Aldh1L1 is expressed by postnatal neural stem cells in vivo

Mature astrocytes as source for astrocyte repopulation after deletion in the medial prefrontal cortex: Implications for depression

The adult brain retains a high repopulation capacity of astrocytes after deletion, and both mature astrocytes in the neocortex and neural stem cells in neurogenic regions possess the potential to generate astrocytes. However, the origin and the repopulation dynamics of the repopulating astrocytes after deletion remain largely unclear. The number of astrocytes is reduced in the medial prefrontal cortex (mPFC) of patients with depression, and selective elimination of mPFC astrocytes is sufficient to induce depression-like behaviors in rodents. However, whether astrocyte repopulation capacity is impaired in depression is unknown. In this study, we used different transgenic mouse lines to genetically label different cell types and demonstrated that in the mPFC of normal adult mice of both sexes, mature astrocytes were a major source of the repopulating astrocytes after acute deletion induced by an astrocyte-specific toxin, L-alpha-aminoadipic acid (L-AAA), and astrocyte regeneration was accomplished within two weeks accompanied by reversal of depression-like behaviors. Furthermore, re-ablation of mPFC astrocytes post repopulation led to reappearance of depression-like behaviors. In adult male mice subjected to 14-day chronic restraint stress, a well-validated mouse model of depression, the number of mPFC astrocytes was reduced; however, the ability of mPFC astrocytes to repopulate after L-AAA-induced deletion was largely unaltered. Our study highlights a potentially beneficial role for repopulating astrocytes in depression and provides novel therapeutic insights into enhancing local mature astrocyte generation in depression.

Dynamic local mRNA localization and translation occurs during the postnatal molecular maturation of perivascular astrocytic processes

Astrocytes are highly ramified and send out perivascular processes (PvAPs) that entirely sheathe the brain's blood vessels. PvAPs are equipped with an enriched molecular repertoire that sustains astrocytic regulatory functions at the vascular interface. In the mouse, PvAP development starts after birth and is essentially complete by postnatal day (P) 15. Progressive molecular maturation also occurs over this period, with the acquisition of proteins enriched in PvAPs. The mechanisms controlling the development and molecular maturation of PvAPs have not been extensively characterized. We reported previously that mRNAs are distributed unequally in mature PvAPs and are locally translated. Since dynamic mRNA localization and local translation influence the cell's polarity, we hypothesized that they might sustain the postnatal maturation of PvAPs. Here, we used a combination of molecular biology and imaging approaches to demonstrate that the development of PvAPs is accompanied by the transport of mRNA and polysomal mRNA into PvAPs, the development of a rough endoplasmic reticulum (RER) network and Golgi cisternae, and local translation. By focusing on genes and proteins that are selectively or specifically expressed in astrocytes, we characterized the developmental profile of mRNAs, polysomal mRNAs and proteins in PvAPs from P5 to P60. We found that some polysomal mRNAs polarized progressively towards the PvAPs. Lastly, we found that expression and localization of mRNAs in developing PvAPs is perturbed in a mouse model of megalencephalic leukoencephalopathy with subcortical cysts. Our results indicate that dynamic mRNA localization and local translation influence the postnatal maturation of PvAPs.

A toolbox of astrocyte-specific, serotype-independent adeno-associated viral vectors using microRNA targeting sequences

Astrocytes, one of the most prevalent cell types in the central nervous system (CNS), are critically involved in neural function. Genetically manipulating astrocytes is an essential tool in understanding and affecting their roles. Adeno-associated viruses (AAVs) enable rapid genetic manipulation; however, astrocyte specificity of AAVs can be limited, with high off-target expression in neurons and sparsely in endothelial cells. Here, we report the development of a cassette of four copies of six miRNA targeting sequences (4x6T) which triggers transgene degradation specifically in neurons and endothelial cells. In combination with the GfaABC1D promoter, 4x6T increases astrocytic specificity of Cre with a viral reporter from <50% to >99% in multiple serotypes in mice, and confers astrocyte specificity in multiple recombinases and reporters. We also present empty vectors to add 4x6T to other cargo, independently and in Cre/Dre-dependent forms. This toolbox of AAVs allows rapid manipulation of astrocytes throughout the CNS, is compatible with different AAV serotypes, and demonstrates the efficacy of using multiplexed miRNA targeting sequences to decrease expression in multiple off-target cell populations simultaneously.

Loss of Astrocytic µ Opioid Receptors Exacerbates Aversion Associated with Morphine Withdrawal in Mice: Role of Mitochondrial Respiration

Astrocytes express mu/µ opioid receptors, but the function of these receptors remains poorly understood. We evaluated the effects of astrocyte-restricted knockout of µ opioid receptors on reward- and aversion-associated behaviors in mice chronically exposed to morphine. Specifically, one of the floxed alleles of the Oprm1 gene encoding µ opioid receptor 1 was selectively deleted from brain astrocytes in Oprm1 inducible conditional knockout (icKO) mice. These mice did not exhibit changes in locomotor activity, anxiety, or novel object recognition, or in their responses to the acute analgesic effects of morphine. Oprm1 icKO mice displayed increased locomotor activity in response to acute morphine administration but unaltered locomotor sensitization. Oprm1 icKO mice showed normal morphine-induced conditioned place preference but exhibited stronger conditioned place aversion associated with naloxone-precipitated morphine withdrawal. Notably, elevated conditioned place aversion lasted up to 6 weeks in Oprm1 icKO mice. Astrocytes isolated from the brains of Oprm1 icKO mice had unchanged levels of glycolysis but had elevated oxidative phosphorylation. The basal augmentation of oxidative phosphorylation in Oprm1 icKO mice was further exacerbated by naloxone-precipitated withdrawal from morphine and, similar to that for conditioned place aversion, was still present 6 weeks later. Our findings suggest that µ opioid receptors in astrocytes are linked to oxidative phosphorylation and they contribute to long-term changes associated with opioid withdrawal.

The ribosome-associated protein RACK1 represses Kir4.1 translation in astrocytes and influences neuronal activity

The regulation of translation in astrocytes, the main glial cells in the brain, remains poorly characterized. We developed a high-throughput proteomics screen for polysome-associated proteins in astrocytes and focused on ribosomal protein receptor of activated protein C kinase 1 (RACK1), a critical factor in translational regulation. In astrocyte somata and perisynaptic astrocytic processes (PAPs), RACK1 preferentially binds to a number of mRNAs, including Kcnj10, encoding the inward-rectifying potassium (K⁺) channel Kir4.1. By developing an astrocyte-specific, conditional RACK1 knockout mouse model, we show that RACK1 represses production of Kir4.1 in hippocampal astrocytes and PAPs. Upregulation of Kir4.1 in the absence of RACK1 increases astrocytic Kir4.1-mediated K⁺ currents and volume. It also modifies neuronal activity attenuating burst frequency and duration. Reporter-based assays reveal that RACK1 controls Kcnj10 translation through the transcript's 5' untranslated region. Hence, translational regulation by RACK1 in astrocytes represses Kir4.1 expression and influences neuronal activity.

