Biology subjects

Brown, L. S.

Publications and source records attributed to Brown, L. S..

10 recordsLinked to original sources

bioRxiv2025

Transfer of graded information through gated receptivity to widely broadcast signals

Making accurate decisions requires the brain to maintain evolving representations of the evidence supporting different possible choices. The population of neurons that maintains this evolving representation may change over the course of the decision-making process. Such changes may result from a switch in the set of neurons representing the relevant behavioral output or a switch in the set of neurons receiving task-relevant information. Recent work has shown how intervening actions like eye movements or navigation to a new location can shift the set of neurons that encode subsequent inputs and outputs. Here, we present a computational model of how analog-valued information that supports an evolving decision can be flexibly transferred between populations of neurons without changing the synaptic connectivity of the underlying network. This is accomplished through a dynamic gating mechanism in which information is widely broadcast throughout a network, but only those neurons receiving a gating signal are receptive to this information. Using this framework, we provide a mechanistic explanation for recent experimental results in which information supporting an evolving decision was shown to be transferred between different populations of parietal cortex neurons during intervening smooth pursuit and saccadic eye movements. This mechanism enables organisms to maintain a continuous decision process despite changing frames of reference, offering a potentially general framework for cognitive continuity in dynamic environments.

neuroscience↗

bioRxiv2025

Striatal pathways oppositely shift cortical activity along the decision axis

The cortex and basal ganglia are organized into multiple parallel loops that serve motor, limbic, and cognitive functions. The classic model of cortico-basal ganglia interactions posits that within each loop, the direct pathway of the basal ganglia activates the cortex and the indirect pathway inhibits it1-3. While this model has found support in the motor domain4,5, whether opponent control by the two pathways extends to the cognitive domain remains unknown. Here, we record from anterior cingulate cortex (ACC) and dorsomedial striatum (DMS) while inhibiting direct or indirect pathway neurons in DMS, as mice perform an accumulation-of-evidence task6-10. Inconsistent with the classic model, the manipulations do not produce opponent changes in overall ACC activity. Instead, the pathways exert opponent influence over a subpopulation of ACC neurons that encode accumulated sensory evidence, the task-relevant decision variable. The direction of the modulation depends on a neurons tuning to ipsilateral versus contralateral evidence, such that the two pathways generate opponent shifts in coding specifically along the decision axis. Thus, our results uncover unexpected specificity in the effects of basal ganglia pathways on the cortex, with the two pathways of the DMS exerting opponent control not on overall activity but on coding of the relevant task variable. This functional specificity may extend to other basal ganglia loops to support different aspects of adaptive behavior, with the pathways serving a general role in selecting and shifting cortical representations to subserve circuit-specific functions.

neuroscience↗

bioRxiv2024

Rapamycin Treatment Reduces Brain Pericyte Constriction in Ischemic Stroke

The contraction and subsequent death of brain pericytes may play a role in microvascular no-reflow following the re-opening of an occluded artery during ischemic stroke. Mammalian target of rapamycin (mTOR) inhibition has been shown to reduce motility/contractility of various cancer cell lines and reduce neuronal cell death in stroke. However, the effects of mTOR inhibition on brain pericyte contraction and death during ischemia have not yet been investigated. Cultured pericytes exposed to simulated ischemia for 12 hours in vitro contracted after less than 1 h, which was about 7h prior to cell death. Rapamycin significantly reduced the rate of pericyte contraction during ischemia, however, it did not have a significant effect on pericyte viability at any time point. Rapamycin appeared to reduce pericyte contraction through a RhoA-dependent pathway, independent of changes in intracellular calcium. Using a mouse model of middle cerebral artery occlusion, rapamycin significantly increased the diameter of capillaries underneath pericytes and increased the number of open capillaries 30 minutes following recanalization. Our findings suggest rapamycin may be a useful adjuvant therapeutic to reduce pericyte contraction and improve cerebral reperfusion post-stroke.

