Striosome-dendron bouquets highlight a unique striatonigral circuit targeting dopamine-containing neurons.

The dopamine systems of the brain powerfully influence movement and motivation. We demonstrate that striatonigral fibers originating in striosomes form highly unusual bouquet-like arborizations that target bundles of ventrally extending dopamine-containing dendrites and clusters of their parent nigral cell bodies. Retrograde tracing showed that these clustered cell bodies in turn project to the striatum as part of the classic nigrostriatal pathway. Thus, these striosome-dendron formations, here termed "striosome-dendron bouquets," likely represent subsystems with the nigro-striato-nigral loop that are affected in human disorders including Parkinson's disease. Within the bouquets, expansion microscopy resolved many individual striosomal fibers tightly intertwined with the dopamine-containing dendrites and also with afferents labeled by glutamatergic, GABAergic, and cholinergic markers and markers for astrocytic cells and fibers and connexin 43 puncta. We suggest that the striosome-dendron bouquets form specialized integrative units within the dopamine-containing nigral system. Given evidence that striosomes receive input from cortical regions related to the control of mood and motivation and that they link functionally to reinforcement and decision-making, the striosome-dendron bouquets could be critical to dopamine-related function in health and disease.

Optogenetic Mapping of Intracortical Circuits Originating from Semilunar Cells in the Piriform Cortex.

Despite its comparatively simple trilaminar architecture, the primary olfactory (piriform) cortex of mammals is capable of performing sophisticated sensory processing, an ability that is thought to depend critically on its extensive associational (intracortical) excitatory circuits. Here, we used a novel transgenic mouse model and optogenetics to measure the connectivity of associational circuits that originate in semilunar (SL) cells in layer 2a of the anterior piriform cortex (aPC). We generated a mouse line (48L) in which channelrhodopsin-2 (ChR) could be selectively expressed in a subset of SL cells. Light-evoked excitatory postsynaptic currents (EPSCs) could be evoked in superficial pyramidal cells (17.4% of n = 86 neurons) and deep pyramidal cells (33.3%, n = 9) in the aPC, but never in ChR- SL cells (0%, n = 34). Thus, SL cells monosynaptically excite pyramidal cells, but not other SL cells. Light-evoked EPSCs were also selectively elicited in 3 classes of GABAergic interneurons in layer 3 of the aPC. Our results show that SL cells are specialized for providing feedforward excitation of specific classes of neurons in the aPC, confirming that SL cells comprise a functionally distinctive input layer.

Dicer maintains the identity and function of proprioceptive sensory neurons.

Neuronal cell identity is established during development and must be maintained throughout an animal's life (Fishell G, Heintz N. Neuron 80: 602-612, 2013). Transcription factors critical for establishing neuronal identity can be required for maintaining it (Deneris ES, Hobert O. Nat Neurosci 17: 899-907, 2014). Posttranscriptional regulation also plays an important role in neuronal differentiation (Bian S, Sun T. Mol Neurobiol 44: 359-373, 2011), but its role in maintaining cell identity is less established. To better understand how posttranscriptional regulation might contribute to cell identity, we examined the proprioceptive neurons in the dorsal root ganglion (DRG), a highly specialized sensory neuron class, with well-established properties that distinguish them from other neurons in the ganglion. By conditionally ablating Dicer in mice, using parvalbumin (Pvalb)-driven Cre recombinase, we impaired posttranscriptional regulation in the proprioceptive sensory neuron population. Knockout (KO) animals display a progressive form of ataxia at the beginning of the fourth postnatal week that is accompanied by a cell death within the DRG. Before cell loss, expression profiling shows a reduction of proprioceptor specific genes and an increased expression of nonproprioceptive genes normally enriched in other ganglion neurons. Furthermore, although central connections of these neurons are intact, the peripheral connections to the muscle are functionally impaired. Posttranscriptional regulation is therefore necessary to retain the transcriptional identity and support functional specialization of the proprioceptive sensory neurons.NEW & NOTEWORTHY We have demonstrated that selectively impairing Dicer in parvalbumin-positive neurons, which include the proprioceptors, triggers behavioral changes, a lack of muscle connectivity, and a loss of transcriptional identity as observed through RNA sequencing. These results suggest that Dicer and, most likely by extension, microRNAs are crucially important for maintaining proprioception. Additionally, this study hints at the larger question of how neurons maintain their functional and molecular specificity.

