High levels of Cre expression in neuronal progenitors cause defects in brain development leading to microencephaly and hydrocephaly

NAD depletion in skeletal muscle does not compromise muscle function or accelerate aging

Nicotinamide adenine dinucleotide (NAD) is a ubiquitous electron carrier essential for energy metabolism and post-translational modification of numerous regulatory proteins. Dysregulations of NAD metabolism are widely regarded as detrimental to health, with NAD depletion commonly implicated in aging. However, the extent to which cellular NAD concentration can decline without adverse consequences remains unclear. To investigate this, we generated a mouse model in which nicotinamide phosphoribosyltransferase (NAMPT)-mediated NAD+ biosynthesis was disrupted in adult skeletal muscle. The intervention resulted in an 85% reduction in muscle NAD+ abundance while maintaining tissue integrity and functionality, as demonstrated by preserved muscle morphology, contractility, and exercise tolerance. This absence of functional impairments was further supported by intact mitochondrial respiratory capacity and unaltered muscle transcriptomic and proteomic profiles. Furthermore, lifelong NAD depletion did not accelerate muscle aging or impair whole-body metabolism. Collectively, these findings suggest that NAD depletion does not contribute to age-related decline in skeletal muscle function.

A Study on Potential Sources of Perineuronal Net-Associated Sema3A in Cerebellar Nuclei Reveals Toxicity of Non-Invasive AAV-Mediated Cre Expression in the Central Nervous System

Semaphorin 3A (Sema3A) is an axon guidance molecule, which is also abundant in the adult central nervous system (CNS), particularly in perineuronal nets (PNNs). PNNs are extracellular matrix structures that restrict plasticity. The cellular sources of Sema3A in PNNs are unknown. Most Sema3A-bearing neurons do not express Sema3A mRNA, suggesting that Sema3A may be released from other neurons. Another potential source of Sema3A is the choroid plexus. To identify sources of PNN-associated Sema3A, we focused on the cerebellar nuclei, which contain Sema3A+ PNNs. Cerebellar nuclei neurons receive prominent input from Purkinje cells (PCs), which express high levels of Sema3A mRNA. By using a non-invasive viral vector approach, we overexpressed Cre in PCs, the choroid plexus, or throughout the CNS of Sema3Afl/fl mice. Knocking out Sema3A in PCs or the choroid plexus was not sufficient to decrease the amount of PNN-associated Sema3A. Alternatively, knocking out Sema3A throughout the CNS induced a decrease in PNN-associated Sema3A. However, motor deficits, microgliosis, and neurodegeneration were observed, which were due to Cre toxicity. Our study represents the first attempt to unravel cellular sources of PNN-associated Sema3A and shows that non-invasive viral-mediated Cre expression throughout the CNS could lead to toxicity, complicating the interpretation of Cre-mediated Sema3A knock-out.

Detailed phenotyping of Tbr1-2A-CreER knock-in mice demonstrates significant impacts on TBR1 protein levels and axon development

Cre recombinase knock-in mouse lines have served as invaluable genetic tools for understanding key developmental processes altered in autism. However, insertion of exogenous DNA into the genome can have unintended effects on local gene regulation or protein function that must be carefully considered. Here, we analyze a recently generated Tbr1-2A-CreER knock-in mouse line, where a 2A-CreER cassette was inserted in-frame before the stop codon of the transcription factor gene Tbr1. Heterozygous TBR1 mutations in humans and mice are known to cause autism or autism-like behavioral phenotypes accompanied by structural brain malformations, most frequently a reduction of the anterior commissure (AC). Thus, it is critical for modified versions of Tbr1 to exhibit true wild-type-like activity. We evaluated the Tbr1-2A-CreER allele for its potential impact on Tbr1 function and complementation to Tbr1 loss-of-function alleles. In mice with one copy of the Tbr1-2A-CreER allele, we identified reduction of TBR1 protein in early postnatal cortex along with thinning of the AC, suggesting hypersensitivity of this structure to TBR1 dosage. Comparing Tbr1-2A-CreER and Tbr1-null mice to Tbr1-null complementation crosses showed reductions of TBR1 dosage ranging from 20% to 100%. Using six combinatorial genotypes, we found that moderate to severe TBR1 reductions (≥44%) were associated with cortical layer 5 expansion, while only the complete absence of TBR1 was associated with reeler-like "inverted" cortical layering. In total, these results strongly support the conclusion that Tbr1-2A-CreER is a hypomorphic allele. We advise caution when interpreting experiments using this allele, considering the sensitivity of various corticogenic processes to TBR1 dosage and the association of heterozygous TBR1 mutations with complex neurodevelopmental disorders.

Overexpression of the schizophrenia risk gene C4 in PV cells drives sex-dependent behavioral deficits and circuit dysfunction

Fast-spiking parvalbumin (PV)-positive cells are key players in orchestrating pyramidal neuron activity, and their dysfunction is consistently observed in myriad brain diseases. To understand how immune complement pathway dysregulation in PV cells drives disease pathogenesis, we have developed a transgenic line that permits cell-type specific overexpression of the schizophrenia-associated C4 gene. We found that overexpression of mouse C4 (mC4) in PV cells causes sex-specific alterations in anxiety-like behavior and deficits in synaptic connectivity and excitability of PFC PV cells. Using a computational model, we demonstrated that these microcircuit deficits led to hyperactivity and disrupted neural communication. Finally, pan-neuronal overexpression of mC4 failed to evoke the same deficits in behavior as PV-specific mC4 overexpression, suggesting that perturbations of this neuroimmune gene in fast-spiking neurons are especially detrimental to circuits associated with anxiety-like behavior. Together, these results provide a causative link between C4 and the vulnerability of PV cells in brain disease.

Overexpression of the schizophrenia risk gene C4 in PV cells drives sex-dependent behavioral deficits and circuit dysfunction

Fast-spiking parvalbumin (PV)-positive cells are key players in orchestrating pyramidal neuron activity, and their dysfunction is consistently observed in myriad brain diseases. To understand how immune complement dysregulation - a prevalent locus of brain disease etiology - in PV cells may drive disease pathogenesis, we have developed a transgenic mouse line that permits cell-type specific overexpression of the schizophrenia-associated complement component 4 (C4) gene. We found that overexpression of mouse C4 (mC4) in PV cells causes sex-specific behavioral alterations and concomitant deficits in synaptic connectivity and excitability of PV cells of the prefrontal cortex. Using a computational network, we demonstrated that these microcircuit deficits led to hyperactivity and disrupted neural communication. Finally, pan-neuronal overexpression of mC4 failed to evoke the same deficits in behavior as PV-specific mC4 overexpression, suggesting that C4 perturbations in fast-spiking neurons are more harmful to brain function than pan-neuronal alterations. Together, these results provide a causative link between C4 and the vulnerability of PV cells in brain disease.

In vivo tracing of the Cytokeratin 14 lineages using self-cleaving guide RNAs and CRISPR/Cas9

The current gold-standard for genetic lineage tracing in transgenic mice is based on cell-type specific expression of Cre recombinase. As an alternative, we developed a cell-type specific CRISPR/spCas9 system for lineage tracing. This method relies on RNA polymerase II promoter driven self-cleaving guide RNAs (scgRNA) to achieve tissue-specificity. To demonstrate proof-of-principle for this approach a transgenic mouse was generated harbouring a knock-in of a scgRNA into the Cytokeratin 14 (Krt14) locus. Krt14 expression marks the stem cells of squamous epithelium in the skin and oral mucosa. The scgRNA targets a Stop cassette preceding a fluorescent reporter in the Ai9-tdtomato mouse. Ai9-tdtomato reporter mice harbouring this allele along with a spCas9 transgene demonstrated precise marking of the Krt14 lineage. We conclude that RNA polymerase II promoter driven scgRNAs enable the use of CRISPR/spCas9 for genetic lineage tracing.

