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An Integrin Approach to Axon Regeneration

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An Integrin Approach to Axon Regeneration Eye (2017) 31, 206–208 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved 0950-222X/17 www.nature.com/eye CAMBRIDGE OPHTHALMOLOGICAL SYMPOSIUM An integrin approach JW Fawcett to axon regeneration Abstract endosomes. Regenerating axons must penetrate the glial extracellular matrix (ECM), and the key Axon regeneration in the CNS is blocked by receptors for ECM molecules are integrins. inhibitory molecules in the environment and Within the adult CNS the main ECM by a developmental loss of regenerative potential in CNS axons. Axon growth is a glycoprotein is Tenascin-C (TN-C), which is specialized form of cell migration, and for upregulated at sites of injury in the brain, spinal any cell to migrate there must be an adhesion cord, and optic nerve. The integrin that allows α β molecule at the growth tip that recognizes a cells to migrate on TN-C is alpha9 beta1 ( 9 1), ligand in the environment, and which is and this integrin is expressed in the CNS during linked to signaling and cytoskeletal embryonic development but then mechanisms. The reasons for this loss of downregulated in adulthood and not regenerative ability in CNS axons are several, upregulated after injury. When we started to fi fi but important contributors are the investigate this eld, our rst question was α β developmental loss of integrins that recognize whether expression of 9 1-integrin in adult ligands in the mature CNS environment, and neurons would promote axon regeneration. in vitro selective trafficking of integrins and other We found that neurons transfected grew molecules to exclude them from axons and axons for long distances on TN-C, but when the in vivo direct them to dendrites. Regeneration of same idea was tried there was much less sensory axons in the spinal cord can be regeneration. An adeno-associated virus (AAV) achieved by expression of tenascin-binding vector was used to transduce sensory neurons α β α9-integrin together with the integrin with 9 1 and either the spinal cord or the dorsal activator kindlin-1. This works because roots were cut, in both cases seeing only a integrins are transported into sensory axons. modest increase in regeneration. The reason for in vitro in vivo Transport of integrins into retinal ganglion the difference between and cell axons is seen in the retina, but may regeneration turned out to be because the major become more restricted in the optic nerve, inhibitory molecules of the adult CNS (not present in the in vitro model) all inactivate John van Geest Centre for with a subset of axons containing expressed Brain Repair, Department of integrins. Transduction of ganglion cells with integrins, so that they can no longer bind to Clinical Neurosciences, α9-integrin and kindlin-1 should promote TN-C and signal. Integrin activation is regulated University of Cambridge, regeneration of this subset of axons, but by talin and kindlin, which bind to the Cambrige, UK attention to transport may be required for intracellular tail of β1-integrin. The next step was regeneration of the remaining axons. therefore to overcome the inactivation of Correspondence: Eye – integrins by chondroitin sulphate proteoglycans JW Fawcett, John van (2017) 31, 206 208; doi:10.1038/eye.2016.293; Geest Centre for Brain published online 23 December 2016 (CSPGs) and NogoA by transducing neurons Repair, Department of with kindlin-1 to prevent integrin inactivation.1 Clinical Neurosciences, This, together with transduction with α9β1 University of Cambridge, allowed the axons to overcome inhibition by Robinson Way, Cambrige Axon growth is a specialized form of cell both NogoA and CSPGs, and to regenerate for CB2 0PY, UK migration, but all forms of cell migration must 2 Tel: +44 1223331160; long distances in the spinal cord. The C6 and Fax: +44 1223331174. obey the same biological rules. For any cell to C7 dorsal root ganglia were transduced with E-mail: [email protected] migrate, it must have on its surface a receptor α9β1 and kindlin-1 and the roots crushed. that can bind to a molecule in the surrounding Regenerating sensory axons were able to grow Received: 11 November 2016 environment, and this receptor must be linked all the way up the spinal cord to the medulla, Accepted in revised form: to intracellular signaling pathways and to the 22 November 2016 making connections in the correct layers of Published online: dynamic cytoskeleton. The cell surface receptors the dorsal horn along the way. This is an 23 December 2016 are cycled on and off the surface by trafficking unprecedented degree of regeneration, and the Integrins and axon regeneration JW Fawcett 207 regenerated connections were functional, with animals transport and this inhibits axon growth and regeneration. recovering pain and touch sensation and sensory-motor The situation in cortical neurons is different, because their function. axons contain integrins during their developmental If extensive regeneration of sensory axons in the CNS is growth phase, but