Astrocytic TDP-43 dysregulation impairs memory by modulating antiviral pathways and interferon-inducible chemokines

Transactivating response region DNA binding protein 43 (TDP-43) pathology is prevalent in dementia, but the cell type-specific effects of TDP-43 pathology are not clear, and therapeutic strategies to alleviate TDP-43-linked cognitive decline are lacking. We found that patients with Alzheimer's disease or frontotemporal dementia have aberrant TDP-43 accumulation in hippocampal astrocytes. In mouse models, induction of widespread or hippocampus-targeted accumulation in astrocytic TDP-43 caused progressive memory loss and localized changes in antiviral gene expression. These changes were cell-autonomous and correlated with impaired astrocytic defense against infectious viruses. Among the changes, astrocytes had elevated levels of interferon-inducible chemokines, and neurons had elevated levels of the corresponding chemokine receptor CXCR3 in presynaptic terminals. CXCR3 stimulation altered presynaptic function and promoted neuronal hyperexcitability, akin to the effects of astrocytic TDP-43 dysregulation, and blockade of CXCR3 reduced this activity. Ablation of CXCR3 also prevented TDP-43-linked memory loss. Thus, astrocytic TDP-43 dysfunction contributes to cognitive impairment through aberrant chemokine-mediated astrocytic-neuronal interactions.

In Vivo Targeting of the Neurovascular Unit: Challenges and Advancements

The blood-brain barrier (BBB) is essential for the homeostasis of the central nervous system (CNS). Functions of the BBB are performed by the neurovascular unit (NVU), which consists of endothelial cells, pericytes, astrocytes, microglia, basement membrane, and neurons. NVU cells interact closely and together are responsible for neurovascular coupling, BBB integrity, and transendothelial fluid transport. Studies have shown that NVU dysfunction is implicated in several acute and chronic neurological diseases, including Alzheimer's disease, multiple sclerosis, and stroke. The mechanisms of NVU disruption remain poorly understood, partially due to difficulties in selective targeting of NVU cells. In this review, we discuss the relative merits of available protein markers and drivers of the NVU along with recent advancements that have been made in the field to increase efficiency and specificity of NVU research.

Astrocytic SARM1 promotes neuroinflammation and axonal demyelination in experimental autoimmune encephalomyelitis through inhibiting GDNF signaling

Astrocytes are important components of the innate immune response in the central nervous system (CNS), involving in the inflammatory and neurotoxic responses that occur in CNS diseases, such as multiple sclerosis (MS). Recent studies have shown that SARM1 plays a critical role in axonal degeneration and inflammation. However, the detailed role of astrocytic SARM1 in MS remains unclear. Here, we established the MS model of mice - experimental autoimmune encephalomyelitis (EAE) and found that SARM1 was upregulated in astrocytes of the spinal cords of EAE mice. Moreover, conditional knockout of astrocytic SARM1 (SARM1GFAP-CKO mice, SARM1Aldh1L1-CKO mice) delayed EAE with later onset, alleviated the inflammatory infiltration, and inhibited the demyelination and neuronal death. Mechanically, RNA-seq revealed that the expression of glial-derived neurotrophic factor (GDNF) was upregulated in SARM1-/- astrocytes. Western blot and immunostaining further confirmed the upregulation of GDNF in spinal cord astrocytes of SARM1GFAP-CKO EAE mice. Interestingly, the downregulation of GDNF by streptozotocin (STZ, a drug used to downregulate GDNF) treatment worsened the deficits of SARM1GFAP-CKO EAE mice. These findings identify that astrocytic SARM1 promotes neuroinflammation and axonal demyelination in EAE by inhibiting the expression of GDNF, reveal the novel role of SARM1/GDNF signaling in EAE, and provide new therapeutic ideas for the treatment of MS.

Astroglial and oligodendroglial markers in the cuprizone animal model for de- and remyelination

Myelin loss with consecutive axon degeneration and impaired remyelination are the underlying causes of progressive disease in patients with multiple sclerosis. Astrocytes are suggested to play a major role in these processes. The unmasking of distinct astrocyte identities in health and disease would help to understand the pathophysiological mechanisms in which astrocytes are involved. However, the number of specific astrocyte markers is limited. Therefore, we performed immunohistochemical studies and analyzed various markers including GFAP, vimentin, S100B, ALDH1L1, and LCN2 during de- and remyelination using the toxic murine cuprizone animal model. Applying this animal model, we were able to confirm overlapping expression of vimentin and GFAP and highlighted the potential of ALDH1L1 as a pan-astrocytic marker, in agreement with previous data. Only a small population of GFAP-positive astrocytes in the corpus callosum highly up-regulated LCN2 at the peak of demyelination and S100B expression was found in a subset of oligodendroglia as well, thus S100B turned out to have a limited use as a particular astroglial marker. Additionally, numerous GFAP-positive astrocytes in the lateral corpus callosum did not express S100B, further strengthening findings of heterogeneity in the astrocytic population. In conclusion, our results acknowledged that GFAP, vimentin, LCN2, and ALDH1L1 serve as reliable marker to identify activated astrocytes during cuprizone-induced de- and remyelination. Moreover, there were clear regional and temporal differences in protein and mRNA expression levels and patterns of the studied markers, generally between gray and white matter structures.

Regional heterogeneity of astrocyte morphogenesis dictated by the formin protein, Daam2, modifies circuit function

Astrocytes display extraordinary morphological complexity that is essential to support brain circuit development and function. Formin proteins are key regulators of the cytoskeleton; however, their role in astrocyte morphogenesis across diverse brain regions and neural circuits is unknown. Here, we show that loss of the formin protein Daam2 in astrocytes increases morphological complexity in the cortex and olfactory bulb, but elicits opposing effects on astrocytic calcium dynamics. These differential physiological effects result in increased excitatory synaptic activity in the cortex and increased inhibitory synaptic activity in the olfactory bulb, leading to altered olfactory behaviors. Proteomic profiling and immunoprecipitation experiments identify Slc4a4 as a binding partner of Daam2 in the cortex, and combined deletion of Daam2 and Slc4a4 restores the morphological alterations seen in Daam2 mutants. Our results reveal new mechanisms regulating astrocyte morphology and show that congruent changes in astrocyte morphology can differentially influence circuit function.

