Strong Feedback Inhibition Research Articles

Early deficits in dentate circuit and behavioral pattern separation after concussive brain injury

Traumatic brain injury leads to cellular and circuit changes in the dentate gyrus, a gateway to hippocampal information processing. Intrinsic granule cell firing properties and strong feedback inhibition in the dentate are proposed as critical to its ability to generate unique representation of similar inputs by a process known as pattern separation. Here we evaluate the impact of brain injury on cellular decorrelation of temporally patterned inputs in slices and behavioral discrimination of spatial locations in vivo one week after concussive lateral fluid percussion injury (FPI) in mice. Despite posttraumatic increases in perforant path evoked excitatory drive to granule cells and enhanced ΔFosB labeling, indicating sustained increase in excitability, the reliability of granule cell spiking was not compromised after FPI. Although granule cells continued to effectively decorrelate output spike trains recorded in response to similar temporally patterned input sets after FPI, their ability to decorrelate highly similar input patterns was reduced. In parallel, encoding of similar spatial locations in a novel object location task that involves the dentate inhibitory circuits was impaired one week after FPI. Injury induced changes in pattern separation were accompanied by loss of somatostatin expressing inhibitory neurons in the hilus. Together, these data suggest that the early posttraumatic changes in the dentate circuit undermine dentate circuit decorrelation of temporal input patterns as well as behavioral discrimination of similar spatial locations, both of which could contribute to deficits in episodic memory.

Strong Feedback Inhibition of Key Enzymes in the Morphine Biosynthetic Pathway from Opium Poppy Detectable in Engineered Yeast.

Systematic screening of morphine pathway intermediates in engineered yeast revealed key biosynthetic enzymes displaying potent feedback inhibition: 3'-hydroxy-N-methylcoclaurine 4'-methyltransferase (4'OMT), which yields (S)-reticuline, and the coupled salutaridinol-7-O-acetyltransferase (SalAT) and thebaine synthase (THS2) enzyme system that produces thebaine. The addition of deuterated reticuline-d1 to a yeast strain able to convert (S)-norcoclaurine to (S)-reticuline showed reduced product accumulation in response to the feeding of all four successive pathway intermediates. Similarly, the addition of deuterated thebaine-d3 to a yeast strain able to convert salutaridine to thebaine showed reduced product accumulation from exogenous salutaridine or salutaridinol. In vitro analysis showed that reticuline is a noncompetitive inhibitor of 4'OMT, whereas thebaine exerts mixed inhibition on SalAT/THS2. In a yeast strain capable of de novo morphine biosynthesis, the addition of reticuline and thebaine resulted in the accumulation of several pathway intermediates. In contrast, morphine had no effect, suggesting that circumventing the interaction of reticuline and thebaine with 4'OMT and SalAT/THS2, respectively, could substantially increase opiate alkaloid titers in engineered yeast.

Visual Deprivation Selectively Reduces Thalamic Reticular Nucleus-Mediated Inhibition of the Auditory Thalamus in Adults.

Sensory loss leads to widespread cross-modal plasticity across brain areas to allow the remaining senses to guide behavior. While multimodal sensory interactions are often attributed to higher-order sensory areas, cross-modal plasticity has been observed at the level of synaptic changes even across primary sensory cortices. In particular, vision loss leads to widespread circuit adaptation in the primary auditory cortex (A1) even in adults. Here we report using mice of both sexes in which cross-modal plasticity occurs even earlier in the sensory-processing pathway at the level of the thalamus in a modality-selective manner. A week of visual deprivation reduced inhibitory synaptic transmission from the thalamic reticular nucleus (TRN) to the primary auditory thalamus (MGBv) without changes to the primary visual thalamus (dLGN). The plasticity of TRN inhibition to MGBv was observed as a reduction in postsynaptic gain and short-term depression. There was no observable plasticity of the cortical feedback excitatory synaptic transmission from the primary visual cortex to dLGN or TRN and A1 to MGBv, which suggests that the visual deprivation-induced plasticity occurs predominantly at the level of thalamic inhibition. We provide evidence that visual deprivation-induced change in the short-term depression of TRN inhibition to MGBv involves endocannabinoid CB1 receptors. TRN inhibition is considered critical for sensory gating, selective attention, and multimodal performances; hence, its plasticity has implications for sensory processing. Our results suggest that selective disinhibition and altered short-term dynamics of TRN inhibition in the spared thalamic nucleus support cross-modal plasticity in the adult brain.SIGNIFICANCE STATEMENT Losing vision triggers adaptation of the brain to enhance the processing of the remaining senses, which can be observed as better auditory performance in blind subjects. We previously found that depriving vision of adult rodents produces widespread circuit reorganization in the primary auditory cortex and enhances auditory processing at a neural level. Here we report that visual deprivation-induced plasticity in adults occurs much earlier in the auditory pathway, at the level of thalamic inhibition. Sensory processing is largely gated at the level of the thalamus via strong cortical feedback inhibition mediated through the thalamic reticular nucleus (TRN). We found that TRN inhibition of the auditory thalamus is selectively reduced by visual deprivation, thus playing a role in adult cross-modal plasticity.

