BRaf signaling principles unveiled by large-scale human mutation analysis with a rapid lentivirus-based gene replacement method

Architectural organization of ∼1,500-neuron modular minicolumnar disinhibitory circuits in healthy and Alzheimer's cortices

Acquisition of neuronal circuit architectures, central to understanding brain function and dysfunction, remains prohibitively challenging. Here I report the development of a simultaneous and sequential octuple-sexdecuple whole-cell patch-clamp recording system that enables architectural reconstruction of complex cortical circuits. The method unveils the canonical layer 1 single bouquet cell (SBC)-led disinhibitory neuronal circuits across the mouse somatosensory, motor, prefrontal, and medial entorhinal cortices. The ∼1,500-neuron modular circuits feature the translaminar, unidirectional, minicolumnar, and independent disinhibition and optimize cortical complexity, subtlety, plasticity, variation, and redundancy. Moreover, architectural reconstruction uncovers age-dependent deficits at SBC-disinhibited synapses in the senescence-accelerated mouse prone 8, an animal model of Alzheimer's disease. The deficits exhibit the characteristic Alzheimer's-like cortical spread and correlation with cognitive impairments. These findings decrypt operations of the elementary processing units in healthy and Alzheimer's mouse cortices and validate the efficacy of octuple-sexdecuple patch-clamp recordings for architectural reconstruction of complex neuronal circuits.

Oncogenic BRAFV600E induces microglial proliferation through extracellular signal-regulated kinase and neuronal death through c-Jun N-terminal kinase

Activating V600E in v-Raf murine sarcoma viral oncogene homolog B (BRAF) is a common driver mutation in cancers of multiple tissue origins, including melanoma and glioma. BRAFV600E has also been implicated in neurodegeneration. The present study aims to characterize BRAFV600E during cell death and proliferation of three major cell types of the central nervous system: neurons, astrocytes, and microglia. Multiple primary cultures (primary cortical mixed culture) and cell lines of glial cells (BV2) and neurons (SH-SY5Y) were employed. BRAFV600E and BRAFWT expression was mediated by lentivirus or retrovirus. Blockage of downstream effectors (extracellular signal-regulated kinase 1/2 and JNK1/2) were achieved by siRNA. In astrocytes and microglia, BRAFV600E induces cell proliferation, and the proliferative effect in microglia is mediated by activated extracellular signal-regulated kinase, but not c-Jun N-terminal kinase. Conditioned medium from BRAFV600E-expressing microglia induced neuronal death. In neuronal cells, BRAFV600E directly induces neuronal death, through c-Jun N-terminal kinase but not extracellular signal-regulated kinase. We further show that BRAF-related genes are enriched in pathways in patients with Parkinson's disease. Our study identifies distinct consequences mediated by distinct downstream effectors in dividing glial cells and in neurons following the same BRAF mutational activation and a causal link between BRAF-activated microglia and neuronal cell death that does not require physical proximity. It provides insight into a possibly important role of BRAF in neurodegeneration as a result of either dysregulated BRAF in neurons or its impact on glial cells.

Co-dependent regulation of p-BRAF and potassium channel KCNMA1 levels drives glioma progression

BRAF mutations have been found in gliomas which exhibit abnormal electrophysiological activities, implying their potential links with the ion channel functions. In this study, we identified the Drosophila potassium channel, Slowpoke (Slo), the ortholog of human KCNMA1, as a critical factor involved in dRaf^GOF glioma progression. Slo was upregulated in dRaf^GOF glioma. Knockdown of slo led to decreases in dRaf^GOF levels, glioma cell proliferation, and tumor-related phenotypes. Overexpression of slo in glial cells elevated dRaf expression and promoted cell proliferation. Similar mutual regulations of p-BRAF and KCNMA1 levels were then recapitulated in human glioma cells with the BRAF mutation. Elevated p-BRAF and KCNMA1 were also observed in HEK293T cells upon the treatment of 20 mM KCl, which causes membrane depolarization. Knockdown KCNMA1 in these cells led to a further decrease in cell viability. Based on these results, we conclude that the levels of p-BRAF and KCNMA1 are co-dependent and mutually regulated. We propose that, in depolarized glioma cells with BRAF mutations, high KCNMA1 levels act to repolarize membrane potential and facilitate cell growth. Our study provides a new strategy to antagonize the progression of gliomas as induced by BRAF mutations.