Astrocytic ApoE underlies maturation of hippocampal neurons and cognitive recovery after traumatic brain injury in mice

Polymorphisms in the apolipoprotein E (ApoE) gene confer a major genetic risk for the development of late-onset Alzheimer's disease (AD) and are predictive of outcome following traumatic brain injury (TBI). Alterations in adult hippocampal neurogenesis have long been associated with both the development of AD and recovery following TBI and ApoE is known to play a role in this process. In order to determine how ApoE might influence hippocampal injury-induced neurogenesis, we generated a conditional knockout system whereby functional ApoE from astrocytes was ablated prior to injury. While successfully ablating ApoE just prior to TBI in mice, we observed an attenuation in the development of the spines in the newborn neurons. Intriguingly, animals with a double-hit, i.e. injury and ApoE conditionally inactivated in astrocytes, demonstrated the most pronounced impairments in the hippocampal-dependent Morris water maze test, failing to exhibit spatial memory after both acquisition and reversal training trials. In comparison, conditional knockout mice without injury displayed impairments but only in the reversal phase of the test, suggesting accumulative effects of astrocytic ApoE deficiency and traumatic brain injury on AD-like phenotypes. Together, these findings demonstrate that astrocytic ApoE is required for functional injury-induced neurogenesis following traumatic brain injury.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Distribution of Aldh1L1-CreERT2 Recombination in Astrocytes Versus Neural Stem Cells in the Neurogenic Niches of the Adult Mouse Brain

In the adult central nervous system, neural stem cells (NSCs) reside in two discrete niches: the subependymal zone (SEZ) of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus (DG). Here, NSCs represent a population of highly specialized astrocytes that are able to proliferate and give rise to neuronal and glial progeny. This process, termed adult neurogenesis, is extrinsically regulated by other niche cells such as non-stem cell astrocytes. Studying these non-stem cell niche astrocytes and their role during adult neuro- and gliogenesis has been hampered by the lack of genetic tools to discriminate between transcriptionally similar NSCs and niche astrocytes. Recently, Aldh1L1 has been shown to be a pan-astrocyte marker and that its promoter can be used to specifically target astrocytes using the Cre-loxP system. In this study we explored whether the recently described Aldh1L1-CreER^T2 mouse line (Winchenbach et al., 2016) can serve to specifically target niche astrocytes without inducing recombination in NSCs in adult neurogenic niches. Using short- and long-term tamoxifen protocols we revealed high recombination efficiency and specificity in non-stem cell astrocytes and little to no recombination in NSCs of the adult DG. However, in the SEZ we observed recombination in ependymal cells, astrocytes, and NSCs, the latter giving rise to neuronal progeny of the rostral migratory stream and olfactory bulb. Thus, we recommend the here described Aldh1L1-CreER^T2 mouse line for predominantly studying the functions of non-stem cell astrocytes in the DG under physiological and pathological conditions.

Loss of Quaking RNA binding protein disrupts the expression of genes associated with astrocyte maturation in mouse brain

Quaking RNA binding protein (QKI) is essential for oligodendrocyte development as myelination requires myelin basic protein mRNA regulation and localization by the cytoplasmic isoforms (e.g., QKI-6). QKI-6 is also highly expressed in astrocytes, which were recently demonstrated to have regulated mRNA localization. Here, we define the targets of QKI in the mouse brain via CLIPseq and we show that QKI-6 binds 3'UTRs of a subset of astrocytic mRNAs. Binding is also enriched near stop codons, mediated partially by QKI-binding motifs (QBMs), yet spreads to adjacent sequences. Using a viral approach for mosaic, astrocyte-specific gene mutation with simultaneous translating RNA sequencing (CRISPR-TRAPseq), we profile ribosome associated mRNA from QKI-null astrocytes in the mouse brain. This demonstrates a role for QKI in stabilizing CLIP-defined direct targets in astrocytes in vivo and further shows that QKI mutation disrupts the transcriptional changes for a discrete subset of genes associated with astrocyte maturation.

Slc1a3-2A-CreERT2 mice reveal unique features of Bergmann glia and augment a growing collection of Cre drivers and effectors in the 129S4 genetic background

Genetic variation is a primary determinant of phenotypic diversity. In laboratory mice, genetic variation can be a serious experimental confounder, and thus minimized through inbreeding. However, generalizations of results obtained with inbred strains must be made with caution, especially when working with complex phenotypes and disease models. Here we compared behavioral characteristics of C57Bl/6—the strain most widely used in biomedical research—with those of 129S4. In contrast to 129S4, C57Bl/6 demonstrated high within-strain and intra-litter behavioral hyperactivity. Although high consistency would be advantageous, the majority of disease models and transgenic tools are in C57Bl/6. We recently established six Cre driver lines and two Cre effector lines in 129S4. To augment this collection, we genetically engineered a Cre line to study astrocytes in 129S4. It was validated with two Cre effector lines: calcium indicator gCaMP5g-tdTomato and RiboTag—a tool widely used to study cell type-specific translatomes. These reporters are in different genomic loci, and in both the Cre was functional and astrocyte-specific. We found that calcium signals lasted longer and had a higher amplitude in cortical compared to hippocampal astrocytes, genes linked to a single neurodegenerative disease have highly divergent expression patterns, and that ribosome proteins are non-uniformly expressed across brain regions and cell types.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

Diacylglycerol lipase alpha in astrocytes is involved in maternal care and affective behaviors

Genetic deletion of cannabinoid CB1 receptors or diacylglycerol lipase alpha (DAGLa), the main enzyme involved in the synthesis of the endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG), produced profound phenotypes in animal models of depression-related behaviors. Furthermore, clinical studies have shown that antagonists of CB1 can increase the incidence and severity of major depressive episodes. However, the underlying pathomechanisms are largely unknown. In this study, we have focused on the possible involvement of astrocytes. Using the highly sensitive RNAscope technology, we show for the first time that a subpopulation of astrocytes in the adult mouse brain expresses Dagla, albeit at low levels. Targeted lipidomics revealed that astrocytic DAGLa only accounts for a minor percentage of the steady-state brain 2-AG levels and other arachidonic acid derived lipids like prostaglandins. Nevertheless, the deletion of Dagla in adult mouse astrocytes had profound behavioral consequences with significantly increased depressive-like behavioral responses and striking effects on maternal behavior, corresponding with increased levels of serum progesterone and estradiol. Our findings therefore indicate that lipids from the DAGLa metabolic axis in astrocytes play a key regulatory role in affective behaviors.

Aldehyde dehydrogenases contribute to skeletal muscle homeostasis in healthy, aging, and Duchenne muscular dystrophy patients

BACKGROUND

Aldehyde dehydrogenases (ALDHs) are key players in cell survival, protection, and differentiation via the metabolism and detoxification of aldehydes. ALDH activity is also a marker of stem cells. The skeletal muscle contains populations of ALDH-positive cells amenable to use in cell therapy, whose distribution, persistence in aging, and modifications in myopathic context have not been investigated yet.

METHODS

The Aldefluor® (ALDEF) reagent was used to assess the ALDH activity of muscle cell populations, whose phenotypic characterizations were deepened by flow cytometry. The nature of ALDH isoenzymes expressed by the muscle cell populations was identified in complementary ways by flow cytometry, immunohistology, and real-time PCR ex vivo and in vitro. These populations were compared in healthy, aging, or Duchenne muscular dystrophy (DMD) patients, healthy non-human primates, and Golden Retriever dogs (healthy vs. muscular dystrophic model, Golden retriever muscular dystrophy [GRMD]).