NMDA receptor–BK channel coupling regulates synaptic plasticity in the barrel cortex

Postsynaptic N-methyl-D-aspartate receptors (NMDARs) are crucial mediators of synaptic plasticity due to their ability to act as coincidence detectors of presynaptic and postsynaptic neuronal activity. However, NMDARs exist within the molecular context of a variety of postsynaptic signaling proteins, which can fine-tune their function. Here, we describe a form of NMDAR suppression by large-conductance Ca2+- and voltage-gated K+ (BK) channels in the basal dendrites of a subset of barrel cortex layer 5 pyramidal neurons. We show that NMDAR activation increases intracellular Ca2+ in the vicinity of BK channels, thus activating K+ efflux and strong negative feedback inhibition. We further show that neurons exhibiting such NMDAR-BK coupling serve as high-pass filters for incoming synaptic inputs, precluding the induction of spike timing-dependent plasticity. Together, these data suggest that NMDAR-localized BK channels regulate synaptic integration and provide input-specific synaptic diversity to a thalamocortical circuit.

The ASM Journals Committee Values the Contributions of Black Microbiologists.

Abstract Oscillations are ubiquitous features of brain dynamics that undergo task-related changes in synchrony, power, and frequency. The impact of those changes on target networks is poorly understood. In this work, we used a biophysically detailed model of prefrontal cortex (PFC) to explore the effects of varying the spike rate, synchrony, and waveform of strong oscillatory inputs on the behavior of cortical networks driven by them. Interacting populations of excitatory and inhibitory neurons with strong feedback inhibition are inhibition-based network oscillators that exhibit resonance (i.e., larger responses to preferred input frequencies). We quantified network responses in terms of mean firing rates and the population frequency of network oscillation; and characterized their behavior in terms of the natural response to asynchronous input and the resonant response to oscillatory inputs. We show that strong feedback inhibition causes the PFC to generate internal (natural) oscillations in the beta/gamma frequency range (>15 Hz) and to maximize principal cell spiking in response to external oscillations at slightly higher frequencies. Importantly, we found that the fastest oscillation frequency that can be relayed by the network maximizes local inhibition and is equal to a frequency even higher than that which maximizes the firing rate of excitatory cells; we call this phenomenon population frequency resonance. This form of resonance is shown to determine the optimal driving frequency for suppressing responses to asynchronous activity. Lastly, we demonstrate that the natural and resonant frequencies can be tuned by changes in neuronal excitability, the duration of feedback inhibition, and dynamic properties of the input. Our results predict that PFC networks are tuned for generating and selectively responding to beta- and gamma-rhythmic signals due to the natural and resonant properties of inhibition-based oscillators. They also suggest strategies for optimizing transcranial stimulation and using oscillatory networks in neuromorphic engineering. Author Summary The prefrontal cortex (PFC) flexibly encodes task-relevant representations and outputs biases to mediate higher cognitive functions. The relevant neural ensembles undergo task-related changes in oscillatory dynamics at beta- and gamma frequencies. Using a computational model of the PFC network, we show that strong feedback inhibition causes the PFC to generate internal oscillations and to prefer external oscillations at similar frequencies. The precise frequencies that are generated and preferred can be flexibly tuned by varying the synchrony and strength of input network activity, the level of background excitation, and neuromodulation of intrinsic ion currents. We also show that the peak output frequency in response to external oscillations, which depends on the synchrony and strength of the input as well as the strong inhibitory feedback, is faster than the internally generated frequency, and that this difference enables exclusive response to oscillatory inputs. These properties enable changes in oscillatory dynamics to govern the selective processing and gating of task-relevant signals in service of cognitive control.

Delayed Antidepressant Efficacy and the Desensitization Hypothesis.

