CRISPR/Cas9-engineered Gad1 elimination in rats leads to complex behavioral changes: implications for schizophrenia

Rats show up to 72 h of significant retention for spatial memory in the radial maze

This study explored long-term retention of spatial memory in rats using an eight-arm radial maze. Crystal and Babb (Learning and motivation, 39(4), 278-284, 2008) previously demonstrated that rats could retain spatial memory for up to 25 h in the radial maze. Notably, they found performance improved with 48-h intertrial intervals compared with 24-h intervals. Our study investigated the effects of extending intertrial intervals on long-term retention of spatial memory by reducing the potential for proactive interference. Each trial comprised a learning phase, during which subjects were required to sequentially visit four randomly selected arms, followed by a free-choice test that included all eight arms, conducted after increasing the retention and intertrial intervals. The retention intervals were systematically increased from 1 h to 24, 48, and, ultimately, 72 h, with corresponding intertrial intervals expanding from 24 to 48, 120, and 144 h. Performance significantly surpassed chance levels across all conditions, demonstrating that rats are capable of retaining spatial memory for up to 72 h.

Pentylenetetrazole kindling induces dynamic changes in GAD65 expression in hippocampal somatostatin interneurons

INTRODUCTION

One of the mechanisms of epileptgenesis is impairment of inhibitory neural circuits. Several studies have compared neural changes among subtypes of gamma-aminobutyric acid-related (GABAergic) neurons after acquired epileptic seizure. However, it is unclear that GABAergic neural modifications that occur during acquisition process of epileptic seizure.

METHODS

Male rats were injected with pentylenetetrazole (PTZ kindling: n = 30) or saline (control: n = 15) every other day to observe the development of epileptic seizure stages. Two time points were identified: the point at which seizures were most difficult to induce, and the point at which seizures were most easy to induce. The expression of GABAergic neuron-related proteins in the hippocampus was immunohistochemically compared among GABAergic subtypes at each of these time points.

RESULTS

Bimodal changes in seizure stages were observed in response to PTZ kindling. The increase of seizure stage was transiently suppressed after 8 or 10 injections, and then progressed again by the 16th injection. Based on these results, we defined 10 injections as a short-term injection period during which seizures are less likely to occur, and 20 injections as a long-term injection period during which continuous seizures are likely to occur. The immunohistochemical analysis showed that hippocampal glutamic acid decarboxylase 65 (GAD65) expression was increased after short-term kindling but unchanged after long-term kindling. Increased GAD65 expression was limited to somatostatin-positive (SOM+) cells among several GABAergic subtypes. By contrast, GAD, GABA, GABAAR α1, GABABR1, and VGAT cells showed no change following short- or long-term PTZ kindling.

CONCLUSION

PTZ kindling induces bimodal changes in the epileptic seizure stage. Seizure stage is transiently suppressed after short-term PTZ injection with GAD65 upregulation in SOM+ cells. The seizure stage is progressed again after long-term PTZ injection with GAD65 reduction to baseline level.

A comprehensive update on genetic inheritance, epigenetic factors, associated pathology, and recent therapeutic intervention by gene therapy in schizophrenia

Schizophrenia is a severe psychological disorder in which reality is interpreted abnormally by the patient. The symptoms of the disease include delusions and hallucinations, associated with extremely disordered behavior and thinking, which may affect the daily lives of the patients. Advancements in technology have led to understanding the dynamics of the disease and the identification of the underlying causes. Multiple investigations prove that it is regulated genetically, and epigenetically, and is affected by environmental factors. The molecular and neural pathways linked to the regulation of schizophrenia have been extensively studied. Over 180 Schizophrenic risk loci have now been recognized due to several genome-wide association studies (GWAS). It has been observed that multiple transcription factors (TF) binding-disrupting single nucleotide polymorphisms (SNPs) have been related to gene expression responsible for the disease in cerebral complexes. Copy number variation, SNP defects, and epigenetic changes in chromosomes may cause overexpression or underexpression of certain genes responsible for the disease. Nowadays, gene therapy is being implemented for its treatment as several of these genetic defects have been identified. Scientists are trying to use viral vectors, miRNA, siRNA, and CRISPR technology. In addition, nanotechnology is also being applied to target such genes. The primary aim of such targeting was to either delete or silence such hyperactive genes or induce certain genes that inhibit the expression of these genes. There are challenges in delivering the gene/DNA to the site of action in the brain, and scientists are working to resolve the same. The present article describes the basics regarding the disease, its causes and factors responsible, and the gene therapy solutions available to treat this disease.

