With growing interest in tau immunotherapy, researchers are exploring new antibodies that may have therapeutic potential for Alzheimer’s disease. In particular, recent findings suggest that antibodies against tau’s mid-region are the most effective at stopping the cell-to-cell propagation of aggregates (Apr 2018 conference news). In the December 4 issue of Structure, researchers led by Ian Wilson at The Scripps Research Institute in La Jolla, California, in collaboration with scientists at Janssen Pharmaceuticals, describe one such antibody that was isolated from human blood.

  • New tau antibody binds to the protein’s mid-region.
  • In vitro, it recognizes unphosphorylated and recombinant tau.
  • In AD brain sections, it appears specific for tau aggregates.

The antibody, CBTAU-24.1, bound to both monomeric and aggregated tau in vitro, and had only slightly higher affinity for phosphorylated than unphosphorylated tau. However, on AD brain sections, it recognized only aggregates. The reason for this remains unclear.

“This is an elegant and comprehensive structural study, which will help guide future studies,” noted Rakez Kayed at the University of Texas Medical Branch in Galveston in an email to Alzforum. However, he pointed out that the study did not examine any control brains, or tauopathies other than AD. “The biochemical and immunohistochemical data fall short of demonstrating the antibody’s specificity for pathological tau,” Kayed added.

Caught Midstream. Tau antibody CBTAU-24.1 binds to proline-rich region P2, just before the four repeat domains. [Courtesy of Zhang et al., Structure.]

Janssen researchers previously exposed pooled memory B cells taken from healthy donors to phosphorylated tau, and recovered 52 different tau-binding antibodies (Pascual et al., 2017).  CBTAU-24.1 was one of them. In vitro binding studies determined that it recognized residues in the second proline-rich region of tau, just before the first repeat domain (see image above). The antibody bound recombinant tau, as well as paired helical filaments of tau isolated from AD brain.

To pin down the binding site, first author Heng Zhang at the Beijing Synchrotron Radiation Facility in China crystallized CBTAU-24.1 in complex with a tau fragment, then measured the electron density to 2.6 angstrom resolution. This showed the antibody interacted with residues 235–243 via a combination of hydrogen bonds, hydrophobic interactions, and van der Waals electrostatic forces (see image below). Phosphorylation of Ser237 disrupted these interactions and prevented antibody binding. However, a phosphate group on Ser238 pointed away from the antibody and had no effect on binding. Because phosphorylation at these residues occurs late in AD, it may have a minimal effect on the antibody’s ability to recognize hyperphosphorylated tau (Hanger et al., 2007). 

Phosphorylated? No Problem. Mid-section tau fragment (yellow) binds specific residues (green, blue) on CBTAU-24.1, while a phospho group (orange) on S238 points away from the binding pocket. [Courtesy of Zhang et al., Structure.]

In sections from AD brain, Janssen researchers found that CBTAU-24.1 decorated neurofibrillary tangles and neuritic plaques, although it bound more weakly to them than did the phospho-tau-specific antibody AT8. Janssen has applied for a patent on CBTAU-24.1.

Einar Sigurdsson at New York University School of Medicine wondered if this antibody was the same as Janssen’s mid-tau antibody JNJ-63733657, now in clinical trials for AD. “It would also be interesting to find out how the Janssen antibody(ies) against this region differ from the UCB tau antibody in clinical trials,” Sigurdsson added. UCB0107 recognizes residues 235-246, almost the same region as CBTAU-24.1 does. The authors could not be reached for comment.—Madolyn Bowman Rogers

Comments

  1. This article describing the structure and binding properties of the CBTAU-24.1 antibody is a nice addition to similar reports in recent years of various tau antibodies against different epitopes. It would be informative to know if this antibody is the same one that Janssen has in clinical trials targeting the mid-region of tau as a potential therapy for Alzheimer’s disease. If not, how do they differ? It would also be interesting to find out how the Janssen antibody(ies) against this region differ from the UCB tau antibody in clinical trials. That one has been reported to recognize tau 235–246, which is similar to the epitope of the one reported here.

    The affinity of the CBTAU-24.1 antibody appears to be rather low (0.3-2.5 µM) but our studies indicate that higher affinity does not necessarily translate into better efficacy (Congdon et al., 2016). A word of caution is that it is not accurate to describe this antibody as recognizing the entire population of (aggregated) tau subspecies as mentioned in the last paragraph of the article. As reported, its binding to tau is partially phospho-dependent, with about eightfold higher affinity for its phospho- versus non-phospho-epitope but interestingly is blocked by phospho-serine 237. Therefore, like all tau antibodies, it will only recognize a certain pool of tau proteins, which is the best we can do.

    Xiangpeng Kong is a co-author of this comment. 

    References:

    . Affinity of Tau antibodies for solubilized pathological Tau species but not their immunogen or insoluble Tau aggregates predicts in vivo and ex vivo efficacy. Mol Neurodegener. 2016 Aug 30;11(1):62. PubMed.

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References

News Citations

  1. To Block Tau’s Proteopathic Spread, Antibody Must Attack its Mid-Region

Therapeutics Citations

  1. JNJ-63733657
  2. UCB0107

Paper Citations

  1. . Immunological memory to hyperphosphorylated tau in asymptomatic individuals. Acta Neuropathol. 2017 May;133(5):767-783. Epub 2017 Mar 24 PubMed.
  2. . Novel phosphorylation sites in tau from Alzheimer brain support a role for casein kinase 1 in disease pathogenesis. J Biol Chem. 2007 Aug 10;282(32):23645-54. PubMed.

External Citations

  1. clinical trials

Further Reading

Primary Papers

  1. . Structural Basis for Recognition of a Unique Epitope by a Human Anti-tau Antibody. Structure. 2018 Sep 12; PubMed.