Many conventional antidepressants can quickly raise the levels of extracellular serotonin, yet their positive effects on mood ensues only weeks later. This delay in efficacy is a clinical problem that has proven difficult to overcome. Early investigation noted that the initial increases in extracellular serotonin engaged strong feedback inhibition of serotonin neurons via 5-HT1A autoreceptors, resulting in a profound reduction in their firing rate. Over the course of chronic treatment, however, firing rate returned to normal and the inhibition via 5-HT1A receptor agonists was attenuated. The coincident timeline of these phenomena led to the influential hypothesis that the relationship was causal and that gradual loss of feedback inhibition mediated by 5-HT1A receptors was critical to the delayed therapeutic onset. Simple and appealing, the desensitization hypothesis has taken strong hold, yet much of the supporting evidence is circumstantial and there are several observations that would refute a causal relationship. In particular, even though 5-HT1A receptors may desensitize, there is evidence that feedback inhibition mediated by remaining receptors persists. That is, baseline serotonin firing rate returns to normal not because of 5-HT1A desensitization but rather despite ongoing feedback inhibition. Thus, while 5-HT1A receptors remain important for emotional behavior, it may be other slow-adaptive changes triggered by antidepressants that allow for therapeutic effects, such as those involving glutamatergic synaptic plasticity.

Mechanisms underlying contrast-dependent orientation selectivity in mouse V1

Recent experiments have shown that mouse primary visual cortex (V1) is very different from that of cat or monkey, including response properties-one of which is that contrast invariance in the orientation selectivity (OS) of the neurons' firing rates is replaced in mouse with contrast-dependent sharpening (broadening) of OS in excitatory (inhibitory) neurons. These differences indicate a different circuit design for mouse V1 than that of cat or monkey. Here we develop a large-scale computational model of an effective input layer of mouse V1. Constrained by experiment data, the model successfully reproduces experimentally observed response properties-for example, distributions of firing rates, orientation tuning widths, and response modulations of simple and complex neurons, including the contrast dependence of orientation tuning curves. Analysis of the model shows that strong feedback inhibition and strong orientation-preferential cortical excitation to the excitatory population are the predominant mechanisms underlying the contrast-sharpening of OS in excitatory neurons, while the contrast-broadening of OS in inhibitory neurons results from a strong but nonpreferential cortical excitation to these inhibitory neurons, with the resulting contrast-broadened inhibition producing a secondary enhancement on the contrast-sharpened OS of excitatory neurons. Finally, based on these mechanisms, we show that adjusting the detailed balances between the predominant mechanisms can lead to contrast invariance-providing insights for future studies on contrast dependence (invariance).

Flexible resonance in prefrontal networks with strong feedback inhibition.

Oscillations are ubiquitous features of brain dynamics that undergo task-related changes in synchrony, power, and frequency. The impact of those changes on target networks is poorly understood. In this work, we used a biophysically detailed model of prefrontal cortex (PFC) to explore the effects of varying the spike rate, synchrony, and waveform of strong oscillatory inputs on the behavior of cortical networks driven by them. Interacting populations of excitatory and inhibitory neurons with strong feedback inhibition are inhibition-based network oscillators that exhibit resonance (i.e., larger responses to preferred input frequencies). We quantified network responses in terms of mean firing rates and the population frequency of network oscillation; and characterized their behavior in terms of the natural response to asynchronous input and the resonant response to oscillatory inputs. We show that strong feedback inhibition causes the PFC to generate internal (natural) oscillations in the beta/gamma frequency range (>15 Hz) and to maximize principal cell spiking in response to external oscillations at slightly higher frequencies. Importantly, we found that the fastest oscillation frequency that can be relayed by the network maximizes local inhibition and is equal to a frequency even higher than that which maximizes the firing rate of excitatory cells; we call this phenomenon population frequency resonance. This form of resonance is shown to determine the optimal driving frequency for suppressing responses to asynchronous activity. Lastly, we demonstrate that the natural and resonant frequencies can be tuned by changes in neuronal excitability, the duration of feedback inhibition, and dynamic properties of the input. Our results predict that PFC networks are tuned for generating and selectively responding to beta- and gamma-rhythmic signals due to the natural and resonant properties of inhibition-based oscillators. They also suggest strategies for optimizing transcranial stimulation and using oscillatory networks in neuromorphic engineering.

Interactions between Inhibitory Interneurons and Excitatory Associational Circuitry in Determining Spatio-Temporal Dynamics of Hippocampal Dentate Granule Cells: A Large-Scale Computational Study.