Tissue-specific Cre driver mice to study vascular diseases

Vascular diseases, including atherosclerosis and abdominal aneurysms, are the primary cause of mortality and morbidity among the elderly worldwide. The life quality of patients is significantly compromised due to inadequate therapeutic approaches and limited drug targets. To expand our comprehension of vascular diseases, gene knockout (KO) mice, especially conditional knockout (cKO) mice, are widely used for investigating gene function and mechanisms of action. The Cre-loxP system is the most common method for generating cKO mice. Numerous Cre driver mice have been established to study the main cell types that compose blood vessels, including endothelial cells, smooth muscle cells, and fibroblasts. Here, we first discuss the characteristics of each layer of the arterial wall. Next, we provide an overview of the representative Cre driver mice utilized for each of the major cell types in the vessel wall and their most recent applications in vascular biology. We then go over Cre toxicity and discuss the practical methods for minimizing Cre interference in experimental outcomes. Finally, we look into the future of tissue-specific Cre drivers by introducing the revolutionary single-cell RNA sequencing and dual recombinase system.

PTEN knockout using retrogradely transported AAVs transiently restores locomotor abilities in both acute and chronic spinal cord injury

Restoring function in chronic stages of spinal cord injury (SCI) has often been met with failure or reduced efficacy when regenerative strategies are delayed past the acute or sub-acute stages of injury. Restoring function in the chronically injured spinal cord remains a critical challenge. We found that a single injection of retrogradely transported adeno-associated viruses (AAVrg) to knockout the phosphatase and tensin homolog protein (PTEN) in chronic SCI can effectively target both damaged and spared axons and transiently restore locomotor functions in near-complete injury models. AAVrg's were injected to deliver cre recombinase and/or a red fluorescent protein (RFP) under the human Synapsin 1 promoter (hSyn1) into the spinal cords of C57BL/6 PTENFloxΔ/Δ mice to knockout PTEN (PTEN-KO) in a severe thoracic SCI crush model at both acute and chronic time points. PTEN-KO improved locomotor abilities in both acute and chronic SCI conditions over a 9-week period. Regardless of whether treatment was initiated at the time of injury (acute), or three months after SCI (chronic), mice with limited hindlimb joint movement gained hindlimb weight support after treatment. Interestingly, functional improvements were not sustained beyond 9 weeks coincident with a loss of RFP reporter-gene expression and a near-complete loss of treatment-associated functional recovery by 6 months post-treatment. Treatment effects were also specific to severely injured mice; animals with weight support at the time of treatment lost function over a 6-month period. Retrograde tracing with Fluorogold revealed viable neurons throughout the motor cortex despite a loss of RFP expression at 9 weeks post-PTEN-KO. However, few Fluorogold labeled neurons were detected within the motor cortex at 6 months post-treatment. BDA labeling from the motor cortex revealed a dense corticospinal tract (CST) bundle in all groups except chronically treated PTEN-KO mice, indicating a potential long-term toxic effect of PTEN-KO to neurons in the motor cortex which was corroborated by a loss of β-tubulin III labeling above the lesion within spinal cords after PTEN-KO. PTEN-KO mice had significantly more β-tubulin III labeled axons within the lesion when treatment was delivered acutely, but not chronically post-SCI. In conclusion, we have found that using AAVrg's to knockout PTEN is an effective manipulation capable of restoring motor functions in chronic SCI and can enhance axon growth of currently unidentified axon populations when delivered acutely after injury. However, the long-term consequences of PTEN-KO on neuronal health and viability should be further explored.

Tomosyn affects dense core vesicle composition but not exocytosis in mammalian neurons

Tomosyn is a large, non-canonical SNARE protein proposed to act as an inhibitor of SNARE complex formation in the exocytosis of secretory vesicles. In the brain, tomosyn inhibits the fusion of synaptic vesicles (SVs), whereas its role in the fusion of neuropeptide-containing dense core vesicles (DCVs) is unknown. Here, we addressed this question using a new mouse model with a conditional deletion of tomosyn (Stxbp5) and its paralogue tomosyn-2 (Stxbp5l). We monitored DCV exocytosis at single vesicle resolution in tomosyn-deficient primary neurons using a validated pHluorin-based assay. Surprisingly, loss of tomosyns did not affect the number of DCV fusion events but resulted in a strong reduction of intracellular levels of DCV cargos, such as neuropeptide Y (NPY) and brain-derived neurotrophic factor (BDNF). BDNF levels were largely restored by re-expression of tomosyn but not by inhibition of lysosomal proteolysis. Tomosyn's SNARE domain was dispensable for the rescue. The size of the trans-Golgi network and DCVs was decreased, and the speed of DCV cargo flux through Golgi was increased in tomosyn-deficient neurons, suggesting a role for tomosyns in DCV biogenesis. Additionally, tomosyn-deficient neurons showed impaired mRNA expression of some DCV cargos, which was not restored by re-expression of tomosyn and was also observed in Cre-expressing wild-type neurons not carrying loxP sites, suggesting a direct effect of Cre recombinase on neuronal transcription. Taken together, our findings argue against an inhibitory role of tomosyns in neuronal DCV exocytosis and suggests an evolutionary conserved function of tomosyns in the packaging of secretory cargo at the Golgi.

Sex-specific effects of Cre expression in Syn1Cre mice

The Cre-loxP system has been used to generate cell-type specific mutations in mice, allowing researchers to investigate the underlying biological mechanisms of disease. However, the Cre-recombinase alone can induce phenotypes that confound comparisons among genotypes if the appropriate Cre control is not included. In this study, we characterised behavioural, morphological and metabolic phenotypes of the pan-neuronal Syn1Cre line. We found that these mice possess intact neuromuscular parameters but have reduced exploratory activity and a male-specific increase in anxiety-like behaviour. Moreover, we observed a male-specific deficit in learning and long-term memory of Syn1Cre mice that could be a result of decreased visual acuity. Furthermore, we found that over-expression of human growth hormone (hGH) from Syn1Cre results in a male-specific reduction in body weight and femur length, potentially through decreased hepatic Igf1 expression. However, metabolic characteristics of Syn1Cre mice such as glucose metabolism, energy expenditure and feeding were unaffected by the presence of Syn1Cre. In conclusion, our data demonstrate that Syn1Cre expression has effects on behavioural and morphological traits. This finding highlights the importance of including the Cre control in all comparisons, while the male-specific effects on some phenotypes highlight the importance of including both sexes.

Perspective: The current state of Cre driver mouse lines in skeletal research: Challenges and opportunities

The Cre/Lox system has revolutionized the ability of biomedical researchers to ask very specific questions about the function of individual genes in specific cell types at specific times during development and/or disease progression in a variety of animal models. This is true in the skeletal biology field, and numerous Cre driver lines have been created to foster conditional gene manipulation in specific subpopulations of bone cells. However, as our ability to scrutinize these models increases, an increasing number of issues have been identified with most driver lines. All existing skeletal Cre mouse models exhibit problems in one or more of the following three areas: (1) cell type specificity-avoiding Cre expression in unintended cell types; (2) Cre inducibility-improving the dynamic range for Cre in inducible models (negligible Cre activity before induction and high Cre activity after induction); and (3) Cre toxicity-reducing the unwanted biological effects of Cre (beyond loxP recombination) on cellular processes and tissue health. These issues are hampering progress in understanding the biology of skeletal disease and aging, and consequently, identification of reliable therapeutic opportunities. Skeletal Cre models have not advanced technologically in decades despite the availability of improved tools, including multi-promoter-driven expression of permissive or fragmented recombinases, new dimerization systems, and alternative forms of recombinases and DNA sequence targets. We review the current state of skeletal Cre driver lines, and highlight some of the successes, failures, and opportunities to improve fidelity in the skeleton, based on successes pioneered in other areas of biomedical science.

Deletion of histone demethylase Lsd1 (Kdm1a) during retinal development leads to defects in retinal function and structure

Purpose

The purpose of this study was to investigate the role of Lysine specific demethylase 1 (Lsd1) in murine retinal development. LSD1 is a histone demethylase that can demethylate mono- and di-methyl groups on H3K4 and H3K9. Using Chx10-Cre and Rho-iCre75 driver lines, we generated novel transgenic mouse lines to delete Lsd1 in most retinal progenitor cells or specifically in rod photoreceptors. We hypothesize that Lsd1 deletion will cause global morphological and functional defects due to its importance in neuronal development.

