- Email: [email protected]
Novel Direct Conversion of Microglia to Neurons
Acknowledgments C.S. receives financial support from the European Commission (ITN EU-GliaPhD; ERA-NET NEURON, BrIE), Bundesministerium für Bildung und Forschung (CONNEXIN), and Deutsche Forschungsgemeinschaft (SPP1757, STE 552/5-2). 1
Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Sigmund-Freud-Strasse 25, 53105 Bonn, Germany *Correspondence: [email protected] (C. Steinhäuser).
https://doi.org/10.1016/j.molmed.2018.12.001 References
changes and proliferative stages resulting from de-differentiation. Recently, Matsuda et al. (Pioneer factor NeuroD1 rearranges transcriptional and epigenetic profiles to execute microglia-neuron conversion; Neuronin in press) demonstrated that expression of transcription factor NeuroD1 can convert mouse microglia to neurons, both in vitro and in vivo.
1. Araque, A. et al. (2014) Gliotransmitters travel in time and space. Neuron 81, 728–739
The generation of neurons from human induced pluripotent stem cells (hiPSCs) represents a breakthrough for transplan3. Tian, G.-F. et al. (2005) An astrocytic basis of epilepsy. Nat. Med. 11, 973–981 tation paradigms and for studying mech4. Gómez-Gonzalo, M. et al. (2010) An excitatory loop with anisms of neurodegenerative diseases; it astrocytes contributes to drive neurons to seizure threshalso provides a novel system for drug old. PLoS Biol. 8, e1000352 5. Santello, M. et al. (2011) TNFa controls glutamatergic development [1]. However, the reproggliotransmission in the hippocampal dentate gyrus. Neuramming process requires an intermediron 69, 988–1001 6. Habbas, S. et al. (2015) Neuroinflammatory TNFa ate proliferative stage and is very time impairs memory via astrocyte signaling. Cell 163, 1730– intensive and laborious. Moreover, the 1741 process of reprogramming to generate 7. Nikolic, L. et al. (2018) Blocking TNF a-driven astrocyte purinergic signaling restores normal synaptic activity dur- hiPSCs removes the epigenetic changes ing epileptogenesis. Glia Published online November 5, that reflect cell aging. Recent advances 2018. http://dx.doi.org/10.1002/glia.23519 8. Engel, J. (2001) Mesial temporal lobe epilepsy: what have in cell reprogramming have generated we learned? Neuroscientist 7, 340–352 methods for direct reprogramming (also 9. Balosso, S. et al. (2013) The dual role of TNF-a and its termed direct conversion or transdifferreceptors in seizures. Exp. Neurol. 247, 267–271 10. Bedner, P. et al. (2015) Astrocyte uncoupling as a entiation) of fibroblasts to neurons using cause of human temporal lobe epilepsy. Brain 138, defined transcription factors (TFs) and/or 1208–1222 microRNAs [2–5]. Direct reprogramming of glial cells to neurons may have therapeutic implications, allowing cell replace[57_TD$IF] Focus on Glial Cells in Health and ment in vivo [6]. While direct conversion Disease of astrocytes to neurons has been reported previously [7], using this Spotlight approach as a potential treatment for neurodegenerative disorders raises some concerns that these astrocytes may be gliotic, meaning reactive, hypertrophic, or dysfunctional. To provide another option, Matsuda et al. [8] Dorit Trudler1 and endeavor to convert microglia directly Stuart A. Lipton1,2,* to neurons, and their new approach is the first study to accomplish this, albeit in Direct cell reprogramming, the pro- mouse. 2. Seifert, G. et al. (2006) Astrocyte dysfunction in neurological disorders: a molecular perspective. Nat. Rev. Neurosci. 7, 194–206
that the TF NeuroD1 (ND1), which has been previously used to convert astrocytes to neurons [9], had the highest efficiency of converting microglia to neurons (Figure 1). The previous report of direct reprogramming of astrocytes used retroviral expression of ND1 under an astrocyte-specific promoter, whereas Matsuda et al. [8] expressed ND1 using lentiviral expression under a microgliaspecific promoter. Matsuda et al. [8] report a conversion efficiency of about 25–35%, which is considered a reasonably high rate for direct conversion. By immunostaining, the resulting cells were positive for the neuronal markers bIIItubulin- and Map2ab, representing wellestablished markers for neurons. By additional immunocytochemical criteria, most of the cells were excitatory (75%), but there was a large percentage of inhibitory neurons as well (25%). Further characterization of neuronal cells types will be important to determine how cell-type distribution will affect attempts at cell replacement therapy for various brain regions. In vitro, the authors also demonstrated that the cells were functional by calcium imaging and electrophysiological analysis. Global gene expression analysis showed that microglial-derived neurons displayed a similar gene expression profile to that of primary neurons by principal component analysis.