This paper reports on findings from a million-cell granule cell model of the rat dentate gyrus that was used to explore the contributions of local interneuronal and associational circuits to network-level activity. The model contains experimentally derived morphological parameters for granule cells, which each contain approximately 200 compartments, and biophysical parameters for granule cells, basket cells, and mossy cells that were based both on electrophysiological data and previously published models. Synaptic input to cells in the model consisted of glutamatergic AMPA-like EPSPs and GABAergic-like IPSPs from excitatory and inhibitory neurons, respectively. The main source of input to the model was from layer II entorhinal cortical neurons. Network connectivity was constrained by the topography of the system, and was derived from axonal transport studies, which provided details about the spatial spread of axonal terminal fields, as well as how subregions of the medial and lateral entorhinal cortices project to subregions of the dentate gyrus. Results of this study show that strong feedback inhibition from the basket cell population can cause high-frequency rhythmicity in granule cells, while the strength of feedforward inhibition serves to scale the total amount of granule cell activity. Results furthermore show that the topography of local interneuronal circuits can have just as strong an impact on the development of spatio-temporal clusters in the granule cell population as the perforant path topography does, both sharpening existing clusters and introducing new ones with a greater spatial extent. Finally, results show that the interactions between the inhibitory and associational loops can cause high frequency oscillations that are modulated by a low-frequency oscillatory signal. These results serve to further illustrate the importance of topographical constraints on a global signal processing feature of a neural network, while also illustrating how rich spatio-temporal and oscillatory dynamics can evolve from a relatively small number of interacting local circuits.

Abstract A41: Oncogenic RAF mutants that signal as functional dimers are resistant to current RAF inhibitors but sensitive to a novel inhibitor of RAF dimer kinase activity

Abstract Dysregulation of ERK signaling is a common event in malignancy. Receptor activation of RAS leads to dimerization and activation of RAF kinase, which in turn activates ERK. Physiologic activation of the pathway is limited by ERK dependent feedback inhibition of RAS activation and other components of the pathway. Oncogenic mutations and translocations of RAF family members have been identified in many human tumors and serve as drivers of tumor growth. We now show that these mutants are active despite feedback inhibition of RAS and that this is associated with dysregulated ERK output. All translocations and almost all of the RAF mutants tested so far form RAS-independent constitutive dimers, the activity of which is dependent on their dimerization. Mutations at one site, V600E and V600K allow RAF to function as a monomer in cells with low RAS activity and as a RAS-dependent dimer in cells with adequate RAS activity. In tumors expressing any of these mutants, RAF/MEK/ERK activity is elevated despite feedback inhibition of RAS. Currently available RAF inhibitors bind to one protomer of the RAF dimer and cause negative cooperativity of binding to the second site. This is sometimes associated with transactivation of the unbound protomer. These inhibitors cannot be used to inhibit ERK signaling in tumors in which ERK is driven by RAF-dimers (those with mutant RAS, NF1 loss, atypical RAF mutations, RAF translocations.) In contrast, in tumors with V600E BRAF mutations, ERK causes strong feedback inhibition of RAS and V600E BRAF exists as a monomer that is inhibited by these drugs. Acquired resistance to RAF inhibitors usually results from a molecular lesion that causes RAS-dependent or independent dimerization of V600E (NRAS mutation, V600E BRAF amplification, V600E aberrantly spliced isoform.) Current RAF inhibitors inhibit V600E BRAF dimers at concentrations 10-100 fold greater than those required to inhibit the corresponding monomer in cells. We take this as a provisional measure of the degree of negative cooperativity of binding to the second site. We have now identified inhibitors that inhibit V600E dimers at concentrations close (1-3 fold higher) than those required to inhibit the monomer. They do not cause dissociation of dimers and their inhibition is abolished by gatekeeper mutations. Phenotypically, therefore, binding of these compounds to dimers is associated with only minimal negative cooperativity. These drugs inhibit ERK signaling and the proliferation of tumor cells with atypical BRAF mutations and translocations as well as V600E melanoma models with acquired resistance to Vemurafenib. However, they are much less active in cancer cells with KRAS mutation or receptor-dependent activation of RAF. Selective inhibitors of subsets of RAF dimers may allow effective ERK inhibition of large subset of tumors without inhibiting signaling in normal cells. Citation Format: Zhan Yao, Torres N Merna, Wei Min, Hanrahan Aphrothiti, Abdel-Wahab Omar, Solit David, Luo Lusong, Poulikakos Poulikos, Rosen Neal. Oncogenic RAF mutants that signal as functional dimers are resistant to current RAF inhibitors but sensitive to a novel inhibitor of RAF dimer kinase activity. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr A41. doi: 10.1158/1557-3125.RASONC14-A41