Methods

We tested the retinal function of young adult mice by electroretinogram (ERG) and assessed retinal morphology by in vivo imaging by fundus photography and SD-OCT. Afterward, eyes were enucleated, fixed, and sectioned for subsequent hematoxylin and eosin (H&E) or immunofluorescence staining. Other eyes were plastic fixed and sectioned for electron microscopy.

Results

In adult Chx10-Cre Lsd1fl/fl mice, we observed a marked reduction in a-, b-, and c-wave amplitudes in scotopic conditions compared to age-matched control mice. Photopic and flicker ERG waveforms were even more sharply reduced. Modest reductions in total retinal thickness and outer nuclear layer (ONL) thickness were observed in SD-OCT and H&E images. Lastly, electron microscopy revealed significantly shorter inner and outer segments and immunofluorescence showed modest reductions in specific cell type populations. We did not observe any obvious functional or morphological defects in the adult Rho-iCre75 Lsd1fl/fl animals.

Conclusion

Lsd1 is necessary for neuronal development in the retina. Adult Chx10-Cre Lsd1fl/fl mice show impaired retinal function and morphology. These effects were fully manifested in young adults (P30), suggesting that Lsd1 affects early retinal development in mice.

Cardiac-Specific Expression of Cre Recombinase Leads to Age-Related Cardiac Dysfunction Associated with Tumor-like Growth of Atrial Cardiomyocyte and Ventricular Fibrosis and Ferroptosis

Transgenic expression of Cre recombinase driven by a specific promoter is normally used to conditionally knockout a gene in a tissue- or cell-type-specific manner. In αMHC-Cre transgenic mouse model, expression of Cre recombinase is controlled by the myocardial-specific α-myosin heavy chain (αMHC) promoter, which is commonly used to edit myocardial-specific genes. Toxic effects of Cre expression have been reported, including intro-chromosome rearrangements, micronuclei formation and other forms of DNA damage, and cardiomyopathy was observed in cardiac-specific Cre transgenic mice. However, mechanisms associated with Cardiotoxicity of Cre remain poorly understood. In our study, our data unveiled that αMHC-Cre mice developed arrhythmias and died after six months progressively, and none of them survived more than one year. Histopathological examination showed that αMHC-Cre mice had aberrant proliferation of tumor-like tissue in the atrial chamber extended from and vacuolation of ventricular myocytes. Furthermore, the αMHC-Cre mice developed severe cardiac interstitial and perivascular fibrosis, accompanied by significant increase of expression levels of MMP-2 and MMP-9 in the cardiac atrium and ventricular. Moreover, cardiac-specific expression of Cre led to disintegration of the intercalated disc, along with altered proteins expression of the disc and calcium-handling abnormality. Comprehensively, we identified that the ferroptosis signaling pathway is involved in heart failure caused by cardiac-specific expression of Cre, on which oxidative stress results in cytoplasmic vacuole accumulation of lipid peroxidation on the myocardial cell membrane. Taken together, these results revealed that cardiac-specific expression of Cre recombinase can lead to atrial mesenchymal tumor-like growth in the mice, which causes cardiac dysfunction, including cardiac fibrosis, reduction of the intercalated disc and cardiomyocytes ferroptosis at the age older than six months in mice. Our study suggests that αMHC-Cre mouse models are effective in young mice, but not in old mice. Researchers need to be particularly careful when using αMHC-Cre mouse model to interpret those phenotypic impacts of gene responses. As the Cre-associated cardiac pathology matched mostly to that of the patients, the model could also be employed for investigating age-related cardiac dysfunction.

Microglial motility is modulated by neuronal activity and correlates with dendritic spine plasticity in the hippocampus of awake mice

Microglia, the resident immune cells of the brain, play a complex role in health and disease. They actively survey the brain parenchyma by physically interacting with other cells and structurally shaping the brain. Yet, the mechanisms underlying microglial motility and significance for synapse stability, especially in the hippocampus during adulthood, remain widely unresolved. Here, we investigated the effect of neuronal activity on microglial motility and the implications for the formation and survival of dendritic spines on hippocampal CA1 neurons in vivo. We used repetitive two-photon in vivo imaging in the hippocampus of awake and anesthetized mice to simultaneously study the motility of microglia and their interaction with dendritic spines. We found that CA3 to CA1 input is sufficient to modulate microglial process motility. Simultaneously, more dendritic spines emerged in mice after awake compared to anesthetized imaging. Interestingly, the rate of microglial contacts with individual dendritic spines and dendrites was associated with the stability, removal, and emergence of dendritic spines. These results suggest that microglia might sense neuronal activity via neurotransmitter release and actively participate in synaptic rewiring of the hippocampal neural network during adulthood. Further, this study has profound relevance for hippocampal learning and memory processes.

Aberrant glial activation and synaptic defects in CaMKIIα-iCre and nestin-Cre transgenic mouse models

Current scientific research is driven by the ability to manipulate gene expression by utilizing the Cre/loxP system in transgenic mouse models. However, artifacts in Cre-driver mouse lines that introduce undesired effects and confound results are increasingly being reported. Here, we show aberrant neuroinflammation and synaptic changes in two widely used Cre-driver mouse models. Neuroinflammation in CaMKIIα-iCre mice was characterized by the activation and proliferation of microglia and astrocytes in synaptic layers of the hippocampus. Increased GFAP and Iba1 levels were observed in hippocampal brain regions of 4-, 8- and 22-month-old CaMKIIα-iCre mice compared to WT littermates. Synaptic changes in NMDAR, AMPAR, PSD95 and phosphorylated CaMKIIα became apparent in 8-month-old CaMKIIα-iCre mice but were not observed in 4-month-old CaMKIIα-iCre mice. Synaptophysin and synaptoporin were unchanged in CaMKIIα-iCre compared to WT mice, suggesting that synaptic alterations may occur in excitatory postsynaptic regions in which iCre is predominantly expressed. Finally, hippocampal volume was reduced in 22-month-old CaMKIIα-iCre mice compared to WT mice. We tested the brains of mice of additional common Cre-driver mouse models for neuroinflammation; the nestin-Cre mouse model showed synaptic changes and astrocytosis marked by increased GFAP+ astrocytes in cortical and hippocampal regions, while the original CaMKIIα-Cre T29-1 strain was comparable to WT mice. The mechanisms underlying abnormal neuroinflammation in nestin-Cre and CaMKIIα-iCre are unknown but may be associated with high levels of Cre expression. Our findings are critical to the scientific community and demonstrate that the correct Cre-driver controls must be included in all studies using these mice.

Cre toxicity in mouse models of cardiovascular physiology and disease

The Cre-LoxP system provides a widely used method for studying gene requirements in the mouse as the main mammalian genetic model organism. To define the molecular and cellular mechanisms that underlie cardiovascular development, function and disease, various mouse strains have been engineered that allow Cre-LoxP-mediated gene targeting within specific cell types of the cardiovascular system. Despite the usefulness of this system, evidence is accumulating that Cre activity can have toxic effects in cells, independently of its ability to recombine pairs of engineered LoxP sites in target genes. Here, we have gathered published evidence for Cre toxicity in cells and tissues relevant to cardiovascular biology and provide an overview of mechanisms proposed to underlie Cre toxicity. Based on this knowledge, we propose that each study utilising the Cre-LoxP system to investigate gene function in the cardiovascular system should incorporate appropriate controls to account for Cre toxicity.

