. The BDNF val66met polymorphism affects activity-dependent secretion of BDNF and human memory and hippocampal function. Cell. 2003 Jan 24;112(2):257-69. PubMed.

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  1. We were impressed by the research presented in Egan et al., which clinches a decisive role for BDNF in cognitive processes in humans. In animal models there has been an increasingly strong basis supporting an important role of BDNF in cognitive function, and the Egan study has translated this effect to humans. Essentially, the message is that when BDNF is not functioning properly, cognition suffers in the long term. Importantly, the study demonstrates that these cognitive effects occur in young/middle-aged, cognitively intact individuals. It is especially interesting that their cell culture results indicate that the effect is not in the synthesis of BDNF, but rather in the processing and release. The polymorphism is in the proBDNF (thus, on the preprocessed protein rather than in the active protein), and is affecting intracellular trafficking and also activity-dependent release.

    View all comments by Nicole Berchtold
  2. In the present studies, Daniel Weinberger and colleagues have reported that valine (val) substitution to methionine (met) in BDNF was associated with a failure to localize in secretory granules and synapses. The studies using transfected neurons appeared to translate to some degree into the human condition, where the val/met polymorphism was associated with poorer episodic memory.

    This is a difficult series of studies and the authors have generated some interesting data. Their imaging data were complicated by the numbers of subjects in the met/met group, but nonetheless, the authors have shown that there was no correlation between BDNF genotype and memory dysfunction in schizophrenia. In addition, the studies indicated that possession of the met/met genotype was associated with a significant episodic memory deficit relative to val/val or val/met in all three patient subgroups.

    These data strengthen rodent data indicating that BDNF plays a key role in memory and learning. While the molecular mechanisms of memory formation are complex and the role of long-term potentiaton widely debated, it seems possible that the initial rapid events (calcium entry, early LTP) then lead to a secondary phase of protein-synthesis dependent (late LTP) and later phenomena. The recruitment of cAMP and CREB signalling pathways could clearly result in increased secretion of BDNF, and there is an accumulating body of evidence that this neurotrophic factor plays a key role in synaptic processes that underlie memory formation.

    The lack of effect in schizophrenia is of interest. A recent postmortem study by Chen and coworkers (Chen et al., 2001) reported increased BDNF levels in the hippocampus of depressed patients treated with antidepressants (chronic antidepressant treatment is well known to increase cAMP, CREB and BDNF in rodents), but no such increase was observed in schizophrenia patients. Taken together, these data may suggest that BDNF plays a key role in learning, memory and depression, but different additional factors may come into play in schizophrenia.

    It is clear from preclinical studies that tetanic stimulation and memory tasks increase BDNF gene expression, and gene knockout studies and infusion of anti-sense to BDNF impairs spatial memory tasks. Therefore, the data seem to show BDNF is pivotal in cognitive processes. What is less clear is how this links to activity-dependent processes and how BDNF is packaged and secreted at the correct time and in the correct synaptic locations. The data from Weinberger and colleagues suggest that BDNF with the met polymorphism may be processed differently, have a low synaptic level, and may be released in lower levels in response to depolarization. These data suggest that BDNF does play a key role in memory formation in humans.

    It is possible that, in the aging brain or in disease situations (AD, depression), the met polymorphism would be associated with a more severe or rapid onset of symptoms. The data also suggest that viral vector delivery of neurotrophins or small-molecule approaches that increase synaptic BDNF levels may have potential for treating symptoms and perhaps the neuronal abnormalities in these disease states.

    View all comments by Michael O'Neill
  3. Several lines of evidence show that BDNF is implicated in hippocampal long-term potentiation, learning, and memory in nonhuman species, but until now the involvement of this factor in human memory and hippocampal function has not been examined directly.

    In this respect, Egan and colleagues provide evidence that a valine/methionine substitution polymorphism at codon 66 (V66M) in the 5’ pro-region of the human BDNF gene affects intracellular distribution, packaging, and release of BDNF protein in vivo. Furthermore, they elucidated the effect of different genotypes, in human, on verbal episodic memory, hippocampal physiological activation, neuronal integrity and synaptic abundance. The authors did not find evidence that V66M polymorphic system was associated with schizophrenia, but suggest a possible role in other neurological and psychiatric disorders. Last year, several papers reported a genetic association of V66M polymorphism with different neurological diseases. In particular, the val allele has been associated with susceptibility to Alzheimer’s disease (AD) (Ventriglia et al., 2002) and, in two family-based independent studies, with an increased risk to develop bipolar disorder (Neves-Pereira et al., 2002; Sklar et al., 2002). Similarly, the met allele was associated with Parkinson’s disease by (Momose et al.., 2002) and recently the V66M has been associated with susceptibility to anorexia nervosa (Ribasés et al., 2002).

    In light of these results, further studies are needed to clarify the effective role of the two BDNF alleles in the pathogenesis and symptomatology of each disease.

    View all comments by Massimo Gennarelli
  4. The exciting paper by Egan et al. is a far-reaching collection of experiments ranging from cell culture to human behavior that convincingly demonstrate the differing properties of proBDNF molecules carrying val66 or met66 polymorphic substitutions and their role in human episodic memory. It is very clear from the transfection experiments in hippocampal neurons that valBDNF localizes to dendrites, whereas metBDNF localizes to cell bodies. The green fluorescence is of a different character (punctate for valBDNF-GFP, diffuse for metBDNF-GFP), reflecting localization in different subcellular compartments, and only valBDNF, not metBDNF, is released at the synapse by regulated secretion. The authors have made an important discovery, namely, that the polymorphism in the pro region of BDNF is important for intracellular trafficking and activity-dependent release of BDNF. However, the number and size of the proteins, and more importantly their processing to the mature form, are not affected by the polymorphism.