To understand how ND1 acts on the genome to convert microglia to neurons, Matsuda et al. [8] performed ChIP-seq analysis and demonstrated that ND1 occupied genes that are associated with neuronal development and differentiation. Next, using ChIP-seq and whole-genome bisulfite sequencing, the authors studied epigenetic changes, including DNA methylation and histone modification, and reported that ND1 preferentially bound unmethylated CpG-rich regions. Howcess by which a somatic cell is conever, whether this contributes to changes verted to another cell type, can Matsuda et al. [8] tested various TFs, in gene expression is not clear as there potentially circumvent epigenetic using an unbiased approach, and found was no observed difference between
Novel Direct Conversion of Microglia to Neurons
72
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
Microglia
Neuron NeuroD1
Healthy mouse brain Are they funcƟonal?
Diseased mouse brain Will it be possible to convert microglia to neurons in mouse models of neurodegeneraƟve diseases?
3D organoids Will microglia converted to neurons become funcƟonally integrated?
Human brain Can human microglia undergo direct conversion to funcƟonal neurons?
Figure 1. Direct Conversion of Microglia to Neurons. Forced expression of NeuroD1 in mouse microglia directly converted the cells to neurons, as evidenced by immunostaining for bIII-tubulin, Map2ab, and other neuronal markers, and downregulated microglial markers. Moreover, in vitro the directly converted neurons displayed action potentials and synaptic currents by patch-clamp recording. Questions for the future are outlined at the bottom of the figure.
upregulated and unchanged genes. Further analysis showed that ND1 accessed closed chromatin with bivalent modifications and induced the expression of neuronal development- and differentiationrelated genes. The observed histone modifications in the induced neurons were similar to those observed in primary neurons but less frequent, suggesting that the conversion time may need to be extended to reach complete epigenetic change to more closely simulate primary neurons. The authors also identified 20 TFs that bear a bivalent domain that contains both H3K4 and H3K27 methylations, two modifications that have opposing effects, positively or negatively regulating transcription, respectively [10]. Matsuda et al. [8] demonstrated that three of these TFs, namely, Bhlhe22, Prdm8, and Myt1l, are individually sufficient to
convert microglia to neurons. In addition, the authors identified several microglial TFs that are downregulated following ND1 expression and showed that ND1 suppressed the expression of two of these TFs, Mafb and Lyl1, which are known to regulate microglial cell identity. To show that direct conversion of microglia to neurons can also be achieved in vivo, Matsuda et al. [8] expressed ND1 in the adult mouse brain under the microglial-specific CD68 promoter using a lentiviral vector injected into the striatum. Two weeks post-transduction, 33-50% of the transduced cells displayed the neuronal markers bIII-tubulin and Map2ab. Four weeks post-transduction 75% of the transduced cells were positive for DARPP32, a marker for striatal projection neurons.
