Sensory perception is not a simple feed-forward process and higher brain areas can actively modulate information processing in “lower” areas. We used optogenetic methods to examine how cortical feedback projections affect circuits in the first olfactory processing stage, the olfactory bulb. Selective activation of back projections from the anterior olfactory nucleus/cortex (AON) revealed functional glutamatergic synaptic connections on several types of bulbar interneurons. Unexpectedly, AON axons also directly depolarized mitral cells, enough to elicit spikes reliably in a time window of a few milliseconds. Mitral cells received strong disynaptic inhibition, a third of which arises in the glomerular layer. Activating feedback axons in vivo led to suppression of spontaneous as well as odor-evoked activity of mitral cells, sometimes preceded by a temporally-precise increase in firing probability. Our study indicates that cortical feedback can shape the activity of bulbar output neurons by enabling precisely timed spikes and enforcing broad inhibition to suppress background activity.
Although TAM receptor tyrosine kinases play key roles in immune regulation, cancer metastasis, and viral infection, the relative importance of the two TAM ligands – Gas6 and Protein S – has yet to be resolved in any setting in vivo. We have now performed a genetic dissection of ligand function in the retina, where the TAM receptor Mer is required for the circadian phagocytosis of photoreceptor outer segments by retinal pigment epithelial cells. This process is severely attenuated in Mer mutant mice, which leads to photoreceptor death. We find that retinal deletion of either Gas6 or Protein S alone yields retinae with a normal number of photoreceptors. However, concerted deletion of both ligands fully reproduces the photoreceptor death seen in Mer mutants. These results demonstrate that Protein S and Gas6 function as independent, bona fide Mer ligands, and are, to a first approximation, interchangeable with respect to Mer-driven phagocytosis in the retina.
Mertk; phagocytosis; retinal pigment epithelium; Gas6; Protein S; retinal degeneration
Although excitatory mossy cells of the hippocampal hilar region are known to project both to dentate granule cells and to interneurons, it is as yet unclear whether mossy cell activity’s net effect on granule cells is excitatory or inhibitory. To explore their influence on dentate excitability and hippocampal function, we generated a conditional transgenic mouse line, using the Cre/loxP system, in which diphtheria toxin receptor was selectively expressed in mossy cells. One week after injecting toxin into this line, mossy cells throughout the longitudinal axis were degenerated extensively, theta wave power of dentate local field potentials increased during exploration, and deficits occurred in contextual discrimination. By contrast, we detected no epileptiform activity, spontaneous behavioral seizures, or mossy-fiber sprouting 5–6 weeks after mossy cell degeneration. These results indicate that the net effect of mossy cell excitation is to inhibit granule cell activity and enable dentate pattern separation.
Cre recombinase; dentate excitability; diphtheria toxin; genetic cell ablation; inducible transgenic mice; mossy fiber sprouting; pattern separation; temporal lobe epilepsy
Muscle synergies have been proposed as a mechanism to simplify movement control. Whether these coactivation patterns have any physiological reality within the nervous system remains unknown. Here we applied electrical microstimulation to motor cortical areas of rhesus macaques to evoke hand movements. Movements tended to converge towards particular postures, driven by synchronous bursts of muscle activity. Across stimulation sites, the muscle activations were reducible to linear sums of a few basic patterns—each corresponding to a muscle synergy evident in voluntary reach, grasp, and transport movements made by the animal. These synergies were represented non-uniformly over the cortical surface. We argue that the brain exploits these properties of synergies—postural equivalence, low dimensionality, and topographical representation—to simplify motor planning, even for complex hand movements.
Humans can see and name thousands of distinct object and action categories, so it is unlikely that each category is represented in a distinct brain area. A more efficient scheme would be to represent categories as locations in a continuous semantic space mapped smoothly across the cortical surface. To search for such a space, we used functional magnetic resonance imaging (fMRI) to measure human brain activity evoked by natural movies. We then used voxel-wise models to examine the cortical representation of 1705 object and action categories. The first few dimensions of the underlying semantic space were recovered from the fit models by principal components analysis. Projection of the recovered semantic space onto cortical flat maps shows that semantic selectivity is organized into smooth gradients that cover much of visual and non-visual cortex. Furthermore, both the recovered semantic space and the cortical organization of the space are shared across different individuals.
fMRI; vision; category; semantic; regression; natural stimuli
Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time-scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.