Gad1 knock-out rats exhibit abundant spike-wave discharges in EEG, exacerbated with valproate treatment

Objective

To elucidate the functional role of gamma-aminobutyric acid (GABA)-ergic inhibition in suppressing epileptic brain activities such as spike-wave discharge (SWD), we recorded electroencephalogram (EEG) in knockout rats for Glutamate decarboxylase 1 (Gad1), which encodes one of the two GABA-synthesizing enzymes in mammals. We also examined how anti-epileptic drug valproate (VPA) acts on the SWDs present in Gad1 rats and affects GABA synthesis in the reticular thalamic nucleus (RTN), which is known to play an essential role in suppressing SWD.

Methods

Chronic EEG recordings were performed in freely moving control rats and homozygous knockout Gad1 (-/-) rats. Buzzer tones (82 dB) were delivered to the rats during EEG monitoring to test whether acoustic stimulation could interrupt ongoing SWDs. VPA was administered orally to the rats, and the change in the number of SWDs was examined. The distribution of GABA in the RTN was examined immunohistochemically.

Results

SWDs were abundant in EEG from Gad1 (-/-) rats as young as 2 months old. Although SWDs were universally detected in older rats irrespective of their Gad1 genotype, SWD symptom was most severe in Gad1 (-/-) rats. Acoustic stimulation readily interrupted ongoing SWDs irrespective of the Gad1 genotype, whereas SWDs were more resistant to interruption in Gad1 (-/-) rats. VPA treatment alleviated SWD symptoms in control rats, however, counterintuitively exacerbated the symptoms in Gad1 (-/-) rats. The immunohistochemistry results indicated that GABA immunoreactivity was significantly reduced in the somata of RTN neurons in Gad1 (-/-) rats but not in their axons targeting the thalamus. VPA treatment greatly increased GABA immunoreactivity in the RTN neurons of Gad1 (-/-) rats, which is likely due to the intact GAD2, another GAD isozyme, in these neurons.

Discussion

Our results revealed two opposing roles of GABA in SWD generation: suppression and enhancement of SWD. To account for these contradictory roles, we propose a model in which GABA produced by GAD1 in the RTN neuronal somata is released extrasynaptically and mediates intra-RTN inhibition.

Gene Therapy, A Potential Therapeutic Tool for Neurological and Neuropsychiatric Disorders: Applications, Challenges and Future Perspective

Neurological and neuropsychiatric disorders are the main risks for the health care system, exhibiting a huge socioeconomic load. The available range of pharmacotherapeutics mostly provides palliative consequences and fails to treat such conditions. The molecular etiology of various neurological and neuropsychiatric disorders is mostly associated with a change in genetic background, which can be inherited/triggered by other environmental factors. To address such conditions, gene therapy is considered a potential approach claiming a permanent cure of the disease primarily by deletion, silencing, or edition of faulty genes and by insertion of healthier genes. In gene therapy, vectors (viral/nonvial) play an important role in delivering the desired gene to a specific region of the brain. Targeted gene therapy has unraveled opportunities for the treatment of many neurological and neuropsychiatric disorders. For improved gene delivery, the current techniques mainly focus on designing a precise viral vector, plasmid transfection, nanotechnology, microRNA, and in vivo clustered regulatory interspaced short palindromic repeats (CRISPR)-based therapy. These latest techniques have great benefits in treating predominant neurological and neurodevelopmental disorders, including Parkinson's disease, Alzheimer's disease, and autism spectrum disorder, as well as rarer diseases. Nevertheless, all these delivery methods have their limitations, including immunogenic reactions, off-target effects, and a deficiency of effective biomarkers to appreciate the effectiveness of therapy. In this review, we present a summary of the current methods in targeted gene delivery, followed by the limitations and future direction of gene therapy for the cure of neurological and neuropsychiatric disorders.

