Alzheimer's disease-associated U1 snRNP splicing dysfunction causes neuronal hyperexcitability and cognitive impairment.

Recent proteome and transcriptome profiling of Alzheimer's disease (AD) brains reveals RNA splicing dysfunction and U1 small nuclear ribonucleoprotein (snRNP) pathology containing U1-70K and its N-terminal 40-KDa fragment (N40K). Here we present a causative role of U1 snRNP dysfunction to neurodegeneration in primary neurons and transgenic mice (N40K-Tg), in which N40K expression exerts a dominant-negative effect to downregulate full-length U1-70K. N40K-Tg recapitulates N40K insolubility, erroneous splicing events, neuronal degeneration and cognitive impairment. Specifically, N40K-Tg shows the reduction of GABAergic synapse components (e.g., the GABA receptor subunit of GABRA2), and concomitant postsynaptic hyperexcitability that is rescued by a GABA receptor agonist. Crossing of N40K-Tg and the 5xFAD amyloidosis model indicates that the RNA splicing defect synergizes with the amyloid cascade to remodel the brain transcriptome and proteome, deregulate synaptic proteins, and accelerate cognitive decline. Thus, our results support the contribution of U1 snRNP-mediated splicing dysfunction to AD pathogenesis.

Age-dependent expression pattern in the mammalian brain of a novel, small peptide encoded in the 22q11.2 deletion syndrome region.

22q11.2 deletion syndrome (22q11.2DS) carries increased risk for both physical and psychiatric symptoms, including a high risk for schizophrenia. Understanding the genetic elements within the deletion region therefore has the potential to unlock the mysteries of both diseases. While most of the protein-coding genes in this region have been characterized, novel elements, such as non-coding RNAs and small Open Reading Frames (sORFs) remain unstudied. We have identified a novel, highly-conserved mouse sORF in a region of the mouse genome that is orthologous to a portion of the 22q11.2 deletion. This region was previously associated with age-dependent synaptic plasticity abnormalities. We refer to it as the Plasticity Associated Neural Transcript Short, or Pants. In developing and aging mouse brain, Pants expression is strongest in hippocampus, especially in areas CA3 and CA2, throughout the dorsoventral axis. The Pants peptide is expressed throughout the hippocampus, with an age-dependent increase in stratum lucidum at 16 weeks of age. This expression pattern suggests a potential role for Pants in many hippocampal behaviors, as well as a potential role in the age-dependent neurologic deficits displayed by 22q11.2DS model mice and patients.

SCYL2 Protects CA3 Pyramidal Neurons from Excitotoxicity during Functional Maturation of the Mouse Hippocampus.

Neuronal death caused by excessive excitatory signaling, excitotoxicity, plays a central role in neurodegenerative disorders. The mechanisms regulating this process, however, are still incompletely understood. Here we show that the coated vesicle-associated kinase SCYL2/CVAK104 plays a critical role for the normal functioning of the nervous system and for suppressing excitotoxicity in the developing hippocampus. Targeted disruption of Scyl2 in mice caused perinatal lethality in the vast majority of newborn mice and severe sensory-motor deficits in mice that survived to adulthood. Consistent with a neurogenic origin of these phenotypes, neuron-specific deletion of Scyl2 also caused perinatal lethality in the majority of newborn mice and severe neurological defects in adult mice. The neurological deficits in these mice were associated with the degeneration of several neuronal populations, most notably CA3 pyramidal neurons of the hippocampus, which we analyzed in more detail. The loss of CA3 neurons occurred during the functional maturation of the hippocampus and was the result of a BAX-dependent apoptotic process. Excessive excitatory signaling was present at the onset of degeneration, and inhibition of excitatory signaling prevented the degeneration of CA3 neurons. Biochemical fractionation reveals that Scyl2-deficient mice have an altered composition of excitatory receptors at synapses. Our findings demonstrate an essential role for SCYL2 in regulating neuronal function and survival and suggest a role for SCYL2 in regulating excitatory signaling in the developing brain. Significance statement: Here we examine the in vivo function of SCYL2, an evolutionarily conserved and ubiquitously expressed protein pseudokinase thought to regulate protein trafficking along the secretory pathway, and demonstrate its importance for the normal functioning of the nervous system and for suppressing excitatory signaling in the developing brain. Together with recent studies demonstrating a role of SCYL1 in preventing motor neuron degeneration, our findings clearly establish the SCY1-like family of protein pseudokinases as key regulators of neuronal function and survival.

