BDNF: Brain-derived Neurotrophic Factor

BDNF Blog Post:

The stimulus for attempting to present information about BDNF came from listening to a podcast between Andrew Huberman and Wendy Suzuki, Huberman Lab Episode 73. It also comes from my wish to understand physiologically why exercise can greatly benefit us psychologically.

Brain-derived Neurotrophic Factor (BDNF) is a neurotrophic factor protein that helps stimulate and control neurogenesis. Neurotrophic factors are proteins responsible for the growth and survival of nerve cells for the development and maintenance of adult nerve cells (Liou, 2010). Undergoing several different pathways, BDNF is thought to assist in memory, brain plasticity, and alleviating depression or anxiety.

Several studies indicate that decreased levels of BDNF are associated with depression, while increased levels of BDNF are associated with antidepressant treatments (Sleiman et al.). All of us have a great amount of BDNF circulating in our brains, specifically the hippocampus dentate gyrus region. Much of the research done on the neurotrophic factor looks at how the concentrations of BDNF are affected in response to exercise. As Colucci-D’Amato et al. point out, “Adult neurogenesis in the dentate gyrus is enhanced by voluntary exercise, exposure to an enriched environment, and chronic antidepressant administration.” Defects associated with BDNF signaling are thought to contribute to Huntington’s Disease, Alzheimer’s Disease, depression, schizophrenia, bipolar, and anxiety disorders. In this excerpt from Sleiman et al:

In animal models, exercise induces Bdnf mRNA expression in multiple brain regions (Cotman et al., 2007), most prominently in the hippocampus. BDNF production provides trophic support and increases synaptogenesis and dendritic and axonal branching and spine turnover. Blocking BDNF signaling attenuates the exercise-induced improvement of spatial learning tasks (Vaynman et al., 2004), as well as the exercise-induced expression of synaptic proteins (Vaynman et al., 2006).

As we see in the above excerpt, the hippocampus, responsible for short-term memory storage and the transfer of short-term memory into long-term memory, sees the biggest increase in Bdnf mRNA expression during exercise. Blocking a portion of the BDNF signaling pathway weakens the effect normal exercise has on inducing the improvement of synaptic development (Sleiman et al.).

The production of BDNF, the neurotrophic factor, stems from the usual processes of DNA replication, transcription, and translation, as well as the maturation of BDNF which is also worth noting. As a highly conserved (not mutated often) protein of 247 amino acids, which undergoes synthesis and proper protein folding in the ER, it makes its way into the Golgi where a signal sequence section of the protein is cleaved, generating a proBDNF isoform. In contrast to a mature BDNF isoform (mBDNF), the balance of proBDNF and mBDNF is important to regulate neuroprotection and synaptic plasticity (Sleiman et al.).

Experiment: BDNF levels in response to an exercise bout:

Two different groups of mice: one a control group, one an experimental group. The difference between the two groups was the presence of a running wheel that would allow mice to exercise voluntarily.

All mice were not forced to run on wheels, since the experimenters wanted to limit the amount of stress on the mice.

Results showed that voluntary exercise for four weeks significantly induced Bdnf promoter I and II expression in the hippocampus. This was shown via RT-PCR analysis and in Western Blot analysis of a striking increase in the concentration of BDNF expression.

Fig. 1 — Comparing BDNF concentrations between control trial (no voluntary exercise) and experimental trial (mice engaging in voluntary exercise on a running wheel). The cranial tissue samples were collected after a 30 day period (Sleiman et al.).

DBHB:

In assessing the expression of Bdnf, researchers also tracked D-β-hydroxybutyrate (DBHB), which is increased in the hippocampus in response to exercise. DBHB is considered a ketone body released from the liver that is increased in response to exercise, caloric restriction, fasting, ketogenic diets, and β-oxidation of fatty acids. The pathway that DBHB takes is from the liver into the blood, past the blood-brain barrier, and into the hippocampus. Could this be the pathway that initiates how BDNF is released that eventually leads to those positive effects of exercise?