Beyond the γ-aminobutyric acid hypothesis of schizophrenia

Abnormalities in the γ-aminobutyric acid (GABA) system have been reported in the postmortem brains of individuals with schizophrenia. In particular, the reduction of one of the GABA-synthesizing enzymes, the 67-kDa isoform of glutamate decarboxylase (GAD67), has garnered interest among researchers because of its role in the formation of γ-oscillations and its potential involvement in the cognitive dysfunction observed in schizophrenia. Although several animal models have been generated to simulate the alterations observed in postmortem brain studies, they exhibit inconsistent behavioral phenotypes, leading to conflicting views regarding their contributions to the pathogenesis and manifestation of schizophrenia symptoms. For instance, GAD67 knockout rats (also known as Gad1 knockout rats) exhibit marked impairments in spatial working memory, but other model animals do not. In this review, we summarize the phenotypic attributes of these animal models and contemplate the potential for secondary modifications that may arise from the disruption of the GABAergic nervous system.

Peripheral Nerve Injury Induces Changes in the Activity of Inhibitory Interneurons as Visualized in Transgenic GAD1-GCaMP6s Rats

Peripheral nerve injury induces cortical remapping that can lead to sensory complications. There is evidence that inhibitory interneurons play a role in this process, but the exact mechanism remains unclear. Glutamate decarboxylase-1 (GAD1) is a protein expressed exclusively in inhibitory interneurons. Transgenic rats encoding GAD1–GCaMP were generated to visualize the activity in GAD1 neurons through genetically encoded calcium indicators (GCaMP6s) in the somatosensory cortex. Forepaw denervation was performed in adult rats, and fluorescent Ca2+ imaging on cortical slices was obtained. Local, intrahemispheric stimulation (cortical layers 2/3 and 5) induced a significantly higher fluorescence change of GAD1-expressing neurons, and a significantly higher number of neurons were responsive to stimulation in the denervated rats compared to control rats. However, remote, interhemispheric stimulation of the corpus callosum induced a significantly lower fluorescence change of GAD1-expressing neurons, and significantly fewer neurons were deemed responsive to stimulation within layer 5 in denervated rats compared to control rats. These results suggest that injury impacts interhemispheric communication, leading to an overall decrease in the activity of inhibitory interneurons in layer 5. Overall, our results provide direct evidence that inhibitory interneuron activity in the deprived S1 is altered after injury, a phenomenon likely to affect sensory processing.

Abnormal synaptic plasticity and impaired cognition in schizophrenia

Schizophrenia (SCZ) is a severe mental illness that affects several brain domains with relation to cognition and behaviour. SCZ symptoms are typically classified into three categories, namely, positive, negative, and cognitive. The etiology of SCZ is thought to be multifactorial and poorly understood. Accumulating evidence has indicated abnormal synaptic plasticity and cognitive impairments in SCZ. Synaptic plasticity is thought to be induced at appropriate synapses during memory formation and has a critical role in the cognitive symptoms of SCZ. Many factors, including synaptic structure changes, aberrant expression of plasticity-related genes, and abnormal synaptic transmission, may influence synaptic plasticity and play vital roles in SCZ. In this article, we briefly summarize the morphology of the synapse, the neurobiology of synaptic plasticity, and the role of synaptic plasticity, and review potential mechanisms underlying abnormal synaptic plasticity in SCZ. These abnormalities involve dendritic spines, postsynaptic density, and long-term potentiation-like plasticity. We also focus on cognitive dysfunction, which reflects impaired connectivity in SCZ. Additionally, the potential targets for the treatment of SCZ are discussed in this article. Therefore, understanding abnormal synaptic plasticity and impaired cognition in SCZ has an essential role in drug therapy.