Non-coding RNA regulation of synaptic plasticity and memory: implications for aging.

Advancing age is associated with the loss of cognitive ability and vulnerability to debilitating mental diseases. Although much is known about the development of cognitive processes in the brain, the study of the molecular mechanisms governing memory decline with aging is still in its infancy. Recently, it has become apparent that most of the human genome is transcribed into non-coding RNAs (ncRNAs) rather than protein-coding mRNAs. Multiple types of ncRNAs are enriched in the central nervous system, and this large group of molecules may regulate the molecular complexity of the brain, its neurons, and synapses. Here, we review the current knowledge on the role of ncRNAs in synaptic plasticity, learning, and memory in the broader context of the aging brain and associated memory loss. We also discuss future directions to study the role of ncRNAs in the aging process.

A Synaptic Function Approach to Investigating Complex Psychiatric Diseases.

The 22q11 deletion syndrome (22q11DS) is the most common microdeletion syndrome in humans and presents with a complex and variable psychiatric phenotype. Patients show cognitive impairment and have a higher probability of psychiatric disorders. As much as 30% of patients with 22q11DS suffer from schizophrenia, the strongest association between any mutation and the disease. Schizophrenia is a complex psychiatric disease that affects multiple brain regions, giving rise to a constellation of seemingly unrelated symptoms including hallucinations, social withdrawal, and memory deficits. Synaptic or neuronal malfunctions within certain physiological circuits appear to be at the core of these symptoms. Understanding disease at the synaptic level requires genetic model systems where intact neural circuits can be interrogated for functional deficits. Because of the overlap between 22q11DS and schizophrenia, models of 22q11DS may be key genetic tools for investigating both diseases. Here we discuss the advantages of using a synaptic function approach to studying mouse models of 22q11DS, review recent findings, and discuss them in the broader context of 22q11DS and schizophrenia.

Age-dependent microRNA control of synaptic plasticity in 22q11 deletion syndrome and schizophrenia.

The 22q11 deletion syndrome (22q11DS) is characterized by multiple physical and psychiatric abnormalities and is caused by the hemizygous deletion of a 1.5-3 Mb region of chromosome 22. It constitutes one of the strongest known genetic risks for schizophrenia; schizophrenia arises in as many as 30% of patients with 22q11DS during adolescence or early adulthood. A mouse model of 22q11DS displays an age-dependent increase in hippocampal long-term potentiation (LTP), a form of synaptic plasticity underlying learning and memory. The sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA2), which is responsible for loading Ca(2+) into the endoplasmic reticulum (ER), is elevated in this mouse model. The resulting increase in ER Ca(2+) load leads to enhanced neurotransmitter release and increased LTP. However, the mechanism by which the 22q11 microdeletion leads to SERCA2 overexpression and LTP increase has not been determined. Screening of multiple mutant mouse lines revealed that haploinsufficiency of Dgcr8, a microRNA (miRNA) biogenesis gene in the 22q11DS disease-critical region, causes age-dependent, synaptic SERCA2 overexpression and increased LTP. We found that miR-25 and miR-185, regulators of SERCA2, are depleted in mouse models of 22q11DS. Restoration of these miRNAs to presynaptic neurons rescues LTP in Dgcr8(+/-) mice. Finally, we show that SERCA2 is elevated in the brains of patients with schizophrenia, providing a link between mouse model findings and the human disease. We conclude that miRNA-dependent SERCA2 dysregulation is a pathogenic event in 22q11DS and schizophrenia.

Homologs of genes expressed in Caenorhabditis elegans GABAergic neurons are also found in the developing mouse forebrain.

BACKGROUND: In an effort to identify genes that specify the mammalian forebrain, we used a comparative approach to identify mouse homologs of transcription factors expressed in developing Caenorhabditis elegans GABAergic neurons. A cell-specific microarray profiling study revealed a set of transcription factors that are highly expressed in embryonic C. elegans GABAergic neurons. RESULTS: Bioinformatic analyses identified mouse protein homologs of these selected transcripts and their expression pattern was mapped in the mouse embryonic forebrain by in situ hybridization. A review of human homologs indicates several of these genes are potential candidates in neurodevelopmental disorders. CONCLUSIONS: Our comparative approach has revealed several novel candidates that may serve as future targets for studies of mammalian forebrain development.