Modulation of in vitro epileptiform activity by optogenetic stimulation of parvalbumin-positive interneurons

GABA_A signaling is surprisingly involved in the initiation of epileptiform activity since increased interneuron firing, presumably leading to excessive GABA release, often precedes ictal discharges. Field potential theta (4-12 Hz) oscillations, which are thought to mirror the synchronization of interneuron networks, also lead to ictogenesis. However, the exact role of parvalbumin-positive (PV) interneurons in generating theta oscillations linked to epileptiform discharges remains unexplored. We analyzed here the field responses recorded in the CA3, entorhinal cortex (EC) and dentate gyrus (DG) during 8 Hz optogenetic stimulation of PV-positive interneurons in brain slices obtained from PV-ChR2 mice during 4-aminopyridine (4AP) application. This optogenetic protocol triggered similar field oscillations in both control conditions and during 4AP application. However, in the presence of 4AP, optogenetic stimuli also induced: (i) interictal discharges that were associated in all regions with 8 Hz field oscillations; and (ii) low-voltage fast onset ictal discharges. Interictal and ictal events occurred more frequently during optogenetic activation than during periods of no stimulation. 4AP also increased synchronicity during PV-interneuron activation in all three regions. In opsin-negative mice, optogenetic stimulation did not change the rate of both types of epileptiform activity. Our findings suggest that PV-interneuron recruitment at theta (8 Hz) frequency contributes to epileptiform synchronization in limbic structures in the in vitro 4AP model.

Toxic and Phenotypic Effects of AAV_Cre Used to Transduce Mesencephalic Dopaminergic Neurons

A popular approach to spatiotemporally target genes using the loxP/Cre recombination system is stereotaxic microinjection of adeno-associated virus (AAV) expressing Cre recombinase (AAV_Cre) in specific neuronal structures. Here, we report that AAV_Cre microinjection in the ventral tegmental area (VTA) of ErbB4 Cyt-1-floxed (ErbB4 Cyt-1fl/fl) mice at titers commonly used in the literature (~1012-1013 GC/mL) can have neurotoxic effects on dopaminergic neurons and elicit behavioral abnormalities. However, these effects of AAV_Cre microinjection are independent of ErbB4 Cyt-1 recombination because they are also observed in microinjected wild-type (WT) controls. Mice microinjected with AAV_Cre (1012-1013 GC/mL) exhibit reductions of tyrosine hydroxylase (TH) and dopamine transporter (DAT) expression, loss of dopaminergic neurons, and they behaviorally become hyperactive, fail to habituate in the open field and exhibit sensorimotor gating deficits compared to controls microinjected with AAV_GFP. Importantly, these AAV_Cre non-specific effects are: (1) independent of serotype, (2) occur with vectors expressing either Cre or Cre-GFP fusion protein and (3) preventable by reducing viral titers by 1000-fold (1010 GC/mL), which retains sufficient recombination activity to target floxed genes. Our studies emphasize the importance of including AAV_Cre-injected WT controls in experiments because recombination-independent effects on gene expression, neurotoxicity and behaviors could be erroneously attributed to consequences of gene ablation.

Duplex Labeling and Manipulation of Neuronal Proteins Using Sequential CRISPR/Cas9 Gene Editing

CRISPR/Cas9-mediated knock-in methods enable the labeling of individual endogenous proteins to faithfully determine their spatiotemporal distribution in cells. However, reliable multiplexing of knock-in events in neurons remains challenging because of cross talk between editing events. To overcome this, we developed conditional activation of knock-in expression (CAKE), allowing efficient, flexible, and accurate multiplex genome editing in rat neurons. To diminish cross talk, CAKE is based on sequential, recombinase-driven guide RNA (gRNA) expression to control the timing of genomic integration of each donor sequence. We show that CAKE is broadly applicable to co-label various endogenous proteins, including cytoskeletal proteins, synaptic scaffolds, ion channels and neurotransmitter receptor subunits. To take full advantage of CAKE, we resolved the nanoscale co-distribution of endogenous synaptic proteins using super-resolution microscopy, demonstrating that their co-organization depends on synapse size. Finally, we introduced inducible dimerization modules, providing acute control over synaptic receptor dynamics in living neurons. These experiments highlight the potential of CAKE to reveal new biological insight. Altogether, CAKE is a versatile method for multiplex protein labeling that enables the detection, localization, and manipulation of endogenous proteins in neurons.Significance StatementAccurate localization and manipulation of endogenous proteins is essential to unravel neuronal function. While labeling of individual proteins is achievable with existing gene editing techniques, methods to label multiple proteins in neurons are limiting. We introduce a new CRISPR/Cas9 strategy, CAKE, achieving faithful duplex protein labeling using sequential editing of genes. We use CAKE to visualize the co-localization of essential neuronal proteins, including cytoskeleton components, ion channels and synaptic scaffolds. Using super-resolution microscopy, we demonstrate that the co-organization of synaptic scaffolds and neurotransmitter receptors scales with synapse size. Finally, we acutely modulate the dynamics of synaptic receptors using labeling with inducible dimerization domains. Thus, CAKE mediates accurate duplex endogenous protein labeling and manipulation to address biological questions in neurons.

Efficient spatially targeted gene editing using a near-infrared activatable protein-conjugated nanoparticle for brain applications

Spatial control of gene expression is critical to modulate cellular functions and deconstruct the function of individual genes in biological processes. Light-responsive gene-editing formulations have been recently developed; however, they have shown limited applicability in vivo due to poor tissue penetration, limited cellular transfection and the difficulty in evaluating the activity of the edited cells. Here, we report a formulation composed of upconversion nanoparticles conjugated with Cre recombinase enzyme through a photocleavable linker, and a lysosomotropic agent that facilitates endolysosomal escape. This formulation allows in vitro spatial control in gene editing after activation with near-infrared light. We further demonstrate the potential of this formulation in vivo through three different paradigms: (i) gene editing in neurogenic niches, (ii) gene editing in the ventral tegmental area to facilitate monitoring of edited cells by precise optogenetic control of reward and reinforcement, and (iii) gene editing in a localized brain region via a noninvasive administration route (i.e., intranasal).

Spatial control of gene expression allows precise control over biological processes. Here, the authors develop an efficient light-responsive formulation based on upconversion nanoparticles, and demonstrate on-demand genetic manipulation in deep brain tissue.

EZH2-Mediated H3K27me3 Targets Transcriptional Circuits of Neuronal Differentiation

The Polycomb Repressive Complex 2 (PRC2) plays important roles in the epigenetic regulation of cellular development and differentiation through H3K27me3-dependent transcriptional repression. Aberrant PRC2 activity has been associated with cancer and neurodevelopmental disorders, particularly with respect to the malfunction of sits catalytic subunit EZH2. Here, we investigated the role of the EZH2-mediated H3K27me3 apposition in neuronal differentiation. We made use of a transgenic mouse model harboring Ezh2 conditional KO alleles to derive embryonic stem cells and differentiate them into glutamatergic neurons. Time course transcriptomics and epigenomic analyses of H3K27me3 in absence of EZH2 revealed a significant dysregulation of molecular networks affecting the glutamatergic differentiation trajectory that resulted in: (i) the deregulation of transcriptional circuitries related to neuronal differentiation and synaptic plasticity, in particular LTD, as a direct effect of EZH2 loss and (ii) the appearance of a GABAergic gene expression signature during glutamatergic neuron differentiation. These results expand the knowledge about the molecular pathways targeted by Polycomb during glutamatergic neuron differentiation.

Chemogenetics as a neuromodulatory approach to treating neuropsychiatric diseases and disorders

Chemogenetics enables precise, non-invasive, and reversible modulation of neural activity via the activation of engineered receptors that are pharmacologically selective to endogenous or exogenous ligands. With recent advances in therapeutic gene delivery, chemogenetics is poised to support novel interventions against neuropsychiatric diseases and disorders. To evaluate its translational potential, we performed a scoping review of applications of chemogenetics that led to the reversal of molecular and behavioral deficits in studies relevant to neuropsychiatric diseases and disorders. In this review, we present these findings and discuss the potential and challenges for using chemogenetics as a precision medicine-based neuromodulation strategy.

