Some people succumb to the effects of Alzheimer’s pathology sooner than others. Scientists do not understand this variability. Some believe that a common Val66Met substitution in brain-derived neurotrophic factor, aka BDNF, may play a role, since it can speed cognitive decline. In the January 31 JAMA Neurology, researchers led by Yen Ying Lim at Monash University in Clayton, Australia, reported that the Met allele also accelerates phosphorylation of tau. In people with autosomal-dominant AD, the Met allele came with more p-tau217 and p-tau181 in cerebrospinal fluid at presymptomatic stages than the Val allele, and with more p-tau205 and total tau at symptomatic stages. In addition, non-demented Met carriers had worse episodic memory, while those with dementia had worse global cognition.

  • The BDNF Val66Met mutation associates with cognitive decline.
  • Before dementia, Met66 BDNF carriers have more p-tau217 and p-tau181 in the brain, and worse memory.
  • After symptoms start, Met66 carriers have more p-tau205, total tau, and more cognitive loss.
  • The Met allele fuels progression of disease.

The results support a model in which the Met allele hastens symptomatic disease onset, Lim told Alzforum. Met66 lowers BDNF protein levels, raising the question of whether boosting BDNF could slow disease progression. Lim plans to explore ways of doing just that.

Others cautiously agreed the approach has potential. “The data are consistent with the general view that BDNF is important to the prevention of AD,” Helen Scharfman at the Nathan Kline Institute in New York wrote to Alzforum. However, she noted that simply stimulating BDNF production could backfire, since the immature form of the protein can also promote cell death (full comment below).

Found in about a third of Caucasians and 70 percent of Asians genotyped thus far, the Met polymorphism interferes with BDNF secretion, in effect lowering the amount of this growth factor (Jan 2003 news). BDNF drops with age, and low levels associate with memory loss (May 2009 news; Apr 2010 news). Perhaps unsurprisingly, then, people who carry the allele tend to have worse episodic memory, and if they also have amyloid plaques, their cognition declines faster than it does in those without Met (Lim et al., 2013; Lim et al., 2014; Oct 2014 news).

Lim had previously linked Met with higher CSF total tau and p-tau181, as well as with worse memory and a shrinking hippocampus, among presymptomatic mutation carriers in the Dominantly Inherited Alzheimer Network (Aug 2016 news; Lim et al., 2018). Since then, assays for other phospho-tau markers have been developed, allowing scientists to tie p-tau217 to amyloid plaques and p-tau205 to atrophy, which, in turn, helps them characterize distinct stages of the disease (Mar 2020 news; Apr 2020 conference news).

To parse how BDNF genotypes affect these newer markers, Lim and colleagues analyzed CSF from 374 DIAN participants. Of these, 144 had no AD mutation, 156 had a mutation but were presymptomatic, and 74 carriers were symptomatic. About one-third of each group carried the Met allele. At the presymptomatic stage, Val carriers were indistinguishable from healthy controls in memory, global cognition, and hippocampal volume, although they had more CSF p-tau217 and p-tau181. Met carriers fared worse, with smaller hippocampi and worse episodic memory than Val carriers. Presymptomatic Met carriers also had more of the early AD markers p-tau217 and p-tau181 in their CSF, and slightly more total tau, but, as might be expected, no difference in the later AD marker p-tau205.

At symptomatic stages, the picture changed. Both Met and Val carriers had similarly impaired episodic memories and shrunken hippocampi. Both groups had slipped on global cognition, but Met carriers more so. On tau markers, Met carriers had much more of the late markers p-tau205 and total tau than did Val carriers, but only slightly more p-tau217, and no difference in p-tau181. When the authors compared participants based on their estimated year of symptom onset, Met carriers fared worse than their EYO would suggest.

Overall, the data suggested an earlier onset of neurodegeneration, with Met carriers reaching each pathological benchmark sooner than Val carriers.