Cell therapy in general and neuronal replacement in particular are important avenues for future therapy for neurodegenerative diseases. Matsuda et al. [8] provide an efficient process to use existing microglia, which may also be increased in number due to recruitment, activation, and proliferation [11] during the neurodegenerative process, to replace neuronal populations. Further work needs to address the functionality of these converted neuronal cells in vivo and improve the conversion efficiency in mouse models of neurodegeneration. It will be important to compare the yield of microglia from healthy versus diseased mouse brains as well as potential functional differences of the converted cells under these disparate circumstances. Critically, it will be important to examine whether these microglialconverted neurons can become functional neurons and integrate into neural networks using calcium imaging and patch-clamp recording methods in 3D cerebral organoid models as well as in vivo models. Moreover, future research should aim to validate this work in a human context and investigate whether human microglia can also undergo direct conversion to functional neurons. Altogether, Matsuda et al. [8] demonstrate that direct conversion of microglia to neurons is feasible with a single transcription factor, thus contributing to the approach of direct conversion and cell replacement therapy. Along these lines, many exciting studies remain to be performed. 1
Neuroscience Translational Center and Departments of Molecular Medicine and Neuroscience, The Scripps Research Institute, La Jolla, CA 92037, USA 2 Department of Neurosciences, School of Medicine, University of California–San Diego, La Jolla, CA 92093, USA *Correspondence: [email protected] (S.A. Lipton). https://doi.org/10.1016/j.molmed.2018.12.005 References 1. Takahashi, K. et al. (2007) Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell 131, 861–872 2. Vierbuchen, T. et al. (2010) Direct conversion of fibroblasts to functional neurons by defined factors. Nature 463, 1035–1041
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
73
3. Ambasudhan, R. et al. (2011) Direct reprogramming of adult human fibroblasts to functional neurons under defined conditions. Cell Stem Cell 9, 113–118
by neurovascular dysfunction, accumulation of amyloid-b (Ab) in brain paren4. Pang, Z.P. et al. (2011) Induction of human neuronal cells chyma and vasculature, tau pathology, by defined transcription factors. Nature 476, 220–223 and neuronal loss [1]. Prions are mis5. Yoo, A.S. et al. (2011) MicroRNA-mediated conversion of folded proteins that have been implicated human fibroblasts to neurons. Nature 476, 228–231 6. Kim, J. et al. (2012) Direct lineage reprogramming to neural in several diseases, including transmissicells. Curr. Opin. Neurobiol. 22, 778–784 ble spongiform encephalopathies, 7. Heinrich, C. et al. (2010) Directing astroglia from the cerebral cortex into subtype specific functional neurons. PLoS Creutzfeldt-Jakob disease, and GerstBiol. 8, e1000373 mann-Sträussler-Scheinker syndrome, 8. Matsuda, T. et al. Pioneer factor NeuroD1 rearranges transcriptional and epigenetic profiles to execute micro- and recently in AD [2–4]. The cellular prion glia-neuron conversion. Neuron (in press) protein (PrPC[37_TD$IF]) is a small, cell-surface gly9. Guo, Z. et al. (2014) In vivo direct reprogramming of reactive coprotein that has several physiological glial cells into functional neurons after brain injury and in an functions in brain. For example, PrPC proAlzheimer’s disease model. Cell Stem Cell 14, 188–202 10. Bernstein, B.E. et al. (2006) A bivalent chromatin structure tects against neuronal stress (e.g., apomarks key developmental genes in embryonic stem cells. ptosis, oxidative stress, and endoplasmic Cell 125, 315–326 11. Perry, V.H. and Holmes, C. (2014) Microglial priming in reticulum stress), maintains myelin, modneurodegenerative disease. Nat. Rev. Neurol. 10, 217–224 ulates neuronal excitability, and can induce neurite outgrowth and extension [5]. Importantly, PrPC is one of the cellular receptors for the oligomeric form of Ab Spotlight (Abo) [2,3]. Previous studies have shown that PrPC and Abo interaction on neurons leads to neuronal dysfunction, suppression of synaptic plasticity, and cognitive impairment [6]. It has also been shown that blocking PrPC–Abo interactions with peripherally administered antibodies improves learning and memory in mouse models of amyloidosis [2,7]. Here, we Abhay P. Sagare,1,2 Melanie highlight a recent study by Gunther D. Sweeney,1,2 Amy et al. demonstrating that an orally adminR. Nelson,1,2 Zhen Zhao,1 and istered PrPC antagonist rescues synaptic 1, and behavioral deficits in a APPswe/ Berislav V. Zlokovic * PS1DE9 mouse model of Ab amyloidosis Recent studies revealed that cellu- [8], suggesting its potential therapeutic lar prion protein on neurons bind implications for AD.
Prion Protein Antagonists Rescue Alzheimer’s Amyloidb-Related Cognitive Deficits
Alzheimer’s amyloid-b oligomers, causing neurotoxic effects. A new article in Cell Reports by Gunther and colleagues (Cell Rep. 2019; 26:145–158) shows that an orally administered cellular prion protein antagonist can rescue synaptic and cognitive deficits in Alzheimer’s mice overexpressing amyloid-b.
