Jessie Ebeling ’21 and Jolie Stocki ’20
Vaping has become a quickly growing epidemic among teens in America, with over 25% of all high school students having used a vape in the past month in 2019 – an increase of more than 1800% the rate in 2011. Rates have similarly jumped among middle schoolers, rising from 0.6% reporting using an e-cigarette in the past month in 2011 to 10.5% in 20192. While the use of these products is growing, especially among youth, no long-term effects are known.
So are the products even harmful? After all, they’re marketed as a safe alternative to cigarettes. In reality, e-cigarettes often contain nicotine in even higher concentrations than regular cigarettes. Adolescents actually often smoke more nicotine than they would with a cigarette, and the products appeal to youth with a variety of flavors. We are already seeing the impact of these products, with the CDC reporting over 2,800 hospitalizations and 68 deaths in the US due to vaping products as of mid-February 20201. Additionally, epigenetic research has shown that the effects of nicotine are far more severe for adolescents than for adults, and even impact learning, brain development, and behavior for this age group. Perhaps these effects inspired the government to increase the sale of tobacco and vaping products that contain nicotine from 18 to 21. But does this three-year difference really matter? Read this article and see for yourself.
Genetics as a Ball of Yarn
In order to really dive into the effects of nicotine on the adolescent brain, we first need to understand the various terms related to genetics. To better visualize the diverse concepts, it helps to imagine a ball of yarn. What can you do with this yarn? You could make a scarf or a sweater or socks. No matter what you decide to make, the makeup of the yarn, how soft it is, what color it is, does not change. This relates to the idea of epigenetics. The makeup of the yarn resembles our genetics, or the specific sequence of genes. On the other hand, what we decide to make out of the yarn resembles the epigenetics. The sequence of genes stays the same, but the way the genes are expressed may vary, just as the makeup of the yarn is the same, but what we decide to do with the yarn can change.
One mechanism that changes the expression of genes, or in our analogy what we decide to make out of the yarn, is histone modification. Just as yarn is wrapped around a ball, our DNA is wrapped around structures called histones. The DNA can be wrapped tightly or loosely, depending on the epigenetic signals it receives from the cell. Let’s go back to our analogy and imagine we have a multicolored yarn, with a specific pattern of colors. If we want to use the pink color to make a sweater, but it is tightly wrapped around the ball of yarn, we will have a difficult time accessing it and making a pink colored sweater. This is similar to DNA, since when it is wrapped more tightly around the histones, less genes are accessible, and cannot be expressed to perform their function. On the other hand, when the DNA is loosely wrapped around the histones, it is easier to access and the specific genes in that region can be expressed.
Epigenetics and Developmental Nicotine Use
Methylation is a type of histone modification that can change the structure of DNA by loosening or tightening DNA around the histones, or how tightly the yarn is wrapped. One specific type of methylation is Histone 3 Lysine 4 trimethylation (H3K4me3), meaning the amino acid Lysine on histone 3 has 3 methyl groups added. This type of modification loosens the DNA and makes it more accessible. Jung and colleagues discovered that H3K4me3 occurs at specific genes upon developmental nicotine use and uncovered how the methylation mechanism works3. They administered mice nicotine from 0-21 days after birth, then examined their brains to determine the specific proteins, called Ash2l and Mef2c, that are activated by developmental nicotine use and perform the H3K4me3 that loosens the DNA.
Unwinding the DNA through H3K4me3 at specific sites increases expression of the genes at those sites. In our analogy, we can think about unwinding the pink color segment of the yarn from the tightly wound yarn ball. This makes the entire pink color segment more accessible and we can now use it to make a pink sweater. Translated, this means the H3K4me3 makes the gene of interest more accessible, and the gene can now get expressed and do its specific function.
When nicotine was administered to 0-21 day old developing mice, H3K4me3 changed gene expression in brain cells, also known as neurons, in ways that impact the dendrites. Dendrites are the part of neuronal cells that receive messages from other neurons. These dendrites have many branches connecting to other neurons, forming a tree-like structure which allows our brain cells to communicate. Nicotine intake results in an increase in the number of “branches” and ways it communicates to other cells, in both adolescent and developing mice. Having more dendritic connections is the result of H3K4me3 and DNA unwinding due to nicotine, just as making a pink sweater is the result of unwinding the pink yarn. This same mechanism that is occurring in developing mice could also be affecting adolescents.