Gain fields, the eye-position modulation of visual responses, are thought to provide a mechanism by which the motor system can accurately calculate target position in space despite a constantly moving eye. Current gain-field models assume that the modulation of visual responses by eye position is accurate at all times, even around the time of a saccade. Here we show that for at least 150 ms after a saccade, gain fields in the lateral intraparietal area (LIP) are unreliable. The majority of LIP cells with steady-state gain fields reflect the presaccadic eye position. The remainder of the cells have responses that cannot be predicted by their steady-state gain fields. Nonetheless, a monkey’s oculomotor performance is accurate during this time. These results suggest that current models built upon a simple gain-field algorithm cannot be used to calculate the position of a target in space that flashes briefly after a saccade.
M-type K+ channels, encoded by the KCNQ2-5 (Kv7) gene family, play key roles in regulation of neuronal excitability; however, less is known about the mechanisms controlling their transcriptional expression. Here, we discovered a novel mechanism regulating KCNQ2/3 transcriptional expression by neuronal activity in rodent neurons, involving activation of calcineurin and Nuclear Factor of Activated T-cells (NFAT) transcription factors, orchestrated by A-kinase-anchoring protein (AKAP)79/150. The signal requires Ca2+ influx through L-type Ca2+ channels and both local and global Ca2+ elevations. We postulate increased M-channel expression to act as a negative-feedback to suppress hyper-excitability of neurons, demonstrated by profoundly up-regulated KCNQ2/3 transcription in hippocampi from wild-type mice after drug-induced seizures, an effect nearly eliminated in AKAP150−/− mice. Thus, we suggest a distinct role of AKAP79/150 and the complex it organizes in activity-dependent M-channel transcription, which may potentially serve throughout the nervous system to limit over-excitability associated with disease states such as epilepsy.
K+ channels; transcription; calcineurin; nuclear factor of elevated T-cells; Ca2+ signals; epilepsy
Olfactory cortex pyramidal cells integrate sensory input from olfactory bulb mitral and tufted (M/T) cells and project axons back to the bulb. However, the impact of cortical feedback projections on olfactory bulb circuits is unclear. Here, we selectively express channelrhodopsin-2 in olfactory cortex pyramidal cells and show that cortical feedback projections excite diverse populations of bulb interneurons. Activation of cortical fibers directly excites GABAergic granule cells, which in turn inhibit M/T cells. However, we show that cortical inputs preferentially target short axon cells that drive feedforward inhibition of granule cells. In vivo, activation of olfactory cortex that only weakly affects spontaneous M/T cell firing strongly gates odor-evoked M/T cell responses: cortical activity suppresses odor-evoked excitation and enhances odor-evoked inhibition. Together, these results indicate that although cortical projections have diverse actions on olfactory bulb microcircuits, the net effect of cortical feedback on M/T cells is an amplification of odor-evoked inhibition.
Brain function is shaped by postnatal experience and vulnerable to disruption of Methyl-CpG-binding protein, Mecp2, in multiple neurodevelopmental disorders. How Mecp2 contributes to the experience-dependent refinement of specific cortical circuits and their impairment remains unknown. We analyzed vision in gene-targeted mice and observed an initial normal development in the absence of Mecp2. Visual acuity then rapidly regressed after postnatal day P35–40 and cortical circuits largely fell silent by P55-60. Enhanced inhibitory gating and an excess of parvalbumin-positive, perisomatic input preceded the loss of vision. Both cortical function and inhibitory hyperconnectivity were strikingly rescued independent of Mecp2 by early sensory deprivation or genetic deletion of the excitatory NMDA receptor subunit, NR2A. Thus, vision is a sensitive biomarker of progressive cortical dysfunction and may guide novel, circuit-based therapies for Mecp2 deficiency.