Distinctiveness and continuity in transcriptome and connectivity in the anterior-posterior axis of the paraventricular nucleus of the thalamus.

The paraventricular nucleus of the thalamus (PVT) projects axons to multiple areas, mediates a wide range of behaviors, and exhibits regional heterogeneity in both functions and axonal projections. Still, questions regarding the cell types present in the PVT and the extent of their differences remain inadequately addressed. We applied single-cell RNA sequencing to depict the transcriptomic characteristics of mouse PVT neurons. We found that one of the most significant variances in the PVT transcriptome corresponded to the anterior-posterior axis. While the single-cell transcriptome classified PVT neurons into five types, our transcriptomic and histological analyses showed continuity among the cell types. We discovered that anterior and posterior subpopulations had nearly non-overlapping projection patterns, while another population showed intermediate patterns. In addition, these subpopulations responded differently to appetite-related neuropeptides, with their activation showing opposing effects on food consumption. Our studies unveiled the contrasts and the continuity of PVT neurons that underpin their function.

Increased PDGFRB and NF-κB signaling caused by highly prevalent somatic mutations in intracranial aneurysms.

Intracranial aneurysms (IAs) are a high-risk factor for life-threatening subarachnoid hemorrhage. Their etiology, however, remains mostly unknown at present. We conducted screening for sporadic somatic mutations in 65 IA tissues (54 saccular and 11 fusiform aneurysms) and paired blood samples by whole-exome and targeted deep sequencing. We identified sporadic mutations in multiple signaling genes and examined their impact on downstream signaling pathways and gene expression in vitro and an arterial dilatation model in mice in vivo. We identified 16 genes that were mutated in at least one IA case and found that these mutations were highly prevalent (92%: 60 of 65 IAs) among all IA cases examined. In particular, mutations in six genes (PDGFRB, AHNAK, OBSCN, RBM10, CACNA1E, and OR5P3), many of which are linked to NF-κB signaling, were found in both fusiform and saccular IAs at a high prevalence (43% of all IA cases examined). We found that mutant PDGFRBs constitutively activated ERK and NF-κB signaling, enhanced cell motility, and induced inflammation-related gene expression in vitro. Spatial transcriptomics also detected similar changes in vessels from patients with IA. Furthermore, virus-mediated overexpression of a mutant PDGFRB induced a fusiform-like dilatation of the basilar artery in mice, which was blocked by systemic administration of the tyrosine kinase inhibitor sunitinib. Collectively, this study reveals a high prevalence of somatic mutations in NF-κB signaling pathway-related genes in both fusiform and saccular IAs and opens a new avenue of research for developing pharmacological interventions.

Opposing roles in neurite growth control by two seven-pass transmembrane cadherins.

The growth of neurites (axon and dendrite) should be appropriately regulated by their interactions in the development of nervous systems where a myriad of neurons and their neurites are tightly packed. We show here that mammalian seven-pass transmembrane cadherins Celsr2 and Celsr3 are activated by their homophilic interactions and regulate neurite growth in an opposing manner. Both gene-silencing and coculture assay with rat neuron cultures showed that Celsr2 enhanced neurite growth, whereas Celsr3 suppressed it, and that their opposite functions were most likely the result of a difference of a single amino acid residue in the transmembrane domain. Together with calcium imaging and pharmacological analyses, our results suggest that Celsr2 and Celsr3 fulfill their functions through second messengers, and that differences in the activities of the homologs results in opposite effects in neurite growth regulation.

Deletion of Stk11 and Fos in mouse BLA projection neurons alters intrinsic excitability and impairs formation of long-term aversive memory.

Conditioned taste aversion (CTA) is a form of one-trial learning dependent on basolateral amygdala projection neurons (BLApn). Its underlying cellular and molecular mechanisms remain poorly understood. RNAseq from BLApn identified changes in multiple candidate learning-related transcripts including the expected immediate early gene Fos and Stk11, a master kinase of the AMP-related kinase pathway with important roles in growth, metabolism and development, but not previously implicated in learning. Deletion of Stk11 in BLApn blocked memory prior to training, but not following it and increased neuronal excitability. Conversely, BLApn had reduced excitability following CTA. BLApn knockout of a second learning-related gene, Fos, also increased excitability and impaired learning. Independently increasing BLApn excitability chemogenetically during CTA also impaired memory. STK11 and C-FOS activation were independent of one another. These data suggest key roles for Stk11 and Fos in CTA long-term memory formation, dependent at least partly through convergent action on BLApn intrinsic excitability.