Recent Advances in the Production of Genome-Edited Rats

The rat is an important animal model for understanding gene function and developing human disease models. Knocking out a gene function in rats was difficult until recently, when a series of genome editing (GE) technologies, including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the type II bacterial clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated Cas9 (CRISPR/Cas9) systems were successfully applied for gene modification (as exemplified by gene-specific knockout and knock-in) in the endogenous target genes of various organisms including rats. Owing to its simple application for gene modification and its ease of use, the CRISPR/Cas9 system is now commonly used worldwide. The most important aspect of this process is the selection of the method used to deliver GE components to rat embryos. In earlier stages, the microinjection (MI) of GE components into the cytoplasm and/or nuclei of a zygote was frequently employed. However, this method is associated with the use of an expensive manipulator system, the skills required to operate it, and the egg transfer (ET) of MI-treated embryos to recipient females for further development. In vitro electroporation (EP) of zygotes is next recognized as a simple and rapid method to introduce GE components to produce GE animals. Furthermore, in vitro transduction of rat embryos with adeno-associated viruses is potentially effective for obtaining GE rats. However, these two approaches also require ET. The use of gene-engineered embryonic stem cells or spermatogonial stem cells appears to be of interest to obtain GE rats; however, the procedure itself is difficult and laborious. Genome-editing via oviductal nucleic acids delivery (GONAD) (or improved GONAD (i-GONAD)) is a novel method allowing for the in situ production of GE zygotes existing within the oviductal lumen. This can be performed by the simple intraoviductal injection of GE components and subsequent in vivo EP toward the injected oviducts and does not require ET. In this review, we describe the development of various approaches for producing GE rats together with an assessment of their technical advantages and limitations, and present new GE-related technologies and current achievements using those rats in relation to human diseases.

Impact of GAD65 and/or GAD67 deficiency on perinatal development in rats

GABA is a major neurotransmitter in the mammalian central nervous system. Glutamate decarboxylase (GAD) synthesizes GABA from glutamate, and two isoforms of GAD, GAD65, and GAD67, are separately encoded by the Gad2 and Gad1 genes, respectively. The phenotypes differ in severity between GAD single isoform-deficient mice and rats. For example, GAD67 deficiency causes cleft palate and/or omphalocele in mice but not in rats. In this study, to further investigate the functional roles of GAD65 and/or GAD67 and to determine the contribution of these isoforms to GABA synthesis during development, we generated various kinds of GAD isoform(s)-deficient rats and characterized their phenotypes. The age of death was different among Gad mutant rat genotypes. In particular, all Gad1^-/- ; Gad2^-/- rats died at postnatal day 0 and showed little alveolar space in their lungs, suggesting that the cause of their death was respiratory failure. All Gad1^-/- ; Gad2^-/- rats and 18% of Gad1^-/- ; Gad2^+/- rats showed cleft palate. In contrast, none of the Gad mutant rats including Gad1^-/- ; Gad2^-/- rats, showed omphalocele. These results suggest that both rat GAD65 and GAD67 are involved in palate formation, while neither isoform is critical for abdominal wall formation. The GABA content in Gad1^-/- ; Gad2^-/- rat forebrains and retinas at embryonic day 20 was extremely low, indicating that almost all GABA was synthesized from glutamate by GADs in the perinatal period. The present study shows that Gad mutant rats are a good model for further defining the role of GABA during development.