Research during the past decade has seen significant progress toward a model for the genetic architecture of autism spectrum disorders (ASD), with gene discovery accelerating as the characterization of genomic variation has become increasingly comprehensive. At the same time this research has highlighted ongoing challenges. Here we address the enormous impact of high throughput sequencing (HTS) on ASD gene discovery, outline a consensus view for leveraging this technology, and describe a large multi-site collaboration developed to accomplish these goals. Similar approaches could prove effective for severe neurodevelopmental disorders more broadly.
Neural circuits are shaped by activity-dependent elimination of redundant synapses during postnatal development. In many systems, postsynaptic activity is known to be crucial, but the precise mechanisms remain elusive. Here we report that the immediate early gene Arc/Arg3.1 mediates elimination of surplus climbing fiber (CF) to Purkinje cell (PC) synapses in the developing cerebellum. CF synapse elimination was accelerated when activity of channelrhodopsin-2 expressing PCs was elevated by 2-day photo-stimulation. This acceleration was suppressed by PC-specific knockdown of either P/Q-type voltage-dependent Ca2+ channel (VDCC) or Arc. PC-specific Arc knockdown had no appreciable effect until around postnatal day 11, but significantly impaired CF synapse elimination thereafter, leaving redundant CF terminals on PC somata. The effect of Arc knockdown was occluded by simultaneous knockdown of P/Q-type VDCC in PCs. We conclude that Arc mediates the final stage of CF synapse elimination downstream of P/Q-type VDCC by removing CF synapses from PC somata.
Newly generated neuroblasts from the subventricular zone of the adult brain migrate as neuronal chains within a network of astroglial tubes in the rostral migratory stream. This highly directed, rapid migration channels new neurons to the olfactory bulb. In this issue of Neuron, Kaneko et al. demonstrate that migrating neurons dynamically remodel the morphology and organization of astroglial tubes to promote long distance, directional migration of neurons in the adult brain.
In this study a K+ current, IKx, in isolated salamander rod photoreceptors was characterized and its role in shaping small photovoltages was examined. IKx is a standing outward current of about 40 pA at –30 mV that deactivates slowly when the cell is hyperpolarized (τmax = 0.25 s). The voltage and time dependence of IKx are similar to that of M-current, but IKx can be distinguished from M-current because it is not suppressed by acetylcholine and is “blocked” by external Ba2+ in a surprising manner: the activation range of IKx is shifted strongly in the positive direction. Using current-clamp recordings and a computer simulation of the photoresponse, we show that IKx figures prominently in setting the dark resting potential and accelerates the voltage response to small photocurrents.
The biochemical means through which multiple signaling pathways are integrated in navigating axons is poorly understood. Semaphorins are among the largest families of axon guidance cues and utilize Plexin (Plex) receptors to exert repulsive effects on axon extension. However, Semaphorin repulsion can be turned-off by other distinct cues and signaling cascades, raising questions of the logic underlying these events. We now uncover a simple biochemical switch that controls Semaphorin/Plexin repulsive guidance. Plexins are Ras family GTPase activating proteins (GAPs) and we find that the PlexA GAP domain is phosphorylated by the cAMP-dependent protein kinase (PKA). This PlexA phosphorylation generates a specific binding site for 14-3-3ε, a phospho-binding protein that we find to be necessary for axon guidance. These PKA-mediated Plexin-14-3-3ε interactions prevent PlexA from interacting with its Ras family GTPase substrate and antagonize Semaphorin repulsion. Our results indicate that these interactions switch repulsion to adhesion and identify a point of convergence for multiple guidance molecules.
