Darcie L. Moore

KLF Family Members Regulate Intrinsic Axon Regeneration Ability

Containing Neuronal Exuberance In rats and mice, around the time of birth, neurons of the central nervous system switch from a growth mode and lose their ability to regenerate. Studying retinal ganglion cells of the rat, Moore et al. (p. 298; see the Perspective by Subang and Richardson) identified a gene, Krüppel-like factor-4 (KLF4), that seems to contribute to the switch. The KLF4 gene belongs to a family of related transcription factors that possess repressive or enhancing effects on axon growth. The combinatorial effect of this family of transcription factors before and after birth may fine-tune the ability of the neurons to extend axons. The regenerative capacity of mouse retinal ganglion cells after injury is regulated by the KLF family of transcription factors. Neurons in the central nervous system (CNS) lose their ability to regenerate early in development, but the underlying mechanisms are unknown. By screening genes developmentally regulated in retinal ganglion cells (RGCs), we identified Krüppel-like factor–4 (KLF4) as a transcriptional repressor of axon growth in RGCs and other CNS neurons. RGCs lacking KLF4 showed increased axon growth both in vitro and after optic nerve injury in vivo. Related KLF family members suppressed or enhanced axon growth to differing extents, and several growth-suppressive KLFs were up-regulated postnatally, whereas growth-enhancing KLFs were down-regulated. Thus, coordinated activities of different KLFs regulate the regenerative capacity of CNS neurons.

Multiple transcription factor families regulate axon growth and regeneration.

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP‐responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c‐Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ‐light chain enhancer of activated B cells (NFκB), and Krüppel‐like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

High Content Screening of Cortical Neurons Identifies Novel Regulators of Axon Growth

Neurons in the central nervous system lose their intrinsic capacity for axon regeneration as they mature, and it is widely hypothesized that changes in gene expression are responsible. Testing this hypothesis and identifying the relevant genes has been challenging because hundreds to thousands of genes are developmentally regulated in CNS neurons, but only a small subset are likely relevant to axon growth. Here we used automated high content analysis (HCA) methods to functionally test 743 plasmids encoding developmentally regulated genes in neurite outgrowth assays using postnatal cortical neurons. We identified both growth inhibitors (Ephexin, Aldolase A, Solute Carrier 2A3, and Chimerin), and growth enhancers (Doublecortin, Doublecortin-like, Kruppel-like Factor 6, and CaM-Kinase II gamma), some of which regulate established growth mechanisms like microtubule dynamics and small GTPase signaling. Interestingly, with only one exception the growth-suppressing genes were developmentally upregulated, and the growth-enhancing genes downregulated. These data provide important support for the hypothesis that developmental changes in gene expression control neurite outgrowth, and identify potential new gene targets to promote neurite outgrowth.

Kruppel-Like Transcription Factors in the Nervous System: Novel players in neurite outgrowth and axon regeneration

The Krüppel-like family of transcription factors (KLFs) have been widely studied in proliferating cells, though very little is known about their role in post-mitotic cells, such as neurons. We have recently found that the KLFs play a role in regulating intrinsic axon growth ability in retinal ganglion cells (RGCs), a type of central nervous system (CNS) neuron. Previous KLF studies in other cell types suggest that there may be cell-type specific KLF expression patterns, and that their relative expression allows them to compete for binding sites, or to act redundantly to compensate for anothers function. With at least 15 of 17 KLF family members expressed in neurons, it will be important for us to determine how this complex family functions to regulate the intricate gene programs of axon growth and regeneration. By further characterizing the mechanisms of the KLF family in the nervous system, we may better understand how they regulate neurite growth and axon regeneration.

A mechanism for the segregation of age in mammalian neural stem cells

Youthful damage limitation in stem cells Every day brings more risk of damage to stem cells, which could have consequences for the whole organism. Moore et al. observed that dividing neural stem cells in rodents establish a diffusion barrier that restricts damaged proteins to one daughter cell, leaving the other with intact molecules. But with age this diffusion barrier weakens, so that replicating stem cells of older animals are less able to exclude damaged proteins than are the stem cells of younger rodents. Science, this issue p. 1334 Neural stem cells segregate age-related damage by establishing a diffusion barrier during cell division. Throughout life, neural stem cells (NSCs) generate neurons in the mammalian brain. Using photobleaching experiments, we found that during cell division in vitro and within the developing mouse forebrain, NSCs generate a lateral diffusion barrier in the membrane of the endoplasmic reticulum, thereby promoting asymmetric segregation of cellular components. The diffusion barrier weakens with age and in response to impairment of lamin-associated nuclear envelope constituents. Weakening of the diffusion barrier disrupts asymmetric segregation of damaged proteins, a product of aging. Damaged proteins are asymmetrically inherited by the nonstem daughter cell in embryonic and young adult NSC divisions, whereas in the older adult brain, damaged proteins are more symmetrically distributed between progeny. Thus, these data identify a mechanism of how damage that accumulates with age is asymmetrically distributed during somatic stem cell division.

