The role of PV+ basket cell axon myelination in temporal synchrony of the hippocampal CA2 network

Project Information

MyeSync

Grant agreement ID: 101025627

DOI

10.3030/101025627

Project closed

EC signature date 22 February 2021

Start date 15 March 2021

End date 14 March 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 175 572,48

EU contribution

€ 175 572,48

Coordinated by

KONINKLIJKE NEDERLANDSE AKADEMIE VAN WETENSCHAPPEN - KNAW
Netherlands

Periodic Reporting for period 1 - MyeSync (The role of PV+ basket cell axon myelination in temporal synchrony of the hippocampal CA2 network)

Reporting period: 2021-03-15 to 2023-03-14

Summary of the context and overall objectives of the project

Neuronal communication takes place at a time scale of milliseconds, which illustrates the importance of neuronal precision and speed for proper brain function. The axon is a subcellular neuronal structure that is responsible for conveying so called action potentials (APs) in a unidirectional manner, from one neuron to the next. To efficiently serve as the highway for inter-neuronal information traffic, axons can be covered up with compact sheaths of myelin. The general belief dominating the field is that the speed benefit provided by myelin is exclusive to long-range neuronal projections of large diameter. Recently though, it has been shown that this “fatty insulator” can be found as well around the relatively short and thin axons of locally projecting parvalbumin positive basket cells (PV+ BCs).

This specific class of interneuron provides potent, fast, and ultra-precise perisomatic inhibition onto neighboring excitatory pyramidal cells. By doing so PV+ BCs contribute to the synchronized activity of populations of principal neurons during well-defined brain oscillations. One such type of oscillation they entrain is highly specific to the hippocampus and is called the Sharp Wave Ripple complex (SWRs). It Consists out of sharp waves and high frequency ripples (> 200 Hz), is observed during non-REM sleep, and has been demonstrated to fulfill a pivotal role in memory formation and consolidation.

The overarching goal of this MSCA fellowship was to investigate whether myelination of the PV+ BC axon contributes to inhibitory precision within the hippocampus (Objective A) and whether it contributes to fast hippocampal ripple oscillations (Objective B) that are important for mnemonic functions.

Loss of myelin within the central nervous system is the underlying cause of Multiple Sclerosis (MS). Besides motor deficits do MS patients often suffer from a decline in memory and reduced cognitive abilities. The exact mechanisms behind this phenomenon remain up to present largely unknown. This fellowship can be expected to give novel insights in the basic principles underlying alterations in the hippocampal circuitry in MS. The aim is to do so by complementing subcellular and synaptic observations with microcircuit measurements and to carefully link structure to cellular function. Such information can be expected to contribute to future translational studies and the development of novel therapies to ultimately try and relieve the suffering of patients living with MS.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

To achieve objective A, we performed targeted paired patch-clamp recordings between inhibitory PV+ BCs and connected excitatory pyramidal neurons in the Cornu Ammonis (CA) areas of the hippocampus. This allowed us to elicit highly controlled presynaptic action potentials (APs) at the level of the PV+ BC and record inhibitory postsynaptic currents (IPSCs) at the soma of the pyramidal neuron. To test how myelination affects this form of inhibitory neurotransmission, we compared a control with a de- and hypomyelinated rodent model. While the hypomyelinated model lacks only compact myelin, does the demyelination model lack both compact as well as non-compact forms of myelin. Altogether our findings indicate that IPSCs between controls and the de- and hypomyelination model are very comparable in terms of their basic features (e.g. amplitudes and kinetics). When we looked at the timing between the pre- and the postsynaptic signal, we were able to conclude that only the complete loss of myelin (demyelination) leads to small but significant synaptic delays. Our dataset is limited to connected neuronal pairs located within approximately 50 m of distance. The reason being that low connection probabilities at larger distances make electrophysiological measurements in the paired configuration increasingly difficult. We are currently developing novel ways to elicit presynaptic APs from PV+ BCs at varying distances from a patched pyramidal neuron by optical means.

To achieve objective B, we established local field potential (LFP) measurements from thick acute hippocampal slices stored in interface chambers. This configuration keeps the hippocampal microcircuit functionally intact and allows the measurement of the local generation and propagation of Sharp Wave Ripple (SWRs) complexes. Since synaptic precision was exclusively affected by a complete loss of myelin (explained above), for this set of experiments we focused on the full demyelination model. In a nutshell, we found that demyelination led to significant reductions in the frequency of fast hippocampal ripple oscillations. Experiments are currently ongoing to validate these findings in vivo.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Altogether this two-year fellowship led to the discovery that myelin (even in its non-compact form) does contribute to inhibitory precision within the hippocampus. We provided evidence that by driving inhibitory precision, myelin supports locally generated networks. This shows that, in contrary to the existing general belief, myelin around short and thin axons can provide a speed advantage to locally projecting PV+ BC interneurons. Our study is unique in the sense that it managed to link subcellular observations (axonal myelination) to synaptic function and how this has implications on the behavior of populations of neurons that form microcircuits. Our description of the dependency of inhibitory latencies on the presence of myelin provides novel parameters required to build realistic in silico models. Our data moreover offer preliminary cues to a potential mechanism that might explain memory deficits in MS patients.

As a training grant, this MSCA fellowship allowed me to learn novel techniques (holographic optogenetics, Local Field Potential measurements, Ca2+-imaging) and related analysis methods. It contributed to the broadening of my interests (into structural- and glia cell biology), provided me with mentoring and supervision skills, boosted my managerial capabilities, and created unique networking opportunities that could be of invaluable importance for my future career. In return I was able to transfer my specific knowledge about hippocampal physiology and skills in synaptic electrophysiology to the host lab. Preliminary scientific results have been presented and discussed at conferences and workshops, a manuscript including the findings of both objectives of this fellowship is in its final stages of preparation.

graphical summary MyeSync