche in which adult RSCs reside.

CONCLUSIONS

Together, our results identify glucocorticoids as novel regulators of retinal stem and progenitor cell proliferation in culture and provide evidence that GCs may activate endogenous RSCs.

The leading edge: Emerging neuroprotective and neuroregenerative cell-based therapies for spinal cord injury

Spinal cord injuries (SCIs) are associated with tremendous physical, social, and financial costs for millions of individuals and families worldwide. Rapid delivery of specialized medical and surgical care has reduced mortality; however, long-term functional recovery remains limited. Cell-based therapies represent an exciting neuroprotective and neuroregenerative strategy for SCI. This article summarizes the most promising preclinical and clinical cell approaches to date including transplantation of mesenchymal stem cells, neural stem cells, oligodendrocyte progenitor cells, Schwann cells, and olfactory ensheathing cells, as well as strategies to activate endogenous multipotent cell pools. Throughout, we emphasize the fundamental biology of cell-based therapies, critical features in the pathophysiology of spinal cord injury, and the strengths and limitations of each approach. We also highlight salient completed and ongoing clinical trials worldwide and the bidirectional translation of their findings. We then provide an overview of key adjunct strategies such as trophic factor support to optimize graft survival and differentiation, engineered biomaterials to provide a support scaffold, electrical fields to stimulate migration, and novel approaches to degrade the glial scar. We also discuss important considerations when initiating a clinical trial for a cell therapy such as the logistics of clinical-grade cell line scale-up, cell storage and transportation, and the delivery of cells into humans. We conclude with an outlook on the future of cell-based treatments for SCI and opportunities for interdisciplinary collaboration in the field.

Constraint-induced movement therapy promotes motor recovery after neonatal stroke in the absence of neural precursor activation

Neonatal stroke is a leading cause of long-term disability and currently available rehabilitation treatments are insufficient to promote recovery. Activating neural precursor cells (NPCs) in adult rodents, in combination with rehabilitation, can accelerate functional recovery following stroke. Here, we describe a novel method of constraint-induced movement therapy (CIMT) in a rodent model of neonatal stroke that leads to improved functional outcomes, and we asked whether the recovery was correlated with expansion of NPCs. A hypoxia/ischemia (H/I) injury was induced on postnatal day 8 (PND8) via unilateral carotid artery ligation followed by systemic hypoxia. One week and two weeks post-H/I, CIMT was administered in the form of 3 botulinum toxin (Botox) injections, which induced temporary paralysis in the unaffected limb. Functional recovery was assessed using the foot fault task. NPC proliferation was assessed using the neurosphere assay and EdU immunohistochemistry. We found that neonatal H/I injury alone expands the NPC pool by >2.5-fold relative to controls. We determined that using Botox injections as a method to provide CIMT results in significant functional motor recovery after H/I. However, CIMT does not lead to enhanced NPC activation or migration into the injured parenchyma in vivo. At the time of functional recovery, increased numbers of proliferating inflammatory cells were found within the injured motor cortex. Together, these findings suggest that NPC activation following CIMT does not account for the observed functional improvement and suggests that CIMT-mediated modification of the CNS inflammatory response may play a role in the motor recovery.

Segregation of caffeine reward and aversion in the rat nucleus accumbens shell versus core

Caffeine, the most commonly consumed psychoactive drug in the world, is readily available in dietary sources, including soft drinks, chocolate, tea and coffee. However, little is known about the neural substrates that underlie caffeine's rewarding and aversive properties and what ultimately leads us to seek or avoid caffeine consumption. Using male Wistar rats in a place conditioning procedure, we show that systemic caffeine at a low intraperitoneal dose of 2 mg/kg (or 100 µM injected directly into the rostral, but not caudal, portion of the ventral tegmental area) produced conditioned place preferences. By contrast, high doses of systemic caffeine at 10 and 30 mg/kg produced conditioned place aversions. These aversions were not recapitulated by a caffeine analog restricted to the periphery. Both caffeine reward and aversion were blocked by systemic D1-like receptor antagonism using SCH23390, while systemic D2-like receptor antagonism with eticlopride had smaller effects on caffeine motivation. Most important, we demonstrated that pharmacological blockade of dopamine receptors using α-flupenthixol injected into the nucleus accumbens shell, but not core, blocked caffeine-conditioned place preferences. Conversely, α-flupenthixol injected into the nucleus accumbens core, but not shell, blocked caffeine-conditioned place aversions. Thus, our findings reveal two dopamine-dependent and functionally dissociable mechanisms for processing caffeine motivation, which are segregated between nucleus accumbens subregions. These data provide novel evidence for the roles of the nucleus accumbens subregions in mediating approach and avoidance behaviours for caffeine.

Cell competition during reprogramming gives rise to dominant clones

The ability to generate induced pluripotent stem cells from differentiated cell types has enabled researchers to engineer cell states. Although studies have identified molecular networks that reprogram cells to pluripotency, the cellular dynamics of these processes remain poorly understood. Here, by combining cellular barcoding, mathematical modeling, and lineage tracing approaches, we demonstrate that reprogramming dynamics in heterogeneous populations are driven by dominant “elite” clones. Clones arise a priori from a population of poised mouse embryonic fibroblasts derived from Wnt1-expressing cells that may represent a neural crest–derived population. This work highlights the importance of cellular dynamics in fate programming outcomes and uncovers cell competition as a mechanism by which cells with eliteness emerge to occupy and dominate the reprogramming niche.