Abstract 3706: Oncogenic RAF mutants that signal as functional dimers are resistant to current RAF inhibitors but sensitive to a novel inhibitor of RAF dimer kinase activity

Abstract Dysregulation of ERK signaling is a common event in malignancy. Receptor activation of RAS leads to dimerization and activation of RAF kinase, which in turn activates ERK. Physiologic activation of the pathway is limited by ERK dependent feedback inhibition of RAS activation and other components of the pathway. Oncogenic mutations and translocations of RAF family members have been identified in many human tumors and serve as drivers of tumor growth. We now show that these mutants are active despite feedback inhibition of RAS and that this is associated with dysregulated ERK output. All translocations and almost all of the RAF mutants tested so far form RAS-independent constitutive dimers, the activity of which is dependent on their dimerization. Mutations at one site, V600E and V600K allow RAF to function as a monomer in cells with low RAS activity and as a RAS-dependent dimer in cells with adequate RAS activity. In tumors expressing any of these mutants, RAF/MEK/ERK activity is elevated despite feedback inhibition of RAS. Currently available RAF inhibitors bind to one protomer of the RAF dimer and cause negative cooperativity of binding to the second site. This is sometimes associated with transactivation of the unbound protomer. These inhibitors cannot be used to inhibit ERK signaling in tumors in which ERK is driven by RAF-dimers (those with mutant RAS, NF1 loss, atypical RAF mutations, RAF translocations.) In contrast, in tumors with V600E BRAF mutations, ERK causes strong feedback inhibition of RAS and V600E BRAF exists as a monomer that is inhibited by these drugs. Acquired resistance to RAF inhibitors usually results from a molecular lesion that causes RAS-dependent or independent dimerization of V600E (NRAS mutation, V600E BRAF amplification, V600E aberrantly spliced isoform.) Current RAF inhibitors inhibit V600E BRAF dimers at concentrations 10-100 fold greater than those required to inhibit the corresponding monomer in cells. We take this as a provisional measure of the degree of negative cooperativity of binding to the second site. We have now identified inhibitors that inhibit V600E dimers at concentrations close (1-3 fold higher) than those required to inhibit the monomer. They do not cause dissociation of dimers and their inhibition is abolished by gatekeeper mutations. Phenotypically, therefore, binding of these compounds to dimers is associated with only minimal negative cooperativity. These drugs inhibit ERK signaling and the proliferation of tumor cells with atypical BRAF mutations and translocations as well as V600E melanoma models with acquired resistance to Vemurafenib. However, they are much less active in cancer cells with KRAS mutation or receptor-dependent activation of RAF. Selective inhibitors of subsets of RAF dimers may allow effective ERK inhibition of large subset of tumors without inhibiting signaling in normal cells. Citation Format: Zhan Yao, Merna Torres, Min Wei, Aphrothiti Hanrahan, Omar Abdel-Wahab, David Solit, Lusong Luo, Poulikos Poulikakos, Neal Rosen. Oncogenic RAF mutants that signal as functional dimers are resistant to current RAF inhibitors but sensitive to a novel inhibitor of RAF dimer kinase activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3706. doi:10.1158/1538-7445.AM2014-3706

Erratum to: Effects of neuromodulation in a cortical network model of object working memory dominated by recurrent inhibition

Experimental evidence suggests that the maintenance of an item in working memory is achieved through persistent activity in selective neural assemblies of the cortex. To understand the mechanisms underlying this phenomenon, it is essential to investigate how persistent activity is affected by external inputs or neuromodulation. We have addressed these questions using a recurrent network model of object working memory. Recurrence is dominated by inhibition, although persistent activity is generated through recurrent excitation in small subsets of excitatory neurons. Our main findings are as follows. (1) Because of the strong feedback inhibition, persistent activity shows an inverted U shape as a function of increased external drive to the network. (2) A transient external excitation can switch off a network from a selective persistent state to its spontaneous state. (3) The maintenance of the sample stimulus in working memory is not affected by intervening stimuli (distractors) during the delay period, provided the stimulation intensity is not large. On the other hand, if stimulation intensity is large enough, distractors disrupt sample-related persistent activity, and the network is able to maintain a memory only of the last shown stimulus. (4) A concerted modulation of GABA A and NMDA conductances leads to a decrease of spontaneous activity but an increase of persistent activity; the enhanced signal-to-noise ratio is shown to increase the resistance of the network to distractors. (5) Two mechanisms are identified that produce an inverted U shaped dependence of persistent activity on modulation. The present study therefore points to several mechanisms that enhance the signal-to-noise ratio in working memory states. These mechanisms could be implemented in the prefrontal cortex by dopaminergic projections from the midbrain.

