EphA4 is emerging as a key obstacle blocking the regeneration of axons damaged by neurological diseases including ALS (Van Hoecke et al., 2012). But developing strategies that reduce the signaling of this receptor tyrosine kinase is tricky to do. Peptide antagonists such as “KYL” degrade within minutes upon administration – at least in mice. Small molecule-based strategies struggle with specificity (see February 2017 news). And, antibodies can be challenging to deliver into the brain and spinal cord.
Now, a research team led by VIB’s Wim Robberecht in Belgium, introduce highly potent nanobodies that block ephA4 activation in vitro. These single-domain antibodies, generated in alpacas and llamas, are emerging as a key alternative to conventional therapeutic antibodies due to their increased thermostability, improved tissue distribution, reduced immunogenencity and at least in some cases, the ability to enter the central nervous system including the brain (Hussack et al., 2011; Li et al., 2016).
The study is published on May 19 in the Journal of Biological Chemistry.
The nanobodies, known as Nb39 and Nb53, which target the ligand-binding domain of the receptor, bind EphA4 at nanomolar affinity (KD = 23-30 nM) and are more stable in circulation – at least in mice (t½ > 72 hours). What’s more, pre-treatment with these single-domain antibodies prevents the ephrin-b3 mediated collapse of growth cones of cortical neurons and therefore, may facilitate axon growth. This potency and stability is comparable to cyclic peptides, recently developed by Sanford Burnham Prebys’ Elena Pasquale in California and University of Texas Southwestern’s Eric Olsen, which also block EphA4 activation (Olson et al., 2016).
The study is a key first step in developing these single-domain antibodies as a potential therapy for ALS. This is the first nanobody-based strategy reported for the disease.
Meanwhile, University of California’s Maurizio Pellechia in Riverside is developing a different therapeutic approach in hopes to enable the repair of motor neurons in people with ALS damaged by the disease. Pellechia’s team is developing highly potent and selective small molecule agonists that target EphA4 (see February 2017 news; Wu et al., 2017). The approach, being developed in collaboration with Iron Horse Therapeutics in San Diego, California, is thought to eliminate EphA4 from the surface of damaged motor neurons by stimulating receptor-mediated endocytosis. The strategy is now being optimized and is at the preclinical stage.
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