Tracing Neuronal Circuits


    One of the hottest topics in neuroscience today lies in the visualization of neuronal connectivity in order to fully understand the nervous system. A variety of neuronal networks are connected through synapses that convey electrical signals between each system of the CNS and PNS. Specific groups of neurons populate different areas of brain and spinal cord architecture, giving rise to motor, sensory, behavioural and cognitive functions. This recent paper from the George Smith lab, by my colleague Dr. Ying-peng Liu, demonstrates a new method of tracing such a neuronal network, specifically to study regenerating axons in the spinal cord of rat models.




    He essentially injects self-complementary Adeno-Associated Virus (sc-AAV-2), tagged with GFP into different nuclei of the brain or the dorsal root ganglia in order to label motor and sensory fiber tracts. Neuronal cell bodies and axons fluoresce bright green three weeks after injection, following the expression of the sc-AAV-2. Any damage or lesions to the tract can be easily visualized since the virus is anterogradely transported along the axon.



    Schematic shows the pathway traced by injecting the sc-AAV-2 virus into the sensorimotor cortex to trace the corticospinal tract (A), injecting into the Raphe Nucleus to trace the rubrospinal tract (B) and injecting into the dorsal root ganglia to trace the sensory motorneurons of the dorsal column (C)


    What makes this sc-AAV-2 tracer different from previous anterograde tracers, such as BDA or CTB or single stranded AAV is that transduction efficiency is so high and it is so specific to large cell body neurons. It is clearly demonstrated by this paper that this is a faster, more effective way to visualize damaged axons. The fact that AAV plasmids can be cloned to carry therapeutic genes is also a bonus since it allows us to inject the green fluorescence tag together with genes of interest in one virus and study their distributions in the animal after transfection.



    This figure shows a lesion cavity in the cervical spinal cord, labeled by GFAP (BLUE), the corticospinal tract labeled by BDA tracer (RED) and the sensory axons labeled by sc-AAV-2 GFP (GREEN)