We often describe neural connections in the brain as if they were a cellular version of Michelangelo’s famous painting “Hand of God Giving Life to Adam,” on the ceiling of the Sistine Chapel. Like the painting, the neural explanations usually invoke 2 outstretched limbs nearly touching each other—one presynaptic, one postsynaptic—that are separated by their 20-nanometer synaptic cleft.
Such descriptions hardly depict the neurological reality of the brain, of course. A better metaphor for 2 neurons might be 2 trees that have been uprooted and turned 90 degrees so their root systems face each other. Then, some Paul Bunyan–type character jams both ends together. Thousands of connections from 1 tree now face thousands of connections from the other. Multiply those 2 neurons by thousands while all their root systems are still jammed together with the same Bunyanesque enthusiasm and you can visualize an approximation of the real world of brain wiring. Not as elegant as 2 limbs—and not as simple either.
How can we understand the way all those myriad connections work together to produce the various neurological abilities of the brain? Given its complexity, the task is enormous. Since we are only in the initial stages, it will take a long time before we will be able to map structure to function.
Like any exploration in its initial stages, we first need a good map—a schematic that shows how each slender dendritic branch interacts with a specific nerve cell. From Nissl and Weigert stains to the canonical Golgi stain, we have traditionally used dye technology to help us visualize these interactions. However, there are severe limitations to most of these staining technologies that center around their inability to make visible all the connections that actual neurons possess. We need something with far greater resolution and, perhaps, with a bit more elegance.
The topic of this column and the next is a technology that promises to deliver just such circuit diagrams at a very high level of resolution. The technology involves the exploration of viruses, which in their native form cause some pretty tough diseases (eg, rabies, cancer). Thoughtful genetic engineering has transformed these viruses’ job description from fearful disease inducer to doughty cartographer. We are going to follow how this transformation has occurred.
I give you fair warning that it is a complex manipulation. I encourage you to closely follow the Figure that accompanies this column as well as the figure in Part 2. Both diagrams describe the story of hypothetical “Neuron A” and our attempts to understand all its many intricacies and connections. To explain Neuron A’s interactions, I need to review not only 2 pathogenic viruses but also several manipulated proteins, genetically engineered neural cells, and protein-based fluorescent dyes. In Part 1, I will discuss some background, which I have divided into 3 parts. In Part 2, I will explain the data.
The actual experiment that took place asks and answers the simple question: Are the neighbors green yet?
Why rabies virus?
The rabies virus has 2 characteristics that make it an ideal “truck” for the delivery of molecular products to mammalian neural interiors. The first has to do with its legendary life cycle. Once inside a nerve cell, the virus sets up a manufacturing site to create more viruses, like any typical virus. At maturity, these progeny can jump to other neurons, spreading the virus along specific neural routes. If you could find a way to follow the virus, you could identify the routes.
The second characteristic has to do with access to the CNS, which is a topic of great concern to researchers who are interested in drug delivery, for example. The rabies virus is not a respecter of neural borders. It can infect neurons in the peripheral nervous system and then jump the neurological border to enter the CNS. That is why a bite anywhere on the body can result in a catastrophic brain infection. By investigating how this transfer occurs at the molecular level, biologists can exploit the viral life cycle to understand neural interactions.
I will now discuss how a modified rabies virus could confront our Neuron A described previously. We will “genetically engineer” a rabies virus and, at the same time, genetically engineer its neural target to make this happen.