RNA Binding Motif 5 Gene Deletion Modulates Cell Signaling in a Sex-Dependent Manner but not Hippocampal Cell Death

RNA-binding motif 5 (RBM5) is a pro-death tumor suppressor gene in cancer cells. It remains to be determined if it is neurotoxic in the brain or rather if it plays a fundamentally different role in the central nervous system (CNS). Brain-specific RBM5 knockout (KO) mice were given a controlled cortical impact (CCI) traumatic brain injury (TBI). Markers of acute cellular damage and repair were measured in hippocampal homogenates 48 h post-CCI. Hippocampal CA1/CA3 cell counts were assessed 7 days post-CCI to determine if early changes in injury markers were associated with histological outcome. No genotype-dependent differences were found in the levels of apoptotic markers (caspase 3, caspase 6, and caspase 9). However, KO females had a paradoxical increase in markers of pro-death calpain activation (145/150-spectrin and breakdown products [SBDP]) and in DNA repair/survival markers. (pH2A.x and pCREB). CCI-injured male KOs had a significant increase in phosphorylated calcium/calmodulin-dependent protein kinase II (pCaMKII). Despite sex/genotype-dependent differences in KOs in the levels of acute cell signaling targets involved in cell death pathways, 7 day hippocampal neuronal survival did not differ from that of wild types (WTs). Similarly, no differences in astrogliosis were observed. Finally, gene analysis revealed increased estrogen receptor α (ERα) levels in the KO hippocampus in females and may suggest a novel mechanism to explain sex-dimorphic effects on cell signaling. In summary, RBM5 inhibition did not affect hippocampal survival after a TBI in vivo but did modify targets involved in neural signal transduction/Ca²⁺ signaling pathways. Findings here support the view that RBM5 may serve a purpose in the CNS that is dissimilar from its traditional pro-death role in cancer.

Emerging strategies for the genetic dissection of gene functions, cell types, and neural circuits in the mammalian brain

The mammalian brain is composed of a large number of highly diverse cell types with different molecular, anatomical, and functional features. Distinct cellular identities are generated during development under the regulation of intricate genetic programs and manifested through unique combinations of gene expression. Recent advancements in our understanding of the molecular and cellular mechanisms underlying the assembly, function, and pathology of the brain circuitry depend on the invention and application of genetic strategies that engage intrinsic gene regulatory mechanisms. Here we review the strategies for gene regulation on DNA, RNA, and protein levels and their applications in cell type targeting and neural circuit dissection. We highlight newly emerged strategies and emphasize the importance of combinatorial approaches. We also discuss the potential caveats and pitfalls in current methods and suggest future prospects to improve their comprehensiveness and versatility.

Neuroregenerative gene therapy to treat temporal lobe epilepsy in a rat model

Temporal lobe epilepsy (TLE) is a common drug-resistant epilepsy associated with abundant cell death in the hippocampus. Here, we develop a novel gene therapy-mediated cell therapy that regenerates GABAergic neurons using internal hippocampal astrocytes to suppress seizure activity in a rat TLE model. We discovered that TLE-induced reactive astrocytes in the hippocampal CA1 region can be efficiently converted into GABAergic neurons after overexpressing a neural transcription factor NeuroD1. The astrocyte-converted neurons showed typical markers of GABAergic interneurons, fired action potentials, and formed functional synaptic connections with other neurons. Following NeuroD1-mediated astrocyte-to-neuron conversion, the number of hippocampal interneurons was significantly increased, and the spontaneous recurrent seizure (SRS) activity was significantly decreased. Moreover, NeuroD1 gene therapy treatment rescued total neuronal loss in the CA1 region and ameliorated the cognitive and mood dysfunctions in the TLE rat model. These results suggest that regeneration of GABAergic interneurons through gene therapy approach may provide a novel therapeutic intervention to treat drug-resistant TLE.

Progress of genome editing technology and developmental biology useful for radiation research

Following the development of genome editing technology, it has become more feasible to create genetically modified animals such as knockout (KO), knock-in, and point-mutated animals. The genome-edited animals are useful to investigate the roles of various functional genes in many fields of biological science including radiation research. Nevertheless, some researchers may experience difficulty in generating genome-edited animals, probably due to the requirement for equipment and techniques for embryo manipulation and handling. Furthermore, after obtaining F0 generation, genome-edited animals generally need to be expanded and maintained for analyzing the target gene function. To investigate genes essential for normal birth and growth, the generation of conditional KO (cKO) animals in which a tissue- or stage-specific gene mutation can be introduced is often required. Here, we describe the basic principle and application of genome editing technology including zinc-finger nuclease, transcription-activator-like effector nuclease, and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR associated protein (Cas) systems. Recently advanced developmental biology methods have enabled application of the technology, especially CRISPR/Cas, to zygotes, leading to the prompt production of genome-edited animals. For pre-implantation embryos, genome editing via oviductal nucleic acid delivery has been developed as an embryo manipulation- or handling-free method. Examining the gene function at F0 generation is becoming possible by employing triple-target CRISPR technology. This technology, in combination with a blastocyst complementation method enables investigation of even birth- and growth-responsible genes without establishing cKO strains. We hope that this review is helpful for understanding and expanding genome editing-related technology and for progressing radiation research.

SOCS3 Expression by Thymic Stromal Cells Is Required for Normal T Cell Development

The suppressor of cytokine signaling 3 (SOCS3) is a major regulator of immune responses and inflammation as it negatively regulates cytokine signaling. Here, the role of SOCS3 in thymic T cell formation was studied in Socs3^fl/flActin-creER mice (Δsocs3) with a tamoxifen inducible and ubiquitous Socs3 deficiency. Δsocs3 thymi showed a 90% loss of cellularity and altered cortico-medullary organization. Thymocyte differentiation and proliferation was impaired at the early double negative (CD4-CD8-) cell stage and apoptosis was increased during the double positive (CD4+CD8+) cell stage, resulting in the reduction of recent thymic emigrants in peripheral organs. Using bone marrow chimeras, transplanting thymic organoids and using mice deficient of SOCS3 in thymocytes we found that expression in thymic stromal cells rather than in thymocytes was critical for T cell development. We found that SOCS3 in thymic epithelial cells (TECs) binds to the E3 ubiquitin ligase TRIM 21 and that Trim21^−/− mice showed increased thymic cellularity. Δsocs3 TECs showed alterations in the expression of genes involved in positive and negative selection and lympho-stromal interactions. SOCS3-dependent signal inhibition of the common gp130 subunit of the IL-6 receptor family was redundant for T cell formation. Together, SOCS3 expression in thymic stroma cells is critical for T cell development and for maintenance of thymus architecture.

Evidence for the involvement of caspases in establishing proper cerebrospinal fluid hydrodynamics

A large number of cells undergo apoptosis via caspase activation during and after neural tube closure (NTC) in mammals. Apoptosis is executed by either intrinsic or extrinsic apoptotic pathways, and inhibition of each pathway causes developmental defects around NTC stages, which hampers the physiological roles of apoptosis and caspases after NTC. We generated transgenic mice in which a broad spectrum of caspases could be suppressed in a spatiotemporal manner by pan-caspase inhibitor protein p35 originating from baculovirus. Mice with nervous system-specific expression of p35 (Nestin-Cre (NCre);p35V mice) exhibited postnatal lethality within 1 month after birth. They were born at the expected Mendelian ratio, but demonstrated severe postnatal growth retardation and hydrocephalus. The flow of cerebrospinal fluid (CSF) between the third and fourth ventricles was disturbed, whereas neither stenosis nor abnormality in ciliary morphology was observed in the pathway of CSF flow. Hydrocephalus and growth retardation of NCre;p35V mice were not rescued by the deletion of RIPK3, an essential factor for necroptosis which occurs in the absence of caspase-8 activation during development. The CSF of NCre;p35V mice contained a larger amount of secreted proteins than that of the controls. These findings suggest that the establishment of proper CSF dynamics requires caspase activity during brain development after NTC.