Induction of rod versus cone photoreceptor-specific progenitors from retinal precursor cells

During development, multipotent progenitors undergo temporally-restricted differentiation into post-mitotic retinal cells; however, the mechanisms of progenitor division that occurs during retinogenesis remain controversial. Using clonal analyses (lineage tracing and single cell cultures), we identify rod versus cone lineage-specific progenitors derived from both adult retinal stem cells and embryonic neural retinal precursors. Taurine and retinoic acid are shown to act in an instructive and lineage-restricted manner early in the progenitor lineage hierarchy to produce rod-restricted progenitors from stem cell progeny. We also identify an instructive, but lineage-independent, mechanism for the specification of cone-restricted progenitors through the suppression of multiple differentiation signaling pathways. These data indicate that exogenous signals play critical roles in directing lineage decisions and resulting in fate-restricted rod or cone photoreceptor progenitors in culture. Additional factors may be involved in governing photoreceptor fates in vivo.

Mutations in the guanylate cyclase gcy-28 neuronally dissociate naïve attraction and memory retrieval

The molecules and mechanisms that are involved in the acquisition, storage, and retrieval of memories in many organisms are unclear. To investigate these processes, we use the nematode worm Caenorhabditis elegans, which is attracted naïvely to the odorant benzaldehyde but learns to avoid it after paired exposure with starvation. Mutations in the receptor-like guanylate cyclase GCY-28 have previously been thought to result in a behavioral switch in the primary chemosensory neuron AWC^ON , from an attractive state to an aversive (already-learned) state. Here, we offer a different interpretation and show that GCY-28 functions in distinct neurons to modulate two independent processes: naïve attraction to AWC^ON -sensed odors in the AWC^ON neuron, and associative learning of benzaldehyde and starvation in the AIA interneurons. Consequently, mutants that lack gcy-28 do not approach AWC^ON -sensed odors and cannot associate benzaldehyde with starvation. We further show that this learning deficit lies in memory retrieval, not in its acquisition or storage, and that GCY-28 is required in AIA for sensory integration only when both AWC neurons (ON and OFF) are activated by chemical stimuli. Our results reveal a novel role of GCY-28 in the retrieval of associative memories and may have wide implications for the neural machineries of learning and memory in general.

Single-Cell Tumbling Enables High-Resolution Size Profiling of Retinal Stem Cells

Retinal stem cells (RSCs) are promising candidates for patient-derived cell therapy to repair damage to the eye; however, RSCs are rare in retinal samples and lack validated markers, making cell sorting a significant challenge. Here we report a high-resolution deterministic lateral displacement microfluidic device that profiles RSCs in distinct size populations. Only by developing a chip that promotes cell tumbling do we limit cell deformation through apertured channels and thereby increase the size-sorting resolution of the device. We systematically explore a spectrum of microstructures, including optimized notched pillars, to study and then rationally promote cell tumbling. We find that RSCs exhibit larger diameters than most ciliary epithelial cells, an insight into RSC morphology that allows enrichment from biological samples.

Quiescent Oct4+ Neural Stem Cells (NSCs) Repopulate Ablated Glial Fibrillary Acidic Protein+ NSCs in the Adult Mouse Brain

Adult primitive neural stem cells (pNSCs) are a rare population of glial fibrillary acidic protein (GFAP)^- Oct4⁺ cells in the mouse forebrain subependymal zone bordering the lateral ventricles that give rise to clonal neurospheres in leukemia inhibitory factor in vitro. pNSC neurospheres can be passaged to self-renew or give rise to GFAP⁺ NSCs that form neurospheres in epidermal growth factor and fibroblast growth factor 2, which we collectively refer to as definitive NSCs (dNSCs). Label retention experiments using doxycycline-inducible histone-2B (H2B)-green fluorescent protein (GFP) mice and several chase periods of up to 1 year quantified the adult pNSC cell cycle time as 3-5 months. We hypothesized that while pNSCs are not very proliferative at baseline, they may exist as a reserve pool of NSCs in case of injury. To test this function of pNSCs, we obtained conditional Oct4 knockout mice, Oct4^fl/fl ;Sox1^Cre (Oct4^CKO ), which do not yield adult pNSC-derived neurospheres. When we ablated the progeny of pNSCs, namely all GFAP⁺ dNSCs, in these Oct4^CKO mice, we found that dNSCs did not recover as they do in wild-type mice, suggesting that pNSCs are necessary for dNSC repopulation. Returning to the H2B-GFP mice, we observed that the cytosine β-d-arabinofuranoside ablation of proliferating cells including dNSCs-induced quiescent pNSCs to proliferate and significantly dilute their H2B-GFP label. In conclusion, we demonstrate that pNSCs are the most quiescent stem cells in the adult brain reported to date and that their lineage position upstream of GFAP⁺ dNSCs allows them to repopulate a depleted neural lineage. Stem Cells 2017;35:2071-2082.

