Once again, BACE1 is the baddie. But in this instance, it is in nerve regeneration, not Alzheimer’s disease, where β-site amyloid precursor protease (APP) cleaving enzyme 1 (BACE1) seems to get in the way. BACE1 knockout mice regenerate crushed peripheral nerves faster, according to an April 13 paper in the Journal of Neuroscience. Although the mechanism at play is unclear, macrophages without the enzyme do a better job of clearing away debris from broken nerves, setting the stage for new growth. The study authors, at Johns Hopkins University in Baltimore, Maryland, also report that small molecule BACE1 inhibitors similarly promoted axon growth.

First author Mohamed Farah led the study with senior author John Griffin. Griffin succumbed April 16 to bladder cancer. He was the “mastermind of the whole project,” Farah said. “Jack Griffin was a beloved and much admired mentor,” he added in an e-mail to ARF. Griffin spent most of his four-decade career at Johns Hopkins School of Medicine, where he headed the department of neurology and founded the Brain Science Institute. Both the Brain Science Institute and the Baltimore Sun ran obituaries.

The researchers were inspired to examine BACE1’s potential role in nerve regeneration by reports that the enzyme cleaves neuregulin, which is involved in myelination during development (see ARF conference story on Willem et al., 2006; Michailov et al., 2004; Taveggia et al., 2005). Later studies showed that N-APP, another BACE1 cleavage product, triggers axon pruning and neural degeneration during development (see ARF related news story on Nikolaev et al., 2009).

Farah first examined the repair of sciatic nerve injury in BACE1 knockout mice, in which axons are hypomyelinated (Hu et al., 2006). Fixing a crushed or severed nerve is a multistep process. First, the damaged axons retract from the injury site. Macrophages clean up debris, particularly myelin. Finally, new axons grow into the site. Young rodents are able to re-innervate after crush injury to peripheral nerves, but humans are rarely so lucky: Distances are long, and the axons regrow so slowly that Schwann cells and target tissues atrophy and die before the new axons reach them (reviewed in Höke, 2006).

Within days of sciatic nerve crush, axons disintegrate in a process called Wallerian degeneration. Farah observed no difference in this process between wild-type and BACE1 knockout mice. But in the knockout animals, leftover fragments disappeared faster. Five days after the crush, knockout mice had less degraded myelin basic protein at the injury site. Their macrophages rapidly infiltrated the crushed nerves and accumulated cholesterol droplets, giving them a foamy appearance that indicates digestion of myelin. Two weeks after the nerve crush, the macrophages had mostly completed their task and left the site injury of BACE1 knockouts, whereas the macrophages were still present in the injury of wild-type mice at that time point.

The quick clearance of myelin was no great surprise; in part, it could have been because the BACE1 knockout mice have less myelin to begin with. Farah experimented on the macrophages in vitro to better understand the cleanup process. BACE1 knockout macrophages, he found, take up more small beads than do wild-type macrophages. Thus, the absence of BACE1 in these cells revs up their phagocytic appetite. The researchers suggest that macrophages must contain a BACE1 substrate, although they are not sure what that might be. The protein triggering receptor expressed on macrophage 2 (TREM2) is one attractive candidate, since it can improve the rate of myelin phagocytosis (Takahashi et al., 2007).

Having cleared a path, the BACE1 knockout mice then grew more and bigger axons, and did so faster. Axonal sprouts had grown farther in knockout than wild-type mice just three days after nerve crush. Five days into recovery, knockouts had more axonal sprouts in the distal stump, and more of them were greater than a micron in diameter, than in wild-type mice. Finally, foot and leg muscles were re-innervated faster in BACE1 mice.

This could be a result of neurons themselves growing faster, or simply a downstream result of the macrophages’ quick clearance of old fragments. To test the effects of BACE1 knockout in neurons alone, Farah and colleagues prepared explants of dorsal root ganglia (DRG). By four days of culture, axons in the knockout DRG were one and a half times as long as those in wild-type DRG, suggesting the loss of BACE1 directly affects axonal regrowth within the neuron. Perhaps, Farah speculated, N-APP acts as a “brake” to stop axon growth.