The Property-Based Practical Applications and Solutions of Genetically Encoded Acetylcholine and Monoamine Sensors

Neuromodulatory communication among various neurons and non-neuronal cells mediates myriad physiological and pathologic processes, yet defining regulatory and functional features of neuromodulatory transmission remains challenging because of limitations of available monitoring tools. Recently developed genetically encoded neuromodulatory transmitter sensors, when combined with superresolution and/or deconvolution microscopy, allow the first visualization of neuromodulatory transmission with nanoscale or microscale spatiotemporal resolution. In vitro and in vivo experiments have validated several high-performing sensors to have the qualities necessary for demarcating fundamental synaptic properties of neuromodulatory transmission, and initial analysis has unveiled unexpected fine control and precision of neuromodulation. These new findings underscore the importance of synaptic dynamics in synapse-, subcellular-, and circuit-specific neuromodulation, as well as the prospect of genetically encoded transmitter sensors in expanding our knowledge of various behaviors and diseases, including Alzheimer's disease, sleeping disorders, tumorigenesis, and many others.

Organizational principles of amygdalar input-output neuronal circuits

The amygdala, one of the most studied brain structures, integrates brain-wide heterogeneous inputs and governs multidimensional outputs to control diverse behaviors central to survival, yet how amygdalar input-output neuronal circuits are organized remains unclear. Using a simplified cell-type- and projection-specific retrograde transsynaptic tracing technique, we scrutinized brain-wide afferent inputs of four major output neuronal groups in the amygdalar basolateral complex (BLA) that project to the bed nucleus of the stria terminals (BNST), ventral hippocampus (vHPC), medial prefrontal cortex (mPFC) and nucleus accumbens (NAc), respectively. Brain-wide input-output quantitative analysis unveils that BLA efferent neurons receive a diverse array of afferents with varied input weights and predominant contextual representation. Notably, the afferents received by BNST-, vHPC-, mPFC- and NAc-projecting BLA neurons exhibit virtually identical origins and input weights. These results indicate that the organization of amygdalar BLA input-output neuronal circuits follows the input-dependent and output-independent principles, ideal for integrating brain-wide diverse afferent stimuli to control parallel efferent actions. The data provide the objective basis for improving the virtual reality exposure therapy for anxiety disorders and validate the simplified cell-type- and projection-specific retrograde transsynaptic tracing method.

Genetically encoded sensors enable micro- and nano-scopic decoding of transmission in healthy and diseased brains

Neural communication orchestrates a variety of behaviors, yet despite impressive effort, delineating transmission properties of neuromodulatory communication remains a daunting task due to limitations of available monitoring tools. Recently developed genetically encoded neurotransmitter sensors, when combined with superresolution and deconvolution microscopic techniques, enable the first micro- and nano-scopic visualization of neuromodulatory transmission. Here we introduce this image analysis method by presenting its biophysical foundation, practical solutions, biological validation, and broad applicability. The presentation illustrates how the method resolves fundamental synaptic properties of neuromodulatory transmission, and the new data unveil unexpected fine control and precision of rodent and human neuromodulation. The findings raise the prospect of rapid advances in the understanding of neuromodulatory transmission essential for resolving the physiology or pathogenesis of various behaviors and diseases.

BRAFV600E expression in neural progenitors results in a hyperexcitable phenotype in neocortical pyramidal neurons

Somatic mutations have emerged as the likely cause of focal epilepsies associated with developmental malformations and epilepsy-associated glioneuronal tumors (GNT). Somatic BRAFV600E mutations in particular have been detected in the majority of low-grade neuroepithelial tumors (LNETS) and in neurons in focal cortical dysplasias adjacent to epilepsy-associated tumors. Furthermore, conditional expression of an activating BRAF mutation in neocortex causes seizures in mice. In this study we characterized the cellular electrophysiology of layer 2/3 neocortical pyramidal neurons induced to express BRAFV600E from neural progenitor stages. In utero electroporation of a piggyBac transposase plasmid system was used to introduce transgenes expressing BRAF wild type (BRAFwt), BRAFV600E, and/or enhanced green fluorescent protein (eGFP) and monomeric red fluorescent protein (mRFP) into radial glia progenitors in mouse embryonic cortex. Whole cell patch-clamp recordings of pyramidal neurons in slices prepared from both juvenile and adult mice showed that BRAFV600E resulted in neurons with a distinct hyperexcitable phenotype characterized by depolarized resting membrane potentials, increased input resistances, lowered action potential (AP) thresholds, and increased AP firing frequencies. Some of the BRAFV600E-expressing neurons normally destined for upper cortical layers by their birthdate were stalled in their migration and occupied lower cortical layers. BRAFV600E-expressing neurons also displayed increased hyperpolarization-induced inward currents (Ih) and decreased sustained potassium currents. Neurons adjacent to BRAFV600E transgene-expressing neurons, and neurons with TSC1 genetically deleted by CRISPR or those induced to carry PIK3CAE545K transgenes, did not show an excitability phenotype similar to that of BRAFV600E-expressing neurons. Together, these results indicate that BRAFV600E leads to a distinct hyperexcitable neuronal phenotype.NEW & NOTEWORTHY This study is the first to report the cell autonomous effects of BRAFV600E mutations on the intrinsic neuronal excitability. We show that BRAFV600E alters multiple electrophysiological parameters in neocortical neurons. Similar excitability changes did not occur in cells neighboring BRAFV600E-expressing neurons, after overexpression of wild-type BRAF transgenes, or after introduction of mutations affecting the mammalian target of rapamycin (mTOR) or the catalytic subunit of phosphoinositide 3-kinase (PIK3CA). We conclude that BRAFV600E causes a distinct, cell autonomous, highly excitable neuronal phenotype when introduced somatically into neocortical neuronal progenitors.