How might the Met allele bring on an earlier onset? Lim noted that low BDNF can push neurons toward death, which might worsen tau pathology. Alternatively, the Met allele could directly promote tau hyperphosphorylation. Frank Longo at Stanford University in Palo Alto, California, and Stephen Massa at the University of California, San Francisco, consider the latter more likely. They note that the Val66Met polymorphism lies within the portion of BDNF that is cleaved off to make the mature protein. The full-length BDNF protein containing this site is a known trigger of tau phosphorylation, synapse loss, and degeneration via the p75 neurotrophin receptor and its sortilin co-receptor (Aguilar et al., 2017; Giza et al., 2018). 

“Therapeutic strategies … [that] reduce p75NTR-mediated signaling promoting excess tau phosphorylation and dendritic spine degeneration might serve to counteract the degenerative effects of the Val66Met allele,” Longo and Massa wrote to Alzforum (full comment below). Longo has been working on such a strategy, developing the p75NTR ligand LM11A-31-BHS for more than a decade (Yang et al., 2020; Yang et al., 2020). 

Indeed, Lim found that the Met allele may primarily wreak havoc via its effect on p-tau, at least in the early stages. A statistical analysis suggested that the higher p-tau217 level in Met carriers fully explained their worse episodic memory. On the other hand, higher levels of p-tau205 did not fully account for worse global cognition at later stages. This may be because later in disease, other processes such as neuroinflammation and neuron death are at work, as well, Lim said.

Do the findings apply to the much more common sporadic AD? The authors examined CSF data from 125 cognitively healthy, amyloid-positive ADNI participants. The 38 Met carriers among them had worse episodic memory and higher total tau and p-tau181 than the 87 Val carriers, matching the findings from ADAD. P-tau217 and p-tau205 data were unavailable. Previously, Lim had reported that Met carriers with presymptomatic sporadic AD developed symptoms sooner than Val carriers (Lim et al., 2021). 

Lim will follow DIAN participants longitudinally to determine whether Met carriers do, in fact, become cognitively impaired sooner than Val carriers. She also wants to find out whether BDNF genotype affects a person’s neurofibrillary tangle load, which rises in the symptomatic stage of the disease.—Madolyn Bowman Rogers

Comments

  1. This study is very interesting in showing that the BDNF Val66Met polymorphism is associated with increasingly abnormal tau levels and cognitive decline, as well as with lower hippocampal volume, in the transition from presymptomatic to symptomatic dominantly inherited AD (DIAD). While the variability in the data and sample size are limiting, as is the study of subjects with DIAD, a subset of AD, the data are consistent with the general view that BDNF is important in the prevention of AD and that interfering with BDNF will promote AD.

    The authors conclude that neurotrophin support is important to potentially prevent the effects of the Val/Met polymorphism, but I think this may not be the best therapeutic approach because supporting all neurotrophins, or even just increasing BDNF, may have negative consequences in AD. The reason being that some of the aspects of BDNF synthesis and action will not necessarily correct what the Val/Met polymorphism impairs.

    One additional negative consequence of increased BDNF synthesis is the production of proBDNF, which can cause cell death. In the setting of a neurodegenerative state, glia are quite different than they are in control conditions, and can interact with neurotrophins and their receptors to negative effect.

  2. This Lim/DIAN report describes their cross-sectional cohort study of mutation carriers who also carry the BDNF Val66Met allele. This allele has been associated with decreased episodic memory, reduced hippocampal volume, and higher CSF p-tau217/tau217, p-tau181/tau181, and total tau. The findings are interpreted to suggest that changes in BDNF function resulting from the Val66Met polymorphisms influence amyloid effects on tau and enhance the ability of amyloid to promote neural degeneration. Given these apparent effects, it is of interest to consider the potential mechanisms triggered by this allele and the ways in which these mechanisms might point to therapeutic strategies.