Advantages and Limitations of Cre Mouse Lines Used in Skeletal Research

The Cre-LoxP technology permits gene ablation in specific cell lineages, at chosen differentiation stages of this lineage and in an inducible manner. It has allowed tremendous advances in our understanding of skeleton biology and related pathophysiological mechanisms, through the generation of loss/gain of function or cell tracing experiments based on the creation of an expanding toolbox of transgenic mice expressing the Cre recombinase in skeletal stem cells, chondrocytes, osteoblasts, or osteoclasts. In this chapter, we provide an overview of the different Cre-LoxP systems and Cre mouse lines used in the bone field, we discuss their advantages, limitations, and we outline best practices to interpret results obtained from the use of Cre mice.

Altered baseline and amphetamine-mediated behavioral profiles in dopamine transporter Cre (DAT-Ires-Cre) mice compared to tyrosine hydroxylase Cre (TH-Cre) mice

RATIONALE

Transgenic mouse lines expressing Cre-recombinase under the regulation of either dopamine transporter (DAT) or tyrosine hydroxylase (TH) promoters are commonly used to study the dopamine (DA) system. While use of the TH promoter appears to have less liability to changes in native gene expression, transgene insertion in the DAT locus results in reduced DAT expression and function. This confound is sometimes overlooked in genetically targeted behavioral experiments.

OBJECTIVES

We sought to evaluate the suitability of DAT-Ires-Cre and TH-Cre transgenic lines for behavioral pharmacology experiments with DA agonists. We hypothesized that DAT-Ires-Cre expression would impact DAT-mediated behaviors, but no impact of TH-Cre expression would be observed.

METHODS

DAT-Ires-Cre and TH-Cre mice bred on mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds were evaluated for novelty-induced, baseline, and amphetamine (AMPH)-induced locomotion, and for AMPH and D1 agonist (SKF-38393)-induced preservative behaviors.

RESULTS

DAT-Ires-Cre mice on both mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds displayed increased novelty-induced activity and decreased AMPH-induced locomotion, with mixed results for AMPH-induced stereotypy. TH-Cre mice on both backgrounds showed typical baseline activity and AMPH-induced stereotypy, with a difference in AMPH-induced locomotion observed only on the mixed background. Both transgenic lines displayed unaltered SKF-38393-induced grooming behavior.

CONCLUSIONS

Our findings indicate that the DAT-Ires-Cre transgenic line may lead to confounds for experiments that are dependent on DAT expression. The TH-Cre transgenic line studied here may be a more useful option, depending on background strain, because of its lack of baseline and drug-induced phenotypes. These data highlight the importance of appropriate controls in studies employing transgenic mice.

Sparse Activity of Hippocampal Adult-Born Neurons during REM Sleep Is Necessary for Memory Consolidation

The occurrence of dreaming during rapid eye movement (REM) sleep prompts interest in the role of REM sleep in hippocampal-dependent episodic memory. Within the mammalian hippocampus, the dentate gyrus (DG) has the unique characteristic of exhibiting neurogenesis persisting into adulthood. Despite their small numbers and sparse activity, adult-born neurons (ABNs) in the DG play critical roles in memory; however, their memory function during sleep is unknown. Here, we investigate whether young ABN activity contributes to memory consolidation during sleep using Ca²⁺ imaging in freely moving mice. We found that contextual fear learning recruits a population of young ABNs that are reactivated during subsequent REM sleep against a backdrop of overall reduced ABN activity. Optogenetic silencing of this sparse ABN activity during REM sleep alters the structural remodeling of spines on ABN dendrites and impairs memory consolidation. These findings provide a causal link between ABN activity during REM sleep and memory consolidation.

Abundant expression of the membrane-anchored protease-regulator RECK in the anterior pituitary gland and its implication in the growth hormone/insulin-like growth factor 1 axis in mice

The tumor suppressor gene Reversion-inducing cysteine-rich protein with Kazal motifs (Reck) encodes a membrane-anchored protease regulator expressed in multiple tissues in mouse embryos and is essential for embryonic development. In postnatal mice, however, physiological roles for the RECK protein remain unclear. We found in this study that Reck is abundantly expressed in growth hormone (GH)-producing cells (somatotrophs) in the anterior pituitary gland (AP). We also found that two types of viable Reck mutant mice, one with reduced RECK expression (Hypo mice) and the other with induced Reck deficiency from 10 days after birth (iKO mice treated with tamoxifen), exhibit common phenotypes including decreases in body size and plasma levels of insulin-like growth factor-1 (IGF1). To gain insights into the function of RECK in the AP, we characterized several somatotroph-associated molecules in the AP of these mice. Immunoreactivity of GH was greatly reduced in tamoxifen-treated iKO mice; in these mice, two membrane receptors involved in the stimulation of GH secretion [growth hormone secretagogue receptor (GHSR) and growth hormone releasing hormone receptor (GHRHR)] were decreased, however, their mRNAs were increased. Decrease in GHSR immunoreactivity and concomitant increase in its mRNA were also found in the other mutant line, Hypo. Furthermore, reduced immunoreactivity of growth hormone receptor (GHR) and concomitant increase in its mRNA was also found in the liver of Hypo mice. These results raise the possibility that RECK supports proper functioning of the GH/IGF1 axis in mice, thereby affecting their growth and metabolism.

Smad4-dependent morphogenic signals control the maturation and axonal targeting of basal vomeronasal sensory neurons to the accessory olfactory bulb

The vomeronasal organ (VNO) contains two main types of vomeronasal sensory neurons (VSNs) that express distinct vomeronasal receptor (VR) genes and localize to specific regions of the neuroepithelium. Morphogenic signals are crucial in defining neuronal identity and network formation; however, if and what signals control maturation and homeostasis of VSNs is largely unexplored. Here, we found transforming growth factor β (TGFβ) and bone morphogenetic protein (BMP) signal transduction in postnatal mice, with BMP signaling being restricted to basal VSNs and at the marginal zones of the VNO: the site of neurogenesis. Using different Smad4 conditional knockout mouse models, we disrupted canonical TGFβ/BMP signaling in either maturing basal VSNs (bVSNs) or all mature VSNs. Smad4 loss of function in immature bVSNs compromises dendritic knob formation, pheromone induced activation, correct glomeruli formation in the accessory olfactory bulb (AOB) and survival. However, Smad4 loss of function in all mature VSNs only compromises correct glomeruli formation in the posterior AOB. Our results indicate that Smad4-mediated signaling drives the functional maturation and connectivity of basal VSNs.

Summary: Genetic disruption of TGFβ/BMP signaling in maturing basal vomeronasal sensory neurons (VSNs) or in all mature VSNs indicates that Smad4 signaling drives maturation and connectivity of basal VSNs.

Heterogeneous Origin of Gonadotropin Releasing Hormone-1 Neurons in Mouse Embryos Detected by Islet-1/2 Expression

In vertebrates, Gonadotropin releasing hormone-1 (GnRH) neuroendocrine cells originate in the olfactory placode and migrate into the forebrain where they regulate reproduction. However, the embryonic lineage of their progenitors remains controversial. Most GnRH neurons are derived from placodal ectodermal progenitor cells, but data from lineage tracing in zebrafish (Whitlock et al., 2003) and mouse (Forni and Wray, 2012) indicate that some GnRH progenitor cells have a neural crest (NC) origin. In contrast, a recent study in zebrafish (Aguillon et al., 2018), using Islet-1/2 expression, identified this LIM-homeodomain protein in all developing GnRH neuroendocrine cells, and the authors concluded a homogenous origin from progenitors within the preplacodal ectoderm. Evidence in different animal models and systems suggests that expression of Islet-1 plays a pivotal role in cell fate specification and differentiation. Thus, expression of Islet-1/2 in all GnRH cells in the nasal placode may not be lineage dependent but rather initiated locally in the placode as part of the program for GnRH cell specification and/or differentiation. This study addresses this issue and shows two populations of olfactory derived GnRH neurons in embryonic mouse: Islet-1/2(+) and Islet-1/2(−). Notably, triple-label immunofluorescence using the NC lineage tracer Wnt1, showed that GnRH neurons derived from Wnt1 progenitors are Islet-1/2(−). These results are consistent with two separate origins of GnRH neuroendocrine cells and suggest that either (1) NC-derived GnRH cells differentiate earlier than PE-derived GnRH cells or (2) different programs are used for cell specification in NC- vs. PE-derived GnRH cells.

