- Email: [email protected]
Enhanced Actin Dynamics: A Therapeutic Strategy for Axonal Regeneration?
Neuron
Previews Enhanced Actin Dynamics: A Therapeutic Strategy for Axonal Regeneration? Hauke B. Werner1 and Klaus-Armin Nave1,* 1Department of Neurogenetics, Max Planck Institute of Experimental Medicine, 37075 Go ¨ ttingen, Germany *Correspondence: [email protected] https://doi.org/10.1016/j.neuron.2019.09.008
Spinal cord injury causes permanent paralysis due to the inability of neurons in the central nervous system to regenerate transected axons. In this issue of Neuron, Tedeschi et al. (2019) report that axonal regrowth can be stimulated by actin-depolymerizing proteins, at least in mice. The mature mammalian central nervous system (CNS) constitutes an inhibitory milieu for the regeneration of injured neurons. Indeed, the regrowth of axons after a nerve transection in adults requires overcoming both inhibitory myelin proteins and a scar mainly formed by extracellular matrix secreted from astrocytes (Courtine and Sofroniew, 2019). The quest for therapeutic strategies toward axonal regeneration in patients suffering from acute or chronic spinal cord injury thus includes counteracting these glia-derived biochemical and physical barriers. Alternative strategies aim to enhance the naturally insufficient intrinsic capacity of axons to regrow. Cytoskeletal proteins have previously received attention as possible therapeutic targets, as exemplified by the observation that inhibitors of microtubule destabilization improve axonal regeneration in both optic nerve (Sengottuvel et al., 2011) and spinal cord of rodent models (Hellal et al., 2011; Ruschel et al., 2015). However, the mechanism of action remained undefined considering that the therapeutic effect may have been secondary to stabilizing microtubules in non-neuronal cells beside the injured axons themselves. During the developmental growth of neuronal processes, microtubules provide a stabilizing cytoskeletal backbone along the entire length of the neurite, i.e., the future axon. Conversely, the vibrant extension and retraction of filopodia and lamellipodia at the leading edge of the growth cone that results in net axonal growth is facilitated by the dynamic polymerization and depolymerization of actin filaments (Flynn et al., 2012; Lowery and Van Vactor, 2009). It is the key concept of the current paper from the Bradke lab (Tedeschi et al., 2019) that experimentally
restoring actin dynamics in mature neurons is beneficial to the regrowth of axons after nerve transection. As a model, Tedeschi et al. (2019) use an established paradigm termed ‘‘conditioning lesion’’ (Richardson and Issa, 1984). In this model, one of the (peripheral) sciatic nerves is cut, thereby transecting the peripheral axonal branches of dorsal root ganglion (DRG) neurons. One week later, this is followed by the transection of the central axonal branches of the DRG neurons on both the previously severed and the intact contralateral side of the body, the latter serving as a control. Remarkably, the central axonal branches of DRG neurons are up to 100-fold more likely to regrow if their peripheral branches had been transected before (Richardson and Issa, 1984). This led to the concept that neurons can be induced to regenerate their axons by enhancing their intrinsic growth capacity, which overcomes, at least in part, the extrinsic inhibitory tissue environment (Yang and Yang, 2012). To explore actin turnover as a therapeutic strategy toward axonal regrowth after a conditioning lesion, Tedeschi et al. (2019) focus on a group of actinassociated proteins, the ‘‘actin depolymerizing factor’’ (ADF)/Cofilin family, which binds filamentous actin (F-actin) and thereby induces both depolymerization and splitting (also referred to as severing) of actin filaments. The authors generated triple mutant mice lacking three functionally related ADF/Cofilin proteins, namely cofilin-1 (CFL1), cofilin-2 (CFL2), and destrin (DSTN), all of which are expressed in neurons (Zhang et al., 2014). Importantly, axonal regrowth upon a conditioning lesion was markedly reduced when ADF/Cofilin proteins were lacking (Tedeschi et al., 2019). This