RESULTS

ALDEF⁺ cells persisted through muscle aging in humans and were equally represented in several anatomical localizations in healthy non-human primates. ALDEF⁺ cells were increased in dystrophic individuals in humans (nine patients with DMD vs. five controls: 14.9 ± 1.63% vs. 3.6 ± 0.39%, P = 0.0002) and dogs (three GRMD dogs vs. three controls: 10.9 ± 2.54% vs. 3.7 ± 0.45%, P = 0.049). In DMD patients, such increase was due to the adipogenic ALDEF⁺ /CD34⁺ populations (11.74 ± 1.5 vs. 2.8 ± 0.4, P = 0.0003), while in GRMD dogs, it was due to the myogenic ALDEF⁺ /CD34^- cells (3.6 ± 0.6% vs. 1.03 ± 0.23%, P = 0.0165). Phenotypic characterization associated the ALDEF⁺ /CD34^- cells with CD9, CD36, CD49a, CD49c, CD49f, CD106, CD146, and CD184, some being associated with myogenic capacities. Cytological and histological analyses distinguished several ALDH isoenzymes (ALDH1A1, 1A2, 1A3, 1B1, 1L1, 2, 3A1, 3A2, 3B1, 3B2, 4A1, 7A1, 8A1, and 9A1) expressed by different cell populations in the skeletal muscle tissue belonging to multinucleated fibres, or myogenic, endothelial, interstitial, and neural lineages, designing them as potential new markers of cell type or of metabolic activity. Important modifications were noted in isoenzyme expression between healthy and DMD muscle tissues. The level of gene expression of some isoenzymes (ALDH1A1, 1A3, 1B1, 2, 3A2, 7A1, 8A1, and 9A1) suggested their specific involvement in muscle stability or regeneration in situ or in vitro.

CONCLUSIONS

This study unveils the importance of the ALDH family of isoenzymes in the skeletal muscle physiology and homeostasis, suggesting their roles in tissue remodelling in the context of muscular dystrophies.

Parallel astrocyte calcium signaling modulates olfactory bulb responses

Astrocytes are the most abundant glial cell in the central nervous system. They modulate synaptic function through a variety of mechanisms, and yet remain relatively understudied with respect to overall neuronal circuit function. Exploiting the tractability of the mouse olfactory system, we manipulated astrocyte activity and examined how astrocytes modulate olfactory bulb responses. Toward this, we genetically targeted both astrocytes and neurons for in vivo widefield imaging of Ca2+ responses to odor stimuli. We found that astrocytes exhibited odor response maps that overlap with excitatory neuronal activity. By manipulating Ca2+ activity in astrocytes using chemical genetics we found that odor-evoked neuronal activity was reciprocally affected, suggesting that astrocyte activation inhibits neuronal odor responses. Subsequently, behavioral experiments revealed that astrocyte manipulations affect both odor detection threshold and discrimination, suggesting that astrocytes play an active role in olfactory sensory processing circuits. Together, these studies show that astrocyte calcium signaling contributes to olfactory behavior through modulation of sensory circuits.

The optic nerve lamina region is a neural progenitor cell niche

Retinal ganglion cell axons forming the optic nerve (ON) emerge unmyelinated from the eye and become myelinated after passage through the optic nerve lamina region (ONLR), a transitional area containing a vascular plexus. The ONLR has a number of unusual characteristics: it inhibits intraocular myelination, enables postnatal ON myelination of growing axons, modulates the fluid pressure differences between eye and brain, and is the primary lesion site in the age-related disease open angle glaucoma (OAG). We demonstrate that the human and rodent ONLR possesses a mitotically active, age-depletable neural progenitor cell (NPC) niche, with unique characteristics and culture requirements. These NPCs generate both forms of macroglia: astrocytes and oligodendrocytes, and can form neurospheres in culture. Using reporter mice with SOX2-driven, inducible gene expression, we show that ONLR-NPCs generate macroglial cells for the anterior ON. Early ONLR-NPC loss results in regional dysfunction and hypomyelination. In adulthood, ONLR-NPCs may enable glial replacement and remyelination. ONLR-NPC depletion may help explain why ON diseases such as OAG progress in severity during aging.

GLAST-CreERT2 mediated deletion of GDNF increases brain damage and exacerbates long-term stroke outcomes after focal ischemic stroke in mouse model

Focal ischemic stroke (FIS) is a leading cause of human death. Glial scar formation largely caused by reactive astrogliosis in peri-infarct region (PIR) is the hallmark of FIS. Glial cell-derived neurotrophic factor (GDNF) was originally isolated from a rat glioma cell-line supernatant and is a potent survival neurotrophic factor. Here, using CreER^T2 -LoxP recombination technology, we generated inducible and astrocyte-specific GDNF conditional knockout (cKO), that is, GLAST-GDNF^-/- cKO mice to investigate the effect of reactive astrocytes (RAs)-derived GDNF on neuronal death, brain damage, oxidative stress and motor function recovery after photothrombosis (PT)-induced FIS. Under non-ischemic conditions, we found that adult GLAST-GDNF^-/- cKO mice exhibited significant lower numbers of Brdu+, Ki67+ cells, and DCX+ cells in the dentate gyrus (DG) in hippocampus than GDNF floxed (GDNF^f/f ) control (Ctrl) mice, indicating endogenous astrocytic GDNF can promote adult neurogenesis. Under ischemic conditions, GLAST-GDNF^-/- cKO mice had a significant increase in infarct volume, hippocampal damage and FJB+ degenerating neurons after PT as compared with the Ctrl mice. GLAST-GDNF^-/- cKO mice also had lower densities of Brdu+ and Ki67+ cells in the PIR and exhibited larger behavioral deficits than the Ctrl mice. Mechanistically, GDNF deficiency in astrocytes increased oxidative stress through the downregulation of glucose-6-phosphate dehydrogenase (G6PD) in RAs. In summary, our study indicates that RAs-derived endogenous GDNF plays important roles in reducing brain damage and promoting brain recovery after FIS through neural regeneration and suggests that promoting anti-oxidant mechanism in RAs is a potential strategy in stroke therapy.

Genetic interactions between INPP4B and RAD50 is prognostic of breast cancer survival

RAD50 is commonly depleted in basal-like breast cancer with concomitant absence of INPP4B and several tumor suppressors such as BRCA1 and TP53. Our previous study revealed that INPP4B and RAD50 interact and such an interaction is associated with breast cancer survival at the transcriptional, translational and genomic levels. In the present study, we explored single nucleotide polymorphisms (SNPs) of these two genes that have synergistic effects on breast cancer survival to decipher mechanisms driving their interactions at the genetic level. The Cox's proportional hazards model was used to test whether SNPs of these two genes are interactively associated with breast cancer survival, following expression quantitative trait loci (eQTL) analysis and functional investigations. Our study revealed two disease-associating blocks, each encompassing five and two non-linkage disequilibrium linked SNPs of INPP4B and RAD50, respectively. Concomitant presence of any rare homozygote from each disease-associating block is synergistically prognostic of poor breast cancer survival. Such synergy is mediated via bypassing pathways controlling cell proliferation and DNA damage repair, which are represented by INPP4B and RAD50. Our study provided genetic evidence of interactions between INPP4B and RAD50, and deepened our understandings on the orchestrated genetic machinery governing tumor progression.