Getting "Ras"-ults: Solving Molecular Promiscuity through Microdomain-Selective Targeting

In this issue of Neuron, Zhang et al. (2018) report a powerful new method for probing subcellular microdomain-specific signaling in cellular function. Through a microdomain-targeting approach, they delineate how Ras-family GTPases balance signaling diversity with specificity required for various forms of hippocampal synaptic plasticity.

RAS GTPase-dependent pathways in developmental diseases: old guys, new lads, and current challenges

Deregulated RAS signaling is associated with increasing numbers of congenital diseases usually referred to as RASopathies. The spectrum of genes and mutant alleles causing these diseases has been significantly expanded in recent years. This progress has triggered new challenges, including the origin and subsequent selection of the mutations driving these diseases, the specific pathobiological programs triggered by those mutations, the type of correlations that exist between the genotype and the clinical features of patients, and the ancillary genetic factors that influence the severity of the disease in patients. These issues also directly impinge on the feasibility of using RAS pathway drugs to treat RASopathy patients. Here, we will review the main developments and pending challenges in this research topic.

CRISPR-Cas9 human gene replacement and phenomic characterization in Caenorhabditis elegans to understand the functional conservation of human genes and decipher variants of uncertain significance

Our ability to sequence genomes has vastly surpassed our ability to interpret the genetic variation we discover. This presents a major challenge in the clinical setting, where the recent application of whole-exome and whole-genome sequencing has uncovered thousands of genetic variants of uncertain significance. Here, we present a strategy for targeted human gene replacement and phenomic characterization, based on CRISPR-Cas9 genome engineering in the genetic model organism Caenorhabditis elegans, that will facilitate assessment of the functional conservation of human genes and structure-function analysis of disease-associated variants with unprecedented precision. We validate our strategy by demonstrating that direct single-copy replacement of the C. elegans ortholog (daf-18) with the critical human disease-associated gene phosphatase and tensin homolog (PTEN) is sufficient to rescue multiple phenotypic abnormalities caused by complete deletion of daf-18, including complex chemosensory and mechanosensory impairments. In addition, we used our strategy to generate animals harboring a single copy of the known pathogenic lipid phosphatase inactive PTEN variant (PTEN-G129E), and showed that our automated in vivo phenotypic assays could accurately and efficiently classify this missense variant as loss of function. The integrated nature of the human transgenes allows for analysis of both homozygous and heterozygous variants and greatly facilitates high-throughput precision medicine drug screens. By combining genome engineering with rapid and automated phenotypic characterization, our strategy streamlines the identification of novel conserved gene functions in complex sensory and learning phenotypes that can be used as in vivo functional assays to decipher variants of uncertain significance.

A genetically encoded fluorescent acetylcholine indicator for in vitro and in vivo studies

The neurotransmitter acetylcholine (ACh) regulates a diverse array of physiological processes throughout the body. Despite its importance, cholinergic transmission in the majority of tissues and organs remains poorly understood owing primarily to the limitations of available ACh-monitoring techniques. We developed a family of ACh sensors (GACh) based on G-protein-coupled receptors that has the sensitivity, specificity, signal-to-noise ratio, kinetics and photostability suitable for monitoring ACh signals in vitro and in vivo. GACh sensors were validated with transfection, viral and/or transgenic expression in a dozen types of neuronal and non-neuronal cells prepared from multiple animal species. In all preparations, GACh sensors selectively responded to exogenous and/or endogenous ACh with robust fluorescence signals that were captured by epifluorescence, confocal, and/or two-photon microscopy. Moreover, analysis of endogenous ACh release revealed firing-pattern-dependent release and restricted volume transmission, resolving two long-standing questions about central cholinergic transmission. Thus, GACh sensors provide a user-friendly, broadly applicable tool for monitoring cholinergic transmission underlying diverse biological processes.