indicated that either depolymerization or splitting of actin filaments is critical for axonal regrowth. Using mutant variants of CFL1, it is possible to distinguish between the functional relevance of actin filament depolymerization versus splitting. Interestingly, adeno-associated-virus (AAV)-mediated expression of wildtype CFL1 or a CFL1 variant that stimulates actin filament splitting (but not depolymerization) partly restored axonal regrowth in triple mutant DRG neurons. Conversely, a CFL1 variant that causes actin depolymerization (but not splitting) was ineffective. This comparison demonstrates that splitting of actin filaments rather than depolymerization supports axonal regrowth, at least in the conditioning lesion paradigm. Therapeutic concepts for spinal cord injury in humans cannot be based on prior conditioning lesions. It was therefore an important discovery that AAV-mediated expression of CFL1 in DRG neurons significantly enhanced axonal regrowth also after lesioning the non-conditioned spinal cord of wild-type mice (Tedeschi et al., 2019). So far, fewer than 5% of axons in the dorsal column that expressed the virus reached a length of over 1 mm beyond the lesion site. Thus, it will require more experiments to tell whether functional recovery can be achieved, for example, by combining CFL1-induced actin-mediated regrowth of axons with additional measures, such as microtubule stabilization and/or training/rehabilitation protocols. As healthy people, we tend to take all our sensory and motor functions for granted, but accidents do happen. As a result, worldwide more than 250,000 people experience acute spinal cord injuries per year, as estimated by the World
Neuron 103, September 25, 2019 ª 2019 Elsevier Inc. 949
Neuron
Previews Health Organization (Courtine and Sofroniew, 2019), and neurological recovery remains very limited. Indeed, no clinical trial so far has proven efficacy of a therapeutic repair strategy to improve functional recovery after spinal cord injury (Courtine and Sofroniew, 2019). Interventions modulating the cell biology of neurons and glial cells provide great hope toward developing novel therapy concepts. Transiently targeting proteins that modulate actin dynamics, e.g., by small-molecule drugs, RNA interference, or viral expression, may emerge as a promising strategy to induce actin-driven regrowth of axons after injury. REFERENCES Courtine, G., and Sofroniew, M.V. (2019). Spinal cord repair: advances in biology and technology. Nat. Med. 25, 898–908.
Flynn, K.C., Hellal, F., Neukirchen, D., Jacob, S., Tahirovic, S., Dupraz, S., Stern, S., Garvalov, B.K., Gurniak, C., Shaw, A.E., et al. (2012). ADF/cofilin-mediated actin retrograde flow directs neurite formation in the developing brain. Neuron 76, 1091–1107. Hellal, F., Hurtado, A., Ruschel, J., Flynn, K.C., Laskowski, C.J., Umlauf, M., Kapitein, L.C., Strikis, D., Lemmon, V., Bixby, J., et al. (2011). Microtubule stabilization reduces scarring and causes axon regeneration after spinal cord injury. Science 331, 928–931. Lowery, L.A., and Van Vactor, D. (2009). The trip of the tip: understanding the growth cone machinery. Nat. Rev. Mol. Cell Biol. 10, 332–343. Richardson, P.M., and Issa, V.M.K. (1984). Peripheral injury enhances central regeneration of primary sensory neurones. Nature 309, 791–793. Ruschel, J., Hellal, F., Flynn, K.C., Dupraz, S., Elliott, D.A., Tedeschi, A., Bates, M., Sliwinski, C., Brook, G., Dobrindt, K., et al. (2015). Axonal regeneration. Systemic administration of epothilone B
promotes axon regeneration after spinal cord injury. Science 348, 347–352. Sengottuvel, V., Leibinger, M., Pfreimer, M., Andreadaki, A., and Fischer, D. (2011). Taxol facilitates axon regeneration in the mature CNS. J. Neurosci. 31, 2688–2699. Tedeschi, A., Dupraz, S., Curcio, M., Laskowski, C.J., Schaffran, B., Flynn, K.C., Santos, T.E., Stern, S., Hilton, B.J., Larson, M.J.E., et al. (2019). ADF/Cofilin-mediated actin turnover promotes axon regeneration in the adult CNS. Neuron 103, this issue, 1073–1085. Yang, P., and Yang, Z. (2012). Enhancing intrinsic growth capacity promotes adult CNS regeneration. J. Neurol. Sci. 312, 1–6. Zhang, Y., Chen, K., Sloan, S.A., Bennett, M.L., Scholze, A.R., O’Keeffe, S., Phatnani, H.P., Guarnieri, P., Caneda, C., Ruderisch, N., et al. (2014). An RNA-sequencing transcriptome and splicing database of glia, neurons, and vascular cells of the cerebral cortex. J. Neurosci. 34, 11929–11947.