Deletion of α5 nicotine receptor subunits abolishes nicotinic aversive motivational effects in a manner that phenocopies dopamine receptor antagonism

Nicotine addiction is a worldwide epidemic that claims millions of lives each year. Genetic deletion of α5 nicotinic acetylcholine receptor (nAChR) subunits has been associated with increased nicotine intake, however, it remains unclear whether acute nicotine is less aversive or more rewarding, and whether mice lacking the α5 nAChR subunit can experience withdrawal from chronic nicotine. We used place conditioning and conditioned taste avoidance paradigms to examine the effect of α5 subunit-containing nAChR deletion (α5 -/-) on conditioned approach and avoidance behaviour in nondependent and nicotine-dependent and -withdrawn mice, and compared these motivational effects with those elicited after dopamine receptor antagonism. We show that nondependent α5 -/- mice find low, non-motivational doses of nicotine rewarding, and do not show an aversive conditioned response or taste avoidance to higher aversive doses of nicotine. Furthermore, nicotine-dependent α5 -/- mice do not show a conditioned aversive motivational response to withdrawal from chronic nicotine, although they continue to exhibit a somatic withdrawal syndrome. These effects phenocopy those observed after dopamine receptor antagonism, but are not additive, suggesting that α5 nAChR subunits act in the same pathway as dopamine and are critical for the experience of nicotine's aversive, but not rewarding motivational effects in both a nondependent and nicotine-dependent and -withdrawn motivational state. Genetic deletion of α5 nAChR subunits leads to a behavioural phenotype that exactly matches that observed after antagonizing dopamine receptors, thus we suggest that modulation of nicotinic receptors containing α5 subunits may modify dopaminergic signalling, suggesting novel therapeutic treatments for smoking cessation.

A single administration of the hallucinogen, 4-acetoxy-dimethyltryptamine, prevents the shift to a drug-dependent state and the expression of withdrawal aversions in rodents

Despite several studies suggesting the therapeutic use of 5-hydroxytryptamine receptors type 2A (5-HT_2A ) agonists in the treatment of substance use disorders, the neurobiological basis accounting for such effects are still unknown. It has been observed that chronic exposure to drugs of abuse produces molecular and cellular adaptations in ventral tegmental area (VTA) neurons, mediated by brain-derived neurotrophic factor (BDNF). These BDNF-induced adaptations in the VTA are associated with the establishment of aversive withdrawal motivation that leads to a drug-dependent state. Growing evidence suggests that 5-HT_2A receptor signaling can regulate the expression of BDNF in the brain. In this study, we observed that a single systemic or intra-VTA administration of a 5-HT_2A agonist in rats and mice blocks both the aversive conditioned response to drug withdrawal and the mechanism responsible for switching from a drug-naive to a drug-dependent motivational system. Our results suggest that 5-HT_2A agonists could be used as therapeutic agents to reverse a drug dependent state, as well as inhibiting the aversive effects produced by drug withdrawal.

A Hyaluronan-Based Injectable Hydrogel Improves the Survival and Integration of Stem Cell Progeny following Transplantation

The utility of stem cells and their progeny in adult transplantation models has been limited by poor survival and integration. We designed an injectable and bioresorbable hydrogel blend of hyaluronan and methylcellulose (HAMC) and tested it with two cell types in two animal models, thereby gaining an understanding of its general applicability for enhanced cell distribution, survival, integration, and functional repair relative to conventional cell delivery in saline. HAMC improves cell survival and integration of retinal stem cell (RSC)-derived rods in the retina. The pro-survival mechanism of HAMC is ascribed to the interaction of the CD44 receptor with HA. Transient disruption of the retinal outer limiting membrane, combined with HAMC delivery, results in significantly improved rod survival and visual function. HAMC also improves the distribution, viability, and functional repair of neural stem and progenitor cells (NSCs). The HAMC delivery system improves cell transplantation efficacy in two CNS models, suggesting broad applicability.

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An injectable biomaterial improves rod survival/integration into adult retina

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The same material improves neural stem cell distribution/survival into adult brain

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Functional repair is demonstrated after cell transplantation in both retina and brain

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Hyaluronan-CD44 interaction is implicated in the pro-survival effect on stem cell progeny

Diabetes enhances the proliferation of adult pancreatic multipotent progenitor cells and biases their differentiation to more β-cell production

Endogenous pancreatic multipotent progenitors (PMPs) are ideal candidates for regenerative approaches to compensate for β-cell loss since their β-cell-producing capacities as well as strategic location would eliminate unnecessary invasive manipulations. However, little is known about the status and potentials of PMPs under diabetic conditions. Here we show that β-cell metabolic stress and hyperglycemia enhance the proliferation capacities of adult PMP cells and bias their production of progeny toward β-cells in mouse and human. These effects are dynamic and correlate with functional β-cell regeneration when conditions allow.

Local acting Sticky-trap inhibits vascular endothelial growth factor dependent pathological angiogenesis in the eye

Current therapeutic antiangiogenic biologics used for the treatment of pathological ocular angiogenesis could have serious side effects due to their interference with normal blood vessel physiology. Here, we report the generation of novel antivascular endothelial growth factor-A (VEGF) biologics, termed VEGF “Sticky-traps,” with unique properties that allow for local inhibition of angiogenesis without detectable systemic side effects. Using genetic and pharmacological approaches, we demonstrated that Sticky-traps could locally inhibit angiogenesis to at least the same extent as the original VEGF-trap that also gains whole-body access. Sticky-traps did not cause systemic effects, as shown by uncompromised wound healing and normal tracheal vessel density. Moreover, if injected intravitreally, recombinant Sticky-trap remained localized to various regions of the eye, such as the inner-limiting membrane and ciliary body, for prolonged time periods, without gaining access either to the photoreceptors/choriocapillaris area or the circulation. These unique pharmacological characteristics of Sticky-trap could allow for safe treatment of pathological angiogenesis in patients with diabetic retinopathy and retinopathy of pre-maturity.