Regulation of dendritic maintenance and growth by a mammalian 7-pass transmembrane cadherin.

Drosophila Flamingo is a 7-pass transmembrane cadherin that is necessary for dendritic patterning and axon guidance. How it works at the molecular level and whether homologs of Flamingo play similar roles in mammalian neurons or not have been unanswered questions. Here, we performed loss-of-function analysis using an RNAi system and organotypic brain slice cultures to address the role of a mammalian Flamingo homolog, Celsr2. Knocking down Celsr2 resulted in prominent simplification of dendritic arbors of cortical pyramidal neurons and Purkinje neurons, and this phenotype seemed to be due to branch retraction. Cadherin domain-mediated homophilic interaction appears to be required for the maintenance of dendritic branches. Furthermore, expression of various Celsr2 forms elicited distinct responses that were dependent on an extracellular subregion outside the cadherin domains and on a portion within the carboxyl intracellular tail. Based on these findings, we discuss how Celsr2 may regulate dendritic maintenance and growth.

ATAC-seq on Sorted Adult Mouse Neurons.

Transcription regulation is a key aspect of cellular identity established during development and maintained into adulthood. Molecular and biochemical assays that probe the genome are critical tools in exploring mechanisms of transcription regulation and cell type identity. The mammalian brain is composed of a huge diversity of cell types with distinct properties and functions. To understand these specific roles, it is necessary to selectively target cell populations for study. However, the need to selectively study restricted cell populations poses a challenge in neurobiology. It is often difficult to collect sufficient cellular input for many standard biochemical and molecular assays. Recently, important advances have been made to scale assays down, opening up new frontiers to explore molecular mechanisms in neurons. Concurrently, methodologies for preparing neurons for such assays has advanced taking into consideration specific methods to preserve the cell biology meant to be assayed. Here we describe a method for preparing live neurons from adult brain tissue for the Assay for Transposase Accessible Chromatin (ATAC).

Mapping the transcriptional diversity of genetically and anatomically defined cell populations in the mouse brain.

Understanding the principles governing neuronal diversity is a fundamental goal for neuroscience. Here, we provide an anatomical and transcriptomic database of nearly 200 genetically identified cell populations. By separately analyzing the robustness and pattern of expression differences across these cell populations, we identify two gene classes contributing distinctly to neuronal diversity. Short homeobox transcription factors distinguish neuronal populations combinatorially, and exhibit extremely low transcriptional noise, enabling highly robust expression differences. Long neuronal effector genes, such as channels and cell adhesion molecules, contribute disproportionately to neuronal diversity, based on their patterns rather than robustness of expression differences. By linking transcriptional identity to genetic strains and anatomical atlases, we provide an extensive resource for further investigation of mouse neuronal cell types.

Slingshot-3 dephosphorylates ADF/cofilin but is dispensable for mouse development.

Actin-depolymerizing factor (ADF) and cofilin constitute a family of key regulators of actin filament dynamics. ADF/cofilin is inactivated by phosphorylation at Ser-3 by LIM-kinases and reactivated by dephosphorylation by Slingshot (SSH) family phosphatases. Defects in LIM kinases or ADF/cofilin have been implicated in morbidity in human or mice; however, the roles of mammalian SSH in vivo have not been addressed. In this study, we examined the endogenous expression of each mouse SSH member in various cell lines and tissues, and showed that SSH-3L protein was strongly expressed in epithelial cells. Our structure-function analysis of SSH-3L suggested the possibility that the C-tail unique to SSH-3L negatively regulates the catalytic activity of this phosphatase. Furthermore we made ssh-3 knockout mice to examine its potential in vivo roles. Unexpectedly, ssh-3 was not essential for viability, fertility, or development of epithelial tissues; and ssh-3 did not genetically modify the corneal disorder of the corn1/ADF/destrin mutant.

Distinct projection patterns of different classes of layer 2 principal neurons in the olfactory cortex.