Simplicity, Flexibility, and Interpretability in a Model of Dendritic Protein Distributions Cian O’Donnell1,* 1Bristol Computational Neuroscience Unit, School of Computer Science, Electrical and Electronic Engineering, and Engineering Mathematics, University of Bristol, Bristol, UK *Correspondence: [email protected] https://doi.org/10.1016/j.neuron.2019.09.010
In this issue of Neuron, Fonkeu et al. (2019) present a mathematical model of mRNA and protein synthesis, degradation, diffusion, and trafficking in neuronal dendrites. The model can predict the spatial distribution and temporal dynamics of proteins along dendrites. The authors use the model to account for in situ imaging data of CaMKII⍺ mRNA and protein in hippocampal neurons.
The goal of computational modeling in biology is not to capture every last detail in whatever system we are studying, but to help us understand how the system works. As a result, a model should be judged by its practical usefulness toward answering a particular set of questions, not by whether it includes this or that protein or mechanism. Three desirable properties of models that affect their practical usefulness are simplicity, flexibility, and interpretability. Simplicity enables mathematical analysis and quick simulation on a
computer. Flexibility allows the model to capture a wide range of phenomena. Interpretability allows us to relate model parameters and variables to things measurable in the lab. Frustratingly, however, these properties often seem to trade off against each other; it is usually difficult to meet all three demands in one model. In this issue of Neuron, Fonkeu et al. (2019) present that rare thing: a mathematical model of mRNA and protein synthesis, diffusion, and trafficking in neuronal dendrites that is simple, flexible, and interpretable.
950 Neuron 103, September 25, 2019 Crown Copyright ª 2019 Published by Elsevier Inc.
Why was the model by Fonkeu et al. (2019) needed? Learning and memory in the brain is largely due to modifications of synapses distributed across each neuron’s dendritic tree, hundreds of microns from the cell’s nucleus in the soma. Because these dendritic and synaptic structures need proteins to operate and their makeup changes during synaptic plasticity, to understand learning and memory, we need to understand how protein distribution is controlled across the dendritic tree.
Previews Enhanced Actin Dynamics: A Therapeutic Strategy for Axonal Regeneration? Hauke B. Werner1 and Klaus-Armin Nave1,* 1Department of Neurogenetics, Max Planck Institute of Experimental Medicine, 37075 Go ¨ ttingen, Germany *Correspondence: [email protected] https://doi.org/10.1016/j.neuron.2019.09.008
Spinal cord injury causes permanent paralysis due to the inability of neurons in the central nervous system to regenerate transected axons. In this issue of Neuron, Tedeschi et al. (2019) report that axonal regrowth can be stimulated by actin-depolymerizing proteins, at least in mice. The mature mammalian central nervous system (CNS) constitutes an inhibitory milieu for the regeneration of injured neurons. Indeed, the regrowth of axons after a nerve transection in adults requires overcoming both inhibitory myelin proteins and a scar mainly formed by extracellular matrix secreted from astrocytes (Courtine and Sofroniew, 2019). The quest for therapeutic strategies toward axonal regeneration in patients suffering from acute or chronic spinal cord injury thus includes counteracting these glia-derived biochemical and physical barriers. Alternative strategies aim to enhance the naturally insufficient intrinsic capacity of axons to regrow. Cytoskeletal proteins have previously received attention as possible therapeutic targets, as exemplified by the observation that inhibitors of microtubule destabilization improve axonal regeneration in both optic nerve (Sengottuvel et al., 2011) and spinal cord of rodent models (Hellal et al., 2011; Ruschel et al., 2015). However, the mechanism of action remained undefined considering that the therapeutic effect may have been secondary to stabilizing microtubules in non-neuronal cells beside the injured axons themselves. During the developmental growth of neuronal processes, microtubules provide a stabilizing cytoskeletal backbone along the entire length of the neurite, i.e., the future axon. Conversely, the vibrant extension and retraction of filopodia and lamellipodia at the leading edge of the growth cone that results in net axonal growth is facilitated by the dynamic polymerization and depolymerization of actin filaments (Flynn et al., 2012; Lowery and Van Vactor, 2009). It is the key concept of the current paper from the Bradke lab (Tedeschi et al., 2019) that experimentally