When the proteins that loosen the DNA in the presence of nicotine were inhibited, nicotine-administered rats showed no increases in dendritic complexity or passive avoidance behavior3. This behavior is another effect of nicotine use during adolescence which will be elaborated on later. These results suggest that the proteins which unwind the DNA at the specific target gene were not being made, so our gene of interest was not expressed. We can compare this to our yarn analogy: if the pink yarn is not unwound, we cannot make a pink sweater.
Additionally, increasing the amount of these proteins in rats that were not exposed to nicotine caused nicotine-like effects on gene expression and behavior, or resulted in pink sweater production. The amount of DNA-loosening proteins decreases in adulthood, making the H3K4me3 effect due to nicotine specific to the developing brain3. Although this study was conducted in rats 21 days old while rat adolescence typically starts around 30 days, the effects of nicotine in 30 day old mice, which we will discuss in the next section, were nearly identical to those measured in this study. This means that H3K4me3 DNA modifications could also be responsible for the gene expression changes seen in adolescents.
Gene Expression Changes Induced by Nicotine
Two research groups conducted studies measuring arc mRNA, dendrin mRNA and Dendrin protein levels in the brains of adolescents and adults after nicotine treatment. mRNA is the link between the DNA-coded gene and its specific function, oftentimes being a protein. In our analogy, mRNA can be described as a blueprint of what the sweater will look like. With the blueprint, we can make a sweater out of the yarn, just as we can make a protein from DNA using mRNA as a blueprint. The arc gene is linked to neuronal plasticity, or how the brain develops and changes, as well as addiction, emotional regulation, and decision making4. The exact role of Dendrin is still unknown, but it is linked to dendritic modification (the number of branches), plasticity, learning, and memory5. Potentially, both genes could be regulated by the Ash2l/Mef2c complex, described above, since both are specific to dendritic changes, although there is not yet any evidence for a direct link.
The researchers compared the mRNA levels of nicotine-injected mice to saline-injected controls and found that arc and dendrin mRNA are expressed at higher levels in adolescents than adults after nicotine treatment in the prefrontal cortex, which is the brain region for cognitive behavior. The increase in dendrin mRNA directly correlated to an increase in Dendrin protein in adolescents, which was not visible in adults. The control adolescents injected with saline had no visible levels of Dendrin at all, which shows that nicotine has a significant impact on Dendrin levels.
The results of both papers suggest that the prefrontal cortexes of adolescent brains are more vulnerable to changes in plasticity after nicotine exposure. Adolescents had much higher changes in both genes which modify the connections between neurons, arc and dendrin, and an increase in Dendrin protein compared to adults, after nicotine treatment. These significant brain modifications may severely impact emotional and cognitive processing, as well as learning and memory, as the studies suggest.
It is also important to examine long-term effects of nicotine on adolescent Dendrin levels. Even though the Dendrin study does mention that nicotine has been seen to have long-lasting effects on the adolescent brain in other studies, it fails to test the long-term effect of nicotine on adolescent Dendrin levels. This limitation makes it difficult to determine whether the potential changes in plasticity are significant and long-lasting.
With the changes in plasticity-related genes, adolescents are perhaps also more likely to have significant changes in neuronal connections and signaling due to nicotine exposure. However, we cannot be certain, since the arc mRNA study does not measure Arc protein levels. Changes in mRNA do not always correlate to changes in protein levels, just as having a blueprint does not ensure that we will make a perfect sweater. Thus, we cannot be sure whether the Arc protein levels are the same between adults and adolescents or if there is a direct change in the plasticity. However, mRNA levels are still good estimations of cellular changes. Since there was such a significant difference in arc mRNA expression between adolescents and adults after nicotine exposure, we can suggest there is also a significant difference in protein levels and thus plasticity. We can also use behavioral studies as a good indicator of true plasticity changes, which will be discussed next.
Behavioral Changes Induced by Nicotine
So, why does the color of the sweater matter? What difference does it make to create a pink sweater, as opposed to a blue sweater, or no sweater at all? These changes in gene expression and plasticity, or pink sweater production, coincide with significant behavioral consequences in adolescents. Researchers Trauth, Seidler, and Slotkin observed behavioral changes in adolescent rats due to nicotine and found that female adolescents show decreased grooming and decreased movement6. Both male and female adolescent rats exposed to nicotine displayed passive avoidance behavior. Passive avoidance behavior is characterized by rats avoiding a darkened chamber where they are given a small shock. Since this small shock would normally be ignored by rats, passive avoidance behavior upon nicotine exposure could be connected to an inability to screen out minor stimuli. The scientists link this inability to increased attention disorders in kids developmentally exposed to nicotine.
