Islet-to-LMO stoichiometries control the function of transcription complexes that specify motor neuron and V2a interneuron identity

Conditionally immortalized stem cell lines from human spinal cord retain regional identity and generate functional V2a interneurons and motorneurons

Introduction

The use of immortalized neural stem cells either as models of neural development in vitro or as cellular therapies in central nervous system (CNS) disorders has been controversial. This controversy has centered on the capacity of immortalized cells to retain characteristic features of the progenitor cells resident in the tissue of origin from which they were derived, and the potential for tumorogenicity as a result of immortalization. Here, we report the generation of conditionally immortalized neural stem cell lines from human fetal spinal cord tissue, which addresses these issues.

Methods

Clonal neural stem cell lines were derived from 10-week-old human fetal spinal cord and conditionally immortalized with an inducible form of cMyc. The derived lines were karyotyped, transcriptionally profiled by microarray, and assessed against a panel of spinal cord progenitor markers with immunocytochemistry. In addition, the lines were differentiated and assessed for the presence of neuronal fate markers and functional calcium channels. Finally, a clonal line expressing eGFP was grafted into lesioned rat spinal cord and assessed for survival, differentiation characteristics, and tumorogenicity.

Results

We demonstrate that these clonal lines (a) retain a clear transcriptional signature of ventral spinal cord progenitors and a normal karyotype after extensive propagation in vitro, (b) differentiate into relevant ventral neuronal subtypes with functional T-, L-, N-, and P/Q-type Ca²⁺ channels and spontaneous calcium oscillations, and (c) stably engraft into lesioned rat spinal cord without tumorogenicity.

Conclusions

We propose that these cells represent a useful tool both for the in vitro study of differentiation into ventral spinal cord neuronal subtypes, and for examining the potential of conditionally immortalized neural stem cells to facilitate functional recovery after spinal cord injury or disease.

Lmo4 establishes rostral motor cortex projection neuron subtype diversity

The mammalian neocortex is parcellated into anatomically and functionally distinct areas. The establishment of area-specific neuronal diversity and circuit connectivity enables distinct neocortical regions to control diverse and specialized functional outputs, yet underlying molecular controls remain largely unknown. Here, we identify a central role for the transcriptional regulator Lim-only 4 (Lmo4) in establishing the diversity of neuronal subtypes within rostral mouse motor cortex, where projection neurons have particularly diverse and multi-projection connectivity compared with caudal motor cortex. In rostral motor cortex, we report that both subcerebral projection neurons (SCPN), which send projections away from the cerebrum, and callosal projection neurons (CPN), which send projections to contralateral cortex, express Lmo4, whereas more caudal SCPN and CPN do not. Lmo4-expressing SCPN and CPN populations are comprised of multiple hodologically distinct subtypes. SCPN in rostral layer Va project largely to brainstem, whereas SCPN in layer Vb project largely to spinal cord, and a subset of both rostral SCPN and CPN sends second ipsilateral caudal (backward) projections in addition to primary projections. Without Lmo4 function, the molecular identity of neurons in rostral motor cortex is disrupted and more homogenous, rostral layer Va SCPN aberrantly project to the spinal cord, and many dual-projection SCPN and CPN fail to send a second backward projection. These molecular and hodological disruptions result in greater overall homogeneity of motor cortex output. Together, these results identify Lmo4 as a central developmental control over the diversity of motor cortex projection neuron subpopulations, establishing their area-specific identity and specialized connectivity.

The LIM-Homeodomain transcription factor Islet-1 is required for the development of sympathetic neurons and adrenal chromaffin cells