restoring actin dynamics in mature neurons is beneficial to the regrowth of axons after nerve transection. As a model, Tedeschi et al. (2019) use an established paradigm termed ‘‘conditioning lesion’’ (Richardson and Issa, 1984). In this model, one of the (peripheral) sciatic nerves is cut, thereby transecting the peripheral axonal branches of dorsal root ganglion (DRG) neurons. One week later, this is followed by the transection of the central axonal branches of the DRG neurons on both the previously severed and the intact contralateral side of the body, the latter serving as a control. Remarkably, the central axonal branches of DRG neurons are up to 100-fold more likely to regrow if their peripheral branches had been transected before (Richardson and Issa, 1984). This led to the concept that neurons can be induced to regenerate their axons by enhancing their intrinsic growth capacity, which overcomes, at least in part, the extrinsic inhibitory tissue environment (Yang and Yang, 2012). To explore actin turnover as a therapeutic strategy toward axonal regrowth after a conditioning lesion, Tedeschi et al. (2019) focus on a group of actinassociated proteins, the ‘‘actin depolymerizing factor’’ (ADF)/Cofilin family, which binds filamentous actin (F-actin) and thereby induces both depolymerization and splitting (also referred to as severing) of actin filaments. The authors generated triple mutant mice lacking three functionally related ADF/Cofilin proteins, namely cofilin-1 (CFL1), cofilin-2 (CFL2), and destrin (DSTN), all of which are expressed in neurons (Zhang et al., 2014). Importantly, axonal regrowth upon a conditioning lesion was markedly reduced when ADF/Cofilin proteins were lacking (Tedeschi et al., 2019). This
indicated that either depolymerization or splitting of actin filaments is critical for axonal regrowth. Using mutant variants of CFL1, it is possible to distinguish between the functional relevance of actin filament depolymerization versus splitting. Interestingly, adeno-associated-virus (AAV)-mediated expression of wildtype CFL1 or a CFL1 variant that stimulates actin filament splitting (but not depolymerization) partly restored axonal regrowth in triple mutant DRG neurons. Conversely, a CFL1 variant that causes actin depolymerization (but not splitting) was ineffective. This comparison demonstrates that splitting of actin filaments rather than depolymerization supports axonal regrowth, at least in the conditioning lesion paradigm. Therapeutic concepts for spinal cord injury in humans cannot be based on prior conditioning lesions. It was therefore an important discovery that AAV-mediated expression of CFL1 in DRG neurons significantly enhanced axonal regrowth also after lesioning the non-conditioned spinal cord of wild-type mice (Tedeschi et al., 2019). So far, fewer than 5% of axons in the dorsal column that expressed the virus reached a length of over 1 mm beyond the lesion site. Thus, it will require more experiments to tell whether functional recovery can be achieved, for example, by combining CFL1-induced actin-mediated regrowth of axons with additional measures, such as microtubule stabilization and/or training/rehabilitation protocols. As healthy people, we tend to take all our sensory and motor functions for granted, but accidents do happen. As a result, worldwide more than 250,000 people experience acute spinal cord injuries per year, as estimated by the World
Neuron 103, September 25, 2019 ª 2019 Elsevier Inc. 949
Neuron
Previews Health Organization (Courtine and Sofroniew, 2019), and neurological recovery remains very limited. Indeed, no clinical trial so far has proven efficacy of a therapeutic repair strategy to improve functional recovery after spinal cord injury (Courtine and Sofroniew, 2019). Interventions modulating the cell biology of neurons and glial cells provide great hope toward developing novel therapy concepts. Transiently targeting proteins that modulate actin dynamics, e.g., by small-molecule drugs, RNA interference, or viral expression, may emerge as a promising strategy to induce actin-driven regrowth of axons after injury. REFERENCES Courtine, G., and Sofroniew, M.V. (2019). Spinal cord repair: advances in biology and technology. Nat. Med. 25, 898–908.