Expression Patterns of Inducible Cre Recombinase Driven by Differential Astrocyte-Specific Promoters in Transgenic Mouse Lines

Astrocytes are the most abundant cell type in the central nervous system (CNS). They provide trophic support for neurons, modulate synaptic transmission and plasticity, and contribute to neuronal dysfunction. Many transgenic mouse lines have been generated to obtain astrocyte-specific expression of inducible Cre recombinase for functional studies; however, the expression patterns of inducible Cre recombinase in these lines have not been systematically characterized. We generated a new astrocyte-specific Aldh1l1-CreER^T2 knock-in mouse line and compared the expression pattern of Cre recombinase between this and five widely-used transgenic lines (hGfap-CreER^T2 from The Jackson Laboratory and The Mutant Mouse Resource and Research Center, Glast-CreER^T2, Cx30-CreER^T2, and Fgfr3-iCreER^T2) by crossing with Ai14 mice, which express tdTomato fluorescence following Cre-mediated recombination. In adult Aldh1l1-CreER^T2:Ai14 transgenic mice, tdTomato was detected throughout the CNS, and five novel morphologically-defined types of astrocyte were described. Among the six evaluated lines, the specificity of Cre-mediated recombination was highest when driven by Aldh1l1 and lowest when driven by hGfap; in the latter mice, co-staining between tdTomato and NeuN was observed in the hippocampus and cortex. Notably, evident leakage was noted in Fgfr3-iCreER^T2 mice, and the expression level of tdTomato was low in the thalamus when Cre recombinase expression was driven by Glast and in the capsular part of the central amygdaloid nucleus when driven by Cx30. Furthermore, tdTomato was clearly expressed in peripheral organs in four of the lines. Our results emphasize that the astrocyte-specific CreER^T2 transgenic lines used in functional studies should be carefully selected.

Regional methylome profiling reveals dynamic epigenetic heterogeneity and convergent hypomethylation of stem cell quiescence-associated genes in breast cancer following neoadjuvant chemotherapy

Background

Neoadjuvant chemotherapy (NAC) induces a pathological complete response (pCR) in ~ 30% of patients with breast cancer. However, aberrant DNA methylation alterations are frequent events during breast cancer progression and acquisition of chemoresistance. We aimed to characterize the inter- and intra-tumor methylation heterogeneity (MH) in breast cancer following NAC.

Methods

DNA methylation profiles of spatially separated regions of breast tumors before and after NAC treatment were investigated using high-density methylation microarray. Methylation levels of genes of interest were further examined using multiplexed MethyLight droplet digital PCR (ddPCR).

Results

We have discovered different levels of intra-tumor MH in breast cancer patients. Moreover, NAC dramatically altered the methylation profiles and such changes were highly heterogeneous between the patients. Despite the high inter-patient heterogeneity, we identified that stem cell quiescence-associated genes ALDH1L1, HOPX, WNT5A and SOX9 were convergently hypomethylated across all the samples after NAC treatment. Furthermore, by using MethyLight ddPCR, we verified that the methylation levels of these 4 genes were significantly lower in breast tumor samples after NAC than those before NAC.

Conclusions

Our study has revealed that NAC dramatically alters epigenetic heterogeneity in breast cancer and induces convergent hypomethylation of stem cell quiescence-associated genes, ALDH1L1, HOPX, WNT5A and SOX9, which can potentially be developed as therapeutic targets or biomarkers for chemoresistance.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0278-y) contains supplementary material, which is available to authorized users.

Astrocyte Specificity and Coverage of hGFAP-CreERT2 [Tg(GFAP-Cre/ERT2)13Kdmc] Mouse Line in Various Brain Regions

Astrocyte is the most abundant cell type in the central nervous system and its importance has been increasingly recognized in the brain pathophysiology. To study in vivo function of astrocyte, astrocyte-specific gene-targeting is regarded as a powerful approach. Especially, hGFAP-CreERT2, which expresses tamoxifen-inducible Cre recombinase under the human GFAP promoter, has been developed and characterized from several research groups. However, one of these mouse lines, [Tg(GFAP-Cre/ERT2)13Kdmc] from Ken McCarthy group has not been quantitatively analyzed, despite its frequent use. Here, we performed comprehensive characterization of this mouse line with quantitative analysis. By crossing this mouse line with Ai14 (RCL-tdTomato), a very sensitive Cre reporter mouse line, we visualized the Cre-expressing cells in various brain regions. For quantitative analysis, we immunostained S100β as an astrocytic marker and NeuN, tyrosine hydroxylase or calbindin as a neuronal marker in different brain regions. We calculated ‘astrocyte specificity’ as the proportion of co-labelled S100β and tdTomato positive cells in the total number of tdTomato positive cells and the ‘astrocyte coverage’ as the proportion of co-labelled S100β and tdTomato positive cells in the total number of S100β positive cells. Interestingly, we found varying degree of astrocyte specificity and coverage in each brain region. In cortex, hypothalamus, substantia nigra pars compacta and cerebellar Purkinje layer, we observed high astrocyte specificity (over 89%) and relatively high astrocyte coverage (over 70%). In striatum, hippocampal CA1 layer, dentate gyrus and cerebellar granule layer, we observed high astrocyte specificity (over 80%), but relative low astrocyte coverage (50–60%). However, thalamus and amygdala showed low astrocyte specificity (about 65%) and significant neuron specificity (over 30%). This hGFAP-CreERT2 mouse line can be useful for genetic modulations of target gene either in gain-of-function or loss-of-function studies in the brain regions with high astrocyte specificity and coverage. However, the use of this mouse line should be restricted to gain-of-function studies in the brain regions with high astrocyte specificity but low coverage. In conclusion, hGFAP-CreERT2 mouse line could be a powerful tool for gene-targeting of astrocytes in cortex, striatum, hippocampus, hypothalamus, substantia nigra pars compacta and cerebellum, but not in thalamus and amygdala.

Astrocytes and microglia: Models and tools

An amazing array of tools both old and new are available to investigate the function of astrocytes and microglia. Guttenplan and Liddelow discuss tools available to study the physiology and pathophysiology of these cells both in vivo and in culture systems.

Glial cells serve as fundamental regulators of the central nervous system in development, homeostasis, and disease. Discoveries into the function of these cells have fueled excitement in glial research, with enthusiastic researchers addressing fundamental questions about glial biology and producing new scientific tools for the community. Here, we outline the pros and cons of in vivo and in vitro techniques to study astrocytes and microglia with the goal of helping researchers quickly identify the best approach for a given research question in the context of glial biology. It is truly a great time to be a glial biologist.

Cancer Stem Cells: Emergent Nature of Tumor Emergency

A functional analysis of 167 genes overexpressed in Krebs-2 tumor initiating cells was performed. In the first part of the study, the genes were analyzed for their belonging to one or more of the three groups, which represent the three major phenotypic manifestation of malignancy of cancer cells, namely (1) proliferative self-sufficiency, (2) invasive growth and metastasis, and (3) multiple drug resistance. 96 genes out of 167 were identified as possible contributors to at least one of these fundamental properties. It was also found that substantial part of these genes are also known as genes responsible for formation and/or maintenance of the stemness of normal pluri-/multipotent stem cells. These results suggest that the malignancy is simply the ability to maintain the stem cell specific genes expression profile, and, as a consequence, the stemness itself regardless of the controlling effect of stem niches. In the second part of the study, three stress factors combined into the single concept of "generalized cellular stress," which are assumed to activate the expression of these genes, were defined. In addition, possible mechanisms for such activation were identified. The data obtained suggest the existence of a mechanism for the de novo formation of a pluripotent/stem phenotype in the subpopulation of "committed" tumor cells.

