BDNF Axonal Transport: Mechanisms and Neuropathophysiology – BATMAN

1) To study the molecular and cellular mechanisms regulating the active transport of BDNF to the synapse, cortical and striatal neurons are seeded onto a microfluidic device mounted on electrodes chip to create a neural network in vitro in which each compartment can be specifically manipulated. Several types of micro-fluidic chambers are developed to address the different issues. The impact on the transport of BDNF and synaptic connectivity are simultaneously analyzed by confocal videomicroscopy and electrophysiological recordings.

2) The evolution of the transport and release of BDNF by the cortico-striatal axon is analyzed during axonal development and is correlated with the establishment of the synapse and the effectiveness of the transmission.

3) Specific patterns of electrical stimulation can be applied to establish a mathematical model of axonal transport of BDNF to the active synapse, in a physiological context and after pharmacological and genetic manipulations.

4) The modulation of BDNF transport by neuronal activity and the molecular mechanisms identified on the microfluidic system will be analyzed in vivo. The effects on axonal remodeling in the frontal cortico-circuit in normal and pathological conditions (murine models hdhQ111, hdhS421A, hdhS421D) will also be characterized after manipulating the different molecular actors identified above.

1 / For this project, several biological and technical tools have been developed:
- Various microfluidic devices to create functional cortico-striatal networks and to study/manipulate the different compartments (cortex and striatum cell body, dendrites, axons, synaptic contacts, synaptic axon split for pre or post manipulation, etc ...)
- A microelectrodes device associated with microfluidic chambers to manipulate the pre-or postsynaptic electrical activity and to record synaptic responses while analyzing axonal transport by videomicroscopy
- BDNF plasmids with different fluorescent markers with mutations for the study of mature versus immature BDNF

2 / Some microfluidic systems developed during this project are still being characterized and will be subject to patent application

3 / morphological, functional and live imaging showed that the axonal transport of BDNF depends on the development of the axon and is conditioned by the establishment of functional and active synapses. Electrophysiological analysis and electrical manipulation of pre-and postsynaptic elements are ongoing.

The main perspective of the project is to establish a new concept of synaptic plasticity regulation by BDNF and its contribution in the genesis of cognitive impairment

No scientific production to date. Some microfluidic systems will be patented.

Previously thought to be a rigid and ageing organ the adult brain shows an incredibly high level of plasticity. Dynamic remodelling of axonal connections is a sine qua none condition to adapt proper response to environmental stimuli. What makes the active rewiring of axonal circuitry efficient in the adult brain? How is it regulated?

Although subject to intense scrutiny, the mechanisms that control circuitry refinement and plasticity are not fully understood. BDNF (Brain-Derived Neurotrophic Factor), the most abundant neurotrophin in the adult brain, has recently emerged as a key regulator of synaptic plasticity, axonal remodelling and dendritic sprouting. These functions are critical for high-order cognitive functions, long-term memory and behavioural adaptations to environmental changes. However, several questions remain unanswered concerning the regulation of its actions. How does neuronal activity dictate the transport and release of BDNF in an activated axonal circuitry? Recently, the host lab (F. Saudou, PI) has proposed that the transport of BDNF is regulated by huntingtin (htt) in neurons. Signalling pathways sensitive to neuronal activity could gate and translate specific stimulatory patterns into a “go/no go” signal for htt-dependent transport of BDNF. Why is this message specific to BDNF? The presence of specific regulatory complex associated to BDNF vesicles could detect neuronal activity and produce a quick and appropriate response. Whereas the transport of mature BDNF is regulated by neuronal activity immature pro-BDNF is constitutively secreted. How are these two forms differentially recognized? In addition, if the functional role of BDNF has been extensively studied in the peripheral nervous system and in the hippocampus, very little is known about its role in the fronto-striatal circuitry. The prefrontal cortex is known to underlie complex behavioural responses such as cognition, task planning, behavioural flexibility and working memory. Is the functional role of BDNF observed in other regions true for the fronto-striatal circuitry? What are the mechanisms? Finally, a defect in cortical BDNF transport has been reported in the context of Huntington’s disease, when htt is mutated. What is the consequence on the fronto-striatal circuitry? Is this defective transport responsible for the cognitive and psychiatric symptoms observed in HD patients?

The present project aims to better understand the regulatory mechanisms that control BDNF actions in the fronto-striatal circuitry. More specifically, I want to determine how huntingtin gates neuronal activity to direct BDNF transport to the active synapse. I want to determine the functional role of activity-regulated BDNF transport on the plasticity of the fronto-striatal circuitry. Finally, I want to determine whether a defect in BDNF transport can generate fronto-striatal disconnectivity and early cognitive and psychiatric symptoms of HD patients.