A Selective Review of the Excitatory-Inhibitory Imbalance in Schizophrenia: Underlying Biology, Genetics, Microcircuits, and Symptoms

Schizophrenia is a chronic disorder characterized by specific positive and negative primary symptoms, social behavior disturbances and cognitive deficits (e.g., impairment in working memory and cognitive flexibility). Mounting evidence suggests that altered excitability and inhibition at the molecular, cellular, circuit and network level might be the basis for the pathophysiology of neurodevelopmental and neuropsychiatric disorders such as schizophrenia. In the past decades, human and animal studies have identified that glutamate and gamma-aminobutyric acid (GABA) neurotransmissions are critically involved in several cognitive progresses, including learning and memory. The purpose of this review is, by analyzing emerging findings relating to the balance of excitatory and inhibitory, ranging from animal models of schizophrenia to clinical studies in patients with early onset, first-episode or chronic schizophrenia, to discuss how the excitatory-inhibitory imbalance may relate to the pathophysiology of disease phenotypes such as cognitive deficits and negative symptoms, and highlight directions for appropriate therapeutic strategies.

GABAergic System Dysfunction and Challenges in Schizophrenia Research

Despite strenuous studies since the last century, the precise cause and pathology of schizophrenia are still largely unclear and arguably controversial. Although many hypotheses have been proposed to explain the etiology of schizophrenia, the definitive genes or core pathological mechanism remains absent. Among these hypotheses, however, GABAergic dysfunction stands out as a common feature consistently reported in schizophrenia, albeit a satisfactory mechanism that could be exploited for therapeutic purpose has not been developed yet. This review is focusing on the progress made to date in the field in terms of understanding the mechanisms involving dysfunctional GABAergic system and loops identified in schizophrenia research.

Epilepsy and Migraine Shared Genetic and Molecular Mechanisms: Focus on Therapeutic Strategies

Epilepsy and migraine are both episodic disorders and share clinical as well as pathophysiological mechanisms. The prevalence of epilepsy in migraine patients is generally higher than normal as compared to general population and vice versa. Various environmental risk factors and genetic factors have been reported to be associated with susceptibility of these comorbid diseases. Specific genes have been implicated in the pathogenesis of the two diseases. However, the shared genetic susceptibility has not been explored extensively. Previous studies have reported that the alterations in the genes encoding ion channel proteins are common risk factors for both the diseases. The alterations in ion channel-encoding genes CACNAIA (T666M) and SCNIA (Q1489K and L1649Q) have been found to be involved in the development of familial hemiplegic migraine (FHM) as well as generalized epilepsy and some cases of focal epilepsy as well. The fact that both these disorders are treated with anti-epileptic drugs (AEDs) strongly supports common underlying mechanisms. This review has been compiled with an aim to explore the alterations in common genes involved in various pathways regulating neuronal hyperexcitability, a common risk factor for both these conditions. The avenue for future treatment strategies targeting common genes and molecular mechanisms has also been discussed.

CRISPR/Cas: a Nobel Prize award-winning precise genome editing technology for gene therapy and crop improvement

Since it was first recognized in bacteria and archaea as a mechanism for innate viral immunity in the early 2010s, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) has rapidly been developed into a robust, multifunctional genome editing tool with many uses. Following the discovery of the initial CRISPR/Cas-based system, the technology has been advanced to facilitate a multitude of different functions. These include development as a base editor, prime editor, epigenetic editor, and CRISPR interference (CRISPRi) and CRISPR activator (CRISPRa) gene regulators. It can also be used for chromatin and RNA targeting and imaging. Its applications have proved revolutionary across numerous biological fields, especially in biomedical and agricultural improvement. As a diagnostic tool, CRISPR has been developed to aid the detection and screening of both human and plant diseases, and has even been applied during the current coronavirus disease 2019 (COVID-19) pandemic. CRISPR/Cas is also being trialed as a new form of gene therapy for treating various human diseases, including cancers, and has aided drug development. In terms of agricultural breeding, precise targeting of biological pathways via CRISPR/Cas has been key to regulating molecular biosynthesis and allowing modification of proteins, starch, oil, and other functional components for crop improvement. Adding to this, CRISPR/Cas has been shown capable of significantly enhancing both plant tolerance to environmental stresses and overall crop yield via the targeting of various agronomically important gene regulators. Looking to the future, increasing the efficiency and precision of CRISPR/Cas delivery systems and limiting off-target activity are two major challenges for wider application of the technology. This review provides an in-depth overview of current CRISPR development, including the advantages and disadvantages of the technology, recent applications, and future considerations.