neuroscience↗

bioRxiv2023

Pericyte ablation causes hypoactivity and reactive gliosis in adult mice

Pericytes are contractile cells that enwrap capillaries allowing them to control blood flow, maintain the blood-brain barrier and regulate immune cell trafficking in the CNS. Pericytes are lost or become dysfunctional in neurodegenerative diseases such as Alzheimers disease, stroke, and multiple sclerosis, but their role in health and disease is poorly understood. Our aim was to evaluate blood-brain barrier integrity and glial reactivity, and to assess behavioural phenotypes that emerge following pericyte ablation in adult mice. The delivery of tamoxifen to PDGFR{beta}-CreERT2 :: Rosa26-DTA transgenic mice produced a dose-dependent ablation of pericytes. A single low dose of tamoxifen ablated approximately half of all brain pericytes, and two consecutive daily high doses ablated more than 80% of brain pericytes. To determine whether pericyte ablation could induce a behavioural phenotype, we assessed patterns of voluntary movement, as well as balance and coordination using the open field and beam walk tasks. Mice with [~]50% pericyte loss travelled half the distance and spent half as much time moving in the open field as control mice. Mice with more than 80% pericyte ablation also slipped more frequently in the beam walk task than control mice. In brain cryosections from pericyte-ablated mice, blood vessel structure was unchanged, but lumen area was increased. Pericyte-ablated mice also experienced blood-brain barrier leakage, hypoxia and increased microgliosis and astrogliosis compared to control mice. Our results highlight the importance of pericytes for brain health, as pericyte loss can directly drive brain injury and behavioural alterations in mice.

neuroscience↗

bioRxiv2023

Induced pluripotent stem cell derived pericytes respond to endogenous mediators of proliferation and contractility

BackgroundPericytes are multifunctional contractile cells that reside on capillaries. Pericytes are critical regulators of cerebral blood flow and blood-brain barrier function, and pericyte dysfunction may contribute to the pathophysiology of human neurological diseases including Alzheimers disease, multiple sclerosis, and stroke. Induced pluripotent stem cell (iPSC)-derived pericytes (iPericytes) are a promising tool for vascular research. However, it is unclear how iPericytes functionally compare to primary human brain vascular pericytes (HBVPs). We differentiated iPSCs into iPericytes of either the mesoderm or neural crest lineage using established protocols. We compared iPericyte and HBVP morphologies, quantified gene expression by qPCR and bulk RNA sequencing, and visualised pericyte protein markers by immunocytochemistry. To determine whether the gene expression of neural crest iPericytes, mesoderm iPericytes or HBVPs correlated with their functional characteristics in vitro, we quantified EdU incorporation following exposure to the key pericyte mitogen, platelet derived growth factor (PDGF)-BB and, contraction and relaxation in response to the vasoconstrictor endothelin-1 or vasodilator adenosine, respectively. iPericytes were morphologically similar to HBVPs and expressed canonical pericyte markers. However, iPericytes had 1864 differentially expressed genes compared to HBVPs, while there were 797 genes differentially expressed between neural crest and mesoderm iPericytes. Consistent with the ability of HBVPs to respond to PDGF-BB signalling, PDGF-BB enhanced and PDGF receptor-beta inhibitors impaired iPericyte proliferation. Administration of endothelin-1 led to iPericyte contraction and adenosine led to iPericyte relaxation, of a magnitude similar to the response evoked in HBVPs. We determined that neural crest iPericytes were less susceptible to PDGFR beta inhibition, but responded most robustly to vasoconstrictive meditators. iPericytes express pericyte-associated genes and proteins and, exhibit an appropriate physiological response upon exposure to a key endogenous mitogen or vasoactive mediators. Therefore, the generation of functional iPericytes would be suitable for use in future investigations exploring pericyte function or dysfunction in neurological diseases.

cell biology↗

bioRxiv2023

Neural circuit models for evidence accumulation through choice-selective sequences

