A connectivity-constrained computational account of topographic organization in primate high-level visual cortex

Inferotemporal (IT) cortex in humans and other primates is topographically organized, containing multiple hierarchically organized areas selective for particular domains, such as faces and scenes. This organization is commonly viewed in terms of evolved domain-specific visual mechanisms. Here, we develop an alternative, domain-general and developmental account of IT cortical organization. The account is instantiated in interactive topographic networks (ITNs), a class of computational models in which a hierarchy of model IT areas, subject to biologically plausible connectivity-based constraints, learns high-level visual representations optimized for multiple domains. We find that minimizing a wiring cost on spatially organized feedforward and lateral connections, alongside realistic constraints on the sign of neuronal connectivity within model IT, results in a hierarchical, topographic organization. This organization replicates a number of key properties of primate IT cortex, including the presence of domain-selective spatial clusters preferentially involved in the representation of faces, objects, and scenes; columnar responses across separate excitatory and inhibitory units; and generic spatial organization whereby the response correlation of pairs of units falls off with their distance. We thus argue that topographic domain selectivity is an emergent property of a visual system optimized to maximize behavioral performance under generic connectivity-based constraints.

Dual in Utero Electroporation in Mice to Manipulate Two Specific Neuronal Populations in the Developing Cortex

Precise regulation of gene expression during development in cortical neurons is essential for the establishment and maintenance of neuronal connectivity and higher-order cognition. Dual in utero electroporation provides a precise and effective tool to label and manipulate gene expression in multiple neuronal populations within a circuit in a spatially and temporally regulated manner. In addition, this technique allows for morphophysiological investigations into neuronal development and connectivity following cell-specific gene manipulations. Here, we detail the dual in utero electroporation protocol.

An extended toolkit for production and use of RVdG-CVS-N2c rabies viral vectors uncovers hidden hippocampal connections

From the large collection of molecular tools used to investigate neuronal connectivity, envA-pseudotyped rabies viral vectors (RVdGenvA) uniquely enable cell-type specific, trans-synaptic retrograde labeling. However, widespread use of the powerful and flexible method is to date hindered by low-yield and cumbersome production pipelines. Here, we report the development of new cell lines, which significantly reduce production time while increasing viral titer and eliminating background contamination from native-coat particles. We further show that RVdGenvA-CVS-N2c vectors produced using this system retain their enhanced retrograde-trafficking when compared with SAD-B19 vectors, allowing us to uncover undescribed cortico-hippocampal connections and to monitor activity in a cortical microcircuit of behaving animals. Along with new suites of AAV and RVdG-CVS-N2c vectors, developed to enable retrograde labeling from a wide range of neuronal populations and tailored for diverse experimental requirements, we present here an optimal system for mapping, manipulating and imaging of neuronal circuits.

Necroptosis inhibition counteracts axonal degeneration, cognitive decline and key hallmarks of aging, promoting brain rejuvenation.

Age is the main risk factor for cognitive impairment and the development of neurodegenerative diseases. In the aged brain, axonal degeneration is an early pathological event, preceding neuronal dysfunction and brain disabilities in humans, primates, rodents, and invertebrates. Necroptosis activation mediates degeneration of mechanical and chemically injured axons, but whether this pathway triggers axonal degeneration and cognitive impairment during brain aging has not been studied. Here we show that necroptosis is activated in the hippocampus during aging, especially in axonal tracts. Loss of the main necroptotic effector, Mlkl, was sufficient to delay age-associated axonal degeneration. Accordingly, aged Mlkl-KO mice also displayed a youthful phenotype at the synaptic and functional level, protecting against decreased synaptic transmission and memory decline. Short-term pharmacologic inhibition of necroptosis by targeting RIPK3 in aged mice, proved to be extraordinarily effective at reverting axonal degeneration and hippocampal-dependent functional impairment at the electrophysiological and behavioral level. Remarkably, a comprehensive quantitative proteomic analysis uncovered a set of aging hallmarks that were recovered in both, the genetic and pharmacologic models of necroptosis inhibition, including molecular biofunctions associated with brain rejuvenation. Taken together, these findings demonstrate that necroptosis contributes to the age-associated deterioration of axonal integrity, affecting hippocampal neuronal connectivity and cognitive function in aged individuals. We therefore propose necroptosis as an attractive target for the future development of geroprotective tools to treat age-related disabilities.