Glial Cell Expansion Coincides with Neural Circuit Formation in the Developing Auditory Brainstem

Neural circuit formation involves maturation of neuronal, glial and vascular cells, as well as cell proliferation and cell death. A fundamental understanding of cellular mechanisms is enhanced by quantification of cell types during key events in synapse formation and pruning and possessing qualified genetic tools for cell type-specific manipulation. Acquiring this information in turn requires validated cell markers and genetic tools. We quantified changing proportions of neurons, astrocytes, oligodendrocytes, and microglia in the medial nucleus of the trapezoid body (MNTB) during neural circuit development. Cell type-specific markers, light microscopy and 3D virtual reality software, the latter developed in our laboratory, were used to count cells within distinct cell populations at postnatal days (P)3 and P6, bracketing the period of nerve terminal growth and pruning in this system. These data revealed a change from roughly equal numbers of neurons and glia at P3 to a 1.5:1 ratio of glia to neurons at P6. PCNA and PH3 labeling revealed that proliferation of oligodendrocytes contributed to the increase in glial cell number during this timeframe. We next evaluated Cre driver lines for selectivity in labeling cell populations. En1-Cre was specific for MNTB neurons. PDGFRα-Cre and Aldh1L1-Cre, thought to be mostly specific for oligodendrocyte lineage cells and astrocytes, respectively, both labeled significant numbers of neurons, oligodendrocytes, and astrocytes and are non-specific genetic tools in this neural system.

BMP-Responsive Protease HtrA1 Is Differentially Expressed in Astrocytes and Regulates Astrocytic Development and Injury Response

Astrocytes perform a wide array of physiological functions, including structural support, ion exchange, and neurotransmitter uptake. Despite this diversity, molecular markers that label subpopulations of astrocytes are limited, and mechanisms that generate distinct astrocyte subtypes remain unclear. Here we identified serine protease high temperature requirement A 1 (HtrA1), a bone morphogenetic protein 4 signaling regulated protein, as a novel marker of forebrain astrocytes, but not of neural stem cells, in adult mice of both sexes. Genetic deletion of HtrA1 during gliogenesis accelerates astrocyte differentiation. In addition, ablation of HtrA1 in cultured astrocytes leads to altered chondroitin sulfate proteoglycan expression and inhibition of neurite extension, along with elevated levels of transforming growth factor-β family proteins. Brain injury induces HtrA1 expression in reactive astrocytes, and loss of HtrA1 leads to an impairment in wound closure accompanied by increased proliferation of endothelial and immune cells. Our findings demonstrate that HtrA1 is differentially expressed in adult mouse forebrain astrocytes, and that HtrA1 plays important roles in astrocytic development and injury response.SIGNIFICANCE STATEMENT Astrocytes, an abundant cell type in the brain, perform a wide array of physiological functions. Although characterized as morphologically and functionally diverse, molecular markers that label astrocyte subtypes or signaling pathways that lead to their diversity remain limited. Here, after examining the expression profile of astrocytes generated in response to bone morphogenetic protein signaling, we identify high temperature requirement A 1 (HtrA1) as an astrocyte-specific marker that is differentially expressed in distinct adult mouse brain regions. HtrA1 is a serine protease that has been linked to cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy, a small blood vessel disease in humans. Understanding the role of HtrA1 during development and after injury will provide insights into how distinct astrocyte populations are generated and their unique roles in injury and disease.

Three-Dimensional Environment Sustains Morphological Heterogeneity and Promotes Phenotypic Progression During Astrocyte Development

Astrocytes are critical for coordinating normal brain function by regulating brain metabolic homeostasis, synaptogenesis and neurotransmission, and blood-brain barrier permeability and maintenance. Dysregulation of normal astrocyte ontogeny contributes to neurodevelopmental and neurodegenerative disorders, epilepsies, and adverse responses to injury. To achieve these multiple essential roles, astrocyte phenotypes are regionally, morphologically, and functionally heterogeneous. Therefore, the best regenerative medicine strategies may require selective production of distinct astrocyte subpopulations at defined maturation levels. However, little is known about the mechanisms that direct astrocyte diversity or whether heterogeneity is represented in biomaterials. In vitro studies report lack of normal morphologies and overrepresentation of the glial scar type of reactive astrocyte morphology and expression of markers, questioning how well the in vitro astrocytes represent glia in vivo and whether in vitro tissue engineering methods are suitable for regenerative medicine applications. Our previous work with neurons suggests that the three-dimensional (3D) environment, when compared with standard two-dimensional (2D) substrate, yields cellular and molecular behaviors that more closely approximately normal ontogeny. To specifically study the effects of dimensionality, we used purified glial fibrillary acidic protein (GFAP)-expressing primary cerebral cortical astrocyte cultures from single pups and characterized the cellular maturation profiles in 2D and 3D milieu. We identified four morphological groups in vitro: round, bipolar, stellate, and putative perivascular. In the 3D hydrogel culture environment, postnatal astrocytes transitioned from a population of nearly all round cells and very few bipolar cells toward a population with significant fractions of round, stellate, and putative perivascular cells within a few days, following the in vivo ontogeny. In 2D, however, the population shift from round and bipolar to stellate and perivascular was rarely observed. The transition to distinct cellular morphologies in 3D corresponded to the in vivo expression of phenotypic markers, supporting the generation of mature heterogeneous glial populations in vitro. This study presents quantitative data supporting that 3D culture is critical for sustaining the heterogeneity of astrocytes in vitro and for generating a representation of the in vivo portfolio of heterogeneous populations of astrocytes required for therapeutic interventions in neurodevelopmental disorders, epilepsy, and brain injury.

Astrocyte-Secreted Glypican 4 Regulates Release of Neuronal Pentraxin 1 from Axons to Induce Functional Synapse Formation

The generation of precise synaptic connections between developing neurons is critical to the formation of functional neural circuits. Astrocyte-secreted glypican 4 induces formation of active excitatory synapses by recruiting AMPA glutamate receptors to the postsynaptic cell surface. We now identify the molecular mechanism of how glypican 4 exerts its effect. Glypican 4 induces release of the AMPA receptor clustering factor neuronal pentraxin 1 from presynaptic terminals by signaling through presynaptic protein tyrosine phosphatase receptor δ. Pentraxin then accumulates AMPA receptors on the postsynaptic terminal forming functional synapses. Our findings reveal a signaling pathway that regulates synaptic activity during central nervous system development and demonstrates a role for astrocytes as organizers of active synaptic connections by coordinating both pre and post synaptic neurons. As mutations in glypicans are associated with neurological disorders, such as autism and schizophrenia, this signaling cascade offers new avenues to modulate synaptic function in disease.