In the search for a novel PrPC antagonist, Gunther et al. screened 2560 known drugs and 10 130 diverse small molecules for inhibition of the PrPC interaction with Ab1–42 oligomers [8]. The screen initially identified a promising cephalosporin antibiotic, cefixime, but further validation suggested that the inhibitory action may have resulted from a cephalosporin degradation product. The authors then screened cephalosporin degradation Alzheimer’s disease (AD) is the most products and discovered conditions common form of dementia, characterized under which ceftazidime degradation 74
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
resulted in an active antagonist termed compound Z [8]. Compound Z not only disrupted the interaction between PrPC and Ab1–42 oligomers but also mitigated behavioral deficits in APPswe/PS1DE9 mice when administrated centrally into the brain [8]. While compound Z was effective in reducing cognitive dysfunction, it did not cross the blood–brain barrier (BBB), which limits its therapeutic potential. To overcome the BBB hurdle, the authors employed a directed approach for inhibitor development with insight from the chemical nature of compound Z and sought to identify molecules with greater potential to cross the BBB. A range of negatively charged polymers were identified with specific PrPC affinity in the low to sub-nanomolar range, from both biological (melanin) and synthetic Poly (4-styrenesulfonic acid-co-maleic acid) (PSCMA) origins [8]. They identified that polystyrene sulfonate and PSCMA, like compound Z, function to provide potent inhibition of Abo binding and protection of dendritic spines from Abo-induced loss [8]. When delivered orally, PSCMA (20 kDa) can cross the BBB and functions to inhibit PrPC binding to Abo in vivo, as shown in 12-month old APPswe/PS1DE9 mice with established Ab plaque accumulation, synaptic loss, and learning and memory deficits. In these mice, PSCMA rescued learning and memory on the Morris water maze behavior test. It also repaired synaptic loss, as shown by increased expression of presynaptic antisynaptic vesicle glycoprotein 2A in the hippocampus, but did not alter Ab metabolism or gliosis [8]. A previous study showed that Aboinduced PrPC activation of metabotropic glutamate receptor-5 (mGluR5) coreceptors and downstream signaling through Fyn-kinase phosphorylation of N-methylD-aspartate receptor subunit NR2B and tau leads to synaptic dysfunction [3]. Whether PSCMA can alter function of
Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Sigmund-Freud-Strasse 25, 53105 Bonn, Germany *Correspondence: [email protected] (C. Steinhäuser).
https://doi.org/10.1016/j.molmed.2018.12.001 References
changes and proliferative stages resulting from de-differentiation. Recently, Matsuda et al. (Pioneer factor NeuroD1 rearranges transcriptional and epigenetic profiles to execute microglia-neuron conversion; Neuronin in press) demonstrated that expression of transcription factor NeuroD1 can convert mouse microglia to neurons, both in vitro and in vivo.
1. Araque, A. et al. (2014) Gliotransmitters travel in time and space. Neuron 81, 728–739
The generation of neurons from human induced pluripotent stem cells (hiPSCs) represents a breakthrough for transplan3. Tian, G.-F. et al. (2005) An astrocytic basis of epilepsy. Nat. Med. 11, 973–981 tation paradigms and for studying mech4. Gómez-Gonzalo, M. et al. (2010) An excitatory loop with anisms of neurodegenerative diseases; it astrocytes contributes to drive neurons to seizure threshalso provides a novel system for drug old. PLoS Biol. 8, e1000352 5. Santello, M. et al. (2011) TNFa controls glutamatergic development [1]. However, the reproggliotransmission in the hippocampal dentate gyrus. Neuramming process requires an intermediron 69, 988–1001 6. Habbas, S. et al. (2015) Neuroinflammatory TNFa ate proliferative stage and is very time impairs memory via astrocyte signaling. Cell 163, 1730– intensive and laborious. Moreover, the 1741 process of reprogramming to generate 7. Nikolic, L. et al. (2018) Blocking TNF a-driven astrocyte purinergic signaling restores normal synaptic activity dur- hiPSCs removes the epigenetic changes ing epileptogenesis. Glia Published online November 5, that reflect cell aging. Recent advances 2018. http://dx.doi.org/10.1002/glia.23519 8. Engel, J. (2001) Mesial temporal lobe epilepsy: what have in cell reprogramming have generated we learned? Neuroscientist 7, 340–352 methods for direct reprogramming (also 9. Balosso, S. et al. (2013) The dual role of TNF-a and its termed direct conversion or transdifferreceptors in seizures. Exp. Neurol. 