How are sensory representations in the brain influenced by the state of an animal? Here we use chronic two-photon calcium imaging to explore how wakefulness and experience shape odor representations in the mouse olfactory bulb. Comparing the awake and anesthetized state, we show that wakefulness greatly enhances the activity of inhibitory granule cells and makes principal mitral cell odor responses more sparse and temporally dynamic. In awake mice, brief repeated odor experience leads to a gradual and long-lasting (months) weakening of mitral cell odor representations. This mitral cell plasticity is odor-specific, recovers gradually over months and can be repeated with different odors. Furthermore, the expression of this experience-dependent plasticity is prevented by anesthesia. Together, our results demonstrate the dynamic nature of mitral cell odor representations in awake animals, which is constantly shaped by recent odor experience.
Microtubule nucleation is essential for proper establishment and maintenance of axons and dendrites. Centrosomes, the primary site of nucleation in most cells, lose their function as microtubule organizing centers during neuronal development. How neurons generate acentrosomal microtubules remains unclear. Drosophila dendritic arborization (da) neurons lack centrosomes and therefore provide a model system to study acentrosomal microtubule nucleation. Here we investigate the origin of microtubules within the elaborate dendritic arbor of class IV da neurons. Using a combination of in vivo and in vitro techniques, we find that Golgi outposts can directly nucleate microtubules throughout the arbor. This acentrosomal nucleation requires gamma-tubulin and CP309, the Drosophila homologue of AKAP450, and contributes to the complex microtubule organization within the arbor, and dendrite branch growth and stability. Together, these results identify the first direct mechanism for acentrosomal microtubule nucleation within neurons, and reveal a previously unknown function for Golgi outposts in this process.
Postrhinal cortex, the rodent homolog of the primate parahippocampal cortex, processes spatial and contextual information. Our hypothesis of postrhinal function is that it serves to encode context, in part, by forming representations that link objects to places. We recorded postrhinal neuronal activity and local field potentials (LFPs) in rats trained on a two-choice, visual discrimination task. As predicted, a large proportion of postrhinal neurons signaled object-location conjunctions. In addition, postrhinal LFPs exhibited strong oscillatory rhythms in the theta band, and many postrhinal neurons were phase locked to theta. Although correlated with running speed, theta power was lower than predicted by speed alone immediately before and after choice. However, theta power was significantly increased following incorrect decisions, suggesting a role in signaling error. These findings provide evidence that postrhinal cortex encodes representations that link objects to places and suggest that postrhinal theta modulation extends to cognitive as well as spatial functions.
Brain networks are commonly defined using correlations between blood oxygen level-dependent (BOLD) signals in different brain areas. Although evidence suggests that gamma band (30–100 Hz) neural activity contributes to local BOLD signals, the neural basis of inter-areal BOLD correlations is unclear. We first defined a visual network in monkeys based on converging evidence from inter-areal BOLD correlations during a fixation task, task-free state and anesthesia, and then simultaneously recorded local field potentials (LFPs) from the same four network areas in the task-free state. Low frequency oscillations (< 20 Hz), and not gamma activity, predominantly contributed to inter-areal BOLD correlations. The low frequency oscillations also influenced local processing by modulating gamma activity within individual areas. We suggest that such cross-frequency coupling links local BOLD signals to BOLD correlations across distributed networks.
Multiple electrode recordings; Neural oscillations; Cross-frequency coupling; Functional connectivity; Resting state FMRI
Lesions in human posterior parietal cortex can cause optic ataxia (OA), in which reaches but not saccades to visual objects are impaired, suggesting separate visuomotor pathways for the two effectors. In monkeys, one potentially crucial area for reach control is the parietal reach region (PRR), in which neurons respond preferentially during reach planning as compared to saccade planning. However, direct causal evidence linking the monkey PRR to the deficits observed in OA is missing. We thus inactivated part of the macaque PRR, in the medial wall of the intraparietal sulcus, and produced the hallmarks of OA, misreaching for peripheral targets but unimpaired saccades. Furthermore, reach errors were larger for the targets preferred by the neural population local to the injection site. These results demonstrate that PRR is causally involved in reach-specific visuomotor pathways, and reach goal disruption in PRR can be a neural basis of OA.