    The mature form of the BDNF protein is derived from the pro-form of BDNF, and the Val66Met site lies within the prodomain, which is cleaved from the mature domain and may have independent functions. Mature BDNF binds to the p75 neurotrophin receptor (p75NTR) and TrkB co-receptor to promote neuronal survival, dendritic spine density and maturation, and synaptic plasticity (Brown et al., 2020). In contrast, in many systems, pro-BDNF binds to p75NTR and the sortilin co-receptor to promote degenerative patterns of signaling, including alteration of RhoA/Rac/cofilin mechanisms, which promote loss of dendritic spines as well as excess tau phosphorylation (Aguilar et al., 2017). 

    In preclinical models, the Val66Met polymorphism has been associated with reduced secretion of mature BDNF (Egan et al., 2003) and compromised synaptic plasticity (Ninan et al., 2010). In addition to this loss-of-function model, a gain-of-function model has been suggested by the demonstration that the Val66Met prodomain interacts with the SorCS2 receptor to inhibit Rac activity and promote growth cone retraction (Anastasia et al., 2013). Moreover, in in vitro and mouse models, the BDNF Met prodomain has been found to act through SorCS2 in association with p75NTR to perturb Rac function and disrupt actin regulation and thereby promote disassembly of mature mushroom spines on hippocampal neuron dendrites resulting in loss of synapses (Giza et al., 2018). 

    Future BDNF Val66Met human studies that include assessment of synapses and spines will be of great interest, but the present findings regarding changes in tau phosphorylation, hippocampal volume, and cognition begin to create a picture that is at least consistent with the prior preclinical studies and proposed mechanisms of loss and gain of function triggered by this prodomain alteration. Therapeutic strategies targeting promotion of TrkB activation to accommodate BDNF deficiency in the context of the Val66Met allele (Warnault et al., 2016), or to reduce p75NTR-mediated signaling promoting excess tau phosphorylation and dendritic spine degeneration (Yang et al., 2020; Yang et al., 2020), might serve to counteract the degenerative effects of the Val66Met allele.

    References:

    . The BDNF Val66Met Polymorphism Modulates Resilience of Neurological Functioning to Brain Ageing and Dementia: A Narrative Review. Brain Sci. 2020 Mar 25;10(4) PubMed.

    . Rho GTPases as therapeutic targets in Alzheimer's disease. Alzheimers Res Ther. 2017 Dec 15;9(1):97. PubMed.

    . 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.

    . The BDNF Val66Met polymorphism impairs NMDA receptor-dependent synaptic plasticity in the hippocampus. J Neurosci. 2010 Jun 30;30(26):8866-70. PubMed.

    . Val66Met polymorphism of BDNF alters prodomain structure to induce neuronal growth cone retraction. Nat Commun. 2013;4:2490. PubMed.

    . The BDNF Val66Met Prodomain Disassembles Dendritic Spines Altering Fear Extinction Circuitry and Behavior. Neuron. 2018 Jul 11;99(1):163-178.e6. Epub 2018 Jun 14 PubMed.

    . The BDNF Valine 68 to Methionine Polymorphism Increases Compulsive Alcohol Drinking in Mice That Is Reversed by Tropomyosin Receptor Kinase B Activation. Biol Psychiatry. 2016 Mar 15;79(6):463-73. Epub 2015 Jun 12 PubMed.

    . Small molecule modulation of the p75 neurotrophin receptor inhibits multiple amyloid beta-induced tau pathologies. Sci Rep. 2020 Nov 23;10(1):20322. PubMed.

    . Small-molecule modulation of the p75 neurotrophin receptor inhibits a wide range of tau molecular pathologies and their sequelae in P301S tauopathy mice. Acta Neuropathol Commun. 2020 Sep 5;8(1):156. PubMed.

  3. This very interesting paper expands on the author’s previous work (Lim et al., 2018) providing support for the importance of early therapeutic boosting of brain BDNF levels in AD mutation carriers. The next question is whether physical exercise, which is currently the best known way of increasing brain BDNF levels, can increase BDNF sufficiently to slow cognitive decline specifically in presymptomatic Met66 carriers.