Ectopic germline recombination activity of the widely used Foxp3-YFP-Cre mouse: a case report

Regulatory T (Treg) cell-specific deletion of a gene of interest is a procedure widely used to study mechanisms controlling Treg development, homeostasis and function. Accordingly, several transgenic mouse lines have been generated that bear the Cre recombinase under control of the Foxp3 promoter either as a random transgene insertion or knocked into the endogenous Foxp3 locus, with the Foxp3^YFP-Cre strain of mice being one of the most widely used. In an attempt to generate Treg cells that lacked expression of the insulin receptor (Insr), we crossed Foxp3^YFP-Cre mice with Insr^fl/fl mice. Using a conventional two-band PCR genotyping method we found that offspring genotypes did not correspond to the expected Mendelian ratios. We therefore developed a quantitative PCR-based genotyping method to investigate possible ectopic recombination outside the Treg lineage. With this method we found that ~50% of the F₁ -generation mice showed evidence of ectopic recombination and that ~10% of the F₂ -generation mice had germline Cre recombination activity leading to a high frequency of offspring with global Insr deletion. Use of the quantitative PCR genotyping method enabled accurate selection of mice without ectopic recombination and only the desired Treg cell-specific Insr deletion. Our data highlight the need to use genotyping methods that allow for assessment of possible ectopic recombination driven by the Foxp3^YFP-Cre allele, particularly when studying genes that are systemically expressed.

Cre recombinase toxicity in podocytes: a novel genetic model for FSGS in adolescent mice

Here, we show that inducible overexpression of Cre recombinase in glomerular podocytes but not in parietal epithelial cells may trigger focal segmental glomerulosclerosis (FSGS) in juvenile transgenic homocygous Pod-rtTA/LC1 mice. Administration of doxycycline shortly after birth, but not at any other time point later in life, resulted in podocyte injury and development of classical FSGS lesions in these mice. Sclerotic lesions were formed as soon as 3 wk of age, and FSGS progressed with low variability until 13 wk of age. In addition, our experiments identified Cre toxicity as a potentially relevant limitation for studies in podocytes of transgenic animals. In summary, our study establishes a novel genetic model for FSGS in mice, which exhibits low variability and manifests already at a young age.

Unlimited Genetic Switches for Cell-Type-Specific Manipulation

Gaining independent genetic access to discrete cell types is critical to interrogate their biological functions as well as to deliver precise gene therapy. Transcriptomics has allowed us to profile cell populations with extraordinary precision, revealing that cell types are typically defined by a unique combination of genetic markers. Given the lack of adequate tools to target cell types based on multiple markers, most cell types remain inaccessible to genetic manipulation. Here we present CaSSA, a platform to create unlimited genetic switches based on CRISPR/Cas9 (Ca) and the DNA repair mechanism known as single-strand annealing (SSA). CaSSA allows engineering of independent genetic switches, each responding to a specific gRNA. Expressing multiple gRNAs in specific patterns enables multiplex cell-type-specific manipulations and combinatorial genetic targeting. CaSSA is a new genetic tool that conceptually works as an unlimited number of recombinases and will facilitate genetic access to cell types in diverse organisms.

Mitochondrial Complex I Function Is Essential for Neural Stem/Progenitor Cells Proliferation and Differentiation

Neurogenesis in developing and adult mammalian brain is a tightly regulated process that relies on neural stem cell (NSC) activity. There is increasing evidence that mitochondrial metabolism affects NSC homeostasis and differentiation but the precise role of mitochondrial function in the neurogenic process requires further investigation. Here, we have analyzed how mitochondrial complex I (MCI) dysfunction affects NSC viability, proliferation and differentiation, as well as survival of the neural progeny. We have generated a conditional knockout model (hGFAP-NDUFS2 mice) in which expression of the NDUFS2 protein, essential for MCI function, is suppressed in cells expressing the Cre recombinase under the human glial fibrillary acidic protein promoter, active in mouse radial glial cells (RGCs) and in neural stem cells (NSCs) that reside in adult neurogenic niches. In this model we observed that survival of central NSC population does not appear to be severely affected by MCI dysfunction. However, perinatal brain development was markedly inhibited and Ndufs2 knockout mice died before the tenth postnatal day. In addition, in vitro studies of subventricular zone NSCs showed that active neural progenitors require a functional MCI to produce ATP and to proliferate. In vitro differentiation of neural precursors into neurons and oligodendrocytes was also profoundly affected. These data indicate the need of a correct MCI function and oxidative phosphorylation for glia-like NSC proliferation, differentiation and subsequent oligodendrocyte or neuronal maturation.

Viral vector-mediated Cre recombinase expression in substantia nigra induces lesions of the nigrostriatal pathway associated with perturbations of dopamine-related behaviors and hallmarks of programmed cell death

Cre/loxP recombination is a widely used approach to study gene function in vivo, using mice models expressing the Cre recombinase under the control of specific promoters or through viral delivery of Cre-expressing constructs. A profuse literature on transgenic mouse lines points out the deleterious effects of Cre expression in various cell types and tissues, presumably by acting on illegitimate loxP-like sites present in the genome. However, most studies reporting the consequences of Cre-lox gene invalidation often omit adequate controls to exclude the potential toxic effects of Cre, compromising the interpretation of data. In this study, we report the anatomical, neurochemical, and behavioral consequences in mice of adeno-associated virus (AAV)-mediated Cre expression in the dopaminergic nuclei substantia nigra, at commonly used viral titers (3 × 10⁹ genome copies/0.3 μL or 2 × 10⁹ genome copies/0.6 μL). We found that injecting AAV-eGFP-Cre into the SN engendered drastic and reproducible modifications of behavior, with increased basal locomotor activity as well as impaired locomotor response to cocaine compared to AAV-eGFP-injected controls. Cre expression in the SN induced a massive decrease in neuronal populations of both pars compacta and pars reticulata and dopamine depletion in the nigrostriatal pathway. This anatomical injury was associated with typical features of programmed cell death, including an increase in DNA break markers, evidence of apoptosis, and disrupted macroautophagy. These observations underscore the need for careful control of Cre toxicity in the brain and the reassessment of previous studies. In addition, our findings suggest that Cre-mediated ablation may constitute an efficient tool to explore the function of specific cell populations and areas in the brain, and the impact of neurodegeneration in these populations.

Conditional Mutagenesis in Oligodendrocyte Lineage Cells

Cell-type-specific gene targeting with the Cre/loxP system has become an indispensable technique in experimental neuroscience, particularly for the study of late-born glial cells that make myelin. A plethora of conditional mutants and Cre-expressing mouse lines is now available to the research community that allows laboratories to readily engage in in vivo analyses of oligodendrocytes and their precursor cells. This chapter summarizes concepts and strategies in targeting myelinating glial cells in mice for mutagenesis or imaging, and provides an overview of the most important Cre driver lines successfully used in this rapidly growing field.

Podocyte-specific expression of Cre recombinase promotes glomerular basement membrane thickening

Conditional gene targeting using Cre recombinase has offered a powerful tool to modify gene function precisely in defined cells/tissues and at specific times. However, in mammalian cells, Cre recombinase can be genotoxic. The importance of including Cre-expressing control mice to avoid misinterpretation and to maximize the validity of the experimental results has been increasingly recognized. While studying the role of podocytes in the pathogenesis of glomerular basement membrane (GBM) thickening, we used Cre recombinase driven by the podocyte-specific podocin promoter (NPHS2-Cre) to generate a conditional knockout. By conventional structural and functional measures (histology by periodic acid-Schiff staining, albuminuria, and plasma creatinine), we did not detect significant differences between NPHS2-Cre transgenic and wild-type control mice. However, surprisingly, the group that expressed Cre transgene alone developed signs of podocyte toxicity, including marked GBM thickening, loss of normal foot process morphology, and reduced Wilms tumor 1 expression. GBM thickening was characterized by altered expression of core structural protein laminin isoform α₅β₂γ₁. RNA sequencing analysis of extracted glomeruli identified 230 genes that were significant and differentially expressed (applying a q < 0.05-fold change ≥ ±2 cutoff) in NPHS2-Cre mice compared with wild-type control mice. Many biological processes were reflected in the RNA sequencing data, including regulation of the extracellular matrix and pathways related to apoptosis and cell death. This study highlights the importance of including the appropriate controls for potential Cre-mediated toxicity in conditional gene-targeting experiments. Indeed, omitting the Cre transgene control can result in critical errors during interpretation of experimental data.

