Protease regulation: the Yin and Yang of neural development and disease

Abstract

The formation, maintenance, and plasticity of neural circuits rely upon a complex interplay between progressive and regressive events. Increasingly, new functions are being identified for axon guidance molecules in the dynamic processes that occur within the embryonic and adult nervous system. The magnitude, duration, and spatial activity of axon guidance molecule signaling are precisely regulated by a variety of molecular mechanisms. Here we focus on recent progress in understanding the role of protease-mediated cleavage of guidance factors required for directional axon growth, with a particular emphasis on the role of metalloprotease and γ-secretase. Since axon guidance molecules have also been linked to neural degeneration and regeneration in adults, studies of guidance receptor proteolysis are beginning to define new relationships between neurodevelopment and neurodegeneration. These findings raise the possibility that the signaling checkpoints controlled by proteases could be useful targets to enhance regeneration.

Figure 1. The Yin and Yang of neural development and disease

(A) The formation, maintenance and repair of neural circuits rely on the complex interplay of progressive and regressive events at multiple hierarchical levels from circuits, to cells, to molecules. As depicted in the Taiji diagram, these Yin and Yang events are interdependent, interconnected and transformable. (B) A number of signaling molecules required for axon guidance during development are also linked to neural degeneration and regeneration in adults. Studies of guidance molecule proteolysis are beginning to uncover exciting molecular connections in the pathways controlling neural wiring, degeneration and regeneration.

Figure 2. Extracellular cleavage of guidance molecules

(A) Metalloprotease-mediated termination of guidance signaling. Upon Netrin ligand binding, DCC is cleaved, generating an ectodomain segment and a membrane-tethered “stub”. The DCC stub is quickly eliminated, terminating Netrin attraction. Inhibition of metalloprotease (MP) activity increases DCC receptor levels, potentiating Netrin attraction. (B) Metalloprotease-mediated activation of guidance signaling. Slit binding to Robo results in cleavage of Robo by metalloprotease Kuz (ADAM10). Release of the ectodomain may cause a conformational change of the Robo stub that activates downstream signaling. In kuz mutants, Slit-mediated repulsion is disrupted. (C) Metalloprotease-mediated cell detachment. Following the formation of Ephrin-A5/EphA3 complexes, ADAM10 cleaves the Ephrin-A5 ligand, which breaks the cell-cell adhesion, allowing for growth cone retraction. Disrupting ADAM10 cleavage prolongs the adhesion and delays axon withdrawal. (D) Regulated proteolysis controls guidance signaling. Prior to crossing the floor plate, commissural neurons are unable to respond to Sema-3B, because Plexin-A1 protein is cleaved by Calpain1. As commissural axons enter the floor plate, NrCAM suppresses Calpain1 activity, allowing Plexin-A1/Neuropilin receptor complex accumulation in the growth cone and sensitization to Sema-3B.

Figure 3. Sequential cleavage of intermembrane proteins

(A) In the amyloidogenic pathway, APP is first cleaved by β-secretase (β-sec, BACE), leading to a secreted ectodomain, sAPPβ and a membrane bound fragment, C99. C99 is subsequently cleaved by γ-secretase (γ-sec), generating the APP intracellular domain (AICD) and Aβ, prone to aggregation. In the non-amyloidogenic pathway, α-secretase (α-sec, ADAMs) cleaves APP to generate the secreted sAPPα and membrane bound C83 fragment. Then C83 is cleaved by γ-secretase, yielding the P3 fragment and the AICD. (B) Upon ligand binding, Notch is cleaved by ADAM, releasing the extracellular domain, leaving a membrane-tethered “stub”. This stub is the substrate of γ-secretase, which releases the Notch intracellular domain (NICD) from the membrane, allowing it to translocate to the nucleus where it acts as a transcriptional regulator. (C–E) Sequential processing of DCC. (C) DCC is cleaved by metalloprotease that leads to “shedding” of the ectodomain segment, generating a membrane-tethered DCC stub. Following γ-secretase cleavage, DCC-ICD is released from the membrane. (D) Inhibition of metalloprotease cleavage increases the level of full-length DCC receptor on the membrane. (E) Blocking γ-secretase activity leads to the accumulation of DCC stubs, which form a heterogeneous receptor complex with full-length DCC and are able to signal Netrin attraction. (F) Synaptic activity induces sequential processing of EphA4 by metalloprotease and γ-secretase. The protease-generated EphA4 intracellular domain (EICD) can enhance the formation of dendritic spines by activating the Rac signaling pathway. (G) EphB stimulates a metalloprotease cleavage of Ephrin-B2, producing a carboxy-terminal fragment that is further processed by γ-secretase to produce Ephrin-B2/CTF2. This fragment can activate Src-family kinases, contributing to reverse signaling. Likewise, EphB2 receptor is also sequentially processed by metalloprotease and γ-secretase. (H) Following cleavage of Robo by Kuz, Robo “stub” is eliminated by γ-secretase in cancer cells. Nevertheless, it is unclear whether γ-secretase cleavage of Robo also occurs in neurons, because Robo stubs have never been detected in PS1 mutants (Bai and Pfaff, unpublished observations).