The same research group participated in an experiment looking at DBHB levels in the hippocampus. This group found that DBHB did very well increase in mice after voluntary exercise, as expected. Except, the group also decided to treat the mice being tested with DBHB itself, to see if BDNF was, in fact, related to the concentration of DBHB that was administered. As a result, “overnight treatment with DBHB significantly induced the coding and pI-driven Bdnf transcripts consistent with the effects of exercise” (Sleiman et al.) As you can see in Fig. 4B, both the Bdnf coding and promoter sequence concentrations were enhanced in primary cortical neurons and the hippocampus.

Fig. 4 — Tracking or Administering DBHB (D-β-hydroxybutyrate) to cortical neurons as a means to measure Bdnf coding and promoter regions concentrations in different areas of the brain. 4A serves to measure DBHB after the 30 day period. 4B is the result taken from administering the 5mM DBHB to primary cortical neurons. 4C does the same thing, except to cross-sectional slices of the hippocampus. 4D looks at the overall hippocampus (Sleiman et al.)

So, there is an association between DBHB and BDNF, but what can we make of this relationship? Is there a receptor in between that stimulates further downstream signaling? Does DBHB inhibit some inhibitor of BDNF that usually exists when the individual is not undergoing exercise? Since these two pathways are usually apparent in many biochemical processes, the answer, maybe unsurprisingly, comes more from a molecular biology point of view, which you may have guessed since Bdnf was under italics, hinting that DNA or RNA was relevant to this discussion.

Another way to look at this is to view the amount of gene expression from the context of histone and chromatin remodeling. Figure 5 of Sleiman et al. notes how Histone deacetylase (HDAC) Histone H3 levels increase compared to Acetylated Histone H3 levels under variable conditions of DBHB, using BRD3308. BRD3308, acts as a selective inhibitor for a type of Histone deacetylase 3, where HDAC2 and HDAC3 work in tandem to occupy Histone H3 binding sites. Histone deacetylases, themselves, function to condense the histone and DNA binding so that DNA transcription is less accessible to transcription machinery.

This image presents an overall schematic of the impact that acetyl groups binding to POSITIVE histones that are attracted to NEGATIVE phosphate backbone groups seen on the DNA strands (not shown). If an acetyl group binds to a histone tail, the histone DNA partnership loosens, allowing transcription factors (activators in most cases) to activate transcription that will eventually lead to protein production. Deacetylases serve to prevent this from occurring (Journal of Ophthalmology).

As such, an inhibition of histone deacetylases will serve to INCREASE transcription machinery accessibility. “Taken together, these results suggest that inhibition of HDAC2 and HDAC3 in the hippocampus induces Bdnf expression.”

(A lot is going on here, but only parts A-C matter) Fig. 5: (A) Here, we see that DBHB, at higher concentrations compared to control, will decrease the Histone Deacetylase (HDAC) occupancy, thereby reducing its function of limiting transcription access. As such, a lower HDAC2/3 concentration means the acetyl groups are less inhibited in opening up the DNA regions for further transcriptional processing. (B) This is a western blot that is specific for Acetylated Histone H3 levels, verifying that higher DBHB leads to higher acetylated Histone H3 levels by reducing HDAC2/3 concentrations. (C) The BRD3308 is the HDAC inhibitor, which is another check to see that Bdnf transcription increases (Sleiman et al.).

What does all this mean, then?

To put it succinctly, D-β-hydroxybutyrate (DBHB) levels influence BDNF levels, if they take place in our hippocampi (we have two housed in our craniums). So, if we know that DBHB can come from a ketogenic diet, would that necessarily mean that a ketogenic diet could possibly result in a spike in BDNF that would be synonymous with how exercise can make us feel? Not exactly…

But how can DBHB do this? Like always in biology, an experiment is needed.