Decision making is traditionally thought to be mediated by neurons that accumulate evidence through persistent activity. However, recent decision-making experiments in rodents have observed neurons across the brain that fire sequentially, rather than persistently, with the subset of neurons in the sequence depending on the animals choice. We developed two new candidate circuit models in which neurons are active sequentially and transfer evidence faithfully to the next active population. One model encodes evidence in the relative firing of two competing chains of neurons, and the other in the network location of a stereotyped pattern ("bump") of neural activity. Neural recordings from four brain regions during an evidence accumulation task revealed that different regions displayed evidence tuning consistent with different candidate models. This work provides a mechanistic explanation for how graded information may be precisely accumulated within and transferred between neural populations, and suggests that different brain regions may accumulate evidence through different circuit mechanisms.

neuroscience↗

bioRxiv2022

Microglia associations with brain pericytes and the vasculature are reduced in Alzheimer's disease

Cerebral blood flow is important for the maintenance of brain function and its dysregulation has been implicated in Alzheimers disease (AD). Subpopulations of microglia have well-characterised associations with the vasculature in the central nervous system but the precise relationship between microglia and cells which exist on the vasculature is not yet clear. In this study we explored the relationship between microglia and pericytes, a vessel-resident cell type that has a major role in the regulation of cerebral blood flow and maintenance of the blood brain barrier. Using fixed tissue sections and in vivo live imaging, we discovered a subset of microglia that closely associated with pericytes, termed PEricyte-associated Microglia (PEM). PEM are present throughout all regions of the brain and spinal cord in NG2DsRed x CX3CR1+/GFP mice, and in the human frontal cortex. They reside adjacent to pericytes at all levels of the capillary tree and can maintain their position for at least 28 days. PEM associate with pericytes lacking astroglial endfeet coverage but are segregated from pericytes by capillary basement membranes and capillary vessel width is similarly increased beneath pericytes with or without an associated PEM. Deletion of the microglia fractalkine receptor (CX3CR1) did not disrupt the association between pericytes and PEM, suggesting the association is not reliant on fractalkine signalling. Finally, we found that the proportion of microglia that are capillary-associated and PEM declines in the superior frontal gyrus (SFG) in AD, which is exacerbated by the APOE {varepsilon}3/{varepsilon}4 genotype. In summary, we identify and characterise a subpopulation of microglia that specifically associate with pericytes and find this population is reduced in the SFG in AD. This reduction may be a novel mechanism contributing to vascular dysfunction in diseases such as AD.

neuroscience↗

bioRxiv2021

Kalium rhodopsins: Natural light-gated potassium channels

We report a family of K+ channels, kalium channelrhodopsins (KCRs) from a fungus-like protist. Previously known potassium channels, widespread and mainly ligand- or voltage-gated, share a conserved pore-forming domain and K+-selectivity filter. KCRs differ in that they are light-gated and they have independently evolved an alternative K+ selectivity mechanism. The KCRs are potent, highly selective of K+ over Na+, and open in less than 1 millisecond following photoactivation. Their permeability ratio PK/PNa of [~] 20 make KCRs powerful hyperpolarizing tools that suppress excitable cell firing upon illumination, demonstrated here in mouse cortical neurons. KCRs enable specific optogenetic photocontrol of K+ gradients promising for the study and potential treatment of potassium channelopathies such as epilepsy, Parkinsons disease, and long-QT syndrome and other cardiac arrhythmias. One-Sentence SummaryPotassium-selective channelrhodopsins long-sought for optogenetic research and therapy of neurological and cardiac diseases.