Islet-1 is a LIM-Homeodomain transcription factor with important functions for the development of distinct neuronal and non-neuronal cell populations. We show here that Islet-1 acts genetically downstream of Phox2B in cells of the sympathoadrenal cell lineage and that the development of sympathetic neurons and chromaffin cells is impaired in mouse embryos with a conditional deletion of Islet-1 controlled by the wnt1 promotor. Islet-1 is not essential for the initial differentiation of sympathoadrenal cells, as indicated by the correct expression of pan-neuronal and catecholaminergic subtype specific genes in primary sympathetic ganglia of Islet-1 deficient mouse embryos. However, our data indicate that the subsequent survival of sympathetic neuron precursors and their differentiation towards TrkA expressing neurons depends on Islet-1 function. In contrast to spinal sensory neurons, sympathetic neurons of Islet-1 deficient mice did not display ectopic expression of genes normally present in the CNS. In Islet-1 deficient mouse embryos the numbers of chromaffin cells were only mildly reduced, in contrast to that of sympathetic neurons, but the initiation of the adrenaline synthesizing enzyme PNMT was abrogated and the expression level of chromogranin A was diminished. Microarray analysis revealed that developing chromaffin cells of Islet-1 deficient mice displayed normal expression levels of TH, DBH and the transcription factors Phox2B, Mash-1, Hand2, Gata3 and Insm1, but the expression levels of the transcription factors Gata2 and Hand1, and AP-2β were significantly reduced. Together our data indicate that Islet-1 is not essentially required for the initial differentiation of sympathoadrenal cells, but has an important function for the correct subsequent development of sympathetic neurons and chromaffin cells.

Islet1 deletion causes kidney agenesis and hydroureter resembling CAKUT

Islet1 (Isl1) is a transcription factor transiently expressed in a subset of heart and limb progenitors. During studies of limb development, conditional Isl1 deletion produced unexpected kidney abnormalities. Here, we studied the renal expression of Isl1 and whether it has a role in kidney development. In situ hybridization revealed Isl1 expression in the mesenchymal cells surrounding the base of the ureteric bud in mice. Conditional deletion of Isl1 caused kidney agenesis or hypoplasia and hydroureter, a phenotype resembling human congenital anomalies of the kidney and urinary tract (CAKUT). The absence of Isl1 led to ectopic branching of the ureteric bud out from the nephric duct or to the formation of accessory buds, both of which could lead to obstruction of the ureter-bladder junction and consequent hydroureter. The abnormal elongation and poor branching of the ureteric buds were the likely causes of the kidney agenesis or hypoplasia. Furthermore, the lack of Isl1 reduced the expression of Bmp4, a gene implicated in the CAKUT-like phenotype, in the metanephric region before ureteric budding. In conclusion, Isl1 is essential for proper development of the kidney and ureter by repressing the aberrant formation of the ureteric bud. These observations call for further studies to investigate whether Isl1 may be a causative gene for human CAKUT.

LMO4 is an essential cofactor in the Snail2-mediated epithelial-to-mesenchymal transition of neuroblastoma and neural crest cells

Neuroblastoma is an embryonic tumor derived from cells of the neural crest. Taking advantage of a newly developed neural crest lineage tracer and based on the hypothesis that the molecular mechanisms that mediate neural crest delamination are also likely to be involved in the spread of neuroblastoma, we were able to identify genes that are active both in neural crest development and neuroblastoma tumor formation. A subsequent search of the neuroblastoma gene server for human orthologues of genes differentially expressed in the chick embryo neural crest screen retrieved the LIM domain only protein 4 (LMO4), which was expressed in both cell types analyzed. Functional experiments in these two model systems revealed that LMO4 activity is required for neuroblastoma cell invasion and neural crest delamination. Moreover, we identified LMO4 as an essential cofactor in Snail2-mediated cadherin repression and in the epithelial-to-mesenchymal transition of both neural crest and neuroblastoma cells. Together, our results suggest that the association of high levels of LMO4 with aggressive neuroblastomas is dependent on LMO4 regulation of cadherin expression and hence, tumor invasiveness.

Distinct cis regulatory elements govern the expression of TAG1 in embryonic sensory ganglia and spinal cord

Cell fate commitment of spinal progenitor neurons is initiated by long-range, midline-derived, morphogens that regulate an array of transcription factors that, in turn, act sequentially or in parallel to control neuronal differentiation. Included among these are transcription factors that regulate the expression of receptors for guidance cues, thereby determining axonal trajectories. The Ig/FNIII superfamily molecules TAG1/Axonin1/CNTN2 (TAG1) and Neurofascin (Nfasc) are co-expressed in numerous neuronal cell types in the CNS and PNS – for example motor, DRG and interneurons - both promote neurite outgrowth and both are required for the architecture and function of nodes of Ranvier. The genes encoding TAG1 and Nfasc are adjacent in the genome, an arrangement which is evolutionarily conserved. To study the transcriptional network that governs TAG1 and Nfasc expression in spinal motor and commissural neurons, we set out to identify cis elements that regulate their expression. Two evolutionarily conserved DNA modules, one located between the Nfasc and TAG1 genes and the second directly 5′ to the first exon and encompassing the first intron of TAG1, were identified that direct complementary expression to the CNS and PNS, respectively, of the embryonic hindbrain and spinal cord. Sequential deletions and point mutations of the CNS enhancer element revealed a 130bp element containing three conserved E-boxes required for motor neuron expression. In combination, these two elements appear to recapitulate a major part of the pattern of TAG1 expression in the embryonic nervous system.

