Dual Roles for Spike Signaling in Cortical Neural Populations

A prominent feature of signaling in cortical neurons is that of randomness in the action potential. The output of a typical pyramidal cell can be well fit with a Poisson model, and variations in the Poisson rate repeatedly have been shown to be correlated with stimuli. However while the rate provides a very useful characterization of neural spike data, it may not be the most fundamental description of the signaling code. Recent data showing γ frequency range multi-cell action potential correlations, together with spike timing dependent plasticity, are spurring a re-examination of the classical model, since precise timing codes imply that the generation of spikes is essentially deterministic. Could the observed Poisson randomness and timing determinism reflect two separate modes of communication, or do they somehow derive from a single process? We investigate in a timing-based model whether the apparent incompatibility between these probabilistic and deterministic observations may be resolved by examining how spikes could be used in the underlying neural circuits. The crucial component of this model draws on dual roles for spike signaling. In learning receptive fields from ensembles of inputs, spikes need to behave probabilistically, whereas for fast signaling of individual stimuli, the spikes need to behave deterministically. Our simulations show that this combination is possible if deterministic signals using γ latency coding are probabilistically routed through different members of a cortical cell population at different times. This model exhibits standard features characteristic of Poisson models such as orientation tuning and exponential interval histograms. In addition, it makes testable predictions that follow from the γ latency coding

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease.

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action

Neurosystems: brain rhythms and cognitive processing

Neuronal rhythms are ubiquitous features of brain dynamics, and are highly correlated with cognitive processing. However, the relationship between the physiological mechanisms producing these rhythms and the functions associated with the rhythms remains mysterious. This article investigates the contributions of rhythms to basic cognitive computations (such as filtering signals by coherence and/or frequency) and to major cognitive functions (such as attention and multi-modal coordination). We offer support to the premise that the physiology underlying brain rhythms plays an essential role in how these rhythms facilitate some cognitive operations.098352 - Wellcome Trust; 5R01NS067199 - NINDS NIH HH

Building Neural in vitro Models with Human Pluripotent Stem Cells : Neuronal Functionality and the Role of Astrocytes in the Networks