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Astrocyte-secreted Gpc4 induces release of NP1 from neurons

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Release of NP1 is mediated through Gpc4 interaction with presynaptic RPTPδ

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Gpc4 or RPTPδ KO causes presynaptic NP1 retention and decreased synapse number

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Astrocytic release of Gpc4 provides spatial and temporal cues for synaptogenesis

ALDH1L1 variant rs2276724 and mRNA expression predict post-operative clinical outcomes and are associated with TP53 expression in HBV-related hepatocellular carcinoma

Aldehyde dehydrogenase 1 family member L1 (ALDH1L1) is downregulated in hepatocellular carcinoma (HCC) tumors, and its decreased expression is associated with the poor prognosis of HCC patients. We, therefore, evaluated the effect of single nucleotide polymorphisms (SNPs) of ALDH1L1, and its mRNA expression on the survival of hepatitis B virus (HBV)-related HCC patients and the association with tumor protein p53 (TP53) expression. ALDH1L1 SNPs in 415 HBV-related HCC patients were genotyped via direct sequencing. Expression profile chip datasets and survival information were obtained from GSE14520. The C allele (CT/CC) carriers of rs2276724 were significantly associated with a favorable prognosis [adjusted P=0.040; adjusted hazard ratio (HR)=0.725; 95% confidence interval (CI)=0.533–0.986]. Joint-effect analyses suggested that the CT/CC genotype of rs2276724 in TP53-negative patients was significantly associated with a decreased risk of death, compared to the TT genotype of rs2276724 in TP53-positive patients (adjusted P=0.037; adjusted HR=0.621; 95% CI=0.396–0.973). Furthermore, low expression of ALDH1L1 predicted a poor prognosis for the HBV-related HCC patients (adjusted P=0.04 for disease-free survival; adjusted P=0.001 for overall survival). Patients with high ALDH1L1 expression and low TP53 expression were significantly associated with a decreased risk of recurrence and death, and patients with a high TP53 expression were also significantly associated with a decreased risk of death in HBV-related HCC, compared with low ALDH1L1 and low TP53 expression. Our results suggest that ALDH1L1 may be a biomarker for predicting postoperative clinical outcomes. Moreover, ALDH1L1-rs2276724 and mRNA expression were associated with TP53 expression in HBV-related HCC patients.

A Comprehensive Analysis of Cell Type-Specific Nuclear RNA From Neurons and Glia of the Brain

BACKGROUND

Studies in psychiatric genetics have identified >100 loci associated with disease risk, yet many of these loci are distant from protein coding genes. Recent characterization of the transcriptional landscape of cell lines and whole tissues has suggested widespread transcription in both coding and noncoding regions of the genome, including differential expression from loci that produce regulatory noncoding RNAs that function within the nucleus; however, the nuclear transcriptome of specific cell types in the brain has not been previously investigated.

METHODS

We defined the nuclear transcriptional landscape of the three major cellular divisions of the nervous system using flow sorting of genetically labeled nuclei from bacTRAP mouse lines. Next, we characterized the unique expression of coding, noncoding, and intergenic RNAs in the mature mouse brain with RNA-Seq and validation with independent methods.

RESULTS

We found diverse expression across the cell types of all classes of RNAs, including long noncoding RNAs, several of which were confirmed as highly enriched in the nuclei of specific cell types using anatomic methods. We also discovered several examples of cell type-specific expression of tandem gene fusions, and we report the first cell type-specific expression of circular RNAs-a neuron-specific and nuclear-enriched RNA arising from the gene Hnrnpu.

CONCLUSIONS

These data provide an important resource for studies evaluating the function of various noncoding RNAs in the brain, including noncoding RNAs that may play a role in psychiatric disease.

Inducible targeting of CNS astrocytes in Aldh1l1-CreERT2 BAC transgenic mice

Background: Studying astrocytes in higher brain functions has been hampered by the lack of genetic tools for the efficient expression of inducible Cre recombinase throughout the CNS, including the neocortex. Methods: Therefore, we generated BAC transgenic mice, in which CreERT2 is expressed under control of the Aldh1l1 regulatory region. Results: When crossbred to Cre reporter mice, adult Aldh1l1-CreERT2 mice show efficient gene targeting in astrocytes. No such Cre-mediated recombination was detectable in CNS neurons, oligodendrocytes, and microglia. As expected, Aldh1l1-CreERT2 expression was evident in several peripheral organs, including liver and kidney. Conclusions: Taken together, Aldh1l1-CreERT2 mice are a useful tool for studying astrocytes in neurovascular coupling, brain metabolism, synaptic plasticity and other aspects of neuron-glia interactions.

New Transgenic Mouse Lines for Selectively Targeting Astrocytes and Studying Calcium Signals in Astrocyte Processes In Situ and In Vivo

Astrocytes exist throughout the nervous system and are proposed to affect neural circuits and behavior. However, studying astrocytes has proven difficult because of the lack of tools permitting astrocyte-selective genetic manipulations. Here, we report the generation of Aldh1l1-Cre/ERT2 transgenic mice to selectively target astrocytes in vivo. We characterized Aldh1l1-Cre/ERT2 mice using imaging, immunohistochemistry, AAV-FLEX-GFP microinjections, and crosses to RiboTag, Ai95, and new Cre-dependent membrane-tethered Lck-GCaMP6f knockin mice that we also generated. Two to three weeks after tamoxifen induction, Aldh1l1-Cre/ERT2 selectively targeted essentially all adult (P80) brain astrocytes with no detectable neuronal contamination, resulting in expression of cytosolic and Lck-GCaMP6f, and permitting subcellular astrocyte calcium imaging during startle responses in vivo. Crosses with RiboTag mice allowed sequencing of actively translated mRNAs and determination of the adult cortical astrocyte transcriptome. Thus, we provide well-characterized, easy-to-use resources with which to selectively study astrocytes in situ and in vivo in multiple experimental scenarios.

A murine toolbox for imaging the neurovascular unit

The neurovascular unit (NVU) coordinates many essential functions in the brain including blood flow control, nutrient delivery, and maintenance of BBB integrity. These functions are the result of a cellular and molecular interplay that we are just beginning to understand. Cells of the NVU can now be investigated in the intact brain through the combined use of high-resolution in vivo imaging and non-invasive molecular tools to observe and manipulate cell function. Mouse lines that target transgene expression to cells of the NVU will be of great value in future work. However, a detailed evaluation of target cell specificity and expression pattern within the brain is required for many existing lines. The purpose of this review was to catalog mouse lines available to cerebrovascular biologists and to discuss their utility and limitations in future imaging studies.