The broadly-distributed, non-topographic projections to and from the olfactory cortex may suggest a flat, non-hierarchical organization in odor information processing. Layer 2 principal neurons in the anterior piriform cortex (APC) can be divided into 2 subtypes: semilunar (SL) and superficial pyramidal (SP) cells. Although it is known that SL and SP cells receive differential inputs from the olfactory bulb (OB), little is known about their projections to other olfactory regions. Here, we examined axonal projections of SL and SP cells using a combination of mouse genetics and retrograde labeling. Retrograde tracing from the OB or posterior piriform cortex (PPC) showed that the APC projects to these brain regions mainly through layer 2b cells, and dual-labeling revealed many cells extending collaterals to both target regions. Furthermore, a transgenic mouse line specifically labeling SL cells showed that they send profuse axonal projections to olfactory cortical areas, but not to the OB. These findings support a model in which information flow from SL to SP cells and back to the OB is mediated by a hierarchical feedback circuit, whereas both SL and SP cells broadcast information to higher olfactory areas in a parallel manner.

A Mammalian enhancer trap resource for discovering and manipulating neuronal cell types.

There is a continuing need for driver strains to enable cell-type-specific manipulation in the nervous system. Each cell type expresses a unique set of genes, and recapitulating expression of marker genes by BAC transgenesis or knock-in has generated useful transgenic mouse lines. However, since genes are often expressed in many cell types, many of these lines have relatively broad expression patterns. We report an alternative transgenic approach capturing distal enhancers for more focused expression. We identified an enhancer trap probe often producing restricted reporter expression and developed efficient enhancer trap screening with the PiggyBac transposon. We established more than 200 lines and found many lines that label small subsets of neurons in brain substructures, including known and novel cell types. Images and other information about each line are available online (enhancertrap.bio.brandeis.edu).

Upregulation of µ3A Drives Homeostatic Plasticity by Rerouting AMPAR into the Recycling Endosomal Pathway.

Synaptic scaling is a form of homeostatic plasticity driven by transcription-dependent changes in AMPA-type glutamate receptor (AMPAR) trafficking. To uncover the pathways involved, we performed a cell-type-specific screen for transcripts persistently altered during scaling, which identified the µ subunit (µ3A) of the adaptor protein complex AP-3A. Synaptic scaling increased µ3A (but not other AP-3 subunits) in pyramidal neurons and redistributed dendritic µ3A and AMPAR to recycling endosomes (REs). Knockdown of µ3A prevented synaptic scaling and this redistribution, while overexpression (OE) of full-length µ3A or a truncated µ3A that cannot interact with the AP-3A complex was sufficient to drive AMPAR to REs. Finally, OE of µ3A acted synergistically with GRIP1 to recruit AMPAR to the dendritic membrane. These data suggest that excess µ3A acts independently of the AP-3A complex to reroute AMPAR to RE, generating a reservoir of receptors essential for the regulated recruitment to the synaptic membrane during scaling up.

Convergence of pontine and proprioceptive streams onto multimodal cerebellar granule cells.

Cerebellar granule cells constitute the majority of neurons in the brain and are the primary conveyors of sensory and motor-related mossy fiber information to Purkinje cells. The functional capability of the cerebellum hinges on whether individual granule cells receive mossy fiber inputs from multiple precerebellar nuclei or are instead unimodal; this distinction is unresolved. Using cell-type-specific projection mapping with synaptic resolution, we observed the convergence of separate sensory (upper body proprioceptive) and basilar pontine pathways onto individual granule cells and mapped this convergence across cerebellar cortex. These findings inform the long-standing debate about the multimodality of mammalian granule cells and substantiate their associative capacity predicted in the Marr-Albus theory of cerebellar function. We also provide evidence that the convergent basilar pontine pathways carry corollary discharges from upper body motor cortical areas. Such merging of related corollary and sensory streams is a critical component of circuit models of predictive motor control. DOI:http://dx.doi.org/10.7554/eLife.00400.001.

A resource of Cre driver lines for genetic targeting of GABAergic neurons in cerebral cortex.

A key obstacle to understanding neural circuits in the cerebral cortex is that of unraveling the diversity of GABAergic interneurons. This diversity poses general questions for neural circuit analysis: how are these interneuron cell types generated and assembled into stereotyped local circuits and how do they differentially contribute to circuit operations that underlie cortical functions ranging from perception to cognition? Using genetic engineering in mice, we have generated and characterized approximately 20 Cre and inducible CreER knockin driver lines that reliably target major classes and lineages of GABAergic neurons. More select populations are captured by intersection of Cre and Flp drivers. Genetic targeting allows reliable identification, monitoring, and manipulation of cortical GABAergic neurons, thereby enabling a systematic and comprehensive analysis from cell fate specification, migration, and connectivity, to their functions in network dynamics and behavior. As such, this approach will accelerate the study of GABAergic circuits throughout the mammalian brain.