Surfaceome profiling reveals regulators of neural stem cell function

The composition of cell-surface proteins changes during lineage specification, altering cellular responses to their milieu. The changes that characterize maturation of early neural stem cells (NSCs) remain poorly understood. Here we use mass spectrometry-based cell surface capture technology to profile the cell surface of early NSCs and demonstrate functional requirements for several enriched molecules. Primitive NSCs arise from embryonic stem cells upon removal of Transforming growth factor-β signaling, while definitive NSCs arise from primitive NSCs upon Lif removal and FGF addition. In vivo aggregation assays revealed that N-cadherin upregulation is sufficient for the initial exclusion of definitive NSCs from pluripotent ectoderm, while c-kit signaling limits progeny of primitive NSCs. Furthermore, we implicate EphA4 in primitive NSC survival signaling and Erbb2 as being required for NSC proliferation. This work elucidates several key mediators of NSC function whose relevance is confirmed on forebrain-derived populations and identifies a host of other candidates that may regulate NSCs.

Bone morphogenetic proteins and secreted frizzled related protein 2 maintain the quiescence of adult mammalian retinal stem cells

Rare retinal stem cells (RSCs) within the ciliary epithelium at the retinal margin of the adult mouse and human eyes can divide in vitro in the absence of growth factors to generate clonal, self-renewing spheres which can generate all the retinal cell types. Since no regenerative properties are seen in situ in the adult mammalian eye, we sought to determine the factors that are involved in the repression of endogenous RSCs. We discovered that factors secreted by the adult lens and cornea block the proliferation of adult RSCs in vitro. Bone morphogenetic protein (BMP)2, BMP4, and secreted frizzled related protein 2 were identified as principal effectors of the anti-proliferative effects on RSCs. As a similar induced quiescence was observed in vitro on both mouse and human RSCs, targeting these molecules in vivo may reactivate RSCs directly in situ in the eyes of the blind.

BDNF signaling in the VTA links the drug-dependent state to drug withdrawal aversions

Drug administration to avoid unpleasant drug withdrawal symptoms has been hypothesized to be a crucial factor that leads to compulsive drug-taking behavior. However, the neural relationship between the aversive motivational state produced by drug withdrawal and the development of the drug-dependent state still remains elusive. It has been observed that chronic exposure to drugs of abuse increases brain-derived neurotrophic factor (BDNF) levels in ventral tegmental area (VTA) neurons. In particular, BDNF expression is dramatically increased during drug withdrawal, which would suggest a direct connection between the aversive state of withdrawal and BDNF-induced neuronal plasticity. Using lentivirus-mediated gene transfer to locally knock down the expression of the BDNF receptor tropomyosin-receptor-kinase type B in rats and mice, we observed that chronic opiate administration activates BDNF-related neuronal plasticity in the VTA that is necessary for both the establishment of an opiate-dependent state and aversive withdrawal motivation. Our findings highlight the importance of a bivalent, plastic mechanism that drives the negative reinforcement underlying addiction.

Primitive neural stem cells in the adult mammalian brain give rise to GFAP-expressing neural stem cells

Adult forebrain definitive neural stem cells (NSCs) comprise a subpopulation of GFAP-expressing subependymal cells that arise from embryonic fibroblast growth factor (FGF)-dependent NSCs that are first isolated from the developing brain at E8.5. Embryonic FGF-dependent NSCs are derived from leukemia inhibitory factor (LIF)-responsive, Oct4-expressing primitive NSCs (pNSCs) that are first isolated at E5.5. We report the presence of a rare population of pNCSs in the periventricular region of the adult forebrain. Adult-derived pNSCs (AdpNSCs) are GFAP⁻, LIF-responsive stem cells that display pNSC properties, including Oct4 expression and the ability to integrate into the inner cell mass of blastocysts. AdpNSCs generate self-renewing, multipotent colonies that give rise to definitive GFAP⁺ NSCs in vitro and repopulate the subependyma after the ablation of GFAP⁺ NSCs in vivo. These data support the hypothesis that a rare population of pNSCs is present in the adult brain and is upstream of the GFAP⁺ NSCs.

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Rare, multipotent, self-renewing, Oct4⁺ AdpNSCs in the adult brain

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AdpNSCs lie upstream of definitive, GFAP-expressing adult NSCs

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AdpNSCs repopulate the SE after ablation of GFAP-expressing NSCs

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The AdpNSC pool is activated and expands after injury or LIF infusion in vivo

The neural stem cell lineage reveals novel relationships among spermatogonial germ stem cells and other pluripotent stem cells