Silencing neuronal activity is required for developmental circuit remodeling

Postnatal refinement of neuronal connectivity shapes the mature nervous system. Pruning of exuberant connections involves both cell autonomous and non-cell autonomous mechanisms, such as neuronal activity. While the role of neuronal activity in the plasticity of excitatory synapses has been extensively studied, the involvement of inhibition is less clear. Furthermore, the role of activity during stereotypic developmental remodeling, where competition is not as apparent, is not well understood. Here we use the Drosophila mushroom body as a model to show that regulated silencing of neuronal activity is required for developmental axon pruning of the γ-Kenyon cells. We demonstrate that silencing neuronal activity is mechanistically achieved by cell autonomous expression of the inward rectifying potassium channel (irk1) combined with inhibition by the GABAergic APL neuron. These results support the Hebbian-like rule 'use it or lose it', where inhibition can destabilize connectivity and promote pruning while excitability stabilizes existing connections.

Environmental Enrichment Rescues Social Behavioral Deficits and Synaptic Abnormalities in Pten Haploinsufficient Mice

Pten germline haploinsufficient (Pten+/−) mice, which model macrocephaly/autism syndrome, show social and repetitive behavior deficits, early brain overgrowth, and cortical–subcortical hyperconnectivity. Previous work indicated that altered neuronal connectivity may be a substrate for behavioral deficits. We hypothesized that exposing Pten+/− mice to environmental enrichment after brain overgrowth has occurred may facilitate adaptation to abnormal “hard-wired” connectivity through enhancing synaptic plasticity. Thus, we reared Pten+/− mice and their wild-type littermates from weaning under either standard (4–5 mice per standard-sized cage, containing only bedding and nestlet) or enriched (9–10 mice per large-sized cage, containing objects for exploration and a running wheel, plus bedding and nestlet) conditions. Adult mice were tested on social and non-social assays in which Pten+/− mice display deficits. Environmental enrichment rescued sex-specific deficits in social behavior in Pten+/− mice and partially rescued increased repetitive behavior in Pten+/− males. We found that Pten+/− mice show increased excitatory and decreased inhibitory pre-synaptic proteins; this phenotype was also rescued by environmental enrichment. Together, our results indicate that environmental enrichment can rescue social behavioral deficits in Pten+/− mice, possibly through normalizing the excitatory synaptic protein abundance.

A Role for Frizzled and Their Post-Translational Modifications in the Mammalian Central Nervous System

The Wnt pathway is a key signalling cascade that regulates the formation and function of neuronal circuits. The main receptors for Wnts are Frizzled (Fzd) that mediate diverse functions such as neurogenesis, axon guidance, dendritogenesis, synapse formation, and synaptic plasticity. These processes are crucial for the assembly of functional neuronal circuits required for diverse functions ranging from sensory and motor tasks to cognitive performance. Indeed, aberrant Wnt–Fzd signalling has been associated with synaptic defects during development and in neurodegenerative conditions such as Alzheimer’s disease. New studies suggest that the localisation and stability of Fzd receptors play a crucial role in determining Wnt function. Post-translational modifications (PTMs) of Fzd are emerging as an important mechanism that regulates these Wnt receptors. However, only phosphorylation and glycosylation have been described to modulate Fzd function in the central nervous system (CNS). In this review, we discuss the function of Fzd in neuronal circuit connectivity and how PTMs contribute to their function. We also discuss other PTMs, not yet described in the CNS, and how they might modulate the function of Fzd in neuronal connectivity. PTMs could modulate Fzd function by affecting Fzd localisation and stability at the plasma membrane resulting in local effects of Wnt signalling, a feature particularly important in polarised cells such as neurons. Our review highlights the importance of further studies into the role of PTMs on Fzd receptors in the context of neuronal connectivity.

Extracellular detection of neuronal coupling

We developed a method to non-invasively detect synaptic relationships among neurons from in vitro networks. Our method uses microelectrode arrays on which neurons are cultured and from which propagation of extracellular action potentials (eAPs) in single axons are recorded at multiple electrodes. Detecting eAP propagation bypasses ambiguity introduced by spike sorting. Our methods identify short latency spiking relationships between neurons with properties expected of synaptically coupled neurons, namely they were recapitulated by direct stimulation and were sensitive to changing the number of active synaptic sites. Our methods enabled us to assemble a functional subset of neuronal connectivity in our cultures.

Stimulation-mediated reverse engineering of silent neural networks

Reconstructing connectivity of neuronal networks from single cell activity is essential to understanding brain function, but the challenge of deciphering connections from populations of silent neurons has been largely unmet. We demonstrate a protocol for deriving connectivity of realistic silent neuronal networks using stimulation combined with a supervised learning algorithm, that enables inferring connection weights with high fidelity and predicting spike trains at the single-spike and single-cell level with high accuracy. These testable predictions about the number and protocol of the required stimulations is expected to enhance future efforts for deriving neuronal connectivity and drive new experiments to better understand brain function.