Ymmärryksemme ihmisaivojen kehityksestä ja toiminnasta on edelleen puutteellista. Valitettavasti neurotieteiden alalta puuttuu edustavia ihmisspesifisiä malleja, jotka täydentäisivät eläinmalleilla tehtäviä tutkimuksia. Ihmisaivokudoksen saatavuus tutkimustarkoituksiin on rajoitettua, mikä kannustaa uusien lähestymistapojen, kuten ihmisen monikykyisistä kantasoluista johdettujen hermosolujen hyödyntämistä. Muutaman viime vuosikymmenen aikana ihmisen monikykyisiin kantasoluihin liittyvä tutkimusala on laajentunut valtavasti. Kasvavat odotukset kohdistuvat kantasolujen ja niiden johdannaisten soveltamiseen regeneratiivisessa lääketieteessä, lääkkeiden seulonnassa ja tautien mallinnuksessa. Tämän tutkimuksen pääpainona oli arvioida ihmisen monikykyisistä kantasoluista johdettujen hermosoluviljelmien potentiaalia jäljitellä tiettyjä keskushermoston kehityksen ja toiminnallisuuden tunnusmerkkejä. Tulosten on tarkoitus auttaa validoimaan kantasolumallien hyödyllisyys soveltavan neurotieteen käyttötarkoituksissa. Tätä varten verrattiin kahdella yleisesti käytetyllä erilaistusmenetelmällä tuotettujen ihmisen monikykyisistä kantasoluista johdettujen hermosolujen erilaistumiskykyä. Hermosolujen toiminnallista kypsymistä arvioitiin sekä pidennetyn erilaistusajan jälkeen, että viljelyolosuhteiden optimoinnin seurauksena verkostotason analyyseillä. Tuloksia täydennettiin vertailemalla näiden verkostojen toiminnallisuutta laajalti käytettyyn jyrsijäperäiseen solumalliin. Koska astrosyytit ovat hermosoluja ympäröiviä soluja ja ne tukevat keskushermoston toiminnallisuutta, erityistä huomiota kiinnitettiin myös astrosyyttien rooliin sekä normaaleissa että tulehduksellisissa olosuhteissa, joista jälkimmäinen on tyypillinen tila keskushermoston vaurioissa. Tämän väitöskirjan tulokset viittaavat siihen, että ihmisen monikykyisistä kantasoluista johdetut hermosoluviljelmät toistavat useita in vivo olosuhteissa tapahtuvia keskushermoston kehityksen vaiheita. Erityinen kemiallinen induktio tehosti hermosoluerilaistusta johtaen korkeaan solupuhtauteen ja saantoon. Erilaistusajan pidennys lisäsi endogeenisesti muodostuvien astrosyyttien osuutta viljelmässä ja edisti hermosolujen toiminnallisuudessa havaittua kypsää aktiivisuustyyppiä. Lisäksi valitsemalla viljelypinnoitteeksi tietty laminiini-isoformi saavutettiin vahva, pitkäkestoinen hermosoluaktiivisuus muodostuneissa verkostoissa. Tämän kehitystyön ansiosta kantasoluista johdetut hermoverkot osoittivat samankaltaista ajallista ja vaiheittaista toiminnallisuuden kehitystä, kuin vastaavat jyrsijöistä eristetyt hermosolut. Jyrsijä- ja ihmisverkostojen toiminnallisuuden muodoissa havaittiin kuitenkin myös huomattavia eroavaisuuksia, mikä saattaa liittyä eroihin niiden kypsyysasteissa tai lajien välisiin eroavaisuuksiin. Lopuksi ihmisen monikykyisistä kantasoluista johdetut astrosyytit altistettiin tietyille tulehduksellisille tekijöille. Niiden vaste osoitti keskushermoston sairauksissa tyypillisesti havaitun astroglioosin erityispiirteitä, ja tutkitut hermosoluihin kohdistuvat vaikutukset viittasivat polarisaatioon hermosoluja tukevaksi fenotyyppiksi. Tutkimuksessa luodut kontrolloidut ihmisen hermosolujen ja astrosyyttien yhteisviljelmät tarjoavat vaihtoehtoisen kantasolupohjaisen alustan solujen vuorovaikutusten mallintamiseksi sekä terveessä että sairauskonteksteissa. Yhteenvetona voidaan todeta, että tämän väitöskirjan tulokset edistävät toiminnallisten ihmisen monikykyisistä kantasoluista johdettujen hermosoluverkkojen kehittämistä, ne vahvistavat astrosyyttien roolia merkittävinä kumppaneina näissä verkostoissa sekä rohkaisevat kantasolujen soveltamiseen ihmisspesifisinä malleina neurotieteen tutkimuksissa.Our understanding of human brain development and function is still incomplete. Unfortunately, the field of neuroscience lacks representative human-specific models to accompany the animal studies. Access to human brain tissue for research purposes is limited, encouraging the utilization of novel approaches such as human pluripotent stem cell (hPSC)-derived neurons. Within the last few decades, research on hPSCs has undergone enormous expansion. Growing expectations are aimed at the application of stem cells and their derivatives in regenerative medicine, drug screening and disease modeling. The main focus of this thesis was to evaluate the potential of hPSC-derived neural cultures in mimicking certain characteristics of central nervous system (CNS) development and functionality. The results were intended to help validate the utility of stem cell models for translational neuroscience applications. For this purpose, the differentiation capacities of hPSC-derived neuronal cells generated with two generally used differentiation methodologies were compared. The functional maturation of neurons following a prolonged differentiation time and optimization of culture conditions was assessed in network-level analyses. The results were complemented with a functional comparison to the widely used rodent in vitro model. Since astrocytes are the cells surrounding neurons and supporting neuronal functionality in the CNS, special focus was also placed on their role in both normal and neuroinflammatory conditions, the latter of which is typical of CNS insults. The results of this thesis suggest that hPSC-derived neuronal cultures recapitulate many of the characteristics of CNS development in vivo. Specific chemical induction accelerated neural differentiation, leading to high cell purity and yield. Prolongation of the differentiation time increased the proportion of endogenously formed astrocytes and promoted the functionally mature activity type of neurons. Furthermore, the emergence of robust neuronal activity and the long-term maintenance of functional networks were achieved with the selection of defined laminin isoform as a culture substrate. With this improvement, the hPSC-derived networks exhibited time frames and stages of activity development similar to those of their rodent counterparts. However, marked variability was detected in the activity patterns between the rodent and human networks, which could relate to differences in their maturation stage or interspecies dissimilarities. Finally, hPSC-derived astrocytes were exposed to specific inflammatory stimuli. Their response showed distinct characteristics of astrogliosis observed in CNS diseases, and the studied neuronal effects suggested polarization into a neurosupportive phenotype. Established controlled co-cultures with human neurons and astrocytes provide an alternative hPSC-based platform for modeling cell interactions in the context of health and disease. In conclusion, the work presented in this thesis advances the development of functional hPSC-derived neuronal networks, confirms the role of astrocytes as significant partners in these networks, and encourages their translation into human- specific models for neuroscience research

Disrupted CXCR2 Signaling in Oligodendroglia Lineage Cells Enhances Myelin Repair in a Viral Model of Multiple Sclerosis.