    The authors carefully studied the biological activity of the mostly mature BDNF resulting from the processing and secretion of either valBDNF or metBDNF by COS cells (Fig. 7A). Not surprisingly, since the polymorphism is in the pro region, the mature forms exhibit no differences in TrkB activation or neurite outgrowth activity. What is important to determine next, however, is whether there are any differences in biological activity between the valproBDNF or metproBDNF precursors themselves. Both proBDNF and mature BDNF are present in the human CNS (Fahnestock et al., 2002; Michalski & Fahnestock, Mol. Brain Res., in press), both are secreted by transfected cells, including neurons (Egan et al., Figure 6E; Mowla et al., 2001), and both are capable of activating TrkB (Mowla et al., 2001). Thus, secreted proBDNF may exert some biological effects in the brain separate from its role as a precursor. Constitutively released proBDNF (val or met), which may regulate neuronal growth and survival for example, has yet to be investigated in the absence of mature BDNF.

    The discovery reported in this paper—that the regulated release of BDNF at synapses is compromised by the met polymorphism and contributes to BDNF’s effects on learning and memory—is exciting and significant. It is of note that there was no correlation between the met polymorphism and schizophrenia. A genetic association has been reported, however, between the val66 allele of the BDNF gene and bipolar disorder (Sklar et al., 2002; Neves-Pereira et al., 2002), and between other polymorphisms of the BDNF gene and Alzheimer’s disease (Kunugi et al., 2001; Ventriglia et al., 2002). This paper will surely encourage further examination of the role of BDNF in human cognitive function.

    View all comments by Margaret Fahnestock
  5. The study by Egan and colleagues deserves much attention because it attempts to track the role of BDNF in hippocampal function from human (!) memory traces down to the underlying molecular mechanisms. The observed polymorphism in position 66 of the pro domain, in fact, shows very interesting consequences for hippocampus-related memory tasks and hippocampal metabolism. Besides these very valuable advancements, some additional issues brought up by the authors would be interesting to focus on in future studies. Among these are:

    1. During the N-back memory task (which, as the authors mention, primarily relies on neocortical function), this study observes an unsual hyperactivation (i.e., hyperoxygenation) in the hippocampal region, thus indicating increased hippocampal neuronal activity in the met mutants. In contrast, the N-acetyl-aspartate measurements indicate a decreased metabolism of the val/met heterozygotes in the hippocampus. These experiments seem to indicate both increased and decreased BDNF-dependent hippocampal function, depending on the experimental protocol used. It will be interesting to determine which effect on hippocampal neuronal function is the more dominant one.

    2. Localization of overexpressed rat BDNF-GFP (which relates to the human val-BDNF individuals) in rat neurons has been shown to vary significantly between different cells of the same preparation (own unpublished results), and to show dissimilar subcellular localization in closely related neuronal preparations (compare e.g., Kohara et al., 2001 and Hartmann et al., 2001). Given this relatively large scatter of BDNF localization in seemingly similar neurons, it will be interesting to find out whether the met mutant is consistently mistargeted in all neuronal populations, and whether this effect is species-independent.

    3. Last, but not least, the authors seem to have expressed human BDNF in rat neurons. Although human BDNF is known to activate rodent BDNF receptors (i.e., TrkB and p75), perhaps we can’t just assume that human BDNF is targeted similarly to rat BDNF in these rodent neurons. This issue is of particular importance, given that even a single amino acid substitution in this study seems to change completely the cellular targeting of BDNF.

    In this respect, the paper by Egan et al. opens new avenues to linking BDNF biochemistry and BDNF-dependent neuronal function. However, additional studies (e.g., with rat BDNF mutated in the respective valine position) are clearly needed to settle the issue that targeting is ruled by the pro domain of these secreted molecules.

    View all comments by Volkmar Lessmann
  6. Egan et al. have performed an extraordinary study ranging from population to molecular levels. BDNF val66met polymorphism was shown first to affect episodic memory and hippocampal activation. Met-BDNF was revealed to fail in trafficking into secretory vesicles, resulting in the reduction of its release when synapses were depolarized. Thus, this SNP leads to dysfunction of BDNF in synaptic plasticity, causing memory impairment, although the biological activity of met-BDNF itself is as potent as that of val-BDNF.

    This story seems to be beautifully verified and convincing. BDNF SNPs have attracted much interest in other stories, too. The val66met polymorphism was reported to occur more frequently in patients with Parkinson’s disease (Momose et al., 2002). Other BDNF polymorphisms, such as 270C/T and 196A/G, were suggested to be associated with Alzheimer’s disease (Riemenschneider et al., 2002; Ventriglia et al., 2002). The pathological significance of BDNF SNPs should be pursued further.

    In the study by Egan et al., the met allele was revealed to be associated with poorer episodic memory, but not with altered semantic or working memory. This observation was made only with young subjects. If old met/met homozygotes were tested, the results could be different. As decreases in general cognitive function are seen in aging people, deficits in cognitive ability other than episodic memory may occur, and they might be more manifested in met/met homozygotes.

    BDNF is known to act as a factor for stimulating neuronal regeneration. The major effects of BDNF are ascribed to enhance axonal outgrowth and neuronal survival. Many studies have reported that BDNF promotes recovery from neural lesions (Blesch et al., 2001; Heibert et al., 2002; Ando et al., 2002). According to Egan et al., the release of mBDNF is much lower than that of vBDNF when synapses are depolarized. Therefore, mBDNF may be less efficient at neuronal restoration. Although Egan et al. mentions that mBDNF has the same potency for synaptic plasticity as vBDNF, it would be interesting to examine whether any delay in the recovery from neuronal damage such as ischemic insults could be seen in met/met homozygotes.

    View all comments by Susumu Ando