biochemistry↗

bioRxiv2021

Cation and anion channelrhodopsins: Sequence motifs and taxonomic distribution

Cation and anion channelrhodopsins (CCRs and ACRs, respectively) primarily from two algal species, Chlamydomonas reinhardtii and Guillardia theta, have become widely used as optogenetic tools to control cell membrane potential with light. We mined algal and other protist polynucleotide sequencing projects and metagenomic samples to identify 75 channelrhodopsin homologs from three channelrhodopsin families, including one revealed in dinoflagellates in this study. We carried out electrophysiological analysis of 33 natural channelrhodopsin variants from different phylogenetic lineages and 10 metagenomic homologs in search of sequence determinants of ion selectivity, photocurrent desensitization, and spectral tuning in channelrhodopsins. Our results show that association of a reduced number of glutamates near the conductance path with anion selectivity depends on a wider protein context, because prasinophyte homologs with the identical glutamate pattern as in cryptophyte ACRs are cation-selective. Desensitization is also broadly context-dependent, as in one branch of stramenopile ACRs and their metagenomic homologs its extent roughly correlates with phylogenetic relationship of their sequences. Regarding spectral tuning, two prasinophyte CCRs exhibit red-shifted spectra to 585 nm, although their retinal-binding pockets do not match those of previously known similarly red-shifted channelrhodopsins. In cryptophyte ACRs we identified three specific residue positions in the retinal-binding pocket that define the wavelength of their spectral maxima. Lastly, we found that dinoflagellate rhodopsins with a TCP motif in the third transmembrane helix and a metagenomic homolog exhibit channel activity. IMPORTANCEChannelrhodopsins are widely used in neuroscience and cardiology as research tools and are considered as prospective therapeutics, but their natural diversity and mechanisms remain poorly characterized. Genomic and metagenomic sequencing projects are producing an ever-increasing wealth of data, whereas biophysical characterization of the encoded proteins lags behind. In this study we used manual and automated patch clamp recording of representative members of four channelrhodopsin families including a family that we report in this study in dinoflagellates. Our results contribute to a better understanding of molecular determinants of ionic selectivity, photocurrent desensitization, and spectral tuning in channelrhodopsins.

biochemistry↗

bioRxiv2020

RubyACRs, non-algal anion channelrhodopsins with highly red-shifted absorption

Channelrhodopsins are light-gated ion channels widely used to control neuronal firing with light (optogenetics). We report two previously unknown families of anion channelrhodopsins (ACRs), one from the heterotrophic protists labyrinthulomycetes and the other from haptophyte algae. Four closely related labyrinthulomycete ACRs, named RubyACRs here, exhibit a unique retinal binding pocket that creates spectral sensitivities with maxima at 590-610 nm, the most red-shifted channelrhodopsins known, long-sought for optogenetics, and more broadly the most red-shifted microbial rhodopsins so far reported. We identified three spectral tuning residues critical for the red-shifted absorption. Photocurrents recorded from the RubyACR from Aurantiochytrium limacinum (designated AlACR1) under single-turnover excitation exhibited biphasic decay, the rate of which was only weakly voltage-dependent, in contrast to that in previously characterized cryptophyte ACRs, indicating differences in channel gating mechanisms between the two ACR families. Moreover, in A. limacinum we identified three ACRs with absorption maxima at 485, 545, and 590 nm, indicating color-sensitive photosensing with blue, green and red spectral variation of ACRs within individual species of the labyrinthulomycete family. We also report energy transfer from a cytoplasmic fluorescent protein domain to the retinal chromophore bound within RubyACRs, not seen in similar constructs in other channelrhodopsins. Significance StatementOur identification and characterization of two ACR families, one from non-photosynthetic microorganisms, shows that light-gated anion conductance is more widely spread among eukaryotic lineages than previously thought. The uniquely far red-shifted absorption spectra of the subset we designate RubyACRs provide the long-sought inhibitory optogenetic tools producing large passive currents activated by long-wavelength light, enabling deep tissue penetration. Previously only low-efficiency ion-pumping rhodopsins were available for neural inhibition by the orange-red region of the spectrum. The unusual amino acid composition of the retinal-binding pocket in RubyACRs expands our understanding of color tuning in retinylidene proteins. Finally, energy transfer from the fluorescent protein used as a tag on RubyACRs opens a potential new dimension in molecular engineering of optogenetic tools.

biochemistry↗