2 November 2011. It does not take much to tilt the biochemical balance needed for optimal brain function. Take brain-derived neurotrophic factor (BDNF); changes in the levels of this trophic factor, or its activity, can make the difference between a neuron’s death or survival. Now, a study by Ahmad Salehi’s group at the Stanford University School of Medicine, California, shows that pilots who have a single nucleotide polymorphism in the BDNF gene perform less well in flight simulation as they age than do people without the genetic blip; their hippocampus also shrinks a little faster. Several studies already drew connections between decreased BDNF function and memory and cognitive deficits, including those seen in Alzheimer’s disease (see ARF Live Discussion). “But the interesting aspect of this paper is that they are looking at a more complex task, expected to involve more widely dispersed brain areas than just hippocampus,” said Margaret Fahnestock of McMaster University in Hamilton, Ontario, who was not involved in the work. “It widens the focus of BDNF from memory to more global effects on brain function.”
If the results, which appeared in the October 25 Translational Psychiatry online, are reproduced in larger studies, it may have implications for people carrying the polymorphism, which is believed to reduce BDNF activity. And they might benefit from a BDNF boost. “There are simple ways to increase BDNF levels, such as increasing physical activity, or caloric restriction,” said Salehi (see ARF related news story on Cotman and Berchtold, 2002). “This polymorphism could be used as a biomarker for targeting certain people for these interventions.”
The polymorphism consists of a G-to-A substitution in the BDNF gene, culminating in a valine for methionine amino acid swap at position 66 in the proBDNF protein, a precursor to mature BDNF. The methionine variant has not been shown to be a risk factor to AD. There is, however, ample evidence that people with AD have less BDNF (see ARF related news story on Li et al., 2009), and that boosting BDNF levels might improve AD symptoms (see ARF related news story on Erickson et al., 2010; Massa et al., 2010 and ARF related news story on Nagahara et al., 2009). Fahnestock’s lab has reported that large amyloid-β oligomers in particular are responsible for depleting BDNF levels in mouse models (see ARF related news story on Peng et al., 2009). “We think that the amyloid causes BDNF to decrease, and in turn, that contributes to the memory deficit,” said Fahnestock. The methionine substitution in BDNF also causes a decrease in BDNF function, but it does not, however, seem to play any role in the pathogenic changes leading to amyloid deposits and AD.
In 2003, Daniel Weinberger and Bai Lu at the National Institutes of Health in Bethesda, Maryland, showed that the methionine version of the protein affects proBDNF processing, thereby decreasing the amount of mature BDNF that is secreted from neurons in response to their activation. This defect is associated with episodic memory loss and hippocampal shrinkage in schizophrenic patients and family members, but not with changes in word recall, semantic memory, or working memory (see ARF related news story on Egan et al., 2003). Other labs reproduced the basic findings (see, e.g., Cathomas et al., 2010 and Hajek et al., 2011).
In Salehi’s lab, Marth Millan Sanchez and Devsmita Das, joint first authors on the paper, asked whether the decline in BDNF activity due to the methionine mutation could also be affecting other brain functions in people who have apparently normal memory. “Here we did not look at cognition per se, but rather at a large skill set involved in flying a plane,” said Salehi. The group recruited 144 pilots, including 55 with the methionine polymorphism and 89 without, ranging in age from 44 to 69 years (about 57 years old on average). They then asked the pilots to perform in a flight simulator test for 75 minutes, in which they faced flight scenarios with emergency situations, such as engine malfunctions or incoming air traffic. The pilots performed a version of this test annually for three years; they were also examined using a cognitive test called CogScreen-AE, which evaluates cognitive abilities relative to flying. One-third of participants (43 people) also underwent magnetic resonance imaging (MRI); 65 percent of the pilots screened at baseline returned for follow-up imaging at least once.
After three years, Sanchez and colleagues found that people with the methionine polymorphism had a twofold greater decline in their flight simulation test scores compared to those without the polymorphism; there was no difference in flight test scores based on ApoE gene status, which is a known risk factor for AD. The CogScreen-AE scores of the two groups of pilots, with or without the methionine polymorphisms, were the same. “They remember normally if you give them lists of words. We did not find a correlation between cognition and the polymorphism,” said Salehi. “But that does not mean that if we continue to follow these people for 10 years, their cognition might become affected.” The hippocampus of all pilots age 65 and over showed some shrinking but the shrinkage was greater in people with the methionine variant. In this study, hippocampus volume correlated with the ability to perform a complex task. “The presence or absence of the polymorphism can be used as a predictive way to understand loss of brain function, both structurally and functionally,” said Salehi. One question that this study raises is why the effects of the BDNF polymorphism only become evident later in life and not at earlier stages. “You might need an interaction between reduced BDNF activity and age-related decline in other mechanisms to see any effects in this test,” suggested Fahnestock.
A related question is how BDNF affects brain function. BDNF is a jack of all trades, promoting neuronal survival and differentiation, and synaptic plasticity. Recent work by Erik Dent’s group at the University of Wisconsin in Madison suggests that BDNF achieves the latter by influencing microtubules (Hu
et al., 2008). In the October 26 Journal of Neuroscience, Dent’s group reports that as they telescope in and out of dendritic spines, microtubules deliver post-synaptic density 95 (PSD-95), a protein that boosts synaptic strength. This mechanism is regulated by BDNF, reports first author Xindao Hu and colleagues. Agents that inhibit microtubule polymerization prevent PSD-95 from accumulating in the spines in response to BDNF. “This could be a new pathway by which materials are brought into spines as they change morphologically,” said Dent. “Microtubule invasion only occurs in certain spines, probably the more active ones. Getting molecules to these active spines helps reinforce the changes induced by synaptic activity,” he added. Salehi welcomed the result. “BDNF may indeed be involved in regulating the transport machinery in spines,” he said.—Laura Bonetti.
Sanchez MM, Das D, Taylor JL, Noda A, Yesavage JA and Salehi A. BDNF Polymorphism Predicts the Rate of Decline in Skilled Task Performance and Hippocampal Volume in Healthy Individuals. Translational Psychiatry. Published online 25 October 2011.
Hu X, Ballo L, Pietila L, Viesselmann C, Ballweg J, Lumbard D, Stevenson M, Merriam E, Dent EW. BDNF-Induced Increase of PSD-95 in Dendritic Spines Requires Dynamic Microtubule Invasions. J Neurosci. 2011 Oct 26;31(43):15597-603. Abstract