Flynn, K.C., Hellal, F., Neukirchen, D., Jacob, S., Tahirovic, S., Dupraz, S., Stern, S., Garvalov, B.K., Gurniak, C., Shaw, A.E., et al. (2012). ADF/cofilin-mediated actin retrograde flow directs neurite formation in the developing brain. Neuron 76, 1091–1107. Hellal, F., Hurtado, A., Ruschel, J., Flynn, K.C., Laskowski, C.J., Umlauf, M., Kapitein, L.C., Strikis, D., Lemmon, V., Bixby, J., et al. (2011). Microtubule stabilization reduces scarring and causes axon regeneration after spinal cord injury. Science 331, 928–931. Lowery, L.A., and Van Vactor, D. (2009). The trip of the tip: understanding the growth cone machinery. Nat. Rev. Mol. Cell Biol. 10, 332–343. Richardson, P.M., and Issa, V.M.K. (1984). Peripheral injury enhances central regeneration of primary sensory neurones. Nature 309, 791–793. Ruschel, J., Hellal, F., Flynn, K.C., Dupraz, S., Elliott, D.A., Tedeschi, A., Bates, M., Sliwinski, C., Brook, G., Dobrindt, K., et al. (2015). Axonal regeneration. Systemic administration of epothilone B
promotes axon regeneration after spinal cord injury. Science 348, 347–352. Sengottuvel, V., Leibinger, M., Pfreimer, M., Andreadaki, A., and Fischer, D. (2011). Taxol facilitates axon regeneration in the mature CNS. J. Neurosci. 31, 2688–2699. Tedeschi, A., Dupraz, S., Curcio, M., Laskowski, C.J., Schaffran, B., Flynn, K.C., Santos, T.E., Stern, S., Hilton, B.J., Larson, M.J.E., et al. (2019). ADF/Cofilin-mediated actin turnover promotes axon regeneration in the adult CNS. Neuron 103, this issue, 1073–1085. Yang, P., and Yang, Z. (2012). Enhancing intrinsic growth capacity promotes adult CNS regeneration. J. Neurol. Sci. 312, 1–6. Zhang, Y., Chen, K., Sloan, S.A., Bennett, M.L., Scholze, A.R., O’Keeffe, S., Phatnani, H.P., Guarnieri, P., Caneda, C., Ruderisch, N., et al. (2014). An RNA-sequencing transcriptome and splicing database of glia, neurons, and vascular cells of the cerebral cortex. J. Neurosci. 34, 11929–11947.
Simplicity, Flexibility, and Interpretability in a Model of Dendritic Protein Distributions Cian O’Donnell1,* 1Bristol Computational Neuroscience Unit, School of Computer Science, Electrical and Electronic Engineering, and Engineering Mathematics, University of Bristol, Bristol, UK *Correspondence: [email protected] https://doi.org/10.1016/j.neuron.2019.09.010
In this issue of Neuron, Fonkeu et al. (2019) present a mathematical model of mRNA and protein synthesis, degradation, diffusion, and trafficking in neuronal dendrites. The model can predict the spatial distribution and temporal dynamics of proteins along dendrites. The authors use the model to account for in situ imaging data of CaMKII⍺ mRNA and protein in hippocampal neurons.
The goal of computational modeling in biology is not to capture every last detail in whatever system we are studying, but to help us understand how the system works. As a result, a model should be judged by its practical usefulness toward answering a particular set of questions, not by whether it includes this or that protein or mechanism. Three desirable properties of models that affect their practical usefulness are simplicity, flexibility, and interpretability. Simplicity enables mathematical analysis and quick simulation on a
computer. Flexibility allows the model to capture a wide range of phenomena. Interpretability allows us to relate model parameters and variables to things measurable in the lab. Frustratingly, however, these properties often seem to trade off against each other; it is usually difficult to meet all three demands in one model. In this issue of Neuron, Fonkeu et al. (2019) present that rare thing: a mathematical model of mRNA and protein synthesis, diffusion, and trafficking in neuronal dendrites that is simple, flexible, and interpretable.
950 Neuron 103, September 25, 2019 Crown Copyright ª 2019 Published by Elsevier Inc.
Why was the model by Fonkeu et al. (2019) needed? Learning and memory in the brain is largely due to modifications of synapses distributed across each neuron’s dendritic tree, hundreds of microns from the cell’s nucleus in the soma. Because these dendritic and synaptic structures need proteins to operate and their makeup changes during synaptic plasticity, to understand learning and memory, we need to understand how protein distribution is controlled across the dendritic tree.