The embryonic stem cell (ESC) derived from the inner cell mass is viewed as the core pluripotent cell (PC) type from which all other cell types emanate. This familiar perspective derives from an embryological time line in which PCs are ordered according to their time of appearance. However, this schema does not take into account their potential for interconversion, thereby excluding this critical quality of PCs. The persistence of bona fide pluripotent adult stem cells has garnered increasing attention in recent years. Adult pluripotent spermatogonial germ stem cells (aSGSCs) arise from primordial germ cells (pGCs) that emerge from the epiblast during gastrulation. Adult definitive neural stem cells (dNSCs) arise clonally from pluripotent embryonic primitive neural stem cells (pNSCs), which can also be derived clonally from ESCs. To test for stem cell-type convertibility, we employed differentiation in the clonal lineage from ESCs to pNSCs to dNSCs, and revealed the relationships and lineage positioning among various PC populations, including spermatogonial germ cells (aSGSCs), epiblast-derived stem cells (Epi-SCs) and the bFGF, Activin, and BIO-derived stem cell (FAB-SC). Adult, murine aSGSCs assumed a 'pseudo-ESC' state in vitro, and then differentiated into dNSCs, but not pNSCs. Similarly, Epi-SCs and FAB-SCs only gave rise to dNSCs and not to pNSCs. The results of these experiments suggest a new pluripotency lineage model describing the relationship(s) among PCs that better reflects the transitions between these cell types in vitro.

The aggregation and inheritance of damaged proteins determines cell fate during mitosis

Recent evidence suggests that proliferating cells polarize damaged proteins during mitosis to protect one cell from aging, and that the structural conformation of damaged proteins mediates their toxicity. We report that the growth, resistance to stress, and differentiation characteristics of a cancer cell line (PC12) with an inducible Huntingtin (Htt) fused to enhanced green fluorescent protein (GFP) are dependent on the conformation of Htt. Cell progeny containing inclusion bodies have a longer cell cycle and increased resistance to stress than those with diffuse Htt. Using live imaging, we demonstrate that asymmetric division resulting from a cell containing a single inclusion body produces sister cells with different fates. The cell that receives the inclusion body has decreased proliferation and increased differentiation compared with its sister cell without Htt. This is the first report that reveals a functional consequence of the asymmetric division of damaged proteins in mammalian cells, and we suggest that this is a result of inclusion body-induced proteasome impairment.

Oct4 is required ~E7.5 for proliferation in the primitive streak

Oct4 is a widely recognized pluripotency factor as it maintains Embryonic Stem (ES) cells in a pluripotent state, and, in vivo, prevents the inner cell mass (ICM) in murine embryos from differentiating into trophectoderm. However, its function in somatic tissue after this developmental stage is not well characterized. Using a tamoxifen-inducible Cre recombinase and floxed alleles of Oct4, we investigated the effect of depleting Oct4 in mouse embryos between the pre-streak and headfold stages, ∼E6.0–E8.0, when Oct4 is found in dynamic patterns throughout the embryonic compartment of the mouse egg cylinder. We found that depletion of Oct4 ∼E7.5 resulted in a severe phenotype, comprised of craniorachischisis, random heart tube orientation, failed turning, defective somitogenesis and posterior truncation. Unlike in ES cells, depletion of the pluripotency factors Sox2 and Oct4 after E7.0 does not phenocopy, suggesting that ∼E7.5 Oct4 is required within a network that is altered relative to the pluripotency network. Oct4 is not required in extraembryonic tissue for these processes, but is required to maintain cell viability in the embryo and normal proliferation within the primitive streak. Impaired expansion of the primitive streak occurs coincident with Oct4 depletion ∼E7.5 and precedes deficient convergent extension which contributes to several aspects of the phenotype.

Embryogenesis is an intricate process requiring that division, differentiation and position of cells are coordinated. During mammalian development early pluripotent populations are canalized or restricted in potency during embryogenesis. Due to considerable interest in how this fundamental state of pluripotency is maintained, and the requirement of the transcription factor Oct4 to maintain pluripotency, Oct4 has been intensively studied in culture. However, it is not clear what role Oct4 has during lineage specification of pluripotent cells. Oct4 removal during lineage specification indicates that it is required in the primitive streak of mouse embryos to maintain proliferation. The consequences of Oct4 removal diverge from the consequences of removing another factor required for pluripotency between preimplantation development and early cell fate specification suggesting that the network Oct4 acts within is altered between these stages.

Notch signaling induces retinal stem-like properties in perinatal neural retina progenitors and promotes symmetric divisions in adult retinal stem cells

Understanding the mechanisms regulating retinal stem cell (RSC) activity is fundamental for future stem cell-based therapeutic purposes. By combining gain and loss of function approaches, we addressed whether Notch signaling may play a selective role in retinal stem versus retinal progenitor cells in both developing and adult eyes. Inhibition of either Notch or fibroblast growth factor signaling reduced proliferation of retinal stem and retinal progenitor cells, and inhibited RSC self-renewal. Conversely, exogenous Delta-like 3 and direct intrinsic Notch activation stimulated expansionary symmetric divisions in adult RSCs with the concomitant upregulation of Hes5. Knocking down Hes5 expression specifically decreased the numbers, but not the diameters, of adult RSC primary spheres, indicating that HES5 is the downstream effector of Notch receptor in controlling adult RSC proliferation. In addition, constitutive Notch activation induced retinal stem-like asymmetric self-renewal properties, with no expansion (no symmetrical division) in perinatal neural retina progenitor cells. These findings highlight central roles of Notch signaling activity in regulating the modes of division of retinal stem and retinal progenitor cells.