Characterization of the Gbx1-/- mouse mutant: a requirement for Gbx1 in normal locomotion and sensorimotor circuit development

The Gbx class of homeobox genes encodes DNA binding transcription factors involved in regulation of embryonic central nervous system (CNS) development. Gbx1 is dynamically expressed within spinal neuron progenitor pools and becomes restricted to the dorsal mantle zone by embryonic day (E) 12.5. Here, we provide the first functional analysis of Gbx1. We generated mice containing a conditional Gbx1 allele in which exon 2 that contains the functional homeodomain is flanked with loxP sites (Gbx1(flox)); Cre-mediated recombination of this allele results in a Gbx1 null allele. In contrast to mice homozygous for a loss-of-function allele of Gbx2, mice homozygous for the Gbx1 null allele, Gbx1(-/-), are viable and reproductively competent. However, Gbx1(-/-) mice display a gross locomotive defect that specifically affects hindlimb gait. Analysis of embryos homozygous for the Gbx1 null allele reveals disrupted assembly of the proprioceptive sensorimotor circuit within the spinal cord, and a reduction in ISL1(+) ventral motor neurons. These data suggest a functional requirement for Gbx1 in normal development of the neural networks that contribute to locomotion. The generation of this null allele has enabled us to functionally characterize a novel role for Gbx1 in development of the spinal cord.

The LIM-homeodomain protein islet dictates motor neuron electrical properties by regulating K(+) channel expression

Neuron electrical properties are critical to function and generally subtype specific, as are patterns of axonal and dendritic projections. Specification of motoneuron morphology and axon pathfinding has been studied extensively, implicating the combinatorial action of Lim-homeodomain transcription factors. However, the specification of electrical properties is not understood. Here, we address the key issues of whether the same transcription factors that specify morphology also determine subtype specific electrical properties. We show that Drosophila motoneuron subtypes express different K⁺ currents and that these are regulated by the conserved Lim-homeodomain transcription factor Islet. Specifically, Islet is sufficient to repress a Shaker-mediated A-type K⁺ current, most likely due to a direct transcriptional effect. A reduction in Shaker increases the frequency of action potential firing. Our results demonstrate the deterministic role of Islet on the excitability patterns characteristic of motoneuron subtypes.

Onecut transcription factors act upstream of Isl1 to regulate spinal motoneuron diversification

During development, spinal motoneurons (MNs) diversify into a variety of subtypes that are specifically dedicated to the motor control of particular sets of skeletal muscles or visceral organs. MN diversification depends on the coordinated action of several transcriptional regulators including the LIM-HD factor Isl1, which is crucial for MN survival and fate determination. However, how these regulators cooperate to establish each MN subtype remains poorly understood. Here, using phenotypic analyses of single or compound mutant mouse embryos combined with gain-of-function experiments in chick embryonic spinal cord, we demonstrate that the transcriptional activators of the Onecut family critically regulate MN subtype diversification during spinal cord development. We provide evidence that Onecut factors directly stimulate Isl1 expression in specific MN subtypes and are therefore required to maintain Isl1 production at the time of MN diversification. In the absence of Onecut factors, we observed major alterations in MN fate decision characterized by the conversion of somatic to visceral MNs at the thoracic levels of the spinal cord and of medial to lateral MNs in the motor columns that innervate the limbs. Furthermore, we identify Sip1 (Zeb2) as a novel developmental regulator of visceral MN differentiation. Taken together, these data elucidate a comprehensive model wherein Onecut factors control multiple aspects of MN subtype diversification. They also shed light on the late roles of Isl1 in MN fate decision.

