Post-translational regulation of neuronal differentiation

Glycosphingolipids (GSLs) are amphipathic lipid moieties that make up only 3% of the total lipid content of the cell and are almost exclusively expressed at the Plasma Membrane(PM). They are key drivers of signalling hotspots known as âlipid-raftsâ as well as acting as primary signalling molecules themselves. Pathologically, perturbations in their synthesis (Knock-Out mice models) and disorders in their metabolism(Lysosomal storage disorders), both converge on neurological symptoms. In fact, during neuronal development, stem cells have been known to completely remodel their glycosphingolipid profiles, so much so, that the developmental stages of the nervous system have been associated with the expression of different glycosphingolipids. This coordinated change in the expression of glycosphingolipids has been previously attributed to the transcriptional shift in the the expression of genes encoding specific Glycosphingolipid Synthesizing Enzymes (GSEs) observed over neuronal development and, more recently, shown to be internally regulated by the GSLs themselves whereby globosides negatively regulate the expression of the ganglioside synthesizing enzyme GM3S via a neuronal transcription factor AUTS2. In this thesis I show that there exists an extra layer of post-translational regulation that is active over neuronal development that destabilizes the Gb3 synthesizing enzyme, Gb3S, on one hand while stabilizing the GM3S on the other. The regulatory mechanism achieves this effect by doubling the rate of degradation of Gb3S in neuronal cells, probably via membrane trafficking events. The exact details of the stabilization effect of GM3S remains to be determined as the results could not ascertain if it was the same mechanism or a completely different one. My study goes on to show that this regulatory mechanism is activated early during development, preceding the changes in canonical stem and neuronal markers. Morphologically, the study points towards a post-compaction but pre-cavitation (E2.5-E3.5) point of activation for the mechanism of Gb3S destabalization, relative to early mouse embryological development. Furthermore, I found that the Gb3S interactor and AUTS2 transcriptional target UCHL1 is a key regulatory unit of the machinery such that inhibiting its activity results in a specific rescue of the Gb3S enzyme.

Preserving axosomatic spiking features despite diverse dendritic morphology

Henry Markram, Idan Segev

Throughout the nervous system, cells belonging to a certain electrical class (e-class)-sharing high similarity in firing response properties-may nevertheless have widely variable dendritic morphologies. To quantify the effect of this morphological variability on the firing of layer 5 thick-tufted pyramidal cells (TTCs), a detailed conductance-based model was constructed for a three-dimensional reconstructed exemplar TTC. The model exhibited spike initiation in the axon and reproduced the characteristic features of individual spikes, as well as of the firing properties at the soma, as recorded in a population of TTCs in young Wistar rats. When using these model parameters over the population of 28 three-dimensional reconstructed TTCs, both axonal and somatic ion channel densities had to be scaled linearly with the conductance load imposed on each of these compartments. Otherwise, the firing of model cells deviated, sometimes very significantly, from the experimental variability of the TTC e-class. The study provides experimentally testable predictions regarding the coregulation of axosomatic membrane ion channels density for cells with different dendritic conductance load, together with a simple and systematic method for generating reliable conductance-based models for the whole population of modeled neurons belonging to a particular e-class, with variable morphology as found experimentally.

American Physiological Society2013

Developmental and spatial expression pattern of syntaxin 13 in the mouse central nervous system

Harald Hirling, Juan-Carlos Sarria

Vesicular transport involves SNARE (soluble- N-ethylmaleimide-sensitive-factor-attachment-protein-receptor) proteins on transport vesicles and on target membranes. Syntaxin 13 is a SNARE enriched in brain, associated with recycling endosomes; its overexpression in PC12 cells promotes neurite outgrowth. This suggests an important role for receptor recycling during neuronal differentiation. Here we describe the spatiotemporal pattern of syntaxin 13 expression during mouse brain development. During early embryogenesis (E12-E15), it was found in the forebrain ventricular zone and in primary motor and sensory neurons in the brainstem, spinal cord and sensory ganglia. In the forebrain at E15, syntaxin 13 was not detected in neuroblasts in the intermediate zone of the embryonic hemispheric wall, while there was labeling in cortical neurons in deeper layers starting at E15-18, and progressively in later-generated neurons up to layer II around P6. Syntaxin 13 reached maximal expression in all brain divisions at about P7, followed by a decrease, with heterogeneous neuron populations displaying various staining intensities in adult brain. While usually restricted to the soma of neurons, we transiently detected syntaxin 13 in dendrites of pyramidal neurons during the first postnatal week. In conclusion, the developmentally regulated syntaxin 13 expression in various neuronal populations is consistent with its involvement in endocytic trafficking and neurite outgrowth.

2002

The Neuron Phenotype Ontology: A FAIR Approach to Proposing and Classifying Neuronal Types

Sean Lewis Hill, Mohameth François Sy

The challenge of defining and cataloging the building blocks of the brain requires a standardized approach to naming neurons and organizing knowledge about their properties. The US Brain Initiative Cell Census Network, Human Cell Atlas, Blue Brain Project, and others are generating vast amounts of data and characterizing large numbers of neurons throughout the nervous system. The neuroscientific literature contains many neuron names (e.g. parvalbumin-positive interneuron or layer 5 pyramidal cell) that are commonly used and generally accepted. However, it is often unclear how such common usage types relate to many evidence-based types that are proposed based on the results of new techniques. Further, comparing different types across labs remains a significant challenge. Here, we propose an interoperable knowledge representation, the Neuron Phenotype Ontology (NPO), that provides a standardized and automatable approach for naming cell types and normalizing their constituent phenotypes using identifiers from community ontologies as a common language. The NPO provides a framework for systematically organizing knowledge about cellular properties and enables interoperability with existing neuron naming schemes. We evaluate the NPO by populating a knowledge base with three independent cortical neuron classifications derived from published data sets that describe neurons according to molecular, morphological, electrophysiological, and synaptic properties. Competency queries to this knowledge base demonstrate that the NPO knowledge model enables interoperability between the three test cases and neuron names commonly used in the literature.

HUMANA PRESS INC2022