Entwined engrams: The evolution of associative and non-associative learning

The nematode Caenorhabditis elegans displays a surprisingly sophisticated behavioral repertoire that includes the utilization of both associative and non-associative forms of learning. Elucidating the molecular basis of learning remains a fundamental, yet daunting, challenge of modern neuroscience. In Pereira and van der Kooy (ref. 2), we described the use of a two input—two output stimuli system to dissociate between associative and non-associative learning and between memory acquisition and retrieval processes. Briefly, one finding indicated that after training with the odorant isoamyl alcohol (IsoA), we could preferentially retrieve either associative or non-associative memory with a choice of either a benzaldehyde (Bnz) or IsoA retrieval stimulus, respectively. Here, we describe how that apparently enigmatic molecular cross wiring of the two forms of memory examined could represent an evolutionary relic of the ancient divergence between non-associative and associative learning. In addition, we extrapolate on the utility and subtleties of using such a system to dissociate and decipher the components of memory in C. elegans.

Social defeat stress switches the neural system mediating benzodiazepine conditioned motivation

Benzodiazepines have been demonstrated to have a high abuse liability in persons suffering from anxiety but have demonstrated mixed abuse liability findings in preclinical models. We hypothesized that by modeling anxiety in a male C57BL/6 mouse model it would be possible to reveal a preference for benzodiazepines within this subpopulation through negative reinforcement. Using the Tube Test of Social Dominance and the Resident/Intruder Paradigm we investigated whether animals identified as dominant or submissive/defeated would differentially display a preference for midazolam (a short acting benzodiazepine) in a conditioned place preference paradigm. Consistent with our hypotheses, benzodiazepine conditioned motivation was mediated by negative reinforcement as submissive but not dominant mice displayed a preference for midazolam. Furthermore, different neural systems mediated midazolam conditioned motivation depending on the stress status of the animal (single vs. repeated stress-as induced by the Resident/Intruder Paradigm). Singly stressed animals showed midazolam place preferences through a dopamine-independent pathway, whereas the place preferences of repeatedly stressed animals were mediated through a dopamine-dependent pathway. This demonstrates that stress is sufficient for switching the neural system mediating midazolam conditioned motivation. Finally, midazolam reinforcement in the conditioned place preference paradigm was shown to be predictive for dominance/submission status.

Ventral tegmental area GABA neurons and opiate motivation

RATIONALE

Past research has demonstrated that when an animal changes from a previously drug-naive to an opiate-dependent and withdrawn state, morphine's motivational effects are switched from a tegmental pedunculopontine nucleus (TPP)-dependent to a dopamine-dependent pathway. Interestingly, a corresponding change is observed in ventral tegmental area (VTA) GABAA receptors, which change from mediating hyperpolarization of VTA GABA neurons to mediating depolarization.

OBJECTIVES

The present study investigated whether pharmacological manipulation of VTA GABAA receptor activity could directly influence the mechanisms underlying opiate motivation.

RESULTS

Using an unbiased place conditioning procedure, we demonstrated that in Wistar rats, intra-VTA administration of furosemide, a Cl(-) cotransporter inhibitor, was able to promote a switch in the mechanisms underlying morphine's motivational properties, one which is normally observed only after chronic opiate exposure. This behavioral switch was prevented by intra-VTA administration of acetazolamide, an inhibitor of the bicarbonate ion-producing carbonic anhydrase enzyme. Electrophysiological recordings of mouse VTA showed that furosemide reduced the sensitivity of VTA GABA neurons to inhibition by the GABAA receptor agonist muscimol, instead increasing the firing rate of a significant subset of these GABA neurons.

CONCLUSIONS

Our results suggest that the carbonic anhydrase enzyme may constitute part of a common VTA GABA neuron-based biological pathway responsible for controlling the mechanisms underlying opiate motivation, supporting the hypothesis that VTA GABAA receptor hyperpolarization or depolarization is responsible for selecting TPP- or dopamine-dependent motivational outputs, respectively.

The asymmetric segregation of damaged proteins is stem cell-type dependent

Asymmetric segregation of damaged proteins is detectable in three different adult tissue stem cells in Drosophila melanogaster but does not always favor segregation away from the stem cell.

Asymmetric segregation of damaged proteins (DPs) during mitosis has been linked in yeast and bacteria to the protection of one cell from aging. Recent evidence suggests that stem cells may use a similar mechanism; however, to date there is no in vivo evidence demonstrating this effect in healthy adult stem cells. We report that stem cells in larval (neuroblast) and adult (female germline and intestinal stem cell) Drosophila melanogaster asymmetrically segregate DPs, such as proteins with the difficult-to-degrade and age-associated 2,4-hydroxynonenal (HNE) modification. Surprisingly, of the cells analyzed only the intestinal stem cell protects itself by segregating HNE to differentiating progeny, whereas the neuroblast and germline stem cells retain HNE during division. This led us to suggest that chronological life span, and not cell type, determines the amount of DPs a cell receives during division. Furthermore, we reveal a role for both niche-dependent and -independent mechanisms of asymmetric DP division.

The adult mammalian pancreas contains separate precursors of pancreatic and neural crest developmental origins

The developmental origin of a pancreatic precursor cell could provide clues to properties that may be crucial to its molecular regulation and therapeutic potential. Previously, lineage tracing experiments showed that multipotent precursors in mouse islets had a pancreatic and not a neural crest developmental origin. However, a different Cre reporter system reveals that there is, in fact, a rare population of proliferative cells in the pancreas that is descended from the Wnt1 neural crest lineage, in addition to the majority population descended from the Pdx1 pancreatic lineage. These two proliferative cell populations are distinct in their gene expression and differentiation potential. This evidence suggests that there are at least two distinct types of precursors present in adult pancreatic islets, one of pancreatic origin and one of neural crest origin.