247, 267–271 10. Bedner, P. et al. (2015) Astrocyte uncoupling as a entiation) of fibroblasts to neurons using cause of human temporal lobe epilepsy. Brain 138, defined transcription factors (TFs) and/or 1208–1222 microRNAs [2–5]. Direct reprogramming of glial cells to neurons may have therapeutic implications, allowing cell replace[57_TD$IF] Focus on Glial Cells in Health and ment in vivo [6]. While direct conversion Disease of astrocytes to neurons has been reported previously [7], using this Spotlight approach as a potential treatment for neurodegenerative disorders raises some concerns that these astrocytes may be gliotic, meaning reactive, hypertrophic, or dysfunctional. To provide another option, Matsuda et al. [8] Dorit Trudler1 and endeavor to convert microglia directly Stuart A. Lipton1,2,* to neurons, and their new approach is the first study to accomplish this, albeit in Direct cell reprogramming, the pro- mouse. 2. Seifert, G. et al. (2006) Astrocyte dysfunction in neurological disorders: a molecular perspective. Nat. Rev. Neurosci. 7, 194–206
that the TF NeuroD1 (ND1), which has been previously used to convert astrocytes to neurons [9], had the highest efficiency of converting microglia to neurons (Figure 1). The previous report of direct reprogramming of astrocytes used retroviral expression of ND1 under an astrocyte-specific promoter, whereas Matsuda et al. [8] expressed ND1 using lentiviral expression under a microgliaspecific promoter. Matsuda et al. [8] report a conversion efficiency of about 25–35%, which is considered a reasonably high rate for direct conversion. By immunostaining, the resulting cells were positive for the neuronal markers bIIItubulin- and Map2ab, representing wellestablished markers for neurons. By additional immunocytochemical criteria, most of the cells were excitatory (75%), but there was a large percentage of inhibitory neurons as well (25%). Further characterization of neuronal cells types will be important to determine how cell-type distribution will affect attempts at cell replacement therapy for various brain regions. In vitro, the authors also demonstrated that the cells were functional by calcium imaging and electrophysiological analysis. Global gene expression analysis showed that microglial-derived neurons displayed a similar gene expression profile to that of primary neurons by principal component analysis.
To understand how ND1 acts on the genome to convert microglia to neurons, Matsuda et al. [8] performed ChIP-seq analysis and demonstrated that ND1 occupied genes that are associated with neuronal development and differentiation. Next, using ChIP-seq and whole-genome bisulfite sequencing, the authors studied epigenetic changes, including DNA methylation and histone modification, and reported that ND1 preferentially bound unmethylated CpG-rich regions. Howcess by which a somatic cell is conever, whether this contributes to changes verted to another cell type, can Matsuda et al. [8] tested various TFs, in gene expression is not clear as there potentially circumvent epigenetic using an unbiased approach, and found was no observed difference between
Novel Direct Conversion of Microglia to Neurons
72
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
Microglia
Neuron NeuroD1
Healthy mouse brain Are they funcƟonal?
Diseased mouse brain Will it be possible to convert microglia to neurons in mouse models of neurodegeneraƟve diseases?
3D organoids Will microglia converted to neurons become funcƟonally integrated?
Human brain Can human microglia undergo direct conversion to funcƟonal neurons?
Figure 1. Direct Conversion of Microglia to Neurons. Forced expression of NeuroD1 in mouse microglia directly converted the cells to neurons, as evidenced by immunostaining for bIII-tubulin, Map2ab, and other neuronal markers, and downregulated microglial markers. Moreover, in vitro the directly converted neurons displayed action potentials and synaptic currents by patch-clamp recording. Questions for the future are outlined at the bottom of the figure.