Age-dependent alterations in expression and co-localization of Pax6 and Ras-GAP in brain of aging mice

As the brain ages, the survival and plasticity of neurons and glia are compromised. The data-mining and in silico studies suggest interactions of Pax6 with Ras and binding sites in Ras-GAP promoter. The Pax6 also shows age-dependent alterations. Therefore, it is presumed that Pax6 may be associated with the Ras-GAP, a synaptic protein, either directly or indirectly in brain. The expression, co-localization and interaction of Pax6 and Ras-GAP in different regions of brain of mice during aging were investigated through immunofluorescence assay, co-immunoprecipitation and western blotting, respectively. The co-localization of Pax6 and Ras-GAP were observed in dentate gyrus (DG) and sub-granular zone (SGZ) of hippocampus, in glomerular (GlLa) and mitral cells (MiCe) of olfactory lobe, granular cells (GrCe), Purkinje cell (PuCe) and molecular cell layer (MoLa) of cerebellum, internal plexiform layer (InPl), molecular layer (MoLa) of cerebral cortex and in intercalated cells of amygdala (ITC), caudate nucleus regions in brain of aging mice. The expression of Pax6 and Ras-GAP was altered in hippocampus, amygdala, caudate nucleus, olfactory lobe, cerebral cortex and cerebellum from young to old mice. The Pax6 interacts with Ras-GAP in brain of mice. Results indicate impact of Pax6 on Ras-GAP-mediated activities of synapses, learning and memory, emotions and fear as well as motor functions. Alterations in expression and co-localization of Pax6 and Ras-GAP during aging may be responsible for age-associated compromised survival and plasticity of neurons and glia.

Ras and Rap Signal Bidirectional Synaptic Plasticity via Distinct Subcellular Microdomains

How signaling molecules achieve signal diversity and specificity is a long-standing cell biology question. Here we report the development of a targeted delivery method that permits specific expression of homologous Ras-family small GTPases (i.e., Ras, Rap2, and Rap1) in different subcellular microdomains, including the endoplasmic reticulum, lipid rafts, bulk membrane, lysosomes, and Golgi complex, in rodent hippocampal CA1 neurons. The microdomain-targeted delivery, combined with multicolor fluorescence protein tagging and high-resolution dual-quintuple simultaneous patch-clamp recordings, allows systematic analysis of microdomain-specific signaling. The analysis shows that Ras signals long-term potentiation via endoplasmic reticulum PI3K and lipid raft ERK, whereas Rap2 and Rap1 signal depotentiation and long-term depression via bulk membrane JNK and lysosome p38MAPK, respectively. These results establish an effective subcellular microdomain-specific targeted delivery method and unveil subcellular microdomain-specific signaling as the mechanism for homologous Ras and Rap to achieve signal diversity and specificity to control multiple forms of synaptic plasticity.

Piconewton-Scale Analysis of Ras-BRaf Signal Transduction with Single-Molecule Force Spectroscopy

Intermolecular interactions dominate the behavior of signal transduction in various physiological and pathological cell processes, yet assessing these interactions remains a challenging task. Here, this study reports a single-molecule force spectroscopic method that enables functional delineation of two interaction sites (≈35 pN and ≈90 pN) between signaling effectors Ras and BRaf in the canonical mitogen-activated protein kinase (MAPK) pathway. This analysis reveals mutations on BRaf at Q257 and A246, two sites frequently linked to cardio-faciocutaneous syndrome, result in ≈10-30 pN alterations in RasBRaf intermolecular binding force. The magnitude of changes in RasBRaf binding force correlates with the size of alterations in protein affinity and in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-sensitive glutamate receptor (-R)-mediated synaptic transmission in neurons expressing replacement BRaf mutants, and predicts the extent of learning impairments in animals expressing replacement BRaf mutants. These results establish single-molecule force spectroscopy as an effective platform for evaluating the piconewton-level interaction of signaling molecules and predicting the behavior outcome of signal transduction.