Control of sensory habituation by an ultra-conserved calcium sensor

Animal nervous systems continually sense the external environment, generating a huge volume of sensory data that is transmitted to central neural circuits. Habituation is an important neural process that allows animals to filter out irrelevant sensory data, leading to ever-diminishing responses to repeated sensory stimuli and allowing organisms to focus primarily on salient stimuli. This form of non-associative memory appears ubiquitous across Metazoan (multi-cellular animal) species and is disrupted in a range of human neuropsychiatric and neurodevelopmental disorders, including Schizophrenia, autism spectrum disorder, and intellectual disability. Uncovering the molecular and neural circuit basis of habituation therefore promises to advance our understanding of conserved memory mechanisms and a number of highly debilitating diseases of high socio-economic importance.

We aim to do so using the fruit fly, Drosophila, as a model system. We present two key advances - one technical, and one data-driven - that will help further our understanding of habituation. Firstly, we have developed a new technique to rapidly and robustly quantify habituation of adult Drosophila to mechanical stimuli. Secondly, we have used this method to uncover a novel role for protein called Neurocalcin in promoting habituation.

Neurocalcin is a member of a class of proteins called neuronal calcium sensors, which upon binding calcium, translocate to membrane compartments and change their spectrum of binding partners, leading to alterations in neural excitability, neurotransmitter release, synaptic plasticity, and gene expression. In preliminary experiments, we have precisely mapped the function of Neurocalcin in habituation to a small number of neurons in the fly brain: mushroom body alpha'/beta' neurons. We propose a series of experiments to understand how Neurocalcin and mushroom body alpha'/beta' neurons act to influence habituation.

A notable feature of Neurocalcin is that it is incredibly well-conserved across Metazoan genomes, with >90% of amino-acids identical between Drosophila Neurocalcin and its homologue from the Cnidarian Nematostella vectensis. We therefore further propose an exciting cross-species gene replacement strategy to demonstrate that diverse Neurocalcin homologues can integrate into cellular pathways driving habituation.

Collectively, the above approaches promise to shed light on conserved molecular pathways influencing a critical and fundamental form of memory. Since mutations in the human Neurocalcin homologue Hippocalcin have been linked to intellectual disability, our findings will also have important implications for the understanding of highly debilitating and socio-economically damaging neurological disorders linked to defective memory and habituation.

Habituation is a ubiquitous form of non-associative learning that has been proposed to be prerequisite for higher cognitive function and which is perturbed in neuropsychiatric disorders. Thus, identifying conserved molecular constituents of cellular pathways promoting habituation promises to advance our understanding of both the evolution of memory and the patho-mechanisms underlying a variety of important neurological diseases.

In our Case for Support, we present three key advances. Firstly, we have developed a novel platform to study mechanosensory habituation in Drosophila that possesses substantial technical advantages over current set-ups. Secondly, we have used this platform to uncover a novel role for Neurocalcin, a neuronal calcium sensor, in promoting habituation. This calcium sensor is incredibly well-conserved, with >90% of amino-acids identical between Drosophila Neurocalcin and its homologue in the Cnidarian Nematostella vectensis. Furthermore, mutations in the human Neurocalcin homologue Hippocalcin cause intellectual disability, a disorder characterised by defective memory. Thirdly, we have precisely mapped the function of Neurocalcin to a small number of neurons in the fly brain: mushroom body alpha'/beta' neurons.

By combining optogenetic neural circuit manipulations, optical measurements of neural excitability, cell-specific genetic perturbations, and CRISPR-Cas gene editing, we aim to 1. confirm a role for mushroom body alpha'/beta' neurons in regulating habituation; 2. define the cellular pathway in which Neurocalcin acts to promote habituation within mushroom body alpha'/beta' neurons; and 3. show that Neurocalcin homologues from diverse Metazoan species can successfully integrate into habituation-promoting cellular pathways.

Through these approaches we aim to identify conserved molecular cascades modulating a fundamental and universal form of memory.