Behavioral Consequences of a Combination of Gad1 Haplodeficiency and Adolescent Exposure to an NMDA Receptor Antagonist in Long-Evans Rats

Glutamate decarboxylase 67-kDa isoform (GAD67), which is encoded by the GAD1 gene, is one of the key enzymes that produce GABA. The reduced expression of GAD67 has been linked to the pathophysiology of schizophrenia. Additionally, the excitatory glutamatergic system plays an important role in the development of this disorder. Animal model studies have revealed that chronic blockade of NMDA-type glutamate receptors can cause GABAergic dysfunction and long-lasting behavioral abnormalities. Based on these findings, we speculated that Gad1 haplodeficiency combined with chronic NMDA receptor blockade would lead to larger behavioral consequences relevant to schizophrenia in a rat model. In this study, we administered an NMDAR antagonist, MK-801 (0.2 mg/kg), to CRISPR/Cas9-generated Gad1^+/− rats during adolescence to test this hypothesis. The MK-801 treated Gad1^+/− rats showed a shorter duration in each rearing episode in the open field test than the saline-treated Gad1^+/+ rats. In contrast, immobility in the forced swim test was increased and fear extinction was impaired in Gad1^+/− rats irrespective of MK-801 treatment. Interestingly, the time spent in the center region of the elevated plus-maze was significantly affected only in the saline-treated Gad1^+/− rats. Additionally, the MK-801-induced impairment of the social novelty preference was not observed in Gad1^+/− rats. These results suggest that the synergistic and additive effects of Gad1 haplodeficiency and NMDA receptor blockade during adolescence on the pathogenesis of schizophrenia may be more limited than expected. Findings from this study also imply that these two factors mainly affect negative or affective symptoms, rather than positive symptoms.

Insights: fear of GAD

Welcome to Insights, a new series in which articles published in FEBS Open Bio are summarised for the wider community. We hope that this series will help make the findings we publish more accessible to the general public and encourage greater engagement. In this first article of the series, we introduce a research paper on fear in rats, authored by Professor Yuchio Yanagawa and colleagues and published in this issue. Photo: Young rat and cat in front of black background. Cynoclub/Shutterstock.com.

Genetic deletion of the 67-kDa isoform of glutamate decarboxylase alters conditioned fear behavior in rats

The GABAergic system is thought to play an important role in the control of cognition and emotion, such as fear, and is related to the pathophysiology of psychiatric disorders. For example, the expression of the 67‐kDa isoform of glutamate decarboxylase (GAD67), a GABA‐producing enzyme, is downregulated in the postmortem brains of patients with major depressive disorder and schizophrenia. However, knocking out the Gad1 gene, which encodes GAD67, is lethal in mice, and thus, the association between Gad1 and cognitive/emotional functions is unclear. We recently developed Gad1 knockout rats and found that some of them can grow into adulthood. Here, we performed fear‐conditioning tests in adult Gad1 knockout rats to assess the impact of the loss of Gad1 on fear‐related behaviors and the formation of fear memory. In a protocol assessing both cued and contextual memory, Gad1 knockout rats showed a partial antiphase pattern of freezing during training and significantly excessive freezing during the contextual test compared with wild‐type rats. However, Gad1 knockout rats did not show any synchronous increase in freezing with auditory tones in the cued test. On the other hand, in a contextual memory specialized protocol, Gad1 knockout rats exhibited comparable freezing behavior to wild‐type rats, while their fear extinction was markedly impaired. These results suggest that GABA synthesis by GAD67 has differential roles in cued and contextual fear memory.