Available methods for differentiating human embryonic (ES) and induced pluripotent stem (iPS) cells into neurons are often cumbersome, slow and variable. Alternatively, human fibroblasts can be directly converted into induced neuronal (iN) cells. However, with present techniques conversion is inefficient, synapse formation is limited, and only small amounts of neurons can be generated. Here, we show that human ES and iPS cells can be converted into functional iN cells with nearly 100% yield and purity in less than two weeks by forced expression of a single transcription factor. The resulting ES-iN or iPS-iN cells exhibit quantitatively reproducible properties independent of the cell line of origin, form mature pre- and postsynaptic specializations, and integrate into existing synaptic networks when transplanted into mouse brain. As illustrated by selected examples, our approach enables large-scale studies of human neurons for questions such as analyses of human diseases, examination of human-specific genes, and drug screening.
Targeting genetically encoded tools for neural circuit dissection to relevant cellular populations is a major challenge in neurobiology. We developed a new approach, Targeted Recombination in Active Populations (TRAP), to obtain genetic access to neurons that were activated by defined stimuli. This method utilizes mice in which the tamoxifen-dependent recombinase CreERT2 is expressed in an activity-dependent manner from the loci of the immediate early genes Arc and Fos. Active cells that express CreERT2 can undergo recombination only when tamoxifen is present, allowing genetic access to neurons that are active during a time window of less than 12 h. We show that TRAP can selectively provide access to neurons activated by specific somatosensory, visual, and auditory stimuli, and by experience in a novel environment. When combined with tools for labeling, tracing, recording, and manipulating neurons, TRAP offers a powerful new approach for understanding how the brain processes information and generates behavior.
Calcium/calmodulin-dependent kinase II has been suggested to produce input-specific long-term potentiation of synaptic strength. This idea has been complicated by results from Rose, Jin, and Craig demonstrating that spatiotemporally restricted NMDA receptor excitation at contiguous synapses can result in the translocation of activated CaMKII throughout the dendritic arbor.
Senile plaques consisting of β-amyloid (Aβ) and neurofibrillary tangles composed of hyperphosphorylated tau are major pathological hallmarks of Alzheimer’s disease (AD). Elucidation of factors that modulate Aβ generation and tau hyperphosphorylation is crucial for AD intervention. Here we identify a novel mouse gene Fg01 that originated through retroposition of ribosomal protein S23. We demonstrate that FG01 protein reduces the levels of Aβ and tau phosphorylation by interacting with adenylate cyclases to activate cAMP/PKA and thus inhibit GSK-3 activity. The function of Fg01 is demonstrated in cells of various species including human, and in transgenic mice overexpressing FG01. Furthermore, the AD-like pathologies of triple transgenic AD mice were improved and levels of synaptic maker proteins increased after crossing them with Fg01 transgenic mice. Our studies reveal a new target/pathway for regulating AD pathologies and uncover a novel retrogene and its role in regulating protein kinase pathways.
Cellular interactions between neighboring axons are essential for global topographic map formation. Here we show that axonal interactions also precisely instruct the location of synapses. Motoneurons form en passant synapses in Caenorhabditis elegans. While axons from the same neuron class significantly overlap, each neuron innervates a unique and tiled segment of the muscle field by restricting its synapses to a distinct subaxonal domain—a phenomenon we term “synaptic tiling”. Using DA8 and DA9 motoneurons as a model, we found that the synaptic tiling requires Plexin(PLX-1) and two transmembrane Semaphorins. In the Plexin or Semaphorin mutants, synaptic domains from both neurons expand and overlap with each other without guidance defects. In a Semaphorin-dependent manner, PLX-1 is concentrated at the synapse-free axonal segment, delineating the tiling border. Furthermore, Plexin inhibits presynapse formation by suppressing synaptic F-actin through its cytoplasmic GAP domain. Hence, contact-dependent, intra-axonal Plexin signaling specifies synaptic circuits by inhibiting synapse formation at the subcellular loci.