“A BACE1 inhibitor could be useful for conditions in which peripheral axons regenerate,” Farah said. He examined the effects of two chemical BACE1 inhibitors and found they promoted myelin clearance, regeneration, and re-innervation—though less than a BACE1 knockout did. Given within a month of injury, he suggested, such an inhibitor might promote regeneration. Then, removing the inhibitor would allow the axons to become normally myelinated.

Alzheimer’s researchers have been working on BACE1 inhibitors for a decade, with a few early-stage clinical candidates to show for the effort (for a recent update, see ARF AD/PD 2011 conference story). Many of the earlier candidates fail to cross the blood-brain barrier, but that would be unnecessary to treat peripheral nerve injury, Farah noted. More challenging would be treating spinal cord injury, which Farah is currently studying.

In addition, he suggested BACE1 inhibition might be helpful for people with neuropathy caused by diabetes or chemotherapy. As a proof of principle, Farah is currently crossing BACE1 knockout mice with mice that model the neuropathy amyotrophic lateral sclerosis. However, he noted the treatment would only regrow nerves that still have a cell body.

Therapeutic possibilities aside, the study offers a more general message about myelin and axon regeneration, wrote Rudolf Martini of the University of Würzburg in Germany, in an e-mail to ARF: “The study indirectly supports the concept of myelin removal as an important prerequisite for axon regrowth. This process is extremely delayed in the central nervous system, and may at least partially explain the chronically poor outcome of spinal cord injury and other traumata of the CNS.” (See full comment below.)—Amber Dance

Comments

Make a Comment

To make a comment you must login or register.

Comments on News and Primary Papers

  1. This work deals with the role of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) in nerve lesions and toxic neuropathies in mice. The enzyme generates the well-known amyloid-β peptide, which is a pathogenetically relevant component of plaques in Alzheimer’s disease (AD) brains, and N-APP, which can trigger axon degeneration. In the present study, the consequences of the absence of BACE1 were investigated in injured peripheral nerves, a topic remote to AD.

    In contrast to axons of the central nervous system (CNS), axons of the peripheral nervous system (PNS) can regrow for long distances after injury. The reasons for this clinically highly relevant difference are manifold and imply neuronal and glial properties. Of particular relevance is the role of a third cell type, nerve-borne and infiltrating macrophages that remove growth-inhibiting myelin from the injured nerve (Vargas and Barres, 2007).

    How might BACE1 relate to peripheral nerve pathology and repair? Initially, the authors might have expected that—due to the absence of the axonopathic BACE1 products—BACE1-deficiency might preserve injured axons. In fact, this was neither the case in the nerve lesion model nor in toxic neuropathy. Instead, the authors observed by in vitro approaches a higher axonal outgrowth rate of BACE1-deficient neurons. Even more spectacular was a substantially increased removal of degenerating myelin and a more robust axonal regrowth and restoration of muscle synapses after nerve injury in the BACE1-deficient mice. Pharmacological inhibition of BACE1 produced similar results as genetic inactivation.

    How might BACE1 deficiency or inhibition improve removal of myelin and axon regrowth? By an in vitro approach, the authors showed that BACE1-deficient macrophages have a higher capacity for phagocytosis, which could lead to an increased removal of myelin, thus “clearing” pathways for axon regrowth. This issue could be further characterized experimentally by testing the increased clearance capacity of BACE1-/- macrophages when transplanting them into normal (i.e., BACE1+) mice before nerve injury.

    The authors’ favored interpretation of increased myelin phagocytosis by macrophages is that the triggering receptor expressed on myeloid cells-2 (TREM-2), a cell surface protein that drives macrophages into a phagocytic activation state (Takahashi et al., 2007), might be a substrate of BACE1: More preserved TREM-2 in the absence of BACE1 could lead to more phagocytosis of myelin. Together with the increased outgrowth properties of BACE1-deficient neurons, the removal of myelin of lesioned fibers may substantially increase axonal regeneration and reinnervation of denervated muscle fibers.