While adult rats typically show behavioral changes during nicotine administration, the behavioral effects of nicotine in adolescents actually increase after nicotine exposure ends. Adolescents also have more long-lasting increases of nicotine receptors than adults, and in different areas of the brain. Thus, the DNA-loosening caused by nicotine use, the resulting gene expression changes, and consequential changes in dendritic complexity and connections between brain cells, could result in long-term behavioral consequences for adolescents.
Implications
The studies we’ve discussed have shown that many effects, like H3K4me3, are thought to be long-lasting, even into adulthood. Additionally, the expression of several genes that modify the connections between neurons increase in adolescents after nicotine exposure but are suppressed in adults. The behavioral effects of nicotine use specific to adolescents continue and even increase after stopping nicotine use. These pieces of evidence suggest that nicotine works differently in the adult and adolescent brains, and its effects are probably more enduring in adolescents, especially with the already increased vulnerability to changes in brain plasticity.
The effects of nicotine use during adolescence on long-term gene expression and behavior are especially relevant given today’s teen smoking patterns. Adolescent nicotine exposure continues to increase in the US with the introduction of vaping. Teens are told to believe that vaping is safer, yet vaping products still contain significant concentrations of nicotine, and teens often consume more nicotine with e-cigs than they would by smoking a cigarette. Nicotine is a highly addictive drug and is shown to affect gene expression, dendritic complexity, brain plasticity and learning, and behavior. The studies we discussed show that nicotine exposure has especially harmful impacts for adolescents and the developing brain, posing a heightened risk for this age group. Although we cannot directly say that adolescents are more vulnerable to brain changes from nicotine, these studies present convincing evidence to suggest a strong correlation. The evidence also suggests that these changes are long-lasting and may even persist into adulthood, jeopardizing the health of America’s youth today.
Links to Further Readings
References
- Centers for Disease Control and Prevention. (2021, August 3). Outbreak of lung injury associated with the use of e-cigarette, or vaping, products. Centers for Disease Control and Prevention. Retrieved September 11, 2022, from https://www.cdc.gov/tobacco/basic_information/e-cigarettes/severe-lung-disease.html
- E-cigarettes: Facts, stats and regulations. Truth Initiative. (n.d.). Retrieved September 11, 2022, from https://truthinitiative.org/research-resources/emerging-tobacco-products/e-cigarettes-facts-stats-and-regulations
- Jung, Y., Hsieh, L. S., Lee, A. M., Zhou, Z., Coman, D., Heath, C. J., … Picciotto, M. R. (2016). An epigenetic mechanism mediates developmental nicotine effects on neuronal structure and behavior. Nature Neuroscience, 19(7), 905–914. doi: 10.1038/nn.4315
- Schochet, T., Kelley, A. E., & Landry, C. (2005). Differential expression of arc mRNA and other plasticity-related genes induced by nicotine in adolescent rat forebrain. Neuroscience, 135(1), 285–297. doi: 10.1016/j.neuroscience.2005.05.057
- Schochet, T. L., Bremer, Q. Z., Brownfield, M. S., Kelley, A. E., & Landry, C. F. (2008). The dendritically targeted protein Dendrin is induced by acute nicotine in cortical regions of adolescent rat brain. European Journal of Neuroscience, 28(10), 1967–1979. doi: 10.1111/j.1460-9568.2008.06483.x
- Trauth, J. A., Seidler, F. J., & Slotkin, T. A. (2000). Persistent and delayed behavioral changes after nicotine treatment in adolescent rats. Brain Research, 880(1-2), 167–172. doi: 10.1016/s0006-8993(00)02823-7
Image Sources
https://signaltribunenewspaper.com/44369/news/lb-long-beach-city-council-to-discuss-banning-e-cigarette-sales-during-oct-1-meeting/
https://www.nature.com/articles/nn.4330/figures/1
https://www.vecteezy.com/vector-art/358962-diagram-of-neuron-anatomy
Audio Sources
NBC news clip on illnesses linked to vaping
ABC news clip on vape health cases
CNN news clip on vaping epidemic in high schools
ABC news clip on Trsiten’s case
Intro Music: Madness is Everywhere by Lobo Loco
Featured Image Source
Centers for Disease Control and Prevention. (2022, June 23). Quick facts on the risks of e-cigarettes for kids, teens, and young adults. Centers for Disease Control and Prevention. Retrieved September 7, 2022, from https://www.cdc.gov/tobacco/basic_information/e-cigarettes/Quick-Facts-on-the-Risks-of-E-cigarettes-for-Kids-Teens-and-Young-Adults.html