An ambitious attempt to link cognitive tests, functional brain imaging, and cell biological assays implicates a polymorphism in brain-derived neurotrophic factor (BDNF) as the source for highly specific memory deficits, alterations in hippocampal activity, and protein processing defects. The study, led by researchers at the National Institutes of Health, appeared in the January 24 Cell.
Beyond its well-documented role in supporting neuronal growth and survival (see related news item), BDNF plays a significant role in hippocampal long-term potentiation (LTP), and in learning and memory (see Lu and Gottschalk, 2000; Poo, 2001). Given the presence of a frequent polymorphism in the human BDNF gene (a valine for methionine substitution), and some evidence of hippocampal dysfunction in schizophrenia patients and their siblings, Daniel Weinberger of the National Institute of Mental Health and Bai Lu of the National Institute of Child Health and Human Development, both in Bethesda, Maryland, led a team to test whether BDNF genotype might be a risk factor for hippocampal dysfunction in these groups. The researchers also explored whether BDNF polymorphisms might affect the cellular processing of the peptide.
The cohort (n = 641) for the memory experiments included schizophrenia patients, their unaffected siblings, and normal controls, all drawn from a large sibling study of schizophrenia. The intent of the experiment was to determine whether the BDNF genotype was related to memory dysfunction in schizophrenia, but the researchers found no such correlation. Instead, they found that possession of the met/met genotype was associated with a significant, and similar, episodic memory deficit relative to val/val or val/met in all three patient subgroups. Interestingly, there was no such difference in tests of word recall, semantic memory, or working memory/executive function-memory domains that may depend less upon the hippocampus.
In two imaging experiments, the researchers were able to correlate this suggestion of hippocampal dysfunction with hippocampal activity differences. They used fMRI to monitor brain function in subjects performing the N-back test, a memory trial that involves primarily neocortex while tending to "disengage" the hippocampus. In two independent, small cohorts of normal controls (13 and 17 subjects, respectively), the researchers found that val/val subjects had the expected deactivation of the hippocampi, whereas val/met subjects had an inappropriate overactivation of both hippocampi. (There were not enough met/met subjects to make a valid assessment of this subgroup.)
In the second imaging study, the researchers used MRI spectroscopy to measure n-acetyl-aspartate (NAA), which has been proposed as an indirect marker of neuronal integrity and synaptic abundance (Maier et al., 1995). Again studying all three groups (schizophrenia patients, siblings, and controls), they found that the NAA signal was significantly reduced in the left hippocampus of val/met subjects relative to val/val subjects. In the right hippocampus, the researchers noted only trends in this same direction. Again, the number of met/met subjects was too small to allow direct comparison to the other groups, but a multiple regression analysis yielded a linear reduction in NAA levels in the left hippocampus with an increasing number of met alleles, suggesting an allele dose effect.
The third level of experiments looked at the effects of BDNF genotype on cultured rat hippocampal neurons. Neurons were transfected with either val- or met-BDNF, and the polymorphisms did not appear to affect the production of the protein, or its ability to function as a neurite-inducing growth factor. However, fluorescence and double-labeling techniques indicated that, whereas val-BDNF occurred both in the cell body and on dendrites, met-BDNF was confined to the cell body. Fluorescence analysis suggested that met-BDNF levels were lower than val-BDNF levels, and that the met-BDNF accumulated near the nucleus, whereas the val-BDNF showed a punctate distribution in the cell body and dendrites.
In response to a depolarizing challenge, val-BDNF-transfected, but no met-BDNF-transfected neurons markedly increased their secretion of the protein into the culture medium. In contrast to this activity-dependent secretion, there was no difference in the amounts of regularly (constitutively) secreted protein between the two groups of cells.
Hypothesizing that the met polymorphism may lead to sorting of the protein into the wrong secretory pathways, the researchers examined its subcellular locations. Markers for various intracellular organelles indicated that only val-BDNF is successfully sorted from the Golgi apparatus to secretory vesicles. This would seem to explain why val-BDNF, but not met-BDNF, is found near synapses and is secreted in response to the depolarization challenge.
The accumulated results from this collaboration lead the authors to suggest that these problems with intracellular trafficking and activity-dependent secretion of the met-BDNF could play a role both in the altered hippocampal function seen in imaging scans and the deficits on episodic memory tests. Further, "it is reasonable to speculate that the gene will impact the manifestation of diseases where function of the hippocampus and memory are impaired by the disease. Thus, one can imagine that a condition such as Alzheimer's disease, which destroys the hippocampus, may produce more dramatic effects or have a worse or more rapid course in individuals who have the met-BDNF genes in comparison to individuals with the val form of the gene. Similar phenomena may also occur with normal aging and in depression," said Weinberger in a Cell press release.—Hakon Heimer