as they mature integrins are achievable by transducing them with the correct integrin progressively excluded from the axons and at the same and activating it, would this same method work for time the axons lose the ability to regenerate. We analyzed retinal ganglion cells (RGCs) and corticospinal (CST) the factors that exclude integrins from cortical axons, and neurons? A requirement is that the neurons can transport as for many complex biological controls, there are several integrins and kindlin down their axons to the site of influences. Chief amongst these is the developmental axotomy in order that they can participate in the upregulation of the Arf6 GEF Efa6, which activates Arf6 regeneration and progression of the axon growth cone. and causes integrin transport to become predominantly For dorsal root ganglion (DRG) neurons this is not a retrograde, excluding integrins from the axons and problem, because almost all molecules expressed in the reducing levels at the axon tip. However, the axon initial cell bodies appear to be transported down axons. CNS segment and postsynthetic modifications of tubulin also neurons however are partitioned into a somatodendritic play a part.4 Knockdown of Efa6 in mature cortical domain and an axonal domain, and there is a selective neurons causes large changes, with the axon now transport mechanism to direct some molecules to becoming full of Rab11 vesicles that contain integrin. dendrites, others to axons. For instance very few of Importantly these axons now have an increased ability to the many molecules that make up postsynaptic regenerate when they are cut by a laser in vitro. specializations are transported into axons. Integrins, trks, From these findings it is now possible to construct integrin-like growth factor receptors are three key methods for stimulating axon regeneration in the CNS. receptor types that are excluded from the axons of cortical For sensory axons the situation is straightforward. neurons,3–5 while kindlin is transported for long Expression of α9β1-integrin and kindlin-1 through viral distances. RGCs have some ability to transport integrins: vector delivery will introduce these molecules into the when RGCs were transduced with α9β1-V5 many of their axons, and they are then able to regenerate in the spinal axons contained integrin within the retina, but transport cord. For cortical neurons the requirements are more into the optic nerve was more limited, although some demanding, because expression of α9β1 only introduces axons contained integrin. Moreover RGCs in adulthood in integrin into the somatodendritic compartment. It is now mice continue to express some integrins at a level greater known that preventing Arf6 activation will allow some than cortical neurons but less than DRG neurons. A study integrin transport, but it may be necessary to add a of integrins in adult RGCs in vitro revealed that several further intervention to ensure both transport and integrins are expressed, and that they are present in all the signaling sufficient to allow long-distance regeneration. cell processes.6 For RGC axons the situation is not so clear. RGCs are able From the above studies it is clear that transport and to transport integrins into their proximal axons, but only a trafficking of growth-promoting molecules is a key issue subset may be able to transport them down the optic in axon regeneration. We therefore worked out how nerve where they are needed for regeneration. The first integrins are transported into regenerating axons through need is for experiments to be performed to find out studying adult DRG neurons. In these cells integrins are whether expression of α9β1 and kindlin-1 will stimulate found particularly in recycling endosomes marked by the regeneration in the same way as in the sensory system. small GTPases Rab11 and Arf6.7 This implies that that If this is not successful it will be necessary to add a integrins enter axons by first being inserted into the cell treatment to increase long-distance anterograde transport membrane in the somatodendritic compartment, then of the integrin into the RGC axons, probably via they are recycled by transcytosis in recycling endosomes knockdown of Efa6. In addition increasing PIP3 by and transported down axons. The control of endosomal knockdown of PTEN or overexpression of PI3Kδ transport is complex, but it depends on the state of stimulates axon growth and also enhances integrin activation of Arf6 and Rab11, which are linked together transport. with the transport adaptor JIP3: when Arf6 is activated Overall, axon regeneration research has progressed JIP3 links to the retrograde motor dynein so that all the from the time when the mechanisms governing Arf6/Rab11 cargos travel retrogradely and are excluded regeneration were very mysterious to the present from axons.4,8 The state of activation of Arf6 is controlled situation, where there are several interventions that can particularly by the GEF Efa6, which together with Arf6 is promote regeneration in predictable and comprehensible the master regulator of transport direction.
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