The mammalian brain is made of billions of neurons. In order for these neurons to communicate, they must extend thin projections, called axons, over long distances and to exact locations. The precision with which this occurs is particularly vital during development. However, many developing neurons initially overextend their axons to inappropriate regions of the brain. Later, these extensions are removed through a process known as axon pruning. Defects in pruning have been implicated in neuropsychiatric disorders such as schizophrenia (Brent et al., 2014) and autism (N. Barnea-Goraly et al., 2004). Understanding the mechanisms that regulate pruning in mammals might therefore provide therapeutic insight for treating these disorders. The aim of my project is to study the mechanisms underlying axon pruning in mammalian vertebrates.
At the Hwai-Jong Cheng Laboratory at the University of California, Davis, we are particularly interested in a type of pruning known as stereotyped axon pruning. Stereotyped axon pruning refers to the process in which a specific and predictable axon branch (or branches) is cut back from an area. A classic example of this occurs during the development of the corticospinal tract (CST) in the vertebrate visual system. In this pathway, neurons extend axons from the visual cortex – the area of the brain that processes vision – to regions of the hindbrain, and then overextend into the spinal cord. This overextension into the spinal cord is then predictably pruned back to the pons (a hindbrain region).
The Cheng Laboratory is interested in determining the mechanisms that regulate visual CST pruning during development. One of the current focuses is the neural activity from the eyes, which can be divided into two phases during development: intrinsic and extrinsic activity. Intrinsic activity refers to early spontaneous retinal waves, which are spontaneous bursts of correlated and localized activity that propagate within the developing vertebrate retina before eye opening (Huberman et al, 2008). Extrinsic activity is the retinal activity triggered by visual experience after eye opening. The Cheng Laboratory has found that the intrinsic, rather than extrinsic, activity is likely to regulate visual CST pruning in mice (Luu and Cheng, unpublished data).
In order to determine whether these findings are evolutionary conserved processes (that is, processes that are common among different species and therefore may occur in humans), I am examining the neural activity mechanisms that underlie visual CST pruning in the ferret. In contrast to the mouse, the ferret is a higher-order carnivorous mammalian species. Its highly structured visual pathways resemble those of primates, while the composition of its retina is similar to that of humans. Thus, the ferret is a useful model for connecting results found in mice to higher-order mammals like humans. Given that such a comparison between ferret and mouse visual CST pruning mechanisms presents a step toward understanding human neuropsychiatric disorders, my project focuses on establishing the existence of a visual CST in the ferret and then determining the time point of its refinement.
However, many developing neurons initially overextend their axons to inappropriate regions of the brain.”
To do so, I am tracing visual CST axons to understand the tract’s progression throughout development. To trace these axons, I inject a protein known as biotinylated dextran amine (BDA) into the visual cortex of ferrets of different ages. The BDA is taken up by neurons surrounding the site of injection and transported through the entire length of the axons. When coupled with another enzymatic reaction, these BDA-labeled axons undergo a color change that allows for their visualization under a microscope. As a result, I can locate the visual CST of ferrets of various ages and quantify the number of axons that extend into specific regions of the brain. I can then compare this data with the timeline of neural activity mechanisms in the ferret to determine if the time point of extension and refinement coincides with the time point of intrinsic or extrinsic neural activity mechanisms.
So far, I have found that axons project from the visual cortex into the pons until the ferret is ten days old (post-natal day 10, referred to as “P10”). They then extend into the spinal cord until P15, which confirms the presence of a visual CST in the ferret. These axons subsequently retract toward the pons by P20, and terminate there by P35. Overall, this data indicate that ferret visual CST pruning occurs from P15 to P20. This is prior to ferret eye opening (P30), indicating that visual experience has not yet occurred and therefore extrinsic neural activity mechanisms do not mediate this pruning. Likewise, since the time point of pruning overlaps with the time point of intrinsic activity on the ferret neural activity timeline, it is likely that spontaneous retinal waves mediate visual CST pruning, though further studies are currently being done to eliminate intrinsic activity in order to causally relate it with visual CST pruning. So far, since these results are similar to those found in the mouse, this is likely a conserved evolutionary mechanism that may occur in humans.
- Brent BK, Seidman LJ, Thermenos HW, Holt DJ, Keshavan MS. Self-disturbances as a possible premorbid indicator of schizophrenia risk: A neurodevelopmental perspective. Schizophr Res. 2014; 152(1):73-80.
- Barnea-Goraly N, Kwon H, Menon V, Eliez S, Lotspeich L, Reiss AL. White matter structure in autism: preliminary evidence from diffusion tensor imaging. Biol Psychiatry. 2004; 55:323-326.
- Huberman AD, Feller MB, Chapman B. Mechanisms underlying development of visual maps and receptive fields. Annu Rev Neurosci. 2008; 31:479-509.
Atrin Toussi is a Brevia Primary Research Co-editor. She can be reached at email@example.com.