Dopamine D1 receptors are not critical for opiate reward but can mediate opiate memory retrieval in a state-dependent manner

Although D1 receptor knockout mice demonstrate normal morphine place preferences, antagonism of basolateral amygdala (BLA) D1 receptors only during drug-naive rat conditioning has been reported to inhibit the expression of a morphine place preference. One possible explanation for this result is state-dependent learning. That is, the omission of the intra-BLA infusion cue during testing - which acts as a potent discriminative stimulus - may have prevented the recall of a morphine-environment association and therefore, the consequent expression of a morphine place preference. To examine this possibility, we tested whether intra-BLA infusion of the D1-receptor antagonist SCH23390 during both training and testing might reveal a morphine place preference. Our results suggest that in previously drug-naive animals, D1 receptor antagonism during testing restores the opiate conditioned place preference that is normally absent when D1 receptors are blocked only during training, suggesting that BLA D1 receptors can mediate state-dependent memory retrieval.

Nicotine-motivated behavior in Caenorhabditis elegans requires the nicotinic acetylcholine receptor subunits acr-5 and acr-15

Signaling at nicotinic acetylcholine receptors in Caenorhabditis elegans controls many behaviors, including egg-laying and locomotor activity. Here, we show that C. elegans approaches a point source of nicotine in a time-, concentration- and age-dependent manner. Additionally, nicotine paired with butanone under starvation conditions prevented the reduced approach to butanone that is observed when butanone is paired with starvation alone and pairing with nicotine generates a preference for the tastes of either sodium or chloride over baseline. These results suggest nicotine acts as a rewarding substance in C. elegans. Furthermore, the nicotinic receptor antagonist mecamylamine, the smoking cessation pharmacotherapy varenicline, mutation of the dop-1 and dop-2 dopamine receptors, and mutations of either acr-5 or acr-15, two nicotinic receptor subunit genes with sequence homology to the mammalian α7 subunit, all reduced the nicotine approach behavior. These two mutants also were defective at associating the presence of nicotine with butanone under starvation conditions and acr-5 mutation could obviate the effect of pairing nicotine with salts. Furthermore, the approach deficit in acr-15 mutants was rescued by selective re-expression in a subset of neurons, but not in muscle. Caenorhabditis elegans may therefore serve as a useful model organism for nicotine-motivated behaviors that could aid in the identification of novel nicotine motivational molecular pathways and consequently the development of novel cessation aids.

Infusion of brain-derived neurotrophic factor into the ventral tegmental area switches the substrates mediating ethanol motivation

Recent work has shown that infusion of brain-derived neurotrophic factor (BDNF) into the ventral tegmental area (VTA) promotes a switch in the mechanisms mediating morphine motivation, from a dopamine-independent to a dopamine-dependent pathway. Here we showed that a single infusion of intra-VTA BDNF also promoted a switch in the mechanisms mediating ethanol motivation, from a dopamine-dependent to a dopamine-independent pathway (exactly opposite to that seen with morphine). We suggest that intra-VTA BDNF, via its actions on TrkB receptors, precipitates a switch similar to that which occurs naturally when mice transit from a drug-naive, non-deprived state to a drug-deprived state. The opposite switching of the mechanisms underlying morphine and ethanol motivation by BDNF in previously non-deprived animals is consistent with their proposed actions on VTA GABAA receptors.

The neurobiology of opiate motivation

Opiates are a highly addictive class of drugs that have been reported to possess both dopamine-dependent and dopamine-independent rewarding properties. The search for how, if at all, these distinct mechanisms of motivation are related is of great interest in drug addiction research. Recent electrophysiological, molecular, and behavioral work has greatly improved our understanding of this process. In particular, the signaling properties of GABA(A) receptors located on GABA neurons in the ventral tegmental area (VTA) appear to be crucial to understanding the interplay between dopamine-dependent and dopamine-independent mechanisms of opiate motivation.

The generation of definitive neural stem cells from PiggyBac transposon-induced pluripotent stem cells can be enhanced by induction of the NOTCH signaling pathway

Cell-based therapies using neural stem cells (NSCs) have shown positive outcomes in various models of neurological injury and disease. Induced pluripotent stem cells (iPSCs) address many problems associated with NSCs from various sources, including the immune response and cell availability. However, due to inherent differences between embryonic stem cells (ESCs) and iPSCs, detailed characterization of the iPS-derived NSCs will be required before translational experiments can be performed. Murine piggyBac transposon iPSCs were clonally expanded in floating sphere colonies to generate primitive NSCs initially with serum-free media (SFM) containing the leukemia inhibitory factor and followed by SFM with the fibroblast growth factor-2 (FGF2) to form colonies of definitive NSCs (dNSCs). Primitive and definitive clonally derived neurospheres were successfully generated using the default conditions from iPSCs and ESCs. However, the iPSC-dNSCs expressed significantly higher levels of pluripotency and nonectoderm lineage genes compared to equivalent ESC-dNSCs. The addition of the bone morphogenetic proteins antagonist, Noggin, to the media significantly increased primary neurosphere generation from the iPSC lines, but did not affect the dNSC sphere colonies generated. The induction of the NOTCH pathway by the Delta-like ligand 4 (DLL4) improved the generation and quality of dNSCs, as demonstrated by a reduction in pluripotency and nonectodermal markers, while maintaining NSC-specific gene expression. The iPS-dNSCs (+DLL4) showed functional neural differentiation by immuncytochemical staining and electrophysiology. This study suggests the intrinsic differences between ESCs and iPSCs in their ability to acquire a dNSC fate that can be overcome by inducing the NOTCH pathway.