Experiment with Hippocampal slices:

The same research group sliced the hippocampus of a mouse into thin cross-section slices, incubating them with DBHB and then carrying out a field recording of post-synaptic potentials. Through a lot of neurological testing that includes measuring EPSP slopes compared to pulses sent through these slices, DBHB was thought to impact pre-synaptic modulation mediated by TrKB signaling. TrkB is a presynaptic receptor in the hippocampus that responds to DBHB. Hence, a theory comes about: DBHB binds to the TrkB receptor in the presynaptic region that will inhibit HDAC2 and HDAC3, thereby activating transcription factors that can bind to open Bdnf DNA, further translated into mature BDNF into the hippocampus. BDNF, in the meantime, also has a presynaptic modulatory role that results in increases in synaptic transmission when TrkB is also present. “Hence the presynaptic enhancement by DBHB is dependent upon the BDNF TrkB receptors” (Colucci-D’Amato et al.). Much of molecular biology relies on the presence and number of receptors for many different pathways, rather than the ligands which bind those receptors.

Taken from Sleiman et al.

During Exercise:

Understanding the different variables between how BDNF expression is released in the hippocampus still needs further exploration. It is still unclear how exercise induces Bdnf expression in the hippocampus and not in all the other brain regions. Perhaps, the ketone body transporters migrate to specific regions in the brain (hippocampus) that are a product of downstream signaling events as a result. One player, the Mct2 transporter, sees increased expression levels in the hippocampus immediately after and up to 10 hours post-exercise. This correlates with increased BDNF and TrkB levels (Sleiman et al.).

The takeaway:

Andrew Huberman mentions BDNF as an important neurotrophic factor that is released in the hippocampus to positively enhance our focus, memory, and mood long-term. Implications of the dysregulation of this factor seem to negatively impact our health in these ways, judging by the studies cited and ongoing assessments of BDNF, there are actionable habits we can take to positively regulate this physiology. Multiple habits we can develop, as Huberman in many podcasts have pointed out, that getting sunlight in our eyes for as many as 5–20 minutes, depending on the weather after waking, does also upregulate helpful pathways that do regulate not only our circadian rhythms but our moods as well, based on a timed cortisol release (Huberman, 2022). This seems to be the easiest, practical way to cultivate a measured impact on our well-being.

Yet, as 2023 begins, many are hoping to establish a routine of creating a habit to exercise regularly. I support people’s goals in doing so, not only for physical well-being but promoting our cognitive functions. This post served to hopefully elucidate one mechanism of how exercise can promote mental well-being, showing that there is physiological backing behind developing an exercise protocol.

Works Cited

Colucci-D’Amato, L., Speranza, L., & Volpicelli, F. (2020). Neurotrophic Factor BDNF, Physiological Functions and Therapeutic Potential in Depression, Neurodegeneration and Brain Cancer. International journal of molecular sciences, 21(20), 7777. https://doi.org/10.3390/ijms21207777

Journal of Ophthalmology. (2014). Figure 1: The role of epigenetics in the fibrotic processes associated with glaucoma. Hindawi. Retrieved January 5, 2023, from https://www.hindawi.com/journals/joph/2014/750459/fig1/

Huberman, A. (2022, May 23). Dr. Wendy Suzuki: Boost Attention & Memory with Science-Based Tools | Huberman Lab Podcast #73 [Podcast Dr. Wendy Suzuki: Boost Attention & Memory with Science-Based Tools | Huberman Lab Podcast #73]. In Huberman Lab. Retrieved May 23, 2022, from https://www.youtube.com/watch?v=099hgtRoUZw

Liou, S. (2010, June 26). Brain-derived neurotrophic factor (BDNF) — HOPES Huntington’s Disease Information. HOPES Huntington’s Disease Information. https://hopes.stanford.edu/brain-derived-neurotrophic-factor-bdnf/

Sleiman, S. F., Henry, J., Al-Haddad, R., El Hayek, L., Abou Haidar, E., Stringer, T., Ulja, D., Karuppagounder, S. S., Holson, E. B., Ratan, R. R., Ninan, I., & Chao, M. V. (2016). Exercise promotes the expression of brain derived neurotrophic factor (BDNF) through the action of the ketone body β-hydroxybutyrate. eLife, 5, e15092. https://doi.org/10.7554/eLife.1β5092