Cellular Imaging May Tie Neurodegeneration to Specific Disease Symptoms

Lay Summary

Cellular imaging may tie neurodegeneration to specific disease symptoms

A new animal model study may help determine how the abnormal protein tau contributes to disease. Several degenerative brain diseases involve an abnormal form of the protein tau but little is known about how this protein leads to the symptoms specific to each disease. That may change with use of novel imaging. Scientists are learning more about the molecular basis of tau accumulation within cells that blocks cellular communication in Alzheimer’s (AD), post-encephalitic Parkinson’s, and other tauopathies like supranuclear palsy (PSP). How, though, does tau chemistry produce effects that differ in each disease? Early AD symptoms, for instance, are often short-term memory problems. Early symptoms of PSP are frequent falls arising from problems maintaining balance and looking up and down (called “gaze palsy”); with the disease’ s progression, patients rarely live past age 10.

Neurons responsible for maintaining balance and gaze are in brainstem circuits. These populations of neurons, called “vestibular circuits” ordinarily sense when the body is tilting and transform those signals into reflex commands that stabilize posture and eye movements. The neurons are found in all vertebrates, from fish to humans. The investigators hypothesize that when abnormal tau protein begins to accumulate in these vestibular neurons in PSP, it disrupt s the network creating balance and gaze problems as the neurons progressively degenerate.

Scientists cannot yet image and correlate PSP disease progression in patients’ vestibular circuits with their posture and gaze problems, but they can do so using a zebrafish model they developed. These “transgenic” fish have modifications to their genome that mimic the biochemistry of PSP. This allows the investigators to model this deadly disease in specific vestibular neurons. At the larval stage, the transgenic zebrafish are almost totally transparent, making this an ideal model to use optical microscopy to monitor brain activity. The investigators plan to measure brain activity using a novel microscope design, called “ SCAP E.” SCAPE imaging is fast enough to measure the rapid changes in neuronal activity in vestibular neurons as they stabilize balance and gaze. Simultaneously, the investigators will measure progressive failures in the function of synapses that connect vestibular neurons. This way they will track the time-course of tau-mediated degeneration that affects molecules, function (neuronal loss), and behavior (balance and eye movement problems).

Abstract

A novel whole-brain method for in vivo imaging of progressive neurodegeneration

Considerable effort has gone towards understanding the biochemical basis of neurodegeneration, but our understanding of how these molecular events give rise to clinical symptoms lags behind. Progress requires longitudinal evaluation of disease progression in the context of relevant neural circuits. Here we propose to use a cutting-edge imaging technique to measure the cellular/synaptic correlates and consequences of progressive neurodegeneration. PSP presents first with a set of symptoms that implicate an evolutionarily ancient vestibular circuit responsible for balance [1–4]. The key functional population in this circuit are brainstem neurons that transform sensed body tilts into stabilizing reflex commands [5, 6]. These neurons are compromised early in PSP [7] making them an excellent cellular substrate to study neurodegeneration. Balance circuits have long served as an excellent model to address fundamental questions of brain function [8], but the lack of molecular control has limited their applicability to questions of neurodegeneration. We have established the larval zebrafish as a new preparation to study normal development of gaze-stabilization and postural reflexes [9–13]. Our system affords unprecedented genetic and optical access to the vestibular neurons responsible. These advantages permit us to use cutting-edge imaging methodologies to make longitudinal measures of progressive neurodegeneration and associated behavioral and synaptic failures. This work will establish a tractable model to elucidate general mechanisms of tauopathic neurodegeneration. PSP is distinguished by two cardinal clinical findings emerging early in the course of the disorder: vertical gaze palsy and falls [14]. Currently, there is no known cure, and as treatments do not prevent disease progression, relatively poor prognosis (median 7 years survival) [15]. PSP is one of a family of neurodegenerative disorders characterized by neurofibrillary tangles. The main constituent of these tangles is the microtubule-associated protein tau [16]. While considerable progress has been made in understanding tau biochemistry, relating accumulation to specific clinical symptoms remains challenging [17]. Progress requires evaluating disease progression in the context of neural circuits responsible for particular symptoms. The early falls and vertical gaze palsy that distinguish PSP implicate an evolutionarily ancient neural circuit responsible for balance [2–4]. This circuit contains brainstem vestibular neurons that process otolith-derived tilt sensations to enable posture and gaze stabilizing reflexes [6]. While the motoneuron targets downstream are spared, these vestibular neurons are compromised in PSP [7]. To date, cellular evaluation of tau-related pathologies is largely limited to post-mortem evaluation [18]. A non-invasive means to image vestibular neurons during early pathogenesis in vivo would provide insight into the mechanisms of degeneration. The model vertebrate zebrafish (Danio rerio) permits imaging tau-related degeneration in vestibular neurons. We have achieved three key milestones that allow us to directly examine the effects of tau accumulation on vestibular neuron degeneration. First, we have developed a transgenic zebrafish that permits molecular-level control of gene expression in a set of ~200 vestibular interneurons. We have developed assays to demonstrate that these neurons are necessary and sufficient for the gaze and posture stabilization that is disrupted in PSP [9, 11]. Second, we have built our own swept, confocally-aligned planar excitation (SCAPE) microscope [19] that allows us to make volumetric measurements of vestibular neuron activity. SCAPE’s unprecedented spatiotemporal resolution allows imaging of vestibular neuron activity in response to body tilts. Finally, we have established a line of zebrafish in our laboratory that permits targeted over-expression of a mutant form of human Tau [20, 21]. Here we propose to use SCAPE microscopy to make longitudinal measurements of functional degeneration in a circuit-level model of PSP. Concomitantly, we will assay behavioral dysfunction and interrogate synaptic capacity as tau accumulates. Our aim is to illuminate the mechanisms of tau-mediated progressive neural dysfunction and the associated synaptic and behavioral failures. This work stands to uncover general principles of how neurodegeneration comes to impair function, guiding earlier diagnosis and one day facilitating therapeutic discovery.