Crossveinless 2 regulates bone morphogenetic protein 9 in human and mouse vascular endothelium

AbstractThe importance of morphogenetic proteins (BMPs) and their antagonists in vascular development is increasingly being recognized. BMP-4 is essential for angiogenesis and is antagonized by matrix Gla protein (MGP) and crossveinless 2 (CV2), both induced by the activin receptor like-kinase 1 (ALK1) when stimulated by BMP-9. In this study, however, we show that CV2 preferentially binds and inhibits BMP-9 thereby providing strong feedback inhibition for BMP-9/ALK1 signaling rather than for BMP-4/ALK2 signaling. CV2 disrupts complex formation involving ALK2, ALK1, BMP-4, and BMP-9 required for the induction of both BMP antagonists. It also limits VEGF expression, proliferation, and tube formation in ALK1-expressing endothelial cells. In vivo, CV2 deficiency translates into a dysregulation of vascular BMP signaling, resulting in an abnormal endothelium with increased endothelial cellularity and expression of lineage markers for mature endothelial cells. Thus, mutual regulation by BMP-9 and CV2 is essential in regulating the development of the vascular endothelium.

Abstract 210: Crossveinless 2 Regulates Bone Morphogenetic Protein 9 in Human and Mouse Vascular Endothelium

The importance of morphogenetic proteins (BMPs) and their antagonists in vascular development is increasingly being recognized. BMP-4 is essential for angiogenesis and is antagonized by matrix Gla protein (MGP) and crossveinless 2 (CV2), both induced in a staged fashion by the activin-like kinase receptor 1 (ALK1) after stimulation by BMP-9. In this study, however, we show that CV2 preferentially binds and inhibits BMP-9 thereby providing strong feedback inhibition for BMP-9/ALK1 signaling rather than for BMP-4/ALK2 signaling. CV2 disrupts complex formation by ALK2, ALK1, BMP-4 and BMP-9 required for the induction of both BMP antagonists. It also limits VEGF expression and proliferation of ALK1-expressing endothelial cells. In vivo, CV2 deficiency translates into a dysregulation of vascular BMP signaling, resulting in a thickened, abnormal endothelium with increased markers of endothelial differentiation. Thus, mutual regulation by BMP-9 and CV2 is essential in regulating the development of the vascular endothelium.

Improving putrescine production by Corynebacterium glutamicum by fine-tuning ornithine transcarbamoylase activity using a plasmid addiction system

Corynebacterium glutamicum shows a great potential for the production of the polyamide monomer putrescine (1,4-diaminobutane). Previously, we constructed the putrescine-producing strain PUT1 by deletion of argF, the gene for ornithine transcarbamoylase (OTC), and argR, encoding the L-arginine repressor, combined with heterologous expression of the Escherichia coli gene for L-ornithine decarboxylase SpeC. As a consequence of argF deletion, this strain requires supplementation of L-arginine and shows growth-decoupled putrescine production. To avoid costly supplementation with L-arginine and the strong feedback inhibition of the key enzyme N-acetylglutamate kinase (ArgB) by L-arginine, a plasmid addiction system for low-level argF expression was developed. By fine-tuning argF expression through modifications of the promoter, the translational start codon and/or the ribosome binding site, high productivity and titer could be obtained. OTC activity varied almost thousandfold between 960 and 1 mU mg⁻¹ resulting in putrescine yields on glucose from less than 0.001 up to 0.26 g g⁻¹, the highest yield in bacteria reported to date. The most promising strain, designated PUT21, was characterized comprehensively. PUT21 strain grew with a rate of 0.19 h⁻¹ in mineral salt medium without the need for L-arginine supplementation and produced putrescine with a yield of 0.16 g g⁻¹ glucose at a volumetric productivity of 0.57 g L⁻¹ h⁻¹ and a specific productivity of 0.042 g g⁻¹ h⁻¹. The carbon balance suggested that no major unidentified by-product was produced. Compared to the first-generation strain PUT1, the putrescine yield observed with PUT21 was increased by 60%. In fed-batch cultivation with C. glutamicum PUT21, a putrescine titer of 19 g L⁻¹ at a volumetric productivity of 0.55 g L⁻¹ h⁻¹ and a yield of 0.16 g g⁻¹ glucose could be achieved. Moreover, while plasmid segregation of the initial strain required antibiotic selection, plasmid segregation in C. glutamicum PUT21 was fully stable for more than 60 generations without antibiotic selection even in the presence of L-arginine. The ornithine decarboxylase gene speC was expressed from this argF addiction plasmid ensuring stable putrescine production by the engineered C. glutamicum strain.