Four steps to optic nerve regeneration.

The failure of the optic nerve to regenerate after injury or in neurodegenerative disease remains a major clinical and scientific problem. Retinal ganglion cell (RGC) axons course through the optic nerve and carry all the visual information to the brain, but after injury, they fail to regrow through the optic nerve and RGC cell bodies typically die, leading to permanent loss of vision. There are at least 4 hurdles to overcome in preserving RGCs and regenerating their axons: 1) increase RGC survival, 2) overcome the inhibitory environment of the optic nerve, 3) enhance RGC intrinsic axon growth potential, and 4) optimize the mapping of RGC connections back into their targets in the brain.

A Fatty Acid Oxidation-Dependent Metabolic Shift Regulates Adult Neural Stem Cell Activity

Summary Hippocampal neurogenesis is important for certain forms of cognition, and failing neurogenesis has been implicated in neuropsychiatric diseases. The neurogenic capacity of hippocampal neural stem/progenitor cells (NSPCs) depends on a balance between quiescent and proliferative states. Here, we show that the rate of fatty acid oxidation (FAO) regulates the activity of NSPCs. Quiescent NSPCs show high levels of carnitine palmitoyltransferase 1a (Cpt1a)-dependent FAO, which is downregulated in proliferating NSPCs. Pharmacological inhibition and conditional deletion of Cpt1a in vitro and in vivo leads to altered NSPC behavior, showing that Cpt1a-dependent FAO is required for stem cell maintenance and proper neurogenesis. Strikingly, manipulation of malonyl-CoA, the metabolite that regulates levels of FAO, is sufficient to induce exit from quiescence and to enhance NSPC proliferation. Thus, the data presented here identify a shift in FAO metabolism that governs NSPC behavior and suggest an instructive role for fatty acid metabolism in regulating NSPC activity.

KLF9 and JNK3 Interact to Suppress Axon Regeneration in the Adult CNS

Neurons in the adult mammalian CNS decrease in intrinsic axon growth capacity during development in concert with changes in Krüppel-like transcription factors (KLFs). KLFs regulate axon growth in CNS neurons including retinal ganglion cells (RGCs). Here, we found that knock-down of KLF9, an axon growth suppressor that is normally upregulated 250-fold in RGC development, promotes long-distance optic nerve regeneration in adult rats of both sexes. We identified a novel binding partner, MAPK10/JNK3 kinase, and found that JNK3 (c-Jun N-terminal kinase 3) is critical for KLF9s axon-growth-suppressive activity. Interfering with a JNK3-binding domain or mutating two newly discovered serine phosphorylation acceptor sites, Ser106 and Ser110, effectively abolished KLF9s neurite growth suppression in vitro and promoted axon regeneration in vivo. These findings demonstrate a novel, physiologic role for the interaction of KLF9 and JNK3 in regenerative failure in the optic nerve and suggest new therapeutic strategies to promote axon regeneration in the adult CNS. SIGNIFICANCE STATEMENT Injured CNS nerves fail to regenerate spontaneously. Promoting intrinsic axon growth capacity has been a major challenge in the field. Here, we demonstrate that knocking down Krüppel-like transcription factor 9 (KLF9) via shRNA promotes long-distance axon regeneration after optic nerve injury and uncover a novel and important KLF9–JNK3 interaction that contributes to axon growth suppression in vitro and regenerative failure in vivo. These studies suggest potential therapeutic approaches to promote axon regeneration in injury and other degenerative diseases in the adult CNS.

Erratum: Role of Mitochondrial Metabolism in the Control of Early Lineage Progression and Aging Phenotypes in Adult Hippocampal Neurogenesis (Neuron (2017) 93(3) (560–573.e6) (S089662731630959X), (10.1016/j.neuron.2016.12.017))

Ruth Beckervordersandforth,* Birgit Ebert, Iris Sch€ affner, Jonathan Moss, Christian Fiebig, Jaehoon Shin, Darcie L. Moore, Laboni Ghosh, Mariela F. Trinchero, Carola Stockburger, Kristina Friedland, Kathrin Steib, Julia von Wittgenstein, Silke Keiner, Christoph Redecker, Sabine M. Hölter, Wei Xiang, Wolfgang Wurst, Ravi Jagasia, Alejandro F. Schinder, Guo-li Ming, Nicolas Toni, Sebastian Jessberger, Hongjun Song, and D. Chichung Lie* *Correspondence: ruth.beckervordersandforth@fau.de (R.B.), chi.lie@fau.de (D.C.L.) http://dx.doi.org/10.1016/j.neuron.2017.03.008

All astrocytes are not created equal—the role of astroglia in brain injury

In two recent papers published in Nature Neuroscience and Cell Stem Cells, Magdalena Gotz and colleagues shed new light on the in vivo response of glial cells to brain injury and characterize a highly heterogeneous behavior of astrocytes to chronic and acute brain injury.