Maternal metabolic adaptations are necessary for normal offspring growth and brain development

Several metabolic adaptations emerge during pregnancy and continue through lactation, including increases in food intake and body weight, as well as insulin and leptin resistance. These maternal adaptations are thought to play a role in offspring viability and success. Using a model of attenuated maternal metabolic adaptations induced by ablation of the Socs3 gene in leptin receptor expressing cells (SOCS3 KO mice), our study aimed to investigate whether maternal metabolic changes are required for normal offspring development, and if their absence causes metabolic imbalances in adulthood. The litters were subjected to a cross‐fostering experimental design to distinguish the prenatal and postnatal effects caused by maternal metabolic adaptations. Males either born or raised by SOCS3 KO mice showed reduced body weight until 8 weeks of life. Both adult males and females born or raised by SOCS3 KO mice also had lower body adiposity. Despite that, no significant changes in energy expenditure, glucose tolerance or insulin resistance were observed. However, males either born or raised by SOCS3 KO mice showed reduced brain mass in adulthood. Furthermore, animals born from SOCS3 KO mice also had lower proopiomelanocortin fiber density in the paraventricular nucleus of the hypothalamus. In conclusion, these findings indicate that the commonly observed metabolic changes in pregnancy and lactation are necessary for normal offspring growth and brain development.

Glutamate Dehydrogenase-Deficient Mice Display Schizophrenia-Like Behavioral Abnormalities and CA1-Specific Hippocampal Dysfunction

Brain imaging has revealed that the CA1 subregion of the hippocampus is hyperactive in prodromal and diagnosed patients with schizophrenia (SCZ), and that glutamate is a driver of this hyperactivity. Strikingly, mice deficient in the glutamate synthetic enzyme glutaminase have CA1 hypoactivity and a SCZ-resilience profile, implicating glutamate-metabolizing enzymes. To address this further, we examined mice with a brain-wide deficit in the glutamate-metabolizing enzyme glutamate dehydrogenase (GDH), encoded by Glud1, which should lead to glutamate excess due to reduced glutamate metabolism in astrocytes. We found that Glud1-deficient mice have behavioral abnormalities in the 3 SCZ symptom domains, with increased baseline and amphetamine-induced hyperlocomotion as a positive symptom proxy, nest building and social preference as a negative symptom proxy, and reversal/extradimensional set shifting in the water T-maze and contextual fear conditioning as a cognitive symptom proxy. Neuroimaging of cerebral blood volume revealed hippocampal hyperactivity in CA1, which was associated with volume reduction. Parameters of hippocampal synaptic function revealed excess glutamate release and an elevated excitatory/inhibitory balance in CA1. Finally, in a direct clinical correlation using imaging-guided microarray, we found a significant SCZ-associated postmortem reduction in GLUD1 expression in CA1. These findings advance GLUD1 deficiency as a driver of excess hippocampal excitatory transmission and SCZ symptoms, and identify GDH as a target for glutamate modulation pharmacotherapy for SCZ. More broadly, these findings point to the likely involvement of alterations in glutamate metabolism in the pathophysiology of SCZ.

In Vivo Genetic Strategies for the Specific Lineage Tracing of Stem Cells

BACKGROUND

Characterization of the fate changes of stem cells is essential to understand the roles of certain stem cells both during development and in diseases, such as cancer. In the past two decades, more and more importance has been paid to the studies of in vivo lineage tracing, because they could authentically reveal the differentiation, migration and even proliferation of stem cells. However, specific genetic tools have only been developed until recently.

OBJECTIVE

To summarize the progresses of genetic tools for specific lineage tracing with emphasis on their applications in investigating the stem cell niche signals.

RESULTS

Three major genetic strategies have been reviewed according to the development of technique, particularly the advantages and disadvantages of individual methods.

CONCLUSION

In vivo specific lineage tracing of stem cells could be achieved by comprehensive application of multiple genetic tools.

Differential susceptibility of striatal, hippocampal and cortical neurons to Caspase-6

Active cysteinyl protease Caspase-6 is associated with early Alzheimer and Huntington diseases. Higher entorhinal cortex and hippocampal Caspase-6 levels correlate with lower cognitive performance in aged humans. Caspase-6 induces axonal degeneration in human primary neuron cultures and causes inflammation and neurodegeneration in mouse hippocampus, and age-dependent memory impairment. To assess whether Caspase-6 causes damage to another neuronal system, a transgenic knock-in mouse overexpressing a self-activated form of Caspase-6 five-fold in the striatum, the area affected in Huntington disease, and 2.5-fold in the hippocampus and cortex, was generated. Detection of Tubulin cleaved by Caspase-6 confirmed Caspase-6 activity. The Caspase-6 expressing mice and control littermates were subjected to behavioral tests to assess Huntington disease-relevant psychiatric, motor, and cognitive deficits. Depression was excluded with the forced swim and sucrose consumption tests. Motor deficits were absent in the nesting, clasping, rotarod, vertical pole, gait, and open field analyzes. However, Caspase-6 mice developed age-dependent episodic and spatial memory deficits identified by novel object recognition, Barnes maze and Morris water maze assays. Neuron numbers were maintained in the striatum, hippocampus, and cortex. Microglia and astrocytes were increased in the hippocampal stratum lacunosum molecular and in the cortex, but not in the striatum. Synaptic mRNA profiling identified two differentially expressed genes in transgenic hippocampus, but none in striatum. Caspase-6 impaired synaptic transmission and induced neurodegeneration in hippocampal CA1 neurons, but not in striatal medium spiny neurons. These data revealed that active Caspase-6 in the striatal medium spiny neurons failed to induce inflammation, neurodegeneration or behavioral abnormalities, whereas active Caspase-6 in the cortex and hippocampus impaired episodic and spatial memories, and induced inflammation, neuronal dysfunction, and neurodegeneration. The results indicate age and neuronal subtype-dependent Caspase-6 toxicity and highlight the importance of targeting the correct neuronal subtype to identify underlying molecular mechanisms of neurodegenerative diseases.

The transcription factor Tfap2e/AP-2ε plays a pivotal role in maintaining the identity of basal vomeronasal sensory neurons

The identity of individual neuronal cell types is defined and maintained by the expression of specific combinations of transcriptional regulators that control cell type-specific genetic programs. The epithelium of the vomeronasal organ of mice contains two major types of vomeronasal sensory neurons (VSNs): 1) the apical VSNs which express vomeronasal 1 receptors (V1r) and the G-protein subunit Gαi2 and; 2) the basal VSNs which express vomeronasal 2 receptors (V2r) and the G-protein subunit Gαo. Both cell types originate from a common pool of progenitors and eventually acquire apical or basal identity through largely unknown mechanisms. The transcription factor AP-2ε, encoded by the Tfap2e gene, plays a role in controlling the development of GABAergic interneurons in the main and accessory olfactory bulb (AOB), moreover AP-2ε has been previously described to be expressed in the basal VSNs. Here we show that AP-2ε is expressed in post-mitotic VSNs after they commit to the basal differentiation program. Loss of AP-2ε function resulted in reduced number of basal VSNs and in an increased number of neurons expressing markers of the apical lineage. Our work suggests that AP-2ε, which is expressed in late phases of differentiation, is not needed to initiate the apical-basal differentiation dichotomy but for maintaining the basal VSNs' identity. In AP-2ε mutants we observed a large number of cells that entered the basal program can express apical genes, our data suggest that differentiated VSNs of mice retain a notable level of plasticity.