Islet1 regulates establishment of the posterior hindlimb field upstream of the Hand2-Shh morphoregulatory gene network in mouse embryos

How divergent genetic systems regulate a common pathway during the development of two serial structures, forelimbs and hindlimbs, is not well understood. Specifically, HAND2 has been shown to regulate Shh directly to initiate its expression in the posterior margin of the limb mesenchyme. Although the Hand2-Shh morphoregulatory system operates in both the forelimb and hindlimb bud, a recent analysis suggested that its upstream regulation is different in the forelimb and hindlimb bud. A combination of all four Hox9 genes is required for Hand2 expression in the forelimb-forming region; however, it remains elusive what genetic system regulates the Hand2-Shh pathway in the hindlimb-forming region. By conditional inactivation of Islet1 in the hindlimb-forming region using the Hoxb6Cre transgene, we show that Islet1 is required for establishing the posterior hindlimb field, but not the forelimb field, upstream of the Hand2-Shh pathway. Inactivation of Islet1 caused the loss of posterior structures in the distal and proximal regions, specifically in the hindlimb. We found that Hand2 expression was downregulated in the hindlimb field and that Shh expression was severely impaired in the hindlimb bud. In the Hoxb6Cre; Islet1 mutant pelvis, the proximal element that is formed in a Shh-independent manner, displayed complementary defects in comparison with Pitx1(-/-) hindlimbs. This suggests that Islet1 and Pitx1 function in parallel during girdle development in hindlimbs, which is in contrast with the known requirement for Tbx5 in girdle development in forelimbs. Our studies have identified a role for Islet1 in hindlimb-specific development and have revealed Islet1 functions in two distinct processes: regulation upstream of the Hand2-Shh pathway and contributions to girdle development.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Fusion protein Isl1-Lhx3 specifies motor neuron fate by inducing motor neuron genes and concomitantly suppressing the interneuron programs

Combinatorial transcription codes generate the myriad of cell types during development and thus likely provide crucial insights into directed differentiation of stem cells to a specific cell type. The LIM complex composed of Isl1 and Lhx3 directs the specification of spinal motor neurons (MNs) in embryos. Here, we report that Isl1-Lhx3, a LIM-complex mimicking fusion, induces a signature of MN transcriptome and concomitantly suppresses interneuron differentiation programs, thereby serving as a potent and specific inducer of MNs in stem cells. We show that an equimolar ratio of Isl1 and Lhx3 and the LIM domain of Lhx3 are crucial for generating MNs without up-regulating interneuron genes. These led us to design Isl1-Lhx3, which maintains the desirable 1:1 ratio of Isl1 and Lhx3 and the LIM domain of Lhx3. Isl1-Lhx3 drives MN differentiation with high specificity and efficiency in the spinal cord and embryonic stem cells, bypassing the need for sonic hedgehog (Shh). RNA-seq analysis revealed that Isl1-Lhx3 induces the expression of a battery of MN genes that control various functional aspects of MNs, while suppressing key interneuron genes. Our studies uncover a highly efficient method for directed MN generation and MN gene networks. Our results also demonstrate a general strategy of using embryonic transcription complexes for producing specific cell types from stem cells.

Islet1-mediated activation of the β-catenin pathway is necessary for hindlimb initiation in mice

The transcriptional basis of vertebrate limb initiation, which is a well-studied system for the initiation of organogenesis, remains elusive. Specifically, involvement of the β-catenin pathway in limb initiation, as well as its role in hindlimb-specific transcriptional regulation, are under debate. Here, we show that the β-catenin pathway is active in the limb-forming area in mouse embryos. Furthermore, conditional inactivation of β-catenin as well as Islet1, a hindlimb-specific factor, in the lateral plate mesoderm results in a failure to induce hindlimb outgrowth. We further show that Islet1 is required for the nuclear accumulation of β-catenin and hence for activation of the β-catenin pathway, and that the β-catenin pathway maintains Islet1 expression. These two factors influence each other and function upstream of active proliferation of hindlimb progenitors in the lateral plate mesoderm and the expression of a common factor, Fgf10. Our data demonstrate that Islet1 and β-catenin regulate outgrowth and Fgf10-Fgf8 feedback loop formation during vertebrate hindlimb initiation. Our study identifies Islet1 as a hindlimb-specific transcriptional regulator of initiation, and clarifies the controversy regarding the requirement of β-catenin for limb initiation.