upregulated and unchanged genes. Further analysis showed that ND1 accessed closed chromatin with bivalent modifications and induced the expression of neuronal development- and differentiationrelated genes. The observed histone modifications in the induced neurons were similar to those observed in primary neurons but less frequent, suggesting that the conversion time may need to be extended to reach complete epigenetic change to more closely simulate primary neurons. The authors also identified 20 TFs that bear a bivalent domain that contains both H3K4 and H3K27 methylations, two modifications that have opposing effects, positively or negatively regulating transcription, respectively [10]. Matsuda et al. [8] demonstrated that three of these TFs, namely, Bhlhe22, Prdm8, and Myt1l, are individually sufficient to
convert microglia to neurons. In addition, the authors identified several microglial TFs that are downregulated following ND1 expression and showed that ND1 suppressed the expression of two of these TFs, Mafb and Lyl1, which are known to regulate microglial cell identity. To show that direct conversion of microglia to neurons can also be achieved in vivo, Matsuda et al. [8] expressed ND1 in the adult mouse brain under the microglial-specific CD68 promoter using a lentiviral vector injected into the striatum. Two weeks post-transduction, 33-50% of the transduced cells displayed the neuronal markers bIII-tubulin and Map2ab. Four weeks post-transduction 75% of the transduced cells were positive for DARPP32, a marker for striatal projection neurons.
Cell therapy in general and neuronal replacement in particular are important avenues for future therapy for neurodegenerative diseases. Matsuda et al. [8] provide an efficient process to use existing microglia, which may also be increased in number due to recruitment, activation, and proliferation [11] during the neurodegenerative process, to replace neuronal populations. Further work needs to address the functionality of these converted neuronal cells in vivo and improve the conversion efficiency in mouse models of neurodegeneration. It will be important to compare the yield of microglia from healthy versus diseased mouse brains as well as potential functional differences of the converted cells under these disparate circumstances. Critically, it will be important to examine whether these microglialconverted neurons can become functional neurons and integrate into neural networks using calcium imaging and patch-clamp recording methods in 3D cerebral organoid models as well as in vivo models. Moreover, future research should aim to validate this work in a human context and investigate whether human microglia can also undergo direct conversion to functional neurons. Altogether, Matsuda et al. [8] demonstrate that direct conversion of microglia to neurons is feasible with a single transcription factor, thus contributing to the approach of direct conversion and cell replacement therapy. Along these lines, many exciting studies remain to be performed. 1
Neuroscience Translational Center and Departments of Molecular Medicine and Neuroscience, The Scripps Research Institute, La Jolla, CA 92037, USA 2 Department of Neurosciences, School of Medicine, University of California–San Diego, La Jolla, CA 92093, USA *Correspondence: [email protected] (S.A. Lipton). https://doi.org/10.1016/j.molmed.2018.12.005 References 1. Takahashi, K. et al. (2007) Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell 131, 861–872 2. Vierbuchen, T. et al. (2010) Direct conversion of fibroblasts to functional neurons by defined factors. Nature 463, 1035–1041
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
73
3. Ambasudhan, R. et al. (2011) Direct reprogramming of adult human fibroblasts to functional neurons under defined conditions. Cell Stem Cell 9, 113–118
by neurovascular dysfunction, accumulation of amyloid-b (Ab) in brain paren4. Pang, Z.P. et al. (2011) Induction of human neuronal cells chyma and vasculature, tau pathology, by defined transcription factors. Nature 476, 220–223 and neuronal loss [1]. Prions are mis5. Yoo, A.S. et al. (2011) MicroRNA-mediated conversion of folded proteins that have been implicated human fibroblasts to neurons. Nature 476, 228–231 6. Kim, J. et al. (2012) Direct lineage reprogramming to neural in several diseases, including transmissicells. Curr. Opin. Neurobiol. 22, 778–784 ble spongiform encephalopathies, 7. Heinrich, C. et al. (2010) Directing astroglia from the cerebral cortex into subtype specific functional neurons. PLoS Creutzfeldt-Jakob disease, and GerstBiol. 