The adult brain is a plastic place. To ensure that the mature nervous system's control of behaviour is flexible in the face of a varying environment, morphological and physiological changes are possible at many levels, including that of the entire cell. In two areas of the adult brain - the olfactory bulb and the dentate gyrus - new neurons are generated throughout life and form an integral part of the normal functional circuitry. This process is not fixed, but highly modulated, revealing a plastic mechanism by which the brain's performance can be optimized for a given environment. The functional benefits of this whole-cell plasticity, however, remain a matter for debate.

In neurons, the axon initial segment (AIS) is a specialised region near the start of the axon that is the site of action potential initiation [1][2][3][4][5][6] . The precise location of the AIS varies across and within different neuronal types 7,8 , and has been linked to cells' information-processing capabilities 8 ; however, the factors determining AIS position in individual neurons remain unknown. Here we show that changes in electrical activity can alter the location of the AIS. In dissociated hippocampal cultures, chronic depolarization with high potassium moves multiple components of the AIS, including voltage-gated sodium channels, up to 17 μm away from the soma of excitatory neurons. This movement reverses when neurons are returned to non-depolarized conditions, and depends upon the activation of T-and/or L-type voltage-gated calcium channels. The AIS also moved distally when we combined long-term LED (light-emitting diode) photostimulation with sparse neuronal expression of the light-activated cation channel channelrhodopsin-2; here, burst patterning of activity was successful where regular stimulation at the same frequency failed. Furthermore, changes in AIS position correlate with alterations in current thresholds for action potential spiking. Our results show that neurons can regulate the position of an entire subcellular structure according to their ongoing levels and patterns of electrical activity. This novel form of activity-dependent plasticity may fine-tune neuronal excitability during development.In dissociated hippocampal neurons, global depolarization with 15 mM extracellular potassium from 12 to 14 days in vitro (DIV) produced a significant distal shift in AIS location ( Fig. 1a-d and Supplementary Figs 1-3). Labelling for the AIS scaffolding protein ankyrin G 9 , showed that start, maximum, and end AIS positions were all significantly relocated away from the soma (Fig. 1a,b; start: Mann-Whitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 885 cells, 36 coverslips), leaving the length of the AIS unchanged ( Fig. 1a,b; P = 0.11). We also observed significant activity-dependent distal shifts in other AIS-specific proteins 10,11 , including the scaffolding protein βIV spectrin ( Fig. 1c,d; start: Mann-Whitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 1065 cells, 44 coverslips), the extracellular matrix binding protein neurofascin ( Fig. 1d; start: Mann-Whitney U-test, P < 0.0001; maximum: P = 0.002; end: P = 0.01; n = 96 cells, 4 coverslips), the ion channel-associated protein FGF14 ( Fig. 1d; start: MannWhitney U-test, P < 0.0001; maximum: P < 0.0001; end: P < 0.0001; n = 89 cells, 4 coverslips), and, vitally, the voltage-gated sodium channels (VGSCs) essential for actionCorrespondence and requests for materials should be addressed to M.G. (matthew.grubb@kcl.ac.uk) or J.B. (juan.burrone@kcl.ac.uk).. Author contributions M.G. planned and performed all experiments and analysis, and wrote the paper. J.B. produced simulation data, planned experiments,...

Abstract:Altered olfactory function is a common symptom of COVID-19, but its etiology is unknown. A key question is whether SARS-CoV-2 (CoV-2) – the causal agent in COVID-19 – affects olfaction directly, by infecting olfactory sensory neurons or their targets in the olfactory bulb, or indirectly, through perturbation of supporting cells. Here we identify cell types in the olfactory epithelium and olfactory bulb that express SARS-CoV-2 cell entry molecules. Bulk sequencing demonstrated that mouse, non-human primate and human olfactory mucosa expresses two key genes involved in CoV-2 entry, ACE2 and TMPRSS2. However, single cell sequencing revealed that ACE2 is expressed in support cells, stem cells, and perivascular cells, rather than in neurons. Immunostaining confirmed these results and revealed pervasive expression of ACE2 protein in dorsally-located olfactory epithelial sustentacular cells and olfactory bulb pericytes in the mouse. These findings suggest that CoV-2 infection of non-neuronal cell types leads to anosmia and related disturbances in odor perception in COVID-19 patients.

Recent reports suggest an association between COVID-19 and altered olfactory function. Here we analyze bulk and single cell RNA-Seq datasets to identify cell types in the olfactory epithelium that express molecules that mediate infection by SARS-CoV-2 (CoV-2), the causal agent in COVID-19. We find in both mouse and human datasets that olfactory sensory neurons do not express two key genes involved in CoV-2 entry, ACE2 and TMPRSS2. In contrast, olfactory epithelial support cells and stem cells express both of these genes, as do cells in the nasal respiratory epithelium. Taken together, these findings suggest possible mechanisms through which CoV-2 infection could lead to anosmia or other forms of olfactory dysfunction.

No abstract

Recent developments have used light-activated channels or transporters to modulate neuronal activity. One such genetically-encoded modulator of activity, channelrhodopsin-2 (ChR2), depolarizes neurons in response to blue light. In this work, we first conducted electrophysiological studies of the photokinetics of hippocampal cells expressing ChR2, for various light stimulations. These and other experimental results were then used for systematic investigation of the previously proposed three-state and four-state models of the ChR2 photocycle. We show the limitations of the previously suggested three-state models and identify a four-state model that accurately follows the ChR2 photocurrents. We find that ChR2 currents decay biexponentially, a fact that can be explained by the four-state model. The model is composed of two closed (C1 and C2) and two open (O1 and O2) states, and our simulation results suggest that they might represent the dark-adapted (C1-O1) and light-adapted (C2-O2) branches. The crucial insight provided by the analysis of the new model is that it reveals an adaptation mechanism of the ChR2 molecule. Hence very simple organisms expressing ChR2 can use this form of light adaptation.

Complete and dorsal hippocampal lesions impaired spatial performance on 2 working memory tasks: rewarded alternation on the T maze and matching to position in the water maze. In contrast, ventral hippocampal lesions had no effect on these tasks, even when task difficulty was increased by the introduction of delays. Ventral lesions did resemble complete lesions in reducing anxiety in 3 commonly used tests of anxiety (social interaction, plus-maze, and hyponeophagia). Dorsal lesions also appeared to be anxiolytic in the social interaction and plus-maze tests, but they did not affect hyponeophagia. Complete- and dorsal-lesioned rats displayed hyperactivity, whereas ventral-lesioned rats did not. These results show a double dissociation between dorsal and ventral hippocampal lesions (hyponeophagia vs. spatial memory), suggesting differentiation of function along the septotemporal axis of this structure.

SummaryNeurons implement a variety of plasticity mechanisms to alter their function over timescales ranging from seconds to days. One powerful means of controlling excitability is to directly modulate the site of spike initiation, the axon initial segment (AIS). However, all plastic structural AIS changes reported thus far have been slow, involving days of neuronal activity perturbation. Here, we show that AIS plasticity can be induced much more rapidly. Just 3 hr of elevated activity significantly shortened the AIS of dentate granule cells in a calcineurin-dependent manner. The functional effects of rapid AIS shortening were offset by dephosphorylation of voltage-gated sodium channels, another calcineurin-dependent mechanism. However, pharmacological separation of these phenomena revealed a significant relationship between AIS length and repetitive firing. The AIS can therefore undergo a rapid form of structural change over timescales that enable interactions with other forms of activity-dependent plasticity in the dynamic control of neuronal excitability.