    The study has important therapeutic implications, particularly since regrowth of axons over long distances is clearly a limiting factor of nerve regeneration in humans. Thus, inhibiting BACE1 after nerve injury might be a promising approach to accelerate regeneration and to reduce the risk of denervated organs being irreversably lost by atrophy due to delayed contact with regrown axons. In a more general take-home message, the study indirectly supports the concept of myelin removal as an important prerequisite for axon regrowth. This process is extremely delayed in the CNS and may partially explain the chronically poor outcome of spinal cord injury and other traumata of the CNS (Vargas and Barres, 2007). Thus, it should be a future challenge to investigate BACE1 inhibition in models for CNS injuries.

    All in all, the paper by the Baltimore group around John W. Griffin is a gem with strong therapeutical and conceptual implications. It profoundly extends our knowlege about possible therapeutic intervention cues in nerve injury. The clinical application using existing BACE1 inhibitors originally designed for treatment of AD would honor this and other pivotal work by Jack Griffin, who sadly passed away a few days after the appearence of this paper.

    View all comments by Rudolf Martini

References

News Citations

  1. Madrid: BACE Found to Have Big Job in Wrapping Motoneurons
  2. Keystone: Death Receptor Ligand—New Role for APP, New Model for AD?
  3. Barcelona: Out of Left Field—Hit to The Eye Kills BACE Inhibitor

Paper Citations

  1. . Control of peripheral nerve myelination by the beta-secretase BACE1. Science. 2006 Oct 27;314(5799):664-6. PubMed.
  2. . Axonal neuregulin-1 regulates myelin sheath thickness. Science. 2004 Apr 30;304(5671):700-3. PubMed.
  3. . Neuregulin-1 type III determines the ensheathment fate of axons. Neuron. 2005 Sep 1;47(5):681-94. PubMed.
  4. . APP binds DR6 to trigger axon pruning and neuron death via distinct caspases. Nature. 2009 Feb 19;457(7232):981-9. PubMed.
  5. . Bace1 modulates myelination in the central and peripheral nervous system. Nat Neurosci. 2006 Dec;9(12):1520-5. Epub 2006 Nov 12 PubMed.
  6. . Mechanisms of Disease: what factors limit the success of peripheral nerve regeneration in humans?. Nat Clin Pract Neurol. 2006 Aug;2(8):448-54. PubMed.
  7. . TREM2-transduced myeloid precursors mediate nervous tissue debris clearance and facilitate recovery in an animal model of multiple sclerosis. PLoS Med. 2007 Apr;4(4):e124. PubMed.
  8. . Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system. J Neurosci. 2011 Apr 13;31(15):5744-54. PubMed.

External Citations

  1. Brain Science Institute
  2. Baltimore Sun

Further Reading

Papers

  1. . Genetic deletion of BACE1 in mice affects remyelination of sciatic nerves. FASEB J. 2008 Aug;22(8):2970-80. PubMed.
  2. . Long-distance axon regeneration in the mature optic nerve: contributions of oncomodulin, cAMP, and pten gene deletion. J Neurosci. 2010 Nov 17;30(46):15654-63. PubMed.
  3. . PTEN deletion enhances the regenerative ability of adult corticospinal neurons. Nat Neurosci. 2010 Sep;13(9):1075-81. PubMed.
  4. . Large-scale in vivo femtosecond laser neurosurgery screen reveals small-molecule enhancer of regeneration. Proc Natl Acad Sci U S A. 2010 Oct 26;107(43):18342-7. PubMed.
  5. . Amyloid precursor protein cleavage-dependent and -independent axonal degeneration programs share a common nicotinamide mononucleotide adenylyltransferase 1-sensitive pathway. J Neurosci. 2010 Oct 13;30(41):13729-38. PubMed.

Primary Papers

  1. . Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system. J Neurosci. 2011 Apr 13;31(15):5744-54. PubMed.