Expression, identification and purification of Dictyostelium acetoacetyl-coa thiolase expressed in Escherichia coli.

Acetoacetyl-CoA thiolase (AT) is an enzyme that catalyses the CoA-dependent thiolytic cleavage of acetoacetyl-CoA to yield 2 molecules of acetyl-CoA, or the reverse condensation reaction. A full-length cDNA clone pBSGT-3, which has homology to known thiolases, was isolated from Dictyostelium cDNA library. Expression of the protein encoded in pBSGT-3 in Escherichia coli, its thiolase enzyme activity, and the amino acid sequence homology search revealed that pBSGT-3 encodes an AT. The recombinant AT (r-thiolase) was expressed in an active form in an E. coli expression system, and purified to homogeneity by selective ammonium sulfate fractionation and two steps of column chromatography. The purified enzyme exhibited a specific activity of 4.70 mU/mg protein. Its N-terminal sequence was (NH2)-Arg-Met-Tyr-Thr-Thr-Ala-Lys-Asn-Leu-Glu-, which corresponds to the sequence from positions 15 to 24 of the amino acid sequence deduced from pBSGT-3 clone. The r-thiolase in the inclusion body expressed highly in E. coli was the precursor form, which is slightly larger than the purified r-thiolase. When incubated with the cell-free extract of Dictyostelium cells, the precursor was converted to the same size to the purified r-thiolase, suggesting that the presequence at the N-terminus is removed by a Dictyostelium processing peptidase.

Differential expression of the seven-pass transmembrane cadherin genes Celsr1-3 and distribution of the Celsr2 protein during mouse development.

Drosophila Flamingo (Fmi) is an evolutionally conserved seven-pass transmembrane receptor of the cadherin superfamily. Fmi plays multiple roles in patterning neuronal processes and epithelial planar cell polarity. To explore the in vivo roles of Fmi homologs in mammals, we previously cloned one of the mouse homologs, mouse flamingo1/Celsr2. Here, we report the results of our study of its embryonic and postnatal expression patterns together with those of two other paralogs, Celsr1 and Celsr3. Celsr1-3 expression was initiated broadly in the nervous system at early developmental stages, and each paralog showed characteristic expression patterns in the developing CNS. These genes were also expressed in several other organs, including the cochlea, where hair cells develop planar polarity, the kidney, and the whisker. The Celsr2 protein was distributed at intercellular boundaries in the whisker and on processes of neuronal cells such as hippocampal pyramidal cells, Purkinje cells, and olfactory neurons. Celsr2 is mapped to a distal region of the mouse chromosome 3. We discussed possible functions of seven-pass transmembrane cadherins in mouse development.

Differential activities, subcellular distribution and tissue expression patterns of three members of Slingshot family phosphatases that dephosphorylate cofilin.

BACKGROUND: Cofilin, a key regulator of actin filament dynamics, is inactivated by phosphorylation at Ser-3 by LIM-kinases and is reactivated by dephosphorylation by a family of protein phosphatases, termed Slingshot (SSH). RESULTS: We have identified two novel isoforms of SSHs, termed SSH-2L and SSH-3L and characterized them in comparison with SSH-1L that was previously reported. SSH-1L and SSH-2L, but not SSH-3L, tightly bound to and co-localized with actin filaments. When expressed in cultured cells, SSH-1L, SSH-2L and SSH-3L decreased the level of Ser-3-phosphorylated cofilin (P-cofilin) in cells and suppressed LIM-kinase-induced actin reorganization, although SSH-3L was less effective than SSH-1L and SSH-2L. In cell-free assays, SSH-1L and SSH-2L efficiently dephosphorylated P-cofilin, whereas SSH-3L did do so only weakly. Using deleted mutants of SSH-1L and SSH-2L, we found that the N-terminal and C-terminal extracatalytic regions are critical for cofilin-phosphatase and F-actin-binding activities, respectively. In situ hybridization analyses revealed characteristic patterns of expression of each of the mouse Ssh genes in both neuronal and non-neuronal tissues; in particular, expression of Ssh-3 in epithelial tissues is evident. CONCLUSION: SSH-1L, SSH-2L and SSH-3L have the potential to dephosphorylate P-cofilin, but subcellular distribution, F-actin-binding activity, specific phosphatase activity and expression patterns significantly differ, which suggests that they have related but distinct functions in various cellular and developmental events.