Dysregulation of presynaptic calcium and synaptic plasticity in a mouse model of 22q11 deletion syndrome.

The 22q11 deletion syndrome (22q11DS) is characterized by cognitive decline and increased risk of psychiatric disorders, mainly schizophrenia. The molecular mechanisms of neuronal dysfunction in cognitive symptoms of 22q11DS are poorly understood. Here, we report that a mouse model of 22q11DS, the Df(16)1/+ mouse, exhibits substantially enhanced short- and long-term synaptic plasticity at hippocampal CA3-CA1 synapses, which coincides with deficits in hippocampus-dependent spatial memory. These changes are evident in mature but not young animals. Electrophysiological, two-photon imaging and glutamate uncaging, and electron microscopic assays in acute brain slices showed that enhanced neurotransmitter release but not altered postsynaptic function or structure caused these changes. Enhanced neurotransmitter release in Df(16)1/+ mice coincided with altered calcium kinetics in CA3 presynaptic terminals and upregulated sarco(endo)plasmic reticulum calcium-ATPase type 2 (SERCA2). SERCA inhibitors rescued synaptic phenotypes of Df(16)1/+ mice. Thus, presynaptic SERCA2 upregulation may be a pathogenic event contributing to the cognitive symptoms of 22q11DS.

Coenzyme Q protects Caenorhabditis elegans GABA neurons from calcium-dependent degeneration.

Mitochondria are key regulators of cell viability and provide essential functions that protect against neurodegenerative disease. To develop a model for mitochondrial-dependent neurodegeneration in Caenorhabditis elegans, we used RNA interference (RNAi) and genetic ablation to knock down expression of enzymes in the Coenzyme Q (CoQ) biosynthetic pathway. CoQ is a required component of the ATP-producing electron transport chain in mitochondria. We found that reduced levels of CoQ result in a progressive uncoordinated (Unc) phenotype that is correlated with the appearance of degenerating GABA neurons. Both the Unc and degenerative phenotypes emerge during late larval development and progress in adults. Neuron classes in motor and sensory circuits that use other neurotransmitters (dopamine, acetylcholine, glutamate, serotonin) and body muscle cells were less sensitive to CoQ depletion. Our results indicate that the mechanism of GABA neuron degeneration is calcium-dependent and requires activation of the apoptotic gene, ced-4 (Apaf-1). A molecular cascade involving mitochondrial-initiated cell death is also consistent with our finding that GABA neuron degeneration requires the mitochondrial fission gene, drp-1. We conclude that the cell selectivity and developmental progression of CoQ deficiency in C. elegans indicate that this model may be useful for delineating the role of mitochondrial dysfunction in neurodegenerative disease.

RGS9-2 modulates D2 dopamine receptor-mediated Ca2+ channel inhibition in rat striatal cholinergic interneurons.

Regulator of G protein signaling (RGS) proteins negatively regulate receptor-mediated second messenger responses by enhancing the GTPase activity of Galpha subunits. We describe a receptor-specific role for an RGS protein at the level of an individual brain neuron. RGS9-2 and Gbeta(5) mRNA and protein complexes were detected in striatal cholinergic and gamma-aminobutyric acidergic neurons. Dialysis of cholinergic neurons with RGS9 constructs enhanced basal Ca(2+) channel currents and reduced D(2) dopamine receptor modulation of Cav2.2 channels. These constructs did not alter M(2) muscarinic receptor modulation of Cav2.2 currents in the same neuron. The noncatalytic DEP-GGL domain of RGS9 antagonized endogenous RGS9-2 activity, enhancing D(2) receptor modulation of Ca(2+) currents. In vitro, RGS9 constructs accelerated GTPase activity, in agreement with electrophysiological measurements, and did so more effectively at Go than Gi. These results implicate RGS9-2 as a specific regulator of dopamine receptor-mediated signaling in the striatum and identify a role for GAP activity modulation by the DEP-GGL domain.