Generation and clonal isolation of retinal stem cells from human embryonic stem cells

Retinal stem cells (RSCs) are present within the pigmented ciliary epithelium (CE) of the adult human eye and produce progeny that differentiate in vitro into all neural retinal subtypes and retinal pigmented epithelium (RPE). We hypothesized that a RSC population, similar to the adult CE-derived RSC, is contained within pigmented colonies that arise in long-term cultures of human embryonic stem cells (hESCs) suggested to recapitulate retinal development in vitro. Single pigmented hESC-derived cells were isolated and plated in serum-free media containing growth factors and, after 2 weeks, clonal sphere colonies containing both pigmented and non-pigmented cells were observed. These colonies expressed the early retinal transcription factors Rx, Chx10 and Pax6, and could be dissociated and replated as single cells to form secondary clonal colonies. When allowed to differentiate, expression of markers for both RPE and neurons was observed. Rhodopsin expression was detected after explant co-culture and transplantation into the developing mouse eye as well as following treatment with soluble factors in vitro. We show that RSCs emerge in an in vitro model of retinal development and are a potential source of human photoreceptors for use in transplantation.

Critical evaluation of imprinted gene expression by RNA-Seq: a new perspective

In contrast to existing estimates of approximately 200 murine imprinted genes, recent work based on transcriptome sequencing uncovered parent-of-origin allelic effects at more than 1,300 loci in the developing brain and two adult brain regions, including hundreds present in only males or females. Our independent replication of the embryonic brain stage, where the majority of novel imprinted genes were discovered and the majority of previously known imprinted genes confirmed, resulted in only 12.9% concordance among the novel imprinted loci. Further analysis and pyrosequencing-based validation revealed that the vast majority of the novel reported imprinted loci are false-positives explained by technical and biological variation of the experimental approach. We show that allele-specific expression (ASE) measured with RNA–Seq is not accurately modeled with statistical methods that assume random independent sampling and that systematic error must be accounted for to enable accurate identification of imprinted expression. Application of a robust approach that accounts for these effects revealed 50 candidate genes where allelic bias was predicted to be parent-of-origin–dependent. However, 11 independent validation attempts through a range of allelic expression biases confirmed only 6 of these novel cases. The results emphasize the importance of independent validation and suggest that the number of imprinted genes is much closer to the initial estimates.

Typically both copies of mammalian genes are expressed, but in some cases, “imprinting” restricts expression to the maternal or paternal copy. Having two copies of each gene is considered advantageous since in enables compensation when one does not function properly. Why imprinting evolved and its utility to each sex is widely debated, and having a complete catalog of imprinted genes and their functions is essential for fully characterizing this phenomenon. 25 years of screening has revealed about 130 imprinted genes, and the slowing rate of discovery suggests that we are reaching saturation. Two recent studies based on high-throughput sequencing of RNA reported more than 1,300 imprinted genes. To understand the basis of this paradigm shift, we first attempted to reproduce these results. Unable to do so, we performed additional analyses that show that most of these discoveries are due to noise in the experimental approach and assay. We remedy this with new methods that account for this noise and applied them to identify 50 novel putative imprinted genes. These methods will be useful for identifying genuine novel cases of imprinted expression as this type of screening approach becomes broadly utilized.

The adult mouse dentate gyrus contains populations of committed progenitor cells that are distinct from subependymal zone neural stem cells

There is currently a debate as to whether or not a neural stem cell (NSC) exists in the adult mammalian hippocampus. Clonal colony-forming assays allow single cells to cells to be evaluated for stem cell properties: self-renewal and multipotentiality. In these in vitro assays, single cells from the subependymal zone (SEZ) of the adult lateral ventricle yield large colonies which self-renew and are multipotential, while single cells from the adult dentate gyrus (DG) produce small, unipotent, and nonself-renewing colonies. We find that multipotential and long-term self-renewing colonies can be isolated only from the early embryonic hippocampus, before the formation of the DG. No movement of progenitors from the postnatal SEZ to the newly forming DG subgranular zone is detected and adult DG colonies in vitro originate from the embryonic hippocampal primordium. These data support a model where embryonic hippocampal NSCs change their properties as the organism ages. When adult DG spheres are cocultured with embryonic brain slices, self-renewal (but not multipotentiality) is restored and maintained for several passages off of slices. Adult clonal DG spheres grown on embryonic brain slices or transplanted into brains of neonatal mice do not give rise to neurons. Neurons arise from separate, small clones that are approximately 10 times more frequent than sphere colonies in vitro and may be responsible for maintaining neurogenesis in the adult in vivo. We propose that there are separate glial and neuronal clones in the adult hippocampus, with glial progenitors being the most proliferative in culture.