    The paper also implies a possible bi-directional interaction: higher phosphorylated tau levels are associated with BDNF-Met66, which reduces BDNF levels compared to BDNF-Val66; and pathological tau reduces BDNF expression (as shown by our lab). In future, examination of kinase activation by BDNF Val66 vs. Met66 and its pro-peptide may illuminate AD mechanisms.

    References:

    . Effect of BDNFVal66Met on disease markers in dominantly inherited Alzheimer's disease. Ann Neurol. 2018 Sep;84(3):424-435. Epub 2018 Aug 25 PubMed.

  4. The results in this paper are indeed interesting and suggest that CSF pTau-181 levels could be included in the workflow for clinical studies addressing therapeutic effects of molecules targeting the BDNF/TrkB pathway. Furthermore, the results are well in line with previously published data on the role of the BDNF-Val66Met polymorphism in sporadic or familial AD (Lim et al., 2013; Lim et al., 2016; Boots et al., 2017) on cognitive performance in older adults (Kennedy et al., 2015) and on the increased risk of dementia and increased risk of progression into AD when Val66Met is combined with positive Aβ status (van den Bosch et al., 2021). 

    One important aspect of the Val66Met polymorphism is the combined effects with ApoE4 status on increased amyloid load (Adamczuk et al., 2013; Lim et al., 2014; Stonnington et al., 2020) and reduced cognitive function (Ward et al., 2014; Cechova et al., 2020), suggesting that ApoE4/Val66Met carriers are at higher risk for developing AD and dementia.

    Pharmacological approaches to modulate pro-BDNF or BDNF signaling via p75NTR or TrkB, respectively, are promising options for the treatment of cognitive dysfunction in AD. Modulation of the receptor activity by a small molecule offers many advantages in comparison to direct delivery of growth factors or the use of agonist. These include easier route of administration, increased signaling regionally where NGF or BDNF are released but sufficient signaling is not reached, and less side effects than a pure agonist of Trk receptors.

    A positive allosteric modulator of Trk receptors was described to increase the kinase activity of Trk receptors and to improve cognition in a TrkB-dependent manner in scopolamine-induced memory impairment, and to improve long-term memory in aged animals with a natural decline in cognition (Dahlström et al., 2021). The described molecule ACD856 is a triazinetrione derivative identified in an extensive lead optimization program using a HTS hits, a known and well-tolerated veterinary anti-parasitic medicine, as starting point (Kennedy et al., 2001). 

    The use of small molecules acting as pan-Trk positive allosteric modulators is novel and has not been described before. A drug that increases signaling of the neurotrophins could be a beneficial therapy for the treatment of neurodegenerative disorders. The ongoing Phase 1 clinical trial with ACD856, or the results obtained in the Phase 2 trial with LM11A-31-BHS, will lend further support to modulators of neurotrophin receptors as being a valid mechanism for the treatment of Alzheimer’s disease.

    References:

    . BDNF Val66Met, Aβ amyloid, and cognitive decline in preclinical Alzheimer's disease. Neurobiol Aging. 2013 Nov;34(11):2457-64. PubMed.

    . BDNF Val66Met moderates memory impairment, hippocampal function and tau in preclinical autosomal dominant Alzheimer's disease. Brain. 2016 Oct;139(Pt 10):2766-2777. Epub 2016 Aug 12 PubMed.

    . BDNF Val66Met predicts cognitive decline in the Wisconsin Registry for Alzheimer's Prevention. Neurology. 2017 May 30;88(22):2098-2106. Epub 2017 May 3 PubMed.

    . BDNF val66met polymorphism affects aging of multiple types of memory. Brain Res. 2015 Jul 1;1612:104-17. Epub 2014 Sep 28 PubMed.

    . BDNF-Met polymorphism and amyloid-beta in relation to cognitive decline in cognitively normal elderly: the SCIENCe project. Neurobiol Aging. 2021 Dec;108:146-154. Epub 2021 Sep 4 PubMed.