Network dynamic regime controls the structure of the V1 extra-classical receptive field

Event Abstract Back to Event Network dynamic regime controls the structure of the V1 extra-classical receptive field Daniel B. Rubin1* and Kenneth D. Miller2 1 Columbia University, Department of Neurosciences, United States 2 Columbia University, United States Recent work has shown that the V1 extra-classical receptive field (eCRF or "surround"), the region that surrounds the classical receptive field (CRF or "center") and exerts a subthreshold influence over the response to stimuli, has a well-defined spatial structure (Tanaka and Ohzawa 2009). To map out the spatial structure of the CRF and eCRF, these authors presented a large grating of the CRF’s preferred spatial frequency (SF) and orientation (ORI), and superimposed a periodic contrast modulation. They studied the dependence of the cell’s response to the modulation SF and ORI, and found that the combined CRF and eCRF typically resembled a large simple cell, with a preferred modulation ORI unrelated to the preferred luminance ORI of the CRF and a preferred modulation SF about 2-6 times lower than the preferred luminance SF of the CRF. We have previously shown (Ozeki et al. 2009) that surround suppression in V1 is a "de-amplification" rather than an inhibition - a lowering of the gain in a network with strong, destabilizing recurrent excitation stabilized by strong feedback inhibition (we term this an "inhibition-stabilized network" or ISN). Here we show that such a network, along with spatial connectivity typical of cortex, creates inhibitory activity that is periodic over retinotopic space. This network "resonates" over a narrow range of spatial frequencies, resulting in the observed tuning for modulation SF. We study a linear firing-rate-model of V1 comprised of recurrently connected excitatory and inhibitory neuronal populations positioned on an array of retinotopic space. As in cortical networks, inhibitory connections are localized while excitatory connections are longer range. Using analytic and numeric modeling, we find that in order for inhibitory firing rates in a generic network with non-periodic connectivity to oscillate as a function of space, several requirements must be met. In particular, the network must be an ISN. Only in the ISN regime are both excitatory and inhibitory neurons tuned for a nonzero modulation SF of a contrast-modulated grating. The preferred modulation frequency is controlled by the spatial extents of the different synaptic connections (E–>E, E–>I, I–>E, I–>I). Using estimates of the extent of the lateral connections in layer II/III of cat V1, we observe preferred SF’s matching closely those recorded experimentally; we also show more generally that the preferred modulation SF will be several times lower than the CRF preferred luminance SF, as observed. Lastly, once the nonlinearity of cortical networks is considered, dynamic regime (ISN or non-ISN) becomes an activity-dependent feature that changes with input strength; different contrasts drive the network into different dynamic regimes. This input-driven shift in network regime may underlie a number of contrast-dependent properties of the eCRF. In particular, this mechanism can explain the reduction in summation field size and the emergence of spatially-periodic length-tuning curves of firing rate and inhibitory conductance (Anderson et al. 2001) with increasing stimulus contrast. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Poster Presentation Topic: Poster session II Citation: Rubin DB and Miller KD (2010). Network dynamic regime controls the structure of the V1 extra-classical receptive field. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00330 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 08 Mar 2010; Published Online: 08 Mar 2010. * Correspondence: Daniel B Rubin, Columbia University, Department of Neurosciences, New York, United States, dbr2107@columbia.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. 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The thyromimetic T-0681 protects from atherosclerosis

This report describes studies in hyperlipidemic New Zealand White (NZW) rabbits investigating the impact of the liver-selective thyromimetic T-0681 on lipoprotein metabolism and the development of atherosclerosis. Prolonged treatment with T-0681 increased the hepatic expression of both LDL receptor and scavenger receptor class B, type I without affecting cholesteryl ester transfer protein activity. Upregulation of hepatic lipoprotein receptors was accompanied by a marked decrease of apolipoprotein B-containing lipoproteins, reflected by a 60% reduction of plasma cholesterol and a >70% reduction of plasma triglyceride levels. Most importantly, T-0681 reduced the development of atherosclerosis by 80% in NZW rabbits on high-cholesterol chow. Our data suggest that liver-selective thyromimetics, such as T-0681, may prove to be useful therapeutic agents against the development of atherosclerosis in humans.

