Distinct connectivity of newly-generated dopaminergic neurons in the adult brain?

Lead Research Organisation: King's College London
Department Name: Developmental Neurobiology

Abstract

A healthy brain requires its constituent cells, or neurons, to be properly connected to each other. These connections can involve thousands of different inputs arriving onto a single cell, before that cell integrates the incoming information and sends an output signal to a range of highly specific downstream partners. In this way, neuronal networks transform input information into a suitably-processed output signal, so that sensory experience, for example, can control our behaviour. Understanding the way in which neurons are connected, then, is crucial in understanding their function. This is particularly true for a unique type of neuron: those born in the adult brain. These cells are in the great minority, because most of our neurons are generated before birth and lack the ability to regenerate after injury or disease. However, in a few specialised zones of the mature mammalian brain, new neurons are continually produced throughout life in a process known as adult neurogenesis. This generates entirely new neurons which must integrate into existing networks, receiving inputs and sending outputs to contribute to circuit function.

The goal of our proposal is to study these newly-formed connections made by a particular type of adult-generated neuron. In the olfactory bulb - the first part of the brain to process smell information arriving from the nose - dopaminergic neurons play a key role in modulating sensory signals at their earliest stages, and these cells can be generated throughout life. However, it is entirely unknown whether the input and output connections, and therefore the circuit function of these neurons, is unique to dopaminergic cells produced in adulthood. By using a combination of cutting-edge approaches, we will ask whether adult neurogenesis produces cells with unique connectivity in this sensory-processing circuit.

In addressing this important basic biological question, our work has the potential to impact on healthcare in the UK. Our study of the olfactory system may inform future approaches to treat debilitating smell disorders such as anosmia and hyposmia, which have a huge impact on quality of life and affect at least 20% of the population. Our focus on dopaminergic neurons may inform treatments for disorders where these cells are lost in later life, such as Parkinson's Disease. And our study of new neurons in old circuits has the wider promise of informing any attempt to repair brain damage by adding freshly-generated cells to perturbed networks.

Technical Summary

Adult neurogenesis generates newborn neurons in specialised regions of the mature mammalian brain. These newly-generated cells integrate into existing circuitry, where their functional role is determined to a large extent by their patterns of input and output connectivity. But is this connectivity, and therefore function, unique to adult-born cells? Do they provide connections that are missing in existing mature circuits? We aim to address these questions by studying the connectivity of a unique population of adult-generated neurons: dopaminergic neurons in the olfactory bulb. Our approach involves the synthesis of multiple state-of-the-art techniques for uncovering anatomical and functional connectivity, including morphological reconstructions of sparsely-labelled individual neurons in intact cleared brain tissue, rabies virus-mediated monosynaptic tracing of input populations, and electrophysiological recordings coupled with targeted optogenetic photostimulation to reveal functional connectivity. By combining these parallel approaches we will compare and contrast the connectivity of dopaminergic neurons that are born in embryonic development versus those generated in adulthood, and will ask whether adult neurogenesis produces a specialised pool of new cells with distinct contributions to functional circuit architecture.

Planned Impact

Although the proposed project is primarily one of basic scientific research, there are a number of potential non-academic beneficiaries from the data, results, and knowledge we will produce. These beneficiaries can be divided into 2 broad groups: 1) public sector, commercial enterprises and policy makers with a stake in the provision of healthcare, and 2) the general public.

1) The proposed research has the potential to contribute to the nation's health and wellbeing, directly meeting the BBSRC's Key Strategic Research Priority of 'Bioscience for Health'. By studying the functional implications of new cells in old circuits, our work directly impacts on therapeutic attempts to repair damaged or diseased brain tissue through the replacement of newly-generated neurons. By studying the novel circuit-level function(s) associated with new neurons generated in the adult brain, and thereby uncovering the full plastic potential of mature neuronal networks, we also address the Council's specific priority of 'Healthy ageing across the lifecourse'. And, as well as providing basic knowledge which can then be applied to a broad range of issues surrounding normal ageing and mental health, the proposed work will also impact on research aiming to treat specific disorders. Our study of dopaminergic neuron function and plasticity may influence attempts to treat disorders based on dopaminergic cell malfunction such as Parkinson's Disease, while novel insights into the function of olfactory bulb networks could impact on attempts to improve recovery rates and quality of life in the significant proportion of the population - 20% of us, rising to >50% in over-75s - that suffers from debilitating smell dysfunction. Finally, the potential benefits for public health produced by the proposed project can also lead to advances in evidence-based policy making, if, for example, eventual effective treatments can be included in NICE guidelines. In the more immediate term, we additionally hope that our findings can contribute to efforts to persuade policy makers that basic bioscience research is a just, important, and profitable use of public funds.