Structural basis for partial redundancy in a class of transcription factors, the LIM homeodomain proteins, in neural cell type specification

Combinations of LIM homeodomain proteins form a transcriptional "LIM code" to direct the specification of neural cell types. Two paralogous pairs of LIM homeodomain proteins, LIM homeobox protein 3/4 (Lhx3/Lhx4) and Islet-1/2 (Isl1/Isl2), are expressed in developing ventral motor neurons. Lhx3 and Isl1 interact within a well characterized transcriptional complex that triggers motor neuron development, but it was not known whether Lhx4 and Isl2 could participate in equivalent complexes. We have identified an Lhx3-binding domain (LBD) in Isl2 based on sequence homology with the Isl1(LBD) and show that both Isl2(LBD) and Isl1(LBD) can bind each of Lhx3 and Lhx4. X-ray crystal- and small-angle x-ray scattering-derived solution structures of an Lhx4·Isl2 complex exhibit many similarities with that of Lhx3·Isl1; however, structural differences supported by mutagenic studies reveal differences in the mechanisms of binding. Differences in binding have implications for the mode of exchange of protein partners in transcriptional complexes and indicate a divergence in functions of Lhx3/4 and Isl1/2. The formation of weaker Lhx·Isl complexes would likely be masked by the availability of the other Lhx·Isl complexes in postmitotic motor neurons.

LMO4 functions as a co-activator of neurogenin 2 in the developing cortex

The proneural protein neurogenin 2 (NGN2) is a key transcription factor in regulating both neurogenesis and neuronal radial migration in the embryonic cerebral cortex. However, the co-factors that support the action of NGN2 in the cortex remain unclear. Here, we show that the LIM-only protein LMO4 functions as a novel co-factor of NGN2 in the developing cortex. LMO4 and its binding partner nuclear LIM interactor (NLI/LDB1/CLIM2) interact with NGN2 simultaneously, forming a multi-protein transcription complex. This complex is recruited to the E-box containing enhancers of NGN2-target genes, which regulate various aspects of cortical development, and activates NGN2-mediated transcription. Correspondingly, analysis of Lmo4-null embryos shows that the loss of LMO4 leads to impairments of neuronal differentiation in the cortex. In addition, expression of LMO4 facilitates NGN2-mediated radial migration of cortical neurons in the embryonic cortex. Our results indicate that LMO4 promotes the acquisition of cortical neuronal identities by forming a complex with NGN2 and subsequently activating NGN2-dependent gene expression.

Isl1 is required for multiple aspects of motor neuron development

The LIM homeodomain transcription factor Islet1 (Isl1) is expressed in multiple organs and plays essential roles during embryogenesis. Isl1 is required for the survival and specification of spinal cord motor neurons. Due to early embryonic lethality and loss of motor neurons, the role of Isl1 in other aspects of motor neuron development remains unclear. In this study, we generated Isl1 mutant mouse lines expressing graded doses of Isl1. Our study has revealed essential roles of Isl1 in multiple aspects of motor neuron development, including motor neuron cell body localization, motor column formation and axon growth. In addition, Isl1 is required for survival of cranial ganglia neurons.

Motor and dorsal root ganglion axons serve as choice points for the ipsilateral turning of dI3 axons

The axons of the spinal intersegmental interneurons are projected longitudinally along various funiculi arrayed along the dorsal-ventral axis of the spinal cord. The roof plate and the floor plate have a profound role in patterning their initial axonal trajectory. However, other positional cues may guide the final architecture of interneuron tracks in the spinal cord. To gain more insight into the organization of specific axonal tracks in the spinal cord, we focused on the trajectory pattern of a genetically defined neuronal population, dI3 neurons, in the chick spinal cord. Exploitation of newly characterized enhancer elements allowed specific labeling of dI3 neurons and axons. dI3 axons are projected ipsilaterally along two longitudinal fascicules at the ventral lateral funiculus (VLF) and the dorsal funiculus (DF). dI3 axons change their trajectory plane from the transverse to the longitudinal axis at two novel checkpoints. The axons that elongate at the DF turn at the dorsal root entry zone, along the axons of the dorsal root ganglion (DRG) neurons, and the axons that elongate at the VLF turn along the axons of motor neurons. Loss and gain of function of the Lim-HD protein Isl1 demonstrate that Isl1 is not required for dI3 cell fate. However, Isl1 is sufficient to impose ipsilateral turning along the motor axons when expressed ectopically in the commissural dI1 neurons. The axonal patterning of dI3 neurons, revealed in this study, highlights the role of established axonal cues-the DRG and motor axons-as intermediate guidepost cues for dI3 axons.