CXCR2 is a chemokine receptor expressed on oligodendroglia that has been implicated in the pathogenesis of neuroinflammatory demyelinating diseases as well as enhancement of the migration, proliferation, and myelin production by oligodendroglia. Using an inducible proteolipid protein (Plp) promoter-driven Cre-loxP recombination system, we were able to assess how timed ablation of Cxcr2 in oligodendroglia affected disease following intracranial infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV). Generation of Plp-Cre-ER(T)::Cxcr2flox/flox transgenic mice (termed Cxcr2-CKO mice) allows for Cxcr2 to be silenced in oligodendrocytes in adult mice following treatment with tamoxifen. Ablation of oligodendroglia Cxcr2 did not influence clinical severity in response to intracranial infection with JHMV. Infiltration of activated T cells or myeloid cells into the central nervous system (CNS) was not affected, nor was the ability to control viral infection. In addition, the severity of demyelination was similar between tamoxifen-treated mice and vehicle-treated controls. Notably, deletion of Cxcr2 resulted in increased remyelination, as assessed by g-ratio (the ratio of the inner axonal diameter to the total outer fiber diameter) calculation, compared to that in vehicle-treated control mice. Collectively, our findings argue that CXCR2 signaling in oligodendroglia is dispensable with regard to contributing to neuroinflammation, but its deletion enhances remyelination in a preclinical model of the human demyelinating disease multiple sclerosis (MS).IMPORTANCE Signaling through the chemokine receptor CXCR2 in oligodendroglia is important for developmental myelination in rodents, while chemical inhibition or nonspecific genetic deletion of CXCR2 appears to augment myelin repair in animal models of the human demyelinating disease multiple sclerosis (MS). To better understand the biology of CXCR2 signaling on oligodendroglia, we generated transgenic mice in which Cxcr2 is selectively ablated in oligodendroglia upon treatment with tamoxifen. Using a viral model of neuroinflammation and demyelination, we demonstrate that genetic silencing of CXCR2 on oligodendroglia did not affect clinical disease, neuroinflammation, or demyelination, yet there was increased remyelination. These findings support and extend previous findings suggesting that targeting CXCR2 may offer a therapeutic avenue for enhancing remyelination in patients with demyelinating diseases

Functional characterization of human pluripotent stem cell-derived cortical networks differentiated on laminin-521 substrate : comparison to rat cortical cultures

Human pluripotent stem cell (hPSC)-derived neurons provide exciting opportunities for in vitro modeling of neurological diseases and for advancing drug development and neurotoxicological studies. However, generating electrophysiologically mature neuronal networks from hPSCs has been challenging. Here, we report the differentiation of functionally active hPSC-derived cortical networks on defined laminin-521 substrate. We apply microelectrode array (MEA) measurements to assess network events and compare the activity development of hPSC-derived networks to that of widely used rat embryonic cortical cultures. In both of these networks, activity developed through a similar sequence of stages and time frames; however, the hPSC-derived networks showed unique patterns of bursting activity. The hPSC-derived networks developed synchronous activity, which involved glutamatergic and GABAergic inputs, recapitulating the classical cortical activity also observed in rodent counterparts. Principal component analysis (PCA) based on spike rates, network synchronization and burst features revealed the segregation of hPSC-derived and rat network recordings into different clusters, reflecting the species-specific and maturation state differences between the two networks. Overall, hPSC-derived neural cultures produced with a defined protocol generate cortical type network activity, which validates their applicability as a human-specific model for pharmacological studies and modeling network dysfunctions.Peer reviewe

Dual-Transmitter Systems Regulating Arousal, Attention, Learning and Memory

An array of neuromodulators, including monoamines and neuropeptides, regulate most behavioural and physiological traits. In the past decade, dramatic progress has been made in mapping neuromodulatory circuits, in analysing circuit dynamics, and interrogating circuit function using pharmacogenetic, optogenetic and imaging methods This review will focus on several distinct neural networks (acetylcholine/GABA/glutamate; histamine/GABA; orexin/glutamate; and relaxin-3/GABA) that originate from neural hubs that regulate wakefulness and related attentional and cognitive processes, and highlight approaches that have identified dual transmitter roles in these behavioural functions. Modulation of these different neural networks might be effective treatments of diseases related to arousal/sleep dysfunction and of cognitive dysfunction in psychiatric and neurodegenerative disorders

Alcohol and the Developing Brain: Why Neurons Die and How Survivors Change

The consequences of alcohol drinking during pregnancy are dramatic and usually referred to as fetal alcohol spectrum disorders (FASD). This condition is one of the main causes of intellectual disability in Western countries. The immature fetal brain exposed to ethanol undergoes massive neuron death. However, the same mechanisms leading to cell death can also be responsible for changes of developmental plasticity. As a consequence of such a maladaptive plasticity, the functional damage to central nervous system structures is amplified and leads to permanent sequelae. Here we review the literature dealing with experimental FASD, focusing on the alterations of the cerebral cortex. We propose that the reciprocal interaction between cell death and maladaptive plasticity represents the main pathogenetic mechanism of the alcohol-induced damage to the developing brain