8, e1000373 mann-Sträussler-Scheinker syndrome, 8. Matsuda, T. et al. Pioneer factor NeuroD1 rearranges transcriptional and epigenetic profiles to execute micro- and recently in AD [2–4]. The cellular prion glia-neuron conversion. Neuron (in press) protein (PrPC[37_TD$IF]) is a small, cell-surface gly9. Guo, Z. et al. (2014) In vivo direct reprogramming of reactive coprotein that has several physiological glial cells into functional neurons after brain injury and in an functions in brain. For example, PrPC proAlzheimer’s disease model. Cell Stem Cell 14, 188–202 10. Bernstein, B.E. et al. (2006) A bivalent chromatin structure tects against neuronal stress (e.g., apomarks key developmental genes in embryonic stem cells. ptosis, oxidative stress, and endoplasmic Cell 125, 315–326 11. Perry, V.H. and Holmes, C. (2014) Microglial priming in reticulum stress), maintains myelin, modneurodegenerative disease. Nat. Rev. Neurol. 10, 217–224 ulates neuronal excitability, and can induce neurite outgrowth and extension [5]. Importantly, PrPC is one of the cellular receptors for the oligomeric form of Ab Spotlight (Abo) [2,3]. Previous studies have shown that PrPC and Abo interaction on neurons leads to neuronal dysfunction, suppression of synaptic plasticity, and cognitive impairment [6]. It has also been shown that blocking PrPC–Abo interactions with peripherally administered antibodies improves learning and memory in mouse models of amyloidosis [2,7]. Here, we Abhay P. Sagare,1,2 Melanie highlight a recent study by Gunther D. Sweeney,1,2 Amy et al. demonstrating that an orally adminR. Nelson,1,2 Zhen Zhao,1 and istered PrPC antagonist rescues synaptic 1, and behavioral deficits in a APPswe/ Berislav V. Zlokovic * PS1DE9 mouse model of Ab amyloidosis Recent studies revealed that cellu- [8], suggesting its potential therapeutic lar prion protein on neurons bind implications for AD.
Prion Protein Antagonists Rescue Alzheimer’s Amyloidb-Related Cognitive Deficits
Alzheimer’s amyloid-b oligomers, causing neurotoxic effects. A new article in Cell Reports by Gunther and colleagues (Cell Rep. 2019; 26:145–158) shows that an orally administered cellular prion protein antagonist can rescue synaptic and cognitive deficits in Alzheimer’s mice overexpressing amyloid-b.
In the search for a novel PrPC antagonist, Gunther et al. screened 2560 known drugs and 10 130 diverse small molecules for inhibition of the PrPC interaction with Ab1–42 oligomers [8]. The screen initially identified a promising cephalosporin antibiotic, cefixime, but further validation suggested that the inhibitory action may have resulted from a cephalosporin degradation product. The authors then screened cephalosporin degradation Alzheimer’s disease (AD) is the most products and discovered conditions common form of dementia, characterized under which ceftazidime degradation 74
Trends in Molecular Medicine, February 2019, Vol. 25, No. 2
resulted in an active antagonist termed compound Z [8]. Compound Z not only disrupted the interaction between PrPC and Ab1–42 oligomers but also mitigated behavioral deficits in APPswe/PS1DE9 mice when administrated centrally into the brain [8]. While compound Z was effective in reducing cognitive dysfunction, it did not cross the blood–brain barrier (BBB), which limits its therapeutic potential. To overcome the BBB hurdle, the authors employed a directed approach for inhibitor development with insight from the chemical nature of compound Z and sought to identify molecules with greater potential to cross the BBB. A range of negatively charged polymers were identified with specific PrPC affinity in the low to sub-nanomolar range, from both biological (melanin) and synthetic Poly (4-styrenesulfonic acid-co-maleic acid) (PSCMA) origins [8]. They identified that polystyrene sulfonate and PSCMA, like compound Z, function to provide potent inhibition of Abo binding and protection of dendritic spines from Abo-induced loss [8]. When delivered orally, PSCMA (20 kDa) can cross the BBB and functions to inhibit PrPC binding to Abo in vivo, as shown in 12-month old APPswe/PS1DE9 mice with established Ab plaque accumulation, synaptic loss, and learning and memory deficits. In these mice, PSCMA rescued learning and memory on the Morris water maze behavior test. It also repaired synaptic loss, as shown by increased expression of presynaptic antisynaptic vesicle glycoprotein 2A in the hippocampus, but did not alter Ab metabolism or gliosis [8]. A previous study showed that Aboinduced PrPC activation of metabotropic glutamate receptor-5 (mGluR5) coreceptors and downstream signaling through Fyn-kinase phosphorylation of N-methylD-aspartate receptor subunit NR2B and tau leads to synaptic dysfunction [3]. Whether PSCMA can alter function of