2) Our research also has the potential to benefit the UK public, by generating openly-available novel data on brain function and plasticity, and by public engagement in our discoveries. The brain is a fascinating organ, and our research into its inner workings could benefit, amongst others, school students deciding where to take their careers, patient and carer groups interested in the implications for particular disorders, and adults with well-honed scientific curiosity.

Publications

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Description We have discovered important differences between brain cells born during embryonic development and those born in adulthood. Specifically, among a particular type of cell - neurons in the olfactory bulb that release the neurotransmitter dopamine - we have found that those born embryonically have very different structural and functional features to those born postnatally (see Galliano et al. eLife 7:e32373, which is fully Open Access).

Recent advances in the project have uncovered key differences in the connectivity of these two types of olfactory bulb dopaminergic neuron. In terms of the inputs to these cells, we have - after a long period of technical optimisation, necessary because of the cutting-edge nature of our approach - obtained a series of successful experiments in which the input cells immediately presynaptic to either adult-born or embryonically-generated olfactory bulb dopaminergic cells have been anatomically labelled. We are currently in the process of analysing these data, which promise to generate a large and very revealing connectomic dataset for these neuron types.

In terms of outputs from olfactory bulb dopaminergic cells, we have both structural and functional data suggesting that they are very different between adult-born and embryonically-generated neurons. Electrophysiological recordings in acute slices have shown that adult-born dopaminergic neurons are able to release the neurotransmitter GABA from their dendrites, but embryonically-generated cells cannot. These functional data are being supplemented by ongoing anatomical investigations into the presence or absence of dendritic neurotransmitter release sites in these neuron types, using labelling for endogenous synaptic vesicle-associated proteins as well as cell-specific overexpression of fluorophore-tagged presynaptic proteins. Together, we are looking to combine these analyses of dopaminergic cell inputs and outputs to generate a complete picture of the connectivity of adult-born versus embryonically-generated cells, ready for submission later in 2020.
Exploitation Route These findings show that the properties of brain cells depend on how & when they were generated. They need to be taken into consideration when attempting to 'reprogram' cells in efforts to repair brain regions damaged by insult or disease.
Sectors Healthcare

 
Title Raw data for 'Embryonic and postnatal neurogenesis produce functionally distinct subclasses of dopaminergic neuron' 
Description We deposited, and made freely available under a CC-BY licence, all of the raw data associated with our 2018 eLife article entitled 'Embryonic and postnatal neurogenesis produce functionally distinct subclasses of dopaminergic neuron' 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact 57 downloads and 1 citation of the dataset itself, as of March 2020 
URL https://datadryad.org/stash/dataset/doi:10.5061/dryad.b5hg8d6
 
Description Dendritic GABA release in embryonically-generated olfactory bulb dopaminergic neurons? 
Organisation National Institutes of Health (NIH)
Department National Institute of Neurological Disorders and Stroke (NINDS)
Country United States 
Sector Public 
PI Contribution We are sharing structural and functional data we have on the potential lack of dendritic GABA release sites in embryonically-generated olfactory bulb dopaminergic neurons
Collaborator Contribution Kevin Briggman and his graduate student Kara Fulton are undertaking specific analyses to address the question of dendritic release sites in specific subtypes of olfactory bulb dopaminergic neuron. They are doing this using a large connectomic dataset they have generated, using electron microscopy to map the structural features of defined cell types in a single mouse olfactory bulb glomerular processing unit.
Impact None to date
Start Year 2019
 
Description In2Science placement 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact We host an In2Science summer project student almost every year, and I have now become an ambassador for this fantastic scheme. Placement students are Year12 and come from underpriveledged backgrounds, and are given the opportunity to experience a real scientific research environment - and to undertake research themselves - in our laboratory. We know of at least one of our placement students who has gone on to successfully complete a BioMedical Sciences degree here at King's.
Year(s) Of Engagement Activity 2012,2013,2014,2015,2016,2017,2019
URL https://in2scienceuk.org/