A Fluctuation-Driven Mechanism for Slow Decision Processes in Reverberant Networks

The spike activity of cells in some cortical areas has been found to be correlated with reaction times and behavioral responses during two-choice decision tasks. These experimental findings have motivated the study of biologically plausible winner-take-all network models, in which strong recurrent excitation and feedback inhibition allow the network to form a categorical choice upon stimulation. Choice formation corresponds in these models to the transition from the spontaneous state of the network to a state where neurons selective for one of the choices fire at a high rate and inhibit the activity of the other neurons. This transition has been traditionally induced by an increase in the external input that destabilizes the spontaneous state of the network and forces its relaxation to a decision state. Here we explore a different mechanism by which the system can undergo such transitions while keeping the spontaneous state stable, based on an escape induced by finite-size noise from the spontaneous state. This decision mechanism naturally arises for low stimulus strengths and leads to exponentially distributed decision times when the amount of noise in the system is small. Furthermore, we show using numerical simulations that mean decision times follow in this regime an exponential dependence on the amplitude of noise. The escape mechanism provides thus a dynamical basis for the wide range and variability of decision times observed experimentally.

Association of 5‐HT1B receptor polymorphisms with the loudness dependence of auditory evoked potentials in a community‐based sample of healthy volunteers

The terminal autoreceptor 5-HT1B is centrally involved in the regulation of the brain serotonergic system and in several psychiatric disorders including depression, addiction, and obsessive-compulsive disorder. The loudness dependence of the auditory evoked N1/P2-component (LDAEP; primary auditory cortex) is currently considered as one of best-validated indicators of serotonergic neurotransmission, especially for synaptically released serotonin. Since the 5-HT1B receptor is involved in the release of serotonin at terminal endings of cortical neurons, this study addressed the question whether single nucleotide polymorphism (SNP) in the gene coding for this receptor (HTR1B) are related to LDAEP of the primary auditory cortex (tangential dipole) investigating a community-based sample of 127 healthy subjects randomly selected from the general population. In this carefully recruited sample, a G-G haplotype (rs1213368-rs6296) and the respective G-alleles were found to be related to a strong LDAEP response of the left tangential dipole, indicating low serotonergic activity. Apart from the fact that this is the first study which relates HTR1B SNPs to a measure of serotonergic function, it can be speculated that LDAEP may reflect parts of the release mechanism of serotonin at cortical synapses, although the lateralized finding cannot be entirely explained. Carriers of the G-alleles may be characterized by a particularly strong feedback inhibition of serotonin release at cortical terminals in the primary auditory cortex, possibly mediated by higher sensitivity of 5-HT1B receptors associated with low serotonergic activity.

On the mechanism of cerebral accumulation of cholestanol in patients with cerebrotendinous xanthomatosis

The most serious consequence of sterol 27-hydroxylase deficiency in humans [cerebrotendinous xanthomatosis (CTX)] is the development of cholestanol-containing brain xanthomas. The cholestanol in the brain may be derived from the circulation or from 7alpha-hydroxylated intermediates in bile acid synthesis, present at 50- to 250-fold increased levels in plasma. Here, we demonstrate a transfer of 7alpha-hydroxy-4-cholesten-3-one across cultured porcine brain endothelial cells (a model for the blood-brain barrier) that is approximately 100-fold more efficient than the transfer of cholestanol. Furthermore, there was an efficient conversion of 7alpha-hydroxy-4-cholesten-3-one to cholestanol in cultured neuronal and glial cells as well as in monocyte-derived macrophages of human origin. It is concluded that the continuous intracellular production of cholestanol from a bile acid precursor capable of rapidly passing biomembranes, including the blood-brain barrier, is likely to be of major importance for the accumulation of cholestanol in patients with CTX. Such a mechanism also fits well with the observation that treatment with chenodeoxycholic acid, which normalizes the level of the bile acid precursor, results in a reduction of cholestanol-containing xanthomas even in the brain.