SSDP cofactors regulate neural patterning and differentiation of specific axonal projections

The developmental activity of LIM homeodomain transcription factors (LIM-HDs) is critically controlled by LIM domain-interacting cofactors of LIM-HDs (CLIM, also known as NLI or LDB). CLIM cofactors associate with single-stranded DNA binding proteins (SSDPs, also known as SSBPs) thereby recruiting SSDP1 and/or SSDP2 to LIM-HD/CLIM complexes. Although evidence has been presented that SSDPs are important for the activity of specific LIM-HD/CLIM complexes, the developmental roles of SSDPs are unclear. We show that SSDP1a and SSDP1b mRNAs are widely expressed early during zebrafish development with conspicuous expression of SSDP1b in sensory trigeminal and Rohon-Beard neurons. SSDP1 and CLIM immunoreactivity co-localize in these neuronal cell types and in other structures. Over-expression of the N-terminal portion of SSDP1 (N-SSDP1), which contains the CLIM-interaction domain, increases endogenous CLIM protein levels in vivo and impairs the formation of eyes and midbrain-hindbrain boundary. In addition, manipulation of SSDP1 via N-SSDP1 over-expression or SSDP1b knock down impairs trigeminal and Rohon-Beard sensory axon growth. We show that N-SSDP1 is able to partially rescue the inhibition of axon growth induced by a dominant-negative form of CLIM (DN-CLIM). These results reveal specific functions of SSDP in neural patterning and sensory axon growth, in part due to the stabilization of LIM-HD/CLIM complexes.

Loss of LMO4 in the retina leads to reduction of GABAergic amacrine cells and functional deficits

BACKGROUND

LMO4 is a transcription cofactor expressed during retinal development and in amacrine neurons at birth. A previous study in zebrafish reported that morpholino RNA ablation of one of two related genes, LMO4b, increases the size of eyes in embryos. However, the significance of LMO4 in mammalian eye development and function remained unknown since LMO4 null mice die prior to birth.

METHODOLOGY/PRINCIPAL FINDINGS

We observed the presence of a smaller eye and/or coloboma in ∼40% LMO4 null mouse embryos. To investigate the postnatal role of LMO4 in retinal development and function, LMO4 was conditionally ablated in retinal progenitor cells using the Pax6 alpha-enhancer Cre/LMO4flox mice. We found that these mice have fewer Bhlhb5-positive GABAergic amacrine and OFF-cone bipolar cells. The deficit appears to affect the postnatal wave of Bhlhb5+ neurons, suggesting a temporal requirement for LMO4 in retinal neuron development. In contrast, cholinergic and dopaminergic amacrine, rod bipolar and photoreceptor cell numbers were not affected. The selective reduction in these interneurons was accompanied by a functional deficit revealed by electroretinography, with reduced amplitude of b-waves, indicating deficits in the inner nuclear layer of the retina.

CONCLUSIONS/SIGNIFICANCE

Inhibitory GABAergic interneurons play a critical function in controlling retinal image processing, and are important for neural networks in the central nervous system. Our finding of an essential postnatal function of LMO4 in the differentiation of Bhlhb5-expressing inhibitory interneurons in the retina may be a general mechanism whereby LMO4 controls the production of inhibitory interneurons in the nervous system.

LIM domain only 4 protein promotes granulocyte colony-stimulating factor-induced signaling in neurons

Granulocyte colony-stimulating factor (GCSF) is currently in clinical trials to treat neurodegenerative diseases and stroke. Here, we tested whether LIM domain only 4 protein (LMO4), a hypoxia-inducible gene that protects neurons from ischemic injury, could modulate the neuroprotective effect of GCSF. We showed that GCSF treatment acetylates and phosphorylates Stat3, activates expression of a Stat3-dependent anti-apoptotic gene, p27, and increases neuron survival from ischemic injury. LMO4 participates in Stat3 signaling in hepatocytes and associates with histone deacetylase 2 (HDAC2) in cancer cells. In the absence of LMO4, GCSF fails to rescue neurons from ischemic insults. In wild-type neurons, inhibition of HDAC promoted Stat3 acetylation and the antiapoptotic effect of GCSF. In LMO4 null cortical neurons, expression of wild-type but not HDAC-interaction-deficient LMO4 restored GCSF-induced Stat3 acetylation and p27 expression. Thus, our results indicate that LMO4 enhances GCSF-induced Stat3 signaling in neurons, in part by sequestering HDAC.