    . Polymorphism of brain derived neurotrophic factor influences β amyloid load in cognitively intact apolipoprotein E ε4 carriers. Neuroimage Clin. 2013;2:512-20. PubMed.

    . APOE and BDNF polymorphisms moderate amyloid β-related cognitive decline in preclinical Alzheimer's disease. Mol Psychiatry. 2014 Oct 7; PubMed.

    . Interaction Between BDNF Val66Met and APOE4 on Biomarkers of Alzheimer's Disease and Cognitive Decline. J Alzheimers Dis. 2020;78(2):721-734. PubMed.

    . APOE and BDNF Val66Met polymorphisms combine to influence episodic memory function in older adults. Behav Brain Res. 2014 Sep 1;271:309-15. Epub 2014 Jun 16 PubMed.

    . Impact of APOE and BDNF Val66Met Gene Polymorphisms on Cognitive Functions in Patients with Amnestic Mild Cognitive Impairment. J Alzheimers Dis. 2020;73(1):247-257. PubMed.

    . Identification of Novel Positive Allosteric Modulators of Neurotrophin Receptors for the Treatment of Cognitive Dysfunction. Cells. 2021 Jul 23;10(8) PubMed.

    . Safety of ponazuril 15% oral paste in horses. Vet Ther. 2001;2(3):223-31. PubMed.

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References

News Citations

  1. From Protein Trafficking to Episodic Memory: Tracing BDNF Genotypes
  2. Research Brief: Low Spinal Fluid BDNF a Prelude to Memory Decline?
  3. Research Brief: BDNF Data Speak Volumes, Offer Therapeutic Target
  4. Do Neurotrophin and ApoE Together Exacerbate Alzheimer's?
  5. Monogenetic and Sporadic Alzheimer’s Disease: Same or Different?
  6. Different CSF Phospho-Taus Match Distinct Changes in Brain Pathology
  7. 217—The Best Phospho-Tau Marker for Alzheimer’s?

Therapeutics Citations

  1. LM11A-31-BHS

Paper Citations

  1. . BDNF Val66Met, Aβ amyloid, and cognitive decline in preclinical Alzheimer's disease. Neurobiol Aging. 2013 Nov;34(11):2457-64. PubMed.
  2. . Effect of BDNF Val66Met on memory decline and hippocampal atrophy in prodromal Alzheimer's disease: a preliminary study. PLoS One. 2014;9(1):e86498. Epub 2014 Jan 27 PubMed.
  3. . Effect of BDNFVal66Met on disease markers in dominantly inherited Alzheimer's disease. Ann Neurol. 2018 Sep;84(3):424-435. Epub 2018 Aug 25 PubMed.
  4. . Rho GTPases as therapeutic targets in Alzheimer's disease. Alzheimers Res Ther. 2017 Dec 15;9(1):97. PubMed.
  5. . The BDNF Val66Met Prodomain Disassembles Dendritic Spines Altering Fear Extinction Circuitry and Behavior. Neuron. 2018 Sep 19;99(6):1356. PubMed.
  6. . Small-molecule modulation of the p75 neurotrophin receptor inhibits a wide range of tau molecular pathologies and their sequelae in P301S tauopathy mice. Acta Neuropathol Commun. 2020 Sep 5;8(1):156. PubMed.
  7. . Small molecule modulation of the p75 neurotrophin receptor inhibits multiple amyloid beta-induced tau pathologies. Sci Rep. 2020 Nov 23;10(1):20322. PubMed.
  8. . BDNF VAL66MET polymorphism and memory decline across the spectrum of Alzheimer's disease. Genes Brain Behav. 2021 Jun;20(5):e12724. Epub 2021 Jan 6 PubMed.

Further Reading

Primary Papers

  1. . Association of BDNF Val66Met With Tau Hyperphosphorylation and Cognition in Dominantly Inherited Alzheimer Disease. JAMA Neurol. 2022 Mar 1;79(3):261-270. PubMed.