Genetic control of astrocyte function in neural circuits

During the last two decades numerous genetic approaches affecting cell function in vivo have been developed. Current state-of-the-art technology permits the selective switching of gene function in distinct cell populations within the complex organization of a given tissue parenchyma. The tamoxifen-inducible Cre/loxP gene recombination and the doxycycline-dependent modulation of gene expression are probably the most popular genetic paradigms. Here, we will review applications of these two strategies while focusing on the interactions of astrocytes and neurons in the central nervous system (CNS) and their impact for the whole organism. Abolishing glial sensing of neuronal activity by selective deletion of glial transmitter receptors demonstrated the impact of astrocytes for higher cognitive functions such as learning and memory, or the more basic body control of muscle coordination. Interestingly, also interfering with glial output, i.e., the release of gliotransmitters can drastically change animal’s physiology like sleeping behavior. Furthermore, such genetic approaches have also been used to restore astrocyte function. In these studies two alternatives were employed to achieve proper genetic targeting of astrocytes: transgenes using the promoter of the human glial fibrillary acidic protein (GFAP) or homologous recombination into the glutamate-aspartate transporter (GLAST) locus. We will highlight their specific properties that could be relevant for their use.

Astrocyte development: A Guide for the Perplexed

Astrocytes are the predominant cell type in the brain and perform key functions vital to CNS physiology, including blood brain barrier formation and maintenance, synaptogenesis, neurotransmission, and metabolic regulation. To fully understand the contributions of astrocytes to brain function, it will be important to bridge the existing gap between development and physiology. In this review, we provide an overview of Astrocyte development, including recent insights into molecular mechanisms of astrocyte specification, regional patterning and proliferation. This developmental perspective is complemented with recent findings that describe the functional maturation of astrocytes and their prospective diversity. Future progress in understanding Astrocyte development will depend on the development of astrocyte- stage specific markers and tools for manipulating astrocytes without affecting neuron production. Ultimately, a mechanistic approach to Astrocyte development will be crucial to developing new treatments for the many neurodevelopmental, neurodegenerative, neuroimmune, and neoplastic diseases involving astrocyte dysfunction.

Acetaldehyde and retinaldehyde-metabolizing enzymes in colon and pancreatic cancers

Colorectal cancer (CRC) and pancreatic cancer are two very significant contributors to cancer-related deaths. Chronic alcohol consumption is an important risk factor for these cancers. Ethanol is oxidized primarily by alcohol dehydrogenases to acetaldehyde, an agent capable of initiating tumors by forming adducts with proteins and DNA. Acetaldehyde is metabolized by ALDH2, ALDH1B1, and ALDH1A1 to acetate. Retinoic acid (RA) is required for cellular differentiation and is known to arrest tumor development. RA is synthesized from retinaldehyde by the retinaldehyde dehydrogenases, specifically ALDH1A1, ALDH1A2, ALDH1A3, and ALDH8A1. By eliminating acetaldehyde and generating RA, ALDHs can play a crucial regulatory role in the initiation and progression of cancers. ALDH1 catalytic activity has been used as a biomarker to identify and isolate normal and cancer stem cells; its presence in a tumor is associated with poor prognosis in colon and pancreatic cancer. In summary, these ALDHs are not only biomarkers for CRC and pancreatic cancer but also play important mechanistic role in cancer initiation, progression, and eventual prognosis.

Deimination level and peptidyl arginine deiminase 2 expression are elevated in astrocytes with increased incubation temperature

Astrocytes respond to environmental cues, including changes in temperatures. Increased deimination, observed in many progressive neurological diseases, is thought to be contributed by astrocytes. We determined the level of deimination and expression of peptidyl arginine deiminase 2 (PAD2) in isolated primary astrocytes in response to changes on either side (31°C and 41°C) of the optimal temperature (37°C). We investigated changes in the astrocytes by using a number of established markers and accounted for cell death with the CellTiter-Blue assay. We found increased expression of glial fibrillary acidic protein, ALDH1L1, and J1-31, resulting from increased incubation temperature and increased expression of TSP1, S100β, and AQP4, resulting from decreased incubation temperature vs. optimal temperature, suggesting activation of different biochemical pathways in astrocytes associated with different incubation temperatures. Mass spectrometric analyses support such trends. The PAD2 level was increased only as a result of increased incubation temperature with a commensurate increased level of deimination. Actin cytoskeleton and iso[4]LGE, a lipid peroxidase modification, also showed an increase with higher incubation temperature. Altogether, these results suggest that temperature, as an environmental cue, activates astrocytes in a different manner on either side of the optimal temperature and that increase in deimination is associated only with the higher temperature side of the spectrum.

Finding degrees of separation: experimental approaches for astroglial and oligodendroglial cell isolation and genetic targeting

The study of CNS glial cell function requires experimental methods to detect, purify, and manipulate each cell population with fidelity and specificity. With the identification and cloning of cell- and stage-specific markers, glial cell analysis techniques have grown beyond physical methods of tissue dissociation and cell culture, and become highly specific with immunoselection of cell cultures in vitro and genetic targeting in vivo. The unique plasticity of glial cells offers the potential for cell replacement therapies in neurological disease that utilize neural cells derived from transplanted neural stem and progenitor cells. In this mini-review, we outline general physical and genetic approaches for macroglial cell generation. We summarize cell culture methods to obtain astrocytes and oligodendrocytes and their precursors, from developing and adult tissue, as well as approaches to obtain human neural progenitor cells through the establishment of stem cells. We discuss popular targeting rodent strains designed for cell-specific detection, selection and manipulation of neuroglial cell progenitors and their committed progeny. Based on shared markers between astrocytes and stem cells, we discuss genetically modified mouse strains with overlapping expression, and highlight SOX-expressing strains available for targeting of stem and progenitor cell populations. We also include recently established mouse strains for detection, and tag-assisted RNA and miRNA analysis. This discussion aims to provide a brief overview of the rapidly expanding collection of experimental approaches and genetic resources for the isolation and targeting of macroglial cells, their sources, progeny and gene products to facilitate our understanding of their properties and potential application in pathology.

The complex morphology of reactive astrocytes controlled by fibroblast growth factor signaling

Astrocytes are the most abundant cell-type of the human brain and play a variety of roles in brain homeostasis and synaptic maturation, under normal conditions. However, astrocytes undergo dramatic pathological changes in response to brain injury, such as reactive gliosis and glial scar formation. Although abnormal hypertrophy and massive proliferation of astrocytes are obvious, the molecular identity and cues that dictate the structural changes in reactive astrocytes remain unclear. This study proposes that fibroblast growth factor (FGF) signaling is responsible for making astrocyte morphology more complex and hypertrophic in response to an inflammatory stimulus such as lipopolysaccharide. Primary astrocytes isolated from perinatal brains developed more branches in the presence of FGF8 or lesser branches in the presence of FGF2. Introduction of the constitutively active form of the FGF receptor 3 (caFGFR3) into the brain increases the structural complexity, with greater glial fibrillary acidic protein level in astrocytes, while overexpression of a dominant-negative form of FGFR3 (dnFGFR3) reduces it. Treatment of FGF8 facilitated the wound-healing process of primary astrocytes in vitro by changing their morphology, indicating that the FGF signal may control the responsiveness of astrocytes in injury conditions. Finally, the blockade of FGF signaling by introducing dnFGFR3 at the site of reactive gliosis reduces astrocyte branch formation and minimizes hypertrophic responses during reactive gliosis. Taken together, these results indicate that FGF8-FGFR3 signaling controls structural changes in astrocytes during reactive gliosis, under pathogenic conditions.